K/T Boundary: Fossilized Trees Transported to a Deep Sea Environment in Baja California, Mexico

Abstract

The Peninsular Ranges Forearc Basin crops out along the western margin of the Baja California Peninsula, Mexico. The Late Cretaceous Rosario Formation in the central portion of the basin represents a deep marine, steep slope environment, with intricate depositional architecture, including mass transport deposits and debrites, incised by several submarine canyons and entrenched slope channel systems. The San Fernando and San Carlos Channels are examples of such channels, that cut the fine-grained background slope sediments and contain coarse grained turbidites filling the channel itself. The K/T boundary occurs within the upper part of the Rosario Formation (between the San Fernando and Sierrita channel systems). Within this interval a large number of fossilized tree trunks were found within the slope mudstones. Allowing for the regional structural dip of these deposits, the K/T boundary can be inferred across the slope sediments between the San Fernando Channel and Mesa San Carlos, and coincides with this outcrop. The tree fossils preserve original structures, such as growth rings, nodes from branches and bark, and some show also burnt and carbonized portions. Their diameter can exceed 50cm, and the total preserved length may be several meters, although smaller pieces are common. They are found in two main intervals, stratigraphically less than 5m apart, interbedded with mudstones. A persistent bioclast-rich debrite, with thickness varying from 30cm to 1m, occurs above them. The transport mechanism of these fossilized trees is still uncertain, but given their occurrence within slope mudstones they are likely to have floated over the sea surface and sunk when saturated. We suggest they may be associated with a catastrophic event related to the Chicxulub impact itself, in the Yucatán Peninsula, 2700 km distant from this locality. The next steps of this research include the gathering of biostratigraphic data on the slope mudstone deposits, to place the K/T boundary precisely, and understand better the facies architectures and depositional processes involved.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015