Stratigraphic and Environmental Significance of Sedimentary Structures in Cayugan (Silurian) Tidal Flat Carbonates, Northwestern Ohio

摘要

Detailed studies of outcropping rocks of Cayugan age in the Findlay arch area disclose numerous and diverse types of sedimentary structures, the majority of which have not been recognized or described previously in these strata. The recognition of these structures provides a new approach to the lithostratigraphic and environmental analysis of Cayugan rocks in the Findlay arch region. The 170-ft (52-m) sequence of Cayugan rocks which crops out in the Waterville, Ohio, area is the thickest sequence of Cayugan rocks exposed in Ohio along the southern margin of the Michigan basin. These rocks include the uppermost part of the Greenfield Formation, nearly all the Tymochtee Formation, and the lower part of the Put-in-Bay Formation. The 130-ft (40-m) section of the Tymochtee Formation is the thickest surface section of this unit and the only section in which both the upper and lower contacts are exposed in the Findlay arch region. The section of the Tymochtee Formation is designated as a principal reference section for the Findlay arch region; portions of this section can be correlated physically with the type section of the Tymochtee Formation. Sedimentary structures provide criteria for the recognition of four new members in the Tymochtee Formation, which are, in ascending Stratigraphic order, the Granger Island, Ovitt Road, Maumee River, and Roche de Boeuf Members. The principal types of rocks developed from Cayugan sedimentation in northwestern Ohio are fine-grained, mostly unfossiliferous dolomites and shaly dolomites, which contain algal stromatolites, shrinkage cracks, ripple marks, tidal channels, gypsum, pseudomorphs after gypsum, celestite, and molds and casts of halite. Collapse has occurred in some rocks due to the solution of evaporite minerals. By analogy with Holocene sediments, it is concluded that the Cayugan rocks were formed on an evaporite-carbonate tidal flat wherein the conditions of sedimentation were not unlike those found today in the Persian Gulf. Subenvironments in the Cayugan tidal flat were mainly intertidal, but ranged from subtidal to supratidal. During Cayugan time northwestern Ohio was an evaporite-carbonate tidal flat rather than a reef bank complex as has been suggested previously. Strandlines within the tidal flat were probably oriented mainly northwestward, in opposition to the present northeastward trend of the Findlay arch through northwestern Ohio. It is probable that the Findlay arch did not affect sedimentation in northwestern Ohio during Cayugan time.