The Noonday Dolomite and Equivalent Stratigraphic Units, Southern Death Valley Region, California
Eugene G. Williams,1 Lauren A. Wright,1 Bennie W. Troxel2
The Noonday Dolomite of the Death Valley region of eastern California is generally assigned a Precambrian age because it lies as much as 1,200 m stratigraphically beneath the lowest beds that have yielded Lower Cambrian fossils. It was defined by Hazzard (1937) from exposures near the Noonday mine at the southern end of the Nopah Range. It rests upon various units of the late Precambrian Pahrump Group and also upon a pre-Pahrump complex of crystalline rocks that has yielded radiometric dates as old as 1.7 b.y. Most of the exposures of the Noonday, including those described herein, lie within an elongate area, about 50 km wide, that extends from the southern part of the Panamint Range east-southeastward for about 130 km. The Noonday also has been traced discontinuously northward in the Panamint Range to the vicinity of Tucki Mountain (Hunt and Mabey, 1966), but these more northerly occurrences remain relatively uninvestigated.
In the northern and eastern part of the east-southeast-trending area of exposure, the Noonday consists almost entirely of dolomite. As it contains abundant structures of algal and cryptalgal origin, it qualifies as a shallow-water platform deposit. It passes southward and very abruptly, however, into a deeper water facies of clastic carbonate, breccia, siliceous sandstone, and shale. Because these lithologies are totally unlike those of the Noonday Dolomite as originally defined, this facies warrants and will eventually be given a new formational name. But for the purpose of this report, it will simply be referred to as "the basin facies" and will be divided informally into four members.
In the east-southeast-trending belt of exposure, and for an undetermined distance northward in the Panamint Range, the Noonday Dolomite or "platform facies" comprises two well-defined members. Each has been previously noted and mapped (Wright and Troxel, 1966, 1967; Wright, 1973) but has remained unstudied in detail. These two members will be referred to as the lower and upper dolomite members or as the platform dolomite members. In most of the present exposures of the Noonday Dolomite, the two members display a combined thickness within the range of 60 to 400 m. They terminate southward, generally within a distance of several tens of meters, against units of the basin facies in the manner shown in Figures 2, 3, and 5.
The lower algal dolomite is very finely crystalline and laminated. It is pinkish gray to grayish orange on fresh surfaces and weathers to very light gray or grayish orange cliff-forming exposures. The laminations, which commonly are obscured by minute fractures, consist of alternating light and dark layers. Discontinuous accumulations of banded cherty dolomite are common in the upper part of the member. Elsewhere the lower dolomite contains less than 1 percent of insoluble material.
Specimens of the lower dolomite member, as observed in thin section, consist of dark cryptocrystalline dolomite and dolomite spar in varying proportions, and exhibit a spongelike texture. The dark material commonly comprises spherical bodies distributed in a cement of dolomite spar. Variations in the percentages of these two materials account for the laminations.
At many localities, and especially those within 2 km of the platform-basin facies change, the lower dolomite member forms concentrically banded mounds as much as 200 m high and 600 m long (see sketch by Wright and others, 1974b, this volume). These can be interpreted only as algal growth structures and will be described in detail in later publications. The observation that the mound-forming dolomite is identical in physical and chemical characteristics with the more evenly structured occurrences of the member, together with the presence in the latter of smaller scale structures that are apparently of algal origin, has led us to view the entire member as having formed in the presence of algae.
The between-the-mound low areas in the paleotopographic depressions outlined by the top of the lower algal dolomite are filled with onlapping younger strata in sharp contact with the underlying dolomite. These compose the lowest part of the upper member and ordinarily consist of dolomite with wavy laminations. Some of the larger low areas, however, are filled with deposits of very evenly laminated rock, composed of interbedded finely crystalline clastic dolomite and dolomitic siltstone, which grades upward into the wavy, laminated dolomite. The clastic dolomite and siltstone are generally dark gray on fresh surfaces, but weather to various shades of red, orange, and yellow. Even where the mounds are absent or only a few feet in relief, such strata commonly directly overlie the lower algal dolomite.
The upper dolomite member, apart from the discontinuous bodies of evenly laminated rock, is generally from 45 to 90 m thick. It is a fine-to-medium crystalline pale gray dolomite, which weathers to light brownish gray and persistently contains from 2 to 10 percent of insoluble residues, consisting of chert, clay, and silt-size quartz. The upper dolomite is less resistant than the underlying member. The lower half consists of internally laminated beds, 2 to 4 cm thick, and commonly displays wavy structures that are generally 10 to 20 cm in amplitude and cuspate to undulating in cross section. Some are clearly cross-laminations and others are slump folds, but most seem best interpreted as laterally linked algal stromatolites.
