L .. .. a. . , hmfifimmg _ . $5.”... . E . l n. W" : AWJh éL: a :33. .. .¢ ‘ .5. I . , .thtbb Iqu:. .5 .LS . L...» A 2: iv. . “9512., . ? I 532.... . ‘ t. 13:56.} .9 fig. New; TIM 0.3 1.. I» ‘ . ‘.l 5N... v .au “.03.... .nl? . , an? .3 “Gun! gag»; V ‘ V . . . 4 2.1:, :u .11 o. is “In”; a; £2 ’ll ‘ IHESIB 2 aIlllllllllllllllllllllllllllll a 31293 017141593 This is to certify that the thesis entitled The Composition of Impact Breccias from the Chicxulub Impact Crater, Yucatan Peninsula, Yucatan, Mexico presented by Catherine M. Corrigan has been accepted towards fulfillment of the requirements for Masters degree in Geological Sciences / / g/ Major professor Date ‘5 7 9 0.7639 MSU is an Ajfirmalive Action/Equal Opportunity Institution ——.7 ,, .___— LIBRARY Michigan State University PLACE IN RETURN BOX to remove this checkout from your record. TO AVOID FINES return on or before date due. MTE DUE DATE DUE DATE DUE 1/93 chlRC/DatoDmpGS—p.“ THE COMPOSITION OF IMPACT BRECCIAS FROM TI-IE CHICXULUB IMPACT CRATER, YUCATAN PENINSULA, YUCATAN, MEXICO BY Catherine M. Corrigan A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Geological Sciences 1 998 ABSTRACT THE COMPOSITION OF IMPACT BRECCIAS FROM THE CHICXULUB IMPACT CRATER, YUCATAN PENINSULA, YUCATAN, MEXICO By Catherine M. Corrigan The Chicxulub impact crater, a 65 million year old impact crater centered on the northern coast of the Yucatan peninsula, Mexico, was formed by impact of an asteroid or comet into Tertiary sediments overlying a silicate basement. This study examines the brecciated products of this impact to in order to determine more about their compositions and textures and how they are formed and emplaced. The majority of samples for this project came from the UNAM drill cores 5, 6 and 7. Samples from these cores were analyzed using X- ray Fluorescence, Electron Microscopy, petrologic microscopy and image analysis. Breccias have been divided into two types based on the Ries Crater, Germany: those rich in deep target material containing textures associated with high shock (suevitic breccias), and those rich in CaO and MgO containing textures associated with lower shock temperatures (Bunte-type breccias). The stratigraphy within the crater is inverted, with suevitic breccias overlying Bunte-type breccias. Inverted stratigraphy also exists within the U5 well and is exemplified by changing composition and texture with depth. The existence of suevitic breccias at a radial distance of 125 km and its absence at 150 km radius may place constraints on the crater’s diameter of 250-300 km. For my parents. iii ACKNOWLEDGMENTS I would like to thank my parents, grandparents and family, without whose encouragement and support I would never even have made it this far! Thanks to my committee members, Dr. Buck Sharpton (without whom this project would never have existedl), Dr. Tom Vogel, Dr. Michael Velbel, Dr. Bill Cambray, and Dr. Duncan Sibley, for all of their guidance and support, both academically and financially. I would especially like to thank the members of the Crisis Control Team, Dr. Ben Bussey, Wee Ann Feher, Laurel Pruski, Jeff Sisson, Dave “Pants” Szymanski, Karen Stockstill, Ed Wilson, Nate Mellott, room 114 Natural Science Bldg, all those who “could have been big” and the residents of the ‘97l’98 FOG House. Special thanks to the Houston crew for their support and crazy times: Dr. Jeff Gillis, Leanne Woolley, David Gwynn, Mary Frybarger, Mary Sue Bell, the Martinez family (thanks for letting me stay!) and Yampa, Graham and Abigail Ryder, Mike Zolensky, Burkhard Dressler, Robbie Herrick and Carol Howard (thank for all of your help, Carol! ll). Special thanks, again, to MAV, Mike 2, Buck and Dr. David Black for allowing me the opportunity to become involved in Planetary Geology in the first place. iv TABLE OF CONTENTS LISTOFTABLES .............................................................................................................. iv LISTOFFIGURES ............................................................................................................. v INTRODUCTION ............................................................................................................... 1 Previouswork ........................................................................................................ 1 Purpose .................................................................................................................. 9 Regional Geology ................................................................................................. 9 Methods ............................................................................................................... 13 WHOLE ROCK CHEMICAL COMPOSITIONS OF CHICXULUB IMPACT BRECCIAS ....................................................................................................................... 22 Two types of Chicxulub impact breccias ....................................................... 22 Major and trace element chemical compositions ....................................... 23 Major elements ...................................................................................... 23 Trace elements ...................................................................................... 32 Electron Microprobe Results ............................................................... 41 TEXT URES AND PETROLOGY OF THE CHICXULUB IMPACT BRECCIAS ....................................................................................................................... 42 Hand sample analyses .................................................................................... 43 Petrography ......................................................................................................... 46 Mineralogy ............................................................................................... 46 Textures ................................................................................................... 47 INTERPRETATION AND DISCUSSION ................................................................... 59 Textural variations within drill core samples ................................................ 59 Chemical variations within drill core samples ............................................. 64 CONCLUSIONS ............................................................................................................. 68 APPENDIXA .................................................................................................................. 71 APPENDIXB ................................................................................................................... 88 APPENDIXC .................................................................................................................. 93 APPENDIXD ................................................................................................................ 112 BIBLIOGRAPHY ............................................................................................................ 121 Table 1. Table 2. Table 3. Table 4. Table 5. Table 6. Table 7. Table 8. Table 9. LIST OF TABLES Description of wells, including their sites, depths, surface geologies and other comments (from Sharpton et al., unpublished) ......................................................................................... 16 Detection Limits (ppm) for glass disks for trace elements ........... 18 Relative standard deviations (%) for glass disks for trace elements .................................................................................................. 18 Statistical Data for Major-element Analyses on Two Standards Treated as Unknowns ........................................................................... 19 Modal point count analysis shown in percent .................................. 21 Whole rock XRF major element chemical analyses (normalized to 100%) ....................................................................................................... 89 Whole rock XRF trace element chemical analyses ......................... 91 Comparison of samples from this study (those followed by “C”) and Sharpton et al. (unpublished) ..................................................... 3O Conversions of data from Sharpton et al. (unpublished) from F9203IOF90 ........................................................................................... 30 vi Figure 1. Figure 2. Figure 3. Figure 4. Figure 53-h. Figure 6. Figure 7. Figure 8a-d. Figure 9a-c. Figure 10. LIST OF FIGURES Map of the Gulf of Mexico area, showing the location of the Chicxulub impact crater (from Ocampo et al., 1996) ......................... 2 Close-up map of the Chicxulub impact crater showing the approximate location of crater rings. The center of the crater is represented by the X north of the city of Merida. Well locations are as marked (from Sharpton et al., unpublished) ................................. 3 The lunar Orientale Basin and its concentric rings (from Spudis, 1993) ........................................................................................................... 6 The Mead basin of Venus, which may be the best planetary analogue for the Chicxulub crater (from Sharpton et al., 1997) ....... 7 a) An aerial view of Meteor Crater, Arizona (from Melosh, 1989); b) This figure shows the overturned stratigraphy of Meteor Crater, An'zona (from Melosh, 1989) .................................................................. 8 Topographic map of the Yucatan peninsula, Mexico, showing the possible topographic evidence of a fourth ring corresponding to the Chicxulub crater (from Sharpton et al., 1997) ............................ 14 Ternary diagram of XRF data from Chicxulub impact breccias. Triangles = U5 samples; diamonds = U7 samples; squares = U6 samples; cross = Y6 sample; star = Y4 sample and circle = Y1 sanple ..................................................................................................... 25 Variations of composition with depth in the U5 well ....................... 26 Ternary diagrams comparing the data from this study (a) and that from Sharpton et al. (unpublished) (b): The large ternary diagram (0) shows the fields of data from the Sharpton et al. (unpublished) data overlain on the data from this study showing the two distinct trends that exist in the data .................................................................. 28 Plot of samples from this study against samples of the same wells and depth intervals from the Sharpton et al. (unpublished) data set to show correlations .............................................................. 29 vii Figure 11. Ternary plot of XRF data showing variations in K20 with CaO and SiOz. Triangles = U5 samples; diamonds = U7 samples; squares = U6 samples; cross = Y6 sample; star = Y4 sample and circle=Y1 sample ................................................................................. 33 Figure 12a-b. Plots of XRF data: a) MgO vs. Ca0 + MgO showing the two Figure 13. distinct trends that exist in the data; b) NaZO + K20 vs. SiOz. Symbols for both figures are as follows: triangles = U5 samples; diamonds = U7 samples; squares = U6 samples; cross = Y6 sample; star = Y4 sample and circle = Y1 sample ......................... 35 Plot of XRF data showing trend of decreasing TiOz with increasing Ca0 + MgO. Triangles = U5 samples; diamonds = U7 samples; squares = U6 samples; cross = Y6 sample; star = Y4 sample and circle = Y1 sample .......................................................... 