Kirsten M. Bannister¹, Todd A. Thompson², Brian D. Keith²

(1) Dept. of Geological Sciences, Indiana University, Bloomington, IN
(2) Indiana Geological Survey, Indiana University, Bloomington, IN

ABSTRACT: Three-Dimensional Model of Bedform Geometry in a Carbonate Shoal, Salem Limestone (Middle Mississippian), South-Central Indiana

Ancient carbonate shoals are poorly understood because of the lack of extensive vertical and lateral exposures combined with their complex internal structures and sequences. Exposures of the Salem Limestone (Mississippian, Valmeyeran) shoal facies in sawed quarry walls offer a unique opportunity to model the three-dimensional geometry of bedforms and their bounding surfaces. Quarry walls are mapped from digital photographs and enhanced with paleocurrent, porosity, and permeability measurements. These data are used to make a three- dimensional actualistic model to represent the shoal delineating bedform geometry, bounding surface relationships, and spatial distribution of rock properties.

Four levels of bounding surfaces were recognized that define a hierarchy of bedform geometries within the shoal complex. The levels are based on increasing degrees of internal complexity and are recognized by changes in sedimentary structures, lithology, and paleocurrent direction. First order-surfaces (foreset laminae) show an alternation in grain size and composition resulting from grain falls, grain flows, and ripple migration. Second-order surfaces are erosional bounding surfaces between individual bedforms. Third-order surfaces are erosional bedding surfaces that define laterally continuous mesoforms or suites of similar bedforms. Fourth-order surfaces correspond to lithofacies changes between shoal environments and often truncate first-, second-, and third-order bounding surfaces having irregular relief up to 1.5 m. Paleocurrent measurements identify changes in past flow conditions within trough cross-stratification from NE to ENE, which represents onshore sediment transport during the Mississippian on the eastern margin of the ancestral Illinois Basin.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado