Rift Destabilization of a Proterozoic Epicontinental Pediment: A Model for the Belt–Purcell Basin, North America
Published:January 01, 2007
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James W. Sears, 2007. "Rift Destabilization of a Proterozoic Epicontinental Pediment: A Model for the Belt–Purcell Basin, North America", Proterozoic Geology of Western North America and Siberia, Paul K. Link, Reed S. Lewis
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In the absence of land plants, broad pediments may have formed stable landforms that beveled Proterozoic continents. Braided streams would have transported a thin layer of clastic sediment across such Proterozoic epicontinental pediments. The Proterozoic pediment–braidplain system may be represented by extremely flat regional unconformities beneath locally preserved, supermature, braidplain sandstones. Continental rifting would have destabilized Proterozoic epicontinental pediments by funneling runoff along rift axes to create large rivers, which otherwise were not favored in the Proterozoic landscape. The sedimentological history and detrital–zircon provenance of the intracratonic Mesoproterozoic Belt–Purcell basin of western North America may be described in terms of destabilization of a late Paleoproterozoic to early Mesoproterozoic epicontinental pediment by a three–armed rift system with the Belt–Purcell basin at its center. A model using a Siberia–Laurentia–Australia paleocontinental reconstruction implies that the sedimentary veneer of the pediment washed down the western branch of the rift system to enter the Belt–Purcell basin at a point source on its western side. Capture of clastic sediment in delta fans on the western side of the basin permitted clean carbonate to precipitate on the northeast side. Reconfiguration of the basin by renewed rifting appears to have changed composition, grain size, and sedimentary provenance during deposition of the Missoula Group (upper Belt–Purcell Supergroup).
Prior to the colonization of land by Devonian plants, vast, low–relief plains appear to have characterized tectonically stable parts of continents. An example of such a surface is the profound angular unconformity between smoothly truncated basement rocks and overlying Cambrian sandstones, imaged in seismic– reflection profiles in many areas of the North American platform and exposed over thousands of square kilometers in the Colorado Plateau (cf. Marshak et al., 2000). Such low–relief planation surfaces may represent continent–wide pediments—broad, smooth, stable plains that descended exponentially from tectonic lands to sea level in the barren pre–Devonian world.
Rainbird et al. (1992) showed that distinctive late Meso– proterozoic detrital–zircon suites were fluvially transported some 3000 km across the North American continent from the Grenville tectonic highlands in the southeast to depositional sites in the Canadian Arctic in Proterozoic marine quartz arenites of the Shaler Group. The system they described may embody a continent–wide pediment–braidplain grading to the sea. This paper proposes that such a surface may provide a structural datum for interpreting tectonic activity in the interior of a continent, specifically the history of the Mesoproterozoic BeltPurcell basin of western North America.
The paper employs the Siberia–Laurentia–Australia troika of Sears and Price (2003), which connects numerous diverse basement piercing points and structures between the correlative rifted margins of western North America and the northern and eastern Siberian craton (Fig. 1). It modifies the AUSWUS (Karlstrom et al., 2001; Burrett and Berry, 2000) and AUSMEX (Wingate et al., 2002) models to juxtapose the northern Australian, southwestern North American, and the southern Siberian cratons.
In the proposed model, a late Paleoproterozoic–early Mesoproterozoic pediment descended northward from a tectonic divide near the southern margin of the continent, which was undergoing tectonic accretion for much of late Paleoproterozoic time (Karlstrom and Houston, 1984; Karlstrom and Bowring, 1993), toward the future site of the Belt–Purcell basin. Supermature quartz arenites of the late Paleoproterozoic–early Mesoproterozoic Baraboo interval (Medaris et al., 2003) occur across Siberia and North America; some of these may represent vestiges of braidplain sandstones that were deposited on the pediment. In eastern Siberia, these may comprise widespread supermature quartz– arenites of the basal Uchur Group (cf. Khudoley et al., 2001; Khudoley and Guriev, 2003). In North America, remnants may include quartz arenites strewn from South Dakota, Montana, and Washington to northern Saskatchewan and the Canadian Northwest Territories; these may comprise parts of the Sioux, Neihart, Steptoe Butte, Athabasca, Thelon, Muskwa, and Hornby Bay assemblages (Stockwell et al., 1970; Southwick et al., 1986; Ross et al., 2001; Medaris et al., 2003). The Chewings Group (Meyers et al., 1996) may be the Australian equivalent.
The sandstones overlie a very smooth, profound angular unconformity that truncates crystalline basement rock ranging in age from Archean to late Paleoproterozoic. The unconformity is here taken to represent the erosional surface of the pediment. The unconformity is bracketed between about 1750 and 1700 Ma in several regions. A paleosol 10–50 m thick commonly mantles the unconformity. In northern Canada, the Thelon paleosol involved rock as young as 1760 Ma, and diagenetic apatite yielded Pb–Pb ages of 1719–1648 Ma (Kotzer et al., 1992). In Montana, the Neihart Quartzite overlies basement rock having a biotite cooling age of 1690 Ma (Hayden and Wehrenberg, 1959). In places, the unconformity merged with earlier planation surfaces, as recorded by the preservation of biotite cooling ages of the Canadian Shield as old as Archean (Stockwell et al., 1970). In many areas, the paleosol is a highly depleted saprolite that retains the texture of the parent basement rock. The Athabasca and Thelon regions exhibit 50–m–thick, mature paleosols at the basement unconformity (Kotzer et al., 1992). Hydrothermal fluids diagenetically altered these paleosols and deposited hematite and uranium– and thorium–rich apatite (Miller et al., 1989).
The arenites are commonly red and trough–crossbedded, and are interpreted to represent continental deposition, mainly by braided–fluvial systems. The compositional maturity of the quartz– ites is partly diagenetic, due to multiple generations of silica cements that replaced chemically unstable clasts and matrix (Cox et al., 2002b; Kotzer et al., 1992). Diagenetic maturation of the quartzites was locally accompanied by phosphatic alteration of the paleosols as well, resulting from hydrothermal circulation within the sediment and soil zone. The Athabasca hydrothermal event is dated at 1477 ± 57 Ma (Kotzer et al., 1992).
A hallmark feature of the sandstones is the occurrence of abundant detrital zircons as young as 1710 Ma, even where they overlie older basement. The detrital–zircon assemblage of the Neihart Quartzite of Montana is dominated by 1.9–1.7 Ga grains, with surprisingly few Archean grains (Ross and Villeneuve, 2003; Mueller et al., 2003), considering the abundance of Archean basement in the region of the deposits. Although there are local 1.9–1.8 Ga zircon sources in the Great Falls tectonic zone (Mueller et al., 2003), the uniform maturity of the sandstones may indicate little admixture of newly eroded local basement grains. In the pediment model, the probable source for the 1.9–1.7 Ga zircons is a 1200–km–wide tract of felsic volcanics and terranes in the Mojave, Yavapai, and Mazatzal provinces that accreted along the southern margin of the Laurentian craton (Hoffman, 1989; Condie, 1993; Karlstrom et al., 2001; Duebendorfer et al., 2001; Tyson et al.,2002).These provinces were denuded to a depth of 15–20 km (Karlstrom and Houston, 1984; Chamberlain et al., 1993).
Similarly, the Penokean orogenic belt and Southern province of Canada underwent metamorphism and plutonism at 1.8 Ga, followed by deep erosion to a low surface by 1.7 Ga (Medaris et al., 2003). The accretionary tract may have formed a drainage divide at the head of the pediment; the Mazatzal, Ortega, and Baraboo quartzites may have been shed southward from the divide into a marginal–marine environment, and were involved in later metamorphism and deformation associated with continued accretion to the southeast (cf. Soegaard and Eriksson, 1989; Cox et al., 2002a; Karlstrom and Bowring, 1993; Medaris et al.,2003).
North of the proposed divide, sediment may have remained in flux across the pediment, undisturbed by tectonic activity until captured by rifting that opened the Belt–Purcell basin. The system is somewhat unusual from a stratigraphic standpoint sediment could stream past a position on the pediment for hundreds of millions of years; the stratigraphic age of a pedi– ment–veneer deposit would correspond to the time at which it was removed from the flux, buried, and lithified. Some of the deposits are kilometers in thickness; in the present model, these represent anomalous parts of the craton that subsided to capture sediment in flux across the pediment.
