ABSTRACT

Microbiohermal carbonate bodies constructed by an enigmatic tubular microorganism have been found in two cores of the ?Oxfordian Smackover Formation, in Washington and Escambia Counties, Alabama. The cores (Alabama State Oil and Gas Board Permit No. 2943, IJAMS 19-2 No. 2 and Alabama State Oil and Gas Board Permit No. 4971, T. R. Miller Mill Co. Unit 17-4 No. 1) are located on the northern flank of the Wiggins arch and the northern flank of the Conecuh embayment, respectively. Most examples that we have found of the tubular organism come from the IJAMS core.

The IJAMS core is 18.6 m (61 ft) thick and consists of dolomitic mixed-particle wackestone and dolomitic peloid pack-grainstone containing undolomitized microbioherms (for which we propose the term microherm) up to 0.32 m (1 ft) thick and 5 cm (2 in) to more than 9 cm (3.5 in) wide (the width of the core). Microherms average 16 percent of the core by volume and range from 5 to 100 percent locally. Microherms grew on the sea floor and were constructed chiefly by tubular microorganisms. Microherms and their enclosing matrix in the IJAMS core may have been deposited in a carbonate mound analogous to grainy Carboniferous non-Waulsortian mounds. The microherms and possible decameter-scale carbonate mound described here represent a previously unknown Jurassic bioherm type.

Most Smackover biostromes and bioherms in Alabama are stromatolitic or thrombolitic and contain no remains of the constructing organisms. By contrast, most or all microherms in the IJAMS core consist of micritic tubules embedded in radial fibrous calcium-carbonate cement. Preservation of the tubules is attributed to early cementation that insulated the interiors of the microherms from subsequent diagenesis. The tubules are cylindrical, sparsely branched with "Y"-shaped junctions, and average 42 to 57 micrometers in diameter. Tubules are septate; septa are convex upward in longitudinal section and are spaced 14 to 42 micrometers apart. Tubule walls and septa average 10 micrometers thick; cement coats range from about 15 to 35 micrometers in thickness. Some septa appear to consist of two parallel microcrystalline laminae separated by a thin layer of calcium-carbonate cement; each of the three layers is about 5 micrometers thick. The tubules resemble Renalcis and other microorganisms that have been interpreted as cyanobacteria or foraminifera.