10: Transect across the northeastern margin of the Mesoproterozoic Belt Basin and Lewis thrust, Glacier National Park, Montana
Published:September 07, 2016
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James W. Sears, John S. MacLean, Stuart D. Parker, 2016. "Transect across the northeastern margin of the Mesoproterozoic Belt Basin and Lewis thrust, Glacier National Park, Montana", Unfolding the Geology of the West, Stephen M. Keller, Matthew L. Morgan
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The Lewis thrust plate in Glacier National Park contains the displaced margin of the Mesoproterozoic Belt Basin, which was transported some 130 km (~80 mi) northeastward during the Late Cretaceous to late Paleocene Laramide orogeny. This two-day field trip examines the Lewis plate on an eastward transect along the Going-to-the-Sun Road, crossing the Continental Divide at Logan Pass. The trip begins in basinal facies of the Belt Supergroup, and crosses eastward into shelf facies. The shelf-to-slope transition coincides with the axis of the Akamina syncline along the Continental Divide. The trip visits classic exposures of the Lewis thrust along the Rocky Mountain front, where the lower Belt Supergroup overlies a duplex zone in the Late Cretaceous marine section.
The purpose of the field trip is to explore the tectonic evolution of the northeastern margin of the Mesoproterozoic Belt Basin on a transect across Glacier National Park (GNP), Montana (Fig. 1). We will stop at numerous classic outcrops of the spectacularly preserved and exposed supergroup. We will address the structural configuration of the Lewis thrust plate and consider the original palinspastic position of the northeastern margin of the Belt Basin. This field guide incorporates new findings on the Belt Basin reported in GSA Special Paper 522 (MacLean and Sears, 2016). Previous geologic field guides cover many classic sites along the spectacular Going-to-the-Sun Road. The reader is referred to Winston and Sears (2013), Link (1997), and Raup et al. (1983) for additional sedimento-logical details at these sites.
The two-day trip begins on the west side of GNP at West Glacier, Montana, crosses the park on Going-to-the-Sun Road over the Continental Divide at Logan Pass, and returns to West Glacier on U.S. Highway 2, by way of Marias Pass (aka Theodore Roosevelt Pass). The driving distance totals ~240 km (150 mi).
Weather may be capricious in GNP in September. Expect cold wind and possible snow at the higher elevations. We cross Logan Pass at 2026 m (6646 ft) elevation. On the other hand, autumn in the Rockies can be splendid. The trip does not require extensive hiking, but surfaces may be wet and slippery, especially near creeks. Many exposures are near cliffs with possible loose rock. Going-to-the-Sun Road has numerous narrow sections, and some stops are near the roadside. There are rest rooms at several of the stops. Lunches will be provided. We will stay overnight in the village of East Glacier Park.
The Belt Supergroup of GNP is entirely allochthonous, having been transported ~130 km (~80 mi) northeastward on the Lewis thrust (Bally, 1984; Price and Sears, 2000). Bailey Willis first recognized the spectacularly well-exposed Lewis thrust in 1902. Willis mapped the fault on the east side of GNP in glacially scoured valleys that cut deep re-entrants through the Precambrian Belt rocks and into underlying Cretaceous strata. Willis also recognized windows through the Lewis thrust on the footwall of the Flathead normal fault in Canada. One of the first overthrusts documented anywhere in the world, the Lewis fault became a central element of a long controversy between physicists and geologists concerning the mechanical viability of such faults. In a classic paper, Hubbert and Ruby (1959) finally demonstrated that overthrusts were theoretically possible, using the Lewis thrust as a prime example. Plate tectonics and GPS surveys have since confirmed that large horizontal translations of massive crustal plates do indeed occur.
Willis (1902) conservatively estimated that the Lewis plate moved as much as 64 km (~40 mi), based on erosional remnants and windows through the plate. Bally (1984), an early champion of balanced cross sections, crafted a detailed regional section across the Lewis plate from the Alberta foreland southwest to the Montana-Idaho border. He based his section on geologic mapping as well as on industry seismic and borehole data. He calculated that the plate had moved northwesterly some 130 km (~80 mi). Bally derived the measurement from painstaking retro-deformation of the complex imbricated thrust systems in the Alberta foreland basin in front of the Lewis plate, as well as thrusts within the duplex structure underlying the plate. Figure 2, a cross section through Logan Pass modified from Harris (1985), 40 km (~25 mi) south of Bally’s section line, is consistent with Bally’s estimate for displacement of the Lewis plate.
Price and Sears (2000) demonstrated that Bally’s measurement restores the leading edge of the Lewis plate into the Libby trough, a rectangular depression in NW Montana having the same vertical and horizontal dimensions as the rectangular salient of the Lewis plate preserved in Waterton-Glacier International Peace Park of Montana and Canada. The inset map in Figure 2B outlines the Libby trough and Lewis salient, compares their map dimensions, and shows their separation of 130 km (~80 mi) in the direction of transport, parallel to cross section A-A’-A”. Large lateral ramps bound the Lewis salient on the north and south. The Libby trough preserves Cambrian strata at the surface, but is flanked on three sides by anticlines cored by the Prichard Formation. These plunge into the Libby trough from the north and the south, and the west limb of the Purcell anticlinorium dips into the trough from the east (Fig. 2, inset map).
