Cobble Conglomerates Research Articles

Upper Cretaceous alluvial fan deposits in the Jianglangshan Geopark of Southeast China: implications for bedrock control on Danxia landform evolution

The Jianglangshan Geopark in the western Zhejiang Province of Southeast China is well-known for its spectacular red-colored sandstone landforms. Little is known about the depositional processes of the conglomerate-dominated Fangyan Formation, the lithologic base of the Danxia landforms in this geopark. Based on detailed field investigation of lithology, sedimentary structures, bed thickness and geometry, five facies are recognized: facies A (matrix-supported cobble conglomerate), facies B (pebble conglomerate), facies C (pebbly sandstone), facies D (fine-grained sandstone) and facies E (mudstone). The results show that streamflow-dominated fans were the main depositional processes responsible for the accumulation of the Fangyan Formation along the mountain fronts. These fan conglomerates form the base for the evolution of the Danxia landscapes owing to the uplift and erosion of the study area. In contrast, the fine-grained sedimentary successions produced by fluvial floodplains in the distal part of the fans were thinner and more easily weathered. Such sedimentary facies distribution patterns were thought to be similar during Late Cretaceous across Southeast China. The Danxia landforms are largely the geographical expressions of the conglomerate-dominated redbeds in the proximal-middle fans.

Stratigraphy and vertebrate paleoecology of Upper Cretaceous–?lowest Paleogene strata on Vega Island, Antarctica

The Upper Cretaceous (Maastrichtian) Sandwich Bluff Member of the López de Bertodano Formation is well exposed on Vega Island in the James Ross Basin off the northeastern coast of the Antarctic Peninsula. Although this unit is one of the richest sources of end-Cretaceous vertebrate fossils in Antarctica, it is also one of the least sedimentologically and stratigraphically characterized units in the basin. New facies and stratigraphic analyses of the Sandwich Bluff Member and the underlying Cape Lamb Member of the Snow Hill Island Formation were performed in tandem with intensive prospecting for fossil vertebrates and stratigraphic assessment of historic paleontological localities on Vega Island. This effort has led to a revised stratigraphy for the Sandwich Bluff Member and the precise stratigraphic placement of important terrestrial and marine vertebrate fossil localities.Facies analysis reveals a fining and shallowing upward trend through the section that culminates in a newly recognized sequence boundary near the top of the Sandwich Bluff Member, followed by the deposition of a previously unrecognized, 6m-thick, matrix-supported pebble–cobble conglomerate of probable alluvial origin. Immediately overlying this unit, well-developed Thalassinoides burrow networks in fine-grained transgressive sandstones and siltstones indicate a rapid return to marine conditions. A similar stratigraphic pattern is well documented at the top of the López de Bertodano Formation and the base of the overlying (Paleocene) Sobral Formation on Seymour Island in the southern part of the basin. Although no fossils were recovered to constrain the age of the upper 10–15m of the succession on Vega Island that preserves the newly recognized upper sequence boundary, strata below this level can be confidently placed within the Manumiella bertodano interval zone, which extends to a short distance below the K–Pg boundary on Seymour Island. Hence, based on sequence stratigraphic and lithostratigraphic evidence, the uppermost 10–15m of the succession on Vega Island may encompass the Cretaceous–Paleogene boundary together with a few meters of the Paleocene Sobral Formation.

Procesos gravitacionales-fluviales y preservación de vegetación boscosa en un área volcánica: Formación Rancahué (Oligoceno), Andes neuquinos, Argentina

The siliciclastic deposits of the Rancahue Formation (Oligocene, Alumine Basin), formed in the eastern side of the Andean Cordillera (70oW y 39oS) are studied to evaluate the relationship among different depositional systems in volcanic settings and to disclose the processes that contributed to the preservation of a forest developed at the foot of a volcanic relief. In the alluvial volcano-sedimentary record of the Rancahue Formation an upland petrified forest with 33 logs in life position was identified. This flora, with Nothofagoxylon as the main taxon, was developed under temperate and humid climatic conditions. Three facies associations were defined in the study region. Facies association A was accumulated in subaerial talus and foot-talus slopes, and is composed of monomictic (basaltic) boulder and cobble conglomerates formed by debris floods, hyperconcentrated flows, avalanches, landslides and debris flows. The source of these sediments is local and is related to the growth of an intrabasinal basaltic volcanic edifice. Facies association B consists of cross-stratified, plane bedded and massive conglomerates, pebbly sandstones and coarse-grained sandstones accumulated in a regional braided fluvial system with source areas located in the Andean Cordillera. Facies association C is characterised by fine-grained sediments accumulated in subaerial and subaqueous environments, with periodic contribution of ash falls and development of palaeosols. The forest rooted in deposits of facies association C, and the logs preserved in life position are located within the overlying coarse-grained volcaniclastic deposits of facies association A. The burial and preservation of this vegetation was favoured by the highly aggrading rhythm and comparative low erosive power of the gravitational flows that operated in alluvial and colluvial fans of the riparian hydrogeomorphic zone. Conversely, abundant transported and broken trunks included in the deposits of facies association B clearly indicate the strong erosive power of the currents of the associated braided fluvial system.

