Ripple Cross-laminated Sandstones Research Articles

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.

Limnopus trackways from the Conemaugh Group (Late Pennsylvanian), southern West Virginia

Abundant and well-preserved tetrapod footprints have been discovered in the Glenshaw Formation (Lower Conemaugh Group) in Wayne County, West Virginia. The tracks occur along at least three horizons within a 30-cm-thick stratigraphic interval about 15 m (50 ft) above the Brush Creek Limestone; they are of Missourian age. The tracks are preserved mainly as casts on the underside of thin-bedded, ripple cross-laminated sandstones and less commonly as molds in intervening dark-gray shales. Associated body fossils include Spirorbis worm tubes and washed-in plant debris. Facies characteristics indicate the tracks were formed along the margins of an ephemeral lake in a flood basin setting adjacent to delta plain fluviodeltaic channel systems. Short-lived lacustrine conditions were likely to have resulted from a seasonal tropical to subtropical climate.Most of the tracks can be assigned to the ichnogenus Limnopus, making them one of the earliest known occurrences. At least five trackways are discernible with an external width ranging from 320 to 400 mm. Limnopus glenshawensis, a new ichnospecies, is herein proposed and various morphologic and locomotion parameters are quantified. Mean values include stride/manus, 407 mm; stride/pes, 411 mm; oblique pace/manus, 313 mm; oblique pace/pes, 317 mm; pace angle/manus, 82.4 degrees; pace angle/pes, 80.5 degrees; glenoacetabular distance, 329 mm; manus length, 87.2 mm; manus width, 104.2 mm; pes length, 121.4 mm; pes width, 112.7 mm. The tracemaker was most likely an eryopoid amphibian with a total length of slightly over 1 m.

Coastal plain sedimentation in the late Devonian of Southern Ireland; hummocky cross-stratification in fluvial deposits?

The late Devonian Toe Head Formation crops out extensively in southwest Ireland and has been considered previously as the topmost Old Red Sandstone (terrestrial) formation in a conformable non-marine to marine sequence. The most characteristic lithofacies, the flat laminated and inclined parallel laminated sandstones, displays a range of structures from flat to gently inclined laminae and includes sets which meet the four criteria of Harms et al. (1982) for hummocky cross-strata. Associated facies include desiccated mudrocks, palaeosols and ripple cross-laminated sandstones which lack the flaser-linsen dominated bedsets characteristic of the overlying marine strata. Only non-marine fossils are recorded and numerous palynological preparations lack the marine indicators which appear at the top of the formation. Palaeocurrents show considerable spread with indications of overall easterly transport of sand. There is no preferred vertical sequence of lithofacies. The balance of evidence strongly favours a non-marine environment with the implication that hummocky cross-strata, as presently defined, are not good environmental or even process indicators. The fine to very fine sand sizes and possibly the amount of suspended load were important controls on the resultant structures. A depositional model of essentially fluvial deposition on a low-gradient coastal plain is proposed.

Tidal Influence in Fluvial Strata--a Key Element in High-Resolution Sequence Stratigraphic Correlation: ABSTRACT

Recognition of tidal influence in fluvial, estuarine, and alluvial strata is critical to establishing of high-resolution sequence stratigraphic correlations between marine and nonmarine strata. In Turonian through Campanian strata of the Kaiparowits Plateau of southern Utah, tidally influenced facies were deposited during at least two periods of relatively rapid base level rise. These facies form part of transgressive systems tracts. Landward of the transgressive maxima, these tidal facies are present in channels that are otherwise encased in alluvial strata. Recognition of subtle sedimentary structures resulting from tidal processes allows an accurate chronostratigraphic framework to be developed between marine deposits and strata deposited up to 50 km inland. Tidally influenced channel deposits near the shoreline are sandstone dominated. They have well-developed sigmoidal bedding, thin mudstone drapes, and contain multiple reactivation surfaces and scattered marine trace fossils. Thirty kilometers inland, channel deposits contain wavy to lenticular bedded fine-grained sandstones and mudstones interbedded with thin sigmoidal cross-stratified sandstones. Teredolites, bidirectional current orientations, and the heterolithic nature suggest a tidally influenced system. Twenty kilometers farther inland lateral accretion deposits consisting of alternating ripple cross-laminated sandstones and mudstones are interpreted as meandering river deposits. The persistence of many mud drapes from the base to themore » top of lateral accretion surfaces, the presence of lenticular bedded sandstones, and trace fossil evidence suggest a tidal influence. Tidal facies landward of transgressive maxima are correlated to condensed sections representing periods of maximum flooding in the marine shales.« less

Barrier-Island Coastline Deposition and Paleogeographic Implications of the Upper Cretaceous Horsethief Formation, Northern Disturbed Belt, Montana

