GERTA KELLER PUBLICATIONS

Platinum Group Element (PGE) Geochemistry of Brazos Sections, Texas, U.S.A.

TitlePlatinum Group Element (PGE) Geochemistry of Brazos Sections, Texas, U.S.A.
Publication TypeBook Chapter
Year of Publication2011
AuthorsGertsch, B, Keller, G, Adatte, T, Bartels, D
Book TitleThe End-Cretaceous Mass Extinction and the Chicxulub Impact in Texas
VolumeNo. 100
EditionSEPM, Special Publication
Pagination227-249
PublisherSEPM (Society for Sedimentary Geology)
CityTulsa
ISBN978-1-56576-308-1
KeywordsBrazos River, Iridium, K-T boundary, PGE, Texas
Abstract

Geochemical and sedimentological analyses of Platinum Group Element (PGE) patterns across the Cretaceous/Tertiary (K-T) transition of 8 sections along the Brazos River, Texas, reveal possible sources and processes responsible for PGE enrichments. Of the five global KT boundary-defining characteristics (mass extinction in planktic foraminifera, first appearance of Danian species, negative δ13C excursion, Ir anomaly, thin (0.5 cm) red clay layer), the Ir anomaly and red clay layer are not present at the KTB in the Brazos sections. Instead, PGEs and especially Ir show several minor enrichments within the sandstone complex with the largest peak at the top or just above it. Possible mechanisms of PGE enrichments include, low sedimentation rates or sediment starvation that concentrates Ir and other PGEs. Absence of Ir at the KTB is likely linked to dilution effects caused by high sedimentation rates, and other still unknown processes. The source of PGEs remains elusive, but may be linked to an increased input of extraterrestrial dust during the late Maastrichtian, or reworked PGEs from the Chicxulub impact that predates the KTB in these sections.  PDF

Trace-Element Geochemistry of Brazos Sections, Texas, U.S.A.

TitleTrace-Element Geochemistry of Brazos Sections, Texas, U.S.A.
Publication TypeBook Chapter
Year of Publication2011
AuthorsGertsch, B, Keller, G, Adatte, T, Bartels, D
Book TitleThe End-Cretaceous Mass Extinction and the Chicxulub Impact in Texas
VolumeNo. 100
EditionSEPM, Special Publication
Pagination251-279
PublisherSEPM (Society for Sedimentary Geology)
CityTulsa
ISBN978-1-56576-308-1
KeywordsBrazos, K-T boundary, Texas, Trace Elements
Abstract

A geochemical study of major (MEs) and trace (TEs) elements across the Cretaceous-Tertiary (KT) transition was carried out on 2 sections of the Brazos area to investigate signals of the Chicxulub impact, as well as redox conditions and weathering processes in these shallow-water environments. Results show that ME and TE patterns are primarily affected by the type of lithology, including claystones, mudstones, a 3 cm thick yellow clay layer, a sandstone complex with 2-3 spherule-rich layers and alternating hummocky cross-bedded and laminated sandstone layers. In the yellow clay layer, TEs (Mn, Ni, Cr, Na) concentrations are comparable to the spherules-rich layers and geochemistry of spherules. Relative abundance of MEs (Al, Ca, Fe, Mg) in both spherule-rich layers and the yellow clay layer shows also a good correlation. This indicates that the yellow clay layer is likely an alteration product of a spherule-rich layer. No other evidence of the Chicxulub impact could be determined based on MEs and TEs values. In the claystone/mudstone intervals, TE concentrations are constant and slightly enriched in redox-sensitive TE (Mo, U, V), which indicates that dysoxic conditions prevailed. Occurrence of rare large pyrite framboids (30-50 μm) below the sandstone complex confirms that redox conditions were dysoxic in the shallow-water Brazos environments. High values for Al and weathering indices show high detrital input dominated by chemical weathering. These results reveal that persistent high stress conditions and high continental runoff prevailed through the late Maastrichtian-early Danian transition. No significant geochemical and environmental change due to the Chicxulub impact is detected. Sudden increases in trace elements (Co, Cr, Ni) possibly related to an impact, are observed only in reworked intervals within the sandstone complex.  PDF

The Sandstone Complex in the Brazos Riverbed Section: Geochemical Contraints on Genesis and Depositional Conditions

TitleThe Sandstone Complex in the Brazos Riverbed Section: Geochemical Contraints on Genesis and Depositional Conditions
Publication TypeBook Chapter
Year of Publication2011
AuthorsMunsel, D, Berner, Z, Stuben, D
Book TitleSEPM, Special Publication
VolumeNo. 100
Pagination281-295
ISBN978-1-56576-308-1
Abstract

