Deep Water Gulf of Mexico High Gamma Ray Shales and Their Implications for Flooding Surfaces, Source Rocks and Extinctions

William J. Sercombe¹ and Thomas W. Radford^2
1GOM, BP, Houston, TX
²BP, Sunbury on Thames, United Kingdom

High gamma ray ‘hot’ Eocene shales were encountered in southern deepwater Gulf of Mexico wells. The two prominent shales were in the age range of 40 and 50 MYA. The shales were originally deposited in basin floor settings that palinspastically restore to water depths of greater than 20,000’ sstvd. The occurrence of ‘hot’ shales deposited at such water depths forces the questioning of the association of ‘hot’ shales and maximum flooding surfaces. The influence from eustatic sea level changes in very deep water would be minimal. The deep water ‘hot’ shales alternatively suggest very high global temperature excursions which created extensive algal blooms and anoxic oceanic conditions. The high temperatures would cause the related cascade effects of hydrate melting, global anoxic flooding events, global alginate source rock occurrences, atmospheric compositional change, marine and terrestrial stress/extinctions and faunal lagerstattens. The age of these shales correlates with lagerstatten death assemblages in the Green River shales, the Messel fossil beds of Germany and Whale Valley in Egypt. The vertebrate death assemblages contain multiple species that died together in the same place. The articulated and unscavenged skeletal remains indicate post death predation was non existent as lethal atmospheric temperatures most likely suppressed predators and scavengers. The large number of specimens also suggests a rapid onset of thermal change. The Whale Valley lagerstatten includes a red bed containing crabs that are found in a hibernation position after mass simultaneous burrowing suggesting a rapid transition from healthy faunal community to mass death. Periods of high temperatures punctuated by brief episodes of extreme heat would explain the association of marine deep water ‘hot’ organic shales and terrestrial death assemblages.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery