Conglomerate Facies Research Articles

Detrital record of the Gravina arc, southeastern Alaska: Petrology and provenance of Seymour Canal Formation sandstones

Sandstones of the Upper Jurassic to mid-Cretaceous Seymour Canal Formation (northern Gravina belt, southeastern Alaska) are volcaniclastic wackes. They exhibit a variably developed solution cleavage and zeolite- to low-greenschist-grade metamorphic assemblages. Point counts of 37 lithofeldspathic sandstones, spanning the unit9s stratigraphic range, yield framework modes typical of detritus shed from undissected arc sources (Q 4 F 40 L 56 , Qm 3 F 40 Lt 57 , Qm 8 P 91 K 1 , Qp 3 Lvm 91 Lsm 6 ) with only minor detritus characteristic of older terranes or continental-margin sources. Sandstones of the dominant thinly bedded turbidite lithology were derived almost exclusively from the contemporaneous Gravina arc. In contrast, sandstones of channelized conglomeratic facies are richer in quartz, sedimentary, metasedimentary, and plutonic detritus derived from basement terrane(s). The compositional heterogeneity and grain size of conglomeratic facies are consistent with derivation from local source areas, and we interpret them to reflect minor unroofing of the Alexander terrane and the superposed Gravina arc, mostly during Early Cretaceous time. A distinctive, although volumetrically minor, clinopyroxene-rich sandstone petrofacies signals a change in modal expression of source-area volcanism during the Cretaceous, and it probably records initial eruption of pyroxene-rich basalts of the mid-Cretaceous Douglas Island Volcanics. Supracrustal records preserved in the Nutzotin Mountains sequence, Dezadeash flysch, and Gravina sequence are also dominated by shallow-level arc detritus, although coeval supracrustal units on the Insular terrane of British Columbia reflect deeper incision of arc basement, possibly due to along-strike variation in arc geometries or tectonics of the Jura- Cretaceous Insular terrane.

Alluvial Fan Qrigin of the Bagra Formation (Mesozoic Gondwana) and Tectono-Stratigraphic Implications

Abstract The Bagra Formation of the Mesozoic Gondwana sequence of Satpura basin has been considered as a facies equivalent of the underlying Denwa, and assigned a late Triassic age. The present study reviews the above contention, prompted by distinctive lithofacies assemblage, depositional setting and tectonic framework of the Bagra Formation, and its variable relationship with the underlying strata. The Bagra Formation as a whole comprises conglomerate assemblage in the lower and middle parts and sandstones with intercalated lenses of shale in the upper part. The lower conglomerate facies occurring northward is clast - to matrix supported and poorly sorted; it is finergrained, better sorted, including thin lenses of cross-bedded sandstone southward in middle part of the formation. The overlying cross-bedded sandstone and shale facies is the most important assemblage in the distal south. The coarse grained nature of the sediments, the upward- fining distribution and lateral changes in gross lithofacies and stratification types, and paleocurrent data suggest that the Bagra conglomeratic sequence was deposited as an alluvial fan system that prograded southward from a highland source located immediately to the north. In sharp contrast, the fluvial system was directed dominantly northward and source area located southward for the underlying Permo-Triassic Gondwana sediments, including Denwa. It is argued that a late Triassic age of the Bagra Formation is questionable, in as much as a substantial stratigraphic break in sedimentation is called for after the deposition of Middle Triassic Denwa. The Bagra may well be late Jurassic-early Cretaceous in age based on its relationship with the overlying JabaIpur Formation.

Carboniferous calcretes in the Canadian Arctic

ABSTRACTCalcrete palaeosols have been found in the Upper Carboniferous Canyon Fiord Formation of southwestern Ellesmere Island, Canadian Arctic. These calcretes were developed in fluvial and shallow marine sediments that accumulated within two adjacent subbasins, in which the tectono‐sedimentary environment led to the deposition of five different sedimentary facies: (i) floodbasin sandstones; (ii) alluvial fan sandstones; (iii) alluvial fan conglomerates; (iv) braided fluvial sandstones; and (v) shallow marine limestones.Nodular/massive palaeosol profiles, consisting of cryptic, nodular, massive and laminar horizons, occur within the floodbasin sandstone and alluvial fan sandstone facies. Plugged palaeosol profiles, consisting of cryptic, plugged and laminar horizons, are restricted to the alluvial fan conglomerate facies. Massive/brecciated palaeosol profiles, consisting of cryptic, massive/brecciated and laminar horizons, occur essentially within the shallow marine limestone facies.The relationships between calcrete profiles and sedimentary facies suggest that profile types were controlled mainly by the texture and composition of the parent material: nodular/massive profiles are restricted to silicate‐rich sandstone hosts, plugged profiles are restricted to carbonate‐rich conglomerate hosts and massive/brecciated profiles are restricted to limestone hosts. Important relationships also exist between the maturity levels of nodular/massive profiles and sedimentary facies: profiles are generally mature in the floodbasin sandstones, invariably immature in the alluvial fan sandstones and absent from the braided fluvial sandstones. These different maturity levels were probably controlled mainly by exposure time, vegetation and substrate composition.

Limestone-Pebble Conglomerate Facies of the Eocene Ghazij Formation, Balochistan, Pakistan: Evidence for Collison-Related Tectonism on the Northwestern Margin of the Indian Plate

Limestone-Pebble Conglomerate Facies of the Eocene Ghazij Formation, Balochistan, Pakistan: Evidence for Collison-Related Tectonism on the Northwestern Margin of the Indian Plate

Early angiosperm fossil leaves in Chubut Group, Cretaceous, Argentina

The oldest angiosperm leaves from Patagonia were reported from Baquero Formation (Barremian-Aptian) in a mesofitic flora of more than one hundred gymnosperm and fern species. The next younger records are from the Chubut Group, a thick assemblage of pyroclastic sediments, with sand and conglomerate facies and frequent paleosols. In the Sierra de San Bernardo area sediments are mainly tufaceous and fluvial, deposited under an arid climate in shallow lakes and swamps of the alluvial plains. The sea was hundreds of kilometers to the West. Four Formations were described: Matasiete (Aptian), Castillo and Bajo Barreal (Senonian) and Laguna Palacios (Campanian-Maastrichtian). We explored in the Senonian Formations, and studied a few outcrops that can be sorted as:a) Monospecific, with only Onychiopsis sp., in palustral sediments. b) Strongly dominated by Eauisetum sp, and few remains of two dicot species with large, entire margined, low rank leaves, in palustral sediments. c) Strongly dominated by two species of angiosperms with small leaves, associated with remains of conifers in tufaceous beds deposited in the alluvial plain. d) Dominated by a few species of dicots, with medium size, entire margined leaves. Also with about 10 species of ferns, gymnosperms and lobate angiosperms. They are in alluvial plains sediments. e) More balanced associations, with several abundant species, including angiosperms with lobate and leaves. They are in fluvial sediments.Although angiosperm radiation and taphonomy undoubtedly accounts for differences of plant composition between outcrops, it seems apparent that the record of early angiosperms in Chubut Group is represented by remains of many different plant associations, with few species, adapted to different environments, that probably coexisted, and developed under an arid climate, far from the sea.

Evolving alluvial stratigraphy and thrust front development in the West Jaca piggyback basin, Spanish Pyrenees

In the southwestern Pyrenees, distinctive alluvial mega-sequences correlate with the development of a shallow, mountain front thrust structure, the Peña flexure. The flexure evolved during the Oligocene to early Miocene and is interpreted as a duplex marking the limit of thrust front advance. Three main depositional facies, preserved completely across the Peña flexure, are described: a pre-thrust emergence, high-sinuosity fluvial sequence (the c. 2300 m thick Sheet Sandstone Facies of the Campodarbe Formation); a syn-thrust emergence, conglomeratic alluvial fan sequence (the c. 1100 m thick Conglomeratic Facies of the Bernués Formation); and a post-thrust emergence, low sinuosity fluvial sequence (the > 700 m thick Ribbon Sandstone Facies of the Uncastillo Formation). Collectively, their continuous exposure across the Peña flexure provides a more or less complete record of the sedimentary evolution of a mountain front. Syn-deformational features in the alluvial fan sequence of the Bernués Formation and marked contrasts in palaeochannel morphology, sandstone body geometry and palaeocurrent patterns between the two fluvial sequences are best understood by invoking a strong tectonic control over their deposition. An increased rate of vertical sediment accretion was a key process in determining fluvial style. The onset of major thrusting led to more rapid basin floor subsidence, thereby promoting a change from lateral to vertical accretion as channels responded to the rising base level.