The west side of the Lewis plate in GNP comprises the east edge of the broad Purcell anticlinorium, which covers most of northwestern Montana and continues into British Columbia. As illustrated in Figure 2B, the Purcell anticlinorium represents the hanging-wall anticline of the Lewis plate and carries the thick basinal facies of the Belt-Purcell Supergroup (Price and Sears, 2000). It represents the inverted profile of the northeast part of the original Belt Basin. The west limb of the anticlinorium drapes an ~15-km (~50,000 ft)-high basement ramp that descends into the Libby trough and marks the autochthonous northeast margin of the Belt Basin (Fig. 2B). The northeast limb of the anticlinorium results from the abrupt eastward stratigraphic thinning of the Belt Supergroup across the original basin slope. The northeasterly dips are accentuated by a large footwall duplex beneath the Lewis thrust and by northeastward tilt against Paleogene listric normal faults, such as the Rocky Mountain trench and, in GNP, the Flathead fault. The northeastern edge of the Purcell anticlino-rium follows the axial trace of the Akamina syncline in GNP, and coincides with the thrust-transported shelf margin of the Belt Basin. As shown in Figure 2A, the thinner shelf facies on the northeast limb of the syncline were tilted southwest by the underlying imbricated thrusts of the duplex.
Regional section A-A -A” (Fig. 2B), after Harris (1985), extends southwestward through the Paul Gibbs #1 borehole, which penetrated 5.4 km (~17,700 ft) of lower Belt strata on the crest of the Purcell anticlinorium (Boberg et al., 1989). The section, palinspastically restored to the Cambrian datum (Fig. 2B, lower), shows that the Paleozoic strata rested on an ~15 km (~9.32 mi) thickness of Belt rocks in the west, but rested directly on pre-Belt basement gneiss to the east.
Bedrosian and Box (2016) interpreted magnetotelluric surveys in NW Montana to show that pre-Belt basement was likely involved in the Cordilleran thrusting west of GNP. Balanced and restored cross sections demonstrate that displacement of the Belt Basin increases systematically northward from a few kilometers near Helena, Montana, to ~250 km (~155 mi) in southeastern British Columbia (Sears, 2000; Price and Sears, 2000). Sears (1994) proposed that the northern part of the Belt Basin rotated clockwise about an Euler pole near Helena, Montana, during Cretaceous and Paleocene thrust displacement. A palinspastic map based on retro-deformation of numerous balanced cross sections shows that the basin was originally segmented into several discrete fault blocks in a large triangular graben system (Fig. 3), likely at a triple-rift junction (Sears, 2007a). The portion of the basin exposed at GNP occupied the roll-over edge of a deep halfgraben that was faulted on the south and west sides and had a triangular plan view. This graben was infused with massive mafic sills and contains the thickest Belt sections, as well the large rift-related Sullivan sedimentary exhalative (SEDEX) deposit (Lydon, 2000). The fill of the graben underwent burial metamorphism associated with high heat flow from the great volume of syndepositional mafic sills (Norwick, 1972). During Cordilleran orogenesis, the fill of the graben was driven northeastward into the foreland basin as part of the strong wedge that now comprises the Lewis thrust plate. The foreland basin subsided under the load of the Lewis plate, as shown by the configuration of the Cretaceous and Paleogene foreland basin deposits of Montana and Alberta. From Helena to Alberta, the Lewis thrust moved during the classic Laramide orogeny, from ca. 75 Ma to ca. 60 Ma (Sears, 2007a).
Belt Basin Margin
The field trip will investigate evidence that the Lewis thrust plate in GNP contains the displaced northeastern depositional margin of the Belt Basin. The trip starts in thick basinal facies of the Belt Supergroup, and crosses northeastward, rather abruptly, into thin shelf facies, across the axial trace of the Akamina syncline at Logan Pass on the Continental Divide.
