OCCURRENCE AND FEATURES OF FOSSILIFEROUS SEDIMENTS OF THE PIPE CREEK SINKHOLE (LATE NEOGENE, GRANT COUNTY, INDIANA)

The Pipe Creek Sinkhole (PCS) of northern Indiana has yielded a diverse fossil assemblage of latest Miocene or earliest Pliocene plants and vertebrates. Sinkhole stratigraphy was complicated in places by post-depositional slumping/sagging of sediment into presumed drainage conduits, but a characteristic vertical sediment sequence could be recognized. The main fossiliferous layer (Zone A) is a dark-colored, poorly-sorted, organic-rich sediment. A highly variable percentage of Zone A sediment comprises coarse material (pieces of limestone, siderite nodules, quartzite pebbles, large bones). Of the non-coarse fraction, the weight percentages of material retained on 2-mm, 1-mm, and 0.5-mm fractions are subequal (c. 4-5 % of the non-coarse fraction); about 6 % by weight of the non-coarse fraction is retained on 0.25-mm screens, 5 % on 0.125-mm screens, and about 2 % on 0.063-mm screens. About 75 % by weight of the non-coarse fraction passes through 0.063-mm screens. The textural fabric of Zone A sediment includes reworked lithoclasts. Frog, fish, turtle, snake and rodent bones, bivalve and gastropod internal molds, charophytes, wood, leaves, stems, and seeds are regularly found in Zone A sediments.

Beneath Zone A sediments typically are orange to yellow-brown clays (Zone B) that have a few fossils. Zone B clay does not have reworked lithoclasts, but does show fine laminae, root structures, burrows, and illuviated clay with meniscae. Beneath Zone B are thick terra rossa clays (Zone C) that are devoid of fossils but contain pieces of calcite crystals and limestone fragments of variable size, and that extend as deeply into the sinkhole as we were able to penetrate. The depositional fabric of Zone C indicates that the clays are largely reworked lithoclasts. Parts of Zone C have been altered from red to yellow. The < 1 micron fraction of Zone A-C clays is mineralogically similar, and dominated by an expandable kaolinite phase, discrete illite, and goethite.

Zone C constitutes sediments transported into a cave prior to opening of the sinkhole to the surface. Zone B is a soil, the end product of weathering of Zone C clay after the sinkhole opened. Zone A sediments were deposited in a pond. PCS sediment organic carbon stable isotope ratios are fairly positive (c. -20 to -24 per mil), suggesting a mixed C3/C4 vegetation on the landscape.