Geology of the Late Neogene Pipe Creek Sinkhole ( Grant County , Indiana )

The Pipe Creek Sinkhole (Pipe Creek Jr. Quarry, Irving Materials, Inc., Grant County, Indiana) preserves a diverse paleobiota of late Neogene (late Hemphillian or early Blancan) age. Part of the site was destroyed by quarry operations before it could be studied, but intensive interdisciplinary field and laboratory work has revealed much about the microstratigraphic context and characteristics of the fossiliferous deposit and associated units. Despite frequent complications of postdepositional sagging around large limestone boulders in the sinkhole, a typical stratigraphic sequence of distinct layers (zones) was recognized. The fossiliferous sediment (Zone A) was a dark-colored, mud-rich, organic-rich diamicton overlying a yellow-brown clay (Zone B), which in turn overlay (in situ sediments no longer exist in the sinkhole) thick red clays (Zone C). Zone C was so thick that its lower limit could not be detected, either by mechanical digging or by ground penetrating radar. Sediment microfabrics of Zones A and C were similar in containing reworked mudballs presumably transported into the sinkhole from the surrounding landscape. Zone B appeared to be a product of weathering of the uppermost parts of the red clays; Zone B and parts of Zone C preserved root traces, and appear to have undergone alternating oxidizing and reducing conditions. In contrast, Zone A experienced reducing conditions and poor drainage through most of its history, as indicated by its dark color and abundant siderite nodules, and by stable carbon and oxygen isotope ratios of the siderites. Such reducing conditions presumably were associated with large quantities of organic matter (5 to 10 percent dry weight of the noncoarse sediment fraction) that accumulated in the Zone A depositional environment. Reducing conditions in turn facilitated preservation of plant and other fossils. In one part of the sinkhole, Zone A 2 iNd iaNa GeoloGical surve y sPec ial rePort 69 contained a laterally extensive layer of wood, seeds, and other detritus. The organic content of Pipe Creek Sinkhole fossil plant material (as a percentage of dry weight) is very little, less than that of modern plant tissues. In addition to large plant fragments, fossils preserved in Zone A included vertebrates, invertebrates, and charophyte gyrogonites. Because Zone A was mostly unconsolidated, screen-washing was used to concentrate fossils. Vertebrate bones occurred in concentrates collected on 4-mm, 2-mm, 1-mm, and 0.5-mm mesh screens. The most common small vertebrates were frogs and turtles, but snakes, small mammals, and fishes were also present. Small molluscs, seeds, and charophyte gyrogonites were abundant in the 1-mm and 0.5-mm fractions. With few exceptions, large mammals were represented by isolated bones. Zone C probably derived from terra rossa clays on the surrounding landscape that filtered into the future site of the sinkhole while it was still a cave that was largely closed to the surface. Zone B constitutes the end product of weathering of the uppermost portions of Zone C after the sinkhole had opened, permitting plants to grow and pedogenesis to occur. Along with sedimentary features of Zone A, the dominance of the biota by aquatic taxa indicates deposition in a wetland at least and, more likely, a pond.