AN INTEGRATED PETROLEUM EVALUATION OF NORTHEASTERN NEVADA
Type Section Information
The Strathearn Formation is named for exposures at the ranch headquarters for the former Strathearn Cattle Co. and has a type section in Secs. 19 and 24, T. 33 N., R. 55 E. (Dott, 1955).
The Strathearn is considered Upper Pennsylvanian and Lower Permian (Missourian to Wolfcampian) in age by Dott (1955). It has a gradational upper contact with Permian strata such as the Carlin Sequence which is locally included with it, and overlies the Diamond Peak Formation along an angular unconformity. The Strathearn is correlated with the Sunflower Formation of Coash (1967) which has locally been included with the Strathearn in mapping within Elko County. The Sunflower Formation is described separately.
In the Carlin-Pinon Range area, the Strathearn Formation is composed of clastic calcareous lithologies which are dominated by gray, thin to thick-bedded limestone in beds 1 to 4 feet in thickness (Smith and Ketner, 1975). Quartz sand, silt, and chert pebbles are common in the limestones which are locally crossbedded on a small scale. Lenses of conglomerate are composed of well-rounded green, red, black and gray chert, and gray quartzite clasts which are commonly 0.25 to 2 inches in diameter and up to 8 inches across (Smith and Ketner, 1975). The conglomerates grade laterally as well as vertically into the limestones. Yellow weathering platy siltstone and brown fine-grained sandstone are also abundant in the Strathearn.
In the Windermere Hills and southern HD Range, the Strathearn Formation was lumped with the Permian Carlin Sequence in mapping (Oversby, 1969; Coats, 1985). The Strathearn unconformably overlie both Chainman and Diamond Peak Formations and is overlain by the Buckskin Mountain Formation of the Carlin Sequence (Oversby, 1969). The Strathearn consists of alternating 1 to 10 foot thick beds of quartz siltite and medium-bedded bioclastic dark gray, light gray weathering limestone (calcarenite and calcirudite). Graded beds of brownish-gray quartz-chert arenite and rudite and quartz siltite are present as lenses up to 80 feet thick (Oversby, 1969).
In the northern Snake Mountains, the Strathearn consists of about 100 feet of sandy fossiliferous limestone and chert pebble conglomerate, overlain by siltstones and limestones of the Ferguson Mountain Formation (Gardner, 1968). Bezzerides (1967) assigned this sequence to the Oquirrh Formation in the O'Neill Pass area of the Snake Mountains. Where mapped by Gardner (1968), the Strathearn unconformably overlies the Valmy Formation and consists of bioclastic limestone with bryozoans and fusulinids, and shell fragments forming as much as 20 percent of the limestone. Brachiopods, gastropods, pelmatazoans and echinoderm spines have also been collected from the limestone which is massive to well-bedded, fine-grained and dark gray, and contains detrital quartz and chert grains. The basal conglomerate is about 50 feet thick and is composed of poorly sorted, angular to subrounded chert and quartzite pebbles and boulders.
The Strathearn is also present in a poorly exposed and undivided Pennsylvanian and Permian section in the Pilot and Southern Silver Island and Leppy Ranges (Schaeffer and Anderson, 1960; Blue, 1960; O'Neill, 1968). In the Leppy Range, it is composed of nearly 300 feet of brown-gray chert pebble-conglomerate with a pink silty limestone matrix, which is interbedded with gray thin-bedded limestone, yellow-brown siltstone, and minor dolomite (Schaeffer and Anderson, 1960). The Strathearn rests with angular unconformity on the Ely Limestone in the Southern Silver Island and Pilot Ranges (O'Neill, 1968; Blue, 1960).
The Strathearn is about 1,200 to 1,500 feet thick at its type section in the northern Carlin-Pinon Range, more than 2,000 feet thick just north of Carlin Canyon, and is only 300 feet thick at Buckskin Mountain (Smith and Ketner, 1975). These variations are primarily the result of the upper gradational boundary of the Strathearn with overlying Permian, which has been mapped differently by different workers. The Strathearn is 80 to 200 feet thick in the Windermere Hills and southern HD Range (Oversby, 1969, 1972), 110 feet in the northern Snake Mountains (Gardner, 1968), about 287 feet thick in the Leppy Range (Schaefer and Anderson, 1960) and is several hundred feet in the Silver Island and Pilot Ranges (Blue, 1960; O'Neill, 1968).
The Strathearn has been mapped in the Carlin-Pinon Range area, the Windermere Hills, Snake Mountains, southern HD, Pilot, southern Silver Island and Leppy Ranges. Several unnamed or undivided sections in other ranges have been correlated with the Strathearn or the Sunflower Formations.
The Strathearn Formation was deposited as a shallow marine shelf sequence which received periodic influx of coarse detrital sediment from stream and delta systems draining the eroding Roberts Mountains allochthon to the west. These clastics were probably deposited on broad submarine flats and banks (Smith and Ketner, 1975).