Point-bar Deposits Research Articles

SEDIMENTOLOGICAL AND ECOLOGICAL CONTROL OF WESTPHALIAN B PLANT ASSEMBLAGES FROM WEST YORKSHIRE

SUMMARY The lowest Middle Coal Measures (Upper Carboniferous, Westphalian B) at five localities in West Yorkshire (Swillington, Thorpe, Lodge Hill, Pugneys and Lowther North) expose strata from the Thornhill rock to the Two Foot Marine Band. The sections show three major facies associations interpreted as representing lake, delta and floodplain. These are divided into nine subfacies associations, each characterised by its particular lithology, sedimentary structures, fauna and flora. Facies 1A, medium grey and fissile black shales with non-marine bivalves and Lepidodendron leafy shoots, is interpreted as lacustrine/prodelta clays. Facies 1B, upward-coarsening silty shales to silts containing rare Calamites, represents distributary mouthbar sequences. Facies 2A, erosionally based sheet sands showing small scale trough cross bedding with horizons of comminuted plant debris, is interpreted as a proximal distributary mouthbar. Facies 2B is dominated by laminated silty shales with thin units of silty sandstone, contains abundant comminuted plant debris, and represents interdistributary lake deposits. Rarely, Facies 2A and 2B are overlain by a thin coal seam rich in Calamites, Facies 2C, which represents marginal swamp deposits. Facies 3A consists of thick coals and seatearths which were formed mainly by lycopods (shown by the palynology) and were dominated by different groups in succession; the facies is broadly interpreted as the “swamp-association”. Facies 3B, poorly laminated shales with abundant plants (often species rich) and thin siltstones, represents true floodplain/crevasse splay deposits. Facies 3C, thick, variable, upward-fining sandstones showing lateral accretion surfaces, is interpreted as pointbar deposits of meandering rivers which often have silty shale plugs. Abandoned channels are frequently filled with silty shales (Facies 3D) and have slumped banks, and often contain large contorted fronds of fern-like foliage. The fossil plant assemblages are in part related to the sedimentary environment in which they are found and in part to the communities from which they have come. Preliminary reconstruction of the sedimentary environment allows hypotheses of the original plant communities to be made. The differences between the flora forming the coal and the flora found in other lithologies are emphasised.

Upper permian coal measures of the Central Sydney Basin, N.S.W. – seismic data analysis and drilling results

The results of a preliminary eight hole coal drilling program in the Dural-Spencer-St. Albans region are presented. The data from the South Colah, Frederick and St. Albans holes are discussed with respect to lithologies, stratigraphy, depositional environments, coal intersections and qualities. The holes were drilled beyond 800m. The north western bores intersected coking coal seams, the thicknesses of which varied between 0.69m and 4.85m. A study was made of the regional seismic reflection data available from petroleum prospecting operations. The data was very useful in the drilling programme in that it enabled the recognition of, and estimation of depths to, the coal measures as a gross unit. However it was not possible to determine seam configurations owing to fundamental limitations in the resolving power of the reflection method. The coal measure sediments intersected in the drill holes comprise: (a) A lower unit in which sandstones are mostly thin; shales, dirty coals and thin tuff beds are common. The beds appear to be lenticular. Evidence of salinity (indicated by burrowing) is common; (b) An upper unit in which sandstone and conglomerates are the dominant rock types. The sandstones appear to be sheet-like bodies. The coals are cleaner and burrowing in the finer sediments is rare. Coking quality coals are developed in this unit, they are thickest in the northwest. Assuming a thick tuff bed to be a time plane, the facies change is diachronous becoming younger from windeyer's Hawkesbury Bore southwards. The relationships in the South Colah drill hole may be more complex. Environmentally the lower is almost entirely deltaic ranging from bay to upper delta plain deposits becoming more distal westwards and downwards. One possible point bar sequence is present in the South Colah hole. The upper unit is mainly fluvial, probably deposited by braided streams but some point bar and deltaic deposits are present in South Colah. The sediments of the upper sequence could have been derived from the northeast or the north to northwest. The simplest model is that the upper and lower units are the downstream deposits of the streams that laid down the coal measures in the Newcastle-Lake Macquarie area.

