Geological Survey Bulletin 1386 Silurian, Devonian, and Mississippian Formations of the Funeral Mountains in the Ryan Quadrangle, Death Valley Region, California

HIDDEN VALLEY DOLOMITE

The Hidden Valley Dolomite, of Silurian and Devonian age, lies concordantly on the Ordovician Ely Springs Dolomite and under dolomite of the Devonian Lost Burro Formation. The Hidden Valley is distinguished by marker beds at the boundaries and by rocks of a characteristic sequence of colors visible on the mostly bare mountainsides. The lower boundary is marked by a dark layer of cherty dolomite below less resistant, brown-tinged beds and above contrastingly lighter gray chertless dolomite at the top of the Ely Springs. The upper boundary is at the top of a bench-forming pinkish- and yellowish-gray dolomite and at the base of the brown-weathering Lippincott Member of the Lost Burro Formation. The Hidden Valley Dolomite is divided into a medium- to dark-gray lower member and a lighter gray much thicker upper member. The lower member consists of well-bedded dolomite containing chert and widely distributed Silurian fossils, whereas the upper member consists in general of obscurely bedded dolomite that is apparently barren except for Devonian fossils in the uppermost part. The Hidden Valley Dolomite and the underlying sequence down to the Bonanza King formation are clearly displayed along the skyline of Schwaub Peak (fig. 2) and can be easily viewed from the road to Dantes View.

FIGURE 2.—Northward view of Schwaub Peak. Skyline profile shows stratigraphic sequence from lower part of Hidden Valley Dolomite (DShl) down through Ely Springs Dolomite (Oes), Eureka Quartzite (Oe), Pogonip Group (Op), Nopah Formation (∈n), to Bonanza King Formation (∈b).

A complete section of the Hidden Valley Dolomite (fig. 3) that contains the characteristic lithologic sequence of the formation in the quadrangle is designated a reference section and is used in the stratigraphic column on plate 1. The base of the section is 2.07 miles N. 51° W. of Pyramid Peak and the top is 1,500 feet S. 74° E. of the base. The Hidden Valley Dolomite in the reference section is 1,440 feet thick; the lower member is 520 feet, and the upper member is 920 feet. The stratigraphic column includes information from a supplementary measured section of the lower member (1.6 miles north of the reference section and about 3 miles northwest of Pyramid Peak) and some fossils from other localities.

FIGURE 3.—Hidden Valley Dolomite 2 miles northwest of Pyramid Peak; reference section for the Funeral Mountains. Oes, Ely Springs Dolomite; DShl, lower member of Hidden Valley Dolomite, containing Silurian fossils; DShu, upper member of Hidden Valley Dolomite, containing Lower Devonian (upper Emsian) fossils near boundary with Lippincott Member of Lost Burro Formation (Dll) in saddle.

LOWER MEMBER (SILURIAN)

The lower member of the Hidden Valley Dolomite lies above light-gray and medium-gray chertless dolomite at the top of the Ely Springs Dolomite, which contains a sandy dolomite marker about 50 feet below the boundary. The member's basal unit (h1, pl. 1), which is about 20 feet thick, consists of medium-dark-gray dolomite that contains nodular chert. Generally the lower part of the unit is exposed as a ledge, and the upper part is concealed by rubble from overlying beds.

The middle unit (h2), about 100 feet thick, is less resistant and produces a generally concealing rubble that is a distinctive reddish brown and gray. The unit consists of thin-bedded medium- to light gray very fine grained dolomite and subordinate interstratified chert (fig. 4). The chert characteristically is in very thin beds and in flat nodules. Argillaceous and silty material, which weathers to a distinctive reddish brown, is dispersed in some of the dolomite and it lines partings of flaggy beds. Intraformational conglomerate or sedimentary breccia in an 8-foot bed near the top of the unit consists of lithologically varied dolomite and chert fragments in a dolomite matrix. This bed contains conspicuous silicified Favosites and Halysites, characteristic fossils of the Silurian part of the Hidden Valley.

