Fluvial Sheet Sandstones Research Articles

Long-range, high-resolution stratigraphic correlation of Rotliegend fluvial-fan deposits in the central Dutch offshore

The Rotliegend feather-edge area in the central part of the endorheic Southern Permian Basin in the Dutch offshore is characterized by a predominance of mud-prone, evaporite-bearing playa and lake deposits with a subordinate amount of interbedded, thin, fluvial sheet sandstones. The distribution and lateral facies changes of the sandstone bodies have been analyzed by generating a long-range, high-resolution chronostratigraphic correlation framework. The correlation technique of pattern matching of GR logs was applied, supported by calculating spectral trend curves. Flooding events are the primary near-synchronous correlation surfaces, which can be traced up to and over 100 km. The basin setting of the Southern Permian Basin, the studied sandstone depositional architecture (logs) and sedimentary characteristics (core) are analogous to the depositional setting of laterally-amalgamated terminal lobes of dryland-river systems in an endorheic basin, such as the Holocene Altiplano Basin in Bolivia, present-day Lake Eyre (Australia) and the Miocene Ebro Basin (Spain). The integrated approach has yielded a stratigraphic reservoir-architecture framework in which the reservoir sandstones, with net sand up to 10 m, have been identified as amalgamated terminal-splay sandstone sheets formed at the end of dryland-river pathways, alternating with lacustrine mudstone layers deposited during short-duration, high-magnitude flooding in intermittent wet climate periods.

Evolving stratigraphy of a Middle Triassic fluvial‐dominated sheet sandstone: The Otter Sandstone Formation of the Wessex Basin (UK)

The Middle Triassic (Anisian) Otter Sandstone Formation of the Wessex Basin (UK) has long been considered a single undivided geological formation, but building on previous work, it is shown here to have a remarkably well‐defined internal stratigraphy that is evident not only in the fluvial sedimentary architecture but also in the colour, grain size, sorting, and whole‐rock geochemistry of the sandstones. The Otter Sandstone Formation is here divided into four named members of very different character (West Down Member, Otterton Ledge Member, Chiselbury Bay Member, and Pennington Point Member). The study highlights the importance of outcrop‐to‐subsurface correlation in stratigraphic studies, which suggests several major unconformities within the Otter Sandstone Formation. The Otter Sandstone Formation provides evidence for an increase in humidity throughout the early Middle Triassic, with an upward decrease in the proportion of calcrete and an apparent increase in channel size. Whole‐rock major and trace geochemistry (chemostratigraphy) is widely used in the subdivision and correlation of red‐bed sheet sandstones, and it is shown here that data, even when used in a very raw and unprocessed way, can be a useful stratigraphic discriminant. Fluvial sheet sandstones are often regarded as relatively homogeneous aquifer units, but this study shows that changes in fluvial style and background climate can introduce major changes to the geometry and physical properties of the aquifer or reservoir. The recognition of thin marker beds such as palaeosols can be particularly critical for the correlation of thick fluvial sheet sandstones.

The Kaapvaal Craton, South Africa: no evidence for a supercontinental affinity prior to 2.0 Ga?

