Bar Deposits Research Articles

Depositional environments, architecture, and controls of Early Cretaceous non-marine successions in the northwestern part of Kyongsang Basin, Korea

In the northwestern part of Kyongsang Basin, southeast Korea, non-marine successions of interbedded sandstone and mudstone are divided into successive stratigraphic units on the basis of facies assemblages and sandstone architecture. Parts of the successions (Sinpyong–Anpyong and Jotap units), formed in southward- or southeastward-draining fluvial systems, are documented in detail in terms of macroform-scale to stratigraphic-level architecture for the assessment of fluvial styles and evolution of non-marine depositional environments. The Sinpyong–Anpyong unit consists of thick sandstone bodies (2–47 m thick), thin sandstone bodies (generally <2 m thick), and mudstone-dominated bodies. The thick sandstone bodies are characterized by superposition of numerous bars and channels, indicative of braided-channel systems. The channels are estimated at a few to 10 m deep and 9 m to tens of meters wide. The thin sandstone bodies comprise planar beds of massive, horizontally stratified, and trough cross-stratified sandstones, interpreted as sand sheets and splays. The mudstone-dominated bodies largely consist of purple siltstone in the northern (proximal) part and gray mudstone in the southern (distal) part, representing well-drained floodplains and poorly drained floodplains with local shallow lakes, respectively. In the basal and uppermost parts of Sinpyong–Anpyong unit, the distal poorly drained facies (gray mudstone) expands to the northern basin margin. The Jotap unit, overlying the Sinpyong–Anpyong unit, comprises thick sandstone bodies (2–10 m thick), interbedded sandstone/siltstone bodies, and siltstone-dominated bodies. The thick sandstone bodies are dominated by the deposits of small bars and dunes with common lenticular beds of purple siltstone and internal scour surfaces, suggestive of bedform-dominated channels with ephemeral discharges. The channels are estimated at generally a few meters in depth and a few tens to hundreds of meters in width. The interbedded sandstone/siltstone bodies comprise thin, sheet-like and lenticular sandstone beds (<2 m thick) interbedded with purple siltstone, interpreted as channel-proximal floodplains where sand was frequently deposited as sand sheets and splays, levees, and crevasse-channel fills. The siltstone-dominated bodies consist mostly of purple siltstone with subordinate thin sandstone beds, representing channel-distal, well-drained floodplains. Along with the marked change in fluvial styles, the successions show systematic variations in the proportion and connectedness of channel sandstone bodies and the distribution of floodplain/lake facies. The basal and uppermost parts of the Sinpyong–Anpyong unit are characterized by the sourceward expansion of distal, poorly drained floodplain/shallow lake facies and the low proportion and connectedness of channel bodies. Following these gray mudstone-dominated sequences, the middle part of Sinpyong–Anpyong unit and Jotap unit show increases in the proportion, connectedness, and grain size of channel bodies and basinward expansion of proximal, well-drained floodplain facies. Such a stratigraphic architecture can be attributed to the fluctuations in the ratio of accommodation space/sediment supply, regulated by repeated basin subsidence.

A Geomorphic Explanation for a Meander Cutoff Following Channel Relocation of a Coarse-Bedded River

The Veteran's Fishing section of the Blackledge River in central Connecticut was relocated in the late 1950s. The relocation resulted in an unstable channel despite extensive efforts to prevent erosion. Overbank erosion and meander cutoffs were investigated using detailed survey data, characterizations of sediment deposits, flow modeling, and a moment-stability analysis. Limited reworking of revetment boulders indicate that riprap bank material was immobile during a 1979 flood event responsible for the formation of the cutoff channel. A moment-stability analysis factor-of-safety value of 1.1 supports the conclusion that riprap was not directly eroded from the banks. Alluvial particles with d(95) values ranging up to 120 mm were deposited along a bar downstream from the cutoff channel at flows estimated to be below a 1.5-year recurrence interval flow. Development of the bar deposit resulted in locally elevated water surfaces at high flow. The resulting overbank flow across the meander neck to the adjacent downstream bend led to the creation of an upstream migrating knickpoint, the erosion of approximately 16,000-year-old sediments, and the subsequent meander cutoff. The results of the study indicate that traditional erosion-control measures cannot prevent extreme channel adjustments if the geomorphic processes that control sediment continuity also are not considered.

Late Pleistocene–Holocene hydrologic changes in the interfluve areas of the central Ganga Plain, India

Abandoned channel belts, ponds and point bar deposits of palaeochannels in the interfluve regions of the central Ganga Plain suggest changes in the morphohydrologic conditions during the Latest Pleistocene–Holocene. Stratigraphy of these ponds comprises channel sand at the base overlain by shell-bearing clayey silt. The contact of the two facies marks the phase when channels converted into standing water bodies. Point bar deposits of some palaeochannels are overlain by oxidised aeolian sand, indicating that the channel abandonment possibly occurred due to the desiccation and aridity in the region. Optically stimulated luminescence (OSL) chronometry of the pond sediments suggests that the deposition of the basal channel sand started before 13 ka and continued up to ∼8 ka. The ponds formed around 8–6 ka when the channel activity ceased. Evidence from the point bar deposits also indicates that the fluvial activity in the region ended sometime during 7–5 ka. This was followed by aeolian aggradation. The present study thus suggests that the hydrologic conditions in the Gangetic plains, i.e. initiation of channels and their abandonment, formation of microgeomorphologic features such as ponds and their eventual siltation, were controlled largely by climatic changes (i.e. monsoon changes) supported by tectonic activity. For the past 2 ka, increasing human and related agricultural activity has substantially accentuated the natural siltation rate of ponds.

Mercury contamination of alluvial sediments within the Essequibo and Mazaruni River Basins, Guyana

Small- and medium-scale mining operations in Guyana have increased significantly since the late 1980s. The majority of these gold mining operations utilize mercury (Hg) amalgamation methods in the recovery process, raising the question as to the significance of Hg inputs to the environment from mining activities. In March and April, 2001, 168 samples were collected from floodplain, sand bar, and channel bed deposits along a 350 km reach of the Mazaruni River and a 160 km reach of the Essequibo River. Distinct trends in the geochemical data suggest that much of the Hg found in the alluvial deposits is related to anthropogenic sources, including (1) Hg concentrations in floodplain, channel bed and sand bar deposits locally exceed background values defined by ferralitic soils; (2) core data reveal that Hg concentrations within floodplain deposits have increased in recent years; and (3) high Hg concentrations along the channels can be attributed to the influx of material from tributaries affected by mining operations, or to mining activities along the rivers. Recent investigations in Amazonia have argued that Hg from amalgamation mining represents a small portion of the total Hg load to riverine systems, the majority coming from the erosion of Hg enriched upland soils within deforested terrain. Geochemical data from the Essequibo and Mazaruni Rivers suggest that Hg from mining may be a more significant source in Guyana where large-scale deforestation is limited. However, it is unclear whether the increased Hg represents the direct input associated with the amalgamation process, or Hg associated with the erosion of soils and sediments that results from activities that accompany mining.

Early Permian silt-bed fluvial sedimentation in the Orogrande basin of the Ancestral Rocky Mountains, New Mexico, USA

The Lower Permian (Wolfcampian) Abo Formation of south-central New Mexico was deposited by a silt-dominated fluvial system along the western half of the Orogrande basin a few degrees north of the equator in western Pangaea. Fluvial channel deposits consist primarily of: (1) inclined siltstone stratasets up to 4.5 m thick and 25 m wide interpreted as point bar deposits, and (2) symmetrically infilled siltstone stratasets up to 2.4 m thick and 14 m long that may represent avulsion crevasse channels. Both types of channels are dominated by climbing ripple cross-laminae and plane bed laminae, but trough cross-beds are also present, as are several types of soft-sediment deformation structures and desiccation cracks. Red silty mudstones interpreted as floodplain deposits comprise up to 70% of the formation and are interbedded with thin (<2 m) tabular siltstone beds of crevasse-splay/levee origin, as well as rare lacustrine carbonates (∼0.5 m thick) and structureless siltstones (<1 m thick) interpreted as loessites. Common pedogenic features in floodplain strata and on the tops of some fluvial channel beds include root traces, peds, and calcic nodules/tubules, whereas gypsum, gley colour mottling, and translocated clay and/or iron oxides are rare. Also present is an ichnofauna dominated by opportunistic arthropods that colonized the moist upper surfaces of recently deposited crevasse-splay/levee beds and the uppermost surfaces of fluvial channels following avulsion. The relative abundance of channel and crevasse-splay/levee deposits increases eastward in response to moderate asymmetrical subsidence of the basin, whereas calcic paleosols are more numerous on the more slowly subsiding western margin of the basin. A semi-arid to sub-humid palaeoclimate with seasonal precipitation favoured relatively deep rivers whose point bars were seasonally exposed, the formation of calcic and vertic soils, a sparse ichnofauna and flora, periodic desiccation of small carbonate lakes, and perhaps reworking of loess from the floodplain and/or upstream catchments.

Architecture of channel-belt deposits in an aggrading shallow sandbed braided river: the lower Niobrara River, northeast Nebraska

Architecture of recent channel-belt deposits of the Niobrara River, northeast Nebraska, USA, records the response of a sandy braided river to rapid base-level rise. Up to 3 m of aggradation has occurred within the lower 14 km of the Niobrara River since the mid-1950s as a result of base-level rise at the confluence of the Niobrara and Missouri Rivers. Aerial photographs and channel surveys indicate that the lower Niobrara has evolved from a relatively deep, stable channel with large, bank-attached braid bars to a relatively shallow, aggrading channel with braid bars and smaller secondary channels. Architecture of channel-belt deposits associated with the recent aggradation has been defined using ground-penetrating radar (GPR) and vibracores. The channel-belt deposits exhibit a series of amalgamated channel fills and braid bar complexes (i.e., macroforms). Radar facies identified in the GPR data represent architectural elements of the braid bar complexes, large and small bedforms [two-dimensional (2-D) and three-dimensional (3-D) dunes], and channels. Individual braid bars appear to consist of basal high-flow and upper low-flow components. Preservation of the complete, high-flow bar geometry is generally incomplete due to frequent migration of smaller scale, secondary channels within the channel belt (i.e., braided channel network) at low discharges. The large-scale stratification of the braid bar deposits is dominated by cross-channel and upstream accretion. Elements of downstream accretion are also recognized. These accretion geometries have not been documented previously in similar sandy braided rivers. Braid bar deposits with low-flow modification (e.g., incision by secondary channels) are recognized in the deeper portions of the deposits imaged by GPR. Preservation of braid bars, with both high- and low-flow components, is a result of the rapid base-level rise and channel-bed aggradation experienced by the Niobrara River over the past 45 years. Recent avulsion of the river channel allowed preservation of the upper, low-flow component of the braid bar deposits (i.e., bar-top sequences). The relative abundance and stratigraphic position of the amalgamated channels and braid bar complexes within the channel-belt deposits constitute a “signature” of the recent base-level rise.

Geological controls on the formation of alluvial meanders and floodplain wetlands: the example of the Klip River, eastern Free State, South Africa

AbstractFloodplain wetlands are common features of rivers in southern Africa, but they have been little studied from a geological or geomorphological perspective. Study of the upper Klip River, eastern Free State, South Africa, indicates strong geological controls on the formation of alluvial meanders and associated floodplain wetlands. Along this river, pronounced and abrupt changes in valley width are strongly linked to lithological variations. Where weakly cemented sandstone crops out, the Klip has laterally eroded bedrock and carved valleys up to 1500 m wide. In these valleys, the river meanders (sinuosity up to ∼1·75) on moderate gradients (&lt;0·001) within extensive floodplains marked by numerous oxbow lakes, backswamps and abandoned channels, many of which host substantial wetlands. In contrast, where highly resistant dolerite crops out, lateral erosion of bedrock is restricted, with the Klip tending instead to erode vertically along joints or fractures. Here, valleys are narrower (&lt;200 m), channel‐bed gradients are steeper (&gt;0·003), the river follows a much straighter course (sinuosity ∼1·10–1·34), and floodplains are restricted in width.Long‐term landscape development in the Klip and numerous similar catchments depends on the interaction between fluvial processes in the sandstone and dolerite valleys. In the sandstone valleys, vertical erosion rates are controlled by erosion rates of the more resistant dolerites downstream. Hence, in the short‐ to medium‐term (decades to tens of thousands of years), lateral erosion dominates over vertical erosion, with the river concomitantly planing sandstone in the channel floor and reworking floodplain sediments. The thickness of alluvial fill in the sandstone valleys is limited (&lt;4 m), but the resultant meanders are naturally dynamic, with processes such as point bar deposition, cutoff formation and channel avulsion resulting in an assemblage of fluvial landforms. In the longer term (greater than tens of thousands of years), however, vertical erosion will occur in the sandstone valleys as the downstream dolerites are lowered by erosion, resulting in channel incision, floodplain abandonment, and desiccation of the wetlands. Identification of the geological controls on meander and wetland formation provides information vital for the design of effective management guidelines for these ecologically rich habitats, and also contributes to a better understanding of rivers that are intermediate between fully alluvial and fully bedrock. Copyright © 2002 John Wiley &amp; Sons, Ltd.

Climatic and human influence on river systems and glacier fluctuations in southeast Spain since the Last Glacial Maximum

The semi-desert environments of the Mediterranean region are most sensitive to climatic changes and human land exploitation. This paper is focused on the Late Pleistocene and Holocene geomorphic evolution of southeast Spain, one of the driest regions in Europe. Late Pleistocene and Holocene chronosequences of river terraces were established for the Antas and Aguas valleys, respectively. The deposition of up to four fluvial terraces during isotope stage 2 points to important erosion and accumulation processes at the end of Late Pleistocene in the Vera basin. In contrast to the stage 2 deposits, the Holocene terraces are composed of point bar deposits accumulated by meandering rivers with less sediment discharge. According to the obtained 14C-dating and extracted pottery fragments, Holocene terrace deposition occurred during the Atlantic period, early Middle Ages, Little Ice Age (LIA) and the 20th century. The most outstanding site in southern Spain for studying Holocene glacier shifts (up to five end-moraines) is the Veleta cirque in Sierra Nevada, where the southernmost glacier of Europe persisted during the Little Ice Age. The climate changes of the LIA probably are recorded by two glacier fluctuations, as indicated by 210Pb dating and historical data. The reconstruction of the evolution of the Aguas and Antas valleys stress that the aggradations of the lower terraces is mostly climate-induced although during the last 500 years, human interference in the landscape (Christian conquest, mining boom of the 19th century, etc.) may have played an important role to river dynamics.