FIGURE 4.—Thin-bedded dolomite and dark-gray chert near base of Hidden Valley Dolomite. This unusually well exposed part of unit h2 is in a measured section about 1-1/2 miles north of the reference section. Scale at left is 6 inches long.

The upper unit (h3) is by far the thickest unit of the member; it is 400 feet thick in the reference section, northwest of Pyramid Peak, and 480 feet thick in the supplementary section about 1-1/2 miles farther north. The difference in thickness may result from the nature of the upper boundary, a gradational change in color and texture, which transects the bedding at the supplementary section. The unit consists of medium- to dark-gray dolomite that contains nodular chert. Some of the darkest dolomite in the bottom part has vugs, clots, and networks of coarse white dolomite, resembling some dolomite in the lower part of the Ordovician Ely Springs Dolomite. Chert nodules of very irregular shape are sparse 100 feet above the base of the unit, become abundant about 80 feet higher, and disappear between 60 and 80 feet from the top. Fossils, which generally are silicified, occur throughout unit h3 but are more conspicuous in the upper part.

Collections of fossils from the lower member of the Hidden Valley Dolomite at widely scattered localities in the Funeral Mountains verify the Silurian age of the member. The collections include fossils definitely of Middle Silurian (Wenlock) age and a few that extend the range in age from possibly Early Silurian (Llandovery) into probably Late Silurian (Ludlow).

The lowest collection (JFM6659B, pl. 1), which is from thin-bedded unit h2 at a place less than 100 feet above the base of the formation southeast of Schwaub Peak, contains a diagnostic graptolite fragment. W. B. N. Berry (written commun., 1966) identifies the fragment of a rhabdosome as Monograptus sp. (M. priodon type) and comments

The form is slender and may be a portion of the proximal region of M. priodon itself or it may be a part of a more slender form such as M. parapriodon. Age: Monograptids of the M. priodon type range in age from the Late Llandovery into the Wenlock. The more slender forms of this type and the thinner, shorter forms of M. priodon itself are essentially restricted to the Late Llandovery. Inasmuch as this specimen does appear to have come from either a slender member of the M. priodon group or from relatively slender form of M. priodon itself, I would suggest the age of the beds from which it came is Late Llandovery.

The same specimen is referred to by Berry and Boucot (1970, p. 162) as "indicative of an age in the span of Late Llandovery-Wenlock."

Higher in the same thin-bedded unit, 90-115 feet above the base of the Hidden Valley, a varied fauna that includes conspicuous Favosites and Halysites occurs in the dolomite matrix of sedimentary breccia and in closely associated silty dolomite. A collection (JFM6354Ca) from the supplementary section about 1-1/2 miles north of the reference section includes Ryderophyllum, Pycnactis, Brachyelasma, favositids, Heliolites, Hesperorthis, the cephalopods Huronia and Huroniella, and the large dasycladacean Verticillopora. Another collection (JFM661022J) from this persistently fossiliferous interval, at a locality about 3 miles east of the main section, includes Halysites (Cystihalysites) cf. Halysites (Cystihalysites magnitubus) Buehler, 1955, Favosites sp., Brachyelasma sp. B, and Ryderophyllum n. sp. The fossils were identified by C. W. Merriam (written commun., 1971) who comments as follows:

This fossil assemblange is that characterizing Great Basin coral zone B of early Middle Silurian (early Wenlockian) age. In the Panamint Range reference section north of Ubehebe Peak it occurs in McAllister's [1952] Hidden Valley unit 1 about 325 feet above the base of the formation. Its most distinctive coral is the solitary lykophyllid Ryderophyllum n. sp. which I have given a species name in manuscript. Similar species occur in the Wenlockian of Gotland, Sweden. The Brachyelasma is a primitive solitary rugose coral occurring also in Late Ordovician rocks.

