Regional variations in the fluvial Upper Devonian and Lower Mississippian(?) Kanayut Conglomerate, Brooks Range, Alaska

Sedimentary Geology, 38 (1984) 465-497

465

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

REGIONAL

VARIATIONS

LOWER

MISSISSIPPIAN(?)

RANGE,

ALASKA

IN THE

FLUVIAL

KANAYUT

UPPER

DEVONIAN

CONGLOMERATE,

AND

BROOKS

THOMAS E. MOORE and TOR H. NILSEN

U.S. Geological Survey, 345 MiddlefieM Road, Menlo Park, CA 94025 (U.S.A.) (Accepted for publication June 23, 1983)

ABSTRACT Moore, T.E. and Nilsen, T.H., 1984. Regional variations in the fluvial Upper Devonian and Lower Mississippian(?) Kanayut Conglomerate, Brooks Range, Alaska. Sediment. Geol., 38: 465-497. The wholly allochthonous Upper Devonian and Lower Mississippian(?) Kanayut Conglomerate is one of the most extensive fluvial deposits in North America. It crops out for 950 km along the crest of the Brooks Range in a series of thrust plates and is as thick as 2615 m. The Kanayut forms the fluvial part of a large, coarse-grained delta. The lower part of the Kanayut (the Ear Peak Member) overlies marginal-marine and prodelta turbidite deposits and consists of fining-upward meandering-stream-channel cycles of conglomerate and sandstone within black to maroon floodplain shale deposits. The middle part of the Kanayut (the Shainin Lake Member) lacks shale and consists of fining-upward couplets of channelized conglomerate and parallel- to cross-stratified sandstone interpreted as braidplain deposits. These deposits contain the largest clasts (23 cm) and were deposited during maximum progradation of the fluvial sequence. The upper part of the Kanayut (the Stuver Member), which consists of fining-upward meandering stream cycles similar to those of the lower part, grades upward into overlying Lower Mississippian tidal and marginal-marine deposits. Paleocurrent data and distribution of largest clasts indicate that the Kanayut was deposited by southwest-flowing streams fed by at least two major trunk streams that drained a mountainous region to the north and east. Comparison of stratigraphic and sedimentologic data collected at three selected locations representative of proximal, intermediate and distal parts of the Kanayut basin reveal regional variations in its fluvial character. These include a decrease in total thickness of fluvial strata, an increase in total thickness of associated marine sandstone, the pinch-out of the coarse-grained middle part of the Kanayut and decreases in the conglomerate/sandstone and sandstone/shale ratios from proximal to distal areas of the basin. The coarse-grained parts of the fluvial cycles decrease in thickness and lateral extent from proximal to distal areas of the basin. In more distal areas of sedimentation, the middle parts of some fluvial cycles consist of calcareous and bioturbated marine sandstone. Although thinner than in more proximal areas, the associated fine-grained upper parts of some cycles also contain marine features and suggest that these strata represent the deposits of interdistributary bays. These features are interpreted to indicate that the proximal deposits of the Kanayut Conglomerate were deposited by large, stable fine-grained meandering rivers (the Ear Peak and Stuver Members) and gravelly braided rivers (Shainin Lake Member) on the upper delta plain of the Kanayut delta. Sedimentation in more distal locations, interpreted to represent lower delta plain deposits, was by smaller 0037-0738/84/$03.00

© 1984 Elsevier Science Publishers B.V.

466

distributary rivers with characteristics of both braided and meandering streams. Near their interface with marginal marine deposits the fluvial deposits were locally strongly influenced bv tidal or estuarine conditions.

INTRODUCTION

The Upper Devonian and Lower Mississippian(?) Kanayut Conglomerate is one of the most extensive fluvial units in North America. It crops out along the crest of the Brooks Range in northern Alaska over an east-west distance of 950 km and a north-south distance of about 65 km (Fig. 1). It forms the fluvial part of a large, coarse-grained delta that prograded to the southwest in the Late Devonian and retreated in the Late Devonian and Early Mississippian (Nilsen et al., 1981a). The Kanayut Conglomerate is wholly allochthonous and is present in imbricate, generally east-striking, north-vergent thrust sheets. The original nonmarine depositional basin was therefore much wider than the width of the present exposure belt. Stratigraphic and sedimentologic data, including 26 measured sections from the Kanayut Conglomerate and associated units in the Brooks Range, were presented in a series of open-file reports (Nilsen et al., 1980b, 1981b, 1982; Nilsen and Moore, 1982, in press a). The purpose of this report is to summarize sedimentologic data from the Kanayut Conglomerate throughout the Brooks Range. We will also discuss possible basin-wide changes in its sedimentology through comparison of three selected measured sections that are representative of proximal, intermediate and distal parts of the Kanayut depositional basin. G E O L O G I C SETTING A N D PREVIOUS WORK

The Kanayut Conglomerate forms part of the Endicott Group, a thick sequence of Upper Devonian and Mississippian clastic units bounded below by platform carbonate of the Baird Group and above by platform carbonate, carbonaceous shale and black chert of the Lisburne Group (Tailleur et al., 1967). The Endicott Group in the central part of the Brooks Range (Fig. 2) consists in ascending order of the marine Hunt Fork Shale, marine Noatak Sandstone, nonmarine Kanayut Conglomerate, and marine Kayak Shale (Bowsher and Dutro, 1957; Chapman et al., 1964; Porter, 1966). This sequence of strata forms a series of allochthonous thrust plates thought to have been transported northward during late Mesozoic orogenesis (Mull et al., 1976; Mull and Tailleur, 1977; Roeder and Mull, 1978). A second sequence of the Endicott Group is autochthonous or parautochthonous and may structurally underlie the allochthonous sequence of the Endicott Group. It consists primarily of the Kekiktuk Conglomerate and Kayak Shale (Brosge et al., 1962; Reed, 1968; Martin, 1970; Ellersieck et al., 1979), which rest unconformably on deformed and intruded pre-Upper Devonian metasedimentary and metavolcanic

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469

rocks (Fig. 2). The autochthonous sequence crops out in the Brooks Range to the south, east and northeast of the allochthonous sequence. It also underlies parts of the North Slope and Arctic Foothills north of the allochthonous sequence. Relations between the allochthonous and autochthonous sequences of the Endicott Group are not wholly clear because of uncertain palinspastic reconstructions within the Brooks Range and between the Brooks Range and other circum-Arctic orogenic belts (Nilsen, 1981). The Kanayut Conglomerate has been mapped in some areas in considerable detail (Brosge and Reiser, 1962, 1964, 1965, 1969, 1971; Brosge et al., 1976, 1979a,b). In the central and eastern Brooks Range it consists of three members, in ascending order: (1) the Ear Peak Member, as thick as 1150 m, consisting chiefly of fining-upward cycles thought to have been deposited by meandering streams; (2) the Shainin Lake Member, as thick as 526 m, consisting chiefly of conglomerate-sandstone couplets thought to have been deposited by braided streams; and (3) the Stuver Member, as thick as 1310 m, consisting chiefly of fining-upward cycles thought to have been deposited by meandering streams (Nilsen and Moore, 1982b, in press-b; Fig. 3). In the east-central Brooks Range the Kanayut is as thick as 2615 m. It thins and fines markedly to the south and west. The Kanayut overlies the marine Upper Devonian Noatak Sandstone or, where it is missing, the marine Upper Devonian Hunt Fork Shale. It is overlain by the marine Mississippian Kayak Shale. The Kanayut Conglomerate in the western Brooks Range consists of well-defined fining-upward fluvial cycles with interbedded floodplain deposits that contain plant debris and paleosols. It rests gradationally on the Noatak Sandstone and is overlain gradationally by the Kayak Shale (Nilsen and Moore, 1982). It records a major offlap-onlap cycle within the Endicott Group similar to that of the central and eastern Brooks Range. Based on paleocurrent measurements ,and size distribution of quartz clasts, Donovan and Tailleur (1975) suggested southerly transport of the sedimentary detritus of the Endicott Group from a northern source area. Nilsen et al. (1980a) determined regional sediment transport to the southwest. Although brachiopods of latest l)evonian age (J.T. Dutro, pers. commun., 1982) are present in its uppermost beds about 15 km south of Anaktuvuk Pass and plant fossils generally indicate a Late Devonian age, some plant fossils from the upper parts of the Kanayut Conglomerate suggest a probable Early Mississippian age (S.H. Mamay, pers. commun., 1980, 1982). We therefore consider the Kanayut Conglomerate to be of Late Devonian and Early Mississippian(?) age. Brachiopods from the underlying Hunt Fork Shale and Noatak Sandstone are of Frasnian and Famennian (Late Devonian) age and those from the overlying Kayak Shale are of Kinderhookian and Osagean (Early Mississippian) age (Nilsen et al., 1980b). The Kanayut Conglomerate is compositionally very mature, containing about 82% chert, 15% vein quartz and 3% quartzite clasts. Point-count estimates from thin

