HIGH-FREQUENCY SYNTECTONIC UPLIFT IN OUTCROP AND SUBSURFACE: EXAMPLES FROM THE LOWER BELLE FOURCHE MEMBER OF THE FRONTIER FORMATION, WY

Subtle changes in base level due to fault-related block movements can have an overriding effect on depositional system architecture in low accommodation shallow marine environments. Topographic lows formed by block downwarping can become “mud sinks” and infill with poor reservoir quality muddy to heterolithic sediment successions. Topographic highs caused by block uplift may raise seabeds to fair-weather wave conditions and cause local erosion. Four such tectonically-forced erosional events are mapped in the Lower Belle Fourche Member of the Frontier Formation, Wyoming. Recurrence of these events is of sub-million year frequency, it results in parasequence-scale stratigraphic architecture, and dominates reservoir distribution. The truncation observable in the Second Frontier Sandstone offers the best example. In a continuous outcrop exposure, syntectonic truncation results in an “abrupt” pinch-out of a shallow marine coarsening-up shoreface succession. Over a lateral distance of several hundred meters, over 10m of stratigraphic erosion is suggested by a single pebble lag bed that truncates into increasingly deeper water facies. The pebble lag is everywhere overlain by bioturbated pebble-bearing muddy-sandstones and sandy-mudstones topped by offshore mud. The same erosional feature can be traced for over 60 km into subsurface data and is interpreted to be related to wave erosion on top of a proto-Laramide anticlinal structure. Based on outcrop and subsurface data, similar erosional features can also be mapped within the Harlan, Willow, and Posey allomembers of the Lower Belle Fourche.