Sedimentology of the Upper Triassic Chinle Formation Southeastern Utah: Paleoclimatic Implications

Home , Chinle Formation, Moenkopi Formation, Owl Rock, Petrified Forest Member, Wingate Sandstone

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1987

Russell F. Dubiel U.S. Geological Survey

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Dubiel, Russell F., "Sedimentology of the Upper Triassic Chinle Formation Southeastern Utah: Paleoclimatic Implications" (1987). USGS Staff -- Published Research. 216. https://digitalcommons.unl.edu/usgsstaffpub/216

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RUSSELLF. DUBIEL U.S. Geological Survey MS 919 Box 25046, Federal Center Denver, Colorado 80225

ABSTRACT

The UpperTriassic Chinle Formationin southeasternUtah was depositedin a complex fluvial-deltaic-lacustrinesystem. The Chinle records the evolution of a continental system in response to variations in climate, tectonics, and sediment supply.Chinle stratarepresent deposits of fluvial channels and floodplains,-lacustrine deltas,-lacustrine basins,- and lacustrine and playa mudflats. These rocks include a variety of vertebrate,invertebrate, and plant fossils, trace fossils, and paleosols that provide information on depositional environments, water tables, and paleoclimate. Sedimentologic and paleontologic interpretations both support an interpretation of abundant lakes, streams, and marshes with high, but fluctuating water tables for all but the last phase of Chinle deposition. This final phase represents a transition to eolian deposition of the Wingate erg. The Chinle climate is interpretedto have been characterisedby tropical monsoons, with abundant precipitation and seasonally drier periods. This interpretation agrees with Late Triassic paleoclimates predicted from theoretical models.

INTRODUCTION (1972b) contains a summary of Chinle nomenclature development. Lupe (1977, 1979, 1984) investigated Chinle In southeastern Utah, erosion by the Colorado River depositional environments in the San Rafael Swell, Utah and its tributarieshas producedextensive three-dimensional and in the vicinity of Moab, Utah. Gubitosa (1981) and exposures of the Upper Triassic Chinle Formation. Sedi- Blakey and Gubitosa (1983, 1984) interpreteddepositional mentological examination of the Chinle Formationthrough environments and fluvial architectureof the Chinle, based measured stratigraphic sections (Figure 1) and recon- primarilyon detailed work in Canyonlands National Park, struction of stratigraphic cross-sections has enabled an Utah and in Arizona. Working with extensive three- interpretation of its complex continental depositional dimensional exposures in southeasternUtah, Dubiel (1982, environments. These interpretations are based on sedi- 1983a, 1983b, 1984, 1985, 1986)developed a detaileddeposi- mentary structures, lithofacies variability of process- tional model for Chinle environments depicting a complex controlled genetic units, and paleosol horizons. The fluvial-deltaic-lacustrine sequence in southeastern Utah. resulting depositional model provides a depositional Krausand Middleton (1987)investigated the sedimentology frameworkfor the formationthat accounts for the observed of the Chinle Formation in Petrified Forest National Park, lithofacies variation, and depicts the paleogeographyat the Arizona. Dubiel et al. (1987) discussed the occurrence of time of deposition. The sedimentologic investigation lungfish burrows in the Chinle and related Dolores For- incorporatesobservations of the fauna,flora, and trace fossil mation. assemblages. While many of these fossils are not age While there is a general concensus on the interpreta- diagnostic, the assemblages and their mode of formation tion of continental depositional environments and the impose restrictions on the model. The model, in turn, depositional history of the Chinle Formation, there is a provides valuable information for the interpretationof the correspondinglack of agreement regardingthe interpreta- Late Triassic paleoclimate. tion of the climate during Chinle deposition. Climatic Several publications in recent years have addressed interpretations, based on a variety of stratigraphic and various aspects of the stratigraphy and depositional paleontologic evidence, range from arid and semi-arid environmentsof the Chinle Formation.An inclusive review conditions (Stewartet al. 1972a,-Lupe 1979) with through- of previous investigations of the Chinle Formation on the flowing streams (Daugherty 1941) to a humid tropical Colorado Plateau and interpretations of the stratigraphy, climate (Ash 1967, 1972, 1978,-Gottesfeld 1972)or a humid depositional environments, fossils, and paleoclimate of the climate with increasing aridity with time (Blakey and Chinle is provided by Stewart et al. (1972a). Stewart et al. Gubitosa 1983). Recent sedimentological studies (Bownet

Dubiel, R. F. 1987. Sedimentologyof the Upper Triassic Chinle Formation,Southeastern Utah: Paleoclimatic Implications. Journalof the Arizona-NevadaAcademy of Science 22:35-45.

Journal of the Arizona-Nevada Academy of Science, Vol. 22, No. 1, Triassic Continental Deposits of the American Southwest (1987), pp. 35-45 36 JOURNALOF THE ARIZONA-NEVADAACADEMY OF SCIENCE VOL. 22

Figure 1. Map showing location of measured sections and geographic features referredto in the text.