Geology of Stephens Island and the age of Stephens formation

On Stephens Island, northernmost Marlborough Sounds, New Zealand, 1100 m of southeast-dipping sedimentary rocks, ranging from mudstone with limestone lenses to volcanogenic sandstone and conglomerate, comprise Stephens Formation, the youngest formation within the Maitai Group. The rocks form the most northerly exposure of the Nelson Regional Syncline; younging directions show that they are overturned and therefore are on the eastern limb of the syncline. They are well indurated with widespread metamorphic prehnite, pumpel1yite and lawsonite. No top or bottom to the formation is exposed. The formation is subdivided into Queens Beach, Incline, Takapourewa, and Titapua Members. The oldest rocks (Queens Beach Member) comprise 150 m of rounded cobble conglomerate which encloses angular blocks of coarse volcanogenic sandstone. One such block, 4 m long and up to 1.5 m thick, contains a diverse but indifferently preserved marine fossil assemblage dominated by brachiopods and pectinid bivalves of Late Permian age....

Magnetostratigraphy of the Suerkuli Basin indicates Pliocene (3.2 Ma) activity of the middle Altyn Tagh Fault, northern Tibetan Plateau

The left-lateral strike-slip Altyn Tagh Fault (ATF) forming the northern boundary of the Tibetan Plateau accommodates parts of the overall convergence between the colliding Indian and Eurasian plates. Precise dating of the ATF activity is essential for understanding possible mechanisms of Tibetan Plateau deformation and uplift. Here we report a magnetostratigraphic study of the Suerkuli Basin deposits recording depositional changes during the ATF activity. Field investigations reveal a remarkable and widespread change in depositional environment in the Suerkuli Basin, i.e. a transformation from low-energy lacustrine deposits (grayish-green mud-siltstone and brown mud-siltstone) into high-energy alluviul fan deposits (poorly sorted gray pebble and cobble conglomerates). Detailed magnetostratigraphy of the 390-m-thick Daban section, at the southeastern margin of the Suerkuli Basin (38°43.09′N, 90°58.84′E), shows that this change in depositional facies occurred at ∼3.2 Ma, accompanied by a remarkable increase in sediment accumulation rate. We attribute this depositional change to the Piocene tectonic activity of the middle ATF although the contribution of the Pliocene global climate deterioration cannot be excluded.

Sediment waves and depositional implications for fine-grained rocks in the Cerro Toro Formation (upper Cretaceous), Silla Syncline, Chile

The Cerro Toro Formation contains a variety of lithofacies including pebble and cobble conglomerate, coarse-grained sandstone, thin-bedded mudstone and sandstone, and slumped to chaotic mudstone. Coarse-grained rocks in the Cerro Toro have been described and well documented, whereas characterization and description of mudstone-prone lithofacies are not well documented. These mudstone-prone rocks exhibit two architectural patterns: 1) broad (>200 m) undulating or wavy-bedded elements that laterally terminate by onlap, truncation (toplap) and downlap patterns, and 2) laterally persistent (>400 m), horizontal, thin-bedded mudstone and sandstone. Adjacent to Channel Complex 3 (Paine C Member), these facies exhibit a stratigraphic transition from horizontal to wavy and curved beds concurrent with pronounced aggradation of laterally equivalent, and possibly coeval, channel facies. Sandstone and mudstone beds within the wavy-bedded facies exhibit turbidite lithofacies that include current-ripple lamination (Tc), planar lamination (Tb) massive, graded intervals (Ta), and laminated to structureless silt- and clay-rich beds (Tde). Typically, these beds are a few centimeters thick, but locally, sandstone beds form bedsets over 1 m thick. These thick sandstone bedsets display inclined bedding or lamination associated with mudstone rip-up clasts and are confined to troughs or swales within large-scale wavy-bedded units. Erosion surfaces within this thin-bedded fine-grained lithofacies are spaced vertically at 10–15 m, commonly associated with the crest of curved bedding and display at least 5 m of relief. Based on map patterns, channel-form architectural elements, and stratal-stacking relationships, the coarse-grained rocks in the Cerro Toro have been interpreted as the fill within deep-water channels. In contrast, the thin-bedded mudstone and sandstone lithofacies located adjacent to and eroded into by the channel facies are interpreted as coeval levee facies. Planar, tabular bedsets located at the base of this fine-grained lithofacies are interpreted as initial overbank deposition associated with the upper part of Channel Complex 3, whereas the curved, wavy beds in the fine-grained lithofacies are interpreted as sediment waves developed on the backside of a levee. The curved bedding, lenticular, medium-grained sandstone bedsets, scattered erosional surfaces, and onlap-downlap stratal geometries in the Cerro Toro bear resemblance to sediment waves associated with Quaternary coarse-grained, channel-levee systems. In contrast, sediment waves generated in other deep-water settings, such as upper slope, continental rise or basinal settings, usually are dominated by silt- and very fine-grained sand, very continuous layering (over 1 km), relatively long wavelength (>1 km) and relatively high amplitudes (>10 m). A possible analog for the coarse-grained and fine-grained rocks in the Cerro Toro is the Quaternary Var Ridge and associated fan valley located in the Mediterranean Sea. Like the Cerro Toro, the Var system has coarse-grained material within the fan valley, pebble- to cobble-size clasts, and thin-bedded mud and sand in levees with documented sand bed thickness up to 40 cm and up to medium-grained sand. Architectural details described from the Var Ridge indicated sediment waves make up a significant portion of the levee material. Beds within the sediment waves terminate laterally by onlap downlap, and toplap, similar to the Cerro Toro. The fine-grained material can be linked to deposition from unconfined turbidity currents that were generated via overbank flows from the Var fan channel. The development of these architectural elements requires out of channel turbidity currents and relatively thick flows. Growth of the sediment waves in the Var system represents active sedimentation and turbidity currents within a coeval channel rather than development during periods of channel inactivity or degradation.

