Interchannel Deposits Research Articles

Depositional environments and taphonomy of the bone-bearing beds of the Lower Cretaceous Kuwajima Formation, Tetori Group, Japan

Thebone-bearing beds of the Lower Cretaceous Kuwajima Formation (Tetori Group) are described. Three facies of bone-bearing beds (Facies I: carbonaceous sandstones; Facies II: dark grey fine-grained silty sandstones; Facies III: dark greenish-grey mudstones) are present in inter-channel deposits that originated on a floodplain. The grain size of the sediments, and plant and molluscan fossils occurring in each bone-bearing bed, indicate that Facies I was deposited in a peat marsh, Facies II in a shallow lake, and Facies III in a vegetated swamp. Isolated small bones and teeth are the most abundant vertebrate fossils. Common elements in Facies II are aquatic vertebrates such as fishes and turtles. Facies III is characterized by the occurrence of terrestrial lizards, tritylodontid synapsids and mammals. Vertebrate fossil assemblages in Facies II and III are not mixed with each other even though they both represent parautochthonous assemblages. In contrast, Facies I is allochthonous, and is composed mostly of reworked sediments from Facies II. Depositional environments of the bone-bearing beds are strongly correlated with the composition of their fossil assemblages, indicating that different facies preserve the original faunal differences that existed between the shallow lake and vegetated swamp environments.

Deep-lacustrine sediment gravity flow channel-lobe complexes on a stepped slope: An example from the Chengbei Low Uplift, Bohai Bay Basin, East China

Many stepped slopes are present at deep-lacustrine basin margins in China, especially in the Paleogene rift basin margins of East China. Sediment gravity-flow deposits on these stepped slopes have the potential to form excellent reservoir quality sediments that are targeted for hydrocarbon exploration, but still remain poorly understood. In this study we use the Ed3 member of Oligocene-aged Dongying Formation deposited over the Chengbei Low Uplift, as an example for investigating facies types, distribution and main controlling factors of sediment gravity-flow deposits on stepped slopes at deep-lacustrine basin margins. Paleotopographical reconstructions indicate that the study area displayed a three-level stepped slope that were dissected by three large canyons and many associated smaller-scale channels prior to the deposition of the Ed3 member. On the basis of facies identification, six facies associations are further distinguished: (1) fan valley fills, (2) confined channel deposits, (3) poorly confined channel deposits, (4) levee deposits, (5) interchannel deposits, and (6) lobe deposits. These facies associations are grouped into four systems, namely fan valley fills, confined channel-levee, poorly confined channel-levee and lobe systems, as architectural elements constituting two types of channel-lobe complexes in the study area, namely anastomosing channel-lobe complexes and meandering channel-lobe complexes. The interaction between paleotopography and lake level fluctuations likely represented the direct control on the morphology, pattern and spatial-temporal distribution of channel-lobe complexes in the study area. Moreover, lake level fluctuations can also influence the volume and grain size of sediment supply to the stepped slope systems. Hyperpycnal flood events, as trigger factors, caused the generation of subaqueous sediment gravity flows that fed the stepped slope systems. Climate variations appear to be a contributing driver of lake-level fluctuations, and also indirectly control sediment supply into the deep lake and development scale of the stepped slope systems. These controls therefore determined the sediment supply, trigger and spatial-temporal variations of the stepped slope systems. This study improves the understanding of facies types and their distribution across stepped slopes at deep-lacustrine basin margins, as well as the controlling factors on the development of these sediment gravity-flow deposits.

The Upper Permian volcanic-sedimentary succession in northern Qamdo Block, central Qinghai-Tibet Plateau and its sedimentary, paleogeographic and tectonic significance

A combined sedimentological and geochemical study has been carried out on a volcanic-sedimentary succession from a borehole newly drilled in the northern Qamdo Block (QMB, in central Qinghai-Tibet Plateau) in order to improve the knowledge of the sedimentary and tectonic environment. The lithologic data allow to separate four main lithofacies, including massive volcanic lithofacies (LF1), tuffaceous lithofacies (LF2), sandstone lithofacies (LF3) and mudstone lithofacies (LF4). LF1 likely reflects the products of felsic lava flows that were formed during the process of magma ascent onto the seafloor at an earlier stage. LF2 contains two sublifhofacies that respectively represents the proximal and distal accumulation of tuffaceous turbidity flows. LF3 could be the arc-sourced submarine fan deposits, and the sublithofacies LF3a and LF3b represent mid-fan distributaries and interchannel deposits in fan lobes. LF4 is mainly composed of suspension deposits, which represents the sedimentation of post-volcanic activities. Furthermore, the lithofacy associations reveal the evolution of a volcano-sourced submarine fan in the back-arc basin. Geochemically, LF1 and LF2 are characterized by LREE-enriched patterns with negative Eu anomalies, enrichment in large ion lithophile elements (LILEs) and depletion in high-field-strength elements (HFSEs), which correspond with the previously published island arc environment. However, LF2 shows more distinctive patterns, which represents a later evolution stage than that of LF1. This volcanic-sedimentary succession reveals that the sedimentary-tectonic setting could be consistent with the subduction of Jinshajiang oceanic slab in the late Permian period.

