Braid Plain Research Articles

Early Proterozoic gold placers of the Moeda Formation within the Gandarela Syncline, Minas Gerais, Brazil

Gold-bearing placer conglomerates, dated between 2.2 and 2.8 Ga, have been identified at the base of the Moeda Formation in the Gandarela syncline. Intensive investigation of outcrop and underground exposures, both old and new, and borehole intersections around the western and southern margin of the syncline, has defined a paleoenvironment consisting of an upland fluvial drainage system in the north, leading southward to an alluvial fan and braided plain depository. Heavy mineral concentrates, composed predominantly of coarse rounded pyrite, are concentrated in the matrix of cobble conglomerates that occur in the form of discrete bars and sheets. The mineralogy and geochemistry of the ore closely resembles that of Witwatersrand placers as does the composition of the gold. Gold concentrations are equal to those of the average Witwatersrand gold deposit and therefore these deposits may make a significant contribution to Brazil9s future gold production.

Braided Fluvial to Marine Transition: The Basal Lower Cambrian Wood Canyon Formation, Southern Marble Mountains, Mojave Desert, California

ABSTRACT The Lower Cambrian Wood Canyon Formation, recognized over a 39,000 km2 belt through the southern Great Basin and Mojave Desert, was examined in the southern Marble Mountains, eastern Mojave Desert, southeastern California. There it crops out as a well-exposed 90-130 m thick, feldspathic sandstone nonconformably overlying Proterozoic basement. Previous investigators have suggested a tide-dominated, nearshore, shallow marine origin for the entire unit in the eastern Mojave, based on broad-scale stratigraphic trends and localized fossil occurrences in the upper member of the formation. In contrast to a regional approach, the present study uses a detailed lithofacies analysis in a smaller study area to suggest that a fluvial to marine transition occurs within these strata. The middle and upper members of the Wood Canyon Formation are recognized in the southern Marble Mountains and are divisible into six distinct lithofacies that possess gradational contacts where exposed: cross-stratified sandstone (Facies A); mediumpebble conglomerate (Facies B); planar-stratified sandstone (Facies C); sandstone and thin mudstone (Facies D); mudstone with intercalated sandstone (Facies E); and horizontally laminated siltstone (Facies F). Facies A-D, contained within the middle member, comprise the bulk of Wood Canyon sediments and record the depositional characteristics of a transition zone from a distal alluvial braid plain to a tidally influenced, fluvial-dominated braid-delta complex. Cross stratification in the braid-plain sediments displays a strongly unimodal pal ocurrent pattern, suggestive of downstream-migrating sinuous-crested megaripples. Braid-delta sediments, however, show a more dispersed paleocurrent pattern and contain a sparse trace fossil assemblage. A change from fluvial-dominated to marine-dominated processes is recorded in upper member rocks (Facies E and F). Facies E contains many of the traits characteristic of a low- to mid-tidal flat sequence, such as coarsely interlayered bedding, flaser and lenticular bedding, microripple marks, and locally abundant Rusophycus and Planolites trace fossils. Facies F, which dominantly consists of horizontally laminated siltstone, is interpreted as the product of an estuarine lagoon that developed behind and within a discontinuous barrier of stranded braid bars.

Turbidites and iron formations, Beardmore‐Geraldton, Ontario: application of a combined ramp/fan model to Archaean clastic and chemical sedimentation