The upper half of the upper dolomite member consists mostly of interbeds, 20 to 40 cm thick, of dolomite of two types; (1) massive, medium crystalline, and (2) laminated and banded finely crystalline. Distributed through this unit, although more abundant near the top, are moundlike structures with amplitudes of 20 cm to 5 m. Some of these are clearly algal stromatolites of hemispherical form. Other structures are less regular and commonly are surrounded by dolomite clasts. We view these as cryptalgal, yet recognize the possibility that they may be caused by slumping or differential loading.
This mound-forming dolomite high in the upper member is overlain by and intertongued with an intensely cross-laminated dolomitic quartzite unit, which marks a transition into the overlying Johnnie Formation. In the 15 to 30 m of sandstone in which this gradational contact is ordinarily recorded, and in the sandstone that intertongues with the upper dolomite, the quartz grains vary in size from medium to very coarse, and are well rounded, although poorly sorted. The sandstone contains both igneous and metamorphic types of quartz; feldspar in proportions of 5 percent or less also is present and consists of albite, microcline, and orthoclase.
The generally southward change from the Noonday Dolomite to the basin facies is so abrupt that the transition is represented in Figure 1 by single lines rather than by gradational symbols. South of these lines the two members of the Noonday are replaced by generally thicker clastic units of the basin facies which generally thicken abruptly. The abruptness of these facies and thickness changes is strongly indicative of vertical movements on fault-bounded blocks. Indeed, through-going faults that were active in Cenozoic and earlier timethe Butte Valley fault in the Panamint Range and the Sheephead fault in the Black Mountainslie parallel with and close to the lines of facies change. In addition, abrupt variations in the thickness of the platform facies are controlled by vertical movements on contemporaneous faults, some of the faults being detectable in present exposures (Wright and others, 1974b, this volume).
The basin facies which generally exceeds 300 m in thickness and has a maximum measured thickness of 488 m, is divisable into four members. In upward succession they are here named (1) the basal breccia member, (2) the arkose member, (3) the clastic carbonate member, and (4) the quartzitic clastic dolomite member.
The basal breccia member, which ranges from 3 m to 10 m or more thick in its observed occurrences, is characterized by a polymict breccia featured by gigantic clasts derived from the lower dolomite member of the platform facies and from conglomerate of the Kingston Peak Formation.
Well and evenly laminated dolomite, similar in lithology to the lower dolomite member, commonly forms a thin layer, no more than a meter or two thick, beneath the breccia. It marks a much thinned and basinward extension of the members. Well laminated dolomite also forms masses within the breccia. There the laminations are very irregular and appear to have been deformed contemporaneously with the transportation and deposition of the other components of the breccia. The basal breccia member has been observed only in a northwest-trending belt that lies with 1 km of the southward termination on the zone of marked thinning of the lower dolomite member (Figs. 1 and 5), and is especially well exposed in the southern part of the Ibex Hills (loc. 10, Fig. 1). Elsewhere the arkose member forms the lowest unit of the basin facies. We interpret the breccia as deposited at the base of the platform-basin slope and composed mostly of material dislodged from the slope.
The most complete, easily accessible, and continuous exposures of the other three members of the basin facies occur in the southern part of the Black Mountains immediately north of the highway between Jubilee Pass and the Ashford Mill site (loc. 11 in Fig. 1; Fig. 2), but they also are well exposed at numerous other localities extending from the southwestern part of the Panamint Range south-southeastward to the Silurian Hills.
The arkose member generally consists of arkose and feldspathic siltstone and shale. It forms a north-tapering clastic wedge with a maximum observed thickness of about 250 m. Its disappearance northward ordinarily occurs slightly north of the southern termination of the lower platform dolomite, the arkose thinning as the underlying dolomite thickens. Thus, when traced to its most northerly exposures, it generally can be observed to rest successively upon the breccia and the lower algal dolomite. In most places, however, the arkose member rests upon the Kingston Peak Formation.