Stratigraphy and Depositional Model of Palaeogene Dongying Braided-River Delta Sandstones, L Field, Bohai Bay Basin, Offshore Eastern China

Fluid production of L field, Bohai Bay Basin, offshore eastern China is mainly from the Palaeogene Dongying D interval. The stratigraphy framework and depositional model of the braided-river delta system within the D interval are investigated using seismic, well log, core data and production response. The D interval is interpreted as having been deposited in a lowstand system tract. And two progradational successions are recognized, including in ascending order the D2 and D1. The younger sandstone in the D1 interval is of greater thickness and larger distribution area than the elder sandstone in the D2 interval. Six core facies and five log patterns are recognized and interpreted to be underwater distributary channels, levees, overbank splays, mouth bars, sheet sand and shalier interchannel deposits. Channel deposits occur along the trend of thickest D2 and D1 sandstones. The sandstones that flank each side of the channel deposits are interpreted to be levee and overbank splay deposits. The sandstones that develop at terminal distributary channel mouth are interpreted to be mouth bar and sheet sand deposits. Channel-flank deposits can form good-quality reservoir sandstones, but they contain interbedded siltstones and thus have lower porosity and permeability than do channel deposits. The facies distributions predicted for the D interval match trends of the daily total fluid production. Knowledge gained from study of the L field has application to the development of other fields with similar depositional and diagenetic histories.

Spatial Discrimination of Complex, Low-Relief Quaternary Siliciclastic Strata Using Airborne Lidar and Near-Surface Geophysics: An Example from the Texas Coastal Plain, USA

Depositional units preserved on coastal plains worldwide control lithologic distribution in the shallow subsurface that is critical to infrastructure design and construction, and are also an important repository of information about the large-scale climate change that has occurred during many Quaternary glacial-interglacial cycles. The lateral and vertical lithologic and stratigraphic complexity of these depositional units and their response to climatic and sea-level change are poorly understood, making it difficult to predict lithologic distribution and to place historical and future climate and sea-level change within a natural geologic context. Mapping Quaternary siliciclastic depositional units on low-relief coastal plains traditionally has been based on their expression in aerial photographs and low-resolution topographic maps. Accuracy and detail have been hindered by low relief and lack of exposure. High-resolution airborne lidar surveys, along with surface and borehole geophysical measurements, are being used to identify subtle lateral and vertical boundaries of lithologic units on the Texas Coastal Plain within Quaternary strata. Ground and borehole conductivity measurements discriminate sandy barrier island and fluvial and deltaic channel deposits from muddy floodplain, delta-plain, and estuarine deposits. Borehole conductivity and natural gamma logs similarly distinguish distinct lithologic units in the subsurface and identify erosional unconformities that likely separate units deposited during different glacial-interglacial stages. High-resolution digital elevation models obtained from airborne lidar surveys reveal previously unrecognized topographic detail that aids identification of surface features such as sandy channels, clay-rich interchannel deposits, and accretionary features on Pleistocene barrier islands. An optimal approach to identify lithologic and stratigraphic distribution in low-relief coastal-plain environments employs ① an initial lidar survey to produce a detailed elevation model; ② selective surface sampling and geophysical measurements based on preliminary mapping derived from lidar data and aerial imagery; and ③ borehole sampling, logging, and analysis at key sites selected after lidar and surface measurements are complete.

Stratigraphic architecture of an Early–Middle Jurassic tidally influenced deltaic system (Plover Formation), Browse Basin, Australian North West Shelf

