Tectonic and Eustatic Controls on Foreland Basin Deposition, The Lower Colorado Group and Equivalent Strata of the Alberta Basin, Canada

Roca, Xavier¹, Jessica Rylaarsdam², Heng Zhang³, Guy Plint¹ (1) University of Western Ontario, London, ON (2) Anadarko Canada Corporation, Calgary, (3) University of Western Ontario, London, ON

Tectonically-driven subsidence and eustatic fluctuations control the distribution of accommodation in foreland basins. The geometry of chronostratigraphic units of regional extent is a proxy for the relative contribution of each control. Flexural subsidence induces thickening towards the tectonic load producing a wedge, whereas eustatic rise generates similar amounts of accommodation throughout the basin, producing more tabular geometries. Correlation of transgressive surfaces throughout the proximal depozone of the Alberta foreland basin has defined eight chronostratigraphic units within the Late Albian Lower Colorado Group and time-equivalent strata. Localized tectonic loading triggered the onset of largely coastal plain deposition in the northwestern corner of the study area. Subsequent eustatic rise led to westward transgression and widespread deposition of marine shale during a period of tectonic quiescence. Overlying shallow marine regressive silty sandstones were eroded during subsequent, probably eustatic fall, although tectonic tilting indicated by the progressive northward beveling of these rocks also contributed to relative sea level fall. Subsequent transgression and deposition of marine shale records both eustatic rise and flexural subsidence in two different depocenters. The cessation of subsidence in the south contrasts with continuous subsidence in the north. The two overlying units indicate continued flexure in the north, while eustatic rise led to progressive southward transgression. By latest Albian time, the basin was partitioned into a southern, low/no accommodation zone characterized by intense condensation, and a northern, high-accommodation zone filled with mudstone. All units show a higher-order depositional cyclicity, probably of eustatic origin.