al. 1983) in Petrified Forest National Park, Arizona and in lungfish burrowsand the recent discovery of lungfish teeth southeastern Utah (Dubiel 1983b, 1984, 1986) have in Utah (Dubiel 1987; Parrishand Good 1987) indicate that characterizedthe climate as being wet but punctuated by lungfish were abundant in the Chinle ecosystem. This seasonally dry periods. In addition, the identification of abundance of lungfish indicates that the climate provided lungfish burrows and superimposed gley paleosols in the sufficient moisture to form extensive lakes, streams, and Chinle and Dolores Formations(Dubiel et al. 1987)supports marshes for lungfish habitats. The occurrence of aquatic these interpretations of a tropical monsoonal climate. vertebratessuch as phytosaursand metoposaurs(Parrish and Paleoclimatic models (Robinson 1971; Parrishet al. 1986) Good 1987) supportsthe previous interpretationsof humid also indicate a tropical monsoonal climate for the Late climatic conditions with extensive lakes, streams, and Triassic. marshes. The recent discovery of unionid bivalves in the Sedimentological study of the Chinle Formation in Chinle of Utah provides additional evidence for both southeastern Utah has provided a variety of evidence that lacustrine and perennial fluvial conditions (Parrishand bears on the interpretationof paleoclimate during the Late Good 1987; Good et al. 1987). Triassic. The Chinle depositional model depicts a complex Paleomagnetic studies indicate that the majorlucus of fluvial-deltaic-lacustrine system replete with extensive Chinle deposition was about 0° to 20° north latitude (Van marshes, bogs, floodplains, and mudflats. The presence of der Voo 1976), that is, clearly within the tropics. Thus, the these environments indicates that water was abundant in sedimentologic and paleontologic evidence are compli- the depositional system. Paleosols developed on subaerial mentary (Good et al. 1987) and indicate that tropical and seasonally flooded portions of the floodplains and lake monsoons with abundantprecipitation and seasonally dry margins indicate that groundwater levels fluctuated in spells characterizedthe climate during Chinle deposition. response to periodic flooding. Trace fossils that have been interpretedas the casts of REGIONALSTRATIGRAPHY lungfish burrows (Dubiel et al. 1987) are found in various The Chinle Formation in southeastern Utah consists lithofacies of all members of the Chinle. The ubiquitous of six formalmembers (Stewartet al. 1972a):the Shinarump, ISSUE 1, 1987 SEDIMENTOLOGYOF THE UPPERTRIASSIC CHINLE FORMATION 37

Monitor Butte, Moss Back, PetrifiedForest, Owl Rock, and Church Rock Members, in ascending order. In the study area, the Chinle Formation unconformably overlies the Lower and Middle (?)Triassic Moenkopi Formation. The Chinle Formationis overlainby the UpperTriassic Wingate Sandstone. Generally,the Chinle Formationfills largepaleovalleys incised into the Moenkopi Formation (Stewartet al. 1972a; Blakey 1974; Blakey and Gubitosa 1983; Dubiel 1983b, 1986);these paleovalleys formed in response to a lowered regional baselevel. A subsequent rise in baselevel resulted in aggradationand the filling of the paleovalleys by the continental Chinle sediments. Specific lithofacies, their distribution within the various members, and their interpreted depositional environments are discussed in the following sections.

SEDIMENTOLOGICEVIDENCE FOR WET ENVIRONMENTS Figure 2. Shinarump Member of the Chinle Formationat The members of the Chinle Formationwere deposited Colt Mesa in the Circle Cliffs, Utah. Shinarump fluvial in a complex fluvial-deltaic-lacustrinesystem (Blakeyand conglomerate and sandstone fill a large paleovalley cut Gubitosa 1983; Dubiel 1983b, 1984, 1985, 1987). Conse- into the underlying Moenkopi Formation. Slope of paleo- quently, the lithofacies include a complex array of valley wall cut into Moenkopiis visible at arrow.Mine adits lithologies and sedimentary structures indicative of the are 3 m high. environment of deposition. The lithofacies and sedimentologic and climatic interpretationsfor each member of the Chinle are discussed in the following sections. removal of portions of the Shinarump in response to locally lowered baselevel, possibly due to lake-level fluc- Lithofacies and Depositional Environments tuations. Similar cuts and fills have been reportedfrom the Petrified Forest Member in Arizona (Krausand Middleton The ShinarumpMember is characterizedby white to 1987). yellow and gray, medium- to coarse-grained and con- Where these cuts are not present, a gradationalcontact glomeraticsandstone. The sandstone is cut by complex cut- exists between the Shinarump Member and the overlying and-fill structures, lenticular internal scour surfaces, and Monitor Butte Member, which may be the most hetero- large-scalelateral accretion bedding.Abundant, large-scale, geneous lithologic unit in the Chinle Formation. The trough cross-stratificationand less abundanttabular planar Monitor Butte contains purple-mottled, yellow to brown, cross-stratificationand horizontal laminations are common and red sandstones and siltstones; green, bentonitic, silty sedimentary structures. Sandstone bodies grade laterally sandstonesand mudstones;red, calcareousmudstone; black, into siltstone and mudstone lenses that contain organic- organic-carbon-richmudstones; and pink and green to tan carbon fragments and whole, carbonized plant fossils. limestones. Because of the complex interfingering and Lithology, sedimentary structures, and isopach maps lateralvariability of these lithofacies and the diversedeposi- (Dubiel 1983b) indicate that the Shinarump Member tional environments that they represent, each will be represents fluvial strata deposited under conditions of discussed in more detail, beginningwith the basal units and rising baselevel in the lowest portions of the paleovalleys working up through the section. cut into the Moenkopi Formation(Figure 2). The transition Directly overlying the ShinarumpMember, but often from massive, conglomeratic, and tabular-planarstratified exhibiting a gradational contact with the rocks below, is sandstone at the base upwardinto medium-grained,trough a purple-, yellow-, and white-mottled sandy siltstone and cross-stratifiedsandstone is thought to represent a change sandstone interval referredto here as the purple-mottled from essentially bedloaddeposition in braidedstreams with unit (PMU) of the Monitor Butte. The PMU is generally transverse bars to mixed-load deposition in more sinuous silicified and is characterized by large, irregularlyshaped fluvial systems with sand waves and point bars. color mottles of dark purple, lavender, yellow, and white. In the Circle Cliffs and near Capitol Reef (Figure1), the The variations in color are the result of varying concentra- fluvial sandstonesof the ShinarumpMember have been cut tions of iron-bearing minerals. Dense concentrations of out by large-scale scours or paleovalleys that have subse- hematite occur in the dark purple areas, there is less quently been filled by green Monitor Butte Member strata. hematite in the lavender areas, a hydrated iron compound These scours are interpreted to represent cuts into and (probablylimonite) colors the yellow areas, and hematite 38 JOURNAL OF THE ARIZONA-NEVADA ACADEMY OF SCIENCE VOL. 22

