Isopach mapping of the BAT2 sequence in the "Basal Quartz" unit (Lower Mannville Group; Early Cretaceous) in southern Alberta reveals that it occupies an integrated network of incised valleys. The single trunk valley is a 4th-order stream at the northern edge of the study area and is fed from the east, south, and west by four 3rd-order and ten 2nd-order tributaries. Sixty-four 1st-order tributaries have been identified. Most of the valley network displays a rectilinear drainage pattern in response to synerosional faulting. A narrow, deeply incised water gap was formed where the trunk valley cut across the most significant uplifted block. The south side of this block was drained by a series of short 1st-order tributaries that define a trellis drainage pattern. A dendritic pattern is locally developed over homogeneous marine shale in the southern part of the study area, while a radial pattern occurs around the northern end of a long-lived structural arch. Within the valley network, localized deep scours occur at tributary confluences and at the outside of bends. These features represent excellent exploration targets.
Our observations show that the BAT2 valley preserves a mature drainage network that lies more than 300 km seaward of the lowstand shoreline. We suggest that slow headward propagation of a knickpoint would not allow this faithful preservation because avulsion remains possible above the knickpoint; such avulsions would interrupt the development of the valley system and reduce the chance of incision of the smaller tributaries. We propose that preservation of entire valley network requires near simultaneous entrenchment, perhaps in response to slow tectonic uplift.
SEPM Research Symposium -- Processes and Images of Incised Valley and Lowstand Deposits