Nd isotopic anatomy of a pebble conglomerate from the Murihiku terrane of New Zealand: Record of a varied provenance along the Mesozoic Gondwanaland margin

The Murihiku terrane is a volcano-sedimentary terrane of Late Permian to Early Cretaceous age that forms part of a collage of accreted terranes in the Eastern Province of New Zealand. These terranes record a history of deposition along the Gondwanaland margin, and include terranes of oceanic character (e.g. Brook Street terrane) and terranes dominated by continental detritus (e.g. Torlesse terrane). The Murihiku terrane is of particular interest because it is relatively large, one of the least structurally deformed terranes, and preserves a long (∼120 m.y) record of sedimentation. Previous studies have suggested that the Murihiku terrane preserves a record of change in provenance from predominantly mafic, juvenile sources to dominantly felsic volcanic sources in Middle to Late Triassic time interpreted as containing contributions from Precambrian continental crust, and modest amounts of detritus supplied directly from this old crust [Frost, C.D., Coombs, D.S., 1989. Nd isotope character of New Zealand sediments: Implications for terrane concepts and crustal evolution. American Journal of Science 289, 744–770.; Roser, B.P., Coombs, D.S., Korsch, R.J., Campbell, J.D., 2002. Whole-rock geochemical variations and evolution of the arc-derived Murihiku Terrane, New Zealand. Geological Magazine 139, 665-685.]. We present Nd isotopic data from a Late Triassic (Rhetian; Otapirian local stage; 200–206 Ma) roundstone cobble conglomerate from near Gore, South Island, New Zealand. The sandstone matrix of this conglomerate has an initial ɛ Nd of + 3.0 and Sm/Nd of 0.219, values that are intermediate between the more radiogenic Nd isotopic and higher Sm / Nd ratios of older Murihiku siltstones and less radiogenic, lower Sm / Nd, younger siltstones. Clasts within this matrix are mainly volcanic: we analyzed basaltic-andesite, andesite and dacite pebbles along with a volcanogenic sandstone pebble and hypabyssal quartz diorite clast. These pebbles have initial ɛ Nd between + 3.7 and + 2.6, and are probably derived from a Late Triassic volcanic arc. A felsic plutonic pebble with an initial ɛ Nd of − 0.5 and crustal residence age of 1.0 Ga may record erosion of Gondwanaland Proterozoic basement. A mudstone rip-up clast may be composed of fine-grained cratonic detritus; it has a Nd crustal residence age of 1.1 Ga. This conglomerate, containing both arc-derived and older cratonic-derived clasts, captures in a single sample the contribution of mafic arc, felsic arc and craton-derived sedimentary rocks to bulk sediment Nd budgets recorded in the Murihiku terrane.

