Erosional Troughs Research Articles

Cenozoic deposition in the Nansen Basin, a first-order estimate based on present-day bathymetry

Considering the age of the underlying oceanic crust, the Eurasia Basin in anomalously shallow, especially the southern Nansen sub-Basin. Particularly large bathymetric anomalies are found outside major erosional troughs on the Barents Sea Shelf, indicating sediment infill from the adjacent shelf as a major cause of the shallowing. To obtain a first order estimate of the sediment isopachs in the Eurasia Basin, the standard age-depth curve for oceanic crust in the North-Atlantic was used as a reference to define a bathymetric anomaly which was then inverted, assuming local isostatic equilibrium. The isopachs reveal a huge delta outside the St. Anna and Voronin troughs. Conservatively estimated this “twin delta” covers ≈ 75,000 km 2 and has a maximum sediment thickness ≈ 7 km. Similar, but smaller, deltas are found both east and west of this delta. The deltas in the Nansen Basin appear analogous to the Bjørnøyrenna and Storfjordrenna deltas, which are located off major submarine troughs in the Western Barents Sea and are comparable to the St. Anna/Voronin Delta in size. The latter deltas contains huge amounts of young, glacigenic sediments, and this is also likely to be the case for the deltas in the Nansen Basin. The huge sediment accumulations off the northeastern Barents Sea, the anomalously large depth of the Barents Shelf, and its trough dominated morphology, suggest that extensive erosion similar to that documented in the Western Barents Sea (0.5–1.5 km) affected the entire Barents Shelf.

Small submarine fans on the eastern margin of Corsica: Sedimentary significance and tectonic implications

The small turbidite systems and their network of distributary channels lying on the base of slope and basin off the eastern margin of Corsica exhibit physiographic and acoustic characteristics similar to those encountered in typical deep-sea fans. In most cases, the fans appear to be connected to sinuous canyons that incise into the shelf break and display varying shapes along the continental slope. They begin as large highly meandering erosional troughs. Then they become progressively depositional and bordered by lateral prominent levees and show evidence of branching. In their most distal part the height of levees decrease and lobes appear unchannelled. Seismically, the lobes are characterized in their middle and lower parts by superposition of channel-levee units, showing high amplitude reflectors with poor penetration in channels, and bedded facies with better penetration on levees. Only the most distal domains located beyond the channelized areas of the lobes are formed entirely by high amplitude reflectors with poor penetration that are interpreted as coarse deposits of sand or silt. The majority of these sand-rich fans, and especially those located in the northern sector of the margin, are related to the insular drainage system of Corsica. The sandrich fans located in the southern sector may have been structuraly controlled. By the lack of any obvious actual connexion with insular drainage, they suggest earlier association with high continental relief, subsequently removed by erosion or tectonic collapse. The southernmost canyon of this area (Caprera canyon) records structural trends of the Sardinian crystalline basement, and is also probably related to tectonic movements linked with the opening of the Tyrrhenian Sea.

Stratigraphic Framework of a Late Pleistocene Shelf-Edge Delta, Northeast Gulf of Mexico

Lithologic, biostratigraphic, and chronostratigraphic data from a 92-m continuous surface boring, in the Main Pass area of the outer Mississippi-Alabama shelf, were used to calibrate high-resolution seismic profiles in a study of a late Pleistocene shelf-edge delta. The boring is the first of its kind through a shelf-edge clinoform wedge and the first ground-truth confirmation that the clinoforms in the study area are deltaic in origin. Chronologic control for the late Pleistocene outer shelf stratigraphy is based on the identification of Ericson Zones X, Y, and Z (alternating warm and cold water planktonic foraminifera zones) in the boring, representing at least the last 130 k.y. During sea level lowering related to the previous glacial maximum, the delta system progra ed onto a carbonate-rich outer shelf and upper slope starved of terrigenous sediments. The resulting seaward-thickening deltaic wedge is more than 100 m thick in localized areas. Stacking patterns of delta lobes demonstrate overall progradation from the northeast to the southwest. The ancestral Mobile River, possibly joined by the Pascagoula River, was the fluvial feeder of the shelf-edge delta. The upper portion of the delta wedge is extensively eroded, primarily by a broad swath of significant fluvial scour centered along the northeast- to southwest-oriented dip axis of the delta, and to a lesser extent by subsequent transgressive truncation. Fluvial scour resulted in a broad erosional trough. Evidence from the boring indicates that the erosional trough is filled with fluvial and estua ine facies. Thin estuarine and overlying marine units reflect transgression of the Lagniappe delta during the late Pleistocene--early Holocene transgression. According to standard sequence stratigraphic definitions and nomenclature, the extent of the sequence boundary, identified as the erosional base of the fluvial facies, places the majority of the outer shelf delta in the systems tract. Although the sequence stratigraphic approach is strictly a methodology, the term highstand is misleading for this systems tract. The portion of the delta thus categorized was built during the falling to lowstand minimum part of the relative sea level curve. Deposits of the actual in sea level that preceded progradation to the shelf edge occur instead on the inner and middle shelf s thin (<10 m) erosional remnants. Although eustatic sea level fall was large and rapid (at least 60 m in 20 ka), fluvial incision of the shelf break did not occur. Incision is instead best developed on the inner to middle shelf (about 20 m of erosional relief). No canyon or channel features occur on the upper slope that can be associated with this shelf-edge delta.

Quaternary sequences and their relations to the pre‐Quaternary in the vicinity of Anholt, Kattegat, Scandinavia

The stratigraphic record from a boring penetrating the 104 m thick Quaternary sequence on the island of Anholt is summarized. The spatial distribution of the pre‐Quaternary formations and the surface topography of these are described on the basis of reflection seismic profiles. It is concluded that Anholt is located in the crestal zone of a southeast–northwest trending anticline in the pre‐Quaternary. The anticline was formed during the Late Cretaceous–Early Tertiary inversion episodes and was later deeply truncated by erosion. A southeast–northwest trending erosional channel, c. 2 km wide and with a maximum depth c. 250 m below sea level, is located southeast of Anholt along the crest of the anticline. This channel is not present at the bore locality. Although no direct correlation from the boring to the seismic profiles could be achieved it is argued that a strong reflection near the base of the Quaternary outside the channel may be correlated with the Saalian–Eemian complex found in the boring. Three younger sequences of probable Early and Middle Weichselian, Late Glacial and Holocene age respectively have been recognized. The Late Glacial and Holocene sediments appear to have been deposited in erosional troughs and channels cut into a sequence of Lower and Middle Weichselian sediments. Post‐Eemian till deposits or other evidence unambiguously indicating the presence of Weichselian glaciers have not been found, either in the boring or in the seismic profiles. It is therefore assumed that the erosion of the Lower‐Middle Weichselian sequence was of fluvial origin and can be ascribed to the lowstand period of the Weichselian glacial period. The western part of Anholt can possibly be regarded as an erosional remnant of the Lower‐Middle Weichselian sequence.

F15-A: a Triassic gas field on the eastern limit of the Dutch Central Graben

The F15-A gas field lies at the eastern margin of the Dutch Central Graben, the reservoirs being found within the overpressured Triassic Main Buntsandstein Formation. The discovery well, F15-4, was drilled in 1985 and was subsequently followed by the two appraisal wells, F15-5 and F15-7. Structurally speaking, the F15 area is located in an intermediate structural compartment between the Central Graben, to the west and the Schill Grund High, to the east. The structure consists of a NNE–SSW-trending turtle back anticline, plunging to the south, with gently dipping flanks to the east and west. Northwards, closure is provided by a NW–SE-oriented erosional trough which cuts deeply into the Lower Triassic and is filled by Upper Triassic/Jurassic sediments. The primary reservoir is the Volpriehausen Sandstone Member, which is very fine to coarse grained and irregularly cemented by dolomite, anhydrite and halite. Four lithofacies have been recognized: dunal, interdunal, ephemeral fluviatile (distal wadi) and playa-lake related. These facies show a coarsening-upward profile, caused by the successive depositional environments becoming increasingly arid as they grade from playa lake to dunes. A seismic amplitude study was carried out, based on the links between porosity and seismic response at the reservoir level, to identify the areas with the best potential reservoir development. The field was originally identified and discovered on the basis of two-dimensional seismic data; however, a three-dimensional survey was subsequently shot and has increased the confidence in the structural interpretation. Based on this, one horizontal production well will be drilled in order to put the field on stream in June 1993.

