Upper Cretaceous) and Modern Himalayan Foreland Basin Systems

Home , Evanston Formation, Himalayan foreland basin

A comparison of fluvial megafans in the Cordilleran (Upper Cretaceous) and modern Himalayan foreland basin systems

P. G. DeCelles* Department of Geosciences, University of Arizona, Tucson, Arizona 85721 W. Cavazza Center for Advanced Studies of Geodynamics, University of Basilicata, 85100 Potenza, Italy

ABSTRACT these types of deposits may be the volumetrically largest gravel accumulations in nonmarine foreland basin systems. The Campanian–Maastrichtian Hams Fork Conglomerate Member of the Evanston Formation in northeastern Utah and southwestern INTRODUCTION Wyoming consists of a widespread (>10 000 km2) boulder to pebble, quartzitic conglomerate that was deposited by east-southeastward– A fluvial megafan is a large (103–105 km2), fan-shaped (in plan view) flowing, gravelly braided rivers on top of the frontal part of the Sevier mass of sediment deposited by a laterally mobile river system that em- fold-thrust belt and in the adjacent foredeep of the Cordilleran foreland anates from the mouth of a gorge at the topographic front of a mountain basin. In northeastern Utah the conglomerate was deposited in a lobate range (Fig. 1; Gohain and Parkash, 1990). Fluvial megafans are especially fan-shaped body, up to 122 m thick, that trends southeastward away prevalent on the proximal sides of nonmarine foreland basin systems, from its principal source terrane in the southern end of the Willard where large antecedent rivers exit the fold-thrust belt and debouch onto the thrust sheet. The Willard sheet contains thick Proterozoic quartzite units low-relief alluvial plain of the foreland basin (e.g., Geddes, 1960; Wells that produced highly durable clasts capable of surviving long-distance and Dorr, 1987a, 1987b; Gohain and Parkash, 1990; Willis, 1993; Sinha fluvial transport. Although the main source of sediment for the Hams and Friend, 1994; Gupta, 1997). Like other fan-shaped depositional sys- Fork Conglomerate was the Willard sheet, the active front of the thrust tems, the morphology of a megafan results from the fact that the upstream belt lay 40–50 km to the east along the Absaroka thrust system. Dis- portion of the main feeder channel is fixed by the location of the exit gorge, placement along the Absaroka system uplifted and topographically reju- whereas downstream reaches of the channel are free to migrate laterally venated the Willard sheet, and antecedent drainages carried detritus over an arc of ~180° (Parkash et al., 1980; Stanistreet and McCarthy 1993; from hinterland source terranes into the proximal foreland basin. Al- Sinha and Friend, 1994). The actual arc of migration is typically much less though topographic ridges associated with fault-propagation anticlines than 180°, however, because adjacent megafans constrict each other later- along frontal thrusts locally influenced transport directions, they pro- ally. Other terms that have been used to describe large subaerial fans in- vided relatively little sediment to the Hams Fork Conglomerate. clude fluvial fans (Collinson, 1996), terminal fans (Friend, 1978; Parkash Lithofacies, paleocurrent, and isopach data indicate that the Hams et al., 1980; Kelly and Olsen, 1993), fluvial distributary systems (Nichols, Fork Conglomerate was deposited in fluvial megafans and stream- 1987), and humid, or wet, alluvial fans (Schumm, 1977). Terminal or near- dominated alluvial fans, similar in scale and processes to megafans and terminal fans, such as those discussed by Mukerji (1976), Friend (1978), alluvial fans in southern Nepal and northern India that are forming Parkash et al. (1980), and Stanistreet and McCarthy (1993), are character- along the proximal side of the Himalayan foreland basin system. The ized by distributary fluvial channels that ultimately run dry because of Himalayan fluvial megafans have areas of 103–104 km2, slopes of evaporation and seepage. Terminal fans described in the Indo-Gangetic 0.05°–0.18°, and are deposited by large transverse rivers that are an- foreland basin are occupied by underfit channels, suggesting that the fans tecedent to frontal Himalayan structures and topography. The main themselves were mainly formed during wetter climatic phases (Parkash fluvial channels on the upper parts of the megafans are anastomosed et al., 1980). and braided at bankfull stage but commonly have braided thalwegs at Fluvial megafans are distinct from typical sediment–gravity low-flow stage. Downstream, these channels become predominantly flow–dominated and stream-dominated alluvial fans in terms of their braided and meandering and ultimately merge with the axial Ganges sizes, slopes, textural ranges, and depositional processes (Singh et al., trunk river system. Stream-dominated alluvial fans in the Himalayan 1993; Stanistreet and McCarthy, 1993). The main distinction is scale: flu- foreland basin system fringe the topographic front of the fold-thrust vial megafans are deposited by sizeable rivers, and therefore have geo- belt in the intermegafan areas. These fans have areas of ~102 km2 and morphic and sedimentologic characteristics that are typical of large fluvial slopes of ~0.5°. The proximal parts of both types of fans are dominated systems, including a predominance of water-laid facies, large areal distri- by extremely coarse (boulder-cobble) bedload that is in transit mainly bution of facies, and low slope (Table 1; Fig. 2; e.g., Stanistreet and during the monsoon. The prevalence of fluvial megafans in the modern McCarthy, 1993; Blair and McPherson, 1994). Although they are perhaps and Miocene Himalayan foreland and in the Upper Cretaceous–lower the paramount depositional elements in the proximal parts of most mod- Tertiary stratigraphic record of the Cordilleran foreland suggests that ern, nonmarine foreland basin systems, the sedimentological literature contains only meager information about possible ancient counterparts of *E-mail: [email protected]. fluvial megafans and little attempt has been made to specifically compare

GSA Bulletin; September 1999; v. 111; no. 9; p. 1315–1334; 16 figures; 1 table.

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EXIT GORGE EXIT FOLD-THRUST BELT GORGE

FRINGE OF STREAM- DOMINATED ALLUVIAL FANS INTER-MEGAFAN AREA

FLUVIAL MEGAFAN 10 s to 100+ km

AXIAL FLUVIAL TRUNK SYSTEM

GRAVELLY SANDY MIXED SANDY-SILTY

Figure 1. Schematic map showing the main large-scale morphological and depositional elements of a typical nonmarine foreland basin system, based mainly on the modern Himalayan and Andean foreland basin systems. Two large transverse rivers deposit fluvial megafans, which are sep- arated by an intermegafan area, the proximal part of which is occupied by stream-dominated alluvial fans. The transverse rivers ultimately join an axial fluvial trunk river system.

modern and ancient megafan deposits (e.g., Willis, 1993; Kelly and Olsen, ies (Oriel and Tracey, 1970; Jacobson and Nichols, 1982; Nichols and 1993; DeCelles et al., 1998). The purposes of this paper are to document Bryant, 1990) and dinosaur fossils (Oriel and Tracey, 1970) indicate a deposits of ancient fluvial megafans in the Upper Cretaceous Hams Fork Campanian–Maastrichtian age for the Hams Fork Conglomerate, and a Conglomerate Member of the Evanston Formation, a widespread synoro- Paleocene age for the Main Body of the Evanston Formation. genic foreland-basin deposit in northeastern Utah and southwestern Throughout the study area in northeastern Utah and southwestern Wyoming (Fig. 3), and to draw some comparisons between these ancient Wyoming, the Evanston Formation rests on a basal angular unconformity deposits and modern fluvial megafans in Nepal and northern India. that represents a hiatus of several million years’ duration (Oriel and Tracey, 1970; Jacobson and Nichols, 1982). The unconformity bevels rocks as old GEOLOGIC AND TECTONIC SETTING as Pennsylvanian and as young as Upper Cretaceous. In the vicinity of the Absaroka, Medicine Butte, and Coalville thrusts, the unconformity is highly The Hams Fork Conglomerate is the middle member of the Upper Cre- angular (>25°). A progressive unconformity (e.g., Anadon et al., 1986) in taceous–lower Paleocene Evanston Formation, which crops out discon- the Hams Fork Conglomerate adjacent to the Absaroka thrust indicates that tinuously over an area of >10 000 km2 in northeastern Utah and south- the thrust was active during deposition of the conglomerate (Oriel and western Wyoming in the southern part of the Idaho-Wyoming-Utah Tracey, 1970; Lamerson, 1982). salient of the Sevier fold-thrust belt (Fig. 3; Oriel and Tracey, 1970). The To fully appreciate the tectonic implications and paleogeography of the Evanston Formation is up to ~650 m thick, entirely fluvial, and consists of Hams Fork Conglomerate, a brief explanation of its regional structural setting an unnamed lower member, the Hams Fork Conglomerate, and an upper is necessary. From west to east, the six major thrust systems in the southern member referred to as the Main Body (Fig. 4). The lower member is pre- part of the Idaho-Wyoming-Utah salient are the Willard, Crawford, Coalville, dominantly mudstone, lignite, and coal. The Hams Fork Conglomerate is Medicine Butte, Absaroka, and Hogsback thrusts (Fig. 3). The Willard thrust a prominent, cliff-forming conglomerate and sandstone unit. The Main carries a >10-km-thick succession of Proterozoic and Paleozoic sedimentary Body is composed of lignitic mudstone, coal, gray siltstone, and lenticu- and low-grade metasedimentary rocks (Yonkee, 1992). The other five thrusts lar conglomeratic sandstone (Oriel and Tracey, 1970). Palynological stud- carry Paleozoic and Mesozoic rocks and generally ramp upsection eastward

