|Original published in D.F. Merriam, ed., 1964, Symposium on cyclic sedimentation: Kansas Geological Survey, Bulletin 169, pp. 265-274|
University of California, Los Angeles, California
Marine Triassic sediments in the Lost Cabin area of Clark County, Nevada, exhibit repetition of a distinctive sequence of lithologies. From bottom to top a generalized repetition consists of the following units: (1) massive limestone beds with abundant Pentacrinus columnals, (2) calcareous shale and mudstone, (3) limestone in which the stratification has been markedly disturbed by mud-ingesting, burrowing organisms, but containing few if any Pentacrinus columnals, and (4) calcareous shale and mudstone. This sequence and slight variations of it recur throughout the section.
As used here, the term "repetitive" means either recurring sequences of lithologies, separated by varying thicknesses of rocks not included within any particular recurrent sequence, or repetitive sequences of markedly different thickness and time value.
Previous work has indicated that Lower and Middle(?) Triassic sediments of western Clark County, Nevada, are at least in part correlative with the Moenkopi Formation at the type section along the Little Colorado River in central Arizona (Longwell, 1925; Hewett, 1931; McKee, 1954; Clark, 1957). Regionally the Moenkopi is characterized by a predominantly continental redbed facies to the east that interfingers with a marine limestone and limy shale facies to the west. At the type section the Moenkopi consists of some 400 feet of red mudstone, siltstone, and sandstone (McKee, 1954). As the unit becomes progressively more marine toward the west, the thickness increases greatly, attaining a maximum of 1,500 to 2,000 feet in southwestern Utah and southern Nevada (McKee, 1954, p. 3). Farther to the west and northwest the Moenkopi is absent, having been removed by subsequent erosion.
The section exposed in the western Spring Mountains represents approximately the western limit of exposure of typical Moenkopi. Clark (1957,1960) studied the section at Lost Cabin (Fig. 1) referring to it as Chambers Ranch; however, because Lost Cabin is the name appearing on the U. S. Geological Survey topographic sheet, it is used in this study. Clark (1957, p. 2212) discusses the Triassic sediments of this area as miogeosynclinal, distinct from the continental redbed facies of the type section and surrounding areas. He also notes that the Meekoceras zone, a zone typical elsewhere in Early Triassic sediments of the Great Basin is absent (Clark, 1960).
Various aspects of the Moenkopi have been discussed by numerous workers in recent years. However, with the exception of widely scattered described sections, little has been done in southern Nevada. Longwell (1928) and Hewett (1931) have described long sections in the Muddy Mountains, Frenchman Mountains, and Spring Mountains. More recently Poborski (1954) raised the Virgin Limestone Member of the Moenkopi Formation in the vicinity of St. George, Utah, to formational rank, thereby raising the Moenkopi to group status. Although he mentions that the other members should also be raised to formational rank, his failure to do so has created a group composed of one formation and five members not assigned to a particular formation. Consequently, the original usage of Moenkopi as a formational name is continued here. Regarding the age of the Moenkopi, Poborski (1954, p. 993) states that "Tirolites in strata about 107 feet above the top of the Virgin formation places the formation definitely in the upper part of the early Triassic and clearly suggests that some, at least, of the overlying 'members' of the Moenkopi group are Middle Triassic."
Poborski (1954, p. 991) also recognized aspects of repetitive sedimentation in the Moenkopi in the St. George, Utah area: "Individual cycles in the Virgin formation consist of an alternation of limestone and siltstone. A single cycle is bounded both below and above by an unconformity."
Acknowledgments--The authors would like to thank Mrs. Peter Kurtz for drafting the text figures. Field work for this study was partially supported by the Research Committee of the Academic Senate, University of California, Los Angeles, and by a field expenses fund of a Richfield Oil Company Fellowship. This paper is part of a continuing study of Triassic rocks in Clark County, Nevada, by the senior author.
Figure 1--Index map of Clark County, Nevada, showing location of Lost Cabin area of the Western Spring Mountains and traverses (dotted lines) along which Triassic rocks were measured.