Tectonic Setting of the Belt–PurcellBasin
The Belt–Purcell basin is the largest Mesoproterozoic basin in western North America, and it has important implications for paleoenvironmental interpretations and paleogeographic reconstructions (Link et al., 1993). Although the Belt–Purcell Supergroup was long considered to be an open–marine miogeoclinal deposit (Harrison, 1972), recent sedimentological, stratigraphic, detrital–zircon, and neodymium–isotope studies favor a deposi– tional setting in an enclosed intracratonic basin with a significant western provenance (Winston, 1986, 1991, 1999; Frost and Winston, 1987; Ross and Villeneuve, 2003). Although Ross and Villeneuve (2003) suggested a collisional tectonic origin, the BeltPurcell basin is generally interpreted as an intracratonic basin rift system. Lydon (2000) proposed that the rift was initiated at about 1510–1485 Ma. Recent U–Pb geochronology indicates that the bulk of the Belt–Purcell Supergroup (Fig. 2) accumulated rapidly in the basin, with the overall accumulation rate declining exponentially with time. The Belt–Purcell basin appears to have shoaled and foundered several times in response to discrete rifting events, which also generated mafic igneous activity (Chamberlain et al., 2003). This paper proposes that the rifting event that opened the Belt–Purcell basin disrupted the pediment, and channeled sediment from the braidplain into the basin, where it accumulated rapidly.
The Belt–Purcell Supergroup is divided into four main units, the “lower Belt–Purcell”, the Ravalli Group, the Piegan Group (cf. Winston, this volume), and the Missoula Group. For the lower units, the western side of the basin was dominated by very thick siliciclastic sediment that was derived from the west, while the northeastern side precipitated some clean stromatolitic carbonates and siliciclastic sediment that was derived from the east (Winston, 1986). The early history of the basin can be viewed in terms of a three–armed rift system branching outward from the heart of the basin (Fig. 3). The rifts appear to have funneled sediment into the Belt–Purcell basin from specific regions identified by ages of detrital–zircon grains within Belt–Purcell sandstones, recently presented by Ross and Villeneuve (2003). In the present model, one sediment source comprised the pediment braidplain, represented locally by the Neihart Quartzite, with prominent detrital–zircon age peaks at 1.7–1.9 Ga and scattered Archean grains (Ross and Villeneuve, 2003; Mueller et al., 2003). A second source comprised the source region for detrital zircons corresponding to the North American magmatic gap, 1610–1490 Ma, possibly Australia (Blewett et al., 1998; Ross and Villeneuve, 2003). These exotic detrital zircons characterize the siliciclastic wedges that entered the basin from the west, but they appear to be absent in eastern facies of the basin (Ross and Villeneuve, 2003). A third source correlates with the 1.47–1.37 Ga Granite– Rhyolite Province (Van Schmus and Bickford, 1993), which was syndepositional with the Belt–Purcell Supergroup. A fourth corresponds to local basement sources that may have been exposed on fault scarps. The Missoula Group records a shift in provenance to the south and a loss or diminishment of exotic grains (Ross and Villeneuve, 2003; Link and Fanning, 2003; Link et al., this volume). In the present model, the shift reflects reorganization of the rift system during the early stages of Missoula Group deposition.
A detailed Laurentia–Siberia reconstruction (Sears et al., 2004) juxtaposes the Belt–Purcell basin of western North America and the Udzha–Khastakh basin of northeast Siberia. The reconstructed Udzha–Belt–Purcell rift basin is triangular, bounded by northeast–, east–, and southeast–trending extensional faults.
The northeast–trending Udzha–Moyie rift system followed a segment of a Paleoproterozoic orogenic belt. Figure 3 links these faults with the Sette–Daban trough of southeast Siberia (Khudoley et al., 2001). Sub–basins along the Moyie system were active during injection of a 1469 Ma sill and coeval exhalation of the Sullivan bedded–sulfide deposit (Hoy et al., 2000). The facies of the syndepositional Udzha trough contrast markedly with those in neighboring parts of northeast Siberia (Sears et al., 2004).Lower Riphean strata thicken into the Sette–Daban trough (Khudoley et al., 2001).
