Minor Sedimentary Structures Research Articles

Tidally influenced barrier island and estuarine sedimentation in the upper carboniferous of southern West Virginia

Upper Carboniferous orthoquartzites exposed in the Guyandotte Valley of southern West Virginia are interpreted as barrier island and backbarrier deposits. They overlie fossiliferous marine limestones and shales, and are overlain in turn by fluvio-deltaic subgraywacke sandstones, siltstones and coals. Only the upper one-third of the 180 m thick orthoquartzitic interval is exposed, but these rocks outcrop over a distance of 16 km, and reveal landward transition of the barrier orthoquartzites into backbarrier and alluvial or delta-plain deposits. Within the exposed orthoquartzitic interval are three major orthoquartzite bodies, separated by subordinate siltstones containing smaller sandstone bodies. The major orthoquartzites are 10–25 m thick and are lenticular in transverse section, with a north-south elongation approximately perpendicular to the palaeoslope. These sandstone units are subdivisible into three facies. Facies 1 is most widespread and comprises mainly upward-thinning sets of cross-stratification, with a silty interval occasionally preserved towards the top. Palaeocurrent indicators reflect increasing dispersal from the base of the sequence upward. These deposits are interpreted as originating within laterally migrating tidal inlets, in which ebb currents were dominant. Facies 2 contains low-angle bedding suggestive of wave swash processes, with patterns of minor sedimentary structures similar to those observed in modern ebb tidal deltas. Facies 3 consists of sandstone with a preponderance of landward-directed cross-stratification, and is attributed to a flood tidal delta, flood-channel and washover berm origin. The orthoquartzites merge eastward and southeastward (landward) into inferred estuarine or lagoonal siltstones, but in places they are in lateral contact with fluvio-deltaic subgraywackes. Thus the barrier sediments are thought to have been derived at least in part by the reworking of abandoned delta lobes. Longshore drift created spit extension towards the south and southwest, thereby enclosing lagoons within which accumulated siltstones and sandstones with brackish or marine fossils and a variety of structures suggestive of tidal processes and local subaerial exposure. Washover sands were deposited to landward of the barrier. Both barrier and backbarrier deposits are incised by small tidal channels. Occurrence of washover features in association with small but well-developed tidal deltas suggests moderate to strong wave activity and a mesotidal (2–4 m) range.

Sediments of the Tay Estuary. III. Sedimentological and Faunal Relationships on the Southern Shore at the Entrance to the Tay.

SynopsisThe southern entrance shores to the Tay estuary form an area of extensive inter-tidal sedimentation. Tidal currents of varying strengths characterise the area and the situation is modified by both wind and wave. The net result is a series of seven sub-environments of sedimentation. The textural characteristics of the sediment and the distribution of major and minor sedimentary structures in each subenvironment is closely linked to stability and changing physical processes. The distribution of organisms, live and dead, provides a good indication of these patterns of sediment and stability.

Instant Peels Using Polyester Resin

ABSTRACT Instant peels made of marine polyester resin are a quick, easy and inexpensive method of obtaining undisturbed samples of sediments. They permit stereomicroscope measurements to be taken of minor sedimentary structures, textures, and fabrics. Peels can be X-rayed to study sedimentary structures or treated and used to prepare thin sections. The technique can also be used to study bottom distributions and orientations of shells. Limitations of the technique are related to the taking of the peel itself. To obtain a reasonably good sample, the surface to be sampled must be nearly flat; it must not contain anomalously large particles mixed with finer particles; it cannot be wet, and it must be at a temperature not much lower than 60° F (15°C). If these conditions are not fulfilled, either the resin will not solidify properly, or the peel will selectively pick up different grain sizes. Some of these difficulties can be overcome by selecting resins that polymerize at low temperatures or are soluble in water.

Minor EBB-structures and shell orientations on a tidal beach (bay of Arcachon, France)

Minor sedimentary structures occurring on a small tidal flat at La Pointe, Bay of Arcachon (Gironde, France) are analyzed in the light of their usefulness for interpreting ancient environments, particularly as indicators of short subaerial periods in the history of a sediment. The structures may be grouped into the categories: ripple breaching, rill marks, ripple erosion, wave sweeping, and “dribble” marks. The relative abundances of pelecypod shells differ significantly in various parts of the beach, reckoned in a straight line perpendicular to the shoreline. The ratio of concave-upwards to concave-downwards oriented valves is unlike for different species across the beach.

Preliminary note on the breamrock formation (upper devonian), county cork, ireland—an ancient tidal deposit

Bipolar current directions allied with a characteristic suite of minor sedimentary structures suggest a tidal environment for the Breamrock Formation. A brief comparison is made with recent tidal deposits.

Sediments and Sedimentary Processes of Eastern Mississippi Cone, Gulf of Mexico

The upper 6-7 m of sediment of the eastern Mississippi cone consists of a repetitious vertical succession of gray silt and silty clay intercalated with a few layers of fine sand and topped by a 20-50-cm layer of yellowish-brown foraminiferal clay. Disequilibrium age determinations indicate that the lower silty layers, representing the deposits of latest low sea-level stand, were deposited more rapidly than the upper foraminiferal clay. These sedimentation rates, which depend primarily on the rate of the detrital influx and sea-level change, average about 30 cm/1,000 years. Sedimentary processes on the deep-sea fan are interpreted from sedimentary structures, textures, and compositions, as well as from bathymetry, bottom photographs, and continuous seismic profiles. The more than 20 varieties of minor sedimentary structures recognized from X-ray radiographs are grouped into five varieties that correlate closely with sediment type. None of the structures is typical of vertical turbidity sequences. On the contrary, the evidence suggests that the primary mechanisms of sediment transport are differential pelagic settling and low-flow-regime bottom currents, with mass movements by sliding or slumping common in channel and slope areas. Statistical evaluation of the occurrence and distribution of minor structures indicates that (1) most of the structures asso iated with coarser materials are analogous to structures formed by traction transport or by ripple migration in shallow water, and (2) the distribution of both bottom-current intensity and internal waves that create small-scale ripples is local. Photographs of the present bottom support this conclusion. The importance of diagenetic solution of carbonate, mostly planktonic foraminifers and pteropods, as verified by laboratory experimentation, is related to the degradation of organic matter in the sediments. The most active solution occurs near the boundary between the upper foraminiferal clay and the lower silty layers and is partly responsible for (1) the abrupt decrease of carbonate downward in the cores, (2) the rearrangement of clay particles into secondary thin laminae, and (3) the shortening of the distance between noncarbonate silt and sand layers or laminae. These results, combined with compaction, accentuate the uniformity of layering. The upper cone is indented by digitate leveed valleys and canyons cut by transverse ridges, whereas the lower section is characteristically smooth. The bathymetry of the cone reflects its underlying structure. Continuous seismic profiles show that the lower cone is composed of relatively uniform flat-lying beds, representing at least five major depositional cycles since Plio-Miocene time and as many as 14 since Late Cretaceous time. In contrast, the upper cone has many internal irregularities, probably caused by gravity sliding, folding, and slumping contemporaneous with deposition, and by diapiric salt intrusion. The cone's depocenter has shifted continuously basinward as the Mississippi delta has prograded gulfward since Late Cretaceous time.

Delineation of some Outer Deltaic Plain Subenvironments Based on Sedimentary Properties in Vertical Section: ABSTRACT

A small wedge of detrital sedimentary rocks (deposits of the outer part of a deltaic plain) was investigated to delineate subenvironments. The rock unit (Middle Pennsylvanian age) is as thick as 40 ft and occupies an area of about 300 sq mi in western Pennsylvania, where intense strip mining provides excellent continuity of outcrop. Criteria for defining rock subfacies are vertical sequences of bed-thickness properties of sandstone and siltstone, together with minor sedimentary structures and fossil content--in brief, those properties observable in smaller exposures or subsurface records. Lateral relations of the vertically defined rock subfacies, exposed in continuous cut faces, delineate End_Page 710------------------------------ five distinct lithologic facies. The composite properties of these facies imply deposition in subenvironments associated with an outer, predominantly subaqueous deltaic plain--distributary-channel, distributary-mouth-bar, crevasse, reworked and probably drifted bar-sand, and open-bay subenvironments. End_of_Article - Last_Page 711------------

Distribution of Minor Structures in Gulf of Mexico Sediments: ABSTRACT

Minor sedimentary structures are studied by means of X-ray radiography from cores collected from various physiographic provinces of the Gulf of Mexico. The Gulf can be divided into two large petrographic provinces meeting at DeSoto Canyon and Campeche Canyon. The western part is a terrigenous clastic province and the eastern is a carbonate province. For a study of the sedimentary structures it is necessary to divide both petrographic areas into their proper physiographic provinces. The sedimentary structures and characteristics distinguished are: thin bedding (regular, irregular, lenticular), lamination (parallel, lenticular, irregular), coarse-grained laminae, turbidites (single, multiple, vague), convolute lamination, load casts, degassing structures, mottled, slump, homogeneous, burrowing, mycelium, shells, and shell fragments. Several of these characteristics can be found in almost any area but some of them are of restricted occurrence. Cores collected from one province reveal only slight variations in their content of sedimentary structures. End_of_Article - Last_Page 1830------------

Minor sedimentary structures in a prograding distributary

Minor sedimentary structures were studied in cores taken at the mouth of a small prograding distributary within the Mississippi River delta. The mouth of Johnson's Pass in Garden Island Bay was mapped and the following environments were recognized: subaerial and subaqueous natural levee, channel, distributary mouth bar, interdistributary bay and marsh. Oriented, undisturbed cores were taken from each environment. These cores were split, dried and photographed and the types of minor sedimentary structures within each environment were tabulated. Natural levee deposits contained abundant current ripple laminations, unidirectional cross-laminations, parallel and wavy laminations, distorted layers and burrowed oxidized silty sands. Channel fill deposits consisted of alternating beds of clay and silt containing trough cross-laminations, scour and fill structures and distorted layers. The distributary mouth bar, composed predominantly of silt and sand, is characterized by a variety of small-scale multi-directional cross-laminations and air-heave structures. Two types of interdistributary bay deposits were recognized, homogeneous clay with scattered brackish-water fauna and a predominantly clay section with thin parallel and lenticular laminations and ripple marks. The structures within these two types are a reflection of availability of coarse detritus. Marsh deposits are characterized by the abundance of peat, carbonaceous clays, calcareous nodules and root disturbances. Each environment is characterized by a distinct assemblage of structures. These assemblages form a valuable tool for use in interpreting palaeo-environments in ancient sedimentary rocks.

Sedimentary Structures: Mississippi River Deltaic Plain: ABSTRACT

Minor sedimentary structures were studied in cores and exposures from the deltaic and marginal deltaic plains of the Mississippi River. Selected active environments were sampled, and the occurrence of sedimentary structures from each was recorded. Individual structures were found to occur in more than one environment; however, suites of structures were characteristic. Within the study area the following twelve depositional environments have been investigated: shelf, prodelta, delta front (distal bar, distributary mouth bar, channel, and subaqueous levee), subaerial levee, marsh and swamp, interdistributary bay, mudflat, and fresh-water lake. Shelf deposits consisted of: (1) fine-grained clastics, burrowed and showing parallel laminations and (2) marine organic debris. Prodelta deposits are similar to clayey shelf deposits, but contain lenticular and parallel lamination with finely divided plant inclusions. The delta front is a complex of sub-environments constituting the advancing locus of active deposition of the prograding delta. The sloping seaward margin on this zone--the distal bar--exhibits current structures such as trough cross-laminations and current ripples as well as parallel and lenticular laminations, wave ripples, and burrows. The silty and sandy distributary mouth bar is characterized by a variety of small-scale, multi-directional cross-laminations and gas-heave structures. Channel deposits exhibit trough c oss-laminations, scour and fill, and distorted laminations, whereas subaqueous levees contain abundant ripple and unidirectional cross-laminations, parallel, wavy, and distorted laminations. In addition, subaerial levees are burrowed and oxidized. Marsh and swamp deposits are distinguished by abundant plant remains, burrows, and parallel laminations. Lenticular laminations, wave ripples, burrows, shell, and plant remains are characteristic of both interdistributary bay and fresh-water lakes. The mudflat assemblage of structures includes lenticular laminations, current and wave ripples, burrows, and shell remains. Not only are associations of sedimentary structures End_Page 521------------------------------ important in recognizing individual environments of deposition; equally important is the association of the environments to one another. This relationship must be understood for correct paleogeographic reconstruction of ancient deltaic deposits. The manner in which a sequence of marine and deltaic deposits might accumulate in a segment of a basin with resulting stratigraphic relationships of associated environmentally determined facies, is illustrated. End_of_Article - Last_Page 522------------