470

sections of sandstone indicate a framework composition of 30-70% quartz, 10-50% chert, lesser amounts of argillite, quartzite, granitic and gneissic rock fragments, minor amounts of feldspar, biotite, mica, tourmaline and quartz-mica tectonite rock fragments, and rare volcanic rock fragments. The composition appears to vary little laterally and vertically. Although the source area must have contained abundant siliceous sedimentary rocks and vein quartz, its original composition volumetrically is difficult to ascertain because of the removal of most labile detritus by processes of sediment transport and chemical weathering. SEDIMENTARY FACIES Hunt Fork Shale

Shale deposited in low-energy and probably deep-marine (at least below wave base) settings forms most of the lower member of the Hunt Fork Shale (Fig. 3). It is as thick as 700 m in the Philip Smith Mountains quadrangle (Brosge et al., 1979a) and contains thin turbidite interbeds that increase in abundance upward. Shale and shaley siltstone interbedded with fine- to medium-grained sandstone that contains abundant brachiopod fossils make up overlying wacke member of the Hunt Fork Shale (Dutro et al., 1977). It is as thick as 700 m in the Philip Smith Mountains quadrangle (Brosge et al., 1979a) and is thought to have been deposited in a marine slope to outer-shelf setting. Noatak Sandstone

The Noatak Sandstone (Dutro, 1952, 1953a,b), previously mapped as the basal sandstone member of the Kanayut Conglomerate (Brosge et al., 1979a,b) in the central and eastern Brooks Range, consists of calcareous sandstone interbedded with black shale. The Noatak is locally conglomeratic and contains marine megafossils in many places. It is as thick as 1000 m in the western Brooks Range (Tailleur et al., 1967) and 600 m in the Philip Smith Mountains quadrangle (Brosge et al., 1979a) but is absent along the northern margin of much of the central and eastern Brooks Range. The Noatak is characterized by thickening- and coarsening-upward cycles deposited in delta-front settings, probably most commonly as channel-mouth bars. Kanayut Conglomerate Ear Peak and Stuoer Members The Ear Peak Member and Stuver Member of the Kanayut Conglomerate consist of a series of thinning- and fining-upward cycles of conglomerate, sandstone and shale. Because of the similarity of the cycles and their sedimentary structures to those described from many modern meandering rivers, we provisionally interpret these members to have been deposited by meandering streams on a floodplain.

471

Prodelta slope deposits

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$trMe:amnd:ri;ogits Braided stream deposits

Meandering /e0" streamdeposits Shoreline deposits /f~ Channel mouth l Q~ bar deposits ::)delt8 slope deposits Fig. 3. Diagrammatic columnar section of the allochthonous sequence of the Endicott Group, central

Brooks Range. Arrows inclined upward to left indicate fining-upward sequences and arrows inclined to right indicate coarsening-upward sequences. Fossil symbols to right of column indicate general vertical position of plant and molluscan fossils. The entire sequence is about 4500 m thick.

The cycles characteristically commence at their base with erosional truncation of underlying shale or paleosols by thick beds of conglomerate or sandstone. The basal beds typically consist of massive or crudely plane-parallel conglomerate or conglomeratic sandstone containing abundant rip-up clasts of shale, siltstone and paleosol material. Overlying the basal conglomeratic beds are parallel-stratified beds of sandstone that are in turn overlain by trough-cross-stratified beds of sandstone. Trough amplitudes gradually decrease upward in the cycles with decreasing grain size of the sandstone. These deposits represent fill of the channel by sand waves that migrate downchannel as the stream channel gradually shifts and migrates laterally by the meander process.

472 The lower coarse-grained parts of the cycles typically are 2 m to as much as 30 m in thickness. A characteristic feature of this part of the cycles is long inclined surfaces that cut across the vertical sequence (epsilon cross-stratification). These surfaces, which are thought to be the original inclined surfaces of the inner parts of meander loops or point-bar surfaces, have been observed in a number of locations in the Kanayut Conglomerate. They are most visible in the sandy rather than conglomeratic cycles and typically have amplitudes on the order of 5-15 m, crest-totrough lengths of 25-75 m and stratification thicknesses of 1-3 m. The upper part of the cycles consists of thinly bedded current-ripple-marked fine-grained sandstone with thin shale interbeds. The ripple-marked sandstone contains abundant mica, clay and carbonaceous material. Climbing ripples are locally common in these deposits, as well as plant fossils and root impressions. These thin beds of sandstone are interpreted to be levees deposited on the inner parts of meander loops by overbanking processes during flood stages. The uppermost part of the cycles consists of interchannel and floodplain shale and siltstone that are 1 m to as much as 60 m in thickness. The shale varies from reddish brown to black in color, probably depending upon the amount of exposure to the atmosphere. Red shale probably was deposited chiefly on higher ground of the floodplain and black shale in lower, swampy areas. Many cycles contain red shale directly over the sandy levee facies, succeeded upward by black shale. Both red and black shale contain abundant fossil plant debris, much of it in situ. Mudcracks, raindrop imprints and features that might represent burrows but are more likely root casts from plants, are common. The Ear Peak Member locally contains very thick sections of reddish-brown shale, particularly in the eastern Brooks Range. These deposits may represent large floodplain areas traversed by few river channels. In these areas, shale deposited by major floods probably accumulated to substantial thicknesses. The fine-grained upper parts of the cycles commonly contain local bright yellow, orange, and red highly oxidized altered horizons that we interpret to be paleosols. These layers are generally massive and argillitic and locally contain abundant pyrite. They range from 10 cm to as much as 200 cm in thickness and are continuous laterally for distances of at least 50 m. They are generally developed in sandstone and siltstone, commonly at the top of coarsening-upward cycles, 1-3 m thick, interpreted to represent levee deposits. Where present, the paleosols commonly are mottled, non-calcareous, and appear to overprint or destroy other sedimentary features such as plant fossils and ripple markings. Cycles of sandstone not characterized by fining- and thinning-upward trends accumulated in parts of the meandering stream facies. These bodies of sandstone are locally channelized, may form symmetrical vertical cycles, and characteristically contain abundant rip-up clasts and fragments of levee, interchannel and floodplain facies. The bodies may be crevasse-splay deposits formed where levees have been broken through during large floods.

473

Shainin Lake Member The Shainin Lake Member consists of interbedded conglomerate and sandstone which we interpret to have been deposited by braided streams. The characteristic feature of these deposits is the vertical stacking of fining-upward couplets of conglomerate and sandstone, with the erosional base of each conglomerate bed truncating the underlying sandstone. In some sections, conglomerate rests on conglomerate to form amalgamated beds, with sandstone absent either as a result of non-deposition or erosion. The conglomerate-sandstone couplets are thought to represent deposition of various kinds of bars within a braided stream complex. Sandstone is deposited on the flanks, tops and downstream edges of gravel bars as thin but wide lens-shaped bodies characterized generally by parallel stratification, low-angle trough cross stratification or very low-angle inclined tabular cross stratification. The sandstone probably accumulated during waning stages of floods and on the protected downstream margins of bars. The largest conglomerate clasts are found in the Shainin Lake Member. The conglomerate is typically well imbricated and characterized by a closed framework with a sandstone or pebbly sandstone matrix. Long axes are oriented parallel to flow and have proved to be useful paleocurrent indicators for the Shainin Lake Member. Paleosols, levee deposits, shale, and siltstone are rarely present and generally less than 1 m in thickness. Where present, the paleosols are generally marked by highly discontinuous red oxidized layers a few centimeters thick that are commonly at the top of conglomerate and sandstone beds. Kayak Shale In its type area, near Shainin Lake, the Kayak Shale is about 300 m thick, rests conformably on nonmarine facies of the Stuver Member and has been subdivided into five members: (1) basal fine-grained sandstone, 40 m thick; (2) lower black shale, 180 m thick; (3) argillaceous limestone, 24 m thick; (4) upper black shale, 40 m thick; and (5) red limestone, 5 m thick (Bowsher and Dutro, 1957). The three lower members can be traced along the entire Brooks Range, despite marked changes in the total thickness of the Kayak from the effects of thrust faulting. However, in the southern and eastern Brooks Range, the Kayak Shale in the allochthonous sequence of the Endicott Group is generally less than 150 m thick and consists mostly of black shale. The basal sandstone member typically consists of thinly cross-stratified and ripple-marked fine-grained quartzose sandstone with abundant burrows. The overlying black shale contains some thin graded beds of fine-grained sandstone that appear to be either turbidites or vertical accumulations of storm-generated sediment overflows. The argillaceous limestone has features that suggest a debris-flow origin (Nilsen and Moore, 1982a).

474

The Kayak Shale in general represents a sequence that was deposited in progressively deeper water, except at its top in the central and eastern Brooks Range, where it shoals upward into platform limestone of the Lisburne Group. The basal sandstone represents nearshore deposition, probably in tidal sand flats. Paleocurrent directions from it are highly variable and indicate flow toward the southwest, southeast and northeast (Nilsen et al., 1980a). The overlying black shale represents deeper marine sedimentation, probably a prodelta slope setting, into which some massive fossiliferous debris flows of argillaceous limestone were resedimented. RE PRESE NTAT IVE M E A S U R E D SECTIONS

The thickness of the Kanayut Conglomerate varies from east to west and from thrust plate to thrust plate. The Kanayut appears to be thickest in the northern part of the Trans-Alaska Pipeline area (Fig. 1) and it generally thins toward the west and south. The coarser Shainin Lake Member pinches out toward the west and south, beyond which the members of the Kanayut Conglomerate cannot be distinguished. We provide here three measured sections representative of proximal, intermediate and distal environments that demonstrate lateral changes in facies. The three sections are: (1) the type sections in the Shainin Lake area, central Brooks Range; (2) a complete section at Siavlat Mountain, west-central Brooks Range; and (3) two incomplete sections in the Husky Mountains of the western Brooks Range (Fig. 1). No complete sections have been measured in the easternmost area of exposure of the Kanayut Conglomerate, but partial sections there contain facies similar to those exposed in the type sections in the Shainin Lake area. Shainin Lake area Ear P e a k M e m b e r

The type section of the Ear Peak Member near Ear Peak, east of Shainin Lake (sec. 13, T. 13 S., R. 5 E., Chandler Lake quadrangle; Figs. 1 and 4), is 510 m thick and consists of thirty-five major fining-upward cycles that average 15 m in thickness. About 20% of the section is covered. The entire sequence generally coarsens upward as it approaches the base of the Shainin Lake Member. Cycles typically begin with beds of conglomerate or conglomeratic sandstone, 1-5 m thick, overlain by troughcross-stratified or flat-stratified sandstone capped in some cycles by ripple-marked siltstone and red shale. The base of some cycles is marked by erosional scour into the underlying unit. The amount of shale, siltstone, and fine-grained sandstone decreases upsection as the amount of coarse-grained sandstone and conglomerate increases. Rare burrows and plant fossils are present within beds of shale and fine siltstone in the lower half of the section. The boundary of the Ear Peak Member with the underlying Hunt Fork Shale is transitional and marked by the disappearance of marine megafossil debris and the initiation of distinct fining-upward cycles. The first distinct fining-upward cycle, at

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476

an elevation of 1610 m, forms the boundary between the two units. We interpret the boundary to mark the change upward from marine-dominated deposition to fluvialdominated deposition. The distinct fining-upward cycles may result from lateral and vertical accretion of meandering-stream channel and point-bar deposits. Epsilon cross-strata, observed at 18 and 255 m above the base of the section, form inclined surfaces transecting the vertical sequence and may represent the actual inclined surfaces of point-bars within the system. Upper point-bar and floodplain deposits consist of shale and ripple-marked siltstone, with thin interbeds of fine-grained sandstone possibly representing crevasse-splay deposition. Shainin Lake Member The type section of the Shainin Lake Member about 8 km south-southeast of Shainin Lake (sec. 33, T. 13 S., R. 5 E. and sec. 5, T. 14 S., R. 5 E., Chandler Lake quadrangle; Fig. 1) is 526 m thick (Fig. 5). The section is relatively uniform, consisting of thick beds of conglomerate that fine upward to conglomeratic sandstone and medium- to very coarse-grained sandstone. Shale is generally absent except for some thin intervals from 450 to 490 m above the base of the section and almost all covered intervals appear to be sandstone. The coarsest and most thickbedded conglomerate is present in the middle part of the section, from 260 to 440 m above the base. Thinner bedded and finer grained conglomerate with greater amounts of interbedded sandstone characterizes the lower and upper parts of the section and marks the transition from the Ear Peak Member and to the Stuver Member, respectively. The conglomerate beds typically have erosional bases characterized by coarser conglomerate resting on finer conglomerate, conglomeratic sandstone, sandstone or shale. The conglomerate beds are most commonly massive and characterized by normal size grading of conglomerate clasts, well-developed imbrication and clast long-axis orientation. The matrix consists of finer conglomerate and sandstone. Clast-supported conglomerate is most typical, although in finer grained conglomerate and conglomeratic sandstone, matrix-supported conglomerate is common. However, the matrix is never mud-rich and there is no other indication of sediment transport and deposition by debris flows or related processes such as reverse grading, vertically oriented clasts, and other distinctive fabrics. Instead, all coarsegrained deposits appear to have resulted from streamflow processes. Finer conglomerate typically has poorly developed parallel stratification and consists of interlayered beds of coarser and finer conglomerate. Large- and medium-scale trough cross-strata and planar cross-strata are present in the upper parts of some conglomerate beds, but are most characteristic of the conglomeratic sandstone units that rest without erosional scour on the lower beds of massive conglomerate. The fining-upward sequences of conglomerate to sandstone or shale average about 2-3 m thick in the lower 260 m of the section, about 7 m thick in the middle

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478 180 m and about 4 m thick in the upper 90 m. The sequences record stream deposition, most likely that of a broad braidplain, in which braided streams transporting very coarse debris spread out over a broad aggrading depositional surface. There is no evidence for deposition by meandering streams and no indication for the development of fining-upward point-bar cycles. The lack of significant thicknesses of shale indicates that fine-grained floodplain deposits did not develop. The massive and parallel-stratified bars of conglomerate probably represent longitudinal gravel bars developed on the braidplain. The trough cross-stratified and planar cross-stratified beds probably represent transverse bars and dunes as well as deposition on the downstream flanks of the longitudinal bars. The thin shale intervals in the upper part of the section probably represent drapes of mud deposited over the bars during post-flood lowering of stream levels. In most of the sections these mud drapes, if deposited, were probably eroded away before or during deposition of the overlying bed of conglomerate. Stuoer Member The type section of the Stuver Member, designated and previously measured by Bowsher and Dutro (1957), is located about 3 km southeast of Shainin Lake (sec. 16, T. 13 S., R. 5 E., Chandler Lake quadrangle). The section is 217 m thick and consists of 14 fining- and thinning-upward cycles that average about 16 m in thickness (Fig. 6). Shale, including most covered intervals, makes up about 95 m of the section~ or 40% of the thickness of the Stuver Member. Almost all of the cycles begin with conglomerate that has an erosional basal contact with shale. The conglomerate grades upward through cross-stratified sandstone into current-ripple-marked finegrained sandstone and siltstone, and at the top, shale with locally abundant paleosols. Epsilon cross-stratification is conspicuous in the lower part of the finingupward cycles in pebble conglomerate and medium-grained sandstone at 140 m above the base of the section. The shale is most typically black with abundant plant fossils, but also includes shale with reddish, orange, yellow and maroon colors probably developed under varying conditions of oxidation. The paleosols are commonly pyritic and siliceous and are developed in sandstone and, locally, in conglomerate. The fining-upward cycles are thicker and better defined in the lower part of the Stuver Member, where there are sequences of shale as thick as 20 m or more. In the interval from 130 to 170 m, the fine-grained upper part of the cycles are thin and the Stuver Member consists of repetitively interbedded conglomerate and sandstone similar in appearance to the Shainin Lake Member. Deposition by braided streams may have become dominant again in this interval. The uppermost 50 m of the Stuver Member is relatively finer grained, consisting mostly of sandstone and shale. However, this uppermost interval contains the coarsest clasts in the Stuver, as large as 13 cm, at the base of a fining-upward cycle about 200 m above the base of the Stuver Member.

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The upper contact with the basal sandstone member of the Kayak Shale is well defined. Marine trace fossils, herringbone cross-strata, reactivation surfaces, oscillation ripple marks, flaser bedding, the absence of plant fossils, conglomerate or paleosols, and the presence of very fine-grained, well-sorted quartzose sandstone suggest marine deposition, probably under tidal influences, for the basal part of the Kayak Shale.

480 Siavlat Mountain

A complete section of the Kanayut Conglomerate was measured along the northeast flank of Siavlat Mountain, about 50 km south of the Lisburne Well site in the west-central Brooks Range (sec. 25, T. 31 N., R. 14 E., Killik River quadrangle: Figs. 1 and 7). Although mapping by Brosge et al. (in press, plate 1) shows the presence of only the Ear Peak Member and Stuver Member at Siavlat Mountain, the Kanayut Conglomerate at Siavlat Mountain can be roughly divided into three units that may correlate with its three members as defined at Shainin Lake in the east-central Brooks Range. These three units, however, do not exactly fit the definitions of the three members of the Kanayut proposed by Nilsen and Moore (in press b). In this report, they will be informally referred to as units 1. 2, and 3 in ascending stratigraphic order. Unit 1 (from 79 to 146 m in the section) can be subdivided into five fining-upward cycles that average 13.4 m in thickness. Shale is almost totally absent except as thin partings associated with ripple-marked very fine-grained sandstone at the tops of the cycles. However, rip-up clasts of the shale are abundant throughout unit 1 and suggest that shale was originally deposited on surrounding floodplains but was stripped away by erosion during channel migration. Unit 1 gradually coarsens upward, containing chiefly fine- to medium-grained sandstone in its lower cycles and medium- to coarse-grained sandstone in its upper cycles. A paleosol is present about 4 m above the base of the lowest cycle and indicates subaerial weathering. Above the soil horizon is a medium-grained, trough cross-stratified sandstone that is 2.7 m thick. It is in turn overlain by about 1 m of ungraded carbonate-cemented sandstone that contains oscillation ripple markings and shale drapes, which may indicate that it has a marine origin (86--87 m above the base of the section). Thus, the lowest fluvial cycle of the Kanayut at Siavlat Mountain contains one marine interbed in its upper part. Unit 2 of the Kanayut Conglomerate (from 146 to 268 m in the section) can be subdivided into nine fining-upward cycles that average 13.6 m in thickness, approximately the same as those from unit 1. The conglomerate in unit 2 has a maximum clast size of 1.5 cm. Beds of massive conglomerate and conglomeratic sandstone at the base of fining-upward cycles are as thick as 9 m. Most cycles in unit 2 consist, in ascending order, of a basal massive conglomerate or conglomeratic sandstone that may contain rip-up clasts of shale, parallel-stratified coarse-grained sandstone, trough cross-stratified fine- to coarse-grained sandstone, laminated and ripple-marked very fine- to fine-grained sandstone and siltstone, and shale that ranges in color from black to red-brown. Shale is present at the top of only five of the nine cycles of unit 2. The basal massive beds are typically channeled into the shale or thinly interbedded very fine-grained sandstone and siltstone. Unit 3 of the Kanayut Conglomerate (from 268 to 307 m in the section) consists of three fining-upward cycles that average 13 m in thickness. Unit 3 has shale

481 Grain Size fern, 5 .2 m . =, C a r b o n a t e r--i , ~ Cement

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Fig. 7. M e a s u r e d s e c t i o n o f the K a n a y u t C o n g l o m e r a t e a t Siavlat M o u n t a i n . See Fig. 6 for e x p l a n a t i o n o f symbols.

482 intervals as thick as 5 m and the lowest conglomerate and sandstone to shale ratio in the Kanayut at Siavlat Mountain. In fact, it contains no conglomerate: the lowest cycle starts with very coarse-grained sandstone that contains abundant rip-up clasts of shale and the two overlying cycles start with trough cross-stratified fine- to medium-grained sandstone. The shale at the tops of the cycles is black. Fine- to medium-grained, trough cross-stratified sandstone is the dominant bed type in unit 3.

