Bar Facies Research Articles

Sedimentation on Gravel Outwash Fans, Malaspina Glacier Foreland, Alaska

ABSTRACT Three depositional facies occur on the outwash fans of Fountain Stream and Alder Stream, which presently drain portions of the Malaspina Glacier along the northeastern Gulf of Alaska. Deposition of longitudinal bars occurs during high-flow stage and consists of plane-bedded, imbricate, very poorly sorted pebble and cobble gravels. The sediment of the bars decreases in mean size downstream. Some bars terminate in avalanche slip faces of poorly sorted, silty, medium to coarse sand. Paleocurrent directions suggest that stream flow diverges across bar surfaces. Deposition in channels occurs in two morphologically distinct areas, riffles and pools. Sediment deposited in pools consists primarily of ripple cross-laminated, poorly sorted, silty, medium to coarse sand, commonly capped by draped lamination. Ripple crests vary from straight to cuspate. Sediment in riffles consists of gravel laid down under upper flow regime conditions as transverse ribs and stone cells. Both apparently are relict antidune bedforms. During late-stage flow, thin patches of horizontally bedded sand are deposited between transverse ribs and adjacent stone-cell borders. The longitudinal bar facies and channel pool facies are common in cutbank sections. The channel riffle facies was not recognized in cutbank sections. Scour pits, commonly observed on some bar surfaces, are produced by currents scouring around grounded ice blocks. Ice-block trails are produced by ice blocks dragged through soft sediment. Neither of these sedimentary structures have been reported in the literature. If found in ancient sediments, both structures would indicate a nearby source of ice.

Middle Silurian Patch Reefs in Gasport Member (Lockport Formation), New York

The Gasport Member is a normal-marine unit of the Lockport Formation, and records a period of patch-reef growth on the southeast flank of a crinoidal, submarine-bar complex along the Algonquin axis in western New York. Reefs, averaging 25 ft high and up to 100 ft in diameter, are exposed in a 60-mi zone in the western part of the state. Associated facies of the Gasport are exposed westward along the Niagara Peninsula of Ontario and eastward almost to Rochester, a distance of 110 mi. Detailed lithologic and paleontologic study of these patch reefs and associated facies has shown that all were deposited under normal-marine conditions in a shallow, subtidal and intertidal area. The reefs first began growing as coral thickets on a crinoidal-sand substrate (crinoidal bar facies) that spread eastward from the barrier-bar complex. These coral thickets (initial reef subfacies) consisted of delicate, branching tabulate and rugose corals, bryozoans, and encrusting girvanellid algae. These were replaced upward by large, massive, and encrusting stromatoporoids which served as the organic framework, as well as the binder of interstitial carbonate mud (reef-core subfacies). Organic debris torn loose from the tops of the reefs was deposited on the flanks (reef-flank subfacies), which grade laterally into the interreef facies of dolomitized biomicrite and sparsely fossiliferous micrite. As the reefs grew into shallower, more agitated water, a slight lowering of sea level resulted in the termination of reef growth, allowing low, domal stromatoporoids to spread laterally over the reef zone and to form a biostromal layer of organic rubble (stromatoporoid cap facies).

Depositional Models from a High-Energy Coast: ABSTRACT

The 2 most common depositional systems along the coast of southern Oregon are the nonbarred nearshore and the longshore bar-rip channel systems. Detailed observation of these systems, largely by scuba diving, has led to delineation of facies of sedimentary structures and recognition of their genetic relation to physical processes operating in the high-energy coastal environment. A 3-dimensional analysis of the geometric interrelation of the facies within each system permits construction of progradational depositional models that can be used to identify deposits of similar origin in the stratigraphic record. In the most simple depositional system, the nonbarred nearshore, the bottom profile extends smoothly seaward into deeper water. Progradation of the facies in this system will produce a distinctive vertical sequence. At the base lies fine sand of the offshore facies, which shows landward-dipping ripple lamination and scour-and-fill structure. The upper part of this facies is likely to contain lenses of crossbedded coarser sand in which foresets dip landward. The facies grades upward into predominantly cross-stratified beds formed by different facies of the surf zone. The upper crossbeds are likely to be gravelly and inclined seaward. They are overlain by planar-bedded swash facies. Progradation of the longshore bar-rip channel system produces a different sequence. An erosion surface separates the offshore facies from overlying rip channel facies in which crossbedding dips seaward. Above the rip channel facies lies a gradational sequence of longshore trough facies, wave-current complex facies, and, at the top, swash facies. In an actual situation, the bar facies, in which cross-stratification dips onshore, may be preserved locally within the deposit. End_of_Article - Last_Page 609------------

