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Upper Cretaceous) and Modern Himalayan Foreland Basin Systems

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Upper Cretaceous) and Modern Himalayan Foreland Basin Systems 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 accu- mulations 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. 1315 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 DECELLES AND CAVAZZA 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
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