is absent in the white areas. Ubiquitous in the PMU are Both the lungfish burrows and the mottled coloration largecylindrical trace fossils (Figure3) interpretedto be the of these rocks are thought to reflect fluctuatingwater tables casts of lungfish burrows (Figure4) (Dubiel et al. 1987). within the sediments, probably in response to seasonal flooding (Dubiel et al. 1987). The lungfish are believed to have formed the burrows for aestivation in response to seasonal dryness. Fluctuating water tables that produced alternating oxidizing and reducing conditions resulted in redistribution of iron within the sediments. As organic matter in the sediment was oxidized, iron was reduced.The iron was then mobilized in the reduced state, and repre- cipitated under oxidizing conditions that developed during subsequent drying.The purple and white mottling extends laterally into the Moenkopi (Figure5) at the same level of development in the Chinle and, in both cases, represents a gleyed paleosol formed in response to fluctuating water tables.

Figure 3. Purple and white-mottled unit of the Monitor Butte Member with distinctive white tubular trace fossils interpretedto be lungfish burrows. Hammer for scale.

Figure5. Purpleand white-mottled paleosol formed on the Moenkopi (^ m) and overlain by Chinle mudstones of the Monitor Butte Member (T^cms). Note the decreasing intensity of the pedogenic alteration with increasing depth in the Moenkopl.

The sandy siltstone and sandstone facies of the PMU can generally be traced laterally into gray and purple siltstones that contain finely comminuted fragments of plant material and organic-carbonfragments. The siltstones can further be traced laterally and distally into black, very thinly and horizontally laminated, organic-carbon-rich mudstones. The black mudstones typically contain abundant, chitinous tests of conchostracans, fish scales, and fragmented fish bones,-whereas gray mudstones that have a lower organic-carbon content contain abundant, cal- careous Darwinulaostracodes and organic-carbonfragments (Dubiel 1983b).The black mudstones are as much as 15 m thick and in places contain lenses of coal as much as Figure 4. Purple and white-mottled siltstones and sand- 20 cm thick. Locally, the mudstones grade into purple- stones of the Monitor Butte Member with very abundant mottled gray mudstones and tan thin-bedded limestones lungfish burrows. that contain ostracodes, thin-shelled unionid bivalves, and ISSUE 1, 1987 SEDIMENTOLOGY OF THE UPPER TRIASSIC CHINLE FORMATION 39

small vertebrate bones (Dubiel 1987; Parrish and Good 1987). These Monitor Butte units represent coarse-grained clastic deposition in fluvial systems and relatedfine-grained clastic and organic-carbon deposition in overbank and lacustrine-marsh and bog wetland environments (Dubiel 1984). Alternation of conchostracan- and ostracode-rich horizons within mudstone units indicates that water levels in the lakes and marshes must have fluctuated somewhat, causing small-scale transgressions and regressions of the laterally adjacent environments (Dubiel 1984). Concho- stracans typically inhabit ephemeral pools and lacustrine marshes that are subjected to seasonal drying, while Darwinula sp. ostracodes are indicative of a more perma- nent lacustrine environment (R. M. Forester, written comm. 1982; Dubiel 1983b). The occurrence of these particularconchostracans and ostracodes within the same mudstone beds suggests that the water chemistry may be 6. Foreset beds of a lacustrine delta in characterized a alkaline of 7.5 to 8 and that Figure Gilbert-type by slightly pH the Monitor Butte Member near Lake Powell, Utah. the of the water was less than 5 thousand salinity parts per Geologist (arrow)for scale. (R. M. Forester,written comm. 1982; Dubiel 1983b). The freshwatersalinity is supportedby the analysis of organic- carbon content of as much as 20 weight percent in the mudstones and the occurrence of coal, which necessitates a continually high, fresh water table in a continental setting to preserveorganic material. The thin-beddedfossil- bearingcarbonates and mudstones represent deposition in small lacustrine systems. Gradationally overlying the PMU and related marsh and lacustrine units are a series of laterally extensive but thin-bedded,burrowed limestones and Darwinulaostracode- bearing mudstones as much as 5 m thick. These, in turn, are overlain by a sequence of green, bentonitic, sandstones, siltstones, and sandy mudstones that commonly exhibit large-scaleforeset beddingas much as 25 m thick (Figure6) (Dubiel 1983b).The foreset beds internally consist of abundant climbing, lunate ripples capped by thin, 2 cm-thick zones of oscillation ripples, llie green beds invariably contain comminuted well-preserved finely black, organic Figure 7. Carbonized plant fossil fern Phlebopteris and whole of several Late (the plant fragments specimens smithii) in Monitor Butte deltaic beds in White Canyon. Triassic plants, including true ferns (Figure7) (Ash et al. and the casts of horsetails 1982) giant (Equisetities sp.) The Moss BackMember occurs in a narrow belt Ash as much as 20 cm outcrop (Figure8) (Holt 1947; 1967, 1972) between White Utah and National in diameter. Canyon, Canyonlands Park,Utah (Blakeyand Gubitosa 1983; Dubiel 1983b).The Based on lithology, sequence of deposition, paleocur- Moss Back consists of brown to gray,medium-grained sand- rent measurements,and isopachmaps, these beds have been stone and carbonate-nodule conglomerate with minor interpreted as an extensive system of fluvial and deltaic mudstone lenses. Sedimentarystructures include abundant distributarychannels and splays, and lacustrine, prodelta, tabularplanar and large-scaletrough crossbeddingwith less and deltaic deposits (Dubiel 1983b, 1985). The fluvial and abundanthorizontal lamination. The sandstone bodies are deltaic systems prograded northwest into a large lake, lenticular and exhibit internal scour surfaces and cut-and- filling in most of the remnant topography of the original fill structure. Generally, the Moss Back erosively overlies Moenkopi paleovalleys. The bentonitic character of these the Monitor Butte, but in White Canyon it exhibits some Monitor Butte rocks and the presence of alteredlithic clasts importantlateral relationships with the MonitorButte. The (Schultz 1963) and relict glass shards (Watersand Granger thick tabular planar- and trough-crossbeddedsandstones 1953) indicate that volcanic ash was a major component interfinger with and grade laterally into red, thin-bedded of the sediment. sandstones, siltstones, and mudstones that contain trans- 40 JOURNALOF THE ARIZONA-NEVADAACADEMY OF SCIENCE VOL. 22

