MINERALOGICAL AND GEOCHEMICAL STUDIES OF THE WORLD'S LONGEST SPELEOTHEM, SNOWY RIVER PASSAGE, FORT STANTON CAVE, NEW MEXICO

Fort Stanton Cave, recently designated as a National Conservation Area within the Bureau of Land Management, is an historic cave in central New Mexico. The cave is 23.8 km in length and includes an important new discovery: the Snowy River passage. This section of the cave is a recently opened extension that contains a creamy-white calcite “river” lining the floor of a borehole-like passage. Currently explored to over 7.5 km in length, with no end in sight, the unique Snowy River passage may host the world's longest continuous speleothem.

The Snowy River formation is a thinly laminated carbonate crust that extends more or less continuously along the entire length of the explored passageway. The calcite formation is a subaqueous coral pool deposit, with a cauliflower-like texture on the surface, and in cross section, consists of thin laminae that vary from microns to millimeters in thickness. Dark colored layers appear to be continuous across a suite of 8 drill cores taken over a distance of several km. The calcite-lined pool has a very low gradient (<0.8 degrees over 7.5 km) and responds to surface meteoric events, filling rapidly within hours, flowing for several months, and then draining and drying over a period of weeks. Each event deposits a thin layer of detrital minerals that settle out during gentle flow and new layer of calcite as the calcite-saturated water dries out.

One core was sectioned and analyzed by electron microprobe for major and minor element chemistry, and laminae were counted under a petrographic microscope. This single core contains 500 individual light and dark laminae, each pair of light and dark laminae making up a couplet of dark detritus-rich calcite at the bottom followed by a lamina of clear calcite. Thus, each couplet likely represents a filling and draining event with 250 such events taking place over the period of deposition of 821 (+/-120) years, as determined by uranium-series dating. The minor element chemistry varies significantly from layer to layer, as does the thickness of each layer. The erratic chemistry and changes in thickness of the layers suggest episodic filling events of variable duration have deposited the Snowy River formation.