We briefly examine the possible antiquity of the supercontinental cycle while noting the likely unreliability of palaeomagnetic data >ca.1.8 Ga, assuming a gradual change from a magmatically dominated Hadean Earth to a plate tectonically dominated Neoarchaean system. A brief review of one of Earth's oldest cratons, Kaapvaal, where accent is placed on the lithostratigraphic and geodynamic-chronological history of its cover rocks from ca. 3.1 to 2.05 Ga, forms the factual basis for this article. The ca. 3.1–2.8 Ga Witwatersrand–Pongola (Supergroups) complex retroarc flexural foreland basin developed while growth and stabilization of the craton were still underway. Accretion of relatively small composite granite-gneiss-greenstone terranes (island arc complexes) from both north and west does not support the formation of a Neoarchaean supercontinent, but may well have been related to a mantle plume which enhanced primary gold sources in the accreted terranes and possibly controlled the timing and rate of craton growth through plate convergent processes. Subsequent deformation of the Witwatersrand Basin fill with concomitant loss of ≤1.5 km of stratigraphy must have been due to far-field tectonic effects, but no known mobile belt or even greenstone belts can be related to this contractional event. At ca. 2714–2709 Ma, a large mantle plume impinged beneath the thinned crust underlying the Witwatersrand Basin forming thick, locally komatiitic flood basalts at the base of the Ventersdorp Supergroup, with subsequent thermal doming leading to graben basins within which medial bimodal volcanics and immature sediments accumulated. Finally (possibly at ca. 2.66–2.68 Ga), thermal subsidence enabled the deposition of uppermost Ventersdorp sheet-like lavas and sediments, with minor komatiites still present. Ongoing plume-related influences are thus inferred, and an analogous cause is ascribed to a ca. 2.66–2.68 Ga dike swarm to the north of the Ventersdorp, where associated rifting allowed formation of discrete ‘protobasinal’ depositories of the Transvaal (ca. 2.6–2.05 Ga Supergroup, preserved in three basins). Thin fluvial sheet sandstones (Black Reef Formation, undated) above these lowermost rift fills show an association with localized compressive deformation along the palaeo-Rand anticline, north of Johannesburg, but again with no evidence of any major terrane amalgamations with the Kaapvaal. From ca. 2642 to 2432 Ma, the craton was drowned with a long-lived epeiric marine carbonate-banded iron formation platform covering much of it and preserved in all three Transvaal Basins (TB). During this general period, at ca. 2691–2610 Ma, the Kaapvaal Craton collided with a small exotic terrane [the Central Zone (CZ), Limpopo Belt] in the north. Although farfield tectonic effects are likely implicit in TB geodynamics, again there is no case to be made for supercontinent formation. Following an 80–200 million years (?) hiatus, with localized deformation and removal of large thicknesses of chemically precipitated sediments along the palaeo-Rand anticline, the uppermost Pretoria Group of the Transvaal Supergroup was deposited. This reflects two episodes of rifting associated with volcanism, and subsequent thermal subsidence within a sag basin setting; an association of the second such event with flood basalts supports a plume affinity. At ca. 2050 Ma the Bushveld Complex intruded the northern Kaapvaal Craton and reflects a major plume, following which Kaapvaal–CZ collided with the Zimbabwe Craton, when for the first time, strong evidence exists for a small supercontinent assembly, at ca. 2.0 Ga. We postulate that the long-lived evidence in favour of active mantle (cf. plume) influences with subordinate and localized tectonic shortening, implicit within the review of ca. 3.1–2.05 Ga geological history of the Kaapvaal Craton, might reflect the influence of earlier Precambrian mantle-dominated thermal systems, at least for this craton.

Sequence architecture within a low-accommodation setting: An example from the Permian of the Galilee and Bowen basins, Queensland, Australia

The coal-bearing, Upper Permian Betts Creek Beds (BCB) of the northeastern intracratonic Galilee Basin in north-central Queensland, Australia, record sedimentation within an alluvial-coastal plain in a low-accommodation basin-margin setting. Six third-order depositional sequences are preserved within the BCB and are characterized by a sequence architecture that is overall relatively thin (15 m; 49 ft), condensed, and top truncated. Sequences have a lowstand systems tract composed of extensive braided fluvial sheet sandstones, a transgressive systems tract characterized by tidally influenced fluvial sheet sandstones and bioturbated estuarine mudrocks, and a highstand systems tract composed of coastal-plain mudrocks and coals. Sequences can be correlated throughout the entire Galilee Basin and related to the lithostratigraphy established in the adjacent Bowen Basin, a foreland basin to the east. The overall Late Permian fill is considered a second-order sequence. The stacking patterns show an overall retrogradational to progradational architecture suggestive of relatively low accommodation space. Sequences show an extremely laterally continuous geometry (1000 km; 600 mi) and are relatively uniform in thickness. Sequences do not show evidence of onlap, toplap, or erosional truncation, but instead are thin in areas of low accommodation and expand into areas of higher accommodation. That these third-order sequences can be traced across basins with differing tectonic regimes, together with the persistent, sheetlike nature of the sequences, suggests that tectonic effects on the sequence architecture were minimal. The basin fill covers a time interval during which two glacial events occurred in the final stages of the late Paleozoic Ice Age, suggesting that changes in glacioeustasy may have been the dominant controls on the sequence architecture of the Late Permian succession in the Galilee Basin of northeastern Australia.

Architecture and origin of an amalgamated fluvial sheet sand, lower Castlegate Formation, Book Cliffs, Utah

