Three-Dimensional Delta Lobe Geometry and Stratigraphic Architecture of the Upper Cretaceous Peay Sandstone (Frontier Formation), Northeast Bighorn Basin, Wyoming
Several detached, isolated deltaic sandstone bodies, encased by marine mudrocks, are preserved within the Upper Cretaceous (Cenomanian- Turonian) Frontier Formation in the Bighorn Basin of Wyoming, where they are active hydrocarbon-producing reservoirs. These bodies are thought to have been deposited in coastal to shallow marine environments via eastward and southward delta progradation into the Cretaceous Western Interior Seaway. The Peay member is a top-truncated, composite deltaic sandstone body continuously exposed along outcrops in the northeast Bighorn basin. Analysis of outcrop and subsurface data reveals the axial body is elongate (over 35 km) along depositional dip (SSE) and maintains a constant thickness (<60 m) of fluvial dominated, tidally influenced delta mouth bar facies with a sparsely distributed trace assemblage (Skolithos ichnofacies). Across depositional strike, (ENE-WSW) the Peay has a lensoid geometry with both flanks abruptly (<15 km) pinching out to thin (<5 m) sheet-like sandstone bodies. Well logs show the unexposed western flank to finger out into several thin discrete bodies, eventually pinching out completely (35+ km). However, outcrop sections show the eastern flank thins to a single laterally continuous sandstone body (30+ km). Depositional facies change laterally off-core from tidal/storm influenced proximal delta front (<20 m) (Cruziana/Skolithos ichnofacies) to a medial-distal delta front at the flank margins (<5 m) (stressed Cruziana ichnofacies). Physical structures show paleoflow direction orthogonal to the SSE axial trend. The similar geometry of both flanks suggests deposition before the influence of Bighorn structural growth (Laramide Orogeny) to the East. However, the contrasting character of the flanks suggests different depositional conditions, possibly shoreline reworking on the eastern flank from the gyre circulation in the KWIS. Evidence indicates the Peay was deposited in an aggradational context because both the sandstone and underlying shale (Stucco member) are thickest in the axis and thin symmetrically toward the flanks. This argues against erosional down-cutting during relative sea level fall. Moreover, the transition from the underlying shale into the Peay is gradational, rather than sharp-based. The Peay sandstone thus shows a complete delta front facies assemblage with lateral pinch outs in both directions.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California