Cephalopods and bivalves of the genus Daonella occur at certain levels throughout the Middle Triassic section in the Humboldt Range, northwestern Nevada. These fossiliferous strata are assigned to the Fossil Hill Member and upper member of the Prida Formation, which here forms the oldest part of the Star Peak Group. The distribution and abundance of fossils within the section is uneven, partly because of original depositional patterns within the dominantly calcareous succession and partly because of diagenetic secondary dolomitization and hydrothermal metamorphism in parts of the range.Lower and middle Anisian fossil localities are restricted to the northern part of the range and are scattered, so that only three demonstrably distinct stratigraphic levels are represented. Cephalopods from these localities are characteristic of the Caurus Zone and typify the lower and upper parts of the Hyatti Zone, a new zonal unit whose faunas have affinity with those from the older parts of the Varium Zone in Canada.The upper Anisian and lowermost Ladinian, as exposed in the vicinity of Fossil Hill in the southern part of the range, are extremely fossiliferous. Cephalopod and Daonella shells form a major component of many of the limestone interbeds in the calcareous fine-grained clastic section here. Stratigraphically controlled bedrock collections representing at least 20 successive levels have been made from the Fossil Hill area, which is the type locality for the Rotelliformis, Meeki, and Occidentalis Zones of the upper Anisian and the Subasperum Zone of the lower Ladinian. Above the Subasperum Zone fossils are again scarce; upper Ladinian faunas representing the Daonella lommeli beds occur at only a few places in the upper member of the Prida Formation.Although unevenly fossiliferous, the succession of Middle Triassic cephalopod and Daonella faunas in the Humboldt Range is one of the most complete of any known in the world. Newly collected faunas from this succession provide the basis for revising the classic monograph on Middle Triassic marine invertebrates of North America published in 1914 by J. P. Smith and based largely on Stratigraphically uncontrolled collections from the Humboldt Range. Taxonomic treatment of these collections, old and new, from the Humboldt Range provides the documentation necessary to establish this Middle Triassic succession as a biostratigraphic standard of reference.Of the 68 species of ammonites described or discussed, 4 are from the lower Anisian, 20 from the middle Anisian, 39 from the upper Anisian, 4 from the lower Ladinian, and 1 from the upper Ladinian. A few additional ammonite species from other localities in Nevada are also treated in order to clarify their morphologic characteristics and stratigraphic occurrence. Other elements in the Middle Triassic molluscan faunas of the Humboldt Range comprise five species of nautiloids and three of coleoids from the middle and upper Anisian parts of the section. Eight more or less Stratigraphically restricted species of Daonella occur in the upp...
The Star Peak Group in and near the Sonoma Range one-degree quadran gle, northwestern Nevada, is redefined to include the mainly calcareous strata that overlie the predominantly volcanic rocks of the Koipato Group and in turn are overlain by the terrigenous clastic rocks of the Auld Lang Syne Group. The Star Peak Group ranges in age from late Spathian (latest Early Triassic) to late Karnian (middle Late Triassic), and it is as much as 1,200 m thick. Although the paleogeography of the group has been affected locally by post-Triassic low-angle faulting, widespread formational units within the Star Peak genetically relate all parts of its outcrop region and obviate the need to separate these rocks into different major structural blocks, as has been done in the past.In the older parts of the Star Peak Group, complex stratigraphic patterns among a variety of carbonate and terrigenous clastic rocks resulted from localized relative uplift, first early and then late in Middle Triassic time. Widespread diagenetic secondary dolomitization of calcareous rocks in the older part of the section further complicates the primary stratigraphic patterns. Following the earlier of these intra-Star Peak tectonic episodes, regionally uniform calcareous rocks of late Anisian age blanketed most of the outcrop area. These rocks contain a rich pelagic fauna and were evidently deposited below wave base. The more western of these rocks grade upward into dark cherty limestone which was deposited in basinal and slope environments concomitantly with uplift and erosion, during Ladinian time, in the central part of the Star Peak outcrop area. South and southeast of the uplifted area, subsidence and peritidal deposition took place followed by deposition of supratidal algal-laminate dolomite. This dolomite was deposited across the beveled uplift and correlates with platform-margin and basinal limestone farther west. Thereafter, lower Karnian platform limestone thickly blanketed the outcrop area and built regressively westward out over the basinal deposits. Following a widespread mid-Karnian break in deposition, local erosion, and deposition of terrigenous clastic rocks, platform carbonate rocks again blanketed the outcrop area during late Karnian time and extended an unknown distance westward, perhaps as far as the original site of the Sierran-eastern Klamath belt. l 118° 0 0 ' 1170 00' Figure 1. Index map showing location oi Star Peak Group exposures (heavy stipple) in and near the Sonoma Range one-degree quadrangle (outlined by lightly stippled border). Location oi maps shown in Figure 3 are indicated by labeled rectangles.INTRODUCTION 5 Although the correlations, stratigraphie relations, and rank of previously named formations of the Star Peak Group have been revised, their lithologie boundaries have not been changed from those embodied in their originaldescriptions. Our revision has entailed incorporating some previously named units into others and dropping one name, the "Natchez Pass Formation." REGIONAL GEOLOGIC SETTING As first described by ...
I Lower Triassic limestone is part of the unique stratigraphic suc-I Gne of the most unusual features of the Chulitna cession of the Chulitna terrane. This terrane is believed to be a greatly displaced lithosphere fragment whose exposures are limited to a 400 km2 area on the south flank of the central Alaska Range in Alaska. The nearest known correlative Lower Triassic marine strata are located hundreds of kilometers distant in northern Alaska and northwestern British Columbia; in general, they are not represented elsewhere among the oceanic-volcanic pre-Tertiary sequences of the entire circumpacific margin. Ammonites are the dominant megafossils in the Lower Triassic limestone of the Chulitna terrane and are represented in a single bed by 13 identifiable species assigned to 13 genera, of which Meekoceras, Dieneroceras, Arctoceras, and Paranannites are the most abundant. Ammonites in the Chulitna fauna are entirely of species previously described from the western conterminous United States from rocks of early Smithian age that were deposited along the margin of the North American craton a t paleolatitudes of about 10" north. The Chulitna fauna is much less similar to faunas of this age from localities in western Canada and the North American arctic, representing paleolatitudes greater than 20" north. Moreover, remnants of the original marine carbonate cement in the ammonite-bearing rock are acicular and evidently formed in warm water; their presence may indicate that the Lower Triassic limestone and associated rocks of the Chulitna terrane were tectonically transported from a site much farther south than their present location.
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