Detection and prediction of Lower Cretaceous sandstone distribution in the Scapa Field, North Sea

Abstract The Scapa Field is a combination structural/stratigraphic trap located in UK North Sea Block 14/19, 112 miles northeast of Aberdeen. The productive intervals consist of turbidite sandstones in the lower part of the early Cretaceous Valhall Formation. Three discrete sandstone bodies have been identified during the development of the Scapa Field, ranging in age from early Valanginian to late Hauterivian. The sandstones are interbedded with marls, limestones and conglomerates that filled the early Cretaceous northwest-southeast trending syncline between the Halibut Shelf and Claymore tilt block, on the southwest flank of the Witch Ground Graben. The combination of lithofacies, wireline log correlation, palaeontological, reservoir production and pressure studies led to the recognition of a five-fold subdivision (‘SA’ to ‘SF’) of the Scapa Sandstone Member. Detailed seismic studies, in particular amplitude extraction mapping using the 3D data set, have enhanced the interpretation of the distribution of the sandstone bodies. Tight conglomerate facies dominate the Lower Valhall sequence to the south in a narrow fringe bordering the Halibut Shelf. Reservoir sandstones interfinger with these conglomerates and thin to the north, changing facies to sandy marls onto the Claymore tilt block. Within the main Scapa syncline, sandstone distribution was controlled by clastic input associated with subtle structural control. The gravity-driven turbidite system concentrated deposition in the bathymetric lows. Lime muds were deposited in a constant hemipelagic rain, both over the intervening highs and interbedded with the sands.

The Numidian Sequence between Guelma and Constantine (Eastern Tell, Algeria)*

AbstractThe Numidian Sequence represents one of the main tectono‐stratigraphic units involved in the geological evolution of the Maghrebian orogen during the Upper Oligocene‐Lower Miocene. Geo‐structural aqalysis led to the reconstruction of tectonc‐stratigraphic units and recognition of two main tectonic phases (Eocene and Lower Miocene). Sedimentological analysis of the studied sections has resulted in the distinction of arenaceous and conglomeratic facies generated by debris flows or high‐density turbidity currents on a submarine slope, present within the pelitic‐arenaceous and arenaceous‐pelitic sequences which also contain slumped units in places.Previous and new micropalaeontological data indicate Late Oligocene‐Early Burdigalian ages for the Numidian sequences of the Constantine Mountains. On the basis of petrographic study the Numidian rocks can be classified as poorly sorted quartzarenites containing siliciclastic matrix. The overall petrographic data indicate that the Continental intercalaire (Hoggar, Tassili and Fezzan) and Series Pharusienne (Hoggar and Eglab) are possible supply areas for the Numidian quartz.

Sedimentology and tectonic implications of Cretaceous fan‐delta conglomerates, Queen Charlotte Islands, Canada

