ANALYSIS OF TWO FOSSIL CONCENTRATIONS IN THE TOMBIGBEE SAND MEMBER CAPROCK, EUTAW FORMATION, MONTGOMERY COUNTY, ALABAMA

Determining the genesis of shellbeds and other fossil concentrations can play a key role in sedimentologic, paleontologic, and stratigraphic studies of a particular rock unit. Skeletal concentrations may be biogenic, sedimentologic, or have a mixed origin. Biogenic concentrations may result from rapid burial of dense autochthonous populations or by the activities of predators or scavengers. Sedimentologically generated concentrations can be caused by time averaging, due to either winnowing or a low sedimentation rate, allochthonous organisms being deposited during a storm or other high-energy event, or by ravinement during the early phase of a transgressive systems tract. Orientation and preservation state of skeletal elements can indicate which of the above process were involved in the generation of the unit.

Two fossil concentrations are found in the Tombigbee Sand Member caprock of the Eutaw Formation in Montgomery, Alabama. The caprock consists of four beds of calcite-cemented, quartzose sandstone. The concentrations include a molluscan-echinoid dominated shellbed in the lowermost bed and an assemblage of pen shells (Pinna? sp.) stratigraphically higher. Bulk samples of the shellbed were sectioned perpendicular to bedding and were point-counted to determine the percentage of fossil material in the concentration. Pen shells in the upper bed were mapped over a ten square-meter area, and orientations of the hinge and plane of commissure were measured.

Dominated by disarticulated, recrystallized bivalves and including whole and partial tests of the irregular echinoid Hardouinia bassleri, the shellbed has 7-11 times the fossil density of the adjacent beds. Bivalve shells, some of which are nested, are in concordant, oblique, and vertical orientations. These features point to a sedimentologic origin for this bed. The pen shell concentration consists of over 180 articulated individuals in a single bedding plane in an area of 2x5 m. Most individuals appeared to be in the life position (upright): only 23% deviated by more than 45º from vertical.The combination of these two fossil concentrations supports the previous interpretation of the Tombigbee Sand Member as a lower shoreface environment in a barrier island sequence. This setting is a common modern pen shell and echinoid habitat which is subject to storms.