McLaughlin, Patrick I.¹, Carlton E. Brett¹ 
(1) University of Cincinnati, Cincinnati, OH

ABSTRACT: Faunal Gradient Analysis of “Shoal” Carbonates: Uncovering the Role of Siliciclastic Sediment Starvation in Producing Widespread Skeletal Sediments

Carbonate facies models attribute many thick successions of carbonate sand to winnowing of sediments through wave agitation in high energy “shoals”. However, recent studies of ancient mixed carbonate-siliciclastic successions suggest that certain widespread skeletal limestones require an alternative explanation. Skeletal grains within these widespread limestones contain not only important taphonomic information (e.g. sorting, abrasion, and fragmentation), but also distinctive faunal assemblages that have been shown to be sensitive bathymetric indicators. For example faunal analysis of an Upper Ordovician skeletal grainstone succession (~7m) suggests that despite little apparent vertical change in lithofacies, the faunal assemblages vary substantially upward through the succession. Multivariate analysis of faunal census data from this grainstone interval suggests deepening upward. Indeed, many of the taxa in the census overlap with those found in the overlying shaly-nodular wackestone-packstone succession (~75m) that would be inferred to represent substantially deeper, quieter water based on sedimentology. However, faunal gradient analysis indicates similar paleobathymetry for portions of these two successions, suggesting that interpreting paleobathymetry of a sedimentary succession based simply on mud content may be misleading. We propose that, while winnowing certainly plays a part in the formation of skeletal grainstone-packstone successions, siliciclastic sediment starvation on the shelf is the primary control on the formation of certain, widespread subtidal skeletal limestones. Thereby, during sea level rise, skeletal sediment blankets may cover large portions of the shelf, and are not just restricted to high-energy “shoals”.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.