Multistorey Sand Bodies Research Articles

Sedimentary characteristics of sandy braided river deposits and factors controlling their deposition: A case study of the lower Shihezi Formation in the northern ordos basin, China

Fluvial sandstones have been identified as significant reservoirs due to their importance in global hydrocarbon exploration. The current study investigated the sedimentary characteristics and temporal evolution of sandy braided river systems as well as the factors controlling their deposition in the Lower Shihezi Formation (H1-1 to H1-4 sub-member) of the Xinzhao area in the northern Ordos Basin using well logs, core intervals, whole rock geochemical data, and other measured reservoir parameters. Within the braided river deposits, five architectural elements were identified: multistory braided channel, single-story braided channel, sandy braid bar, overbank, and floodplain. The evolution of architectural elements in the Lower Shihezi Formation is closely related to paleoclimatic conditions and tectonic activity. Climate influences sediment accumulation through its effect on terrestrial detrital input, and tectonism influences stacking patterns and architectural style by influencing subsidence rates and accommodation changes. High sediment deliveries and high-energy hydrodynamic conditions in warm and humid climates, combined with low subsidence rates and low accommodation/sediment supply (A/S) ratios, can result in the formation of highly amalgamated braided channels and braid bars as well as complex vertical stacking patterns and aggradational processes in the H1-2 and H1-3 sub-members. Conversely, the dry and hot climates, high subsidence rates and associated high A/S ratios are responsible for the low detrital influx input, stacking of isolated sand bodies, and high preservation of overbank and floodplain deposits in the H1-4 sub-member. Considering the accommodation-based systems tracts, we contend that climate and tectonism both have a significant impact on the evolution and architectural styles of sandy braided river deposits. It is suggested that climates control the stacking pattern of fluvial sand bodies during high accommodation systems tracts, whereas both climates and accommodation spaces influence architectural styles and the degree of sand body amalgamation during low accommodation systems tracts. Considering the thick, highly amalgamated sand bodies with better reservoir properties and high vertical and lateral connectivity in the H1-2 and H1-3 sub-members, we posit that these stacked multistory sand bodies should be the preferential targets for hydrocarbon exploration. The findings of this study can further elucidate the architectural styles and factors that influence the deposition of sandy braided river systems under complex tectonic and climatic conditions.

Litologie a paleopůdy klikovského souvrství ve vrtu 2140 01T1 Dunajovice, třebonská pánev

