Coarse Member Research Articles

Using ichnological relationships to interpret heterolithic fabrics in fluvio‐tidal settings

AbstractIn fluvio‐tidal settings, the sediment is dominantly derived from the river systems. However, the importance of landward tidal transport of sediment in tidally influenced sedimentary environments is difficult to assess, particularly in the rock record. This problem is addressed using two intervals within the Lower Cretaceous McMurray Formation, each representing a distinct inclined heterolithic stratification motif. The ichnological variation between the heterolithic intervals is analyzed to determine which lithosomes are associated with brackish‐water (tidally influenced) colonization windows. From this, the relative fluvial influence responsible for the deposition of the fine and coarse members can be determined. Both of the inclined heterolithic stratification fabrics studied record the deposition of fluvio‐tidal point bars wherein the heterolithic bedding represents variations in river discharge. The first fabric comprises inclined heterolithic stratification in which bioturbation only occurs in mudstone beds. This fabric indicates that deposition occurred in more proximal positions within a fluvio‐tidal system (i.e. the outermost inner to middle estuary or distributary channels). In this example sand deposition is interpreted to represent high‐energy, freshwater dune migration within a fluvial‐dominated setting, whereas mud beds reflect brackish‐water suspension deposition during times of low river discharge. The second fabric, which is interpreted to have developed in more distal depositional positions (i.e. the middle estuary or seaward of the turbidity maximum in deltas), consists of inclined heterolithic stratification with laminated mudstone and bioturbated sandstone. In these inclined heterolithic stratification successions the mudstone beds were deposited under the influence of freshwater and heightened sedimentation rates, whereas bioturbated sandstone was colonized under brackish‐water conditions and in the presence of tidally facilitated sediment transport. In both examples, the bioturbated lithosomes are related to colonization windows that indicate the predominance at that time of marine or tidally influenced processes over fluvial processes.

Morphology and Depositional History of Exhumed Permian Point Bars in the Southwestern Karoo, South Africa

ABSTRACT A large planimetric exposure of an exhumed, high-sinuosity channel sandstone in the Permian Beaufort Group of the southwestern Karoo, South Africa, reveals the original geometry and eroded topography of four consecutive point bars. An abandoned channel fill separates the crescentic point-bar sandstone bodies, clearly defining the paleomeander loops. The point-bar coarse member is made up of trough-cross-bedded and horizontally laminated, fine- to medium-grained sandstone with patchily developed basal mudrock conglomerate. Low-angle (< 10°) discontinuities divide the single-storied sandstone into epsilon units that are expressed on the eroded upper surface as irregular accretion ridges. Remnant channel-fill deposits comprise two units, a lower active fill similar to the basal point-bar sequence, overlain by a passive fill of numerous thin beds of bioturbated, ripple-cross-laminated, fine-grained sandstone separated by maroon mudrock veneers. A tabular crevasse-splay sandstone sheet emanating from the apex of meander 2 is composed of horizontally laminated, fine-grained sandstone. The irregularity of lateral units and the continuity of discontinuity surfaces to the base of the point-bar coarse member is indicative of highly fluctuating, possibly ephemeral discharge. Measurements of meander wavelength (2,400-4,000 m), being one of the least altered paleomorphological features, are used to calculate average bankfull width (approx. 277 m), depth (approx. 11 m), mean annual discharge (approx. 850 m3/s) and mean annual flood (approx. 12,000 m3/s). Paleocurrent data and the pattern of ridges are considered in the reconstruction of migration behavior. The meanders expanded and increased in curvature while migrating at a rate of approximately 0.85 m/year over a period of some 1,500 years before an upstream avulsion caused their abandonment.

Sedimentary stratigraphy of Eocene sheetflood deposits, Southern Pyrenees, Spain

AbstractThe Eocene Monllobat Formation of the southern Pyrenees accumulated under continental (non-marine) conditions. The total thickness of 180 m consists of seven cycles, defined by coarse member concentrations (often sheets) at the bases. The lateral extent of the sheets defines the fixed positions of eight parallel-trending alluvial systems, separated by narrow zones of fine sediment.Sheetflooding dominated the sedimentation of these eight contemporaneous alluvial systems. The sediment was supplied from the rising axial zone of the Pyrenees. The cycles were caused by variations in tectonic activity. The positions and forms of coarse members were locally determined by contemporaneous faulting and folding of the alluvium.

Fluvial conglomerates in continental red beds of the Buntsandstein (Lower Triassic) in the Eifel (F.R.G.) and their palaeoenvironmental, palaeogeographical and palaeotectonic significance

Fluvial conglomerates occur in a variety of facies within the continental red bed sequence of the Buntsandstein (Lower Triassic) in the Eifel at the western margin of the Mid-European Triassic Basin. The alluvial rudites are classified into channel-floor lag, channel-bar, channel-top lag, crevasse-splay and floodplain-lag conglomerates. The conglomerates were predominantly deposited in Scott-type, and in subordinate amounts also in Donjek-type and South Sasketchewan-type braided rivers.The vertical distribution of clast size and frequency within the megacycles reflects the palaeotectonic sequence of uplift events in the source area, resulting in supply of large amounts of coarse detritus which cause recurrences in fluvial style. Gradually upwards diminishing size and abundance of gravel indicate continuous lowering of source area relief during weakening endogenic activity or even tectonic quiescence after final peneplanation. The palaeoenvironmental interpretation of the individual members and the palaeotectonic analysis of the conglomerate successions are combined with a reconstruction of the depositional history of the Eifel Buntsandstein which comprises an evolution of fluvial deposition. The terminal stage of the palaeoenvironmental evolution is highlighted by a climax of regional diversification of conglomerate sedimentary milieu. The horizontal distribution of clast size and abundance delineates the palaeogeographical evolution of the elongated Eifel Basin, giving evidence of a progressive sidewards extension of the depositional area with time. Lateral supply from parts of the western margin and mixing of proximally derived coarse clasts with distantly delivered fine detritus result in highly inhomogeneous gravel distributions within the sedimentary region often not being characterized by gradually decreasing size and abundance of gravel in downstream direction.The stepwise southwestwards shift of the marginal centre of supply of coarse gravel with time during the course of the lateral extension of the basin results in ascension of the fluvial conglomerate wedges into successively younger formations comprising consecutively Middle Buntsandstein, Upper Buntsandstein, Lower Muschelkalk, Middle Muschelkalk and finally Middle Keuper. The extraformational conglomerates mainly delineate the palaeogeographical framework, whereas the intraclastic rudites primarily give evidence of processes operating within the depositional area, reflecting erosion and reworking of overbank sequences with floodplain fines, palaeosols and aeolian dune sands during lateral channel shift. The Bröckelbank intraformational carbonate breccias to conglomerates originating from reworking of calcrete palaeosols are a peculiar facies element with particularly high palaeoenvironmental significance. They document the frequency, spatial extension and temporal persistence of pedogenesis within the alluvial plain. The size of the extraformational clasts gives evidence of the range of current velocities during aggradation of the channels. The distribution of conglomerate type and clast characteristics within cyclothems indicates the presence of stage fluctuations and variations in discharge in parts of the sequence. The preservation of complete cyclothems or the multistoreying of channel bar conglomerates reflects the differential importance of the two main mechanisms controlling accumulation of sediments in the alluvial plain: primary-depositional restriction to suppression of formation and secondary-erosional removal to destruction of finer-grained channel sediments, floodplain deposits and aeolian dune sands or palaeosols. Condensation of a series of depositional events within monotonous stacked conglomerate sequences obliterates the full range of sedimentary processes and mimics periods of long-term stability of conglomerate formation. Waning-flow mud drapes are particularly significant instruments to split up the individual genetical units of formation of fluvial conglomerates, often comprising discrete flood phases separated by stage fluctuations of different magnitude. The homogeneous or microcyclic composition of the coarse member reflects uniphase or multiphase aggradation of watercourses with gradually declining or abruptly waning individual flood periods. The fluvial rudites thus are of high significance in the reconstruction of the depositional milieu in the Eifel Buntsandstein and provide a key to detailed interpretation of palaeoenvironmental processes, palaeogeographical constellations and palaeotectonic events which could not have been identified to such an extent in exclusively sandy sequences.