A varied fauna in the most fossiliferous part of unit h3 in the supplementary section is represented by a collection (JFM6354F2) from about 300 feet above the base of the Hidden Valley Dolomite. C. W. Merriam (written commun., 1971) states that this fauna, also assigned to coral zone B, is "* * *characterized by Tryplasma, Halysites, Alveolites, Romingerella, and Cladopora. Associated brachiopods include Leptaena, Fardenia, Rhipidium, Atrypa, Atrypina, and Esopirifer (Striispirifer). Lykophyllids, usually to be expected in coral zone B, were not found in this upper horizon." In the supplementary section Halysites is abundant in the uppermost part of the lower member.

A collection (JFMS6120A) from about 450 feet above the base of the formation and near the top of the lower member in the reference section contains Heliolites sp., favositids, abundant ?Syringaxon sp., a pentameroid resembling Cymbidium, and abundant Conchidium sp. (C. W. Merriam written commun., 1962, reviewed 1971). A. J. Boucot and J. G. Johnson (J. G. Johnson, written commun., 1965; Berry and Boucot, 1970, p. 161) report Rhipidium sp., indicating late Wenlock age, from an unspecified horizon at the same general locality.

Conodonts were recovered by J. W. Huddle from the matrix of megafossil collections (7299—SD, 7300—SD) from a 20-foot interval high in the lower member of the Hidden Valley Dolomite on spur 4298 (fig. 1, pl. 2). The associated, crudely silicified megafossils, as reported by W. A. Oliver, Jr. (written commun., 1964), include Favosites sp., Syringopora sp., thamnoporoid corals, horn corals, and Silurian halysitids. Huddle's report (written commun., 1965, amended 1971) giving the identification, number of specimens, and stratigraphic range of the conodonts, with comments, follows:

The O. aff. O. crassa in this collection looks like a form illustrated by Walliser, 1964, from the Ludlow of Germany. He considered it an aberrant form but included it in O. crassa.

This collection seems to be Silurian in age and adds weight to the probable Late Silurian age of the previous collection.

The age of silicified fossils from the lower member at two other localities is reported as merely Silurian. About 2 miles east-northeast of the reference section, a collection (6843—SD) from unit h3 consists of Cladopora sp., Syringaxon? sp., and Tryplasma? sp., according to W. A. Oliver, Jr. (written commun., 1963). He reports Cladopora sp. also in a collection (6842—SD) about 55 feet stratigraphically lower, and another (colln. 6844—SD) about 100 feet higher in a biostrome of Halysites sp. The stratigraphically lowest of these collections was taken about 75 feet above the lower contact of thin-bedded unit h2, or an estimated 100 feet above the base of the Hidden Valley Dolomite. About 6 miles north of the reference section, a collection (JFM70518G) from an undetermined horizon in unit h3 is described by C. W. Merriam (written commun., 1970) thus:

Halysites sp., Heliolites sp., Favosites sp. (massive form), Lissocoelina? n. sp. B. This assemblage is of Silurian age, as indicated by Heliolites and the abundant large, smooth pentameroid brachiopod Lissocoelina? n. sp. B. The large brachiopod externally resembles Pentamerus or Pentameroides, but differs from each internally. It is almost identical to a species occurring abundantly at Bare Mountain near Beatty, Nevada, * * * in the dark-gray limestones in the lower part of the Silurian section below the Silurian light-gray dolomite which has been called Lone Mountain Dolomite.

UPPER MEMBER (SILURIAN? AND DEVONIAN)

Dolomite in the upper member of the Hidden Valley Dolomite is, in general, conspicuously lighter gray and coarser grained than dolomite in the lower member. The change in color and texture is gradational and at some places transects the bedding. The upper member in the reference section is 920 feet thick; it is divided into three units (pl. 1). Although some characteristics of these lithologic units persist laterally, the boundaries between the units are difficult to trace consistently at the same stratigraphic horizons.

