THE ROLE OF SUBCUTANEOUS FLUID ESCAPE IN THE FORMATION OF A CAVITY SYSTEM WITHIN A MID-ORDOVICIAN (WHITEROCKIAN) MUD MOUND AT MEIKLEJOHN PEAK, SOUTHWESTERN NEVADA

Mid-Ordovician (Whiterockian) sedimentary rocks of the Antelope Valley Formation of southwestern Nevada contain a large carbonate mud mound. The mud mound is exposed as a 300 m-wide, up to 80 m-high, slice along the west-facing slope of Meiklejohn Peak. The mud mound contains a variety of features related to the presence of a complex cavity system during its formation, including stromatactis, zebra limestone and vertical conduits. Analysis of field relationships, thin sections and polished slabs suggest the cavity system formed as the result of the escape of subcutaneous fluids. Zebra limestone (cm-scale couplets comprised of alternating layers of micrite and fibrous calcite cement) is found in a m-scale stratum along the base of the mud mound, and probably formed early in its history. The regular spacing of zebra limestone laminae and a high degree of interconnection imply that the zebra limestone formed as fluids were forced to flow laterally, possibly as the result of confinement between stacks of submarine crusts. Above the zebra limestone, stromatactis (flat-bottom cavities with irregular tops that are filled with fibrous calcite) are quite common, in places making up more than 50% of the rock. Patchy cementation of the mud mound appears to have resulted in an irregular distribution of the stromatactis. Formation as the result of fluid flow is suggested by the reticulate nature and high degree of interconnection between the stromatactis. Evidence from sedimentary rocks underlying the mud mound reveal that the area had a long history of subcutaneous fluid escape prior to growth of the mud mound, including: 1) brecciated lenses of micritic limestone that are most prevalent under the center of the mud mound and become smaller and less common toward the flanks of the mud mound; 2) injection pipes with brecciated borders and a groundmass that contains chaotically-oriented chert nodules; 3) the presence of stacks of hardgrounds and insipient hardgrounds; and 4) stromatactis occurrences in the encrinite that directly underlies the mud mound. Formation of the mud mound and its cavities as the result of a petroleum seep or due to freezing and thawing of methane clathrates is not supported by isotopic data, which reveal a normal marine composition for cavity cements. Instead dewatering sediments may have been the source of fluids.