Amalgamated, fluvial sheet sandstones in the stratigraphic record are often interpreted as the deposits of braided rivers. Alluvial sequence stratigraphic models show these sheet sandstones overlying sequence boundaries and deposited during low rates of base level rise and fall. Since sequence stratigraphic models rely on the balance between creation/destruction of accommodation and sediment supply, is the sheet-like nature of these units controlled by channel style, by aggradation rates tied to base level changes or a combination of both? The lower member of the Castlegate Formation in the Book Cliffs of eastern Utah, U.S.A., was selected to answer these questions regarding the origin and architecture of amalgamated sheet sandstones. The lower Castlegate is 60 m thick and is underlain by a regional sequence boundary. Delineation of the alluvial architecture utilized a combination of outcrop photography, measured sections, and paleochannel dimension estimation at two outcrops in the type area. Architectural components within the outcrops were defined and include 1) basal, thalweg-fill units, 2) inclined sandy or heterolithic barform deposits, and 3) overbank siltstone and mudstone. Thalweg-fill units represent deposition in the topographically lowest part of the channel and are preferentially preserved. Thalweg units exist as single units or as a composite of multiple thalweg units. Overlying thalweg units, inclined barform deposits are in places preserved and genetically linked to an underlying thalweg-fill unit. Bar accretion directions, when combined with paleocurrent indicators, show that lateral, downstream and upstream accretion all occur in the lower Castlegate. Overbank deposits are rarely preserved, but where present they typically overlie bar deposits. The architectural components stack to form a threefold, cyclic hierarchy of 1) channel-bar systems (2–12 m thick), 2) channel belts (2–16 m thick), and 3) stacked channel-belt complexes (17–40 m thick). Complete channel-bar systems are rarely preserved and occur locally as a succession of multiple, stacked thalweg-fill units. The lack of barforms within a composite thalweg unit suggests that channel migration is the dominant process controlling preservation within channel-bar systems, possibly associated with a component of local avulsion. Channel belts are composed of stacked channel-bar systems capped by a barform. The barform is preserved due to a regional avulsion moving the active channel to another location within the floodplain, isolating it from potential reworking. Stacked channel-belt complexes are characterized by alternations of thalweg-dominated and barform-dominated channel belts. The mechanism responsible for deposition at the stacked channel-belt complex scale may be creation/destruction of accommodation associated with base level changes. Paleochannel reconstructions show that channel flow depths using bar and underlying thalweg thickness were up to 7.6 m. Channel depth estimates utilizing a cross-bed set thickness of 26 cm averaged 4.6–7.7 m. These values were used to calculate channel widths which were estimated at 143–365 m up to a maximum of approximately 1 km. Regional paleoflow within the lower Castlegate was dominantly southeast directed. The architectural components, paleochannel characteristics, and paleocurrent data suggest the lower Castlegate member was deposited in low to intermediate-sinuosity rivers influenced by autocyclic processes at smaller scales and allocyclic processes at the larger outcrop scale.

Stratigraphic signatures of translation of thrust‐sheet top basins over low‐angle detachment faults

Abstract Low‐angle detachment faults and thrust‐sheet top basins are common features in foreland basins. However, in stratigraphic analysis their influence on sequence architecture is commonly neglected. Usually, only eustatic sea level and changing flexural subsidence are accounted for, and when deformation is considered, the emphasis is on the generation of local thrust‐flank unconformities. This study analyses the effects of detachment angle and repetitive detachment activation on stratigraphic stacking patterns in a large thrust‐sheet top basin by applying a three‐dimensional numerical model.Model experiments show that displacement over low‐angle faults (2–6°) at moderate rates (∼5.0 m kyr−1) results in a vertical uplift component sufficient to counteract the background flexural subsidence rate. Consequently, the basin‐wide accommodation space is reduced, fluvio‐deltaic systems carried by the thrust‐sheet prograde and part of the sediment supply is spilled over towards adjacent basins. The intensity of the forced regression and the interconnectedness of fluvial sheet sandstones increases with the dip angle of the detachment fault or rate of displacement. In addition, the delta plain is susceptible to the formation of incised valleys during eustatic falls because these events are less compensated by regional flexural subsidence, than they would be in the absence of fault displacement.

Sedimentation in a discharge dominated fluvial-lacustrine system: the Neogene Productive Series of the South Caspian Basin, Azerbaijan

The largely lower Pliocene Productive Series and its regional equivalents contain the major hydrocarbon reservoirs of the South Caspian Basin. Examination of outcrops in the Apsheron region, Azerbaijan, has resulted in re-interpretation of the depositional environments of the Productive Series. The lower part of the Productive Series consists of sandstones and mudstones interpreted as channelised and sheetflood fluvial deposits, intercalated with mudstones that represent short-lived lacustrine transgressions of a regional Caspian lake. The upper Productive Series (Pereriva and younger suites) comprises sandstone-prone intervals, interpreted as deposition during periods of increased fluvial discharge and sediment supply, and mudstone-prone intervals, interpreted as deposition during periods of decreased discharge and therefore coarse-grained sediment starvation. Sand-prone reservoir intervals in the Pereriva and overlying Balakhany suites mainly comprise amalgamated low sinuosity, braided fluvial sheet sandstones. Mudstone-rich intervals form inter- and intra -suite seals, comprising alluvial plain and lacustrine facies. The succession displays an overall fining-up trend, which is controlled by long-term climatic cooling, decreased fluvial discharge and reduced coarse clastic input. The modern Volga/Caspian system is a partial analogue for the lower Productive Series in the Apsheron region, where fluvial and lacustrine conditions alternate on a low-gradient ramp, without evidence for a shelf-break. Lake Eyre, Australia, may be a modern analogue for the upper Productive Series, where there are fewer lacustrine influences discernible in the sedimentology and the overall setting resembles a dryland river/terminal fan.

