The Vinini Formation was named for exposures in a major thrust sheet exposed along Vinini Creek on the eastern side of the Roberts Mountains, 25 miles northwest of Eureka (Merriam and Anderson, 1942).

The Vinini is Middle and Upper Ordovician in age and is a time equivalent of the Ninemile Formation and Antelope Limestone, and a time and facies equivalent of the Valmy Formation.

The stratigraphic sequence of western assemblage allochthonous strata in the Vinini Formation is complicated by intricate faulting, folding, and poor exposure within eroded thrust slivers. Detailed faunal work on continuous exposures indicates both repetition and deletion of section (Churkin, 1963). Exposures of the Vinini are commonly isolated patches of isoclinally to openly folded and shattered beds which are cut by both small and large-scale thrust faults. The structural base of the Vinini Formation lies along the Roberts Mountains thrust.

In general, the Vinini is characterized by bedded chert and interbedded quartzite and shale, alternating carbonaceous shale and quartz siltstone, and irregularly interbedded shale, siltstone, sandstone, and limestone, and tholeiitic volcanic rocks (Stanley and others, 1977). In general, the lower, Early Ordovician portion of the formation is quartzite, siltstone, limestone and calcareous sandstone with minor amounts of andesitic flows and tuffs. The upper, Middle Ordovician portion is dominantly bedded chert, quartzite and siliceous siltstone and mudstone, and shale which is locally rich in organic matter.

Much of the Vinini is represented by black, gray, tan, and green, bedded cherts commonly in beds 2 to 6 inches thick with 1 millimeter thick laminations, and gray, black or varicolored siltstones and siliceous shales or argillites. In most outcrops the shales or mudstones are siliceous or argillaceous and are massive or packed with burrows, are occasionally well laminated, and weather to a grey, brown or white color. Greenish-brown calcareous siltstones interstratified with the shales are well-sorted and sandy, commonly in layers less than 5 centimeters thick, and locally contain sponge spicules (Stanley and others, 1977; Coats, 1985).

The siltstones contain well sorted, rounded to angular quartz grains, and both silica and carbonate cement. A variety of sedimentary structures are present in the siltstones which have sharp bottom and top surfaces. The upper surface is often burrowed and scoured while bottom surfaces have load casts, burrows and various sole markings. Internally, horizontal laminations are overlain by climbing-ripple stratification which is truncated by or abruptly grades into shale (Stanley and others, 1977).

Vinini cherts are pale green to black in layers from 1 inch to several feet in thickness. They are well bedded and contain graptolites, algal material and radiolarians. Cherts compose most of the section in many Vinini exposures.

Dark gray limestones are micritic, massive or horizontally laminated with varying amounts of quartz silt and pellets. Sedimentary structures in these limestones include both horizontal and climbing-ripple cross stratification, as well as various types of slump structures. Some limestones are starkly graded turbidites with basal bioclastic calcarenite and intraformational conglomerates. The larger exposures of limestone are present in and near the northern portion of the Adobe Range such as Swales Mountain and in the Coal Mine Basin Quadrangle, and near Lone Mountain in the southern Independence Mountains (Ketner, 1973a; Coats, 1985). Most limestones in the Vinini, however, occur as 2 to 5 inch thick lenses in the cherts. The larger masses of limestone have given some Late Devonian conodont ages while the typical thinner interbedded lenses appear to be Ordovician in age.

Well bedded, grey to brown, quartz sandstones are locally present in the lower portion of the Vinini. Both horizontal and cross stratification are present in these sands, and often the bottom of sand layers have burrow or flute casts. The tops of the sands are often burrowed as well. Nereites type trace fossils are abundant in the Vinini, particularly burrows in the upper portions of limestone and sandstone layers.

In the Roberts Mountains, the Vinini Formation is divided into two members. The lower portion is characterized by dark-grey brownish weathering quartz sandstone (quartzite), gray sandy limestone, and brownish-gray to green graptolitic and cherty shales and siltstones, with andesitic flows and breccias near the top of the member (Merriam and Anderson, 1942, Winterer, 1968). The upper member is interlayered brown to cream weathering petroliferous shales which are easily ignited, and subordinate black, gray and green bedded cherts (Roberts and others, 1967).

In the Mahogany Hills, at Devils Gate, and on the eastern and southern slopes of Lone Mountain, black shale and minor dark grey to black chert and limestone make up most of the Vinini (Merriam, 1963). At Yahoo Canyon in the northern Mahogany Hills, dark-grey to black, bluish-grey to white weathering graptolitic shales are interbedded with brown weathering, poorly sorted sandstones composed of angular grey chert fragments, well-rounded quartz grains, and poorly preserved algal shreds. Bedded dark-grey to green resistant cherts containing radiolarians, graptolites, and algal debris are also abundant (Merriam, 1963).