Understanding the internal stratigraphic architecture of sand-dominated deltas is critical to assessing the extent and distribution of petroleum reservoirs. The stratigraphic architecture and evolution of a major Early–Middle Jurassic fluvio-deltaic system (Plover Formation) on the Australian North West Shelf has been established through integrated analysis of core, borehole image logs and wireline logs for the Calliance field in the Browse Basin. Six facies associations identified in cored intervals are interpreted as tidally influenced channel- and tidal channel-fill complexes (FA1–FA2), crevasse-splay deposits and interchannel marshes (FA3), heterolithic mouthbars and sandflats (FA4), sandy mouthbars (FA5) and offshore transition to offshore (FA6). Therefore, the overall depositional system in the study area is proposed to be a tidally influenced deltaic system, in which FA6 represents prodelta deposits, FA5 and FA4 constitute distributary mouthbar deposits of the delta front and tidally influenced channel and interchannel deposits represent the lower delta plain (FA2, FA1, FA3). Analysis of image lithology and fabric are used to extend interpretation to uncored intervals and to identify intrusive and extrusive igneous units and associated volcaniclastic facies within the formation. Five third-order stratigraphic sequences (S1–S5) record progradational (S1, S2 and S4) and retrogradational (S3 and S5) phases of delta evolution. Paleocurrent indicators derived from borehole image logs indicate common southerly directed sediment dispersal in S2 and S3 and increasingly complex with westerly directions in S4 and S5. Two rift-related depositional phases are recognised separated by a phase of uplift between S3 and S4. The stratigraphic succession of S2 is consistent with the depositional pattern expected in a synrift setting and deposition of the over-thickened sandy succession (FA5 in S4), which is also the major reservoir interval, was most likely controlled by syn-tectonic faulting.

Tight gas sandstone reservoir delineation through channel-belt analysis, Late Jurassic Monteith Formation, Alberta Deep Basin

Abstract The Late Jurassic–Early Cretaceous Monteith Formation (Minnes Group) records incipient foredeep deposition in the Deep Basin of northwestern Alberta and British Columbia. The uppermost lithostratigraphic unit of the Monteith Formation, informally referred to as the Monteith A, is investigated with data from 550 wells, which includes wireline log suites from each well and approximately 540 m of core from 29 wells, across a 17,500 km2 area (T58–72, R6-20W6M). These data provide the basis for characterization of fine-grained, organic-rich interchannel deposits and channel-belt sandstone bodies. Channel-belt sandstone body dimensions are estimated to range from 4.1–12.6 m thick and 514–2851 m wide. Channel-belt sandstone body stacking patterns are variable laterally and vertically with the most amalgamated architecture present in the plains, in the southern portion of the area studied (T58–70) and within the lower section of the Monteith A interval. Stratigraphic architecture imparts a significant control on natural gas production from the Monteith A as almost all of the current production is coincident with amalgamated channel-belt units. The most prolific production from the Monteith A is in the Foothills where natural fractures enhance permeability within highly amalgamated channel-belts. In the Plains where channel-belts are highly amalgamated, gas distribution and producibility are controlled by inter-and intra-channel-belt architecture and associated zones of enhanced permeability. These zones of enhanced permeability are likely controlled by primary reservoir properties, diagenetic alteration and natural fractures. Heterogeneous reservoir quality and a relatively thin reservoir interval (typically 20–60 m thick) make horizontal wells and hydraulic fracturing a more efficient development strategy than vertical wells. An area >3000 km2 characterized by amalgamated channel-belt architecture is present in the lower portion of the Monteith A and may provide a widespread, ideal target suitable for horizontal drilling.

Facies distribution and stratigraphic architecture of the Lower Cretaceous McMurray Formation, Lewis Property, northeastern Alberta

Abstract Within the Lewis study area (Townships 91–92, and Ranges 6-8W4), McMurray Formation strata comprise four facies associations that form a depositional continuum of braided-fluvial (FA1), tidally-influenced braided- or low-accommodation meandering-fluvial and meandering-tidal channel-fills (FA2), associated overbank (FA3), and open-estuarine tidal flat deposits (FA4). The primary reservoir occurs in transgressive FA2 deposits. Lower-FA2 channels incise older water-saturated FA1 sand and consist of medium to locally-coarse, bitumen-saturated sand with rare to locally common pin-stripe laminated and/or 1-5 cm thick mud beds. Channels were initially confined by steep valley walls formed along the sub-Cretaceous unconformity. As a consequence, coeval interchannel sediment (FA3) was cannibalized by lateral channel migration and occurs only as common mud-clasts. The net result was the accumulation of a sand-rich, sheet-like deposit with locally-preserved fine-grained interchannel deposits, suggesting a high rate of lateral versus vertical accretion. With the filling and elimination of the irregular paleotopography along the unconformity, upper-FA2 channels became unconfined and formed thick, areally extensive inclined heterolithic stratification (IHS) deposits. Similarly, coeval interchannel deposits are more widely-distributed and thickly preserved compared to underlying strata. The position of upper-FA2 channels was indirectly influenced by paleotopography along the unconformity because IHS strata are thickest and sandiest along trends that overlie earlier channel systems, suggesting that syn-depositional salt dissolution controlled channel position and infilling history. Stratigraphically-upward there is a gradational change to tidal-flat deposits (FA4), suggesting abandonment of the main north-south channel system and a condition of coarse-grained bedload sediment starvation. Although most point-bar deposits within the Lewis study area consist of 10-30 m thick vertically-stacked successions of incomplete IHS deposits, several complete successions from channel base to overbank deposits are preserved. The geometric similarity between meandering-fluvial channels of different scales, climatic regimes, and substrate conditions suggest that the geometry of meanders is governed by mechanical principles. Although the applicability of these relationships to meandering-tidal channels is equivocal, using these formulas on IHS deposits of the McMurray Formation can provide a first order approximation of channel width (W=1.5h/tanβ), depth (0.595h), meander wavelength (λ=10.9w1.01), and point-bar length (λ). For two examples of complete point-bar deposits, channel widths were estimated to be at 148 m and 336 m; channel depth 12.6 m and 18.7 m; meander wavelength 1718 m and 3930 m; and point-bar length 859 m and 1965 m.

