Allostratigraphy of an Abbreviated Section: The Muscatatuck Group (Middle Devonian) in Southern Indiana
LEONARD, KARL W., Department of Geological Sciences, Indiana University, Bloomington, IN
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.
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