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

LOST BURRO FORMATION

The Lost Burro Formation consists mainly of dolomite in the lower part and limestone in the upper part, both striped dark and light gray; and it includes brown-weathering sandy and quartzitic rocks that outline the boundaries of the formation. At the lower boundary, these brown-weathering rocks are in the Lippincott Member (McAllister, 1955, p. 12), and at the upper boundary they are in the Quartz Spring Sandstone Member of Langenheim and Tischler (1960, p. 92). The only complete exposure of the formation in the quadrangle, clearly visible from California Highway 190 about 7 miles west of Death Valley Junction, is in the Bat Mountain block (fig. 1) at the southeast end of the Funeral Mountains. A composite section of the Lost Burro Formation, measured from the underlying Hidden Valley Dolomite on the northwest flank of the Bat Mountain block (fig. 5) to the overlying Tin Mountain Limestone on the southwest flank (fig. 6), is 2,460 feet thick (pl. 1). Lithologic units above the Lippincott Member in the section are designated 1b2 to 1b5.

The Lippincott Member, at the base of the Lost Burro Formation, is characterized by sandy and silty light-gray dolomite, nodular chert, sandstone, and quartzite, interbedded with some dolomite. Much of the siliceous matter weathers brown in contrast to pinkish or yellowish tinges of the underlying argillaceous and silty dolomite in the uppermost part of the Hidden Valley. The siliceous components are generally in the following stratigraphic order, although proportions and sequences vary in detail from place to place: silt and fine-grained quartz sand dispersed in dolomite and concentrated in "fucoids" (perhaps fillings of straight, branching, and clustered burrows); nodular chert, increasingly abundant upward in dolomite interbedded with some sandstone or quartzite, partly lenticular; thick sandstone or quartzite near the middle of the member, above the highest chert; sandstone or quartzite interbedded in dolomite up to the top. The thickness of the Lippincott Member, measured to include the uppermost sandstone or quartzite, is 260 feet on the northwest side of the Bat Mountain block (fig. 5), 295 feet on spur 4298, 290 feet in a section 1.8 miles northwest of Pyramid Peak (fig. 3), and 320 feet about 1-1/2 miles east-southeast of Schwaub Peak. The Lippincott Member has not been dated by fossils in it, but because it lies above fossils of late Early Devonian (late Emsian) age and well below fossils of late Middle Devonian (Givetian) age, its age is considered to be Middle Devonian.

FIGURE 5.—Lost Burro Formation on Hidden Valley Dolomite, northwest side of Bat Mountain block. DSh, Hidden Valley, containing Lower Devonian (upper Emsian) fossils near top; Dll, Lippincott Member of Lost Burro; S, Stringocephalus-bearing dark dolomite. The lowest part of composite measured section of the Lost Burro extends from base of the Lippincott Member to bed at X.

Unit 1b2 above the Lippincott Member consists of light-gray and very light gray dolomite containing some poorly defined but distinctive layers of medium-gray to dark-gray dolomite. The thickness is 675 feet in the composite section on the Bat Mountain block (figs. 5, 6). Two persistent sets of darker dolomite beds are in the middle of the unit and about 45 feet from the top; below them, several dark beds generally 1-5 feet thick are widely spaced in the lighter dolomite. The lower dark set is 100 feet thick, including 10-25 feet of light-gray dolomite about 15 feet below the top. The upper dark set is about 60 feet thick. Both sets contain silicified hemispherical stromatoporoids, brachiopod fragments, and broken-spaghettilike forms that are possibly Amphipora?, as in collection 7278—SD (W. A. Oliver, written commun., 1964). Stringocephalus sp., identified by J. T. Dutro, Jr. (written commun., 1964), occurs in the upper set of dark beds (colln. 7275—SD, pl. 1), and 55 feet lower in a 3-foot-thick bed of medium-gray dolomite (colln. 7277—SD); these beds are 845 and 790 feet above the base of the formation. Merriam's Stringocephalus zone in the Great Basin is a widespread indicator of late Middle Devonian (Givetian) age (Merriam, 1940, p. 58-59; 1963a, p. 14-17; 1963b, 45-49; Poole and others, 1967, fig. 2a—b, p. 901-902).

FIGURE 6.—Lost Burro Formation capped by Tin Mountain Limestone, Mt, southwest side of Bat Mountain block. S, Stringocephalus-bearing dark dolomite; F, dolomite marker containing low Upper Devonian (Frasnian) fossils. Composite measured section of the Lost Burro continues upward from bed at X.

