BASINWARD: REGRESSIVE DEPOSITIONAL ARCHITECTURE IN THE OLDMAN AND WAPITI FORMATIONS (CAMPANIAN, WESTERN INTERIOR SEAWAY)

The study of the Belly River Group of southern Alberta provides an unparalleled example on how clastic lobes forming a sedimentary prism display remarkable compositional, geographic and temporal variations. A basin-scale analysis of time-equivalent, prograding systems across Alberta and northern Montana document the complex interaction of tectonic uplift, sediment supply and climatic fluctuation in terms of fluvial response, accommodation changes, and paleoenvironmental shifts.

The Oldman Formation represents part of a southwest-thickening unit that accumulated across northern Montana and southern Alberta during the Campanian maximum progradation of non-marine systems. In fact, the Oldman Formation has an overall thickness of 40 m in central Alberta, a value that doubles near the Canada–US border.

Recent field activities resulted in a comprehensive, cross-border section of the Oldman sedimentary wedge. The Herronton Sandstone Zone, a sandy interval overlaid by stacked paleochannels on top of the paralic deposits of the Foremost Formation, marks an abrupt depositional shift at a regional scale and the onset of the regressive phase of the Belly River wedge (lower Oldman, ≈40 m). Similarly, the Comrey Sandstone Zone marks the maximum extend of the clastic system basinward and most likely deposition during the earliest stages of transgression (≈30 m). In addition, this sandy interval overlay to the west and south a prominent, white-colored, leached interval that crops out extensively in the Milk River valley. Finally, the upper Oldman, which is time equivalent to the lower part of the Dinosaur Park Formation, reflects the regional reorganization of terrestrial deposits associated with the onset of transgressive deposition (≈50 m). Discrete volcanic ash beds, representing a reliable tool for lateral correlations, and well-developed paleosoils also characterize this interval. Finally, the nature of the Oldman-Dinosaur Park formational contact near the Canada-US border further support the diachronous southeastward migration of the DPFm clastic wedge.

This wedge has an equivalent in the Wapiti Formation of northern Alberta where the alluvial regressive system is bounded by massive fluvial sands at the base and by a regional, subaerial unconformity at the top documenting a depositional gap of 1.5 Ma.