Basal Erosion Surface Research Articles

Forced regressions: recognition, architecture and genesis in the Campanian of the Bighorn Basin, Wyoming

Abstract The Cretaceous Mesaverde Group of the Bighorn Basin, northwestern Wyoming, is comprised of two major clastic wedges that record the progradation and retrogradation of deltaic depositional systems within the Cretaceous Western Interior Seaway. Within the Campanian Virgelle and Judith River Formations 16 sand-rich clastic tongues, deposited in mixed wave/storm-dominated shallow marine shoreface depositional environments, have been studied and traced into equivalent updip non-marine and downdip offshore facies. Each tongue is typically a massive shoreface sandbody that pinches-out and correlates basinward (east) with fine-grained offshore heterolithic progradational parasequences. Regional correlation reveals the sandstone tongues to be sharp-based. Their lower bounding surfaces are characterized by: (1) a marked basinwards shift in facies, (2) an abrupt increase in sand: shale ratio, (3) missing/eroded facies below, (4) a change in parasequence stacking patterns, (5) local development of Glossifungites firmground ichnofabrics, (6) deposition of precursor gutter cast facies, (7) widespread soft-sediment deformation and growth faulting, (8) changes in palaeocurrent orientation, (9) regional truncation of older parasequences and systems tracts, (10) regional depositional-dip correlation of 20–40 km. The basal sharp-based surfaces of the shoreface sandstones are interpreted to be regressive surfaces of marine erosion (RSME) formed during falls in relative sea-level, with the shallow marine successions deposited during forced regression of the shoreline. Internal hetrogeneities and erosion surfaces within the massive shoreface sandstones are interpreted to record stepwise progradation during relative sea-level falls. These relatively steep seaward-dipping erosion surfaces reflect the overall trend of shoreface deposition and amalgamation during periods of decreasing accommodation space. Downdip, the internal erosion surfaces amalgamate with the basal RSME and provide evidence that this basal surface is composite in nature and as such diachronous in its development. Within the studied interval, four examples of forced regressive deposits are confidently correlated updip to correlative incised valley fills. In each case, the basal erosion surface to the incised valleys truncates strandplain deposits, and ties laterally to a subaerial exposure surface (interfluve) developed across the top of the strandplain. These surfaces, formed in response to a fall in relative sea-level, are interpreted as sequence boundaries. Traced basinward their expression is commonly lost as the upper strandplain and capping interfluve are eroded by transgressive ravinement at the base of tidal inlets. However, the interfluve is thought to correlate downdip to the final seaward dipping erosion surface that separates the massive amalgamated shoreface sandstones of the falling stage system tract, deposited during overall relative sea-level fall, from the more heterolithic parasequences of the low-stand systems tract, deposited under conditions of stillstand to relative sea-level rise. Within sediments deposited during relative sea-level fall the critical transition from the subaerial to submarine expression of the sequence boundary is recognized as the main factor in the ongoing controversy regarding the identification of a finite chronostratigraphic sequence-bounding surface. Drawbacks exist in making a simple choice between the subaerial exposure surface or the RSME as the sequence boundary because they are normally diachronous and at the same time form contemporaneously. In updip areas two separate important stratigraphic surfaces may be distinguished; the RSME and an overlying subaerial exposure surface. In these areas the subaerial exposure surface must be regarded as the main sequence bounding surface. Basinward exists a critical transition zone where (i) storm-related erosion surfaces and shoreface amalgamation during deposition of the strandplain inhibit correlation and (ii) transgressive ravinement may erode part of the subaerial expression of the sequence boundary. In this area choice of surfaces proves difficult. In constrast, basinward of the last sharp-based shoreface, the RSME would be the principal (and most obvious) stratal surface, and must be interpreted as a sequence boundary. By considering the evolution of the RSME to occur at the same time as the fluvial erosion/subaerial exposure surfaces, the massive sharp-based shorefaces and their distal equivalents can be observed to be the response of an linked and dynamic system to relative sea-level fall.

Large‐scale structure of Calamus River deposits (Nebraska, USA) revealed using ground‐penetrating radar

The large‐scale (i.e. bar‐scale) structure of channel deposits of the braided, low‐sinuosity Calamus River, Nebraska, is described using ground‐penetrating radar (GPR) profiles combined with vibracores. Basal erosion surfaces are generally overlain by medium‐scale, trough‐cross‐stratified (sets 3–25 cm thick), very coarse to medium sands, that are associated with relatively high amplitude, discontinuous GPR reflectors. Overlying deposits are bioturbated, small‐scale cross‐stratified (sets <3 cm thick) and vegetation‐rich, fine to very‐fine sands, that are associated with low‐amplitude discontinuous reflectors. Near‐surface peat and turf have no associated GPR reflectors. In along‐stream profiles through braid and point bars, most GPR reflectors dip downstream at up to 2° relative to the basal erosion surface, but some reflectors in the upstream parts of bars are parallel to the basal erosion surface or dip upstream. In cross‐stream profiles through bars, GPR reflectors are either approximately parallel to bar surfaces or have low‐angle inclinations (up to 6°) towards cut banks of adjacent curved channels. Basal erosion surfaces become deeper towards cut banks of curved channels. These structures can be explained by lateral and downstream growth of bars combined with vertical accretion. Convex upwards forms up to 0·5 m high, several metres across and tens of metres long represent episodic accretion of unit bars (scroll bars and bar heads). Stratal patterns in channel fills record a complicated history of erosion and deposition during filling, including migration of relatively small bars. A revised facies model for this type of sandy, braided river has been constructed based on this new information on large‐scale bedding structure.

