Thierry Adatte

Intertrappean beds in the main part of the Deccan Traps volcanic province of peninsular India are generally considered to be terrestrial deposits of late Maastrichtian age, lthough the precise position of the Cretaceous-Tertiary (K-T) boundary event has remained speculative. Recent investigations of the outlying Deccan Traps exposures around Rajahmundry near the southeastern coast, however, revealed the K-T event in intertrappean

Recent advances in Deccan volcanic studies suggest that the main phase of eruptions occurred rapidly over tens of thousands of years near the end of the Maastrichtian (Chenet et al. 2007, 2008) and may have caused the mass extinction as initially discovered in intertrappean sediments exposed in quarries of Rajahmundry, SE India. In these shallow marine sediments early Danian zone P1a planktic foraminifera were deposited in C29r immediately above the last mega eruption of the main volcanic phase (Keller et al. (2008). At Jhilmili in central India (Madhya Pradesh), early Danian zone P1a assemblages were also discovered in intertrappean sediments, which mark a marine incursion in a predominantly terrestrial sequence which signals a major seaway existed at K-T time. In Meghalaya, NE India, about 600 km from the Deccan volcanic province the K-T boundary and mass extinction identified from planktic foraminifera, calcareous nannofossils and palynomorphs is marked by very large Ir (11.8 ppb...

The response of shallow-water sequences to oceanic anoxic event 2 and mid-
Cenomanian events 1a and 1b was investigated along the west African margin
of Morocco north of Agadir (Azazoul) and correlated with the deep-water
sequence of the Tarfaya Basin (Mohammed Beach) based on biostratigraphy,
mineralogy, phosphorus and stable isotopes. In the deeper Mohammed Beach
section results show double peaks in d13Corg for mid-Cenomanian events 1a
and 1b (Rotalipora reicheli biozone, lower CC10a biozone), the characteristic
oceanic anoxic event 2 d13C excursion (Rotalipora cushmani extinction, top of
CC10a biozone) and laminated (anoxic) black shale. In the shallow
environment north of Agadir, a fluctuating sea-level associated with dysoxic,
brackish and mesotrophic conditions prevailed during the middle to late
Cenomanian, as indicated by oyster biostromes, nannofossils, planktonic and
benthonic foraminiferal assemblages. Anoxic conditions characteristic of
oceanic anoxic event 2 (for example, laminated black shales) did not reach
into shallow-water environments until the maximum transgression of the early
Turonian. Climate conditions decoupled along the western margin of Morocco
between mid-Cenomanian event 1b and the Cenomanian–Turonian boundary,
as also observed in eastern Tethys. North of Agadir alternating humid and dry
seasonal conditions prevailed, whereas in the Tarfaya Basin the climate was
dry and seasonal. This climatic decoupling can be attributed to variations in
the Intertropical Convergence Zone and in the intensity of the north-east trade
winds in tropical areas.

Multidisciplinary studies, including stratigraphy, sedimentology, mineralogy and geochemistry, of the new core Mullinax-1 and outcrops along the Brazos River and Cottonmouth Creek, Falls County, Texas, reveal the complex history of the Chicxulub impact,
the event deposit and the K–T boundary event. The K–T boundary, as identified by the negative δ13C shift, first occurrence of Danian planktic foraminifera and palynomorphs occurs 80 cm above the event deposit in core Mullinax-1. The underlying 80 cm interval was deposited in a shallow low oxygen environment during the latest Maastrichtian, as indicated by high stress microfossil
assemblages, small shells and burrows infilled with framboidal pyrite. The underlying event deposit, commonly interpreted as K–T impact tsunami, consists of a basal conglomerate with clasts containing Chicxulub impact spherules, repeated upward fining units of spherule-rich sands, followed by hummocky cross-bedded and laminated sands, which are burrowed by Thalassinoides, Planolites and Ophiomorpha and truncated by erosion. This suggests a series of temporally separated storm events with recolonization
of the ocean floor by invertebrates between storms, rather than a series of waning tsunami-generated waves. The lithified clasts with impact spherules at the base of the event deposit provide strong evidence that the Chicxulub impact ejecta layer predates the event deposit, but was eroded and re-deposited during the latest Maastrichtian sea level lowstand. The original Chicxulub ejecta layer was discovered in a 3 cm thick yellow clay layer interbedded in undisturbed late Maastrichtian clay- and mudstones 40 cm below the base of the event deposit and near the base of planktic foraminiferal zone CF1, which spans the last 300 kyr of the Maastrichtian. The yellow clay consists of cheto smectite derived from alteration of impact glass, as indicated by rare altered glass spherules with similar chemical compositions as reworked spherules from the event deposit and Chicxulub impact spherules from NE Mexico and Haiti. The Brazos sections thus provide strong evidence that the Chicxulub impact predates the K–T boundary by about 300 kyr, consistent with earlier observations in NE Mexico and the Chicxulub crater core Yaxcopoil-1.