36 Figure 14a-b. a) Plot of XRF data showing the trend of increasing Zr with Figure 15. Figure 16. Figure 17. Figure 18. Figure 19. increasing SiOz. Triangles = U5 samples; diamonds = U7 ; squares = U6 samples; cross = Y6 sample; star = Y4 sample and circle = Y1 sample; b) Variation of Zr with depth in the U5 well..38 Plot of XRF data showing trend of increasing Sr with increasing CaO. Triangles = U5 samples; diamonds = U7 samples; squares = U6 samples; cross = Y6 sample; star = Y4 sample and circle=Y1 sample ................................................................................. 39 Plot of XRF data showing trend of increasing Rb with increasing K20. Triangles = U5 samples; diamonds = U7 samples; squares = U6 samples; cross = Y6 sample; star = Y4 sample and circle = Y1 sample ............................................................................................... 40 Images of drill cores from the U5 well and the subunits designated by Sharpton et al. (unpublished) ................................... 45 Image of U5-450-b, a suevitic breccia, showing heterogeneous fine grained matrix of suevites, anhydrite clasts and crystals, rimmed clasts and clasts altered to clay minerals. Image was taken from thin section under crossed polars at 2.5x power. Field of view=3.5x2.5 mm ........................................................................... 49 Image of U5-450-b, a suevitic breccia, showing heterogeneous matrix, shocked quartz with planar deformation features, rimmed clasts, and clasts altered to clay minerals. Image was taken from thin section under uncrossed polars at 2.5x power. Field of view = 3.5x2.5mm ............................................................................................ 49 viii Figure 20. Figure 21. Figure 22. Figure 23. Figure 24. Figure 25. Figure 26. Figure 27. Figure 28. Figure 29. Image of U5-450-b, a suevitic breccia, showing a large , rimmed, melt clast exhibiting flow structures. Other rimmed clasts are also seen. Image was taken from thin section under uncrossed polars at 2.5x power. Field of view = 3.5 x 2.5 mm ...................................... 52 Image of U5-450-b, a suevitic breccia, showing a larger melt clast exhibiting flow textures. Image was taken from thin section under crossed polars at 2.5x power. Field of view = 3.5 x 2.5 mm ......... 52 Image of U7-658-b, a Bunte—type breccia, showing different types of matrix, fossils, anhydrite crystals, and clasts altered to clay minerals. Image was taken from thin section under crossed polars at 2.5x power. Field of view = 3.5 x 2.5 mm ......................... 54 Image of U7-658-b, a Bunte-type breccia, showing different types of matrix, anhydrite clasts, individual anhydrite crystals, and clasts altered to clay minerals. Image was taken from thin section under crossed polars at 2.5x power. Field of view = 3.5 x 2.5 mm ......... 54 Image of U7-658-b, a Bunte-type breccia, showing fossils and typical clasts, including anhydrite and calcite. Image was taken from thin section under crossed polars at 2.5x power. Field of view=3.5x2.5 mm ............................................................................... 56 Image of U7-658-b, a Bunte-type breccia, showing homogeneous matrix next to a large anhydrite clast, which exhibits large, individual, bladed crystals. Image was taken from thin section under crossed polars at 2.5x power. Field of view = 3.5 x 2.5 mm ............................................................................................................ 56 Image of U7—658-b, a Bunte-type breccia, showing separate foraminifera fossils as well as a fossiliferous matrix. Anhydrite clasts and crystals also exist. Image was taken from thin section under crossed polars at 6.5x power. Field of view = 1.5 x 1.0 mm ............................................................................................................ 58 Image of U7-658-b, a Bunte-type breccia, showing foraminifera fossils, anhydrite clasts and crystals, a silicate clast and clasts altered to clay minerals. Image was taken from thin section under crossed polars at 2.5x power. Field of view = 3.5 x 2.5 mm ......... 58 Similar fossils found in KIT impact deposits from Belize (from Ocampo etal., 1996) ............................................................................. 60 Photograph of the hand sample Y6-1196 (scale shown) .............. 73 ix Figure 30. Figure 31. Figure 32. Figure 33. Figure 34. Figure 35. Figure 36. Figure 37. Figure 38. Figure 39. Figure 40. Figure 41. Figure 42. Figure 43. Figure 44. Figure 45. Figure 46. Figure 47. Photograph of the hand sample U5-338 (scale shown) ................ 73 Photograph of the hand sample U5-360 (scale shown) ................ 75 Photograph of the hand sample U5—390 (scale shown) ................ 75 Photograph of the hand sample U5-483 (scale shown) ................ 77 Photograph of the hand sample U5-489 (scale shown) ................ 77 Photograph of the hand sample U7-318 (scale shown) ................ 79 Photograph of the hand sample U7-658 (scale shown) ................ 79 Photograph of the hand sample U6—302 (scale shown) ................ 81 Photograph of the hand sample U6-508 (scale shown) ................ 81 Thin section views (crossed and uncrossed polars) of the thin section U5-360-b. Field of view = ~ 40 x 25 mm ............................. 95 Thin section views (crossed and uncrossed polars) of the thin section U5-365-b. Field of view = ~ 40 x 25 mm ............................. 97 Thin section views (crossed and uncrossed polars) of the thin section U5-383-2a. Field of view = ~ 40 x 25 mm ........................... 99 Thin section views (crossed and uncrossed polars) of the thin section U5-400-b. Field of view = ~ 40 x 25 mm ........................... 101 Thin section views (crossed and uncrossed polars) of the thin section U5-420-b. Field of view = ~ 40 x 25 mm ........................... 103 Thin section views (crossed and uncrossed polars) of the thin section U5-450-b. Field of view = ~ 40 x 25 mm ........................... 105 Thin section views (crossed and uncrossed polars) of the thin section U5-483-a. Field of view = ~ 40 x 25 mm ........................... 107 Thin section views (crossed and uncrossed polars) of the thin section U6-302-b. Field of view = ~ 40 x 25 mm ........................... 109 Thin section views (crossed and uncrossed polars) of the thin section U7-335-b. Field of view = ~ 40 x 25 mm ........................... 111 Figure 48. Figure 49. Figure 50. Figure 51. Elemental maps corresponding to a breccia boundary in the upper portion of the U5-420-b thin section (Figure 43). Colored scale represents counts. Analyzed element is shown at the bottom ofeach frame .......................................................................... 114 Elemental maps corresponding to a breccia boundary in the upper portion of the U5-420-b thin section (Figure 43). Colored scale represents counts. Analyzed element is shown at the bottom ofeach frame .......................................................................... 116 Elemental maps corresponding to a breccia boundary in the upper portion of the U5-420-b thin section (Figure 43). Colored scale represents counts. Analyzed element is shown at the bottom ofeach frame .......................................................................... 118 Elemental maps corresponding to a breccia boundary in the upper portion of the U5-420-b thin section (Figure 43). Colored scale represents counts. Analyzed element is shown at the bottom ofeach frame .......................................................................... 120 INTRODUCTION The 65 Ma Chicxulub Crater, Yucatan peninsula, Mexico (Figure 1,2), with a diameter of at least 200 km (and possibly over 300 km) is one of the largest recognized impact basins on the earth. Compared to the other two large, known impact structures, the Sudbury (~1.85 Ga, 140 km diameter) and Vredefort (~2 Ga, 160 km in diameter) structures, it is geologically young and relatively unmodified, making it an ideal area for study. This study focuses on breccias deposited on the flanks of the Chicxulub impact crater. This material seems to land within approximately 2 crater radii from the center of crater, including within the transient crater. Any material landing outside of this transient crater is termed ejecta. The most important question addressed in this study is “how does this ejected material vary in composition and structure with distance from the craters center?” Other relevant questions include: 1) What is the origin of this material? 2) How deep below the surface does the material originate? 3) How is it ejected and emplaced along the flanks of the crater? These questions will be addressed by evaluation of the nature and composition of impact breccias from three cores drilled by the Universidad Nacional Autonoma de Mexico (UNAM). Previous Work Because the large Chicxulub structure is buried beneath sediments and lacks topographic expression, it was discovered by gravity and magnetic N 90W 0 200 km CHICXULUB ‘ ' Q CRATER CENTER I —- - \ C/:/‘—~\\ CUBA ’ / "‘ \ canons RING / I b// x \ a \ \/ I l I v HOLBOX ‘ :,' FRACTURE zone: {$01 CARIBBEAN SEA r. r ‘1‘ : :/ ALBION vs I ISLAND \ ‘ 6" 0’31“ p 0 G «r I F ------ J “o// / z, i Z Figure 1. Map of the Gulf of Mexico area, showing the location of the Chicxulub impact crater (from Ocampo et al., 1996). I0 22°30' 22° 21 °- 20%- ‘I 9°30' 90°30' 90° 89° 88° Figure 2. Close-up map of the Chicxulub impact crater showing the approximate location of crater rings. The center of the crater is represented by the X north of the city of Merida. Well locations are as marked (from Sharpton et al., unpublished). methods. In the early 1950’s, Petroleos Mexicanos (Pemex) drilled test wells into the structure. Unusual breccias and crystalline silicate rocks were intercepted in three of these wells. These breccias were originally interpreted to be of volcanic origin, but now most workers agree that they are ejecta deposits created by the collision of an extraterrestrial body (an asteroid, meteorite or comet) with the Earth (Hildebrand et al., 1991; Sharpton et al., 1992) The Chicxulub basin has been linked to the Cretaceous / Tertiary (KIT) boundary by many methods. The 4oAr-agAr ages of the melt rocks taken from the center of the crater reflect a melting/recrystallization age of 65 my (lzett et al., 1991; Swisher et al., 1992; Sharpton et al., 1992). Magnetic properties indicate that the basin was formed during a geomagnetic reversal period, which is consistent with the geomagnetics of the KIT boundary (magnetochron 29R; Sharpton et al., 1992; Urrutia-Fucugauchi et al., 1994). Melt rock compositions from the crater itself are the same as impact glass spherules in Haiti and NE Mexico, in terms of their isotopic properties, trace elements and major elements (Koeberl et al., 1994); and fragments of unmelted basement in breccia samples reveal a similar composition to shocked lithic fragments found at KIT sites worldwide (Sharpton et al., 1990). These shocked lithic fragments are believed to be impact induced. U-Pb determinations on zircons indicate that the age of the Chicxulub basement matches the age (545 Ma) of unmelted shock debris at the KIT boundary in Haiti and the western interior of North America (Krogh et al., 1993a, b; Bohor et al., 1993). Craters that have three or