Transregional Sequence-Stratigraphic Correlation of the Maastrichtian Fox Hills Sandstone: Colorado, Wyoming, South Dakota, North Dakota and Montana

Abstract

The Campanian to Maastrichtian Fox Hills Sandstone records the terminal retreat of the Western Interior Seaway (WIS) in which shoreline sandstones were forced to regress >1300 km. This regressive sandstone is important for clarifying paleogeographic interpretations for the WIS, and understanding the source-to-sink processes that produce large-scale regressive sequences that can form important reservoir intervals. Despite numerous local studies of the Fox Hills Sandstone, the regional “source-to-sink” connection of the fluvial-to-marginal marine depositional systems is poorly understood. This study correlates three main unconformities, and uses biostratigraphic, and geochronologic data to develop a transregional stratigraphic framework of the Fox Hills S.s. from CO to MT that elucidates the fluvial feeder systems, shoreline orientations, and architectural elements of regressive shoreline sandstones. Nineteen facies define ten depositional environments within the Fox Hills S.s. including: offshore marine, lower shoreface, middle shoreface, upper shoreface, pro-delta, delta front delta plain, distributary channel, fluvial channel, and flood plain. Facies shifts highlight three transregional unconformities (i.e., sequence boundaries) that define 2 incised valley systems responsible for providing sediment to regressive shorelines of the Fox Hills S.s. The oldest incised valley system in Casper, WY feeds three parasequences that form an E-prograding, flat-to-falling, progradational parasequence set. By contrast, the youngest incised valley system feeds five parasequences that form a NE-prograding, more vertically-stacked progradational parasequence set in Glendive, MT; NE to E of Glendive, MT the number of parasequences increases up to eight. In general, parasequencess are more numerous and more vertically stacked from SW-W to NE-E, indicating a NE-E increase in the ratio of accommodation to sediment supply. Accommodation in the Fox Hills system could be driven by eustasy, Sevier and Laramide tectonics, and dynamic subsidence migration (Liu et al., 2014). However, we infer a main control of migrating dynamic subsidence because: 1) the Fox Hills S.s. progrades a distance far greater than any Sevier- or Laramide-related flexural subsidence, and 2) Fox Hills shorelines generally parallel dynamic basin-margins.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017