Middle Miocene to early Pliocene beds of the Muddy Creek Formation near Mesquite in southeastern. Nevada are at least 200 meters thick. The formation ...
Utah Geological Association Publication 15, 1986 Thrusting and Extensional Structures and Mineralization in the Beaver Dam Mountains, Southwestern Utah
SEDIMENTARY ENVIRONMENTS OF THE MUDDY CREEK FORMATION NEAR MESQUITE, NEVADA Bart J. Kowallis and Brent H. Everett Department of Geology, Brigham Young University, Provo, Utah 84602 ABSTRACT
Middle Miocene to early Pliocene beds of the Muddy Creek Formation near Mesquite in southeastern Nevada are at least 200 meters thick. The formation consists of discontinuous beds of sandstone, siltstone, mudstone, and claystone deposited as intrabasinal fill. The sediments were derived locally from adjacent block-faulted mountains. The beds have a general strike of about N 50 E and a dip of3-5 to the northwest. The beds are not continuous, but pinch and swell laterally, due to the many _stream channels that are superimposed upon each other. Mesa-capping limestones, thought perhaps to be lake deposits in the Muddy Creek Formation, are younger and unconformably sit on top of the Muddy Creek Formation. The limestones contain well developed pisolites and appear to be caliche deposits. Sedimentary structures in the formation include clay rip-up clasts, armored mud-balls, trough crossbeds, mudcracks, and ripple marks. The ripple marks range in wavelength from 2 cm to 40 cm and in amplitude from 1 cm to 5 cm. They occur in a number of ripple types. Many of the sandstone beds are relatively unconsolidated. In places, thin mudstone and claystone beds are present within channels, along with coarse sandstone and occasionally larger mud rip-up clasts indicating a highly variable flow regime. Thin overbank deposits of green mudstone are laterally continuous with channel sandstones in many places. Thicker deposits (1-2 m) oflight green mudstone and siltstone are laterally continuous with some of the larger channels and may represent oxbow lake deposits in abandoned meanders. Fossils and trace fossils in the Muddy Creek Formation are locally abundant, such as "camel" tracks and bones, bird and lizard tracks, many types of burrows, and impressions of grass or small sticks in what may have been tufa deposits. Sedimentary structures and fossils seem to indicate an environment of deposition not too dissimilar to the climate of today with some periods of higher rainfall and more moderate climate producing a savannah-like environment. Both braided and meandering stream deposits occur in the formation, along with associated pond, lake, and flood plain deposits. may even be over 600 meters thick in some of the larger basins (Metcalf, 1982). Near Mesquite, Nevada where our studies were carried out (Fig. 1), the exposed part of the formation is about 200 meters thick, and there may be much more still buried beneath the surface. The age ofthe Muddy Creek Formation was not well constrained for many years. The first geologists to study the formation placed it in the early Pliocene based on indirect evidence (Spurr, 1903) and sparse mammalian fossils (Stock, 1921). Isotopic dating of basalts capping the formation by Anderson (1972) and Damon and others (1978) confirmed that the upper portion of the Muddy
INTRODUCTION The Muddy Creek Formation is an important, but little studied group of sediments that occur as interior basin fill deposits in the Basin and Range. The formation was first named by Stock (1921) as the "Muddy Valley Beds" or the "Muddy Creek Beds" and was later formalized by Longwell (1928, 1936) as the Muddy Creek Formation. The formation varies considerably in thickness from the edges to the centers of each basin and also from basin to basin. It has been reported to be over 300 meters thick in some locations (Longwell, 1928; Hunt and others, 1942) and 69
BARTJ. KOWALLIS, etal.
70
METHODS
NEVADA
o I
100
Stole
!
