Sedimentation in a tectonically partitioned, nonmarine foreland basin: The Lower Cretaceous Kootenai Formation, southwestern Montana

PETER G. DECELLES* Department of Geology, Indiana University, Bloomington, Indiana 47405

ABSTRACT creased tectonism, influx of siliceous volcanic ash, and change in source lithology. The Lower Cretaceous Kootenai Formation in southwestern 4. Renewed tectonic activity in the fold-thrust belt generated a Montana was deposited in the nonmarine, Cordilieran foreland basin second episode of coarse-grained alluvial sedimentation, producing in the United States during a period of intensified uplift in the west- the Kootenai Second Sandstone Member. Basin partitioning was ac- ward adjacent, but increasingly impingent, Sevier fold-thrust belt. centuated, but drainage directions remained essentially unchanged. Concurrently, the foreland basin was partitioned by uplift of intra- Initial activation of the Blacktail-Snowcrest uplift dammed the south- foreland structural elements and incipient plutonism. Kootenai fluvial ern portion of the Second Sandstone fluvial system. Base level was and fluviolacustrine depositional systems developed and evolved in elevated, and a major anastomosed-stream system developed up- response to changing basin architecture. The Kootenai thus provides a stream from the structural dam. Downstream from the axis of the case study of nonmarine sedimentary responses to tectonic partition- uplift, a transport-efficient, probably entrenched, meandering system ing in a foreland basin. developed. The following criteria for recognition of tectonic partitioning of a 5. The remainder of Kootenai siliciclastic deposition was domi- nonmarine foreland basin are demonstrated by the Kootenai: (1) com- nated by muddy fluvial and fluviolacustrine systems. The foreland- plex, internalized drainage patterns which locally diverge from the basin configuration remained essentially unchanged as drainage over-all regional paleoslope direction and a typical longitudinal pat- became ponded in response to further elevation of base level as basin tern; (2) intraformational and interformational unconformities, both partitioning continued. An extensive carbonate lacustrine interval proximal to the fold-thrust belt and within the foreland basin; (3) punctuated this period of sedimentation. evidence for cannibalization and redistribution of foreland-basin sed- 6. Gradual southward transgression of the Cretaceous sea left a iments; (4) abrupt deflections of paleodrainage axes in response to transitional fluviolacustrine to marine imprint on the sedimentary rec- initiation of intraforeland uplifts; and (5) dramatic base-level altera- ord of the uppermost Kootenai and overlying Blackleaf Formation. tions and consequent changes of fluvial geomorphology as a result of Rejuvenation of orogenesis in the fold-thrust belt, augmented by tectonic partitioning and segmentation of the foreland basin. cratonic and major intraforeland volcanic and sedimentary lithic Kootenai sedimentation can be divided into six phases. sources, renewed the supply of coarse-grained detritus to the foreland 1. A tectonic pulse in the Sevier fold-thrust belt produced a thin basin during deposition of the Blackleaf. but widespread sheet of gravel alluvium, referred to as the "Basal Conglomerate Member," which was deposited by a shallow, perhaps INTRODUCTION AND OBJECTIVES OF THE STUDY ephemeral, gravel-bed, braided-stream system. Drainage was predom- inantly eastward, across the axis of the foreland basin. Retroarc and peripheral basins are elongated, pericratonic basins 2. Subsequently, initial activation of intra-foreland structures which develop on continental crust in response to tectonic and sedimen- over the present Tobacco Root, Madison, Gravelly, and Beartooth tary loading adjacent to active fold-thrust belts (Dickinson, 1974; Beau- Archean blocks and the Boulder batholith caused reworking of the mont, 1981; Jordan, 1981). In this sense, both can be considered as Basal Conglomerate in all but the westernmost foreland basin. Al- foreland basins (Dickinson, 1977). Because foreland basins are formed on though the topographic manifestations of these uplifts were subtle, the cratons and are genetically coupled with adjacent orogens, they are glo- foreland basin was effectively partitioned into at least four distinct bally the major loci of preservable nonmarine sedimentation (Allen and drainage basins. The resulting deposit, commonly called the "Pryor others, 1967; Van Houten, 1969; Dickinson, 1974; Miall, 1981). Member," but here referred to as the "First Sandstone Member," was Foreland-basin fills are generally considered to be wedge-shaped deposited by sand- and gravel-dominant, braided-stream systems in packages (thinning toward the craton) of detritus derived primarily from the west and east, respectively. Paleodrainage directions in the west- the adjacent fold-thrust belt (Dickinson, 1977). Miall (1978a, 1981), how- ern foreland basin were southward, but over-all drainage was ever, called attention to the possibility of intraforeland tectonism, sug- northward. gested probable sedimentary manifestations, and noted modern examples. 3. A period of mud-dominant sedimentation followed deposition Despite the abundance of nonmarine foreland-basin fills preserved in the of the First Sandstone, probably resulting from a combination of de- rock record, however, few studies have been devoted specifically to under- standing the evolution of nonmarine depositional systems in tectonically * Present address: Department of Geological Sciences, University of Rochester, broken, or partitioned, foreland basins. Rochester, New York 14627. The Cretaceous Kootenai and overlying Blackleaf Formations were

Geological Society of America Bulletin, v. 97, p. 911 -931, 18 figs., 1 table, August 1986.

911

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deposited in the western Montana portion of the Cordilleran retroarc basin and (2) to provide a set of generally applicable, diagnostic criteria for foreland basin during a period of intensified uplift in the westward adja- the recognition of ancient nonmarine sedimentary responses to intrafore- cent, but increasingly impingent, Sevier fold-thrust belt. Concurrently, land tectonic episodes. The Kootenai was examined at more than 75 nascent uplift of inherent, local, intraforeland structural elements and in- localities, and 41 stratigraphic sections were measured. More than 1,800 cipient volcanism and plutonism occurred (Suttner and others, 1981; measurements of limbs of large-scale trough cross-strata, according to Schwartz, 1983; Schwartz and others, 1983; DeCelles, 1984). Within this Method I of DeCelles and others (1983), were gathered at 130 different tectonic-plutonic framework, sedimentological study of the Blackleaf has outcrops. All paleocurrent data used in directional interpretations were been undertaken, and many of the results are in the literature (Schwartz, gathered from major-channel lithofacies. Cobble imbrications, maximum 1983; Schwartz and others, 1983). The Blackleaf consists largely of marine conglomerate-clast sizes, and longitudinal erosional furrows were mea- deposits, however, and sedimentary responses to tectonics (for example, sured at a few localities. Finally, -120 thin sections of Kootenai sand- sediment-dispersa l patterns) were modified by complex marine processes. stones were point-counted (medium-grained sandstones, 400 counts per The Kootenai, on the other hand, is entirely nonmarine, and the types, section, according to the methods of the "traditional school" of Ingersoll directions of flow, and facies distributions of the various alluvial systems and others, 1984) to determine framework mineralogy. can be interpreted in terms of tectonic driving forces. The area covered by this study includes most of southwestern and With respect to intraforeland tectonism, the Kootenai of southwest- portions of west-central Montana (Fig. 2). The Kootenai thickens west- ern Montana is of special interest because it was deposited across the zone ward to an erosional zero edge, where thrusting and uplift have removed of transition between the structural provinces of the central Rocky Moun- most Phanerozoic rocks and exposed the Proterozoic Belt Supergroup. tain foreland (to the south), which is characterized by large, Laramide Kootenai outcrops are generally confined to the flanks of major Laramide basement-cored uplifts, and the northern Rocky Mountain disturbed belt mountain ranges (Fig. 2), the intervening fault-block basins being filled by (to the north), characterized by thin-skinned folding and thrusting of Tertiary sedimentary material. Upper Cretaceous intrusive bodies; (most supra-crustal sedimentary rocks (Fig. 1). In addition, this transition zone is notably the Boulder, Tobacco Root, and Pioneer batholiths) are present the site of a major eastward salient in the Mesozoic plutonic belt of throughout the central and northern study area. western North America (Figs. 1, 2). Greenwood and others (1979), Suttner and others (1981), Schwartz (1983), and Schwartz and others STRATIGRAPHY AND AGE (1983) have suggested that Kootenai and Blackleaf sedimentation in southwestern Mo ntana was influenced by possible precursors of the Lara- The Kootenai in southwestern Montana can be subdivided into eight mide foreland blocks and plutons. If this was the case, depositional genetically distinct lithologic members (Fig. 3): the Basal Conglomerate, patterns similar to those found in this study should also occur in Lower First Sandstone, Lower Fine-Grained, Second Sandstone, Middle Fine- Cretaceous rocks to the south, where more foreland uplifts are present, but Grained, Lower Calcareous, Upper Fine-Grained, and Upper Calcareous not the north, where foreland uplifts and plutons are absent. members. In general, the entire suite of members is present in the southern The objectives of this report are twofold: (1) to document the evolu- and western study area, and the units thin eastward, where the Second tion of sedimentaiy styles in a tectonically partitioned, nonmarine foreland Sandstone and both calcareous members pinch out. Fossils (nonmarine mollusks, ostracodes, charophytes) indicate that the Upper Calcareous Member is Aptian (Yen, 1949,1951; McGookey, 1972) and the overlying Blackleaf is Albian (Gwinn, 1960; McGookey, 1972). James (1977) concluded that the Kootenai-Blackleaf contact in the study area is gradational, diachronous, and therefore, probably not located precisely at the Aptian-Albian boundary, as generally assumed. According to James (1977, p. 17), no evidence exists for a "distinct, laterally persist- ent unconformity between the Kootenai and Blackleaf' in the study area. The nature of the Morrison-Kootenai contact is obscured by poor expo- sure and has been a subject of debate for many years. Fossils from the Morrison indicate a Kimmeridgian age (Yen, 1952; Imlay, 1952), and on this basis, a regional unconformity between the Morrison and Kootenai has been proposed. Most of the Kootenai fossils, however, are from the Upper Calcareous Member, and -430 m of Kootenai strata, possibly representing Purbeckian-Neocomian time, lie between it and the top of the Morrison at some western localities. New evidence from this study and re-evaluation of previously reported evidence strongly suggests that a major unconformity, locally angular, separates the Morrison from the Kootenai. The Kootenai First Sandstone is absent at several localities in the western study area (not because of faulting). Along the eastern front of the Pioneer Mountains, the Basal Conglomerate and First Sandstone can be traced for several kilometres until they gradually pinch out to the north. The northward attenuation of stratigraphic units can be explained ai! depo- Figure 1. Map showing generalized paleotectonic framework of sitional onlap against the flank of a large hill on the Morrison unconfor- the western United States during the Early Cretaceous and the Rocky mity surface. Apparently, the underlying Morrison surface in the western Mountain foreland structural province (after Dickinson, 1976). FB, study area was dissected and had considerable relief (locally on the order forearc basin; FTB, Sevier fold-thrust belt. Black area represents of 50 m) prior to deposition of the First Sandstone. Franciscan subduction complex. A similar stratigraphic problem was reported by Cobban (1945) in

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/8/911/3445061/i0016-7606-97-8-911.pdf by guest on 28 September 2021 Figure 2. A. Map showing the study area and distribution of Cretaceous batholiths and Kootenai outcrops. Numbered mountain ranges are: 1, Flint Creek; 2, Elkhorn; 3, Bridger; 4, Tobacco Root; S, Pioneer; 6, Ruby; 7, Blacktail; 8, Snowcrest; 9, Gravelly; 10, Madison; 11, Gallatin. Outlined subareas (referred to in text) are: N, northern; C, central; E, eastern; S, southern; W, western. B. Map showing generalized structural geology of the study area, with major faults, Upper Cretaceous batholiths, and Precambrian crystalline and sedi- mentary (Belt Supergroup) rocks. HP refers to the Horse Prairie fault zone. SWT refers to southwest Montana transverse zone, the north- ernmost extent of the foreland structural province. City symbols: D, Dillon; B, Butte; Dr, Drummond; Bz, Bozeman; H, Helena. After Ruppel and others (1981) and Schmidt and O'Neill (1983).

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WEST

ALBIAN- UPPER CALCAREOUS

Figure 3. Generalized, composite strati- EAST graphic columns of the Kootenai in the west- UPPER FINE-GRAINED ern and eastern study area, ages, and strati- \ graphic nomenclature used in this report. UPPER CALCAREOUS Thicknesses are approximate.

k LOWER CALCAREOUS MIDDLE FINE-GRAINED

SECOND SANDSTONE SECOND SANDSTONE

LCWER FINE-GRAINED LOWER FINE-GRAINED

FIRST SANDSTONE FIRST SANDSTONE BASAL CONGLOMERATE KIMMERDGIAN MORRISON FM.

north-central Montana, where the base of the Kootenai is represented by TECTONIC-STRUCTURAL SETTING the Cut Bank Sandstone. The Cut Bank is present only locally because it fills paleovalleys which were formed as a result of gentle folding of the During Late Jurassic-Late Cretaceous time, the western margin of underlying Morrison beds. Not until deposition of the overlying Sunburst North America was occupied by a subduction zone and its associated Sandstone did the entire area receive a Cretaceous sedimentary cover. magmatic arc (Fig. 1; Dickinson, 1976). By the end of the Jumssic, a A 10° angular unconformity between the Morrison and Kootenai is largely nonmarine, retroarc foreland basin had developed to the eist of a present at an outcrop in the Gravelly Range (Fig. 4; Mann, 1954). At the fold-thrust belt which approximately paralleled the magmatic arc and a same outcrop, there occurs a thin (<0.25 m thick) zone of very angular complex zone of accreted terranes (Eisbacher and others, 1974; Dickinson, cobbles of gray, thinly bedded, sparry limestone. These clasts resemble 1976; Saleeby, 1983). The Kootenai and its correlatives were deposited in lithologies of the Jurassic Rierdon Formation, suggesting that local erosion the foreland basin during a major period of uplift in the margin,il fold- of pre-Morrison strata occurred during deposition of the First Sandstone. thrust belt, commonly termed the "Sevier orogeny" in the northwestern The Kootenai rests in angular unconformity upon the Triassic Thaynes, United States (Roberts and Crittenden, 1973; Dickinson, 1976). Woodside, and Dinwoody Formations in the vicinity of the Horse Prairie As summarized by Schmidt and others (1986) and shown in Figure fault (Fig. 2B), indicating a major pre-Kootenai (possibly pre-Morrison) 2, the foreland basin in southwestern Montana is now overprinted, by the uplift along the fault (Scholten, 1982). interaction and overlap of several tectonic styles, including thrusting (both basement-involved and supracrustal), foreland-block uplift, and exten- sional normal faulting. The timing of structural and plutonic evenls in the foreland basin of southwestern Montana, particularly the inception of foreland-block uplifts and the initial intrusion of the Boulder batholith and its satellites, is of critical importance in evaluating the possibility of Early Cretaceous intra-foreland tectonic activity. As summarized in Figure 5, thrusting within the foreland basin began earliest in the west, -100 m.y. B.P., and migrated eastward. Thrusts im-

TABLE I. MECHANISMS FOR ELEVATING BASE LEVEL. CAUSING FLUVIAL ANASTOMOSIS

General mechanism Specific mechanism Authors

Damming of the fluvial Cross-valley alluvial Smith and Smith, 1980 system fan progradation Smith and Putnam, 1980

Cross-channel neo- Burnett and Schumm, 1983 tectonic doming

Sea-tevel rise Cretaceous transgression Smith and Putnam, 1980 of the North American Figure 4. Morrison-Kootenai angular unconformity in the Grav- western interior elly Range (SE'A, NE%, Sec. 25, T. 10 S., R. 3 W.). The zone of tilted Subsidence Tectonic downwarping Rust, 1981 Morrison beds (arrows) can be traced for more than 80 m to the west Smith, 1986 Smith and Putnam. 1980 (right).

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pinged upon coevally and/or previously uplifted foreland blocks. The Basal Conglomerate major episode of igneous intrusion and extrusion took place between 83 and 68 m.y. B.P., but igneous activity occurred sporadically as early as Facies Assemblage. The Basal Conglomerate is characterized by a 106 m.y. B.P. dominance of massive and crudely horizontally stratified chert-clast con- Rare siliceous, epiclastic-tuff beds are present in the Kootenai fine- glomerate. The conglomerate occurs as a thin (<4 m thick) sheet, locally grained members, but direct evidence for proximal volcanism and/or with a capping zone of large-scale, trough cross-stratified sandstone, or as unroofing of plutonic sources within the foreland basin in southwestern isolated lenses of conglomerate surrounded by mudstone. The conglomer- Montana during Kootenai deposition has not been documented. On the ate bodies are clast supported, with coarse-grained sandstone matrix. Both other hand, abundant volcaniclastic material, derived from intraforeland a(t)b(i) [a-axis transverse to paleoflow, a-b plane dipping upstream] and sources, is present in the Blackleaf Formation (Schwartz, 1983). Thus, a(p)a(i) [a-axis parallel to paleoflow and dipping upstream] imbrications subsurface thermal events, as suggested by Suttner and others (1981), are present (Fig. 6A). Erosional furrows occur at the bases of conglomer- occurred as early as early Albian time, possibly during deposition of the ates which lie atop fine-grained rocks (Fig. 6B). Multiple scour surfaces upper Kootenai. and lateral discontinuity characterize the internal architecture of the Basal Conglomerate. DEPOSITIONAL SYSTEMS AND TECTONIC CONTROLS Interpretation. The Basal Conglomerate is interpreted as a proximal, flashy, gravel-bed, braided-stream deposit. This conglomerate closely re- Introduction sembles the idealized Scott Type facies model of Miall (1978b) and the

Analyses of paleocurrent and isopach data and facies associations and distributions have led to the following interpretations of the depositional systems of the Kootenai and the tectonic factors which controlled the development of these systems. The Basal Conglomerate, First and Second Sandstones, and fine-grained members are treated separately. Interpreta- tions of the calcareous members are drawn from the detailed studies of Holm and others (1977) and James (1977).

THRUSTING

60

70 BOULDER BATHOLITH 80 S SATELLITES

90 1.2.I 3 I «-12 100 6.7,8 20.21 110 THIS STUDY

120

Figure 5. Chart showing the timing of Cretaceous tectonic and igneous events in southwestern Montana. Numerals refer to the fol- lowing sources of data: (1) Ruppel and others, 1981; (2) Ruppel and Lopez, 1984; (3) Klepper and others, 1971; (4) Robinson and others, 1968; (5) Schmidt and Hendrix, 1981; (6) Ryder and Scholten, 1973; Figure 6. A. Clast imbrication [a(p)a(i)] in the Kootenai Basal (7) Nichols and others, 1985; (8) DeCelles and others, 1986; (9) Conglomerate Member at an outcrop on the north side of Trapper Schmidt and others, 1986; (10) Schmidt and Garihan, 1983; (11) Creek in the northern Pioneer Mountains (SWV4, SWW, Sec. 23, T. 2 Schmidt and O'Neill, 1983; (12) Perry and others, 1983; (13) Haley, S., R. 10 W.). Scale is 20 cm long. B. Conglomerate-filled furrows on 1983; (14) Tilling and others, 1968; (15) Zen and others, 1975; (16) the underside of an outcrop of the Basal Conglomerate, directly over- Zen, 1977; (17) Klepper and others, 1957; (18) Young, 1985; (19) lying fine-grained rocks of the Morrison Formation. Jacob staff is Daugherty and Vitaliano, 1969; (20) Wiltschko and Dorr, 1983; (21) parallel to furrow axes and 1.5 m long. Note the smaller furrows Jordan, 1981. superimposed on larger furrows.