In the Monitor Range, bedded light-bluish weathering cherts, light grey vitreous quartzites, and dark-gray graptolitic shales are common in the Vinini (Merriam, 1963). Cherts and quartzites dominate the Vinini and much of the shale is strongly silicified. In the Copenhagen Canyon area of the Monitor Range, the Vinini is up to 500 feet of green and black bedded vitreous chert, light-gray dense argillite, yellowish-gray to brown calcareous mudstone and shale and platy argillaceous limestone and thick bedded dark-gray limestone (Bortz, 1959). Near Dobbin Summit, the formation is about 130 feet of black and light gray, well-bedded chert and varicolored shale and thin sandstone (Wise, 1977).

In the Simpson Park Mountains and Toquima Range, the Vinini consists of green and black chert beds and nodules, massive brown weathering, gray quartzite and sandstone, yellow, pink, red, and black siltstones, small amounts of dark thin-bedded limestone, dark brown gray silty shale which is largely argillite, and mafic flows up to 20 feet in thickness (Roberts and others, 1967; McKee and Ross, 1969). In the Simpson Park Mountains, the Vinini is overlain by conglomerates of the Pennsylvanian-Permian Antler Sequence. In the Toquima Range, a relatively undeformed section can be determined by a series of relatively undeformed graptolite zones which reveal a Late Ordovician (Arenigian to Caradocian) age (McKee, 1976). In the Toquima Range, Kay and Crawford (1964) used the designation of Clipper Canyon Group, and divided what is now the Vinini into four formations with a total thickness of 3,300 feet. These are the Charcoal Canyon, Petes Summit, Sams Spring, and Joes Canyon Formations in ascending order.

In the Cortez Mountains, Gilluly and Masursky (1965) suggested that more than half of the Vinini is composed of carbonaceous, light-gray, sandy dolomitic and siliceous siltstone, or poorly fissile shale. Most of the siltstone shows well developed current cross-bedding. Interbedded with the siltstone are sandstone, chert, quartzite and conglomerate commonly on the order of 10 to 20 feet in thickness. The sandstones are light gray to black and are composed of well rounded quartz and subangular chert grains, with minor amounts of plagioclase and potassium feldspar. The cherts are laminated, organic-rich, and dark blue-grey to black or turquoise in color, in beds up to 5 inches in thickness. Quartzites are dark-gray to brown in beds rarely more than 20 feet in thickness with the quartzites near the top of the formation often brecciated with zones of reddish jasperoid (Gilluly and Masursky, 1965; Roberts and others, 1967). Thin conglomerate interbeds contain poorly rounded to angular chert fragments up to 2 cm across, in a matrix of finer grained chert and quartz. Also reported in the Cortez Mountains Vinini section are thin massive blue to black limestone, and a few thin greenstone layers (Roberts and others, 1967).

In the Sulphur Spring Range, the Vinini is poorly exposed and is often mapped based on float covered hills. Our observations suggest it is primarily siliceous black, brown and green siltstones which commonly weather to a light brown or orange; gray and green, massive and thinly bedded chert; white to grey, brown weathering massive vitreous quartzite; and minor amounts of platy finely laminated silty shale. Shales are best exposed through Bruffey Canyon, particularly near Union Summit, where the Vinini forms low hills composed of soft, black, light brown to gray, platy to blocky weathering, silty shale. Organic-rich shales are also present at Garden Pass Summit where gray to black fissile shale contains abundant graptolites. The section in the Garden Valley area also contains medium-grained blue limestone in 1 to 4 inch thick beds interbedded with evenly bedded red-brown, dark green and black cherts, and thin lenses of fine-grained, brown-gray, siltstone and sandstone (Forman, 1951).

In the Carlin-Pinon Range, the Vinini is poorly exposed in small thrust slices in the southern portion of the range near Papoose Canyon and along Willow Creek, and in the northern portion of the range east of Carlin along the Humboldt River. Most of the section is formed by grey, black and green bedded cherts, dark gray to black, yellowish-brown weathering vitreous quartzite, grey to black and light greenish, platy laminated siltstone, and silty, silicified, green to light brown weathering mudstone and shale. Also present in the exposures near Carlin are minor amounts of coarse-grained light brown sandstone with abundant subangular green and gray chert fragments and brown to orange weathering chert pebble conglomerate. Shales are best exposed along Indian Pony Road along Willow Creek, and just south of Carlin along Woodruff Creek. Along Woodruff Creek we found and dug out poorly exposed shales in the Vinini which are greenish gray where fresh, and weather to a crumbly orange-brown greasy regolith.