ABSTRACTSandy turbidites, grain flows, conglomeratic mass‐flows and oxide‐facies iron formation are present in the late Archaean Beardmore‐Geraldton terrain, a metasedimentary belt which extends for at least 80 km in an E‐W direction. The marine portion of this basin contains four lithofacies associations (LA): (1) Thinbedded, iron formation‐clastic sediment association. This association represents a continuum of deposit types containing iron formation; subtypes are defined on the basis of bedding attributes and the proportion of iron formation to sand/silt. (2) Thin‐bedded, turbidite‐dominated association. These sediments consist mostly of silt/sand beds which either show no vertical trends, or thin and fine upwards over a few metres. (3) Medium‐bedded, turbidite‐dominated association. Most of these sediments are medium to coarsegrained, vertically unstructured sand sequences with occasional structured intervals. (4) Thick‐bedded association. This is dominated by poorly graded sands up to 7–8 m in thickness. Sand beds are characterised by a thin basal zone of coarse sand and pebbles, a large central interval containing a mixture of medium and coarse sand, and a thin upper zone of fine sand/silt.The overall depositional system was initiated by transport of sediment by braided streams to the strand area where it accumulated in distributary mouth bars. We infer a nearshore break in slope, locally with large channels (LA 4) extending from close to the strand line across deltaic surfaces to the deeper portions of submarine fans (structured portions of LA 3). However, many deltaic surfaces probably were not tapped by major channels, but merged downslope into a submarine ramp. Sediment was transported across the ramp by slump events and sheet‐like grain flows (unstructured portions of LA 3). Iron formation and LA 2 sediments probably accumulated both in upper‐mid ramp areas with low sediment delivery rates, and distal to fan‐ramp successions.As major streams on the braid plain changed position, associated submarine channels and slump‐fed ramp deposits also would have shifted laterally. This produced overlap of different facies associations in both the fan and ramp environments, which may explain why observed vertical trends in bedding are limited to several metres. We suggest that on narrow, active Archaean cratonic margins, additional non‐regular processes such as variations in sediment supply related to periods of heightened pyroclastic activity, and seismic activity associated with the arc, also contributed to the lack of vertically structured sequences. Turbidite sequences in such environments in general may contain important contributions from both submarine ramp and submarine fan sedimentation.

"Drainage systems" associated with mid-ocean channels and submarine yazoos: Alternative to submarine fan depositional systems

Submarine drainage associated with mid-ocean channels and Yazoo River-type tributaries in small ocean basins represent a contrast to deep-sea fan depositional systems. Deep-sea fans are diverging sediment-dispersal systems of distributary fan valleys. Deep-sea channel-submarine-yazoo systems, on the other hand, form centripetally converging patterns of tributaries and yazoo-type satellite channels that join a major basin-draining (mid-ocean) channel. The facies model for such systems is characterized by randomly stacked fining-upward, gravelly, and sandy channel-fill and submarine point-bar sequences of the main channel encased in fine-grained overbank deposits. Second-order channels contain sandy proximal overbank deposits, whereas the levees of the main channel are predominantly composed of silt and clay. Second-order channels may be braided and may broaden into braid plains. Morphology and surficial sediment distribution have been studied within the Northwest Atlantic Mid-Ocean Channel of the Labrador Sea and its associated levees and yazoo-type (and other) tributaries.

Marginal marine sedimentation in the basal alpine Buntsandstein (Scythian) in the western part of the Northern Calcareous Alps (Tyrol/Salzburg, Austria)

The beginning of the Scythian Alpine Buntsandstein marks a transitional change in sedimentation style at the P/T-boundary. In the investigated area the continental Permian Gröden Formation is overlain by marginal marine sandstones of the Alpine Buntsandstein, as indicated by their sedimentary structures. Shallow subaquatic conditions alternated with emergence phases, pointing to a microtidal environment. During long-term supratidal phases pedocretes developed. They indicate a semi-arid climate at least at the beginning of the Alpine Buntsandstein sedimentation. South-eastward progradation of the coastline produced sandstones of a fluvial braided plain. This advance of the fluvial system was caused by a shift to more humid climatic conditions. New sulphur isotope data from gypsum support the assignment of the Alpine Buntsandstein into the Scythian, and indicate a Late Permian age for the underlying shales of the Gröden Formation.