200
300
I • Kilometers
The information presented here was collected during the 1985 summer field camp held by Brigham Young University in southwestern Utah and southern Nevada. Nine partial stratigraphic sections were measured in the Muddy Creek formation using a Jacob's staff. The students recorded sediment colors using a GSA rock color chart, grain size, bedding thicknesses, sedimentary structures, and any other information that might be useful in interpreting the paleoenvironmental and sedimentation history of the formation. In addition, they collected any fossils and trace fossils that they encountered while measuring the sections. SEDIMENTATION AND STRATIGRAPHY
Figure 1. Index map of the study area. The dotted areas are bedrock in the Mormon and Virgin Mountains, the lined areas are the Muddy Creek Formation, and the open areas are Quaternary alluvium.
Creek Formation was indeed early Pliocene (5-6 myoId). Later dating of tephra layers by Metcalf (1982) has demonstrated, however, that a large portion of the formation is probably Miocene in age (6-12 myoId). Hintze (personal communication) obtained an age of 9 my on a basalt flow in Muddy Creek sediments near Motoqua, Utah. The formation does contain some fossils and trace fossils, but the only published description of fossils from the Muddy Creek is Stock's (1921) description of a few camel and horse bones. We found camel bones and tracks, bird and lizard tracks, and abundant burrows. The environment of deposition for these sediments has, in a general sense, been understood from the first studies of this area. Spurr (1903) is the first reference we can find for these sediments and he proposed that they were lake and stream deposits that accumulated in the intermontane basins and were derived from the erosion of the adjacent mountain ranges. Subsequent workers have agreed with this assessment, adding only that the climate must have fluctuated somewhat betwe~n dry, with playa lakes and evaporite deposition to more moist with lakes containing standing water for longer periods of time. They also showed that as you follow the sediments from the center of the basins to the edges the sediments become coarser and more poorly sorted, typical of alluvial fan deposition (Longwell, 1928, 1936, 1963; Hunt and others, 1942; Hunt, 1956; Lucchitta, 1972, 1979). The sediments we studied were mostly lacustrine clays and silts and fluvial sands, silts, and clays, both from meandering and braided streams, deposited in the central part of the Virgin River valley near Mesquite, Nevada.
The Muddy Creek Formation is well exposed in only a few localities in southern Nevada. Some of the best exposures are those we studied just off of Interstate 15 near Mesquite. Here the formation is capped by a thick (3-5 meters) bed of pisolitic limestone that was formed as a caliche. The limestone is flat-lying and unconformable on top of the gently dipping Muddy Creek beds below. Because of the durability of this caliche limestone, it has formed considerable talus slopes and cover over the less resistant Muddy Creek beds. Even where the Muddy Creek Formation is exposed, it is frequently so poorly consolidated that sedimentary structures cannot be seen. However, we were able to measure several overlapping partial sections through the formation, generally in washes and road cuts where exposures were good. Stratigraphy Figure 2 shows one of the sections we measured that is typical of the formation in this area. The sediments are mostly (80-90%) immature, poorly sorted, coarse to fine grained sand and sandstone with a characteristic pinkish or pale orange color. Siltstones and light brown to pale green mudstones make up the rest of the section. The better-cemented sandstones and siltstones stand out as ledges (Fig. 3). Calcite and clay are the major cementing agents in the sandstones. The calcite cement is often quite unevenly distributed, so that different parts of the same bed of sand may weather quite differently. The cemented part standing up as a ledge and the uncemented part forming a slope. The calcite cement in some places in these sandstones may be zones where caliche developed while the sandstones were exposed at the surface. None of the sandstones, siltstones, or mudstones are laterally continuous; they all pinch out over relatively short distances of a few 10's of meters for the most continuous beds and much less for some smaller beds. This makes correlation between sections difficult. Coarse detritus, anything larger than sand size particles, is almost completely ab-
SEDIMENTARY ENVIRONMENTS MUDDY CREEK FORMATION
71
Sm
o Scale Meters
Sed imentary Structures and Fossils b- Burrows
I - Load Casts m- Mud Cracks 0- Camel Bone r - Ripple Marks t - Tracks u - Rip-up Clasts x- Cross Bedding
Figure 2. Schematic stratigraphic section through a part of the Muddy Creek Formation. The section was measured below Flat Top Mesa about 200 meters north of Interstate 15, two tenths of a mile west of Milepost 117. Field units and sedimentary structures in the formation are shown. In some of the slope-forming sandstones, sedimentary structures could not be seen but crossbedding is probably still present. The ledge forming sandstones are all cemented with calcite and may represent surfaces that were exposed where calcium carbonate precipitated as caliche. Only about 60 meters of section are shown, but approximately 200 meters are present in this area. Flat Top Mesa is capped by a pisolitic caliche limestone that is quite resistant to erosion and protects the more unconsolidated Muddy Creek Formation below.
