The Valmy Formation was named by Roberts (1951) for several thousand feet of chert, quartzite and argillite, slate and greenstone on the north side of Battle Mountain, in the Antler Peak Quadrangle, Humboldt County. The name was derived from the railroad station at Valmy about 4 miles to the north.

The Valmy is considered a western facies of the Vinini Formation. Like the Vinini it occurs in thrust sheets within the upper plate of the Roberts Mountains thrust, commonly lying on transitional and eastern carbonate assemblage rocks, and best exposed on the eastern slope of the Shoshone Range and western flank of the Cortez Mountains. The Valmy is locally overthrust by Silurian and Devonian western assemblage rocks such as the Slaven Chert, and in some places is unconformably overlapped with Pennsylvanian and Permian conglomerates such as the Antler Sequence (Stewart and McKee, 1977).

Most fossils (primarily graptolites) indicate a Middle Ordovician age for the Valmy Formation, although Early, Middle, and Late Ordovician faunas are present (Roberts, 1964; Gilluly and Gates, 1965; Gilluly and Masursky, 1965; Stewart and McKee, 1977).

Distinction between the Valmy and Vinini Formations is classically based upon the relative proportion of lithologies which are present in both formations. The Valmy is considered to contain a higher relative proportion of massive quartzites, cherts, and volcanic flows than the Vinini which contains relatively more siltstones, shales, and limestones. These criteria are general and very subjective, particularly since on a regional scale neither formation contains significant amounts of shale or limestone, but are nearly everywhere dominated by siltstone, chert and quartzites.

The Valmy Formation is composed of thousands of meters of thick-bedded, massive, vitreous and quartz-rich (98%) quartzite and sandstone interbedded with grey, green, or black chert, black shale, siltstone, greenstone, and minor limestone (Roberts, 1964; Gilluly and Gates, 1965; Roberts and others, 1967; Stewart and Poole, 1974). Distinction between the Valmy and Vinini are based on the general presence in the Valmy of much more greenstone and quartzite and sandstone, with less shale and limestone than the Vinini Formation (Gilluly and Masursky, 1965; Gilluly and Gates, 1965; Stewart and Poole, 1974; Stewart and McKee, 1977). Regionally, the Valmy Formation appears to become coarser grained with a higher percentage of quartzite and volcanic detritus to the west (Roberts, 1964b; Gilluly and Gates, 1965).

In general, thick-bedded, massive, gray to brownish quartzite beds are often 10 to 15 feet in thickness and may be as much as 50 feet thick. The quartzites are often thinner bedded where they grade laterally and vertically into poorly exposed gray to black shales and finely laminated and regularly bedded, green, gray, or black cherts. The cherts are organic-rich and occasionally contain rod-like structures which may be worm trails (Roberts, 1964b). Pillow lavas of basaltic composition are often 20 to 100 feet in thickness but occasionally may be as much as 200 feet in thickness. Associated with the greenstones are greenish-brown breccias and thin tuffaceous shales or tuff composed of clay minerals, sericite, chlorite, serpentine, and biotite (Roberts, 1964b).

Near Beowawe in the Dry Hills, the Valmy Formation consists of thin-bedded black and brown chert, and dark grey to black shale with thin-bedded vitreous quartzites and thick beds of chert conglomerate (Roberts and others, 1967). In the Toiyabe Range the Valmy is perhaps 1,000 feet in thickness. It consists mainly of dark-grey chert, dark-grey argillite and shale, and a few thin layers of grey to yellowish-grey quartzite (Stewart and McKee, 1977).

Within the Cortez Mountains the Valmy is composed of varying percentages of contorted, thin-bedded, dark chert, dark-grey medium to coarse-grained, poorly sorted quartzite with detrital chert grains; nearly pure (99 percent silica) white quartzite and light-grey, fine-grained sandstone dominantly composed of quartz and detrital chert grains; abundant but poorly exposed thin-bedded gray and black dolomitic siltstone; gray angular to sub-rounded chert-pebble conglomerate from a few feet to locally (Cortez Mine section) several hundred feet in thickness; very rare limestone lenses up to 10 feet in thickness and a few hundred yards in length containing abundant trilobite, brachiopod, and gastropod fragments; and highly altered andesitic greenstone up to 150 feet in thickness (Gilluly and Masursky, 1965; Roberts and others, 1967).