Fluviatile evidence for a period of late-quaternary pluvial climate in coastal southeastern Australia

The Cranebrook Terrace near Sydney, New South Wales, yields sound evidence of a period of exceptional fluvial activity in lowland basins of southeastern coastal Australia well prior to the last glacial maximum. This episode, titled here the “Cranebrook Pluvial”, is dated from radiocarbon and thermoluminescence age determinations as ending about 40,000–45,000 yr B.P. It was characterised along the lower Nepean River by abundant gravel transported over a braid plain proximal to confining sandstone gorges. Thick basal gravels were deposited almost contemporaneously with a sandy-clay overburden until this time when the river quite quickly became confined to two laterally stable channels. Additional overbank deposition probably accompanied this stable phase. The easternmost channel was abandoned about 34,000–37,000 yr B.P. and the Nepean River appears to have occupied only the western channel since then. Isolated from eustatic changes occurring downstream, the Nepean River has left no significant alluvial deposits from the last glacial maximum or from the Holocene, periods of time which, in this fluvial environment, appear quiescent by comparison with preglacial pluvial. However, stripping and replacement of the fine overburden appears to have occurred over the western part of the terrace about 14,000 yr B.P., or somewhat younger from 14C determinations. These conclusions are at odds with the proposal that the main period of alluvial deposition in coastal valleys of southeastern Australia was triggered by widespread slope instability during the last glacial maximum. Alluvial and pollen data suggest the glacial was a cool dry period of relatively little fluvial activity in basins not affected by glaciation. The Cranebrook Pluvial was probably the most important erosional-depositional episode in the non-glacially effected areas of southeastern Australia in the last 50,000 years.

Styles of Alluvial Fan - Braid Plain Sedimentation in Orogenic Foredeeps -- Examples from the Canadian Cordilleran Orogen

ABSTRACT Alluvial deposits are an important component of orogenic foredeeps because they indicate periods of rapid uplift and erosion of the orogen, and proximity to the leading edge of the deformed belt. Stratigraphic and sedimentologic studies of three examples from the Canadian Cordillera: the Pocaterra Creek Member (Columbian Orogeny of southern Alberta), the Summit Creek Formation (Laramide Orogeny of Fort Norman area), and the Monster Formation (probably Laramide Orogeny of west-central Yukon) show a remarkable similarity in lithofacies despite large temporal and spatial separations. The over-all similarity in depositional processes is also reflected in the common forms of cyclicity: thick, allocyclic coarsening- and fining-upward sequences that record major changes in uplift, rapid basin subsidence and sediment supply, and autocyclic sequences that reflect more local facies changes. Conglomerate and sandstone facies were deposited by low-sinuosity braided streams in the upper braid plain - fan environments. Down-slope changes in facies are preserved in the Summit Creek and Monster formations; they record increased channel sinuosity and, in some cases, meandering streams and point bars. Facies that are unique to each alluvial sequence include red beds and caliche in the Pocaterra Creek Member, coal seams in the Summit Creek Formation, and a distinct marine shelf to nonmarine transition in the Monster Formation that includes hummocky crossbedded sandstone. Thus, three different alluvial settings are envisaged: the Pocaterra, representing a semi-arid braid plain and containing possible lacustrine deposits, that acted as a local precursor to the more extensive Cadomin Formation and prograded eastward into a foredeep of the Rocky Mountain foreland basin; a humid Summit Creek fan - braid plain system that accumulated within a foredeep of Peel Trough; and the Monster coastal fan system that, following an earlier Cenomanian transgression, prograded into a foredeep situated along the northern rim of the Cordillera. DES STYLES DE SEDIMENTATION DE CONE DE DEJECTION PLAINE ANASTOMOSEE DANS DES AVANT FOSSES OROGENIQUES -- DES EXAMPLES DE L'OROGENE CORDILLERE AU CANADA RESUME Des alluvions sont un composant important des avant-fosses orogeniques car elles signalent des periodes de soulevement rapide et d'erosion de l'orogene, et proximite du front de la zone deformee. Des etudes stratigraphiques et sedimentologiques de trois exemples dans la Cordillere Canadienne ont etabli un rapprochement frappant dans le lithofacies malgre de grandes differences temporales et spatiales. Le membre Pocaterra Creek (orogenese columbienne de l'Alberta du Sud), la formation de Summit Creek (orogenese laramienne de la region de Fort Norman) et la formation Monster (probablement orogenese laramienne du Yukon ouest-central). Les ressemblances generales dans les procedes stratigraphiques sont evidentes aussi dans les formes communes de cyclicite; des sequences epaisses allocycliques grossieres et affinees vers le haut, qui enregistrent d'importants changements de soulevement, d'effondrement de bassin et de disponibilite de sediments et des sequences autocycliques qui refletent des changements de facies plus locales. Des facies de conglomerat et de gres furent deposes par des ruisseaux anastomoses a faible sinuosite dans les milieux de plaines anastomosees superieures-cones de dejection. Des changements de facies de l'aval pendage sont conserves dans les formations de Summit Creek et Monster et montrent d'avantage de sinuosite dans les chenaux et parfois des fleuves a meandres et des barres de meandre. Certains facies sont uniques a chaque sequence alluviale et sont: des couches rouges et du caliche dans le membre de Pocaterra Creek, des couches de carbon dans la formation de Summit Creek et une nette transition entre plateforme marine et continentale dans la formation de Monster, qui comprend du gres qui presente une stratification entrecroisee du type Hummocky. Donc trois milieux alluviaux sont envisages; la formation Pocaterra represente une plaine anastomosee semi-aride avec des depots vraisemblablement d'origine lacustre, qui ont annonce localement la plus vaste formation Cadomin et qui s'avancerent dans une direction est vers la mer dans un avant-fosse du bassin d'avant-pays des Montagnes Rocheuses; un systeme de cone de dejection-plaine anastomosee humide a Summit Creek qui s'accumula dans un avant-fosse de Peel Trough; et le systeme de cone de dejection littoral Monster, qui, suivant une transgression anterieure Cenomanienne, progressa dans un avant-fosse situe sur l'arete nord de la Cordillere. Traduit par Joan Lovell End_Page 1--------------------------