The bottom unit h4, which is 420 feet thick, consists rather uniformly of massive, very light gray and light-gray dolomite, tinged yellowish on the surface. The texture is diversely coarse, and the bedding is obscure if not effaced by recrystallization.

The middle unit (h5), which is 420 feet thick, consists of predominantly light-gray dolomite, tinged pale olive, and some very light gray dolomite in the middle and uppermost parts. The dolomite appears less massive and the stratification becomes progressively more distinct above the middle of the unit. Some distinctive clastic beds are in the lowest part of unit h5 in the reference section, but their lateral persistence is not known. The base of the unit is marked by a 2-foot-thick set of moderate-orange-pink to pale-reddish-brown beds, consisting of fine-grained dolomite sandstone, or dolarenite, and siltstone. About 45 feet above the base of the unit, a breccia of gray dolomite fragments in a brownish matrix is 6-12 inches thick.

The top unit (h6), which is 80 feet thick in the reference section, changes upward from medium-gray and light-gray dolomite in well-defined beds to light-gray somewhat argillaceous and silty dolomite in poorly exposed beds. Its base is a 5-foot-thick ledge-forming bed of medium-gray dolomite that contains silicified corals and a few brachiopods. Overlying fossiliferous dolomite in an interval of about 10 feet is a contrasting light gray to moderate orange pink. Muddy dolomite constituting the upper part of the unit is generally concealed by pale-red or pale-yellowish-gray rubble.

Fossils in the top unit (h6) of the Hidden Valley are late Early Devonian (late Emsian) in age, as determined by C. W. Merriam (written communs., 1965, 1971). In the reference section, a collection (JFM62611B) from the ledge-forming dark dolomite in unit h6 contains silicified Favosites (small form), a member of the Halliidae resembling Odontophyllum, and fragments of Halliidae resembling Aulacophyllum. Float from this, but mostly from the basal 10 feet of the overlying lighter dolomite, has, in addition, (colln. JFM62611B1) Favosites (large form), Siphonophrentis (Breviphrentis) invaginatus (Stumm), Papiliophyllum elegantulum Stumm, and Atrypa sp. From the next spur 0.4 mile southward (colln. JFM62427E) Gypidula cf. G. loweryi Merriam occurs with S. invaginatus and Favosites (small and large forms). Collections (7301—SD, JFM65313D, JFM6547D) from unit h6 at the southeast end of the Funeral Mountains include an abundance of a large Meristella similar to M. robertsensis occurring with abundant S. invaginatus and less common P. elegantulum. From trimmings of collections 7301—SD, J. W. Huddle obtained 12 specimens of Icriodus symmetricus Branson and Mehl and a fragment of Spathognathodus sp.; these fossils sustain an Emsian if not a younger Devonian age (J.W. Huddle, written commun., 1965).

The fossil assemblages examined by Merriam are indicative of the lowest part of his Devonian coral zone D in the Great Basin, specifically subzone D₁ of late Early Devonian (late Emsian) age. He regards coral zone D as being practically equivalent to the Eureka spirifer pinyonensis Zone and straddling the boundary between Lower and Middle Devonian (C. W. Merriam, written communs., 1965, 1971). His next lower coral zone C is equivalent to the Acrospirifer kobehana Zone (early Emsian); the A. kobehana Zone occurs in the uppermost unit of the Hidden Valley Dolomite at the Andy Hills locality near the type locality (McAllister, 1952, p. 17) but has not been found in the Funeral Mountains. Further collecting at the Andy Hills locality, but in sequence for comparison with occurences in the Funeral Mountains, shows that the stratigraphically highest fossils from the Hidden Valley Dolomite include S. invaginatus and P. elegantulum (C. W. Merriam, written commun., 1971) in both areas. These corals in the Andy Hills occur about 35 feet above the A. kobehana-bearing beds and at least 20 feet below the contact with the Lippincott Member of the Lost Burro Formation. So, according to the same evidence, the lowest part of the E. pinyonensis Zone (Devonian coral subzone D₁) is represented in the Hidden Valley Dolomite near the type locality and in the Funeral Mountains.