stones contain rare lungfish burrows that are difficult to discern due to their infilling by clastic material identical in size to the surroundingmatrix (Dubiel et al. 1987).Inter- fingeredwith the sandstones and mudstones are bentonitic sandstone and siltstone strata that contain abundant vertebrate remains, lungfish toothplates, gastropods, and thin-shelled unionid bivalves (Dubiel 1987). Isolated out- crops of black, organic-carbon-richand conchostracan- bearing mudstone are present in the Petrified Forest Member. These strata are interpretedto represent fluvial sandstones and floodplain mudstones interfingeredwith fossil- bearing splay deposits and laterally restricted marsh mudstones. The abundance of lateral accretion bedding, numerous splay deposits, and the presence of floodplain mudstones that completely encase the sandstone units suggest the strata were deposited by sinuous streams that were subject to numerous avulsion events. The remains of Figure 8. Pith casts of giant horsetails (arrows) in the aquatic phytosaurs, lungfish, and lacustrine unionid MonitorButte Memberin Capitol Reef National Park,Utah. bivalves (Parrishand Good 1987) support the interpreta- The casts were formed when the hollow trunks of the tions of rivers, lakes, and marshes for the Petrified Forest horsetails were broken off and filled with sediment during strata.Observations of apparentaltered glass shards(Waters a flood event. and Granger1953) and bentonitic mudstones (Schultz 1963) in the Petrified Forest Member indicate that volcanic ash continued to form a significant component of the clastic ported, thick-shelled unionid bivalves (Dubiel 1987; Par- input. rish and Good 1987), isolated carbonate nodules, and The Petrified Forest Member interfingers with and abundant, 1 cm-diameter trace fossils with a distinctive gradesupward into the pink and green limestones and red external, ropey texture. to orange siltstones of the Owl Rock Member. The lime- This assemblageof lithology and structuresin the Moss stones vary from 10 cm to 2 m thick, can be tracedlaterally Backsandstone indicates depostion by fluvial processes.The for several miles, and typically display a mottled coloration lenticular, coarse-graineddeposits represent deposition in and knobby-weatheredtexture. The cylindricaltrace fossils sinuous fluvial channels. The red, thin-bedded units are interpreted to be lungfish burrows (Dubiel et al. 1987) are interpretedas levee, crevasse splay, and overbank deposits locally abundant in the limestones. These burrows often that exhibit carbonate-rich paleosol horizons. These extend down into the adjacent siltstones and are probably paleosols, probably vertisols, were the source of the car- responsible for the extensive bioturbation and knobby tex- bonate nodules within the Moss Back conglomerates. The ture of the rocks. Locallythe limestones contain ostracodes, thick-shelled unionids were washed out of a perennial but no other body fossils have been observed.The siltstones but fluvial system duringflood events (Parrishand Good 1987). are massive, exhibit no sedimentary structures, locally The interfingeredlateral relationship of these fluvial do contain lungfish burrows and other small trace fossils. and floodplainunits, and the fact that they overlie and also The presence of extensive carbonate units in a con- appearto gradedistally into the deltaic units of the Monitor tinental setting and the occurrence of lungfish burrowsin Butte, indicate that at least the White Canyon portion of both the limestones and siltstones indicate that deposition the Moss Back was the distributary fluvial system to the occurred in lacustrine basins and on lacustrine margins, MonitorButte deltaic system. The exact relationshipof this respectively. Both the carbonateand the fine-grainedclastic Moss Backto the Moss Back of CanyonlandsNational Park deposition imply that significant clastic detritus was not to the north and to the overlying Petrified Forest Member supplied to these environments. in both areas is under investigation. Interfingered with and generally overlying the Owl Overlying the Moss Back are the lavender and brown Rock Member are the orange to red and brown sandstones sandstonesand variegatedmudstones of the PetrifiedForest and siltstones of the Church Rock Member. The Church Member. The bentonitic sandstones and thin lenses of Rock has a ratherlimited distribution (Stewartet al. 1972a) carbonate-noduleconglomerate typically exhibit large-scale and is not present at every locality in the study area. Sand- trough cross-stratification, larger-scale internal scour stones are fine- to medium-grainedand are either structure- surfaces,and lateral accretion bedding. The sandy portions less or contain faint, large-scaletrough cross-stratification of the units can be traced laterally into red-brownmud- and minor lateral accretionbedding. Many of the units con- stones that contain pedogenic carbonatenodules identical tain small but abundant,meniscate back-filledtrace fossils. to those in the Moss Back floodplain paleosols. The mud- Mudstones locally contain dessication cracks as much as ISSUE 1, 1987 SEDIMENTOLOGYOF THE UPPERTRIASSIC CHINLE FORMATION 41

