Small-scale Cross-bedding Research Articles

Mineralogy and Geochemistry of Sandstones from the Tanjero Formation, Bekhme Gorge Northeastern Iraq: Implications for Paleoweathering and Provenance

The Tanjero Formation is exposed on the Northern flank of the Perat anticline near the Northern entrance to the Bakhme gorge through which the waters of the Upper Zab flow. As for tectonics, it is located within the High Folded Zone of the Unstable Shelf. The Tanjero Formation is about 80 m. thick, it is bounded from the bottom by the Shiranish Formation. The formation consists of gray sandstone that contains gravel at the bottom followed by beds of marl, hard sandstone, and black claystone containing cushioning structures and small-scale cross-bedding are present. According to the Williams classification, theTanjero sandstones drop at the lithic wacke field. The provenance of these sandstones was investigated based on their minerals and geochemical composition. The geochemical analyses showed low silica, a negative correlation of silica with the most major oxides due to recycling sedimentary sources or weathering of silicates. The index of compositional variability indicates that the sandstones of the Tanjero Formation are immature clastic sediments and the high index of compositional variability values confirm that the clastic sediments occurred in the tectonically-active-settings area. Moderate chemical weathering has dominated in the studied area, which was inferred from the chemical index of alteration, the chemical index of weathering and the plagioclase index of alteration. Based on the tectonic discrimination diagrams, the Tanjero sandstones are quartzose sedimentary provenance, felsic igneous rocks source, transitional arc–lithic recycled setting, and located in the collision zone between the passive Arab margin and the effective Iranian-Turkish margin that formed the orogeny belt along the collision zone.

Sedimentary Characteristics of Fuyu Oil Layer in the Southern Fuxin Uplift of Songliao Basin

Based on 22 well cores and logging data of nearly 800 well, sedimentary characteristics of Fuyu oil layer in the southern Fuxin uplift was analyzed. It is shown that types of sandstone in the Fuyu oil layer are main feldspathic litharenite, debris arkose and minor litharenite. Granularity probability curve is displayed as a typical of two-stage or three-stage. A large number of parallel bedding, one-way oblique bedding, small-scale cross bedding, the bottom scour structure and syngenetic deformation structure can be seen in the Fuyu oil layer. Mudstone color is main fuchsia, grey purple massive mudstone and gray-green, gray, dark gray massive mudstone. Combined with the analysis of the sedimentary facies sign, Fuyu oil layer in the southern Fuxin uplift is large fluvial-dominated shallow water delta deposition. The skeletal sand bodies in the delta system are distributary channelsand bodies that are large, dense, narrow strip, continuous distribution. The overall is south-southwest trending. Sand body is continuous and far underwater extension until it disappears into thin sandsheet. multi-branch fluvial-dominated ribbon-like bodies are formed.

Lithofacies Characters and Significance of the Submarine Fan of the Liufengguan Group in Qinling

Abstract:Field investigation and laboratory research on flysch of the Liufengguan Group in Qinling indicate the following: (1) Sandstone of the Liufengguan Group is categorized as feldspathic lithic graywacke with a minor amount of lithic graywacke in the QFR triangular diagram. Grain size ≤0.3 mm. Bedding plane structures such as groove casts and suspected flute casts can be found at the bottom of the sandstone. It is inferred that currents may have come from the southeast during deposition. Bedding structures such as ripple marks, graded bedding, parallel bedding, small‐scale cross bedding, climbing bedding, suspected convolute bedding, microlamination and sliding structures have also been observed, which are of indicative significance. It is thought that the Liufengguan Group has the sedimentary characteristics of bedding, bedding plane structures and lithological assemblages of deep‐sea low‐density turbidity current deposits. The vertical succession of the Bouma sequence in the inner fan subfacies zone is generally incomplete: the assemblage of Ta and Tabc is commonly seen; the succession of the middle fan subfacies zone is relatively complete; and divisions Te and Tb are common in the outer fan subfacies zone. (2) The flysh of the Liufengguan Group is a sequence of deep‐sea argillaceous‐arenaceous submarine fan deposits, in which the authors recognize the inner, middle and outer fan subfacies and also nine types of lithofacies: normal graded sandstone (A1), medium‐ to thick‐bedded, fine‐grained sandstone (A2), medium‐ to thick‐bedded and massive siltstone (A3), thin‐bedded, fine‐grained sandstone and mudstone (B1), irregular interbeds of thin‐bedded, fine‐grained sandstone and siltstone (B2), thin‐bedded, fine‐grained sandstone (C1), very thin‐bedded, fine‐grained sandstone (D1), olistostromes (E1) and deep‐sea mudstone (F). The inner fan consists of four microfacies: natural levee (A1), water channel (A2, A3) and olistostromc (E1); in the middle fan there also occur four microfacies, i.e., branch channel (B1), branch channel (B2), interdistributary bay (D1) and olistostrome. The outer fan is made up of the branch channel (C1) and sheet sand (D1) microfacies, which alternate vertically with sediments of deep‐sea plain subfacies (F). There occur fining‐ and thin ning‐upward channel deposits in the outer‐fan subfacies zone of the submarine fan of the Liufengguan Group observed in this study. The quartz content of the graywacke of the deposits is all higher than 40% and may reach as high as 60%. Therefore, on the basis of the aforementioned features, this flysh should be formed in a passive continental‐margin tectonic environment.

The interrelation and distinction between flood-tidal delta and washover deposits in a transgressive barrier island

The distinction and interrelation between flood-tidal delta and washover deposits is described by comparing back-barrier sands associated with an active and a relict tidal inlet system on the south shore of Long Island. Guidelines for comparison include three-dimensional morphology, sedimentary structures, sediment texture, and facies sequences. The active, modern flood-tidal delta is covered by reversing small-scale ripples represented by bidirectional small-scale cross-bedding in the subsurface. Large-scale flood-oriented cross-beds are associated with non-reversing sand waves on the flood ramp. The relict flood-tidal delta is represented by medium- to fine-grained sands characterized by small-scale cross-bedding or massive bedding. Relict flood-tidal delta deposits are overlain by distinct washover deposits separated by thin units of salt-marsh peat. Dominant washover structures are parallel laminations. Well-preserved roots are commonly found in washover sands, but are largely absent in flood-tidal delta sands. Facies sequences of the relict inlet system indicate that flood-tidal delta deposits extend the width of the barrier island. Overwashing serves to infill abandoned channels of the delta and contributes to vertical buildup.

Late Cretaceous to Early Tertiary Deltaic to Marine Sedimentation, North Slope, Alaska: ABSTRACT

Along the lower Colville River near Ocean Point, Alaska, Late Cretaceous to early Tertiary sediments (Colville Group and the Sagavanirktok Formation) record depositional environments from delta plain to prograding delta to shallow marine shelf. The unit hosts the northernmost known dinosaur remains and is less than 100 m thick with numerous tephra deposits in its lower sections. Furthermore, it is characterized by cyclic, relatively fine-grained sediments indicating mostly depositional and few erosional events. The major depositional elements of the delta plain are 3.5 to 5.45-m thick tabular fining-upward cycles that are cut by sand-filled fluvial channels (up to 10 m thick). The cyclic sediments contain abundant roots and grade upward from small-scale cross-beds to laminated and then structureless silt and clay terminating in organic-rich layers. The channel-fill sequences fine upward and change vertically from large to small-scale cross-beds. Over-bank flooding as well as lateral migration of small meandering fluvial channels formed the cyclically interbedded deposits, meandering rivers deposited the thick cross-bedded sands, and soil development or marsh deposits formed the organic-rich horizons that cap each cycle. Plant debris, nonmarine invertebrates, and vertebrate fossils are locally concentrated in the delta plain sediments. Subsidence related to compaction of the deltaic sediments alongmore » with possible delta lobe switching resulted in repeated progradation of the delta front over the delta plain. Delta front sediments are 3 to 10-m thick tabular deposits of large and small-scale cross-bedded sands and silt bounded by organic-rich beds. Also, there are abundant roots, rare channels and invertebrate fossils that suggest a transitional environment from sand-flats to estuarine or bay.« less

Physical and biogenic aspects of backbarrier sedimentary sequences, Georgia Coast, U.S.A.

Configuration of the present Georgia coast, with its broad expanses of salt marsh and intricate tidal drainages behind Pleistocene—Holocene barrier islands, implies the filling of an extensive backbarrier lagoonal system. However, riverine and salt marsh estuaries dominate the modern depositional setting and reconstructed vertical sequences suggest that backbarrier filling was accomplished by estuarine processes. Backbarrier sediments exhibit considerable diversity in grain size and physical and biogenic sedimentary structures. Principal sediment textures include clean to muddy coarse and fine sand, mud and, to a lesser extent, shells, pebbles, and mud and bedrock clasts. Physical sedimentary structures, many modified by bioturbation, include large-scale cross-bedding (trough and planar) with graded sets, small-scale crossbedding (ripple lamination and flaser, wavy and lenticular bedding), laminated to interbedded sand and mud, and rare pebble or shell beds. Ebb-oriented structures predominate over flood-oriented ones. Except in upper reaches of riverine estuaries, biogenic sedimentary structures typically are diverse and abundant. characteristic lebensspuren include polychaete dwelling tubes, decapod and polychaete burrows, ray holes, and bioturbate textures imparted by amphipods, ophiuroids, bivalves and anemones. Patterns of bioturbation are controlled to some extent by sediment type, salinity levels and rates of deposition and physical reworking of sediments. Although intensity of bioturbation generally increases seaward, bioturbate textures are more readily preservable in the lower-energy, middle reaches of estuaries. Because of numerous local Pleistocene sediment sources and local variations in hydrographic regimes, the stratigraphic record of salt marsh estuaries differs relatively little from that of riverine estuaries. The basic sequence, in ascending order, typically consists of: (1) channel-lag and (or) channel-fill deposits; (2) estuarine accretionary beds; (3) point bar, tidal flat and shoal sediments; and (4) channel margin and salt marsh deposits.

Early Diagenetic History of Late Pennsylvanian Calclithite Beds, Hueco Mountains, El Paso County, Texas: ABSTRACT

Calclithite beds exposed on the western escarpment of the Hueco Mountains were deposited in a fan-delta system featuring fluvial channels, marine bars or beach sands, shallow-marine shale and limestone, and tidal flats and lagoons. Two distinct calclithite types occur. One is a coarse, poorly sorted gravelly sand with angular to subrounded grains. This type occurs in discontinuous or channel-shaped beds. Sedimentary structures include fining-upward sets, imbrication, and trough cross-bedding. These characteristics indicate sporadic unidirectional flow, as would be expected in ephemeral streams. The second calclithite type is fine to medium-grained well-sorted sand with very well-rounded grains. This type crops out as thin, relatively continuous units. Sedimentary structur s include ripples, small-scale cross-beds, low-angle and horizontal or planar bedding. The early history of the calclithites reflects their depositional environment. The coarse calclithite was deposited in undersaturated freshwater conditions, shown by the absence of early cement. Early compaction of the coarse calclithite, indicated by intergranular microstylolitization and shale-clast deformation, is the most commonly observed texture. The fine-grained calclithite exhibits a markedly different history. The first recognizable diagenetic event is micritization of marine-derived fossils and calclithite grains. Early cementation in the marine phreatic environment resulted in isopachous rims of fibrous aragonite and bladed Mg-calcite cement. Pore centers contain a later equant calcite cement. Little or no early compaction occurred in the fine calcli hite. Freshwater flushing is indicated by the replacement of former aragonite cement rims by finely crystalline equant calcite. End_of_Article - Last_Page 297------------

Sedimentation and Tectonic Implications of Cambrian-Ordovician Clastics, Renville County, North Dakota: ABSTRACT

