Marine transgressions in the mid-Cretaceous of the Cordilleran foreland basin re-interpreted as orogenic unloading deposits

Abstract

Abstract A detailed stratigraphic and sedimentologic study of the Early Cenomanian Fish Scales Formation and the Barons Sandstone in southern Alberta, based on detailed core measurement and well log correlation, was undertaken in order to assess the principal controlling factors on the mid-Cretaceous stratigraphic fill of the Cordilleran foreland basin. Fine-grained units in the basin, such as the Fish Scales Formation (Mowry equivalent) and the Early Turonian Second White Specks Formation (Greenhorn equivalent), have traditionally been attributed to deposition during episodes of high eustatic sea levels. However, facies, allostratigraphy, and isopach patterns of the latest Albian Westgate, Fish Scales, mid–late Cenomanian Belle Fourche and Second White Specks formations in southern Alberta suggest that strata were deposited alternately during orogenic loading periods and unloading periods. We interpreted the Fish Scales Formation, a deposit formed during the marine transgression of the Mowry Seaway in the latest Albian–earliest Cenomanian, to have been deposited in a wide shallow interior seaway during an orogenic unloading period. The Barons Sandstone, interbedded with the mudstone of the Fish Scales Formation, is interpreted as a regressive shoreface, distributary channel and barrier island sandstone deposited during the unloading period. We also tentatively interpreted the Second White Specks Formation, a regional stratigraphic marker deposited during the so-called peak eustatic sea level in the Early Turonian, to have been deposited in a wide shallow interior seaway during a period of regional orogenic unloading. This study questions the significance of the Mowry Shale and Greenhorn Formation as indicators of eustatic highstands of sea level, and suggests regional tectonism as the primary control on basin evolution.