The origin and deposition of spherule-bearing, dominantly sandy beds in a sandstone complex (also called ‘‘event deposit’’) below the biostratigraphic Cretaceous–Tertiary (KT) boundary plays a key role in models linking the KT mass extinction to the Chicxulub impact. This study, which focuses on the chemostratigraphy of this complex exposed in a ca. 60-cm-thick succession along the Brazos River, Falls County, Texas, U.S.A., aims to constrain the source of the material as well as the depositional conditions and postdepositional history of this highly controversial stratigraphic unit. Major and trace elements, as well as the isotopic composition of the Ca carbonate, contrast sharply with the underlying Corsicana Formation, indicating a dramatic change in the source of material and depositional conditions. Evaluation of geochemical data by principal-component analysis permits identification of (1) siliciclastic components, (2) ejecta material, consisting of altered impact-glass spherules, and (3) Ca carbonate. The ejecta material, originally represented chiefly by glass spherules with carbonate infill, is strongly altered to clay minerals with dominantly smectitic composition and is characterized by the element association Al2O3, TiO2, Fe2O3, P2O5, and SO2 and the trace elements (TE) V, Cr, Ni, Cu, Zn, Ga, and Mo. The occurrence of two moderately high Ir peaks (0.2 and 1.1 μg/kg) suggests the presence of tiny amounts of extraterrestrial material within the sandstone complex. Based on the contrasting abundance of Ni and Cu in chondritic meteorites and middle crust, the Ni/Cu ratio was used to trace the portion of extraterrestrial material in the sequence. The distribution of this ratio reflects changes in the amount of siliciclastic components added during deposition of the sandstone complex rather than variations in the amount of meteoritic material. The disagreement between evidence suggesting a prevalence of reducing conditions during the alteration of the ejecta material (pyrite inclusions in spherules; accommodation of Mn2+ by secondary calcite) and sedimentologic features which indicate that the sandstone complex was deposited in a dominantly oxic, high-energy environment strongly supports the case that the ejecta material in these deposits was subjected to reworking. PDF

 

Stratigraphy of the Cenomanian–Turonian Oceanic Anoxic Event OAE2 in shallow shelf sequences of NE Egypt

TitleStratigraphy of the Cenomanian–Turonian Oceanic Anoxic Event OAE2 in shallow shelf sequences of NE Egypt
Publication TypeJournal Article
Year of Publication2011
AuthorsEl-Sabbagh, A, Tantawy, AAAM, Keller, G, Khozyem, H, Spangenberg, J, Adatte, T, Gertsch, B
JournalCretaceous Research
Volume32
Issue6
Pagination705 - 722
Date PublishedJan-12-2011
ISSN01956671
Abstract

Two shallow water late Cenomanian to early Turonian sequences of NE Egypt have been investigated to evaluate the response to OAE2. Age control based on calcareous nannoplankton, planktic foraminifera and ammonite biostratigraphies integrated with δ13C stratigraphy is relatively good despite low diversity and sporadic occurrences. Planktic and benthic foraminiferal faunas are characterized by dysoxic, brackish and mesotrophic conditions, as indicated by low species diversity, low oxygen and low salinity tolerant planktic and benthic species, along with oyster-rich limestone layers. In these subtidal to inner neritic environments the OAE2 δ13C excursion appears comparable and coeval to that of open marine environments. However, in contrast to open marine environments where anoxic conditions begin after the first δ13C peak and end at or near the Cenomanian–Turonian boundary, in shallow coastal environments anoxic conditions do not appear until the early Turonian. This delay in anoxia appears to be related to the sea-level transgression that reached its maximum in the early Turonian, as observed in shallow water sections from Egypt to Morocco.  PDF

URLhttp://linkinghub.elsevier.com/retrieve/pii/S0195667111000474
DOI10.1016/j.cretres.2011.04.006
Short TitleCretaceous Research

Biostratigraphy, Age of Chicxulub Impact, and Depositional Environment of the Brazos River KTB Sequences

TitleBiostratigraphy, Age of Chicxulub Impact, and Depositional Environment of the Brazos River KTB Sequences
Publication TypeBook Chapter
Year of Publication2011
AuthorsKeller, G, Abramovich, T, Adatte, T, Berner, Z
Book TitleThe End-Cretaceous Mass Extinction and the Chicxulub Impact in Texas
VolumeNo. 100
EditionSEPM, Special Publication
Pagination81-122
PublisherSEPM (Society for Sedimentary Geology)
CityTulsa
ISBN978-1-56576-308-1
KeywordsBiostratigraphy, Brazos, Chicxulub impact, Cretaceous-Tertiary, d13C shift, Evolution, Ir anomaly, mass extinction, Sandstone complex, Sea level, Texas
Abstract