Architecture of fluvial‐dominated valley‐fill deposits in the Cretaceous Fall River Formation

ABSTRACTThe Fall River Formation is a 45 m thick layer of fluvial‐dominated valley‐fills and shore‐zone strata deposited on the stable cratonic margin of the Cretaceous Western Interior Seaway. Fall River deposits in Red Canyon, in the south‐west corner of South Dakota (USA), expose a cross‐section of a 3.5 km wide valley‐fill sandstone and laterally adjacent marine deposits. The marine deposits comprise three 10 m thick upward‐shoaling sequences; each composed of multiple metres‐thick upward‐coarsening successions. The lower two of these sequences are laterally cut by the valley‐fill sandstone, and are capped by metres‐thick muddy palaeosols. The upper sequence spans the top of the valley‐fill sandstone, and is overlain by the Skull Creek Shale. The 30 m thick valley sandstone is partitioned into four distinct fills by major erosion surfaces, and each of these fills contain many metres‐thick channel‐form bodies. Deposits in the lower parts of these fills are sheet‐like, top‐truncated channel bodies, whereas deposits in the upper parts of fills are upward‐concave, laterally amalgamated channel bodies, more completely preserved heterolithic channel bodies, or wave‐deposited sheets. Each valley‐fill basal erosion surface records an episode of valley incision and relative sea‐level fall, and the gradual progression from fluvial to more estuarine deposits upwards within each fill records relative sea‐level rise. All fills are dominantly channel deposits and are capped by marine flooding surfaces. The dominance of channel deposits, the gradual change to more estuarine facies in the upper parts of fills, and the location of flooding surfaces at valley‐fill tops all suggest that sediment supply initially kept pace with relative sea‐level rise and valleys filled during late marine lowstand and transgression, not during subsequent highstands. Recently proposed facies models have focused on variations in the relative strength of tide, wave and river currents as controls on valley‐fill deposits. However, relative rates of sediment supply and basin accommodation change, and the shift in this ratio along the depositional profile during multiple‐scale cycles in relative sea‐level, are equally important controls on the style of valley‐fill deposits.

Scour in large braided rivers and the recognition of sequence stratigraphic boundaries

Alluvial scour into shallow marine sediments may be caused by the incision of a river adjusting to a new base level1–4 following a fall in sea level. The identification of such erosion surfaces1–3 has therefore been pivotal in the reconstruction of past sea-level changes from ancient sedimentary sequences1–14. Here we report data from a study of the Jamuna river, Bangladesh, one of the world's largest modern braided rivers15, which illustrate that bed scour associated with channel confluences and bends alone can be substantial—as much as five times greater than the mean channel depth. Indeed, the basal erosion surfaces produced by such deep scours have characteristics similar to those of boundaries in some ancient sedimentary sequences that have been assumed to result from sea-level fall1–14, potentially leading to radically different interpretations of past variation in base level and climate. We suggest that, to discount unambiguously the influence of fluvial scour in ancient sediments, the erosive boundary should be greater than five times the mean channel depth and extend for distances greater than the floodplain width. Ideally, it should be traceable between different basins.

Tectonic Control on Paleovalley Incision at the Distal Margin of the Early Tertiary Alpine Foreland Basin, Southeastern France

ABSTRACT Paleovalley systems preserved in the stratigraphic record provide useful information on the evolution of ancient landscapes. I investigate the controls on erosion of a set of bedrock-confined paleovalleys at the distal margin of the early Tertiary Alpine foreland basin in southeastern France. The paleovalleys are incised into pre-Tertiary subcrop along the basal unconformity of the foreland basin. They have been identified on the basis of: (i) they form incised paleotopographic features that erosionally truncate bedrock; (ii) they comprise a basal erosion surface that is locally characterized by paleosols along valley margins; (iii) valley-fill strata onlap the margins of the paleo-erosional features; (iv) they show abrupt lateral thickness and facies variations. Paleosol remnants and erosional scour indicate subaerial exposure and erosion of the foreland plate bedrock. The paleovalleys are located adjacent to or on the hanging wall of a set of pre-late Eocene reverse faults that bound a major basement massif. Provenance studies indicate that paleovalley-fill conglomerates were locally derived from erosion of paleo-fault scarps. Incision of the paleovalleys is linked to fluvial erosion of bedrock during surface uplift and tilting related to growth of basement-cored fault-propagation anticlines. The pre-Priabonian paleodrainage drained hanging-wall uplands created by uplift of these structures. Catchment basins were small and stream lengths relatively short. Paleovalley incision in response to growth of intraforeland uplifts contrasts with flexural forebulge uplift and eustatic sea-level fall as an explanation for development of paleodrainage at the d stal margin of foreland basins.

