Alluvial-Fan Sedimentation of the Cutler Formation (Permo-Pennsylvanian) Near Gateway, Colorado

Alluvial-fan sedimentation of the Cutler Formation (Permo-Pennsylvanian) near Gateway, Colorado

GREG H. MACK Department of Earth Sciences, New Mexico State University, Las Cruces, New Mexico 88003 KEITH A. RASMUSSEN Phillips Petroleum Company, 101 N. Robinson, Oklahoma City, Oklahoma 73102

ABSTRACT esses (Hubert, 1960; Baars, 1962; Mack, 1977; Campbell, 1979, 1980). Alluvial-fan sediment can provide important paleogeographic and paleo- The Permo-Pennsylvanian Cutler Formation near Gateway, Colo- tectonic information, not only about conditions within the basin of deposi- rado, is the most complete (1,334 m) proximal section of alluvial-fan tion, but also about conditions in the source area. Experimental fan sediments deposited along the western flank of the Ancestral Uncom- studies and observations on modern alluvial fans indicate fan morphology, pahgre uplift. Cutler facies can be correlated with depositional processes and surface processes are controlled by both allocyclic variables, such as observed on modern "dry" alluvial fans. Proximal Cutler facies include climate and tectonism, and autocyclic variables, such as channel diversion matrix-supported bouldery debris-flow and channel-form streamflood and bar migration, that are inherent to fan depositional systems (Eckis, conglomerates. Midfan sedimentation in the Cutler is represented by 1928; Hooke, 1967; Williams, 1973; Steel and Wilson, 1975; Schumm, trough-cross-bedded, granular, braided-stream sandstones, laterally con- 1977). The ability to differentiate between these variables is critical to the tinuous streamflood conglomerates, and sheetflood deposits. Laterally con- interpretation of geologic events. One problem is converting changes in fan tinuous streamflood conglomerate was deposited at the mouth of large morphology and depositional processes observed on modern and experi- channels near the intersection point and consists of a cross-bed set as much mental fans into vertical changes in facies and grain size in an ancient as 2 m thick with a basal boulder-cobble lag. Rippled and laminated sedimentary sequence. With the exception of the relationship between siltstone with gravel channels represents distal sheetflood sedimentation. Pedogenic features include rhizocretions and calcareous nodules. Vertical changes in facies and maximum clast size delineate three megasequences on the scale of hundreds of metres thick. Each megasequence is composed of a coarsening-upward sequence of proximal facies overlain by a fining-upward sequence of more distal facies. Coarsening- upward sequences record periods of tectonic uplift and fan progradation, whereas fining-upward sequences result from tectonic quiescence and weathering-back of the mountain front. Small-scale cycles on the scale of ten metres occur within the larger megasequences and represent changes inherent to the alluvial-fan system (autocyclic). Sedimentologic data on Cutler alluvial-fan sediments at Gateway support previous interpretations of semiarid or arid paleoclimate during Permian time along the western flank of Uncompahgria and may act as a standard of comparison for tests of the role of tectonism on sedimentation trends in the Paradox basin.

INTRODUCTION

Permo-Pennsylvanian sedimentation in Colorado was controlled by uplift of two separate blocks of the Ancestral Rocky Mountains: Uncom- pahgria on the west and Frontrangia-Apishapaia on the east (Mallory, 1972a; Fig. 1). Clastic wedges deposited as an apron around the uplifts include the Cutler Formation on the west flank of Uncompahgria, the Fountain and Lyons Formations on the east flank of Frontrangia and Apishapaia, and the Maroon, Minturn, and Sangre de Cristo Formations in the intermontane basin (Mallory, 1972a, 1972b). The thickness and coarse grain size of the terrigenous detritus indicate that the source terrain Figure 1. Generalized paleogeography of the Ancestral Rocky had high relief, and arkosic composition reflects derivation from uplifts Mountains and position of the Permo-Pennsylvanian equator. Large cored by crystalline rocks (Werner, 1974). Deposition directly adjacent to arrows represent inferred trade-wind direction; adapted from Mallory the Ancestral Rocky Mountain front was the result of alluvial-fan proc- (1972a) and Mack and others (1979).