The section studied in the Lost Cabin area is situated on the southwest limb of a large, north-trending syncline. An alternating sequence of limy shale, limestone, and minor sandstone beds is well exposed on the ridges above the cabin. Traverses for the measured sections are indicated on the index map (Fig. 1).
The completely marine section in this area suggests miogeosynclinal deposition (Clark, 1957), with terrigenous material being derived from a source to the east. Although present only a few miles to the east in the lower plate of the Keystone thrust, no redbeds occur in this area indicating a very abrupt change from cratonal to miogeosynclinal environment (Clark, 1957). This change may be further accentuated by major west to east thrust faulting in the Spring Mountains. Some 9 to 12 miles east of the Lost Cabin area, both marine limestone and red shale are present in the Moenkopi exposed on the lower plate of the Keystone thrust. Here, west of Blue Diamond, in the eastern Spring Mountains, red shale is restricted to the upper part of the unit.
Limestone and limy shale predominate throughout the Lost Cabin section although thin sandstone beds are present. Fossils are locally abundant in the limestone, but the section as a whole is only sparingly fossiliferous. The limestone units occur as strongly resistant beds increasing in thickness and number towards the top of the section (Fig. 2). Commonly the limestone is fine grained, but in some beds the grain size increases so that the rock becomes a fine breccia.
Figure 2--A, View of repetitive resistant limestone ledges and nonresistant shale and mudstone units in Moenkopi Formation, Lost Cabin area, looking west from Lost Cabin. B, View southwest from Lost Cabin, lowest part of measured section exposed at "a," upper part of measured section in gully at "b." C, Close-up of unit 27 of measured section showing irregular laminations caused by mud-ingesting organisms. Hand sledge indicates scale.
Several types of limestone, based on field criteria, can be distinguished: (1) massive-bedded, unfossiliferous or sparingly fossiliferous limestone; (2) limestone with abundant Pentacrinus columnals; (3) limestone in which the stratification has been disturbed by mud-ingesting, burrowing organisms; (4) argillaceous limestone; (5) sandy limestone, and (6) nodular limestone.
In the massive-bedded limestone (type 1) fossils are only locally abundant. With the exception of small, high-spired gastropods abundant near the top of the section, megafossils are generally poorly preserved. Terebratulid brachiopods, pelecypods, and echinoid plates and spines occur sparingly through portions of the more thickly bedded limestone, the latter commonly in association with the Pentacrinus plates.
The star-shaped columnal plates of Pentacrinus occur in such abundance in some beds that the limestone is locally a crinoidal breccia; elsewhere Pentacrinus plates are rare or absent.
The feeding paths of mud-ingesting, burrowing organisms appear as dark-gray, very fine grained blebs and meandering trails in a lighter colored, yellow-brown, coarse-grained matrix. Where the limestone is thinly bedded or laminated, these organisms have had the effect of markedly distorting the stratification. The apparent absence of echinoid spines in the beds disturbed by burrowing organisms suggests that the feeding paths are probably not directly traceable to these particular spine-bearing echinoids. In fact, no skeletal remains of the mud-ingesting organisms were found. Less resistant beds of nodular and argillaceous limestone commonly are without evidence of burrowing organisms.
Nodular and argillaceous limestone occurs as thin, weakly resistant beds. Massive-bedded sandy limestone appears rarely and is locally fossiliferous.
Shale and mudstone units occur throughout the section as distinct nonresistant units. Both are usually calcareous and apparently unfossiliferous. No distinct limits characterize either the shale or the mudstone and the two lithologies commonly grade vertically into one another. Shaly mudstone is probably the most common detrital lithology; typically it weathers into blocky chips and plates or soft chalky powder. The thickness of individual units ranges from stringers less than an inch thick between limestone beds to massive units more than 50 feet thick. Thin beds of buff-green nodular shale occur occasionally, and shades of green, brown, and red brown are the common colors for the shale and mudstone.
