Print this pageDiagenetic Complexities of the Middle Ordovician Antelope Valley Limestone, Lone Mountain, Eureka County, Nevada
The Antelope Valley Limestone (AVL, upper Pogonip Group) consists of laminated mudstones and skeletal wackestones and packstones deposited on the western Laurentian passive margin carbonate shelf during Middle Ordovician time. The AVL is exposed on the west side of Lone Mountain and is unconformably overlain by the Ordovician Eureka Quartzite and a sequence of Upper Ordovician through Devonian carbonates. For this study, the diagenetic history of the AVL was interpreted using thin-section petrography.
The AVL has undergone a long and complex diagenetic history. Following deposition, initial marine diagenesis is represented by the presence of micritic envelopes surrounding skeletal allochems. Evidence for the first stage of meteoric diagenesis (both vadose and phreatic zones) is the dissolution of skeletal allochem cores and peloids, and the formation of early calcite isopachous, blocky and meniscus cements. Fine-grained euhedral dolomite locally replaced micrite during meteoric or early burial diagenesis. Burial diagenetic silica (chalcedony and chert) cements locally occluded secondary porosity formed during meteoric diagenesis. The void-filling textures of the silica cements are consistent with pore filling and not replacement. Further evidence of burial diagenesis is the presence of sutured (locally stylolitic) contacts and fractured grains. Several generations of spar-occluded cross-cutting fractures are present. Sparry calcite crystals containing inclusions of the silica burial cement formed either during late-stage burial or post-burial meteoric diagenesis. Tertiary porosity was formed by dissolution of earlier calcite cements during a second stage of meteoric diagenesis, possibly during exhumation as calcite grains show strain, an indication of tectonism. A final stage of fine-grained splotchy dolomitization partially replaces depositional, early meteoric, burial, and post-burial calcite. This diagenetic history is consistent with the multiple phases of Late Paleozoic and Mesozoic compressional tectonism and Miocene to recent basin-and-range extension documented in the Lone Mountain area.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009