The east–trending Khastakh–Perry rift system defines the southern edge of the Khastakh trough in northeast Siberia and the southern edge of the Helena embayment of the Belt–Purcell basin in Montana. The Khastakh trough abuts the Udzha fault system. A syndepositional normal fault of the Perry system was injected by sedimentary breccia that rose to erupt into mud volcanoes at 1469 Ma (Sears et al., 1998). The Perry fault is a prominent syndeposi– tional extensional structure that shed subaqueous debris flows of the Lahood diamictite into the Helena embayment (McMannis, 1965; Link et al., 1993). In the subsurface, the Khastakh–Perry fault system may follow a major basement lineament southeast to Missouri (cf. Marshak et al., 2000). The lineament marks the northern edges of the Gallatin Mountains, the Beartooth Plateau, the Bighorn Mountains, and the Black Hills; Belt Supergroup rocks have been drilled along this trend in the central Montana trough (Mallory, 1972). In South Dakota, the fault system drops the > 1–km–thick, low–grade Sioux Quartzite down to the north against basement rocks (Reed, 1993). In Missouri, the fault system offsets the 1.47–1.45 Ga Granite–Rhyolite Province Reed, 1993).
The main axis of the Belt–Purcell basin trends southeast on a palinspastic map that restores Cordilleran tectonic movements (Price and Sears, 2000). A major syndepositional sill followed the main Belt–Purcell basin axis at 1469 Ma during an episode of tectonic collapse of the basin (Sears et al., 1998). A 1469 Ma dike swarm trends southeastward from the basin to central Wyoming (Chamberlain et al., 2000). Cook and Van der Velden (1993) identified a subsurface Proterozoic rift zone continuing northwest along the Cordilleran trend from the Belt–Purcell basin to the Yukon and Northwest Territories. This rift zone comprises the third arm of the three–armed rift system that centered on the Belt–Purcell basin. It may be conjugate to the Mesoproterozoic component of the Taimyr trough on the Siberian side (Sears and Price, 2003). Its age and sedimentology are not well constrained.
Within a 150–km–wide zone adjacent to and south of a lineament along the southern edge of the Wind River Mountains and the northern edge of the Granite and Laramie Mountains, rocks with Archean zircon Pb–Pb ages and Rb–Sr whole rock ages yield early Mesoproterozoic (1560–1300 Ma) biotite K–Ar ages (Hills and Armstrong, 1974). Peterman and Hildreth (1978) concluded that the dates record uplift and erosion of 10–16 km of crust south of the line during a time generally correlative with deposition of the Belt–Purcell Supergroup. Chamberlain et al. (1993) presented cross sections that indicate approximately 8 km of uplift and denudation at the south end of this zone after 1560 Ma. The 1.44 Ga Laramie layered anorthosite was emplaced at shallow, perhaps nearly eruptive levels (Graff et al., 1982) on the southern edge of this belt (Fig. 4). This constrains most of the uplift to have occurred before 1.44 Ga. Just south of the Laramie anorthosite, the shallow Sherman rapakivi granite was generated in an exten– sional environment at 1.43 Ga (Frost and Frost, 1997).
Facies and Provenance of the Belt–Purcell Supergroup
Disruption of the proposed pediment by the rifts may explain both the facies distribution and the detrital–zircon provenance of the Belt–Purcell Supergroup. The rifts segmented a triangular region of the proposed pediment that appears to have fed the western side of the basin along a tectonic gutter against the Sette– Daban–Udzha–Moyie fault system (Fig. 3). Because the Khastakh– Perry fault system abuts the Udhza–Moyie fault system, displacement on the Udzha–Moyie system produced plunge on the other system. This geometry produced a knickpoint at the entrance to the Udzha basin. A great quantity of sediment may have been eroded from the pediment braidplain of the triangular region and funneled through this knickpoint into the Belt–Purcell basin. Tectonic activity may have shifted the sediment input point back and forth between the Udzha and Khastakh troughs to feed different delta fans during deposition of the western clastic wedges of the Belt–Purcell Supergroup (Sears et al., 2004).