Computer simulation of sedimentation in meandering streams

ABSTRACTA dynamic mathematical model for simulation of sedimentation in meandering streams is briefly described. This is composed of component mathematical models which are formulated to predict the following aspects of the system for a given physical situation and a single time increment. (1) The characteristics of the plan form of the meander; (2) the movement of the meander in plan, and definition of cross‐sections across the meander in which erosion and deposition are considered in detail; (3) the hydraulic properties of the channel in the bend and the erosional and depositional activity within the channel as defined in specific cross‐sections; (4) the nature and occurrence of cut‐off; (5) a relative measure of the discharge during a seasonal high water period, which is used in (3) and (4); (6) aggradation.The model, in the form of a FORTRAN IV computer program, has been used to simulate various aspects of sedimentation in meandering streams by performing a set of experiments with the program under different input conditions.The geometry of simulated point bar sediments, as controlled by channel migration over floodplains with variable sediment type, agrees broadly with the natural situation, however extensive sheets of point bar sediment cannot be simulated because large scale meander‐belt movements are not accounted for.In the simulated sediments, successive surfaces of the point bar before falling stage deposition (lateral and vertical) may be picked out, and these delineate the epsilon cross‐stratification of Allen (1963b). The epsilon unit thickness is that measured from bankfull stage down to the lowest channel position existing prior to deposition.The model records the characteristic fining upwards of grain sizes in the point bar, and the systematic distribution of sedimentary structures. Channel migration combined with seasonal scouring and filling across the channel section produces a characteristic relief in the basal scoured surfaces and facies boundaries (as defined by variation in grain size and sedimentary structure). A related lensing and inter‐fingering of the facies may also be present. The model also records large‐scale lateral changes in grain size and sedimentary structure associated with changes in the shape of developing meanders.When channel migration is combined with a constant aggradation rate the model predicts a general slope (relative to the land surface) of facies boundaries and scoured basal surfaces upward in the direction of channel movement. If aggradation sufficiently increases the thickness of fine‐grained overbank material, there is a channel stabilization effect. It is shown that a complete sequence of point bar sediments capped by overbank sediments would rarely be preserved in the moving‐phase situation. Such preservation only becomes likely when an aggrading section lies out of range of an eroding channel for a considerably longer time span than it takes a meander to move one half‐wavelength downvalley. Deep channel scours have a higher preservation potential than contemporary shallower ones.Where appropriate field data exist the model can be used in the more accurate recognition of ancient fluviatile sediments. Inferences may be made about the erosion‐deposition processes operating in the ancient channel system, and the geometry and hydraulics of the system can be alluded to. A representative application of the model to the quantitative interpretation of an ancient point bar deposit is illustrated. There is reasonable agreement between the natural and the simulated deposits, and a broad quantitative picture of the palaeoenvironment of sedimentation is obtained.

Overbank Sedimentation in the Delaware River Valley during the Last 6000 Years

A thick sequence of floodplain sediments has accumulated in the Delaware River Valley by the process of overbank deposition. Textures in the sediments indicate that the sequence contains no point-bar deposits and is unbroken by periods of erosion. Fourteen radiocarbon dates show that deposition began at least 6000 years ago and has continued to the present. Because the Delaware River shifts its position laterally at a very slow rate, overbank deposition becomes dominant in the construction of its floodplain.