Sedimentology of Basal Potsdam Sandstone in Adirondack Border Region, New York, Southeastern Ontario, and Southwestern Quebec: ABSTRACT

Field evidence supports the relatively widespread presence of nonmarine facies within the basal Potsdam formation of the Adirondack border areas of northern New York, southeastern Ontario, and southwestern Quebec. Detailed observations of areal extent, analysis of sedimentary structures and paleocurrent directions, and petrographic studies have been combined with the paleomagnetic determination of the temporal relationships of these strata to establish depositional patterns and facies trends within basal Potsdam units. Four distinct nonmarine lithofacies have been identified: massive matrix-supported conglomerate, stratified framework-supported conglomerate, conglomerate-arkose, and pebble conglomerate-arkose fining-upward sequences, interpreted to represent debris flows, proximal gravelly braided-stream deposits, intermediate to distal gravelly braided-stream deposits, and proximal sandy braided-stream deposits, respectively. Facies of eolian or possibly tidal, and shallow marine origin have also been identified at the base of the Potsdam sequence. Most basal Potsdam sediments are compositionally and texturally immature, derived directly from the crystalline detritus of the extensively weathered Precambrian surface and regoliths that may have locally developed on the craton. The desert-like environment of the Precambrian surface allowed for rapid transport and deposition of relatively unweathered sediments and the subsequent construction of a braided alluvial plain system. Field relations and evidence derived from consideration of the paleomagnetic properties inmore » five localities of fine-grained alluvium suggest that terrestrial Potsdam deposition in the Early and Middle Cambrian largely preceded the marine transgression that deposited the thick, shallow marine units typifying most of the Potsdam sequence.« less

Sedimentological Evolution of Mississippian Kekiktuk Formation, Sagavanirktok Delta Area, North Slope, Alaska: ABSTRACT