The Prichard (USA) and Aldridge (Canada) formations constitute the lower Belt–Purcell Supergroup in the western part of the basin. These formations have an exposed thickness of 6 km (Cressman, 1989), with an additional 8 km in the subsurface, interpreted from seismic reflection profiles (Cook and Van der Veldon, 1995). They are dominated by deep–water turbidites, with two shoaling–upward cycles. Lydon (2000) calculated an accumulation rate of 57 cm/kyr.
Cressman (1989) divided the Prichard Formation into members A–H, with A at the base of the exposed section in western Montana. Turbidites of members A–D define a delta fan that spread from a point source on the southwest side of the basin (Cressman, 1989). In Figure 3, that source is the Khastakh trough. Member D shoaled into mud–cracked and ripple–marked quartzite of member E. A sample of Prichard E quartzite contains detrital zircons that match the “pediment veneer “population (1.9–1.7 Ga), a new set that matches the Granite–Rhyolite Province (1.47 Ga), and an exotic set that falls within the North American magmatic gap (1.6–1.55 Ga) but matches sources in Australia (Ross and Villeneuve, 2003; Link et al., this volume). At 1469 Ma, a major rifting event deepened the basin, collapsing the Prichard E member into deep water, where it was succeeded by the deep basinal turbidites of Prichard F (Sears et al., 1998). Cressman (1989) estimated that this rift event deepened the basin by 3 km. The deepening was accompanied by syndeposi– tional faulting, sill injection, and eruption of sulfide–rich mud volcanoes of member F (Cressman, 1989). The sills trend northwest, but a major syndepositional fault, marked by sedimentary breccia dikes, trended east, parallel to the Perry–Khastakh trend (Sears et al., 1998). The upper Prichard and Aldridge formed a new delta fan that may have entered the basin from the Udzha trough (Fig. 3).
Lower Belt–Purcell strata on the northeast side of the basin, equivalent to the Prichard and Aldridge formations, include clean stromatolitic carbonates of the Altyn, Waterton, and Newland formations (Winston, 1986). These formations are an order of magnitude thinner than correlative members of the western facies. In the rifted–pediment model, most of the clastic load was baffled in the Prichard and Aldridge turbidites on the upslope side of the basin, with carbonate precipitated on the sediment–starved downstream side.
Along the Helena embayment, the uplifted footwall of the Khastakh–Perry system underwent erosion that may have partly reset biotite cooling ages north of “Giletti’s line” (cf. Giletti, 1966). The footwall scarp shed subaqueous debris flows into the Lahood diamictite (McMannis, 1963). These contain large clasts of basement rock as well as quartz sand likely derived from the pediment veneer. Detrital zircons from the Lahood point to sources in the Paleoarchean basement of the Dillon block south of the Perry line as well as recycled “pediment veneer” (cf. Ross and Villeneuve, 2003). The diamictite thins northward and changes facies into turbidite, the distal ends of the delta fans that entered the embay– ment from the west. Detrital zircons from the Greyson Shale on the northeast margin of the basin exhibit a large recycled “pedi– ment–veneer” peak, plus some Neoarchean grains (Ross and Villeneuve, 2003).
The Fort Steele Formation is the lowest exposed unit of the Purcell Supergroup in southeastern British Columbia. This 800– m–thick, fine–grained, supermature, fluvial quartz arenite has paleocurrents indicating northward flow (Ross and Villeneuve, 2003). Although commonly correlated with the Neihart Quartzite because of its lithology, its detrital–zircon assemblage differs. Bimodal detrital–zircon ages suggest a mix of the late Paleo– proterozoic “pediment–veneer” zircon population (1.9–1.7 Ga) and a local Neoarchean population (Ross and Villeneuve, 2003). In the rifted–pediment model, recycled pediment veneer and underlying basement and paleosol shed the Fort Steele sediment into the rift trough. The Fort Steele Formation indicates that initial rifting was paced by fluvial deposition in this part of the basin.
