TIMING AND ORIGIN OF CAVITIES WITHIN MEIKLEJOHN PEAK, A MID-ORDOVICIAN MUDMOUND FROM SOUTHWESTERN NEVADA

Meiklejohn Peak, a mid-Ordovician (Llanvirnian) carbonate mudmound contained within the Antelope Valley Limestone of southwestern Nevada, has long been of interest to carbonate sedimentologists because of the wide variety of cavities found there. Meiklejohn Peak is a classic locality for both stromatactis (flat-bottom cavities with irregular tops that are filled with fibrous calcite) as well as the enigmatic zebra limestone (comprised of alternating layers of mudstone and fibrous calcite cement). Other types of cavities are also present within the mound, including vertical conduits, and irregular cavities or depressions. Analysis of these various forms of cavities in the field and in thin section reveal the timing of emplacement of each, and provides clues to their origins. The presence of fossil debris, as well as bioturbated sediments within the irregularly-shaped cavities and depressions suggest that they formed while the mound was being constructed, probably in semi-lithified mud. They may represent scour fills (depressions), or former cavities. Stromatactis cavities are believed to have formed after the majority of the mound was constructed, but before burial based on the presence of fossil debris and cryptic fauna (organisms which live in cavities) within stromatactis along the margins of the mound, but not in the core facies. Zebra limestone formation is more difficult to explain, but may represent either shear failure, or possibly fluid flow after much of the mound was lithified, and the stromatactis network became "plugged" with cement. Cavity formation within the mound is believed to have been the result of fluids migrating upwards through the mound from below. Isotopic studies of cement reveal that the fluids were probably not related to petroleum seep activity or the freezing and thawing of gas hydrates, but may instead represent dewatering of underlying sediments.