Cambrian-Ordovician clastics of the Deadwood Formation were studied in detail from Newporte field in Renville County, North Dakota. This small Cambrian-Ordovician oil pool was extensively cored, often to the Precambrian basement, allowing close examination of clastic deposition influenced by local basement tectonics. In Renville County, the basal unit consists of a well-rounded, fine to medium-grained glauconitic quartz sandstone. Paleohighs appear to have had a pronounced effect on Deadwood sedimentation. Sands, from quiet water settings, show poor to moderate sorting, are commonly finely laminated, and/or show traces of minor small-scale cross-bedding. In places, bedding planes are highly disrupted, suggesting intervals of intense bioturbation (Skolithos). Sands associated with paleohighs are clean, well sorted, and commonly friable. Their association with basement structure is suggestive of beach-barrier-bar sequences related to irregularly upthrown basement blocks. In one example, this clean basal sand is associated with an upthrown basement block and is sharply truncated by the pre-Winnipeg ( arly Ordovician) unconformity. The first unit above the basal sandstone in structurally lower wells is an anomalous conglomerate unit. Large angular basement clasts up to cobble size were viewed in core. This unit grades upward into a fine sand sequence and distally grades into a marine sand. It terminates abruptly in upthrown wells and indicates rapid fault movement and offset during middle Deadwood deposition, with development of localized fanglomerate sequences associated with fault scarps. Immediately capping this sequence is a dark-gray marine shale that thins depositionally toward paleohighs. End_of_Article - Last_Page 288------------

Sedimentology and morphodynamics of a macrotidal beach, Pendine Sands, SW Wales

The foreshore of Pendine Sands forms the seaward part of an extensive, sandy coastal barrier in a shallow Carmarthen Bay, SW Wales. The sedimentological features of the macrotidal foreshore reflect a tide-induced modification of nearshore wave characteristics. As the tide ebbs, the breaker height may decrease, the surf zone widens and becomes increasingly dissipative, and swash/backwash velocities diminish. A concomitant change from plunging to spilling breakers and increasingly symmetrical swash zone flows are associated with a decreasing beach gradient. A zero net transport model demonstrates that the beach profile is self-stabilising in the short-term, and periodic levelling has shown that the beach is in long-term equilibrium with prevailing conditions, though this does not preclude a significant dynamic response to changing tides and waves. The flow regimes of wave-generated currents decline as the tide ebbs, and normal beach processes do not usually affect the lower foreshore. Accordingly, there is an overall seaward-fining of the primary framework component of the sands. In more detail, this framework component displays a slight seaward-coarsening across an upper foreshore dominated by high water swash and surf; a rapid seaward-fining across the mid-foreshore in response to the ebb-attenuating swash zone flow velocities; and a slight seaward-fining across the lower foreshore under the action of nearshore shoaling waves. Bedforms vary from a swash/backwash emplaced flat bed across the upper foreshore to the small ripples of nearshore asymmetric oscillatory flows across the lower foreshore. The surface sediment veneer is not representative of the subsurface sediments which form in response partly to fairweather conditions, partly to storms. The upper foreshore is characterised by swash/backwash emplaced plane bedding in fine sands frequently disrupted by bubble cavities. The mid-foreshore is composed of coarser-grained shelly traction clogs arranged as landward- and seaward-dipping large-scale cross bedding and/or plane bedding; these are probably storm breaker/surf deposits. The lower foreshore, though partially and sometimes totally bioturbated, shows landward-dipping small-scale cross bedding in very fine sands sorted by nearshore shoaling waves. Tide- and storm-induced modification of the nearshore flow regimes therefore produces a distinctive shore-normal array of sedimentary facies. Each facies is characterised by diagnostic textural and structural signatures. A prograding sequence of such macrotidal deposits would be similar to, but more extensive than, a comparable microtidal sequence.