Near the crest and along the northeastern flank of the Sulphur Spring Range, Smith and Ketner (1975) described a section of Valmy Formation composed of about 50 feet of black to gray, silty and poorly fissile shale overlain by about 100 feet of interbedded fine-grained and well rounded, white to brown quartzite in beds 6 to 12 inches thick and units 10 feet in thickness, and green equigranular greenstone. This section may be better assigned to the Vinini Formation rather than Valmy based upon its geographic position and abundance of shale which we found to be very rare in the Valmy.

In the Shoshone Range, Gilluly and Gates (1965) divided the formation into older, intermediate and younger portions. The older (Late Tremadocian to Late Arenigian age) section is composed of platy, thin-bedded, silty sandstone about 100 feet thick, overlain by interbedded quartzite and sandstone about 250 feet thick, thin-bedded sandstone with interlayered chert about 100 feet thick, and greenstone breccia about 400 feet in thickness. A tectonically higher sheet of the older section consists of 1,000 feet of chert and interbedded sandstone, siltstone, and chert pebble conglomerate, overlain by greenstone breccia which grades laterally into pillow lavas containing pockets of limestone which is about 300 feet in thickness; in turn overlain by 250 feet of massive sandstone, 30 feet of yellowish weathering massive quartzite, interbedded sandstone, micaceous-siltstone, thin platy limestone 200 feet in thickness, 75 feet of massive quartzite, and about 900 feet of well-bedded sandstone with a few interbeds of chert and quartzite. The aggregate thickness for the older Valmy section is about 2,300 feet.

The intermediate (Llanvirnian to Llandeilian age) portion of the formation is described as over 2,200 feet of interbedded sandstone, chert, quartzite and very minor shale. The highest or youngest portion in the Shoshone Range (Caradocian age) is exposed in a series of imbricate thrust slices. The Caradocian part of the Valmy consists of sandstone, greenstone, quartzite, shale and chert at least 2,700 feet in thickness. Between Horse and Pipe Canyons, Gilluly and Gates (1965) describe the following Caradocian thrust slice; 100 feet of chert and cherty shale overlain by about 500 feet of pillow lavas, followed by 500 feet of interbedded sandstone, shale, and thin chert beds about 600 feet thick, and interlayered greenstone and sandstone as much as 1,000 feet thick. The highest beds described in the Valmy are Ashgillian and represent about 1,000 feet of interbedded sandstone, shale, and greenstone.

In the Tuscarora Mountains, the lithologies examined by Western Cordillera geologists, for instance in the Chicken Spring and Mack Creek areas, are best assigned to the Valmy Formation. The principal lithologies are thin-bedded gray, green and black cherts which are often interbedded and contain 1 to 4 inch thick interbeds of siliceous shale, and gray, caramel, and black massive to platy quartzites, and minor lenses of limestone which have locally undergone secondary replacement to barite. On the eastern side of the Tuscarora Range just west of Maggie Creek, Coats (1985) has reported the occurrence of limestone lenses which contain Devonian brachiopods. The structural relationship between these limestones and the surrounding western assemblage lithologies is unclear, but is quite clear that the overwhelming majority of the unit is Ordovician in age.

In the northern Independence Mountains, highly folded and internally thrusted western assemblage rocks have been assigned to the Seetoya Sequence by Kerr (1962) who broke the unit into two informal members. The lower member is brown to black, thin to medium-bedded chert with interbedded quartz siltstone, mudstone and dark green to grey mafic flow rocks. The upper member is a poorly bedded, light olive-gray to medium-gray, fine to medium-grained quartzite composed of subrounded quartz grains. These rocks however, are considered Valmy Formation by other workers such as Miller and others (1980) and are so considered here based upon our field observations.

Churkin and Kay (1967) divide the western assemblage rocks in the northern Independence Mountains into three formations which they considered part of the Valmy Group. These formations are the Snow Canyon Formation, McAfee Quartzite and Jacks Peak Formation. The Snow Canyon Formation is a thin-bedded dark gray chert with argillite partings and very thin siliceous shale and siltstone interbeds which are interbedded with several fine-grained light-gray quartzite beds several tens of feet in thickness. Basaltic lavas are also abundant with both pillow structures and vesicles as are lithic breccias, and local lenses of coarse-grained light-gray limestone. The formation is estimated to be 1,200 feet thick. The McAfee Quartzite is mostly fine-grained, white to light gray quartzite in beds 10 to more than 100 feet in thickness with a few thin graptolitic shale and siltstone beds up to 4 inches thick, and minor bedded chert near the top of the unit. The McAfee quartzite was estimated by Churkin and Kay (1967) to be about 1,387 feet thick. The Jacks Peak Formation is divided into a lower unit composed of about 300 feet of thin-bedded black chert and an upper member of about 200 feet of fine-grained, very thick-bedded and massive white quartzite.