Yaxcopoil-1 (Yax-1), drilled within the Chicxulub crater, was expected to yield the final proof that this impact occurred precisely 65 Myr ago and caused the mass extinction at the
Cretaceous-Tertiary (K/T) boundary. Instead, contrary evidence was discovered based on five independent proxies (sedimentologic, biostratigraphic, magnetostratigraphic, stable isotopic, and iridium) that revealed that the Chicxulub impact predates the K/T boundary by about 300,000 years and could not have caused the mass extinction. This is demonstrated by the presence of five bioturbated glauconite layers and planktic foraminiferal assemblages of the latest Maastrichtian zone CF1 and is corroborated by magnetostratigraphic chron 29r and characteristic late Maastrichtian stable isotope signals. These results were first presented in Keller et al. (2004). In this study, we present more detailed evidence of the presence of late Maastrichtian planktic foraminifera, sedimentologic, and mineralogic analyses that demonstrate that the Chicxulub impact breccia predates the K/T boundary and that the sediments between the breccia and the K/T boundary were
deposited in a normal marine environment during the last 300,000 years of the Cretaceous.

Since the early l990s the Chicxulub crater on Yucatan, Mexico, has been hailed as the smoking gun that proves the hypothesis that an asteroid killed the dinosaurs and caused the mass extinction of many other organisms at the Cretaceous-Tertiary (K-T) boundary 65 million years ago. Here, we report evidence from a previously uninvestigated core, Yaxcopoil-1, drilled within the Chicxulub crater, indicating that this impact predated the K-T boundary by 300,000 years and thus did not cause the end-Cretaceous mass extinction as commonly believed. The evidence supporting a pre-K-T age was obtained from Yaxcopoil-1 based on five independent proxies, each with characteristic signals across the K-T transition: sedimentology, biostratigraphy, magnetostratigraphy, stable isotopes,
and iridium. These data are consistent with earlier evidence
for a late Maastrichtian age of the microtektite deposits in northeastern Mexico.

Profound biotic changes accompanied the late Cenomanian d13C excursion and OAE2 in planktic foraminifera in the Tarfaya Basin of Morocco. Planktic foraminifera experienced a severe turnover, though no mass extinction, beginning with the rapid d13C excursion and accelerating with the influx of oxic bottom waters during the first peak and trough of the excursion. Species extinctions equaled the number of evolving species, though only the disaster opportunists Guembelitria and Hedbergella thrived along with a low oxygen tolerant benthic assemblage. The succeeding d13C plateau and organic-rich black shale deposition marks the anoxic event and maximum biotic stress accompanied by a prolonged drop in
diversity to just two species, the dominant (80–90%) low oxygen tolerant Heterohelix moremani and surface dweller Hedbergella planispira. After the anoxic event other species returned, but remained rare and sporadically present well into the lower Turonian, whereas Heterohelix moremani remained the single dominant species. The OAE2 biotic turnover suggests that the stress to calcareous plankton was related to changes in the watermass stratification, intensity of upwelling, nutrient flux and oxic levels in the water column driven by changes in climate and oceanic circulation. Results presented here
demonstrate a 4-stage pattern of biotic response to the onset, duration, and recovery of OAE2 that is observed widely across the Tethys and its bordering epicontinental seas.