The transition is most notable between the Prichard Formation in the southwest and the Altyn Formation in the northeast. The Prichard Formation comprises a > 6-km (~20,000 ft)-thick, basinal, sulfidic, turbiditic, synrift succession in the interior of the Belt Basin (Cressman, 1989; Chandler, 2000; Hoy et al., 2000), where its base is not exposed. The Prichard Formation is laced with syndepositional mafic sills aggregating 2 km (~6500 ft) in exposed thickness, underlain by an additional volume of thick mafic sills interpreted from seismic sections (Cook and van der Velden, 1995). The Paul Gibbs #1 well (Fig. 2B) penetrated over a dozen mafic sills in the Prichard Formation (Boberg et al., 1989). The northeastern edge of the Prichard Formation crops out on the southwest side of GNP at Stop 1. The Prichard Formation passes vertically and laterally into the Appekunny Formation, which thins northeastward across the Akamina syncline (Fig. 2) (Whipple, 1992). The Appekunny Formation overlies the Prichard Formation in the southwest, but overlies the Altyn Formation in the northeast due to the depositional pinch-out of the Prichard. The Altyn Formation comprises a relatively thin, clean, shallow-water, stromatolitic dolomite with lenses of quartz arenite and argillite. The Altyn Formation does not occur southwest of the Akamina syncline, but allodapic lenses of dolomite within the Prichard Formation may represent carbonate debris flows from the Altyn shelf. The Altyn may comprise part of an extensive pre-Belt platform cover sequence that was inset by the Belt rift basin. A spectacular 30-km (~19 mi)-long cross-sectional view of the Lewis plate along Marias Pass (Stop 14) exposes the southwest edge of the Altyn Formation, where it is faulted against thick argillite of the Prichard and Appekunny Formations (Whipple, 1992).
The Belt Basin has become a central part of the argument that a craton, the conjugate partner of Laurentia, rifted away and now occupies a distant continent. Cressman (1989), Winston (1991), Frost and Winston (1987), Ross et al. (1992), and Ross and Villeneuve (2003) demonstrated from sedimentological, neodymiun systematics of argillites, and detrital zircon distributions that the Belt Supergroup was deposited in an intracratonic basin with some sources to the southwest. That basin was broken apart in late Neoproterozic time when the Windermere rift system established the first continuous deposits mapped along the length of the North American Cordillera (Price, 1964; Ross, 1991; Burchfiel et al., 1992; Link, 1993). Consequently, geologists have used the Belt Basin as a pin to propose links to Meso-proterozoic basins on other continents (Sears and Price, 1978; Burrett and Berry, 2000; Li et al., 2008; Jones et al., 2015).
Sears and Price (1978) originally proposed the Siberian cra-ton as the conjugate partner of western Laurentia. They tightened their Siberia-Laurentia connection with new data from Siberia that became available after the collapse of the Soviet Union (Sears and Price, 2003). Their reconstruction seamlessly links several pre-Belt basement terranes between the cratons (Fig. 5). It fits the Belt Basin against the correlative Udzha Basin of NE Siberia, and aligns several major syndepositional fault systems and dike swarms of the Belt Basin with similar and correlative ones mapped in NE Siberia. New U-Pb data from Mesoproterozoic rocks of the Olenek region of NE Siberia date a mafic sill to 1473 ± 24 Ma, a close correlative to the 1469 Ma Moyie mafic sills of the Belt Basin (Wingate et al., 2009). The site plots directly against the Belt Basin on Figure 5.
Platform deposits in the Anabar shield region of northern Siberia lithologically match correlative ones (Neihart, Chamberlain, and Newland Formations) in the Little Belt Mountains of Montana, east of the Belt Basin. The Newland Formation likely corresponds to the Altyn Formation of GNP (Harrison, 1972). Perhaps the Belt Basin opened as a complex rift system that cut across the platform cover of the Siberia-Laurentia craton during and/or after deposition of the Altyn Formation and correlatives, insetting the deep Prichard turbidite basin into the platform (Sears, 2007b).
Sears (2012) proposed a model for Paleozoic displacement of the Siberian craton from west Laurentia along a coast-parallel transform fault system. In that model, the cratons rifted in Neoproterozoic time, opening the Windermere basins of North America, but they did not drift apart until Cambrian time. In GNP, a prominent diabase sill dated to 780 Ma represents the Neoproterozoic rift phase; it is part of the Gunbarrel large igneous province, which has been dated from Wyoming to NW Canada (Harlan et al., 2003). In Washington and British Columbia, the Belt-Purcell Basin is truncated and overlapped by the Windermere rift system, but a second major rift phase began in latest Precambrian and Early Cambrian time and advanced to seafloor spreading (Bond et al., 1985; Price and Sears, 2000).
Day 1. West Glacier to East Glacier Park Village—Crossing Glacier National Park, West to East, On Going-to-the-Sun Road
Part of the road log for Day 1 is adapted from a Belt V Symposium field trip by Winston and Sears (2013), which emphasized Belt stratigraphy and sedimentary structures. That trip ran from east to west, rather than from west to east. The stop numbers given in brackets (i.e., WS 3) refer to stops discussed in Winston and Sears (2013).
Driving instructions given in cumulative distances.
km (mi) Directions
0.0 (0.0) West entrance gate to Glacier National Park (48.505552° N, 113.98541° W).
1.9 (1.2) Turn right on Going-to-the-Sun Road.
3.2 (2.0) Lake McDonald on left. Glaciers excavated the lake basin into the Eocene Kishenehn Formation, which had filled the subsiding Flathead half-graben (Constenius, 1996).