大規模れき質フォーセットベッド れきし‐沖合れき州モデル

A gravelly spit-offshore gravel bar model convincingly explains the depositional process of gravel beds in a transgressive systems tract. According to this model, large-scale gravelly foreset beds are deposited on the slope of a spit platform, which is the subaqueous portion of a gravelly spit. Massive, horizontally stratified, or planar cross-stratified subhorizontal sets (topset beds) overlie the foreset beds, and exhibit a foreset and topset structure. The topset beds include nearshore deposits on the spit platform and offshore gravel bar deposits, which are reworked sediments from the pre-existing spit-beach gravels that had formed during a transgression. Large-scale foreset beds of gravelly spits have a high preservation potential, because gravelly spits are likely to form in fluvially incised valleys, in which gravelly spit deposits are preserved from truncation by shoreface erosion during shoreface retreat. The gravelly spit provides a new example of large-scale foreset beds. The foreset beds of the gravelly spit are easily differentiated from those of fluvial gravel bars and tidally influenced dunes in a marine environment, but are similar to those of Gilbert-type deltas. Sequencestratigraphic interpretations differ depending on the depositional model that is applied to the foreset beds : a delta during a highstand, lowstand, or shelf-margin systems tracts; a gravelly spit in a transgressive systems tract. At this point we need to find a criterion to differentiate the foreset beds of gravelly spits from those of deltas.

Architectural analysis of fluvial sequences in the northwestern part of Kyongsang Basin (Early Cretaceous), SE Korea

Abstract The northwestern part of Kyongsang Basin (Early Cretaceous) largely comprises interbedded sandstones and mudstones of fluvial depositional environments with local basin-margin conglomeratic deposits of alluvial fan systems. These non-marine successions are divided into 5 successive stratigraphic units, each of which is unique in facies assemblages and architecture of sandstone bodies. One stratigraphic unit (Sinpyong–Anpyong unit) is documented in this paper with detailed architectural analysis of sandstone bodies. The deposits of the Sinpyong–Anpyong unit are divided into three major components: thick sandstone, thin sandstone, and mudstone-dominated bodies. The thick sandstone bodies (2–47 m thick) comprise stacked element groups bounded by laterally persistent erosional surfaces, probably representing superposition of channel belts. Within individual element groups, elements are laterally superposed, indicating growth and superposition of bars and movement of associated channels. The thickness of bar deposits and dip angles of laterally inclined strata suggest that channels were a few to 10 m in depth and 9 m to several tens of meters in width. Paleocurrent data indicate southward- or southeastward-draining channel systems. The thin sandstone bodies (generally The thick sandstone bodies, formed in major channels and channel belts, are characterized by lateral superposition of numerous bar and channel deposits, limited extent of individual lateral-accretion sets, and the presence of mid-channel bar deposits, suggesting deposition in braided rivers. Detailed reconstruction of bars and channels reveals a range of processes including growth, coalescence, and erosion of bars and migration, switching, and filling of channels. Lateral accretion of bars and the associated channel migration might have been principal processes in the Sinpyong–Anpyong river systems. Bars were commonly coalesced by lateral superposition, forming large bar complexes.

Camadas de tempestito grosso (coarse grained storm beds): exemplos do Permiano da bacia do Paraná

Tempestitos grossos constituem camadas delgadas de conglomerado gradando a arenito, com estratificação cruzada seguida de laminação ondulada truncante a simétrica e de drape/flaser de siltito/folhelho. Cinco exemplos extraídos do Permiano da bacia do Paraná ilustram esse tipo de depósito: três deles são de rochas siliciclásticas, contendo bioclastos de bivalves e vertebrados (Formação Rio Bonito-Membro Triunfo e Formação Palermo), enquanto os outros dois são de rochas carbonática e fosfática (respectivamente, formações Teresina e Corumbataí do Grupo Passa Dois). O componente tracional da base do tempestito grosso apresenta-se como arenite quartzoso/lítico ou grainstone oolítico com cimento calcífero preenchendo poros (casos das formações Palermo e Teresina). Em sua maioria, os tempestitos grossos constituem pavimentos transgressivos intercalados em folhelhos ou tempestitos finos (arenitos muito finos a folhelhos com estratificação ondulada truncante-hummocky). Em outro caso, extraído de subsuperfície, o pavimento transgressivo ocorre na base de uma sucessão progradante de barra de plataforma. O tempestito grosso da Formação Teresina constitui um evento transgressivo sobreposto a depósitos de barra de plataforma.