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Donjek River Fades Assemblage G II of Rust (1978), both of which are The erosional furrows at the bases of Kootenai Basal Conglomerate representative of some proximal gravel-bed, braided-stream deposits. bodies are similar to those reported by Allen (1967) and Massari (1983). Deposits of nodern gravel-bed, braided systems are commonly sheet- Flood (1983) asserted that sedimentary furrows develop in environments like and display a high degree of lateral variability and discontinuity as a that are subject to highly episodic but directionally stable currents. The result of the relative ease and high frequency with which braided channels furrows are initiated by the action of longitudinal helicoidal flow vortices shift back and forth across the alluvial plain (Doeglas, 1962; Fahnestock, in the boundary layer above a cohesive substrate. Coarse material is swept 1963; Rust, 1972,a, 1975; Eynon and Walker, 1974; Smith, 1974). Nu- into ribbons along streamwise lines of helicoidal convergence. Continued merous studies have shown that massive and crudely horizontally strati- transport of coarse material in these ribbons abrades and scours the under- fied, imbricated gravels are deposited in longitudinal bars and in shallow lying surface, producing flow-parallel furrows. The coarse grain size and channels between and on top of bars (see Miall, 1977, for a review). chaotic nature of the material filling the Kootenai furrows suggest (hat the The Kootenai Basal Conglomerate has a marked lack of cross- depositional flows were highly competent, decreased suddenly, and were stratification, indicating that angle-of-repose slip faces did not develop. followed by rapid deposition. All of these criteria are met by fluvial According to He:'.n and Walker (1977), slip faces should develop when systems with flashy discharges. both sediment loud and fluid discharges decrease quickly after a flood. Regional Distribution and Dispersal Directions. The Base l Con- Their assumption was that with waning flow, bars should be able to glomerate is present only in the western study area. Paleocurrent data are aggrade vertically at a higher rate than laterally. If flow during flood stage, sparse, but cobble imbrications and longitudinal erosional furrows indicate however, never reaches substantial depth and then quickly subsides to eastward dispersal (Fig. 7). North-striking conglomerate bodies in the virtually no flow at all (for example, in an ephemeral system), slip faces do western study area possess channel geometry, suggesting that the present not form (Rust, 1978). Instead, only relatively thin sheets of imbricated outcrop trend is approximately normal to the direction of paleoflow. gravel have time to equilibrate with the flow. Owing to the rapidity of Tectonic Implications. Suttner (1969) suggested that the c asts of flow-stage change, falling-stage modification of these sheets (that is, verti- the Basal Conglomerate were derived from upper Paleozoic rocks ( Quad- cal aggradation arid slip-face development) does not occur. rant and Phosphoria Formations) which were uplifted in the Sevier fold- Flashy discharge is suggested by the presence of a(p)a(i) clast imbri- thrust belt to the west. This provenance interpretation has been offered for cation and the large erosional furrows at the bases of some conglomerate lithologically similar, roughly correlative rocks to the north, south, and east bodies (Figs. 6A, 6B). The pebbles and cobbles with a(p)a(i) orientations (Lammers, 1939; Stokes, 1944, 1950; Moberly, 1960; MacKenzie and could not have rolled along the bed. Rather, they were probably trans- Ryan, 1962; Rapson, 1965; Furer, 1970), and the few paleocurrent data ported in a dense dispersion above the bed until a sufficient decrease in from the Basal Conglomerate of this study support Suttner's conclusion. In flow strength occurred. Maintenance of such a coarse-grained dispersion the past, however, all of the chert-rich conglomerates and conglomeratic must have required considerable flow velocity and sediment load, such as sandstones which occur atop the Morrison Formation have been consid- might be attained during a flash flood (for example, McKee and others, ered to be time-correlative, having resulted from the same tectonic pulse in 1967). Documentation of a(p)a(i) fabric in fluvial deposits is relatively the Sevier fold-thrust belt. This scheme puts the Basal Conglomerate of sparse. Krumbein (1940, 1942) noted examples in very coarse-grained the western study area and the First Sandstone (or Pryor Member) of the flood deposits. Rust (1972b) suggested that a(p)a(i) imbrication results eastern area (Fig. 3) in the same chronostratigraphic unit and, by implica- from high-magnitude flow in fluvial systems. Harms and others (1982) tion, genetic tectonic episode. On the contrary, the two appear to be implied that this type of fabric should be expected in alluvial flash-flood stratigraphically distinct. deposits.

Figure 7. Map showing isopach and paleocurrent pat- terns of the Basal Conglomerate Member. Isopach contours are in metres. Small arrows repre- sent paleocurrent directions ob- tained from clast imbrications and gravel-furrow orientations (with numbers of measurements indicated). Large arrows show inferred over-all paleocurrent directions. Dark circles indicate section locations. Shaded, por- tions represent gaps produced by palinspastic restoration of major thrust plates. D, Dillon.

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Paleocurrent data from the First Sandstone in the area east of Dillon Interpretation. The First Sandstone of the western and central study indicate westward and southwestward dispersal (Fig. 8A). If the First area is interpreted as the deposit of a distal, sand-dominant braided fluvial Sandstone of the eastern study area is considered to be the distal equivalent system. Miall's (1978b) South Saskatchewan Type model serves as a of the Basal Conglomerate of the western study area, the resultant conver- useful generalization for comparison with the First Sandstone in the west- gent paleocurrent patterns are difficult to explain. Furthermore, chert peb- ern and central study area. bles are rare in the First Sandstone in the central study area but abundant Cant (1978) showed that deposits of the braided South Saskatche- at localities farther east. It seems unlikely that pebbles derived from a wan River are dominated by large-scale, trough cross-stratified, horizon- western source would have bypassed the central area and been deposited tally laminated, and rippled sand, with minor amounts of fine-grained farther east. material. The deposits are arranged in generally upward-fining sequences, If the Basal Conglomerate of the western study area is equivalent to with variations in the proportion of planar cross-stratified sand depending the First Sandstone in the eastern study area, paleocurrent data indicate on the relative amounts of channel versus sand-flat deposition. Sand-flat that the source of the chert pebbles in the eastern and central study area deposits are characterized by planar cross-strata and channel deposits by must have been uplifted patches of Phosphoria chert beds, presumably trough cross-strata. The relative paucity of large-scale, planar cross- atop intraforeland structural elements. Middle Jurassic erosion of the stratification in the Kootenai First Sandstone suggests that minimal vol- Phosphoria occurred in southwestern Montana, producing a local chert- umes of transverse-bar and sand-flat deposits were preserved. pebble conglomerate in the Ellis Group and erosional onlap against the The First Sandstone contains virtually no clay- or silt-sized material. Belt Arch (see inset, Fig. 2). Conceivably, Kootenai chert clasts could have Individual channel-fill sequences are stacked directly upon each other, been derived from intraforeland sources of reworked, second-cycle chert with no intervening vertical-accretion deposits. The overlying Lower Fine- pebbles in the Ellis Group, but there is no known stratigraphic evidence Grained Member is probably composed of genetically unrelated deposits (for example, a Phosphoria- or Ellis-Kootenai unconformity) for Early of a different type of depositional system. Based on modal petrographic Cretaceous erosion of chert-rich source lithologies within the foreland data (Fig. 10), fine-grained detritus, in the form of Proterozoic Belt Super- basin (Cressman and Swanson, 1964; Peterson, 1972). The source of chert group argillite rock fragments, was available during deposition of the First clasts probably was extra-basinal. Sandstone. The lack of vertical-accretion deposits probably resulted from The Basal Conglomerate of the western study area is thus not time- the erosive capability of numerous concurrently active, shifting channels. correlative with the First Sandstone. Instead, the Basal Conglomerate is No storage space existed for fine-grained material, and so mud was passed probably a remnant of a once more-widespread sheet of braided gravel through the system by continuous reworking. alluvium that was shed eastward from the active Sevier fold-thrust belt. Facies Assemblage: Southern and Eastern Study Area. The The conglomerate filled paleovalleys on the dissected Morrison surface in Kootenai First Sandstone of the southern and eastern study area forms a front of the fold-thrust belt and spread eastward across an essentially flat laterally persistent sheet of conglomerate, conglomeratic sandstone, and Morrison surface in the foreland basin. To the east, the Basal Conglomer- sandstone. The conglomerate clasts are composed of chert and quartzite. ate was later reworked and incorporated into the First Sandstone, thereby This unit is dominated by large-scale, trough cross-stratified sandstone destroying any evidence reflecting original dispersal directions and deposi- (42%); massive and trough cross-stratified chert-pebble conglomerate tional systems. (13%); and large-scale, planar cross-stratified sandstone (10%); with varia- ble but minor amounts of mudstone and horizontally laminated, rippled, First Sandstone and small-scale, cross-stratified sandstone (Fig. 11). Lateral facies conti- nuity and channel geometry are absent. Introduction. Because of textural and lithologic differences, the First Grain-size changes are typically abrupt, but crude upward-fining se- Sandstone of the western and central study area is described and inter- quences with erosional bases are present. In general, conglomerate occurs preted separately from that of the southern and eastern study area. in lenses and tabular sheets ~ 1 m thick (Fig. 12), or in lags at the bases of Facies Assemblage: Western and Central Study Area.The Koote- trough cross-stratified sandstone units. These units are overlain by upward- nai First Sandstone of the western and central study area consists of a fining (very coarse- to medium-grained sandstone), large-scale, trough prominent, laterally persistent sheet of sandstone, dominated by large- cross-stratified sandstone. Rippled, horizontally bedded, and small-scale, scale, trough cross-stratification (65%); small-scale, planar and trough trough and planar cross-stratified zones occur in the upper portions of cross-stratification (20%); and large-scale, planar cross-stratification (3%); upward-fining sequences. Large-scale, planar cross-strata occur as 0.5- to with subordinate amounts of horizontally laminated and rippled sand- 1-m-thick, isolated cross-sets (surrounded by trough cross-stratified sand- stone, and minor mudstone (Fig. 9). Sandstone bodies are laterally and stone) and in multiply stacked units up to 5 m thick. vertically discontinuous, channel geometry is absent, and internal facies Interpretation. The First Sandstone in the southern and eastern organization is minimal. study area is interpreted as the deposit of a gravel-dominant, distal, Crude upward-fining sequences of very coarse- to medium-grained braided-stream system (see Rust, 1978). Miall's (1978b) Donjek Type sandstone are present locally. Erosional bases are overlain, in ascending facies model closely resembles the typical vertical profile of the First order, by large-scale, trough cross-stratified; small-scale, trough cross- Sandstone in this area. stratified; rippled; and horizontally laminated sandstone. Generally, the Williams and Rust (1969) and Rust (1972a, 1978) noted that depos- bulk of the sequence is composed of trough cross-stratified sandstone. its in the downstream reaches of the Donjek River are characterized by an Isolated, and locally multiple, sets of planar cross-strata occur within these assemblage of trough cross-stratified gravel and sand; massive or horizon- sandstone bodies. The planar cross-sets may persist laterally for as much as tally bedded gravel; horizontally stratified sand; and horizontally lami- 50 m but are usually truncated by deposits of trough cross-stratified sand- nated, very fine-grained sand and mud. Gravel and sand dominate. stone. Large-scale cross-sets commonly are overlain by zones of small- Upward-fining sequences are produced by the filling of minor bartop and scale structures. Paleocurrent data from planar cross-sets diverge major interbar channels. In the Donjek, trough cross-stratified gravel is considerably from the average orientations of associated large-scale trough deposited in major channels, and massive to horizontally stratified gravels axes. are deposited by migrating longitudinal bars. Wedges of planar cross-

