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Abstract

The Ferron Sandstone Member of the Mancos Shale is divided informally into upper and lower units. The upper Ferron has been divided by earlier workers into seven or eight delta-front units. These units correspond to parasequence sets. We suggest one additional delta-front or parasequence set, bringing the total to nine for the upper Ferron. The opportunity to study many of these stratal units in detail on outcrop has led to the further division into numerous parasequences. These parasequences often contain distinct, mappable genetically related packages of beds which are clearly not bounded by marine-flooding surfaces and have hence been labeled bedsets. Nineteen parasequences and four bedsets are named and described along with several undivided parasequence sets. The landward and seaward pinchouts of the nearshore marine facies of most of these units are mapped, enhancing our ability to predict geometries of associated reservoir facies. Parasequences tend to follow an evolution of delta types from initial regression to maximum regression. This evolution begins with a wave-dominated shoreline, passing through a transitional wave-modified shoreline, and typically ending with a fluvial-dominated shoreline. Parasequences in seaward-stepping parasequence sets have an average dip length of nearshore marine facies of 4.3 mi (6.9 km) and average maximum thickness of 55 ft (17 m). Although less detailed information was gathered on the aggradational parasequences, their average dip length of nearshore marine facies is 6 mi (10 km) and average maximum thickness of 56 ft (17 m). The best reservoir facies are found in the wave-dominated deltas and distributary channels. Wave dominated delta facies are consistently found in the initial regression of each parasequence, and range in shoreline strike orientation from N60W to N5E. Ambiguities in the definitions of sequence stratigraphic units and differing application of those definitions have lead to a variety of stratigraphic schemes from different workers on the same Ferron outcrops.

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