Point Bar Complexes Research Articles

Climate and anthropogenic impacts on North American erosion and sediment transport since the Last Glacial Maximum: Evidence from the detrital zircon record of the Lower Mississippi Valley, USA

The Mississippi River provides an opportunity to examine models of sediment transport in large alluviated floodplain systems. We test the idea that sources of sandy sediment in such settings are invariable on timescales <104 y because of storage and recycling in the floodplains. To reconstruct the development of the Mississippi sediment load over the past 2500 years we collected sediment from an abandoned point bar complex nearby at False River, Louisiana, USA. We also took annual samples from the lower reaches between 2015 and 2021 to assess changes on that timescale. Optically stimulated luminescence dating indicated that the point bar accreted between 2460 and 860 years ago. Detrital zircon U-Pb dating was used to assess sediment source and variability over time. We confirm a dominant sediment flux from the Rocky Mountain foreland but with higher relative erosion from the Superior Province during the Last Glacial Maximum (LGM) based on existing data from the Gulf of Mexico. There have been resolvable changes in the sources of sediment particularly since the LGM and after 860 years ago, but also over shorter, even sub-annual timescales in the recent past. These changes may reflect seasonal weather or storm events in the headwater regions and imply limited floodplain buffering of the sand load. In recent times this may reflect the installation of levees in the lower reaches, suppressing reworking. Changes over 102−103 y time periods may be related to changes in climate (e.g., the Medieval and Roman warm periods) and to the development of agriculture across North America after ∼2000 years ago. Detrital zircon dating is an effective provenance tool and does not appear to be strongly biased by the grain size of the sediment in this setting.

Integrating facies analysis with dipmeter data to characterize point bars of the Lower Cretaceous McMurray Formation, Christina River, AB, Canada

The Lower Cretaceous McMurray Formation contains crude bitumen reserves of the Athabasca Oilsands that formed by channel amalgamation and lateral migration of fluvio-tidal point bars. These point bars are widely accepted as incised valley fills during episodic base-level rises in tidally influenced estuary (Mathison, 2004; Hein et al., 2013). Historically, well logs such as gamma-ray and spontaneous potential were used in combination with core data to interpret facies and to conduct stratigraphic correlations of the McMurray Formation. Microresistivity image logs, dipmeter logs, and seismic data have greatly improved the identification of facies and recognition of point-bar geometry. In comparison to microresistivity image logs and seismic data, dipmeter data is more regionally accessible; the tool indicates the orientations of fractures, bedding planes and contacts and it is commonly used to delineate boundaries between different point bars in the McMurray Formation. This work provides examples of dipmeter tadpole analyses and discusses how dipmeter-log interpretations can be supported with core-based facies analyses.This study integrates dipmeter tadpole patterns, sedimentological and ichnological characteristics from 47 sedimentary cores. Vintage dipmeter tadpole patterns were observed and digitized from 52 wells to establish a detailed facies characterization with a particular emphasis on McMurray point-bar complexes and sheltered shoreface/delta deposits. In particular, dipmeter data are useful in indicating paleocurrent directions and lateral accretion orientation of point bars, which helps to reconstruct paleodepositional history and stratigraphic relationships. Point bars in the study area are characterized by these sedimentological and dipmeter characteristics: 1) a lower point-bar section characterized by trough cross-bedded sandstone (locally inclined stratified master bedding) with local mud clasts and disorganized, low-to high-angle dipmeter readings; 2) a middle section represented by inclined heterolithic stratified lateral accretionary deposits that exhibit consistent low-angle dips (5°–18°), and display subtle shallow-to-steep-to-shallow dipmeter pattern; and 3) an upper section of the point bar or in some cases abandoned channel strata comprising laminated and lenticular mudstone or bioturbated heterolithic intertidal strata, each showing low-angle (<4°) dipmeter measurements.The dipmeter characteristics of estuarine point bars are integrated with core observations to delineate distinct point-bar stacking patterns and stratigraphic units. In particular, dipmeter tadpole patterns help to refine facies interpretations, such as cross-stratified sandstone facies. Similar point bar scales and stacking patterns are observed in four channel sets. Point bars show either NW-SW or NE-SE lateral accretion surfaces, suggesting northward paleocurrent flow direction.The analyzed datasets support a transgressive fill incised valley fill model that is consistent with earlier McMurray studies in different areas. The four channel sets descend from different stratigraphic levels, but share similarities in scale, orientation and sedimentology/ichnology. The similarity attests to persistent transgression over McMurray time punctuated with regular sea-level fall and rise until the McMurray Formation strata were replaced by Clearwater Formation sedimentation.