1316 Geological Society of America Bulletin, September 1999

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TABLE 1. LIST OF SELECTED MODERN AND ANCIENT FLUVIAL MEGAFANS IN FORELAND BASIN SYSTEMS River-Megafan Location Catchment Megafan Slope Reference(s) (103 km2) area (103 km2) Modern Kosi Nepal, India 59 ~16.5 0.06° Sinha and Friend (1994) (Himalaya) Wells and Dorr (1987a, 1987b) Gandak Nepal, India 45 ~17.5 0.03° Sinha and Friend (1994) (Himalaya) Karnali Nepal, India 44 ~1.0 0.18° This paper (Himalaya) Tista Nepal, India ~12 ~16 0.04° Geddes (1960) (Himalaya) This paper Pastaza-Marañon Peru, Brazil ? 60 ? Rasänen et al. (1991) (Andes [north]) Jachal Argentina 27.7 ~1.4 ? Damanti (1993) (Andes [south]) Huaco Argentina 7.1 ~0.7 ? Damanti (1993) (Andes [south]) Ancient Hams Fork Utah ? >2.5 ? This paper (Cretaceous–Paleocene) (Sevier belt) Harebell-Pinyon Wyoming ? >10 ? Lawton et al. (1994) (Cretaceous–Paleocene) (Sevier belt) Lindsey (1972) Central Utah Utah ? >3.6 ? Lawton et al. (1994) (Cretaceous–Paleocene) (Sevier belt) Southern Utah Utah ? >4.0 ? Schmitt et al. (1991) (Cretaceous–Paleocene) (Sevier belt) Lawton et al. (1994) Luna Spain ? ~2.5 ? Nichols (1987) (Miocene) (Pyrenees)

from a regional décollement in Cambrian shale. The Cambrian décollement for the five frontal thrusts roots to the west beneath the Wasatch anticlinorium, 5.0 a large antiformal duplex consisting of several horses of Precambrian crystalline basement rocks and overlying Paleozoic–Mesozoic cover rocks that Typical sediment- were imbricated by the Ogden thrust system (Bruhn et al., 1983; Schirmer, gravity flow fans 3.0 1988; Yonkee, 1992; Yonkee et al., 1997). The present trace of the Willard thrust is broadly synformal because the fault was folded by uplift of the Wasatch anticlinorium. Because the Crawford, Absaroka, and Hogsback 1.0 thrusts root beneath the Wasatch anticlinorium, displacements on each of these frontal thrusts were accompanied by displacements on the Ogden thrust system and coeval growth of the anticlinorium. Thus, the Wasatch anticlinorium and the overlying Willard thrust sheet served as the principal sources of Stream-dominated alluvial fans sediment in this part of the fold-thrust belt from Early Cretaceous through along Himalayan front 0.4 Paleogene time, a period of ~90 m.y. (DeCelles, 1994). The western part of

Fan slope (degrees) Fan the Willard thrust system and the western flank of the Wasatch anticlinorium were dropped into the subsurface by the Wasatch normal fault system during Karnali megafan Miocene time (Yonkee et al., 1997; Fig. 3). 0.2 Okavango fan The Willard thrust has a minimum east-southeastward displacement of Tista, Kosi, and ~50 km, and was active during Late Jurassic–Early Cretaceous time (Yonkee, Gandak megafans 0 1997). The Crawford,Absaroka, and Hogsback thrusts accommodated a total 0 10 20 of ~100 km of east-west shortening in this part of the Sevier fold-thrust belt. Fan area (×1000 km2) The Coalville and Medicine Butte thrusts are relatively minor faults (Royse et al., 1975; Lamerson, 1982). The ages of main-phase displacements on these Figure 2. Plot showing planimetric area of fan-shaped nonmarine thrusts generally decrease eastward from ca. 90 Ma to ca. 55 Ma, although the depositional systems vs. slope. Range of values for typical sediment– Coalville and Medicine Butte thrusts had several periods of out-of-sequence gravity flow fans is taken from Blair and McPherson (1994). The Oka- displacement (Lamerson, 1982; DeCelles, 1994). The Hams Fork Conglom- vango fan is from Stanistreet and McCarthy (1993). Diamonds indi- erate was deposited during the main phase of Absaroka thrusting (Royse et al., cate fluvial megafans from the Himalayan foreland basin system. 1975; Lamerson, 1982), but much of the conglomerate was derived from the Stream-dominated alluvial fans are from the region between the Kar- structurally higher Willard thrust sheet, which was passively folded and up- nali and Kali Rivers in southwestern Nepal. Note break in scale along lifted above the Wasatch anticlinorium (Oriel and Tracey, 1970; Crawford, vertical axis. 1979; Schmitt, 1987; DeCelles, 1994). Post-Maastrichtian displacements on

Geological Society of America Bulletin, September 1999 1317

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Figure 3. Tectonic map of the Idaho-Wyoming-Utah salient of the Sevier fold-thrust belt, showing major thrust systems (barbed lines); outcrops of Archean–early Proterozoic crystalline basement (jackstraw pattern) and Proterozoic sedimentary (finely stippled) rocks; and region of deposition of the Hams Fork Conglomerate Member of the Evanston Formation (coarsely stippled). LCR—Lost Creek Reservoir area; ECR—East Canyon Reservoir; OTS—Ogden thrust system; MBT—Medicine Butte thrust; CT—Coalville thrust. After Royse et al. (1975), Lamerson (1982), Coogan (1992), and Yonkee et al. (1997).

1318 Geological Society of America Bulletin, September 1999

SEDIMENTOLOGY AND REGIONAL DISTRIBUTION

Description Ma AGE FORMATION Lithofacies Description. The Hams Fork Conglomerate is predomi- 50 nantly composed of various conglomeratic lithofacies, with subordinate EOC. WASATCH FM. amounts of sandstone and extremely minor amounts of finer-grained litho- THANETIAN facies. The most abundant lithofacies is well-organized, clast-supported, imbricated (a[t]b[i]) pebble to cobble conglomerate. Clasts are commonly SELANDIAN MAIN BODY marked by crescentic percussion scars (Fig. 5, A and B). Average maximum 60 clast size ranges from 14 to >40 cm. Crude horizontal stratification is present in some of these units. Less common are trough and large-scale planar PALEOGENE DANIAN cross-stratified conglomerate. In the Chalk Creek section (Fig. 6), individ- PALEOCENE ual sets of planar cross-stratification are up to 12 m thick. Individual beds of MAASTRICHTIAN conglomerate are typically 50–150 cm thick, and amalgamated units 5–15 70 HAMS FORK m thick are common. Bedding is characteristically lenticular over lateral CGL. distances of tens of meters, and most conglomerate units have erosional basal surfaces (Fig. 5C). Many conglomerate units in the Hams Fork fine LOWER MEMBER FM. EVANSTON upward on a vertical scale of several meters. CAMPANIAN Sandstone in the Hams Fork Conglomerate is limited to thin (<50 cm) 80 lenticular beds and wedges of medium- to very coarse-grained sandstone WEBER CYN. CGL. within large conglomerate bodies. Internal structures within these sandstone SANTONIAN ECHO CYN. CGL. wedges include plane-parallel lamination, ripple cross-lamination, trough cross-stratification (Fig. 5D), and low-angle planar cross-stratification.