more concentric rings (topographic or structural) are a class of impact features on planetary surfaces termed “multi- ringed impact basins.” Examples of these include the Orientale Basin (Figure 3), which is located on the far side of the earth’s moon (Spudis, 1993) and the Mead basin (Figure 4) of Venus (270 km diameter, located 12.5°N; 57.0°E), which may be the best planetary analog of the Chicxulub crater (Sharpton et al., 1997). Other than the Chicxulub crater, there are only two craters exceeding 150 km on Earth. These are the Sudbury structure in Ontario, Canada (Dressler et al., 1994), and the Vredefort structure in South Africa (Reimold, 1994), which are ~1850 Ma (Krogh et al., 1984) and ~2 Ma (Reimold, 1994), respectively. In contrast, Chicxulub is young and structurally unmodified. This gives us an excellent opportunity to study a young, unmodified feature of this magnitude. Very little is presently known about these features and the highly energetic collisions that form them. There is mounting evidence that the events creating such large features have the potential to bring about geological, biological and environmental change of global proportions (Sharpton et al., 1996) resulting in, for example, major extinction events such as that at near the Cretaceous/Tertiary boundary. Ejecta studies have been conducted on other craters, and, though smaller in diameter, these can be of use in this investigation. Meteor Crater, Arizona, USA (Figure 5a), and the Ries Crater, Nordlingen, Germany, both approximately 1 km in diameter, have been examined in great detail. The ejected materials nearest to the rim of both craters possess an inverted The lunar Orientale Basin and its concentric rings (from Spudis, Figure 3. ). 1993 Figure 4. The Mead basin of Venus, which may be the best planetary analogue for the Chicxulub crater (from Sharpton et al., 1997). W Kmbabdebns cwenwnedflap Moenkopi debris of Moenkopi Moenkopi lm. Airlall breccia Lake beds 0- .m- "unto-o _~<~u— ——_—/‘. .'-. Iflflfllfllllbw Coconino ss -," -a°° . I O £9,/ \\°~‘: ’ o ’, H4_L__i_i 0 500m 500m Figure 5a-b. a) An aerial view of Meteor Crater, Arizona (from Melosh, 1989); b) This figures shows the overturned stratigraphy of Meteor Crater, Arizona (from Melosh, 1989). stratigraphy, where an over-tumed flap of older, deeper target material lies on top of younger, shallower target material (Melosh, 1989)(Figure 5b). Purpose This research proposes to closely examine the products of the Chicxulub impact by taking a close look at the chemistries and textures of impact breccias recovered from three wells drilled into the feature. Knowing something about the pre-existing stratigraphy of the area in question, it is possible to reconstruct, in a simplified manner, how that material may have been redistributed by the impact 65 million years ago. It is not known exactly what the compositions of the rocks within the structure are, or how they vary with distances from the center of the crater. Studying the ejecta from the Chicxulub crater in this and concurrent studies (Sharpton et al., unpublished) will aid greatly in the future understanding of this phenomenon by characterization not only of the breccias themselves, but of their vertical and lateral spatial variations around the crater. Regional Geology The Yucatan peninsula, has a complicated geologic history. It is now believed that before the late Paleozoic break-up of Pangea, the Yucatan block was positioned somewhere between the North American plate and the South American plate (Dietz and Holden, 1970), and was somewhat stretched in a north-northwest! south-southeast direction during the breakup of this supercontinent (Salvador, 1987, 1991). The original location of the peninsula is also unclear. Buffler and Sawyer (1985) indicate that it may have once been part of the South American plate. Salvador (1991) does not believe that any affinity exists between the Yucatan and the South American plate. He suggests, instead, a relation with the northeastern portion of Mexico, indicating that both have metamorphic sequences overlain by unmetamorphosed upper Paleozoic rocks intruded by Permian-Triassic granitic plutons. Salvador (1991) also notes that the possibility of the Yucatan having originally been a part of the North American continent still exists. During the late Triassic to middle Jurassic, the Yucatan peninsula began to move away from the North American continent in the early rift stages of the Gulf of Mexico’s formation (Pindell, 1985; Buffler and Sawyer, 1985). During this separation the area encountered stretching of anywhere from 470 km (Buffler and Sawyer, 1985) to 520 km (Pindell, 1985). This crustal extension (which, according to Salvador (1991 ), may represent 50-55% of the displacement of the Yucatan block) was accompanied by rotation of anywhere from 10 degrees (Salvador, 1987) to 50 degrees (Ross and Scotese, 1988) in a counterclockwise direction to 25 degrees in a clockwise direction (Hall, 1982). These values are dependent on where the authors place the Yucatan block during late Paleozoic to early Mesozoic time. Movement during this time (late Triassic to middle Jurassic) most likely took place along transform boundaries, though where these were located is an unresolved issue. Salvador (1987) proposed that there existed two major 10 parallel rift zones trending N-NEIS-SW, one along the east coast of present-day Mexico and the other running the length of the modem-day Florida Escarpment. Pindell (1985) proposed a western transform fault, called the “Tamaulipas- Golden Lane-Chiapas” transform fault. This fault had a SW trend along the Chiapas massif. Buffler and Sawyer (1985), though not indicating a present- day location of the faults, suggested a western transform with a smaller transform in the southeastern Gulf of Mexico. The movement along these faults and the accompanying rotations of the Yucatan block have Buffler and Sawyer (1985) and Salvador (1987) placing the block against the western Florida Platform during the existence of Pangea, and Pindell (1985) placing it farther west, against Texas and Louisiana. During the early to late Jurassic, the Yucatan block had reached its current position east of southern Mexico where it remained an emergent landform, and was most likely still connected to the Florida Platform (Salvador, 1991) until the late Kimmeridgian. During the earliest Cretaceous, a major transgression occurred, and by the late Aptian or early Albian, the peninsula was covered by shallow water leading to the deposition of carbonates and evaporites in the area (Salvador, 1991 ). During the Cenomanian, the margin of the peninsula facing the deep portions of the Gulf of Mexico basin was dominated by reefs of rudist corals. During the mid-Cenomanian, a major unconforrnity exists along the margins of the platform, indicating a major lowering of sea level. By the end of the Cenomanian, the water level transgressed once again, but the platform edges were now bordered by open 11 marine shelves depositing carbonates, instead of corals and organic banks (Salvador, 1991). At the end of the Cretaceous, the Yucatan peninsula was struck by an asteroid (or comet) of at least 10 km in diameter, and possibly as much as 15 km diameter, (Sharpton et al., 1996), throwing target material (ejecta) hundreds of kilometers laterally from the impact site. At the time of impact, three major compositional components of the target were in existence: 1) Upper Cretaceous limestones and dolostones, 2) Lower Cretaceous dolomites and anhydrites 3) Silicate basement (Sharpton, 1997). The region was covered by a shallow sea at the time of impact, possibly shielding the resulting basin, somewhat, from further erosion after formation. However, wave action has most likely intensely modified the high-standing rim and ejecta facies along the basin’s flanks (Sharpton et al., 1996). The center of the crater is located approximately 30 km north of Mérida, the capital city of Yucatan, Mexico (Figure 2), leaving the northern half of the crater buried not only by sediment, but by water. Since the end of the Cretaceous, the Yucatan peninsula has remained a stable area of shallow water carbonate and evaporite deposition (Salvador, 1991). The entire basin is now buried by up to a kilometer of Cenozoic sediments, which include limestones, dolostones, calcareous dolostones, and both calcareous and dolomitic shales (Sharpton, 1997). The only easily recognizable topographic indicators of the crater include an arc of water-filled sinkholes (termed cenotes) located ~80 km from the 12 crater’s center (Figure 2), that lie across the northwest edge of the Yucatan peninsula. There is a possibility that topographic evidence of a crater ring is in existence (Sharpton, 1997)(Figure 6). This ring, which would be the basin’s fourth structural ring, is located farther out from the center of the crater than the ring of cenotes, and if verified, will be the only surface effect of a structural ring known to exist from the Chicxulub crater. Methods The study of impact craters on Earth is extremely important to the understanding of the impact processes that have operated in our Solar System since its birth. Though small scale craters like Meteor Crater in Arizona and the Ries Crater in Germany are well understood, the mechanics of large impact events remain unclear, despite much research in the field. Studying the Chicxulub impact crater may better enable us to constrain the events that take place during large scale impact, especially that of ejecta origin and subsequent emplacement. On a smaller scale, little is known about the actual composition of the rocks (breccias) in question. This study consists of a combination of numerous analytical methods. Breccia samples from the Chicxulub impact structure, obtained from logged cores at various distances from the crater's center, were analyzed to determine major and trace element compositions. This was done using X-ray fluorescence spectrometry (XRF) conducted at Michigan State University, Electron Microprobe (EMP) conducted at the NASA Johnson Space Center, and 13 Figure 6. Topographic map of the Yucatan peninsula, Mexico, showing the possible topographic evidence of a fourth ring corresponding to the Chicxulub crater (from Sharpton et al., 1997). image processing techniques for petrographic thin sections developed for this project conducted both at the Lunar and Planetary Institute and Michigan State University. The samples used in this study originate from a combination of cores drilled by the Universidad Nacional Autonoma de Mexico (UNAM) and the Pemex Corporation, Mexico City, Mexico (the “Yucatan” wells). Samples were obtained from the cores listed in (Table 1), focusing primarily on three wells (UNAM 5, 6 and 7) drilled into the southern flank of the basin. These wells lie at distances of ~110 km south, ~150 km south-southeast and ~125 km south of the center of the crater, respectively (Figure 2). These samples will be discussed from here on in the order of their increasing distance from the center of the crater (i.e., U5, U7, U6). The samples from the Yucatan wells, however, will always be discussed after the UNAM wells, as they are not the main focus of this study. Additional samples were taken from the remaining cores as availability allowed (for example, Pemex core samples of breccias from the Y6 well, and basement rock samples from the Pemex cores Y1 and Y2). The sampling strategy