At regular intervals throughout the section a very distinctive and peculiar type of shale appears, the recurrence of which is one of the more striking indications that the units of this sequence are repetitive. This shale characteristically weathers into long, thin, brittle chips closely resembling pine needles. The size of these needles ranges from approximately one-half inch in length to about 3 inches. Typically the needles form in a calcareous olive-green shale, but occasionally develop in red-brown shale which grades into the olive shale. Possibly these needles are formed by the intersection of bedding planes and two or more directions of cleavage. However, shaly units immediately adjacent fail to show this unusual weathering phenomenon.
Unfossiliferous, calcareous, fine- to medium-grained sandstone occurs in a few places in the lower half of the section. The gray or light reddish-brown color of fresh surfaces weathers to a distinct dark blackish-brown. The beds are massive, thin-bedded or laminated, and shale partings are common throughout. Cross-bedding is present in some of the more massive beds. The upper surfaces of sandstone beds are commonly ripple marked or undulatory, and the contacts with overlying strata are sharp.
Dunbar and Rodgers (1957, p. 107), in discussing repetitive stratification, state that some ". . . sequences of strata that show a consistent repetition of two or more kinds of rock. . . are known to be cyclic, the product of rhythmic changes in environment. . . ." Rhythm, as defined in Webster's New Collegiate Dictionary is ". . . movement marked by regular recurrence of, or regular alternation in, features, elements, phenomena etc., hence periodicity."
Thus both "rhythmic" and "cyclic" incorporate the concept of periodicity or regular alternation. The question as to whether a sequence is cyclic or rhythmic, then, generally centers upon the degree of regularity necessary for an alternating sequence to be considered regular.
In this instance the problem is not encountered. Distinct lithologies are repeated in a definite sequence, but the depositional environments recorded by these lithologies were not periodic or regular in their distribution through time. Rather, specific environments are represented irregularly throughout this section. Consequently, the term "repetitive," while rather noncommital, is most applicable in this case. So used, the term indicates either recurring sequences of lithologies, separated by varying thicknesses of rocks not included within any specific recurrent sequence, or repetitive sequences of markedly different thickness and time value.
A generalized repetitive sequence in the Triassic rocks in the Lost Cabin area consists of the following units in order from bottom to top: (1) massive limestone with abundant Pentacrinus columnal plates; (2) calcareous shale and mudstone; (3) massive limestone within which the bedding has been markedly disturbed by mud-ingesting, burrowing organisms, and (4) an overlying unit of calcareous shale or mudstone. The distinctive "pine-needle" shale, mentioned above, occurs throughout the section but does not appear to occupy a distinctive position in successive repetitions.
An example of this repetitive sequence may be seen between units 29 and 32 on the measured section (Fig. 3). Unit 29 is a 3-foot thick unit of very fine grained, medium-gray limestone with scattered Pentacrinus columnals and small shell fragments. Large, calcite-filled casts of pelecypods and gastropods are abundant in the upper 2 inches of the unit. Above the limestone is 6 feet of light-green, chalky powder, representing poorly exposed calcareous green shale. Unit 31, overlying the shale, consists of 5 feet of massive, dark graybrown limestone weathering mottled with abundant evidence of the presence of burrowing organisms. The feeding trails appear on bedding surfaces as dark-gray, fine-grained, meandering tubes, one-eighth to one-fourth inch in width, contrasting with the light brownish-gray matrix of the surrounding limestone. These tubular structures are especially abundant in the upper part of the unit; some are nearly vertical, terminating at the upper surface of the bed. The sequence ends with a thin, calcareous green shale bed.
Figure 3--Columnar section of repetitive Triassic rocks at Lost Cabin locality. Numbers at right of graphic section refer to bed numbers in detailed measured sections.
This sequence represents in a short section the lithologic repetition that characterizes the section as a whole. Elsewhere, the repetitions are not so perfect, but the pattern of events is obvious. The section from unit 37 to unit 43 illustrates basically the same sequence, but in this case, instead of a unit of limy shale, a unit of nodular and shaly lime. stone occurs between Pentacrinus columnal-bearing limestone and limestone with feeding trails and burrows.
