NEW EVIDENCE FOR SPELEOGENESIS CA. 50 TO 30 KA FROM PORTER CAVE, CENTRAL INDIANA: POTENTIAL GLACIAL HYDROLOGIC AND DYNAMIC CONTROLS

Questions remain on the role of hydrologic changes during Quaternary interglacial and glacial cycles on speleogenesis in mid-continent North America. Study of Porter Cave in central Indiana provides new insight on hydrologic and glacier dynamic controls on karst development. Glacial-lacustrine sedimentary sequences within Porter Cave and in the adjacent watershed indicate a significant rise in hydrologic base level with glacier advance ca. 50 ka ago. Chronologic control on this proglacial system is by radiocarbon dating of allochthonous organic matter and optically stimulated luminescence dating of glacial-lacustrine sediments. This karst area remained flooded with Porter Cave a lacustrine depositional center until ca. 40 ka when ice receded and the proglacial lake rapidly drained, eroded deposited lacustrine sediments, and emplaced fluvial gravels. The presence of glacial-lacustrine silts over fluvial gravels indicate that the proglacial-lake reformed with the advancing ice margin ca. 37 ka and persisted to ca. 31 ka, again flooding the karst system. The presence of waterfalls at Porter and other caves in the area indicates a significant erosive event post ca. 31 ka, potentially associated with the advancing glacier margin and/or rapid draining of a proglacial lake. Post-deglaciation, nick-point migration at the waterfall is the principle mechanism for lowering of the cave floor by <1 m. Speleogenesis appears to be retarded during glacials with higher base levels and the development of proglacial lake systems in central Indiana, which flooded caves. Maximum hydrologic variability occurs with ice marginal fluctuations, particularly upon deglaciation, which would have reversed the hydrologic gradient, removing deposited proglacial sediments, and reoccupying karst systems.