AN INTEGRATED PETROLEUM EVALUATION OF NORTHEASTERN NEVADA
RIEPE SPRING LIMESTONE
Type Section Information
The Riepe Spring Limestone was given to massive coralline and fusulinid bearing limestones originally included in the Ely Limestone of Spencer (1917). The type section is at the north end of Ward Mountain in Sec. 7, T. 15 N., R. 63 E., in the central Egan Range (Steele, 1960). The type section assigned by Steele (1960) is faulted and considerably repeated structurally (Barosh, 1964).
The Riepe Spring Limestone is Permian (Middle Wolfcampian) in age based upon abundant colonial coral and fusulinid faunas. This age assignment allowed the Riepe Spring to be broken out from the disconformably underlying and lithologically identical Ely Limestone of Pennsylvanian age.
In general, the Riepe Spring Limestone is a medium to dark-gray, finely to moderately crystalline, thin-bedded to massive bioclastic limestone. Prominent ledge-forming limestones are composed of colonial coral biostromes at the top of the formation, which makes up more than 10 percent by volume of many sections (Steele, 1960; Brokaw and Heidrick, 1966). A fusulinid coquina underlain by thin chert-pebble conglomerate is commonly present at the base of most sections (Brokaw and Heidrick, 1966).
The Riepe Spring Limestone is lithologically impossible to separate from the Ely Limestone in non-fossiliferous sections. Temporally, the two are separated by a time interval represented by upper Des Moines to Virgil time. The Riepe Spring Limestone differs from the Ely by being more massive, slightly more dolomitized, containing less slope forming siltier limestones, and where fossiliferous, more abundant fusulinids and light gray lens shaped bodies composed of colonial corals with hexagonal outlines than the underlying Ely Limestone (Hose and Blake, 1976).
In the Carbon Ridge area, south of Eureka in the Fish Creek Range, Steele (1960) described the Riepe Spring as a medium gray, fine to medium-grained, medium to massive-bedded, foraminiferal limestone. It rests with angular discordance on calcareous siltstones and mudstones and is overlain by the Riepetown Formation. The entire interval described was named the Carbon Ridge Formation by Nolan and others (1956) but Steele (1960) suggests recognizable units of formational rank are present in the Carbon Ridge and that it should therefore be raised to group rather than formational status.
In the Pancake Range east of Duckwater, several hundred feet of light-gray sublithographic limestone, which locally contains lenses and nodules of brown chert, disconformably overlie the Ely Formation, and have been assigned to the Riepe Spring Limestone by Kleinhampl and Ziony (1985). Bissell (1964) described the section in the Pancake Range as a basal 85 feet of thick-bedded to massive, micritic and skeletal, gray limestone overlain by 158 feet of thin to medium-bedded, dolomitic and sandy, micritic limestones, thin-bedded argillaceous limestones, and 70 feet of medium to thick-bedded encrinal limestone interbedded with thin-bedded argillaceous limestone.
Bissell (1964) has described various sections of the Riepe Spring within the Egan Range. These sections are composed of various thicknesses of massive, gray, micritic and skeletal limestones, and pink-gray silty and sandy, and argillaceous limestone which is locally dolomitic and contains thin chert lenses. Algae, corals, brachiopods, and fusulinids are abundant in most sections (Bissell, 1964). Barosh (1964) divided the Riepe Spring into three mappable members. The lower member is composed of a basal olive-gray chert breccia with a light gray limestone matrix, overlain by gray-brown fusulinid coquinas and interbedded light gray-brown, medium-bedded limestone and minor interbedded fine to medium-grained, yellowish-gray dolomitic limestone. The overlying coralline member is brown-gray, yellowish weathering, fine to medium-grained, thick-bedded limestone with as much as 25 percent coralline remains (Barosh, 1964). The upper member is thin-bedded cherty limestone, silty limestone and calcareous siltstone which weather yellow-gray and commonly contain abundant fusulinids and brachiopods as well as corals. Barosh (1964) recognized these members within the Butte Mountains as well.