Some differences in lithology and thickness of the Hidden Valley Dolomite are noteworthy in its few exposures at the southeast end of the Funeral Mountains. These exposures, which are only around spur 4298 and on the northwest side of the Bat Mountain block (fig. 1), are structurally separated from the main occurences of the Hidden Valley in the quadrangle by major faults, including a thrust fault northwest of spur 4298 (McAllister, 1971). The main lithologic differences are in the lower member, where apparently much of the dolomite is recrystallized to light-gray dolomite, some of the nodular chert is recrystallized to a granular or quartzitic texture, and the basal marker beds, including the reddish-brown-weathering flaggy beds of unit h2, are lacking. Consequently, designation of the boundary with the underlying Ely Springs Dolomite on spur 4298 is tentative. The thickness of the Hidden Valley Dolomite here is no more than 870 feet, measured from the lowest possible boundary, or 800 feet, measured from the tentative boundary (pl. 2). This thickness is 570 or 640 feet less than that of the reference section (pl. 1).

PLATE 2. COLUMNAR SECTION OF THE HIDDEN VALLEY DOLOMITE ON SPUR 4298, SOUTHEAST END OF THE FUNERAL MOUNTAINS, RYAN QUADRANGLE, CALIFORNIA (click on image for a PDF version)

The only complete exposure of the Hidden Valley Dolomite at the southeast end of the Funeral Mountains is on the west flank of spur 4298, where the base of the measured section shown on plate 2 is 4-1/2 miles southeast of Pyramid Peak. The exposure, which is visible from California Highway 190 about 9 miles west of Death Valley Junction, includes some of the dark-gray part of the Ely Springs Dolomite at the base of the spur and the brown-weathering Lippincott Member of the Lost Burro Formation at the top. The dark-gray dolomite of the Ely Springs contains a meagerly sampled Ordovician fauna that includes Catenipora sp. cf. C. rubra (W. A. Oliver, written commun., 1964) and a few conodonts, such as Cordylodus sp., Drepanodus sp., and Gothodus sp. (J. W. Huddle, written commun., 1965). The overlying lighter gray dolomite contains a sandy dolomite marker of the upper Ely Springs. The stratigraphically lowest possible place for the boundary with the Hidden Valley is about 15 feet above this marker and at the base of a 70-foot-thick unit of somewhat variegated medium-gray chertless dolomite. The tentative boundary, 70 feet higher, is at the base of a 215-foot-thick unit of light-gray dolomite that contains the lowest nodular chert and vestiges of fossils. The chert nodules are sparse, and the chert, like the dolomite, is recrystallized and bleached. Large, elongate nodules of recrystallized chert are conspicuously abundant in very light gray dolomite of the next higher, 45-foot-thick unit. The base of this unit is readily traced and was expediently used as the lower boundary of the Hidden Valley Dolomite at spur 4298 on the preliminary geologic map of the quadrangle (McAllister, 1971). The unit, which is above vestiges of fossils and 50 feet below diagnostic Silurian fossils, probably represents the chertiest beds between the horizons of collections JFM6354F2 and JFM56120A in unit h3 of the unmodified Hidden Valley (pl. 1). Silurian fossils (collns. 7299— SD, 7300—SD, p. 9, 10, pl. 2), above very light gray chertless dolomite 50 feet thick, occur along with dark chert nodules in the lowest part of generally medium-gray dolomite, which is 110 feet thick at the top of the lower member.

The upper member here is 380 feet thick and consists generally of light-gray dolomite that contains indistinctly interlayered very light gray and medium-gray dolomite. At the top of the member, a 110-foot-thick unit is lithologically characteristic of unit h6 at the top of the Hidden Valley (pl. 2), and it contains fossils (collns. 7301—SD, JFM6547D, p. 11, pl. 2) indicative of Merriam's coral subzone D₁ of late Early Devonian age (C. W. Merriam, written commun., 1965).