Mesozoic

AN INTEGRATED PETROLEUM EVALUATION OF NORTHEASTERN NEVADA

REGIONAL PALEOGEOGRAPHY

MESOZOIC

Although Triassic and Jurassic rocks are now only exposed in very local and isolated patches, it has been suggested that they may represent part of a once continuous sheet in Elko and White Pine Counties which was eroded in the Cretaceous or Tertiary (Hose and Blake, 1976). The lower Triassic sediments represent the final marine transgression in northeastern Nevada. This transgressing sea encountered considerable relief and deposited coarse conglomerates on the upper surface of underlying Permian units (Collinson and Hasenmueller, 1977).

The Triassic rocks are commonly bounded by an unconformity at the top so that the maximum thickness exposed is often about 150 feet, with as much as 500 feet in the Medicine Range (Collinson, 1968). Wheeler and others (1949) mapped 3,100 feet of the Triassic Dinwoody and Thaynes Formations conformably overlian by Early Jurassic sediments near the townsite of Currie. This suggests a considerable thickness of Mesozoic sediments may have been locally deposited in eastern White Pine and Elko Counties.

The disconformity between the Upper Permian and Lower Triassic rocks in northeastern Nevada is marked by conglomeratic channels at the base of the Thaynes Formation and represents a significant temporal break along which uppermost Permian and lower Triassic sediments have been locally removed. This erosional break coincides with the initiation of the Sonoma Orogeny to the west (Collinson and others, 1976).

Triassic deposition appears to have been somewhat restricted and extensively modified by later erosion. The Lower Triassic Dinwoody and overlying Lower Triassic Thaynes Formations, as well as the unnamed Lower Triassic greenstone-bearing sedimentary sequence to the west in the Adobe Range, represent regressive shallow-marine sequences. The limestones and mudstones of the Thaynes Formation represent open marine, shallow shelf and shoal sediments with thin strandline-channel sands and conglomerates (Collinson and Hasenmueller, 1977). Thin water-lain tuffs are also intercalated in the Dinwoody (Coats, 1985) suggesting local Triassic volcanism. The unnamed Triassic rocks in the Adobe Range also contain greenstone flows up to 20 feet thick which are intercalated with graded, and sole marked limestones and shales. Ketner and Ross (1983) suggested these rocks represent olistostromes, in large part derived from the Permian Gerster Formation, and deposited in a deeper Triassic slope to basin setting to the west of the Dinwoody and Thaynes.

The Middle Triassic Panther Canyon and Augusta Mountain Formations were deposited along the western margin of the area in the Shoshone and Fish Creek Ranges. These bioclastic, sandy and pebbly limestones, chert pebble conglomerate, siliceous sandstones and shale represent a coarsening and shallowing upward deltaic sequence deposited in small shallow basins overlapping rocks within the Golconda allochthon.

Above the Lower Triassic Thaynes Formation in northeastern Elko County, channel conglomerates of the Shinarump Conglomerate and the overlying red-bed sandstones of the Upper Triassic Chinle Formation were locally deposited. Although widespread to the east in Utah and Arizona, and to the south in southernmost Lincoln County, the only Chinle Formation in the evaluation area is exposed in the Currie Hills and Dolly Varden Mountains in central Elko County. The Jurassic cross-bedded, rippled, and raindrop imprinted clastics of the Nugget Sandstone unconformably overlie the Chinle Formation. The fluvial and eolian dune sandstones of the Chinle and Nugget signaled the end of marine deposition in northeastern Nevada.

Smith and Ketner (1976) suggest that regional uplift during the Triassic brought marine deposition to a close farther to the west in the Carlin-Pinon area. Following Triassic uplift, a thick sequence of the Jurassic Pony Trail Group volcanics and volcaniclastic sediments were deposited in a basin centered in what are now the Cortez Mountains. This thick basinal sequence was apparently not subjected to extensive erosion. Although the Pony Trail Group represents the only Mesozoic volcanic sequence in the evaluation area, silicic and minor basic plutons were also emplaced throughout the area during the Jurassic and Cretaceous.

Begining in the early Cretaceous and continuing into the Early Tertiary, basins began forming on an erosional surface which locally had considerable relief. During the Lower Cretaceous, lacustrine and fluviatile sediments such as the interfingering siltstones, sandstones, limestones, and conglomerates of the Lower Cretaceous Newark Canyon Formation were deposited in local basins in the Eureka area. In the Diamond Range, a major post-Permian and Pre-Early Cretaceous unconformity is present at the base of the Cretaceous Newark Canyon Formation which rests on Permian sediments and locally on rocks as old as Ordovician in age (Nolan and others, 1956).