The reservoir interval in the Endicott field, located under the delta of the Sagavanirktok River east of Prudhoe Bay, is the Mississippian Kekiktuk formation. The rocks are quartzose, and lithofacies and sequence analysis of core material shows that the Kekiktuk in that area can be subdivided into three sedimentologically distinct intervals, each of which reflects a different fluviatile depositional environment. The lowermost interval (zone 1) rests directly on metamorphic basement and comprises interbedded coal, mudstones, siltstones, and fine-grained sandstones. These rocks were deposited in a very low-lying swamp plain containing local lakes and sluggish, highly sinuous streams. Zone 1 is overlain sharply by zone 2, which comprises medium to coarse-grained, multistory andstone that was deposited within an unconstrained braided river system. The braid plain contained both ephemeral and permanent lakes, and periodic gravity flows deposited coarser sediment into the latter. That sequence passes gradationally upward into zone 3, which is composed of coarse-, medium-, and fine-grained sandstones, as well as siltstones, mudstones, and coals. The lower part of zone 3 is dominated by upward-fining sandstone sequences, interpreted as channel bars deposited within a moderately sinuous fluvial system. They pass gradationally upward into other, distinct upward-fining sequences, which differ in having higher proportions of siltstone, mudstone, and coal. Those rocks were deposited in a more highly sinuous (meandering) fluvial environment. The rocks grade upward int shallow marine sediments of the Kayak/Itkilyariak formations. The vertical sequence within the Kekiktuk in this area permits an interpretation of the structural history of the basin during Kekiktuk time. End_of_Article - Last_Page 669------------

Sedimentation in Pull-Apart Basins: Active Examples in Eastern Turkey

The Erzincan, Susehri, Niksar, and Lake Hazar basins are actively evolving pull-apart basins in eastern Turkey. Their structural, drainage, and depositional systems generally support a pull-apart basin sedimentation model based on ancient pull-apart basin deposits. This model involves marginal coarse-grained sedimentation adjacent to master faults, axial fine-grained sedimentation away from master faults, and high sedimentation rates. However, studies of the Turkish pull-apart basins suggest the following refinements in the model. (1) Detailed sedimentary facies patterns and the dominant facies of a pull-apart basin vary according to basin shape, climate, drainage character, and volcanic activity. Depositional environments near fault-bounded margins include alluvial fan, fan delta, and alluvial apron. Depositional environments in central areas away from fault-bounded margins include active braid plain, inactive braid plain, meander plain, salt playa, marsh, lacustrine, mouth-bar delta, and volcanic. (2) If internally drained, a pull-apart basin is likely to contain axial and transverse drainages. If externally drained, the drainage pattern varies and may exit the basin axially or transversely. (3) Pull-apart basin shapes are not necessarily rhomboidal. Comparison of data from the Turkish pull-aparts with other modern and ancient pull-apart basins suggests that sediment thickness (y) is related to length of a pull-apart basin parallel to master fault overlap (x) by the equation y = .08x + .26. This relationship predicts sediment thickness in the Erzincan, Susehri, Niksar, and Lake Hazar pull-apart basins is 2.9, 2.1, 1.5, and 0.68 km, respectively.

Tectonic Significance of Depositional Patterns in Nonmarine North Horn Formation, Central Utah: ABSTRACT