Reservoir architecture and evolution of meandering belt: A subsurface case in the Jiyang Depression, eastern China

The reservoir architecture of a meandering river deposition can indicate the heterogeneity of the subsurface reservoirs and deposition and evolution of the paleo-river. Although outcrop study has been useful in characterizing reservoir architecture, the study of the subsurface reservoir is still a challenge due to its complicated reservoir geometry and forming process. In this work, a mature oil field in Bohai Bay Basin, eastern China was investigated to understand the reservoir architecture and evolution of a Neogene meandering belt using drilling cores, well logging, 3D seismic data, and oil production data. In the study area, five types of litho-facies were observed indicating the specific hydrodynamic conditions. The results obtained two types of point bar combination models focused on (1) the point bars migration in the same direction at equal elevation and (2) erosion and superimposition of the multi-phase point bars. Abandoned channels in study area are classified into two types: Type Ⅰ is the “Early and long-term abandoned channel” and Type Ⅱ is the “Late and short-term abandoned channel”. Well log facies of Type Ⅰ presents a low sandstone/mudstone ratio while that of Type Ⅱ has a high sandstone/mudstone ratio. Both models exhibited the weakening of the seismic reflection amplitude and changes in the waveform. Based on the well log curves, stratal slicing, and seismic facies analysis, single point bars are interpreted more accurately. The meandering river in the study area, which was previously interpreted as a large single point bar, was comprehended as a meandering belt formed by a multi-phase point bar complex with three single point bars in the meander. Erosion and overlapping can be observed between the first and second phases of the point bars. In modern river deposition, similar meander reservoir architecture and its evolution can be observed. Additionally, reservoir engineering analysis showed that there are flow barriers between point bars of different phases. The reservoir architecture of a meandering belt was characterized and the depositional evolution of the paleo-river was reconstructed. The methods proposed is significant for qualitative interpretations of reservoirs such as morphology channel recognition, reservoir connectivity analysis, and lithology and interlayer prediction.

Upstream perturbation and floodplain formation effects on chute-cutoff-dominated meandering river pattern and dynamics.

A sustained dynamic inflow perturbation and bar–floodplain conversion are considered crucial to dynamic meandering. Past experiments, one‐dimensional modelling and linear theory have demonstrated that the initiation and persistence of dynamic meandering require a periodic transverse motion of the inflow. However, it remains unknown whether the period of the inflow perturbation affects self‐formed meander dynamics. Here, we numerically study the effect of the inflow perturbation period on the development and meander dynamics of a chute‐cutoff‐dominated river, which requires two‐dimensional modelling with vegetation forming floodplain on bars. We extended the morphodynamic model Nays2D with growth and mortality rules of vegetation to allow for meandering. We tested the effect of a transversely migrating inflow boundary by varying the perturbation period between runs over an order of magnitude around typical modelled meander periods. Following the cutoff cascade after initial meander formation from a straight channel, all runs with sufficient vegetation show series of growing meanders terminated by chute cutoffs. This generates an intricate channel belt topography with point bar complexes truncated by chutes, oxbow lakes, and scroll‐bar‐related vegetation age patterns. The sinuosity, braiding index and meander period, which emerge from the inherent biomorphological feedback loops, are unrelated to the inflow perturbation period, although the spin‐up to dynamic equilibrium takes a longer time and distance for weak and absent inflow perturbations. This explains why, in previous experimental studies, dynamic meandering was only accomplished with a sustained upstream perturbation in flumes that were short relative to the meander wavelength. Our modelling of self‐formed meander patterns is evidence that scroll‐bar‐dominated and chute‐cutoff‐dominated meanders develop from downstream convecting instabilities. This insight extends to many more fluvial, estuarine and coastal systems in morphological models and experiments, which require sustained dynamic perturbations to form complex patterns and develop natural dynamics. © 2019 The Authors. Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd.