CRETACEOUS CONIACIAN 90 HENEFER FM. Minor amounts of siltstone at some localities (Fig. 6) are typically <1 m TURONIAN thick, lenticular, gray, carbonaceous, and massive. Occasional root traces, FRONTIER lignitic horizons, plant fragments, and small burrows are present in some of CENOMANIAN FM. these units. Regional Distribution and Isopachs. The Hams Fork Conglomerate is Figure 4. Chronostratigraphic chart of Upper Cretaceous–Paleogene present throughout most of northeastern Utah and southwestern Wyoming. rocks in northeastern Utah and southwestern Wyoming. After Jacobson It overlaps the southeastern part of the Willard thrust sheet in northeastern and Nichols (1982) and Nichols and Bryant (1990). Utah and pinches out toward the southwest near East Canyon Reservoir (Figs. 3 and 7A). Toward the southeast the conglomerate has been erosionally stripped from the Uinta Mountains region. The conglomerate is thick- the Coalville, Medicine Butte, and Hogsback thrusts have locally folded and est and coarsest along a southeastward-trending zone in northeastern Utah, uplifted the Hams Fork Conglomerate (Lamerson, 1982). During deposition just southwest of the Wyoming corner. The conglomerate is ~120 m thick of the Hams Fork Conglomerate, the structural front of the thrust belt was at along the axis of this zone, and its thickness decreases gradually toward the the Absaroka thrust, but the region of high topography was generally farther northeast and more abruptly toward the southwest (Fig. 7A). The maximum west, associated with the Crawford and Willard thrust sheets and the eastern clast-size pattern generally adheres to the isopach pattern, with the coarsest flank of the Wasatch anticlinorium. conglomerates (clasts 32–44 cm in diameter) located in the northwestern part of the southeast-trending isopach axis (Fig. 7B). In western Wyoming, METHODS the conglomerate is generally less than 50 m thick, but poor exposures in- hibit a reconstruction of its overall geometry. Complete, well-exposed stratigraphic sections of the Hams Fork Con- Detailed geologic mapping in the Lost Creek Reservoir area (Fig. 8) il- glomerate were measured at 14 localities. Lithofacies were recorded on a lustrates important kinematic and paleogeographic relationships between bed-by-bed scale. At many localities, the lateral continuity of exposure is the Hams Fork Conglomerate and the two major thrust systems in this part sufficient to allow for observations of large-scale bed geometries over dis- of the Sevier belt—the Willard and Crawford thrusts. In this area, the con- tances of hundreds of meters. Paleocurrent data were collected by measur- glomerate is absent along the trace of the Crawford thrust (Fig. 8). The ing ten imbricated clasts within small outcrop areas (<4 m2) of individual Crawford thrust at Lost Creek Reservoir is expressed at the surface as a genetic lithofacies units. Within a given stratigraphic section, paleocurrent tight, eastward vergent fault-propagation anticline involving Jurassic and data sets were measured at several stations distributed more or less evenly Cretaceous rocks. The steeply dipping eastern limb of the anticline is cut by throughout the section. Large-scale planar cross-stratified conglomerate the thrust (not exposed), which juxtaposes limestone of the Jurassic Twin foresets were also measured at a few localities. A total of 746 paleocurrent Creek Formation against the Campanian Weber Canyon Conglomerate, indicators were measured. The petrologic composition of the Hams Fork which is a thick, proximal alluvial fan deposit (DeCelles, 1994). The Weber Conglomerate was documented by determining the composition of at least Canyon Conglomerate adjacent to the thrust contains a progressive uncon- 100 clasts at 16 stations at 11 locations throughout the study area and formity in which bedding flattens upsection from vertical to subhorizontal, throughout the thickness of the Hams Fork Conglomerate. Maximum clast and individual beds decrease in thickness westward toward the thrust. The size was determined by measuring the a-axes of the ten largest clasts within Weber Canyon Conglomerate occurs only in the footwall of the Crawford a given bed, and measurements were made in most conglomerate beds thrust in this area, and consists of sediment derived from the Crawford and throughout individual sections. Willard thrust sheets. Overlying the Weber Canyon Conglomerate, the gen-

Geological Society of America Bulletin, September 1999 1319

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/111/9/1315/3383263/i0016-7606-111-9-1315.pdf by guest on 01 October 2021 Figure 5. (A) Coarse, imbricated, well-rounded quartzite-clast conglomerate typical of the Hams Fork Conglomerate. Conglomeratic portion of outcrop is ~4 m thick. (B) Crescentic impact scars on a clast of Proterozoic quartzite in the Hams Fork Con- glomerate. Such scars provide evidence for saltation in powerful currents. (C) Cliffs of Hams Fork Con- glomerate along western side of Lost Creek Reser- voir. Note broadly lenticular bedding geometry and overall upward fining of the unit. Thickness of the unit is ~40 m in this view. (D) Trough cross-stratified sandstone in Hams Fork Conglomerate channel deposits. Hammer is 30 cm long.

erally flat-lying Hams Fork Conglomerate contains no evidence for struc- against the Weber Canyon Conglomerate (Fig. 8). We hypothesize that the tural growth. Unlike the Weber Canyon Conglomerate, the Hams Fork is normal fault joins the Crawford thrust in the subsurface, and structurally in- present on both sides of the Crawford thrust trace, but it has been dropped verted most of the thrust sheet sometime after deposition of the Paleocene down ~240 m to the west along a steep, west-dipping normal fault in the upper member of the Evanston Formation (which is cut by the normal fault). hanging wall of the Crawford sheet. A thin sliver of Jurassic Twin Creek Similar relationships between late normal faults and thrusts in the Sevier Formation in the Crawford hanging wall remains structurally perched belt have been widely documented (Lamerson, 1982; Constenius, 1996).

1320 Geological Society of America Bulletin, September 1999

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l 1 i W I-80 I-84 Cg LOWER MEMBER LOWER Sc Sf F 0 40 50 30 10 20 3: SAWMILL CREEK SAWMILL 3: Cg Sc Conglomerate, large-scale Conglomerate, planar cross-stratified Siltstone, gray and carbonaceous, gray Siltstone, locally lignitic Conglomerate, imbricated, Conglomerate, horizontally stratified Sandstone, cross-stratified or cross-stratified Sandstone, horizontally stratified Paleocurrent direction, based on Paleocurrent at least 10 imbricated clasts Plant fossils Sf F RESERVOIR 2: LOST CREEK LOST 2: 0 EXPLANATION 40 30 10 20 Cg Sc Sf F 1: PINE CREEK 1: 0 40 90 70 50 30 10 80 60 20 110 120 100 Figure 6. Representative measured stratigraphic sections of Hams Fork Conglomerate throughout the study area. Inset shows locat Inset shows the study area. Conglomerate throughout stratigraphic sections of Hams Fork measured 6. Representative Figure sections.

Geological Society of America Bulletin, September 1999 1321 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/111/9/1315/3383263/i0016-7606-111-9-1315.pdf by guest on 01 October 2021 A. PALEOCURRENT AND ISOPACH MAP 15 K SOLID ARROWHEADS = IMBRICATIONS 7 OPEN ARROWHEADS = AVERAGE TROUGH AXES FROM TROUGH LIMBS T 7 S U n = 746 R H ISOPACH STATION T D T R

T FO S

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Figure 7. (A) Map showing paleocurrent and isopach data from Hams Fork Conglomerate. Solid arrows represent vector means of imbrication data from multiple levels at individual measured sections. Isopachs are in meters. The zero-isopach contour represents the approximate location of depositional (or pre-Paleocene erosional) pinch-out of the Hams Fork Conglomerate. (B) Map showing distribution of maximum clast sizes determined from averaging multiple stratigraphic levels at individual stratigraphic sections. SLC—Salt Lake City; E—Evanston; K—Kemmerer; I-80—Interstate highway 80; I-84—Interstate highway 84.