The Maestrichtian through Paleocene(?) North Horn Formation is a nonmarine sequence of interbedded and intertonguing synorogenic conglomerate, sandstone, algal limestone, and claystone. Deposition occurred within an interior drainage system restricted by the Sevier highlands to the west and the San Rafael swell to the east. It extended south to at least Salina Canyon in central Utah and north to the southern margin of the Uinta basin. The base of the formation is diachronous, unconformably overlying Campanian rocks. Earliest deposition began along the eastern Wasatch Plateau, then shifted westward until late Paleocene beds onlapped deformed Mesozoic rocks associated with Sevier thrusting. Paleocurrent measurements indicate a general west to east transport direction. Four major depositional facies are recognized: alluvial fan, proximal braid plain, distal braid plain, and lacustrine. Work has been concentrated on the alluvial fan and braid plain facies west of the Wasatch Plateau. Massive clast-supported cobble to boulder conglomerate characterizes alluvial-fan facies. Deposition on fan surfaces was primarily by sheet-flood or braided stream processes. Conglomerate shows skewed clast populations. Northeast of Mt. Nebo, 60-70% of clasts are carbonate rocks. Clasts range up to 1 m (3 ft) in diameter. Thick Paleozoic carbonate beds associated with the Nebo fold nappe furnished most of them. Upward-fining sequences of massive to crudely stratified clast-supported conglomerate, conglomeratic sandstone, and sandstone characterize proximal and distal braid plain facies. During transport down the braid plain, mixing of Eocambrian and Cambrian quartzite, Paleozoic carbonate, and Mesozoic sandstone occurred. Conglomerate was deposited in bars within braided stream channels. It displays imbrication of flattened clasts with long axes transverse (^cong40-70°) and intermediate axes subparallel to flow direction. Sandstone displays poorly developed cross-stratification, planar stratification, and massive bedding. Rapidly fluctuating episodes of sedimentation with high sediment loads on the proximal braid plain, and extensive bioturbation by infauna on the distal braid plain, ar indicated. End_of_Article - Last_Page 953------------

A spectrum of alluvial deposits in the Lower Carboniferous Bonaventure Formation of western Chaleur Bay area, Gaspé and New Brunswick, Canada

The Lower Carboniferous Bonaventure Formation of western Chaleur Bay, Gaspé and New Brunswick, is a terrestrial redbed succession with abundant calcretes, deposited in a semi-arid paleoclimate. Facies can be grouped into three associations, conglomeratic, sandstone, and mud-dominated, within two 100–150 m upward-fining megasequences. The megasequences are attributed to alluvial fan progradation due to tectonic rejuvenation.Vertical facies relationships and internal structures indicate that varied alluvial environments are represented. Alluvial fans formed on steep slopes adjacent to fault scarps and are dominated by deposits of the conglomeratic association. Lateral and downslope coalescence of fans into a braid plain is represented by transition from the conglomeratic to the sandstone facies association. Distally, the braid plain is transitional into deposits of the mud-dominated association.Paleocurrents and clast compositions show that sediment in the Gaspé outcrops was derived from the northwest, and that in New Brunswick from the southwest. This indicates that Chaleur Bay is an exhumed Carboniferous paleovalley, with axial drainage to the east.

Gas Reservoirs, Deep Basin, Western Canada

Introduction The "Deep Basin" straddles the Alberta - British Columbia border in front of the over thrust belt in the western edge a/the Alberta Sedimentary Basin, covering some 26,000 square miles (67,000 km2). Its geology is complex: at least 18 different zones from the 15,000-ft (4600-m) section of clastic rock, ranging in age from Permian to Upper Cretaceous. These multiple stacked gas bearing reservoir sandstones are found downdip of strati graphically equivalent water-bearing zones. The "Deep Basin" consists of two basic reservoir types: conventional and tight. Conventional reservoirs consist of medium-grained sandstones to conglomerates with porosities of 8% to 12% and permeabilities in the millidarcy to darcy range. Unstimulated flows in the range of 1 million to 28 million cubic feet per day are common (28,000 m3/d to 78,000 m3/d). The Spirit River Formation, a conventional reservoir, is a thick clastic sequence composed of a series of transgressive marine and regressive coastal/deltaic cycles. The major hydrocarbon reservoirs are mature, pea-sized openwork conglomerates deposited on the beach face during either an initial transgressive event or a regression of a shoreline. Tight reservoirs or low-permeability fine-grained sandstones (generally less than 100 microdarcies), have porosities In the 4% to 7% range with natural flows of 100 to 750 thousand cubic feet per day (2,800 to 21,000 m3/d). The Nikanassin Formation, a tight reservoir is a sequence of sands, shales and siltstones and minor coals that thin from west to east. The Upper Nikanassin sands are composed of fine- to medium-grained quartzose sandstones with abundant chert and carbonaceous particles and were deposited on a distal braided plain. The main characteristic that distinguishes a tight gas sand reservoir from the more common conventional type is its extremely low permeability, requiring a massive hydraulic fracture stimulation for sustained commercial development. Through fracturing and other technological advances, significant quantities of gas from various tight horizons will be delineated and the "Deep Basin's contribution to Canada's natural gas reserves will be recognized and produced. Introduction The largest natural gas field in North America was discovered in 1976 in the "Deep Basin" of Alberta. Many aspects of this area have already been discussed in previous talks at other conventions. John Masters of Canadian Hunter(1) in 1978 postulated that the Deep Basin could contain 440 trillion cubic feet of recoverable natural gas, almost seven times the amount then recognized as recoverable in Alberta and British Columbia. Dave Sandmeyer(2) and Richard Flury(3) of Amoco Canada have discussed the evolution of the word "resource" as distinct and separate from reserves when discussing recoverable natural gas. However, both recognized the importance of the Deep Basin play. The Deep Basin is located to the east of the disturbed belt along the front of the Rocky Mountain Foothills. It's major portion as shown on Figure 1, is approximately 430 miles (692 km) long and encompasses approximately 26,000 square miles, (6,734,000 hectares). The area outlined indicates Amo

