Abstract: Monteagle Limestone (Upper Mississippian)--Case for Oolitic Tidal-Bar Sedimentation in Southern Cumberland Plateau
C. Robertson Handford
Most of the thick clastic strata of Carboniferous age in the Black Warrior basin in Alabama was deposited in various subenvironments of a northeast-prograding deltaic complex. Further offshore and toward the north, Upper Mississippian carbonate formations were deposited as banks and shoals. The Monteagle Limestone was deposited contemporaneously with this prograding clastic sequence; it grades toward the southwest for 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/mudstone; (5) pellet packstone/grainst ne; and (6) clay shale. The lithofacies commonly are together in discrete stratigraphic packages, and, when complete, a vertical sequence most commonly includes, from bottom to top, lithofacies 1 to 4. This sequence records 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 a basal set 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.
Generation of tidal currents may have been aided by the presence of the deeper Ouachita trough or an area of possibly deeper water in southeastern Alabama. Emergent areas on the Nashville dome and on the east in the Appalachian tectonic belt delineated the shoreline and bathymetric boundaries of a narrow body of water or strait oriented northeast-southwest through which tidal currents swept.
AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC