- © The Society of Canadian Petroleum Geologists
The Lower Cretaceous Basal Quartz (BQ) of Southern Alberta (Townships 1–40, Ranges 1W5–5W5) can be informally divided into seven mappable units (A Sandstone, composed of the Regional A, Carmangay, Mesa IV, Valley and Terrace; Horsefly, BAT, Ellerslie). The study area is considered to be in an accommodation-limited setting due to the presence of multiple, closely spaced unconformities, and the general absence of marine deposits. Multiple levels of cyclicity exist in the BQ. There are two cycles of increasing-upward mineralogical and textural maturity, the first associated with the A Sandstone and the second associated with the Horsefly-BAT-Ellerslie succession. There are multiple southward marine transgressive events, both on the unit scale and on the cycle scale, indicative of a backstepping stacking geometry.
The high resolution subdivision of the BQ allows for the recognition of changing BQ paleodrainage through time. There is both a progressive spatial and stratigraphic change in incised valley organization, from thin and wide valley forms in the south and at the base of the cycles, to thicker, narrower and more deeply cut systems toward the northwest and top of the cycles. In addition, there is spatial variation in tributary systems for the upper cycle, from no tributaries associated with the thin, wide valley forms associated with the Horsefly, to narrow and thin tributaries in the BAT south of the Vulcan Low, deeply cut complex tributary systems across the Vulcan Low, and linear deep tributaries north of the Vulcan Low for both the BAT and Ellerslie. The style of depositional fill also changes stratigraphically and spatially, from braided to coarse meandering “sheet” deposits south of the Vulcan Low associated with the Carmangay and Horsefly, and low accommodation Valley and Terrace and BAT north of the Vulcan Low; meandering deposits associated with the low accommodation Mesa IV, and higher accommodation Valley and Terrace, Horsefly and BAT; and fluvial-estuarine deposits associated with the Valley and Terrace, BAT and Ellerslie north of the Vulcan Low in higher accommodation setting.
The dominant control on the BQ depositional patterns is the interplay between eustasy and heterogeneous basement subsidence. The tectonic influences on sedimentation are most obvious in the sediments immediately overlying long-duration unconformities. Fluvial erosion on the unconformity surface amplifies the tectonic signal by accentuating the technically produced relief in a low accommodation setting. The accommodation-limited conditions occurring during BQ deposition resulted in sequence boundaries amalgamating on the interfluve, and it is only by detailed correlation and petrographic analysis that BQ units can be differentiated.