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The AAPG/Datapages Combined Publications Database
CSPG Special Publications
Permian-Triassic Boundary in the Eastern Great Basin Area
In the northeastern Great Basin (northeastern Nevada and northwestern Utah) youngest exposed Permian miogeosynclinal sediments are in the uppermost Gerster Formation. Judged by the conodont Neogondolella rosenkrantzi Assemblage Zone and the Neospathodus divergens Assemblage Zone in the upper few tens to hundreds of feet in the Gerster, a late Guadalupian or younger age is represented. The Neospathodus divergens Zone also occurs in the European Zechstein I.
The Thaynes Formation (and locally the Dinwoody beneath it) represents the Lower Triassic miogeosynclinal strata (or, possibly, epicontinental sea accumulations) in this same general area. Here, the Meekoceras gracilitatis Zone occurs from about 20 ft to as much as 300 ft above the base of the Thaynes, thereby establishing a lower Smithian age for the strata which contain these cephalopods. The conodont Gondolella planata Zone is below the Meekoceras Zone, whereas the Neoprioniodus bransoni Zone overlaps the Meekoceras Zone at the top and occurs in stratigraphically younger beds. To date, therefore, neither time equivalents to the uppermost Permian West Texas Ochoan nor Griesbachian and Dienerian (Scythian of European Stages) have been found in this part of the eastern Great Basin. Seemingly a paraconformity embraces this hiatus with little or no evidence of erosional vacuity. Likely, the top of the Permian was a surface of marine planation.
The southern region of the eastern Great Basin lies in Arizona, Utah, and Nevada, but mainly within southern Nevada. Youngest Permian marine rocks here (shelf to miogeosynclinal) are included in the Kaibab Formation which consists of the Fossil Mountain (and younger) Harrisburg Members. Faunas of the Fossil Mountain Member include the sponge Actinocoelia maeandrina, and various brachiopods, all of which contain elements of the highly-distinctive Road Canyon fauna, thus indicating a late Leonardian age. To date, a precise stratigraphic age assignment for the Harrisburg Member has not been established. The Moenkopi Formation represents the Lower Triassic in this area; its various members change facies and increase in thickness from the shelf of southwestern Utah - northwestern Arizona westward into the miogeosyncline (or, possibly an epicontinental sea) of southern Nevada. The Virgin Member, about 115 ft thick on the shelf within a 2,000-ft, mostly nonmarine section of the Moenkopi Formation, thickens to more than 2,500 ft in thickness in the miogeosyncline where the nonmarine members are absorbed through a westerly transition into the marine Virgin. To date, conodonts have not been reported from the Virgin or other members. The Timpoweap Member, oldest of the Moenkopi, is reported to contain elements of the Meekoceras Zone. The lower 170 to 350 ft of the Virgin Member west of Las Vegas, Nevada contain mostly a bivalve fauna, dominated by such forms as Permophorus, Promyaliana, Monotis, Eumorphotis, and Gervilleia. At one horizon 170 ft above the base of the Virgin in the Blue Diamond Hill area west of Las Vegas, collections are dominated by the large Promyalina cf. P. groenlandica; in the Cape Stosch Formation of Greenland, this species occurs with Claraia stachei, which is a widespread Lower Scythian form present in both the Dinwoody and Thaynes Formations. Evidently these are younger than elements of the Meekoceras Zone, and may range as high as the Owenitan (basal upper Scythian). Thus, the pre-Triassic unconformity in southern Nevada may involve a longer hiatus than it does farther north; it is, nonetheless, interpreted as a paraconformity.
Although elements of youngest Permian or oldest Triassic conodonts and cephalopods have not been reported for the eastern Great Basin, this does not negate an interpretation that marine waters occupied much of the Cordilleran miogeosyncline throughout late Permian and early Triassic times, and that only locally were Permian rocks subjected to subaerial erosion. An erosional vacuity is present here and there, but this does not necessarily indicate that the Permian-Triassic boundary is other than a paraconformity, the hiatus, it is believed, was not one of great time span.
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