Integrated biostratigraphy, sedimentology and stable isotopes of 11 outcrops and wells along the Brazos River of Falls County, Texas, U.S.A., reveal the stratigraphic separation and sequential depositional history of the Chicxulub impact, followed by the sandstone complex and associated sea-level fall, which in turn was followed by the Cretaceous-Tertiary boundary (KTB). The KTB was identified up to 1 m above the sandstone complex based on three global standard criteria: the mass extinction in planktic foraminifera, the evolution of first Danian species and the negative d13C shift. No Ir anomaly is associated with the KTB or the Chicxulub impact-ejecta layers. Late Maastrichtian sediment deposition occurred in a middle-shelf environment that shallowed to inner-shelf depth at the time of deposition of the sandstone complex. At this time, Brazos sections show distinct shallowing from inner neritic in the north to subtidal and lagoonal at Cottonmouth Creek, with further shallowing to intertidal swamp or marsh conditions in the Darting Minnow Creek area to the south. The sandstone complex is the most prominent feature of the Brazos sections. At the base of this unit are reworked Chicxulub impact spherules and lithified clasts with impact spherules and mud cracks that bear witness to erosion of an older primary spherule deposit. This primary Chicxulub impact-ejecta layer was discovered between 45 and 60 cm below the sandstone complex in a (3 cm thick) impact-glass layer that is diagenetically altered to yellow clay. The sandstone complex, the reworked impact spherules, the spherule-rich clasts, and the yellow clay layer all clearly predate the KTB. PDF

Nature of the KTB Controversy

TitleNature of the KTB Controversy
Publication TypeBook Chapter
Year of Publication2011
AuthorsKeller, G, Adatte, T
Book TitleThe End-Cretaceous Mass Extinction and the Chicxulub Impact in Texas
VolumeNo. 100
EditionSEPM, Special Publication
ISBN978-1-56576-308-1
Abstract

One of the liveliest debates among scientists concerns potential causes of catastrophic extinction events, but none have garnered the imagination of scientists and public alike as the Cretaceous–Tertiary boundary (KTB) mass extinction including the demise of the dinosaurs 65 million years ago. Over three decades ago the discovery of anomalous concentrations of iridium in a thin clay layer between Cretaceous limestones and Tertiary claystones led Alvarez and collaborators to propose that a large meteorite crashed into Earth and caused the KTB mass extinction (Alvarez et al., 1980). Because iridium is rare on Earth’s surface, relatively common deep in Earth’s interior where it can surface via volcanic eruptions, but most abundant in some meteorites, this hypothesis rapidly gained support. With the discovery of the 175 km diameter Chicxulub impact crater on Yucata´n in 1991 (Hildebrand et al., 1991), followed by discoveries of impact glass spherule ejecta throughout the Caribbean, Central America, and North America in stratigraphic proximity of the KTB mass extinction (Izett et al., l99l; Swisher et al., l992; Smit et al., 1992) there seemed little doubt that the smoking gun had been found in the Chicxulub impact crater and that the impact-kill hypothesis was all but proven. For many scientists, the impact-kill hypothesis became a Eureka moment—a beautiful theory that could be expanded with many corollaries to account for virtually all observations. It was reconfirmed by 41 scientists in a recent Science article (Schulte et al., 2010) and expressed well by Birger Schmitz (2011) in his review of Ted Nield’s new book Incoming—Or why we should stop worrying and learn to love the meteorite. Nield (2011) writes a riveting account on meteorites that begins with fascinating historical facts, heresy, and beliefs through the ages before leading into the scientific geological account of the meteorite theory and an objective treatment of the controversy based on evidence inconsistent with this theory. There is nothing worse than destroying a beautiful theory with facts. Schmitz takes issue with Nield’s suggestion that doubters like Gerta Keller and her small team may have a point—the impact harmed nature, but the mass extinction had more varied causes. Schmitz considers this a compromise that belongs in politics, not in science. He goes on to state that he started his career in the 1980s as a non-believer of the impact theory, but has now seen the KTB clay layer in over 50 localities ‘‘where the iridium enriched layer always occurs exactly at the level at which the microscopic foraminifera typical of Cretaceous oceans disappear almost completely . . . The precise coincidence of these two events is so compelling that it is difficult to understand how anyone can doubt the direct relationship between them’’ (Schmitz, 2011).  PDF