Depositional Systems in Woodbine Formation (Upper Cretaceous), Northeast Texas: ABSTRACT

The Woodbine Formation (Upper Cretaceous) in northeast Texas is a sequence of terrigenous clastic rocks derived largely from Paleozoic sedimentary and mildly metamorphosed sedimentary rocks exposed in the Ouachita Mountains of southern Oklahoma and Arkansas and deposited in a complex of nearshore environments along the margins of a broadly subsiding basin. On the basis of a regional outcrop and subsurface investigation in which external geometry of framework sands was integrated with observations of lithology, sedimentary structures, fossil occurrence, and bounding relationships, 2 principal depositional systems are recognized in Woodbine rocks--a fluvial system and a highly destructive delta system. The tributary-channel facies and the highly meandering channel facies, both components of the fluvial system represented by massive sand and gravel bodies of the lower Woodbine (Dexter) lithosome, are dominant north and northeast of a line from Dallas to Tyler. On the south and southwest, the highly destructive delta system is persistent throughout the entire section. The 4 component facies of the delta system includes: progradational distributary-mouth bar facies; coastal-barrier sand facies, developed either lateral to or basinward of the distributary mouth; prodelta mud facies; and embayment-strandplain facies, developed laterally adjacent to principal deltaic facies. Following or near the end of deposition of Woodbine rocks and before their transgression by Eagle Ford Shale, emergence of the Sabine uplift resulted in erosion of Woodbine material and its redeposition along the margins of the uplift in a lithosome designated the Harris Sand. End_of_Article - Last_Page 1788------------

Depositional and Dispersal Dynamics of Intertidal Sand Bars

ABSTRACT The intertidal sand bars of the Minas Basin, Nova Scotia, are asymmetrical in cross-section and linear in plan. They are formed and reworked by tidal currents which are characterized by an asymmetric time-velocity profile. A causal connection between time-velocity asymmetry of tidal currents and bar topography is demonstrated. Average bottom ebb current velocities (90 cm/sec) were found to exceed average bottom flood current velocities (65 cm/sec) over gently-sloping (2 to 3 degree) bar surfaces, whereas over steep-sloping (8 degree) bar surfaces, average bottom flood current velocities (90 cm/sec) were found to exceed average bottom ebb current velocities (65 cm/sec). Both bar topography and zones of flood- or ebb-dominated bottom tidal currents control the distribution of sediment texture and sedimentary structures. On steeper bar slopes, the sediment is fine-grained sand and is characterized by airholes, planar lamination, and surface dunes and ripples. On the gentler bar slopes, the texture of the sediment is medium-grained and coarse-grained sand which has been fashioned into current ripples, simple dunes, complex dunes, simple sand waves and complex sand waves. The internal organization of some dunes and sand waves is extremely complex, inasmuch as thickness of internal sets of cross-stratification is considerably less than dune and sand wave height. A total of 14 sedimentary bar facies are defined from combinations of texture, internal cross-stratification and surface bedforms. Both topography and zones of ebb- and flood-dominated bottom tidal currents control the orientation of directional current structures. Over steep faces of bars, the slip faces of sand waves are oriented in the same direction as flow directions of bottom flood tidal currents, although they do show evidence of reworking by ebb currents. Over gently sloping bar surfaces, slip faces of both dunes and sandwaves are oriented parallel to the direction of flow of bottom ebb current systems. Because dunes and sand waves migrate only during a single phase (ebb or flood) of a tidal cycle, their orientation is generally unimodal. Current ripples are formed in depths of water less than 0.6 meters, during late-stage sheet-like runoff that is controlled by local slope changes on the bar. Indi idual tracer grains are dispersed in a radial-elliptical pattern form a point source. The size of directional properties is hierarchically sensitive to dispersal history. Tracer grains (modeling mineral indicator grains) are dispersed in a radial-elliptical pattern from a point source. Trimodal, highly variant, current ripple slip face orientation develops during late stage sheet runoff at low water depths; this flow is controlled by local bar slope. Unimodal, low variant dune slip faces, sand wave slip faces and associated maximum dip direction of cross-stratification are aligned parallel to tidal current flow and basinal topographic trend. The sand bars are aligned parallel to basinal topographic trend and in turn segregate zones of flood- and ebb-dominated bottom current systems. The tidal sand bars of the Minas Basin are equilibrium forms. Sand is dispersed alternately around the bar through zones of ebb- and flood-dominated bottom tidal currents. As a consequence, the sedimentary facies distribution on the bars has remained essentially unchanged since 1938. Comparison of low-tide airphotos taken in 1938, 1947 and 1963 in one of the study areas shows a slight erosional regime. This erosional regime controls the extreme reworking of sediments, and the formation of an erosional sand wave bedform by wave processes. Physical criteria characteristic of tide-dominated sand bodies include: sharp erosional contacts between sets of cross-stratification; rounded upper set boundaries of cross-stratification; unimodal and bimodal distributions of orientation of cross-stratification; bimodal distributions of set thickness and dip angles of cross-stratification; orientation of dunes, sand waves and cross-stratification in the dominant direction of flood or ebb tidal flow, basinal topographic trend and sand body axis; trimodal orientation of current ripples; oblique or 90° superposition of smaller current ripples on larger current ripples; double-crested current ripples; superposition of current ripples at 90° or 180° on slip faces and crests of dunes and sand waves; complex organization of inte nal cross-stratification in sand waves; etch marks on slip faces of dunes and sand waves; and alignment of the long axis of sand bodies parallel to tidal current flow, basinal topographic trend and basin axis.

