THE CRETACEOUS UNIT OF THE ICDP DRILL CORE YAX-1, CHICXULUB IMPACT CRATER, MEXICO: IS IT PART OF THE ALLOCHTHONOUS MEGABRECCIA OR MERELY DEFORMED DURING CRATER COLLAPSE?

The Yaxcopoil-1 bore hole (Yax-1) was drilled 60 km south of the center of the 180 km-diameter Chicxulub impact structure. A sequence of 615 m of deformed Cretaceous carbonates and sulfates was recovered below a 100 m-thick unit of suevitic breccias and 795 m of post-impact Tertiary rocks. The Cretaceous sequence displays variable degrees of brittle damage and is cut by different types of impact-induced dikes. Unequivocal shock effects are restricted to the dike injections and the so called “paraconglomerate” [1]. Structural and dip-meter log data suggest, that the sedimentary sequence represents a number of structural units that are tilted and moved with respect to each other. However, the lack of biostratigraphic information does not allow a precise correlation with the normal target stratigraphy and even does not exclude an upside down position of parts of the sequence. The deformation pattern is consistent with a collapse scenario of the Chicxulub transient crater cavity. Consequently, [1] suggested that the Cretaceous sequence was originally located outside the transient crater cavity and upon its collapse moved downward and toward the center to its present position between the peak ring and the crater rim, thereby breaking apart into blocks. An alternative interpretation of the emplacement process of the Cretaceous sequence was given by [2]: The sequence represents a stack of displaced mega-blocks that were transported more or less horizontally and radially outward at the final stage of the transient cavity excavation process and moved somewhat back inward during the subsidence of the outer parts of the transient crater. In this case, the Cretaceous sequence is part of the continuous ejecta and probably part of the “overturned flap”. This interpretation is in agreement with estimates of the ejecta thickness, which, according to results of numerical models [2], should be some 800 to 1000 m thick. This paper lists the arguments for both hypothesis and gives a rationale for a further proof. The answer to this problem is not just of academic value, but is crucial (1) to test the correctness of current numerical models, (2) to refine the size of the transient cavity and thereby (3) to estimate the energy of the Chicxulub impact.

References: [1] Kenkmann, et al., 2004, MAPS 39/6 [2] Stöffler et al., 2004, MAPS, 39/6.