The 2140 01T1 borehole sampling the Late Cretaceous non-marine strata of the Klikov Formation in the central part of the Třeboň Basin in southern Bohemia provided ~ 70 m thick alluvial succession composed of alternating sandstone and mudstone intervals that overlays deeply weathered crystalline basement. The mudstone interval in the lower part of the Klikov Formation is separated by sandy layers into several 2–4 m thick fining-upward units. The sharp-based sandy layers with fusain, carbonized stems and rip-up clasts of underlying mudstone are interpreted as avulsion deposits. They grade upwards into heterolithic sand-silt sequence and mottled massive mudstone with burrows, rhizohalos and variably developed pedogenic features. Such cyclic arrangement probably reflects autogenic processes on distal floodplain with episodic deposition and upward increasing subaerial exposition and pedogenic overprint. In the middle to upper parts of the Klikov Formation up to 6 m thick channel sandbodies with sharp erosional bases cutting into the underlying laminated sand-silt heteroliths of channel levee deposits occur. The thickness of the single-storey sandbodies corresponds usually to c. 2 m which suggests up to 3 m deep channels during bankfull discharge. The thicker multi-storey sandbodies record channel amalgamation on the alluvial ridge or shallow paleovalley fillings. The sandbodies grade upwards to massive mudstones of abandoned channel fills and floodplain deposits. These floodplain units are up to 8 m thick and consist of variegated massive mudstones interrupted by discrete up to 1 m thick fining upward layers of rippled sandstone representing crevasse splay deposits. Depositional context including preserved levee deposits, abandoned channel fillings and floodplain finegrained deposits suggests rather higher sinuosity of the formative rivers. Palaeosol profiles represent pedogenically modified floodplain fines with gradational bases and sharp erosive top surfaces. At least 10 palaeosols were identified in the measured section, forming mostly discrete profiles up to 2 m thick consisting of several B-horizons that differ in the degree of the soil structure development, colour, mottling, grain size, presence of clay hypocoatings and bioturbation index. The most common Bw horizons form lower parts of the profiles and have no or only a weakly developed angular blocky structure (ABS). The typical mottling is caused by a hierarchical system of vermicular greenish-grey reduction rhizohalos, that may be filled by silt to sand or illuviated clay hypocoatings and probably follow the original plant root system. Some patches have diffuse orange rims of iron oxides, probably represented by goethite. Mudstone of Bsst horizons displays well-developed ABS with rare wedge-shaped peds and contains a number of illuviated clay skins and redox depletions. Vertic features like fine cracks filled by sand are common, and slickensides with clay hypocoatings occur rarely. Mudstones of Bt horizons contain a number of illuviated clay hypocoatings filling root channels or interpedal cracks, while the Bss horizons exhibit well-developed ABS with slickensides and contain a number of vertical cracks filled by sandy grains. Palaeosol profiles represent mainly argillisols and vertic argillisols with transitions to argillic vertisols. Poorly developed argillic to vertic protosols are also common. Palaeosol profiles differ mainly in the degree of structural development/maturity, which is given by various lengths of exposition to similar climatic conditions. Vertical distribution of palaeosols reflects their position in the alluvial landscape with locally specific hydromorphic conditions rather than a distinct climatic trend. Most of palaeosols exhibits downprofile translocation of clay (illuviation) leading to the formation of argillic horizon, which typically develop on stabilized landscape in humid climate with seasonal moisture deficit.

Controls on the deposition and preservation of architectural elements within a fluvial multi-storey sandbody

Architectural elements of fluvial multi-storey sandbodies provide principal controls on the distribution of meso-scale (100–101 m scale) heterogeneity and reservoir quality. Consequently, it is valuable to understand the deposition and preservation of sedimentary architecture in such systems in relation to autogenic (stream capture and avulsion) and allogenic controls (subsidence rates, climate and sediment supply). The aims of this study are to quantify the architectural and erosional nature of a fluvial multi-storey sandbody and to establish the effects of downstream distance and subsidence rates upon the preservation of architectural elements, using the Lower Castlegate Sandstone, Utah, USA, as an example. Quantitative architectural element analysis and palaeodischarge reconstructions were undertaken from eight locations using sedimentary logs and three terrestrial photogrammetric outcrop datasets along a 150 km down-dip profile. These observations were supplemented by burial history analysis of ten wells across the same profile. Results show the Lower Castlegate comprises channel-fill, downstream accretion, lateral accretion, upstream accretion and overbank elements. From these observations, calculations of sinuosity and flow depths along with architectural geometric analysis provide evidence of stream capture contemporaneous with foreland basin subsidence. The preservation of lateral accretion and overbank elements is limited within the distal portion of the multi-storey sandbody, as a result of local avulsion and limited subsidence rates. Results demonstrate that complex sedimentary architecture can form in fluvial multi-storey sandbodies as a product of variable discharge rates, the fluvial graded profile and spatially variable aggradation rates, driven principally by subsidence rates. The use of meso-scale architectural analysis, with analysis of in-channel sinuosity and hydrodynamics, along with erosional bounding surfaces, has helped to complemented basin-scale interpretations of fluvial architecture.

Discriminating stacked distributary channel from palaeovalley fill sand bodies in foreland basin settings

Stacked fluvial distributary channel deposits and palaeovalley fills can form major, multi storey sand bodies with similar thicknesses, and with lateral extents often greater than a single exposure. Consequently, they can be difficult to tell apart from one another using outcrop data. This study addresses this problem by quantitatively analysing the architecture of five stacked fluvial distributary channel deposits and two palaeovalley fills from the Pennsylvanian Pikeville and Hyden formations of the central Appalachian Basin, USA. The a priori interpretation of the sand bodies as stacked distributary channels and palaeovalley fills is possible because a robust in-place coal seam correlation framework allows for the recognition of different basin-scale architectures for each type – aspect ratios <1000 and envelopes of fluvial and deltaic strata for stacked distributary channels, and aspect ratios >1000 and a regional basinward facies shift at the bases of palaeovalley fills. Sand body thickness, storey thickness, position and length of storey contacts within the sand body are similar in both types. However, they can be distinguished by different up-system to down-system changes in their respective architectures. Stacked distributary channel sand bodies thin down system, display a decrease in storey thickness, an increase in the mean position of storey heights in the sand body and a decrease in the length of storey contacts. These trends are the result of down-system decrease in channel size, and confinement associated with radially distributive fluvial systems. Palaeovalley fill sand bodies thicken down-system, display an increase in storey thickness, a decrease in the mean position of storey heights, and a decrease in the length of storey contacts. The increase in sand body and storey thickness are the result of down-system increases in original channel size, consistent with trunk axial fluvial systems fed by tributaries that predominate during valley-formation. The down-system increase in amalgamation reflects a down-system decrease in accommodation, from the higher subsidence rate active margin of the basin, and is therefore not necessarily characteristic of palaeovalley fill architectures in all basin settings. This study emphasises the requirement for detailed correlation work and quantitative analysis of external and internal architectures before the interpretation of sand bodies as stacked distributary channels or palaeovalley fills.

Confluence Scours Versus Incised Valleys: Examples From the Cretaceous Ferron Notom Delta, Southeastern Utah, U.S.A

Abstract Confluence scours in braided rivers occur where channel threads join together, producing erosional relief that may be considerably deeper than average channel depth. Based on studies of the continental-scale Ganges–Brahmaputra river system, it has been observed that the maximum depth of confluence scours by autocyclic processes may reach up to four to five times the average depth of the incoming channels. Considering the possibility of such massive scours in an ancient fluvial system, it was argued that allogenically produced incised valleys at sequence boundaries should only be properly defined in ancient systems if the erosional relief is more than five times average channel depth. Based on cross-sectional geometries, a number of confluence scour fills were interpreted on well-exposed fluvial outcrops in a Cretaceous-age compound incised-valley system in the Notom delta complex of the Ferron Sandstone Member, Utah. Cross-sectional observations from outcrops and comparisons with modern rivers reveal that confluence scours have diagnostic fill facies (single set of large steep foresets) and do not produce multistory sand bodies. A confluence scour fill produces a single-story body in which a fifth-order scour is filled with unit bar foresets, which in turn are overlain by a fourth-order surface capped by compound bar deposits. The story thickness in the confluence scour does not represent the average channel depth because confluence scours allow preservation of the deepest parts of channels as well as fully preserved abnormally thick stories. Therefore, we argue that interpretation of incised valleys in the ancient system associated with a sequence boundary should not be based on the depth of the erosional surface versus the number of average preserved channel stories. Rather, it should be defined by the erosional relief that is significantly deeper than the thickest fully preserved stories, which in a braided stream are likely to represent confluence scour fills.

Vertical structure and dominating factors of sand body during Late Triassic Chang 9 time of Yanchang Formation in Ordos Basin, NW China

Based on a synthetic geological study of drilling, well logging and core observations, two main genetic types of Chang 9 sand body in Odors Basin were recognized, which included two effects, that is, delta environment and tractive current effects that lead to the development of mouth bar, distal bar, sheet sand and other sand bodies of subaerial and subaqueous distributary channel, natural levee, flood fan and delta front, and shore-shallow lake environment and lake flow transformation effects that result in the development of sandy beach bar, sheet sand and other sand bodies. Chang 9 sand body mainly developed five basic vertical structures, namely box shape, campaniform, infundibuliform, finger and dentoid. The vertical stacking patterns of multilayer sand body was complex, and the common shapes included box shape + box shape, campaniform + campaniform, campaniform + box shape, infundibuliform + infundibuliform, campaniform + infundibuliform, box shape + campaniform, box shape + infundibuliform, and finger + finger. Based on the analysis on major dominating factors of vertical structure of sand body, sedimentary environment, sedimentary facies and rise, fall and cycle of base level are identified as the major geological factors that control the vertical structure of single sand body as well as vertical stacking patterns and distribution of multistory sand bodies.

Insights into Pleistocene palaeoenvironments and biostratigraphy in southern Buenos Aires province (Argentina) from continental deposits

The coastal cliffs of the Buenos Aires province (Argentina) have been the subject of intense paleontological studies since the XIX century. Therefore, many of the type localities in which is based the late Cenozoic Pampean biostratigraphic/chronostratigraphic scheme are located in this area. In this context, the sedimentites that crop out near the mouth of the Chocorí Creek contain a set of palaeontological sites that, because of their richness and well-preserved fossil content, hold high national and international importance. The aims of the present contribution are: 1) to make a stratigraphic and sedimentological characterization of the study area; 2) to list the fauna outcropped at these palaeontological sites and establish a biostratigraphic framework; 3) to elaborate a palaeoenvironmental model for the area.The study interval was informally subdivided into a lower, middle and upper interval. Interpretation was based on the presence of a number of key features such as architectural elements; channel:overbank ratio and palaeosol occurrence. The first two intervals were interpreted as continental deposits of a fluvio-alluvial nature and are the focus of this paper. The upper interval was related to foreshore marine deposits and will be studied in a future contribution. The lower interval is characterized mainly by overbank architectural elements in which calcisols and argillic protosols were identified. Channel-fill deposits are isolated and surrounded by fine-grained overbank successions and sedimentary structures are suggestive of mixed-load transport. The contact between the lower and middle intervals is an irregular, highly erosive surface characterized by a significant vertical change in the facies. This surface defines the base of multistorey sandbodies which's internal arrangement alongside with the low participation of overbank deposits suggests deposition by a braided fluvial system.Palaeosols and vertebrate fossils were used as palaeoclimatic, palaeoenvironmental and biostratigraphical proxies. Calcisol profiles, displaying Stages II to V morphologies (Machette, 1985), can be interpreted as evidence of periods of geomorphological stability that occurred under semi-arid to sub-humid climatic conditions. The occurrence of argillic Protosols stacked amongst the Calcisols evidence periods of relatively less stability, higher sediment supply and aggradation rates in the system. The vertebrate fossil assemblage and the invertebrate trace fossils also indicate semi-open landscapes under a seasonal, semi-arid climate. The presence of Platygonus, Glyptodon and Tolypeutes fossil remains in the lower interval suggest an Ensenadan age (middle Pleistocene) while the presence of Arctotherium bonariense in the V1 layer indicates post-Ensenadan (late Pleistocene) times for the middle interval.It is concluded that during accumulation of the Chocorí succession, glacio-eustasy and/or climate controlled the balance between generation of accommodation space and sediment supply. Analysis of the architectural elements indicates a general reduction in accommodation space. The lower interval represents the unconfined reaches of a large distributive system, more specifically, a low hierarchy, secondary drainage system inset in a high accommodation alluvial environment. The erosive surface identified at the base of the middle interval can be interpreted as representative of a period of negative accommodation in the system, when general erosion took place. The gradual restoration of accommodation in the fluvial system was accompanied by a low sediment accumulation rate and the development of a braided fluvial system in the middle interval.