Fluvial, evaporitic and shallow-marine facies architecture, depositional evolution and cyclicity in the Sivas Basin (Lower to Middle Miocene), Central Turkey

Lower to Middle Miocene rocks in the Karayün region of the Sivas Basin are represented by the Agilkaya and Egribucak Formations, forming a 4 km thick continuous section. Each formation is subdivided into three members, representing similar lithofacies. The lower members are made up of fluvial sheet-sandstone and red mudstone that migrated laterally on a flood basin within a semi-arid fan system. The middle members are composed of bedded to massive gypsum and red-green mudstone of a coastal and/or continental sabkha environment. They are intercalated with lagoonal dolomitic limestone and bituminous shale in the Agilkaya Formation and fluvial red-pink sandstone–red mudstone in the Egribucak Formation. The upper members are made up of shallow-marine fossiliferous mudstone and sandy limestone. Rapid vertical and horizontal facies changes in both formations reflect the locally subsiding nature of this molassic basin. The overall section can be also subdivided into three orders of fining-upward cycles. Type I cycles correspond to the Agilkaya and Egribucak Formations (each around 2 km thick) and indicate two long-term transgressive phases, controlled by tectono-eustatic factors. Type II cycles (140 m thick on average) are formed by alternations of fluvial sheet-sandstone and red mudstone capped by up to 50 m thick red flood basin mudstone. A 400 ka period Milankovitch cyclicity related to astronomical climatic fluctuations is suggested as a governing mechanism, although subsidence cannot be excluded. The existence of laterally persistent, fluvial sheet-sandstone overlain by red mudstone that forms fining-upward Type III cycles (10 m thick on average) is attributed to changes in subsidence rate. Rhythmic alternations between gypsum beds and red-green mudstone and/or dolomitic limestone and bituminous shale, observed in the middle members, might be related to minor periodical climatic changes rather than to tectonics.

Late Archaean superplume events: a Kaapvaal–Pilbara perspective

The 2714–2709 Ma Ventersdorp Supergroup overlies Mesoarchaean basement rocks and sedimentary strata of the Neoarchaean Witwatersrand Supergroup. The latter basin was inverted by tectonic shortening and suffered the loss of up to 1.5 km of stratigraphy prior to deposition of the Ventersdorp volcanics. Thermal uplift and fluvial incision prior to the basal Klipriviersberg Group flood basalts appear to have been limited, but this could also reflect a hot dry palaeoclimate acting on a peneplained plateau. Rapid ascent of ponded magma beneath thinned sub-Witwatersrand lithosphere, transported laterally from a mantle plume starting head possibly situated marginally to the Kaapvaal craton is inferred for this unit of up to 2 km of predominantly tholeiitic basalts with subordinate, basal komatiites. Crustal extension related to ascent of the ponded magma followed, leading to the formation of a set of graben and half-graben basins, in which immature clastic sedimentary, and felsic to mafic lavas and pyroclastics of the Platberg Group were laid down. The Platberg basins show no evidence for reactivation of pre-existing crustal structures. The Fortescue Group of the Pilbara craton has an analogous lower flood basaltic succession, followed by graben-fills similar to those of the Platberg Group. Differences in the Fortescue include evidence for significant thermal uplift prior to the onset of volcanism, subaqueous basalts in the south of the Pilbara craton, evidence for two episodes of flood basaltic volcanism, possibly related to two plumes at c. 2765 and 2715 Ma, and graben basins aligned along existing cratonic structures. Both Kaapvaal and Pilbara flood basalts and graben-related sedimentary-volcanic deposits are thought to have been part of a c. 2.7 Ga global superplume event. The plume inferred for the Fortescue Group flood basalts was probably related to rifting and the breakup of a plate larger than the preserved Pilbara craton. Uppermost Ventersdorp units (Bothaville Formation terrestrial clastic and Allanridge Formation tholeiitic rocks) suggest a combination of thermal subsidence, allied to continued plume (minor komatiites) and graben basin influences. In the Kaapvaal craton, the Transvaal Supergroup lies unconformably above the Ventersdorp. Basal “protobasinal” successions reflect discrete fault-bounded basin-fills, analogous to those of the Platberg Group; however, it is inferred that the former depositories were related to craton marginal plate tectonic influences, specifically the c. 2.6 Ga Limpopo orogeny. Thin fluvial sheet sandstones of the Black Reef Formation unconformably succeed the protobasinal rocks and reflect the transition to an epeiric drowning of much of the Kaapvaal craton. A shallow shelf carbonate-banded iron formation platform succession (Chuniespoort-Ghaap Groups) developed in two sub-basins on the Kaapvaal craton. They are mirrored by the approximately coeval Hamersley chemical epeiric sediments on the Pilbara craton, and both Kaapvaal and Pilbara transgressive successions are related here to a possible second, c. 2.5 Ga superplume event, which raised sea levels globally. Evidence for the younger superplume event is less clear than for the c. 2.7 Ga event.