Deccan volcanism linked to the Cretaceous-Tertiary boundary mass extinction: New evidence from ONGC wells in the Krishna-Godavari Basin

TitleDeccan volcanism linked to the Cretaceous-Tertiary boundary mass extinction: New evidence from ONGC wells in the Krishna-Godavari Basin
Publication TypeJournal Article
Year of Publication2011
AuthorsKeller, G, Bhowmick, PK, Upadhyay, H, Dave, A, Reddy, AN, Jaiprakash, BC, Adatte, T
JournalJournal of the Geological Society of India
Volume78
Issue5
Pagination399 - 428
Date PublishedJan-11-2011
ISSN0016-7622
Abstract

A scientific challenge is to assess the role of Deccan volcanism in the Cretaceous-Tertiary boundary (KTB) mass extinction. Here we report on the stratigraphy and biologic effects of Deccan volcanism in eleven deep wells from the Krishna-Godavari (K-G) Basin, Andhra Pradesh, India. In these wells, two phases of Deccan volcanism record the world’s largest and longest lava mega-flows interbedded in marine sediments in the K-G Basin about 1500 km from the main Deccan volcanic province. The main phase-2 eruptions (∼80% of total Deccan Traps) began in C29r and ended at or near the KTB, an interval that spans planktic foraminiferal zones CF1–CF2 and most of the nannofossil Micula prinsii zone, and is correlative with the rapid global warming and subsequent cooling near the end of the Maastrichtian. The mass extinction began in phase-2 preceding the first of four mega-flows. Planktic foraminifera suffered a 50% drop in species richness. Survivors suffered another 50% drop after the first mega-flow, leaving just 7 to 8 survivor species. No recovery occurred between the next three mega-flows and the mass extinction was complete with the last phase-2 mega-flow at the KTB. The mass extinction was likely the consequence of rapid and massive volcanic CO2 and SO2 gas emissions, leading to high continental weathering rates, global warming, cooling, acid rains, ocean acidification and a carbon crisis in the marine environment.

Deccan volcanism phase-3 began in the early Danian near the C29R/C29n boundary correlative with the planktic foraminiferal zone P1a/P1b boundary and accounts for ~14% of the total volume of Deccan eruptions, including four of Earth’s longest and largest mega-flows. No major faunal changes are observed in the intertrappeans of zone P1b, which suggests that environmental conditions remained tolerable, volcanic eruptions were less intense and/or separated by longer time intervals thus preventing runaway effects. Alternatively, early Danian assemblages evolved in adaptation to high-stress conditions in the aftermath of the mass extinction and therefore survived phase-3 volcanism. Full marine biotic recovery did not occur until after Deccan phase-3. These data suggest that the catastrophic effects of phase-2 Deccan volcanism upon the Cretaceous planktic foraminifera were a function of both the rapid and massive volcanic eruptions and the highly specialized faunal assemblages prone to extinction in a changing environment. Data from the K-G Basin indicates that Deccan phase-2 alone could have caused the KTB mass extinction and that impacts may have had secondary effects.

URLhttp://link.springer.com/article/10.1007%2Fs12594-011-0107-3
DOI10.1007/s12594-011-0107-3
Short TitleJ Geol Soc India

Defining the Cretaceous-Tertiary Boundary: A Practical Guide and Return to First Principles

TitleDefining the Cretaceous-Tertiary Boundary: A Practical Guide and Return to First Principles
Publication TypeBook Chapter
Year of Publication2011
AuthorsKeller, G
Book TitleThe End-Cretaceous Mass Extinction and the Chicxulub Impact in Texas
VolumeNo. 100
Number of Volumes23-42
EditionSEPM, Special Publication
PublisherSEPM (Society for Sedimentary Geology)
CityTulsa
ISBN978-1-56576-308-1
KeywordsCretaceous-Tertiary, d13C shift, Evolution, Guide, Ir anomaly, KT Definition, mass extinction
Abstract