10 cm across and up to 1 m deep that are filled with sand- or near the level of the water table, a water table that was stone of the overlying Wingate Sandstone. progressivelyrising and drowning the fluvial systems. The These sandstones and mudstones are interpreted to water table rise caused the expansion of the lacustrine have been deposited on lacustrine or playa mudflats system that lay to the west. Seasonalflooding of fluvial and crossed by small fluvial systems with laterally restricted floodplain systems, probablyrelated to seasonal precipita- floodplains. The large-scale cross-stratification indicative tion, is indicated by the ubiquitous lungfish burrows and of eolian deposition and the mudcracks indicate that the gleyed paleosols. Thin lacustrine carbonatesand mud- dessication was more prominent during deposition of the stones were deposited as the water table rose high enough upper part of this member These environments may, in to form the lake. The area was inhabited by aquatic fact, reflect the progradationof and transition to eolian vertebrates, lacustrine molluscs, and lacustrine erg deposition of the overlying Wingate Sandstone. The microinvertebrates. generally reddercoloration of these rocks comparedto the An increase in the rate of sedimentation, associated variegated colors of the lower Chinle is due to a greater with an increase in volcanic activity and ash production, developmentof diagenetichematite. The increaseddevelop- resulted in the progradationof the Monitor Butte and Moss ment of hematite in this part of the Chinle section is Back systems. The Monitor Butte represents deposits oi thought to be related to the lack of organic carbon in the Gilbert-type deltas, distributary mouth bars, distributary rocks. Both hematite development and lack of organic channels, subaqueous and subaerial levees, and splays of carbon are thought to reflect deposition in the more a high-constructionallacustrine delta (Dubiel 1983b).Rapid oxygenatedlacustrine mudflat and playa environments.The sedimentation due to seasonal influx of runoff and clastic red coloration by itself is not a direct indicator of arid and volcanic sediment resulted in overloading and defor- environments, but merely reflects the increased hematite mation on portions of the delta front (Dubiel 1985). Ferns content. and giant horsetails flourished in the lower delta plain environment, and were buried by rapid sedimentation Depositional Model during flood events. As the system continued to prograde, sediment was deposited in the fluvial channels, splays, The model for Chinle deposition (Figure9) depicts the floodplains, and mudflats of the Moss Back delta plain. At complex fluvial-lacustrine system. Shinarump fluvial this time, the pre-Shinarump paleovalleys were essen- deposition, which initially filled the lowest portions of the tially filled with sediment, producing a flat depositional paleovalleysincised into the Moenkopi,progressed upvalley plain. Moss Back fluvial systems were inhabited by fluvial in response to rising baselevel. As headward deposition unionids that indicate the streams were perennial. proceededup the paleovalleys, erosion was continuing in The ash-ladenPetrified Forest sinuous fluvial channels, the headwaters of the drainage basin. The PMU and splays, and floodplains prograded over the area. The associated wetlands environments represent deposition at presence of terrestrial vertebrate remains indicates that

Figure 9. Model showing schematic depositional environments and history of deposition for the Chinle Formation. See text for detailed explanation of the model. Thin white crosshairs in each panel denote the Four Corners area. 42 JOURNALOF THE ARIZONA-NEVADAACADEMY OF SCIENCE VOL. 22