Sedimentary Structures and Discriminant Analysis of Textural Characteristics of Recent and Ancient Point Bars: ABSTRACT

Point-bar deposits of the Raton-Vermejo (Cretaceous) Sandstone are analogous to recent point-bar deposits of the Brazos River of Texas. Sedimentary structures in the two bars are similar; both contain abundant trough (festoon) crossbedding and climbing ripples. Avalanche foreset bedding is prominent at the maximum curvature of the meander in the Brazos River; reverse climbing ripples formed at the toe of the foresets. The upper and lower surfaces of the bars, measured perpendicular to flow direction at the maximum curvature of the meander, converge shoreward in both the Brazos and the Raton-Vermejo bars. Grain-size analyses of the Brazos River and Raton-Vermejo point bars are similar. In comparison with known suites of eolian, fluvial, and beach sands, all of 33 samples of Brazos River sands are classified as fluvial sands by multiple-group discriminant analysis. Ten of the initial 50 variables calculated for each sample were removed to decrease provenance effects. Classifications were performed from the remaining 40 by using the 16 most discriminating variables. Individual sedimentary structures in both the recent and ancient bars have similar grain-size distributions. Discriminant analysis indicates that grain-size variables alone are sufficient to separate the samples into 5 different sedimentary-structure groups (trough crossbedding, avalanche foresets, climbing ripples, reverse-climbing ripples, and horizontal laminations). End_of_Article - Last_Page 808------------

Depositional Environments of Paleocene Tongue River Formation, Western North Dakota

Four basic lithologic types are present in the Tongue River Formation in western North Dakota. These are arranged in a basic cyclic unit consisting, from bottom to top, of (1) gray clay and silt, (2) lignite, (3) yellow silt and sand that may be clayey, and (4) sand. This cyclic unit probably originated during the formation and filling of flood basins on an alluvial plain. The units of gray clay and silt show irregular stratification, and are commonly lignitic and fossiliferous. These units are interpreted as flood-basin deposits. As the flood basins filled, swamps became established and lignite beds formed. Crevassing and expansion of natural levees across the flood basins would have destroyed the swamps and deposited yellow silt and sand over the lignite. The yellow silt and sand commonly show climbing ripples and remnants of vertical tree trunks, indicating high rates of deposition characteristic of natural levees and crevasse splays. Two types of sand bodies are present. Tabular sand bodies generally are about 10 ft thick and thousands of feet wide; they become finer grained upward and have a vertical sequence of sedimentary structures that shows a decrease in the flow regime upward through the deposit. These sand bodies probably are point-bar deposits formed by lateral accretion in high-sinuosity streams. Linear sand bodies are straight and show deeply eroded bases. Paleocurrent indicators trend parallel with the axes of these bodies, which are interpreted as low-sinuosity channel deposits. Elongate concretions, visible on air photographs also trend parallel with the axes of the sand bodies and can be used to map paleochannel patterns.

Sedimentology of the Upper Devonian to Lower Carboniferous Finke Group, Amadeus and Warburton Basins, central Australia

Abstract The Finke Group in central Australia was divided into four formations by Wells et al. (1966), and the Santo Sandstone (Wells et al., 1967) is now included in the group as a synonym of the Idracowra Sandstone. The distribution of the latter has been amended following detailed stratigraphic work south of Black Hill Range. The age of the Finke Group is Upper Devonian to ?Lower Carboniferous, based on correlation with the Pertnjara Group. The relative abundance and significance of sedimentary features are described from each formation, and permit the following environmental deductions: Polly Conglomerate—proximal braided stream alluvial fan deposits; Langra Formation—fluviatile sediments consisting largely of point-bar deposits; Horseshoe Bend Shale—playa lake siltstone deposits; and Idracowra Sandstone—fluviatile sediments. The inter-relationship of these facies and their interdigitation with formations of the Pertnjara Group is discussed in terms of a large terrestrial basin in which through-flowin...

On the bedding in the natural-levee and the point-bar deposits of the Gomti river, Uttar Pradesh, India

Genesis of channel-fill “cross-bedding” and ripple-bedding units in the natural-levee and the point-bar deposits of the Gomti River is discussed. Channel-fill cross-bedding is produced because of development of and deposition in the crevasse-splays on the levees. Occasionally small-ripple bedding occurs, intricately interlayered with horizontal bedded units of upper-flow regime origin. Some of the horizontal bedding near the top of the point bars and levee is probably produced by setting of suspension clouds under low-energy conditions as a result of decrease in turbulence with receding water.