The Prichard shoaled through members G and H and eventually passed upward into the fluvial–alluvial Ravalli Group. The Revett Formation of the Ravalli Group has a detrital–zircon assemblage virtually identical to that of Prichard E (Ross and Villeneuve, 2003). Winston (2002) interpreted tabular sandstones of the Revett to have been deposited by massive sheet floods entering the Belt–Purcell basin from the west. Runoff events that were funneled into the Belt–Purcell basin along the rift through the knickpoint of Figure 3 could have concentrated such floods.
The Piegan Group (Winston, this volume), formerly called the middle Belt carbonate, is a useful lithologic division within the dominantly clastic supergoup (Harrison, 1972). It has distinct western clastics–rich facies, and eastern carbonate–rich facies. The western facies has a detrital–zircon assemblage very similar to the Prichard E member and Revett Formation, with recycled “pediment veneer”, Granite–Rhyolite, and Australian (?) provenance (cf. Ross and Villeneuve, 2003). The thick western facies contains sedimentary breccias and slumps along a zone parallel with the Perry–Khastakh trend (cf. Wallace, 1999). It may record tectonic foundering following Ravalli Group deposition, although the basin remained within reach of storm waves (Winston, 2003). Zircon from a bentonite layer near the top of the unit yielded a depositional age of 1454 ± 9 Ma (Evans et al., 2000). A mafic sill in the Prichard Formation dated at 1456.8 ± 2.7 Ma (Sears et al., 1998) and others dated to 1455 Ma within the Hauser Lake gneiss of northern Idaho, a probable Prichard equivalent, may signal rifting associated with the basin foundering (Chamberlain et al.,2003). The Piegan Group shoals into the basal Missoula Group Snowslip Formation, signaling a return to terrestrial conditions (Link et al., 1993).
The lower Belt–Purcell formations are laced with mafic sills, some of which intruded and disrupted unlithified sediments (Sears et al., 1998; Hoy et al., 2000). Exposed sills have a composite thickness of two kilometers; much of the seismically imaged lower section comprises additional sills that form bright reflectors (Cook and Van der Veldon, 1995). Rapid sedimentation and the injection of mafic magma may have promoted abnormally hot diagenesis. A minimum of eight kilometers of exposed section, up through the Burke Formation, achieved the biotite zone (Norwick, 1972). The entire supergroup achieved the chlorite zone (Ryan, 1991).
The rift may have channeled runoff from the pediment into a major river; such channelized rivers were otherwise not favored on a barren Proterozoic braidplain. The river may have flowed into a lake impounded in the Belt–Purcell basin (Winston, 1986). Deltaic turbidite fans of the Prichard and Aldridge built out on the upstream end of the lake, while carbonates such as the Altyn dolomite accumulated on the relatively sediment–starved northeast side. The rift–basin lake filled rapidly, because the existing pediment veneer could rapidly erode from large regions to concentrate in the narrow rift. Turbidites passed upward into fluvial sediments upon shoaling of the lake. The fluvial facies may represent times when the basin floor was graded to the equilibrium profile of the pediment. Subsidence of the basin captured sand sheets that may have entered the rift basin as immense sheet floods funneled through the knickpoint from the large drainage basin. When subsidence exceeded the sedimentation rate—for example, in response to renewed rifting—lacus– trine conditions returned. The stratigraphic sequence in the basin may thus record the tectonic pulse of the rift (Fig. 2).
Figure 3 provides a large catchment basin consistent with rapid deposition and the provenance of detrital zircons from recycled “pediment veneer”, new detrital zircon sources in Australia that are not recognized in the pediment veneer, and new sources in the Granite–Rhyolite Province. The restoration (Sears and Price, 2003) juxtaposes Queensland, Australia, with the Sette–Daban trough of southeastern Siberia. Two prominent belts of igneous rocks in Queensland have the age range of exotic detrital zircons found in the western Belt–Purcell clastic wedge (Blewett et al., 1998). Propagation of the rift across the pediment drainage divide may have tapped into the uplands of Queensland and shunted detrital zircons on a 2000 km journey toward the Belt–Purcell basin. While the distance is great, it is significantly less than the 3000 km documented for transport of Grenville detrital zircons across Canada into the Shaler Group (Rainbird et al., 1992).