In the Tuscarora Mountains at and near Marys Mountain, the Vinini Formation is composed of white to light brown and green laminated siltstones and massive quartzites and sandstones. These lithologies are structurally interwoven with thoroughly contorted gray to black and brown cherts, and minor amounts of thin, dark brown, grey, and black, soft tuffaceous shales and mudstones, and dark grey limestones. The siltstones which compose most of the section are strongly altered and hematically and/or limonitically stained.

To the northeast along Maggie and Cottonwood Creeks, and near Lone Mountain in the southern Independence Range, the Vinini Formation is composed of a high percentage of gray to brown weathering, moderately soft and fissile, platy to blocky, black silty shale and black, and gray weathering platy to blocky laminated siltstone, which vary in the degree of silicification. Black and gray or green, thinly bedded cherts are also abundant with thin limestone interbeds that have locally been replaced by barite. At Swales Mountain and near Lone Mountain in the southern Independence Range, the Vinini is composed of hard, black massive chert in beds from 1 inch to several feet thick, and black, gray and light brown siliceous shale which is interbedded with and grades into the chert (Evans and Ketner, 1971). Also present are silty and sandy limestones beds and minor amounts of massive gray, brown-weathering quartzite.

In the northern Adobe Range and Peko Hills area, the Vinini is exposed in a series of thrust slices in contact with, and virtually indistinguishable from, allochthonous Silurian cherts (Ketner, 1973a, 1973b; Ketner and Ross, 1984). As a map unit, the Vinini is composed of thin-bedded and massive cherts, siliceous brittle shales, and minor amounts of quartzite, and conglomerate. The cherts are thinly to moderately bedded or massive, and are commonly black or gray with lesser amounts of red and green cherts. The black and dark green-gray shales are strongly silicified and shattered with conchoidal fracture, and grade laterally and vertically into thin-bedded cherts. The gray, brown, and green quartzites are massive and faintly laminated. The chert pebble and cobble conglomerates are brown weathering, poorly to moderately sorted and form less than 5 percent of the unit.

Thicknesses measured or estimated in the Vinini are strictly structural or tectonic. Minnick (1975) measured about 4,200 feet of Vinini in the Roberts Mountains while Nolan and others (1956) estimated a thickness of 500 feet along Vinini Creek, Bortz (1959) estimated about 500 feet in the Copenhagen Canyon area of the Monitor Range, and Merriam (1963) approximated 1,000 feet at Devils Gate. Kay and Crawford (1964) estimated 3,300 feet of Vinini Formation in the Toquima Range, Smith and Ketner (1978) estimated a minimum thickness of 3,280 feet in the Carlin-Pinon Range, Evans and Ketner (1971) estimated 4,700 feet in the Swales Mountain area of the southern Independence Range, and McKee (1976) estimated a total thickness of about 6,000 feet with a measured partial thickness of 1,000 feet in the Petes Summit area of the Toquima Range.

The Vinini Formation is exposed in the Roberts, Cortez, Simpson Park, Tuscarora and Independence Mountains, Monitor, Sulphur Spring, Pinon, Adobe, Antelope, Toquima, Shoshone, and Fish Creek Ranges, Mahogany Hills and Lone Mountain.

The Vinini Formation represents the eastern facies of the Valmy Formation and contains much less greenstone and quartzite and more shale and mudstone than the Valmy Formation. The Vinini grades eastward from primarily chert and shale into more limestone and shale, indicating shallower water from the Valmy in the west to the Vinini in the east. The Valmy has been interpreted as a deep-water abyssal-plain facies, with the Vinini Formation representing an upper continental slope unit (Stanley and others, 1977). Sarniak (1979) interpreted the upward coarsening repetitive sequences of shale, climbing rippled and current cross-bedded and laminated siltstones and cherts overlain by orthoquartzites in the Vinini, as delta front and delta plain sediments. The specifics of depositional environment for the Vinini, particularly water depth, are poorly understood.

The Vinini was deposited in deeper water than time-equivalent shelf carbonates to the east. Trace fossil assemblages suggest deep-water bathyal lower slope depositional environments (Stanley and others, 1977; Ketner, 1977). Modern sedimentation rates suggest accumulation rates of less than 10 meters per million years for graptolitic shales of the Vinini and Valmy Formations.

