Trace Fossils Revealed Through X-Radiography in Facies Analysis of Smackover Formation, Southwest Alabama

Richard A. Esposito, David T. King, Jr.

The use of x-radiography has been applied to slabbed cores of Jurassic Smackover limestones from southwestern Alabama to enhance the complete petrologic description of the rocks. Through x-radiography, trace fossils have been revealed in what would otherwise appear to be homogeneous rock. In these biogenic structures, organic material, partly fecal in origin, is concentrated as infill packing in actively filled burrows. A microreducing environment within the burrow results in the mineralization by finely disseminated FeS₂. The density difference between FeS₂, which has a high absorption coefficient, and the surrounding calcium carbonate highlights the burrows in the x-radiographs. This characteristic burrow mineralization is shown well in the Smackov r where a Zoophycus-Thalassinoides trace-fossil assemblage has been identified. Zoophycus, a feeding structure, is characterized by concave-upward traces with whorled peaks, and is best seen in slabs cut perpendicular to bedding. Thalassinoides is a dwelling structure characterized by a box-work burrow system and is best seen in cores cut parallel to bedding. This assemblage is restricted to a facies that is laterally persistent throughout the Smackover in most of Escambia County, Alabama. This trace-fossil assemblage is found in an oolitic pelletal packstone. This unit is overlain by an oolitic grainstone and is stratigraphically above a sparsely fossiliferous, laminated wackestone and packstone. Trace fossils in this horizon are abundant, but the traces are not found in stratigraphical y adjacent lithofacies. Detecting these otherwise unseen trace fossils by x-radiography assisted the paleoenvironmental interpretation of this depositional facies as a low-energy subwave-base carbonate-shelf deposit.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.