THE AERIAL DISTRIBUTION OF THE LOWER TURONIAN UNCONFORMITY IN EASTERN AND CENTRAL MONTANA: INFLUENCE OF SEA FLOOR TOPOGRAPHY AND WATER MASS CIRCULATION

Structural features have long been known to affect the stratigraphic record of unconformities in the mid-Cretaceous Western Interior Basin. It has previously been suggested that the stratigraphy in the northern Western Interior Basin was influenced by local and regional uplifts. Within this structural setting, evaluation of lithofacies is enhanced using micropaleontology; however this is complicated by biofacies changes that are in-part controlled by the migration of warmer, more saline waters from the south. Geophysical examination of cores and calcareous nannofossil studies of cores and outcrops from southeastern Montana, north-central Montana and southern Canada indicate that several lineaments affected deposition in the region. In southeastern Montana, a lower Turonian unconformity (Watinoceras devonense through Collignoniceras woollgari woollgari ammonite zones) places middle Turonian rocks (Collignoniceras woollgari regulare) of the Carlile Formation in contact with rocks of the latest Cenomanian Greenhorn Formation (Neocardioceras juddii ammonite zone). In contrast, calcareous nannofossils in north-central Montana indicate the presence of the lower Turonian, and therefore the magnitude of the unconformity at the Second White Specks – Carlile Formation boundary is reduced or eliminated towards the northwest.