Virgil and Wolfcamp Fluvial, Deltaic, and Inter-Deltaic Embayment Depositional Systems in North and West-Central Texas: ABSTRACT

Virgil and lower Wolfcamp sediments on the Eastern shelf were deposited within 3 depositional systems--Bowie fluvial system, Cisco delta system, and Lake Hubbard interdeltaic embayment system--which are the subjects of this report. Complementary shelf-edge bank, slope, and basin systems, to be discussed by William E. Galloway, complete the facies tract. Bowie fluvial facies compose tabular and belted sandstone bodies characterized by braided and coarse-grained meander-belt sequences and sedimentary structures. Fine-grained meander-belt facies generally becoming finer upward, lateral accretionary point bar deposits, and characteristic sedimentary structures commonly occur high in the Bowie fluvial system, indicating decreasing tectonic activity in source areas. Basal fluvial channel facies normally cut subjacent deltaic facies and may overlie rocks of previous depositional episodes. Cisco deltaic facies are composed of prograding, prodelta mudstones that become coarser upward, delta front, channel-mouth bar and distributary channel sandstones, and aggradational crevasse splay facies, delta-plain mudstones, and coal or organic-rich clays. Local sandstone bars of destructional origin may fringe the delta facies. Lake Hubbard interdeltaic embayment facies flank deltaic lobes and are composed of mudstone, thin extensive sheet sandstone, and thin impure detrital coal. Some sheet sandstones reflect strandplain accretion, but more commonly they are reworked delta-front sands. Deltas prograded rapidly westward across the slowly subsiding Eastern shelf. Sediments from the east supplied crevassing delta lobes until avulsion of over-extended systems occurred. Delta construction, accompanied by deposition of complementary facies within nearby interdelta embayments, restricted open-shelf limestone facies to areas downslope or in areas of abandoned deltas. Channels were cut deeply into subjacent facies as prograding deltas extended fluvial erosion and deposition far across older deltaic facies. Marine processes slowly modified abandoned, compacting, and subsiding deltas. Winnowed sediment was swept into interdeltaic embayment mudflats and strandplains. In the absence of local terrigenous sediment, shelf limestones transgressed upslope and along the coast to coale ce over marsh-stabilized deltaic and interdeltaic areas. All environments could occur simultaneously, shifting with distribution of delta sites to produce repetitive End_Page 151------------------------------ sequences of thin superposed depositional systems. Deposition of fluvial and deltaic facies was rapid; deposition of destructional and transgressive facies represents most of the time consumed by deposition. Depositional systems shifted westward through time as the eastern flank of the basin was filled. End_of_Article - Last_Page 152------------

PHYSIOGRAPHIC FEATURES AND STRATIFICATION TYPES OF COARSE‐GRAINED POINTBARS: MODERN AND ANCIENT EXAMPLES1

SUMMARYPrimary sedimentary structures in modern point bar deposits of the Amite River in Louisiana and the Colorado River in Texas are analogous to features observed in Eocene Simsboro and Pleistocene Colorado River deposits of the Texas Gulf Coastal Plain.Short‐duration peak flow, channel pattern, average stream gradient of about 2–3 ft./mile, and bank stabilization by dense vegetation are major parameters controlling the depositional pattern of coarse sand and pebble gravel of the Amite and Colorado rivers. Stratification is directly related to specific depositional features and consists of: large‐scale trough‐fill cross‐stratification in the scour pool; trough‐fill cross‐stratification and foreset cross‐stratification in the lower point bar; parallel laminae, large foreset cross‐stratification, and trough‐fill cross‐stratification in the chute bar; parallel inclined laminae, climbing ripple laminae, and mud drapes in the chute fill; and parallel inclined laminae, mud drape, and foreset cross‐stratification in overbank, flood‐plain deposits.Fundamental differences between point bars of bed‐load streams (low suspended load/bed load ratio) and mixed‐load streams (high suspended load/bed load ratio) are that upper point bar sediments with small trough sets and parallel inclined laminae occur only in fine‐grained (mixed load) fluvial deposits, and large scale foresets of chute bars are common to coarse‐grained (bed load) fluvial deposits but are not found in fine‐grained fluvial deposits. Upward fining sequences, characteristic of fine‐grained fluvial deposits, are uncommon in sediments deposited by bed‐load streams such as the Amite and Colorado rivers.The Simsboro Sandstone consists mainly of scour pool, lower point bar, and chute bar sediments. Chute fill and flood‐plain deposits are preserved only in the highest stratigraphic sequence. Pleistocene Colorado River deposits display the same sequence of stratification types as the Simsboro but are composed of coarser material.