terrestrial habitats existed (Parrishand Good 1987), but fluvial, floodplain, and marsh Petrified Forest deposits to water table continued to be high, as evidenced by the lacustrine Owl Rock and then playa Church Rock and presence of aquatic vertebrates, lacustrine unionids, and eolian Wingate deposits indicates the development of organic-carbon-richmarshes. Owl Rock carbonates,silt, and another large lacustrine system and its subsequent demise. mud were depositedin an extensive lacustrineenvironment Factorscontrolling these large-scaletrends include rate and that developed in response to continued subsidence and to locus of tectonic subsidence, rate of sediment supply from a reduction in clastic and volcanic sediment input. The tectonic and volcanic sources, and amount and seasonal persistence of lungfish into the Petrified Forest and Owl distribution of precipitation as a reflection of long-term Rock ecosystems indicates that the climate was still climate. sufficiently wet to form lakes, streams, and marshes. Smaller-scalevariations within lithofacies, such as the However, the development of isolated carbonate nodules numerous climbing ripple and oscillation ripple sequences in PetrifiedForest paleosols suggests that precipitationwas in the Monitor Butte (Dubiel 1983b),variations in lithology seasonal. The alternation of siltstone and limestone beds and microfossil content of the marsh deposits, or the in the Owl Rock indicates that there were longer-termfluc- occurrence of lungfish burrows in the purple-mottledunit, tuations in the supply of water to the lake. are evidence of smaller-scaleseasonal variations in climate It is possible that the lacustrine basin was closed that affected the depositional system. The abundance of duringOwl Rock time, and that closure was accomplished wetland marsh and bog environments,fossil ferns,and giant by progradationof the Wingate erg from the northwest. horsetails 30 cm in diameter indicate there was abundant Gradually,lower water table conditions developedand playa water in the system with water tables near or above the mudflat siltstones and mudstones and ephemeral fluvial ground surfacefor much of the time. The lungfish burrows channel sandstones of the Church Rock Member were associated with gleyed paleosols indicate that there was depositedunder more oxygenated conditions that probably seasonal flooding and variationin the moisture supply.The represent extended drier periods, although there were persistent occurrence of lungfish burrows up into the certainly intermittent flood events. The inception of playa Petrified Forest and Owl Rock Members indicates that the mudflat deposition and the presence of dessication cracks climate was wet and stable enough to support lungfish in within these deposits indicate that the environment had extensive lacustrine and marsh habitats until deposition of to be periodicallywet. The overlying deposits of the eolian the Church Rock Member. Wingate erg attest to the transition to more arid climatic The occurrenceof fossil vertebrates,invertebrates, and conditions. plants in the Chinle strata (Dubiel 1983b, 1984, 1987) afford independent but complimentary interpretations of PALEOCLIMATE depositional environments and paleoclimate of the enclos- ing rocks (Parrish and Good 1987; Good et al. 1987). Several lines of sedimentologic evidence discussed in The inferredhabitats of metoposaurs and phytosaurs sup- the preceedingsections bearon the interpretationof climate port interpretationsof extensive aquatic environments, and at the time of Chinle deposition. This evidence includes in fact, indicate that many of these must have been peren- the lithofacies variation and the interpretation of deposi- nial. The perennial nature of at least the Moss Back fluvial tional environments into a depositional model; the vertical systems is suggested by the occurrence of thick-shelled, succession of depositional environments as a reflection of nonaestivating unionids in Moss Back crevasse splay fluctuatingwater tables,-the occurrenceof lungfish burrows deposits (Good et al. 1987). Thin-shelled, lacustrine in gleyedpaleosols in severaldepositional environments and unionids support sedimentologic interpretations for lungfish distribution throughout the Chinle stratigraphic Monitor Butte and Petrified Forest lakes. Paleobotanical section;the distributionand characterof faunalassemblages evidence is interpreted to indicate wet or humid tropical and trace fossils; paleosols developed on fluvial, floodplain, conditions (Ash 1972, 1978; Gottesfeld 1972). Aside from and exposed mudflat deposits as an indication of frequency the actual characterof the plants, their preservationas car- and abundance of precipitation,-the variation in organic- bonized remains and sediment-filledpith casts indicate con- carboncontent and associated color of the rocks as a reflec- ditions of rapid burial beneath the water table. tion of water table at the time of deposition; and considera- Finally, the coloration of the rocks, which is a reflec- tions basedon the paleontologyof vertebrates,invertebrates, tion of present hematite content and original and present and plants. organic-carboncontent, and the paleosol developmentyield The vertical succession of lithofacies and their inter- additionalinsight into water table fluctuations and climate. preted depositional environments discussed in the pre- Gray,black, and green colors of the Monitor Butte Member ceeding sections provides a depositional model charac- reflect its high organic-carboncontent that was the result terized by fluvial, deltaic, and lacustrine systems. The of rapid sedimentation and preservation in subaqueous progressionfrom fluvial Shinarump to marsh, lacustrine, environments or below the water table and removal from and deltaic Monitor Butte and fluvial Moss Back systems the oxidizing effects of the atmosphere. Well-developed points to the developmentby expansionand the subsequent paleosols would not be expected in these subaqueous infilling of a large lake. The overlying succession from environments. However, seasonally flooded floodplains or ISSUE 1, 1987 SEDIMENTOLOGYOF THE UPPERTRIASSIC CHINLE FORMATION 43

