Facies and systems tracts at high-resolution in an organic-rich mudstone: The Duvernay Formation, Kaybob area, Alberta, Canada

Abstract

In organic-rich mudstones, vertical sequences of sedimentary facies record the depositional responses to complex and varying controls: development of accommodation, redox conditions and multiple sources of detrital and biogenic sediments whose flux varies with relative sea-level, and reworking by bottom currents. In this study of the Upper Devonian Duvernay Formation in the Western Canada Sedimentary Basin, we utilize dense core and well log control to characterize lithofacies and develop a sequence stratigraphic model for the formation, accounting for paleogeographic features whose effects on facies varied during three complete 3rd order sea level cycles. We further identify features characteristic of mudstone sequence boundaries, describe how they vary stratigraphically and geographically, and interpret processes responsible for their formation. These observations provide a framework for interpreting vertical successions in mudstones from lower density datasets.Five lithofacies are identified, deposited by combinations of suspension settling, sediment-gravity flows, and bottom currents under anoxic to fully oxygenated bottom water conditions. Lithofacies distribution was strongly affected by proximity to sediment sources, bottom topography, and sea level cyclicity. Relatively coarse-grained, carbonate-rich, bioturbated, and organic-poor facies are common close to large carbonate platforms and reefs; silt- and sand-rich facies deposited by bottom currents and sediment gravity flows are common on the slopes of carbonate platforms; hemipelagic, fine grained, biosiliceous, organic-rich facies are common in distal areas of the basin.The Duvernay was deposited during a 2nd order transgression and early 2nd order highstand comprising three 3rd order depositional sequences and nine 4th order depositional sequences. Third order transgressive deposits are characterized by upward trends of increasingly fine-grained, biosiliceous, organic-rich facies. Third order highstand deposits are characterized by increasingly coarse-grained, carbonate-rich, bioturbated, organic-poor facies. Third order lowstand deposits are characterized by detrital clay-rich, bioturbated facies. The expressions of 3rd order sequence boundaries vary with geographic position in the basin and stratigraphic positions within the 2nd order depositional sequence. In the 2nd order transgression, sequence boundaries are expressed as scoured surfaces with coarse overlying lags that represent sediment starvation and reworking. In the 2nd order highstand, sequence boundaries are underlain by coarse beds with soft sediment-deformed bases deposited during a forced regression.