The third unit (1b3, pl. 1), 300 feet thick, is a transitional sequence between underlying predominantly light-gray dolomite and overlying medium-gray limestone. The unit, which mostly consists of dolomite and limestone, is set apart by less resistant beds of silty and sandy dolomite at the bottom and platy and silty limestone at the top. Silt and fine-grained sand generally are dispersed in the lowest 50 feet but form some laminae at the base of the unit. At 190 feet above the base, three fine-grained sandstone beds, 2, 5, and 7 feet thick, are interbedded with medium-gray limestone in a zone 25 feet thick. The silty and sandy rocks weather light brown to pale orange, and some of the dolomite weathers very pale yellowish brown or yellowish gray, although most of the dolomite, like the limestone, is medium gray where weathered or fresh. A few of the limestone beds contain amphiporoids and small hemispheroidal stromatoporoids. Two beds, at 45 and 70 feet below the top of the unit, contain brachiopod sections and silicified fragments; those that were collected are indeterminate (J. T. Dutro, Jr., written commun., 1964).

The fourth unit of the Lost Burro Formation (1b4, pl. 1) is 1,130 feet thick and consists mostly of interstratified medium-gray and light-gray limestone, which is predominantly darker gray in the lower part of the unit than in the upper part. The limestone contains interbedded dolomite in two stratigraphic marker zones. Several beds of sandstone or quartzite are widely spaced in the limestone. The lowest part of the unit, extending from the base of a steep slope for 160 feet up to the lower dolomite marker zone, consists of medium-dark-gray to medium-gray limestone, which contains conspicuously abundant amphiporoids and massive stromatoporoids. In the lower dolomite marker zone, which is 35 feet thick, dolomite and dolomitic limestone beds 1-2 feet thick are lighter gray than interbedded stromatoporoidal limestone, and they weather yellowish gray to pale grayish orange, making distinctive stripes on the mountainside. About 290 feet higher, the upper dolomite marker zone (F, fig. 6), which is 80 feet thick, is less conspicuously colored than the lower one. Much of the dolomite is medium dark gray, like the interbedded limestone, although some is medium light gray, yellowish gray, or very pale orange. Dark dolomite and limestone in the lower part of the marker zone contain well-preserved fossils in a varied assemblage. Four quartzose zones are widely spaced between the dolomite zones, and two are within 145 feet above the upper dolomite zone. The quartzose zones, which are 1/2—5-1/2 feet thick, contain fine to coarse quartz sand in very sandy limestone, calcite-cemented sandstone, and quartzite. They are from medium light gray to nearly white where fresh and weather light brown or paler. The highest of these zones is the thickest, 5-1/2 feet; it grades upward from quartzite in the lower half, through sandstone, to irregularly sandy limestone at the top. In the upper 500 feet of the unit (1b4), the limestone is generally light gray or very light gray but contains unevenly spaced darker layers lower than about 200 feet below the top.

Except for abundant poorly preserved Amphipora? and massive stromatoporoids, fossils have been found at only a few horizons in thick unit 1b4, and none below the upper dolomite marker zone are useful for placing the boundary between Middle and Upper Devonian rocks. Stromatoporoids, including amphiporoidal forms, are conspicuous in the lowest 230 feet of the unit and occur as high as the upper marker zone. Outlines of unsilicified horn corals occur about 20 feet and about 85 feet above the lower marker. According to W. A. Oliver, Jr. (written commun., 1964), well-preserved horn corals (colln. 7272—SD) that are 140 feet above the lower marker are apparently of a single species, and they are associated with Amphipora sp., a form that strongly suggests a Middle or early Late Devonian age. He adds that the same species of corals is perhaps the common form in collections (7269—SD, 7270—SD) from the upper marker. Traces of high-spired gastropods are common in a 35-foot interval at approximately 10 feet below the top of the unit.

Fossils in the lower part of the upper dolomite marker zone are varied and persistent in the area of outcrop. Collections of them from several localities contain a gastropod species and a brachiopod species that merit comment. E. L. Yochelson (written commun., 1964) reports "Orecopia" ambiguum (Walcott) in collections 7268—SD and 7269—SD, which bracket fossiliferous beds in a 20-foot interval (pl. 1). He favors assigning to these collections an early Late Devonian age rather than a Middle Devonian age. He expresses this conclusion in his following comments on "Orecopia" ambiguum:

The problem with this species is somewhat complicated. Walcott described two species of 'Platyschisma' from the Eureka area. P. mccoyi was obtained from Newark Mountain, and P. ambiguum from the upper horizon of the Devonian at Devils Gate. In 1945 Knight named the genus Orecopia with P. mccoyi as type. Along with the original types Knight figured a specimen from the Goodsprings area. Ever since, essentially any small gastropod in a Devonian limestone from Nevada or Utah has been called O. mccoyi and dated as early Late Devonian.

