Discussion
Reply to ‘Chicxulub impact predates K–T boundary: New evidence from Brazos, Texas’ Comment by Schulte et al.

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Introduction

We appreciate this opportunity for further discussion of the Brazos, Texas, K–T boundary sequences and their timing with respect to the Chicxulub impact. Keller et al. (2007) used a multidisciplinary approach to document the stratigraphy, paleontology, mineralogy and geochemistry of the newly drilled Mullinax-1 core and a new outcrop sequence. Based on this multi-proxy dataset very strong evidence was presented that reveals that the Chicxulub impact predates the K–T mass extinction (Keller et al., 2007). Schulte et al. take issue with this approach and our findings largely because they believe that the Chicxulub impact caused the K–T mass extinction and therefore the K–T boundary must be placed at the impact spherule layer (Schulte et al., 2008-this volume, Schulte et al., 2006, Smit et al., 1996). We welcome this opportunity to clarify misunderstandings, misconceptions and misinterpretations of the K–T record in Texas and elsewhere.

At the heart of our disagreements is the decades old controversy about the cause of the end-Cretaceous mass extinction. Schulte and collaborators have long argued that the sandstone complex, or event deposit, with impact spherules at the base in NE Mexico and Texas mark Chicxulub impact-generated tsunami deposits at the K–T boundary (Schulte et al., 2006, Smit et al., 1996, Smit, 1999, Schulte et al., 2003, Smit et al., 2004). Keller and others have documented that these sandstone complexes were deposited over a long time period, that the K–T boundary is above these deposits and the original Chicxulub impact layer is in late Maastrichtian sediments predating the K–T mass extinction by about 300ky (Keller et al., 1997, Keller et al., 2002a, Keller et al., 2003a, Keller et al., 2004, Gale, 2006). With the stratigraphic sequences in Mexico and Texas in direct conflict with the Chicxulub as K–T impact scenario, Schulte and collaborators now consider these sequences as too complex to reveal the K–T and Chicxulub impact history. Instead, they favor condensed deep-sea and terrestrial sections as the ultimate support for the Chicxulub K–T age claim because they juxtapose the spherule layer and early Danian sediments (Martínez-Ruiz et al., 2002, Olsson et al., 1997, MacLeod et al., 2006). But the ultimate test for any historical sequence of events lies in the expanded records of continental shelf and slope areas where high sedimentation rates reveal stratigraphic separation and normal sedimentation between events, such as we documented for Texas (Keller et al., 2007). We stand by our published data and interpretations and present new data and graphics to clarify their misconceptions and misrepresentations on the placement of the K–T boundary.

Section snippets

Placement of K–T boundary

Schulte et al. claim that the K–T boundary is based on just two criteria: (1) the evidence of an asteroid impact and (2) the mass extinction in planktic foraminifera (Molina et al., 2006). In the early 1990s when the El Kef and Elles sections of Tunisia were studied by the ICS working group (including Keller), for the nomination of the K–T stratotype, the criteria for the placement of this boundary included: (1) the lithological break from marl to clay, (2) the 2–3mm oxidized red layer at the

Iridium

There is no iridium anomaly or impact spherule layer at the K–T boundary as defined by δ13C and standard micropaleontological critera in the Brazos sections. The three Ir profiles of the classic Brazos-1 outcrop were all done on the same section with just a few meters of lateral exposure and where a discontinuous thin rust-colored sand layer is present. Rocchia et al. (1996) show the maximum Ir anomaly in the 1–2cm below this layer and a second anomaly immediately above, just as we show in Fig.

Yellow clay — original Chicxulub ejecta layer

Schulte et al. claim that without “true Chicxulub ejecta spherules, i.e. round- or drop-shaped spherules with internal cavities and vesicles … a volcanic origin for the yellow clay layer is more plausible.” However, restricting Chicxulub impact ejecta to well-preserved spherules and relegating all altered glass to volcanic origin makes little sense, especially since they argue that the K–T clay, iron and glauconite spherules from the Tethyan realm represent Chicxulub spherules. It is well known

Sequence stratigraphy

Sea-level change was not the major topic of our research report and was only used in the discussion and summary Fig. 9 to illustrate the depositional environment of the Brazos sections (Keller et al., 2007). Yet, Schulte et al. accuse us of violating “well-established sequence stratigraphic concepts,” then launch into a lecture on sequence stratigraphy and sea level analysis developed by Baum (one of the authors) and his colleagues at Exxon. They seem shocked that anyone would propose a major

Omission of evidence?

Schulte et al. criticize our Brazos research paper on the basis that it is not a review paper and therefore omitted evidence “in and outside the Gulf of Mexico” that would support their viewpoint. Specifically, they claim “more than 24 recent ODP K–P drillcores all provide strong support for the genetic relationship between the Chicxulub impact event and the worldwide distributed K–P boundary ejecta layer” and conclude “Yet, Keller and her co-workers prefer to keep ignoring nearly all of it.”

Conclusions

Schulte et al. conclude that Keller et al. (2007) “have not made any case for Chicxulub as a pre-K–T impact.” But they made their case by repeatedly resorting to factual misrepresentations, misinterpretations, out of context quotes, selective use of references, ignoring critical studies and bogus arguments. Amazingly, this was done in the most strident tone and accusations of misuse of biostratigraphy, geochemistry, mineralogy and sequence stratigraphy.

In our reply we have addressed the major

Acknowledgement

This material is based upon work supported by the National Science Foundation's Continental Dynamics Program and Sedimentary Geology and Paleobiology Program under NSF Grants EAR-0207407 and EAR-0447171.

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