undertaken involved the selection of samples from the above listed cores in a manner that focused mainly on the U5 well. This afforded us the ability to assess, in detail, the variations within this well, and make general comparisons with the chemical variations in samples from other wells, particularly with respect to variations with radial distance from the center of the crater. Selection efforts focused primarily on matrix-rich breccias lacking clasts of significant size (>~1-2 cm)(Appendix A), as avoidance of large 15 Table 1. Description of wells, including their sites, depths, surface geologies and other comments (from Sharpton et al., unpubfished) Well Site Depth, Surface Objectives, Comments 09M UNAM lzsoutheastot‘centeroninnerrnost 200mTD. attemptedtocore-GOOmtounderstand prominent gravity ring (Sharpton er al.. I993); Boccne~Oligocene basin filling evolution near center. distance from basin center: ~50 km). See carbonates: 45% Drilling terminated when loss of Figure C I . core recovery. circulation and continued caving could not be remedied UNAM 2.; ~85 tun southeast of center on 'ring of 560 m TD. Eocene assess origin of ring of motes and link cenotes' (Pope er al.. I99l). Corresponds to carbonates; ~85!» with crater. Constrain basin size, depth proposed rim in Hildebrand er al. (I991) and core recovery. and infiiling history: evaluate proposed Pilkington and Hildebrand (I994) model. crater models and kill mechanisms; Corresponds approximately to modified transient document impact breccia characteristics at crater boundary in Sharpton er al. (I993) model. this site. TD. did not reach crater at this See Figure CI . site. ‘ UNAM 3; east of center just inside gravity ring 4 70 m TD. Eocene . lost circulation. caving: terminated coring (Figure CI). proposed rim of Sharpton er of. carbonates: 45% (I993). core recovery. UNAM 4; just south of U3 (figure CI). on the 125 m TD. lost circulation. caving; terminated coring proposed rim of Sharpton at at. (I993). Eocene carbonates. 40% core recovery UNAM S; ~I I0 km south of center on ring- 3 at 503 m TD. impact constrain basin size. depth. and infitiing Santa Elena (Figure CI). Proposed rim in melt encountered at history: evaluate crater models. kill Sharpton er al. (I993). 330 m; suevitic mechanisms). and impact breccia crystalline rock characteristics at this site clast breccia at ' 46" rum-- UNAM 6; ~ISO Irrn 888 of center on ring 4 near ~702 m T.D.: ditto ‘ Peto. carbonate- anhydrite-rich breccia to TD UNAM 7; ~125 km south of center between rings ~700 m TD. ditto 3and4nearTekax. . Suevitebrecciaand underlying carbonate- anhydrite-rich impact breccia l6 clasts during sampling reduced the chances of encountering a compositional bias error during the determination of chemical composition. Twenty-two samples were selected for chemical analysis. Detailed descriptions of each hand sample can be found in Appendix A. Photographs of representative hand samples of breccias from each well can also be found in Appendix A. X-ray fluorescence (XRF) spectrometry was conducted to determine the concentrations of major elements and selected trace elements in the breccias. This process involved measuring 1.0 gram splits of very finely ground rock powder, to which was added 9.0 grams of Lithium Tetraborate (for flux) and 0.16 grams of Ammonium Nitrate (to ensure oxidation). The materials were mixed together within platinum crucibles and then heated until they had melted. The melt was agitated for at least 20 minutes to ensure complete homogenization. Upon completion of this process, these melts were then poured into platinum molds and quenched, forming glass disks. The twenty-two samples were analyzed for both major and trace element compositions in the XRF spectrometer at Michigan State University, East Lansing, Michigan, USA. The detection limits (Table 2) and standard deviations (Table 3) for the trace elements as well as the trimmed mean and standard deviations based on 16 repetitions for each major element are those reported in Table 2 of Mills et al. (1997) and Table 4 here. The image processing technique developed (at the Lunar and Planetary Institute (LPI), Houston, Texas, USA) for the purpose of this project involved scanning petrographic thin sections into a Kodak photographic slide scanner 17 Table 2: Detection Limits (ppm) for glass disks for trace elements. Element Glass Disks Ba 100 ppm La 48 Cr 63 Ni 25 Zn 14 Rb 13 Sr 12 Y 14 Nb 15 Zr 14 Cu 40 Table 3: Relative standard deviations (%) for glass disks for trace elements. Element Ba 10% La Cr Ni Rb Sr Y Nb Zr Cu ODQOODCDQODAOIS 18 05655 00 “02.02... 02.09.05 0;... co 000292. EoEo_o.._o_m_2 .8 900 62.0305 6 032. 2.0 000.0 43.0 0on“. 000 0.00.0 0.0.0 00. 00.0 05.0 55 0002 00.00 30.0 5.2 08 «0.0 05.0 00.0 o 2 5.0 400.0 00.0 002 0t? 0000 00.00 0%.. 5.9 R00 «0.00 .92 Be 0000 00.. Nos 00.00 02.0 2.00 N00 0020 .50 .20 8.2 0006 6T5 0-; wow: 00.0 08.0 00.0 moan. 00... 000.0 {a ole. _R..0 .060 00.0 082 _.00.0 90.0 00.0 000 00.0 :00 00.0 002 10:0 08.0 00.0 00% :9 000.0 00.9 00“. 40.0. :00 2.2 8.2 00.0. 000.0 00.0 No: :00 5.0 8.40 60 0020 30.20 gal 026 sans vmom wow: 19 at 1200 dots-per-inch (dpi) resolution in sleeves of polarizing film, both crossed and uncrossed. Each image was then reduced to 900 (dpi) resolution to allow easier handling and manipulation of the images. Each image was then printed on a on a typical desktop 720-dpi printer and for future analysis. Analysis consisted of point counting: A gridded piece of mylar (with 0.25 inch grid cells) was placed over each image descriptions were recorded of materials located beneath each of the grid intersections. Point counting results are summarized in Table 5. In conjunction with standard petrographic analysis conducted both at the LPl and at Michigan State University to determine the different types of minerals, matrix and clasts present, this technique was used to aid in the determination of the distribution of different target materials as clasts during the emplacement of ejecta upon impact, as well as in the determination of the proportions of melt to clasts to matrix involved in the different types of breccias. The method of choosing thin sections for analysis involved the selection of representative samples of the rocks in each well. Two sections are excluded from this rule (U5-383-2a and U5-420-b), in that they were chosen because of the larger clasts within them for Electron Microprobe analysis. These were point counted only for comparative reasons. Electron Microprobe analysis involved obtaining spot analyses of individual grains as points, as well as construction of elemental maps of an entire region of a thin section to obtain a better view of elemental variation within the selected area. Microprobe work was conducted at NASA’s Johnson Space Center, located in Houston, Texas, USA on a Cameca SX-100 Electron 20 00.0 00.0 .00 00.0 00.0 00.0 00.0 00.0 00.0 zaozxz: 00.0 00.0 00.0 00.0 00.0 00.0 00.0 00.0 00.0 000900 00.0 00.0 00.0 00.0 00.0 00.0 00.0 00.0 00.0 .050 0:09.12... 00.0 00.0 00.0 00.0 4.00 00.0 00.00 00.0 00.0 .050 0203.0 00.0 00.0 00.0 00.0 00.0 00.0 00.0 00.0 00. .0 E002... 502.2 00.00 00.00 00. .0 00.00 00.00 .000 00.0. . 00.00 00.3 E0022 0002.2 00.0 .000 00. .0 00.00 00.00 00.00 .00 00.0. 0.0 :02 000053502 00.0 00.0 00.0 00.0 00.0 00.0 00.0 00.0 0.0 20 20.550 00. .0 00.0 00.0 00.0 00.0 00.0 00.0 00.0 00.0 200006 00.0 00.0 00.0 00.0 00.0 00.0 00.0 00.0 00.0 0:09.15 00.0 00.0 0.00 00.. 00.0 00.. 00.0 00.. 00.0 0:02.50 00.0 00.. 0.... 00.0 00.. 00.0 0.0. 00.0 00.0 02200020 00.0 00.0 00.. 00.0 00.. 00.0 00.0 00.. 00.. E> n 00.0 ”0.0800 m> n 0090 ”003.000 03 u 00.030 00.0800 0: .i. 00.5806 ”00.9000 03 u 00.9.05. 006005 .0095 023820 E0... 0.0.0 “Ex .0 E0506 E050... N 059”. 000 25 uidep utdep :03 mb 0:. E 5%.. 53. 22:08:80 00 0:630:00» 0.00 05mm... 002 000 00%. or m 0 cm 00 ow om 0 com _ $ ‘1 ‘ S q I1 f 1 u JJ 4 4 4 4 8.. 00¢ r i i « a 4 4 4 4 it 0 00m F . 0 com . o .2 N90 om or o on 00 om 0v om ON or o 08 d 4 d 4 d 1 GOA-W « 44 4 4 4 4 4 8.. a... 4 4 . 4 4 4 4 0 000 . i 000 tridep uidep 26 Figure 9a-c. Ternary diagrams comparing the data from this study (a) and that from Sharpton et al. (unpublished) (b): The large ternary diagram (c) shows the fields of data from the Sharpton et al. (unpublished) data overlain on the data from this study showing the two distinct trends that exist in the data. 27 000 000 05 o0: cm 0.000 05 .0 00.9000 05000 >030 02. E9. 00_0_E00 .o .20. .00 050E ow 009.50 00 0:20.008 26:0 9. .00 0.00 60:03:00.3 ._0 .0 099005 05 EB. 0.0295 5000 0:0 0:0; 00 om I I e a 1916 uoidieqs I 0 co om 29 00.0 800.. 000 000.0: 0.0 0000.. ..0 0%: 00.0 0000.. 00.0 000.5 00.. 0000.. 20 0.0.5 00.0 0000.. 3.00. 00.10: Iim00 0000.. «MT 000.0: 00”. 0. .3 0.00. 8.22.08 68“. 0. .3 0.0.000 .000 2 n08“. 52. 620.3350 ._0 .0 8.0.20 ea. 0.00 .o 05.8980 0 0.00.. 00.0 30 00.0 00.0 .00 00.00 00.0 00.0 00.0 00... 0095 0.0 00.0 00.0 ..0.. .00 0... .0 .0. 8.0 00.. 00.0. 0000.5 8.0 .00 .00 00.. 3... 0.00 00.0 3.0 0.0 0.00 0.0.5 000 000 00.0 0.... 00.. 00.0.. 00.0. 00.0 .00 0.00 000.5 00.0 . 00.0 8.0 . 00.0 3.0 00.00 00.0. 8.0 0.0 00.0 00000 00.0 00.0 00.0 0.0 00.0 00.00 00.00 00.0 00.0 0.0 0000.0: 8.0 00.0 8.0 0.0 .00 3.... 00.0. 00.0 00.0 .00 000.00 00.. .00 00.0 ....0 00.0 00.00 0.000 00.0 00.0 0.... 0.0000: 00.0 00.0 00.0 00.. .00 0000 00.0 00.0 0... 00.0.. 000.0: 00.0 000 00.0 00.. 0.0 00.00 00.0 .00 00.0 00.00 000.00 00.0 00.0 00.0 00.0 00.0 00.0. 00.0 00.0 00.0 00.00 00000 00.0 0.0 0.0 00.0 .00 .000 00.0 00.0 00... 00.0.. 0.0000: 000 00¢ 0000 00. 0002 000 00.2 00.2 0002 000 0.0500 ..0000__0803 ._0 .0 099000 000 0.0.. .3 0030.0. 0850 >02... 05 52. 00.900... .0 08009.00 0 030. 30 The major element trends in the 4 analyzed samples from U7 exhibit a wide range of values. The normalized SiOz and Al203 values vary from ~12 to ~35 and ~2 to 8 wt.% respectively, and the CaO and MgO values vary from 37 to 81 and 2 to 14 wt.% respectively. The variation in these abundances show a break in the types of breccias that exist (Figure 7). This is verified by the XRF data from Sharpton et al. (unpublished)(Figure 9). The data presented in Appendix B suggests that both the suevitic and Bunte type breccias exist in the U7 well. The major element concentrations in the two U6 samples analyzed exhibit very similar proportions of elemental abundances. Si02 and Al203 are low (3 and 4 wt. %) (Figure 7) and when compared with the XRF data from Sharpton et al. (unpublished) it seems that they remain low throughout the depth of the core (302 to 508 m)(Figure 9). CaO (with values of 3 and 65 wt. %) and MgO (with values of ~33 and 28 wt.