Mississippian Frobisher-Alida-Kisbey Sandstone, North-Central North Dakota: ABSTRACT

Argillaceous and arenaceous marker beds and lenses are common in the Frobisher-Alida interval of the Mississippian Madison. These markers are used to define pay zones in Bottineau, Renville, and Burke Counties. One of these regional markers is the Kisbey sandstone, also referred to as the MC-4 bed or the K-2 marker. The Kisbey sandstone has a characteristic log pattern and can be traced from southeastern Saskatchewan into north-central North Dakota. It occupies a specific stratigraphic position between the Mohall' and Glenburn porosity zones and averages 15 ft (4.5 m) thick. Lithologically similar to other arenaceous units locally present within the Frobisher-Alida, the Kisbey generally is a gray to buff, medium to fine-grained, well-sorted, subangular to rounded, qu rtzose sandstone. Primary structures include lenticular bedding, ripple marks, and small-scale cross-bedding. Dolomite or anhydrite cement are also present locally. The ability of the Kisbey to act as a reservoir rock is demonstrated by several fields in southeastern Saskatchewan and by the North Haas field in Bottineau County, North Dakota. Nineteen wells in the North Haas field produce or have produced from the Kisbey sandstone. Oil migrating through the Glenburn porosity in the Haas field has charged the porous and permeable Kisbey sandstone. Updip, the Mohall and Glenburn zones become progressively more anhydritic until the Kisbey is bounded above and below by anhydrite. An updip porosity loss acts as the final trapping mechanism. The principal factor which determines the reservoir quality of the Kisbey sandstone is the presence or absence of cement or argillaceous material. End_of_Article - Last_Page 940------------

Depositional Model of Sand-Dominated Mesotidal Estuary: ABSTRACT

Depositional environments were surveyed in a large sand-dominated estuary on the south-central South Carolina coast. The estuarine system is characterized by abrupt lateral variations in sediment texture, composition, and physical and biogenic sedimentary structures. Sand-dominated intertidal environments include point bars, ebb sand spits trailing seaward from marsh headlands, tidal sand ridges, and sandy tidal flats. Morphologically, point bars, trailing spits, and tidal ridges are in a continuum of linear to sinuous depositional forms 1 to 3 km long. An idealized vertical section through these sand bodies is a fining upward sequence beginning with channel lags of coarse sand and gravel followed by interbedded and bioturbated sands and muds interpreted as channel-fill deposits. A gradation between high-angle, large-scale cross-bedding and low-angle, small-scale cross-beds is typical of bar platform and vertical accretion bar sands. Capping the sequence are muddy sands to laminated muds grading into salt-marsh deposits. Sand flats that prograde from marsh islands typically display a fining-upward trend beginning with shell material concentrated in medium to coarse sand where high-angle cross-bedding is the dominant sedimentary structure. In the upper part of the tidal-flat sequence, biogenic structures increase and disrupt low-angle, small-scale cross-bedding. Wave-deposited laminated sands, capped by rooted, bioturbated marsh muds, top the idealized sequence. Delta-like deposits within the estuary are morphologically gradational between tidal-sand ridges and true flood-tidal deltas and include physical and biogenic structures common to both. End_of_Article - Last_Page 955------------

Aspects of the reproductive biology of female snow crabs, Chionoecetes bairdi, from Prince William Sound and the adjacent Gulf of Alaska : HILSINGER J. R., 1976 Mar. Sci. Commun., 2 (3/4): 201–225