Units 46 and 47 represent another slight variation of the basic pattern. Here the lower limestone unit, while still containing abundant Pentacrinus columnals, also contains numerous echinoid spines and shell fragments. In addition, shale and nodular, shaly limestone form thin but conspicuous interbeds within the limestone unit. The middle calcareous shale member has been replaced by thin beds of nodular limestone. No crinoid plates occur above this nodular limestone, but tubes and feeding trails are again abundant some 6 feet higher in the section.
Similar repetitions are present between units 3 and 6 and between units 10 and 14. In the former, the upper part of unit 4, indicated as a covered interval almost certainly represents a nonresistant shaly unit. In the latter sequence, the tubes and feeding trails occur in a very limy sandstone unit, not too greatly different from the sandy limestone unit in which they occur in the lower repetitive sequence. Less well developed repetitions may be found throughout the section.
The repetition of the units is distinct and characteristic; however, the thickness of individual repetitions depends upon how many lithologic units are included within a particular sequence. The thickness of the repetitive sequences described above ranges from about 14 to 25 feet. If the entire section is considered to be repetitive and all units are assigned to specific repetitions, the thickness of the repetitions increases greatly to a maximum of about 90 feet. Divided in this manner, successive repetitions would include units 1 through 8, 9, through most of 15, and the top of 15 through 21. For the most part, this increase in thickness is due to the presence of either thick shale or limestone units at the top and bottom of particular repetitive sequences.
The basin of deposition, represented by the Lower and Middle(?) Triassic sediments of Clark County, Nevada, was marine in the western part, becoming shallower to the east. The section a few miles to the east consists of continental redbeds and marine sediments.
In the Lost Cabin area, the section is entirely marine, and beds of limy, ripple-marked and cross-bedded sandstone suggest occasional influx of clastic sediment into fairly shallow water. The alternation of shale and limestone throughout the section indicates that the environment of deposition of these two lithologies differed only slightly. The repetitive sequence described above also appears to have formed from only slight environmental variations within the basin, because thin stringers and lenses of crinoidallimestone occur within the limestone units with abundant tubes and feeding trails. However, nowhere do the two lithologies appear to grade into one another vertically. Generally the contacts between beds are sharp, and although the environmental changes represented by the different lithologies were slight, lithologic changes are quite abrupt.
Section of Lower and Middle(?) Triassic sediments, measured in Lost Cabin area, Clark County, Nevada.
|51||Limestone, massive, medium gray, coarsely crystalline with abundant silicified crinoid columnals, large echinoid spines, and small, high spired gastropods||18.7|
|50||Limestone, coarse grained, medium gray, sandy near top, with stringers of reddish-brown weathering limy sandstone, bedding planes undulating and irregular, sparse crinoid columnals||3.4|
|49||Limestone, fine grained, medium brownish gray, highly irregular nodular beds 4" to 1' thick, irregular laminae that are contorted and in places subcylindrical||6.9|
|48||Limestone, fine grained, shaly, nodular, light yellow brown with blue gray meandering tubes up to 1/2" in diameter and 4" long||5.2|
|Limestone, mottled, light yellow brown and blue gray with irregular tubes||3.0|
|47||Limestone, fine grained, brownish gray, wavy bedding, weathers to mottled appearance||1.5|
|Limestone, medium gray, coarse grained, abundant small organic particulate detritus. No crinoid columnals||4.5|
|Limestone, nodular, blue gray and brown 3-3" beds||0.8|