The Cretaceous-Tertiary boundary (KTB) is one of the easiest epoch boundaries to identify, whether based on lithological changes in the field, geochemical analysis in the laboratory, or fossil content. A set of five KTB-identifying criteria, originally proposed by the ICS working group during 1980 -1990s, have proven globally applicable and independently verifiable: (1) mass extinction of Cretaceous planktic foraminifera, (2) evolution of the first Danian species, (3) KTB clay and red layer, (4) Ir anomaly, and (5) δ13C shift. Despite this successful track record, it was recently proposed to reduce the five KTB-identifying criteria to just two, the mass extinction and impact signals, based on the assumption that the Chicxulub impact caused the mass extinction and therefore defines the KTB. Because this assumption is contradicted by stratigraphic data in many places, this has led to contentious arguments, whereas defining the Chicxulub impact as KTB in age has led to circular reasoning. This study demonstrates the contradictions, pitfalls, and erroneous assumptions that accompany the use of these reduced impact-event-based KTB criteria. Returning the definition of the KTB to its GSSP based on all five criteria, and where this is not possible based on the mass extinction, the first appearance of Danian species, and the δ13C shift provide the most reliable KT boundary markers.   PDF

Nature and timing of extinctions in Cretaceous-Tertiary planktic foraminifera preserved in Deccan intertrappean sediments of the Krishna–Godavari Basin, India

TitleNature and timing of extinctions in Cretaceous-Tertiary planktic foraminifera preserved in Deccan intertrappean sediments of the Krishna–Godavari Basin, India
Publication TypeJournal Article
Year of Publication2011
AuthorsKeller, G, Adatte, T, Bhowmick, PK, Upadhyay, H, Dave, A, Reddy, AN, Jaiprakash, BC
JournalEarth and Planetary Science Letters
Volume341-344
Pagination211 - 221
Date PublishedJan-08-2012
ISSN0012821X
Abstract

In C29r below the Cretaceous-Tertiary boundary (KTB) massive Deccan Trap eruptions in India covered an area the size of France or Texas and produced the world’s largest and longest lava megaflows 1500 km across India through the Krishna–Godavari (K–G) Basin into the Bay of Bengal. Investigation of ten deep wells from the K–G Basin revealed four lava megaflows separated by sand, silt and shale with the last megaflow ending at or near the KTB. The biologic response in India was swift and devastating. During Deccan eruptions prior to the first megaflow, planktic foraminifera suffered 50% species extinctions. Survivors suffered another 50% extinctions after the first megaflow leaving just 7–8 species. No recovery occurred between the next three megaflows and the mass extinction was complete with the last mega-flow at or near the KTB. The last phase of Deccan volcanism occurred in the early Danian C29n with deposition of another four megaflows accompanied by delayed biotic recovery of marine plankton. Correlative with these intense volcanic phases, climate changed from humid/tropical to arid conditions and returned to normal tropical humidity after the last phase of volcanism. The global climatic and biotic effects attributable to Deccan volcanism have yet to be fully investigated. However, preliminary studies from India to Texas reveal extreme climate changes associated with high-stress environmental conditions among planktic foraminifera leading to blooms of the disaster opportunist Guembelitria cretacea during the late Maastrichtian.  PDF

URLhttp://linkinghub.elsevier.com/retrieve/pii/S0012821X12003056
DOI10.1016/j.epsl.2012.06.021
Short TitleEarth and Planetary Science Letters

Environmental effects of Deccan volcanism across the Cretaceous–Tertiary transition in Meghalaya, India

TitleEnvironmental effects of Deccan volcanism across the Cretaceous–Tertiary transition in Meghalaya, India
Publication TypeJournal Article
Year of Publication2011
AuthorsGertsch, B, Keller, G, Adatte, T, Garg, R, Prasad, V, Berner, Z, Fleitmann, D
JournalEarth and Planetary Science Letters
Volume310
Issue3-4
Pagination272 - 285
Date Published01/2011
ISSN0012821X
Abstract

The Um Sohryngkew section of Meghalaya, NE India, located 800–1000 km from the Deccan volcanic province, is one of the most complete Cretaceous–Tertiary boundary (KTB) transitions worldwide with all defining and supporting criteria present: mass extinction of planktic foraminifera, first appearance of Danian species, δ13C shift, Ir anomaly (12 ppb) and KTB red layer. The geochemical signature of the KTB layer indicates not only an extraterrestrial signal (Ni and all Platinum Group Elements (PGEs)) of a second impact that postdates Chicxulub, but also a significant component resulting from condensed sedimentation (P), redox fluctuations (As, Co, Fe, Pb, Zn, and to a lesser extent Ni and Cu) and volcanism. From the late Maastrichtian C29r into the early Danian, a humid climate prevailed (kaolinite: 40–60%, detrital minerals: 50–80%). During the latest Maastrichtian, periodic acid rains (carbonate dissolution; CIA index: 70–80) associated with pulsed Deccan eruptions and strong continental weathering resulted in mesotrophic waters. The resulting super-stressed environmental conditions led to the demise of nearly all planktic foraminiferal species and blooms (> 95%) of the disaster opportunist Guembelitria cretacea. These data reveal that detrimental marine conditions prevailed surrounding the Deccan volcanic province during the main phase of eruptions in C29r below the KTB. Ultimately these environmental conditions led to regionally early extinctions followed by global extinctions at the KTB.

