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Abstract

Upper Pennsylvanian carbonate platform, bank, and reef-mound complexes of the Horseshoe atoll constitute major oil reservoirs within the northern Midland basin of west Texas. Analyses of over 200 mi of seismic data, constrained by fusulinid biostratigraphy, allow seismic sequences and facies to be identified for the eastern and southern portions of the atoll. The reef complex in these regions is composed of four third-order (1–10 m.y.) seismic sequences, including, from oldest to youngest: (1) Strawn (Desmoinesian) sequence; (2) Canyon A (early-early Missourian) sequence; (3) Canyon B (middle-early to early-middle Missourian) sequence; and (4) Canyon C/Cisco (late-middle Missourian-early Virgilian) sequence. The seismic sequences are composed of one to five parasequence sets, and display a retrogradational geometry in cross section and map view. Additional third-order sequences may be present in the Desmoinesian and Virgilian intervals, but are unresolved seismically in the study area.

The Strawn seismic sequence is characterized by the occurrence of discontinuous, mounded reflectors interpreted to represent amalgamated phylloid-algal mound complexes. The lower Strawn sequence boundary represents the eroded surface of the Absaroka I cratonic subsequence, a type 1 sequence boundary. The upper Strawn sequence boundary appears conformable with the overlying Canyon A sequence, although the exact nature of the upper Strawn sequence boundary is equivocal. The Canyon A sequence is characterized by internal sigmoid geometries interpreted as prograding clinoforms, indicative of oolitic and skeletal grainstone-bearing units. The Canyon B and Canyon C/Cisco sequences, which are generally restricted to the topographically highest portions of the atoll, are characterized by coherent mound facies that are interpreted to represent heterogeneous reef complexes. Adjacent to many of the larger reef masses, the presence of reef-debris facies of late Canyon age is indicated by reflector packages that show offlaping geometries and basinward downlap. The upper Canyon B sequence boundary shows evidence of significant erosion associated with mass wasting of the atoll bank margin, perhaps related to an atoll-wide exposure event. The top of the atoll appears to be a maximum-flooding surface that is unconformably overlain by onlapping Upper Pennsylvanian and Lower Permian shales.

The documentation of third-order seismic sequences and facies of the Horseshoe atoll is important, not only because it provides an internally consistent stratigraphic framework for stratigraphic analysis, but also because it serves as an ancient example of a detached, retrogradational carbonate system. Seismic analysis of the atoll illustrates that the geometries of these types of systems differ substantially from that of attached, prograding systems. The arrangement of systems tracts within retrogradational systems may also differ. The stratigraphic architecture of third-order seismic sequences and facies displayed by the Horseshoe atoll may represent a recurring depositional pattern that arose during Late Pennsylvanian time, a consequence of the geographic, climatic, oceanographic, and tectonic setting of the developing Pangea supercontinent. If so, such a pattern may be anticipated in other time-equivalent carbonate regions.

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