1322 Geological Society of America Bulletin, September 1999

EXPLANATION 0 1 km Te Evanston Fm. Upper Member

0 1600 3200 6600 Khf Evanston Fm. ft 41 Hams Fork Conglomerate Weber Canyon

N Kwc CRETACEOUS- PALEOGENE Conglomerate

PPED Jtc Twin Creek Fm. MA T O Te Nugget Fm. N Jn A JURASSIC E R A Thrust fault (barbs on HW)

Khf A 3 Normal fault (balls on HW)

ir (6005') Te eservo R k 6800 Te

Lost CreeJn 53 7000 NORMAL FAULT WITH ~240 m VERTICAL Khf Khf DISPLACEMENT 44 6600 Jtc 6400 on 6400 any 42 8 ek C 33 90 Kwc l Cre 58 Trai

ONLAP OF PALEO- ust 6400 r TOPOGRAPHIC RIDGE h T Te d

o f F raw ra C ncis 87 Cre ek A'

PALEOTOPOGRAPHIC RIDGE IN NUGGET FORMATION Te Khf 7200 A Crawford thrust A' 6800 Te 6400 Te Khf 6000 Khf 5600 Kwc 5200 Jtc Triassic Jn 4800 4400 No vertical exaggeration Undivided Jurassic-Cretaceous feet

Figure 8. Geologic map and cross section (A–A′) of the Lost Creek Reservoir area (see Fig. 3 for location). Arrows indicate paleocurrent directions from imbrications in Hams Fork Conglomerate. See text for discussion.

In the hanging wall of the normal fault, flat-lying beds of the Hams Fork We interpret this breccia as a colluvial deposit that formed along the weath- Conglomerate rest unconformably on quartz arenites of the Jurassic Nugget ering surface of the paleotopographic ridge. The Hams Fork Conglomerate Formation, overlapping the eastern limb and most of the hinge zone of the crops out again along the west side of Lost Creek Reservoir at the same ele- anticline (Fig. 8). On the upper part of the western limb of the fold, however, vation as it does along the east arm of the reservoir, but it is not present in an the flat-lying Hams Fork rests in buttress unconformity directly against an ~1-km-wide belt along the crest of the Nugget paleotopographic ridge for a ~110-m-high paleotopographic ridge of the Nugget Formation (Fig. 8). This distance of at least 10 km along structural strike (Fig. 8). The upper member relationship is clearly visible along the eastern arm of the reservoir. The of the Evanston Formation is flat-lying and overlaps the entire area, includ- Nugget dips ~45–50°W, and its upper surface is mantled by a several-meter- ing the top of the paleotopographic ridge, but is offset by the normal fault. thick, hematite-cemented, monomictic sedimentary breccia of Nugget clasts. The cross-cutting and overlapping relationships at Lost Creek Reservoir

Geological Society of America Bulletin, September 1999 1323

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CLAST-COMPOSITION DATA

Kemmerer

CRT

Evanston I-80

T B M WYOMING UTAH

I-80 40 km

LEGEND Lower Cambrian Tintic Quartzite Proterozoic quartzite Mesozoic sandstone

Precambrian basement Paleozoic sandstone & carbonate

WT— WILLARD THRUST AT— ABSAROKA THRUST CRT— CRAWFORD THRUST MBT— MEDICINE BUTTE THRUST

Figure 9. Conglomerate clast-count data from Hams Fork Conglomerate. Note the relatively greater abundance of Paleozoic sandstone and carbonate clasts and lesser abundance of Proterozoic and lower Cambrian (Tintic) quartzite and Precambrian basement clasts in sections northeast of Evanston, Wyoming. The bold dashed line delineates approximate boundary between these two petrofacies.

and along the trace of the Willard thrust ~10 km to the northwest can be in the vicinity of either thrust. (3) During the Paleocene, the entire area (in- used to deduce the following sequence of events: (1) Displacement along cluding the paleotopographic ridge along the Crawford thrust) was buried the Crawford thrust and simultaneous growth of the fault-propagation an- by the fluvial deposits of the upper member of the Evanston Formation, ticline at Lost Creek Reservoir took place during deposition of the Weber smoothing regional topography. (4) Sometime after deposition of the Canyon Conglomerate in early Campanian time (DeCelles 1994; Yonkee Evanston, the hanging wall of the Crawford thrust was structurally inverted et al., 1997). (2) During Campanian–Maastrichtian time, the Crawford by normal faulting; on the basis of regional patterns of extension, this prob- fault-propagation anticline was erosionally beveled prior to and during ably took place during early Eocene time (Constenius, 1996). deposition of the Hams Fork Conglomerate, but a narrow (~1 km wide, Paleocurrent Directions. The paleocurrent data indicate generally south- 100+ m high) ridge of resistant Nugget Formation sandstone persisted and southeastward paleoflow, with local predominance of eastward paleoflow in locally controlled accumulation of the conglomerate. Both the Willard and the southwesternmost part of the study area (Fig. 7A). At any given locality, Crawford thrusts must have been inactive by late Campanian time because paleoflow directions exhibit an azimuthal range of ~90°. In the Lost Creek the Hams Fork Conglomerate overlaps the Willard thrust and is not folded area, paleoflow was generally southward, parallel to the trend of the paleo-

