ARCHITECTURE AND EVOLUTION OF A REGRESSIVE, TIDE-INFLUENCED MARGINAL MARINE SUCCESSION, DRUMHELLER, ALBERTA, CANADA

Tide-dominated and tide-influenced clastic depositional environments are typically interpreted to be associated with landward-stepping or transgressive systems. Here, we present an example of an overall regressive succession dominated by high-frequency sequences that exhibit abundant sedimentary and ichnological evidence of tidal processes. These strata comprise the Campanian Bearpaw-Horseshoe Canyon Formation transition exposed near Drumheller, Alberta, Canada. The clastic marine to marginal-marine sediments were deposited in mixed-process (wave-, tide-, and fluvial-influenced) depositional environments along the western margin of a foreland basin. The deposits are subdivided into six relatively thin (10 m on average), high-frequency (similar to 140,000 years), transgressive-regressive (T-R) sequences (A to F). Outcrop sedimentary logs and hand-held gamma-ray profiles were correlated to subsurface data (4 cored wells and 75 wells with wireline logs) within an area of 22 km x 16 km in order to generate a 3D geocellular computer model. This model along with paleocurrent data from outcrop were used to analyze the stratigraphic architecture and evolution of the succession. The evolution of the successive T-R sequences was mapped on a subregional scale using the 3D geocellular model. The orientations of the paleoshorelines rotate in a predictable manner during transgression and regression. Transgressive shorelines are oriented approximately S-N and rotate clockwise during regression to approximately SW-NE. The rotations are attributed to the paleogeography of the region, with the study area interpreted to sit on the western flank of a large structurally controlled embayment. The progradational high-frequency sequence set is dominated by tide-influenced strata and clearly demonstrates that tidal deposits can be preserved in both regressive as well as transgressive successions. Tidal influence in this regressive setting is attributed to shallow water depths on a wide shelf, which resulted in amplification of the tidal wave as it moved across the shelf. The embayed character of the coastline may also have augmented this effect. Tidal range is estimated to have been mesotidal to macrotidal. A modern partial analogue for the Horseshoe Canyon systems is identified from the tropical Mitchell River Delta in the Gulf of Carpentaria, Queensland, Australia. This delta system is analogous in terms of its tide-influenced facies, mesotidal range, low-gradient shelf with accompanying shallow water depths, the mixed-process character of its shoreline systems, and its horizontal to falling shoreline trajectory.