Morphology and spacing of river meander scrolls

Many of the world's alluvial rivers are characterised by single or multiple channels that are often sinuous and that migrate to produce a mosaicked floodplain landscape of truncated scroll (or point) bars. Surprisingly little is known about the morphology and geometry of scroll bars despite increasing interest from hydrocarbon geoscientists working with ancient large meandering river deposits. This paper uses remote sensing imagery, LiDAR data-sets of meandering scroll bar topography, and global coverage elevation data to quantify scroll bar geometry, anatomy, relief, and spacing. The analysis focuses on preserved scroll bars in the Mississippi River (USA) floodplain but also compares attributes to 19 rivers of different scale and depositional environments from around the world.Analysis of 10 large scroll bars (median area = 25 km2) on the Mississippi shows that the point bar deposits can be categorised into three different geomorphological units of increasing scale: individual ‘scrolls’, ‘depositional packages’, and ‘point bar complexes’. Scroll heights and curvatures are greatest near the modern channel and at the terminating boundaries of different depositional packages, confirming the importance of the formative main channel on subsequent scroll bar relief and shape. Fourier analysis shows a periodic variation in signal (scroll bar height) with an average period (spacing) of 167 m (range 150–190 m) for the Mississippi point bars. For other rivers, a strong relationship exists between the period of scroll bars and the adjacent primary channel width for a range of rivers from 55 to 2042 mis ~50% of the main channel width. The strength of this correlation over nearly two orders of magnitude of channel size indicates a scale independence of scroll bar spacing and suggests a strong link between channel migration and scroll bar construction with apparent regularities despite different flow regimes. This investigation of meandering river dynamics and floodplain patterns shows that it is possible to develop a suite of metrics that describe scroll bar morphology and geometry that can be valuable to geoscientists predicting the heterogeneity of subsurface meandering deposits.

Orogeny and the collapse of the Devonian Prairie Evaporite karst in Western Canada: impact on the overlying Cretaceous Athabasca Oil Sands

Abstract The Middle Devonian Prairie Evaporite Formation accumulated up to 200 m of halite-dominated beds across Western Canada in a salt basin extending from northern Alberta southeastwards into southern Saskatchewan and western North Dakota. A 1000 km long salt dissolution trend along the eastern basin margin resulted in the removal of 100–150 m of halite–anhydrite beds. A second salt dissolution trend removed up to 200 m of section across southern Saskatchewan. The removal of the halite-dominated beds and the collapse of the overlying strata responded to regional aquifer and vertical water flows driven by compaction up-structure to the NE towards the eastern margins of the foreland Alberta and intracratonic Williston constituent basins of the Western Canadian Sedimentary Basin. Flows resulting in dissolution trends in the deepening Alberta foreland basin were responses to Middle Jurassic–Early Cretaceous Columbian orogenic tectonism, in contrast with the multiple Paleozoic and Mesozoic dissolution stages across the southern Saskatchewan area of the northern Williston basin. Water flows along the Devonian Keg River strata migrated into the overlying Prairie Evaporite salt beds. The dissolution fronts advanced along multi-kilometre long NW- and NE-oriented sets of fault–fracture lineaments resulting from orogenic Precambrian block movements propagated into the overlying Devonian strata. The Athabasca Oil Sands accumulated above a 300 km long segment of the dissolution trend along the eastern margin of the evaporite basin. An unusually low 1:2 thickness ratio of removed salt beds to overlying strata resulted in significant structural controls on the deposition of the overlying Cretaceous McMurray Formation point bar complexes. Fluvial point bars up to 6 km long and tens of metres thick collinearly aligned along fairways tens of kilometres long, following the halite dissolution trends 200 m below. These sand reservoirs trapped Laramide oil migration into the area. Biodegradation followed, resulting in a trillion barrel resource. A final stage of salt removal occurred with the influx of subglacial meltwaters, rejuvenating 10 km 2 Cretaceous collapses across southern Saskatchewan.

Millennial-scale climatic fluctuation in the fluvial record during MIS3: Very high-resolution seismic images from NE Hungary

Alluvial architectures of a meandering river existing in MIS3 were observed on very high-resolution (VHR) single-channel waterborne seismic profiles, 20–30m below the Tisza River in the Pannonian Basin (Hungary). The study investigated the spatial and temporal variations of two, more than 2km-long continuous series of inclined reflections interpreted as laterally accreted point bar complexes. The phases of natural meander migration were reconstructed in 3D based on the changes in the geometry and dip of the inclined reflections. A channel-forming discharge curve extending over approximately 2.5ky was calculated by using paleo-width and depth data derived from the lateral accretion surfaces.Systematic analysis of the geometrical variations coupled with the discharge curve evaluation on each point bar complexes indicates millennial-scale discharge fluctuations 40–50ky ago, to that the river responded principally by incision and infilling. The primary periodicity, comparable to the sub-Milankovitch cycles, is superimposed by shorter periods: ca. 500-yearcycles reflect the phases of unidirectional meander development, while the smallest ones reflect the recurrence interval (150–200years) of the highest floods. River-bed incisions happened step-by-step related to extreme floods, when the meander development also changed. The smaller-scale and more rapid fluctuations within a development unit were represented in variations of the channel width. Although the poor geochronology of the sandy fluvial deposits cannot allow any direct correlation to the climatostratigraphic events, the millennial-scale climate variations of MIS3 were pronouncedly characteristic in the discharge fluctuations.

Spillage sedimentation on large river floodplains

AbstractActive deposition across the floodplains of large rivers arises through a variety of processes; collectively these are here termed ‘spillage sedimentation’. Three groups of 11 spillage sedimentation styles are identified and their formative processes described. Form presences on large river floodplains show different combinations of active spillage styles. Only some large floodplains have prominent levees; some have coarse splays; many have accessory channel dispersion and reworking, while still‐water sedimentation in lacustrine environments dominates some lower reaches. Infills are also commonly funnelled into prior, and often linear, negative relief forms relating to former migration within the mainstream channel belt.Shuttle Radar Topography Mission (SRTM) and Landsat 8 data are used to map spillage form types and coverage along a 1700 km reach of the Amazon that has an active floodplain width of up to 110 km with a systematic character transformation down‐valley. Spillage forms associated directly with mainstream processes rarely account for more than 5% of the floodplain deposits. There is a marked decrease in floodplain point bar complexes (PBC) over 1700 km downstream (from 34% to 5%), and an increase in the prevalence of large water bodies (2% to 37%) and accompanying internal crevasses and deltas (0% to 5%). Spillage sedimentation is likely within the negative relief associated with these forms, depending on mainstream sediment‐laden floodwater inputs.Spillage style dominance depends on the balance between sediment loadings, hydrological sequencing, and morphological opportunity. Down‐river form sequences are likely to follow gradient change, prior up‐river sediment sequestration and the altered nature of spilled loads, but also crucially, local floodplain relief and incident water levels and velocities at spillage times. Considering style distribution quantitatively, as a spatially distributed set of identifiable forms, emphasizes the global variety to spillage phenomena along and between large rivers. © 2016 The Authors. Earth Surface Processes and Landforms published by John Wiley &amp; Sons Ltd.

Late Quaternary Paleohydrology of the Madre de Dios River, southwestern Amazon Basin, Peru

Late Quaternary climatic and hydrologic variability triggered changes in fluvial deposition and erosion along the course of the Madre de Dios River, Peru, the largest tributary basin of the Madeira basin, itself the largest tributary basin of the Amazon. Three laterally extensive, Quaternary-age, terrace tracts are present within the Madre de Dios basin. Analysis of sedimentary facies, present in the modern cut banks and terraced sequences, along with radiocarbon dates on fossil wood and leaf material preserved in the terraced strata, allow reconstruction of the Late Quaternary depositional history of the sedimentary sequences, including determination of the approximate timing of aggradation and downcutting episodes and its relationship to the timing of past climate change in this portion of the Amazon basin and beyond. The Quaternary sediments underlying the terraces most often recorded deposition in a coarse-grained meandering fluvial system. The T3 terrace, the highest terrace, is underlain by the Miocene (?) Ipururi Formation, which is unconformably overlain by the late Miocene–Pleistocene (?) (> 48,000 cal yrs BP) Madre de Dios Formation, a multistory coarse-sandy to gravelly channel and point bar complex. The latter was downcut before 29,850 ± 100 cal yrs BP. This downcut landscape was infilled by meandering fluvial strata characterized by gravelly channel deposits in a sequence dominated by floodplain and lateral accretion deposits. These strata were in turn downcut to form the T2 terrace before 11,970 ± 100 cal yrs BP. A third episode of aggradation resulted in the deposition of a sand-dominated meandering channel complex that infilled the T2 valley and was subsequently downcut after 3780 ± 50 cal yrs BP. This most recent terrace is infilled by the modern fluvial sediment, which has been actively aggrading since at least 870 ± 50 cal yrs BP. Importantly, the Madre de Dios fluvial system actively aggraded between 30,000 and 25,000 cal yrs BP, (and likely much younger, as dated samples were, thus far, only found near the base of the T2 sequence). This observation implies that some combination of (1) increased precipitation and decreased temperature, (2) decreased evapotranspiration and increased runoff, (3) increased Andean glacial erosion and increased sediment supply, and (4) decreased atmospheric CO 2 (hence decreased rain-forest primary productivity and altered rain-forest physiology/ecology), entering the last glacial maximum period brought about increased floodplain deposition in the southwestern Amazon. Elsewhere in the Amazon basin few, if any, fluvial sediments of this age range have been observed. The start of the next major phase of aggradation coincided with the Younger Dryas and suggested that floodplain sedimentation in the lowlands was again related to cold and wet conditions in the adjacent highlands (and perhaps in the lowlands as well) and that Madre de Dios history was also tied to large-scale global climate. This aggradation may have continued throughout the early and mid-Holocene, until at least 3,780 cal yr BP. If so (and this is uncertain), this episode of sedimentation took place during a dry period.

Supervised Landform Classification to Enhance and Replace Photo‐Interpretation in Semi‐Detailed Soil Survey

A method to enhance manual landform delineation using photo‐interpretation to map a larger area is described. Conventional aerial photo‐interpretation (API) maps using a geo‐pedological legend of 21 classes were prepared for six sample areas totaling 111 km2 in the Baranja region, eastern Croatia. Nine terrain parameters extracted from a digital elevation model (DEM) (ground water depth, slope, plan curvature, profile curvature, viewshed, accumulation flow, wetness index, sediment transport index, and the distance to nearest watercourse) were used to extrapolate photo‐interpretation over the entire survey area (1062 km2). The classification accuracy was assessed using the error matrix, calculated by comparing both the whole API maps and point samples, with the results of classification. The first results, using a maximum‐likelihood classifier, were 58.2% (hill land), 39.1% (plain), and 45.3% (entire area) reproducibility of the training set. Six classes in the plain were responsible for a large proportion of the misclassifications, due to an insufficiently detailed DEM and the complex nature of landforms (point bar complexes, levees, active channel banks), which cannot be explained with the terrain parameters only. Reproducibility for a simplified legend of 15 classes over the study area was improved to 65.8% (plain), 58.2% (hill land), and 63.4% (entire area) using the whole‐API training set. After the simplification of legend (15) and with the iterative (3) selection of point‐sample training set, classification was able to reproduce 97.6% (hill land), 86.7% (plain), and 90.2% (entire area) of the training set. The supervised classification showed fine details not achieved by photo‐interpretation. The number of manual photo‐interpretations that had to be prepared was reduced from 84 to 6. The methodology can be applied by soil survey teams to edit and update current maps and to enhance or replace API for new surveys.