16.0 (10.0) Lake McDonald Lodge on left at the approximate trace of the Paleogene Flathead listric normal fault. The Flathead fault is down-to-the-west, and merges downward into the Lewis thrust (Price, 1964).
19.5 (12.1) Park on the north side of the road at the large pull-off.
Stop 1. Prichard Formation (48° 38’ 20” N, 113° 51’ 51” W; WS Stop 9)
Take the trail to the bridge over McDonald Creek and climb down to the creek level. The outcrops are beside the creek.
The Prichard Formation is a dark, turbidititic, argillite/siltite unit that was deposited in the northern half-graben of the Belt Basin. The Prichard Formation has an exposed thickness of 6 km (3.73 mi) in the center of the Belt Basin, with the base not exposed (Cressman, 1989). This site exposes only the uppermost part of the Prichard Formation and lies to the northeast of the main mass. The Prichard Formation was deposited during the initial rift phases of the Belt basin (Fig. 4). To the west, the Prich-ard Formation contains syndepositional sills, faults (Sears et al., 1998; Poage et al., 2000), mud volcanoes, and SEDEX lead-zinc deposits (Lydon, 2000), and is generally in the biotite zone of ‘burial’ metamorphism (Norwick, 1972). It underwent metamorphism to the greenschist facies in the absence of strain. Likely, the heat associated with the mafic sills raised the geothermal gradient of the basin during Prichard deposition.
To the west, Cressman (1989) subdivided the Prichard Formation into eight or nine informal units. In GNP, the Prichard Formation grades upward and laterally northeastward into the Appekunny Formation (Whipple, 1992). Winston and Sears (2013) suggest that the beds that crop out here could be laterally equivalent to the pyritic facies of Appekunny Formation Member 3 or to Appekunny Formation Member 4 near the Belt Basin margin on the northeast side of the Akamina syncline, where the Appekunny overlies the Altyn Formation rather than the Prich-ard Formation. Slotznick et al. (2016) determined that the iron mineralogy and redox conditions during Appekunny deposition indicate that the basin was generally oxygenated.
The rocks at this site palinspastically restore to the edge of the Libby trough, ~130 km (~80 mi) to the southwest (Fig. 2B). They represent the transition between the thin, shallow-water shelf facies of northeastern GNP and the thick, deep-water turbidite facies to the southwest. The northeastern facies include the stromatolitic Altyn Formation (Stop 11). Thin carbonate beds in the Prichard Formation near here may be allodapic carbonate debris of Altyn dolomite shed from the shelf.
Continue east on the Going-to-the-Sun Road.
20.5 (12.8) Road crosses into the Appekunny Formation. McDonald Creek cascades over gray argillite/ siltite beds near the upper part of Appekunny Formation Member 5. Nearly identical beds occur near the top of Appekunny Formation Member 5 on the northeast limb of the Akamina syncline (Stops 9 and 10). These beds may record a filling phase of the muddy basin, which culminated in deposition of the overlying sandstone and argillite of the Grinnell Formation. The basin abruptly sank after Grinnell deposition with deposition of the deeper-water Empire Formation. The basin subsidence was coincident with emplacement of a widespread 1457 Ma mafic sill in the basin to the southwest, which may record a basinal rift phase (Sears, 2007b) (Fig. 4).
Road continues in the Appekunny Formation.
25.3 (15.7) Going-to-the-Sun Road passes from the Appekunny Formation to the overlying, shallow-water Grinnell Formation. Road climbs through the Grinnell Formation for the next 3.7 km (2.3 mi).
29.0 (18.0) Road crosses from the Grinnell Formation to the overlying, deeper-water Empire Formation and follows that formation for next 5.9 km (3.7 mi).
34.9 (21.7) Road crosses from green beds of the Empire Formation into the Helena Formation. Winston (2007) assigned the Helena Formation to the lower part of the Piegan Group (cf. Fenton and Fenton, 1937). It was classically part of the ‘Middle Belt Carbonate’—a major stratigraphic division within the dominantly siliciclastic Belt Supergroup (Harrison, 1972).
36.9 (22.9) Roadcut exposures of Conophyton stromatolites of the Wallace Formation, now assigned to the upper part of the Piegan Group (Winston, 2007), and of a prominent Neoprotero-zoic diabase sill seen throughout the park at this stratigraphic level.
37.2 (23.1) Tunnel through Wallace Formation.
38.0 (23.6) You are now entering ‘The Loop.’ We will not actually stop. Please drive withextreme cautionif you are taking this trip on your own.
Stop 2. ‘The Loop’ (48° 45’ 16” N, 113° 47’ 57” W)
The Loop hairpin switchback crosses the Wallace/Snowslip contact. The Snowslip Formation forms the base of the Missoula Group, which overlies the Piegan Group. This records the shallowing of the basin from subaqueous carbonates to periodically exposed muds, silts, and sands.
Continue east on the Going-to-the-Sun Road.
38.8 (24.1) Pass down-section from the Snowslip Formation back into the Wallace Formation.
