Paleo-Oceanographic Controls of a Source Rock (Thomel Level) Deposition: Anoxic Event Around the Cenomanian-Turonian Boundary in Southeastern France

J. P. Crumiere

The hemipelagic and pelagic sediments of the eastern Vocontian basin show a stratigraphic level of laminated horizons rich in organic matter around the Cenomanian-Turonian boundary (Whiteinella archeocretacea zone) referred to as the Thomel level. The lack of any benthic activity in the sediment, preserving both original lamination and marine organic matter, and the selective scarcity of certain species within the planktonic microfaunal assemblages, which consist only of radiolarians and tiny globular morphotypes of foraminifers characteristic of surficial marine waters, together show the Thomel level was deposited under deep-water anoxic conditions. That level reflects a worldwide anoxic event, known in the Alpine Tethys and in the north Atlantic (referred to as CTOAE [C nomanian-Turonian Oceanic Anoxic Event]), CTBE [Cenomanian-Turonian Boundary Event], E2 Event, Bahloul Horizon, Livello Bonarelli, or CTBSH [Cenomanian-Turonian Black Shale Horizon]). The Thomel level seems to represent the basinal sedimentary record of the lowstand of a major eustatic oscillation, marked by a rapid decrease of input into the basin. The detrital sediments were trapped on the platform, and surficial water planktonic productivity decreased. Poor oceanic circulation and O₂ turnover by deep oceanic currents are believed to have initiated an extended, stable, and intense deep-sea anoxic water column.

AAPG Search and Discovery Article #91032©1988 Mediterranean Basins Conference and Exhibition, Nice, France, 25-28 September 1988.