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stratified sandstons within Kootenai conglomerate bodies were probably deposited on lee sides of gravel bars as small runoff deltas and lateral Figure 8A. Map showing isopach and paleocurrent patterns of accretion deposits during waning and normal stages. the Kootenai First Sandstone Member. Isopach contours ¡ire in Regional Distribution and Dispersal Directions. Paleocurrent and metres. Small arrows indicate mean orientations of axes of large- scale, isopach data indicate the presence of several large paleochannel complexes trough cross-strata determined from 10 to 30 measurements of trough within the First Sandstone (Fig. 8A). Drainage in the central and western limbs per station. Large arrows represent inferred general paleodrain- study area was to the southwest and south, parallel to the Sevier orogenic age directions. Large filled circles depict the relative sizes of the aver- front. In the southern study area, drainage was bipolar (eastward and age maximum clast size in conglomeratic lithofacies, from 2.0 to 0.5 westward), whereiis a major north-northeastward-flowing system existed cm in diameter. Small filled circles indicate section locations. Shaded in the eastern study area. Clast sizes in the conglomeratic units in the areas are gaps produced by palinspastic restoration of major thrust eastern study area decrease systematically toward the northwest, suggest- plates. Stars in western and central study areas highlight areas in ing dispersal directions similar to those obtained from associated large- which the First Sandstone is absent. Dry-well symbol in southern scale, trough cross-strata. study area represents the American Quasar-Peet Creek-Federal well The lateral variability in thickness of the First Sandstone may reflect, no. 29-1 (data from W. J. Perry, 1983, written commun.); empty trian- to some extent, relict topography on top of the Morrison Formation. It is gle in southern study area represents data from M. C. Kremer (1983, unlikely, however, that relict topography controlled drainage patterns, written commun.). See Figure 2 for city symbols. except in areas proximal to the fold-thrust belt, where the First Sandstone is locally absent (Fig. 8A). If topography on the Morrison surface exerted drainage control oil the First Sandstone, Morrison thin zones should coin- Tectonic Implications. Little doubt exists that deposition of the cide with First Sandstone thick zones. Comparison of Morrison and Kootenai was initiated by tectonism within the marginal fold-thrust belt. Kootenai First Sandstone isopachs shows that such a coincidence does not After deposition of the Basal Conglomerate, however, the focus of tectonic exist. Although the Morrison thins toward Belt arch, Suttner (1969) activity shifted from extra-basinal to intrabasinal. Paleodrainage dire:tions showed that the attenuation resulted from depositional onlap rather than and lithofacies distributions of the First Sandstone indicate that the fore- pre-Kootenai erosion. land basin was topographically complex (Fig. 8B). Major drainage systems

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BBH

iV PRECAMBRIAN CRYSTALLINE ROCKS (1981) suggested that an isolated foreland basin existed on the Sapphire W CRETACEOUS INTRUSIVE thrust plate between the fold-thrust belt and a major topographic "high- ROCKS land" over the present site of the Boulder batholith. They inferred that this basin was filled from the north and northeast. Paleocurrent data from Figure 8B. Schematic representation of drainage patterns, stream outcrops in the Drummond area indicate that drainage was, indeed, to the morphology, and local topographic highs (shaded areas) during south in the portion of the foreland basin between the batholith and the deposition of the Kootenai First Sandstone. The positions of present thrust belt (Fig. 8A). Moreover, a concentration of major southwestward- Laramide basement-cored uplifts and Upper Cretaceous batholiths are flowing channels did exist during deposition of the First Sandstone along also shown. BBH, Boulder batholith high; TGH, Tobacco Root- the eastern flank of what is now the Boulder batholith. This suggests that Madison-Gravelly high; BTH, Beartooth high. Dashed line on the the local topographic grain was approximately parallel to the present trend western portion of the map represents the approximate eastern limit of the batholith. Unfortunately, poor exposure of the Kootenai on the of uplift in the Sevier fold-thrust belt. western flank of the batholith hinders rigorous validation of the sugges- tions of Greenwood and others (1979) and Ruppel and others (1981). Farther south, on the eastern limb of the Ruby synclinorium and in occupied compartments within the foreland basin, which were partitioned the Madison Range, paleocurrent data from the First Sandstone indicate from each other by at least four topographically higher areas. These areas that flow diverged from the area presently occupied by the Madison River are today occupied by the Boulder batholith, the Tobacco Root-Ruby valley, between the Gravelly and Madison Ranges (Fig. 8A). This valley is Archean block, the Madison-Gravelly Archean block, and the Beartooth a downdropped Archean block, parts of which are still elevated along the Archean block. eastern flank of the Gravelly Range and western flank of the Madison Greenwood and others (1979) reported that the Kootenai in the Range. Schmidt and others (1986) indicate that the Tertiary normal, vicinity of the Boulder batholith thins toward the batholith. They inferred basin-bounding fault along the western Madison Range reactivates a that heat from the embryonic batholith caused thermal expansion and thrust fault, of probably Late Cretaceous age, which brought up the Ar- uplift during deposition of the Kootenai. Citing lithologie differences be- chean block. The synorogenic Sphinx Conglomerate, which crops out atop tween the Kootenai east and west of the batholith, Ruppel and others the western Madison Range, was shed eastward off this rising Archean

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Figure 9. Pleasured stratigraphie sections of the Kootenai First Sandstone Member in the western and central study area. Grain-site key: C, conglomerate; SS, sandstone; SI, siltstone; M, claystone. Lithofacies codes: Sh, horizontally laminated sandstone; Sr, rippled sandstone; Sts and Sps, small-scale, trough and planar cross-stratified sandstone; Spl, large-scale, planar cross-stratified sandstone; Stl, large-scale, trough cross-stratified sandstone; Gtc, trough cross-stratified chert-pebble-cobble conglomerate; Gmc, massive, clast-supported, chert-pebble-cobble conglomerate; Sm, massive sandstone; Fsm, massive mudstone. Section locations are given in Appendix 1. Detailed measured sections are given in DeCelles (1984).

block during Maastrichtian time (DeCelles and others, 1986). During the Figure 10. Ternary dia- Tertiary, the sense of movement along the fault was inverted, downdrop- gram of Kootenai sandstone ping the previously uplifted Archean block. Apparently, the Madison- compositions. Q, Monocrys- Gravelly block experienced at least nascent uplift during deposition of the talline and polycrystalline First Sandstone. This hypothesis is supported by the angular unconformity quartz, and quartzose silt- between the Kootenai and Morrison in the Gravelly Range, on what stone and very fine-grained would have been the western flank of the arch. sandstone rock fragments; In the eastern study area, paleocurrent, isopach, and conglomerate- C, chert; M, argillite (weakly clast size data indicate north-northwestward dispersal away from the re- foliated, illite-rich), mud- gion of the Beartooth block. Clast sizes in conglomeratic lithofacies of the stone, and carbonate-rock First Sandstone decrease systematically toward the northwest (Fig. 8A), fragments. Note the generally implying a decrease in stream competence as a function of decreased higher proportion of quartz gradient. This gradient and the eastward-trending conglomeratic channel- in samples of the Second bodies reported by Moberly (1960) in the Pryor Member of the Cloverly Sandstone. Formation (Koolenai equivalent in south-central Montana) probably were controlled by the Early Cretaceous initiation of the Beartooth uplift. In the region of the present Tobacco Root Mountains (another FIRST SANDSTONE > foreland-block uplift), southward-flowing drainage was deflected toward SECOND SANDSTONE the southwest (Figs. 8A, 8B), probably in response to early uplift of the Tobacco Root block. The east-west grain of paleodrainage patterns at the

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northern end of the Tobacco Root Mountains reflects the influence on with minor amounts of rippled, horizontally laminated, and planar cross- paleogeography of the northern boundary of the Rocky Mountain fore- stratified sandstone. The lithofacies assemblage of the Second Sandstone land province (Figs. 1, 2). can be divided into five genetic facies: channel, crevasse-splay, levee, lacus- Although obvious thinning trends exist, it is not known if the First trine/pond, and overbank/paleosol deposits. This suite of facies is charac- Sandstone was deposited atop these topographic barriers. The First Sand- teristic of modern and ancient anastomosed- and meandering-river stone is absent at one locality in the central study area (Fig. 8A), suggesting local uplift and erosion during or just after deposition. The lack of a major regional unconformity between the Kootenai and pre-Morrison strata, however, indicates that the First Sandstone (or stratigraphically equivalent material) was deposited throughout most of the foreland basin. With the exception of the Madison-Gravelly block, the topographic highs were probably quite subtle and served merely to control major drainages, rather than supply detritus. The sedimentological differences between the First Sandstone of the western and central study area and that of the southern and eastern study area also reflect intra-foreland partitioning. The sand-dominant system of the central study area was fed from the north, west, and east, whereas the gravel-dominant system of the southern and eastern study area was fed from the south and east. Streams of the central study area were cut off from those of the eastern study area by a subtle drainage divide between the present-day Tobacco Root Mountains and Bozeman.

