OCCURRENCE AND SIGNIFICANCE OF MICROBIALITE STRUCTURES IN LOWER ORDOVICIAN CARBONATES OF SOUTH CENTRAL MISSOURI, U.S.A

Microbialite (“microbolite”) structures represent a significant component of carbonate lithofacies in the Lower Ordovician strata of central and south-central Missouri. Dolomitization and dissolution have overprinted most of the original carbonate facies; however, diagenetic processes were not fabric destructive and primary macro-, meso-, and most micro-scale features can be discerned. Both laminated and clotted microbialite mesostructures (mm- to cm-sized objects) were observed.

Laminated microbial mesostructures include those organosedimentary deposits having primary laminations. These “stromatolites” are associated with a wide variety of lithofacies, including carbonate mudstones, wackestones, and chert. Most laminated fabrics developed into either hemispherical macrostructures (1 m wide, 0.5 m high) or as densely packed, laminated columns within conical bioherms, which reach 2 meters in thickness and 3 meters in diameter. Both macrostructures are preserved in laterally continuous biostrome megastructures (10 to 100s of meters wide). The stacked bioherms, in particular, are quite distinctive and may be important for intrabasinal paleoenvironmental correlation. Petrographic characterization further subdivides the laminated mesostructures into peloidal and micritic microfabrics. These Lower Ordovician stromatolites are interpreted to have formed in intertidal to shallow subtidal settings. These facies are most common in shallow water regressive deposits of the Roubidoux, Jefferson City, and Cotter Formations.

Clotted “thrombolite” mesostructures are composed of peloids of various dimensions and shapes, which clot to millimeter and centimeter sizes. Some peloidal clots are arranged in geometric patterns. Degree of peloid packing results in variation from sparsely packed peloidal wackestone to highly packed and amalgamated, peloidal grainstones to pure clotted thrombolite. The clotted peloidal fabrics, regularly associated with dense micritic crusts, are often heavily bioturbated. While pure clotted thrombolite fabrics are most common in the Gasconade Formation, clotted and peloidal mesostructures are ubiquitous to all formations. These thrombolite and clotted peloidal mesostructures are associated with subtidal environments during transgressive episodes.