Depositional Systems in Woodbine Formation (Upper Cretaceous), Northeast Texas: ABSTRACT

The Woodbine Formation (Upper Cretaceous) in northeast Texas is a sequence of terrigenous clastic rocks derived largely from Paleozoic sedimentary and mildly metamorphosed sedimentary rocks exposed in the Ouachita Mountains of southern Oklahoma and Arkansas and deposited in a complex of nearshore environments along the margins of a broadly subsiding basin. On the basis of a regional outcrop and subsurface investigation in which external geometry of framework sands was integrated with observations of lithology, sedimentary structures, fossil occurrence, and bounding relationships, 2 principal depositional systems are recognized in Woodbine rocks--a fluvial system and a highly destructive delta system. The tributary-channel facies and the highly meandering channel facies, both components of the fluvial system represented by massive sand and gravel bodies of the lower Woodbine (Dexter) lithosome, are dominant north and northeast of a line from Dallas to Tyler. On the south and southwest, the highly destructive delta system is persistent throughout the entire section. The 4 component facies of the delta system includes: progradational distributary-mouth bar facies; coastal-barrier sand facies, developed either lateral to or basinward of the distributary mouth; prodelta mud facies; and embayment-strandplain facies, developed laterally adjacent to principal deltaic facies. Following or near the end of deposition of Woodbine rocks and before their transgression by Eagle Ford Shale, emergence of the Sabine uplift resulted in erosion of Woodbine material and its redeposition along the margins of the uplift in a lithosome designated the "Harris Sand." End_of_Article - Last_Page 1788------------

Petrography and Permeability of Lower Cretaceous Muddy Sandstone, Bell Creek Area, Montana: ABSTRACT

The Cretaceous Muddy Sandstone at Bell Creek field is an elongate sandstone body that has an average thickness of 20 ft, a mean quartz grain size of 0.16 mm (fine-grained), and a quartz content of 85 percent. In vertical sequence, the sandstone can be subdivided on the basis of internal structure into the following depositional units (in ascending order): lower shoreface, middle shoreface, upper shoreface-beach, and eolian. Grain size increases upward from lower shoreface through upper shoreface but decreases slightly in the eolian unit. In horizontal sequence, the sandstone can be subdivided on the basis of dominant rock type into the following facies (from west to east): forebar, bar, and backbar. Based on petrography, bedding, and unit morphology, the Muddy Sandstone appears to have originated as part of a barrier-bar complex in the main Bell Creek producing area. However, on the southwest at Ranch Creek, the barrier pattern is modified by a local divergent trend. Petrologically, the sandstone at Ranch Creek is indistinguishable from that at Bell Creek and End_Page 218------------------------------ probably was deposited under similar environmental conditions. Core analyses demonstrate that permeability is related to mean grain size and content of quartz. Therefore, permeability plots indicate vertical variations in grain size and quartz content which, in turn, may be interpreted in terms of depositional environment. Permeability values in the thousands of millidarcys and quartz content in excess of 90 percent are common in upper shoreface-beach sediments of the bar facies at Bell Creek. Permeability values and quartz content are significantly reduced in lower shoreface or lagoonal sediments of the forebar and backbar facies, respectively. End_of_Article - Last_Page 219------------