|Limestone, massive, coarse grained, gray, abundant silicified crinoid columnals in upper one-half of unit||3.0|
|Limestone, nodular to shaly, blue gray and brown, weathers dark reddish brown||2.0|
|Shale, nodular and limy||0.7|
|Limestone, medium to coarse grained, medium gray, abundant small silicified Pentacrinus columnals, shell fragments and echinoid spines||3.0|
|46||Shale and mudstone, green, blocky, weathers into large coarse, irregular needles||4.0|
|45||Limestone, coarse grained, medium gray with brown specks, weathers reddish brown with small rare brachiopods||1.0|
|44||Limestone, dark blue gray, laminated with thin, wavy, light yellow brown, clayey partings||25.6|
|Limestone, blue gray, nodular||1.0|
|43||Shale, green, blocky to platy, weathers into large needles||5.0|
|42||Limestone, blue gray, thick bedded nodular with irregular tubes||2.0|
|41||Limestone, blue gray, light yellow brown and dark blue gray weathering, massive, nodular with irregular tubes, not distinctly bedded or laminated||11.0|
|40||Limestone, blue gray and light brown, nodular, shaly||2.2|
|39||Limestone, blue gray and brown, shaly at top, common brachiopods and Pentacrinus columnals||1.0|
|38||Limestone, medium to dark gray lime. stone, weathers with finely laminate brown color, fine grained||2.0|
|37||Shale, green, "pine needle," needles large, up to three inches long, one-half to one-quarter inch thick, calcareous||9.5|
|36||Limestone, light yellow brown and medium blue gray mottled, more shaly at top, irregularly disposed fine grained tubular blebs||1.2|
|Shale, green, platy||0.3|
|35||Limestone, medium brownish gray, weathers medium gray and light yellow brown, fine grained, impure, nodular||1.2|
|Shale, green, platy||0.2|
|34||Limestone, gray with brown flecks, weathering brownish gray to medium gray, medium to coarse grained||1.5|
|Shale, calcareous, green, platy||0.3|
|33||Limestone, light gray, crystalline, upper surface with widely scattered, tubular fillings of medium- to coarse-grained organic debris, up to one-fourth inch wide and two and one-half inches long, meandering over bedding surface||1.0|
|32||Shale, green, calcareous||0.5|
|31||Limestone, dark gray, weathers gray brown, mottled, massive, dark gray tubular parts in light brownish gray matrix, upper and lower surfaces smooth and regular||5.5|
|30||Shale, soft, light green, blocky, weathers to soft green chalky powder, calcareous, poorly exposed||6.0|
|29||Limestone, medium gray, very fine grained, scattered crinoid columnals and small shell fragments, large, white calcite filled pelecypods and gastropods near top||3.0|
|28||Limestone, dark gray, shaly, nodular at base, disturbed bedding and laminae, stringers up to one and one-half feet long and one and one-half inches thick of light gray Pentacrinus rich limstone|
|27||Limestone, laminated, irregular alternating blue gray and light yellow brown layers, weathers into irregular platy fragments, bedding distorted irregularly throughout, intercalated layers of light gray crinoidal limestone, nodular at base with irregular dark patches and perpendicular tubular burrows||14.5|
|26||Limestone, gray, weathers light brownish gray to medium gray, crinoidal, especially in lower two-thirds, lower contact highly irregular, upper contact smooth and regular, shaly at top||2.0|
|25||Shale, poorly exposed||0.3|
|24||Limestone, medium gray, light gray weathering, upper four inches, very abundant white Pentacrinus columnals and sparse large shell fragments, lower six-inch bed of limestone breccia with irregular, angular to subrounded fragments of very fine grained homogeneous limstone||0.9|
|23||Limestone, gray, crinoidal||0.1|
|Shale, light green, soft platy, calcareous||0.3|
|22||Limestone, light brownish gray with very abundant, white Pentacrinus columnals||0.5|
|Shale, dark greenish gray, hard, platy, calcareous||0.5|
|21||Shale, dusky red, "pine-needle," needles up to one inch long, fresh shale breaks into needles two inches long||7.0|
|20||Shale, olive green, "pine-needle," needles flat sided and brittle||14.5|