Geological Society of America Bulletin, v. 95, p. 109-116, 5 figs., January 1984.

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tectonism and sedimentation in Permo-Triassic fan deposits in Scotland debris-flow deposits indicate that the channel-form conglomerate facies (Steel and Wilson, 1975) and Devonian alluvial-fan-fluvial deposits in was deposited in fanhead channels. Norway (Steel, 1976; Steel and others, 1977), and the role of climate in Laterally Continuous Streamflood Facies. These cobble-boulder Cambrian fan deposits in Texas (McGowen and Groat, 1971), geologists conglomerates have an erosive base and are clast-supported, tabular, and have failed to utilize a potentially powerful paleogeographic and paleo- laterally extensive for tens or hundreds of metres parallel to and perpen- tectonic tool. dicular to paleoslope. A basal-lag boulder and cobble conglomerate grades The present study examines the most complete proximal alluvial-fan upward into a single thick (2-m) trough cross-bed set composed of pebbles section of the Cutler Formation deposited on the western flank of Uncom- to coarse sand (Fig. 2c). This facies is relatively uncommon and represents pahgria (Mack, 1977). Depositional facies within the Cutler are related to about 2% of the total section. The basal lag and overlying cross-bed set modern alluvial-fan processes, and vertical changes in grain size and facies represent a single depositional event and imply flow depths of at least 4 m are related to allocyclic and autocyclic variables. and flow velocities that initially were high enough to transport boulders but subsequently were in the upper part of the lower flow regime. Thick METHODS (2-m) sets of cross-bedded gravelly sandstone in other ancient alluvial-fan sequences are interpreted to have resulted from streamflood deposition in Coarse arkosic alluvium of the Cutler Formation was deposited fanhead channels (Bluck, 1965; Steel, 1974; Brookfield, 1980). The thick directly adjacent to the ancient Uncompahgre uplift near Gateway, sets of cross-bedded gravelly arkose in the Cutler differ from previously Colorado (Fig. 1). Several measured sections were combined to form a described streamflood facies by being laterally extensive rather than complete section 1,334 m thick. The basal contact between the Cutler and channel-form, a characteristic that is not consistent with deposition in Precambrian crystalline rocks coincides with the Permo-Pennsylvanian steep-sided fanhead channels. Perhaps the laterally continuous streamflood boundary fault system, because northeast of the contact the Triassic Chinle facies of the Cutler was deposited near the intersection point, where chan- Formation lies directly on Precambrian rocks. Owing to the low angle of nels become shallower and wider before they merge with the midfan dip of the Cutler and to modern erosion, the construction of a complete surface. vertical section was possible only by measuring each partial section at a Braided-Stream Facies. Braided-stream sediment is characterized by location progressively southwest of the Cutler-Precambrian contact. This abundant trough cross-beds in sets 10 to 30 cm thick (Fig. 2c). Grain size direction corresponds roughly to fan paleoslope. Each partial section thus ranges from fine sand to granules, with local lenticular to massive bodies of was measured at an increasingly distal location relative to the ancient clast-supported pebbles and cobbles. Horizontal bedding is subordinate mountain front; the most distal partial section was deposited 7 to 10 km but commonly forms laterally extensive horizons 10 to 40 cm thick. The southwest of the basal