Major influences on the evolution of the 2.67–2.1Ga Transvaal basin, Kaapvaal craton

The Transvaal is one of three structural basins of the Transvaal Supergroup preserved on the Kaapvaal craton. The evolution of the Neoarchaean–Palaeoproterozoic Transvaal basin is ascribed predominantly to magmatism, palaeoclimate and eustasy, with plate tectonics playing a sporadic role. The supergroup comprises basal ‘protobasinal’ rocks, followed by the Black Reef Formation, Chuniespoort Group and uppermost Pretoria Group. Immature siliciclastic and bimodal volcanic rocks of the protobasinal unit reflect a wide zone of rifting related to the c. 2.7 Ga Ventersdorp (Supergroup) mantle plume. Individual protobasinal successions were laid down in separate fault-bounded basins, controlled at least partially by greenstone belt orientations in the Kaapvaal basement. Post-magmatic, post-rifting thermal subsidence accommodated Black Reef fluvial sheet sandstones and the subsequent thick carbonate-BIF epeiric platform succession of the Chuniespoort Group. Subordinate mechanical subsidence accompanied this long-lived thermal relaxation. Intense weathering due to Neoarchaean atmosphere composition greatly reduced clastic sedimentation and the greenhouse palaeoclimate further encouraged carbonate sedimentation. Globally enhanced sea levels, due to enhanced mid-ocean ridge growth consequent upon either global magmatic events or supercontinent break-up, also played a pivotal role in Chuniespoort epeiric basin evolution. Pretoria Group sedimentation is ascribed to two cycles of rifting and subsequent thermal subsidence. The first cycle appears to reflect plate tectonically induced rifting, with an epeiric sea drowning the rift basin despite reduced sea levels due to the first major global glaciation. The second Pretoria cycle is most likely related to a major continental flood basalt event, with thermal subsidence allowing a second and probably larger epeiric sea to advance onto the northern Kaapvaal craton. Evaluation of the inferred relatively minor importance of plate tectonics in the evolution of the essentially intracratonic Transvaal basin depends to a large degree on the age assigned to the Limpopo orogeny during which the Zimbabwe and Kaapvaal cratons collided; recent evidence for a c. 2.0 Ga collision supports the concept that catastrophic global magmatic events gradually gave way to plate tectonics during Transvaal basin evolution.

Bogi and Capiron fields, Oriente Basin, Ecuador: Similar Reservoirs but Contrasting Drive Mechanisms and Recoveries: ABSTRACT

Bogi and Capiron fields are being developed under a unit agreement with Petroecuador. These adjoining fields straddle Block 16 in the Oriente Basin and probably share a common oil water contact. Both fields are simple four-way-dip closures which produce heavy oil from Campanian sandstones of similar quality. However, the two fields are remarkably different in terms of oil production and projected recovery as a result of differing structural closures, reservoir distributions and, hence, differing drive mechanisms. The main reservoir at Bogi field is an amalgamation of two fluvial sheet sandstones thought to be low-stand deposits associated with two falls in relative sea level. The reservoir is thick (56-78 ft) and, with an observed oil column of only 38 feet, a bottom-water drive mechanism is ubiquitous. The oil is heavy (18 API) and mobility ratios unfavorable; water production is high and oil recovery from conventional drilling is expected to be 3-5%. In contrast, only the upper fluvial sheet sandstone is present in Capiron field and a reservoir thickness of 32-48 ft combined with an oil column of 99 ft ensures an edge-water drive mechanism over most of the field with concomitant initial low water production and oil recoveries of approximately 30%.more » The contrast between Bogi and Capiron fields highlights the problems and challenges in the Block 16 area. Small structural closures filled with heavy oil are abundant and an accurate seismic depth map coupled with an understanding of reservoir distribution are vital to economic success.« less