Palaeochannels on the North China Plain: stage division and palaeoenvironments

The late Pleistocene and Holocene palaeochannels in the North China Plain were divided into six stages on the basis of seven indicators: basal erosion surface and channel lag deposits, macropalaeontology, organic matter content, ground water quality, heavy minerals, grain size, and sedimentary structures. They were called the stages I–VI palaeochannels from oldest to youngest. On the basis of the percentage area and thickness of fluvial sand bodies of the total area and thickness of sediments of the same period, there were three palaeochannel development stages, or periods of fluvial activity, and two palaeoswamp development stages, or periods of fluvial inactivity, on the North China Plain during the late Pleistocene and the Holocene.

Origin and significance of mud‐filled incised valleys (Upper Cretaceous) in southern Alberta, Canada

ABSTRACTSeven mud‐filled incised valleys (MFIVs) in the paralic facies of the Dinosaur Park and Horseshoe Canyon formations (Upper Cretaceous) of southern Alberta were studied to better understand their morphology, geometry and depositional histories in an estuarine context. Two preservational geometries occur: simple, U‐shaped forms; and internally complex forms. Both types of MFIV record deposition in the central zone of low energy (turbidity) in an estuarine setting.Simple, U‐shaped MFIVs have sharp basal erosional surfaces and consist of mudstone‐dominated heterolithic fills of channel‐wide, concave‐up laminae. Associated fossil assemblages are marine to brackish. Each simple MFIV records a cut‐and‐fill history associated with a cycle of relative sea‐level drop and rise. Low‐energy depositional settings, loss of channel form during infilling, and associated shoreface deposits, as well as the absence of clear tidal indicators suggest a coastal plain estuarine setting, along a wave‐dominated, barred coastline.Complex MFIVs are rarer, and consist of imbricated, wedge‐shaped sets of inclined‐to‐horizontal heterolithic strata. Tidal deposits and/or nonmarine‐to‐marine macrofossils occur locally. Complex MFIVs were infilled in meandering reaches of the central zone of low energy in tide‐dominated estuaries. Their rarity compared to simple MFIVs and their freshwater palaeontological content suggest that they were contiguous landward with extensive fluvial channels. A complex MFIV near Onefour comprises three in‐channel depositional cycles. Each cycle consists of an erosional surface overlain by lateral accretion bedding and a conformable transition to vertically aggraded strata. Each cycle reflects a cut‐and‐fill event under the control of changes in relative sea‐level that culminated in overbank flooding.All MFIVs formed in low‐gradient settings (≤0.03%) where estuarine zones were stretched out over many tens of kilometres. Tide‐dominated estuaries apparently exhibited simple, straight‐to‐meandering upstream transitions and extensive landward penetration (≥200 km) of tidal backwater effects. Few modern estuaries serve as adequate modern analogues to these ancient, tide‐dominated estuaries.Radiometric data indicate that MFIV cut‐and‐fill cycles were 100 000‐400 000 years in maximum duration and thus, equivalent to 4th order sea‐level cycles. However, negative evidence tentatively suggests that these cycles took place over time intervals 1‐2 orders of magnitude smaller (5th order or higher sea‐level cycles).

Depositional controls on coal distribution and quality in the Eocene Brunner Coal Measures, Buller Coalfield, South Island, New Zealand

The Buller Coalfield on the West Coast of the South Island, New Zealand, contains the Eocene Brunner Coal Measures. The coal measures unconformably overlie Paleozoic-Cretaceous basement rocks and are conformably overlain by, and laterally interfinger with, the Eocene marine Kaiata Formation. This study examines the lithofacies frameworks of the coal measures in order to interpret their depositional environments. The lower part of the coal measures is dominated by conglomeratic lithofacies that rest on a basal erosional surface and thicken in paleovalleys incised into an undulating peneplain surface. These lithofacies are overlain by sandstone, mudstone and organic-rich lithofacies of the upper part of the coal measures. The main coal seam of the organic-rich lithofacies is thick (10–20 m), extensive, locally split, and locally absent. This seam and associated coal seams in the Buller Coalfield are of low- to high-volatile bituminous rank (vitrinite reflectance between 0.65% and 1.75%). The main seam contains a variable percentage of ash and sulphur. These values are related to the thickening and areal distribution of the seam, which in turn, were controlled by the nature of clastic deposition and peat-forming mire systems, marine transgression and local tidal incursion. The conglomeratic lithofacies represent deposits of trunk and tributary braided streams that rapidly aggraded incised paleovalleys during sea-level stillstands. The main seam represents a deposit of raised mires that initially developed as topogenous mires on abandoned margins of inactive braidbelts. Peat accumulated in mires as a response to a rise in the water table, probably initially due to gradual sea-level rise and climate, and the resulting raised topography served as protection from floods. The upper part of the coal measures consists of sandstone lithofacies of fluvial origin and bioturbated sandstone, mudstone and organic-rich lithofacies, which represent deposits of paralic (deltaic, barrier shoreface, tidal and mire) and marine environments. The fluvial sandstone lithofacies accumulated in channels during a sea-level stillstand. The channels were infilled by coeval braided and meandering streams prior to transgression. Continued transgression, ranging from tidal channel-estuarine incursions to widespread but uneven paleoshoreline encroachment, accompanied by moderate basin subsidence, is marked by a stacked, back-stepping geometry of bioturbated sandstone and marine mudstone lithofacies. Final retrogradation (sea-level highstand) is marked by backfilling of estuaries and by rapid landward deposition of the marine Kaiata Formation in the late Eocene.