section. The upper contact of the Cutler is an pebble-cobble conglomerates and horizontally laminated granular sand- angular unconformity between the near-horizontal Moenkopi Formation stones result from deposition of longitudinal bars under rapid flow condi- (Triassic) and the Cutler, which dips 4° to the southwest. tions (Williams, 1971). The abundant trough cross-beds form as dune bed forms migrate along the channel floor during intermittent high water CUTLER FORMATION FACIES (Allen, 1963, 1964; Harms and Fahnestock, 1965; Williams, 1968, 1969, 1971). Debris-Flow Facies Red to maroon, massive to poorly laminated, sandy micaceous mud- stone and thinly laminated brown claystone occur as lenticular to planar This facies consists of laterally continuous, extremely poorly sorted, beds within the braided-stream sediments. This fine-grained sediment is matrix-supported boulder conglomerate (Fig. 2a). Large clasts as much as interpreted as local overbank sediment that accumulated along migrating 600 cm in diameter float in a muddy to granular matrix. The basal contact braided channels (Anderson and Picard, 1974). Silt and clay are deposited with the underlying units is abrupt and non-erosive, and stratification is from suspension along bar tops or within cutoff secondary channels during rare. Sequences commonly coarsen upward as large boulders become waning flood stages and may be reworked by subsequent braided-stream concentrated near the top. Debris flows are restricted to the lower one-half processes (Williams and Rust, 1969; Rust, 1972; Costello and Walker, of the section at Gateway and have previously been described by Mack 1972; Smith, 1974). Braided-stream sediment clearly constitutes the (1977), Campbell (1979), and Shultz (1980). Granular arkose exhibiting dominant facies in the Cutler at Gateway, making up about 65% of the channel-form and trough cross-bedding commonly caps each debris flow. total measured section. Debris-flow deposits represent about 6% of the total section at Gateway. The Sheetflood Facies. Horizontally laminated or rippled, sandy relative abundance of debris flows is misleading, however, and is con- micaceous siltstone makes up about 5% of the total section and about 13% trolled by the necessity of measuring the partial sections in an increasingly of the middle and upper parts of the Cutler section at Gateway. These distal direction from the ancient mountain front. In the lower 350 m of the orange-red units range in thickness from 0.5 to 10 m and are laterally section, the most proximal to the ancient mountain front, debris-flow continuous for several kilometres, both parallel to and perpendicular to deposits constitute 25% of the section and 40% of the exposed section paleoslope, with only minor changes in thickness. Granule to pebble con- (exclusive of covered intervals). glomerate forms channels with high length-to-width ratios within the siltstone units (Fig. 2d). The gravel channels range from 5 to 80 cm thick. Water-Laid Deposits Siltstone interbedded with the channel conglomerate is massive to horizontally laminated with rare parting lineation. The top of each siltstone Channel-Form Streamflood Facies. These coarse, clast-supported unit generally consists of asymmetrical ripples with ripple indices from 7 conglomerates have an erosive, channeled base and represent about 2% of to 12. Soft-sediment deformation also is common in the upper part of the the Cutler section at Gateway (Fig. 2b). Imbrication and cross-bedding are siltstone. Burrows and rhizocretions (root casts) are found throughout weakly developed or absent. Channel morphology and association with the units.

