STROMATOLITE-THROMBOLITE ASSOCIATIONS IN THE LOWER ORDOVICIAN COTTER DOLOMITE AT HOLLISTER, MISSOURI, U.S.A

The naming and interpretation of ancient microbialite fabrics has had a long debated history. Yet, these organosedimentary structures can provide an important indicator of the local environmental conditions in which they developed. In Missouri, Lower Ordovician strata contain many examples of both thrombolite and stromatolite fabrics. Thrombolite fabrics are frequently associated with laterally-persistent biostromes. These structures are composed of peloidal wackestone and grainstone along with clotted thrombolite. In general, the thrombolite fabrics are interpreted to have formed in subtidal environments during transgressive episodes. Ordovician stromatolite facies from Missouri occur as either laminated hemispherical structures (1 m wide, 0.5 m high) or as densely packed, laminated columns, 2 meters in thickness and 3 meters in diameter. The stromatolites are interpreted to have formed in intertidal to shallow subtidal settings.

The classification and genesis of microbialite structures in the Cotter Formation, at Hollister, Missouri, prove more problematic. These buildups comprise macroscale features that at once can be described as a bioturbated thrombolite, yet also contain features that suggest a stromatolite fabric. These laterally linked hemispheroids are each about 0.5 meters in thickness and width. The linked hemispheroid layer extends laterally at least 0.5 kilometers. Overlying desiccation cracks and gastropod shells indicate formation in very shallow water. We propose that the thrombolite features in these structures are a modified stromatolite caused by physical disturbances during or shortly after lithification. These disturbances may have included bioturbation, physical modification, micritization, early cementation and dissolution. Syndepositional remodeling in modern Bahamian stromatolites as interpretated by Planavsky and Ginsburg (2009) may be analogous to these Ordovician structures.