%) are abundant and fairly constant and remain so throughout the core (Table 6, Appendix B). It appears from the data presented here that the U6 breccias are similar to the Bunte-type breccias of the Ries Crater. The three samples analyzed from the Yucatan wells Y1, Y2 and Y6 all exhibit very high proportions of Si02 and lower abundances of Al203, CaO, and MgO. These data are also supported by those from Sharpton et al. (unpublished)(Figure 9). On a ternary plot with SiOz, MgO and CaO as the end members (Figure 7), most of the U5 samples fall in the high SiOz, low MgO, low CaO range, with 31 a few of the deeper samples falling in a range of almost 50% CaO. These samples are from the bottom of the well (Figure 8). One sample from the well falls in the very low SiOz, very high CaO range. This sample is thought to be a representative of a carbonate clast within the silicate matrix. On the same plot, samples from the U7 well are divided into two areas; those samples that fall near the deeper samples from the U5 well, and one that falls near the high CaO sample from U5. The U6 samples plot in the very low Si02 range, with intermediate amounts of both CaO and MgO. General trends from these data follow. On a ternary plot of K20, SiOz and C30 (Figure 10), only those samples from the upper portion of the U5 well show an increase in K20 concentrations. Figure 11-a shows a plot of MgO vs. CaO+MgO. This plot shows two trends, a low CaO+MgO trend and a high CaO+MgO trend. Notice that samples from U7 fall on both trends. NaZO and K20 abundances tend to increase with increasing SiOz as seen in Figure 11-b. TiOz exhibits an increase in abundance with increasing SiOz (Table 6), and a decrease in abundance with CaO+MgO (Figure 13). Trace elements The trace element trends from the Chicxulub impact breccias are shown in Table 7 of Appendix B. Apparent trends are as follows. Figure 14—b shows Zr decreasing with depth in the U5 well, while Figure 14—a shows Zr increasing in abundance with increasing SiOz. Sr shows an increase in abundance with 32 0.0.000 ; .i. 0.2.0 000 0.0.000 v> n 00.0 ”0.9000 m> n 0000 ”00.0500 0: u 00.0000 00.0800 0: n 00580.0 ”00.0800 0: u 00.0000... .005 0:0 000 5.2. 00v. 0. 0000009 00.3000 000 “Ex .0 6.0 .0050... .3 050.“. 000 05 33 Figure 12a-b. Plots of XRF data: a) MgO vs. Ca0 + MgO showing the two distinct trends that exist in the data; b) NaZO + K20 vs. SiOz. Symbols for both figures are as follows: triangles = U5 samples; diamonds = U7 samples; squares = U6 samples; cross = Y6 sample; star = Y4 sample and circle = Y1 sample. 34 r l l 1 I r r I T . a I 30 — a I M90 20 . _ . O 10 - fi 9 _ A “ A* ‘ 0 # I l l l I l l J. I 0 20 40 60 100 CaO+MgO 20 I T l l r l A N820+K20 10 ' -i ‘ ‘9‘ t ‘A 6 O M 0 L; f l l t I l 0 10 20 30 40 50 60 70 SiO2 3S .0_0E00 .> u 0.9.0 000 0.0500 v> n 00.0 0.0.000 o> .i. 0000 ”00.00000 03 u 00.00.00 00.00.00 03 n 0000800 00.0800 0: H 00.90.... .005. + 000 00.000000. 0...... 00:. 00.000000 .0 0:0: 00.2.9.0 000 “EX .0 .05 .9. 059”. 092 + ONO 00? cm om 0? ON 0 lg . _ . _ a 0 _ . . 0.0 i i Nd I .m. L .. e 4 .. 0.0 l fi 1 0o: r i 0.0 i i md . r . _ _ _ _ _ . fin 0.0 36 Figure 14a-b. a) Plot of XRF data showing the trend of increasing Zr with increasing SiOz. Triangles = U5 samples; diamonds = U7 ; squares = U6 samples; cross = Y6 sample; star = Y4 sample and circle = Y1 sample; b) Variation of Zr with depth in the U5 well. 37 1 I I I I 300 - ~ 0 Zl‘ 200 -- .4 A =5! 100 - d ‘6 Q 9" ‘9‘ I A O l 1 I I 0 1O 30 4O 50 70 SiO 2 300 , depth 400 r- : .. A A A I 500 ‘ O 50 100 150 200 Zr 38 0.9000 ; u 0.00.0 000 0.9000 ¢> n .000 ”0.9000 o> n 0000 ”00.9000 0: u 00.0300 00.9000 5: n 0000006 ”00.00.00 0: u 00.0000... .000 00.000000. 5.3 00 00.00080. 00 0000 00.2600 0000 max 00 00..". .3 050.“. cm 000 ON 0 4 at nu. l o coop ooom 0w 0000 ooov 39 2 0.9000 ; u 0.2.0 000 0.0000 v> u 0000 ”0.00000 o> u 000.0 00.9000 0: u 00.03.00 00.9000 0: n 0000000 ”00.9000 0: u 00.0000... .08. 00.000000. 5.? 0m 00.00000. 00 000.: 00.3000 000 “Ex v.0 “0.0 0.. 050.“. ow. or l 00.. 9. com an increase in CaO (Figure 15). Rubidium shows an increase in abundance with increasing K20 (Figure 16). Electron Microprobe Results The results from the Electron Microprobe studies show changes in matrix compositions over very small scales. Elemental maps of two areas on the thin section U5-420-b can be seen in Appendix D. The first of six sections in each set (2 images) is a back-scattered image of the region of interest. Each section thereafter is labeled at- the bottom. Scales are included for both length and counts (colored scale, in units of counts). Each image was taken from a region within the thin section that represented a transition from one type of breccia to the other. Figures 48 and 49 represent a region in the upper boundary region of Figure 43, while Figures 50 and 51 represent a more centrally located boundary region. Within these elemental maps, small scale variations in the matrix materials can be seen. The elemental maps most useful for distinguishing these small scale variations are those for Si, Al, Mg and Ca, which exemplify the components most prominent in the two types of breccia present in the samples and the transitions between them. Elemental maps for sulfur are also helpful in determining the presence of sulfate minerals. 41 Textures and Petrology of Chicxulub Impact Breccias In examining the breccias from the Chicxulub crater both in hand sample and in thin section, it is immediately apparent that the two types of breccia (suevite and Bunte-type) have distinctly different textures. The matrix of these breccias generally consists of pieces of brecciated rocks within a very fine grained groundmass, thus, they are termed polymict breccias. Fragments range in size, in the selected samples, from 1 mm to 1 cm. Fragments smaller than 1 mm are plentiful, but are considered part of the groundmass. In this study, clasts are defined as grains over 1 mm in diameter (in at least one dimension). Samples from the U5 and upper portions of the U7 wells contain much more melt rock than do those from the lower portions of U7 and the U6 wells. They tend to have a darker colored matrix, as well. The processes of identifying clast types and distinguishing matrix from clastic material is markedly easier in the Bunte-type breccias, as the overall samples become much less altered. In all of the samples examined, however, clasts occur that are the same color/rock type as the matrix itself. This is especially true in the U7 and U6 wells. It becomes difficult, in this case, to decide whether to classify such features as clasts or matrix. 42 Hand sample analyses The breccias from the Chicxulub impact are extremely complicated in their nature. This is readily apparent when one begins to look at hand samples of these rocks. Many variations exist within and among the hand samples from the three UNAM wells. Detailed descriptions and photographs of representative hand samples of each breccia type can be found in Appendix A. General observations from each well are as follows. Within the U5 well, the number of silicate clasts decreases with depth. The abundance of reaction rims around clasts also tends to decrease with depth. In this well, large fragments tend to be more angular than smaller fragments (2 mm and less), and the smaller fragments tend to become more angular with depth. The existence of cavities within the matrix (from plucked grains) and within clasts themselves tends to increase near the middle of the sampled section of the well, (depths in the high 300 m area), and tends to decrease sharply after approximately 415 m depth. This defines a range of approximately 40-50 m with a large number of cavities. The fine grained matrix material in the well is greenish in color near the top, tan-gray in color in the middle, and a darker brown near the bottom. The abundance of light colored (carbonate and evaporite) clasts tends to increase toward the bottom of the well. Breccias-within-breccias are apparent throughout samples from this well. The trends in the U5 breccias seen in this study are compatible with those seen by Sharpton et al., (unpublished), who have divided the U5 well into 43 subunits, based on changes in the character of the core samples. These subunits are shown in Figure 17. Within the U7 well, some major differences exist. Samples from the shallower portion of this well are similar in appearance to samples in the U5 well. They are tan/gray in matrix color, and, though some darker clasts exist, light colored clasts are most prevalent. The matrix is more fine grained in these samples, lacking many of the sub-millimeter sized fragments found in the U5 samples. Clasts seem to be of a smaller size, in general, near the bottom of the well, though the clasts tend to be more angular in all of the U7 samples relative to the U5 wells as a whole. No cavities exist in these samples and reaction rims are rare. Breccias-within-breccias do occur in this well. The U6 well appears to be fairly uniformly composed of a very fine grained light tan colored matrix. Very few sub-millimeter sized grains exist as part of the matrix. The small clasts that do exist tend to be very angular. Most clasts are light colored though some darker clasts do exist. The Yucatan-6 samples contain large, angular mostly dark, gray and green clasts. The clasts are numerous and this rock type would be considered clast supported. Very few cavities exist. Clasts here tend to contain pre-impact igneous and metamorphic textures. Impact melt Suevite, subunit B. Suevite, subunit C. 500 rn Figure 17. Images of drill cores from the U5 well and the subunits designated by Sharpton et al. (unpublished). 45 Petrography Many thin sections from the Chicxulub impact rocks were examined briefly. Point counting was conducted from nine thin section images in an attempt to quantify the abundances of different minerals and textures occurring in these breccias. Details of this procedure can be found on pages eight and nine. Results can be seen in Table 5. In studying these thin sections, it is apparent that much of the mineralogy is remnant from the pre-impact stratigraphy discussed on page six. Many textures from the pre-impact lithologies are still visible within these rocks, though many new and unusual textures have formed. Mineralogy The major minerals in existence in the Chicxulub cores include quartz, calcite, dolomite, anhydrite, gypsum and halite. These minerals appear not only as individual crystals and grains, but are the major components of the matrix. Minor minerals must have included such minerals as K-feldspar, hornblende and biotite. Gamets have also been identified within individual basement gneiss clasts (Sharpton et al., unpublished). In many cases, melt rocks have been subsequently altered to clay minerals, most frequently chlorite. In examining the breccias, particularly in thin section, it is apparent that the mineralogy changes drastically between the two breccia types. In the Bunte-type breccias, the matrix becomes much easier to distinguish from the clasts, and individual crystals become much easier to identify compared to the suevitic breccias. In the Bunte-type breccias, evaporites become more prevalent, as much more anhydrite appears as clasts and individual grains, and gypsum also appears as clasts, grains and veins. Textures In examining the impact breccias under the microscope, it is apparent that they are even more complicated than they appear in hand sample. Many complex but fascinating textures appear within these sections, each containing a wealth of information. Some of the most important and frequently recurring textures of these sections are discussed below. Their significance will be discussed later. Images of all thin sections examined appear in Appendix C. The suevitic breccias, found throughout the entire U5 well and portions (possibly only upper portions) of the U7 well, contains many distinct textures. These include many associated with the matrix, the melting and mixing of target rocks and the subsequent alteration of these melts to clay minerals. Others include clasts that may not have not been altered whatsoever. Matrix textures are generally fine grained matrix (Figures 18, 19), where grains are so small or so shocked/melted that they are indistinguishable. These matrices can appear homogeneous, as in