Tectonostratigraphy of the Frontier Formation during Sevier and Laramide orogenesis in the greater Green River Basin, Wyoming

The greater Green River Basin (GGRB) has a complex stratigraphic history during the Late Cretaceous. During this time, the basin transitions from a simple foreland to partitioned basins associated with the Laramide Orogeny. This change in deformational styles has been attributed to flat-slab subduction of the Farallon Plate during the Laramide Orogeny. During the Late Cretaceous, there are potentially three modes of subsidence - flexural subsidence associated with the Sevier Thrust Belt, flexural subsidence associated with individual Laramide uplifts, and long-wavelength dynamic subsidence. The last two have been associated with flat-slab subduction. The patterns of subsidence and associated uplifts are a primary driver on sedimentary dispersal patterns as evidenced by isopachs, lithofacies distribution, paleocurrents and provenance. To test for the earliest evidence of Laramide effects, the Cenomanian-Turonian Frontier Formation is being analyzed from outcrop and subsurface data in the GGRB.

In the GGRB, the Frontier Formation is a strongly progradational sandstone-prone unit, capped by a transgression into the offshore marine Baxter and Steele Shales. The Frontier Formation is deposited during emplacement of the Willard and early Crawford Thrusts in the Sevier Orogenic belt. An isopach map of the Frontier documents the development of a 1.5 km foredeep adjacent to the Sevier fold-thrust belt, thinning to less than 100 m in the distal forebulge-backbulge position. However, the Frontier Formation thickens again, to about 300 m, in the northeastern part of the GGRB. This sedimentary thick is adjacent to the Granite Mountains, a Laramide uplift. Preliminary findings show deposition of more proximal facies, including pebble conglomerates, and southward-directed paleocurrents in this area, possibly indicating coeval uplift. U-Pb dating indicates >80% of Frontier zircons are arc-derived grains, providing maximum depositional ages for the formation. The arc component decreases to the west - the dominance of arc-derived grains in the distal Frontier may be related to grain-size controls or oblique sediment dispersal patterns (ongoing analysis). From the evidence cited above, we hypothesize that Frontier sedimentation is influenced by both Sevier and nascent Laramide-style tectonism.