Sharp‐based, tide‐dominated deltas of the Sego Sandstone, Book Cliffs, Utah, USA

The lower part of the Cretaceous Sego Sandstone Member of the Mancos Shale in east‐central Utah contains three 10‐ to 20‐m thick layers of tide‐deposited sandstone arranged in a forward‐ and then backward‐stepping stacking pattern. Each layer of tidal sandstone formed during an episode of shoreline regression and transgression, and offshore wave‐influenced marine deposits separating these layers formed after subsequent shoreline transgression and marine ravinement. Detailed facies architecture studies of these deposits suggest sandstone layers formed on broad tide‐influenced river deltas during a time of fluctuating relative sea‐level. Shale‐dominated offshore marine deposits gradually shoal and become more sandstone‐rich upward to the base of a tidal sandstone layer. The tidal sandstones have sharp erosional bases that formed as falling relative sea‐level allowed tides to scour offshore marine deposits. The tidal sandstones were deposited as ebb migrating tidal bars aggraded on delta fronts. Most delta top deposits were stripped during transgression. Where the distal edge of a deltaic sandstone is exposed, a sharp‐based stack of tidal bar deposits successively fines upward recording a landward shift in deposition after maximum lowstand. Where more proximal parts of a deltaic‐sandstone are exposed, a sharp‐based upward‐coarsening succession of late highstand tidal bar deposits is locally cut by fluvial valleys, or tide‐eroded estuaries, formed during relative sea‐level lowstand or early stages of a subsequent transgression. Estuary fills are highly variable, reflecting local depositional processes and variable rates of sediment supply along the coastline. Lateral juxtaposition of regressive deltaic deposits and incised transgressive estuarine fills produced marked facies changes in sandstone layers along strike. Estuarine fills cut into the forward‐stepped deltaic sandstone tend to be more deeply incised and richer in sandstone than those cut into the backward‐stepped deltaic sandstone. Tidal currents strongly influenced deposition during both forced regression and subsequent transgression of shorelines. This contrasts with sandstones in similar basinal settings elsewhere, which have been interpreted as tidally influenced only in transgressive parts of depositional successions.

Imaging upper tertiary to quaternary deposits from northern Brazil applying ground penetrating radar

Upper Tertiary to Quaternary deposits from northeastern Para State, Brazil, were imaged with ground penetrating radar (GPR) and calibrated with geological data from a local cliff. The procedure allowed for the recognition of five radar facies (i.e., discontinuous parallel, continuous parallel, chaotic and reflection-free, oblique, and mound facies) that are organized into four radar units (referred to here as Units 1 to 4). Unit 1 includes mostly continuous parallel reflections and secondarily, discontinuous parallel and mound reflections, being correlatable at the cliff face to low-energy muddy lagoon and mud flat depositional settings. Unit 2 consists of continuous even parallel, discontinuous parallel, oblique, as well as chaotic and reflection-free facies, being related to low-energy muddy deposits interlayered with sands, which are attributed to tidal flats and channels. Units 3 and 4 include chaotic, reflection-free and, locally, discontinuous parallel radar facies, correlatable at the cliff face to massive sands, mostly representing aeolian coastal dunes. These units are bounded by continuous, high amplitude reflections that can be easily correlatable throughout the GPR profiles, serving as important stratigraphic markers. The GPR survey improved the reconstruction of the depositional environments through the recognition of tidal channel and point bar deposits within Unit 2, not present at the cliff face. The stratigraphic framework was also improved through the recognition of the discontinuity surface between Units 3 and 4, also not defined in previous studies, but which revealed to be a regionally significant stratigraphic marker attributed to a relative sea level fall.

Lagoon–tidal flat sedimentation in an epeiric sea: Proterozoic Bhander Group, Son Valley, India

AbstractThe Bhander Group, the uppermost stratigraphic unit of the Proterozoic Vindhyan Supergroup in Son Valley, exhibits in its upper part a 550 m thick, muddy siliciclastic succession characterized by features indicative of deposition in a wave‐affected coastal, lagoon–tidal flat environment suffering repeated submergence and emergence. The basic architecture of the deposit is alternation of centimetre‐ to decimetre‐thick sheet‐like interbeds of coarser clastics (mainly sandstone) and decimetre‐thick mudstones. The coarser interlayers are dominated by a variety of ripple‐formed laminations. The preserved ripple forms on bed‐top surfaces and their internal lamination style suggest both oscillatory and combined flows for their formation. Interference, superimposed, ladder‐back and flat‐topped ripples are also common. Synsedimentary cracks, wrinkle marks, features resembling rain prints and adhesion structures occur in profusion on bed‐top surfaces. Salt pseudomorphs are also present at the bases of beds. The mudstone intervals represent suspension settlement and show partings with interfaces characterized by synsedimentary cracks. It is inferred that the sediments were deposited on a coastal plain characterized by a peritidal (supratidal–intertidal) flat and evaporative lagoon suffering repeated submergence and emergence due to storm‐induced coastal setup and setdown in addition to tidal fluctuations.The 550 m thick coastal flat succession is surprisingly devoid of any barrier bar deposits and also lacks shoreface and shelfal strata. The large areal extent of the coastal flat succession (c. 100,000 km2) and its great thickness indicate an extremely low‐gradient epeiric basin characterized by an extensive coastal flat sheltered from the deeper marine domain. It is inferred that the Bhander coastal flat was protected from the open sea by the Bundelkhand basement arch to the north of the Vindhyan basin, instead of barrier bars. Such a setting favoured accumulation of a high proportion of terrigenous mud in the coastal plain, in contrast to many described examples from the Proterozoic. Copyright © 2001 John Wiley &amp; Sons, Ltd.