Figure 18, or very heterogeneous, as in Figure 18. Some instances show more than one type of matrix in each thin section (Figure 19), where areas of completely different mineralogy lie 47 Figure 18. Image of U5-450-b, a suevitic breccia, showing heterogeneous fine grained matrix of suevites, anhydrite clasts and crystals, rimmed clasts and clasts altered to clay minerals. Image was taken from thin section under crossed polars at 2.5x power. Field of view = 3.5 x 2.5 mm. Figure 19. Image of U5-450-b, a suevitic breccia, showing heterogeneous matrix, shocked quartz with planar deformation features, rimmed clasts, and clasts altered to clay minerals. Image was taken from thin section under uncrossed polars at 2.5x power. Field of view = 3.5 x 2.5 mm. intermingled with the matrix that makes up the majority. Rimmed clasts occur frequently in the upper portions of the U5 suevites (Figure 18, 19, 20) where temperatures of clasts were different than that of the matrix during formation. Areas around clasts also appear to have reaction characteristics, though not necessarily in coherent rims (Figure 20). Breccias-within-breccias occur somewhat frequently, usually appearing as an irregularly shaped clast containing clasts within its boundaries (Figure 21). Highly shocked and melted materials may result in such things as impact glasses (Figures 19, 20, 21 ), vesicular melt clasts (Figure 21) and melt clasts that appear to have flow structures within them (Figures 20, 21 ). Figure 19 shows a typical carbonate clast in a suevitic breccia, while Figure 18 shows a typical anhydrite clast in a suevitic breccia. Many clasts have been altered to clay minerals. A few of these examples occur in Figure 18, 19. Figure 41 (U5-383-2a) and Figure 43 (U5- 420-b) in Appendix C show the two large silicate clasts that appeared in the thin sections examined. The samples from the lower U7 well tend to be similar in texture types to those from the U5 well, except for samples from the lower portions of the boundary, which are more similar to samples from the U6 well (Sharpton et al., unpublished). These latter samples are of the Bunte-type. There are a few textures of these breccias worth noting. The matrix of the thin sections from U7 seem to be made up of less shocked and altered material than those in U5. The matrix appears to be more consolidated and homogeneous, and contains fewer sub-millimeter fragments. Examples of this matrix can be seen in Figure 20. Image of U5-450-b, a suevitic breccia, showing a large , rimmed, melt clast exhibiting flow structures. Other rimmed clasts are also seen. Image was taken from thin section under uncrossed polars at 2.5x power. Field of view = 3.5 x 2.5 mm. Figure 21. Image of U5-450-b, a suevitic breccia, showing a larger melt clast exhibiting flow textures. Image was taken from thin section under crossed polars at 2.5x power. Field of view = 3.5 x 2.5 mm. 51 Figure 22. Image of U7-658-b, a Bunte-type breccia, showing different types of matrix, fossils, anhydrite crystals, and clasts altered to clay minerals. Image was taken from thin section under crossed polars at 2.5x power. Field of view = 3.5 x 2.5 mm. Figure 23. Image of U7-658-b, a Bunte-type breccia, showing different types of matrix, anhydrite clasts, individual anhydrite crystals, and clasts altered to clay minerals. Image was taken from thin section under crossed polars at 2.5x power. Field of view = 3.5 x 2.5 mm. 53 Figure 24. Image of U7—658-b, a Bunte-type breccia, showing fossils and typical clasts, including anhydrite and calcite. Image was taken from thin section under crossed polars at 2.5x power. Field of vbw=85x25mm. Figure 25. Image of U7—658-b, a Bunte-type breccia, showing homogeneous matrix next to a large anhydrite clast, which exhibits large, individual, bladed crystals. Image was taken from thin section under crossed polars at 2.5x power. Field of view = 3.5 x 2.5 mm. 55 Figure 26. Image of U7-658-b, a Bunte-type breccia, showing separate foraminifera fossils as well as a fossiliferous matrix. Anhydrite clasts and crystals also exist. Image was taken from thin section under crossed polars at 6.5x power. Field of view = 1.5 x1.0 mm. Figure 27. Image of U7-658-b, a Bunte-type breccia, showing foraminifera fossils, anhydrite clasts and crystals, a silicate clast and clasts altered to clay minerals. Image was taken from thin section under crossed polars at 2.5x power. Field of view = 3.5 x 2.5 mm. 57 Figures 22-27. Different types of matrix do occur simultaneously in these samples, also, as shown in Figures 22 and 23. Anhydrite occurs more frequently in the U7 section, examples of which are shown in Figures 23, 24, 25 and 27. Anhydrite crystals tend to be larger in these samples than they were in the U5 samples. Anhydrite crystals also occur as individual grains within the matrix, as seen in Figures 23, 24, 25, and 27. Silicate clasts occur U7 samples, as well, shown here in Figure 27. One of the most important differences in the thin sections of the Bunte-type breccias as opposed to those of the suevitic breccias lies in the fact that fossil fragments appear in the Bunte- type breccias. These fossils (Figures 24, 26, 27) are most likely benthic foraminifera (Anstey, R., personal communication, and Sibley, 0., personal communication) and are similar to those shown from KIT sites in Belize by Ocampo et al. (1996) (Figure 28). Gypsum also occurs in these breccias, particularly as veins, as portrayed in Figure 46. Interpretation and Discussion Textural variations within drill core samples Textural trends with increasing depth in the U5 crater suggest that the materials in this well are allogenic (i.e., have not been transported far from their source location) and have been exposed to conditions harsh enough to cause their melting, shock and thermal alteration. They also imply that the thermal 59 Similar fossils found in KIT impact deposits from Belize (from Ocampo et al Figure 28. 1996). regime within the well decreased towards cooler, less shocked materials with depth. Textures such as the angularity of clasts can indicate the amount of alteration a clast has seen since it was broken from its source rock. Just like a grain in a sedimentary environment, the more rounded the clast, the more action it has seen. Angularity of breccia clasts seems to decrease with the intensity of thermal alteration and shock. Reaction rims around clasts indicate that the temperatures differences existed between the clasts and matrix. These differences were enough to cause a chemical reaction to occur where the two materials met. This is the result of the fact that heat will flow towards colder materials and could have been either cold clasts falling into hot matrix, hot clasts falling into cooler matrix. The fact that these rims are most prevalent in the middle portion of the U5 well indicate that conditions here were a combination of the upper well (too hot for rims to form - everything was melted) and lower well (temperatures were too cool for reactions to occur) temperature regimes. Cavities in the U5 well are also indicative of extreme thermal conditions. These cavities are either a result of melt clasts being dissolved post impact, plucked during sampling or are vesicles containing trapped gases which formed during impact. These cavities occur most frequently in breccias from the intermediate depths of the well containing less overall melt and a more siliceous matrix. 61 The matrix variations within the U5 well hold important implications in themselves. The fact that the amount of melt in the breccias (clasts and matrix combined) varies from high in the upper portions of the well (to about 400 m depth) to lower in the intermediate portions of the well (400 to ~460 m depth) and then again to high amounts of melt in matrix in the lower portions of the well (> ~460 m depth) suggest a layering of types of emplacement of target materials within the topographic basin itself. This layering could be the result of the emplacement of minor amounts of carbonates mixing with the first- encountered highly melted basement rocks, followed by the next layer of basement rocks which were thrown into the air on impact and later redeposited on top of the initially melted materials, allowing them to have experienced lower temperatures. On landing, then, these materials were re-exposed to high temperatures, causing the formation of reaction rims. Deposited on top of these breccias were the most highly shocked and melted materials from the deepest depths of excavation, which were thrown into the air after the earlier deposited rocks, or may have moved over them in a fluid type manner, as indicated by flow textures within these deposits. The final layer to have been deposited in the U5 well was the fluidized impact melt sheet, in which almost every particle has either been highly altered or melted completely, and the matrix is made up of almost entirely melt. Breccias found within other breccias (i.e., breccias making up clasts themselves) are probably formed during the above processes. As one type of material is broken up (brecciated) and thrown into the air, it can easily be 62 caught up in the deposits from the next set of materials to be ejected, as speeds of ascent and descent probably vary among the different materials. Samples from the lower portion of U7 and from all of the U6 well exhibit textures drastically different from those of the U5 and upper U7 wells. Fossil fragments exist, indicating that these rocks could not have been exposed to a large degree of shock. If they were exposed to a large degree of shock, the fossil fragments would have been destroyed instead of just replaced. Large veins of evaporites, especially in the U6 well, where the upper portions of the breccias are riddled with gypsum veins (Sharpton et al., unpublished). The limited occurrence of silicate materials in these breccias indicates that silicate materials (lower target) were not ejected as far out as carbonate and evaporitic materials (upper target). Textural trends moving away from the center of the crater include, generally, decrease in the number of clasts, decrease in highly shocked and melted materials including basement clasts and glass, and decrease in features such as cavities, reaction rims, breccias within breccias and increased angularity in clasts. These trends reinforce the conclusions from the chemical data (discussed below) that upper target materials were transported farther from ground zero than were those from lower target locations. They also indicate that materials from the upper target were less shocked, less melted and less thermally metamorphosed than were those from the lower target. High degrees of shock and thermal alteration are indicated by features discussed above, such as cavities, reaction rims, less clast angularity, flow 63 features in melted clasts, etc. Lack of these features in distal deposits suggests that they have not been subjected to such conditions. Chemical variations within drill core samples There are a number of chemical trends in the major and trace element data (determined from XRF analyses) from the Chicxulub impact breccias. These trends include the decrease in SiOz and Al203 and increase in CaO and MgO with distance from the center of the crater. As the target stratigraphy is composed of carbonates and evaporites overlying a silicate basement, one interpretation is that the silicate materials were thrown higher (or at a steeper angle) into