Interpreting Paleo-Avulsion Dynamics from Multistory Sand Bodies

Abstract Channel avulsion plays a significant role in building alluvial stratigraphy, including fluvial sand bodies with through-going erosion surfaces, called multistory sand bodies (MSBs); consequently, MSB characteristics may be useful for interpreting paleo-avulsion dynamics from fluvial deposits. Here, using a combination of literature review, geometric modeling, and field observations, we explore the degree to which MSB characteristics may be useful for interpreting paleo-avulsion dynamics from ancient deposits. We use published interpretations of MSB origins to identify characteristics that are exclusively associated with avulsion-related MSBs and find that vertical story stacking, irregular MSB bounding surfaces, and floodplain deposits that correlate with individual stories are attributes uniquely associated with avulsion-origin MSBs. To evaluate how avulsion patterns may affect MSB properties, we use a 2D geometric model to build synthetic stratigraphy and compare characteristics for MSBs formed under different avulsion patterns at a range of model-aggradation rates. Avulsion patterns include random (equal probability of a channel relocating anywhere across the model domain), compensational (channels avulse to the lowest elevation in the model domain at each time step), and clustered (where channel relocation is restricted to a zone near a previously occupied location). Model results show that at moderate to high aggradation rates, MSBs formed under clustered avulsion patterns can be differentiated from those formed via random and compensational avulsion patterns. Specifically, MSBs formed by clustered avulsion patterns have many stories and commonly contain remnant floodplain deposits. We apply these insights to the well-exposed lower Williams Fork Formation (Upper Cretaceous, Piceance Basin, Colorado, USA) to demonstrate how they can be used in practice. Fifty-four percent of lower Williams Fork MSBs contain clear evidence for avulsion reoccupation, 32% have characteristics suggesting intra-channel-belt origins, and the remaining 20% have ambiguous evidence or insufficient exposure to permit confident process interpretations. Overall this study demonstrates that despite inherent challenges in eliciting information about avulsion dynamics from the stratigraphic record, some MSB characteristics can be interpreted as clear signals of paleo-avulsion processes.

Complexity in a cellular model of river avulsion

We propose a new model of river avulsion that emphasizes simplicity, self-organization, and unprogrammed behavior rather than detailed simulation. The model runs on a fixed cellular grid and tracks two elevations in each cell, a high elevation representing the channel (levee) top and a low one representing the channel bottom. The channel aggrades in place until a superelevation threshold for avulsion is met. After an avulsion is triggered a new flow path is selected by steepest descent based on the low values of elevation. Flow path depends sensitively on floodplain topography, particularly the presence of former abandoned channels. Several behavioral characteristics emerge consistently from this simple model: (1) a tendency of the active flow to switch among a small number of channel paths, which we term the active channel set, over extended periods, leading to clustering and formation of multistory sand bodies in the resulting deposits; (2) a tendency for avulsed channels to return to their previous paths, so that new channel length tends to be generated in relatively short segments; and (3) avulsion-related sediment storage and release, leading to pulsed sediment output even for constant input. Each of these behaviors is consistent with observations from depositional river systems. A single-valued threshold produces a wide variety of avulsion sizes and styles. Larger “nodal” avulsions are rarer because pre-existing floodplain topography acts to steer flow back to the active channel. Channel stacking pattern is very sensitive to floodplain deposition. This work highlights the need to develop models of floodplain evolution at large time and space scales to complement the improving models of river channel evolution.

Possible climate variability in the Lower Old Red Sandstone Conigar Pit Sandstone Member (early Devonian), South Wales, UK

The Early Devonian Conigar Pit Sandstone Member (CPSM) of the Freshwater West Formation (Lower Old Red Sandstone, Lochkovian age) at Freshwater West in south Wales comprises a heterolithic, predominantly alluvial suite (mudstones, fine to coarse-grained sandstones, conglomerates) with varying depositional architecture. A number of discrete facies associations are observed. The fine-grained, inclined- and non-inclined heterolithic association is dominant within the CPSM. It represents dryland sinuous channelized flow (IHS bedsets) and unconfined flow (non-inclined bedsets) at terminal and intermediate floodouts deposited under semi-arid conditions. Mudstones were deposited in two distinct environments associated with this semi-arid river system. Laminated and burrowed, reddish brown mudstones were deposited in shallow semi-permanent lakes or pools on the floodplain. Massive mudstones were deposited as within-channel muddy braid-bars, with sedimentary structures being lost during subsequent compaction. The mudstones exhibit Vertisol pedogenesis reflecting the seasonal wetting and drying associated with the semi-arid climate. Periods of intense desiccation are indicated by deep desiccation cracks and associated rubbly surfaces. Possible, though unproven tidal conditions influenced a small percentage of the heterolithic channels at Freshwater West. The extent of these channels is uncertain. The coarser-grained multi-storey sandstone association was deposited by low-sinuosity rivers with a fluctuating, but perennial discharge. Associated with these laterally extensive sandbodies was a high water table with surface ponding (wetlands) that enabled the preservation of plant detritus. During high discharge events, flow expanded over the contemporaneous floodplain depositing the sheet sandstone association within splay complexes. The interpretation that perennial discharge may have been triggered by basin-wide climate change challenges the long-held view that the Lower ORS climate was continuously semi-arid in nature. The spatial/temporal extent of this climate change is uncertain, as it is possible that the multi-storey sandbodies were deposited by allogenic rivers draining a distant, possibly uplifted source area. A decrease in the observed frequency and maturity of Vertisol profiles, and a corresponding increase in multi-storey sandbodies upward through the succession may reflect a long-term, episodic pattern of climate change.