Second Sandstone Figure 12. Large-scale trough cross-stratified and massive chert- Introduction. The Kootenai Second Sandstone is dominated by pebble conglomerate and conglomeratic sandstone in Kootenai First large-scale, trough cross-stratified sandstone (46%); mudstone (18%); and Sandstone at Porcupine Ridge in the Madison Range (SE'/i, SE%, Sec. massive and trough cross-stratified limestone-pebble conglomerate (15%); 21, T. 7 S., R. 4 E.). Visible portion of Jacob staff is 1 m long.

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COVERED

LEGEND COVERED

RIPPLH:, HORIZONTALLY LAMINATO SANDSTONE

LARGE-SCALE TROUGH CROSS-STRATIFIED SANDSTONE MASSIVE LIMESTONE-PÖBLE CONGLOMERATE

GJJJFL TROUGH CROSS-STRATIFIED LIMESTONE-PEBBLE CONGLOMERATE | | MASSIVE MUDSTONE

Figure 13. Lithofacies map traced from a photomosaic of a portion of the Kootenai Second Sandstone channel complex exposed on the eastern limb of the Sandy Hollow anticline (SW'/i, SWV4, Sec. 26, T. 4 S., R. 8 W.). Note the dominantly vertical stacking of five discrete channel sandstone/conglomerate bodies (numbered 1 through 5).

deposits (Allen, 1965; Smith and Putnam, 1980; Smith and Smith, 1980; ples on channel floors. The conglomerate clasts were derived from cilcrete Smith, 1983, 1986; Rust and Legun, 1983). The Second Sandstone is nodules in flood-plain sediments (Fig. 15; Thompson, 1984). interpreted as havi ng been deposited by a highly aggradational, transport- The paucity of lateral-accretion foresets (epsilon cross-stratification), inefficient, anastomosed system. the well-preserved channel geometry with little evidence of lateral and Channel Deposits. The over-all geometry of the Second Sandstone is vertical scouring of adjacent channel fills (Fig. 13), and the coarse grain lenticular, with large (as much as 1 km wide by 25 m thick), first-order size suggest that the channels aggraded vertically (as opposed to laterally) sandstone bodies which pinch out laterally in fine-grained rocks. The at a relatively rapid rate. Had the rate of aggradation been low, individual first-order sandstone bodies consist internally of smaller (as much as 100 m channels would have had time to scour into adjacent and underlying wide by 4 m thick), lenticular sandstone bodies dominated by intraforma- channel deposits. A lower rate of aggradation also would have promoted tional limestone- and mudstone-pebble conglomerate and large-scale, the establishment of point bars, scroll bars, and meander loops, evidence trough cross-stratified sandstone (Fig. 13). Crude upward-fining textural for which is lacking. Instead, the high rate of aggradation, to be expected in trends occur in the second-order sandstones, with conglomerate and very a foreland basin, prevented widespread meander development and pro- coarse-grained, trough cross-stratified sandstone in the lower portions and moted channel avulsions. medium- to coarse-grained, rippled, horizontally laminated, and small- Paleochannel Parameters and Sinuosity. Channel deposits of mod- scale, planar cross-stratified sandstone in the upper portions. Individual ern anastomosed-river systems are characterized by relatively low width: second-order sandstone bodies are separated by thin zones of gleyed mud- thickness ratios, and anastomosed rivers exhibit low sinuosity (Smith, stone with limestone nodules. The single observed occurrence of epsilon 1983). Estimation of paleochannel parameters and sinuosity for the Sec- cross-stratification (Allen, 1965) in the Second Sandstone is in a con- ond Sandstone facilitates comparison with modern anastomosed systems. glomeratic channel body in the nose of the Sandy Hollow anticline (Fig. Empirical equations have been developed for the determination of ancient 14). alluvial-channel parameters and hydrologic characteristics (Ethridge and Crude upward-fining sequences, channel geometry, and erosional Schumm, 1978; Gardner, 1983), and although problems exist in applying bases indicate that the second-order sandstone bodies were deposited in most of these equations (Ethridge and Schumm, 1978), those which re- alluvial channels. The predominance of large-scale, trough cross- stratification reflects deposition by migrating three-dimensional large rip-

LEGEND

PHU MASSIVE LIMESTONE-PEBBLE CONGLOMERATE RIPPLED, HORIZONTALLY LAMINATED SANDSTONE

TROUGH CROSS-STRATIFIED LIMESTONE-PEBBLE CONGLOMERATE LARGE-SCALE PLANAR CROSS-STRATIFIED SANDSTONE

LARGE-SCALE TROUGH CROSS-STRATIFIED SANDSTONE EPSILON CROSS-STRATIFIED LIMESTONE-PEBBLE CONGLOMERATE

Figure 14. Lithofacies map traced from a photomosaic of epsilon cross-stratified, coarse-grained, point-bar deposits in the Second Sandstone expoiied near the nose of the Sandy Hollow anticline.

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sinuosity of 1.467. All three values of sinuosity fall within the range of typical channel sinuosities for modern anastomosed systems (Smith, 1983), but the lower values, because they are based on actual measure- ments of paleocurrent indicators, are more realistic. The low values of sinuosity reflect the local directional stability that is characteristic of anastomosed-river channels. Paleocurrent data from channel sandstones exposed in different portions of the anticline indicate variability of at least 109° in channel orientations within the entire complex, reflecting the presence of many interlaced and vertically overlapping channel fills. Paleochannel parameters can also be reconstructed from the epsilon foresets exposed in the anticline nose (Fig. 14). Width of individual fore- sets, on the horizontal projection, is -34 m. The generally accepted equa- tion for bankfull channel width (Wb) as a function of point-bar width (W) is Wb = 1.5(W) (Gardner, 1983), which yields a value of 50 m for original channel width. Bankfull channel depth, if assumed to be approximately represented by the vertical distance from epsilon foreset-toes to the top of the overlying trough cross-stratified channel fill, was about 7.5 m. There- fore, the width:depth ratio for this particular channel reach was -6.7. Channel sinuosity can be calculated using Schumm's (1972) equation, yielding a sinuosity of 2.1. This value is considerably higher than that obtained from more typical Second Sandstone channels and suggests that local meandering channels coexisted with straighter anastomosing chan- nels. Locally meandering reaches in modern, dominantly anastomosed systems are not uncommon (Rust, 1981; Smith, 1983). Crevasse-Splay Deposits. Thin (0.25-2 m thick) tabular bodies of horizontally laminated, rippled, and planar and trough cross-stratified sandstone occur with minor amounts of limestone-pebble conglomerate near the lower and outer margins of first-order sandstone bodies (Fig. 16A). These units have sharp bases and upward-fining trends, but locally they show an upward-coarsening, then fining trend. The typical sequence of sedimentary structures consists of horizontally laminated sandstone at the base; a single unit of large-scale, planar cross-strata in the middle; and medium- to fine-grained, rippled, horizontally laminated, and small-scale, cross-stratified sandstone at the top. Burrowed zones and paleosols also occur at the tops (Fig. 16B), and rare zones of convoluted mudstone beds are present beneath the bases of these sandstone bodies. Figure 15. A small, limestone-clast conglomerate body (resistant ledge) and in situ calcrete nodules in underlying fine-grained facies in The sandstone bodies are interpreted as crevasse-splay deposits. The the Second Sandstone exposed at SE'/o, NEW, Sec. 22, T. 4 S., R. 8 W. crevasse-splay sandstones are commonly truncated by channel deposits, Rock hammer is 33 cm long. suggesting that splay events were ultimately followed by avulsions. Levee Deposits. Units of red or tan, very fine-grained, rippled and horizontally laminated sandstone are present near the bases of first-order quire only width, thickness, and grain-size measurements can yield fairly sandstone complexes (Fig. 16C). Angles of climb in the rippled zones are reliable, first-order approximations. up to 20°. These sandstones bodies, rarely over 2 m thick and 10 m wide, At the Sandy Hollow anticline (see Fig. 14 caption for location), a are typically truncated laterally and vertically by channel-sandstone Second Sandstone channel complex is exposed in three dimensions, allow- bodies. Locally, burrowed zones are present. The units fine upward into ing detailed reconstruction of various channel parameters. The width of mudstone with limestone nodules (Fig. 16D). In some cases, the rippled the channel complex is -1.07 km, and average thickness is 21.2 m, giving sandstone bodies are wedge-shaped. a width:thickness ratio of -50. Width:thickness ratios of individual chan- This facies is interpreted as levee deposits. Modern fluvial deposits of nel bodies fall within the range of 25-30. These values are comparable to climbing-ripple and ripple cross-laminated sand are common where a those reported by Smith (1986; and 1983, written commun.) and Rust and sediment-laden, high-velocity flow is quickly decelerated. Such a situation Legun (1983) in modern and ancient anastomosed-river deposits. If the develops during flood and waning stages on natural levees and on prox- thickness and width parameters of these channel fills can be used as imal crevasse splays. Deposits of climbing-ripples in levees have been approximations of pre-avulsion channel sizes, typical channels were at documented by Fisk (1961), Ray (1976), Allen (1965), and Smith (1983). least 4 m deep and 100-120 m wide at bankfull stage. Lacustrine Deposits. Thin (< 1 m thick and 30 m wide) units of Paleocurrent data from seven closely spaced (vertically and laterally) massive, locally burrowed, calcareous siltstone, very fine-grained sand- channel fills on the eastern limb of the anticline indicate a range of 33° in stone, or silty limestone are isolated within the Second Sandstone and paleocurrent direction. Miall's (1976) equation for channel sinuosity yields adjacent fine-grained deposits (Fig. 16E). These units commonly weather a value of 1.017. Measurements from four channel fills farther west in to a yellow-brown color. Physical sedimentary structures are absent. They Ziegler Gulch indicate a sinuosity of 1.013. Alternatively, Schumm's are surrounded by massive red mudstone or truncated locally by overlying (1972) equation can be utilized, with a width:depth ratio of 25, yielding a or adjacent channel-sandstone bodies. Petrographically, these units show

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/8/911/3445061/i0016-7606-97-8-911.pdf by guest on 28 September 2021 Figure 16. A. An upward-coarsening, then fining (to the right), crevasse-splay sandstone body embedded in fine-grained facies of the Second Sandstone Member. B. Burrowed (sandstone-filled cylindrical tubes) upper surface of a fine-grained, crevasse-splay sandstone body. C. Lower portion of a channel sandstone (above scale) lying in erosional contact upon rippled, fine-grained, levee deposits in the Sec- ond sandstone. D. Photomicrograph of a limestone nodule, showing sparry calcite cement, voids (dark areas), and micritic, compound glae- bules (gray areas). E. Massive, calcareous, lenticular, very fine-grained sandstone body surrounded by mudstone in Second Sandstone exposed at SE1/«, NE'A, Sec. 22, T. 5 N., R. 1 W. Cliff face is -12 m high. Scale in A, B, and C is 20 cm long; hammer in E is 33 cm long.

