A Conceptual Facies Model for Source Rocks in Epicontinental Basins: Application to Cenomanian/Turonian Deposits of the Cretaceous Western Interior Seaway

I present a conceptual facies model that links sedimentary processes and their products for marine shales/source rocks in epicontinental basin such as the Cretaceous Western Interior Seaway. The model is an idealized simplification that facilitates our understanding of the complex natural phenomena active in these settings, and it is not meant to represent all muddy or organic-rich depositional settings. The model incorporates lithology, sedimentology and bulk-geochemical analyses (e.g., Rock-Eval). Other types of analyses (e.g., biomarkers) can also be incorporated into the model.

The model acknowledges two primary types of sediment: siliciclastic sediment supplied by river mouths (silt, clay) and pelagic sediment generated in the water column (coccoliths, foraminifera, etc.). Various processes entrain, transport, deposit and rework these sediments. In a simple basin, the siliciclastic component of mudstones should decrease in an offshore direction, and the pelagic component should increase. Superimposed on these trends are changes in the preservation potential of marine organic matter (MOM). Proximal, siliciclastic-dominated environments have low preservation potential for MOM, as do the most distal, pelagic-dominated settings. The preservation potential of MOM is greatest in areas characterized by deposition of mixed mudstone lithologies (e.g., calcareous mudstone, marlstone) where the downward flux of organo-mineralic aggregates (“marine snow”) is high enough to develop an oxygen-minimum zone near the sea floor.

Together, the sedimentology and the organic and inorganic geochemistry are powerful tools for identifying where various types of deposits fit into a basin’s paleogeography, and for identifying sequence stacking patterns.