Geomorphology, facies architecture, and high-resolution, non-marine sequence stratigraphy in avulsion deposits, Cumberland Marshes, Saskatchewan

Abstract This paper demonstrates field relationships between landforms, facies, and high-resolution sequences in avulsion deposits. It defines the building blocks of a prograding avulsion sequence from a high-resolution sequence stratigraphy perspective, proposes concepts in non-marine sequence stratigraphy and flood basin evolution, and defines the continental equivalent to a parasequence. The geomorphic features investigated include a distributary channel and its levee, the Stage I crevasse splay of Smith et al. (Sedimentology, vol. 36 (1989) 1), and the local backswamp. Levees and splays have been poorly studied in the past, and three-dimensional (3D) studies are rare. In this study, stratigraphy is defined from the finest scale upward and facies are mapped in 3D. Genetically related successions are identified by defining a hierarchy of bounding surfaces. The genesis, architecture, geometry, and connectivity of facies are explored in 3D. The approach used here reveals that avulsion deposits are comparable in process, landform, facies, bounding surfaces, and scale to interdistributary bayfill, i.e. delta lobe deposits. Even a simple Stage I splay is a complex landform, composed of several geomorphic components, several facies and many depositional events. As in bayfill, an alluvial ridge forms as the feeder crevasse and its levees advance basinward through their own distributary mouth bar deposits to form a Stage I splay. This produces a shoestring-shaped concentration of disconnected sandbodies that is flanked by wings of heterolithic strata, that join beneath the terminal mouth bar. The proposed results challenge current paradigms. Defining a crevasse splay as a discrete sandbody potentially ignores 70% of the landform's volume. An individual sandbody is likely only a small part of a crevasse splay complex. The thickest sandbody is a terminal, channel associated feature, not a sheet that thins in the direction of propagation. The three stage model of splay evolution proposed by Smith et al. (Sedimentology, vol. 36 (1989) 1) is revised to include facies and geometries consistent with a bayfill model. By analogy with delta lobes, the avulsion sequence is a parasequence, provided that its definition is modified to be independent from sea level. In non-marine settings, facies contacts at the tops of regional peats, coals, and paleosols are analogous to marine flooding surfaces. A parasequence is redefined here as a relatively conformable succession of genetically related strata or landforms that is bounded by regional flooding surfaces or their correlative surfaces. This broader definition incorporates the concept of landscape evolution between regional flooding surfaces in a variety of depositional settings. With respect to landscape evolution, accommodation space has three spatial dimensions — vertical (x), lateral (y), and down-the-basin (z). A flood basin fills in as landforms vertically (x) and laterally accrete (y), and prograde down-the-basin (z). Vertical aggradation is limited by the elevation of maximum flood stage (local base level). Differential tectonism and geomorphology control the slope of the flood basin floor and the direction of landscape evolution. These processes produce parasequences that include inclined stratal surfaces and oriented, stacked macroforms (clinoforms) that show the magnitude and direction of landscape evolution.

Lowstand deltas in the Frontier Formation, Powder River basin, Wyoming: Implications for sequence stratigraphic models

Deposits of lowstand deltas formed on the floor of the Cretaceous Interior seaway of North America are found in the Cenomanian, lower Belle Fourche Member of the Frontier Formation, central Wyoming. Sandstones located in similar distal basin locations, hundreds of kilometers basinward of highstand shoreline deposits, form important hydrocarbon reservoirs isolated within marine shales, but interpretation of their origin has been highly controversial. The distribution, geometry, and internal facies of these sandstones are documented by an extensive outcrop study and regional subsurface correlations to develop genetic facies models for these deposits. This integrated record of lithofacies, ichnofacies, palynofacies, paleocurrent data, bedding relationships, and isolith maps incorporates observations from nearly 100 measured outcrop sections and about 550 subsurface well logs. Four episodes of sediment progradation and subsequent transgression each left behind gradually upward-coarsening deltaic sandstones that have eroded tops. These deltaic sandstones have a lobate to elongate geometry, basinward-dipping internal clinoform bedding, radiating paleocurrents, a low to moderate degree of shallow-marine burrowing, and show variable wave influence and tidal influence on deposition. Delta plain, paralic, and nonmarine facies have been eroded from the top of deltaic successions. Erosion surfaces capping progradational deltaic successions are the only stratal discontinuities that can be mapped regionally, and they appear to record transgressive ravinement enhanced over areas of structural uplift, compared with lowstand surfaces of erosion, which record the bypass of sediments basinward. Low accommodation during lowstands left little room for sandstones to stack vertically, and successive episodes of delta progradation were offset along strike. More tide-influenced delta deposits formed within shoreline embayments (Begin page 262) defined by the topography of older wave-influenced delta lobes and subtle syndepositional deformation of the basin floor. Standard sequence stratigraphic terminology is difficult to use in broad lowstand systems like the Frontier Formation because sandstones do not show simple vertical stacking patterns, major stratal discontinuities can form by processes other than lowstand fluvial erosion, and minor syndepositional deformation of the basin floor exerts a first-order influence on depositional and sediment preservation patterns. Although many basin-distal sandstones have been interpreted to be deposits of offshore bars, shelf-isolated valley fills, and stranded shorelines, the Frontier Formation examples documented here suggest that many of these deposits may be top-eroded deltas formed where rivers delivered sediment to lowstand coastlines. The external geometry and internal heterogeneities of hydrocarbon reservoirs found in these types of deposits reflect processes active on the low accommodation deltaic shoreline, even in cases where subsequent ravinement has significantly truncated the deposits during transgression.

