Is the transition impact to post-impact rock complete? Some remarks based on XRF scanning, electron-microprobe, and thin section analyses of the Yaxcopoil-1 core in the Chicxulub crater

available online at http://meteoritics.org 1 © Meteoritical Society, 2004. Printed in USA. Is the transition impact to post-impact rock complete? Some remarks based on XRF scanning, electron-microprobe, and thin section analyses of the Yaxcopoil-1 core in the Chicxulub crater J. SMIT,1* S. VAN DER GAAST,2 and W. LUSTENHOUWER1 1Faculty of Earth and Life Sciences, Vrije Universiteit, de Boelaan 1085, 1081hv Amsterdam, Netherlands 2NIOZ, P.O. Box 59 NL-1790 AB Den Burg, Texel, Netherlands *Corresponding author. E-mail: [email protected] (Received 11 September 2003; revision accepted 15 March 2004) Abstract–The transition from impact to post-impact rocks in the Yaxcopoil-1 core is marked by a 2 cm-thick clay layer characterized by dissolution features. The clay overlies a 9 cm-thick hardground, overlying a 66 cm-thick crossbedded unit, consisting of dolomite sandstone alternating with thin micro-conglomerate layers with lithoand bioclasts and the altered remains of impact glass, now smectite. The micro-conglomerates mark erosion surfaces. Microprobe and backscatter SEM analysis of the dolomite rhombs show an early diagenetic, complex-zoned, idiomorphic overgrowth, with Mn-rich zones, possibly formed by hot fluids related to cooling melt sheet in the crater. The pore spaces are filled with several generations of coelestite, barite, K-feldpar, and sparry calcite. XRF core scanning analysis detected high Mn values in the crossbedded sediments but no anomalous enrichment of the siderophile elements Cr, Co, Fe, and Ni in the clay layer. Shocked quartz occurs in the crossbedded unit but is absent in the clay layer. The basal Paleocene marls are strongly dissolved and do not contain a basal Paleocene fauna. The presence of a hardground, the lack of siderophile elements, shocked quartz, or Ni-rich spinels in the clay layer, and the absence of basal Paleocene biozones P0 and Pa all suggest that the top of the ejecta sequence and a significant part of the lower Paleocene is missing. Due to the high energy sedimentation infill, a hiatus at the top of the impactite is not unexpected, but there is nothing in the biostratigraphy, geochemistry, and petrology of the Yaxcopoil-1 core that can be used to argue against the synchroneity of the end-Cretaceous massextinctions and the Chicxulub crater.The transition from impact to post-impact rocks in the Yaxcopoil-1 core is marked by a 2 cm-thick clay layer characterized by dissolution features. The clay overlies a 9 cm-thick hardground, overlying a 66 cm-thick crossbedded unit, consisting of dolomite sandstone alternating with thin micro-conglomerate layers with lithoand bioclasts and the altered remains of impact glass, now smectite. The micro-conglomerates mark erosion surfaces. Microprobe and backscatter SEM analysis of the dolomite rhombs show an early diagenetic, complex-zoned, idiomorphic overgrowth, with Mn-rich zones, possibly formed by hot fluids related to cooling melt sheet in the crater. The pore spaces are filled with several generations of coelestite, barite, K-feldpar, and sparry calcite. XRF core scanning analysis detected high Mn values in the crossbedded sediments but no anomalous enrichment of the siderophile elements Cr, Co, Fe, and Ni in the clay layer. Shocked quartz occurs in the crossbedded unit but is absent in the clay layer. The basal Paleocene marls are strongly dissolved and do not contain a basal Paleocene fauna. The presence of a hardground, the lack of siderophile elements, shocked quartz, or Ni-rich spinels in the clay layer, and the absence of basal Paleocene biozones P0 and Pa all suggest that the top of the ejecta sequence and a significant part of the lower Paleocene is missing. Due to the high energy sedimentation infill, a hiatus at the top of the impactite is not unexpected, but there is nothing in the biostratigraphy, geochemistry, and petrology of the Yaxcopoil-1 core that can be used to argue against the synchroneity of the end-Cretaceous massextinctions and the Chicxulub crater.