Mineralogy and bedding suggest cherts and shale units were deposited as pelagic and hemipelagic sediments under normal bottom conditions that were locally anoxic excluding any kind of burrowing infauna. Siltstones, limestones, and sandstones are commonly laminated and cross-bedded suggesting deposition by clear-water bottom currents which may be of the contour-current type. Limestones in the Vinini contain shallow water trilobites and gastropods indicating shallow water deposition.

Pillow lavas occur in the upper, middle, and lower portions of many Vinini sections although they are much less prevalent than in the Valmy Formation. Excellent exposures of pillow basalts are present just east of Roberts Creek in the Roberts Mountains. Most aggregates of pillows are a few meters thick but can be up to 100 meters thick as in the Independence Mountains (Wrucke and others, 1978). Pillows studied in the Shoshone Range and Independence Range contain very small or no vesicles indicating deep emplacement or degassing of the flows before emplacement (Wrucke and others, 1978).

Trace fossils suggest rapid deposition of limestone and quartz sands. Both chert and shale are laminated and bedded similar to modern deep sea cherts and shales in the Gulf of Mexico. Sands or quartzites in the formation have been regarded as multi-cyclic mature sands derived from the craton and deposited by submarine deep sea fan systems (Ketner, 1968; Wrucke and Churkin, 1974; Wrucke and others, 1978). The good sorting, sharp upper and lower contacts, internal horizontal and cross laminations, incomplete Bouma sequences, and sole markings in siltstones and finer-grained sandstones have been used to suggest deposition was by clear rather than sediment-charged currents (Stanley and others, 1977). Stanley and others (1977) feel Vinini siltstones represent clear-water thermohaline currents which may have been contour-following bottom currents as described in modern ocean basins (Bouma and Hollister, 1973; Heezen and Hollister, 1972). Coles and Snyder (1985) have suggested that phosphates present in the Vinini in the Toquima Range indicate deposition under upper slope conditions within the oxygen minimum zone.

The Vinini Formation may have been deposited on the continental slope or rise of another crustal plate which collided with the western margin of North America. Much of the internal deformation in the Vinini is most probably the result of deformation on the sea floor as a soft sediment, in an accretionary prism-subduction complex setting. Isoclinal and particularly open folds and small-scale thrusts within the Vinini are the result of later Paleozoic and Mesozoic deformation associated with tectonics. Extensive brecciation has occurred within the Vinini where down-dropped along high-angle normal faults, particularly in the more siliceous and cherty sections.

The Miocene Monterey Formation of California may be a good lithologic-depositional analog for the Vinini Formation. The Monterey is a deformed sequence of siliceous sedimentary rocks including highly organic laminated shales, silt and sandstones, thin limestones and abundant bedded cherts. The Monterey was deposited in continental borderland basins with water depths of about 1,500 meters (Ingle, 1981). Thin volcanic flows and volcaniclastics are intimately associated with the formation (Weaver, 1969).

The Monterey provides both source and reservoir in numerous oil fields throughout coastal California with most production related to fracture porosity as result of both diagenetic and structural preparation. Silica transformation from opal A to opal ct, and finally to quartz, occurs at temperatures which coincide with the onset of oil maturation, and probably play a large role in hydrocarbon migration and accumulation as a result of dehydration reactions which release large volumes of water and create excess fluid pressures necessary to fracture rocks. Field evidence, such as tar cemented breccias in the Monterey, suggests hydrocarbon migration occurred at the time of diagenetic transformation and brecciation. If this is the case in the Vinini as well, early generated oil, perhaps during the Ordovician, may have migrated quite early.

The regional extent of areas of anoxic bottom conditions which excluded a homogenizing fauna are unknown for the Vinini Formation. It is important to document such regions, since shales deposited under conditions of an oxygen minimum zone have been shown globally to be excellent source rocks as a result of organic preservation from oxidation. Along Vinini Creek much of the shale is a true oil shale, and in many localities the Vinini will burn when touched with a match. Selected samples have yielded 25 gallons per ton by distillation. Dark chert beds in the Vinini also carry high organic contents.

Geochemical analysis of the Vinini Formation for this evaluation indicates a narrow fairway of organic-rich and mature source facies. The average TOC for 47 samples is 0.94 percent within mature to overmature surface samples. The geochemical data for the Vinini are summarized in the Geochemical and Geothermal Data Volume, and are displayed on Overlays XI and XII.