Stratigraphy and sequence stratigraphic analysis of the Oldman Formation (Campanian, Cretaceous) in the Milk River Valley of Alberta, Canada

The Oldman Formation (Campanian - Upper Cretaceous) is the middle unit of the Belly River Group, widely exposed in the southern plains of Alberta (Canada). This non-marine unit has been the subject of multiple studies focusing on source-to-sink relationships during large-scale regressive events. Recent field surveys in the Milk River Valley along the Canada-U.S. border have led to the identification of a previously unmapped stratigraphic interval characterized by highly cemented, white quartzarenites, nicknamed 'Big White'. In 2022 and 2023, nine new stratigraphic sections representative of the lower 80 meters of the Oldman Formation were acquired in the Milk River Valley. Measured sections extend from the Taber Coal Zone (TCz) - which marks the base of the Oldman Fm. - to the Comrey member sandstone. In this process, 15 sandstone samples were collected for petrographic analyses. Big White, occurring at about 30 m above the TCz, has an overall lenticular geometry, with sharp erosive base, and significant variations in thickness across the study area. It comprises two distinct intervals: the lower part (3 m thick) is characterized by high-angle clinostratification, typical of fluvial environments. The upper portion (2 m thick) shows low-angle, fining-upward beddings overlain by a thin interval of massive deposits interpreted as beach environment. Big White sediments were then compared with sandstones representative of the TCz-Comrey interval. Petrographic data suggest three sediment source areas during the lower Oldman Formation, a thickness interval of c. 80 meters representing a complex source-to-sink relationship for the basal beds of this unit. Similarly, in terms of depositional environments and sequence-stratigraphic interpretations, the identification of smaller-scale and higher frequency depositional cycles delimited by subaerial surfaces change our current understanding of the processes leading to the deposition of the Oldman Formation.