Monteagle Limestone (Upper Mississippian)--Oolitic Tidal-Bar Sedimentation in Southern Cumberland Plateau

Abstract

Most of the thick clastic strata of Carboniferous age in the Black Warrior basin in Alabama were deposited in various subenvironments of a northeast-prograding deltaic complex. Farther offshore and toward the north, Upper Mississippian carbonate sediments were deposited in banks and shoals. The Monteagle Limestone is considered to have been deposited contemporaneously with part of this prograding clastic sequence; it grades southwest over a distance of 30 km or less to the Pride Mountain Formation and Floyd Shale. Six lithofacies can be recognized within the Monteagle Limestone: (1) massive to lenticular bedded, echinoderm-bryozoan packstone-grainstone; (2) cross-bedded oolitic packstone-grainstone; (3) laminated to structureless dolomitic mudstone; (4) lime wackestone-mu stone; (5) pellet wackestone-grainstone; and (6) clayey shale. The sequence of facies and their sedimentary structures record the initial buildup and migration of an oolitic tidal bar into a relatively low-energy environment culminating in the emergence of the bar above sea level where tidal-flat sedimentation, dolomitization, and calichification occurred. Isolith data suggest that oolitic sand was concentrated in a belt along a line trending northeast-southwest in northeastern Alabama and Tennessee. This belt was composed of individual tidal bars, also oriented northeast-southwest. Initially, each bar was influenced directly by either ebb (northeast) or flood (southwest) tidal currents creating basal sets of cross-strata oriented unidirectionally to correspond with flood or ebb-flow current directions. With growth and expansion, each tidal bar came under the increasing influence of the weaker reverse current which resulted in the reversal of paleocurrent azimuths at the top of the bar.