SEDIMENTARY PROCESS OF THE UPPER PART OF THE IMPACTITE IN THE ICDP CHICXULUB DRILL CORE (YAXCOPOIL-1)

Ocean water invasion into the Chicxulub crater immediately after the K/T boundary impact is probably responsible for deposition of the impactite in the crater. To investigate the influence of ocean water invasion into the crater, samples of the impactite from the YAXCOPOIL-1 (YAX-1) site were investigated. The sedimentary sequence of the impactite at YAX-1 is divided into units 0 to 6 in descending order (Dressler et al., 2003; Goto et al., 2004), and we mainly focused on lithology and petrography of the impactite in units 0 and 1 (794.14 to 808.02 m). Unit 1 is composed of 1 to 2 m relatively well-sorted suevite beds that are repeated at least 8 times. The upward fining grain size and the absence of erosional contact of each cycle suggest that each cycle was deposited from resuspended sediment particles transported as fluid suspension. Furthermore, abundant nannofossils of late Campanian to early Maastrichtian age in the matrix suggests that carbonate sediments deposited in the rim margin or around the crater were selectively eroded and transported into the crater, most likely by ocean water invasion into the crater. Unit 0 conformably overlies unit 1 and is mainly composed of dolomitic very fine calcarenite. Unidirectional cross-lamina with climbing structure is observed in the lower part of the unit and repetition of faint parallel lamination and micro-ripples are observed in the middle part, indicating deposition under the influence of currents. Presence of shocked quartz grains in lower cross-laminated part suggests that deposition of unit 0 was associated with the impact. The dolomitic limestone of the upper part of unit 0 shows evidence of brittle deformation such as cracks and fragmentations, and a two-centimeter thick clay layer irregularly covers these brittle deformation features. The observation suggests that unit 0 was consolidated before the deposition of the clay layer, and thus the boundary is unconformable.