Husky Mountains Two sections of the Kanayut Conglomerate, labeled A and B, were measured in the Husky Mountains about 65 km north-northeast of the town of Noatak and northwest of the confluence of the Noatak and Kelly Rivers (Fig. 8). The lowermost 233 m of the Kanayut Conglomerate was measured in section A (sec. 14, T. 31 N., R. 17 W., DeLong Mountains quadrangle) and the uppermost 58 m was measured in section B (sec. 9, T. 31 N.~ R. 17 W., DeLong Mountains quadrangle). Unfortunately, we were not able to measure a complete section of the Kanayut Conglomerate. We believe, however, that the 233 m of the Kanayut measured in section A is close to the total thickness of the Kanayut in the Husky Mountains, which is probably 250-300 m. The stratigraphy of the Kanayut Conglomerate will be discussed in ascending stratigraphic order. The Kanayut Conglomerate consists of a series of repetitive fining-upward cycles that are commonly separated by red or black shale. Twenty-seven cycles from the lower, middle and possibly upper Kanayut, averaging about 8.5 m in thickness, are present in section A and four cycles from the upper part of the Kanayut, averaging 14.5 m in thickness, are present in section B. The cycles typically begin with a conglomerate or sandstone that overlies and is channeled into shale. The lower part of the Kanayut in section A contains interstratified marine rocks in the cycles. These marine rocks, consisting of principally trough cross-stratified sandstone and conglomeratic sandstone, resemble very closely the underlying Noatak Sandstone. The marine rocks have a calcareous cement, locally contain Scolithus burrows and are present in the middle parts of the otherwise fluvial cycles. The interstratified marine rocks, shown in section A with vertical lines to the right of the sections under the heading "carbonate cement" (Fig. 8), are present in the Kanayut as high as at 78 m above the base of the Kanayut in section A. Thus, the intermixed fluvial and marine cycles represent a significant portion of the lower part of the Kanayut Conglomerate in the Husky Mountains area. The mixed marine-fluvial cycles typically begin at their base with channeled conglomerate, conglomeratic sandstone, or medium- and coarse-grained sandstone that resembles fluvial strata at the base of fining-upward cycles in the Kanayut of the central and eastern Brooks Range. The basal beds of the cycles are massive, crudely parallel-stratified, or trough cross-stratified. Downcutting into the underly-

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Fig. 8. Measured sections of the Kanayut Conglomerate in the Husky Mountains. A, Lower part of the Kanayut Conglomerate. B. Upper part of the Kanayut Conglomerate. See Fig. 6 for explanation of symbols.

484 ing shale is generally less than 1 m. The maximum clast size in the conglomerate is 3 cm, although most conglomeratic intervals contain maximum clast sizes that are less than 1 cm. Rip-up clasts of the underlying shale are present in some basal beds. The cycles that do not contain interstratified marine rocks generally fine upward regularly to red or black shale. Finer grained sandstone with lower amplitude trough cross-strata overlies the basal beds of the cycles. Current ripple-marked and laminated very fine-grained sandstone and siltstone that grade upward into shale form the upper parts of the cycles. The shale intervals contain plant fragments and locally have paleosols as in the lowest cycle of the Kanayut in section A. Thin coarseningupward cycles in some of these shaley floodplain deposits are locally capped by paleosols. The cycles that contain interstratified marine rocks generally have a middle part that does not fine upward regularly. The middle part, formed of the calcareous marine strata, consists of interbedded fine-, medium- and coarse-grained sandstone that is locally conglomeratic. These beds are mostly trough cross-stratified, but locally contain tabular cross-strata and wavy bedding. The marine beds that form the middle part of the cycles are not organized into fining- or coarsening-upward trends. In some cycles, the marine beds form more than half of the total thickness of the cycle. These intermixed fluvial and marine cycles in the lower part of the Kanayut Conglomerate in the Husky Mountains suggest invasion of fluvial channels by marine waters. Marine conditions, indicated by calcareous cement in the sandstone, bioturbation and lack of regular fining-upward cyclicity, apparently extended into the mouths of the distal distributary channels of the Kanayut delta. Because marine conditions extend only into the channels, it is clear that the initial cutting of the channels and their establishment was by typical fluvial processes of river channel formation and migration. Following the partial filling of the channels by marine-influenced, intertidal or estuarine deposits, channel migration or abandonment led to development of nonmarine floodplains that form the tops of each cycle. The upper part of the Kanayut Conglomerate, higher than 150 m above the base of the section in section A, consists of repetitive fining-upward cycles generally separated by thin red shale and siltstone intervals. The upper part of the Kanayut contains no interstratified marine beds and is generally finer grained than the lower part of the Kanayut. The maximum clast size of the conglomerate from the upper part of the Kanayut Conglomerate in section A is 0.5 cm. The fining-upward cycles in this part of the Kanayut are generally thinner and the coarser beds consist almost wholly of trough cross-stratified fine- to medium-grained sandstone. The upward transition of the Kanayut Conglomerate into the Kayak Shale is shown in section B. The uppermost part of the Kanayut in this section consists of four fining-upward cycles, two of which contain conglomerate with clasts as large as 1 cm at their bases. The conglomerate beds are channeled into gray or maroon siltstone. The lower parts of the cycles consist of massive to crudely parallel-strati-

485 fied conglomeratic sandstone or sandstone overlain by trough cross-stratified sandstone. Current ripple-marked very fine-grained sandstone and siltstone overlie the coarser beds and grade upward into shale. Paleosols and coal are present within the shale intervals. Within the upper parts of several cycles, burrows, oscillation ripple markings, wavy bedding and shale drapes over beds of sandstone suggest marine influences on the cyclic sedimentation. In contrast to the marine beds in the lower part of the Kanayut, which are medium- to coarse-grained, have a calcareous cement and are present in the middle of the cycles, the marine beds in the uppermost part of the Kanayut are very fine- to fine-grained, thin- to medium-bedded, have a siliceous cement and are present at the tops of the cycles. The beds of very fine- to fine-grained sandstone closely resemble the overlying beds of sandstone in the basal sandstone member of the Kayak Shale. These transitional cycles of the uppermost part of the Kanayut Conglomerate suggest subsidence below sea level of the floodplain surrounding the fluvial channels. Reworking of levee and floodplain sandstone by wave and current activity yielded well-sorted thin beds of quartzitic sandstone similar to that of the basal sandstone member of the Kayak Shale. Marine organisms burrowed into the floodplain shale, which probably was deposited in interdistributary bays. CONGLOMERATE CLAST SIZE DATA The maximum dimension of the largest conglomerate clast was measured at 218 locations (Fig. 9). In order to sample a large thickness of stratigraphic section for the largest clast contained its conglomeratic strata, we collected data primarily from the bouldery fluvial deposits of modern alluvial fans that were deposited by streams draining only exposures of the Kanayut Conglomerate in restricted areas. Although the map distribution of maximum clast sizes must be interpreted cautiously with regard to paleogeography because of significant amounts of structural shortening, several major conclusions can be drawn from the available data. The contour map of clast sizes (Fig. 10) shows the well-defined maxima of clast sizes in the Shainin Lake area and in the easternmost outcrops. The shape of the contour lines around these clast size maxima indicates that sediment transport in both systems was primarily toward the southwest or south. The largest clasts, 23 cm long, are located near Shainin Lake. To the west and south of Shainin Lake, the clast size decreases regularly and dramatically to the Noatak River where conglomerate is rare. Clast size also decreases toward the southeast from Shainin Lake toward Arctic Village, where the largest clasts are 5 cm. The decreasing clast sizes away from the Shainin Lake region suggests that it marks an entry site of a major trunk system into the basin. A second major trunk system may have been located in the north and east where clast sizes decrease southwestward from a maximum of 10 cm in the eastern outcrops to less than 5 cm near Arctic Village. In the western Brooks Range, clast

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sizes appear to decline southward or southeastward from a maximum of 3 cm along the northwestern edge of the outcrop belt. The smallest clasts are present in the southernmost outcrops of the Mulgrave Hills, suggesting sediment dispersal toward the south, southwest and possibly southeast. Several other clast-size maxima are present west of the headwaters of the Junjik River, near the Sagavanirktok River, east of Iteriak Creek and south of the Kuna River (Fig. 10). Minimal southward bowing of the contour lines at these maxima indicate that these systems were probably overwhelmed distally by detritus transported by the major distributary system emanating from the Shainin Lake region. These second-order size maxima may mark the entry sites of smaller river systems into the Kanayut basin. All four of the smaller systems appear to suggest southerly transport of sediment from a northern source. The distribution of discrete clast size maxima along the eastern and northern edges of the belt of exposure of the Kanayut indicates that the highlands from which the Kanayut detritus was derived were located along the northern and eastern margins of the basin. Comparison of the maximum clast sizes with those of other ancient and modern river systems suggests that the coarse clasts in the Shainin Lake area may have been deposited close to the margin of the Kanayut basin. The edge of the basin in that area, however, has either been removed by faulting or is not exposed. The clast size data strongly argue against a southern highland that contributed sediment to the Kanayut basin and suggest that the Kanayut was originally deposited along a south-facing margin of a mountainous belt. PA LEO('U RRENTS