ABSTRACTThe Honna Formation, of Coniacian age, consists of several hundred metres of polymictic clast‐supported conglomerate associated with sandstone and mudstone. Five conglomerate facies are recognized: ungraded beds; inverse graded beds; normal graded beds; inverse‐to‐normal graded beds; and parallel‐stratified beds. These facies are interpreted as the deposits of subaqueous cohesionless debris flows and/or high‐density turbidity currents.The depositional environment was a deep‐water, gravelly fan that draped a fault‐controlled, basin‐margin slope. The fan is inferred to have passed upslope directly into an alluvial fan (unpreserved); hence, the name fan delta can be applied to the overall depositional system. This type of fan delta, of which the Brae oilfield in the North Sea is an example, is defined here as a deep‐water fan delta. The lack of a shelf is in marked contrast to other types of fan delta.Three facies associations are recognized in the Honna Formation: subaqueous proximal‐fan conglomerates, distal‐fan turbiditic sandstones, and pro‐fan/interfan mudstones with thin sandy turbidites. The proximal fan is envisaged as an unchannelled gravel belt with a downslope length of at least 20 km; such a long subaqueous gravel belt lacks a known modern analogue. The distal fan was an unchannelled sandy extension of the proximal gravel belt.It is postulated that the Honna Formation accumulated in a foreland basin which migrated westwards from the Coast Mountains where the Wrangellia‐Alexander terrane was colliding with North America. In this model, the Honna fan delta was sourced by a (west‐verging) thrust sheet whose sole‐thrust was the Sandspit Fault immediately to the east.Deep‐water fan deltas appear to develop preferentially when eustatic sea‐level is relatively high, so that the‘feeder’ alluvial fan is small, and gravelly throughout. In petroleum exploration and field development, care should be taken to distinguish deep‐water fan deltas from base‐of‐slope (canyon‐fed) submarine fans, because the two systems differ significantly in terms of coarse‐sediment distribution.

Temperate water carbonate facies of the early miocene paratethys (Zogelsdorf Formation, Lower Austria)

In the Early Miocene Zogelsdorf Formation of Lower Austria a transgression upon the rugged crystalline morphology of the Bohemian Massif has led to a complicated facies distribution. The sediments are bioclastic limestones and are compared with recent and fossil examples of temperate water carbonates. They have been studied using microfacies techniques including a point counter analysis of thin sections. Three facies dominate: a calcareous rich bryozoan facies, a calcareous rich coralline algal facies and a bivalve-barnacle facies rich in terrigenous components. Subordinate are an echinoderm-foraminiferal, a coralline algalbryozoan, a calcareous sandstone, an oyster and a conglomerate facies. The facies are interpreted with respect to exposure and supply of terrigenous components. The diagnostic features of lesser known fossils (barnacles and serpulids) in thin section are described.

An exceptionally thick upper Proterozoic (Sturtian) glacial succession in the Mount Painter area, South Australia

The Yudnamutana Subgroup is a thick succession of upper Proterozoic (Sturtian) glacigenic rocks preserved in the Yudnamutana trough in the northeastern part of the Adelaide geosyncline. In ascending order, the subgroup comprises the Fitton, Bolla Bollana, and Lyndhurst Formations. The name Creek Member is proposed for a locally developed conglomeratic facies at the base of the Fitton Formation. These rocks are interpreted as the products of two glacial advances. The first is evidenced by the Hamilton Creek Member, which includes sediment gravity flows derived from subaqueous glacial outwash. The Fitton Formation is composed mainly of basinal siltstones and mudstones with associated coarser-grained rocks that are largely sediment gravity flows. Diamictites in the upper part of the formation signify the onset of a second glacial advance. Thick diamictites of the Bolla Bollana Formation are interpreted as rainout deposits from floating ice derived from ice streams debouching from a steep and high hinterland into a fault-controlled basin. The Lyndhurst Formation is mainly fine-grained rocks. Dropstones and sparse diamictites attest to the presence of some floating ice, but some of the diamictites are probably mass-flow deposits. Paleocurrent data from the Yudnamutana Subgroup indicate transport to the west-northwest. The Sturtian glacigenic rocks are overlain by a transgressive shale. Stratigraphic similarities exist between the Yudnamutana Subgroup and thinner successions to the west. These stratigraphic similarities probably reflect paleoclimatic changes. The twice-repeated sequence of diamictite followed by mudstone is attributed to two glacial advance-retreat cycles. Local variations in thickness and stratigraphic succession are probably related to contemporaneous faulting.