Depositional mechanisms and architecture of a pre-early Cambrian mixed sand–mud deepwater ramp (Apiúna Unit, South Brazil)

The Apiúna Unit is located in the south of Brazil, in the State of Santa Catarina. It is a deepwater depositional system, which is part of the sedimentary fill of the Itajaí Basin, pre-early Cambrian in age, interpreted as a foredeep basin. We have described and interpreted the depositional mechanisms according to the terminology proposed by Mulder and Alexander [Mulder, T., Alexander, J., 2001. The Physical Character of Subaqueous Sedimentary Density Flows and their Deposits. Sedimentology 48 (2), 269–299] and have recognized seven architectural elements. The Apiúna Unit is interpreted as a mixed sand–mud deepwater ramp system. The slope deposits are characterized by laminated argillite with slumped beds. The proximal and medial ramp is characterized by channel–levee systems. The distal ramp shows sandstone sheet, which pass distally into interlayered sandstone/pelite. The sequence development of this ramp unit differs from other known ancient ramp systems. The Apiúna Unit shows at least five phases of sandy input, recording times of progradation or retrogradation of the ramp, interstratified with muddy deposits, related to sand-starved phases. In the upper part of the succession, the ramp building was interrupted and the sandy deposits are replaced by pelitic slope deposits. The depositional mechanisms have a direct relationship to the architectural elements and the regions of the depositional system. Settling, very low-density turbidity currents and slumps formed the slope deposits. Channel deposits are formed by debris flows, hyperconcentrated density flows and concentrated density flows, in that vertical sequence order. Levee deposits were made of surge-like turbidity flows. Proximal sandstone sheet deposits were formed by concentrated density flows. Distal sandstone sheet deposits are formed by concentrated density flows and surge-like turbidity flows. Surge-like turbidity flows and quasi-steady (?) turbidity flows formed the interchannel deposits.

Sand-rich turbidite system of the Late Oligocene Northern Apennines foredeep: physical stratigraphy and architecture of the ‘Macigno costiero’ (coastal Tuscany, Italy)

Abstract The ‘Macigno costiero’ turbidite system characterized the oldest foredeep clastic wedge of the Northern Apennines during the Late Oligocene collisional phase. The cropping-out thickness is about 500 m. The features of the ‘Macigno costiero’ indicate a sand-rich, low-efficiency turbidite system. The system developed within a partially confined basin, which was part of a complex foredeep system. The stacking pattern of the turbidite system was determined through the analysis of facies and physical stratigraphy. It consists of a succession organized in sedimentary units, which are characterized by particular associations of facies linked to distinct depositional environments. Several architectural elements are seen: (1) unchannelized and channelized lobes; (2) distributary channels with channel-fill, overbank and channel-margin deposits; (3) main channel with channel-fill, channel-margin and interchannel deposits. Five turbidite stages were identified. From the bottom up they consist of four lobe stages and one proximal channel stage. The lobe stages are characterized by thickening-coarsening upward trends, from distal lobes to proximal lobes up to the channel-lobe transition zone. The uppermost, fifth stage is linked to a main channel complex with stacked channel-fill, channel-margin and interchannel deposits. This final stage also marks the maximum progradation of the system up to its closure due to the synsedimentary overthrusting of the orogenic wedge.