1324 Geological Society of America Bulletin, September 1999

topographic ridge along the trace of the Crawford thrust (Fig. 8). This, to- abundant at some localities. Proterozoic quartzite clasts, including green, gether with the fact that the conglomerate does not overlap the ridge, indi- maroon, and light gray varieties, are most abundant in northeastern Utah, cates that Crawford thrust-related topography, at least locally, controlled the whereas Pennsylvanian–Permian quartz arenite and carbonate are con- distribution and flow directions of Hams Fork channel systems, even though sistently abundant (>45%) in western Wyoming. Precambrian basement the Crawford thrust itself was inactive. clasts are restricted to northeastern Utah sections, and are most abundant (15%) in the western part of the study area closest to the Wasatch anticli- INTERPRETATION norium. In the southwestern part of the study area, clasts of yellow and brown quartzite are abundant. Clasts of Mesozoic rocks, most notably the The Hams Fork Conglomerate consists of lithofacies that are charac- Jurassic Nugget Formation, are locally abundant near the traces of the teristic of gravelly braided rivers (e.g., Hein and Walker, 1977; Rust, 1978; Crawford and Absaroka thrusts. Miall, 1978, 1996; Collinson, 1996). The erosional basal surfaces, lentic- Provenance Interpretation. The only potential sources of Proterozoic ular geometries, and crude upward-fining trends of conglomerate bodies quartzite in northeastern Utah are (1) in the hanging wall of the Willard thrust indicate deposition in migratory channels. Thicknesses of lenticular con- fault, (2) in the Cottonwood arch, and (3) in the Uinta Mountains (Fig. 3). glomerate bodies and large-scale planar cross-sets indicate that channel The Uinta Mountains can be ruled out as a source for the Hams Fork Con- depths were several meters to perhaps as much as 15 m deep. Gravel im- glomerate on the basis of the predominantly southeastward paleoflow direc- brication and crude horizontal stratification are typical features of longi- tions and regional textural and isopach patterns (Fig. 7). Most of the varicol- tudinal bar deposits. Crescentic percussion marks on Hams Fork clasts ored quartzite lithologies can be correlated with lithologies in the Willard (Fig. 5B) are evidence of clast-clast collisions at high velocity. The planar thrust sheet (Oriel and Tracey, 1970; Crawford, 1979; DeCelles, 1994), an cross-stratified conglomerate units probably were deposited on lateral, interpretation supported by the southeastward paleoflow pattern. The clasts transverse, and oblique gravel bars. Trough cross-stratified conglomerate of yellow and brown quartzite that are abundant in the southwestern part of bodies indicate deposition by powerful, confined flows in channels. The the study area may have been derived from the Proterozoic Big Cottonwood sandstone wedges were probably deposited during waning flood and nor- Formation, which crops out ~40 km to the southwest in the Cottonwood arch mal flow stages on the flanks of gravel bars, when bar tops emerged from (Figs. 3 and 10). However, the paleocurrent data do not support this hypoth- the flow and decreasing flow velocity allowed only finer fractions to be esis. The locally abundant Nugget clasts indicate nearby sources in relict transported. The thin, lignitic, and carbonaceous siltstone layers that oc- (e.g., the paleotopographic ridge along the Crawford thrust) or newly devel- cur in some outcrops of the Hams Fork Conglomerate are interpreted as oping (e.g., the frontal Absaroka thrust ridge) topographic ridges (Fig. 10). the deposits of abandoned sloughs and swampy areas on the floodplain. The chert clasts and Pennsylvanian–Permian quartz arenite clasts were de- The coarseness and high degree of organization of Hams Fork gravelly rived from the Weber (or Wells) Formation, a several hundred-meter-thick channel deposits indicate that flows were powerful and sustained, probably section of which crops out along the eastern flank of the Wasatch anticlino- several meters per second at flood stage. Although the degree of lateral re- rium in the trailing part of the Crawford thrust sheet. Some of these clasts working and amalgamation in the Hams Fork channel deposits is high, in- also could have been derived from the hanging wall of the Willard thrust, and dividual channels were hundreds of meters wide, suggesting that discharges in western Wyoming from the frontal part of the Absaroka thrust sheet. were on the order of 103 m3/s. We have found no evidence for ephemeral or Thus, the provenance data implicate three principal sources for Hams highly seasonal discharge in the Hams Fork Conglomerate, such as desic- Fork detritus (Fig. 10). The main source was in the Willard thrust sheet, cation cracks or highly oxidized, calcareous paleosols. which was passively uplifted and topographically rejuvenated during frontal An independent view of paleoclimatic influence on Hams Fork channels Absaroka thrusting. The Precambrian basement clasts indicate that the can be derived from paleobotanical studies. Plant physiognomic analysis of Wasatch anticlinorium also must have supplied substantial amounts of de- Campanian–Maastrichtian paleofloras from the Western Interior coastal plain tritus. The third significant source was local, frontal thrust-related topo- indicates generally subhumid megathermal (>20 °C mean annual tempera- graphic ridges. A fourth potential source, at least in the southwestern part of ture) paleoclimate at the paleolatitude of the study area (Wolfe and Upchurch, the study area, was the Cottonwood arch. 1987). Fossil woods suggest a seasonal, megathermal climate with moderate The relative abundance of Paleozoic quartz arenite clasts in the northeast- precipitation (~1100 mm/yr) and minimal monsoonal influence (Wolfe and ern part of the study area, compared with the abundance of Lower Cambrian Upchurch, 1987). The abundance of coal (Robinson Roberts and and Proterozoic clasts and small amounts of Precambrian basement clasts in Kirschbaum, 1995) and scarcity of calcic, highly oxidized paleosols in Upper the southwestern part of the area, suggests two distinct petrofacies (Figs. 9 and Cretaceous rocks of the coastal plain to the east of the study area also indicate 10). These petrofacies probably reflect differences in source terrane composi- that paleoclimate was generally equable. However, the fluvial record is gen- tion and locations of source drainage basins. Thus, the provenance data sug- erally biased toward flood events, which may remove dry-season deposits. In gest that two separate drainage systems, which spawned two separate addition, because of its hinterland provenance, discharge in the Hams Fork megafans, deposited the Hams Fork Conglomerate in this region (Fig. 10). fluvial system could have been controlled by climatic conditions over the high elevation mountain ranges to the west (Chase et al., 1998), rather than by lo- Regional Paleogeography cal conditions along the coastal plain. Isopach and paleocurrent data from northeastern Utah indicate that the Provenance Hams Fork Conglomerate was deposited in at least one large fan-shaped lobe, the axis of which trended southeastward, with its upslope apex lap- Description. Our clast-composition data accord with Crawford’s ping onto the Willard thrust sheet (Fig. 10). Within this large lobe, grain (1979) data, and indicate a predominance of Proterozoic and Lower size generally decreased in a downcurrent direction, but coarse gravel Cambrian quartzite, with subordinate amounts of well-cemented Penn- (maximum clast sizes in the 18–23 cm range) was still being transported sylvanian–Permian quartz arenite and Permian and/or Mississippian more than 45 km from the upslope end of the preserved depositional sys- chert (Fig. 9). Precambrian crystalline basement clasts (mostly granitic tem. The large size (>2500 km2) and fan-shaped geometry of the Hams Fork mylonite and amphibolite) and Mesozoic sandstone clasts are moderately lithosome in northeastern Utah, with radially dispersed paleoflow and a pre-

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0 10 20 30 mi

0 50 km

RIDGE N RIDGE Active structural front Idaho of orogenic wedge

ABSAROKA THRUST

Utah CRAWFORD THRUST Megafan 2 THRUST

SHEET n WILLARD i la

al P t s

Coa Wyoming Utah

Megafan 1 Drainage across coastal Possible source in plain to Western Interior Seaway WASATCH ANTICLINORIUM Cottonwood Arch area

Figure 10. Schematic, palinspastically restored paleogeographic map for latest Cretaceous time in the proximal part of the Cordilleran foreland basin system in northeastern Utah and southwestern Wyoming. The stippled areas represent fluvial megafans of the Hams Fork Conglomerate. The three principal source terranes for the Hams Fork are labeled Willard thrust sheet, Wasatch anticlinorium, and the frontal ridges of the Crawford and Absaroka thrust faults. The Cottonwood arch (located ~25 km south of the southwestern corner of the map area) may have supplied some of the Hams Fork detritus in the southwesternmost part of the study area.

ponderance of evidence for fluvial depositional processes, are taken as evi- the inactive Crawford thrust, but was otherwise a region of subdued topog- dence that the conglomerate was deposited on a fluvial megafan in this re- raphy and widespread fluvial deposition. East of the Absaroka thrust sys- gion (Fig. 10, megafan 1). Although isopach and paleocurrent data from the tem, a several hundred-kilometer-wide fluvial–coastal plain in the foredeep Hams Fork Conglomerate in southwestern Wyoming are insufficient to de- depozone stretched toward the shoreline of the Western Interior seaway fine the geometry of a second megafan (because of poor exposure), prove- (Robinson Roberts and Kirschbaum, 1995). The chronostratigraphic (Cam- nance data suggest that one may have existed in this region as well (Figs. 9 panian–Maastrichtian) equivalent of the Hams Fork Conglomerate in south- and 10, megafan 2). Megafan 1 received detritus from the southern part of central Wyoming is the Ericson Formation, which consists of thick quartz- the Willard thrust sheet and the Wasatch anticlinorium, whereas megafan 2 ose sandstones that were deposited by southeastward-flowing braided rivers was derived from the central Willard sheet and frontal thrust ridges (Fig. 10). (Shuster, 1986; Robinson Roberts and Kirschbaum, 1995; our unpublished Topography associated with the Crawford thrust locally influenced drainage paleocurrent data). patterns, but the conglomerate seems to have buried most of the frontal oro- The Hams Fork Conglomerate is merely a thin but widespread portion of genic wedge over an area of ~104 km2. a >4.5-km-thick accumulation of quartzitic detritus that developed from Because the structural front of the thrust belt was located along the Ab- Early Cretaceous through Eocene time in the region northeast of Salt Lake saroka thrust during deposition of the Hams Fork Conglomerate (Figs. 3 City (DeCelles, 1994). Was the entire mass of sediment deposited on fluvial and 10; Royse et al., 1975; Lamerson, 1982), the ~50-km-wide belt of con- megafans, similar to those on which the Hams Fork Conglomerate was de- glomerate between the Willard and Absaroka thrust traces was deposited in posited? The conglomerates of the Frontier Formation (>2 km) and the the wedge-top depozone, which consists of foreland basin deposits that ac- Echo Canyon Conglomerate (>500 m) were deposited mainly in braided cumulated on top of the active frontal part of the orogenic wedge (DeCelles streams (Schmitt, 1985) and stream-dominated alluvial fans (DeCelles, and Giles, 1996). This depozone was interrupted by a narrow ridge along 1994), respectively. The Henefer Formation conglomerates (>50 m) were