URLhttp://linkinghub.elsevier.com/retrieve/pii/S0012821X11004791
DOI10.1016/j.epsl.2011.08.015
Short TitleEarth and Planetary Science Letters

Cretaceous-Tertiary Mass Extinction in Marginal and Open Marine Environments: Texas, U.S.A., and Tunisia

TitleCretaceous-Tertiary Mass Extinction in Marginal and Open Marine Environments: Texas, U.S.A., and Tunisia
Publication TypeBook Chapter
Year of Publication2011
AuthorsKeller, G
Book TitleThe End-Cretaceous Mass Extinction and the Chicxulub Impact in Texas
VolumeNo. 100
EditionSEPM, Special Publication
Pagination197-226
PublisherSEPM (Society for Sedimentary Geology)
CityTulsa
ISBN Number978-1-56576-308-1
KeywordsChicxulub impact, d13C shift, Depositional environment, Evolution, high stress, Ir anomaly, KT Mass Extinction, Sandstone complex, Sea level, Shallow Environment, Texas, Tunisia. Cretaceous-Tertiary
Abstract

The Cretaceous-Tertiary boundary (KTB) sequences along the Brazos River, Texas, U.S.A., have been controversial for over two decades. At issue is whether the KTB and the mass extinction should be placed at the base of a sandstone complex based on the presence of Chicxulub impact spherules or at the mass extinction. This issue goes to the very core of the KTB controversy – did the Chicxulub impact cause the KTB mass extinction? Faunal, stable isotope, platinum group elements (PGEs), and lithological analyses of six Brazos cores and outcrop sections, and comparison of these data with the Elles, Tunisia, parastratotype shed empirical light on these issues. The KTB is well marked by the mass extinction of planktic foraminifera, the first appearance of Danian species, and the δ13C negative shift, which occurs up to 1 m above the sandstone complex that contains two to three impact spherule layers at its base. There is no Ir anomaly at the KTB and mass extinction, but minor Ir enrichments are present in condensed intervals within and slightly above the sandstone complex. Clasts at the base of the sandstone complex contain impact spherules that reveal earlier deposition, lithification, erosion and redeposition. The Chicxulub impact thus predates not only the KTB, but also the sandstone complex. A yellow clay layer consisting of altered impact glass 45-60 cm below the sandstone complex (zone CF1) may represent the original Chicxulub impact ejecta fallout.

 The mass extinction pattern in the Brazos sections appears gradual or progressive compared with patterns documented from open-ocean environments. This is largely the result of high sediment accumulation rates in inner-neritic depositional settings coupled with the sea-level fall that culminated with deposition of the sandstone complex. Comparison of various extinction parameters, such as overall species richness, species abundances, life strategies, and separation into opportunists vs. specialists reveals that the shallow Brazos environment excluded the specialized larger and deeper dwelling species (~40%) that suffered the most abrupt mass extinction at the KTB. The Brazos extinction pattern thus reflects the mass extinction in the most hardy and environmentally most tolerant assemblages, which include several KTB survivors. Similar patterns are observed in shallow-water environments of southern Tunisia, Egypt, Denmark and Argentina. These data strongly show that the Chicxulub impact predates the KTB and caused no species extinctions at the KTB or at the earlier time of the impact.   PDF

The Cretaceous-Tertiary Mass Extinction: Theories and Controversies

TitleThe Cretaceous-Tertiary Mass Extinction: Theories and Controversies
Publication TypeBook Chapter
Year of Publication2011
AuthorsKeller, G
Book TitleEnd-Cretaceous Mass Extinction and the Chicxulub Impact in Texas
VolumeNo. 100
EditionSpecial Publication
Pagination7-22
PublisherSEPM (Society for Sedimentary Geology)
CityTulsa
Abstract

The Cretaceous-Tertiary (KT) mass extinction is primarily known for the demise of the dinosaurs, the Chicxulub impact and the frequently rancorous 30 years old controversy over the cause of this mass extinction. Since 1980 the impact hypothesis has steadily gained support that culminated in 1990 with the discovery of the Chicxulub crater on Yucatan as the KT impact site and ‘smoking gun’ that proved this hypothesis. In a perverse twist of fate this discovery also began the decline of this hypothesis because for the first time it could be tested directly based on the impact crater and impact ejecta in sediments throughout the Caribbean, Central and North America. Two decades of multi-disciplinary studies amassed a database with a sum total that overwhelmingly reveals the Chicxulub impact as predating the KT mass extinction in the impact crater cores, in sections throughout NE Mexico and in Brazos River sections of Texas. This chapter recounts the highlights of the KT controversy, the discovery of facts inconsistent with the impact hypothesis and the resurgence of the Deccan volcanism hypothesis as the most likely cause for the mass extinction.  PDF

