New Mexico Geological Society Annual Spring Meeting & Ft. Stanton Cave Conference — Abstracts

One of the most remarkable features of Fort Stanton Cave is the Snowy River calcite deposit, which is likely the world’s longest speleothem (currently over 19.1 km in length). 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 several 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 in 2008 indicate that the deposit thins from 83-25 mm in thickness in a northerly direction (direction of flow). Two more cores were taken in 2018 for compressional strength analysis and other tests. Muddy layers in the cores are continuous and correlate across all ten cores, enclosing a length of over a km in distance. One core was prepared as a conventional petrographic thin section; the section reveals the presence of 500 individual light and dark lamina. A dark laminae forms as a thin layer of detrital minerals settle out after initial flooding; the light layer of calcite precipitates over the top of it, continuing until the water drains out as the flood event ends. Thus, each dark and light pair forms an apparent couplet recording every flood event. The lamina 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 (Land et al. 2010). This yields a calculated average of ~3.6 yrs between filling events, approximating the interval of El Nino-Southern Oscillation events (3-5 yrs) in the southwestern US (Polyak, pers. comm).

In addition to examining the cores, we are conducting a long-term experiment to monitor calcite precipitation by using coupons, placed on the bottom of Snowy River, that are examined after each flooding event. The coupons show a growth rate of calcite of 8-11 um/month during flooding, with thickness apparently dependent on the coupon surface. Data loggers with attached cords, also in the flow path, show a higher rate of growth, upwards of 72 um/month.

Analyses of major and minor elements across one of the cores reveal that the core is a low-Mg calcite, but it is high in sulfate. Generally low in trace elements, the upper 2/3 of the core exhibits a trend of increased Fe, Mn, and Zn. This also correlates with discoloration and an increase of thin mud layers in the upper portion of the core, implying that conditions at Snowy River changed during deposition.

References:

1. Land L, Polyak V, Newton BT (2010) The Snowy River formation in Fort Stanton Cave, New Mexico—Results from radiometric dating and hydrologic observations of the world’s longest speleothem. In: Eaton M, Landres P (eds), Decade of discovery in the National Landscape Conservation System, Proceedings RMRS-P- 000. Fort Collins, Colo., US Dept. of Agriculture.