Silurian Reefs of Northern Indiana: Reef and Interreef Macrofaunas

Abstract

This synthesis of Niagaran (Middle Silurian) geology for the reef-outcrop area of northern Indiana and adjacent westernmost Ohio is the first for this area to integrate H. A. Lowenstam's concept of Silurian reefs in the Great Lakes area with close regional stratigraphic control and with classic paleontology. In the outcrop area the four Niagaran reef-bearing formations (in order of decreasing age: the Salamonie Dolomite, Waldron Formation, Louisville Limestone, and Wabash Formation) consist dominantly of dolomitized carbonate sediments that represent interreef (or nonreef) basin-and-shelf-edge to shallow-shelf deposition. Reefs abound that include incipient lenslike reefs, mature isolated circular to irregular reefs, and a large fringing or barrier reef or bank. High-energy shallow-water skeletal carbonate sediments, free of terrigenous clastic detritus, were deposited in massive to crossbedded essentially nonreef strata during middle and late Salamonie deposition, but, beginning during late Salamonie and Waldron deposition, advent of considerable terrigenous sediments and possibly some increasing depth of water were accompanied by beginning reef growth. Small incipient reefs are known only at that level, whereas nearly all the larger and more mature reef expanses are at higher Silurian levels, some as high as 400 ft above the Waldron. Further, these large reef expanses first began to grow at about the Salamonie and Waldron level. From these observations it follows that an expanded upward, inverted-cone shape applies to most of the discrete reefs. Many incipient reefs are spaced so closely as to have resulted in upward coalescence both above and below the present surface of erosion. Fine-grained carbonate and terrigenous sediments were deposited in quiet, probably reef-protected and relatively deep but yet shallow water in the interreef areas of the shelf during early Wabash (Mississinewa) deposition; cherty carbonate sediments generally were deposited during latest Wabash (Liston Creek) deposition, but these two principal types of interreef sediments have particularly complex relations with each other. Much of the Liston Creek sediment probably was derived from active reefs that had continued to grow from earlier Niagaran time and that apparently had attained wave base or surf level at the approximate time represented by the regional Mississinewa-Liston Creek contact. The nearly 400 identified species of the Niagaran macrofauna belong to either reef or interreef (or nonreef) faunas and have been assigned precise stratigraphic levels. The two kinds of faunas are rather exclusive of one another. The interreef fauna shows little or no community evolution through Niagaran time. In contrast, the reef fauna shows marked progressive increase in community evolution, species diversity, and relatively greater increase in the reef dwellers (versus reef builders) that is in accord with both successively higher stratigraphic position and advancing structural maturity of the reefs. However, the fact that the Salamonie (nonreef) fauna was already a well-evolved fauna with high reef potential before reefs began to grow in the area tempers the import that the reefs have for community and organic evolution. Altogether, these stratigraphic, paleontologic, and reef-genetic relations show that the Lowenstam reef concept is valid and that it applies on a near-epochal or even greater scale in the study area, rather than on any start-and-stop basis for independent segments of time or for isolated parts of the Silurian shelf. Perhaps thousands of Silurian reefs are undiscovered in the study area or were destroyed by pre-Middle Devonian and later erosion. The possibility that the preserved-reef record in northern Indiana and elsewhere formed a restrictive mechanism for deposition of the lower part of the evaporitic sequence of the Salina Group in the Michigan basin already has been strongly advanced. The new estimates made here, on numbers of reefs and amount of erosion, taken together with the projected Late Silurian climax of coalescing reef complexes, suggest a restrictive mechanism that could be related to deposition of the middle and upper parts of the evaporitic Salina sequence in the basin. Further, such a reef climax at the basin edge suggests a progressive expansive basin history and multiple reef elts that cannot be interpreted from the buried-basin-reef sequence alone.