Palaeofluvial estimates from exhumed meander scrolls, Taref Formation (Turonian), Dakhla Region, Western Desert, Egypt

An array of meander scroll sets (point bar complexes) is exposed over a land surface eroded across the dip slope of sandstones of the Nubia Group, Taref Formation (Turonian) around the southern fringe of Dakhla Oasis, Egypt (25.5°N, 29°E). These sandstones are thus interpreted as fluvially deposited. Using published equations, bankfull channel width, depth, as well as sinuosity are estimated from radii of meander curvature. With a reasonable assumption of velocity, bankfull discharge is then estimated. This is found to be comparable with bankfull discharge values estimated from published plots of hydraulic geometry (width and depth) against discharge. Both approaches result in mean bankfull discharge estimates of 4.4–7.0×10 3 m 3s −1. Using mean radius of meander curvature, sinuosity is estimated at 1.66. In this hyperarid climatic region aeolian erosion has carved yardangs into the Taref sandstone, the flanks of which expose uniform, tabular cross-beds of coarse, clean quartz sand. From the palaeofluvial estimates, from other features of the exhumed meander scrolls, and from the character of the sandstone, channels in which the Taref sands were deposited are inferred to have been wide and shallow, of low sinuosity, rapidly migrating laterally, with minimal overbank deposition of muds, and rapid vertical accretion. Published indications of a tropical monsoon climate over Egypt in the Turonian are harmonized with the absence of fossil flora from the Taref with reference to indications of a rapidly changing fluvial landscape.

Optical resetting in large drainage basins: tests of zeroing assumptions using single-aliquot procedures

Optical resetting of previously accumulated charge populations at the time of transportation and burial of sediments remains a critical assumption in luminescence dating. This is particularly the case for fluvial sediments. We have tested the extent of bleaching in fluvial environments by applying the single-aliquot regeneration (SAR) equivalent dose ( D E) determination technique coupled with tests for partial bleaching. In the majority of cases, samples collected from a point bar complex on the Colorado River, Texas exhibit evidence of incomplete bleaching. Use of the minimum SAR D E in these cases resulted in estimated residual ages of c. 100–300 years. In a similar examination of SAR D E's for bed load samples collected along the Loire River we observe a monotonic decrease in effective equivalent dose ( D e) from values of 14, 10.2 and 3.2 (mean, median and minimum) Gy at source, down to 3.3, 3.6 and 1.6 Gy in the first kilometre of transportation, down to levels of 0.3, 0.2 and 0.0 Gy for transportation distances exceeding 300 km. We observed virtually no effect on the D E in the bed load of the introduction of trunk tributary streams and conclude that the likelihood of resetting of OSL in large fluvial systems is somewhat more effective that previously considered when the recycling of sediments down the system are borne in mind.

RIPARIAN FOREST EFFECT ON LATERAL STREAM CHANNEL MIGRATION IN THE GLACIAL TILL PLAINS1

ABSTRACT: Dendrochronology analyses of point bar complexes were used to quantify the effects of riparian forests on local lateral migration of bends in seven streams in the glacial till plains of north central Missouri. Stream bends were paired with similar bank height, midchannel radius of curvature, soil composition, and watershed size. In each pair, one concave bank was forested and one was unforested. Stream bends with unforested concave banks had an average local migration rate three times greater than stream bends that had forested concave banks.

