--> Abstract: The Parasequenoe Boundary, Its Expression and Lateral Relationships: Insight from the Lower Mesaverde Formation, Bighorn Basin, Wyoming, by G. C. Gaynor; #90987 (1993).

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GAYNOR, GERARD C., Reservoir Geosystems, Inc., Dallas, TX

ABSTRACT: The Parasequenoe Boundary, Its Expression and Lateral Relationships: Insight from the Lower Mesaverde Formation, Bighorn Basin, Wyoming

A parasequence is defined as a "relatively conformable succession of genetically related beds or bedsets bounded by marine flooding surfaces and their correlative surfaces." Thus, demarcations between contrasting regressive and transgressive lithostratigraphic elements are required conform to this descriptive definition. However, and more importantly, the bounds of a parasequence must be characterized by the context in which genetically related sedimentary bodies are found. A key element in characterizing parasequence boundaries is the determination of the most basinward position of the strandline associated with regression. Hence, seaward of that point, a fully marine surface of erosion is developed during subsequent transgression. In contrast, the parasequence boundary landward of t e most basinward strandline position is represented by a complex of surfaces associated with unique transgressive coastal environments. These surfaces include ravinement surfaces, channel-form erosional surfaces, hiatal surfaces marked by bioturbation, local marine incursion surfaces and organic accumulation surfaces.

Such surfaces are widespread in the 350 m thick Campanian lower Mesaverde Formation, a deltaic sequence that intertongues with the marine Cody Shale. Each clastic tongue or parasequence (10-20 m thick) is typified by upwards coarsening and thickening shoreface wave-rippled, hummocky and trough cross-stratified sandstones. In basinward settings, each parasequence is disconformably overlain by shales and siltstones. Marine trough cross-stratified sandstones and relatively coarse-grained lag deposits may be associated with these disconformities. In landward settings, transgressive deposits (1-20 m thick), represented by coal, mottled mudstone and heterolithic channel-fill clastics, inlet-fill sandstones and bioturbated nearshore sandstones, correlate with the basinward disconformity and elated marine clastics.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.