An Alluvial Plain and Lacustrine Model for the Precambrian Witwatersrand Deposits of South Africa

ABSTRACT A depositional model is presented for the gold-rich Precambrian deposits within the Witwatersrand basin. It is a closed basin inset in a now stable craton of the southern African Shield with an areal extent of approximately 39,000 km2 The clastic fill has a maximum thickness of 7,500 m and comprises laterally persistent conglomerates, sandstones and shales occurring within thick (up to 800 m), widespread, and continually alternating coarser and finer-grained sequences. The succession as a whole coarsens upward and is divided into a sandstone and shale dominated lower division and a sandstone and conglomerate dominated upper division. Mineralogical maturity also increases upward, the sandstones of the lower division being mainly sub-greywackes, those of the upper division be ng feldspathic quartzites. Analysis of the abundant sedimentologic data gathered during exploration and development of the gold deposits demonstrates that sedimentation within the basin fits a braided alluvial plain and lacustrine model. The origin and lateral relationships of the different rock types are explained by this depositional setting. The thick alternating coarser and finer grained sequences are explained by a shifting depositional system (sourceward and basinward) controlled by tectonism in the basin and source area. The upward increase in mineralogical maturity and grain size is explained by migration inward of the basin margins, contemporaneous with sedimentation, causing marginal reworking of the lower division sequence as well as regression of the sedimentary succession. The lower division become a distal environmental equivalent of the upper division.

Sediments and Bed Forms on a Braided Outwash Plain

Sediments and bed forms from three braided outwash plains and one steep non-braided mountain stream in the Rocky Mountains of Alberta are compared to sediments and structures exposed in a paleo-outwash sequence.Channel sediments are subdivided into (a) pool facies: ripple- and parallel-laminated silty sand, with current crescents around isolated stones; and (b) riffle facies: parallel-stratified gravel and silt or silty sand.'Transverse ribs' constitute a bed form typical of riffle portions of channels. This bed form occurs as a train of regularly spaced pebble, cobble, or boulder ridges oriented transversely to the flow. Locally, the pebbly ribs are separated by, and rest on, a stone-free silty substratum. It is suggested that transverse ribs constitute a common equilibrium bed form in higher gradient riffle portions of gravelly alluvium.Longitudinal bars dominate interchannel areas of the modern outwash plains. They consist of coarse, poorly sorted gravel, either massive or exhibiting crude horizontal stratification. The paleo-outwash, composed of finer-grained and better sorted gravel, is characterized by sets 5 to 30 cm thick of cross-stratified sand and by sets 75 to 82 cm thick of cross-bedded pebble gravel. These may be related to progradation of dunes and bars, respectively.Standard deviations of channel azimuths in modern outwash plains are appreciably less than that recorded by cross-stratification measurements in the time-transgressive sequence of paleo-outwash. This is partly due to the braided system of channels having swept from side to side across the plain through time.