Genetic Sequence Stratigraphy of the Intermontane Paleocene Fort Union Formation, Greater Green River Basin, Southwest Wyoming and Northwest Colorado

Roger Tyler, D. S. Hamilton

Syntectonic fluvial sedimentation within the Paleocene Fort Union Formation occurred in response to thick-skinned, Laramide basement thrusting. Changes in the Fort Union fluvial depositional style are attributed to perturbations in tectonically controlled sediment supply and fluctuating accommodation space in structurally defined subbasins. Multiple unconformities and widespread lacustrine/flood basin flooding surfaces, as well as changes in bedding architecture, were used to define and correlate Fort Union genetic sequences.

Laramide basement thrusting caused the contemporaneous asymmetric uplift and subsidence of the basin. During early thrusting, uplift above the initially blind basement thrusts resulted in the reorientation of the drainage pattern and the formation of the north-flowing intermontane fluvial depositional system. During rapid thrusting, the basin was characterized by increased rates of subsidence that exceeded or equaled the rate of sedimentation. Characteristic non-marine genetic sequences of the rapid thrusting phase include a bed-load fluvial sequence consisting of conglomerates and sandstones; a mixed-load fluvial sequence consisting of sandstones, mudstones, and coals; and a suspended-load fluvial-lacustrine sequence consisting of sandstones and mudstones. The thick, bed- and mixed-l ad sequence is found above the asymmetrical structural and depositional axes of the basin, and the suspended-load fluvial sequence, representing low-gradient, low-energy environments, thickens toward impinging hanging-wall thrusts. During the transition between syntectonic and post-tectonic sedimentation, thrusting slowed and low-energy floodplain, flood basin, and lacustrine environments expanded across the basin.

Sandstone thickness and continuity is greatest in the bed-load fluvial sequence. Fluvial trunk streams flowed northward predominantly in the center of the subbasins and exited in the northeast. In the mixed-load fluvial sequence, coal thickness and coal-seam continuity is greatest. Net coal thicknesses can range from 0 to more than 30 m in as many as 10 seams. Coal beds were deposited along predominantly north-flowing trunk-stream systems, where thick sandstones served as platforms for peat accumulation. During the deposition of the suspended-load fluvial-lacustrine sequence, north-directed drainage was maintained in the center of the basin, whereas thrustward-thickening lacustrine and flood basin mudstone sequences are found near the basin margins. Localization of tributaries, flood asins, and lacustrine environments near the thrusts suggest that the frontal edges of the impinging hanging-wall blocks did not supply most of the sediment to the basin. This genetically defined sequence stratigraphy emphasizes variations in tectonism, sediment supply, and accommodation space.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995