1326 Geological Society of America Bulletin, September 1999

Harebell-Pinyon

t Megafan Montana

e B

t y

w r

a h

e T Hams Fork - Idaho Megafan(s) S d Nevada l r o o Wyoming i F r

e n t

a n r n I Central Utah i e a l Megafan l n l i P r l e d a r t t

s o s Southern Utah a e o C Megafan

C W Utah Colorado Arizona N. Mexico

0 300 km

Figure 11. Paleogeographic sketch map of the Cordilleran fold-thrust belt and adjacent foreland basin system during Late Cretaceous–Paleogene time, when fluvial megafans (stippled) were actively filling the proximal, nonmarine part of the foreland basin system (modified from Lawton et al., 1994). The dot-dash pattern indicates broad coastal plain belt of mires (represented in the rock record by thick coal seams) and low-gradient fluvial channel systems (after Robinson Roberts and Kirschbaum, 1995), and the bold arrows represent major gravelly to sandy fluvial channel systems that are postulated to have been deposited by fluvial megafans. Sources of information: southern Utah megafan (Schmitt et al., 1991; Goldstrand, 1994); central Utah megafan (Lawton, 1986; DeCelles et al., 1995); Hams Fork megafan (this paper); and Harebell-Pinyon megafan (Lindsey, 1972; Love, 1972).

deposited on marine fan deltas. The Weber Canyon Conglomerate (>500 m) deposition of the Frontier and Henefer Formations), fluvial megafans were was deposited on sediment-gravity flow-dominated alluvial fans. Only the prevented from developing because the shoreline of the Western Interior quartzite and chert conglomerates (and associated finer-grained facies) of seaway was located within a few kilometers of the Crawford thrust. During the Kelvin Formation (Lower Cretaceous) and the Wasatch Formation Early Cretaceous and Late Cretaceous–Paleogene time, the entire foreland (Paleocene–Eocene) in this region have lithofacies, thicknesses, and re- basin system at this latitude was nonmarine, and extensive fluvial systems gional distributions that are comparable to the Hams Fork Conglomerate built eastward across the proximal foreland basin. Fluvial megafans require (DeCelles, 1994). However, none of these units has been studied in enough large rivers and large sediment fluxes. Thus, the combination of relatively detail to outline regional isopachs, paleocurrent directions, and detailed low sea level and the presence of an antecedent river system with a large petrofacies in the manner of this study. The most likely explanation for the catchment were probably the principal controls on development of the ~90 m.y. persistence of gravelly, quartzite-rich, depositional systems in northeastern Utah conglomerate accumulations. northeastern Utah is the existence of an antecedent drainage system that ex- Several other concentrations of Upper Cretaceous–Paleocene quartzitic ited the thrust belt in the vicinity of the present southern edge of the Idaho- conglomerate and sandstone along the front of the Sevier fold-thrust belt Wyoming-Utah salient (Fig. 11). The nature of depositional systems was (Fig. 11; Beutner, 1977; Lageson and Schmitt, 1994; Lawton et al., 1994) controlled by relative sea level (particularly during mid-Cretaceous time), have characteristics similar to those documented in the Hams Fork Con- subsidence rates in the proximal foreland basin system, and the locations of glomerate. These include conglomerates in the Harebell and Pinyon Forma- thrust-related topography. During periods of high sea level (e.g., during tions in northwestern Wyoming (Lindsey, 1972; Love, 1972), the Currant

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70°E N AN NEPAL IST BHUTAN K Islamabad A 32°N H T P r i IB e m E iv a T BANGLADESH R la ya n N F INDIA TA o IS ld K -T A s h P Indu r Ka N us EP t B New Delhi AL elt Fig. 14 K G BHUTAN og utra ra ap G G Brahm ang Ko es River T River Fig. 13 BANGLADESH INDIA Dhaka

20°N 500 km 70°E

Figure 12. Map showing the Himalayan foreland basin system (stippled area) in front of the Himalayan fold-thrust belt (barbed line). In northern India and adjacent Nepal, fluvial megafans are labeled as follows: Ka—Kali; K—Karnali; G—Gandak; Ko—Kosi; T—Tista. Locations of Landsat images shown in Figures 13 and 14 are shown by boxes.

Creek Formation in north-central Utah (Bruhn et al., 1986), parts of the Comparison with Modern Fluvial Megafans Canyon Range Conglomerate and equivalent sandy fluvial facies to the east in central Utah (Lawton, 1986; DeCelles et al., 1995; Lawton et al., 1997), Fluvial megafans are one of several types of large-scale geomorphic ele- and Upper Cretaceous–Paleocene conglomerates and sandstones in south- ments that constitute the depositional mosaic of the Himalayan nonmarine western Utah (Schmitt et al., 1991; Goldstrand, 1994). Each of these accu- foreland basin system (Fig. 1). Other important elements include the inter- mulations is associated with a thick, quartzite-bearing thrust sheet in the hin- megafan areas and a large trunk river, the Ganges River, that flows subparal- terland that was topographically rejuvenated during frontal thrusting lel to the topographic front of the thrust belt (Fig. 12; Geddes, 1960; Willis, (Schmitt and Steidtmann, 1990; Lageson and Schmitt, 1994), and each 1993; Singh et al., 1993; Sinha and Friend, 1994). The rivers that deposit grades eastward (down paleoslope) into a thick, quartzose, fluvial sandstone megafans drain vast catchments that extend into the thrust-belt hinterland and deposit (Fig. 11). The rivers that deposited these vast accumulations of encompass areas of 104–105 km2 in area (Singh et al., 1993; Sinha and quartzose detritus generally flowed toward the east, southeast, and northeast, Friend, 1994). These rivers are antecedent to the topography in the frontal across the 400–500-km-wide coastal plain (Robinson Roberts and part of the fold-thrust belt, and they exit the mountain belt through a deep Kirschbaum, 1995). Eisbacher et al. (1974) noted similar concentrations of gorge (Fig. 1; Wells and Dorr, 1987a; Singh et al., 1993; Gupta, 1997). The coarse-grained sediment in the Upper Cretaceous–Paleocene of Alberta, and exit gorge anchors the upstream end of the river, whereas the river is free to suggested that they are related to large antecedent drainages in the Canadian migrate laterally once it reaches the topographically subdued foreland-basin portion of the Cordilleran fold-thrust belt. Lawton et al. (1994) hypothesized system. Rapid aggradation of the floodplain causes frequent avulsions and that long-lived transverse structural features in the thrust belt (e.g., large lat- lateral shifts in the position of the river, and a fluvial megafan is deposited. eral ramps and tear faults) were occupied by antecedent drainage systems Fluvial megafans associated with antecedent drainages are also present in the that transported quartzite detritus long distances from the hinterland to the Andean and Mesopotamian foreland basin systems (Oberlander, 1965; foreland basin. However, it is important to note that transverse structural fea- Damanti, 1993). The largest modern fluvial megafans cover areas of 104–105 tures are not required for antecedent drainages (Oberlander, 1965). For ex- km2 (Geddes, 1960; Ori et al., 1986; Wells and Dorr, 1987; Gohain and ample, in the case of the Himalayan fluvial megafans, the major rivers are not Parkash, 1990; Rasänen et al., 1991; Mohindra et al., 1992; Damanti, 1993; associated with transverse structures. What is required for a fluvial megafan Singh et al., 1993; Sinha and Friend, 1994; Table 1, Fig. 2). These megafans is a large catchment, capable of providing large amounts of fluvial discharge are characterized by numerous divergent fluvial channels, low slopes and sediment. Because the largest catchments in thrust belts are typically as- (0.05°–0.18°), and low topographic relief (5–20 m) above the neighboring sociated with antecedent drainages, it is not surprising that antecedent rivers intermegafan areas (Sinha and Friend, 1994). Detailed descriptions of the commonly deposit fluvial megafans. Himalayan megafans (Fig. 12) are available in Geddes (1960), Wells and

1328 Geological Society of America Bulletin, September 1999

Figure 13. Landsat MSS image of the Kosi River megafan in southeastern Nepal and northeastern India. The Kosi River exits the frontal part of the Himalayan fold-thrust belt near the upper center of the image and arcs toward the west and south. The large river along the lower edge of the image is the Ganges River, which flows eastward (toward the right). The eastern boundary of the megafan is shown by the dashed line, and the western boundary is just west of the present Kosi River. See Figure 12 for general location.