Evolution of an ancient coastal plain: palaeosols, interfluves and alluvial architecture in a sequence stratigraphic framework, Cenomanian Dunvegan Formation, NE British Columbia, Canada

To date, discussion of changes in alluvial style and in the character of palaeosols in relation to changes in accommodation and sediment supply on floodplains has primarily been from a conceptual standpoint: few case studies are available against which to test ideas. One hundred and thirty metres of non‐marine strata of the Dunvegan Formation were examined in 14 closely spaced sections in the canyon of the Kiskatinaw River, NE British Columbia, Canada. This site was located about 120 km inland from the transgressive limit of the contemporary marine shoreline and represents almost exclusively freshwater environments. Fluvial channels in the Kiskatinaw River section are of two types. Small, single‐storey, very fine‐ to fine‐grained sandstone ribbons with W/T ratios &lt;30, encased in fine‐grained floodplain sediments are interpreted as anastomosed channels. Fine‐ to medium‐grained, laterally accreted point‐bar deposits forming multistorey sand bodies with individual W/T ratios &gt;30 are interpreted as the deposits of meandering rivers filling incised valleys. Interchannel facies include the deposits of crevasse channels and splays, lakes, floodplains and palaeosols. Floodplain palaeosols consist of laterally heterogeneous, simple palaeosol profiles and pedocomplexes similar to modern Entisols, Inceptisols and hydromorphic soils. Interfluve, sequence‐bounding palaeosols adjacent to incised valleys are laterally continuous, up to 3 m thick and can be reliably identified using a combination of (1) stratigraphic position; (2) field observations, such as thickness, structure, colour, degree of rooting; and (3) micromorphological features, such as evidence of bioturbation, clay coatings, ferruginous features and sphaerosiderite. Interfluve palaeosols are similar to modern Alfisols and Ultisols. Correlation of the local stratigraphic succession with the regional sequence stratigraphic framework, based on 2340 well logs and 60 outcrop sections, shows that the vertical changes in coastal plain character (more coals and lakes vs. more pedogenesis) can be related to relatively high‐frequency base level cycles (eustatic?) that are expressed as transgressive–regressive marine cycles in downdip areas. Regional isopach maps show that these cycles were progressively overprinted and modified by an increasing rate of tectonic subsidence in the north and west. The character of palaeosols developed on aggrading floodplains primarily reflects local sediment supply and drainage. In contrast, well‐developed interfluve palaeosols record pedogenesis during periods of reduced or negative accommodation (base level fall). Vertical changes in floodplain palaeoenvironments and palaeosol types reflect changes in accommodation rate. The detailed micromorphological analysis of interfluve palaeosols represents a powerful application of an under‐used technique for the recognition of key surfaces in the geological record. This has broad implications for non‐marine sequence stratigraphy.