Conglomerate Facies and Processes in Shallow to Deep-Marine Cretaceous Forearc Basins of Baja California, Mexico: ABSTRACT

Conglomerate Facies and Processes in Shallow to Deep-Marine Cretaceous Forearc Basins of Baja California, Mexico: ABSTRACT

Provenance and Sediment-Dispersal System in Tectonically Active Rapidly Evolving Foreland Basin, Western Interior: ABSTRACT

The Upper Cretaceous Frontier Formation, along the mobile edge of the Western Interior foreland basin, is composed mainly of clastic sediments and was deposited during the initial Late Cretaceous transgressive-regressive phases of the Western Interior seaway across Wyoming. The formation contains many persistent bentonite beds and several sandstone packages in its lower part and a thin, lenticular lithic wacke-polymictic conglomerate association at its upper contact (Torchlight Sandstone Member). Abundant granule to cobble-sized clasts of andesite, granite, chert, and quartzite are set in a poorly sorted sand-to-granule grade volcaniclastic matrix. There is a lithologic continuity of this volcaniclastic unit across the Bighorn Mountains into the Powder River basin. A high-energy distributary complex of sizable areal extent is invoked for the deposition of this linear conglomerate facies. Geochemical investigations of the whole-rock andesite clasts and bentonite allowed more precise definition of character, tectonic setting, and evolutionary stages of sedimentary distributive provinces. Bentonites and andesites are strongly enriched in strontium and barium, but only mildly enriched in heavy rare earth elements and high field-strength elements. These analyzed rocks have trace element characteristics similar in a general way to those of typical orogenic volcanics; they show some significant differences in detail. Composition of volcaniclasts more » and paleocurrent data indicate a proximal sediment source for the extrabasinal detritus within the Frontier Formation. The possibility of a contribution from a Mesozoic volcanic center in the neighborhood of southwestern Montana is strongly favored. The products of this volcanism constitute an assemblage of deep crustal to mantle( ) derived rocks, and their composition record time-integrated enrichment in light over heavy rare earth elements. « less

Submarine fan sedimentation at a convergent margin: the cretaceous mangapokia formation, New Zealand

The middle Cretaceous Mangapokia Formation (Pahaoa Group) near Te Awaiti, southeast North Island, New Zealand, consists of indurated, poorly fossiliferous, alternating sandstone and argillite, minor conglomerate, grit, pebbly-sandstone, and pebbly-mudstone (terrigenous sedimentary assemblage), and minor basalt, coloured argillite, chert, and micritic limestone (ocean-floor assemblage). Seven lithofacies are distinguished in the sedimentary assemblage on the basis of lithology, bed thickness and geometry, sand/mud ratio, grain size and internal sedimentary features. Facies 1 (10–15% of total exposure), which includes all sediments coarser than sand grade, comprises seven subfacies as follows: lenticular and erosive beds of coarse-grained (Subfacies 1Ai), medium-grained (Subfacies 1Aii) and fine-grained (Subfacies 1Aiii) predominantly clast-supported conglomerate, grit (Subfacies 1Aiv) and pebbly-sandstone (Subfacies 1Av) displaying numerous types of graded bedding and sedimentary structures, were all deposited predominantly from high-concentration turbidity currents or bed-load inertia flows. Minor chaotic sand or mud matrix-supported conglomerate lenses (Subfacies 1Bi), and beds which show clear evidence of post-depositional remobilisation (Subfacies 1Bii), represent debris flow deposits. Thick lenses of sandstone and minor argillite interbeds (Facies 2) were deposited from large-volume inertia flows, possibly grainflows. Facies 3, the most common lithofacies, consists of laterally more extensive, medium thickness, graded beds of alternating sandstone and argillite with rare Bouma sequences. These deposits are proximal turbidites which accumulated in environments more distal than Facies 1 and 2. Thin-bedded (Facies 4) and very thin-bedded (Facies 5) alternating sandstone and argillite, and argillite-dominated sequences with minor interbedded sandstone (Facies 6) were deposited in interchannel depressions, on channel levees, or in areas distant from high-concentration flows. Rare discontinuous graded bedded sandstones containing common flame structures (Facies 7) interbedded with conglomerate facies represent within-channel turbidites. A lower inner-submarine fan channel-overbank assemblage (Facies Association A), an inner-middle submarine fan channel-lobe assemblage (Facies Association B), and a mid-fan assemblage (Facies Association C) are distinguished. Two large-scale progradational suites of facies reflecting large-scale migration of the inner-fan channel complex are superimposed upon numerous small-scale depositional cycles resulting from lateral migration of numerous braided channels within the inner and mid-fan environments. The volumetrically minor ocean-floor assemblage is restricted to zones of melange. Chemically the basalts are tholeiitic, with trace element concentrations similar to modern ocean-floor basalts. Radiolarian-bearing cherts and coloured argillites are highly oxidised, and chemically similar to some modern oceanic pelagic sediments. The juxtaposition of relatively coherent, strongly folded and tectonically imbricated trench-fill submarine fan sediments and minor ocean-floor material is consistent with contemporaneous sedimentation and accretion at the base of the i inner trench slope in a convergent plate setting. The Mangapokia Formation is interpreted to represent the youngest part of the Torlesse terrane, and hence in southern North Island, Torlesse deposition and concurrent subduction tectonism were probably continuous at least until the middle Cretaceous.

