ORIGIN OF POROUS LAYERS IN POOL FINGERS, HIDDEN CAVE, NEW MEXICO: RECRYSTALLIZED MOONMILK?

In Hidden Cave, (Guadalupe Mountains, NM) calcitic pool fingers (narrow pendant subaqueous speleothems) extend downward into dry paleo-pools. A knobby and irregular shape is underlain by an alternation of 0.5 to 1.0 cm dense and porous layers. Dense layers have been interpreted as cave stromatolites but porous layers are enigmatic. These are dark peloidal to clotted micrite enclosing needle fiber to dendrite calcite crystals. Fine micrite consists of micro-rods 0.1–0.2 µm in diameter by 1–2 µm long. Dendrite crystals are branching with limbs of needle fiber calcite 10-20 µm by 30-500 µm. Needles widen and coalesce into irregular 200-600 µm poikilotopic crystals, frequently with ghosts of the original needles.

The knobby appearance of pool finger porous layers strongly mimics a cottage cheese-like deposit found in Spider Cave (Guadalupe Mountains, NM). Known as "crisco", it forms a layer up to 2" thick on surfaces of the two lowest passages. "Crisco" has a greasy, clay-like texture and is a "moonmilk", a class of unconsolidated white, mudlike speleothems. "Crisco" may have formed subaqeously because: 1) it is thin or absent within domes on the ceiling, 2) it has a possible paleo-water line, 3) it occurs only in deepest passages, and 4) fine mud covers some surfaces. A variety of active bacteria and fungi may be instrumental in its formation. Internally, "crisco" is composed of very fine calcite filaments 0.1 µm in diameter and 2–3 µm long, apparently formed around bacterial filaments, and larger crystals resembling stacked rhombohedrons. The fibers and crystals strongly resemble the micro-rods within the porous layers of the Hidden Cave pool fingers. The pool finger micro-rods are slightly larger and more crystalline than "crisco" filaments suggesting some recrystallization along with further lithification.

We suggest that recrystallization of bacterial filaments first produces micro-rods, then needle-fiber calcite, then dendrite crystals, and eventually poikilotopic calcite.