Ancient river systems in the Himalayan foredeep, Chinji Village area, northern Pakistan

The Miocene Chinji and Nagri Formations (Siwalik Group) of northern Pakistan record ancient fluvial environments in the Himalayan foredeep basin. Excellent exposures on the Potwar Plateau (Chinji Village area) allowed detailed documentation of the geometry and stacking of sediment bodies that comprise these strata, and of variation of large-scale bedding geometry, grainsize, sedimentary structures and paleocurrent orientations within such bodies. Major sandstone bodies are tens of meters thick and are continuous along strike for many kilometers. They are composed internally of interconnected channel belt deposits, each of which contain several storeys (channel bar and fill deposits). Individual storeys, defined by inclined bedsets dipping down to a major basal erosion surface at up to 11°, are only 5 to 15 m thick within the Chinji Formation but can be up to 30 m thick within the Nagri Formation. Bedsets within storeys reflect sediment accreted during individual flood events. Along-strike variation of bedsets within storeys and the stacking patterns of storeys within channel belt deposits reflect changes in the channel bed through time due to the growth of bars, migration of channels, and channel cutoff. Generally, braided channel patterns are indicated by the relatively large number of storeys within individual channel belt deposits exposed perpendicular to paleoflow, abundant evidence for channel bar superposition due to channel switching, local evidence for mid-channel bars, dominance of coarse-grained channel fills, and low paleocurrent variations. Paleochannel reconstructions from storeys exposed within the Chinji Formation indicate that individual channel segments generally had widths of 80–200 m, maximum depths of 4–13 m, wavelengths of 1.6–2 km and discharges of 400–800 m 3/s. Full channel belt widths (1–2 km) estimated from exposures perpendicular to paleoflow, and evidence for 2–3 coeval channels within channel belts, indicate full channel discharges of 1500–2000 m 3/s. Larger channel segments reconstructed from the Nagri Formation had widths of 200–400 m, maximum depths of 15–30 m, wavelengths of 3–5 km, and discharges of 3000–5000 m 3/s. As Nagri Formation channel systems were also clearly braided, full channel dimensions and discharge estimates are probably at least a factor of two greater than for individual channel segments (i.e. order of 10,000 m 3/s). Strata between major sandstone bodies are dominated by lobate and wedge-shaped bodies (crevasse splay and levee deposits), minor channel-form bodies (deposits of minor floodplain channels), laminated mudstone bodies (lake deposits) and paleosols. These strata are arranged into meters to 10 m thick stratified sequences that were rapidly deposited, bounded by well developed paleosols recording periods when deposition rates were low. The thickness and grainsize of such paleosol bounded sequences are not directly related to the proximity of a major channel deposit along strike. Instead, such sequences within overbank deposits appear to reflect local progradation of splays and levees into low areas on the floodplain.

Sedimentology and paleohydraulics of the meandering river systems of the Fort Union Formation, southeastern Montana

The Fort Union Formation in southeastern Montana is composed of four lithofacies associations: sandstone bodies, sandstone-mudstone interbeds, carbonaceous shales to coals, and sheet sandstones. The sandstone bodies are meters thick, fine upward and comprise cross-stratified, horizontally stratified, and ripple cross-laminated sandstones. Bedsets are typically arranged into erosively based storeys with lateral accretion bedding. Basal breccias above erosion surfaces locally contain turtle, fish and reptile bone fragments of Torrejonian age. Most sandstone bodies are laterally extensive but channel initiation and abandonment margins occur locally. Sandstone bodies are either single- or multi-storeyed and are generally tabular. Some sandstone bodies are locally thickened where the basal erosion surface cuts down several meters (commonly through an immediately underlying coal seam to a subjacent coal seam). The laterally extensive sandstone bodies are interpreted as point-bar deposits of laterally migrating and aggrading single-channel rivers. Single-storey sandstone bodies record deposition from single lateral migration events. Most multi-storey sandstone bodies probably record downvalley migration of meander bends and accumulation of point-bar deposits within an aggrading channel belt prior to an avulsion event. A few multi-storey sandstone bodies are likely the product of superimposed channel belts which returned to an area of the alluvial plain as a result of avulsion events. The locally thickened portions of sandstone bodies are interpreted as the product of scouring during episodic major flood events. The sandstone-mudstone interbeds, carbonaceous shales to coals, and sheet sandstones are interpreted as overbank, swamp and possibly lacustrine deposits accumulating next to the channels on an alluvial plain. Paleohydraulic reconstructions of point-bar deposits suggest three distinct channel sizes. Large channels (Lebo Member only) flowed easterly and had sinuosities, discharges and slopes of about 1.22 to 1.35, 460 to 600 cm, and 0.000026 to 0.000051, respectively. Intermediate channels show an upsection change in paleoflow from southeasterly in the Tullock Member, to easterly in the Lebo Member, to northerly in the lowest Tongue River Member, possibly influenced by the contemporary uplift of the Black Hills. Intermediate channels had sinuosities, discharges and slopes of 1.25 to 1.38, 208 to 351 cm, and 0.000038 to 0.000057, respectively. Small channels flowed in a variety of easterly directions (ranging from N 350° to N 110°), and had sinuosities, discharges and slopes of about 1.2 to 1.35, 140 to 240 cm, and 0.000038 to 0.000057, respectively. Outcrop relationships suggest the smaller channels were tributaries and/or distributaries of the larger channels. Regional paleodrainage studies also indicate a shift in drainage direction from southeast to northeast probably due to uplift of the Black Hills. The change in paleodrainage direction was accompanied by a shift in depocenters from the Black Hills region to the Williston Basin. In middle Tongue River times, the depocenter shifted to the Powder River Basin causing paleodrainage directions to change to a southwesterly direction. During times when paleodrainages were changing orientation, widespread coal swamps developed.

