Paper No. 10
Presentation Time: 4:10 PM


SPILDE, Michael N., Institute of Meteoritics, University of New Mexico, MSC03-2050, Albuquerque, NM 87131 and BOSTON, Penelope J., Dept. of Earth and Environmental Science, New Mexico Institute of Mining and Technology, Socorro, NM 87801,

Fort Stanton Cave is an historic cave in central New Mexico that includes Snowy River passage, a recent discovery now explored to over 17.7 km in length with no end yet in sight. This section of the cave contains a creamy-white calcite crust covering the mud and gravel floor of a borehole-like passage, forming the world's longest known continuous speleothem. The Snowy River formation is a subaqueous coralloid pool deposit with a very low slope (<0.8 degrees over 7.5 km) that responds quickly to large surface meteoric events, filling within hours, flowing for several months, and then draining and drying over a period of weeks.

The carbonate crust has a cauliflower-like texture on the surface, and in cross section, consists of thin laminae that vary from microns to millimeters in thickness. Eight drill cores taken over a distance of 1.1 km indicate that the deposit thins from 83-25 mm in thickness in the direction of flow, although explorers have noted thin fragile areas further upstream that may be a result of variable flow regimes. Muddy layers are continuous and correlate across the eight cores sampled. One core was sectioned and contains 500 individual light and dark laminae. Each dark and light pair forms an apparent couplet of dark detritus-rich calcite at the bottom followed by a lamina of clear calcite. Each time the passage fills with water, a thin layer of detrital minerals settles out. A new layer of clear calcite is then deposited over that when the flow stops and the water becomes increasingly calcite-saturated as the stream dries out. The new layers are very fragile, but recrystallize into a more robust microstructure integrated with the underlying calcite over remarkably short periods of time (months).

Each laminae couplet records a filling-draining event with 250 such events taking place over the period of deposition of 821 (+/-120) years, as determined by uranium-series dating. This yields a calculated average of ~3.6 yrs between filling events, which approximates the interval of El Nino-Southern Oscillation events (3-5 yrs) in the southwestern US. Thus, Snowy River passage preserves the complex history of the cave in stream gravels and mud that underlie the calcite, while local weather-related events are recorded in the laminated calcite of the much younger Snowy River formation and are seen today.