Stratigraphy and Sedimentology of the Wood Canyon Formation, Death Valley Area, California
Nolan (1924) named the Wood Canyon Formation and described it from exposures in the northwest part of the Spring Mountains, Nevada. Hazzard (1937) extended the use of the name to the southern Nopah Range (Fig. 1), where he measured a section of Precambrian to Cambrian strata including those of the Wood Canyon Formation. Later, Stewart (1966, p. C71) divided the formation into lower, middle, and upper informal members.
Still more recently, Stewart (1970), in a systematic regional stratigraphic study of the Wood Canyon and associated formations of the southern Great Basin, California and Nevada, presented measurements of cross-strata dip directions that he interpreted as indicative of a west- to northwest-dipping paleo-slope. The depositional environments of the Wood Canyon, however, remain relatively unstudied.
Although the general physical stratigraphy of the Wood Canyon Formation has been well established, the sedimentary and tectonic environments in which it was deposited are still little understood, and are the subject of the present investigation. Included in this study are observations of small-scale primary sedimentary features within the formation.
The Wood Canyon Formation is of particular interest because it has diverse lithologies, occupies a stratigraphic position athwart or near the Proterozoic-Paleozoic boundary, and its areal distribution is well documented. The section that contains the Wood Canyon and the underlying Stirling Quartzite, Johnnie Formation, and Noonday Dolomite in the southern part of the Nopah Range has been suspected as a possible stratotype for the Proterozoic-Phanerozoic boundary (Cloud, 1973). In addition, environmental information from this formation, combined with environmental data being gathered by other workers in the area, is useful in attempts to reconstruct the late Precambrian-Cambrian evolution of the Death Valley region.
The Wood Canyon Formation has been divided previously into three informal members that are recognizable over a large area of eastern California and western Nevada. The lower member is composed of interbedded siltstone (36 percent), thinly laminated to platy bedded, fine- to medium-grained feldspathic and micaceous sandstone (50 percent), and laminated, siliceous dolomite (14 percent). The latter occurs primarily as three sub-units in the member. The middle member contains a lower sub-unit composed of arkosic conglomerate and grades up-section into cyclically bedded subarkosic sandstone and maroon siltstone. The interbedded light olive-gray to tan siltstone (23 per cent) and dark brown to pale red weathering, fine- to very fine-grained quartzitic sandstone (49 percent) of the upper member are similar to those in the lower member with respect to distribution and primary sedimentary features. Sandstone beds are 1 to 5 ft thick, well laminated to massive with occasional cross-laminations. The dolomite (12 percent) in the upper member is contained mostly in one sub-unit, which is characteristically cyclically bedded. Distinctive millimeter-sized platelets of echinodermal debris are found in the dolomite beds.
In addition to the gross subdivision of the Wood Canyon into three members, a further subdivision into at least 18 sub-units is recognizable. (Detailed descriptions of sub-units recognizable in the southern Nopah Range measured section are shown in Fig. 2).
The lowermost 66 ft of the measured section in the Nopah Range (Fig. 2) are composed of predominantly medium- to fine-grained feldspathic sandstone (76 percent) and siltstone (20 percent) that weathers drab red-brown. In sub-unit SN-3 (Fig. 2), the sandstone content decreases up-section, and sandstone beds form discontinuous wedges and lenses. This decrease in sandstone is accompanied by an increase in siltstone. One of the three dolomite sub-units (SN-2) occurs at 66 ft. The other two are near the top of the member. Sub-unit SN-1 is featured by cyclic bedding, although the scale of Figure 2 is too small to record it. A typical cycle consists of a lower layer of sandstone that is internally massively bedded at its base and contains little mica. It is 1 to 2 ft thick and grades upward into very evenly and thinly laminated, micaceous, very fine grained sandstone. The latter forms the top one-fourth to one-half of each cycle and gives a "shaly look" to the rock that separates the massive beds.
Mudcracks, trace fossils, interference ripples, loading features, scoured surfaces, and discontinuous, convex sandstone bodies that are cross-laminated suggest shallow water deposition. Bimodal and polymodal current rose diagrams are compatible with shallow water marine, probably tidal environments for the lower member.