The Messinian event on the margin of the Mediterranean Sea in the Nice area, southern France

Seismic reflection profiles have been used to map the Messinian erosion surface and associated basin-margin deposits on the continental margin off Nice. A palaeovalley extends seaward from the incised Var palaeovalley mapped on land. The lower part of this valley is filled by an acoustically incoherent unit with a flat top and steep slope into the basin: this unit is interpreted as a coarse-grained deltaic deposit that accumulated synchronously with the upper Messinian evaporites in a ponded lake basin. Two submersible dives on strategic outcrops supported the interpretation of the seismic profiles. We examined and sampled outcrops of probably early Tertiary sandstones underlying the Messinian erosion surface and of a fining-up, predominantly conglomeratic sequence that fills Messinian channels on the continental slope and probably overlies the deltaic deposits. These conglomerates pass up into Early Pliocene marls and have turbidite sedimentary structures. These are prodeltaic deposits formed during the transgression at the end of the Messinian. Based on seismic facies mapping, sampling and submersible observations, a palaeogeographic reconstruction of the area is proposed. A basal erosion surface, including the Var palaeovalley, represents extreme dessication of the western Mediterranean and was followed by accumulation of the main Messinian salt. Following this, a lake was ponded in the Ligurian Basin, with accumulation of evaporites and shales. A Gilbert-type delta prograded into the basin from the Var palaeovalley. Relative sea (lake)-level lowering is estimated to be about 1200 m at this time. There has been little faulting on the margin since the Early Pliocene transgression, but there is estimated to have been about 800 m of flexural subsidence due to both thermal subsidence and water loading.

Tidal Channel, Levee, and Crevasse-Splay Deposits from a Cambrian Tidal Channel System: A New Mechanism to Produce Shallowing-Upward Sequences

ABSTRACT Two cliff exposures, 1 km and 0.7 km long and 10 m high, of the Middle-Upper Cambrian Waterfowl Formation were mapped in detail on the Costigan Thrust in southwestern Alberta. The formation here comprises 5 rock types: intraformational conglomerate; cross-stratified grainstone; wavy-bedded grainstone-mudstone; massive dolomite mudstone; and prism-cracked, cryptmicrobial-laminated mudstone. These rock types form a grainstone body in the upper 7 m of the outcrops and 4 fining-upwards sequences in the lower 3 m. The grainstone body is divided into 5 storeys, each of which has a basal erosion surface overlain by an intraformational conglomerate with interbedded thrombolites. Storeys contain lateral accretion bedding that is composed of decimeter-thick bedsets which are inclined up to 12 d grees relative to the basal erosion surfaces. The fining-upwards sequences comprise basal cross-stratified grainstone overlain by wavy-bedded grainstone-mudstone that is capped by prism-cracked, cryptmicrobial-laminated mudstones. Three fining-upwards sequences are laterally continuous. However, one fining-upward sequence pinches out along the outcrop face. We interpret the grainstone body as the deposits of laterally migrating, sinuous tidal channels. It is not clear whether the stacking of channel deposits into storeys represents superimposed channel-bar and channel-fill deposits from a single channel belt or superimposed channel-belts. The fining-upwards sequences may be shallowing-upwards sequences in the sense of James (1984) and represent aggrading-prograding tidal flat deposits. A ternatively, these sequences may be crevasse-splay and levee deposits analogous to fluvial overbank flood sequences. In this case, these sequences may not be related to eustatic sea level changes, although they resemble classic shallowing-upwards sequences and punctuated aggradational cycles.

