Siliciclastic Overlap Assemblage - Shale, sandstone, and limestone

Shale with interbedded sandstone and minor limestone characterize the Lower Triassic Candelaria Formation (Ferguson, Muller, and Cathcart, 1954). This vertically coarsening sequence grades up into a distal volcanogenic turbidite in the middle and a proximal turbidite and breccia near the top (Stewart, 1980). The basal strata of the Candelaria are earliest Triassic (Griesbachian) and the highest are late Early Triassic (early Spathian) (Speed, Silberling, and others, 1989). It is equivalent in age to the marine Dinwoody Formation of northwestern Utah and southeastern Idaho, and possibly, to the lower part of the predominantly volcanic Koipato Group in northwestern Nevada (Poole and Wardlaw, 1978). The Candelaria Formation is mainly exposed near the old mining camp of Candelaria, located a little more than 20 mi south of Mina, in Mineral County. Another exposure also has been described from the southern Toquima Range in Nye County, and a collection of Early Triassic fauna was recovered from flaggy brown siltstone from the west side of the Toiyabe Range east of Ione (Poole and Wardlaw, 1978). Early Triassic conodonts in clastic rocks in the northern Hot Creek Range near Morey Peak suggest that some of these rocks may also be correlative with the Candelaria. These additional Early Triassic locals suggest that the Candelaria may have been more extensive, or is still unrecognized elsewhere in the central part of the State. The nature of the basal contact is critical to determining the appropriate paleogeographic setting and regional grouping for this unit. If the basal contact is a major structure, then the Candelaria likely represents a section of one of the many Mesozoic terranes that have been emplaced from the west. If the contact is fundamentally sedimentary, albeit disconformable or unconformable, then it constrains an important piece of the paleogeographic tectonic puzzle of Nevada geology. The Candelaria Formation lies on the subjacent Permian Diablo Formation where it is described as unconformable by Ferguson and others (1954), conformable by Speed and others (1977, p. 303), and nearly conformable by Page (1959). The Candelaria near Willow Spring in the Toquima Range is described as a “probable unconformity” by Poole and Wardlaw (1978). The regional map relations for this unit suggest that the base is a disconformity or slight unconformity with the underlying Diablo Formation (Ferguson, Muller, and Cathcart, 1954; Page, 1959), but not a major structure. The Diablo Formation, included here with the Permian siliciclastic overlap assemblage, lies with marked unconformity on lower Paleozoic basinal rocks of chert, argillite, and shale, as discussed below. The Candelaria Formation is unusual in that it is the oldest Mesozoic sedimentary sequence known in Nevada, and is present in a restricted area only over the Permian rocks of the Siliciclastic overlap assemblage, which also suggests that it was originally deposited directly on those rocks. The presence of volcaniclastic rocks in the upper part of the section is an important tectonostratigraphic link to the rocks of adjacent terranes. Rocks near Quinn River, Nevada that are almost as old and contain volcaniclastic rocks in the upper part of the section, belong to the Black Rock-Jackson terrane (Blome and Reed, 1995; Jones, 1990). Triassic rocks of similar age exposed south of Jarbidge in northeastern Elko County are juxtaposed with Permian rocks of the Siliciclastic overlap assemblage and may correlate with the Candelaria, but the base of the section is unknown and no volcanic facies are reported from those rocks, so they are currently included with the Cratonal sequence, TRmt.
State Nevada
Name Siliciclastic Overlap Assemblage - Shale, sandstone, and limestone
Geologic age Lower Triassic
Lithologic constituents
Major
Sedimentary > Clastic > Sandstone (Bed)
Sedimentary > Clastic > Mudstone > Shale (Bed)
Minor
Sedimentary > Carbonate > Limestone (Bed)
References
NGMDB product
Counties Esmeralda - Mineral - Nye