A total of 626 paleocurrent measurements from the Kanayut Conglomerate show consistent westerly or southwesterly sediment transport (Fig. 11). Significant numbers of southerly-directed paleocurrent measurements are present along the Alaska pipeline in the Atigun River area and in a limited area about 30 km west of the Killik River (see insets, Fig. 11). Unidirectional indicators, shown by the darkened area in the rose diagram (Fig. 11), indicate predominantly southwesterly sediment transport. Bidirectional indicators (those giving direction, but not sense of transport), shown by the clear area in the rose diagram, also suggest west-southwest (or east-northeast) sediment transport. The azimuthal vector mean and standard deviation of all measurements from the Kanayut Conglomerate is 242 ° _+ 47 °, but ranges from t72°_+ 28 ° to 304°_+ 30 ° for individual locations having more than four measurements (Fig. 11). The azimuthal vector mean and standard deviation of paleocurrent data from the eastern, central and western Brooks Range are, respectively, 269 ° _+ 46 °, 240 ° + 46 °, and 229 ° + 46 ° (Fig. 9). These data suggest that the average orientation of sediment transport may become somewhat more southerly from east to west. We also calculated the azimuthal vector mean and standard deviation of paleo-

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current data from the three members of the Kanayut Conglomerate in order to quantify any possible shift of sediment transport through time in the Kanayut depositional basin. Paleocurrent data from the thinning and fining-upward cycles of the Ear Peak (214 measurements) and Stuver (131 measurements) Members yield azimuthal vector means and standard deviations of 242°_+ 47 ° and 232°_+ 56 ° , respectively. A total of 237 measurements from the braided stream deposits of the Shainin Lake Member yield 246 ° + 41°. Forty-four measurements from undifferentiated areas of exposure of the Kanayut Conglomerate, mainly in the southern and western Brooks Range, yield 242°_+ 47 ° . These data are generally consistent but suggest that the direction of sediment transport was slightly more southerly during deposition of the Stuver Member. However, the difference is probably statistically insignificant. We believe that the fluvial sediment transport direction was predominantly toward the southwest throughout the period of deposition of the Kanayut Conglomerate. DISCUSSION

We have generally interpreted the finer grained Ear Peak and Stuver Members as having been deposited by meandering streams and the coarser grained Shainin Lake Member to have been deposited by braided streams. However, the major distinction between the units is the lack of shale intervals and absence of thick, complete fining-upward cycles in the coarser Shainin Lake Member. The great variety of fining-upward couplets and cycles, combined with regional variations in coarseness and thickness of cycles, suggests that the fluvial-dominated Kanayut delta orginally contained broad conglomeratic braidplains, individual braided streams surrounded by fine-grained floodplains, meandering streams and extensive finegrained floodplains that graded laterally into muddy interdistributary bays and sandy shoreface deposits at the margins of the subaerial part of the delta (Fig. 12). The type sections of the members of the Kanayut Conglomerate near Shainin Lake are located near the entry point of one of the major river systems into the basin and therefore record proximal sedimentation in the basin during the time of maximum progradation (Fig. 12). In contrast, regional paleocurrent and maximum clast size data indicate that the measured sections in the Husky Mountains record sedimentation in the distal part of the Kanayut basin. The measured section at Siavlat Mountain is located in an intermediate position. The location of the three measured sections on successively higher south-dipping thrust plates suggests that the original predeformational distances between the sections was substantially greater than at present. Comparison of these sections should therefore provide a means of determining down-basin changes in the Kanayut. However, the sections do not necessarily record sedimentation by the same drainage system but present a generalized picture of sedimentation in the various parts of the basin. The thickness of the Kanayut Conglomerate decreases from more than 1250 m in

491 160 °

150 °

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Fig. 12. Paleogeographicmap showing approximate extent of the Kanayut delta. Arrows indicate chief areas of sedimentary input. The delta is shown in its present location, without required palinspastic reconstruction. Abbreviations: S L = location of measured sections near Shainin Lake; S M = location of measured section at Siavlat Mountain; H M ~ location of measured sections in the Husky Mountains. the Shainin Lake area to 228 m at Siavlat Mountain and about 250-300 m in the Husky Mountains (Table I). The coarse-grained Shainin Lake Member, over 500 m thick at Shainin Lake, cannot be differentiated in the other sections, although it may be expressed as a somewhat coarser part of the Siavlat Mountain section. Conversely, the Noatak Sandstone increases in thickness from zero in the Shainin Lake area to about 300 m at Siavlat Mountain and the Husky Mountains. The Noatak Sandstone apparently increases southward and westward in thickness at the expense of the Kanayut, as suggested by others (see Dutro, 1953b). However, the total thickness of coarse-grained clastic strata (about 600 m) in the Husky Mountains is less than that at Shainin Lake (1253 m). Conglomerate clasts decrease in size from 23 cm in the Shainin Lake area to about 2 cm at Siavlat Mountain and 3 cm in the Husky Mountains (Fig. 10). Additionally, the thickness of conglomeratic strata decreases toward the southwest from the Shainin Lake area. The ratio of conglomerate to sandstone plus shale is about 1/1 in the Shainin Lake area, about 1 / 1 0 at Siavlat Mountain and less than 1 / 2 0 in the Husky Mountains. Likewise, the ratio of matrix-supported conglomerate (conglomeratic sandstone) to framework-supported conglomerate increases from

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about 1/1 in the Shainin Lake area and at Siavlat Mountain to 1 / 6 in the Husky Mountains. These data show that the average, as well as the maximum capacity of streams to move coarse detritus decreased substantially from the Shainin Lake area to the Husky Mountains. The character of the fluvial cycles also changes toward the southwest. Conglomerate-sandstone couplets, indicative of deposition by braided streams, make up almost the entire Shainin Lake Member at Shainin Lake, but are nearly absent to the southwest. The thickness of fining-upward cycles decreases from about 15 m in the Shainin Lake area to 12.5 m at Siavlat Mountain and 8.5 m in the Husky Mountains. The thickness of the coarse-grained parts of the cycles also decreases toward the southwest, from an average of about 13 m at Shainin Lake and 12 m at Siavlat Mountain to 6.5 m in the Husky Mountains. The lateral continuity of the coarsegrained part of individual cycles can likewise be observed to decrease southwestward from the Shainin Lake area where individual cycles can be traced for at least several kilometers (Nilsen et al., 1982). Although they are not as well exposed, the lateral continuity of individual strata seems to be about a kilometer or less at Siavlat Mountain and in the Husky Mountains. The lower or coarse-grained parts of the fining-upward cycles also change. In the Shainin Lake area, cycles commence with erosional surfaces that cut downward from several centimeters to as much as 5 m into underlying floodplain deposits. These are overlain by conglomerate-rich fining-upward sequences. These strata typically fine upward over about 3 m into a thick accumulation of floodplain sediment. At Siavlat Mountain, the basal parts of fining-upward cycles are typically composed of medium- to coarse-grained sandstone and conglomeratic sandstone marked by abundant rip-up clasts of shale. These are overlain by thick sequences of parallelstratified and trough cross-stratified sandstone that are locally thick-bedded and fine upward gradually over intervals as thick as 15 m. The fluvial cycles in the Husky Mountains contain thick basal sequences of fineto medium-grained sandstone that locally include conglomeratic sandstone or rip-up clasts at their base. These fine upward over an average of 2 m into thin floodplain deposits. The interior parts of many of these fluvial cycles, however, contain trough cross-stratified beds of carbonate-cemented sandstone. The carbonate cement, good sorting, abundance of large amplitude cross-stratification and locally bi-directionally oriented paleocurrents suggest that these units represent deposition within fluvial channels that had a tidal or estuarine influence. Marine strata are apparently completely absent in the basal coarse-grained part of fluvial cycles in the Shainin Lake and Siavlat Mountain areas and are present only rarely within floodplain intervals. The floodplain intervals also change in character between the sections. Sandstone to shale ratios decrease to the southwest although the floodplain intervals are thinner in the more distal parts of the Kanayut basin. In the Shainin Lake area, the floodplain deposits are typically 2-10 m thick and locally are as thick as 25 m.