Late Pleistocene (Last Interglacial) terrace deposits, Bahia Coyote, Baja California Sur, México

Late Pleistocene age terrace deposits are exposed in the narrow cliffed coastal plain of Bahia Coyote, Baja California Sur, resting unconformably on the lagoonal-shallow water volcaniclastics of the early Miocene Cerro Colorado Member of the El Cien Formation. The terrace is dissected by widely spaced arroyos and partically covered by alluvial fans in the inner and central areas. The marine deposits vary in thickness from 0.5 to 10 m and were laid down in pre-existing erosional channels and depressions in the Pleistocene landscape. The sequence begins with a cobble conglomerate with oyster shells, overlain by poorly bedded molluscan-rich bioclastic sands and coral rubble, beds of massive Porites in growth position and coral-rhodolith sands and marls. Beach sands and gravels and coastal dunes cap the sequence. Samples of Porites panamensis selected for U/Th dating are well-preserved aragonite (>95%). Preliminary results yield U/Th ages of 109–209 ka but the corals have initial δ 234U values in excess of modern seawater values. This indicates open-system behavior and uncertainty associated with the ages. A corrected age for the top of the massive Porites unit suggests that the corals grew during the last interglacial, marine isotope stage (MIS) 5e sea level high stand. Assuming global sea level during MIS 5e was ca. 4–5 m above present-day sea level (McCulloch and Esat, 2000) and the growth position of the corals was 1–5 m below sea level, the terraces have been uplifted between 12 and 25 m (12–15 cm/kyr). This is consistent with other terrace-based uplift rates for the central Baja California peninsula, north of the La Paz fault.

Stratigraphic architecture and depositional setting of the coarse‐grained Upper Cambrian Owen Conglomerate, West Coast Range, western Tasmania

The Upper Cambrian Owen Conglomerate of the West Coast Range, western Tasmania, comprises two upward‐fining successions of coarse‐grained siliciclastic rocks that exhibit a characteristic wedge‐shaped fill controlled by the basin‐margin fault system. Stratigraphy is defined by the informally named basal lower conglomerate member, middle sandstone member, middle conglomerate member and upper sandstone member. The lower conglomerate member has a gradational basal contact with underlying volcaniclastics of the Tyndall Group,while the upper sandstone member is largely conformable with overlying Gordon Group marine clastics and carbonates. The lower conglomerate member predominantly comprises high flow regime, coarse‐grained, alluvial‐slope channel successions, with prolonged channel bedload transport exhibited by the association of channel‐scour structures with upward‐fining packages of pebble, cobble and boulder conglomerate and sandstone, with abundant large‐scale cross‐beds derived from accretion in low‐sinuosity, multiply active braided‐channel complexes. While the dipslope of the basin is predominantly drained by west‐directed palaeoflow, intrabasinal faulting in the southern region of the basin led to stream capture and the subsequent development of axial through drainage patterns in the lower conglomerate member. The middle sandstone member is characterised by continued sandy alluvial slope deposition in the southern half of the basin, with pronounced west‐directed and local axial through drainage palaeoflow networks operating at the time. The middle sandstone member basin deepens considerably towards the north, where coarse‐grained alluvial‐slope deposits are replaced by coarse‐grained turbidites of thick submarine‐fan complexes. The middle conglomerate member comprises thickly bedded, coarse‐grained pebble and cobble conglomerate, deposited by a high flow regime fluvial system that focused deposition into a northern basin depocentre. An influx of volcanic detritus entered the middle conglomerate member basin via spatially restricted footwall‐derived fans on the western basin margin. Fluvial systems continued to operate during deposition of the upper sandstone member in the north of the basin, facilitated by multiply active, high flow regime channels, comprising thick, vertically stacked and upward‐fining, coarse‐grained conglomerate and sandstone deposits. The upper sandstone member in the south of the basin is characterised by extensive braid‐delta and fine‐grained nearshore deposits, with abundant bioturbation and pronounced bimodal palaeocurrent trends associated with tidal and nearshore reworking. An increase in base‐level in the Middle Ordovician culminated in marine transgression and subsequent deposition of Gordon Group clastics and carbonates.

Sedimentological and Petrographical Characteristics of Coal- and Oil Shale-Bearing Strata from the Ilısılık Region, Central Anatolia, Turkey

The present study describes the sedimentology, coal petrology, and geochemistry of a Miocene coal- and oil shale-bearing succession in the Il L s L l L k region in central Anatolia, Turkey. Vertical sequence analysis within the 50-150 m thick coarse-grained succession of the Il L s L l L k oil shale field has allowed 4 sedimentary associations to be distinguished. The facies associations in the Il L s L l L k oil shale field are dominated by fluvial deposits, which document abrupt facies transitions along the axis of the basin. Fluvial poorly-sorted cobble conglomerates of facies association 1, located along the basin margin grade basinward into well-sorted conglomerates of facies association 2. These deposits grade basinward into the laterally continuous sheets of trough cross-bedded fine-grained sandstone and massive siltstone of facies association 3. Further basinward the fine clastics, oil shale, and coal of facies association 4 are common. Reflectances measured on vitrinite are between 0.54 and 0.67% Rmax. This indicates a coalification equivalent to low volatile subbituminous coal rank of the American Society for Testing and Materials classification for the Il L s L l L k coals. The maceral composition makes the Il L s L l L k oil shales of the lamosite type.

