Petrology of the Rockport Quarry Limestone (Middle Devonian Traverse Group) Alpena, Presque Isle and Montmorency Counties, Michigan
The basal unit of the dominantly carbonate Traverse Group, the Bell Shale, is gradationally overlain by the Rockport Quarry Limestone which has a thickness of approximately 14 m. The Rockport Quarry Limestone is composed of a dark unrestricted marine subtidal organic-mud packstone facies, comprised of an algal-mat-bearing coral packstone subfacies and a shallower water crinoid-bryozoan grainstone subfacies; a shoal-forming stromatoporoid biolithite facies; and a lagoonal micrite facies comprised of a subtidal dense subfacies containing gastropods, ostracods, and calcispheres, and an intertidal to supratidal fenestral subfacies. The local facies tract reconstructed for the Rockport Quarry Limestone is interpreted to be analogous to the subtidal facies relationships present today in and around Rodriguez Key and within the intertidal to supratidal environments at Andros Island and Shark Bay, Australia. At Rockport Quarry and Grand Lake, basal Rockport Quarry strata are comprised of extensive biolithites (3 x 1000 m in cross section) composed of laminate to tabulate stromatoporoid sheets interlaminated with subordinate organic-mud packstone. Higher in the section at Rockport Quarry, the stromatoporoid biolithite facies is capped locally by a subtidal back-shoal biolithite-micrite transition facies containing what is thought to be the first recorded occurrence of calcareous algae in the Middle Devonian of Michigan.
Insoluble residues of the Bell Shale and organic-mud packstones from the Rockport Quarry Limestone are mineralogically similar, but the residues from the Rockport Quarry Limestone are darker due to disseminated pyrite and hydrocarbons. The dominant hydrocarbon grain type, 4 micron brown "microspheres," are inferred to be the fossil unicell remnants of ancient subtidal algal mats.
Diagenetic phenomenon observed within the Rockport Quarry Limestone indicate early marine cementation, neomorphism, solution and compaction, followed by later local void formation and dolomitization. Extensive dolomite at Ocqueoc Falls probably reflects dolomitization along a local fracture system.
Twenty-two taxa were encountered that were previously unrecorded in the literature for Rockport Quarry including: calcareous algae, cephalopod conchs, and trilobite fragments. Three families of stromatoporoids were differentiated.