Sedimentology of the Bieumont Member: influence of the Lion Member carbonate mounds (Frasnian, Belgium) on their sedimentary environment

This is a sedimentological study of Middle Frasnian Bieumont Member fore-reef and off-reef carbonate sediments exposed in the Chimay-Couvin region (Belgium). It is based on four stratigraphic sections: the Lompret quarry, the Frasnes railway section, the Leus quarry and southern parts of the Lion quarry. The Lompret section is here described for the first time and represents the main focus of this work. The Bieumont Member consists of a bedded sequence of argillaceous and bioclastic limestone. At Lompret, the Member is exposed with a thickness of 48 m (the basal contact is not exposed). The Member consists of 6 lithological units (in stratigraphic order): 1. argillaceous limestone with episodic intercalations of reef debris (distal reef talus), 2. an alternation of marl and argillaceous limestone (basinal background sediment), 3. thickly-bedded bioclastic limestone (channel deposits), 4. a sequence of argillaceous limestone (inter-channel deposits), 5. bioclastic limestone with sporadic occurrence of framestone (proximal fore-reef deposits) and 6. fine-grained, bioclastic limestone interbedded with prominent layers of reef debris (distal fore-reef deposits). Within unit 5, an isolated reef block transported down the paleoslope is present. Microfacies analysis revealed 16 microfacies units which together with the lithological subdivisions were used to reconstruct the dynamic sedimentary history of the Bieumont Member at Lompret. There are two orders of depositional rhythmicity. The reef growth cycle starts with the first significant influx of reef-related facies at the beginning of unit 3 and from thereon is well differentiated from the Bieumont Member stratotype which largely consists of lithologies equivalent to units 1 and 2. Progradation was directed to the South and related shallowing-upward conditions culminated during the deposition of unit 5. On a lower scale, rhythmic bedding within units 2 and 4 could be related to orbitally forced cycling. The vertical and lateral variations of facies within the Bieumont Member were used to reconstruct a depositional model for the Lompret area. In addition, the documentation of facies variation on a regional scale was enhanced by using data from the Focant borehole and geological surveys of Han-sur-Lesse and Barvaux regions.

Thrust dissection control of deep-water clastic dispersal patterns in the Klematia–Paramythia foreland basin, western Greece

The Klematia–Paramythia basin is an internal part of the middle Ionian zone of the Hellenide orogen in western Greece. It consists of Middle Eocene to Late Miocene turbidites, up to 3300 m thick, which were deposited in a series of submarine fans. Field studies suggest that the configuration and the depositional environments of the basin were affected by two tectonic phases. During the first tectonic phase, in Middle Eocene to Late Oligocene times, a foreland basin was formed west of the Pindos Thrust front. During the second tectonic phase, in the Early Miocene, the Ionian zone (a part of the foreland basin) was subdivided by internal thrusting into three sub-basins (internal, middle and external) and changed to a complex type foreland basin. Comparison of the type and facies associations of the turbidite deposits that accumulated within the basin suggests that these two tectonic phases had a significant effect on sedimentary dispersal patterns. During the first tectonic phase in the Klematia–Paramythia basin (when it was part of the foreland basin), fine-grained turbidites, up to 1050 m thick, accumulated on the distal part of a submarine fan. The lower part (900 m thick) of these deposits consists of thin to thick interbedded sandstone/mudstone beds which are interpreted as lobes and lobe-fringe (outer-fan) deposits. The upper parts (150 m thick) of these deposits are composed of very thin to thin siltstone/mudstone beds, representing a basin plain environment. During the second tectonic phase, sediments up to 2260 m thick were deposited in the Klematia–Paramythia basin. These deposits are interpreted as lobes and lobe-fringe (outer-fan) fine-grained turbidites in the central part of the basin, channel and interchannel deposits (inner-fan) in some areas of the periphery of the basin, and shelf deposits in the northern and southern terminations of the basin.

Late Eocene to Early Miocene depositional environments of the Mesohellenic Basin, North-Central Greece: Implications for hydrocarbon potential

Late Eocene to Early Miocene Mesohellenic Basin deposits accumulated in a tectonically controlled basin with irregular and narrow shelves. The boundaries between the Eocene-Oligocene Krania-Eptachori Formations and the Oligocene-Miocene Eptachori-Pentalophos Formations record the global sea-level falling during the corresponding time intervals. The sediments of the Mesohellenic Basin have characteristics of lowstand fan deposits without coastal onlap. It is suggested that the concept of the hydrocarbon habitat is applicable to the Mesohellenic Basin due to the following characteristics of the studied deposits: (a) The outer fan sandstones lobes, classified in this work as suprafan lobes, are characterized by stacked channel sand bodies with good lateral and vertical communications. These features constitute excellent reservoir facies. (b) The fme-grained, lobe-fringe and basin-plain deposits of the studied marine turbidites may have operated as either fertile source rocks or topseals. (c) The geometry ofthe lobes (stacked channel sand bodies) and the internal unconformities (between the four formations) present excellent stratigraphic traps. (d) The synchronous evolution of these deposits assures the timely relationship among all factors listed above. Since the studied sediments correspond mostly to the western margin deposits it is very possible that basinwards, in the direction of the progradation, the interchannel deposits must laterally pass into, the outer fan sandstone lobe, lobe fringe and basin plain deposits. Therefore, sediments of the basin interior not presently exposed are very likely to support favourable conditions for petroleum play.