Mid-Tertiary Transgressive Rocky Coast Sedimentation: Central Western Cascade Range, Oregon

ABSTRACT Sediments of the Scotts Mills Formation exposed on the flanks of Oregon's central Western Cascade Range accumulated along a storm-dominated rocky coastline during the Oligocene-Miocene transition. During this time, the development of depositional facies was controlled by the physiography of the underlying erosional surface. Lithofacies include a cross-stratified barnacle limestone (headland) facies; a fossiliferous, graded conglomerate (wave-cut platform/sea stack) facies; a burrowed claystone (tidal flat) facies; an amalgamated sandstone (coastal sand tongue) facies; and a graded mudstone (proximal tempestite) facies. The transgressive sedimentary deposits described here are characterized by 1) oligomictic intrabasinal detritus, 2) epilithic invertebrate faunas, 3) mass transport dep sits, and 4) abrupt basinward gradation of conglomeratic nearshore sediments to offshore muds. The development of these depositional environments and their relationship to the physiography of the underlying unconformity has been translated here into a rocky coast facies model.

Submarine Canyon, Slope, and Shelf Sedimentation in an Upper Eocene-Oligocene Progradational System (Limnos Island, North Aegean Sea, Greece): ABSTRACT

The only well-exposed outcrops of a post-Alpine late Eocene-Oligocene basin in the north Aegean Sea are at Limnos Island. These mostly consist of typical slope deposits overlain by remnants of shallow marine shelf and continental (braided-river) deposits. Three main slope lithofacies are distinguished. Canyon deposits consist of thick-bedded, massive, and pebbly sandstones (facies B), conglomerates (facies A), pebbly mudstone where the matrix shows flow (facies F), rock falls (nummulitic limestones), and zones of slump folds in sandstones (facies F). Channelized facies of massive sandstones, classical turbidites, and thin interbeds of sandstone and mudstone (facies E - probably overbank or levee deposits) are associated with canyon deposits. Pelagic and hemipelagic slope deposits consist of mudstone (facies G) and thin-bedded sandstone (facies D) with occasional small to medium-scale slump folds. These mud-rich slope deposits are incised by several channels filled with conglomerates, thick massive sandstones with well-developed dish structures, and turbidites interbedded with thin layers of mudstone (facies C). This unit composes a typical thinning and fining-upward sequence.

Fluvial, tidal and storm sedimentation in the Chilhowee Group (Lower Cambrian), northeastern Tennessee, U.S.A.