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Figure 2. Depositional fades of the Cutler Formation at Gateway, Colorado, a. Matrix-supported debris flow, capped by a thin (20-cm) unit of granular arkose. b. Two channel-form streamflood conglomerates (base of cliff and left center) that truncate debris-flow facies. The recessed debris-flow unit in the center of the photo is 2 m thick, c. Laterally continuous streamflood facies composed of boulder lag overlain by a single cross-bed set of granule and pebble conglomerate. Streamflood facies is overlain by trough-cross-bedded braided-stream granular arkose. d. Gravel channel within siltstones of the sheetflood facies; ruler is 15 cm long. e. Rhizocretion formed by calcite replacing the root and trunk of a plant; ruler is IS cm long.

channels commonly exist in discrete horizons within the sheetflood unit. Furthermore, pedogenic features also occur in distinct intervals. These observations suggest periods of sheetflood deposition followed by long e periods of nondeposition and soil formation. The orange siltstone is interpreted as distal sheetflood deposited on abandoned and distal segments of the fan during major floods. The gravel Pedogenic Features channel-fills probably represent a network of small, braided distributaries radiating out across the fan surface (Davis, 1938; McGowen and Groat, Rhizocretions are the most common pedogenic feature in the Cutler 1971; Heward, 1978; Brookfield, 1980). Two lines of evidence suggest Formation at Gateway. Rhizocretions were observed within sheetflood that each sheetflood unit represents several depositional events. Gravel siltstones and braided-stream conglomerates and sandstones. The greatest

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concentra tion is found in the upper part of the Cutler section. Rhizocre- ing a few hundred years, would inhibit stage III and IV soil development tions range in size from 0.5 to 20 cm in diameter and can be traced (Gile and others, 1981). Steel (1974) recognized mature petrocalcic hori- laterally and vertically for 1 to 3 m in some cases. Thin-section analysis zons in Permo-Triassic alluvial-plain facies in Scotland but noted their reveals micrite traversed by concentrically oriented veinlets of sparry cal- absence in contemporaneous alluvial-fan facies. cite. The core of the root casts contains silt- and sand-sized grains of quartz and feldspar partially replaced by sparry calcite. Similar descriptions of DEPOSITIONAL MODEL Pennsylvanian and Permian rhizocretions have been made by Loope (1980) and Loope and Schmitt (1980) in the Cedar Mesa Sandstone of Cutler facies can be correlated with depositional processes acting on southeastern Utah and the Fountain Formation of central Colorado. The modern arid and semiarid alluvial fans (Fig. 3). The most compelling pedogenic: origin of the rhizocretions is further supported by their presence evidence for "dry" fan deposition is the presence of debris flows, sheet- as clasts in overlying braided-stream facies. The large size of some rhizo- flood facies, and rhizocretions. Debris flows result from the generation of cretions indicates the establishment of trees in areas of little sedimentation landslides and/or slope wash in the catchment area of an alluvial fan (Fig. 2e). during a heavy storm. Steep slopes, loose debris, lack of vegetative cover, At a few localities, carbonate nodules were observed in sheetflood and infrequent heavy rainfall are critical factors in the development of