Shallow-marine sedimentary processes in the Late Devonian Catskill Sea, New York State

Shallow marine strata of the Upper Devonian West Falls Group in south-central New York comprise centimetre- to decimetre-thick, sheetlike to lenticular beds of sandstone and siltstone interbedded with mudstone. Unidirectional current-scour marks and tool marks occur on the basal erosion surfaces of the sandstones: their internal structures are hummocky cross strata of various styles, planar laminae, and wave-ripple cross-laminae, with characteristic vertical and lateral transitions among them. Vertical burrows of the Skolithos ichnofacies dominate upper portions of sandstone beds, whereas mudstones and silt-stones contain mainly horizontal burrows of the Cruziana ichnofacies. Transported and in situ shelly fossils occur throughout, in places concentrated in “coquinite” lenses. Large-scale (15 to 30 in) bedding sequences are characterized by upward thickening and coarsening of sandstone beds, increase in amalgamated hummocky cross-strata, coquinites, plant debris and bivalves, followed by abrupt fining and thinning of beds. The depositional environment is interpreted as a shallow-marine shelf episodically influenced by intense tropical storms (hurricanes). Individual sandstone beds were deposited by spatially decelerating combined flows where an initially strong offshore-directed flow component decreased in time relative to the oscillatory flow component. Large-scale bedding sequences reflect changes in storm (combined) current intensity and deposition rate. Strong eustatic control is unlikely, and there is no evidence of changing wave climate. Instead, these sequences may be associated with delta-lobe progradation and/or offshore bar development.

Estuarine Trace Fossil Assemblages in Tournaisian Clashmelcon Formation, Kerry Head, Southwest Ireland: ABSTRACT

The Tournaisian Clashmelcon Formation comprises lateral-accretion deposits which fine upward from large-scale cross-stratified sandstone to current-dominated heterolithic beds interbedded with current- and wave-dominated heterolithic beds and desiccated cryptalgal laminites. Large-scale cross-stratification occurs in bundles with structures indicative of unsteady and reversing currents. Wave-dominated heterolithic beds and hummocky cross-stratified sandstone become more abundant upsection. A transported brachiopod-crinoid-bryozoan assemblage which is locally abundant above erosion surfaces becomes more diverse upsection. The Clashmelcon Formation is interpreted as deposits of laterally migrating and aggrading estuarine channels associated with tidal flats which gave way to estuarine embayments dominated by waves. Trace fossils are best developed in heterolithic bedding and include vertical and horizontal burrows. Low in the Clashmelcon Formation, tops of lateral-accretion deposits are locally very bioturbated by a few types of trace fossils including Skolithos, Arenicolites, Teichichnus, Cylindrichnus and, rarely, Glossifungites. Upsection, a more diverse trace fossil assemblage occurs. Wave-dominated heterolithic sediments display a moderately diverse trace fossil assemblage including Teichichnus, Monocraterion, Diplocraterion, Fucusopsis, Planolites, Paleophycus, Chondrites, Neonereites and, rarely, Curvolithus, Rhizocorallium, Bifungites, and Treptichnus. Rhizocorallium is also locally abundant immediately below the basal erosion surfaces of lateral-accretion deposits. Arenicolites, Teichichnus, Fucusopsis, Planolites, Paleophycus, and Chondrites are found in variable abundance throughout much ofmore » the succession. Chondrites is locally very abundant in wave-dominated heterolithic beds and the lower parts of channel fills.« less

Sedimentology and morphology of a low‐sinuosity river: Calamus River, Nebraska Sand Hills

ABSTRACTA study reach of the Calamus River, Nebraska Sand Hills, has a low sinuosity (less than 1.3) and braiding parameter (less than 1). Depending on sinuosity, the channel is occupied by alternate bars and point bars, the emergent parts of which form nuclei for midstream bars (islands). Channel migration occurs by bend expansion and translation, downstream and lateral growth of islands, and by chute cutoff.Channel‐bed sediment is mainly medium‐grained sand, but gravel and coarser sand sizes occur in thalweg areas adjacent to cutbanks and upstream parts of bars and islands, and finer sands occur on the downstream parts of bars and filling channels. Curved‐crested dunes cover most of the channel bed at most flow stages, with ripples restricted to shallow areas near banks. Bed material is mostly large‐scale cross‐stratified, with small‐scale cross‐strata interbedded with plant debris occurring in topographically high areas near banks.Vibracores through channel bars show a basal erosion surface overlain by large‐scale cross‐stratified sands, in turn overlain by small‐scale cross‐stratified sand interbedded with plant debris. The overall sequence generally fines upwards, but the large‐scale cross‐stratified portion either fines upwards, coarsens upwards, or shows little grain size variation. Lithofacies distributions vary spatially within and between bars depending on position in the bar and local channel curvature/width, in a similar way to unbraided rivers elsewhere. Lithofacies of bar deposits are similar to those in the active channel, and the elevations of the basal erosion surface and adjacent channel thalweg correspond closely.Channels abandoned by chute cutoff are filled progressively from the upstream end, and comprise deposits similar to the downstream parts of bars (i.e. fining upwards). The downstream extremities of channel fills may contain large proportions of peat relative to sand, but little mud due to the paucity of such fine suspended load in the Calamus.