494

Where thick, they consist of interstratified levee, floodplain and paleosol deposits locally with massive sandstone beds interpreted as crevasse-splay deposits. The floodplain deposits at Siavlat Mountain and in the Husky Mountains are characteristically very thin and are typically capped by a single paleosol if any is present. Near the top of the Husky Mountains section, however, the floodplain and interdistributary bay intervals locally contain crevasse-splay deposits that are less than 1 m thick. Coal is also present near the top of this section. The type sections of the Ear Peak and Stuver Members were probably deposited by fine-grained meandering streams, whereas the Shainin Lake Member was deposited by gravelly braided rivers. The thick-bedded, sandstone-dominated pointbar sequences and thin floodplain deposits of the Siavlat Mountain section suggest deposition by coarse-grained meandering rivers (McGowen and Garner, 1970) or by rivers that had characteristics of both braided and meandering streams. Fluvial deposition in the Husky Mountains area probably occurred in streams very similar to these interpreted for the Siavlat Mountain area, but were commonly influenced by tidal or estuarine conditions or both. The thinner and laterally less continuous cycles at Siavlat Mountain and in the Husky Mountains suggest that rivers in those areas were smaller and more ephemeral than in the Shainin Lake area. It seems likely that the smaller rivers interpreted for the more distal areas of the Kanayut delta were the distributary channels for the more stable and larger rivers interpreted from the measured sections at Shainin Lake. SUMMARY A N D CONCLUSIONS

The Kanayut Conglomerate consists of three fluvial members, in ascending order, the Ear Peak, Shainin Lake, and Stuver Members. The Ear Peak Member overlies calcareous sandstone and conglomerate of the Noatak Sandstone that was deposited in shallow-marine environments. The Ear Peak Member is inferred to have been deposited by meandering streams, the Shainin Lake Member by braided streams, and the Stuver Member by meandering streams. The Stuver Member is overlain by shallow-marine and intertidal fine-grained sandstone at the base of the Kayak Shale. The three members of the Kanayut can be recognized in the central Brooks Range, but are not differentiatable in the western Brooks Range. The coarse-grained Shainin Lake Member, which permits separation of the members in the central and eastern Brooks Range, apparently pinches out to the southwest between Shainin Lake and Siavlat Mountain. The Kanayut Conglomerate in the western Brooks Range consists of as much as 300 m of meandering-stream deposits that contain abundant interbeds of marine strata in the upper and lower parts. The maximum clast size of conglomerate decreases westward, southward and eastward from the Shainin Lake area in the central Brooks Range, suggesting that a major trunk stream originally entered the depositional basin in this area. A second major trunk stream probably entered the depositional basin at its northeastern end

495 and possibly a third influenced deposition in the western Brooks Range. The composition of conglomerate in the Kanayut varies little from place to place or member to member. In most of the conglomerates examined, about 80-95% of the pebbles are chert, about 5-15% quartz and 1-5% quartzite. Pebbles of argillite and other rock fragments are rare. Judging from the abundance of chert, quartz and quartzite clasts in the Kanayut, the source terrane was probably composed mostly of slightly metamorphosed sedimentary rocks or underwent extensive chemical weathering. The orientation of cross-strata, primary current lineations, current ripple marks, and imbrication and long axes of pebbles in the three fluvial members of the Kanayut consistently show sediment transport toward the southwest across most of the central and western Brooks Range. The consistent southwestward direction of paleocurrents in the fluvial deposits, together with the southwestward decrease of grain size, suggests an eastern, northern or northeastern source, although the allochthonous nature of the outcrop belt precludes identification of the source at present. The facies sequence in the Hunt Fork Shale and Kanayut Conglomerate suggests that the Kanayut comprises the fluvial part of a prograding delta system. Proximal deposits were deposited on a broad braidplain by gravel rivers and by large fine-grained meandering rivers that were capable of carrying a coarse-grained bedload. The meandering rivers were associated with extensive floodplains. These depositional environments comprised the upper delta plain of the Kanayut delta. Lower delta-plain sedimentation was by coarse-grained meandering rivers or by rivers that had characteristics of both braided and meandering rivers. These rivers were more sandy and smaller than those of the upper delta plain and probably were more ephemeral. At the interface between the lower delta plain and the delta front, tidal and estuarine conditions probably extended up the distributary channels. ACKNOWLEDGEMENTS We thank W.P. Brosge, J.T. Dutro Jr., I. Ellersieck, C.F. Mayfield, C.G. Mull, H.N. Reiser and I.L. Tailleur for organizational assistance and helpful discussions. David Orchard, Donna Balin and Samuel Johnson provided field assistance. The manuscript was reviewed and substantially improved by Ronald J. Steel, Fred Peterson and William P. Brosge. REFERENCES Bowsher, A.L. and Dutro Jr., J.T., 1957. The Paleozoicsection in the Shainin Lake area, central Brooks Range, Alaska. U.S. Geol. Surv., Prof. Pap. 303-A, pp. 1-39. Brosge, W.P. and Reiser, H.N., 1962. Preliminarygeologic map of Christian quadrangle, Alaska. U.S. Geol. Surv. Open-File Map OF-62-15, scale 1 : 250,000.

