A unique algal stromatolite complex is located southwest of Maumee, Ohio, on the Niagaran reef-bank system that contributed to the isolation of the Michigan basin during the Cayugan Epoch. Overlying strata have been eroded so that the individual features of the complex may be studied in essentially their original paleogeographic setting. Major features in the study area include tidal channels, natural carbonate levees, algal mounds, scour channels, laminations, ripple marks, desiccation cracks, and gypsum and anhydrite molds.

The dolomite rocks in the area may be subdivided into eight microfacies as follows: (1) mound; (2)

End_Page 534------------------------------

mound scour channel; (3) mound transition; (4) intermound; (5) blanket; (6) laterally linked hemispheroid; (7) mud flat; and (8) tidal channel. Each of these microfacies is characterized by a particular algal mat frequency, the degree of development and the nature of laminations, and by the amount of mechanically deposited sediment.

Major rock types are stromatolite-constructed dolomite mudstone, wackestone, and packstone. Nearly all samples are characterized by the partial or pervasive development of a filamentous (and presumably algal) microstructure. Burrows and pellets are uncommon in all of the microfacies. Ostracods are common. Laminations are well developed in all of the microfacies except the mound microfacies. Breccias are common especially in algal mounds and are due to mineralization by sulfate minerals, and dedolomitization, desiccation, and solution collapse.

The general sequence of mineralization was: (1) formation of microcrystalline xenotopic dolomite, probably by replacement of an initial CaCO₃ mud, accompanied by the formation of gypsum and anhydrite; (2) dedolomitization, primarily in the mound microfacies; (3) partial replacement of calcite and dolomite by celestite; (4) replacement of some calcite, dolomite, and celestite by fluorite; and (5) limited formation of medium crystalline, idiotopic, and hypidiotopic dolomite by a process of local source dolomite crystallization and recrystallization.

Comparison with several Recent carbonate analogs in the Persian Gulf and Bahamas suggests that most of the eight microfacies were formed in a supratidal environment.

End_of_Article - Last_Page 535------------