In the thickest occurrences of the arkose member, the lower part consists of purple siltstone, shale, and limestone. These pass upward into a sequence of arkose and shale beds that are characterized by graded bedding and by bottom markings that indicate current directions from south to north. The arkose ranges from medium to very coarse grained and is very poorly sorted. It consists of approximately equal proportions of quartz and feldspar, the quartz grains being angular and the feldspar grains highly altered. The thinner, more northerly occurrences of the member consist of purple arkosic siltstone interbedded with argillaceous dolomite beds that weather yellowish brown.
The clastic carbonate member is 190 m in maximum thickness, and consists of varicolored interbeds of dolomite, limestone, and shale that are generally 2 to 30 cm thick and laminated. Close to the platform margin, however, the carbonate unit contains conglomeratic layers with angular fragments of limestone, limy dolomite, and dolomite. Some of the clasts are as much as 15 m long.
Both the bedded carbonate and the clasts of the carbonate-bearing conglomeratic strata weather reddish brown, yellowish brown, and lavender. The content of insoluble residues in the bedded carbonates ranges from 10 to 60 percent and consists of silt-size quartz and chert, illite, and minor kaolinite. A very small proportion of the beds contains well-rounded medium-grained quartz. Some of the beds exhibit contorted laminations that suggest contemporaneous, soft sediment deformation.
The uppermost member of the basin facies, which is as much as 460 m thick, consists of clastic dolomite mixed with 10 to 50 percent of coarse, well-rounded floating grains of quartz and very minor proportions of feldspar. Most of this member qualifies as quartzitic clastic dolomite. Occurrences that lie close to the line of facies change commonly consist of coarse breccia with clasts apparently derived, in part, from the upper platform dolomite. Some of the clasts are intraformational, consisting of the quartzitic clastic dolomite. Beds are generally within the range of 30 cm to 1 m thick. Most of them are devoid of internal structure but cross-bedding is locally present. In many outcrops, bedding is difficult or impossible to detect. The quartzitic clastic dolomite is gradational with the overlying transitional member of the Johnnie Formation, in which cross-bedding is abundant and well preserved.
The regional stratigraphic relations between the two members of the Noonday Dolomite and equivalent strata of the basin facies are illustrated in Figure 3; the boundaries between the two are shown as lines in Figure 1. Observe that the southern limits of the lower and upper dolomite members of the Noonday are divergent in the eastern two-thirds of the region and coincident in the western third. These lines are interpreted as being controlled primarily by vertical movements along faults and contemporaneous with sedimentation. The faults, thus determining the trend of the basin and the adjacent platform.
Verification of the configuration of the northern edge of the basin boundary is independently obtainable through analysis of cross-bedding in the dolomitic quartzite of the overlying transitional member of the Johnme Formation. A total of 360 measurements were made at the 17 localities shown in Figure 4. The south zero isopach of the upper dolomite marks the platform edge in upper Noonday time. The strike of the paleoslope during the deposition of the lower Johnnie strata, estimated from cross-bedding rosettes, is closely parallel to the trends of the isopachs. The tendency for modal directions in the lower part of the transition member to trend both platformward and basinward probably reflects tidal influences. The generally southwest directions shown by cross-bedding in the higher parts of the transition member may reflect influence of fluvial processes.
The various observations cited thus far are integrated into the composite sequential diagram in Figure 5. In stage one, laminated algal limestone was deposited during transgression of sediment-free marine waters across the beveled fault blocks upon which the Pahrump Group is preserved. Vertical movements on these faults in Pahrump time influenced the thickness and lithologies of the various Pahrump units (Wright and others, this volume). Relief on the surface was generally low, but hills as high as 300 m marked the position of a resistant unit of conglomerate in the upper part of the Kingston Peak Formation. During this first stage of Noonday deposition, the more northerly blocks were depressed slowly, with respect to sea level, and the algal mats grew evenly. We interpret the algal mounds as characterizing blocks that were being depressed less slowly, and that algal growth was significantly favored on highs of the basement topography and retarded on lows. The differential relief of highs and lows was increased by different rates of algal growth as depression proceeded. Blocks that were depressed to still greater depths contain very thin occurrences of the lower dolomite member, or none at allthe growth of the algal mats apparently being inhibited or prevented by insufficient sunlight.