Complex Fluvial Response to Low Gradients at Maximum Regression: A Genetic Link Between Smooth Sequence-Boundary Morphology and Architecture of Overlying Sheet Sandstone

ABSTRACT The Mesa Rica Sandstone is an extensive fluvial sandstone sheet that is separated from underlying marine strata throughout northeastern New Mexico by a regionally smooth and laterally continuous sequence-bounding unconformity. The Mesa Rica Sandstone consists mostly of channel-fill elements, with lesser proportions of lateral-accretion elements, and almost everywhere is of the same thickness as the largest channel fills. This broad sandstone sheet reflects deposition on a coastal plain by frequent avulsion and minor lateral migration of straight and low-sinuosity streams under nearly stable base-level conditions, which are here attributed to rapid progradation of the adjacent Mesa Rica deltaic shoreline relative to change in relative sea level during maximum Kiowa-Skull Creek regressi n. Rapid deltaic shoreline progradation under such conditions caused seaward extension and gradient reduction in the adjacent Mesa Rica coastal plain. Overextension and resultant loss of power in Mesa Rica rivers crossing this coastal plain forced both stream straightening and storage of nontransportable coarser sediment in coastal-plain channels. Because base level was stable, Mesa Rica channels could aggrade only minimally, and were forced to spread excess sediments mostly laterally by avulsion. Frequent avulsion and minor lateral migration of Mesa Rica channels under these conditions caused both regional scouring of a smooth sequence-bounding unconformity and deposition of a sheet sandstone with minimal channel stacking. The Mesa Rica Sandstone and the sequence boundary that underlies it represent a distinctive genetic association of single-story fluvial sheet sandstone and smooth sequence-boundary morphology that is probably not unique to the Lower Cretaceous of northeastern New Mexico. Such unincised sequence boundaries are also implied in conceptual models and flume experiments of previous authors.

Correlation of the major late Jurassic ?early Tertiary low- and highstand cycles of south-west Egypt and north-west Sudan

The mainly continental deposits of northwest Sudan and south-west Egypt have been correlated with coeval shallow marine and marine deposits in northern Egypt along a north-south running cross-section, based on surface and subsurface data. The palaeodepth curve of northern Egypt illustrates the gradual seal-level rise, reaching its maximum during the Late Cretaceous with conspicuous advances during the Aptian and late Cenomanian. A general highstand is also recorded during the Campanian-Maastrichtian in north-west Sudan. A detailed facies correlation is given for the Aptian and late Cenomanian highstand in western Egypt. The correlation of the Cenomanian Bahariya and Maghrabi formations displays short-term relative sealevel fluctuations. The interpretation illustrates the extensiveness of related erosional processes in the hinterland, partly intensified by temporarily uplift of the Uweinat-Aswan High in the south. Regional uplift and constant erosion took place in south-west Egypt during Coniacian and Santonian times. The regional stratigraphic gaps and uncertain interpretation of the Bahariya Uplift are induced by the influence of the Trans-African Lineament, especially during the Late Cretaceous. Low-stand fluvial sheet sandstones characterized by non-cyclic sequence development and high facies stability occur, especially in the Neocomian and early Turonian. During the Barremian and Albian, fluvial architecture changes to more cyclic fluvial sequences and increasing soil formation, due to increasing subsidence, more humid climatic conditions and the generally rising sea level, culminating in the extensive shallow marine Abu Ballas and Maghrabi formations.

Fluvial Sheet Sandstone Development Above a Tectonically-Controlled Sequence Boundary: Canyon Creek Member, Erickson Sandstone (Campanian), Rock Springs Uplift, Wyoming: ABSTRACT

Fluvial Sheet Sandstone Development Above a Tectonically-Controlled Sequence Boundary: Canyon Creek Member, Erickson Sandstone (Campanian), Rock Springs Uplift, Wyoming: ABSTRACT

Sedimentology of the Rough Rock: a Carboniferous braided river sheet sandstone in northern England

Abstract The Rough Rock is a coarse-grained fluvial sheet sandstone which outcrops across northern and central England. It is interpreted as a braided river sandstone on the basis of sedimentary structures, internal erosion and growth surfaces. This study reveals that the Rough Rock represents more than a single phase of deposition and that internal erosion surfaces may indicate anabranch avulsions or more significant river avulsions. Detailed outcrop profiles are presented to try and distinguish the styles of deposition and the nature of internal bounding surfaces and channel geometry. The Rough Rock Formation contains a very large number of first and third order bedding contacts, which represent the migration of dunes and channels respectively, there are relatively few macroforms and bars preserved. The lack of second order surfaces within many outcrops could indicate that bars in the Rough Rock rivers were very large and therefore cannot be resolved at the outcrop scale. Similarly, the rivers which deposited the Rough Rock may have had such high width: depth ratios that the dip of second order bounding surfaces are too low to be identified. The large number of third order bounding surfaces indicates channel stacking which may have occurred in two ways: (1) relocation of rivers in the same area over time, or, (2) anabranch avulsion within a braided river within a hierarchy of channels. The problems associated with differentiating between these two mechanisms are discussed.