Continental marine transition in the Devonian of Prince of Wales Island, Northwest Territories

In eastern Prince of Wales Island the Peel Sound Formation is subdivided into upper and lower members on the basis of lithology. The upper member comprises five laterally equivalent north–south facies belts which are in westward order:1) Conglomerate Facies: cobble and boulder conglomerates deposited on alluvial fans mainly by the action of debris floods.2) Conglomerate–Sandstone Facies: interbedded conglomerate, fine to coarse red sandstone and red siltstone in a repeated fining-upward succession, deposited by low sinuosity braided streams. Cyclic sedimentation is attributed to channel migration and infill.3) Sandstone Facies: the presence of laterally extensive planar cross sets suggests point bar deposition in high sinuosity streams. Tabular bedded sandstone was deposited by stream floods in upper regime flow.4) Sandstone–Carbonate Facies: streams flowing westward formed small deltas, accumulating red, pink, and buff sandstones as topset deposits, interbedded with dolomites containing marine fossils, and clean washed gray sandstones, shales and dolomites with ostracods, pelecypods, gastropods, and Lingula, indicating an estuarine environment.5) Carbonate Facies: dolomites with abundant marine fauna and limited clastic content. Sedimentary structures indicate quiet, shallow water conditions.Paleocurrent evidence indicates an easterly source, confirmed by conglomerate clasts derived from the Boothia Uplift, an area of Lower Paleozoic and Proterozoic rocks extending northwards from the Boothia Peninsula to Somerset Island.

Stratigraphy and Petrology of the Nonmarine Honda Group (Miocene), Upper Magdalena Valley, Colombia

