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Abstract: Allostratigraphy of an Abbreviated Section: The Muscatatuck Group (Middle Devonian) in Southern Indiana

LEONARD, KARL W.

The application of sequence stratigraphic terminology to genetically described intracratonic Paleozoic strata can be problematic. Geographically limited outcrop study areas, often trending along depositional strike, typically lack the downdip information and temporal control necessary to fully utilize this specific terminology. Alternatively, allostratigraphy has proven to be a useful and conservative approach for building a stratigraphic framework based on genetic and physical criteria for intervals of strata that lack the appropriate or required data to determine the sequence stratigraphic significance of surfaces and units. Genetically related intervals of strata bounded by discordant surfaces are designated as informal allo-units. Discordant surfaces or discontinuities are recognized at individual sections by examining facies and their distributions. Allo-units are delineated by correlating discontinuities between closely-spaced localities relative to facies stacking patterns and available marker horizons.

Five informal allo-units are delineated in platform carbonates of the Middle Devonian Muscatatuck Group. The allo-units are correlatable throughout southern Indiana in an interval which reaches a maximum thickness of 40 m and which may have accumulated over a span of time as long as 13 m.y. The vertical and lateral distribution of facies within allo-units suggests that facies type and thickness were influenced by subtle, structurally-related paleotopography. Four of the five allo units have bounding surfaces that exhibit stratal discontinuities. These stratal discontinuities show these surfaces to be unconformities which bound four probable 3rd-order sequences in this abbreviated carbonate section. These unconformities merge with the sub-Kaskaskia unconformity near the platform margins.

AAPG Search and Discovery Article #90939©1997 AAPG Eastern Section and TSOP, Lexington, Kentucky