Proximal to distal correlation of coal-clastic alluvial successions in the Paleocene of the Williston Basin (USA): new insights into forcing mechanisms

The architecture and in particular the regular facies alternations of coal-bearing alluvial stratigraphy might be controlled both by dynamics internal to a sedimentary system (autogenic) as well as by external variables (allogenic). Understanding the forcing mechanisms in these deposits is highly relevant for better understanding the role of peat deposition has in long-term uptake of carbon from the atmosphere into the terrestrial biosphere and eventually lithosphere.

This work focuses on repetitive coal seams of the lowermost Paleocene Tullock Member (Montana, USA) and discusses two things: First, we present a revised depositional model that recognizes vertically consistent and laterally persistent 3–8 meter-thick alternations of coarsening lacustrine silt- and sandstones, followed by incipient paleosols locally eroded by large-scale incisions, and altogether topped by coal seams. These alternations represent a fluvio-lacustrine depositional environment where local accommodation space partly created by peat consolidation is filled by progradational lacustrine avulsion deltas, that may culminate in proximal channel-belt formation. Second, we perform a quantitative analysis on the thickness distribution of coal-clastic alternations in the Tullock along a 300-km long, proximal to distal transect using magnetostratigraphic correlations. This effort shows how the architecture of the Tullock coal-bearing facies is controlled by downstream evolution of fluvial landscape planforms, and what this insight means for understanding the balance between allogenic and autogenic control.