72
BART J. KOWALLIS, et al.
Figure 3. Photo oftypical Muddy Creek exposure in a wash near Riverside, Nevada. Cliff face is about 10 meters high.
Figure 4. Trough cross bed sets in a poorly consolidated sandstone bed about 0.9 m thick. The sets probably represent a braided stream system. At the base of some sets (see arrow) rip up clasts can be seen.
Figure 5. Ripple marks found in the Muddy Creek Formation occur in a variety offorms. (a) Lunate or catenary ripples overprinted with a second ripple set to produce a fairly complex ripple pattern. (b) Sinuous ripples with flat tops.
sent from the Muddy Creek in this area, even though it is only about 10 miles from the Virgin Mountains to the south and the Mormon Mountains to the northwest. Many channels in the sandstones do contain mudstone rip-up clasts, armored mud balls, or large (0.5 m diameter) mud lumps that probably slump~d off of the sides of the channels, but no foreign cobbles or boulders. Coarser material is common in other areas where the Muddy Creek Formation is exposed (Longwell, 1963, Anderson, 1978), so the lack of this type of material may suggest a different source for the Muddy Creek here or less relief in the source areas. The channels are large enough to have carried coarse material if it had been available.
Sedimentary Structures The most common sedimentary structure present in the Muddy Creek Formation is cross bedding (Fig. 4). The cross bed sets range in size from a few centimenters across in rippled siltstone and sandstone to a few meters across in some of the larger channels. Ripple marks are also common throughout the section, usually near the top of cross-bedded channel sandstones. Several types of ripple marks are present, including sinuous, catenary, straight, and linguoid (Fig. 5). The ripple marks range in wavelength from 2-40 cm and in amplitude from 1-5 cm. In some cases the ripples have been disturbed by burrowing organisms.
SEDIMENTARY ENVIRONMENTS MUDDY CREEK FORMATION
73
Figure 6. (a) Armored mudball found in the Muddy Creek Formation. (b) Armored mudballs and rip-up clasts in a sandstone channel.
Figure 7. (a) Abundant chambered burrows on a horizontal surface in a sandstone block. (b) A single burrow with filling material and chambered end.
Rip-up clasts are common in the cross bedded sandstones and in some places armored mud balls were found (Fig. 6). The armored mud balls range in size from 5 cm to 20 cm in diameter. The rip up clasts are generally small (1-10 cm) but occassionally large mud lumps or rip up clasts are found that may be close to one-half meter in diameter. The larger ones may not be rip-up clasts, but may represent mud that fell into the channel as the stream undercut its bank. Mudcracks occur as both sandstone casts and mudstone molds. Some of the mudcracked horizons are spectacular with crack openings 1-2 cm wide and more than 10 cm deep. Fossils and Trace Fossils Certain horizons in the Muddy Creek Formation contain abundant fossils or trace fossils, although the section as a whole is quite unfossiliferous. Burrows, both horizontal and vertical, are found throughout the formation.
Frequently they are chambered as seen in Figures 7a and 7b. A large variety of burrow types are present and their occurrence and identification would make an interesting study. The bioturbated sandstone beds are generally well cemented with a calcite cement. One bed appeared to have small stick or grass impressions on its surface. The sample is strikingly similar to grass and stick impressions in a modern tufa deposit near Mesquite. Several types of track casts and molds were found in the sections we studied. Figure 8a shows casts of camel tracks that weathered out of a mudstone unit. The tracks are composed of greenish fine-grained greenish that apparently flushed in over the mud and filled the tracks, since mudstone is found both below and above the track casts. Figure 8b shows two bird tracks and a possible tail trace from a lizard or some other animal. Figure 8c shows two tracks of a fairly large, unidentified mammal, probably with four toes around a pad.