Dorr (1987a, 1987b), Gohain and Parkash (1990), Mohindra et al. (1992), 1994). The Pastaza-Marañon megafan in northern Peru is the largest known Willis (1993), Singh et al. (1993), and Sinha and Friend (1994). fluvial megafan, with a length of ~400 km (Rasänen et al., 1991). Such An oft-cited example is the Kosi River megafan in eastern Nepal and megafans have been referred to as humid alluvial fans in some literature, but northeastern India (Fig. 13). The Kosi has migrated westward through an they actually are large fluvial systems that distribute gravelly bedload to ar- arc of 50°, over an east-west distance of ~113 km on the middle part of the eas up to ~50 km from the mountain front (Geddes, 1960; Wells and Dorr, megafan, during the past 228 yr, leaving in its wake a vast fluvial floodplain 1987a; Singh et al., 1993), carry predominantly sandy bedload in their (Wells and Dorr, 1987a). The megafan is ~154 km long and has a topo- downstream reaches (Singh et al., 1993), and have very low slopes com- graphic relief of only ~5–20 m above the adjacent intermegafan areas (Wells pared to typical sediment-gravity flow fans (Fig. 2). Fluvial megafans are and Dorr, 1987a; Sinha and Friend, 1994). Similar fluvial megafans, spaced not restricted to humid climatic zones; for example, Damanti (1993) re- ~200–300 km apart along the Himalayan orogenic front, have been de- ported the Rio Jachal and Rio Huaco megafans in northwestern Argentina, posited by the Kali, Karnali, and Gandak Rivers in southern Nepal and which are being deposited by ephemeral sheet floods in front of the Pre- northern India (Fig. 14; Geddes, 1960; Willis, 1993; Sinha and Friend, cordillera thrust belt. These megafans are about an order of magnitude

Geological Society of America Bulletin, September 1999 1329

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/111/9/1315/3383263/i0016-7606-111-9-1315.pdf by guest on 01 October 2021 Figure 14. Landsat MSS image showing the frontal part of the Himalayan fold-thrust belt and the foreland basin in southwestern Nepal and northern India between the Kali and Karnali Rivers. North is to the left. See Figure 12 for location. The Kali and Karnali fluvial megafans and the intervening intermegafan zone are visible. The location of the Khutia Khola stream-dominated alluvial fan (see Fig. 15) is shown by box.

1330 Geological Society of America Bulletin, September 1999

° ′ 80 40 E Elevation contours 2 km in feet

Fig. 16 P L I F 1000 T ′ E °50 N D 28 T E RR ACE

N 900 MFT Dense Sal Forest

800

′N i °50 a 685 28 Dense Sal Forest Kho

l a

700

674

660 80°40′E

Figure 15. Detailed map of the Khutia Khola stream-dominated alluvial fan in the proximal part of the Himalayan foreland basin system of western Nepal. This fan is located in the intermegafan zone between the Kali and Karnali fluvial megafans (see Fig. 14 for location). Topographic contours are elevations in feet (1 ft = 0.3048 m). The approximate location of the frontal trace of the Main Frontal thrust (MFT) is shown by the barbed line. Stippled areas indicate clear-cut zones in an otherwise densely forested region covered by Sal trees. The map was constructed from aerial photographs and 1:50 000 topographic maps of the Indian Survey.

smaller (~1400 km2 and ~700 km2, respectively) than the largest fluvial megafans, but they cover proportionately smaller areas than the main anas- megafans in Nepal and northern India, perhaps because of the arid Argen- tomosed and braided tributaries. Some of these smaller rivers, such as the tine climate and resulting low sediment flux. Burhi Gandak River on the Gandak megafan, do not have a direct connec- The rivers that deposit fluvial megafans are variable in terms of their tion with the Himalayan foothills; instead, they are fed from springs within planform morphology. On the Himalayan megafans of Nepal, the major the megafan and occupy older (~100 yr) channels that were cut by the main rivers are braided and anastomosed on the upper parts of the megafans but megafan river (Wells and Dorr, 1987a; Sinha and Friend, 1994). become predominantly straight and meandering farther downstream before Many of the fluvial megafans in Nepal have channels that carry predom- joining the braided Ganges trunk system (Fig. 13; Wells and Dorr, 1987a; inantly quartzitic gravels derived from low-grade metasedimentary rocks Singh et al., 1993). At low-flow stage during the dry season, even the anas- above the Main Boundary thrust. The Main Boundary thrust is typically lo- tomosed channels have braided thalwegs. The intermegafan areas are occu- cated more than 10–40 km north of the topographic front of the range. Most pied by stream-dominated alluvial fans in their proximal parts and mainly of the active frontal thrusting in the Himalaya is concentrated along the Main meandering rivers and associated floodplains with oxbows and abandoned Frontal thrust and blind thrusts beneath the Indo-Gangetic plains (Nakata, sloughs in their downstream parts (Figs. 1, 13, and 14; Wells and Dorr, 1989; Mugnier et al., 1993; Jackson and Bilham, 1994; Bilham et al., 1997; 1987a; Sinha and Friend, 1994). Meandering rivers are also present on the Powers et al., 1998). Although carbonate rocks and crystalline metamorphic

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Figure 16. Photograph of the apex region of the Khutia Khola stream-dominated alluvial fan, looking toward the topographic front of the Him- alaya. The photograph was taken during the winter dry season, when the river is confined to a narrow (10–20 m wide) shallow braided channel. During the monsoon, this area is immersed in flood flow, and the large boulders are carried as bedload.

and igneous rocks are widespread in the Himalaya, the abundance of Himalayan foreland basin system have persisted for >10 m.y., and that flu- quartzite lithologies indicates that under the humid monsoonal climate of vial megafan and stream-dominated alluvial fan deposits can be abundantly the modern eastern Himalaya, only highly durable and chemically stable preserved in the stratigraphic records of foreland basins. clasts survive the rigors of long-distance fluvial transport. A comparison of the Hams Fork Conglomerate and fluvial megafans of Fluvial megafans are not to be confused with stream-dominated alluvial the modern Himalayan foreland basin system reveals many similarities. The fans (Hayward, 1983; Ori et al., 1986; Evans, 1991; Ridgway and DeCelles, conglomeratic portion of the southern Hams Fork fluvial megafan covered 1993), which are also common in the intermegafan areas along the topo- a minimum area of ~2500 km2, and may have covered more than twice this graphic front of the Himalayan fold-thrust belt (Figs. 14 and 15). These fans area, given that the most distal conglomerates observed in the study area are cover areas of 150–250 km2, have slopes of ~0.4°–0.6°, and deposit coarse composed of coarse cobbles. The >50 km downstream distance of gravel gravels by fluvial and colluvial processes (Fig. 2). An excellent example is transport is comparable to the distance of gravel transport in rivers deposit- the Khutia Khola stream-dominated alluvial fan, which has a length of ~12 ing Himalayan megafans. Lithofacies in the Hams Fork Conglomerate and km, a maximum width of ~12 km, and a perennial braided channel system its downstream equivalent, the Ericson Formation of south-central that is flanked by an inactive, jungle-covered floodplain (Fig. 15). The pre- Wyoming, are similar to those in the proximal and distal parts, respectively, dominant lithofacies visible in natural stream cuts in the Khutia Khola fan is of the Kosi megafan (Singh et al., 1993). imbricated, subhorizontally stratified gravel, probably deposited in longitu- The provenance and structural setting of the Hams Fork Conglomerate dinal gravel bars and gravel sheets (which are common in the active channel). are also similar to those of the Himalayan megafans. Like the gravels on Bedload in Khutia Khola on the proximal part of the fan is dominated by ex- Himalayan megafans, the Hams Fork Conglomerate is composed pre- tremely coarse, well-rounded boulders (Fig. 16). Fluvial modification of col- dominantly of quartzite detritus derived from a major thrust sheet that was luvium and sediment-gravity flow deposits is also common on the upper part located several tens of kilometers toward the hinterland from the active of the Khutia Khola fan. At least 18 similar, stream-dominated alluvial fans structural front of the thrust belt. Carbonate rocks are abundant in the Se- are present along the proximal part of the Himalayan foreland basin between vier fold-thrust belt, but their erosional products did not survive transport the Kali and Karnali Rivers (Fig. 14), and similar fans are ubiquitous along into the Hams Fork megafans. Similarly, in spite of the abundance of car- the Himalayan front in Nepal and India. These fans are traversed by rela- bonate rocks in parts of the Lesser Himalayan zone, the gravels on mod- tively small, perennial braided rivers that experience dramatic changes in dis- ern Himalayan fluvial megafans contain very little carbonate. Both Him- charge from the monsoon to dry season. Where unmodified by human activ- alayan and Cordilleran fluvial megafans were deposited partly on top of ities, the fan surfaces outside of major active channels are covered by dense their respective orogenic wedges. The proximal parts of the Hams Fork jungle. These stream-dominated alluvial fans typically have low (<1°) megafans were deposited on top of the frontal Sevier belt, where the topo- slopes, but their areal dimensions are quite similar to fans deposited by sed- graphic front of the mountain belt lay several tens of kilometers west of iment gravity flows (e.g., Bull, 1963, 1964; Hooke, 1967; Fig. 2). the structural front. This was also the case for the Harebell-Pinyon Deposits of fluvial megafans and stream-dominated alluvial fans are abun- (Schmitt, 1987) and central Utah (DeCelles et al., 1995) megafans. The dant in the middle Miocene–Pliocene Siwalik Group in Pakistan, northern proximal parts of the Himalayan megafans are also being deposited on top India, and Nepal (Parkash et al., 1980; Willis, 1993; DeCelles et al., 1998). of the blind, frontal part of the Himalayan fold-thrust belt (Nakata, 1989; This demonstrates that the modern geomorphology and sedimentology of the Powers et al., 1998).