siltstone. The nodules are elliptical in shape and average 2 cm in diameter. debris flows and are most common in arid and semiarid climates (Black- The concentration of nodules is low, averaging less than 4 per 100 cm2. welder, 1928; Sharp and Nobles, 1953; Beaty, 1963; Bull, 1964; Hooke, These nod ules resemble stage II development of calcic soil horizon in arid 1967; Wasson, 1977). Sheetflood deposits also are dependent upon and semiarid climates (Gile and others, 1981). Rare dense calcite horizons ephemeral, rapid , surface runoff characteristic of arid and semiarid alluvial as much as 5 cm thick also are present close to rhizocretion zones. The fans (McGee, 1897; Davis, 1938; Bull, 1964; Rahn, 1967; Williams, 1973; calcite layers are parallel to bedding and extend tens of metres laterally. Wasson, 1977). Heavy vegetative cover and perennial streamflow on These horizons do not resemble the stage III or IV plugged and laminar "wet" alluvial fans are not conducive to deposition by debris-flow and petrocalcic horizons of Gile and others (1981) and may be diagenetic in sheetflood processes. Calcified roots (rhizocretions) and pedogenic car- origin. Color mottling is very well developed within granule conglomer- bonate nodules also are diagnostic of arid and semiarid climates (Steel, ates of the braided-stream facies. Red-brown to purple "red beds" contain 1974; Gile and others, 1981). greenish-gray zones of variable shape and extent that may represent local The distribution of processes on modern "dry" alluvial fans is a reducing conditions caused by breakdown of plant material. These mottled function of proximity to the source area (Fig. 3). Debris flows and zones are not enriched in calcite. channel-form streamflood processes are diagnostic of proximal fanhead The presence of rhizocretions and scattered carbonate nodules sets deposition (Beaty, 1963; Bull, 1964; Denny, 1965; Hooke, 1967; Steel and constraints on the length of time of soil development. Replacement of Wilson, 1975), whereas deposition in the midfan below the intersection roots by calcite and development of stage II nodules are relatively rapid point is dominated by sheetflood and braided streams that head on the fan processes when compared to more advanced petrocalcic soil features. surface (McGee, 1897; Davis, 1938; Denny, 1965; Lustig, 1965; Bull, Plugged and laminar petrocalcic horizons (stages III and IV of Gile and 1972; Williams, 1973). Distal fan sedimentation is similar to midfan depo- others, 1981) require in excess of 104 yr and perhaps as much as 105 or sition except for a decrease in grain size and the addition of eolian and 106 yr to form (Gile and others, 1981). Conversely, rhizocretions and stage playa-lake facies (Steel, 1974; Steel and Wilson, 1975). 3 II calcic nodules develop in less than 10 yr and probably no more than Several Cutler facies are perhaps less characteristic of "dry" fan sedi- 2 10 yr. Although this time is small compared to the advanced petrocalcic mentation. The laterally continuous streamflood facies indicates relatively horizons, it does imply long periods of little or no sedimentation on the fan deep, moderate-velocity flow, a condition not commonly associated with surface. The lack of advanced-stage soils in the Cutler is probably a func- "dry" fans. This facies may represent deposition near the intersection point tion of sedimentation rate. Rapid deposition, with diastems rarely exceed- by flood currents that initially deposited the boulder lag and then during

