SEDIMENTATION AND FACIES ANALYSIS OF THE GIRVANELLA-CONSTITUTED ONCOLITIC SHOALS AND ASSOCIATED LITHOFACIES IN THE MIDDLE ORDOVICIAN ANTELOPE VALLEY LIMESTONE, CENTRAL NEVADA, USA

In Girvanella-constituted oncolitic packstone and grainstone lithofaci es of the Antelope Valley Limestone, ellipsoidal- or well-rounded, Gir vanella oncoids along with thick-shelled gastropods (Maclurites and Pa lliseria) are the primary sediments. These particles are cemented by c olumnar calcite marine cement within the normal-graded, thin- to mediu m beds. These bedforms overlie one another with a sharp contact and co nstitute the grain-supported basal part of thick, massive lenticular b edforms. The basal amalgamated units are overlain by the upper packsto ne to wackestone subunits, which consist of evenly parallel silty quar tz-rich laminae. All of the indicated sedimentary structures and textu res represent moderately high energy, storm-dominated subtidal oncoida l shoals. Progressively abating, multiple high-energy pulses or storm events in these shoals probably resulted in the normal-graded multiple thin to medium bedforms. The presence of radial ooids with well round ed peloids in the uppermost parts of the Girvanella shoal bars suggest s that the oncoidal shoal bars may have very briefly reached the tide- dominated depositional conditions, but such physical conditions probab ly did not persist for a long time. Thus, the subtidal oncoidal shoal bars represent a sedimentation below the fair weather-wave base. Moreo ver, the dark-gray Girvanella oncolitic shoal units are found in the w esterly basinward localities, and they laterally grade eastward into t he light-gray to Nuia packstone- and -grainstone units. These relation s also suggest that the subtidal Girvanella oncoidal shoals formed in a relatively deeper, broader, and lower energy environment than that o f the subtidal Nuia shoals. Submerged Girvanella oncolitic shoal bars allowed strong storm currents to shoal at a siliciclastic-dominated hi gh-energy paleoshoreline environment. As a result, storm countercurren ts transported fine siliciclastics basinward and deposited them as mas sive siltstone, rhythmical siltstone and lime mudstone interlayers, an d silty quartz-rich, parallel-laminated lime mudstone in the shoreward shelf of a progressively shoaling-, well-circulated middle shelf. Rec eptaculites bioherms are believed to have formed at the crests of onco idal shoal bars. The affinity of Receptaculites with Girvanella, Sphar ecodioum and Maclurites and Palliseria suggests about 6 to 12 meters w ater depths for Girvanella oncoidal shoals in the locality of 10 degre es south of the paleoequator during the deposition of the AVL.