1332 Geological Society of America Bulletin, September 1999

western Cordilleran of North America and the etiology of climate, in Crowley, T. J., and CONCLUSIONS Burke, K., eds., Tectonic boundary conditions for climate reconstructions: Oxford Mono- graphs on Geology and Geophysics, No. 39, p. 73–99. The Upper Cretaceous Hams Fork Conglomerate was deposited by a pair Collinson, J. D., 1996, Alluvial sediments, in Reading, H. G., ed., Sedimentary environments and facies (third edition): Oxford, Blackwell Scientific Publications, p. 37–82. of fluvial megafans, the southernmost of which had an areal extent >2500 Constenius, K. N., 1996, Late Paleogene extensional collapse of the Cordilleran foreland fold and km2. The megafans were deposited by generally east-southeastward-flowing thrust belt: Geological Society of America Bulletin, v. 108, p. 20–39. gravelly braided rivers that became sandier eastward, carrying mainly Coogan, J.C., 1992, Thrust systems and displacement transfer in the Wyoming-Idaho-Utah thrust belt [Ph.D. dissert.]: Laramie, University of Wyoming, 239 p. durable Proterozoic and lower Cambrian quartzite clasts derived from a Crawford, K. A., 1979, Sedimentology and tectonic significance of the Late Cretaceous–Paleocene large, structurally inactive thrust sheet (Willard thrust) that was topographi- Echo Canyon and Evanston synorogenic conglomerates of the north-central Utah thrust belt [Master’s thesis]: Madison, University of Wisconsin, 143 p. cally rejuvenated by uplift above a structurally lower, younger, Late Creta- Damanti, J. F., 1993, Geomorphic and structural controls on facies patterns and sediment com- ceous thrust fault (the Absaroka thrust). Coarse gravel deposited by these flu- position in a modern foreland basin, in Marzo, M., and Puigdefábregas, C., eds., Alluvial vial megafans covered large areas of the wedge-top and proximal foredeep sedimentation: International Association of Sedimentologists, Special Publication 17, p. 221–233. depozones of the Cordilleran foreland basin system. DeCelles, P. G., 1994, Late Cretaceous–Paleocene synorogenic sedimentation and kinematic his- The southern Hams Fork megafan was similar in size, fluvial discharge, tory of the Sevier thrust belt, northeast Utah and southwest Wyoming: Geological Society of bedload caliber and composition, and depositional processes to modern flu- America Bulletin, v. 106, p. 32–56. DeCelles, P. G., Lawton, T. F., and Mitra, G., 1995, Thrust timing, growth of structural culmina- vial megafans in the Himalayan foreland basin system of Nepal and north- tions, and synorogenic sedimentation in the type area of the Sevier orogenic belt, central ern India. The portion of the Himalayan foreland region that is drained by Utah: Geology, v. 23, p. 699–702. DeCelles, P. G., and Giles, K. N., 1996, Foreland basin systems: Basin Research, v. 8, p. 105–123. transverse rivers may be a modern analog for the nonmarine part of the Late DeCelles, P. G., Gehrels, G. E., Quade, J., Ojha, T. P., Kapp, P.A., and Upreti, B. N., 1998, Neo- Cretaceous Cordilleran foreland basin in Utah and Wyoming. gene foreland basin deposits, erosional unroofing, and the kinematic history of the Hima- The modern depositional mosaic of the Himalayan foreland basin system layan fold-thrust belt, western Nepal: Geological Society of America Bulletin, v. 110, p. 2–21. in Nepal and northern India is composed of transversely oriented fluvial Eisbacher, G. H., Carrigy, M. A., and Campbell, R. B., 1974, Paleodrainage pattern and late- megafans, intermegafan areas which have stream-dominated alluvial fans in orogenic basins of the Canadian Cordillera, in Dickinson, W. R., ed., Tectonics and sedi- their proximal parts, and a major axial trunk river system (the Ganges). Flu- mentation: Society of Economic Paleontologists and Mineralogists Special Publication 22, p. 143–166. vial megafans seem to require large fluxes (at least seasonally) of water and Evans, J. E., 1991, Facies relationships, alluvial architecture, and paleohydrology of a Paleogene, sediment, large catchments, and antecedent drainages, but not necessarily humid-tropical alluvial-fan system: Chumstick Formation, Washington State, U.S.A.: Jour- nal of Sedimentary Petrology, v. 61, p. 732–755. humid climatic conditions or long-lived transverse structures in the fold- Friend, P. F., 1978, Distinctive features of some ancient river systems, in Miall, A. D., ed., Fluvial thrust belt. Because they are formed by large transverse rivers that drain the sedimentology: Memoirs of the Canadian Society of Petroleum Geology, v. 5, p. 531–542. largest catchments in fold-thrust belts, fluvial megafans deposit the volumet- Geddes, A., 1960, The alluvial morphology of the Indo-Gangetic plain: Its mapping and geographical significance: Institute of British Geographers, Transactions and Papers, v. 28, rically largest accumulations of coarse sediment in foreland basin systems. p. 253–276. Gohain, K., and Parkash, B., 1990, Morphology of the Kosi megafan, in Rachocki, A. H., and ACKNOWLEDGMENTS Church, M., eds., Alluvial fans: A field approach: Chichester, United Kingdom, John Wiley and Sons, p. 151–178. Goldstrand, P. M., 1994, Tectonic development of Upper Cretaceous to Eocene strata of south- This research was supported by National Science Foundation grants western Utah: Geological Society of America Bulletin, v. 106, p. 145–154. Gupta, S., 1997, Himalayan drainage patterns and the origin of fluvial megafans in the Ganges EAR-9526991 and EAR-9725607. Field work in Nepal was facilitated by foreland basin: Geology, v. 25, p. 11–14. T. P. Ojha in collaboration with the Royal Nepal Department of Mines and Hayward, A. B., 1983, Coastal alluvial fans and associated marine facies in the Miocene of Geology. J. C. Coogan, J. A. Wolfe, and J. Parrish provided useful informa- S.W. Turkey, in Collinson, J. D., and Lewin, J., eds., Modern and ancient fluvial systems: International Association of Sedimentologists, Special Publication 6, p. 323–336. tion about outcrop locations, paleobotany, and Late Cretaceous paleocli- Hein, F. J., and Walker, R. G., 1977, Bar evolution and development of stratification in the grav- mate. We were assisted in the field by U. and I. Palmito. 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