Flow processes and sediment deformation in the Canary Debris Flow on the NW African Continental Rise

The Canary Debris Flow formed an extensive deposit on the NW African Continental Margin west of the Canary Islands. Sidescan sonar images and 3.5-kHz profiles show that the middle part of the debris flow deposit consists of complex channel systems separated by banks and ridges of debris. Channels are typically up to 10 km wide and 10 to 20 m deep, and have little or no debris fill. They appear to feed a more laterally continuous debris flow sheet which is seen further downslope. Interchannel banks and ridges are composed of 5 to 20 m thick debris deposits. This morphology is remarkably similar to that seen in subaerial debris flows, and we therefore infer that the observed submarine debris flow morphology is a primary flow fabric, rather than the result of the debris flow interacting with or exploiting pre-existing channels. High-resolution Sidescan sonar images show that the debris flow surface is covered by sediment blocks up to 300 m in diameter. A single core collected from the flow shows that most of the 4.6-m flow thickness at the core site is composed of a single clast. The clast has been folded, with its upper part consisting of an inverted minor image of the lower part. The same sequence occurs again, in situ, beneath the debris flow, suggesting that the clast may have a local source, rather than having been derived from the debris flow source area, some 200 km upslope. This indicates that the debris flow was capable of substantial seabed erosion in the middle part of its course. In these middle reaches, erosion within the channelled areas probably occurred simultaneously with deposition in the interchannel areas. Interchannel deposits may contain both locally derived and original source area material.

A Southern Antler Foredeep Submarine Fan: The Mississippian Eleana Formation, Nevada Test Site

ABSTRACT Strata of the Eleana Formation in southern Nevada record the history of the Late Devonian through late Mississippian Antler foreland basin in southern Nevada. The Antler foredeep was created in latest Devonian or early Mississippian time, but did not receive significant sediments in southern Nevada until early-middle Mississippian (late Osagean) time. During late Osagean and Meramecian time, sediments deposited in the foredeep were dominantly mature, recycled siliciclastics derived from the uplifted Antler allochthon and foreland basin to the north in central Nevada. At the same time, a heterolithic sediment source, including mafic volcanic rocks and eroded, older limestones, supplied sediment intermittently and sparsely from the west-northwest. In late Mississippian (Chesterian) time siliciclastic sedimentation waned and deposition was dominated by fine-grained, carbonate- and siliciclastic-debris turbidity currents derived from an organically productive shelf or slope, probably to the north and northeast. The Eleana Formation on the Nevada Test Site is preserved in at least two pre-Tertiary thrust sheets that are separated by a fault with potentially large horizontal displacement. Although palinspastic reconstruction is not yet possible, understanding of this thrust juxtaposition allows a preliminary reconstruction of the Eleana basin paleogeography. The stratigraphically lower part of this Mississippian submarine fan comprised mainly channel and interchannel deposits. More proximal fan deposits are found in the upper thrust plate of an east-vergent thrust system, indicating that the axis of the fan was to the west of the present exposures of the Eleana Formation. The Antler foreland basin remained a basin in southern Nevada long after the Antler foredeep had filled in central Nevada. It may have been a peripheral deep marine basin that occupied either a reentrant in the old passive margin or a reentrant in the encroaching allochthon. Foredeep sediments were recycled down the basin axis from central Nevada south into the Eleana basin. Overlying allodapic limestone turbidites in the upper Eleana Formation were deposited as a transgressive deposit and are coeval with a known eustatic sea-level rise in Chesterian time.

Deep marine trace fossil assemblages from the Lower Carboniferous of Menorca, Balearic Islands, western Mediterranean