Boulder clusters in glacial flutes

Former wave action on small flutes and adjacent surfaces in the proglacial region of Austre Okstindbreen, Nordland, Norway, has removed the finer fraction of diamictons revealing a skeletal arrangement of boulders that survive on the present land surface. Analysis of the patterns of these boulders, in relation to the outline of the flutes, indicate a concentration of boulders within the diamicton that formed the flute relative to the adjacent unfluted surface. Within each skeletal flute the pattern of boulders indicates a large boulder or cluster of large boulders at the ice-proximal end, a preferred orientation of elongate blouders which are not in contact with other boulders, and the clustering of packs of randomly orientated boulders at other locations. On the basis of this evidence, along with evidence from detailed particle size and fabric studies of diamicton in adjacent flutes (Rose, 1989), it is suggested that flutes are formed by the transfer of new sediment directly from the glacier, and the transfer of existing sediment from adjacent erosional troughs. Because of their large size, boulders are most susceptible to this process, and are therefore transferred preferentially.

Glacier stress patterns and sediment transfer associated with the formation of superimposed flutes

Glacial flutes superimposed on megaflutes in the foreland of Austre Okstindbreen, north Norway, provide information suitable for the identification of the areas of erosion and deposition during the formation of the superimposed flutes, and the calculation of the volumes of sediment involved in this process. Detailed analysis of the geometry, particle size, and clast fabric of the sediments involved in these landforms indicate that material was transferred to an incremental zone from an adjacent erosional trough, along with additional material derived directly from the glacier. Sediment transfer is restricted to an upper till with a thickness of about 0.40 m, but deformation imposed during the formation of the superimposed flutes can be traced to a depth of around 0.75 m in a lower sheared silty sand. Stress patterns revealed by clast fabric analysis in the superimposed flutes and adjacent parts of the megaflute indicate compressive flow at the proximal end of the bedform in the lee of an apparently obstructing boulder, convergent extending flow in the central part of the bedform with a characteristic “herringbone” pattern, and divergent extending flow at the terminal zone of the feature. The convergent/divergent pattern is considered to be a response to ice flow cells caused by high pressure over the troughs between the flutes, and low pressure over the flute crestline. These ice flow cells are considered to be initiated by factors such as upstanding boulders and bedrock knobs, clusters of boulders within the diamicton, and lateral and longitudinal oscillations developed in response to identifiable perturbations. First-order estimates of the rates of sediment transfer involved in the formation of the superimposed flutes, vary between 0.0077 m 3 m −2 a −1 and 0.38 m 3 m −2 a −1. These figures have been applied in an exploratory fashion to megaflutes and drumlins from elsewhere, in order to estimate the time required for glacier bedforms to develop and test the reliability of the figures derived. The most meaningful results suggest that glacier bedforms are created within a period of between 4 and 400 years irrespective of their size. This suggests that glacier bedforms develop rapidly and are maintained in a condition of steady state that reflects the balance between glacier energy and resisting bed material.

Catastrophic impact of hurricanes on atoll outer reef slopes in the Tuamotu (French Polynesia)

Underwater effects on coral reefs of the six hurricanes which ravaged French Polynesia between December 82 and April 83 were observed by SCUBA diving around high islands and atolls during September and October 1983. Special attention was paid to Tikehau atoll reef formations (Tuamotu archipelago) where quantitative studies on scleractinians, cryptofauna and fishes were conducted in 1982 immediatly prior to the hurricanes. On outer reef slopes coral destruction, varying from 50 to 100%, was a function of depth. Upper slope coral communities composed of small colonies well adapted to high energy level environments, suffered less than deeper formations. However, there is a narrow erosional trough in this zone at a depth of 6 m that was probably the result of storm-wave action (plunge point). Coral destruction was spectacular at depths greater than 12 m: 60 to 80% between 12 m and 30 m and 100% beyond 35 m, whereas earlier living coral coverage ranged from 60 to 75% in these zones. The outer slope was transformed into a scree zone covered with coarse sand and dead coral rubble. Dives on different sites around steep outer slopes (>45°) of the atolls and more gentle slopes (<25°) of some parts of the high islands permitted the formulation of an explanatory hypothesis: direct coral destruction by hurricane-induced waves occurred between the surface and 18–20 m; on low-angle slopes broken colonies were thrown up on reef flats and beaches; on steep slopes avalanches destroyed much of the living corals and left scree slopes of rubble and sand.

