Abstract: Paleoclimate and Paleo-Ocean Circulation Simulations for the Mid-Silurian Organic-Rich Black Shale Lithofacies

George T. Moore, Karen L. Bice

The Silurian is noted for its extensive organic-rich, graptolitic black shales and shelf carbonates, sediments generally interpreted to have been deposited in anoxic conditions. In order to better understand the climate processes that operated and helped control the deposition of these source rocks (types I and Il kerogens), paleoclimate simulations were performed using an atmospheric general circulation model. Results are presented from these simulations and from global ocean circulation simulations forced by the atmospheric model results.

The paleogeography of the mid-Silurian (approx. 425 Ma) was characterized by an oceanic northern hemisphere and a continental southern hemisphere. Gondwana occupied a polar position with much of its extensive margin in the wind-driven upwelling region of the southern mid-latitudes. Laurentia and Baltica occupied a tropical position; Siberia and Kazakh lay in the northern temperate latitudes. Atmospheric circulation model results suggest the presence of winter sea ice north of 70° N lat. This setting, combined with an atmospheric oxygen concentration which may have been lower than the present-day, created conditions favorable for enhanced primary productivity by marine phytoplankton, and the deposition and preservation of organic carbon in sea-floor sediments.

For mid-Silurian global ocean circulation simulations, a seafloor spreading center was positioned to approximately bisect the northern ocean with water depth at the ridge axis set at 3000 m. A water depth of 4000 m was assumed for subduction trenches along the continental margin facing the Paleo-Panthalassa Ocean and within the Rheic Ocean. The ocean model is forced by temperature, moisture flux and wind stress fields from the atmospheric simulations.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994