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AAPG Bulletin

Abstract


Volume: 39 (1955)

Issue: 11. (November)

First Page: 2211

Last Page: 2305

Title: Pennsylvanian Stratigraphy of Elko and Northern Diamond Ranges, Northeastern Nevada

Author(s): Robert H. Dott, Jr. (2)

Abstract:

Mappable formations of late Mississippian, Pennsylvanian, and early Wolfcampian age have been designated in northeast Nevada. Since the Fortieth Parallel Survey (1876-1878), these have been referred to the "Lower Coal Measures limestone," Weber quartzite or conglomerate, and "Upper Coal Measures limestone."

The Tonka formation (2,000-3,000 feet thick) consists of chert-pebble conglomerate and quartzite with fossiliferous limestone and clay-shale interbeds. It conformably overlies and grades eastward into the White Pine? shale except in Carlin Canyon where it appears to rest unconformably on allochthonous cherty shales. Folding and thrusting during the Antler orogeny in central and western Nevada had raised lands from which gravels were shed into the west margin of the miogeosyncline. Though tentatively correlated with the Diamond Peak formation of central Nevada, the Tonka may not have been laterally connected with it. Deposition was essentially continuous during late Mississippian and early Pennsylvanian, and the systemic boundary can not be precisely defined within these formations.

The Moleen formation (1,100-1,400 feet thick) conformably overlies the Tonka in the north and the Diamond Peak formation farther south. It is characterized by widespread cherty limestone and indicates abrupt change to more uniform stable conditions. Algae and abundance of fragmental fossils, detrital quartz and chert, diastems, and rare oolite indicate shallow water and frequent agitation by currents and waves. The widespread diagnostic brachiopod Rhipidomella nevadensis Meek occurs in the lower member, considered Springeran in age. The middle member comprises the Millerella-Paramillerella faunizone of Morrowan age, and the upper member of Atokan or Derryan age has a widespread, synchronous Chaetetes zone associated with the lowest abundant fusulinids, principally Profusulinella.

The Tomera formation (0-2,000 feet thick) conformably overlies the Moleen except where later Pennsylvanian erosion removed it completely. It has much chert-pebble conglomerate interbedded with quartz-sandy limestones and coquinites, presumably indicating slight revival of crustal instability farther west. The lower two-thirds (Atokan) contains Fusulinella and the upper part (early Desmoinesian), where preserved, has rare Wedekindellina and Fusulina. The upper contact is unconformable where observed.

The long established Ely limestone of east-central Nevada is equivalent to, and is an easterly, less clastic facies of, both the Moleen and Tomera formations. The Ely is elevated to group rank in the Elko-Eureka region with these two formational subdivisions. The latter were deposited nearer lands than the more homogeneous type Ely limestone.

The Strathearn formation (0-1,500 feet thick) unconformably bevels the Tomera and older formations, and conformably underlies Wolfcampian siltstones near Elko. It consists of limestone, siltstone, and fine conglomerate with Missourian-Virgilian Triticites (lower part) and primitive Wolfcampian Schwagerina (upper part). Equivalent rocks ocur east (upper Oquirrh?) and west of Elko (Antler Peak), but a short distance south and southeast no Upper Pennsylvanian is known, and presumably was not deposited.

Most of the strata show evidence of cyclic deposition, although cyclothems were not recognized.

End_Page 2211------------------------------

The rocks are lithologically like many shallow-water cratonal deposits, but are thicker and occur in a linear stratigraphic belt.

Principal hiatus in eastern Nevada occurred in late Pennsylvanian. About the end of Desmoinesian, increasing crustal instability differentiated the miogeosyncline into broad, low, emergent areas and adjacent negative basins. Deposition was more continuous in basins, as near Las Vegas (Bird Springs formation) and Elko (Strathearn formation), but transgression of intervening emergent east-central Nevada was not completed until Wolfcampian time. Thus in the latter region Wolfcampian strata disconformably overlap the Ely limestone. Definition of the base of the Wolfcampian is generally difficult without fusulinid information, but is especially so where the very similar Upper Pennsylvanian strata are present.

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