ABSTRACTThe Lower Cambrian Chilhowee Group of northeastern Tennessee consists of the Unicoi, Hampton and Erwin Formations, and is divided into four facies. The conglomerate facies occurs only within the lower 200 m of measured section (the Unicoi Formation) and consists of fine‐grained to pebbly quartz wacke with rare thin beds of laminated siltstone. Low‐angle to horizontally laminated, fine‐grained sandstone with laminae and lenses of granules and pebbles represents upper flow‐regime, overbank deposition within a braided stream system that was close to a coastline. Medium‐scale, planar‐tabular cross‐stratified conglomerate in which megaripple bedforms are preserved is interpreted as representing deposition in interbar pools of braided channels, as flood stage waned and larger bedforms ceased to migrate. Large‐scale, planar‐tabular cross‐stratified conglomerate beds represent migration of large transverse bars within a broad braided stream channel during high flood stage.The sandstone facies occurs throughout the Chilhowee Group, and is therefore interbedded with all other facies. It consists of mainly medium‐ to very coarse‐grained, subarkosic to arkosic arenite. Thinly interbedded, laminated siltstone and sandstone, which may exhibit wavy or lenticular bedding, represents deposition during slack water periods between ebb and flood tides. Large‐scale planar‐tabular and trough cross‐stratification reflects deposition within the deepest areas of subtidal channels, whereas medium‐scale cross‐stratification represents deposition in shallower water on shoals separating channels. Fining‐ and thinning‐upward sequences most likely resulted from the longshore migration of channels and shoals.The hummocky facies occurs only in the Erwin Formation and consists of horizontally laminated to hummocky stratified, fine‐grained arkosic to subarkosic arenite interbedded with equal amounts of bioturbated mudstone. It represents deposition between storm and fairweather wave‐base by combined‐flow storm currents.The quartz arenite facies is characterized by an absence of fine‐grained units and lithologically consists of a super‐mature, medium‐ to coarse‐grained quartz arenite. Large‐scale planar‐tabular cross‐stratification and abundant low‐angle cross‐stratification with rare symmetrical ripples (lower quartz arenite facies) occurs interbedded with the braided fluvial conglomerate facies, and was deposited within either a ridge‐and‐runnel system or a system of nearshore bars. Large‐scale, planar‐tabular cross‐stratification (upper quartz arenite facies), which forms the top of two 40 m‐thick coarsening‐upward sequences of the type: hummocky faciessandstone faciesquartz arenite facies, probably represents deposition on sand ridges that formed on a sand‐starved shelf as transgression caused the detachment and reworking of shoreface channel‐shoal couplets.Palaeocurrent data for the Chilhowee Group are unimodal but widely dispersed from 0° to 180°, and exhibit a minor mode to the west. The data are interpreted to reflect the influence of longshore, tidal and storm currents. The ichnofossil assemblage changes upsection from one characterized only by Paleophycus to a Skolithos ichnofacies and finally to a Cruziana ichnofacies. The facies sequence, biogenic and palaeocurrent data reflect the interaction through time of (I) non‐marine and marine processes; and (2) transgression coupled with shoreline progradation. The Chilhowee Group represents an overall deepening from terrestrial deposition to a marine shoreface that experienced both longshore and tidal currents, and finally to a storm shelf environment that periodically shoaled upward.

Alluvial fan facies in the miocene-pliocene coastal plain sands, Niger Delta, Nigeria

A 30 m thick alluvial fan developed in the lower part of the Coastal Plain Sand (Miocene-Pliocene) in southeastern Nigeria comprises four intergradational lithofacies: quartz pebble conglomerate, pebbly sandstone, interstratified pebbly sandstone and parallel to cross-stratified sandstone. The quartz pebble conglomerate facies consists of massive pebble packed beds set in a matrix of coarse to very coarse sand. Unordered poorly developed fabric with rare imbrication is characteristic. Deposition by debris flow on the proximal zone of the fan is interpreted for this facies. The pebbly sandstone facies is a structureless unit composed of pebbles dispersed through a medium- to coarse-grained sand with no significant pebble to pebble contact. Reversed distribution grading as well as cut-and-fill structures are common. Deposition by debris flow within the medial section of the fan is proposed. The interstratified pebbly sandstone and sandstone facies comprises relatively thin alternations of pebbly sandstone and horizontally stratified sandstone beds. Alternate deposition by sheet floods and/or debris flows in the distal part of the fan is suggested. Parallel- and cross-stratified fine to medium sand facies represents sheet flood deposits in the most distal part of the fan. These facies characteristics are similar to arid, semi-arid, and temperate alluvial fans. Fault scarp and/or non-tectonic base-level change may have provided the necessary relief for the formation of the fan. The absence of braided stream deposits associated with the alluvial fan appears to support the earlier contentions ∗ ∗ Le Blanc (1972) and Brown et al. (1973). that alluvial fan and braided stream deposits could be separate in space at any given time.