Anabranch confluence kinetics and sedimentation processes in gravel-braided streams

Abstract Based on observations of channel changes and bar sedimentation in small-scale hydraulic models, it is argued that confluence zones play the key role in sedimentation patterns and processes in gravel-bed braided streams. The control is exerted because confluences are nodes in the channel network and therefore sedimentation and channel pattern downstream of a confluence are mediated by the response of the confluence to changes in discharge, bedload, and planform of the upstream confluent channels. Examples of confluence kinetics and sedimentation occurring over time-scales equivalent to days or weeks of competent flows in the prototype are described for the first time. Renewed braiding is directly associated with flow expansion downstream of confluences and triggered by migration of unit bars or bedload sheets through the confluence. Lateral translation of confluences may lead to the accretion of point bar complexes, while downstream translation often causes erosion or complete removal of existing bars. The effects of a change in position or orientation of one of the upstream channels are similar to those of confluence translation. In addition to the changes caused by progressive shifting of the confluence zone, a second group of adjustments involving changes in the discharge or bedload transport rate in the confluent channels is also common. These include: rotation of the scour axis redirecting flow and sediment movement downstream of the confluence and leading to downstream channel avulsion in some cases; a change in the total discharge of the confluence causing expansion or contraction of the scour zone; and the virtual abandonment of one of the confluent channels resulting in obliteration of the scour hole. Observing a confluence for several hours in the model leaves the visual impression of sediment being distributed downstream of the confluence rather like water spurting from a hose as it plays about. This descriptive synthesis of confluence kinetics provides the basis for analysis and quantitative modelling of confluence dynamics, which requires the acquisition of field data, as well as further laboratory experiments.

Depositional, Diagenetic, Production, and Seismic Characteristics of Mid-Dip Tuscaloosa Point Bar Complex, Little Creek Field, Mississippi: ABSTRACT

Little Creek field is oil productive from a structurally and stratigraphically trapped sandstone within the Upper Cretaceous lower Tuscaloosa Formation (Mid-Dip trend of southwestern Mississippi). The field was discovered in 1958 based on the seismic interpretation of simple closure at the lower Tuscaloosa horizon at a depth of 10,750 ft. Interpretation of core and log data indicates that the objective is a series of point-bar deposits and channel abandonment facies formed by the lateral migration of a north-to-south-flowing meandering river system. Development drilling, based on geologic concepts of point-bar architectures, occurred on 40-ac spacing with a total of 208 wells drilled; 162 were successful. New seismic data across the field have shown that the sandstone is a mappable soft seismic event; similar techniques have been successfully used for discovery and delineation elsewhere within the Mid-Dip Tuscaloosa trend. Little Creek has undergone primary recovery, waterflood, a carbon dioxide pilot, and recent initiation of a field-scale carbon dioxide flood. Approximately 70% of the original oil in place (102 million bbl) is estimated to be recovered by all processes. Reservoir quality and continuity are generally high and are controlled by both depositional and diagenetic processes. Average porosity and permeability are 24% and 100more » md, respectively. Depositional controls include slightly reduced vertical to horizontal permeability, local shale drapes, and channel abandonment facies. Diagenetic controls include compaction, chlorite grain coatings, and quartz and ankerite cements. The chlorite is most important in controlling the observed petrophysical properties of the sandstone, including the water saturation, porosity, and bimodal pore system.« less

Depositional Environments, Sequence Stratigraphy, and Trapping Mechanisms of Fall River Formation in Donkey Creek and Coyote Creek Oil Fields, Powder River Basin, Wyoming: ABSTRACT

Donkey Creek and Coyote Creek fields contain combined reserves of approximately 35 million bbl of oil and are within a trend of fields on the eastern flank of the Powder River basin that totals over 100 million bbl of reserves. The principal producing formation is the Lower Cretaceous Fall River Sandstone. A study of 45 cores and 248 logs from the three pools in the Donkey Creek and Coyote fields has shown that the Fall River is composed of three progradational deltaic units deposited during a period of rising relative sea level. These are locally eroded and are filled by a fluvial point-bar complex deposited following a lowering of relative sea level. Four important depositional facies have been recognized: the delta-front and distributary-channel sandstone of the highstand deltaic sequence and the point-bar sandstone and channel-abandonment of the lowstand fluvial sequence. Stratigraphic traps in Coyote Creek and south Donkey Creek pools are the result of permeable (250 md) point-bar sandstone (250 bbl oil/day ip) bounded updip by impermeable (0.1 md) channel abandonment mudstone. Most of the oil in the central Donkey Creek pool is produced from permeable (76 md) distributary-channel sandstone (150 bbl oil/day ip), which is restricted to the westernmore » flank of a structural nose. Lesser production, on the crest and upper western flank of the structure, is obtained from the less permeable (2.8 md) delta-front sandstone (50 bbl oil/day ip). Production is possibly limited to the crest and western flank by hydrodynamic processes.« less