ZONE PROCESS DESCRIPTION

matri*-supporte

Streamflood laterally continuous gravel near inter- composed of single cross- section point bed set MID- Braided trough crossbedded sand- Figure 3. Depositional model for arid and FAN Stream stone and conglomerate semiarid (dry) alluvial fan, based on the work of laterally continuous silt Sheetflood and shallow gravel Bull (1964,1972), Denny (1965), and Hooke (1967). channels

Braided troughcrossbedded sand- Stream stone and conglomerate DISTAL laterally continuous silt Sheetflood and shallow gravel channels FAN laminated, gypsiferous Playa DRY ALLUVIAL silty mud and clay crossbedded, well-sorted Eolian FAN MODEL fine to medium sand

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Moenkopi

Braided Stream

Streamf lood

Braided Stream COARSENING

Sheetflood

Debris Flow Streamflood

Sheetflood 2 nd Braided COARSENING Stream

N-49 Laterally 228° Continuous Streamflood

Streamflood

Debris Flow I st COARSENING

Figure 4. Generalized Cutler section showing megasequences delineated by vertical changes in maximum clast size and in facies. Precambrian 0 100 200 300 400 600 Facies trends are generalized. Also shown are paleocurrent rosettes and mean current direction (arrows) for five randomly selected Maximum Clast Size (cm) domains within the Cutler. Paleocurrent data are from trough cross-bed axes, n = number of paleocurrent measurements in each megasequence consists of a coarsening-upward cycle followed by a fining- domain. upward cycle. The first coarsening-upward sequence begins with about 100 m of massive to laminated, poorly sorted granular arkose. Clasts are the waning stage produced the dune bed form. Similarly, the abundance of extremely angular and have a maximum size of 9 cm. This sediment trough-cross-bedded braided-stream facies indicates that transportation of probably represents grus that was transported only a short distance or not sand and gravel as dune bed forms was a common process on the Cutler at all during the incipient stages of uplift. Significant increase in grain size alluvial fan. Although modern arid and semiarid alluvial fans are com- in the first coarsening-upward sequence corresponds to the appearance of monly dissected by braided washes and gullies, ephemeral, high-velocity channel-form streamflood and debris-flow conglomerates, suggesting flow within the channels is not conducive to the formation of dune bed sedimentation in a proximal fan setting. Maximum clast size reaches 600 forms. Velocity and water depth in midfan washes, however, may be cm. The fining-upward part of the first megasequence corresponds to a strongly influenced by fan gradient. Small, steep fans, such as the modern decrease in maximum grain size to only a few centimetres in diameter. fans studied in the western United States and Australia, would favor Debris-flow and streamflood facies are replaced by laterally continuous rapid-flow-and upper-flow-regime bed forms, whereas large, gentle fans streamflood and braided-stream facies, which are indicators of midfan would experience periods of lower-velocity flow. deposition. The upper part of the fining-upward cycle contains a high percentage of overbank mudstones and claystones. TECTONIC CONTROL ON CUTLER ALLUVIAL-FAN The second coarsening-upward sequence, although thinner than the DEPOSITION first sequence, also exhibits a significant increase in grain size to almost 200 cm in diameter. The grain-size increase is accompanied by the appearance Three large-scale megasequences on the scale of hundreds of metres of laterally continuous streamflood facies, reflecting deposition near the thick exist in the Cutler alluvial-fan sediments at Gateway (Fig. 4). Each intersection point, and proximal fan debris-flow facies. The second fining-