Transition from debris flow to hyperconcentrated flow in a submarine channel (the Cretaceous Cerro Toro Formation, southern Chile)

ABSTRACT It is important to understand the exact process whereby very large amounts of sediment are transported. This paper reports peculiar conglomerate beds reflecting the transition of submarine debris flows into hyperconcentrated flows, something that has been well documented only in subaerial debris‐flow events until now. Voluminous debris flows generated along a Cretaceous submarine channel, southern Chile, transformed immediately into multiphase flows. Their deposits overlie fluted or grooved surfaces and comprise a lower division of clast‐supported and imbricated pebble–cobble conglomerate with basal inverse grading and an upper division of clast‐ to matrix‐supported, disorganized conglomerate with abundant intraformational clasts. The conglomerate beds suggest temporal succession of turbidity current, gravelly hyperconcentrated flow, and mud‐rich debris flow phases. The multiphase flows resulted from progressive dilution of gravelly but cohesive debris flows that could hydroplane, in contrast to the flow transitions in subaerial environments, which involve mostly non‐cohesive debris flows. This finding has significant implications for the definition, classification, and hazard assessment of submarine mass‐movement processes and characterization of submarine reservoir rocks.

Long-lived Transfer-Zone Paleovalleys in Mountain Belts: An Example from the Tertiary of the Spanish Pyrenees

ABSTRACT Major fluvial valleys located in sediment-transfer zones that link hinterland drainage basins with their depositional basins are an important, but neglected, element of fluvial systems. Studies of present-day examples are limited and, to date, no examples have been described from the geological record. Tertiary synorogenic fluvial successions in the Spanish Pyrenees include regional-scale, unconformity-based, linear bodies of conglomerate that are interpreted as the fills of transfer-zone paleovalleys. These paleovalleys are sited mainly in the external zones of the mountain belt, between the internal Axial Zone, on which the drainage basin was largely established, and the depositional basins. One body, here termed the Sis conglomerate, is preserved in a growth syncline between two th ust-related, lateral structures. Growth of the syncline during sedimentation permitted a 1400 m succession of clast-supported, pebble and cobble conglomerates to accumulate during a c. 18 m.y. period between the middle Eocene to Oligocene. The paleovalley served not one but a series of evolving thrust-sheet-top and foreland basins during this period. The stability of the paleovalley was governed by its location between long-lived, pre-existing structures that were inverted during compressional deformation. This example demonstrates that fluvial paleovalleys located in transfer zones can, when structurally controlled, be long-term features capable of accumulating and preserving a significant thickness of strata.

Westerly derived Early Cretaceous gold paleoplacers in the Western Canada foreland basin, southwestern Alberta: tectonic and economic implications

Albian-aged (Early Cretaceous) igneous pebble to cobble conglomerates fill multiple, northeast–southwest-oriented, subparallel channels in the upper Blairmore Group (upper Beaver Mines and Mill Creek formations) of the Rocky Mountain foreland basin, southwestern Alberta and southeastern British Columbia. Paleocurrent data show that the conglomerate was derived from the west. Clast petrography implies a provenance that includes granitoids, mafic volcanics, low-grade metamorphic rocks, and shallow-level (ca. 7 km depth) postmetamorphic quartz veins formed from meteoric fluids. The conglomerate was probably derived from the southern Omineca Belt of British Columbia prior to the rise of the Rocky Mountains. The conglomerate contains detrital gold grains up to 150 μm in diameter, and chemical analyses indicate widespread anomalous gold concentrations (up to 910 ppb Au) in conglomerate matrix. Gold content in these igneous-clast conglomerates excèdes that reported from the richest modern placers in Alberta. Less pronounced but persistent As anomalies (up to 260 ppm) occur also. Postdepositional alteration of conglomerate matrix chemically mobilized Au and As from their detrital source grains and redistributed these elements. Gold enrichment in the igneous-clast conglomerate contrasts strongly with background gold concentrations in the underlying conglomeratic Cadomin Formation. The gold concentrations in the igneous-clast conglomerate demonstrate that paleoplacers derived from the Canadian Cordillera have accumulated in the Western Canada foreland basin, a sedimentary succession previously dismissed as a host for detrital gold.