lacustrine mudflats would be expected to display some of the uppermembers of the Chinle, drierand more oxidiz- effects of pedogenesis. The color-mottling in the PMU is ing conditions were prevalent, and these too are reflected the result of pedogenesis, including both precipitation of in the depositional environments, lack of organic matter, hematite under alternating reducing and oxidizing condi- and correspondinglylower water tables. tions in the presence of organic matter, and of lungfish The identified depositionalenvironments and paleosols bioturbation. The redox conditions and the bioturbation suggest that although water was relatively abundantin the reflect the fluctuating water tables present in the environ- depositional system, it was punctuated by seasonally dry ment. The development of singular, isolated carbonate periods. Paleomagnetic reconstructions place this portion nodules in Chinle vertisols probably reflect the seasonal of the Colorado Plateau within the tropics during the Late influx of carbonate with precipitation or flooding. Black, Triassic, and the climate can be characterizedas tropical organic-carbon-richmarsh mudstones indicate that water monsoonal. This interpretationis consistent with indepen- tables must have been consistently near the surfacefor the dent evidence from paleontology, paleobotany, and paleo- plants to grow and to preserve the organic matter in these climatic models. environments. The Petrified Forest Member exhibits primarily ACKNOWLEDGEMENTS lavender coloration within fluvial channel and splay deposits and deeper red colors in floodplain deposits. This study is a result of ongoing investigations into the Organic-carbon-richmudstones are restrictedin extent and sedimentology and basin analysis of the Chinle Formation thickness, and coalified plant material is rare.The distribu- on the ColoradoPlateau. Many people have contributedto tion of carbonreflects the more variablewater tables of the my understandingof various aspects of the Chinle. I would upper delta plain environment that were subjected to like to thank Sidney R. Ash, Weber State College, for seasonal flooding but not to subaqueous conditions. More identifications of and discussions on the Chinle megaplant extensive oxygenated conditions resulted in a propor- fossils. Mike Parrish,University of ColoradoMuseum, pro- tionately higher destruction of organic matter. Conse- vided similar assistance with vertebratefossils; Steve Good, quently;the lack of carbonhas failed to inhibit the develop- University of Coloradoat Boulder,provided information on ment of hematite and the rocks exhibit redder coloration Chinle molluscs; and JudyParrish, U.S. Geological Survey, related to deposition under more oxidizing conditions that contributed to my understanding of monsoonal climates. were the result of periodic subaerial exposure. Appreciationis expressedto Rick Forester,U.S. Geological In the Owl Rock Member,low rates of clastic sedimen- Survey, for identification of and discussions on paleo- tation under oxygenated lacustrine water columns led to climatic implications of Chinle microfossils. During the the destruction of any organic carbon that may have been years, several people have served as both assistants in the deposited in that environment. Thus, the development field and as soundingboards for many of the ideas presented of carbonate sediments and hematite cements resulted in in this report;my thanks to Mark Larson,Paul Milde, Carl limestone beds and red siltstones, respectively. Similarly, Harris,Willy Meyer, and Carmen Guite. I would also like depositionon essentially subaerialor ephemerallywet playa to thank KarenFranczyk, U.S. Geological Survey,and Steve mudflats is responsiblefor the red coloration of the Church Good, for reviews and suggestions to improve this manu- Rock Member. An important point is that the increased script. development of red coloration in the uppermembers of the Chinle is the result of increased development of diagenetic REFERENCESCITED hematite. Eh-pHconsiderations (Garrelsand Christ 1965) demonstrate that hematite can form in either an oxidizing ASH, S. R. 1967. The Chinle (UpperTriassic) megaflora, or a reducing environment dependent upon pH, so that Zuni Mountains, New Mexico. New Mexico Geo- hematite formation by itself does not indicate an arid logical Society 18th Annual Field Conference Guide- environment.Hematite could have formedin a subaqueous book, 125-131. environment if the water was alkaline enough. While . 1972. Plant megafossils in the Chinle Forma- alkalininty may reflect aridity, it is not a prerequisite. tion. In: Breed, C. S., and Breed,W. J. (eds.),Investiga- tions in the Triassic Chinle Formation. Museum of CONCLUSIONS Northern Arizona Bulletin 47:23-44. . 1978. Geology, paleontology, and paleo- Sedimentologicconsiderations of the Chinle Formation ecology of a Late Triassic lake, western New Mexico. in southeasternUtah provideinsight into paleoclimatefrom BrighamYoung University Geology Studies 25:part2, the standpointof depositionalenvironments, organic-carbon 100 p. preservation, water table position and fluctuation, and , R. J. LITWINand A. TRAVERSE.1982. The included fossils and trace fossils. Early in the history of Upper Triassic fern Phlebopteris smithii (Daugherty) Chinle deposition, water was abundant in the system. Arnold and its spores. Palynology 6:202-219. Wetland,lacustrine, and terrestrialenvironments supported BLAKEY, R. C. 1974. Stratigraphic and depositional aquatic and terrestrial fauna and flora. During deposition analysis of the Moenkopi Formation, southeastern 44 JOURNALOF THE ARIZONA-NEVADAACADEMY OF SCIENCE VOL. 22