Models for fossil concentrations: paleobiologic implications

Four basic types of skeletal concentrations are modeled in terms of changes in sedimentation rate alone. The model categorizes fossil concentrations on the relatively objective basis of their bed contacts, and uses this criterion to infer directional shifts in net sedimentation. This radical simplification of accumulation histories, in which hardpart input is held constant, yields a surprisingly powerful model capable of predicting a broad spectrum of taphonomic and paleobiologic phenomena.Type Iconcentrations grade from less fossiliferous sediments and terminate in omission surfaces; if hardpart supply is held constant, they record a slowdown from positive to zero net sedimentation.Type IIconcentrations are the same as Type I but terminate in erosion surfaces (slowdown to negative net sedimentation), andType IIIandIVconcentrations are characterized by basal erosion or omission surfaces, respectively, grade upward into less fossiliferous sediments, and record increases in net sedimentation from negative or zero rates to positive rates. According to the model, samples collected from successive horizons within any of these shell beds will differ in the degree and type of post-mortem bias owing to differing histories of hardpart exposure at the depositional interface. Moreover, because rates of sediment accumulation govern the abundance of hardparts at the depositional interface and thus many of the physical characteristics of the benthic habitat, the dynamics of fossil accumulation have direct consequences for the structure of benthic communities (taphonomic feedback) and for ecologically controlled species morphometry.The model is highly robust to fluctuations in hardpart input, as judged by its ability to correctly infer modes of formation of concentrations in synthetic stratigraphic sections. In addition, field examples of Type I–IV concentrations show independent evidence of formation during intervals of reduced net sedimentation, and many exhibit trends in taphonomic and paleobiologic features expected from the postulated changes in net sedimentation. The model thus provides a testable working hypothesis for the accumulation of fossil material in a wide range of environments, and should be applicable to concentrations of any taxonomic composition, state of preservation, or geologic age. The power and robustness of this heuristic model in fact argue that fossil-rich and fossil-poor strata provide fundamentally different records of past conditions, and that sedimentation rather than hardpart input is the primary control on the nature of the fossil record.

Early Diagenesis, Atherton Formation (Quaternary), Northern Indiana: A Guide to Understanding Early Cement Distribution in Nonmarine Sandstones: ABSTRACT

In the area studied, the Atherton Formation accumulated primarily as outwash deposits dominated by trough and wedge cross-bedding. Within sand (86% of deposits; Q77F8L15) and polymictic pebble gravel (14%) units, local cements of calcite (99%) and limonite/hematite (1%) are present. The distribution of these cements was controlled by at least three factors: (1) position within the deposit relative to the land surface, (2) average grain size, and (3) primary stratification. Over 90% of cement zones are within 7 m (23 ft) of the present land surface. Meteoric waters made more acidic by decaying organic material locally dissolved carbonate framework grains. As pore fluids continued to move downward and laterally, cementation occurred. On a second level, higher permeability zones associated with sediment of larger grain sizes was an important factor influencing the location of cementation. Cemented horizons are present in 48% of pebble and sandy pebble gravels (contact and pore lining types) and 17% of sands and pebbly sands (contact, pore lining, and occluded types). Moreover, within sand units primary stratification was a parameter that influenced the location of cementation sites. In wedge cross-bed sets, cement zones parallel inclined laminations; in tr ugh cross-beds, 79% of the cement is concentrated in the lower one-third of bed sets near trough axes or immediately below a trough's basal erosion surface. Cement zones were preferentially developed along internal curved laminations within cross-bed sets. Higher permeability concordant with stratification, the result of excellent sorting and coarser grain sizes within individual sand laminations, primarily controlled cement distribution at this level. Carbonate cements in the Atherton range from a trace to 6 mole % MgCo3 (X = 3.6 mole %) with traces of Fe in 16% of the samples analyzed (205 total analyses). The calcite and Mg-calcite generally form a mosaic of anhedral crystals, although distinct rhombohedra are present in some pores. The source for Mg and Fe is believed related to the dissolution of dolomite and Fe-bearing silicate framework grains. The limonite/hematite cements are present as rare, pore-filling patches. A comparison of the percent of carbonate grains in uncemented sand (X = 10 ± 1%) and within cemented zones (X = 7 ± 1%) of similar grain size indicates an information loss during very early diagenesis on the order of 20 to 30% of the carbonate framework component. Therefore, the original proportion of carbonate framework grains in ancient calcite-cemented sandstones may be greatly underestimated due to grain destruction during diagenesis. End_of_Article - Last_Page 489------------

Facies Cycles and Related Main Depositional Environments in Ancient Turbidite Systems: ABSTRACT

