Abstract: Three-Dimensional Facies Architecture of the Salem Limestone (Middle Mississippian), Eastern Margin of Illinois Basin

Nadeem Ahmad, Brian D. Keith, Todd A. Thompson

Mapping of sedimentary surfaces in the Middle Mississippian Salem Limestone exposed on sawed quarry walls in south-central Indiana has revealed a hierarchy of depositional units representative of the extremely dynamic hydrographic regime of the upper shoreface zone. The depositional units on the scale of microform and mesoform are represented by the microfacies and the facies respectively. Based on their hierarchy, genetically related depositional units and associated bounding surfaces were grouped together to construct four architectural packages (APs) of the scale of mesoforms.

AP-I is dominantly an echinoderm- and bryozoan-rich grainstone and consists of bedforms ranging from small ripples bounded by first-order surfaces to two- and three- dimensional megaripples bounded by the second-order surfaces. It formed as part of a giant ramp (asymmetric wavefield) within the intrashoal channel setting. AP-II, also a skeletal grainstone, is a complex of giant sandwaves that moved into the area under the influence of a storm and partly filled the basal channel form of AP-I. Large avalanche foresets with tangential toesets prevail. AP-III is a dark-gray spatially discontinuous skeletal grainstone to packstone that laterally grades into a skeletal packstone to wackestone. It locally developed overhangs, rip-ups, and hardground on its upper surface. AP-IV is a skeletal nd oolitic grainstone formed of tabular two dimensional megaripples (planar cross-beds) and three-dimensional oscillatory megaripples (trough cross-beds).

These architectural packages based on the bedform architecture and micro- and mesoscale compositional changes can be used to characterize micro-, meso-, and macroscale heterogeneities. Models of facies architecture from this and similar outcrop studies can be applied to the subsurface Salem reservoirs in the Illinois Basin using cores.

AAPG Search and Discovery Article #90954©1995 AAPG Eastern Section, Schenectady, New York