74
BART J. KOWALLIS, et al.
Figure 8. (a) Casts of camel tracks preserved from the filling of prints left in the mud by sand which was then covered with more mud. (b) Bird tracks (two of them indicated by adjacent arrows) and a possible lizard tail trace (single arrow) occur in this slab. (c) Two unidentified mammal tracks in a large slab. (d) Pieces of camel bone from the section shown in Figure 2. The only body fossils found in the Muddly Creek Formation near Mesquite are mammal bones;' identified as camel by W. E. Miller (personal comm., 1985). Figure 8d shows a few examples of the material found. Camel bones have been reported from other locations in the Muddy Creek Formation by Stock (1921). PALEOENVIRONMENTAL INTERPRETATION As we mentioned in the introduction, the general environmental setting for the Muddy Creek Formation has been understood at least since Spurr (1903). It is basically intrabasinal fill consisting of fluvial (both braided and meandering streams), lacustrine, alluvial, and playa deposits. Our sections consist primarily offluvial deposits with some small lake and pond deposits, but with a lack of alluvial fan and playa (evaporite) deposits. Still, some questions remain. How dry was the climate? Was it similar to southern Nevada today or was it more like the Mrican savannahs or Great Plains in the U. S.? How variable was the climate over the period of its deposition? How different was the topography from the present topography?
From examining the environment that exists at present in the Virgin River valley near Mesquite, we found many striking similarities to the sedimentary structures and depositional patterns of the Muddy Creek Formation. The present day valley contains fluvial deposits, both from meandering streams and braided streams. We also found rippie marks and mudcra-:ks similar in size and form to their fossil counterparts. Mammal, bird, and lizard tracks and trails are also common in mud along the river and in sand in washes. Grass and stick impressions found in local, modern tufa deposits are very similar to those found in the Muddy Creek Formation. Modern burrowing insects and other burrowing animals can also be found along the present river system. All of these features would seem to indicate that the paleoenvironmentfor much of the Muddy Creek Formation could have been quite similar to the present environment and climate. However, several climatic fluctuations must have occurred during the deposition of the Muddy Creek. Keller and Barron (1983) document three periods of warmer
SEDIMENTARY ENVIRONMENTS MUDDY CREEK FORMATION climate between four periods of cooler climate worldwide from the mid-Miocene (about 12 my ago) to the early Pliocene (about 4.5 my ago), the range of ages for the Muddy Creek Formation. The camel bones and tracks may also suggest times of moister, milder, or cooler climate than at present. These camels were probably more closely related to the llamas of South America than to the modern camels of Africa (W.E. Miller, personal comm.). The llamas are not desert dwellers like their African relatives, but plateau dwellers in a cooler, moister climate. In other studies of the Muddy Creek Formation this changing climate has been noted (Longwell, 1928; Hunt and others, 1942; Hunt, 1956). At times during deposition of the sediments, evaporite depOSits that accumulated most likely in playa lakes, are common. At other places in the section, playa deposits are absent and sediments deposited in deeper lakes where water was present for long periods of time are more abundant. In our sections near Mesquite, layered gypsum was not encountered, but some mudstone units did have gypsum crystals 2-5 cm across. As mentioned earlier, most of the sediment in our sections was fluvial in origin, but we interpreted the discontinuous greenish mudstones and sandstones, around which we sometimes found tracks, to be small lakes and ponds with periods of standing fresh water. Perhaps the camels migrated in and out of this area as the climate changed back and forth. Further documentation of where in the section the camel tracks and bone occur in relation to paleoclimatic indicators might help to clarify the climatic relationships. Oxygen isotope data and a careful dating of ash beds in the Muddy Creek Formation would also be beneficial. The topography was probably more subdued during deposition of the Muddy Creek sediments in this area. This conclusion is based on the fact that nowhere in our sections did we find any coarse detritus similar to the material presently being shed from the Virgin and Mormon Mountains into the Virgin River valley. Longwell (1928) suggested that the mountains were more subdued during deposition of the Muddy Creek Formation where he studied it farther to the southwest. CONCLUSIONS The Muddy Creek Formation consists of fluvial and lacustrine sandstones, Siltstones, and mudstones deposited as intrabasinal debriS, at a time when the topography was subdued compared to its present state. The climate was probably not too different from the present climate, though at times it was more moist and perhaps cooler. Fossils and trace fossils found in the formation indicate that at times life was abundant.