Age and Origin of the Chicxulub Impact and Sandstone Complex, Brazos River, Texas: Evidence from Lithostratigraphy and Sedimentilogy

TitleAge and Origin of the Chicxulub Impact and Sandstone Complex, Brazos River, Texas: Evidence from Lithostratigraphy and Sedimentilogy
Publication TypeBook Chapter
Year of Publication2011
AuthorsAdatte, T, Keller, G, Baum, G
Book TitleThe End-Cretaceous Mass Extinction and the Chicxulub Impact in Texas
VolumeNo. 100
EditionSEPM, Special Publication
Pagination43-80
PublisherSEPM (Society for Sedimentary Geology)
CityTulsa
ISBN978-1-56576-308-1
Abstract

Multidisciplinary investigations based on the lithology, sedimentology, mineralogy and biostratigraphy of upper Maastrichtian to lower Danian boundary (KTB) sequences along 3.5 km of the Brazos River in Falls County, Texas, reveal depositonal sequences, including an impact spherule-rich sandstone complex, characteristic of sequence stratigraphic models applied to shallow shelf areas, such as incised valleys, lag conglomerate, storm deposits and repeated bioturbation. The top of the Corsicana Formation coincides with a channel, which we interpret as incised valley. The erosion surface marks a major depositional sequence boundary (SB) associated with the latest Maastrichtian sea-level fall. Initial channel deposits consist of coarse shelly glauconitic sand with large lithified clasts containing impact spherules and large bored and encrusted phosphatized concretions, which we interpret to indicate that the Chicxulub impact occurred well prior to the lithification, erosion and redeposition at the base of the channel. The primary Chicxulub ejecta layer lies about 40-65 cm below the sandstone complex in a 3 cm-thick yellow clay layer that consists of cheto smectitie (altered impact glass) interbedded in claystones of the Corsicana Formation. Above the sandstone complex, claystones and mudstones are burrowed and correspond to a condensed interval interpreted as a maximum flooding surface (MFS).Based on biostratigraphy and the δ13C shift, the KT boundary is up to 1 m (50-100 ky) above the sandstone complex and coincides with increased sediment accumulation during the early Danian sea level rise (HST). No impact signals are observed in KT deposits. These features are inconsistent with a single catastrophic bolide impact on Yucatan and associated mega-tsunami deposition as commonly interpreted.

 The biostratigraphy and KT characteristic δ13C shift of the Brazos sections indicate that the KTB, sandstone complex and the Chicxulub impact occurrred as three different stratigraphic events during the late Maastrichtian planktic foraminiferal zone CF1. These are represented by : 1) the Chicxulub impact sequence deposited about 200-300 ky prior to the KTB ;  2) the sandstone complex with reworked impact spherules deposited in incised valleys during the latest sea level fall about 100-150 ky prior to the KTB, and 3) the KTB event during the subsequent HST and following the condensed MFS.   PDF

Maastrichtian Planktic Foraminiferal Biostratigraphy and Paleoenvironment of Brazos River, Falls County, Texas, U.S.A.

TitleMaastrichtian Planktic Foraminiferal Biostratigraphy and Paleoenvironment of Brazos River, Falls County, Texas, U.S.A.
Publication TypeBook Chapter
Year of Publication2011
AuthorsAbramovich, S, Keller, G, Berner, Z, Cymbalista, M
Book TitleThe End-Cretaceous Mass Extinction and the Chicxulub Impact in Texas
VolumeNo. 100
EditionSEPM Special Publication
Pagination123-156
ISBN978-1-56576-308-1
Abstract

Investigation of late Maastrichtian faunal and environmental changes in three subsurface wells spanning over 3 km along the Brazos River, Texas, reveals similar minimum diversity high-stress assemblages associated with shallow shelf conditions. Upper Maastrichtian sediments recovered span planktic foraminiferal (CF) zones CF1 to CF4 in well Mullinax-1 to the north and well KT3 at Cottonmouth Creek, and zones CF1-CF2 in Mullinax-3 at Darting Minnow Creek. Biotic stress conditions are demonstrated by the minimum species richness, near exclusion of larger specialized species, dwarfing, and dominance of small generalist taxa.  Faunal assemblages dominated by alternating abundances of the disaster opportunist Guembelitria cretacea (Cushman) (particularly in zones CF4 and CF2-CF1) and heterohelicid species [e.g., Heterohelix globulosa (Ehrenberg), H. planata (Cushman), Paraspiroplecta navarroensis (Loeblich)].. Other small surface and subsurface mixed layer dwellers are rare to common (e.g., hedbergellids, globigerinellids, pseudoguembelinids).