Early Carboniferous Transgression on a Passive Continental Margin: Deposition of the Kekiktuk Conglomerate, Northeastern Brooks Range, Alaska

The base of the Ellesmerian sequence in the northeastern Brooks Range is a widespread terrigenous clastic succession assigned to the Kekiktuk Conglomerate (late Tournaisian-Visean). The Kekiktuk is situated above a regional angular unconformity and is the base of a transgressive succession that is thought to record coastal onlap associated with the formation of a passive continental margin. Six depositional units have been recognized in the Kekiktuk Conglomerate. Unit A consists of clast-supported and rare matrix-supported pebble and cobble conglomerate with minor interbeds of pebbly sandstone. It records deposition at the base of incised paleovalleys from high-sediment-concentration flows and debris flows. Units B, C, and D consist of multistory fining-upward cycles that record deposition within incised paleovalleys from low-sinuosity bed-load-dominated streams, low to moderate-sinuosity bed-load-dominated streams, and high-sinuosity mixed-load streams, respectively. Unit E consists of multistory fining-upward cycles and laminated sandstone with thin conglomerate stringers. It records deposition in the distal regions of incised paleovalleys in a variety of marginal-mari e settings, including fluvial channel, channel-mouth bar, beach ridge, and coastal swamp environments. Unit F consists of horizontally bedded sandstone, interbedded sandstone, and organic-rich mudstone. It records deposition over paleotopographic highs on the sub-Mississippian unconformity from sheetfloods and in associated coastal swamp and mud-flat settings. Relative sea level rise is thought to have initiated fluvial deposition in the northeastern Brooks Range by the latest Tournaisian-earliest Visean. Coal-bearing fluvial channel-fill and flood-basin successions and palynologic data suggest that this part of Arctic Alaska was situated in a humid climatic zone during the Early Carboniferous. The stratigraphic and sedimentologic organization of the Kekiktuk Conglomerate indicates that once base-level rise began to affect the northeastern Brooks Range, local and regional variations in paleotopography along the sub-Mississippian unconformity exerted primary control on accommodation and consequent distribution of Kekiktuk depositional environments. The stratigraphic position of the Kekiktuk Conglomerate above a regional angular unconformity nd below marginal-marine and shallow-marine shales, combined with its limited thickness, widespread distribution, and suite of paleoenvironments, suggests deposition in an upland region situated landward of the tectonic hinge zone along a steadily subsiding passive continental margin. Thermal subsidence combined with eustatic sea level rise were probably responsible for relative sea level rise in Arctic Alaska and the resulting regional transgression. End_Page 679------------------------------

Stream‐dominated alluvial fan and lacustrine depositional systems in Cenozoic strike‐slip basins, Denali fault system, Yukon Territory, Canada

ABSTRACTAncient stream‐dominated (‘wet’) alluvial fan deposits have received far less attention in the literature than their arid/semi‐arid counterparts. The Cenozoic basin fills along the Denali fault system of the northwestern Canadian Cordillera provide excellent examples of stream‐dominated alluvial fan deposits because they developed during the Eocene‐Oligocene temperate climatic regime in an active strike‐slip orogen. The Amphitheatre Formation filled several strike‐slip basins in Yukon Territory and consists of up to 1200 m of coarse siliciclastic rocks and coal. Detailed facies analysis, conglomerate: sandstone percentages (C:S), maximum particle size (MPS) distribution, and palaeocurrent analysis of the Amphitheatre Formation in two of these strike‐slip basins document the transition from proximal, to middle, to distal and fringing environments within ancient stream‐dominated alluvial‐fan systems.Proximal fan deposits in the Bates Lake Basin are characterized by disorganized, clast‐supported, boulder conglomerate and minor matrix(mud)‐supported conglomerate. Proximal facies are located along the faulted basin margins in areas where C:S = 80 to 100 and where the average MPS ranges from 30 to 60 cm. Proximal fan deposits grade into middle fan, channelized, well organized cobble conglomerates that form upward fining sequences, with an average thickness of 7 m. Middle fan deposits grade basinward into well‐sorted, laterally continuous beds of normally graded sandstone interbedded with trough cross‐stratified sandstone. These distal fan deposits are characteristic of areas where C:S = 20 to 40 and where the average MPS ranges from 5 to 15 cm. Fan fringe deposits consist of lacustrine and axial fluvial facies. Palaeogeographic reconstruction of the Bates Lake Basin indicates that alluvial‐fan sedimentation was concentrated in three parts of the basin. The largest alluvial‐fan system abutted the strike‐slip Duke River fault, and prograded westward across the axis of the basin. Two smaller, coarser grained fans prograded syntaxially northward from the normal‐faulted southern basin margin.Facies analysis of the Burwash Basin indicates a similar transition from proximal to distal, stream‐dominated alluvial fan environments, but with several key differences. Middle‐fan deposits in the Burwash Basin define upward coarsening sequences 50 to 60 m thick composed of fine‐grained lithofacies and coal in the lower part, trough cross‐stratified sandstone in the middle, and conglomerate in the upper part of the sequence. Upward‐coarsening sequences, 90–140 m thick, also are common in the fan fringe lacustrine deposits. These sequences coarsen upward from mudstone, through fine grained, ripple‐laminated sandstone, to coarse grained trough cross‐stratified sandstone. The upward‐coarsening sequences are basinwide, facies independent, and probably represent progradation of stream‐dominated alluvial‐fan depositional systems.Coal distribution in the Amphitheatre Formation is closely coupled with predominant depositional processes on stream‐dominated alluvial fans. The thickest coal seams occur in the most proximal part of the basin fill and in marginal lacustrine deposits. Coal development in the intervening middle and distal fan areas was suppressed by the high frequency of unconfined flow events and lateral channel mobility.