Sediments of Lower Carboniferous age in eastern Menorca, Balearic Islands contain a diverse and exceptionally well preserved ichnofauna, including Neonereites biserialis, Nereites isp., Arthrophycus isp., Dictyodora liebeana, two ichnospecies of Chondrites, several ichnospecies of Lophoctenium, two ichnospecies of Phycosiphon, Syncoprulus pharmaceus, annulated burrows and a vertical burrow. The host lithologies are conglomerates, sandstones, siltstones and mudstones; most are the product of debris flows, and of high concentration [Ta(bc) intervals] to relatively dilute (Tcd/Tde intervals) turbidity currents. The rocks are interpreted as deposits of an inner- to mid-fan palaeoenvironment. Channelized deposits, sequences of overbank deposition and interchannel deposits interspersed with the deposits of unconfined debris flows and high concentration turbidity currents are present. The ichnofauna is most frequently, and best, preserved within the Td interval of turbidities, which are interpreted as interchannel deposits, produced by low concentration turbidity flows. The distribution of the ichnofauna is partly controlled by the lithologies in which they are preserved. The rarity and poor preservation of trace fossils in the coarser grained facies contrasts with the detailed preservation of very delicate traces in the finer grained lithologies. However, the ichnofauna is also partitioned between different subenvironments of the mid-fan to produce a series of palaeoichnocoenoses. Thin intervals of interchannel deposits, separated by deposits of high concentration turbidity currents, repeatedly contain only Phycosiphon incertum and small (?juvenile)Dictyodora liebeana. These traces are interpreted as the products of opportunistic colonization of near-channel environments during episodes of quiescent deposition. Thicker intervals of interchannel deposits contain diverse assemblages of trace fossils characteristic of more stable environments, in which widespread colonization occurred. Overbank deposits at Cabo de Favaritz are medium- to thick-bedded, fine-grained beds. In these, the ichnofauna occurs in a simple, two-tier profile. The upper tier is dominated by Nereites isp.; this is underlain by a partially bioturbated layer characterized by large Dictyodora liebeana and Arthrophycus isp.

An interplay of syn‐ and intereruption depositional processes: the lower part of the Jangki Group (Miocene), SE Korea

ABSTRACTThe lower part of the Jangki Group (Miocene), SE Korea consists of pyroclastic mass‐flow‐dominated facies and epiclastic stream‐flow‐dominated facies which reflect sedimentation during syn‐ and intereruption periods, respectively. On the basis of pyroclastic composition, sedimentary structures and bed geometry, they are organized into two facies associations: (1) dacitic and basaltic debris‐flow and hyperconcentrated‐flood‐flow deposits of eruption periods, and (2) epiclastic stream‐flow and interchannel deposits of intereruption periods. The lateral relationship between the syn‐ and intereruption deposits varies significantly over short distances (2 km). In the western part of the study area, syneruption deposits are predominant, and fluvial deposits occur as small‐scale channel‐fill gravelstone bodies encased within dacitic debris flow deposits. In the eastern part, however, intereruption deposits are dominated with thick sequences of interbedded channel and interchannel deposits. The abrupt lateral change indicates alternation of epiclastic axial fluvial system with pyroclastic‐rich volcaniclastic aprons. The syneruption deposits are enriched in vitric ash but lack contemporary volcanic rock fragments (dacitic or basaltic). They are sharply differentiated from intereruption deposits that mostly consist of epiclasts and are deficient in vitric ash. The vertical transition suggests that streams drained a hinterland of igneous basement rocks during intereruption periods and became bulked with pyroclasts during syneruption periods.

Lowstand Fan Model, Basal Complex, Barbados: Utility as a Predictor of Reservoir Geometry and Continuity: ABSTRACT

Lowstand fan deposits on Barbados were studied in outcrop to construct a conceptual reservior model for prediction of facies assemblages. Lowstand fan and wedge deposits are found in terrigenous clastic strata of the Basal Complex of Barbados, and are of Eocene age. The Barbados lowstand fan model consists of five parts, discussed below. (1) Residual lag facies with contrasting texture (conglomerate and coarse-grained sandstone beds interbedded with pebbly mudstone and mudstone) and depositional processes (debris flows, high- and low-density turbidity currents, suspended-load deposits) are interpreted to characterize an area largely of sediment bypass (bypass corridor 1) occurring updip of the lowstand fan. Channel-fill complexes, isolated channel-fills and interchannel deposits are recognized by different types of upward-fining vertical successions and associations of lithologies and compose the lowstand fan proper. (2) Conglomerate, pebbly sandstone and high- and low-density turbiditic sandstone forming hierarchical upward-fining successions, characterize channel-fill complexes in the proximal fan, and are typically amalgamated with other channel-fill complexes. (3) High- and low-density sediment gravity-flow deposits in well-developed hierarchial upward-fining packages characterize amalgamated channel-fill deposits, and are interstratified with mud-rich facies strata representing abandoned channel deposits and interchannel deposits. Together, the amalgamated channel-fill deposits interchannel deposits characterize deposition in the middle fan.more » (4) A residual lag facies, similar to that observed in bypass corridor 1, characterized by cross-bedded lenticular tractional deposits, interbedded with low-density sediment gravity flow deposits, defines bypass corridor 2: this occurs at the transition from the middle fan to the distal fan. (5) Low-density sediment gravity-flow deposits in tabular beds are observed in the distal fan.« less