Gold-bearing fluvial and associated tidal marine sediments of Proterozoic age in the Mporokoso Basin, northern Zambia

The structurally defined Mporokoso Basin contains up to 5000 m of continental and marine clastic sediments and minor silicic volcanics which together form the Mporokoso Group. These rocks overlie unconformably a basement of silicic-intermediate igneous rocks and accumulated within the interval 1830-1130 Ma. This sedimentological study was restricted to the eastern end of the basin and was part of an assessment of the potential for palaeoplacer gold in the Mporokoso Group. At the base of the Mporokoso Group, the Mbala Formation consists of 1000–1500 m of purple sandstones and conglomerates deposited in a braided-stream system overlain by 500–1000 m of mature quartz arenites deposited in a tidal marine setting. A general coarsening-upward trend exists within the fluvial sediments. Sandy, distal braided-stream facies passes upwards into more proximal conglomeratic facies. In proximal sections, poorly sorted conglomerates form the top of the coarsening-up sequence which is 500–700 m thick. The overlying fluvial sediments fine upwards. The tidal marine sandstones at the top of the Mbala Formation resulted from reworking of fluvial sediments during a marine transgression. Well-exposed sections with fluvial conglomerates were studied in detail. Individual conglomerate bodies form sheets extending for hundreds of metres downstream and at least one hundred metres across stream, with little sign of deep scouring or channelling. They are generally matrix-supported. The whole fluvial sequence is characterised by a paucity of mud or silt. These conglomerates were deposited by large velocity, sheet flows of water which transported a bed-load of pebbles and sand. Most fine material settling out from suspension was eroded by the next flow. The great lateral and vertical extent and the uniformity of the fluvial sediments suggest that the sediments accumulated over an unconfined alluvial plain and that the tectonic evolution of the source area was relatively continuous and not episodic. These features are characteristic of other Proterozoic fluvial sequences. There are no distinctly channelised fluvial conglomerates nor angular unconformities within the fluvial sequence, both of which would have been potential sites for economic gold concentrations. Reworking of the fluvial sands during the marine transgression may have concentrated gold locally within the marine sandstones.

Submarine Fan Facies of Upper Cretaceous Strata, Southern San Rafael Mountains, Santa Barbara County, California: ABSTRACT

A 2900-m thick Campanian-Maestrichtian(.) turbidite sequence in Upper Mono Creek Canyon is interpreted to be a progradational submarine fan complex comprised of outer fan, middle fan, inner fan, and slope facies. The basal 600 m of the section consists of thinly bedded, laterally continuous fine sandstones, siltstones, and mudstones (mainly Mutti and Ricci Lucci facies D), interpreted to be outer fan interlobe and lobe-fringe deposits. These are punctuated by infrequent medium to very thickly bedded, flat-based, fine to coarse sandstones (facies C and B), which commonly coarsen and thicken upward, and are interpreted to be depositional lobes. Overlying these deposits are approximately 1400 m of middle fan deposits composed of frequent lenticular, commonly channelized and amalgamated, thickly bedded, fine to very coarse sandstones (facies C and B) organized in fining- and thinning-upward sequences, interpreted to be braided-channel deposits. These alternate with less common nonchannelized coarsening- and thickening-upward sequences suggestive of lobe-apical cycles. These multistory sand deposits are nested within thick intervals of fine sandstones, siltstones, and mudstones (facies C and D), interpreted to be levee, crevasse-splay, and interchannel deposits. Interfingered with and overlying these deposits are approximately 500 m of fining- and thinning-upward or noncyclic, erosionally based, commonly amalgamated, verymore » thickly bedded, medium to very coarse sandstones, pebbly sandstones, and conglomerates (facies A and B), interpreted to be inner fan deposits. Intercalated within this facies, infrequent, laterally discontinuous, thin to thickly bedded, fine to coarse sandstones, siltstones, and mudstones exist, interpreted to be interchannel, levee, and possibly channel-fill deposits.« less