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no textural trends or pedogenic structures. Calcite cement, silt- to very fine ganic material is accumulating in the anastomosed Rio Magdalena basin. sand-sized quartz grains, and rare floating siderite rhombs are the sole In dynamic systems with rapid rates of subsidence and sedimentation, as in constituents. the Rio Magdalena, organic deposits do not have time to build up. The Because of their small size, lateral impersistence, fine grain size, and lack of evidence for vegetation in the Kootenai Second Sandstone is close association with overbank mudstones and paleosols, the calcareous therefore not considered a significant argument against its interpretation as siltstone lenses are interpreted as lake and pond deposits. The massive the deposit of an anastomosed system. character of these units probably resulted from pervasive reworking by Regional Distribution and Dispersal Directions. General paleocur- organisms. The presence of siderite suggests locally reducing environments rent and isopach patterns of the Second Sandstone are similar to those of (Blatt and others, 1980, p. 602) on poorly drained flood plains. the First Sandstone (Fig. 17A). Areas of thinning and nondeposition, Smith (1983) noted the presence of abundant, large, shallow (1-3 m especially in the southern, eastern, and south-central study area, are more deep) lakes on the flood plain of the anastomosed Saskatchewan River. prominent, however. The north-south axis of deposition in the central Shallow lakes, marshes, and swamps cover 80% to 90% of the anasto- study area apparently shifted eastward by about 15-20 km, and the major mosed Rio Magdalena basin in Colombia, South America (Smith, 1986). sandy channel system in the Bozeman-Livingston area no longer existed. Wetlands generally occupy 70% to 80% of typical anastomosed-river flood Tectonic Implications. The framework-grain composition of the plains, although arid climates greatly reduce these figures (Rust, 1981). Second Sandstone indicates no significant change in the primary Wetland sedimentary facies consist of laminated, organic-rich mud, and extrabasinal sources of detritus as compared to the First Sandstone pervasive bioturbation is common (Smith, 1983,1986). (Fig. 10). The increase of monocrystalline and polycrystalline quartz and Lakes of anastomosed-river basins occupy the lowest portions of the quartzose sandstone and siltstone grains in the Second Sandstone may flood plains, and hence, are favorable sites for eventual avulsions (Smith, reflect further downcutting through the Phosphoria and into the underly- 1986). The presence of partially scoured lacustrine deposits between ing Quadrant Formation in the fold-thrust belt. Significant quantities of Kootenai channel sandstones therefore indicates that, indeed, these lakes locally derived, intraformational limestone clasts (sand- to gravel-sized) probably did occupy paths of eventual avulsion on the flood plain. are also present. The composition of the Second Sandstone therefore re- Overbank and Paleosol Deposits. The interbedded, fine-grained flects mixed extrabasinal and intrabasinal source terranes. deposits of the Second Sandstone are dominated by red (10R 6/6 to 10R Thompson (1984) documented weak regional trends in clast sizes of 2/2) and green (10G 6/2 to 10G 4/2) siltstone and claystone. These the Second Sandstone intraformational limestone-pebble conglomerates. deposits commonly occur between individual channel bodies or groups of The largest clasts are concentrated in channel deposits at the northern end channel bodies. This facies is characterized by abundant limestone nod- of the Tobacco Root Mountains and in the Pioneer Mountains, suggesting ules, gleying, and massively burrowed and/or rooted zones (Fig. 16F). proximity to intraforeland uplifts. Paleocurrent and isopach data indicate In thin sections, Kootenai fine-grained rocks exhibit a chaotic mix- that these areas were occupied by major channel systems. It should be ture of sesquioxidic matrix; floating silt- and sand-sized grains of quartz, emphasized, however, that because the limestone clasts are reworked cal- chert, and argillite; bits of dark woody material; and floating paleopeds. crete nodules, their sizes are probably a function of local stream compe- Limestone nodules are characterized by zones of sparry calcite-filled craze tence and nodule size, rather than proximity to source areas. Nevertheless, planes, veining, and displacive calcite cement (Fig. 16D). over-all paleocurrent and isopach patterns suggest that intraforeland parti- The characteristic microfabrics, gleying, and abundance of calcrete tioning elements persisted during deposition of the Second Sandstone nodules in Kootenai fine-grained rocks indicate that pedogenic processes (Fig. 17). modified these deposits (Brewer, 1964; Wright, 1982). The bulk of this The absence of the Second Sandstone in the southwesternmost por- material was deposited from suspension in overbank areas and was subse- tion of the study area seems enigmatic. The Second Sandstone in the quently altered by pedogenesis. Dillon area is a prominent and laterally persistent fluvial sandstone body, In many modern anastomosed systems, bank stability is maintained and paleocurrent data indicate that flow was southward into the area by abundant vegetation, and organic material forms a significant propor- under consideration. The Second Sandstone should therefore be promi- tion of the over-all facies assemblage (Smith, 1976, 1983; Smith and nent in the southern study area. This apparent enigma can be explained as Smith, 1980). Mesozoic fluvial deposits of the western Canadian interior, a result of the interplay between tectonic and resultant sedimentological most notably the Mannville Subgroup (Putnam, 1982), the Brazeau- manifestations. Paskapoo Formation (McLean and Jerzykiewicz, 1978), and the Koote- Anastomosed systems require high rates of aggradation and low gra- nay Formation (Jansa, 1972), contain abundant coal and have been dients (Smith, 1983). Low gradient is caused by elevation of base level, suggested as ancient counterparts of modern anastomosed systems by which in turn forces the system to deposit sediment in an unsuccessful Smith and Putnam (1980). Conversely, the Kootenai in southwestern attempt to achieve grade. If the system achieves grade, it becomes trans- Montana contains scant organic material. Although some of the massively port-efficient, the rate of aggradation decreases, and a stable, meander- bioturbated zones in Kootenai mudstones may be the result of rooting, ing system is established (Smith, 1986). Because meandering systems are unequivocal evidence for plant colonization (such as rhizocretions) is lack- transport-efficient and have low aggradation rates, they should leave rela- ing. Work in Australia by Rust (1981) has indicated that anastomosed tively minor deposits in the rock record (N. D. Smith, 1984, personal systems can develop in areas with arid climates and sparse vegetation. commun.). The Second Sandstone fluvial system in the southernmost Bank stability is maintained by the natural cohesiveness of fine-grained study area may have been a meandering (perhaps entrenched), transport- overbank material and duricrust formation. Mudstone and sandstone rip- efficient system that left volumetrically minor deposits. The upstream tran- up clasts are present in conglomeratic channel fills of the Second Sand- sition from meandering to anastomosing morphology occurred in the area stone, suggesting early cementation. Possibly, rapid calcite cementation south of Dillon. resulted from supersaturation of pore waters by evaporation, a process Mechanisms proposed in the literature for the elevation of base level called upon by Goudie (1973) for the origin of some calcrete deposits. and consequent development of anastomosis in fluvial systems can be D. G. Smith (1983, written commun.) reported that very little or- classified into three general types (Table 1): (1) damming of the system,

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Figure 17A. Map showing isopach and paleocurrent patterns of the Kootenai Second Sandstone Member. Isopach contours in metres. See Figure 8 for explsination of arrows, shaded areas, and city symbols.

causing an upstream decrease in stream energy and increase in aggrada- western system and may ultimately have owed its anastomosed chi.racter tion; (2) rapid basin subsidence; and (3) sea-level rise. The Second Sand- to the damming effect of the Blacktail-Snowcrest uplift. Paleocurrent data stone fluvial system was essentially independent of sea level because the and lithofacies distributions indicate that the axis of sedimentation in the Cretaceous western interior sea was located more than 1,000 km to the central study area consisted of a complex of multiple, interwoven channels. north (McGookey, 1972). Most likely, a combination of damming and The high degree of paleocurrent-direction variability is probably a reflec- increased basin subsidence in the Dillon area caused aggradation and tion of the anastomosed morphology of the channel system, rather than anastomosis in the western study area and meandering in the southernmost individual highly sinuous channels. study area. As implied by Perry and others (1983), the Blacktail- Snowcrest uplift (¿. basement-cored Laramide block) probably was tecton- Fine-Grained Members ically active during deposition of the Kootenai. Consequently, a mildly positive topographic feature over the southwestern end of the uplift Introduction. The Kootenai fine-grained members, which attain dammed the soulhward-flowing Second Sandstone fluvial system and thicknesses in excess of 100 m, represent significant intervals of sedimenta- caused upstream anastomosis and downstream meandering (Fig. 17B). tion in the foreland basin. Unfortunately, interpretation of these rocks is Burnett and Schumm (1983) reported that neotectonic domal uplifts in the hindered by poor exposure. At a few localities, however, trenches were southern United States have caused river systems to aggrade and anasto- dug and locally good, natural exposures allow some general conclusions to mose above the axes of uplifts and to incise and meander below the axes. be drawn. In the central study area, the southward-flowing drainage system was Description. The fine-grained members consist of siltstone and clay- deflected westward around the subtly enlarging positive area over the stone. These members are characterized by red and green colors, abundant rising Tobacco Root block (Fig. 17B). This fluvial system probably fed the limestone nodules, gleying, massive bedding, zones of bioturbatioa, and