Palaeoenvironmental research on diatoms in early and middle Holocene deposits in central North Holland (The Netherlands)

In the geological research of the Holocene coastal deposits of The Netherlands, diatoms are used to reconstruct the sedimentary facies, palaeo-tide levels and salinity gradients during the deposition of the sediments. In this paper, the results of diatom research from 4 borings, taken from the early and middle Holocene deposits of central North Holland are presented. The oldest marine influenced sediments in the area are the deposits of the ‘Velsen layer’, a clay layer rich in organic matter and deposited about 8000–700014C years before present (BP) at a depth of 20–12 m below present mean sea-level. This clay layer was formed in a shallow, permanently submerged environment with a limited tidal influence (lagoonal or pond-like conditions). The salinity changed from brackish/freshwater to marine/brackish. The younger sandy and clayey sediments, formed about 7000–4500 BP at a depth of 16–3 m below present mean sea-level, are classified as ‘tidal channel’ and ‘interchannel’ deposits. It is argued, on the base of both diatom and non-diatom criteria, that the lower and middle parts of the interchannel deposits in the central area of the palaeo-tidal basin of North Holland were formed in a subtidal environment. The upper part of the interchannel deposits and the deposits at the fringe of the North Holland tidal basin were formed in the intertidal zone or, at the fringe of the basin, even in the supratidal zone. The salinity in the North Holland tidal basin during the sedimentation of the tidal channel and interchannel deposits was marine/brackish to marine. This study indicates that diatoms, besides their palaeoecological applications, have chronostratigraphical significance (on a regional scale). The diatomsCymatosira belgica andActinoptychus splendens appear to be a useful (eco)stratigraphical marker in the Holocene coastal deposits of The Netherlands and Belgium.

Glacially influenced sedimentation in the Later Proterozoic of the Paraguay belt (Mato Grosso, Brazil)

Late Proterozoic (probably early Vendian, 610-590 Ma) glaciogenic and turbiditic sediments and metasediments are present along the transitional zone between the Amazonian Craton to the northwest and the Paraguay Belt to the southeast, in the Cuiabá region, Mato Grosso, Brazil. A few exposures of diamictites are associated with conglomerates and graded pebbly sandstones on the cratonic fringe and pass southeastwards into fine-grained sediments deposited in the deeper part of the probably marine basin. In the Paraguay Belt, these rocks have been affected by the Brasiliano Orogeny, possibly around 570 Ma. They show increasing deformation and metamorphism from the craton towards the fold belt. The sedimentation scheme proposed for the western border of the Paraguay Belt and the eastern edge of the Amazonian Craton involves three, probably marine, main depositional systems. To the west, on the craton, the platform domain can be subdivided into a western inner shelf and an eastern outer shelf. The deposits on the inner shelf subdomain show an alternation of dominant massive diamictites, sandstones and fine-grained sediments with few dropstones. This diamictite association is tentatively interpreted as glaciomarine in origin. In the outer shelf subdomain, the massive diamictites are progressively replaced southeastwards by an association of massive diamictites, stratified diamictites and fine-grained sediments. This diamictite association is interpreted, as a whole, as resedimented glacial debris deposited by subaqueous debris flows. The slope domain, located at the transition between craton and fold belt, is characterized by strong reworking of the glaciomarine sediments by gravity flows responsible for a type of submarine fan deposition. The coarse clastic facies, which result from a reworking by rapid downslope transport of cohesive debris-flows and gravity flows with high viscosity, fill feeder channels in the upper fan. Eastward, inside the belt, a progressive sorting in the deeper portions of the fan is shown by the transition from diamictites, massive gravels, sandstone intercalations with occasional inverse and/or normal grading (debris flow + turbidity current) to normal graded, finer sediments (turbidity current). Sandstone and siltstone intercalations represent interchannel deposits formed by turbidity currents. East of the slope, well-bedded, fine-grained sediments were generated by low-density turbidity currents. Direct glacial influence is reduced to the presence of a few isolated clasts or dropstones. Two distinct Upper Proterozoic glaciogenic units are known in Brazil. The older and more widespread one, dated at about 950 Ma, is located on the São Francisco Craton and in the adjacent Brasiliano fold belts. The younger one (≈600 Ma) is restricted to the Paraguay Belt and the adjacent border of the Amazonian Craton. In southern Western Gondwana, Late Proterozoic glaciation is of mountain-type and developed at latitudes of 60° on highs probably generated by Pan African-Brasiliano rifting. At the same time, in its northern part, a widespread ice sheet covered the West African Craton.