The coincidence of Guembelitria blooms with lithological changes and oxygen and carbon stable isotope excursions may represent discrete episodes of freshwater runoff related to short -term pulses of the latest Maastrichtian (zone CF1-CF2) global climate warming. Climate warming ended in the upper part of zone CF1 with the return to a cooler climate, lower sea level and the formation of incised valleys in a coastal-lagoonal environment. With the subsequent early transgression, incised valleys were infilled by a sandstone complex with reworked impact spherules, as well as lithified clasts with impact spherules up to 80 cm below the K-Pg Boundary. In this environment the shallow inner neritic setting superimposed by sea level and climate changes is the most probable cause for the observed high biotic stress conditions preceding the K-Pg Boundary in the Brazos area.  PDF

Cretaceous Extinctions: Evidence Overlooked

TitleCretaceous Extinctions: Evidence Overlooked
Publication TypeJournal Article
Year of Publication2010
AuthorsKeller, G, Adatte, T, Pardo, A, Bajpai, S, Khosla, A, Samant, B
JournalScience
Volume328
Pagination974–975
Date Publishedmay
Abstract

In their Review “The Chicxulub Asteroid impact and mass extinction at the Cretaceous-Paleogene boundary” (5 March, p. 1214), P. Schulte et al. analyzed the 30-year-old controversy over the cause of the end-Cretaceous mass extinction and concluded that the original theory of 1980 was right: A large asteroid impact on Yucatan was the sole cause for this catastrophe. To arrive at this conclusion, the authors used a selective review of data and interpretations by proponents of this viewpoint. They ignored the vast body of evidence inconsistent with their conclusion—evidence accumulated by scientists across disciplines (paleontology, stratigraphy, sedimentology, geochemistry, geophysics, and volcanology) that documents a complex long-term scenario involving a combination of impacts, volcanism, and climate change. Here, we point out some of the key evidence that Schulte et al. overlooked.

The underlying basis for Schulte et al.'s claim that the Chicxulub impact is the sole cause for the Cretaceous-Paleogene (K-Pg) mass extinction is the assumption that the iridium (Ir) anomaly at the K-Pg boundary and Chicxulub are the same age. There is no evidence to support this assertion. No Ir anomaly has ever been identified in association with undisputed Chicxulub impact ejecta (impact glass spherules), and no impact spherules have ever been identified in the Ir-enriched K-Pg boundary clay in Mexico or elsewhere (12). In rare deep-sea sites where the Ir anomaly is just above impact spherules, it is due to condensed sedimentation and/or nondeposition.

A Chicxulub impact–generated tsunami is another basic assumption of Schulte et al. to account for the impact spherules in late Maastrichtian sediments (including a sandstone complex) in Mexico and Texas. Multiple lines of evidence contradict this assumption and demonstrate long-term deposition before the K-Pg, including burrowed horizons, multiple impact spherule layers separated by limestone, and spherule-rich clasts that indicate the original deposition predates the K-Pg and excludes tsunami deposition (14).

Evidence of the pre–K-Pg age of the Chicxulub impact can also be found in sediments above the sandstone complex in Texas and northeastern Mexico and above the impact breccia in the Chicxulub crater. Evidence shows that the K-Pg boundary is not linked to the sandstone complex and impact spherules (1247).

Evidence that supports the pre–K-Pg age of the Chicxulub impact is also found in the presence of a spherule layer in late Maastrichtian sediments below the sandstone complex in northeastern Mexico and Texas (248).

Deccan volcanism is dismissed by Schulte et al. as much older and of no consequence in the K-Pg mass extinction. Recent Deccan volcanism studies show the contrary (911). These studies link the mass extinction with the main phase of Deccan eruptions.

When this evidence is taken into account, it is clear that the massive Chicxulub and Deccan database indicates a long-term multicausal scenario and is inconsistent with the model proposed by Schulte et al.   PDF

URLhttps://doi.org/10.1126/science.328.5981.974-a
DOI10.1126/science.328.5981.974-a

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