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kilometer* 40

PRECAMIRIAN CRYSTALLINE ROCKS

CRETACEOUS INTRUSIVE ROCKS

Figure 17B. Schematic representation of drainage patterns, stream morphology, and local topographic highs (shaded) during deposition of the Kootenai Second Sandstone. The present positions of Laramide basement-cored uplifts and Upper Cretaceous batholiths are also shown. BBH, TGH, and BTH represent, respectively, the Boulder Batholith, Tobacco Root-Madison-Gravelly, and Beartooth Highs. BSU indicates the position of the Blacktail-Snowcrest uplift. See Figure 2 for city symbols.

pedogenic microfabrics (Fig. 16E). A bed of coarse, angular, volcanic tuff woven channels with intervening bars made up of silt and very fine sand is present in the upper Kootenai south of Livingston, (SW'/4, NE'/4, Sec. 22, (Fahnestock, 1969). Channels are shallow (<2 m deep) and laterally T. 3 S., R. 9 E.; Roberts, 1972); yellow, purple, and green beds of silicic, unstable. Rust (1978) reported that, despite the fine grain size of Slims epiclastic volcanic material occur at several other localities. Small (-100 River sediments, most of the material is deposited by tractive currents. The m wide, 3 m thick) channel bodies of medium- to coarse-grained, trough deposits are commonly thixotropic and prone to penecontemporaneous cross-stratified sandstone and limestone-pebble conglomerate are isolated deformation, which destroys most original sedimentary structures. within the fine-grained intervals. These channel deposits generally com- The pedogenic characteristics of Kootenai fine-grained members in- prise a single crude, upward-fining textural sequence with hierarchically dicate that a major proportion of the material was deposited in overbank arranged sedimentary structures, but epsilon cross-stratification and lateral areas. Stacked zones and beds of coalesced calcrete nodules, separated persistence are lacking. from each other by thin (0.75 m thick) zones of mudstone, indicate that Interpretation. Several explanations for the origin of the Kootenai these areas were sites of soil formation for extended periods of time [tens of fine-grained members are present in the literature. The most popular has thousands to hundreds of thousands of years (Goudie, 1973; Allen, 1974)]. been that tectonic quiescence in the fold-thrust belt shut off the supply of The coexistence of red mudstone intraclasts with reworked calcrete nod- coarse-grained detritus and decreased the gradient, which, in turn, de- ules in Kootenai channel sandstones indicates that the red coloration of the creased stream competence (Suttner, 1969; Walker, 1974; James, 1977; fine-grained members developed on a time scale commensurate with that Suttner and others, 1981). The implication is that most of the material was of calcrete development. Therefore, the red colors probably resulted from deposited on the flood plains of a low-energy, probably meandering, flu- subaerial exposure and oxidation of iron-rich compounds in overbank vial system. Abundant pedogenic characteristics indicate that much of the areas. Portions of the flood plain that were subject to periodic or relatively fine-grained material consists of paleosols. The rarity of coarse-grained permanent saturation and/or organic accumulation (for example, lakes, channel deposits, however, seems anomalous in view of the abundance of swamps, channels) developed locally reducing environments and the resul- supposed overbank deposits. Moreover, the lack of evidence for lateral tant colors are varieties of gray, green, and blue. accretion and stability in the few channel sandstone bodies suggests that Tectonic Implications. Regardless of the type of depositional sys- the system did not have stable, laterally migrating, meandering channels. tem^) that deposited the Kootenai fine-grained rocks, the source of such a The system may have been mud-dominant. The Slims River, Yukon, plethora of mud, to the near exclusion of sand and gravel, remains a is a modern mud-dominant system, consisting of many intricately inter- problem. Peterson (1966) suggested that the abundance of fine-grained

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material in the Kootenai and Morrison resulted from influx of airborne (particularly in the area of the Helena embayment), whereas Paleozoic volcanic ash. Although smectitic bentonites have been reported in the chert and quartz fragments and the supply of Ca++ were derived from the Morrison througho ut the foreland basin, their presence in the Kootenai has fold-thrust belt. Petrographic data of Schwartz (1983) indicate that intra- not been detected in southwestern Montana (Suttner, 1968; this study). foreland sources of lower Paleozoic detritus were exposed during deposi- Fine-grained, siliceous tuffs, such as those mentioned above, may have tion of the Blackleaf Formation, probably only a few million years after been reworked by liluvial and pedogenic processes on the flood plain to the deposition of the upper Kootenai. The tuff bed exposed south of Living- extent that their volcanic signatures (such as devitrified glass textures and ston implies the existence of nearby volcanic vents, perhaps associated pumice fragments) were destroyed. with early uplift of the Beartooth block or proto-Yellowstone volca nism. The only extrabasinal, sufficient source of fine-grained material The presence of the lacustrine intervals indicates that paleodrainage re- would have been argillites of the Proterozoic Belt Supergroup. Sand-sized, mained internal, similar to the paleodrainage patterns of the First and weakly foliated, illite-rich argillite grains, derived from the Belt, are present Second Sandstones. Therefore, no major changes in the internal architec- in Kootenai sandstone framework populations (Fig. 10; James, 1977). The ture of the foreland basin occurred during the remainder of Kootenai abundance of illite in Kootenai mudstones (Suttner, 1968) suggests that deposition. the breakdown of similar rock fragments may indeed have supplied a substantial proportion of the fine-grained detritus. Accordingly, Belt rocks SUMMARY AND CONCLUSIONS must have been exposed in the fold-thrust belt and/or, as proposed by James (1977), on the crest of Belt Arch. Suttner (1969) attributed an The sedimentological evolution of the Kootenai Formation in re- upsection increase in quartz grains and decrease in chert grains (corrobo- sponse to intrabasinal and extrabasinal tectonic influences can be summar- rated by data from this study) to progressive downcutting through the ized in six phases (Fig. 18). upper Paleozoic Pliosphoria and underlying Quadrant Formations in the 1. A tectonic pulse, probably associated with the Paris thrust system, fold-thrust belt. In turn, according to Suttner, Mississippian carbonates occurred -115 m.y. B.P. in the Sevier fold-thrust belt of western Idaho. ++ were eroded, supplying abundant Ca for the development of the Upper Paleozoic chert and quartzite beds were uplifted, and a thin sheet of Kootenai lacustrine limestones. coarse alluvial gravel, the Kootenai Basal Conglomerate, was spread The mixture of upper Paleozoic chert fragments and Belt argillite across the southwestern Montana foreland basin by a complex of shallow, fragments in Kootenai sandstones indicates that the provenance of the perhaps ephemeral, gravel-bed, braided streams. In the proximal realm, Kootenai may have been more complex. If the fine-grained material was gravel filled erosional valleys and gulleys on the Morrison unconformity derived from Belt xocks, its predominance in the upper Kootenai (Fig. 3) surface. suggests that Paleozoic carbonates had been largely stripped from the 2. Shortly after deposition of the Basal Conglomerate, drainage pat- fold-thrust belt and that the supply Ca++ for the Upper Calcareous terns in the foreland basin were altered by the influence of nascent Member was instead Belt carbonates in the fold-thrust belt. Alternatively, Laramide block uplifts; in particular, the Madison-Gravelly, Tobacco the Belt argillite clasts may have been derived from intraforeland uplifts Root, and Beartooth blocks. A thermal welt over the eventual site of the

