The Pipe Creek Sinkhole Biota, a Diverse Late Tertiary Continental Fossil Assemblage from Grant County, Indiana
Farlow, James O., Sunderman, Jack A., Havens, Jonathan J., Swinehart, Anthony L., et al., The American Midland Naturalist
ABSTRACT.-Quarrying in east-central Indiana has uncovered richly fossiliferous unconsolidated sediment buried beneath Pleistocene glacial till. The fossiliferous layer is part of a sedimentary deposit that accumulated in a sinkhole developed in the limestone flank beds of a Paleozoic reef. Plant and animal (mostly vertebrate) remains are abundant in the fossil assemblage. Plants are represented by a diversity of terrestrial and wetland forms, all of extant species. The vertebrate assemblage (here designated the Pipe Creek Sinkhole local fauna) is dominated by frogs and pond turtles, but fishes, birds, snakes and small and large mammals are also present; both extinct and extant taxa are represented. The mammalian assemblage indicates an early Pliocene age (latest Hemphillian or earliest Blancan North American Land Mammal Age). This is the first Tertiary continental biota discovered in the interior of the eastern half of North America.
Abundant late Tertiary continental biotas have been described from the Far West, Great Plains and coastal margins of North America (Janis et al, 1998), but have hitherto been unknown from the interior of the eastern half of the continent. Here we report a diverse assemblage of plants and animals (mainly vertebrates) -from a buried sinkhole deposit in east-central Indiana. The vertebrate fossils indicate a latest Hemphillian or earliest Blancan (Early Pliocene; 45 million years B.P.; Tedford et al, 1987) age.
The sinkhole deposit was discovered by employees of Irving Materials, Inc. at the Pipe Creek Jr. limestone quarry near Swayzee, Grant County, Indiana (Fig. 1). Workers stripped away a cover of Wisconsinan till while expanding the quarry and unexpectedly found unconsolidated sediments beneath the till. Most of the sinkhole sediments were dumped on a spoil pile at the edge of the quarry, but enough material remained in situ in the sinkhole to permit reconstruction of a significant part of its history.
Because the Pipe Creek Sinkhole has yielded a fossil assemblage that is unique for this part of the continent, it is the focus of a multidisciplinary study. Detailed accounts of the site's stratigraphy, sedimentology and paleontology are in preparation. The present paper puts the site on record and summarizes geological and paleoecological interpretations to date (Farlow et al., 1997, 1998, 1999; Sunderman et al., 1997,1998; Holman, 1998; Swinehart et al, 1999).
Processing of sediments from the Pipe Creek Sinkhole is on-going, and additional taxa may be discovered as work proceeds, but the currently known Pipe Creek Sinkhole biota sheds much light on a previously unsampled portion of the North American late Tertiary fossil record.
GEOLOGIC SETTING OF THE PIPE CREEK SINKHOLE
The sinkhole developed in flank beds of the Pipe Creek Jr. reef (named for the quarry), a 1.6-km wide feature of Late Silurian age (Shaver and Sunderman, 1982). The sinkhole is about 75 m long by 50 m wide by 11 m deep (Fig. 2) with steep sides. It probably originated as a small cave whose roof eventually collapsed. Stream sediment was then deposited in the sinkhole, forming an alluvial fan with rubble beds containing large clasts of limestone and chert derived from the surrounding bedrock, and rounded quartzite pebbles of distant origin. Fan sediments also include reworked saprolite derived from local limestones, as well as clays and silts derived from nearby paleosols (Sunderman et al., 1997). These fan sediments periodically plugged the sinkhole, at least once producing a small ephemeral pond or wetland, in which the fossils accumulated (Sunderman et al., 1998). The.Tertiary sediments were eventually buried beneath Pleistocene glacial outwash and till. The total volume of sinkhole sediments was about 30,000 ms, about one-third of which were Tertiary deposits.
The fossiliferous layer was near the base of the sinkhole deposit and was about 2 m thick before most of it was removed by quarrymen; the total volume of the fossiliferous layer was 50-100 m^sup 3^. …