Research Article| August 01, 1970 Stratigraphy and Petrology of the Nonmarine Honda Group (Miocene), Upper Magdalena Valley, Colombia SAMUEL S WELLMAN SAMUEL S WELLMAN Shell Oil Company, Box 2241, Houston, Texas 77001 Search for other works by this author on: GSW Google Scholar Author and Article Information SAMUEL S WELLMAN Shell Oil Company, Box 2241, Houston, Texas 77001 Publisher: Geological Society of America Received: 07 Jul 1969 Received: 17 Mar 1970 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Copyright © 1970, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. GSA Bulletin (1970) 81 (8): 2353–2374. https://doi.org/10.1130/0016-7606(1970)81[2353:SAPOTN]2.0.CO;2 Article history Received: 07 Jul 1969 Received: 17 Mar 1970 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation SAMUEL S WELLMAN; Stratigraphy and Petrology of the Nonmarine Honda Group (Miocene), Upper Magdalena Valley, Colombia. GSA Bulletin 1970;; 81 (8): 2353–2374. doi: https://doi.org/10.1130/0016-7606(1970)81[2353:SAPOTN]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Miocene Honda Group, more than 3000 m thick in the Upper Magdalena Valley of Colombia, was derived from the volcanically active igneous-metamorphic central range (Cordillera Central) of the Andes, which was mantled with sedimentary and volcanic rocks. The group was deposited by rivers flowing generally normal to the central range, indicating that major uplift of the eastern range occurred after accumulation of the Honda sediments.Two major depositional environments of fluvial sedimentation are interpreted from sedimentologic and petrologic evidence: low gradient-high sinuosity, and high gradient-low sinuosity streams. The former type, favored during low tectonic and/or volcanic activity, resulted in red hematitic floodplain mudstones of montmorillonite, illite-montmorillonite and kaolinite, with subordinate thin tan, nonconglomeratic, subarkosic to arkosic, simple sand-bodies. The sandbodies, elongate with a low width: depth ratio, show the vertical arrangement of sedimentary features and fining-upward grain-size that characterize point bar deposits of low gradient, meandering streams. Directions of cross-stratification within the sandbody and the sandbodies themselves have a high variance.The second type, operative during moderate to high tectonism and/or volcanism, was probably braided and deposited thick sandbodies, commonly multistory and multilateral, with a high width: depth ratio. Sedimentary structures have a predictable vertical arrangement but do not become finer in grain size upward. Directions of cross-strata in sandbodies and the sandbodies themselves have a lower variance than those of the first type. The gray, conglomeratic, arkosic to impure arkosic sandbodies are associated with subordinate drab mudstone of montmorillonite and illite-montmorillonite.The Honda Group consists of two formations. The lower, La Dorada Formation (new name), was deposited during increasing tectonism an P volcanism, which were almost nil during accumulation of the conformably underlying La Cira Formation. The Puerto Salgar Member (new name) of the La Dorada is overlain by the Perico Member (new name), which is capped by an orthoconglomeratic interval. The upper formation of the Honda, the Villavieja (new name), accumulated during mild tectonic activity and episodic volcanism which yielded two sharply contrasting lithofacies: the Baraya Volcanic Member (new name) and the Cerro Colorado Redbed Member (new name). This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Physical Characteristics of Fluvial Deposits: ABSTRACT

The physical processes of sediment transport and deposition by confined unidirectional flow are produced only in a fluvial environment. The physical properties resulting from these processes provide unique criteria for recognition of fluvial deposits. Characteristic physical properties include (1) surface texture, (2) particle shape, (3) texture, (4) fabric, (5) sedimentary structures, (6) bedding, (7) sequence of structures, bedding, and textures, (8) scour surfaces, and (9) local and regional geometric patterns. Other aspects, including mineralogy, detrital clasts and fragments, physical character of the associated sediments, and fauna and flora, may aid in the identification of fluvial environments. Point-bar deposits resulting from channel migration are the most commonly preserved type of fluvial sandstone bodies. These geomorphic features are nearly universal in all meandering streams, and they control clastic deposition. The commonly developed sequence of festoon, current-laminated, and ripple cross-bedded sedimentary units is developed in response to flow across point bars. Other types of fluvial sand bodies, such as those deposited in alluvial fans, braided streams, and deltaic distributaries, exhibit many fluvial characteristics, but they lack the sequence of sedimentary structures related to point-bar deposition. Unidirectional currents produce characteristic grain-size distributions, which suggest a predominance of saltation and suspension modes of particle transport. Current transport produces elliptical-shaped particles with smooth surfaces. Detrital clay clasts commonly are preserved, many altered to clay-ironstone concretions. Minerals chemically stable in fresh, oxidizing, slightly acid water are commonly characteristic, such as kaolinite, feldspar, and ferric iron. The absence of other minerals such as calcite, glauconite, and ferrous iron compounds is significant. The external geometry of fluvial deposits is probably the least characteristic physical attribute. Individual outcrops may not show channeling, and fluvial sand bodies may be of a blanket type. Boundaries of channels, however, show abrupt pinchouts, commonly within a few hundred feet. Trends of sand bodies in connection with paleocurrent and slope indicators provide strong supporting evidence for identifying fluvial environments. End_of_Article - Last_Page 747------------