Quartzite is also the dominant facies in the Valmy exposed to the east at Double Mountain and in the hills to the north of Double Mountain. Coats (1985) suggests that south of the latitude of Double Mountain the Valmy is composed of bedded gray and black cherts and silty cherts, argillite, siltstone, quartzite and greenstone, with small amounts of shale, medium to coarse-grained, well-rounded, quartz sandstone and allodapic limestone. A few of the shale and siltstone beds present in the Independence Mountains may be Silurian (Wenlockian) rather than Ordovician in age (Coats, 1985). Shales make up perhaps 1 percent of the unit and are commonly 1 to 3 inches in thickness as interbeds in the phyllites, quartzites, and cherts.

In northern Elko County, rocks mapped as western facies in the Owyhee, Mountain City and Rowland Quadrangles are here considered the Valmy Formation in accordance with the work of several geologists including Coats (1971). In the Owyhee and adjacent Mountain City Quadrangles, the Valmy is dominantly composed of gray to black or tan, fine-grained quartzite and micaceous quartz calcarenite and calcareous sandstone lenses which contain phyllocarid crustacean shells (Coats, 1985). Thin-bedded, gray, black, green, and red phosphatic and locally baritic cherts with slaty partings are abundant, as are well-bedded micaceous siltstone and black quartzose phyllite. The remainder of the Valmy is comprised of irregular bodies of greenstone, meta-gabbro and diabase, and micaceous phyllitic limestone (Coats, 1985).

In the Mountain City Quadrangle, the Valmy overlies the Ordovician Goodwin Limestone of the Pogonip Group along the Roberts Mountains thrust, and is intruded by Cretaceous granodiorite (Coats, 1985). Near Mountain City, a broad swath of isoclinally folded and internally thrusted Valmy Formation is dominantly composed of light gray to black, massive and vitreous quartzite which is interbedded with gray, silicified, phyllitic siltstone and thin-bedded black chert, and black sooty, splintery to blocky weathering shale to phyllite in beds that are commonly 1 to 3 inches in thickness. To the east in the Rowland Quadrangle, Bushnell (1967) describes the Valmy as black quartzite, dark phyllite, schist, chert and minor amounts of coarsely crystalline gray limestone and siliceous dark shales. In the Marys River Valley area, the Valmy is interbedded sponge spicule and radiolaria-bearing chert, and siltstone with minor interbedded dark-gray massive quartzite, and dark brown to black shale and barite veins up to 30 feet in thickness (Coats and others, 1977).

In the Snake Mountains of northern Elko County, western assemblage lithologies have been assigned to the Valmy Formation based on our observations and those of Peterson (1968) and Gardner (1968). Gardner (1968) felt the Valmy in the northern portion of the range could be divided into a lower portion of shale, siltstone, and chert with discontinuous lenses and pods of limestone and orthoquartzite, and an upper unit of siltstone, chert, and shale with rare orthoquartzite. Our observations suggest that these rocks dominantly consist of interbedded gray to black, and caramel or greenish, sugary to vitreous, massive orthoquartzites, thin-bedded, highly contorted, gray and black, reddish-weathering cherts, and very light green chert, and very minor siliceous siltstone, conglomerate, limestone and phyllite. Peterson (1968) reported chert, shale, and siltstone in the southern portion of the range. Coats (1985) has suggested that allochthonous lithologies exposed in the northern portion of the Snake Mountains are strictly Ordovician while the southern portion of the range may contain both Ordovician and Silurian rocks.

In the HD Range, Riva (1962, 1970) described three new formations in the Ordovician western assemblage rocks exposed there. These Middle to Late Ordovician formations are the Valder Formation, the Agort Chert, and the Tiser Limestone. These units were subsequently correlated to lithologically similar units to the south in the Windermere Hills by Oversby (1972). It is this writer's opinion that these formations do not pass formational tests, and are in fact not mappable as regional units. As formations, they greatly and unnecessarily complicate both the structural and stratigraphic picture and if used at all, are better considered as local members of the Valmy Formation in both localities. For this reason they have been eliminated from detailed stratigraphic discussion. Unfortunately, a brief but careful examination of Riva’s mapping in the HD Range suggests there are some major problems both in stratigraphy, as well as the fascinating complicated thrust relationships. In several instances, rocks mapped as allochthonous western facies are actually younger carbonates. Both the Ordovician Pogonip and Permian carbonates have been extensively replaced by grey, brown and black cherts. Several of the thrust contacts originally mapped by Riva (1962) are simply not present upon examination. It is probable that a thorough re-examination and mapping of the area would greatly simplify an important area which appears to be primarily complicated by error in mapping.