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sediments of the third coarsening sequence, which are eroded away, would be comparable in size to sediments in the first coarsening sequence. The upper contact of the Cutler Formation at Gateway is an angular unconformity (Fig. 4). The Cutler dips about 4° to the southwest below the near-horizontal Lower Triassic Moenkopi Formation. An unknown thickness of upper Cutler was removed by erosion. In Utah, the Cutler- Moenkopi contact is a disconformity (Baars, 1962), implying that the tectonism responsible for the angular unconformity was centered near the Cutler-Precambrian contact. The three megasequences in the Cutler Formation are interpreted to be the result of three tectonic pulses. As the source area was uplifted along the Uncompahgria boundary fault (and/or the basin was downdropped), coarse proximal alluvial-fan facies prograded over more distal facies (Fig. 5). The fining-upward part of each megasequence resulted from source-area retreat during atectonic periods. Fan regression superimposed more distal facies over more proximal facies (Steel and Wilson, 1975; Steel and others, 1977). The possibility that other variables are responsible for the Cutler megasequences must be considered. Bull (1964) and Williams (1973) observed that climate can be a dominant force on Quaternary alluvial-fan processes such as fan building and entrenchment. Periods of semiarid climate corresponded to fan building, whereas periods of drier-than- normal conditions induced fan dissection and deposition of gravel at the fan toe. Wetter-than-normal periods were represented by well-developed paleosols consisting of calcite nodules and cylindroids (rhizocretions?) and a few laminated and plugged petrocalcic horizons (Williams, 1973). There is no evidence in the Cutler that climatic fluctuations were responsible for Figure 5. Schematic diagrams of paleotectonic con- the megasequences. Rhizocretions and nodular calcite horizons are present trol on alluvial-fan deposition of the Cutler Formation throughout the entire Cutler section in both the fining-upward and near Gateway, Colorado. Coarsening-upward sequences coarsening-upward sequences, suggesting that major climatic variations result from uplift along the boundary fault, whereas did not occur during Cutler deposition. fining-upward sequences reflect weathering-back of the The overlapping of adjacent fans also has been cited as a possible source area. mechanism for superimposing more distal and more proximal facies and causing vertical fluctuations in grain size (Steel and Wilson, 1975). Fan coalescence should be represented by a significant and sudden deviation in mean paleocurrent direction, a phenomenon that is absent in the Cutler at upward sequence exhibits a decrease in grain size and is dominated by Gateway (Fig. 4). Paleocurrent measurements from five domains ran- braided-stream and sheetflood facies. domly selected in the Cutler section'reveal that the average current The third megasequence is not nearly so well defined as the previous direction varied by less than 40° through time (Fig. 4). two. About 1,100 m from the base, grain size increases, and streamflood Eckis (1928) considered fanhead entrenchment to be the natural conglomerates break the monotony of interbedded sheetflood and braided- result of decreased sediment supply in the source area. Denny (1967) stream sediment. Several lines of evidence suggest that the third related episodes of fanhead incision to exceptional flood events. Another coarsening-upward sequence is the result of fanhead entrenchment. autocyclic mechanism of Schumm (1977) suggested that progressive ac- Scoured channels backfilled with cobbles and boulders are interpreted as cumulation of sediment near the fan apex results in fanhead entrenchment streamflood facies deposited by the primary channel system. Furthermore, and a shift of the locus of deposition downfan. Although these mechanisms the size and abundance of rhizocretions associated with the third may episodically transport alluvium to the fan toe, rapid backfilling of the coarsening sequence suggest that vegetative cover became established fanhead trench causes deposition to migrate upfan. Autocyclic fanhead during long periods of minor deposition. Such conditions occur as channel entrenchment thus seems incapable of producing coarsening-upward se- incision shifts the locus of deposition downfan and leaves an abandoned quences on the scale of hundreds of metres thick. The megasequences in fan mesa (Blissenbach, 1954). A fining-upward sequence near the top the Cutler Formation at Gateway, therefore, can best be explained by of the section shows a decline in the abundance of streamflood tectonism. conglomerates. There is an over-all decrease in maximum clast size upsection in the AUTOCYCLIC SEDIMENTATION OF THE Cutler (Fig. 4). This trend is largely controlled by the necessity of measur- CUTLER FORMATION ing the section in a progressively distal direction. The third coarsening sequence was measured almost 10 km down paleoslope from the first Small-scale fining and coarsening cycles and/or cyclic changes in coarsening sequence and is naturally finer grained. Perhaps more proximal facies on a scale of 10 m or less are common in Cutler alluvial-fan