Petrology of caliche-derived peloidal calcirudite/ calcarenite in the late triassic maleri formation of the pranhita-Godavari valley, South India

The Maleri Formation (Late Triassic) of the Pranhita-Godavari Valley, South India, is a continental red-bed sequence represented mainly by red clays with a few sheet-like bodies of channel-fill sandstone. Crudely bedded and cross-bedded peloidal calcirudite/calcarenite occurs at the base of fining-upward sequences in multistorey sand bodies and also as discrete solitary lenses within red clay. The calcirudite/calcarenite consists mainly of calcite-cemented spherical peloids which are made up of micrite and microspary calcite with microfabrics characteristic of caliche. Pure calcitic peloids are often reddened and those having a quartzose component are generally not red. The paucity of broken and abraded peloids, close association with intraformational materials and other evidence indicate a local source of the peloids. The microfabric of the peloids suggest their derivation from caliches. Available evidence favours a pedogenic origin for the peloids which mostly developed by displacive precipitation in a set-up of alternate wetting and drying. The lack of in-situ caliche profiles in the Maleri sequence and the rarity of compound grains imply that the peloids were derived from incipient caliche profiles which were localized at or near the surface and were completely stripped off by subsequent erosion. The ghost caliche profiles suggest the presence of periodically stable levels in the Maleri alluvial plain and low to moderate rates of alluviation. The residence intervals of channels were long enough to allow incipient pedogenesis in the temporarily stable inactive areas. In the present context, the predominance of smectite in fines, the poor floral content, the Maleri faunal assemblage and the paucity of evaporites point to a low seasonal rainfall in a semi-arid climatic environment.

Upper Old Red Sandstone sedimentation in the Buchan oilfield, North Sea

Synopsis A fining and maturing upwards fluvial sequence of Devonian to Viséan age forms the reservoir section of the Buchan oilfield. An upward transition occurs from mainly bed-transport deposits to bed-transport plus suspension deposits. The lower part of the sequence comprises multi-storey sand bodies deposited in mobile-channel belts and during sheetfloods. The upper part of the sequence is characterised by a greater number of fining-upward cycles, an increase in the amount of fine grained material and an increase in the number of cornstone horizons. It was deposited in a system of channels and associated flood basins, over which waning-flow sheetflood deposition periodically occurred. The upward change in facies and sedimentation style is interpreted as being due to the combined influences of three processes: a) a rise in sea level that raised river base levels; b) a change in climate and c) a decrease in the sediment supply from the soure area, which was possibly a granitic intrusion to the WNW of the field.

Depositional environments and paleogeography in the Albian Moosebar Formation and adjacent fluvial Gladstone and Beaver Mines formations, Alberta

The marine Moosebar Formation (Albian) has a currently accepted southerly limit at Fall Creek (Ram River area). It consists of marine mudstones with some hummocky and swaley cross-stratified sandstones indicating a storm-dominated Moosebar (Clearwater) sea. We have traced a tongue of the Moosebar southward to the Elbow River area (150 km southeast of Fall Creek), where there is a brackish-water ostracod fauna. Paleoflow directions are essentially northwestward (vector mean 318°), roughly agreeing with turbidite sole marks (329°) in the Moosebar of northeastern British Columbia.The Moosebar sea transgressed southward over fluvial deposits of the Gladstone Formation. In the Gladstone, thick channel sands (4–8 m) are commonly multistorey (up to about 15 m), with well developed lateral accretion surfaces. The strike of the lateral accretion surfaces and the orientation of the walls of channels and scours indicate northwestward flow (various vector means in the range 307–339°). The Moosebar transgression was terminated by construction of the Beaver Mines floodplain, with thick, multistorey sand bodies up to about 35 m thick. Flow directions are variable, but various vector means roughly cluster in the north to northeast segment. This indicates a major change in dispersal direction from the Gladstone and Moosebar formations.A review of many Late Jurassic and Cretaceous units shows a dominant dispersal of sand parallel to regional strike. This flow is mostly north-northwestward (Passage beds, Cadomin, Gladstone, Moosebar, Gates, Chungo), with the southeasterly dispersal of the Cardium being the major exception. Only at times of maximum thickness of clastic input (Belly River and higher units, and possibly Kootenay but there are no published paleocurrent data) does the sediment disperse directly eastward or northeastward from the Cordillera toward the Plains.