Reservoir Characterization of Mesaverde (Campanian) Bedload Fluvial Meanderbelt Sandstones, Northwestern Colorado: ABSTRACT

Reservoir characterization of Mesaverde meanderbelt sandstones is used to determine directional continuity of permeable zones. A 500-m (1,600-ft) wide fluvial meanderbelt in the Mesaverde Group is exposed as laterally continuous 3-10-m (10-33-ft) high sandstone cliffs north of Rangely, Colorado. Forty-eight detailed measured sections through 3 point bar complexes oriented at right angles to the long axis of deposition and 1 complex oriented parallel to deposition were prepared. Sections were tied together by detailed sketches delineating and tracing major bounding surfaces such as scours and clay drapes. These complexes contain 3 to 8 multilateral sandstone packages separated by 5-20 cm (2-8 in.) interbedded siltstone and shale beds. Component facies are point bars, crevasse splays, chute bars, and floodplain/overbank deposits. Two types of lateral accretion surfaces are recognized in the point bar facies. (1) Gently dipping lateral accretions containing fining-upward sandstone packages. Large scale trough cross-bedding at the base grades upward into ripples and plane beds. (2) Steeply dipping lateral accretion surfaces enclose beds characterized by climbing ripple cross laminations. Bounding surfaces draped by shale lags can seal vertically stacked point bars from reservoir communication. Scoured boundaries allow communication in some stacked point bars. Crevasse splays showing climbing ripples form tongues of very fine-grained sandstone which flank point bars. Chute channels commonly cut upper point bar surfaces at their downstream end. Chute facies are upward-fining with small scale troughs and common dewatering structures. Siltstones and shales underlie the point bar complexes and completely encase the meanderbelt system. Bounding surfaces at the base of the complexes are erosional and contain large shale rip-up clasts. End_of_Article - Last_Page 493------------

Initiation of bed forms and meanders in coarse-grained sediment

In a straight plane-bed channel in coarse sediment under natural flow conditions, primary transverse bars were rapidly formed during infrequent high flows, and the accompanying flow modifications led to bank erosion. Primary bars were subsequently incorporated as the cores of point-bar complexes, with additional lateral and tail accretion, chute formation, and lesser erosional and sedimentary modifications. The range of natural flows produced a variety of bed forms; therefore, the channel form at any point in time was not strictly attributable to any single controlling discharge, but the broad features of channel development in general relate to the forms of the pseudo-meandering model. A three-phase model of meander development, with these observations exemplifying the first phase, is adequate and useful.

The Character of Channel Migration on the Beatton River, Northeast British Columbia, Canada

Dendrochronological surveys on ten point-bar complexes on the Beatton River, northeast British Columbia, provide the basis for measurement of lateral migration and incision during the last 250 yr. The rate of channel-bend migration reaches a maximum value where the ratio radius of channel curvature to stream width ( r m / w m ) approximates 3.0. The rate of channel migration rapidly declines for bends with values of r m / w m greater or less than 3.0. In addition, the spacing of flood-plain ridges ( d ) increases with increasing rates of bend migration. Equations for predicting the migration rate ( M , meters/year) on the study reach take the form: (see pdf for equation)

CHANNEL STABILITY ON THE LOWER OHIO RIVER∗

The meander loop pattern of the lower Ohio River has changed very little over the last thousand years. Point bar complexes are deposited and destroyed quite rapidly in comparison.