* * * there are at least three different forms in the Devonian limestones. The Goodsprings form is undescribed and is almost certainly the same as two specimens which [F. J.] Kleinhampl submitted from the lower part of the Guilmette Limestone in Pancake Range of Nevada. It presumably is of Middle Devonian age in that area. So far as I know the species ambiguum has never been identified since Walcott's original description. It is an open question as to how many O. mccoyi which have been cited in past years belong to ambiguum or to the undescribed species. It is also an open question as to whether all three species should be assigned to Orecopia although they are all related.

According to Miss Jean Berdan it is probable that Walcott's locality for ambiguum (what an appropriate name) refers to rocks of early Late Devonian age. Thus I think that collections 7269—SD and 7268—SD are of that age rather than Middle Devonian. The situation is such that additional material from other areas might have an effect on this age determination.

On referring to the highest collection (7268—SD), which contains "O." ambiguum, a collection (7270—SD) about 10 feet stratigraphically lower at a nearby locality, and a collection (7473—SD) from the same horizon at about 1-1/4 miles from the first, J. T. Dutro, Jr. (written communs. 1964 and 1968), reports "Tenticospirifer" cyrtiniformis (Hall and Whitfield). "The occurrence***indicates an early Late Devonian (Frasnian) age. This species occurs in the Mt. Hawk Formation of the Alberta Rockies and in the Hackberry beds of Iowa." He suggests correlation with some part of Merriam's Spirifer argentarius Zone of Late Devonian (Frasnian) age, in the middle part of the Devils Gate Limestone (Merriam, 1963b, p. 53, 54; Poole and others, 1967, fig. 2b, p. 902).

The uppermost unit of the Lost Burro Formation (1b5, pl. 1), 95 feet thick, is diversified in color and composition. Its colors include shades of gray and a more distinctive range from moderate brown, through moderate orange pink, to very pale orange. The unit is composed of limestone, dolomite, and distinctive siliceous clastic rocks. The siliceous clay, silt, and sand are dispersed in some of the carbonate rocks and form beds of mudstone, sandstone, or quartzite; these beds are thin and widely spaced in the lower part of the unit and predominant in the upper third. The lowest sandstone bed, 1-1/2 feet thick and weathering very pale orange, marks the base of the unit. The overlying two-thirds of the unit, composed mostly of gray limestone, contains colorful beds, 1-3 feet thick, of dolomite that is light brownish gray where fresh but weathers grayish orange to very pale orange. In a 35-foot interval at the top of the unit, two thick beds of brown-weathering sandstone and quartzite that are separated by generally concealed beds correspond to the Quartz Spring Sandstone Member of Langenheim and Tischler (1960, p. 92). Calcite-cemented quartz sandstone and vitreous quartzite intergrade in various proportions in these beds. In general, the lower bed, 10-15 feet thick, consists mostly of quartzite that weathers brown; and the upper bed, 5-8 feet thick, consists mostly of sandstone that weathers a darker brown. The intervening weak beds include, along with thin-bedded shaly, silty, and sandy limestone, some thicker mudstone that is light gray tinged pink or orange. A small sample (colln. 8878—SD) from these beds, between 15 and 20 feet below the top of the formation, contains six fragmentary conodonts and a fish tooth, according to J. W. Huddle (written commun., 1964). He reports the following:

One of the bar-type conodont fragments probably is part of an Hindeodella sp. The other two bar fragments are unrecognizable. Three conodont fragments seem to be parts of three specimens of Polygnathus linguiformis. One of these fragments represents the posterior, cross-ribbed tip of the plate, and the identification of this piece is reasonably certain. Polygnathus linguiformis indicates a Middle Devonian to Early Late Devonian age for the sample.

If the few fragments of conodonts in the sample were not reworked, they would indicate an age no younger than the Frasnian age of beds that are 700 feet lower in the section (p. 00). No additional conodonts were obtained by further cursory sampling of the argillaceous beds near the top of unit 1b5. About 25 feet stratigraphically higher, however, Early Mississippian (Kinderhookian) conodonts (J. W. Huddle, written commun., 1964) occur in the Tin Mountain Limestone about 5 feet above the boundary with the Lost Burro.

A better indication of the age of the upper part of unit 1b5 is derived by correlation. Very near the type locality of the Lost Burro Formation, in the northern Panamint Range, the uppermost unit is fossiliferous and lithologically equivalent to the upper part of unit 1b5. The unit contains within 35 feet below the Tin Mountain Limestone, according to Cooper (McAllister, 1952, p. 19), a Cyrtospirifer fauna of Late Devonian (Cassadaga) age. Among conodonts from the interval 18-20 feet below the Tin Mountain, Palmatolepis distorta Branson and Mehl supports a Late Devonian (toIIβ-toIIIβ, Famennian) age (Youngquist and Heinrich, 1966, p. 975).