Dispersal Patterns of Pennsylvanian Sandstones in the Michigan Basin

ABSTRACT Sandstones in the isolated Pennsylvanian section of the Michigan Basin range in age from Morrowan through Desmoinesian. They are dominantly of fluvial origin, as indicated by dispersal pattern geometry and field characteristics. During Morrowan time, the basin was characterized by deltaic sedimentation, with sediment dispersal controlled mainly by pre-Pennsylvanian topography. Sand detritus was localized in high energy lowlands, primarily as distributary channel deposits. The delta originated near Saginaw Bay and prograded across the basin in southerly and westerly directions, subduing much of the initial basin relief. In succeeding Atokan and Desmoinesian time, the basin became the site of a broad alluvial plain, which contained an aggrading stream system that drained down a regional paleoslope to the west or southwest. Sand was concentrated by point-bar deposition in the higher energy stream channels, whereas finer clastics were localized in inter-channel flood plain areas. The alluvial plain was substantially inland from Pennsylvanian seaways, and only occasionally inundated by marine transgressions. Provenance of Pennsylvanian sandstones was relatively constant, with the primary avenue of detrital influx being consistently from the northeast. Sandstone petrology indicates a multicycle origin, with probable derivation from older Paleozoic arenites.

Comparison of Recent and Ancient Coarse-Grained Point Bars: ABSTRACT

Sequences of sedimentary structures in modern point-bar deposits of the Amite River in east Baton Rouge Parish, Louisiana, are analogous to features observed End_Page 731------------------------------ in Eocene Simsboro deposits in Milam County, Texas. Annual rainfall of approximately 60 in., channel pattern slightly to highly meandering, average stream gradient of about 5 ft/mi, and bank stabilization by dense vegetation are major parameters controlling Amite deposition of coarse sand and pebble-gravel sediment. Stratification types are related directly to specific depositional features, and include (from thalweg to overbank): thalweg (large-scale cross-stratification), lower point bar (trough cross-stratification, avalanche beds), chute bar (parallel laminae, avalanche beds, trough fill), chute fill (parallel inclined laminae, climbing ripples), and overbank (parallel inclined laminae, mud drape, avalanche beds). Fundamental differences between point bars of streams transporting coarse-grained bed load and streams having fine-grained bed load are: muddy floodplain deposits are associated only with fine-grained bed-load streams; upper point-bar sediments with ripple cross-stratification and parallel inclined laminae occur only in fine-grained fluvial deposits; chute-front avalanche beds are common in coarse-grained fluvial deposits but are not found in fine-grained fluvial deposits. Coarse-grained fluvial deposits do not show the upward decrease in grain size that is reported to characterize fluvial sediment. The Simsboro consists mainly of thalweg, lower point-bar, and chute-bar deposits; chute-fill and overbank deposits are preserved only in the last depositional sequence. End_of_Article - Last_Page 732------------