In the 1990s the Chicxulub impact was linked to the K–T boundary by impact spherules at the base of a sandstone complex that was interpreted as an impact-generated tsunami deposit. Since that time a preponderance of evidence has failed to support this interpretation, revealing long-term deposition of the sandstone complex, the K–T boundary above it and the primary impact spherule ejecta interbedded in Late
Maastrichtian marls below. Based on evidence from Mexico and Texas we suggested that the Chicxulub impact predates the K–T boundary. Impact-tsunami proponents have challenged this evidence largely on the basis that the stratigraphically lower spherule layer in Mexico represents slumps and widespread tectonic disturbance, although no such evidence has been presented. The decades-old controversy over the cause of the K–T mass extinction will never achieve consensus, but careful documentation of results that are reproducible
and verifiable will uncover what really happened at the end of the Crectaceous. This study takes an important step in that direction by showing (1) that the stratigraphically older spherule layer from El Pen˜on, NE Mexico, represents the primary Chicxulub impact spherule ejecta in tectonically undisturbed sediments and (2) that this impact caused no species extinctions.

The main Deccan volcanic province erupted mainly in terrestrial to lacustrine environments of late Maastrichtian to early Paleocene age and lack of age diagnostic fossils to accurately place the Cretaceous-Tertiary (K-T) boundary. As a result, the precise position of the K-T event within the volcanic province has remained speculative and no realistic assessment of the biotic consequences of Deccan volcanism at K-T time has been possible. Here, we report the discovery of early Danian Zone P1a planktic foraminifera within lacustrine to brackish-marine ostracod assemblages near Jhilmilli, Chhindwara District, India. These foraminiferal assemblages identify the K-T boundary in intertrappean sediments bracketed by basalt traps that are regionally correlative with C29R (Ambenali Formation). The Jhilmilli sequence is thus correlative with the shallow-marine intertrappean Zone P1a assemblage and C29R and C29N of the lower and upper basalt traps exposed in Rajahmundry quarries. The presence of planktic foraminifera in predominantly terrestrial intertrappean sediments some 800 km from the nearest ocean suggests the presence of a seaway along the Narmada and Tapti rift zones with Jhilmilli located at the eastern margin.

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 megaflow 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.

... W. Stinnesbeck, P. Schulte, F. Lindenmaier, T. Adatte, M. Affolter, L. Schilli, G. Keller, D. Stüben, Z. Berner, U. Kramar, SJ Burns, and JG López ... La Sierrita de Nuevo Leon, au Mexique, on retrouve trois couches de sphérules et jusqu'à 6 m de marnes pélagiques les séparent des ...

Yaxcopoil-1 (Yax-1), drilled within the Chicxulub crater, was expected to yield the final proof that this impact occurred precisely 65 Myr ago and caused the mass extinction at the Cretaceous-Tertiary (K/T) boundary. Instead, contrary evidence was discovered based on five independent proxies (sedimentologic, biostratigraphic, magnetostratigraphic, stable isotopic, and iridium) that revealed that the Chicxulub impact predates the K/T boundary by about 300,000 years and could not have caused the mass extinction. This is demonstrated by the presence of five bioturbated glauconite layers and planktic foraminiferal assemblages of the latest Maastrichtian zone CF1 and is corroborated by magnetostratigraphic chron 29r and characteristic late Maastrichtian stable isotope signals. These results were first presented in Keller et al. (2004). In this study, we present more detailed evidence of the presence of late Maastrichtian planktic foraminifera, sedimentologic, and mineralogic analyses that demonstrate that the Chicxulub impact breccia predates the K/T boundary and that the sediments between the breccia and the K/T boundary were deposited in a normal marine environment during the last 300,000 years of the Cretaceous.