The basal, ubiquitously cross-laminated arkosic conglomerate (Fig. 2) composes 99 percent of sub-unit SN-10. The overlying subarkosic and feldspathic sandstone and siltstone are cyclically bedded in fining-upward sequences. These cycles range from 1 to 20 ft thick. Most of them are from 3 to 6 ft thick. The base of a typical cycle is marked by a layer of coarse- to very coarse-grained quartzitic sandstone or pebbly conglomerate that contains clasts of reddish siltstone. The siltstone clasts apparently were derived from the underlying siltstone bed with which the quartzitic sandstone or conglomerate is in sharp erosional contact (Fig. 3b). The basal layer grades upward into coarse-grained sandstone that is either planar or trough cross-laminated and featured by sets or cosets 2 to 5 in. thick. Planar cross-laminations with straight foresets are as much as 9 in. thick. The next higher layer consists of medium-grained feldspathic sandstone exhibiting complexely festooned cross-laminations and cosets of 2 to 4 in. This sandstone, in turn, grades upward into a massive to well-laminated, fine-grained feldspathic sandstone. Topping the cycle is an alternation of massive and fissile maroon siltstone. The beds within the cycle are 1 to 4 ft thick. The sandstone beds are generally thicker than the siltstone beds, which are as much as 6 in. thick.
Beds of the middle member are from 1 to 4 ft thick, but change thickness as well as color and lithology laterally. Beds of cross-laminated sandstone commonly grade laterally into siltstone, but also interfinger with siltstone beds. Some of the beds that display the lateral wedging and gradation persist for only 3 to 4 ft, whereas others persist for 100 ft or more.
The upper member is similar to the lower member in thickness and in the proportion of sandstone, siltstone, and dolomite that it contains. The sandstone becomes more quartzitic upward and also shows an upward transition from even-lamination to cross-lamination. The siltstone content increases up-section, reaching a maximum in sub-unit 18 underlying the dolomite sub-unit. Details of the sub-units, such as first occurrence of fossil hard parts and sand casts of trilobite fragments, are illustrated in Figure 2.
The dolomite sub-unit within the upper member is cyclic in nature. An ideal cycle is illustrated in Figure 3a. This sequence is variable, and one or more lithologies are commonly absent. The cycle, where fully developed, consists of a thin siltstone bed followed vertically by an evenly laminated, well-sorted sandstone that characteristically contains calcareous cement, and locally is faintly cross-laminated. This sandstone is in turn overlain by persistently cross-laminated, dolomitic sandstone in which fossils are common and that grades upward into a cross-laminated to wavy-laminated dolomite displaying abundant quartz laminations and lenses. The sequence is capped by a massive, "elephant hide"weathering, fine-grained dolomite. Cross-laminations in the dolomite are commonly trough shaped, but planar cross-laminations are also abundant. The sequence is commonly 8 ft thick, and the dolomite beds are as thick as 5 ft. The sandstone and siltstone beds, however, are usually less than 1 ft. thick. The siltstone disappears from the sequence upward through the dolomite unit (Fig. 2). The dolomite also contains distinctive platelets of echinodermal debris, which serve to distinguish the upper member dolomite unit from the other dolomite beds both within the Wood Canyon Formation and in the overlying and underlying formations.
Cross-lamination measurements were recorded with close reference to their stratigraphic positions so that vertical variations in their orientation, discussed below, could be detected within each stratigraphic section. The lateral variation of preferred orientation of paleocurrent indicators could be investigated by comparing their trends from section to section within the Death Valley area.
Analysis of the orientation data gathered so far reveals a consistent vertical variation of paleocurrent orientation within each stratigraphic section. This variation can be correlated with the succession of sub-units defined above (Fig. 2). The bimodal and polymodal frequency diagrams of cross-strata measurements taken in the lower member are consistent in each section measured (Fig. 4a) and suggest a northwest-southeast paleocurrent orientation.
In marked contrast to the lower member, the paleocurrent indicated by current rose diagrams taken from the arkosic conglomerate at the base of the middle member, is to the southwest (Fig. 2, SN-10; Fig. 5a). This nearly 90° change in paleocurrent occurs abruptly at the contact between the lower and middle members. Up-section in the middle member, cross-lamination orientations show a re-establishment of a northwest-southeast paleocurrent. This change is gradual; the frequency modes of paleocurrent direction measurements from sub-units in the remainder of the middle member show an up-section clockwise rotation.
Polymodal and bimodal cross-lamination frequency diagrams plotted from data taken in the upper member again reflect a northwest-southeast paleocurrent as defined in the lower member.
Sub-units recognized within the southern Nopah Range can be recognized in sections throughout the Death Valley area, particularly if sub-units five, six, and seven of the lower member are grouped into one, thus leaving seven regionally recognizable sub-units of that member. In general, these sub-units occur with little lithologic variation throughout the Death Valley area, so that distinct lateral facies are not generally recognizable. The current-direction indicators that are measured from correlative sub-units in various stratigraphic sections (Fig. 1) yield consistently oriented paleoslope indicators except where tectonically disturbed (Figs. 4a, 5a). The same vertical pattern of paleoslope variation described above can generally be detected in sections thus far measured.
