AN INTEGRATED PETROLEUM EVALUATION OF NORTHEASTERN NEVADA

ARCTURUS FORMATION

Type Section Information

The Arcturus Formation was defined by Lawson (1906) as a shaley limestone overlying the Ely Limestone in the Ruth Mining District near Ely. Spencer (1917) specified a type locality for the formation at the Arcturus mining claim in Sec. 15, T. 16 N., R. 62 E. Wilson and Langenheim (1962) redefined the Arcturus as a unit above the Riepetown and below the Kaibab Formations.

Geologic Age

As currently designated, the Arcturus Formation lies above the Riepetown Sandstone and below the Kaibab or Loray Formations. It is Early Permian (Wolfcampian to Leonardian) in age based upon fusulinid, mollusk, and colonial coral determinations (Fritz, 1960; Steele, 1960). In the Egan Range, the Arcturus Formation was divided into two unnamed members, which may be equivalent to the Pequop and Loray formations of Steele (1960). Barosh (1964) has suggested that the utility of the Pequop is severely limited because of the poor definition of the unit and highly faulted type section designated by Steele (1960). As a result, several sections called Arcturus by Barosh (1964) would at least in part represent part of the poorly defined Pequop Formation.

General Lithology

Near its type locality, the Arcturus Formation is composed of light pinkish‑gray, platy, dense limestones with thin lenticular interbeds of siltstone to fine‑grained sandstone. It contains fusulinids and colonial corals particularly in the upper portion of the formation (Steele, 1960; Hose and Blake, 1976). The Arcturus is fairly consistent regionally with the major lithologic change being the proportion of fine‑grained sandstone in differing sections (Hose and Blake, 1976). Barosh (1964) divided the unit into a lower member of alternating medium‑bedded limestone and pale brown siltstone, and an upper member of alternating thin‑bedded molluscan‑bearing limestone and silty limestone, brown to red siltstone, and gypsum. The contact between the Riepetown Sandstone and Arcturus is gradational and is placed at the lowest thick bed of massive limestone in the Arcturus, or where limestone becomes dominant over sandstone and siltstone below (Hose and Blake, 1976).

In the Buck Mountain area south of the Ruby Range, the Arcturus is composed of alternating crinoidal limestone which contains scattered fragments of green, gray, brown and black angular to subangular chert grains, and varicolored calcareous siltstone (Barosh, 1964).

In the Egan Range near Giroux Wash, Brokaw and Heidrick (1966) divided the Arcturus into two parts. The lower portion is exposed near Moorman Ranch and is composed of massive ledge‑forming, olive‑gray to cream, sandy and silty limestone, much of which is coarse‑grained and organic‑detrital with abundant fusulinids, and interbedded platy calcareous siltstone and very fine‑grained, yellowish, calcareous sandstone (Brokaw and Heidrick, 1966; Hose and Blake, 1976). The upper part of the Arcturus is mainly very‑fine grained, yellowish‑gray, calcareous siltstone and sandstone which is red and interbedded with silty to sandy limestone and dolomitic limestone in the upper few hundred feet. The base of the upper portion is a thin evaporite sequence of gypsum and anhydrite and impure gray limestone (Brokaw and Heidrick, 1966; Hose and Blake, 1976). As mentioned above, Barosh (1964) and Hose and Blake (1976) suggest that these units may correlate with the Pequop and Loray Formations respectively which are also evaporite bearing. The evaporite bearing beds from 1,380 to 5,750 feet below the surface in the Summit Springs Unit No. 1 well, described by McJannett and Clark (1960), are probably part of the Arcturus interval (Hose and Blake, 1976).

In the southern Cherry Creek‑northern Egan Range, Fritz (1968) described nearly 2,000 feet of Arcturus Formation unconformably overlying the Ely Formation. The Arcturus is predominantly cream to buff siltstone which locally contains pebbles and flattened cobbles of reddish weathering siltstone and sandstone. Thin interbeds of light gray, very fine‑grained, thin to thickly bedded limestone locally containing orangish‑brown chert are localized in the lower potion of the unit.

In the central‑northern Egan Range, Woodward (1962) measured a somewhat thinner section of the Arcturus overlying the Ely Limestone. The basal 150 feet are red, yellow, and light brown, fine to medium‑grained, calcareous sandstone, overlain by about 25 feet of fine‑grained, light gray limestone with purple chert nodules 1 to 2 inches thick. Above the limestone is 500 feet of yellow to brown, fine to medium‑grained, calcareous sandstone overlain by about 170 feet of fine to medium‑grained light‑gray weathering limestone with minor interbedded shaly calcareous sandstone. The upper 850 feet are about equal proportions of shaly calcareous sandstone, and medium‑grained, gray, chert‑bearing limestone.

In the central and southern Butte Mountains, Barosh (1964) described the Arcturus Formation. The lower member is light brown‑gray, bryozoan‑rich, silty limestone and dolomite, and calcareous, gray to orange, siltstone and brownish crossbedded sandstone which is about 1,775 feet thick on the east flank of the range. The limestones are all bioclastic, fine to medium‑grained and in beds 1 to 3 feet thick. The upper member is gray and olive green limestone and red‑weathering siltstone about 1,800 feet thick along the eastern flank of the Butte Mountains (Barosh, 1964).

In the southern Snake Range a small exposure of the Arcturus Formation is described as light brownish‑gray to yellowish‑gray, calcareous sandstone and sandy limestone in beds about 1 to 3 feet in thickness (Whitebread, 1969). The Arcturus overlies the Ely Limestone and may include the Riepe Spring Limestone or Riepetown Sandstone. No fossils or thicknesses are described for the unit (Whitebread, 1969).

Average Thickness

The Arcturus Formation is 2,550 feet thick in the Buck Mountain area (Barosh, 1964), about 2,900 to 3,300 feet thick in the Egan Range (Brokaw and Heidrick, 1966) and 1,700 feet in the northern Egan Range (Woodward, 1962), 3,400 feet in the Maverick Springs Range (Hose and Blake, 1976), 1,975 feet in the southern Cherry Creek Range (Fritz, 1968), about 3,580 feet in the central Butte Mountains (Barosh, 1964) or 2,989 feet according to Sides (1966), 3,290 feet in the Schell Creek Range (Dechert, 1967), and is about 2,700 feet in the Confusion Range of Utah (Hose and Repenning, 1959).

Areal Distribution

In the evaluation area, the Arcturus Formation is exposed in the Buck Mountain area, Butte Mountains, Maverick Springs, Egan, Cherry Creek, Schell Creek, Snake, and Deep Creek Ranges.

Depositional Setting

The Arcturus Formation was deposited under very shallow marine outer shelf conditions in water depths which were probably less than 100 meters. Barosh (1964) suggests that the unit represents skeletal banks and high‑energy oolite shoals which were reworked by moderate currents and shallows upward to tidal or mud flat environments.