Stratigraphy of Pennsylvanian Detrital Reservoirs, Permian Basin

Significant oil reserves have been found to date in stratigraphic traps in Pennsylvanian detrital reservoirs on the Central Basin platform and Reagan uplift of the Permian basin. The 32 MMBOEG Arenoso field area, discovered in 1966, is the largest producing field. Along a 75 mi northwest-southeast trend, 23 other smaller fields will produce an average 850 MBOEG each, for a total estimated ultimate recovery to date in the trend of 52 MMBOEG. These stratigraphic traps are elusive and complex. However, reservoir quality is excellent, and because of the poorly understood trap types, significant reserves remain to be found in the trend. The Pennsylvanian detrital consists of chert cobble conglomerates, coarse sands, red shales, and gray limestones deposited in an environment that grades seaward from alluvial fan to braided stream to shallow marine. The chert cobble conglomerates of the alluvial fan facies and the coarse sands of the braided stream facies are the highest quality pay zones. Porosities range from 5 to 20%, with permeability ranging up to 26 d. The total unit is seldom more than 400 ft thick; reservoir rock thicknesses within the unit range up to 100 ft. Because of the complex nature of the alluvial fan andmore » braided stream deposits, dry development wells can be expected within fields. These Strawn deposits are located adjacent to and overlying the eroded lower Paleozoic uplifts of the southern Central Basin platform. The major source of the chert cobbles is erosion of the Devonian tripolitic chert. Renewed structural uplift combined with sea level drop in the middle Wolfcampian locally truncated some Pennsylvanian detrital alluvial fan deposits, and complicated or destroyed a potential trap by depositing Wolfcamp chert conglomerates on top of the Pennsylvanian conglomerates.« less

The Bouse Formation and bracketing units, southeastern California and western Arizona: Implications for the evolution of the Proto‐Gulf of California and the lower Colorado River

Mio‐Pliocene sediments of the lower Colorado River area represent the northernmost well documented extent of the proto‐Gulf of California, a tectonically enigmatic marine incursion that occupied much of what is now the Gulf of California region—including the lower Colorado River area and the Salton Trough—as much as 8 m.y. prior to the onset of spreading‐and transform‐related subsidence in that area. Fanglomerate and volcanic rocks informally referred to here as the Osborne Wash strata interfinger with the conformably overlying Bouse Formation. The Bouse Formation includes carbonate and coarse terrigenous‐clastic basin margin deposits that interfinger laterally with basin fill material; basin fill strata comprise a basal estuarine carbonate unit overlain by fine‐grained terrigenous‐clastic deltaic deposits. The upper portion of the Bouse interfingers with overlying cobble conglomerates here referred to as the Colorado River gravels. Syndepositional folding of the Bouse Formation and bracketing units is believed to reflect slumping on oversteepened slopes, perhaps exacerbated by episodic tectonic activity. Syndepositional faults show both normal and reverse separation; outcrop relations allow but do not prove a strike‐slip component of motion on some structures. Contemporaneous minor faults seem to reflect mutually incompatible stress orientations; this suggests that they record either localized stress fields or localized anomalous responses to regional stresses. Alternatively, they may reflect extension in two directions in the sediment pile overlying the subsiding basin floor. The Bouse Formation and bracketing units record four stages in the evolution of the northern proto‐Gulf/lower Colorado River area: (1) dissection of preexisting, detachment fault‐controlled topography and localized, interior‐drainage alluvial deposition (about 14–9 Ma); (2) regional subsidence and proto‐Gulf transgression (perhaps as early as about 8 Ma; not later than about 5.5 Ma); (3) progradation of ancestral Colorado River delta into the northern end of the proto‐Gulf basin (prior to about 4.3 Ma); and (4) arrival of throughgoing Colorado fluvial channel (prior to about 3.5–4 Ma). Outcrop relations between the Osborne Wash strata and the Bouse Formation, and the relationship of these units to modern landforms, indicate that topography in the lower Colorado River area has not changed significantly since middle Miocene time. Subsidence of the Bouse basin is believed to have occurred via broad regional downwarping, which may have represented a sag formed as the locus of active proto‐Gulf extension propagated northward into the lower Colorado River area from the block‐faulted southern portion of the proto‐Gulf. Timing suggests that incipient proto‐Gulf extension in the lower Colorado River area was arrested by the approximately 5‐Ma shift to the modern transtensional regime.