Although the nearly 90° rotation of current direction indicators up-section is clearly demonstrated in the Mclain Peaks section, the rose diagrams there are anomalous, as they suggest a northward paleoslope for the lower and upper members and a northwest paleoslope for the arkosic conglomerate at the base of the middle member. The differences between these orientations and the others already described seem best explained by postdepositional tectonic rotation that occurred about a vertical axis and was related to the strike-slip movement of the Sheephead fault, which apparently passes immediately north of the peak.
The dolomite sub-units of the lower and upper members show gradual thickening to the northwest. The dolomite sub-unit of the upper member in the south Salt Spring Hills is 20 ft thick, whereas in Mosaic Canyon, at the north end of the Panamint Range, the correlative sub-unit is 340 ft thick. The three dolomite sub-units of the lower member are recognizable in each of the sections measured (except in the Salt Spring Hills, where the lower member is absent). They show a progressive thickening from southeast to northwest. These thickness trends are demonstrated in Figure 4a and b where thickness of the dolomite sub-units is plotted against distance from a line placed approximately parallel to the strike of the paleocurrent indicated by current rose diagrams for the two members. This line is positioned south of the Salt Spring Hills, where the lower member of the Wood Canyon Formation is absent.
Both the author's data and those of Stewart (1970) indicate that the lower member of the Wood Canyon Formation gradually thins to the southeast and disappears in the area of the Silurian Hills (Fig. 4b). In that area and to the southeast, conglomeratic arkose and arkosic sandstone of the middle member rest upon the Stirling Quartzite. Farther southeast, the Stirling disappears and the Wood Canyon correlative, the Tapeats Sandstone, lies directly upon older Precambrian gneiss and schist (Stewart, 1970, p. 39). This suggests a high to the southeast that is consistent with the paleocurrent indicators for the upper and lower members and is also compatible with the northwest thickening of the dolomite sub-units.
In contrast to the northwestward thickening of dolomite in the lower and upper members, no thickness trend is apparent in the middle member when it is considered in total. However, when the thickness of the arkosic conglomerate unit is plotted against distance southwest from an arbitrarily defined line which is parallel to the average strike of the paleocurrents indicated by the arkosic conglomerate rose diagrams, a marked thinning of sub-units to the southwest is obvious (Fig. 5b). Maximum diameters of pebbles measured in the arkosic conglomerate sub-unit at each section also decrease to the southwest (Fig. 5c).
Bedding features and trace-fossil content, as well as current rose patterns mentioned above (Figs. 2, 3, 4a, 5a) suggest a tidal marine environment for the Wood Canyon Formation.
The vertical changes in paleocurrent directions demonstrable within single stratigraphic sections are consistent regionally and indicate a change in paleoslope during deposition of Wood Canyon strata. That the paleoslope was northwest during deposition of most of the formation is supported both by cross-lamination frequency diagrams and by the northwest increase of dolomite thickness and content in the upper and lower members. The northwest thickening of dolomite units, which is interpreted as a basinward increase in carbonate content, the existence of a southeastern high indicated by the gradual southeasterly disappearance of the lower member, and the paleocurrent data are consistent with a northwest paleoslope during the time of deposition of lower and upper Wood Canyon strata.
On the other hand, the southwestward thinning of the arkosic conglomerate, the decrease of maximum pebble diameter, also to the southwest, and predominantly unimodal paleocurrent frequency diagrams point to a southwest paleoslope during deposition of that coarse sediment sub-unit.
Thus, the data indicate a sharp change in transport direction from northwest to southwest at the base of the middle members with a gradual re-establishment up-section of the northwest trends shown in the lower member. This change in slope is intimately associated with the deposition of the arkosic conglomerate at the base of the middle member and is interpreted as indicating tectonic activity at that time. The arkosic sediments were probably derived from a granitic terrain to the east-northeast. The overlying and underlying beds may then well represent early stages of the Cordilleran miogeosynclinal development with paleoslope here to the northwest.
Cloud, P. E., 1973, Possible stratotype sequences for the local Paleozoic in North America: Am. Jour. Sci., v. 273, no. 3, p. 193-206.
Hazzard, J. C., 1937, Paleozoic section in the Nopah and Resting Springs Mountains, Inyo County, California: California Jour. Mines and Geology, v. 33, p. 273-339.
Nolan, T. B., 1924, Geology of the northwest portion of the Spring Mountains, Nevada [Ph.D. thesis]: New Haven, Conn., Yale Univ.
Stewart, J. H., 1966, Correlations of Lower Cambrian and some Precambrian strata in the southern Great Basin, California and Nevada: U.S. Geol. Survey Prof. Paper 550C, p. C66-C72.
______ 1970, Upper Precambrian and Lower Cambrian strata in the southern Great Basin, California and Nevada: U.S. Geol. Survey Prof. Paper 620.
This research was supported by NSF grant GA16119.
Last Updated: 24-Jul-2009