The foreshore of Pendine Sands forms the seaward part of an extensive, sandy coastal barrier in a shallow Carmarthen Bay, SW Wales. The sedimentological features of the macrotidal foreshore reflect a tide-induced modification of nearshore wave characteristics. As the tide ebbs, the breaker height may decrease, the surf zone widens and becomes increasingly dissipative, and swash/backwash velocities diminish. A concomitant change from plunging to spilling breakers and increasingly symmetrical swash zone flows are associated with a decreasing beach gradient.A zero net transport model demonstrates that the beach profile is self-stabilising in the short-term, and periodic levelling has shown that the beach is in long-term equilibrium with prevailing conditions, though this does not preclude a significant dynamic response to changing tides and waves.The flow regimes of wave-generated currents decline as the tide ebbs, and normal beach processes do not usually affect the lower foreshore. Accordingly, there is an overall seaward-fining of the primary framework component of the sands. In more detail, this framework component displays a slight seaward-coarsening across an upper foreshore dominated by high water swash and surf; a rapid seaward-fining across the mid-foreshore in response to the ebb-attenuating swash zone flow velocities; and a slight seaward-fining across the lower foreshore under the action of nearshore shoaling waves. Bedforms vary from a swash/backwash emplaced flat bed across the upper foreshore to the small ripples of nearshore asymmetric oscillatory flows across the lower foreshore.The surface sediment veneer is not representative of the subsurface sediments which form in response partly to fairweather conditions, partly to storms. The upper foreshore is characterised by swash/backwash emplaced plane bedding in fine sands frequently disrupted by bubble cavities. The mid-foreshore is composed of coarser-grained shelly traction clogs arranged as landward- and seaward-dipping large-scale cross bedding and/or plane bedding; these are probably storm breaker/surf deposits. The lower foreshore, though partially and sometimes totally bioturbated, shows landward-dipping small-scale cross bedding in very fine sands sorted by nearshore shoaling waves.Tide- and storm-induced modification of the nearshore flow regimes therefore produces a distinctive shore-normal array of sedimentary facies. Each facies is characterised by diagnostic textural and structural signatures. A prograding sequence of such macrotidal deposits would be similar to, but more extensive than, a comparable microtidal sequence.

Depositional Features of Braided-Meandering Stream: GEOLOGIC NOTES

The Cimarron River in north-central Oklahoma shows characteristics intermediate between typical braided and meandering streams. In the Perkins area, the gradient is 1.8 ft per mi; the sinuosity is 1.5; and average channel depth at bankfull stage is 15 ft. Accretion ridges on the downstream sides of bars characterize the straighter reaches, whereas accretion ridges on both the upstream and downstream sides are in the more sinuous reaches. The deposits show upward fining; they are generally fine- to medium-grained, well-sorted sand, with scattered quartz and intraformational pebbles. Horizontal bedding and medium-scale and small-scale crossbedding are the dominant sedimentary structures. Crossbedding, parting lineation, and grain orientation define the sand trend and indicate that directional features of this type are useful in estimating reservoir trend. Compared to typical meandering-stream deposits, the Cimarron River deposits are thinner but possibly wider, and they contain less fine-grained clastics in the upper part of the sequence and as clay interbeds. The Cimarron sediments are finer grained and better sorted than sands of typical braided streams. They contain more horizontal bedding than either braided- or meandering-stream deposits. The type of sand deposit represented by the Cimarron River sand may be similar to certain ancient alluvial sandstones which were deposited during either basinal subsidence or eustatic rise in sea level.

Depositional and Directional Features of Braided-Meandering Stream: ABSTRACT

The Cimarron River in north-central Oklahoma shows characteristics intermediate between typical braided and meandering streams. The gradient is 1.8 ft/mi. at Perkins; the sinuosity is 1.5; the monthly average discharge ranges from 2 to 17,800 cu ft/second; and average channel depth at bankful stage is 15 ft. The deposits are generally fine- to medium-grained, well-sorted sand, with scattered quartz and intraformational pebbles and thin beds of coarse-grained sand. Several clay drapes are present as thin discontinuous layers. Medium-scale crossbedding, horizontal bedding, and small-scale crossbedding are the dominant sedimentary structures. Compositionally the sand is an arkose, which suggests the Wichita uplift and the southern Rocky Mountains as ultimate source areas. Irregularities and discontinuities in the sand deposits are due primarily to channel shifts during times of major floods and secondarily to deposition of clay during recession of high-water stages. The irregularities resulting from dissection of transverse dunes and superposition of ripples on dunes are thought to be of minor significance in causing reservoir variations. Crossbedding, parting lineation, and grain orientation all define the sand trend very well and indicate that directional features in this type of sand deposit are useful in estimating reservoir trend. Directional imbibition parallels the dip of the crossbeds and grain orientation (and parting lineation) in horizontal beds. Compared with typical meandering-stream deposits, the Cimarron River sand is thinner and contains less fine-grained clastics in the upper part of the sequence and as clay interbeds. It is finer grained than typical braided-stream sands, and current indicators show a wider directional range. The type of deposit represented by the Cimarron River sand may be similar to certain Pennsylvanian alluvial sandstones which were deposited as the gradient was reduced during the initial stages of eustatic rise in sea level. End_of_Article - Last_Page 1823------------