Determination of Widths of Meander-Belt Sandstone Reservoirs from Vertical Downhole Data: ABSTRACT

Once it has been determined that a meandering fluvial model is applicable to a formation, paleohydrologic reconstructions can be applied to downhole measurements to derive sand body widths. The nonmarine part of the Mesaverde Formation in the east-central part of the Piceance Creek basin of northwestern Colorado was deposited in a predominantly meandering fluvial End_Page 505------------------------------ system. This interpretation is based on the presence of lateral accretion beds and multi-story sand bodies in outcrops along the Grand Hogback. It is supported by fining-upward trends in cores and gamma ray logs from the wells of the U.S. Department of Energy's Multi-Well Experiment being conducted near Rifle, Colorado. Paleochannel depths, recognized from the heights of fining-upward trends in cores, can be converted to channel widths using Leeder's 1973 formula. The resulting channel widths are used to calculate meander belt amplitudes (sandstone body widths) from relationships derived from Leopold and Wolman's 1960 empirical data. These numbers can be compared with sandstone body widths derived by two other methods. (1) Point-bar dimensions measured in outcrops can be used to calculate channel widths, which are then converted to meander-belt amplitudes as described above. (2) The MWX-2 well is offset from MWX-1 by 135 ft (41 m) at the surface, allowing for positive well-to-well correlation of sandstones. The percentage of sandstones which are penetrated by both wells is used as a probability to derive an average sand body width. These last two methods give compatible results. The first method described, however, predicts comparatively narrow widths, suggesting that channel depths derived from fining-upward sequences in cores are not fully representative of ancient channel depths due to incomplete preservation. A preservation potential factor may be added to Leeder's formula for working with ancient sediments, so that the formula gives comparable results to the other two methods. End_of_Article - Last_Page 506------------

Sandy high-energy flood sedimentation — Some criteria for recognition, with an example from the devonian of S.W. England

Many modern sand deposits resulting from episodic floods in semi-arid zones are dominated by parallel laminations, but parallel-laminated sands are not a major feature in perennial streams. Single flood events may deposit over 1.5 m of parallel-laminated sand. Hence, ancient parallel-laminated, sand-dominated alluvial deposits may be the product of ephemeral flows. The Trentishoe Formation (Hangman Sandstone Group — Middle Devonian) of North Devon consists, in part, of laterally extensive beds of parallel-laminated, fine-grained red sandstones that build multistorey sand bodies. Individual beds are little more than 1 m thick, but may extend laterally for hundreds of metres. A vertical sequence of erosion surface, sparse silt clasts, parallel laminations, silt drape, is often found, suggesting high flow stage deposition of plane-bedded sand, followed by a very rapid waning of flow with silt fallout in the final stages of the flood. This sequence bears marked similarity to certain modern ephemeral flow sands.

Origin and Characteristics of Pennsylvanian-Age Multistory Fluvial Sandstones of Illinois Basin: ABSTRACT

Continuing studies of Illinois basin Pennsylvanian stratigraphy have revealed a fluvial sandstone body of Desmoinesian age in the central and southern part of the Illinois basin. The sand body is a series of multistory or stacked sandstones that aggregate more than 300 ft (100 m) thick, as much as 2 mi (3.2 km) wide, and more than 100 mi (160 km) long, making part of a series of interrelated sandstones that constitute the Highland fluvial complex. The sand complex includes as many as End_Page 506------------------------------ four cycles of sedimentation, producing a multistory fluvial sandstone sequence similar to two- and three-story sand bodies described from the middle Carboniferous of the Donets basin (USSR). As noted by D. Swann and P. Potter the Highland and other fluvial complexes represent the Michigan River system that flowed across the basin from the northeast for as much as 250 × 106 years. The long-continued geographic stability of successive, often reestablished, channel courses is evidence of the subsidence history of the Illinois basin, its lack of southern closure, and its relation to the Mississippi Valley embayment. Certain Pennsylvanian channel sandstones are productive in the Illinois basin, but those of the Highland fluvial complex are not, presumably because of high, uninterrupted permeability, shallow burial, and lack of associated source beds. The multistory sand bodies, however, are accompanied by low-permeability over-bank silts and crevasse splays which appear to seal adjacent coals from contact with marine waters and consequent sulfate contamination. Multistory sands are possible exploration targets in other cratonic basins. They are identifiable by detailed facies analysis, core drilling, and high-resolution seismic surveys. End_of_Article - Last_Page 507------------