Zircons correlative with the Granite–Rhyolite Province (Van Schmus and Bickford, 1993) may have been derived from ash falls into the drainage basin. Although only two have been dated, ash beds are more numerous than generally recognized; using detailed clay–mineralogy techniques, Foster (2005) identified numerous K–bentonite beds in the Belt–Purcell Supergroup. Local sources for the zircons are less likely because correlative igneous rocks in the Belt–Purcell basin have mafic compositions that are characteristically zircon–poor.
Sources of clastic sediment on the northeast were limited to a small area adjacent to the rift. All detrital–zircon data reported by Ross and Villeneuve (2003) from eastern–facies samples are consistent with derivation from “pediment veneer” or bedrock within that area. Detrital zircons from the Greyson Formation and the eastern part of the middle Aldridge Formation resemble the Fort Steele Formation, with an Archean group and a recycled “pedi– ment–veneer” group. Distinctive white quartz arenites of the Grinnell Formation of the Ravalli Group of the eastern basin are dominated by recycled “pediment veneer” assemblage. The Siyeh Formation of the eastern facies of the Piegan Group contains Archean plus recycled “pediment–veneer” detrital zircons.
Belt–Purcell Basin Reorganization,Missoula Group
A major reorganization of the Belt–Purcell basin occurred during deposition of the lower part of the Missoula Group (Fig. 4). The sediment source shifted to the south, the potassium– feldspar content increased, grain size increased, granite clasts appeared (Link et al., 1993), and exotic detrital zircons diminished or disappeared (Ross and Villeneuve, 2003; Link and Fanning, 2003). The change may have been triggered by a rifting event recorded by eruption of the Purcell pillow lava at the base of the Shepard Formation at 1443 Ma (Evans et al., 2000). In Canada, the volcanics are associated with an unconformity, syn– depositional grabens, and a basin–deepening episode (Link et al., 1993). The Shepard is locally conglomeratic and contains granitic pebbles dated to 1430–1445 Ma (Ross and Villeneuve, 2003). At 1440 Ma, the distinctive Bonner Formation clastic fan entered the basin from the south (Evans et al., 2000; Winston 1986). At 1370 Ma, an anorogenic igneous outbreak occurred along the western margin of the basin, close to the suggested line of eventual separation of Laurentia and Siberia (Doughty and Chamberlain, 1996), and along the edge of the Udzha trough (Ernst et al., 2000).
The above events can be explained by renewed movement on the fault zones bordering the basin to shunt sediment into the BeltPurcell basin from the southeast and simultaneously cut off the source for exotic zircons from Australia. New uplift and erosion occurred in the southwest, perhaps recording the tectonic unrest. In the Western Granite–Rhyolite Province, granites that intruded between about 1.5 and 1.4 Ga underwent at least 10 km of uplift and erosion before the Southern Granite–Rhyolite Province was em– placed at 1.37–1.35 Ga (Van Schmus and Bickford, 1993). In New Mexico, metamorphic minerals grew in the Ortega Quartzite at pressures equivalent to about 10 km depth at 1.47–1.42 Ga, with slow cooling and gradual unroofing after 1.42 Ga (Williams et al., 1999). Marshak et al. (2000) concluded that regional exhumation stripped 10 km of crust off the surface of this region prior to Cambrian, producing the Great Unconformity noted in the Grand Canyon by Walcott (1914). The entire region from southern Wyoming to central Arizona, central New Mexico, and northern Kansas may therefore have been high–standing and undergoing deep erosion after 1.42 Ga, and much of it before 1.37 Ga.
The Mount Shields and Bonner formations are dominated by recycled “pediment–veneer” detrital–zircon ages, with some Archean grains (cf. Ross and Villeneuve, 2003). Correlative rocks in central Idaho that appear to have been deposited from 1440 to 1410 Ma have a similar detrital–zircon distribution, with a large percentage of 1640–1780 Ma detrital zircons (recycled “pediment veneer”) and a few 2400–2600 Ma (Link et al., this volume). These rocks also contain syndepositional 1440–1410 Ma detrital zircons, indicating connection with the Granite–Rhyolite Province, either through ash airfall into the drainage basin, or fluvial linkage. A small population of 1540–1640 Ma, non–North American, grains may indicate recycling or some residual connection with Australian sources. In Canada, the Mount Nelson Formation of the Missoula Group has detrital zircons consistent with recycled “pediment veneer” (cf. Ross and Villeneuve, 2003), indicating that the eastern margin of the basin continued to receive clastic input from the east.