40.0 (24.9) Cross the prominent diabase sill again. Road traverses the Wallace Formation for next
10.6 km (6.6 mi).
46.5 (28.9) Stop for an excellent view of the glacially excavated valley of McDonald Creek.
Stop 3. Bird Woman Falls Viewpoint (48° 43’ 34” N, 113° 44’ 32” W)
The cliffs across the valley expose the Piegan and Mis-soula Groups, dipping into the Akamina syncline. Logan Pass occupies the gap to the left of the waterfall, between the Garden Wall to the northeast, and Clements Mountain to the southwest. The Shepard Formation of the Missoula Group caps Clements Mountain.
Continue east on the Going-to-the-Sun Road.
50.7 (31.5) Park in the Logan Pass Visitor Center parking lot.
Stop 4. Logan Pass (48° 41’ 44” N, 113° 43’ 04” W; WS Stop 7)
The broad Akamina syncline lies to the northwest, with the Missoula Group folded down above the Piegan Group.
The visitor center lies near the Snowslip/Wallace contact. The Wallace Formation contains stromatolites and a thin K-bentonite bed dated to 1454 Ma (Evans et al., 2000). The bentonite may have erupted from the massive granite-rhyolite province that swept across the southwest edge of Laurentia (Sears, 2007b). A clay mineralogy study suggests that numerous other thin K-bentonite beds occur in the Missoula Group to the southwest (Foster, 2005).
The boardwalk to Hidden Lake overlook, behind the visitor center, crosses red argillite beds of the Snowslip Formation. They display spectacular ripples and mudcracks. The Snowslip Formation forms the ‘Garden Wall’ north of the Going-to-the-Sun Road, as well as most of the glacial horns seen around Logan Pass. Small remnants of the overlying Shepard Formation cap some of the peaks. The Snowslip Formation marks the passage from the sub-aqueous deposits of the Wallace Formation to the intermittently subaerial deposits of the Missoula Group, as the Belt Basin once more filled with sediment. This filling cycle followed the basin collapse at the top of the Grinnell Formation. The basin again subsided at the Snowslip/Shepard contact. That subsidence coincided with eruption of the Purcell pillow basalts and associated faulting farther to the northwest.
A short hike to the west along the Highline Trail (across the Going-to-the-Sun Road from the parking area) provides access to exposures of Wallace Formation dolomite with excellent molar-tooth structures, stromatolites, and other sedimentary structures. Argillite interbeds display slaty cleavage associated with formation of the Akamina syncline. The trail also crosses the prominent Neoproterozoic diabase sill.
Continue east on Going-to-the-Sun Road. The road now generally traverses downward in the Belt section through the Wallace, Helena, Empire, Grinnell, Appekunny, and Altyn Formations on the east limb of the Akamina syncline.
51.2 (31.8) Park along the roadside.
Stop 5. Diabase Sill (48° 41’ 50” N, 113° 42’ 41” W)
This sill and its bleached contacts make prominent marker bands across Glacier National Park. Throughout the park, the sill typically occupies the Wallace Formation. It has been dated to ca. 780 Ma, and is one of a number of regional sills and dikes of this age that have been mapped throughout the Belt Basin (Burtis et al., 2007). It is part of the Gunbarrel LIP that has been traced from NW Wyoming to NW Canada (Harlan et al., 2003). Rogers et al. (2016) discuss implications of the geochemistry of this and other mafic sills in the Belt Basin. The Gunbarrel LIP signifies the rifting event that led to deposition of the Windermere Supergroup, the first deposit of the Cordillera that continues from Death Valley to Alaska. The Windermere rift truncated the Belt Basin in Idaho, Washington, and British Columbia, and eventually led to establishment of the Cordilleran margin of Laurentia.
Continue a short distance down Going-to-the-Sun Road to the east.
51.7 (32.1) Pull off at the viewpoint on the south side of the road.
Stop 6. Baicalia-Conophyton Cycles in the Wallace Formation (48° 41’ 56” N, 113° 42’ 18” W; WS Stop 6)
Cross to the cliffs on the north side of the road. These two cycles of cliff-forming stromatolites form marker beds that Horo-dyski (1983, 1989) traced extensively through GNP. Winston (2007) mapped this horizon across a wide region of the northern half-graben of the Belt Basin, suggesting that this part of the basin experienced a period of significant tectonic stability.
At this site, there are two distinct forms of stromatolites— branching forms called Baicalia and conical forms called Conophyton. The Baicalia pass upward into the Conophytons in two cycles. The Baicalia continue across the basin far to the west, but the Conophytons are confined to eastern GNP. Winston and Lyons (1993) suggested that the resistant Baicalia in the west protected the Conophytons in GNP from waves coming from the west.