75
ACKNOWLEDGEMENTS We would like to thank Craig Baum, Doug Brown, Mike Hansen, Terrel Hopkin, Darwin Payne, John Petersen, and Bill Webber who allowed us the use of their field camp reports and field data in compiling this manuscript. We would also like to thank J. Keith Rigby for his help in the field and in reviewing this manuscript and Lehi F. Hintze for suggesting this area as a good field camp location. We are also grateful to Wade Miller and Ken Stadtman for help in identifying the tracks and bones. REFERENCES CITED Anderson, RE., 1978, Geologic map of the Black Canyon IS-minute quadrangle, Mohave County, Arizona, and Clark, County, Nevada: U.S. Geological Survey Map GQ-1349. Anderson, RE., Longwell, C.R, Armstrong, RL., and Marvin, R.F., 1972, Significance ofK-Ar ages of Tertiary rocks from the Lake Mead Region, Nevada-Arizona: Geological Society of America Bulletin, v. 83, p. 273-288. Damon, P.E., Shafiquellah, M., and Scarborough, RB., 1978, Revised chronology for critical stages in the evolution of the lower Colorado River [abs.]: Geological Society of America Abstracts with Programs, v. 10, p. 101-102. Hunt, C.B., 1956, Cenozoic geology of the Colorado Plateau: U.S. Geological Survey Professional Paper 279, 99 p. Hunt, C.B., McKelvey, V.E., and Weise, J.H., 1942, The Three Kids manganese district Clark County, Nevada: U.S. Geological Survey Bulletin 936-L, p. 297-319. Keller, G. and Barron, lA., 1983, Paleoceanographic implications of Miocene deep-sea hiatuses: Geological Society of America Bulletin, v. 94, p. 590-613. Longwell, C.R, 1928, Geology of the Muddy Mountains, Nevada, with a section through the Virgin Range to the Grand Wash Cliffs in western Arizona: U. S. GeolOgical Survey Bulletin 798, 152 p. Longwell, C. R, 1936, Geology of the Boulder reservoir floor, ArizonaN evada: Geological Society of America Bulletin, v. 47, p. 1393-1476. Longwell, C.R, 1946, How old is the Colorado River?: American Journal of Science, v. 244, p. 817-835. Longwell, C.R, 1963, Reconnaissance geology between Lake Mead and Davis Dam Arizona-Nevada: U. S. Geological Survey Professional Paper 374-E, 51 p. Lovejoy, E.M.P., 1980, The Muddy Creek Formation at Colorado River in Grand Wash: The dilemma of the immovable object: Arizona Geological Society Digest, v. 12, p. 177-192. Lucchitta, 1., 1972, Early history of the Colorado River in the Basin and Range Province: Geological Society America Bulletin, v., 83, p. 1933-1948. Lucchitta, 1., 1979, Late Cenozoic uplift of the southwestern Colorado Plateau and adjacent lower Colorado River region: Tectonophysics, v. 61, p. 63-95. Metcalf, L.A., 1982, Tephrostratigraphy and potassium-argon age determinations of seven volcanic ash layers in the Muddy Creek Formation of southern Nevada [M.S. thesis]: Reno, University of Nevada, 187p. Spurr, J.E., 1903, Descriptive geology of Nevada south of the fortieth parallel and adjacent portions of California: U.S. GeolOgical Survey Bulletin 208, 229 p. Stock, C., 1921, Late Cenozoic mammalian remains from the Meadow Valley region, southwestern Nevada: American Journal of Science, 5th ser., v. 2, p. 250-264.