Cyclic fluvial sedimentation in a rapidly subsiding basin, northwest Washington

The Eocene Chuckanut Formation of northwest Washington comprises as much as 6000 m of alluvial strata and is one of the thickest nonmarine sequences in North America. The Bellingham Bay Member forms approximately the lower half of the Chuckanut Formation; it is characterized by repetitive fining-upward cycles consisting of a lower coarse-grained and an upper fine-grained member. Two types of cycle have similar vertical profiles but can be distinguished by the nature of their coarse member. Coarse members of the first type are thick (> 2–3 m), are commonly multi-storey, have apparent sheet geometry and are interpreted as the deposits of meandering-river channels and point bars. Coarse members of the second type are thinner (< 2–3 m), have apparent ribbon geometry and are interpreted as floodplain (crevasse?) channel deposits. Down-basin changes in the Bellingham Bay Member include a major decrease in grain size and an increase in fine-member thickness and cycle-recurrence interval. These changes reflect decreased flow competence and increased stability of channel banks. Similar trends might be found in other ancient alluvial sequences and could be useful in studies of both basin geometry and coal exploration. Sediment-accumulation rates are a minimum of 50 cm 1000 yrs −1, implying rapid subsidence. This subsidence probably had a major control on the sedimentation style of the Bellingham Bay Member, which resembles computer-generated models of alluvial stratigraphy at high subsidence rates (Allen, 1978).

Sedimentology of Muddy Fine-grained Channel Deposits in Meandering Streams of the American Middle West

ABSTRACT Many tributary valleys to major streams in Illinois, Indiana, and Wisconsin were sites of mud deposition in late Wisconsinan periglacial lakes. Today the flat lacustrine plains of these valleys contain freely meandering streams. Surveys during low-flow conditions have enabled documentation of the geomorphic and sedimentary features of the sinuous channels of several of these streams. The subaqueous bed material is a complex, variable mixture of quartzose sand (main component), mud, mud blocks, equant mud clasts, rock fragments, and plant debris. Point-bar topography is restricted to bends of low or moderate curvature (rc/w2 > 3); bends of greater curvature show dominantly erosional inner banks. An organic-rich ooze lies beneath the stagnation zone that develops near the inner bank of highly curved sectors. Subaerial sediment on depositional surfaces is primarily a muddy sand with variable content of plant detritus. The lateral-accretion deposits near the downstream end of bends were divided into a fine member and a coarse member, the former two to four times as thick as the latter. The fine member consists of three lithofacies: 1) upper interval of centimeter-thick beds of sandy mud and thinner beds of rooted sand with mud and some plant debris; 2) underlying interval of subequal thicknesses and numbers of beds of clean sand (mostly fine, likely with small-scale cross-stratification) and sandy mud; and 3) basal interval very similar to 2 but with distinctive beds of black, finely divided plant debris. The coarse member is dominantly a fine-to-medium quartzose sand, contains mud clasts towards the base, and is inferred to be large-scale trough cross-stratified. Logjam debris appears near the thalweg in places. Lateral-accretion bedding in the form of intercalated sand and mud-sand is inferred to extend throughout the fine member. Substantial local variations in cross-sectional geometry and channel lithofacies, especially near logjams, reduce the precision of existing paleohydrologic relations and the comprehensiveness of a single lithofacies model. This class of meandering stream produces lateral-accretion deposits which are closer analogues in lithofacies to the classical upward-fining model than are deposits from the graveliferous sand-bed streams that have received primary emphasis from sedimentologists.

Structure, generation and preservation of upward fining, braided stream cycles in the Old Red Sandstone of Scotland

ABSTRACTA sequence of upward fining cycles contain braid bar structures in the coarse member. Bars, medially placed in the stream are recognised either by radiating large foresets or by identifying bar tail sediments with converging dips to their foresets. Lateral bars are recognised by the growth of major sandstone sheets oblique to the direction of channel trend and over slough mudstone. The fine member contains structures which indicate a direction of flow in the same direction as the channel, suggesting that only the unlevéed part of the floodplain is preserved.The direction of transport for the Old Red Sandstone sediments is perpendicular to the direction of bar accretion. This is accounted for by assuming the section dealt with to be on the NNW margin of a cone which builds outward to the NE, and where the river building the cone swings from one side to the other. An upward increase in cycle thickness within the section is possibly the result of an increase in the size of the cone and the consequent amalgamation of the coarse members.