This site also exposes excellent molar-tooth structure, a key feature of Proterozoic sediments first described in the Belt Supergroup and now recognized in Proterozoic sedimentary rocks throughout the world. The structures are thought to have formed as carbon dioxide bubbles rose through the unconsolidated sediment, leaving fine-grained calcite trails that were compacted during later burial (Furniss et al., 1998). Smith (2016) gives a detailed summary of current thinking about the origins and significance of molar-tooth structure. Winston and Sears (2013) provide a detailed discussion of the sedimentology of this site.
Continue east on Going-to-the-Sun Road in the Wallace Formation.
52.0 (32.3) Lunch Creek parking lot. Cross Lunch Creek and continue down Going-to-the-Sun Road.
53.1 (33.0) Tunnel through the Wallace Formation.
54.6 (33.9) Pull off on the right side of the road.
Stop 7. Helena/Wallace Contact (48° 41’ 38” N, 113° 40’ 35” W; WS Stop 5)
Cross the road. This site exposes excellent cross-bedded oolites in the basal Wallace Formation. Winston (2007) interprets the cross-bedded oolites to represent low-relief, ephemeral beaches that reached far across the Belt Basin, indicating that the basin was tectonically stable. The tan-weathering outcrops below the gray ones expose undulating dolomitic couplets at the top of the Helena Formation. They extend across the entire Belt Basin and show that the Helena basin was shallow enough to lie within the reach of waves. Winston and Sears (2013) provide a detailed discussion of the sedimentology of this site.
Continue east on the Going-to-the-Sun Road.
54.9 (34.1) The road curves eastward below Logan Pass.
55.4 (34.4) The roadcuts in the lower part of the cliffs to the west rise through the Helena Formation. Stromatolites form some of the higher knobs.
55.7 (34.6) Road swings around Siyeh bend and crosses Piegan Creek. Platy outcrops on the floor of Piegan Creek are in the lower part of the Helena Formation. Road stays in the Helena Formation for the next 2.9 km (1.8 mi).
60.2 (37.4) Green argillite of the Empire Formation crops out on the cliffs north of the road.
63.1 (39.2) Park along the road.
Stop 8. Sunrift Gorge (48° 40’ 43” N, 113° 35’ 43” W)
Roadcuts expose thin sandstone interlayers of white, well-rounded, coarse quartz in bright red argillite. Winston (2016) and Kuhn (1981) concluded that the coarse sand was derived from the Laurentian craton to the east, possibly from the recycling of sand from the Neihart sand blanket. Ross and Villeneuve (2003) found that the eastern facies contained detrital zircon that correlate with basement zircon of Laurentia, whereas western basinal facies of the lower Belt Supergroup contained detrital zircon exotic to Laurentia.
Continue east on the Going-to-the-Sun Road.
63.9 (39.7) Turn right into the Sun Point parking area.
Stop 9. Sun Point (48° 40’ 35” N, 113° 34’ 50” W)
Take the trail to the viewpoint for Saint Mary Lake. In addition to a beautiful view, this well-exposed ridge of Appekunny argillite displays locally developed slaty cleavage associated with the Akamina syncline and Lewis thrust.
Return to the Going-to-the-Sun Road and continue east.
66.0 (41.0) Park on the south side of the road at the large parking area overlooking Saint Mary Lake.
Stop 10. Appekunny Formation (48° 41’ 18” N, 113° 33’ 28” W)
Cross the road to the west end of the large roadcut (Fig. 6). Large soft-sediment folds and well-defined, small syndepositional normal faults in the siltite could have been generated by slumps down the clinoform slope of the basin margin as it subsided.
At Apikuni Falls, 15 km (~9 mi) north of here, Horodyski (1983,1989) discovered strings of circular impressions about the size of a lead pencil eraser dotting surfaces of Appekunny slabs like strings of beads. Their regular sizes and patterns leave little doubt that they are fossils of some sort. As such, they are among the earliest megafossils in the world. Adam et al. (2016) present new findings of microfossils in the Chamberlain Formation of the lower Belt Supergroup from near Helena, Montana.
Continue east on the Going-to-the-Sun Road.
68.7 (42.7) Wild Goose Island viewpoint. Roadcuts of Altyn dolomite.
69.2 (43.0) Park at the pull-out on the right.
Stop 11. Altyn Formation (48° 41’ 19” N, 113° 31’ 38” W; WS Stop 2)
Cross the road to the cliffy outcrop of Altyn dolomite. The Altyn Formation is the basal unit of the Belt Supergroup across most of GNP northeast of the axial trace of the Akamina syncline. Here, it floors the Lewis thrust plate. Near the Canadian border, the basal duplex of the Lewis thrust system cuts down into the underlying Waterton, Tombstone Mountain, and Haig Brook formations, with a thickness of > 1.7 km (~6000 ft) (Fermor and Price, 1983). The cream-colored Altyn dolomite displays cyanobacterial laminae that locally rise into stromatolitic domes (Fig. 7). The dolomite contains well-rounded, coarse quartz sand grains that are concentrated along bedding-plane stylolites. Elsewhere, the Altyn Formation includes sandstone lenses. The sand was likely derived from the Laurentian craton to the east, perhaps recycled from the Neihart Quartzite, which mantled the craton prior to opening of the Belt Basin (Winston and Sears, 2013). The Altyn Formation contrasts with the Prichard Formation turbidites of the axial part of the northern half-graben of the Belt Basin. It may have been truncated by the rift at the NE margin of the Belt Basin.