Sedimentology of the Pleasantview Sandstone, Southern Iowa and Western Illinois

ABSTRACT In both Iowa and Illinois, the Pleasantview Sandstone exhibits a thick, variably bedded, elongate channel phase and a thin, uniformly, thin-bedded, widespread sheet phase. The sheet phase occurs laterally to the upper part of the channel phase and is believed to be unconformably truncated by, and hence older than, the channel phase at any one place. Ripple marks and local organic reworking are present in the sandstone of the sheet phase, which rarely exceeds a thickness of 10 feet. Sands of the sheet phase are considered to have been deposited initially as small distributary mouth bars in shallow marine water of a deltaic fringe environment. Subsequent coalescence of bars and partial redistribution of sand by coastal currents formed a delta-front sheet sand prograding seaward (westwar in western Illinois, south-southwest in southern Iowa), generally conformably, over delta fringe and distal prodelta silts and clays. Channels carrying the Pleasantview detritus were eroded, predominantly subaerially, to depths of 70 to 80 feet, as distributaries across the swamp- or marsh-covered deltaic plain. Channel filling (formation of the channel phase) was by point bar deposition and (or) nonmarine infilling after channel abandonment. Medium-scale crossbedding was formed within the basal point bar sands, whereas current ripple marks predominate in the upper portion of the channel fill. Texturally, the very fine-grained, silty sheet phase sandstones of the Pleasantview are generally finer grained than the sandstones of the channel phase (fine- to very fine-grained). Pleasantview channel sands in southern Iowa, however, contain noticeably more fine-grained sand and are coarser than the channel sands in western Illinois. Carbonaceous siltstone acid silty shale are common to dominant in the upper, thin-bedded section of the channel phase. Sediment transport within channels was parallel to channel margins, being toward the west in western Illinois, and to the southwest in southern Iowa. Excluding matrix, the modal constituents of the Pleasantview Sandstone show negligible variation from Iowa to Illinois and between sheet and channel sands. Moreover, the composition of the Pleasantview (subfeldspathic lithic wacke) differs but little from the other major Pennsylvanian sandstones of the same or younger ages in the Eastern Interior Basin. The garnet content of the Pleasantview Sandstone, however, is several times greater in southern Iowa than in western Illinois. A garnet-contributing source north, northwest, or northeast of Iowa is implied. This source, believed to have been primarily a Cambrian and (or) Precambrian sedimentary terrane, supplied the Pleasantview of Iowa and Illinois with compositionally stable modal and heavy mineral constituents. Sedimentary recycling is especially evident in the heavy minerals. Although there is no conclusive evidence, some garnet-free detritus may have entered Illinois from the east or northeast, diluting the garnet-bearing sediment coming from the north. Apparently no barrier to sediment transport existed between the Eastern Interior and Mid-Continent basins during Pleasantview Sandstone deposition.

Divergence Between Dip Direction of Cross-Strata and Enclosing Cross-Strata Set Boundaries: a Characteristic of Alluvial Point-Bar Sands: ABSTRACT

Repeated post-flood mapping and trenching of laterally accreting point bars along steep, meandering streams in southeastern Louisiana reveal a characteristic divergence between the azimuths of dip of scoured set boundaries and the cross-strata which they enclose. The interrelation offers a potential criterion for recognition of ancient point-bar deposits. After floods, long curving sand waves are observed to be normal to the low-stage channel high on the emergent point-bar surfaces, but curve in the direction of flow to near parallelism with the low-stage channel near low water level. Cross-strata sets exposed in trenches on the emergent point bar are bounded by scoured surfaces which slope radially toward the low-stage channel (i.e., they mimic the radial slope of the point-bar surface in the meander bend). The enclosed cross-strata dip generally in a downstream direction or obliquely toward the higher part of the point bar, diverging from the channelward dip direction of the scoured set boundaries by 80°-135°. Sand waves which generated the cross-strata appear to have migrated downstream parallel with contours on the point bar or obliquely up onto the point bar, as revealed in the frozen post-flood surface sand-wave patterns. The inferred sand-wave migration pattern and growth of the point bar by lateral accretion into the flood-scoured channel were confirmed by sonic sounding during floods. Sandstones of apparent point-bar-lateral accretion origin form part of complex belt-like bodies of sandstone in the Carboniferous of central Pennsylvania. The alleged point-bar sandstones are composed of unidirectionally dipping sets of cross-strata, crudely sigmoid in shape, which thin both up and down the dip of the scour surfaces bounding the sets. Enclosed cross-strata dip, on the average, in directions perpendicular to the dip of the set boundaries, indicating that flow was along the original strike, rather than down the dip, of the scour surfaces. Lateral accretion of the point bar into the channel appears to have progressed by addition of sand deposited in the lee of sand waves migrating parallel with the point-bar surfaces, as observed on the modern point bars. End_of_Article - Last_Page 530------------

Cross-stratification in San Bernard River, Texas, Point Bar Deposit

ABSTRACT Cross-strata, formed by megaripple migration, have been investigated in a point bar deposit of the San Bernard River, Texas. High-angle cross-strata with downstream dip components and low-angle cross-strata with upstream dip components were found. The former originate on lee slopes and the latter on stoss sides of megaripples during flood stages of the river. Sets of cross-strata are about 1 foot thick and are generally non-continuous.