Depositional Models of Sandstones: ABSTRACT

Determination of the depositional environment for sandstone bodies generally improves the accuracy in estimating sandstone reservoir trends. Depositional framework is best understood by comparing the geometric and internal features of each sandstone to those of models, which are sand and sandstone deposits of known depositional environment. The different environments are distinguishable most commonly, not by a unique property, but by diagnostic combinations of the various features, such as trend, width, thickness, nature of contacts, sequence of sedimentary structures, textural sequence, and constituents. These characteristics are catalogued for 18 different environments, ranging from eolian to deep-marine basin floor, and models of alluvial valley, alluvial plain, and de taic environments are illustrated. The Pennsylvanian Kisinger Sandstone, which crops out in north-central Texas, was deposited in a deep and narrow valley by a westerly flowing river. Each textural sequence of massive-bedded conglomerate, crossbedded conglomeratic sandstone, and convolute-bedded sandstone represents a genetic unit formed at one position of the river, which was 200-300 ft wide. The fine-grained sandstone and carbonaceous shale in the upper 30 ft of the 150-ft section were deposited under near coastal conditions as the valley filled in response to a continued rise of sea level. The Pennsylvanian Robinson Sandstone in southern Illinois was deposited by a river flowing west-southwest on an alluvial plain. Unidirectional medium-scale and small-scale crossbedding, upward decrease in grain size, and sharp basal and lateral contacts indicate stream deposition. The absence of allochthonous pebbles suggests deposition on a plain rather than in a valley. The absence of marine indicators suggests alluvial rather than deltaic conditions. The Davis sand (Yegua) in the Hardin field of southeast Texas was deposited by a deltaic distributary. A width-thickness ratio of approximately 30, and abrupt lower and lateral contacts, together with interstratification and the microfauna of equivalent beds, are suggestive of deposition near the mouth. The narrow width of 1,250 ft suggests further that the sand represents a genetic unit, with insignificant lateral migration. Most of the Cretaceous Newcastle Sandstone in North Dakota represents deposition on a broad, slowly subsiding deltaic plain which formed as streams advanced west-northwestward from South Dakota, and southward from Saskatchewan. In the deltaic complex Skull Creek prodeltaic clays underlie delta-margin End_Page 156------------------------------ coastal sands and distributary sands, which together are thought to represent most of the sandstone in the Newcastle. Very gentle slopes, shallow sea floor, and slow rate of subsidence were primary reasons for the widespread distribution of a relatively thin sandstone section and the wide variation in sandstone trends. End_of_Article - Last_Page 157------------

Paleocurrent Analysis of the Mainly Fluviatile Permian and Triassic Beacon Rocks, Beardmore Glacier Area, Antarctica

ABSTRACT The postglacial Beacon sequence in the Beardmore Glacier area comprises 2000 m of nonmarine clastic sediments. Approximately 3000 paleocurrent measurements were taken in sets of about six from stratigraphic intervals less than 30 m thick from the Mackellar, Fairchild, and Buckley Formations of Permian age and the Fremouw and Falla Formations of Triassic age. These data yielded 432 current directions, 168 from small-scale cross-bedding, 162 from medium-scale cross-bedding, 69 from parting lineation, and 33 from logs, ripple marks, and slump folds. Analysis of variance, using vector rather than arithmetic means, was used to indicate the likelihood that differences in current direction for different areas, formations, or types of sedimentary structures are due to variations in a normally distributed population. Four formations have essentially a unimodal current pattern, while the Buckley Formation has a centripetal pattern. There is no significant difference (F.10) between mean directions to the southeast for the three Permian formations, or to the northwest for the two Triassic formations. This reversal implies a laterally constraint of both Permian and Triassic paleocurrent systems. Mean current directions for small- and medium-scale cross-bedding and parting lineation within each formation are not significantly different, and for the Fairchild and Buckley Formations, values for standard deviation are also similar. For these two units, each structure is an equally reliable indicator of paleocurrent direction, and a hierarchy of bedforms like that proposed by Allen (1966) is not indicated. However, such a hierarchy might exist for the Fremouw Formation, where parting lineation is significantly less variable than medium-scale cross-bedding, and medium-scale cross-bedding is significantly less variable than small-scale cross-bedding.