Sequence Stratigraphic Interrelationship of Lower Cretaceous Dakota and Purgatoire Formations in Northeast New Mexico/Southeast Colorado and Correlative Strata (Muddy, Skull Creek, Plainview) of the Denver Basin

The Albian Glencairn Member (Purgatoire Formation) and underlying Dakota Sandstone of southeastern Colorado and northeastern New Mexico are related depositionally to the Tucumacari, Mesa Rica, and Pajarito formations of east-central New Mexico and to the Plainview, Skull Creek, and Muddy formations of central Colorado. Depositional interrelationships of these strata are best understood when placed in a sequence-stratigraphic framework. The Plainview Formation, Long Canyon sandstone bed (basal Glencairn) and Campana sandstone bed (basal Tucumcari) overlie a correlative lowstand surface of erosion (LSE) and represent backfilling of valleys during Kiowa-Skull Creek transgression. These strata are separated from overlying marine transgressive shale deposits of the lower Skull Creek, Glencairn, or Tucumcari, respectively, by a correlative transgressive surface of erosion. Fluvial incision during maximum Kiowa-Skull Creek regression is manifest as an LSE atop Skull Creek and Glencairn marine deposits. Southward-flowing streams debouched into the maximum regressive sea forming a lowstand wedge, the remnants of which are represented by the Mesa Rica, Pajarito, and uppermost Tucumcari formations. Stable base level conditions developed near the maximum regressive shoreline resulting in widening of paleovalleys and deposition of a fluvial lowstand sheet sandstone in southeastern Colorado and northeastern New Mexico. Transgression followed lowstand deposition and resulted in backfillingmore » of paleovalleys represented by portions of the Muddy and Dakota sandstones.« less

Facies associations and sand-body geometries in the Ness Formation of the Brent Group, Brent Field

Summary The Brent Group in the Brent Field contains over 1000 × 10 6 bbl of recoverable oil in a simple gently dipping half-graben structure. It is 243 m (800 ft) thick and represents a sequence amplified by synsedimentary fault-block subsidence close to the axis of the Viking Graben. On a regional scale the depositional model is of a major shoreface and coastal sequence, the Rannoch and Etive Formations, underlying the partly time-equivalent shallow lagoonal to emergent delta-plain deposits of the Ness Formation. However, more localized and detailed sedimentary modelling is required to predict reservoir geometries and production behaviour, particularly in the Ness Formation. The Ness Formation is the largest reservoir-bearing unit in the Brent Field, being up to 160 m (525 ft) thick. Five main facies associations form reservoir-quality rock: varied channel sandstones, fluvial sheet sandstones, major mouth bars, lagoonal shoals (which are usually associated with, and capped by, thin beaches) and lagoonal sheet sandstones. These associations make up 50%–60% of the sand-rich sequence and are embedded in the non-reservoir facies associations: lagoonal mudstones, emergent flood-plain mudstones and allochthonous and autochthonous coals. The coals are often field wide in extent. Within the amplified Ness Formation the most important environments were the lagoonal deltas which were wave and fluvial dominated with minor tidal influence. As a result of this importance the sand bodies produced by these deltas are subjected to a more detailed description in this paper. Wave energies were sufficient to redistribute large amounts of sediment and indicate significant fetch in the brackish lagoons diluted by major freshwater input. Sand-body geometries are very variable, reflecting both localized controls on sediment distribution, particularly immediately above the compacting shoreface pile, and more regional effects including coarse-sediment supply, basinal processes and fault-block subsidence. The dominant controls on sedimentation resulted in regular drowning events on the delta plains and had a significant impact by producing a strongly layered reservoir.

