MAMMOTH CAVE AS A RECORD OF REGIONAL GEOMORPHIC EVOLUTION

Mammoth Cave, Kentucky, contains clues to the fluvial and glacial evolution of southeastern North America. Its passages were formed by groundwater drainage to springs on the Green River, a tributary of the Ohio. This record clarifies erosional and depositional events at both local and regional scales over several million years.

Surveys of stratal and structural influence on the cave show that passages that formed at the water table are strongly strike-oriented, with very low gradients. Vadose passages (typically canyons) follow the stratal dip (<0.5 degree), even minor and local variations; but phreatic passages show no systematic relation to dip. Detrital cave sediments at middle and upper levels have reversed polarity (thus >780 ka; Schmidt 1982, Science 217). Cosmogenic ²⁶Al/¹⁰Be in quartz sediment gives the approximate final active age of a passage (Granger et al. 2001, GSA Bull. 113 #7; dates below are based on this paper, adjusted for recent recalibration of ¹⁰Be).

Evolution of the cave morphology is fairly clear: (1) Lengthy development of wide, dip-oriented passages (mostly canyons) interrupted by periodic sediment aggradation, which correlate with slow, widespread pre-Pleistocene base-level fluctuations (~2.8 to at least 3.8 Ma) that molded the karst plains of Kentucky and Indiana (still partly mantled by thick residual sediment in low-relief interfluvial areas); cave passages show two phases of slowly fluctuating base level separated by an abrupt drop of ~5–10 m. (2) A gradual base-level decline led to a static period at ~1.6 Ma that produced many wide, tubular, strike-oriented passages with sparse sediment, presently at or near 168 m altitude; there is strong localized evidence for catastrophic flooding of the cave from the south, with lesser floods from other directions, and no apparent relation to glacial activity. (3) An abrupt 15 m base-level drop at ~1.3 Ma, apparently caused by knickpoint progression up the Green River, triggered by glacial diversion of the pre-glacial Teays drainage into the Ohio River; wide tubular passages, formed at the present altitude of ~153 m, are connected with stage 2 passages only by steep canyons and vertical shafts. (4) Younger passages show no clear levels; some lie as much as 15 m below present river level, perhaps the result of late Pleistocene and/or Holocene aggradation.