Utah. Utah Geological and Mineral Survey Bulletin Sedimentary Petrology 57:512-521. 104, 81 p. GARRELS,R. M. and C. L. CHRIST. 1965. Solutions, and R. GUBITOSA. 1983. Late Triassic minerals, and equilibria. Harperand Row, New York, paleogeographyand depositional history of the Chinle 450 p. Formation, southeastern Utah and northern Arizona. GOOD, S. C, J. M. PARRISHand R. F. DUBIEL. 1987. In: Reynolds, R. M. and Dolly, E. D., eds., Mesozoic Paleoenvironmentalimplications of sedimentologyand paleogeographyof west-central United States. Rocky paleontology of the Upper Triassic Chinle Formation, MountainSection, Society of Economic Paleontologists southeastern Utah. Four Corners Geological Society, and Mineralogists, Denver, Colorado, 57-76. 1987 Field Conference, 117-118. and . 1984. Controls of sand- GOTTESFELD,A. S. 1972. Paleoecology of the lower part stone body geometry and architecture in the Chinle of the Chinle Formation in the Petrified Forest. In: Formation(Upper Triassic), Colorado Plateau. Sedimen- Breed,C. S. and Breed,W. J.,eds., Investigations in the tary Geology 38:51-86. Triassic Chinle Formation. Museum of Northern BOWN,T. M., M. J.KRAUS and L. T. MIDDLETON.1983. Arizona Bulletin 47, 59-74. Triassicfluvial systems, PetrifiedForest National Park, GUBITOSA, R. 1981. Depositional systems of the Moss Arizona. Abstract, Symposium on Southwestern Back Member, Chinle Formation (Upper Triassic), Geology and Paleontology, Museum of Northern Canyonlands, Utah. Unpublished Masters Thesis, Arizona, 1 p. Northern Arizona University, Flagstaff,Arizona, 98 p. DAUGHERTY, L. H. 1941. The Upper Triassic flora of HOLT, E. L. 1947. Upright trunks of Neocalamites from Arizona.Carnegie Institute of WashingtonPublication the Upper Triassic of western Colorado. Journal of 526, 108 p., 34 plates. Geology 55:511-513. DUBIEL,R. F. 1982. Measuredsections of the Shinarump, KRAUS, M. J. and L. T. MIDDLETON. 1987. Dissected Monitor Butte, and Moss Back Members of the Chinle paleotopographyand base-level changes in a Triassic Formation (UpperTriassic) in the White Canyon and fluvial sequence. Geology 15:18-21. Red Canyon area, southeastern Utah. U.S. Geological LUPE, R. 1977. Depositional environments as a guide to Survey Open-File Report 82-729, 25 p. uranium mineralization in the Chinle Formation,San . 1983a. Stratigraphicsections of the Shina- Rafael Swell Utah. U.S. Geological Survey Journalof rump, Monitor Butte, and Moss Back Members of the Research 5:365-372. Upper Triassic Chinle Formationin the northern part . 1979. Stratigraphicsections of the Upper of the White Canyon, Red Canyon, and Blue Notch Triassic Chinle Formation, San Rafael Swell to the Canyon area, southeastern Utah. U.S. Geological Moab area. U.S. Geological Survey Oil and Gas Survey Open-File Report 83-188, 30 p. Investigations Chart OC-89, 1 plate. . 1983b. Sedimentology of the lower part of . 1984. Stratigraphicsections of the Upper the Upper Triassic Chinle Formation, southeastern Triassic Chinle Formation in the Capitol Reef, Circle Utah. U.S. Geological SurveyOpen-File Report 83-459, Cliffs, and Monument Valley areas,southeastern Utah. 48 p. U.S. Geological Survey Oil and Gas Investigations . 1984. Evidence for wet paleoenvironments, Chart OC-125, 1 plate. Upper Triassic Chinle Formation, Utah. Geological PARRISH, J. M. and S. C. GOOD. 1987. Preliminary Society of America, Rocky Mountain Section, 37th report on vertebrate and invertebrate fossil occur- Annual Meeting, Abstracts with Program 16:220. rences, Chinle Formation (Upper Triassic), south- . 1985. Preliminary report on mudlumps in eastern Utah. FourCorners Geological Society Guide- lacustrine deltas of the Monitor Butte Member of the book, 1987 Field Conference, 109-115. Chinle Formation,southeastern Utah. U.S. Geological , J. T. PARRISHand A. M. ZIEGLER.1986. Survey Open-File Report 85-27, 29 p. Permian-Triassicpaleogeography and paleoclimatology . 1986. Evolution of a fluvial-lacustrine and implications for therapsiddistributions. In Hotton, system: Tectonic and climatic controls on sedimenta- N., Roth J., Roth, C, and McLean, P. (eds.), The tion of the UpperTriassic Chinle Formation,Colorado biology and ecology of mammal-like reptiles. Smithso- Plateau. Geological Society of America, Rocky Moun- nian Press, Washington, D.C., 109-132. tain Section, 39th Annual Meeting, Abstracts with ROBINSON,P. L. 1971. A problem of faunal replacement Program 18:352. on Permo-Triassiccontinents. Paleontology14:131-153. . 1987. Sedimentology and new fossil occur- SCHULTZ, L. G. 1963. Clay minerals in Triassic rocks rences of the Upper Triassic Chinle Formation,south- of the Colorado Plateau. U.S. Geological Survey Bul- eastern Utah. FourCorners Geological Society Guide- letin 1147-C, C1-C47. book, 1987 Field Conference, 99-107. STEWART,J. H., F. G. POOLEand R. F. WILSON. 1972a. , R. H. BLODGETTand T. M. BOWN. 1987. Stratigraphyand origin of the Upper Triassic Chinle Lungfish burrows in the Upper Triassic Chinle and Formation and related Upper Triassic strata in the Dolores Formations, Colorado Plateau. Journal of Colorado Plateau region: U.S. Geological Survey Pro- ISSUE 1, 1987 SEDIMENTOLOGY OF THE UPPER TRIASSIC CHINLE FORMATION 45

fessional Paper 690, 336 p. VAN DER VOO, R., F. J. MAUK and R. B. FRENCH.1976. . 1972b. Changes in nomenclature of the Permian-Triassiccontinental configurations and the Chinle Formationon the southernpart of the Colorado origin of the Gulf of Mexico. Geology 4:177-180. Plateau: 1850's to 1950's with Changes in WATERS, A. C. and H. C. GRANGER. 1953. Volcanic nomenclatureof the Chinle Formationto 1970 by Carol debris in uraniferoussandstones and its possible bear- S. Breed.In Breed,C. S. and Breed,W. J. (eds.),Investi- ing on the origin and precipitation of uranium. U.S. gations in the Triassic Chinle Formation.Museum of Geological Survey Circular 224, 26 p. Northern Arizona Bulletin 47:75-103.