Identifying the Nature and Extent of the Loyd Sandstone Member of the Buck Tongue of the Mancos Shale in Outcrop and Subsurface Data in the Rangley Area of Colorado

Abstract

The Loyd Sandstone member of the Buck Tongue of the Mancos Shale (Loyd) is found in eastern UT and western CO. The Loyd is well exposed in outcrops near Rangley, CO. Because the Loyd is regionally restricted in extent it has proven challenging to identify strata that should be assigned to the Loyd and separate those from strata of the overlying Sego Formation (Sego). Here we take a systematic approach to solve this problem by (1) correlating key surfaces in wells of the Rangley region and (2) comparing facies, architectures, and surfaces in what we believe to be the Loyd and Sego in the Rangley area with those attributed to the Sego along the strike of the Cretaceous coastline in eastern UT. Because the Sego can be identified in wells from the Uinta and Piceance Creek Basins and because facies, architectures, and surfaces that define the Sego are strikingly similar in outcrops of both regions, this method allows us to identify strata that must be attributed to the Loyd in the Rangley area. In the study area the Sego is divided into four sequences (Sq.1-Sq.4) separated by flooding surfaces. Sq. 1 and Sq. 2, which are most commonly encountered in the Rangley area, contain sedimentary structures and sandbody geometries indicative of both tide and wave influence based on the position along the complex paleoshoreline. Generalized paleoenvironments include shorefaces (lower to upper), tide dominated deltas, tidal bars, and estuaries. Notably, surfaces of incision (incised valleys) initiating in Sq. 2 that incise into Sq. 1 are typically filled with confined tidal bar and estuarine facies. The Loyd in UT is expressed in the Buck Tongue as1-2 m of hummocky cross stratified to ripple cross laminated fine sandstone. In contrast, near Rangley, CO the Loyd expands into a succession of interrelated facies up to 55 m thick. Depositional environments of the Loyd are diverse and include middle to upper shoreface, tidal inlets, rare incision (valleys), embayments, and a potential river dominated delta. Sq.1 and Sq.2 of the Sego in the Rangley area are similar to those in UT. Large incised valleys are also found within Sq.2. Recognizing these key surfaces and facies trends is essential to identifying the Loyd and Sego in outcrop. Because the Loyd was deposited along the complex, likely embayed shorelines of the Cretaceous Western Interior Seaway, clarifying the highly variable paleoenvironments of the Loyd is essential to improve predictive regional reservoir models.

AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014