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.

Sedimentology and paleopedology of Miocene alluvial deposits at the Paşalar Hominoid site, Western Turkey

Miocene age alluvial deposits at the Paşalar Hominoid site in Western Turkey consist of a fossil-bearing green sand unit and two drab-colored, calcareous paleosols that sandwich the fossiliferous bed. The green sand unit contains a significant amount of bone chips and teeth. High concentrations of gravel, bone and teeth occur in, and directly above, large pothole shaped bodies which extend from the green sand down into the underlying calcareous paleosol. Gravel pipes emanating upward from the vicinity of these “potholes” are interpreted as fluid escape structures. The fossiliferous green sand bed is interpreted as the deposit of a silty, pebbly sand flow ( en masse emplacement) on the basis of its overall normal grading, vague horizontal layering, grain and matrix supported fabrie, and abundant fluid escape structures. The enigmatic potholes may have been holes produced by the burning out of tree trunks in the soil; they match modern examples well. The two calcareous paleosols, termed the lower and upper calcareous silt, are very much alike and represent a soil-forming environment where both clay formation and calcification took place. Pedogenic features common to both paleosols include green colors, high clay content, clay skins and clay slickensides, an unusually great thickness (about 2 m after compaction) and an abundance of golf-ball-sized calcareous nodules. The gleyed nature of the paleosol indicates a climate with seasonal waterlogging. Abundant calcareous nodules in the paleosol profiles indicate a pronounced dry season and clay formation indicates a warm, humid season. Up-section from the fossil beds is the main alluvial sequence of thick non-calcareous siltstones and cobble conglomerates exposed in gullies to the northeast of the exacavation site. The siltstones are pervasively mottled red and green. They have abundant soil structures including root traces, blocky peds and cutans. These massive, mottled siltstones are interpreted as paleosols of a generally well drained floodplain and the interbedded conglomerates are interpreted as braided stream channel deposits. The main alluvial sequence has dips of less than 3° whereas the fossil beds and their clayey paleosols dip between 7° and 10°. Thus there was an episode of tilting between the deposition of the two sequences. A riverine levee and floodplain depositional environmental is interpreted for the Paşalar fossil beds based on the gleyed paleosol profiles and the onlap of channel fill and levee deposits on both the fosiliferous sand unit and upper calcareous silt. They probably formed in a low-lying stream side location in a wet-dry monsoon climate, similar to semi-arid to semi-humid areas in the Punjab province of India. On the basis of comparison with modern soils from the Punjab, similar Paşalar paleosols are classified as intermediates between Calciorthids and Ustochrepts that have gleyic properties.

Jura-Cretaceous (Oxfordian to Cenomanian) stratigraphy of the north-central Bowser Basin, northern British Columbia

Three new formations of Late Jurassic and Early to mid-Cretaceous age are defined for a 2000 m thick section of Jura-Cretaceous rocks exposed in the north-central Bowser Basin. The Currier Formation (Oxfordian to Kimmeridgian or Tithonian) consists of 350–600 m of interbedded shales, siltstones, sandstones, coals, and carbonates. The McEvoy Formation (Barremian to as young as Albian) consists of 400–800 m of siltstones and shales with minor sandstones, thin coals, limestones, and conglomerates. The Devils Claw Formation (in part mid-Albian to Cenomanian) consists of 300–600 m of strata characterized by thick pebble and cobble conglomerates, with associated coarse sandstones and minor siltstones and shales.Two successive coarsening-upward sequences are identified in the study area. The first begins with Middle Jurassic marine shales of the Jackson unit grading upwards to coarser Upper Jurassic facies of the Currier Formation. The Currier Formation is conformably or unconformably overlain by siltstones and shales of the Lower Cretaceous McEvoy Formation, which forms the base of a second coarsening-upward sequence. Conglomerates appear with increasing frequency in the upper McEvoy and are the dominant lithology of the overlying Devils Claw Formation. The contact between the McEvoy and Devils Claw formations is gradational. The Devils Claw Formation forms the top of the second coarsening-upward sequence.The Currier Formation (Late Jurassic) is equivalent to the upper units of the Bowser Lake Group. The McEvoy and the Devils Claw formations (Barremian to Cenomanian) are coeval with the Skeena Group (Hauterivian? to Cenomanian). A probable unconformity separating the Upper Jurassic Currier Formation from the Lower Cretaceous McEvoy Formation correlates with a hiatus in the southern Bowser Basin and probably represents a regional unconformity.