The break in slope at the base of the Altyn cliff marks the trace of the Lewis thrust, with Upper Cretaceous shale in the footwall.
Continue east across tree-covered Upper Cretaceous shale and sandstone at the footwall of the Lewis thrust for the next 2.8 km (1.7 mi) to Stop 12.
69.8 (43.4) Rising Sun Motel is on the left.
75.3 (46.8) Park at the “Triple Divide Peak Exhibit.”
Stop 12. View of the Lewis Thrust (48° 43’ 46” N, 113° 27’ 55” W; WS Stop 1)
Bailey Willis (1902) first recognized the Lewis thrust in the deeply glaciated valleys on the east side of Glacier Park. The thrust places far-traveled Belt Supergroup rocks over the Late Cretaceous Two Medicine Formation of the Cordilleran foreland basin. The buff-colored dolomite cliffs of Altyn Formation on both sides of Saint Mary Lake form the sole of the Lewis thrust plate. Fault splays from the Lewis thrust cut into the Altyn, forming complex duplexes along the eastern front of the thrust (Yin, 1988). To the south is Divide Mountain, which separates drainage to the Gulf of Mexico from drainage to the Arctic. It is a near-klippen of the Lewis plate (Fig. 8). The hanging-wall beds of the Lewis plate dip gently to the southwest, so that a continuous Belt Supergroup section is exposed in the mountains from here to Logan Pass. To the southwest, the buff-colored Altyn Formation is overlain by the Appekunny Formation, which comprises greenish-gray argillite with distinctive layers of resistant white quartzite in its lower part. In the distance to the southwest lie the overlying redbeds of the Grinnell Formation. In the far distance to the southwest rise tan and gray cliffs of the Helena and Wallace Formations of the Piegan Group (Fenton and Fenton, 1937; Winston, 2007), which Harrison (1972) termed the ‘Middle Belt Carbonate.’
The Akamina syncline follows the Continental Divide in Glacier Park (Fig. 1). The northeast limb of the syncline tilts southwest above a stack of imbricated thrusts of Mesozoic formations of the foreland basin (Fig. 2A). The southwest limb tilts northeast above a footwall duplex of Paleozoic and Mesozoic formations. The southwest limb of the syncline also comprises the northeast limb of the Purcell anticlinorium. This broad regional fold represents the inverted form of the Belt Basin, where the Belt Supergroup was thrust out of its basin, up a basin-margin ramp, and over the flat surface of the western North American craton. Palinspastic restorations indicate that the Lewis plate was translated ~130 km (~80 mi) to the NE from the original site of deposition of the Belt Supergroup (Fig. 2).
Continue east on the Going-to-the-Sun Road.
79.3 (49.3) Saint Mary entrance station to GNP on the Going-to-the-Sun Road.
80.1 (49.8) Saint Mary. Turn right on U.S. 89 south.
85.5 (53.1) Burned forest.
90.6 (56.3) Tight curve.
102.4 (63.6) Cross Cut Bank Creek.
110.4 (68.6) Turn right on Highway 49.
111.5 (69.3) Switchback.
114.1 (70.9) Tight folds in Cretaceous sandstone. Imbricated thrust structures in front of Lewis plate.
122.3 (76.0) Two Medicine Road intersection on right. Continue to East Glacier Park village to end Day 1.
Day 2. Lewis Thrust at Running Eagle Falls and Marias Pass
0.0 (0.0) Intersection of Highway 49 and U.S. Highway 2. Proceed north on Highway 49.
6.4 (4.0) Turn left toward Two Medicine Lake.
7.1 (4.4) Dam on left.
9.8 (6.1) Park entrance station.
14.8 (9.2) Turn right into the parking lot for Running Eagle Falls. Park and take the trail for 0.5 km (0.3 mi) to the viewpoint of Lewis thrust.
Stop 13. Lewis Thrust at Running Eagle Falls (48° 29’ 54” N, 113° 21’ 05” W)
The wooden viewing platform rests on the Upper Cretaceous Marias River Formation of the Cordilleran foreland basin. The falls tumble over cliffs of the Altyn dolomite at the base of the Lewis thrust plate.
Turn right from the parking lot and continue 3.2 km (2 mi) on Two Medicine Road to Two Medicine Lake for an excellent view of the Lewis plate and glaciated landscape.
Return to East Glacier Park village.
0.0 (0.0) Intersection of Highway 49 and U.S. Highway 2. Proceed west (right) on U.S. 2.