|19||Limestone, gray, weathers mottled yellowbrown and dark gray, earthy, nodular, small organic detritus throughout, lenticular, sharp, but wavy upper surface||0.5|
|18||Shale, gray brown, limy, nodular below, silty above||5.0|
|17||Limestone, blue gray and light yellowbrown mottled, impure, grades upward into nodular, limy shale with light blue gray patches and blebs of limestone, bedding distorted, sparse small brachiopods and gastropods||1.5|
|16||Shale, green, slaty, poorly exposed||0.5|
|15||Limestone, gray, abundant, large, white Pentacrinus columnals, ripples on upper surface||0.2|
|Sandstone, gray, weathers dark red brown to black, fine grained, limy, shaly partings, upper surface sharp but wavy, shale at base||3.4|
|14||Sandstone, medium gray, reddish brown or black weathering, massive, medium grained, wavy bedded in one- to four-inch beds||1.9|
|Sandstone, brownish gray finely laminated, weathers dark brownish black, limy, fine grained, with shaly partings, upper six inches cross-bedded, upper surface wavy||2.0|
|12||Shale, green, platy||0.5|
|Limestone, greenish brown weathering, shaly, nodular; lower six-inch bed, crystalline, brown weathering, poorly fossiliferous||1.5|
|11||Sandstone, dark gray to blue gray, weathers light reddish brown to dark blackish brown, thin bedded or laminated, limy, fine to medium grained, ripple marked||6.0|
|10||Limestone, dark gray to medium gray, weathers dark brown to gray brown; lower three-fourths of unit sandy, medium grained; upper one-half foot of unit shaly, crinoid columnals, sparse echinoid spines and small shell fragments||2.3|
|9||Mudstone and shale, greenish gray, calcareous||1.5|
|Limestone, gray, impure, shaly and sandy||1.0|
|Mudstone, green, massive, calcareous, grades upward into platy shale||1.5|
|8||Mudstone, shale, and shaly mudstone, gray and gray brown, weathers to gray brown blocks and chips, calcareous, small dark organic fragments, interbeds of impure, silty limestone, poorly exposed||59.0|
|7||Limestone, dark blue gray, weathers mottled light brown and medium gray, shaly and nodular, especially at top, fine, particulate fossil debris||1.2|
|6||Limestone, dark gray, fine grained, single bed||1.5|
|Limestone, mottled light brown and medium gray, shaly, nodular||0.8|
|5||Limestone, medium gray to gray brown, massive, medium grained sandy, bedding distorted||3.2|
|Limestone, dark blue gray, very fine grained, medium bedded||3.0|
|3||Limestone, dark blue gray, weathers a mottled, patchy, gray and gray brown, massive, very fine grained, lower bedding surface irregular and wavy, upper surface smooth, with scattered crinoid columnals||4.0|
|2||Limestone, dark gray, very fine grained, nodular, shaly||1.5|
|1||Limestone, dark brownish gray, mottled, massive, very fine grained||1.0|
|Shale, green, "pine-needle," needles large||1.0|
|Limestone, dark reddish brown, earthy, nodular, with interbed of red, "pine-needle" shale||0.6|
|Shale, green, "pine-needle," needles up to two inches long and one-fourth inch thick||4.0|
Clark, D. L., 1957, Marine Triassic stratigraphy in eastern Great Basin: Am. Assoc. Petroleum Geologists Bull., v. 41, p. 2192-2222.
Clark, D. L., 1%0, Triassic biostratigraphy of eastern Nevada, in Guidebook to the geology of east central Nevada: Intermountain Assoc. of Petroleum Geologists and East. Nevada Geol. Soc., p. 122-125.
Dunbar, C. O., and Rodgers, John, 1957, Principles of stratigraphy: John Wiley & Sons, New York, 356 p.
Hewett, D. F., 1931, Geology and ore deposits in the Goodsprings Quadrangle, Nevada: U. S. Geol. Survey Prof. Paper 162, 172 p.
Longwell, C. R., 1925, The pre-Triassic unconformity in southern Nevada: Am. Jour. Sci., 5th ser., v. 10, p. 93-106.
Longwell, C. R., 1928, Geology of the Muddy Mountains, Nevada, with a section through the Virgin Range to the Grand Wash Cliffs, Arizona: U. S. Geol. Survey Bull. 798, 152 p.
McKee, E. D., 1954, Stratigraphy and history of the Moenkopi Formation of Triassic Age: Geol. Soc. America Mem. 61, 133 p.
Poborski, S. J., 1954, Virgin Formation (Triassic) of the St. George, Utah area: Geol Soc. America Bull, v. 65, p. 971-1006.