Architecture of the Castissent fluvial sheet sandstones, Eocene, South Pyrenees, Spain

ABSTRACTThe Castissent Formation represents a phase of strong fluvial progradation within the fluvio‐deltaic Montanyana Group, probably enhanced by the late Ypresian sea‐level fall. The structural setting is characterized by the emplacement of the South Pyrenean nappes. The major architectural feature of the Formation is the superposition of three multilateral and multistorey sheet sandstone complexes (A, B and C), interbedded in finer floodplain deposits and dark‐grey mudstones of brackish transitional origin. The structure of the sheet sandstone complexes is exemplified by the detailed analysis of the lower sheet sandstone complex (A), 25 m thick, 6 km wide and exposed over 25 km of its length. Three types of channel‐fill sandstones serve as components: (1) lenticular‐bedded bodies, interpreted as braided stream deposits, (2) lateral accretion‐bedded bodies, intepreted as meandering stream deposits, and (3) ribbons, originating from laterally stable and short‐lived offshoot or crevasse channels. By coalescence and/or stacking of these channel‐fill types, higher‐order architectural units (A1, A2, A3) have been formed that together build the sheet sandstone complex. On the basis of downstream variation in proportion and geometrical relation between the channel‐fill components, three depositional zones have been differentiated within the fluvial system. Each zone is characterized by specific architectural processes: (1) alternation of minor aggradational‐degradational cycles in the proximal valley, (2) generation of new alluvial belts in the intermediate nodal avulsion zone, and (3) successive random avulsion processes with development of semipermanent crevasse channels or minor distributaries in the distal distributive zone. The sedimentation of the Castissent Formation was structurally controlled by an interplay of vertical basement movement due to thrust stacking in the hinterland and surficial thrust displacement to the foreland resulting in alternating southward and northward shift of the fluvial system.

Arumbera Sandstone: A Possible Late Proterozoic-Early Cambrian Deltaic Complex, Central Australia: ABSTRACT

A detailed investigation of the Arumbera Sandstone was undertaken in the northeastern Amadeus basin, central Australia, where the unit forms distinctive strike ridges with orange-white cliffs and dark reddish slopes. The Arumbera is divisible into four informal, readily mappable units. Approximately 80% of the average stratigraphic section is composed of recessive, pale-red, thin to medium-bedded, fine to medium-grained arkose with major proportions of siltstone and mudstone. These sediments are interpreted as a complex assemblage of coastal and nearshore marine environments including tidal flats, tidal channels, estuaries, and beaches. Evidence includes: (1) predominance of alpha, beta, and xi cross-stratification with common herringbone laminae, hummocky cross-strata, planar foreshore stratification, and flaser bedding; (2) bimodal paleocurrents; (3) records of intermittent subaerial exposure; and (4) rare to abundant marine trace fossils. The remaining 20% of the Arumbera is composed of cliff-forming orange-white thick-bedded, fine to medium-grained arkose and lithic arkose with pebble to cobble conglomerate. This facies probably is a fluvial sheet sandstone. It is characterized by: (1) pi and omicron cross-stratification; (2) general paucity of mudrocks, but abundant shale pebbles; (3) unimodal, northeast-oriented paleocurrents; (4) wedging channel-sand bodies; (5) absence or extreme rarity of trace fossils; (6) sheet-like geometry; and (7) decrease in maximum grain size to the northeast. The Arumbera probably was deposited in a coastal environment unrestricted by vascular land plants, but perhaps analogous in other ways to the delta of the modern Godavari River of India. Evidence includes: (1) a pronounced depocenter for the unit in the central part of the study area (thickness northeasterly from 216 to 1,123 m in 80 km); (2) unidirectional paleocurrents from fluvial sheet sands that radiate to the north, northeast, east, and southeast; (3) fluvial and coastal deposits in vertical, repetitive succession; and (4) east and northeast-trending zones of thicker deposits within fluvial sheet sands which may be distributary lobes. End_of_Article - Last_Page 559------------

Reservoir Geometry of a Fluvial Sheet Sandstone

Fluvial sheet sandstone provides an ideal reservoir for hydrocarbons. Such a sandstone body is large enough to contain giant hydrocarbon reserves as well as an aquifer that could flush the hydrocarbons. The outcrops of the Westwater Canyon Member in the Morrison Formation (Jurassic), northwestern New Mexico, show representative attributes of a fluvial sheet sandstone. This sandstone body consists of coalescing fluvial-channel systems that in turn are composed of still smaller coalescing individual channels. The sheet is more than 63 mi (100 km) wide transverse to the paleocurrent direction and averages 200 ft (61 km) in thickness; the downcurrent dimension is not preserved but may have been as much as 100 mi (160 km). Channel systems internal to the sheet average 7 mi (11 km) wide and 50 ft (15 m) deep. The individual channel deposits that form the systems average 600 ft (183 m) wide by 12 ft (4 m) thick. Any one vertical column through the sheet sandstone is usually more than 90 percent sandstone. Further, each vertical column consists of repetitive incomplete (top-missing) fluvial sequences that show upward-decreasing grain sizes and scales of sedimentary structures. Fluvial sheet sandstone probably is deposited by aggrading and coalescing braided-stream systems.