2.1 (1.3) Heart Butte cutoff on left. The highway follows the valley of Summit Creek and crosses several imbricated thrusts of Cretaceous formations of the Cordilleran foreland basin. For the next 4.8 km (3 mi), three imbricated slices of west-dipping Lower Cretaceous Kootenai Formation sandstone form dark-brown cliffs and ridges faulted against Upper Cretaceous Blackleaf Formation black shale.
6.9 (4.3) The highway crosses imbrications of folded and faulted Blackleaf and Marias River Formation shale and sandstone for the next 11 km (7 mi), up to Marias Pass.
18.2 (11.3) Turn left into the parking area for Marias Pass (aka Theodore Roosevelt Pass). Park near the Roosevelt obelisk.
Stop 14. View of Lewis Thrust from Marias Pass (48° 19’ 08” N, 113° 21’ 13” W)
This site provides an excellent cross-sectional view of the Lewis thrust at the SE edge of the Waterton-Glacier salient of the Lewis plate. The thrust follows the base of the prominent cream-colored cliffs of Altyn dolomite to the NE, but follows the base of the Appekunny Formation farther to the SW. The westernmost extent of the Altyn dolomite may mark the faulted boundary between the shelf and platform facies of the Belt Basin to the NE and the slope facies to the SW.
The SE edge of the salient coincides with a major lateral ramp in the stratigraphic level of the Lewis plate that matches a likely footwall lateral ramp at the southeast margin of the Libby trough, 130 km (~80 mi) southwest of here (Fig. 2B, inset map).
Yin (1988) mapped a series of imbricates and duplexes along the base of the Lewis thrust mostly within the Appekunny and Altyn Formations. Boyer and Elliott (1982) highlighted Willis’s (1902) sketch of a duplex in the Altyn Formation on the Chief Mountain klippen. Beneath the Lewis thrust is a highly deformed fault duplex of Paleozoic and Mesozoic formations. West of here, the duplex involves the Cretaceous Kootenai and Blackleaf Formations, and Jurassic shale and sandstone. An industry seismic profile along Highway 2 revealed repetitions of Mississippian and Devonian carbonates beneath surface exposures of Kootenai Formation.
The footwall structures plunge to the NW. Therefore, the deeper parts of the duplex are exposed to the south, where Devonian and Mississippian carbonates emerge (Mudge and Earhart, 1983). Evidently, the plunge of the duplex resulted from a decrease in vertical stacking of the thrust plates beneath the Lewis thrust. Most workers agree that the Lewis plate formerly continued to the south, over our heads at this location, but that it has been removed by erosion. The structural plunge allowed the Lewis plate to be preserved in Glacier National Park, and higher parts of the Belt section are preserved successively to the NW, up through the Mount Shields Formation of the Missoula Group, NW of Logan Pass. In Canada, Lower and Upper Paleozoic rocks are preserved on the Lewis thrust plate.
Continue SW on Highway 2. The next 8 km (5 mi) cross six thrust slices of Mesozoic rocks in the footwall duplex.
27.2 (16.9) Cross Blacktail normal fault, which drops the Missoula Group down to the west ~5 km (~3 mi), against the Appekunny/Prichard Formations. This is an Eocene-Oligocene splay of the Flathead normal fault. It flattens downward to the west and joins the Lewis thrust at a depth of a few km. The hangingwall block mostly dips to the east, and is cut by several smaller, mostly down-to-the-west normal faults.
38.2 (23.7) Highway bends sharply to the NW at the confluence of Bear Creek and the South Fork of the Flathead River. The highway follows the NW strike of the fault system for the next 40 km (25 mi).
44.7 (27.8) Essex on left. Enter the graben containing the Eocene Coal Creek Formation. These lacustrine beds filled the graben as it subsided (Constenius, 1996).
62.8 (39.0) Cross out of the graben into the northeast-dipping Missoula Group. Prominent Bonner Quartzite outcrop. Road bends sharply west.
78.4 (48.7) Highway 2 bends west and enters a canyon cut through the east-tilted Flathead Range. Missoula and Piegan Groups are exposed in the canyon walls.
87.5 (54.4) West Glacier.
End Field Trip.
This field guide would not have been possible without the long and collegial collaboration and participation on field trips to Glacier National Park with Don Winston, Steve Boyer, Chuck Kluth, and Bill Hansen. We thank the reviewers for their suggestions to improve the manuscript.
Figures & Tables
Unfolding the Geology of the West
Prepared in conjunction with the 2016 GSA Annual Meeting in Denver, Colorado, this volume contains sixteen guides to field trips in this rich geologic region. The four “Great Surveys” of the late 1800s ventured west to explore and document the region’s unknown natural resources and collect valuable geologic information. Many of the field guides in this volume, aptly titled Unfolding the Geology of the West, will cover the same hallowed ground as the early geologic expeditions. Organized into four sections, this volume spans some of the major subdisciplines of geology: (1) stratigraphy, sedimentology, and paleontology; (2) structure and metamorphism; (3) Quaternary landscape evolution; and (4) engineering and environmental geology.