the air upon impact than the carbonate platform rocks, causing them to land after, and, thus, on top of, the carbonate materials. Carbonate materials were thrown out at a shallower angle and were thus ultimately located farther out from the crater’s center. This is in addition to the simpler interpretation that, since the silicate materials were thrown out just an instant after the carbonate materials, they would have landed on top of them. The U5 well did not intercept the bottom of the suevite unit during drilling. It is assumed that there is a significant amount of this breccia below the core bottom, and it is possible that it becomes even more carbonate rich. Whether or not the boundary between the suevitic breccias and the Bunte-type breccias would be found deeper in the well is unknown, but it is a possibility. Within the U5 well, there also exists a trend of decreasing Si02 and AI203 and increasing CaO and MgO with increasing well depth. This trend reflects the same process stated above - the deeper target (i.e., silicate material) lands after the shallower target (carbonate materials). There would inevitably be some mixing of the two during this event, resulting in the gradual increase in carbonate with depth as is shown in the Chicxulub breccias in U5. This increase in carbonate can be seen somewhat in the slight increase in clasts of limestone and dolomites, but is likely to be more associated with increasing portions of fine grained carbonates in the matrix material. The increase in carbonate material with depth in the U7 well, and especially the marked contrast in SiOz and CaO values between the upper and lower samples, indicates that there exists a boundary between the suevites and Bunte-type breccias (Corrigan et al., 1997). The nature of this boundary is not evident from these data, but data from Sharpton et al. (unpublished) indicate that it is very sharp. Trends of high K20 abundances in rocks from the upper sections of U5 would be expected if the material being melted contained minerals such as K- feldspar. The most K20 exists where the most basement material was deposited. As the basement rocks in the Chicxulub target were generally gneissic, this is an expected result. Trends of high Na20 abundances would also be expected in rocks from the upper sections of U5, as minerals such as plagioclase feldspars, containing a significant amount of sodium, would be prevalent in the silicate basement rocks. 65 Trends of high Fe and Ti02 abundances would also be expected in rocks from the upper sections of US, as they are typically found in ilmenite, (FeTiOa), a mineral common in silicate basement rocks. Thus, the trends of increasing TiOz with increasing SiOz and decreasing Ti02 with increasing CaO+MgO (Figure 13) would be expected. The MgO trends seen in the Chicxulub rocks may be the result of two competing scenarios. MgO values tend to be somewhat increased in the mafic minerals basement rocks. M90 is also high in the sedimentary mineral, dolomite, which seems to be a large component of the Chicxulub target stratigraphy. The result, as shown in Figures 7, 9, and 11-a, shows two distinct trends in M90. One for the suevitic breccias, where MgO gradually increases as CaO+MgO increases, and one where MgO increases very sharply as CaO+MgO increases. This is a further implication that there is a large distinction between the chemical makeup of the two breccia types, and the distinct break in the trends is another indication of the boundary between the two breccia types within the U7 well (note U7 samples in Figures 7, 9, 11-a). Trace element trends were somewhat expected, as well. As the trace element zirconium is the major constituent of the mineral zircon which has been found in many K/T deposits (Sharpton et al., 1996) associated with basement rich breccias. It is not unexpected, then, that plots of Zr vs. Si02 show a trend of increasing Zr with Si02. Strontium increasing with increasing CaO is also an expected trend. Strontium tends to be an important trace element abundant in carbonate minerals. The same is true for barium, which also shows a general increase with increasing CaO. Rubidium showing a positive correlation with K20 is also to be expected, as it is an incompatible element and would be the first to be transported into the materials melted upon impact. Here the silicate breccias are rich in K20, as exemplified earlier, which would also cause them to be rich in Sr. The results of Electron Microprobe elemental mapping analyses (Appendix D), which show small-scale variations in the matrix compositions are expected. During the emplacement and resultant combining of the many different components present in the target material, interactions between possibly fluidized portions would be expected. Probably the most important implication of these chemical data, however, is the abundance of Si02 in the U5 and U7 wells. Prior to the onset of this study, silica-rich breccias (i.e., suevitic breccias) were not thought to occur as far out as the U5 well, let alone as far out as the U7 well. The fact that a layer of melt rock also exists above the suevites in U5 is even more unexpected. Prior studies (Sharpton et al., 1996) show melt and melt breccia rocks occurring below suevitic breccias in wells C1, S1 and Y6 (approximately 25, 30 and 45 km from the center of the crater, respectively). No suevitic breccias appear in the stratigraphy of the Ticul 1 well at all (~95 km from crater center). The results presented here, which indicate the presence of suevitic breccias in both wells U5 and U7 (110 and 125 km from crater center, respectively) and melt rock occurring on top of U5 suevites is an important discovery. The importance of this finding lies partially in the debate over the 67 size of the Chicxulub impact crater. If the suevitic breccias are allogenic, or crater-fill breccias (Sharpton et al., 1996) (those deposited in place, or here, within the initial crater rim), then their presence at a distance of 125 km (the U7 well) might indicate something about the initial size of the crater’s topographic basin rim. To accommodate these materials as far out as 125 km, the initial diameter of this rim would have had to have been at least 250 km. Since the suevite breccias do not exist in the U6 well (or if they did they have eroded away), which lies at ~150 km from the crater’s center, we can put a bounding limit on the crater’s maximum size of 300 km. This creates, then, a range for the initial size of the Chicxulub crater from 250 km to 300 km in diameter. These interpretations, however, are only applicable for the south, or landward, side of the crater, as these are the only deposits which have been sampled to date. Whether these same characteristics hold true for the rest of the crater, i.e., whether the crater is symmetrical, remains to be seen. Conclusions Studying the breccias from drill cores of the Chicxulub impact crater have given us new insights into the way ejecta is distributed around a crater and into the Chicxulub event and the characteristics of the resulting crater. This study gave an unprecedented look at the breccias formed by this impact and have reshaped some of the ideas about where certain types of materials now reside and what this may imply. The U5 well, in particular, has given a wealth of new insight into the crater’s structure and formation. The variations with depth within this well alone indicate a complex thermal regime, with temperatures and degree of shock decreasing with depth. The materials in this well range from impact melt to melt-clast rich breccias to breccias with less overall melt and finally to breccias with a melt-rich matrix. This implies that materials from different depths in the target rocks end up stacked in an inverted manner, as is evident in the Ries crater. This evidence is supported even further by comparing the chemical compositions of breccias from the U5 well to those in the U6 well. The drastic difference in chemical make-up between the two indicates that the U5, or suevitic breccias, are derived from rocks at depth (silicate basement) and those in U6, or Bunte-type breccias, are derived from rocks at shallow depths in the target stratigraphy (carbonates and evaporites). The breccias from the U7 well indicate that a boundary exists between the two types of breccia, with suevitic breccias lying on top of Bunte-type breccias. The nature of this break (whether it is sharp or graded), however, can not be accurately determined from these data. The nature of this boundary is an important topic, as it holds many implications as to the type of mechanism responsible for emplacement of the two types of breccia (whether it be ballistic emplacement or more of a fluidized mass of material flowing over the Bunte-type breccias). The fact that a large amount of silicate materials exist within the U5 and U7 wells indicates a large (250-300 km) initial topographic rim diameter for the 69 crater. This size is on the upper limit of diameters suggested so far by workers such as Sharpton et al. (1993, 1996) and Urrutia-Fucugauchi et al. (1996). These and other new findings (Sharpton et al., unpublished) have increased the amount known about the rocks from the Chicxulub impact structure. Undoubtedly, these findings will serve as a basis for conducting more research into Chicxulub and other large impact craters in order to gain more insight as to how these impacts have affected the history of earth and the life it supports. 70 APPENDIX A 71 Figure 29. Photograph of the hand sample Y6-1196 (scale shown). Figure 30. Photograph of the hand sample U5-338 (scale shown). 72 05-358 02% 73 Figure 31. Photograph of the hand sample U5-360 (scale shown). Figure 32. Photograph of the hand sample U5-390 (scale shown). 74 75 Figure 33. Photograph of the hand sample U5-483 (scale shown). Figure 34. Photograph of the hand sample U5-489 (scale shown). 76 U5 ~483-‘llfla l .lllilillll lllllllllllllll l‘l'“ l/l5 ~4$9~ 492' :L Figure 35. Photograph of the hand sample U7-318 (scale shown). Figure 36. Photograph of the hand sample U7-658 (scale shown). 78 79 Figure 37. Photograph of the hand sample U6-302 (scale shown). Figure 38. Photograph of the hand sample U6-508 (scale shown). 80 81 HAND SAMPLE DESCRIP'HONS UNAM 5: U5-338m: M green. Texture: unconsolidated friable melt rock fragments. gait; small clasts (< 2 x 2 mm). U5-360m: m: polymict breccia with light gray-green matrix. Texture: consolidated breccia with a very fine grained matrix; a few small cavities exist that are similar in size to the majority of clasts. _C_l_ags_: clasts from 1-3 mm diameter; clasts are dark green, white, gray or tan, many with visible reaction rims. U5-362m-1: _Co_log_ polymict breccia with green matrix. Texture: consolidated breccia with a very fine grained matrix with visible vesicles and flow structures (pores possibly somewhat oriented). C_|a_Sti clasts relatively sparse and small (< ~0.5 mm). U5-362m-2: QM polymict breccia with light gray-green matrix. Texture: consolidated breccia with a very fine grained matrix. we}; clasts from 1-3 mm diameter; clasts are dark green,white, gray or tan, many with visible reaction rims. U5-365m: QM polymict breccia with light gray-green matrix. 