On the east side of the Grant Range, a structurally complicated section of yellow shale, bluish-black limestone and yellow sandstone has been tentatively assigned to the Riepe Spring Limestone by Moores and others (1968). This section probably includes both the Riepe Spring Limestone and overlying Riepetown Formation.
In the Cherry Creek Range, rocks tentatively assigned to the Riepe Spring Limestone are composed of thin to thick-bedded, light gray weathering, fine-grained limestone. The limestone locally contains light-brown nodular chert and elongate algal structures (Fritz, 1968). Minor light brown weathering siltstone and shale are present in the upper portion of the formation.
In the central and northern Butte Mountains the Riepe Spring is composed of basal gray to brown, medium to coarse grained, skeletal-detrital dolomitic limestone and silty-sandy dolomitic limestone, and micritic, encrinal, silty and sandy limestone and dolomitic limestone with local thin interbedded chert-pebble conglomerates (Bissell, 1964). In the southern and central Butte Mountains Barosh (1964) recognized a local lower member of medium gray cherty limestone in 4 inch to 1 foot thick beds with brown and gray chert lenses and nodules; a coralline member with silty and locally cherty coralline limestone; and an upper member of crinoidal and silty limestone.
In the southern Schell Creek Range the Riepe Spring can be divided into four members (Bissell, 1964). The basal 87 feet is thick to massive, silty and slightly cherty, micritic and skeletal, brown-gray, orange weathering limestone. This is overlain by 133 feet of slightly cherty, massive, gray skeletal limestone with a few coral biostromes. The overlying 114 feet is thin to thick-bedded silty, sandy and cherty, gray micritic limestone. The uppermost 66 feet is olive-gray medium to thick-bedded encrinal limestone which weathers to an orange-tan color (Bissell, 1964).
In northern Lincoln County rocks assigned the designation Pennsylvanian Limestone include the Riepe Spring Limestone in the northern Pahroc and Golden Gate Ranges and on Grassy Mountain where it is composed of massive to thin-bedded gray silty limestone with abundant brown and pink or gray chert nodules and beds.
In the Spruce Mountain Quadrangle, the Riepe Spring Limestone is composed of about 1,400 feet of massive to thick-bedded, medium-gray, fine-grained limestone with abundant nodular chert in the lower portion (Hope, 1972). Lenticular beds of chert-pebble conglomerate are also locally present in the northern portion of the quadrangle.
The Riepe Spring Limestone is 410 feet thick at the type locality in the central Egan Range (Steele, 1960) and 336 feet on Radar Ridge (Bissell, 1964), 220 feet just east of Lund (Bissell, 1964), 230 feet in the southern Egan Range (Bissell, 1964), 618 feet in the Carbon Ridge area, south of Eureka (Steele, 1960), 313 feet in the Pancake Range west of Duckwater (Bissell, 1964). It is about 1,000 feet thick in the Grant Range where the Riepetown Formation is probably included (Moores and others, 1968), 156 feet at Limestone Peak on the eastern flank of the White Pine Range (Bissell, 1964), and 810 feet in the southern Cherry Creek-northern Egan Range (Fritz, 1968). Sections of 184, 343 and 402 feet have been measured in the central and northern Butte Mountains (Bissell, 1964). It is 390 feet in the central Schell Creek Range (Bissell, 1964), and 1,400 feet in the Spruce Mountain area (Hope, 1972).
The Riepe Spring Limestone has been reported in the Fish Creek, Pancake, Egan, Grant, Schell Creek, north Pahroc, and Golden Gate Ranges, Buck Mountain, Butte Mountains, Grassy Mountain, and Spruce Mountain Quadrangle.
The massive coralline, algal, and fusulinid-bearing limestones of the Riepe Spring Formation represent shallow open marine outer shelf, carbonate-bank or patch reef type deposition. The coralline member was probably slightly shallower than the lower portion, representing biohermal and perhaps reefal environments. Barosh (1964) has noted the similarity of the Riepe Spring to the Capitan Reef Complex in the Guadalupe Mountains of west Texas.