Permeability characterization of distributary mouth bar sandstones in Prudhoe Bay field, Alaska: How horizontal cores reduce risk in developing deltaic reservoirs

Oil production from upstructure drill sites at Prudhoe Bay field, Alaska, is almost exclusively from fine-grained deltaic sandstones. Distributary channel and distributary mouth bar facies associations in the Triassic Ivishak Formation comprise the pay zones, but wells are preferentially completed in the lower-permeability distributary mouth bar deposits in an attempt to avoid high gas/oil ratio wells. Thin light-oil columns combined with complex stratigraphy and an overlying, highly mobile gas cap make planning, drilling, and completing economic wells challenging. To accurately assess lateral trends in permeability within distributary mouth bar sandstones, three conventional cores were cut (approximately 120 ft [36 m]) along the 1000 ft (304 m) horizontal reach of a recent development well. The cored interval consists of seven lithofacies, all composed of fine-grained sandstone. Comparison of permeability values to lithofacies demonstrates a striking and consistent trend. Six lithofacies possess average horizontal permeabilities ranging from 12 to 40 md. Average horizontal permeability for the seventh lithofacies (lithofacies 7) is 129 md. Porosity and vertical permeability follow similar patterns. Distributary mouth bars in Prudhoe Bay field were deposited in fluvially dominated delta lobes in which sediment distribution at river mouths was controlled by friction between the sediment plume and basin bottom. During flood stage, the best sorted and most permeable sediments (i.e., lithofacies 7) were deposited on the apex or most proximal part of the distributary mouth bars between distributary channels. Sedimentologic and petrographic data corroborate a strong link between lithofacies and permeability. Sandstones deposited by unsteady flow conditions (e.g., turbulent (Begin page 460) scour, intermittent ripple migration) are likely to be less well sorted, and contain more clay and lignitic organic material (commonly as drapes and wisps), than lithofacies 7 sandstones deposited under more uniform high-energy transport conditions. Small amounts of argillaceous and lignitic laminae serve as nucleation sites for siderite cement precipitation and as catalysts for pressure solution of quartz grains, significantly degrading permeability in lithofacies 1 through 6. Insights gained from analyzing these cores can reduce the risks associated with well completions in distributary mouth bar sandstones. Locations targeting distributary mouth bar deposits can be optimized by (1) using existing well data to map time-equivalent deltaic facies associations; (2) identifying transition zones between distributary channels and distributary mouth bars; (3) extrapolating trends of high-permeability lithofacies within distributary mouth bars; and (4) calculating the well trajectory and length, to optimally contact high-permeability rock (or moderate-permeability rock to inhibit gas coning).

Effects of land-use change on channel morphology in northeastern Puerto Rico

Between 1830 and 1950 much of northeastern Puerto Rico was cleared for agriculture. Runoff increased by 50% and sediment supply to the river channels increased by more than an order of magnitude. Much of the land clearance extended to steep valley slopes, resulting in widespread gullying and landslides and a large load of coarse sediments delivered to the stream channels. A shift from agriculture to industrial and residential land uses over the past 50 yr has maintained the elevated runoff while sediment supply has decreased, allowing the rivers to begin removing coarse sediment stored within their channels. The size, abundance, and stratigraphic elevation of in-channel gravel bar deposits increases, channel depth decreases, and the frequency of overbank flooding increases downstream along these channels. This is presumed to be a transient state and continued transport will lead to degradation of the bed in downstream sections as the channel adjusts to the modern supply of water and sediment. A downstream decrease in channel size is contrary to the expected geometry of self-adjusted channels, but is consistent with the presence of partially evacuated sediment remaining from the earlier agricultural period. Reverse (downstream decreasing) channel morphology is not often cited in the literature, although consistent observations are available from areas with similar land-use history. Identification of reverse channel morphology along individual watercourses may be obscured in multiwatershed compilations in which other factors produce a consistent, but scattered downstream trend. Identification of reverse channel morphology along individual streams in areas with similar land-use history would be useful for identifying channel disequilibrium and anticipating future channel adjustments.

