FORMATION OF THE MEGA-BLOCK ZONE DURING THE CHICXULUB CRATERING EVENT

Two mechanisms have been proposed to account for the emplacement of the Cretaceous sequence constituting the megablock zone within the KT boundary Chicxlub crater (Yucatan). 1) The sequence is para-autochtonous and originated from the external zone located near the rim. It moved inward and slightly downward along normal faults during the crater modification phase. 2) The megablock sequence forms part of the excavated or pushed away components during the evolution of the transient cavity. The sequence has moved horizontally outward into the annular through, from the more central part of the crater, as the transient cavity collapsed at the end of the excavation phase. The megablock sequence is allochtonous and stratigraphic inversion is expected, in analogy with observation of overturned blocks at smaller craters such as the Ries (Germany). Around fifty ⁸⁷Sr/⁸⁶Sr data points (including a number of duplicate analyses) were obtained across the 616m of displaced Cretaceous sequence at the bottom of the ICDP Yaxcopoil core (Yax-1), drilled in 2002. The OAE2 event, located near the base of the core forms a stratigraphic anchor point. From this point upward to ~ 1050m depth, the ⁸⁷Sr/⁸⁶Sr trend shows small undulations, ranging between 0.7074 and 0.7073, characteristic of Cenomanian to Santonian values. It is followed around 1000m by a sharp increase to 0.7078, very similar to the one showed by the seawater strontium curve between the Santonian and Late Maastrichtian. Based on these results, the displaced Cretaceous sequence in Yax-1 appear to have preserved its stratigraphic coherence. This observation favors an emplacement inward along normal (possibly listric) faults from the crater rim zone, during the crater modification stage. The mechanical decoupling along subhorizontal zones of weakness resulted in separation of the target-rock sequence into blocks that were somewhat tilted against each other. It is too early to generalize this to the whole ~ 200 km sized crater. However, the megablock material encountered at Yax-1 should be considered para-authochtonous. It most likely originated in the zone near the crater rim and travelled some considerable distance (>20km?) inward towards the inner through zone, at the base of the central peak-ring.