Petrography and Evolution of Niagaran (Silurian) Reefs, Indiana

More than 800 thin sections, collected from reef exposures at Hanging Rock, Mattock, East Lagro, Bluffton, and Delphi, were studied in quantitative and qualitative detail. In addition, 350 slides from the Wabash reef were examined for general petrography. Eight major but closely related microfacies characterize the core, flank, and interreef rock: argillaceous calcisiltite, fossiliferous calcisiltite, Stromatactis calcisiltite, mud-supported biocalcarenite, grain-supported biocalcarenite, mud-supported pelletoidal calcarenite, grain-supported pelletoidal calcarenite, and stromatoporoid-constructed limestone. Relative energy levels were assigned to each microfacies, the argillaceous calcisiltite corresponding with the lowest, the grain-supported calcarenites the highest, and the stromatoporoid-constructed limestone spanning several energy levels. The sequence of diagenetic action has been solution and filling or recrystallization of delicate fossils, silicification, and dolomitization. Initial reef population (Hanging Rock) was by crinoid accumulation on a muddy sea bottom. The core, or central mound, or mound-like part of the reef regardless of composition, consists of three types: (1) massive-appearing but bedded fossiliferous calcisiltite with scattered horizontal Stromatactis, which developed below wave base; (2) bedded fossiliferous calcisiltite with Stromatactis parallel with the arcuate bedding of the mound, which developed near wave base; and (3) bedded stromatoporoid-constructed limestone, which was wave-resistant and developed above wave base. Stromatactis, in the Indiana reefs, consists mainly of Fistulipora colonies which became partly or wholly dissolved and subsequently filled with calcite spar. Flank beds are composed almost exclusively of bioclastics, mainly crinoidal, that are accumulations of debris derived from flank-dwelling organisms, and are not core-derived. They formed against the core after it attained wave base, and then evolved to become the dominant reef element. Since no one exposure displays an entire reef, the collected sections were arranged to form a composite of an "ideal" Niagaran reef. Six stages of reef development are recognized: Stage 1. Argillaceous calcisiltite constitutes the subreef facies. An apparently fortuitous accumulation, mostly of crinoids, forms a mound of fossiliferous calcisiltite below wave base. Stage 2. With the addition of Stromatactis the fossiliferous calcisiltite mound becomes a massive core-like body and continues to enlarge. Stage 3. The mound is built up into the zone of wave base and forms a transitional core as well bedded fossiliferous Stromatactis calcisiltite, alternating with some fossiliferous calcisiltite and mud-supported crinoidal calcarenite, is deposited. Stage 4. The mound is mostly above wave base. Stromatoporoids become abundant enough to form a bioconstructed core, and continue to enlarge the mound. Well bedded dipping crinoidal flank beds accumulate against the core, and consist of noncore-derived alternating fossiliferous calcisiltite and mud- to grain-supported biocalcarenite. Stromatactis becomes rare. End_Page 397------------------------------ Stage 5. The stromatoporoid core enlarges, but noncore-derived accumulated crinoidal flank beds dominate. A current-swept platform develops on the structure which becomes a true wave-resistant reef. The flank beds consist of alternating mud- to grain-supported biocalcarenite and fossiliferous calcisiltite, some scattered overturned tabulate corals and stromatoporoids, and some stromatoporoid-constructed beds. Stage 6. Bioconstructed stromatoporoid and tabulate coral ridges, probably only as core remnants, form along the margins of the ecologically zoned platform. The reef continues to enlarge areally by marginal flank bed accumulations. These differ from those in stage 5 with the addition of mud- to grain-supported pelletoidal calcarenite and more reworked coral debris.