Shallow-water, eruption-fed, mafic pyroclastic deposits along a Paleoproterozoic coastline: Kangerluluk volcano-sedimentary sequence, southeast Greenland

The 200–300 m thick, volcano-sedimentary sequence at Kangerluluk is part of the psammite zone, one of four major zones, which constitute the 1.8 Ga Ketilidian orogen in south Greenland. Three lithofacies are emphasized in the study: (1) the conglomerate-sandstone; (2) the volcanic; and (3) the pyroclastic lithofacies. The 2–40 m thick conglomerate-sandstone lithofacies represents a subaerial to subaqueous fan-delta deposit. Matrix- and clast-supported conglomerates are interpreted as debris flow and longitudinal gravel bar deposits. Erosive-based conglomerate channel fills attest to stream incision. Trough crossbedded sandstone, interpreted as lunate megaripples, planar-bedded sandstone indicative of upper flow regime bar-top sands, and small-scale trough crossbeds reflecting ripples follow up-section, form collectively with the conglomerate, 0.40–2.50 m thick fining-upward sequences. The sandstone-dominated unit, up-section from the conglomerates and composed of planar and low-angle crossbeds, minor ripples and graded beds as well as mudstone is indicative of a lower shoreface deposit below normal wave base. The clastic sedimentary rocks are suggestive of a fan-delta setting. The 100–200 m thick volcanic lithofacies, composed of pillowed and pillow brecciated lava flows, is consistent with shallow-water deposition. Interstratification of lava flows with both conglomerate-sandstone and pyroclastic lithofacies, intrusion of dykes into volcaniclastic rocks, and peperite formation accentuate contemporaneity between volcanism and sedimentation and is a common feature of island arcs. The 1–50 m thick, pyroclastic lithofacies with sharp depositional contacts to the overlying volcanic and underlying conglomerate-sandstone lithofacies, was emplaced in a subaqueous setting. The lithofacies is divided into a planar- to crossbedded tuff-lapilli tuff and a bedded lapilli tuff breccia, whereby both deposits are inferred to result from shallow-water surtseyan-type eruptions. The 5–15 m thick, bedded lapilli tuff breccia with abundant bomb sag structures and graded beds is considered a result of subaqueous eruptions strong enough to form an insulating steam cupola characterized by ballistically emplaced bombs that rapidly collapsed allowing for transport via mass flow processes. The deposits are considered proximal to the vent. The 2–50 cm thick, planar- to crossbedded tuff-lapilli tuff featuring abundant euhedral and broken crystals of feldspar (≤2cm) and minor pyroxene (≤1 cm), are massive, graded, crossbedded and stratified. The planar but laterally discontinuous beds, characterized by abundant low-angle scours, are interpreted as low- to high-concentration sediment gravity flows produced directly from subaqueous tephra jets that collapsed due to massive water ingestion. Local breccia-size pyroclasts disrupting beds are interpreted as bomb sags. The mafic, eruption-fed, Surtseyan-type deposits, postulated to be a subaqueous counterpart of cold, subaerial base surges, originate from subaqueous tuff cones formed along a rugged volcanic-dominated shoreline featuring high-energy fan-deltas.

Gravelly spit deposits in a transgressive systems tract: the Pleistocene Higashikanbe Gravel, central Japan

The Pleistocene Higashikanbe Gravel, which crops out along the Pacific coast of the Atsumi Peninsula, central Japan, consists of well-sorted, pebble- to cobble-size gravel beds with minor sand beds. The gravel includes large-scale foreset beds (5–10 m high) and overlying subhorizontal beds (0·5–3 m thick), showing foreset and topset structure, from which the gravel has previously been interpreted as deposits of a Gilbert-type delta. However, (1) the gravel beds lack evidence of fluvial activity, such as channels in the subhorizontal beds; (2) the foresets incline palaeolandwards; (3) the gravels fill a fluvially incised valley; and (4) the gravels overlie low-energy deposits of a restricted environment, such as a bay or an estuary. The foresets generally dip towards the inferred palaeoshoreline, indicating landward accretion of gravel. Reconstruction of the palaeogeography of the peninsula indicates that the Higashikanbe Gravel was deposited as a spit similar to that developed at the western tip of the present Atsumi Peninsula, rather than as a delta. According to the new interpretation, the large-scale foreset beds are deposits on the slopes of spit platforms and accreted in part to the sides of small islets that are fragments of the submerging spit during relative sea-level rise. The subhorizontal beds include nearshore deposits on the spit platform topsets and deposits of gravel shoals or bars, which are reworked sediments of the spit beach gravels during a transgression. The lack of spit beach facies in the subhorizontal beds results from truncation by shoreface erosion. Dome structure, which is a cross-sectional profile of a recurved gravel spit at its extreme point, and sandy tidal channel deposits deposited between the small islets were also identified in the Higashikanbe Gravel. The Higashikanbe Gravel fills a fluvially incised valley and occupies a significant part of a transgressive systems tract, suggesting that gravelly spits are likely to be well developed during transgressions. The large-scale foreset beds and subhorizontal beds of gravelly spits in transgressive systems tracts contrast with the foreset and topset beds of deltas, characteristic of highstand, lowstand and shelf-margin systems tracts.