Erosional troughs in deep-sea carbonates and their relationship to basement structure

A detailed survey of a small area in the eastern equatorial Pacific with the Marine Physical Laboratory's Deep-Tow instrument package reveals erosionally excavated troughs in an area of relatively rapid accumulation of carbonate. The troughs lie on top of a basement high implying a relationship between the troughs and the basement feature. Differential accumulation and compaction may explain the origin of the troughs. Sediment deposited around the basement high preferentially accumulates on gentle slopes and in troughs. This results in a thick sequence of sediment on the flanks of the basement high and a thin sequence above on top of the hill. Unstable sediment masses and differential compaction generate a tensile-stress field above the peak of the basement high which causes slumping and faulting. The steep slopes generated by faulting and slumping may then serve to intensify tidal currents or act as spillways and locally generate increased turbulence. This increased turbulence results in local erosion by dissolution. A trough will form above a basement high, however, only when a combination of (1) basement steepness, (2) high sedimentation rate, (3) sediment thickness, and (4) sediment erodibility are present.

Geochemistry of phosphatic chalks and hardgrounds from the Santonian to early Campanian (Cretaceous) of northern France

Phosphatic chalks occur in erosional troughs cut into soft white chalks of late Coniacian to Santonian age and based by strongly lithified and mineralized hardgrounds. The dominant mineralogy consists of low-Mg calcite, carbonate-fluorapatite, α-quartz, glauconite and iron oxides. Lithification is geochemically discernible ~90 cm below the surface of the basal hardgrounds and results in a calcite-associated enrichment in Fe, Mn and Mg but depletion in Sr. Factor analysis indicates 4 major controls on the element distributions—carbonate-fluorapatite, clay minerals and silica, lithification, and iron minerals. White chalks are geochemically similar to modern coccolith oozes but hardgrounds have been indurated by the addition of a primary low-Mg calcite cement. Glauconitization was generally restricted to replacement of clay minerals during the early stages of hardground development while lithified carbonate remained in contact with seawater. Phosphatization occurred by the replacement of low-Mg calcite during burial of the hardground by an organic rich sediment. Mineralization was intimately related to organic processes.

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An Erosional Trough on the Remnant of an Elevated Peneplain at Mt. Kenashimori, Kitakami Mountains

A “Frying-pan” type erosion is observed on the flat top of Mt. Kenashimori (941.5 m) Kitakami Mountains, Iwate Prefecture. The flat of the mountain covered with sod is a small remnant of the uplifted peneplain. A “Frying-pan” type erosion area is the upper part of a shallow-and-wide small valley. The minute cliff at the NE edge of the pan is 15-20 cm high and the bottom is 30 cm lower than the flat top. The author thinks that the mechanism of the erosion is a sheet-flood erosion due to a heavy rain, in which the soil under sod, loosened from solifluction in winter, is washed out by the flood.

Many Rocks--Gallup Field, San Juan Basin, Northern New Mexico: ABSTRACT

This discussion is presented in the belief that through its understanding similar fields may be discovered. Favorable economics exist because of shallow depths of the producing sandstone. Reliable field records, a high density of core analyses, and good mechanical logs are of considerable aid in establishing the geologic history of this stratigraphic trap with its hydrodynamic, faulting, and folding complications. The field is located on the northwestern side of the San Juan basin. Although the Upper Cretaceous producing sandstone is present in Colorado, commercial production is found only in New Mexico. The discovery well, Little No. 2-27 Navajo, was completed in November, End_Page 1575------------------------------ 1962, at a depth of 1,240 ft. Cumulative production is more than 1,000,000 barrels and initial potentials have ranged up to 665 BOPD. Depth of the standstone ranges from 1,134 to 1,781 ft. and averages less than 1,500 ft. Dry-hole cost averages less than $5,500; completed wells cost $15,000-$20,000, and pay-out periods can be as short as 4 months. The productive sandstone ranges from 3 to 13 ft. in thickness, has a porosity of 9-24%, and permeability of 1-872 md. Original bottom-hole pressure was 465 psi. The producing body is an elongate, northwest-trending marine sandstone accumulation of Niobraran age. It is 25 mi. long and averages 0.5 mi. in width. The sandstone was deposited in erosional troughs formed on the Niobrara-Carlile unconformity. The unconformity rises toward the southwest across the basin. Usually sandstones are found in troughs formed by longshore currents or at the base and seaward edges of cliffs at the unconformity. Detailed isopach mapping shows irregularities along the trough. Subsequent Laramide folding and faulting and post-Laramide erosion have caused a variety of fluid accumulations within the stratigraphic trap. Zones of fresh water, oil and brackish to salt water, oil, and gas caps occur at different positions along the trend of the sandstone. In Colorado the sandstone dips southeast for 12 mi., from an outcrop elevation of 5,400 ft. to a deep syncline at the Colorado-New Mexico boundary where the oil-water contact is at 3,900 ft. Southeast of the syncline, the sandstone rises 400 ft. in 2½ mi. onto the faulted asymmetrical Blue Hills anticlinal nose and finally descends gradually southeastward into the San Juan basin. A normal fault that divides the sandstone into separate reservoirs is located almost 1 mi. northwest of and essentially parallel ith the axis of the plunging Blue Hills anticline. This fault causes the gas-oil contact to be 85 ft. higher on the upthrown (northwestern) side. End_of_Article - Last_Page 1576------------