82 U5-389m: U5-390m: U5-397m: U5-400m: Texture: Clasts: Color: Texture: Clasts: Color: Texture: Clasts: Color: Texture: Clasts: 9% Texture: Clasts: consolidated breccia with a very fine grained matrix; a few small cavities exist that are similar in size to the majority of clasts. clasts from 1-3 mm diameter; clasts are dark green, white, gray or tan, many with visible reaction rims. polymict breccia with light gray-green matrix. consolidated breccia with a very fine grained matrix and numerous small, open cavities. clasts from 1-3 mm diameter; clasts are dark green, white, gray or tan, many with visible reaction rims. polymict breccia with light gray-green matrix. consolidated breccia with a very fine grained matrix and numerous small, open cavities. clasts from 1-5 mm diameter; larger clasts here, which are cavities and/or exhibit reaction rims, though rimming appears to be decreasing. polymict breccia with light gray-green matrix. consolidated breccia with a very fine grained matrix and numerous small, open cavities. clasts from 1-5 mm diameter; larger clasts here, which are cavities and/or exhibit reaction rims. polymict breccia with light gray-green matrix. consolidated breccia with a very fine grained matrix and numerous small, open cavities. clasts from 1-5 mm diameter; larger clasts here, which are cavities and/or exhibit reaction rims; carbonate clasts are most frequently rimmed. 83 U541 2m: U5418m: U5-420m: U5450m: Color: Texture: Clasts: Color: Texture: Clasts: Color: Texture: Clasts: Color: Texture: Clasts: polymict breccia with light gray-green matrix. consolidated breccia with a very fine grained matrix; fewer cavities exist here. clasts from 1-5 mm diameter; one large silicate basement clast exhibiting primary gneissic textures; less reaction rimming; one breccia-within-a-breccia. polymict breccia with light gray-green matrix. consolidated breccia with a very fine grained matrix; now with even fewer cavities than U5-412. clasts have decreased in size to 1-3 mm; little reaction rimming; clasts are lighter in color - mostly tan, gray and white. polymict breccia with light gray—green matrix. consolidated breccia with a very fine grained matrix and few cavities. clasts from to 1-3 mm; little reaction rimming; clasts are lighter in color - mostly tan, gray and white, though a few darker clasts do exist. polymict breccia with matrix now more gray. consolidated breccia with a very fine grained matrix and few cavities. clasts even smaller now, from 1-2 mm; rimming generally absent; still mostly light colored clasts with a few small darker clasts. U5-480m: U5-483m: U5-489m-1 : U5489m-2: UNAM é: U6-302m: Color: Texture: Clasts: Color: Texture: Clasts: Color: Texture: Clasts: Color: Texture: Clasts: Color: polymict breccia with gray matrix. consolidated breccia with a very fine grained matrix and few cavities; clasts seem to be arranged in a type of flow pattern. clasts small, from 1-2 mm; rims generally absent; mostly light colored clasts, with only one darker colored (green) clast. polymict breccia with gray matrix. consolidated breccia with a very fine grained matrix. clasts small, from 1-2 mm; rims generally absent; mostly light colored clasts, with only one darker clast; a carbonate breccia-within-a-breccia exists in this sample. polymict breccia with very dark gray matrix. consolidated breccia with very fine grained matrix; possible orientation of clasts. most clasts slightly larger - numerous 5 mm diameter clasts; most clasts are light colored (only ~25% darker clasts). dark gray. fine crystalline. none - possibly a clast itself. polymict breccia with light tan matrix. 85 U6-508m: UNAM 7: U7-244m: U7-31 8m: U7-335m: U7-699m: Texture: Clasts: 92k); Texture: Clasts: Color: Texture: Clasts: Color: Texture: Clasts: Color: consolidated very fine grained breccia with 2 different matrices, one a bit darker tan than the other. clasts from 1-3 mm; fewer overall clasts and the vast majority are light colored (tan, gray and white) clasts. polymict breccia with tan matrix. consolidated breccia, though coarser grained than other breccias. clasts 1-3 mm; far less clast abundance. polymict breccia with tan-gray matrix. consolidated breccia with a very fine grained matrix. clasts generally 1-3 mm, though one large breccia- within-a-breccia exists; some clasts exhibit reaction rims; both dark (green, brown and black) and light colored (tan, gray and white) clasts exist. polymict breccia with gray matrix. consolidated breccia with a very fine grained matrix. clasts from 1-5 mm now, most appear to be darker colored (many green clasts) and some melt. polymict breccia with gray matrix. consolidated breccia with a very fine grained matrix. clasts 1-5 mm; 4-5 mm clasts exist all green; a breccia-within-a-breccia exists. alternating gray, tan and white. YUCATAN-1 : Y1 -3151 m: YUCATAN-2: Y2-3330m: YUCATAN-6: Y6-1 1 96m: Texture: Clasts: Color: Texture: Clasts: Color: Texture: Clasts: Color: Texture: Clasts: very fine grained crystalline layers, possibly alternating silicate with carbonate. none; possibly a clast itself. gray. fine grained uniform looking sample. none - possibly an anhydrite clast. tan and orange. fine grained sandstone with some iron staining. none. polymict breccia with tan, gray and green matrix. consolidated clast supported breccia with a very fine grained matrix that changes color between clasts; very few, if any, cavities. clasts vary in size from 1 mm to 1 cm and are very angular; most clasts are silicate and some exhibit primary igneous and metamorphic textures (though some carbonate clasts do exist); reaction rims exist on many clasts; many very opaque clasts exist (possibly sulfides). APPENDIX B 000 00.0 00.0 30.3 00.0 0.10 30.3 00.0 00.3 00.03 000.5 30.0 00.0 00.0 00.3 00.0 00.00 00.: 00.0 00.0 0000 000-5 000 00.0 00.0 03 00.3 00.0.. 0003 00.0 300 0000 030-5 30 00.0 00.0 00.3 00.3 00.00 00.3 00.0 00.0 0000 30.5 00.0 00.0 00.0 000 00.0 00.00 00.00 00.0 00.0 3.0 000.0: 00.3 3.0 00.0 3.0 00.0 00.00 00.00 00.0 00.0 3... 000.0: 00.03 03.0 3.0 00.0 00.0 00.00 00.0 00.0 00.0 00.03 0.00100 3.0 00.0 00.0 00.3 00.0 00.00 00.0 00.0 00.0 0000 3.00.10: 00.0 30.0 00.0 00.3 00.0 00.00 00.0 00.0 000 0000 00.100 00.0 00.0 00.0 0: 0.0 0000 00.0 00.0 00.0 00.00 00.00 000 00.0 3.0 0 3 .0 00.0 00.00 00.0 0 3 .0 00.03 000.. 00100 000 00.0 03.0 00.0 00.0 003 00.0 000 30.: 00.00 00.100 000 00.0 3.0 30.0 00.0 .000 000 00.0 00.03 0.00 00.100 00.0 3.0 000 000 .00 00.00 00.0 00.0 00.3 0000 000.0: 00.0 00.0 000 00.0 00.0 3.00 00.0 00.0 00.: 03.0.. 000-00 00.0 00.0 000 00.0 30.0 0.00 000 00.0 00.3 3.00 000.0: 00.0 00.0 03.0 000 30.3 00.03 00.03 3.0 003 0000 000-00 00.0 00.0 000 3.0 00.3 0003 3.0 00.0 00.: 00.00 0.00000 00... 00.0 00.0 30.03 00.0 00.03 00.0 00.0 00.03 3.00 7000-00 00.0 00.0 3.0 3.0 00.3 0003 00.03 000 00.03 00.00 000.0: 00“. 00: 0000 000. 0002 000 00.). 05>. 0002 00.0 0.0500 .3009 0. 000.0008. 0002000 .0000000 00000.0 0000 “Ex v.00. 0.00>> ”0 0.00... 89 003-00, . 3.0 00.0 00.0 03. 00.0 00.3 00.0 00.0 00.03 00.00 0:. 00.0 000 000 3... 00.03 00... 03.0 30.03 0.00 032-; 30.0 00.0 00.0 00.0 30.0 0000 00.00 00.0 00.0 0.0 3030-; 00“. 00: 00.0 00v. 0002 #000 00.2 00.2 8......“80 20500 .0000008. 0 0.000 .00 .00 0.: 0.00 0.: 02000 00.00 0.: .00 .00 0.0 0.000000 00.003 0.: .00 00.303 0.0 000000 00.00 00.00 0.0 .00 00.003 000.00 0.: .00 .00 00.30 .00 000300 30.00 00.00 0.0 0.0 0.: 0.0000 0.: .00 .00 0000 0.: 00.000 00.03 00.00 .00 .00 .00 00.00 00.003 .00 .00 0:0 .00 00.200 .00 .0... .00 0.: .00 0-00000 0.: .00 0.0 0:00 .00 00.000 00.00 0003 00.003 .00 .00 0-00000 .00 .00 .00 30.: .00 00.3003 00.00 00.00 .00 0.0 .0: 000000 .00 .0: .00 00.00 .00 00000 0.: 00.00 .00 0.0 .00 3.00.00 .00 0.: 0.0 00.30 .00 00.00: 00.00 00.00 0.: .00 .00 00.100 00.000 .00 0.: 00.00 0.0 0.0000 30.30 00.00 0.0 .00 0.: 0-00.000 03.000 .00 .00 000: 00.0. 0003 0.00 00.30 .00 .00 0.: 00.00 00.000 0.0 .00 00.00 00.03 00.300 00.00 00.00 0.: 0.0 .00 000.00 0.0 0.0 0.: 0.00 003 00.000 00.00 3.00 .00 30.03 00.0003 000.00 00003 .00 .00 0000 .00 00.000 00.00 00.00 .00 .00 0.0 00.00 00.000 .00 .00 0.00 00.00 00.300 00.00 30.00 00.00 0.: .00 000-00 00.000 0.: .00 00.30 00.03 00.000 00.00 00.00 .00 .00 .00 0000-00 00000 .00 .00 00.00 30.00 00.000 00.00 00.00 .00 .00 0.: 000.00 0.000 0.: 0.0 :00 00.3 00.000 0000 30.00 0.0 0.0 .00 0-00000 00.000 .00 .00 30.00 .00 00.000 03.0: 00.00 .00 .00 0.: 7000-00 30.00 0.: 0.: 00.00 00.03 00.000 30.00 0000 .00 0.: 0.: 000-00 00 3 02 .0 > .0 00 00 00 .2 .0 0.0500 0002000 00.0000 00000.0 0000 “Ex 0.00. 0.003 K 0.00.. 91 00.000 .00 .00 00.000 00.00 00.000 00.00 00.00 .00 .00 .00 000 70> .00 .00 .00 00.000 .00 0.0000 00.000 00.00 00.000 .00 .00 002-; 00.000 .00 .00 00.000 00.00 00.00 00.000 00.00 .00 .00 .00 0000-; 00 00 02 .0 > 00 00 00 00 .z .0 0.00.00 ..00000000. ”0 0.000 APPENDIX C Figure 39. Thin section views (crossed and uncrossed polars) of the thin section U5-360-b. Field of view = ~ 40 x 25 mm. 94 Figure 40. Thin section views (crossed and uncrossed polars) of the thin section U5-365-b. Field of view = ~ 40 x 25 mm. Figure 41. Thin section views (crossed and uncrossed polars) of the thin section U5-383-2a. Field of view = ~ 40 x 25 mm. Figure 42. Thin section views (crossed and uncrossed polars) of the thin section U5400-b. Field of view = ~ 40 x 25 mm. 100 101 Figure 43. Thin section views (crossed and uncrossed polars) of the thin section U5-420-b. Field of view = ~ 40 x 25 mm. 102 I. 4 . .4" 0.... .u. . Figure 44. Thin section views (crossed and uncrossed polars) of the thin section U5-450-b. Field of view = ~ 40 x 25 mm. 104 Figure 45. Thin section views (crossed and uncrossed polars) of the thin section U5-483-a. Field of view = ~ 40 x 25 mm. 106 Figure 46. Thin section views (crossed and uncrossed polars) of the thin section U6-302-b. Field of view = ~ 40 x 25 mm. 108 Figure 47. Thin section views (crossed and uncrossed polars) of the thin section U7-335-b. Field of view = ~ 40 x 25 mm. 110 111 ' :. I 0' . I . . a u' , - 04' D ‘ - .5. ' I ~.-." APPENDIX D 112 Figure 48. Elemental maps corresponding to a breccia boundary in the upper portion of the U5-420-b thin section (Figure 43). Colored scale represents counts. Analyzed element is shown at the bottom of each frame. 113 114 Figure 49. Elemental maps corresponding to a breccia boundary in the upper portion of the U5-420-b thin section (Figure 43). Colored scale represents counts. Analyzed element is shown at the bottom of each frame. 115 116 Figure 50. Elemental maps corresponding to a breccia boundary in the upper portion of the U5-420-b thin section (Figure 43). Colored scale represents counts. Analyzed element is shown at the bottom of each frame. 117 118 Figure 51. Elemental maps corresponding to a breccia boundary in the upper portion of the U5-420-b thin section (Figure 43). Colored scale represents counts. Analyzed element is shown at the bottom of each frame. 119 120 BIBLIOGRAPHY 121 BIBLIOGRAPHY Bohor, B. F.,Betterton, W. J., and Krogh, T. E., 1993, Impact-shocked zircons: discovery of shock-induced textures reflecting increasing degrees of shock metamorphism: Earth and Planetary Science Letters, v. 119, p. 419-424. Buffler, R. T., and Sawyer, D. S., 1985, Distribution of crust and early history, Gulf of Mexico Basin: Gulf Coast Association of Geological Societies Transactions, v. 35, p. 333-344. Corrigan, C. M., Sharpton, V. L., Vogel, T. A., and Marin, L. E., 1997, Chemical Compositions of Chicxulub Impact Breccias, LPI Contribution No. 922, p. 10-1 1 . Dietz, R. S., and Holden, J. C., 1970, Reconstruction of Pangea: Breakup and dispersion of continents, Permian to Recent: Journal of Geophysical Research, v. 75, p. 4939-4956. 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