Recent Braided-Stream Sedimentation, South-Central, Alaska: ABSTRACT

The Scott Glacier outwash fan, in the Copper River delta area, Alaska, shows a progressive decrease in grain-size and slope away from the glacier terminus. Bar and channel morphology change systematically from proximal to distal areas. Detailed study along a 12-km tract shows that average clast size on bar surfaces decreases downfan from 30 to 3 cm, and slope decreases from a maximum of 15.5 m/km to 2.0 m/km. Upstream, bars are present between channels in sheetlike deposits; downstream, bars are present largely in mid-channel. Bars are composed of poorly sorted, well-imbricated coarse to fine gravel and plane-bedded, better sorted sand. Plane-bedded sand is more abundant in distal areas and may be capped by small-scale cross-bedding. Cobble and pebble imbrications are the most persistent and reliable indicators of flow direction. Modal dip direction is upstream with long axes transverse to current flow. Other directional features measured included lineations, ripples, bar slipfaces, and logs. Linear stripes of large pebbles, which are oriented transverse to flow direction (determined from pebble imbrication) and occur in groups on bar surfaces, are interpreted as relict antidune bedforms. Most deposition, which occurs during high-water stages, takes place on bar surfaces under upper flow-regime conditions. Flow separation along bar edges allows avalanche slipfaces to form. During low-water stages, when flow is mostly confined to channels, lateral erosion removes bar deposits and some channel deposition occurs under lower flow-regime conditions. End_of_Article - Last_Page 836------------

Nature and Recognition of Limestone Turbidites, Marathon Region, Texas: ABSTRACT

The Dimple Limestone (Pennsylvanian) of the Marathon region represents a period of carbonate deposition which interrupted the deposition of a thick terrigenous flysch section in the Ouachita geosyncline. It consists from north to south of laterally adjacent shelf, slope, and basin facies. The shelf facies is characterized by cross-bedded oolith carbonate grainstone. The slope and basin facies consist of sequences of distinctive limestone turbidite. Paleocurrent analysis indicates the existence of a uniform paleoslope dipping southward, with no apparent slope break. Slope-facies turbidites (proximal turbidites) display a wide variety of internal characteristics. They may be graded, reverse graded, or non-graded in their lower parts, but grade abruptly in their upper parts. Basal parts commonly are conglomeratic, and carbonate-mudstone upper parts commonly are absent. Floating pebbles are common, and 3-foot-thick cross-bedded units have been observed. Small-scale cross-bedding is rare, and large-scale convolutions are abundant. Associated rocks are subaqueous slump conglomerate, spicular carbonate mudstone, and black spicular marl. This facies is 5 miles wide. Basin-facies turbidites (distal turbidites) are nearly always graded. Pebbles are scarce, and the coarsest sizes are sand or silt. Carbonate mudstone beds are well developed, and were deposited from turbidity currents because (1) thick mudstone beds overlie thick graded beds, and (2) the normal pelagic sediment is radiolarian-bearing marl. Small-scale cross-bedding is abundant and convolutions are common. Thick beds are commonly massive, becoming laminated and cross-bedded on a small scale upward. Thin beds are commonly laminated. Associated rocks are black radiolarian-bearing marl and spicular chert. Information gained from the Dimple Limestone was applied to lower Paleozoic rocks in the region, and revealed slope-and-basin-facies limestone turbidites in the Ft. Pean Formation and Maravillas Chert (Ordovician). It appears that some geosynclinal limestone turbidite is the product of major tectonism, and is introduced from the cratonic side. End_of_Article - Last_Page 483------------