NEW DATA ON THE TECTONICS OF THE KIROV REGION AND THE UDMURTSK A.S.S.R. IN CONNECTION WITH THEIR PETROLEUM PROSPECTS

The structure of the northern part of the Volga-Ural region is based mainly on geophysical evidence and data obtained from a widely spaced grid of stratigraphic and exploratory boreholes. Absence of clear criteria for interpreting geophysical anomalies has lead to completely different, published interpretations of identical anomalies. Illustrations include maps showing the several major tectonic elements of present-day structure, relief of the present-day surface of the crystalline basement and a contour map drawn on the top of the Assel'sk stage. The complex structure of the area includes vast blocks of basement and basins bounded by fractures. Parts of the blocks which have experienced the greatest uplift have no terrigenous Devonian deposits and hence are of little interest in regard to petroleum. Only older erosional troughs where sandstone reservoirs could be preserved are of real interest. Zones between blocks and adjacent flank areas have complete Devonian sections. These zones, near the tectonic s...

Distribution of uranium deposits in the White Canyon-Monument Valley District, Utah-Arizona

The White Canyon-Monument Valley district of southeastern Utah and northeastern Arizona contains important uranium deposits in the basal portion of the Chinle formation of Middle (?) and Late Triassic age. The basal unit of the Chinle throughout much of the district is a widespread conglomerate, sandstone and mudstone unit resting discomformably on Permian and Early Triassic strata and commonly designated the Shinarump member. In this report the basal unit is subdivided into two units by a second prominent disconformity. The lower portion, which is essentially restricted to channels carved into older rocks, contains all the uranium deposits and is retained as the Shinarump member. The upper part is considered as the basal sandstone of the overlying Monitor Butte member.The concept of a Shinarup channel is somewhat expanded to embrace the larger more continuous erosional troughs within which there may be one or more discontinuous scours, which many previous workers have referred to as Shinarump channels are widely traceable and join to form a complex paleodrainage system trending generally north-ward in the Monument Valley area but trending westward in the White Canyon area. Channel-fill deposits in Monument Valley are mostly coarse clastics but carbonaceous mudstone becomes more abundant toward the north and in places constitutes the dominant channel-fill.Most Shinarump deposits occur in channel segments in an arcuate belt along the southern and western margin of a paleostructural high. Ore deposits are of the vanadium-uranium and copper-uranium types and occur as irregular tabular lenses roughly parallel to the bedding and enlongated parallel to scour trends. Fine-grained black and yellow uranium minerals fill interstices and replace carbonaceous material in sandstone beds in the deeper scours near channel bends or intersections.It is postulated that the uranium was derived from a source to the south of the district and transported northward through the channel system during or soon after deposition of the Shinarump. Uranium was deposited at numerous scattered locations, by H 2 S produced by sulfate reducing bacteria, in the deeper scours within the channels. An erosional period following Shinarump deposition and prior to Monitor Butte deposition caused removal of most Shinarump deposits from upwarped areas and concentration of the dispersed uranium deposits into a belt of larger ore deposits adjacent to the eroded areas.