Stage two began with the abrupt ending of the deposition of lower algal strata throughout the platform area, the paleotopography of the surface of that unit being preserved in the sharp contact between it and the overlying upper dolomite. Because this contact is equally sharp throughout the platform area, regardless of the thickness of the lower dolomite member or the presence or absence of the mounds, it seems attributable to a regional event, tectonic or eustatic, that caused a change in sea level. It probably represents a deepening of the ocean floor that killed the mat-forming algae, as no evidence of erosion has been observed even on the highest parts of the largest mounds, and the very evenly laminated silty dolomite and siltstone that fill some of the interinound areas are devoid of algal structures and of features attributable to waves or currents.
The coarse breccia at the base of the basin facies formed after much, if not all, of the lower dolomite member was deposited, as it contains large clasts of that unit as well as much debris from conglomerate of the Kingston Peak Formation, which, immediately north of the basin margin, underlies the lower dolomite member. The breccia probably records a further tectonic deepening of the basin, somewhat before or at the beginning of stage two, steepening the basin margin and favoring the accumulation of the breccia, through submarine sliding, on slopes at the base of the escarpment. The debris from the Kingston Peak could have been derived only from the escarpment, because to the north of the basin margin all older units were sealed beneath the platform cover of dolomite.
The arkose member rests upon basal breccia and lower algal dolomite near the northern margin of the basin and upon the Kingston Peak Formation farther south. Its superjacent relation with the breccia and lower dolomite along the basin margin, however, indicates that some and perhaps most of the arkosic unit was deposited during stage two.
The arkosic sediment provides a record of the elevation and erosion of a granitic land area to the south. Although the granitic terrane has been obliterated by later deformational and igneous events, its southerly location can be inferred from the following observations: (1) bottom markings on arkose beds indicate sediment transport was from south to north; (2) the crystalline basement to the north was sealed by the Noonday carbonate rocks; and (3) the Noonday carbonate and detrital rocks on the shelf contain very little feldspar.
Later, argillaceous limestone and shale were deposited in stage two in the basin, their formation being interrupted by debris flows and turbidity currents that carried broken material from the platform margin and platform-basin slope and deposited it near the base of the slope. The occurrence in the basin deposits of fragments and blocks of limestone, limy dolomite, and dolomite suggests that the algal reef had been partly dolomitized prior to the breaking away of the fragments.
The return, during stage two, of an environment favoring the growth of algae on the platform is recorded in the upper dolomite, but the original limestone of this unit was deposited during an influx of clay and quartz silt. We interpret the wavy structures as forming in the presence of sediment-entrapping algae. The final event, illustrated in stage 3 in Figure 5, is marked by the influx of large volumes of quartz sand bringing the destruction of the reef-forming algal mats. By the beginning of this influx, the reef had been completely dolomitized, because only dolomite fragments are found in the resulting basin deposits of quartzitic clastic dolomite and breccia.
On the shelf, tidal currents reworked quartz and fragmental dolomite into bars and beaches, whereas in the basin, submarine currents carried the quartz and reef debris below the wave base to be deposited as submarine fans. This process continued until the water was sufficiently shallow to permit action of tidal currents. At this point the paleotopography produced by faulting was obliterated. Roberts (this volume) and Diehl (this volume) have presented evidence that the southwesterly paleoslope of the Noonday platform existed as early as the beginning of Pahrump time and continued well after the Noonday Dolomite was deposited. The close parallelism between major faults that have been active in Cenozoic and earlier time and the inferred Precambrian faults that mark the margins of the ancient basin or trough suggest periodic reactivation of ancient zones of weakness over an immense segment of geologic time.
Hazzard, J. C., 1937, Paleozoic section in the Nopah and Resting Springs Mountain, Inyo County, California: California Div. Mines Jour. Mines and Geology, v. 33, no. 4, p. 273-339.
Hunt, C. B., and Mabey, D. R., 1966, Stratigraphy and structure Death Valley, California: U.S. Geol. Survey Prof. Paper 494A, 162 p.
Wright, L. A., 1973, Geology of the SE1/4 Tecopa 15-minute quadrangle, San Bernardino and Inyo Counties, California: California Div. Mines and Geology, Map Sheet 20.
Wright, L. A., and Troxel, B. W., 1966, Strata of late PrecambrianCambrian age, Death Valley region, California-Nevada: Am. Assoc. Petroleum Geologists Bull., v. 50, no. 5, p. 846-857.
________ 1967, Limitations on right-lateral, strike-slip displacement, Death Valley and Furnace Creek fault zone, California: Geol. Soc. America Bull., v. 78, p. 933-950.
This research was supported by NSF Grant GA16119.
Last Updated: 24-Jul-2009