The Horsethief Formation (upper Campanian) of the Northern Disturbed Belt and western Sweetgrass Arch area of Montana, was deposited along the western margin of the Western Interior Cretaceous Seaway. Investigation of the Horsethief in an area extending northwest of Augusta, Montana and southwest of Choteau, Montana indicates that deposition occurred along a barrier-island coastline. Deposits in the lower part of the Horsethief (Horsethief-Bearpaw Transition Unit) are transgressive in nature whl1e those in the upper part are regressive. Several distinct lithofacies within the Horsethief Formation lend themselves to paleoenvironmental interpretation on the basis of texture, sedimentary structures, paleocurrent indicators, and faunal content. Lower shoreface sediments consist of a thick sequence of interbedded mudstone, siltstone, and horizontally stratified and ripple cross-laminated sandstone. Predominantly trough cross-bedded sandstone with abundant Ophiomorpha characterizes the upper shoreface. Sandstones of the foreshore and backshore are horizontally to subhorizontally bedded. Marsh-tidal flat sediments consist of carbonaceous siltstone, claystone, sandstone and shale; the siltstone and sandstone is commonly ripple cross-laminated. Shoreward-dipping wedge-planar cross-stratification represents deposition within flood-tidal deltas. Horsethief Formation sandstones include: 1) volcarenites, 2) chertarenites, 3) feldspathic volcarenites, and 4) feldspathic chertarenites. Abundance of volcanic rock fragments increases upward in the Horsethief, suggesting that the regressive nature of the Upper Horsethief may be due in part to influx of volcanogenic sediment from the west.

Shallow-Marine Trace Assemblages in a Cambrian Tidal Sand Body: ABSTRACT

Two distinctive assemblages of biogenic structures are present in the Mt. Simon Formation, basal Upper Cambrian quartzarenite in western Wisconsin. A Skolithos trace assemblage is dominated by specimens of Skolithos and Arenicolites, which commonly occur in medium to very coarse-grained, cross-bedded sandstones. Skolithos traces are typically 10 to 100 mm in length, 1 to 5 mm in width, and are oriented normal or slightly inclined to bedding. Specimens are present throughout the formation, even in basal conglomeratic beds, and thus point to a marine origin for the entire unit. Skolithos density increases upward to concentrations of 5 to 7 burrows/sq cm. A Cruziana trace assemblage is dominated by specimens of Cruziana, Rusophycus, and Planolites, which commonly occur in th n beds of very fine to medium-grained, horizontal to ripple cross-laminated sandstones. Rusophycus specimens are typically 10 to 100 mm in length and 6 to 60 mm in width, and are preserved in convex hyporelief on the bases of sandstone beds. Distribution of the two trace assemblages and the associated physical sedimentary structures points to two different environmental regimes present within a shallow subtidal to intertidal setting: (1) a higher-energy tidal channel environment in which coarser-grained, cross-bedded sandstones containing a Skolithos assemblage were deposited; (2) a lower-energy tidal flat environment in which finer-grained, horizontal and ripple cross-laminated sandstones containing a Cruziana assemblage were deposited. The two subenvironments coexisted within a complex environmental mosaic, because the two trace assemblages are observed to intergrade both laterally and vertically, probably the result of lateral migration of each subenvironment. The pronounced upward increase in trace fossil density indi ates a significant upward decrease in energy conditions and a reduced sedimentation rate, probably due to tidal flat progradation. End_of_Article - Last_Page 701------------

Late Pleistocene Fan-Deltas Along the Dead Sea Rift

ABSTRACT Late Pleistocene lacustrine fan-deltas which developed in an arid, disequilibrated submergent environment along the western fault scarp of the Dead Sea Rift are characterized by: 1) alluvial fan deposits of crudely stratified conglomerate beds and horizontal and cross-bedded sandstones. The conglomerates and sandstones are cone-shaped and comprise the entire sequence in the fan head area; 2) fan-front thick and thin interlayers of ripple cross-laminated sandstones and mudstones which constitute a sedimentary belt a few kilometers wide in front of the fan; and 3) fan-influenced detrital laminated chalks. Both the interlayered and the laminated units appear as widespread sheet bodies whose basal contacts are gradational and upper contacts, sharp and disconformable. The rapid rise of lak level resulted in the buildup of upward-fining sequences. At the end of the Pleistocene, the lake level dropped; it began to rise again during the Holocene and, contemporaneously, new fan deltas began building up.

Radiographic Investigation of Laminated and Ripple Cross-Laminated Flysch Sandstones, Ouachita Mountains, Oklahoma

ABSTRACT The radiographs of horizontally laminated and ripple cross laminated sandstones from the Paleozoic flysch of the Ouachita Mountains show the very details of their internal structures. The pattern of horizontal laminae inside flute casts is related to the flow direction and, therefore, can be used as a paleocurrent indicator. The current ripples are of three dimensional, truncated, and mostly nonclimbing type, indicating deposition from a traction current rather than from a suspended load. All samples studied document the conformity of paleocurrent directions deduced from internal structures and those measured on sole marks. This means that sole marks were cut by the same currents that deposited the horizontally and cross-laminated sand. The number of smaller and larger changes in the flow regime, such as the multiple alternation of laminae and the change from horizontal lamination into current ripples, evident on radiographs of a single bed, document that periods of sedimentation alternated with periods of non-sedimentation and erosion. There is an open question whether these breaks in sedimentation resulted from fluctuations within one turbidity current or from the long-term activity of different sedimentary processes.