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sediments at Gateway, Colorado. These small-scale cycles are controlled processes. If minor tectonic pulses are the cause, they are not recognizable by autocyclic variables inherent to alluvial-fan deposition. Three different by vertical grain-size changes. facies assemblages account for most of the small-scale cycles in the Cutler. REGIONAL IMPLICATIONS Debris Flow-Braided Stream Sedimentologic interpretations of the Cutler Formation at Gateway, Many debris-flow units are capped by a thin (

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Vertical changes in facies and maximum clast size delineate three Davis, W. M., 1938, Sheetfloods and streamfloods: Geological Society of America Bulletin, v. 49, p. 1337-1416. Denny, C. S., 1965, Alluvial fans in the Death Valley region, California and Nevada: U.S. Geological Survey Professional megasequences on the scale of hundreds of metres thick. Coarsening- Paper 466, 62 p. upward parts of the megasequences represent periods of tectonic uplift and 1967, Fans and pediments: American Journal of Science, v. 265, p. 81-105. Eckis, R., 1928, Alluvial fans of the Cucamonga district, southern California: Journal of Geology, v. 36, p. 225-247. fan progradation. Fining-upward portions of the megasequences were Gile, L. H., Hawley, J. W., and Grossman, R. B., 1981, Soils and geomorphology in the Basin and Range area of southern New Mexico—Guidebook to the Desert Project: New Mexico Bureau of Mines and Mineral Resources Memoir produced during tectonically quiescent periods and reflect weathering- 39, 222 p. back of the mountain front. Harms, J. C., and Fahnestock, R. K., 1965, Stratification, bedforms, and flow phenomena (with an example from the Rio Grande): Society of Economic Paleontologists and Mineralogists Special Publication 12, p. 82-115. Small-scale (10-m) cyclic sedimentation in the Cutler Formation Heward, A. P., 1978, Alluvial fan and lacustrine sediments from the Stephanian A and B (La Magdalena, Cinera- Matallana and Sabero) Coalfields, northern Spain: Sedimentology, v. 25, p. 451-488. represents changes inherent to the alluvial-fan system (autocyclic). Vertical Hooke, R. LeB., 1967, Processes on arid region alluvial fans: Journal of Geology, v. 75, p. 438-460. Hubert, J. F., 1960, Petrology of the Fountain and Lyons Formations, Front Range, Colorado: Colorado School of Mines alternation of debris flow-braided stream, debris flow-streamflood, and Quarterly, v, 55, p. 1-242. sheetflood-braided stream account for most of the autocycles. Loope, D. B., 1980, Evidence for soil-forming episodes during deposition of the Permian Cedar Mesa Sandstone of Utah: Geological Society of America Abstracts with Programs, v. 12, p. 278. Alluvial-fan sediments of the Cutler Formation at Gateway provide Loope, D. B., and Schmitt, J. G., 1980, Caliche in the late Paleozoic Fountain Formation: Rediscovery and implications: Geological Society of America Abstracts with Programs, v. 12, p. 473. paleoclimatic and paleotectonic evidence. Debris flows, sheetflood facies, Lustig, L. K., 1965, Clastic sedimentation in Deep Springs Valley, California: U.S. Geological Survey Professional Paper and rhizocretions in the Cutler support arid or semiarid climatic interpreta- 352-F, 192 p. Mack, G. H., 1977, Depositional environments of the Cutler-Cedar Mesa fades transition (Permian) near Moab, Utah: tions. The presence in the Cutler of Precambrian quartz monzonite Mountain Geologist, v. 14, p. 53-68. Mack, G. H., Suttner, L. J., and Jennings, J. R., 1979, Permo-Pennsylvanian climatic trends in the Ancestral Rocky porphyry boulders, which today are exposed directly adjacent to Cutler Mountains, in Baars, D. L., ed., Permianland: Four Corners Geological Society Guidebook, 9th Annual Field rocks at Gateway, suggests that movement on the Permo-Pennsylvanian Conference, p. 7-21. Mallory, W. W., 1972a, Regional synthesis of the Pennsylvanian system: Geologic atlas of the Rocky Mountain region: boundary fault at Gateway was primarily dip-slip. Rocky Mountain Association of Geologists, p. 111-127. 1972b, Pennsylvanian arkose and the Ancestral Rocky Mountains: Geologic atlas of the Rocky Mountain region: Rocky Mountain Association of Geologists, p. 131-132. McGee, W. J., 1897, Sheetllood erosion: Geological Society of America Bulletin, v. 8, p. 84-112. ACKNOWLEDGMENTS McGowen, J. H., and Groat, C. G., 1971, Van Horn Sandstone, west Texas: An alluvial fan model for mineral exploration: University of Texas at Austin, Bureau of Economic Geology, Report of Investigations, v. 72, 57 p. Rahn, P. H., 1967, Sheetfloods, streamfloods, and the formation of pediments: Association of American Geographers This study was funded through a grant from the American Chemical Annals, v. 57, p. 593-604. Rust, B. R., 1972, Structures and processes in a braided river: Sedimentology, v. 18, p. 221-245. Society-Petroleum Research Fund (no. 11647-AC2) awarded to Greg Schumm, S. A„ 1977, The fluvial system: New York, John Wiley and Sons, Inc., 338 p. Mack and Lee J. Suttner. Data for this study were taken, in part, from an Sharp, R. P., and Nobles, L. H., 1953, Mudfiow of 1941 at Wrightwood, southern California: Geological Society of America Bulletin, v. 64, p. 547-560. 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G., 1972, Pleistocene sedimentology: Credit River, southern Ontario: A new component REVISED MANUSCRIPT RECEIVED FEBRUARY 3, 1983 of the braided river model: Journal of Sedimentary Petrology, v. 42, p. 389-400. MANUSCRIPT ACCEPTED FEBRUARY 15, 1983

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