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Brosge, W.P. and Reiser, H.N., t964. Geologic Map and section of the Chandalar quadrangle, Alaska. U.S. Geol. Surv. Open-File Map 1-375, scale 1 : 250,000. Brosge, W.P. and Reiser, H.N., 1965. Preliminary geologic map of the Arctic quadrangle. Alaska. U.S. Geol. Surv. Open-File Rep. OF-65-22, scale 1 : 250,000. Brosge, W.P. and Reiser, H.N., 1969. Preliminary geologic map of Coleen quadrangle, Alaska. U.S. Geol. Surv. Open-File Rep. OF-69-25, scale 1 : 250,000. Brosge, W.P. and Reiser, H.N., 1971. Preliminary bedrock geologic map, Wiseman and eastern Survey Pass quadrangles, Alaska. U.S. Geol. Surv. Open-File Rep. O-71-56, 2 sheets, scale 1 : 250,000. Brosge, W.P., Dutro Jr., J.T., Mangus. M.D. and Reiser, H.N., 1962. Paleozoic sequence in eastern Brooks Range, Alaska. Am. Assoc. Pet. Geol. Bull., 46(12): 2174-2198. Brosge, W.P., Reiser, H.N., Dutro Jr., J.T. and Detterman, R.L., 1976. Reconnaissance geologic map of the Table Mountain quadrangle, Alaska. U.S. Geol. Surv. Open-File Map 76-546, 2 sheets, scale 1 : 200,000. Brosge, W.P., Reiser, H.N., Dutro Jr., J.T. and Detterman, R.L., 1979a. Bedrock geologic map of the Philip Smith Mountains quadrangle, Alaska. U.S. Geol. Surv. Misc. Field Stud. Map MF-879B, 2 sheets, scale 1:250,000. Brosge, W.P., Reiser, H.N., Dutro Jr., J.T. and Nilsen, T.H., 1979b. Geologic map of Devonian rocks in parts of the Chandler Lake and Killik River quadrangles, Alaska. U.S. Geol. Surv. Open-File Map OF-79-1224, scale l : 200,000. Brosge, W.P., Nilsen, T.H., Moore, T.E. and Dutro Jr., J.T., in press. Geology of the Upper Devonian and Lower Mississippian(?) Kanayut Conglomerate, Brooks Range, Alaska. U.S. Geol. Surv. Prof. Pap. on the National Petroleum Reserve, Alaska. Chapman, R.M., Detterman, R.L. and Mangus, M.D., 1964. Geology of the Killik-Etivluk Rivers region, Alaska. U.S. Geol. Surv., Prof. Pap. 303-F, pp. 325-40Z Donovan, T.J. and Tailleur, I.L., 1975. Map showing paleocurrent and clast-size data from the DevonianMississippian Endicott Group, northern Alaska. U.S. Geol. Surv. Misc. Field Stud. Map MF-692, scale 1 : 7,500,000. Dutro Jr., J.T., 1952. Stratigraphy and paleontology of the Noatak and associated formations, Brooks Range, Alaska. U.S. Geol. Surv., Geol. Invest., Naval Petroleum Reserve No. 4, Alaska, Spec. Rep. 33, 154 pp. Dutro Jr., J.T., 1953a. Stratigraphy and paleontology of the Noatak and associated formations, Brooks Range, Alaska. Geol. Soc. Am. Bull., 64, p. 1415 (abstract). Dutro Jr., J.T., 1953b. Stratigraphy and paleontology of the Noatak and Associated Formations~ Brooks Range, Alaska. P h . D . Thesis, Yale University, New Haven, Conn., 154 pp. Dutro Jr.. J.T., Brosge, W.P. and Reiser, H.N., 1977. Upper Devonian depositional history, central Brooks Range Alaska. In: K.M. Blean (Editor), The United States Geological Survey in Alaska: Accomplishments during 1976. U.S. Geol. Surv. Circ. 751B, pp. B15-B18. Ellersieck, 1., Mayfield, C.F., Tailleur, I.L. and Curtis, S.M., 1979. Thrust sequences in the Misheguk Mountain quadrangle, Brooks Range, Alaska. In: K.M. Johnson and J.R. Williams (Editors), The United States Geological Survey in Alaska: Accomplishments during 1978. U.S. Geol. Surv. Circ. 804-B, pp. B9-BI0. Martin, A.J., 1970. Structure and tectonic history of the western Brooks Range, De Long Mountains and Lisburne Hills, northern Alaska. Geol. Soc. Am. Bull., 81: 3605-3622. McGowen, J.H. and Garner, L.E., 1970. Physiographic features and stratification types of coarse-grained point bars: modern and ancient examples. Sedimentology, 14: 77-. 111. Mull, C.G. and Tailleur, I.L., 1977. Sadlerochit?? Group in the Schwatka Mountains, south-central Brooks Range. U.S. Geol. Surv. Circ. 751-B, pp. 1327-1329. Mull, C.G., Tailleur, I.L., Mayfield, C.F. and Pessel, G.H., 1976. New structural and stratigraphic interpretations, central and western Brooks Range and Arctic slope. U.S. Geol. Surv. Circ. 773, pp. 24-26.

497 Nilsen, T.H., 1981. Upper Devonian and Lower Mississippian redbeds, Brooks Range, Alaska. In: A.D. Miall (Editor), Sedimentation and Tectonics in Alluvial Basins. Geol. Assoc. Can., Spec. Pap. 23: 187-219. Nilsen, T.H. and Moore, T.E., 1982a. Sedimentology and stratigraphy of the Kanayut Conglomerate, central and western Brooks Range, Alaska--Report of the 1981 field season. U.S. Geol. Surv. Open-File Rep. 82-674, 64 pp. Nilsen, T.H. and Moore, T.E., 1982b. Fluvial-facies model for the Upper Devonian and Lower Mississippian(?) Kanayut Conglomerate, Alaska. In: A.F. Embry and H.R. Balkwill (Editors), Arctic Geology and Geophysics. Can. Soc. Pet. Geol., Mem. 8, pp. 1-12. Nilsen, T.H. and Moore, T.E., in press a. Kanayut Conglomerate in the westernmost Brooks Range, Alaska. In: W. Coonrad (Editor), The United States Geological Survey in Alaska: Accomplishments during 1981. U.S. Geol. Surv. Circ. Nilsen, T.H. and Moore, T.E., in press b. Stratigraphic nomenclature for the Upper Devonian and Lower Mississippian(?) Kanayut Conglomerate, Brooks Range, Alaska. U.S. Geol. Surv. Bull. Nilsen, T.H., Moore, T.E. and Brosge, W.P., 1980a. Paleocurrent maps for the Upper Devonian and Lower Mississippian Endicott Group, Brooks Range, Alaska. U.S. Geol. Surv. Open-File Rep. 80-1066, scale 1 : 1,000,000. Nilsen, T.H., Moore, T.E., Dutro Jr., J.T., Brosge, W.P. and Orchard, D.M., 1980b. Sedimentology and stratigraphy of the Kanayut Conglomerate and associated units, central and eastern Brooks Range, Alaska--Report of the 1978 field season. U.S. Geol. Surv. Open-File Rep. 80-888, 40 pp. Nilsen, T.H., Brosge, W.P., Dutro Jr., J.T. and Moore, T.E., 1981a. Depositional model for the fluvial Upper Devonian Kanayut Conglomerate, Brooks Range, Alaska. In: N.R.D. Albert and T. Hudson (Editors), The United States Geological Survey in Alaska: Accomplishments during 1979. U.S. Geol. Surv. Circ. 823-B, pp. B20-B21. Nilsen, T.H., Moore, T.E., Brosge, W.P. and Dutro, J.T., 1981b. Sedimentology and stratigraphy of the Kanayut Conglomerate and associated units, Brooks Range, Alaska--Report of 1979 field season. U.S. Geol. Surv. Open-File Rep. 81-506, 37 pp. Nilsen, T.H., Moore, T.E., Balin, D.F. and Johnson, S.Y., 1982. Sedimentology and stratigraphy of the Kanayut Conglomerate, central Brooks Range, Alaska--Report of 1980 field season. U.S. Geol. Surv. Open-File Rep. 82-199, 81 pp. Porter, S.C., 1966. Stratigraphy and deformation of Paleozoic section at Anaktuvuk Pass, central Brooks Range, Alaska. Am. Assoc. Pet. Geol. Bull., 50(5): 952-980. Reed, B.L., 1968. Geology of the Lake Peters ar¢a, northeastern Brooks Range, Alaska. U.S. Geol. Surv. Bull. 1236, 132 pp. Roeder, D. and Mull, G.C., 1978. Tectonics of Brooks Range ophiolites, Alaska. Am. Assoc. Pet. Geol. Bull., 62: 1696-1702. Tailleur, I.L., Brosge, W.P. and Reiser, H.N., 1967. Palinspastic analysis of Devonian rocks in northwestern Alaska. In: D.H. Oswald (Editor), International Symposium on the Devonian System. Alberta Soc. Pet. Geol., 2: 1345-1361.