The rift configuration of Figure 4 may have remained stable through deposition of the Garnet Range Formation near the top of the Belt–Purcell Supergroup. At 1370 Ma, anorogenic rapakivi granites and a mafic complex intruded the western side of the Belt–Purcell basin near the proposed eventual line of separation
between Laurentia and Siberia (Doughty and Chamberlain, 1996; Evans et al., 2000), while at the same time dikes intruded along the Udzha trough (Ernst et al., 2000). The Garnet Range Formation records basin deepening (Link et al., 1993). It contains a significant recycled “pediment–veneer” peak, minor Archean grains, 1436–1378 Ma detrital zircons, as well as micas dated to 1370 Ma (cf. Ross and Villeneuve, 2003). This may indicate that the fault zone had propagated into the Southern Granite–Rhyolite Province, although the 1378 Ma grain could represent airfall–ash zircon from Southern Granite–Rhyolite Province. The uppermost layer of the Belt–Purcell Supergroup, the Pilcher fluvial sandstone, may record reequilibration of the basin to the stable pediment profile. The Belt–Purcell basin appears to have then remained static, with no record of sediment accumulation until Neoproterozoic Windermere rifting (Link et al., 1993).
Summary and Conclusions
The rifted–pediment model discussed above explains details of the sedimentological history of the Mesoproterozoic BeltPurcell basin in the context of published detrital–zircon data. The Siberia–Laurentia–Australia paleocontinental troika places the basin in the heart of a large craton (Sears and Price, 2003). An epicontinental pediment may have sloped northwest toward the future site of the basin from a 1.9–1.7 Ga accretionary orogenic belt. A veneer of sediment in flux across the pediment carried a detrital–zircon assemblage dominated by grains from the denuding tectonic highlands.
A three–armed rift propagated outward from the basin beginning at about 1500 Ma and captured the sediment load from two triangular regions of the pediment. The southern triangle swept sediment into clastic fans on the west side of the basin. It had sources in southwestern Laurentia and northeastern Australia. The northeastern triangle provided less sediment to the east side of the basin, allowing deposition of clean carbonates. Its detrital– zircon assemblage came from the recycled pediment veneer and local basement. Reconfiguration of the rift system at 1443 Ma changed the provenance of the basin, cutting off the Australian source and tapping sources in the Central Plains.
The model can be tested with further detrital–zircon analysis of strata here assigned to the pre–Belt–Purcell pediment veneer and strata in the Belt–Purcell Supergroup.
Many thanks to Paul Link, Darryl Cowan, Gerry Ross, and Colin Shaw for valuable reviews and specific suggestions that helped to clarify some concepts presented in an earlier version of the manuscript. A discussion with Ted Doughty many years ago first sparked my interest in the significance of Proterozoic pediments. Discussions and numerous field trips with Don Winston and Dave Alt helped frame the ideas. This research was partially funded by National Science Foundation Grant EAR–0310186.
Figures & Tables
Proterozoic Geology of Western North America and Siberia
This volume is a compendium of research on the Belt Supergroup. It is an outgrowth of Belt Symposium IV, held in Salmon, Idaho, in July, 2003, in conjunction with the Tobacco Root Geological Society annual field conference. Because of the geographic extent and great thickness of the Belt Supergroup, years of work have been required before conclusions are “bona fide”. The Mesoproterozoic Belt Supergroup of western Montana and adjacent areas is geologically and economically important, but it has been frustratingly hard to understand. The previous Belt Symposium volumes offer an historical view of the progress of the science of geology in the western United States. The advent of U-Pb geochronology, especially using the ion microprobe (SHRIMP) and laser-ablation ICPMS, has injected geochronometric reality into long-standing arguments about Belt stratigraphy. Several papers in this volume utilize these new tools to provide constraints on age and correlation of Belt strata (Chamberlain et al., Lewis et al., Link et al., and Doherty et al.)