Ancient turbidite depositional systems--not necessarily equivalent to deep-sea fans--can be defined as systems where channel-fill sediments are replaced in a downcurrent direction by nonchannelized sandstone lobes. Facies types and facies sequences that characterize channels and lobes have become sufficiently understood within the last 10 years to permit their relatively east recognition in both field and core analyses. Channel-fill sequences may differ considerably from one system to another. However, two main types of channel-fill sequences, which may be intergradational, can be recognized in most ancient systems. The first type represents channel-levee complexes, and is made up of highly lenticular bodies of coarse-grained sediments originally deposited in channel-axis zones, and of thin-bedded mudstones and fine-grained sandstones produced by overbank processes both within the channels and in adjacent interchannel regions. The second type consists essentially of broadly lenticular bodies of thick-bedded, graded sandstones that are commonly characterized by an abundance of rip-up shale clasts and scoured surfaces. Most individual sandstone beds typically thin toward and onlap onto channel walls. B th types of channel-fill deposits develop facies sequences with an overall thinning- and fining-upward character. In addition, each sequence is typically bounded by a basal erosional surface. Lobe sequences are characteristically expressed by an alternation of thick-bedded nonchannelized sandstone bodies, commonly between 3 and 15 m (10 and 50 ft) thick, and thinner bedded and finer grained deposits. Also in this case, facies types and facies associations may vary considerably from one system to another. The loci of maximum sand deposition of the lobe environment can either prograde basinward, particularly in small sand-rich systems, or shift laterally with time in systems which develop essentially through vertical aggradation processes. In both places, the resulting facies sequences do not show significant erosional boundaries and can be generally described as thickening upward. Minor thickening-upward sequences, commonly consisting of a limited number of sandstone beds, re virtually ubiquitous within lobe sandstone units and are thought to represent compensation features produced by the progressive smoothing out of the depositional relief during the process of lobe upbuilding. The transition between channels and lobes is still very poorly understood in both modern and ancient turbidite depositional systems. However, evidence provided by ancient sequences suggests that, at least in some systems, the transition is characterized by an abundance of coarse-grained, cross-stratified, and megaripple-shaped sandstone beds that represent either the bypassing zone of turbidity currents still moving basinward or the most proximal depositional product of the same currents before they develop complete Bouma sequences. End_of_Article - Last_Page 523------------

Aspects of the sedimentology, palynology and palaeobotany of the Upper Devonian of southern Kerry Head, Co. Kerry, Ireland

AbstractThe uppermost 220 m of Upper Devonian sedimentary rocks of southern Kerry Head are described in terms of three lithofacies associations. Facies association 1 (about 49 per cent of succession) comprises fine to medium grained sandstones, with basal erosion surfaces and intraformational breccias, dominantly cross‐stratified, and usually displaying a multistorey character. They are interpreted as deposits of laterally migrating and aggrading single‐channel rivers, flowing in a southerly direction. Facies association 2 (43 per cent) comprises interbedded fine to very fine grained sandstones and siltstones, displaying a diversity of sedimentary structures, rapid lateral and vertical facies transitions, and occurring as broad sheets or channel‐filling sequences. They are interpreted as crevasse, levee, and channel‐fill deposits. Association 3 (8 per cent) is dominantly siltstone, with lenses and sheets of very fine sandstone, interpreted as the floodbasin deposits of a semi‐arid alluvial plain.Macrofloras and spores occurring in association 2 indicate: (1) major plants (woody seed ferns) grew in areas very close to palaeochannel margins; (2) a phytogeographic location within the Munster Basin comparable with Hook Head; and (3) a Late Devonian age for the succession.

New Red Sandstone Floodplain and Piedmont Sedimentation in the Hebridean Province, Scotland

ABSTRACT A sequence of coarse clastic sediments of New Red Sandstone age in western Scotland was probably deposited by alluvial fan (mudflow, streamflood and braided stream), playa and floodplain (low and high-sinuosity channels) processes. Within the alluvial fan group the mudflow deposits are usually laterally persistent, flatbedded paraconglomerates. Individual mudflow units only rarely show basal erosion surfaces. The streamflood deposits consist of laterally impersistent orthoconglomerates and coarse sandstones which often fill channels as large or very large-scale cross-stratified sets. The braided stream deposits are usually sandstones and fine-grained, well-sorted orthoconglomerates with an abundance of trough cross stratification and intraformational pebbles. The braided stream conglomerates, unlike the mudflow and streamflood conglomerates, show no significant relationship between maximum particle size and bed thickness. The playa deposits show alternations of thin, mudcracked mudstones and laminated or rippled sandstones. The deposits of ephemeral, low-sinuosity floodplain rivers are identified as relatively thin, coarse-grained fining-upwards cyclothems in which both fine and coarse members are laterally extensive and there is little lateral variation in structure, thickness or lithology. There is usually a caliche profile capping the fine member. In contrast, thicker, finer-grained, structurally and lithologically more complex cyclothems probably accumulated from high-sinuosity floodplain systems. Alluvial fan (piedmont) sequences show a textural and structural evolution from mudflow up through streamflood to braided stream deposits. The time trend through the laterally equivalent floodplain sequences is from low to high-sinuosity and less ephemeral stream systems. The piedmont trend probably reflects a geomorphic maturing of the basin margins while the floodplain trend suggests an additional climatic change towards less arid conditions. Either trend is consistent with conformably overlying marine (Jurassic) deposits.