The Morphology, Magnitude and Regime of a Carboniferous Fluvial-Distributary Channel

ABSTRACT A meandering fluvial-distributary was initiated into the interdistributary bay of a Carboniferous (Yoredale) delta lobe by avulsion. Initiation immediately preceded abandonment of the lobe, and lateral migration of the channel produced a meander belt sandstone in the short period of channel activity. The channel fill comprises an 8 m fining upwards sequence with lateral accretion surfaces in the coarse member which permit calculation of palaeochannel parameters. The coarse member also exhibits inclined, erosive discontinuities which reflect temporal variations in flow. Paleochannel regime was dominated by frequently recurring normal floods which produced the lateral accretion surfaces, and less frequent exceptional floods which were primarily effective in scouring the bar surface, pro ucing the discontinuities.

Origin of red beds in the Ringerike Group (Silurian) of Norway

The Ringerike Group is a meandering fluviatile succession which is about 60% red. Most of the red zones are formed of mudrocks and siltstones and correspond to the fine members of fining-upwards cyclothems. The majority of coarse members are drab coloured. Textural studies of thin and polished sections show that the red colour is caused by finely crystalline hematite as matrix and grain-coatings. This hematite apparently crystallized post-depositionally. Hematite also occurs in other textural sites: within altered phyllosilicates, as detrital grains and as totally pseudomorphed phyllosilicates. This, and the lack of consistency between colour and clay mineralogy, suggests that the red beds have had a long and complex diagenetic history. Iron analyses indicate that the red beds are enriched in Fe 3+ and total iron (FeO) by about 1%. This is thought to have been derived from the pre-depositional weathering of iron minerals and introduced into the sediments as amorphous iron hydroxide or iron-bearing clays. Crystallization of iron hydroxide under oxidizing conditions and the post-depositional alteration of iron-silicates and oxides is thought to be responsible for the formation of the red beds.

Studies in Fluviatile Sedimentation: A Comparison of Fining-Upwards Cyclothems, with Special Reference to Coarse-member Composition and Interpretation

ABSTRACT The fining-upwards cyclothems repeated through thick stratigraphic sections in the Old Red Sandstone (Devonian) of Britain and the Appalachian region of North America comprise a maximum of six major facies. The cyclothems when complete consist each of a lower coarse member and an upper fine member, between which the facies are distributed. An analysis of facies transitions is made to arrive at an understanding of the composition and variability of these cyclical deposits. The coarse members, which as regards the variety of facies and numbers of facies representatives are more variable than the fine members of the cyclothems, are shown to have been formed through processes of lateral deposition in streams with sinuous talwegs. A quantitative physical model, partly based on general hydr ulic principles and partly empirical, is advanced to show how the different facies sequences observed in the coarse members can be understood in terms of the chief variables involved in river flow. Specifically, the model reproduces the vertical patterns of grain size and sedimentary structure observed in the coarse members. The model also shows how the coarse members may be interpreted in terms of stream power and channel sinuosity.

A quantitative model of grain size and sedimentary structures in lateral deposits

AbstractThe secondary flow and transverse changes of water depth in curved stream channels are exploited to develop a quantitative, partly empirical model of the way lithology varies vertically in deposits accumulated through processes of lateral deposition. According to the model, grain size decreases upward in a lateral deposit and the sedimentary structures change from those denoting large stream powers upward to kinds indicative of small powers. The model also shows that the proportion of flat‐bedded sand‐grade material in a lateral deposit is essentially independent of stream power but controlled by channel curvature and hence general sinuosity. The predictions of the model compare satisfactorily with the characteristics of selected coarse members in fining‐upwards cycles of fluviatile origin from Devonian strata in Britain and North America. The model is thought to have value as a tool for environmental interpretations.

FLUVIATILE SEDIMENTARY STRUCTURES IN THE WOOD BAY SERIES (DEVONIAN) OF SPITSBERGEN

SUMMARYA noteworthy feature of parts of the Series is the development of cyclothems which consist of: scoured surface, coarse member, fine member, scoured surface. General environments, suggested by this and fossil evidence, are: erosion in river bed, deposition in river bed, deposition in overbank area, erosion in river bed. Structures within the sets of strata include: no lamination, flat bedding, small and large‐scale cross‐lamination, parting lineation. The significance of each is discussed. Sole structures, occurring on the scoured surfaces, are described. They include: groove casts, crescents, flute casts, a variety of long welts, and unoriented structures. These are mainly scour structures but some may be subaerial marks, and there is also evidence for post‐depositional flowage. The value of these structures as indicators of palaeocurrent and ultimately palaeoslope directions is discussed with examples. Measurements of sole structures, parting lineations and ripples have been more profitable than large‐scale cross‐stratification or contorted lamination.