AN INTEGRATED PETROLEUM EVALUATION OF NORTHEASTERN NEVADA
Type Section Information
The Pioche was recognized by Hague (1883) as a calcareous shale between the Prospect Mountain Quartzite and Eldorado Dolomite along the east side of Prospect Peak near Eureka, Nevada. The Pioche here forms a discontinuous band from Prospect Mountain Tunnel near the base of Prospect Ridge southward to the base of Prospect Mountain. The formation was redefined by Westgate and Knopf (1932).
The Pioche may represent either or both Lower and Middle Cambrian (upper Waucoban and lower Albertan) ages. The Pioche correlates with part of the Gold Hill Formation in the Toquima and Toiyabe Ranges (Kleinhampl and Ziony, 1985).
In the Eureka area, the Pioche has a variable lithologic character which is most commonly a greenish or grey, calcareous or micaceous shale which may locally be reddish or brown in color. Micaceous units often show foliation near thrust faults. Thin beds of reddish-brown micaceous sandstone and quartzite, and thin to thick-bedded dark blue mottled limestones with abundant trilobite hash are common throughout the formation (Nolan and others, 1956; Roberts and others, 1967).
In the Ruby Mountains, the Pioche is a brown hornfels with interbedded and minor quartzite beds and dark to light-gray coarsely crystallized carbonaceous limestone and dark phyllite near the base of the formation (Willden and Kistler, 1979). In White Pine County, exposures in various ranges are commonly dark greenish-gray to olive silty shale and clay shale with interbedded siltstone and minor beds of cross-bedded sandstone and thin limestones.
In the central Grant Range, Hyde and Huttrer (1970) found the Pioche to contain about 75 percent quartzite and shale with about 25 percent limestone. It weathers to form a greenish or reddish soil horizon. In the Grant Range, the Pioche lies between the Geddes Limestone and the underlying Prospect Mountain Quartzite. The base of the Pioche is often a fault, but where undeformed in the Grant Range, it is a gradational contact with the underlying Prospect Mountain Quartzite. In the southern Grant Range near Troy Canyon, the Pioche is composed of dark gray to brown or red and green metasiltstones, green and reddish-green argillites with locally strongly developed cleavage, and thin-bedded, medium gray to blue limestones or marbles (Cebull, 1967). The Pioche is often tectonically eliminated along low-angle faults.
In the White Pine Mountains the Pioche is about 65 percent green, brown and red argillite, 20 percent brown thin-bedded micaceous and quartzose sandstone, and 15 percent sooty black limestone which are concentrated near the top of the formation (Lumsden, 1964; Kellogg, 1963).
In the northern Egan-southern Cherry Creek Range area, Fritz (1968) described an anomalously thick section of about 1,280 feet of Pioche Shale, which almost certainly contains part of the overlying Eldorado Formation or Pole Canyon Limestone. The Pioche is composed of a lower brown micaceous siltstone with interbedded thin quartzite and thick bedded limestone near the base, and an upper member of siltstone, and medium to dark gray, fine-grained limestone containing Girvanella. The lower 420 feet are probably Pioche, with most or all of the overlying 860 feet representing the Pole Canyon Limestone or Eldorado Dolomite which are characterized by abundant Girvanella algae. Kellogg (1960) reported a lithologically similar undifferentiated sequence which includes the the Pioche and Eldorado Dolomite in the southern Egan Range.
In the Snake Range, the Pioche conformably and gradationally overlies the Prospect Mountain Quartzite. It is composed of greenish to gray, red-brown weathering micaceous siltstone to shale with interbedded fine-grained sandstone and quartzite. Near the base, the Pioche contains intercalated thin limestone beds and lenses, one of which is about 20 feet in thickness and has been correlated with the Combined Metals Member of the Pioche in Lincoln County (Drewes and Palmer, 1957; Misch and Hazzard, 1962). Organic markings, most probably worm trails, have been described on the bedding surfaces.
In the northern Schell Creek Range, the Pioche is composed of thin beds of poorly exposed, brown-gray shale and siltstone, limestone and quartzite. The limestones and shale are both laminated and micaceous and are locally interbedded with light greenish-gray, fine-grained, calcareous sandstone upsection (Dechert, 1967). In the northern Toana Range, the Pioche is a brownish-gray, non-fossiliferous and fissile, sericitic argillite to meta-siltstone (Pilger, 1972). In the Pilot Range, the Pioche is a faulted brown-gray fissile and argillitic shale (O'Neill, 1968). Above the Pioche, are 2,775 to 3,000 feet of unnamed limestone and marble in the Toana Range, and limestone, silty and shaley limestone and dolomite in the Pilot Range (Pilger, 1972; O'Neill, 1967; Coats, 1985).
In Lincoln County, the Pioche Shale is exposed in the southern Schell Creek and Bristol Ranges, and Pioche Hills, and has been subdivided into seven informal members in many of the mining districts of the area where it is a main ore horizon. The Pioche is a sequence of interbedded fossiliferous and micaceous sandy or argillaceous, olive-green to brown shales with a few thin limestone and sandstone interbeds (Tschanz and Pampeyan, 1970). The limestones are as much as 50 feet thick and often contain "Girvanella" and trilobite fragments. A white massive quartzite about 90 feet thick occurs about 375 feet from the base of the formation in the southern Schell Creek Range (Tschanz and Pampeyan, 1970).
In thin section the Pioche often shows the effects of metamorphism, particularly in the Snake and Schell Creek Ranges where it has been recrystallized to chlorite and sericite bearing phyllites or garnet-staurolite schists (Misch and Hazzard, 1962).
The thickness of the Pioche Shale is primarily controlled by shearing. The unit is often thinned to a few feet or entirely missing as a result of thrusting which is localized within the unit near Eureka (Nolan and others, 1956). Shaley beds are often crumbled and massive limestones are often lensed indicating any measured thickness is structural rather than truly stratigraphic. Estimates in the Eureka area vary from 60 to 500 feet and from 150 to 600 feet in White Pine County (Nolan and others, 1956; Hose and Blake, 1976).
The following thicknesses have been estimated in other areas: 525 feet in the Ruby Range (Willden and Kistler, 1969); 211 to 642 feet in the Egan-Schell Creek-Cherry Creek Range area (Young, 1960; Drewes, 1967; Kellogg, 1963; Hose and Blake, 1976), 400 feet in the Grant Canyon area of the central Grant Range (Hyde and Huttrer, 1970), 300 feet in the White Pine Mountains (Kleinhampl and Ziony, 1985), 325 to 450 feet in the Snake Range (Whitebread, 1969), 683 feet in the northern Toana Range (Pilger, 1972), 150 feet thick in a faulted section in the Pilot Range (O'Neill, 1968), 750 to 1,000 feet in the southern Schell Creek Range (Tschanz and Pampeyan, 1970), 590 feet in the northern Schell Creek Range (Dechert, 1967), 500 feet in the southern Deep Creek Range (Nelson, 1959), and about 900 feet in the Bristol Range (Tschanz and Pampeyan, 1970).
The Pioche Shale occurs throughout eastern and southeastern Nevada, northwestern Arizona, and western Utah (Tschanz and Pampeyan, 1970). In the evaluation area, it is exposed in the Eureka area, Ruby, Egan, Schell Creek, Cherry Creek, Grant-White Pine, Quinn Canyon, Snake, southern Deep Creek, northern Toana, Pilot, and Bristol Ranges, and just to the south in the Pioche Hills.
The Pioche Shale contains trilobite and Girvanella algae suggesting deposition in a relatively shallow and quiet marine shelf setting. The details of sedimentation, particularly for interbedded clastics within the shale are poorly understood.