Minor Sedimentary Structures in a Prograding Distributary: ABSTRACT

Minor sedimentary structures were studied in cores taken at the mouth of a small prograding distributary within the Mississippi River delta. The mouth of Johnson's Pass in Garden Island Bay was mapped and the following environments were recognized: subaerial and subaqueous natural levee, channel, distributary mouth bar, interdistributary bay, and marsh. Oriented, undisturbed cores were taken from each environment. These cores were split, dried, and photographed and the types of minor sedimentary structures within each environment were tabulated. Natural levee deposits contained abundant current ripple bedding, unidirectional cross-laminations, parallel and wavy laminations, distorted layers and burrowed oxidized silty sands, whereas channel fill deposits consisted of alte nating beds of clay and silt containing trough cross-laminations, scour and fill structures, and distorted layers. The distributary mouth bar, composed predominantly of silt and sand, is characterized by a variety of small-scale multi-directional cross-laminations and air-heave structures. Three types of interdistributary bay deposits were recognized; highly burrowed interbedded silt and clay, homogeneous clay with scattered brackish-water fauna, and a predominantly clay section with thin parallel and lenticular laminations and ripple marks. The structures within these three types are a reflection of availability of coarse detritus. Marsh deposits are characterized by the abundance of peat, carbonaceous clays, calcareous nodules, and root disturbances. Each environment is characterized by a distinct assemblage of structures. These assemblages can be used to interpret paleoenvironments in ancient sedimentary rocks. End_of_Article - Last_Page 353------------

Minor Internal Structures of Some Recent Unconsolidated Sediments

Widespread occurrence of several different minor sedimentary structures in shallow-water Gulf of Mexico sediments has been shown by more than 2,000 cores and bottom samples. Minor internal structures in two areas, one off the east side of the Mississippi River Delta and the other along the central Texas coast, were found to be similar. Deposits in these two areas were subdivided into those containing: (1) regular layers (thin beds or laminations), (2) irregular layers (rough or crude layers and lenses), (3) mottles (discontinuous lumps, tubes, and pockets), and (4) structureless homogeneous sediments, depending on the principal type that occurs. Areal distributions of these different structures in the surficial sediments are mappable. Similar sedimentary structures and sequences of structures occur in the open gulf and in the bays and sounds. In the open gulf the structures occur in wide bands, and the sequences are developed through wide ranges of water depths; in the bays and sounds the bands generally are narrower, and the sequences are compressed into much smaller ranges of water depths. The different types of structures are formed on or near the depositional surface, contemporaneous or nearly contemporaneous with deposition. The differences between depositional environments which produce the various structures are differences in: (1) sediment sources, (2) in physical processes and their intensities, and (3) rates of deposition. Regular layers are characteristic of areas of rapid deposition and/or relatively few bottom-living animals; they are either primary--formed by fluctuations of sediment, or secondary--formed by wave or current winnowing. Irregular layers and mottles are mostly secondary features, formed mainly by bottom-living animals altering existing sediments. Homogeneous deposits are either primary or secondary; they form by extremely rapid deposition, unif rm deposition, or by complete secondary reworking. The appearance of many normal marine sediments is due in large measure to burrowing and crawling organisms. Structures similar to those in presently forming sediments of the Gulf of Mexico apparently were developed in rocks of all ages when environmental conditions were proper. Knowledge of how these structures form, why they vary, and the relationships between them furnishes a valuable key for interpreting deposition of ancient rocks.

Sedimentary Structures in the Upper Carboniferous of North and Central Derbyshire, England

ABSTRACT Coarse siltstones and sandstones recovered from a series of borehole cores in the Upper Millstone Grit and Lower Coal Measures of Derbyshire exhibit a variety of minor sedimentary structures. The least common show affinities with wash-outs and were probably caused by subsurface water flow. More widespread geographically and stratigraphically are contorted structures in fine-grained sandstones which have originated in some cases by penecontemporaneous flow down shallow submarine slopes and in others by secondary load-casting adjustment or possibly rilling. Associated with the above are two further types of phenomena which consist of infilled, near-vertical tubes. The smaller, with uniform diameters of 1 mm to 4 mm have been ascribed to annelid borings, whereas the origin of the larger, with their irregular morphology are more doubtful although they have been provisionally ascribed to molluscan activities.