Depositional Environments and Packages of the Basal Cambrian Sandstone at the Industrial Heartland

The Basal Cambrian Sandstone (BCS) has become the focus of increasing Carbon Capture Utilization and Storage (CCUS) efforts within Western Canada. Shell’s Quest Facility has been operating since 2015 and is actively capturing and sequestering C02 within the BCS near Fort Saskatchewan. In addition, Cambrian intervals exist at Cold Lake, where the Oil Sands Pathways to Net Zero initiative has indicated that they have applied to the government for pore space (DOB, 2022). Despite the increasing activity within this interval, little work has been presented in the public domain on the sedimentological and stratigraphic characteristics within the BCS (Desjardin et al., 2013). Multiple cores (~70m each) collected at Fort Saskatchewan, Redwater, and Cold Lake form a robust dataset from which a facies scheme can be used to erect a depositional model.

Our observations show that the BCS at Fort Saskatchewan and Cold Lake is a tidally-influenced to dominated succession consisting of large compound dunes and associated bayfill deposits. Tidal sedimentation is supported by an abundance of coarse, poorly sorted lithologies and associated tidal sedimentary structures. Grain size commonly ranges from very coarse sand to pebble-sized clasts, which is interpreted to record proximity to braided-delta mouth bars. Coarse grained sediment is subsequently reworked by fluvio-tidal forces into compound dunes. Evidence for tidal deposition includes thick (>10 m) accumulations of high-energy, unidirectional sedimentary structures consisting of tabular (TBS) and trough cross-stratified (TCS) bedding with abundant current ripple trains. Mud drapes (often in couplets) commonly bound decimeter-scale TCS bed sets with abundant mud draped current ripples and dune foresets. Trace fossil diversity and abundance is low to moderate and is interpreted to indicate abundant physico-chemical stresses in the form of rapidly fluctuating salinity and turbidity in addition to rapid bedform migration. Tubular tidalites within large arthropod burrows are abundant within quiescent bay-fill intervals and further support a tidally-influenced environment.

Abrupt flooding surfaces mark a transition into heavily bioturbated heterolithic mudstone and sandstone. Trace fossils including Schaubcylindrichnus and Chondrites indicate a normal marine depositional environment. Physical sedimentary structures include abundant wave ripples and micro-HCS, recording deposition above storm weather wave base. These intercalated mudstone and sandstone deposits are interpreted to record marine deposition within the proximal wave-dominated prodelta. Wave energy is responsible for attenuating riverine-sourced salinity and turbidity stresses, resulting in higher trace fossil diversity. Abundant sharp-based mud beds are interpreted to record hyperpycnites (dense mud flows sourced from a riverine mouth). Sand content increases upward into consistently wave-rippled intervals that are interpreted to record deposition above fair-weather wave base within the delta front. These sand-rich intervals are occasionally capped by massive to mottled iron-rich red mudstones, which are interpreted to record subaerial exposure surfaces within loess regolith as indicated by desiccation cracks and slickensides with sporadic brecciation.