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Chicxulub Drilling debates, Is the Chicxulub crater the KT boundary killer or Not? Query by Markus Harting and Reply by Smit

"Fluidized, squeezed, diapir-like slumped and folded" Méndez-Formation at Rancho Nuevo?

From Markus Harting *

In Smit's recent reply to Keller et al. he claims that the Upper Cretaceous Méndez shales at the Rancho Nuevo section are fluidised and squeezed up, diapir-like. He further claims that no additional spherule layers are present in the obviously bedded and undisturbed Mendez marls below the spherule layer of unit 1 at the base of the siliciclastic deposit (Fig. c of Smit). No data are presented in support of these claims.

We have conducted a detailed investigation of the Rancho Nuevo outcrop based on field examinations and laboratory studies, including sedimentology, geochemistry, mineralogy and microscopy. Our studies revealed no evidence of fluidised or squeezed Méndez Marls, or squeezing at the edges of the sandstone unit. The outcrop feature interpreted by Smit as "diapir-like" appears to be the result of the unique view across, and as a cross section, of the channelised siliciclastic deposit.

Smit's claim that no additional spherule layers are present below the unit 1 spherule deposit is also unconfirmed. Our investigation revealed at least one more spherule horizon approximately 60&endash;70 cm below the base of the spherule-rich deposit (SRD) of Unit 1, which supports the existence of multiple spherule horizons (Keller et al., 2002, 2003).

Our geochemical and mineralogical analyses of the spherule unit 1 (Smit's primary ejecta layer) revealed altered ejecta material and secondary authigenic minerals due to hydrothermal influence, in contrast to well preserved ejecta within the Méndez Marls. These investigations, as well as the stratigraphic position, indicate a reworked origin for the uppermost spherule layer, which is commonly labelled unit 1 of the siliciclastic deposit.

References

Keller G. et al. 2002. Multiple spherule layers in the late Maastrichtian of northeastern Mexico. G eological Society of America Special Paper 356: 145-161.

Keller G., Stinnesbeck W., Adatte T. and Stueben D. 2003a. Multiple impacts across the Cretaceous-Tertiary boundary . Earth-Science Reviews 62, 327-363.

*Institute of Mineralogy and Geochemistry, Karlsruhe University, Germany

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Reply by J. Smit

Sir,

I have my reservations about how to recognize fluidization or and resulting slump-structures in the Mendez marls by geochemistry, microscopy, mineralogy and laboratory studies. It is only in the field that one can recognize these and I invite Markus Harting to again have a closer look at the unmistakable evidence for slumping at Rancho Nuevo (image 1). On the side of the channels there is ample evidence for syn-sedimentary oversteepening of the channel walls. I include here an image of such a slump taken just below the criticized 'diapirs'

The picture (Fig. c of Smit) of the Rancho Nuevo channels was taken as much as possible parallel to the longest axis of the two channels (see inset image 2), in order to avoid such a "result of the unique view across, and as a cross section, of the channelized siliciclastic deposit". The diapiric movements of the top layers of the Mendez are real, not an artifact of the figure.

Also, the presence of "well preserved ejecta" (actually not well preserved, all tektite glass is altered no matter in what layer) in the supposed lower primary layer is also well visible at the Mimbral site. Well preserved (i.e. not flattened) spherules occur adjacent to sublayers with flattened green spherules. All spherule layers in eastern Mexico are reworked, there is no primary layer left. I have no objection against multiple spherule horizons. I have given my interpretation of the 'multiplets': they are alternations of soft Mendez-clast rich and spherule rich aggradational layers, not separate events separated by thousands of years.

image 1: slumpfolds of one of the turbiditic layers in the Mendez shales just underlying the clastic sandstones (see inset and image 2 for location).

(click on image for enlargement)

image 2 Steepened sidewall, oversteepened due to the sinking of the channel into the soft Mendez marls, of the NE-most channel 2, filled with the clastic sandstones. Vertical striations and furrows on the sidewall indicate the upward movements of the Mendez marls against the channel. The horizontal lineations, perpendicular to the furrows, are synsedimentary faults that indicate similar movements within the sandstones itself. The top sandstone layer stretches across both channels and the top of the diapirized Mendez. Inset: fieldsketch of the orientation of the two channels, with the diapirized Mendez in between. Red arrow, image 1 slump. Blue arrow, image 2. (click for enlargement)

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Look here for the Yaxcopoil-1 core segment 793.85 to 794.60 m: the transition of the impact to post-impact crater infill,