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Boulder batholith also may have influenced drainage patterns. Although tions of drainage patterns (Ouchi, 1985). Abrupt deflection of drainage the topographic manifestations of these embryonic uplifts were quite sub- axes in foreland basins strongly suggests the presence of rising subsurface tle, the foreland basin was effectively partitioned into at least four distinct structures. Swamps and anastomosed-river systems indicate base-level ele- drainage basins. The Basal Conglomerate throughout all but the western- vation, possibly owing to local basin subsidence and/or structural dam- most study area was redeposited in the form of the Kootenai First Sand- ming of fluvial systems. The development of major lacustrine intervals stone. Beds of the Morrison Formation were tilted on the flank of the rising may reflect increased subsidence within tectonically partitioned segments Madison-Gravelly block just prior to deposition of the First Sandstone. of the foreland basin, whereas the systematic translation of fluvial axes Continued, but somewhat subdued, uplift in the thrust belt maintained a indicates the presence of a rising adjacent structure. steady supply of sand-sized chert, quartzite, quartz, and argillite detritus to Tectonic partitioning of a nonmarine foreland basin may produce the foreland basin. Both gravel- and sand-dominant, distal braided systems dramatic base-level changes. Changes in base level cause changes in stream existed coevally in partitioned segments of the foreland basin. type and competence, both of which produce significant effects on assem- 3. A period of fine-grained sedimentation followed deposition of the blages and paleogeographic distributions of lithofacies. For example, the First Sandstone, perhaps as a function of decreased supply of detritus from Kootenai fluvial system changed from braided to anastomosed in response the fold-thrust belt, or a change in source lithology, augmented by the to downstream structural uplift and consequent elevation of base level. In influx of siliceous volcanic ash. In either case, most of the material was the Kootenai, the transition through time from braided-fluvial to deposited by a muddy fluvial system. anastomosed-fluvial to lacustrine depositional environments is an indicator 4. Renewed activity in the fold-thrust belt generated another episode of the increasing influence of partitioning tectonic elements on base level. of sandy alluvial deposition in the foreland basin, producing the Second Sandstone. The western and eastern study areas were further partitioned ACKNOWLEDGMENTS from each other by continued uplift of the Madison-Gravelly and Tobacco Root blocks. Detritus was supplied from the north and west. Initial activa- I gratefully acknowledge the support, criticism, and encouragement tion of the Blacktail-Snowcrest uplift dammed the southern end of the of L. J. Suttner and R. K. Schwartz throughout the execution of this study. Second Sandstone fluvial system. Base level was elevated upstream from I was ably assisted in the field for two summers by D. E. Wuellner. Useful the axis of the structural dam, and a major anastomosed-river system information and ideas were given by W. C. James, D. G. Smith, C. J. developed in the western and central study area. Downstream from the Schmidt, C. A. Wallace, P. E. Myers, N. D. Smith, E. T. Ruppel, W. J. dam, a more transport-efficient system developed, and deposition of the Perry, C. J. Vitaliano, G. S. Fraser, M. C. Kremer, and E. Zen. Financial Second Sandstone was relatively minimal. support was provided by Chevron, U.S.A.; Indiana University Department 5. Influx of coarse detritus to the foreland basin was again shut off, of Geology and Geologic Field Station; the Geological Society of America; and mud-dominant fluvial and fluviolacustrine systems replaced the sandy and Sigma Xi. R. V. Ingersoll, J. R. Steidtman, S. A. Graham, and D. W. conglomeratic, anastomosed-fluvial system. Drainage patterns probably Anderson reviewed earlier versions of the manuscript and provided helpful remained internal because the remainder of Kootenai deposition was suggestions. punctuated by two intervals of widespread lacustrine deposition. 6. Gradual southward transgression of the Cretaceous interior sea left a transitional, fluviolacustrine to marine imprint on the sedimentary record APPENDIX 1. SECTION LOCATIONS of the uppermost Kootenai and lowermost Blackleaf. Superimposed upon marine transgression was the rejuvenation of major extrabasinal and in- BRC—Bridger Canyon: SWW of NW1/., NEW, Sec. 34, T. 1 S., R. 6 E.; on north trabasinal tectonic elements, which, together with cratonic sources, re- side of Bridger Canyon road; section overturned. newed the supply of coarse-grained detritus to the foreland basin CGI—Cottonwood Gulch: NWW of SEW, SEW, Sec. 7, T. 2 N., R. 3 E.; on north (Schwartz, 1983). limb of anticline, near nose of fold. CG2—Cottonwood Gulch: SWW of SEW, SEW, Sec. 7, T. 2 N„ R. 3 E.; in nose of The following summary of sedimentological criteria for recognition anticline, -170 m south of CG 1. of the imprint of intraforeland tectonism on nonmarine sediments may CS—Cramp Spring: SWW of NWW, NW1/«, Sec. 15, T. 3 N., R. 2 E.; on ridge just prove useful in future studies. south of gulley; gastropod limestone forms prominent ridgejus t east of section. Nonmarine foreland basins are typically occupied by longitudinal EU1—Eustis: SEW, SEW, Sec. 29, T. 3 N., R. 2 E.; on north side of deep stream drainage systems that parallel the marginal orogenic front. Drainage direc- valley; section overturned. EU2—Eustis: NWW of SEW, SEW, Sec. 29, T. 3 N., R. 2 E.; -500 m north of EU1; tions in tectonically partitioned foreland basins, however, may show local section overturned. divergence from the over-all paleoslope direction. For example, the drain- GR1— Gravelly Range: SWW of NEW, NEW, Sec. 36, T. 10 S., R. 3 W.; on age direction during much of Kootenai deposition in southwestern Mon- prominent cliff face ~ 1.0 km south of Cottonwood Creek Road. tana was southward, within partitioned segments of the foreland basin, but GR2—Gravelly Range: SEW of NEW, NEW, Sec. 25, T. 10 S., R. 3 W.; on the over-all paleoslope direction was toward the north. prominent cliff -850 m north of Cottonwood Creek Road. H10 1—Highway 10: NEW, NWW, Sec. 25, T. 1 N, R. 2 W.; on low, east- Local intraformational and interformational unconformities are pro- west-trending ridge, -300 m south of U.S. Highway 10. duced in areas proximal to the fold-thrust belt (Miall, 1977) and within the H10 3—Highway 10: NWW of NEW, SWW, Sec. 25, T. 1 N., R. 2 W.; on promi- foreland basin-proper. nent east-west-trending ridge, -600 m east of Jefferson River. JR—Jefferson River: SEW of NWW, NWW, Sec. 26, T. 1 N., R. 2 W.; along old During initial activation of intraforeland uplifts, the uplifts may not Chicago-Pacific railroad bed. serve as sources of detritus. The primary source of sediment remains the LH1—London Hills: SEW of NWW, NWW, Sec. 28, T. 1 N., R. 2 W.; on large adjacent fold-thrust belt. Contradictions between paleocurrent and com- flatiron, -300 m south of Jefferson River. positional data indicate that redistribution of foreland-basin sediments LH2—London Hills: NEW of NEW, NWW, Sec. 29, T. 1 N„ R. 2 W.; just west of must have occurred, probably in response to initiation of intraforeland deep gully -500 m southwest of old Chicago-Pacific railroad bed. LM—Lone Mountain: SWW, NEW, Sec. 29, T. 6 S., R. 3 E.; on north side of new uplifts. Middle Fork valley road. Rivers are sensitive and accurate indicators of topographic configura- MC—Milligan Canyon: NWW, SWW, Sec. 24, T. 2 N., R. 1 W.; on hillside, -300 tion. Even subtle topographic changes are quickly manifested by altera- m east of Milligan Canyon road.

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NH—Negro Hollow: SWK, NWK, Sec. 3, T. 2 N., R. 2 W.; along north side of Goudie, A. S., 1973, Duricrusts in tropical and subtropical landscapes: Oxford, England, Clarendon Press, 174 p. Greenwood, W. R., Wallace, C. A., and Sel ve/s tone, J. E, 1979, The structural environment and controls of Cretaceous creek bed, -200 m north of Negro Hollow road. volcanism and plutonism in the Boulder batholith region, Montana: Geological Society of America Abstracts with NM1—Nada Mine: SEK, NW 14, Sec. 9, T. 4 N., R. 1 W.; -50 m north of gravel Programs, v. 11, p. 435. Gwinn, V. E., 1960, Cretaceous and Tertiary stratigraphy and structural geology of the Drummond area, centn l-westem road. Montana [Ph.D. thesis]: Princeton, New Jersey, Princeton University. NM2—Nada Mine: SEK, NWK, Sec. 9, T. 4 N., R. 1 W.; -150 m north of NM1. Haley, J. C., 1983, The sedimentology of a synorogenic deposit: The Beaverhead Formation of Montana a id Idaho: PC—Pole Canyon: NWK of NW K, NWK, Sec. 17, T. 1 S., R. 3 W.; on low, Geological Society of America Abstracts with Programs, v. 15, p. 589. Harms, J. C., Southard, J. B., and Walker, R. G., 1982, Structures and sequences in clastic rocks: Society of 1-xonomic southeast-trending spur and adjacent saddle. Paleontologists and Mineralogists Short Course 9, Calgary, 249 p. PR—Porcupine Ridge: SEK, SEK, Sec. 21, T. 7 S., R. 4 E.; on west-facing ridge, Hein, F. J., and Walker, R. G., 1977, Bar evolution and development of stratification in the gravelly braidec , Kicking Horse River, B.C.: Canadian Journal of Earth Sciences, v. 14, p. 562-570. -2.0 km north of Porcupine Ranch on east side of Gallatin River. Holm, M. R., James, W. C., and Suttner, L. J., 1977, Comparison of the Peterson and Draney limestones, Idaho and RR1—Ruby River: SEK, NEK, Sec. 18, T. 9 S., R. 3 W.; on north side of Ruby Wyoming, and the calcareous members of the Kootenai Formation, western Montana: Wyoming Geological River gap road; section overturned. Association, 29th Annual Conference, Guidebook, p. 259-270. Imlay, R. W., 1952, Correlation of the Jurassic formations of North America, exclusive of Canada: Geological Society of RR2—Ruby River: SEK, NEK, Sec. 18, T. 9 S., R. 3 W.; -300 m north of RR1; America Bulletin, v. 63, p. 953-992. section overturned. Ingersoll, R. V., Bullard, T. F., Ford, R. F„ Grimm, J. P., Pickle, J. D., and Sares, S. W., 1984, The effect of grain size on SB—South Boulder: NEK of NEK, NWK, Sec. 10, T. 1 S., R. 3 W.; on low detrital modes: A test of the Gazzi-Dickinson point-counting method: Journal of Sedimentary Petrokgy, v. 54, p. 103-116. east-west-trending ridge -200 m west of South Boulder River. James, W. C., 1977, Origin of nonmarine-marine transitional strata at the top of the Kootenai Formation, soi thwestem SMI—Silverstar Mine: SWK of NEK, SWK, Sec. 16, T. 4 N., R. 1 W.; -50 m Montana [Ph.D. thesis]: Bloomington, Indiana, Indiana University, 433 p. Jansa, L., 1972, Depositional history of the coal-bearing, Upper Jurassic-Lower Cretaceous Kootenay Formation, north of gravel road. southern Rocky Mountains, Canada: Geological Society of America Bulletin, v. 83, p. 3199-3222. SM2—Silverstar Mine: SWK, NWK, Sec. 21, T. 4 N., R. 1 W.; on low north- Jordan, T. E., 1981, Thrust loads and foreland basin evolution, Cretaceous, western United States: American Association south-trending ridge, -1.2 km south of gravel road. of Petroleum Geologists Bulletin, v. 65, p. 2506-2520. Klepper, M. R., Weeks, R. A., and Ruppel, E. T., 1957, Geology of the southern Elkhorn Mountains, Jeflerson and SN—Sappington North: NEK of NEK, NWK, Sec. 29, T. 1 N., R. 1 W.; on Broadwater Counties, Montana: U.S. Geological Survey Professional Paper 292, 82 p. east-west-trending ridge, -500 m south of U.S. Highway 10. Klepper, M. R,, Robinson, G. D., and Smedes, H. W., 1971, On the nature of the Boulder batholith of Montana: Geological Society of America Bulletin, v. 82, p. 1563-1580. TF—Taylor's Fork: SWK of NEK, SEK, Sec. 8, T. 9 N., R. 4 E.; along north side of Krumbein, W. C., 1940, Flood gravel of the San Gabriel Canyon: Geological Society of America Bulle in, v. 51, Taylor's Fork of the Gallatin River. p. 636-676. 1942, Flood deposits of Arroyo Seco, Los Angeles County, California: Geological Society of Americi Bulletin, v. 53, p. 1355-1402. Lammers, E.C.H., 1939, The origin and correlation of the Cloverly conglomerate: Journal of Geology, v. 47, p. 113-132. REFERENCES CITED MacKenzie, R. T„ and Ryan, J. 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