The Valmy in both the HD Range and Windermere Hills consists of black to gray, fissile to blocky argillite and phyllite, greenish-gray andesitic volcanics, and interbedded light to dark green massive chert ("Valder Formation"), gray to black chert in beds from 1 to 50 feet thick which alternate with gray weathering argillite commonly 20 to 50 feet thick, and local interbeds up to 25 feet thick of white or gray massive quartzite ("Agort Chert"). Riva's Tiser Limestone was only recognized in the HD Range and consists of 90 to 150 feet of thin-bedded gray limestone with stringers and nodules of black chert, and thin interbeds of siliceous and phyllitic mudstone or shale. In both ranges the Ordovician portion of the section is overlain by Silurian allochthonous rocks assigned to, and described under, the Noh Formation.

Severe structural dismemberment makes accurate determination of thicknesses impossible. Given this disclaimer, Stewart and McKee (1977) suggest there may be 1,000 feet of Valmy in the Toiyabe Range, Roberts (1964) estimated a thickness of 8,000 feet of Valmy on Battle Mountain, and Gilluly and Gates (1965) felt that at least 12,000 feet of Valmy can actually be documented in the northern Shoshone Range. Smith and Ketner (1975) reported about 150 feet in a section, with an unexposed base and thrusted upper contact, along the northeastern flank of the Sulphur Spring Range.

In the Marys River Valley area the Valmy is about 1,000 feet thick (Coats and others, 1977), in the northern Independence Mountains it is at least 3,087 feet thick (Churkin and Kay, 1967) and about 10,000 feet were estimated in the northern Snake Mountains (Gardner, 1968). Approximately 450 feet of Valmy is exposed in the Windermere Hills and 1000 to 1500 feet may be present in the HD Range.

The Valmy is exposed within the Simpson Park, Cortez, Independence, and Bull Run Mountains, Dry Hills, Sulphur Spring, Shoshone and Toiyabe Ranges, Battle Mountain, Owyhee, Mountain City and Rowland Quadrangles, Marys River Valley area, HD Range, Windermere Hills, Snake Mountains, and at Mason and Double Mountains.

The original depositional setting for the allochthonous Ordovician Valmy Formation is not well understood. The gradational nature between the Valmy and the Vinini to the east is well documented suggesting deposition within the same general environment. Stanley and others (1977) suggest that the Valmy Formation represents a deeper water facies of the upper slope Vinini, and was deposited in outer slope to bathyal or abyssal-plain environments. Sarniak (1979) on the other hand has suggested that the Valmy represents distal delta front sediments deposited in a subduction complex.

Moderately to poorly sorted, well rounded, fine to coarse-grained, orthoquartzites and sandstones are present in the Valmy, and at least in part, appear to be second cycle sediments. Sole markings present at the base of sands and quartzites suggest turbidite deposition, however much of the sandstone and quartzite show current-type cross-bedding. Chert-rich sandstones containing silt to gravel size angular fragments of chert and siliceous shale are also present, and may represent gravity flows.

Greenstones ranging from pillow lava through pyroclastic breccia and fine ash, grade laterally into one another in the Valmy Formation (Gilluly and Gates, 1965). In the Shoshone Range, Ross (1958) described a rich trilobite, gastropod, and shelly fauna within pillow lavas of the Valmy suggesting relatively shallow water deposition within the photic zone with water depths of perhaps 100 to 200 meters. In contrast, the carbonaceous nature of cherts and minor amounts of shale in the Valmy, as well as the general absence of fossils (except graptolites and those within the limestones), suggest local depositional conditions which were inhospitable for benthic organisms.

Seventeen (17) surface geochemical samples from the Valmy Formation were analyzed during this evaluation. The average TOC for selected lithologies is about 0.83 percent with most surface samples indicating thermal maturity within the gas generation zone to thermal overmaturity. The Valmy may very locally be a source of dry gas. The results of the geochemical analysis of the Valmy are discussed in the Geochemical and Geothermal Data Volume and the data are displayed on Overlays XI and XII.