Stratigraphy, sedimentology, and geochemistry of mudstone-dominated clinoforms and their depositional environments, Carlile Formation, Eastern Alberta, Canada

Abstract

The Carlile and Niobrara formations of the Colorado Group were deposited in a distal shelf environment in the Cretaceous Interior Seaway, during the latter part of the Greenhorn Transgressive-Regressive Cycle and the beginning of the Niobrara Transgressive-Regressive Cycle. They are dominated by mudstone, with variable amounts of interbedded siltstone and sandstone. In the Wildmere area, they form a shoreline detached clastic wedge that has clinoform geometries that resemble “systems tracts” as defined by Van Wagoner et al. (1988). These systems tracts can be recognized and correlated using cross sections of closely spaced induction LAS logs from closely spaced wells. The presence of clinoforms is indicative of bedload transport of sediment, including bedload transport of silt, sand, and hydrodynamically equivalent mudstone aggregates. Such aggregates are well documented in laboratory experiments and modern environments, but are rarely observable in the rock record due to significant compaction of the aggregates. However, the Carlile Formation contains numerous mudstone aggregates observable in thin section. These are likely derived intrabasinally from erosion by subaqueous currents or wave erosion of muddy substrates. Geochemical variations between systems tracts indicate periodic changes in sediment source, likely caused by changing circulation patterns of subaqueous currents transporting mudstone aggregates with distinct compositions from varying parts of the basin. Redox sensitive trace elements present in the clay minerals reflect variable amounts of oxygenation along the dip of the clinoforms, which relates to the amount of preserved organic material. Similar geochemical trends in modern and ancient mudstone clinoform analogues support these interpretations. As a whole, the data from the Carlile Formation and the analogues demonstrate an association between the stratigraphic geometries, sedimentological processes, geochemical trends, and their associated reservoir properties.