SANDY OOLITIC AND SKELETAL GRAINSTONES IN THE MISSISSIPPIAN MONTEAGLE LIMESTONE, CENTRAL AND SOUTH-CENTRAL TENNESSEE

Grainstones containing coarse to medium quartz sand are found at several horizons within the basal 20 meters of the Monteagle Limestone in central and south-central Tennessee. These sandy limestones are found at selected localities throughout the Cumberland Plateau region in three distinct lithofacies: (1) parallel laminated oolitic-skeletal grainstones and (2) crossbedded oolitic-skeletal grainstones containing 1-3% detrital quartz, and (3) limestone pebble conglomerates that contain the coarsest detrital quartz grains in concentrations of 5-15%. The coeval St. Genevieve Limestone of southern Indiana likewise contains sandy grainstones that have been interpreted to be eolian. However, because these Monteagle sandy grainstones are associated with inter- and supratidal mudstones a marine rather than eolian origin is indicated. These sandy limestones are hypothesized to indicate the presence of disconformities and possible sequence boundaries. One of the quartz-bearing, limestone-pebble conglomerates directly overlies a paleokarst surface developed on a subtidal carbonate. Quartz grains were probably introduced via fluvial systems during subaerial exposure of the carbonates and then locally reworked into oolitic and skeletal grainstones during the subsequent transgression. The contact between the Monteagle and the underlying St. Louis Limestone in the Cumberland Plateau region has been arbitrarily placed at the base of the lowest, mappable oolitic grainstone. Where present, these sandy oolitic/skeletal grainstones are the lowest occurring such grainstones; they indicate a disconformity and sequence boundary at the St. Louis/Monteagle contact.