Fluvial Settings Research Articles

The ichnologic record of the continental invertebrate invasion; evolutionary trends in environmental expansion, ecospace utilization, and behavioral complexity

The combined study of continental trace fossils and associated sedimentary facies provides valuable evidence of colonization trends and events throughout the Phanerozoic. Colonization of continental environments was linked to the exploitation of empty or under-utilized ecospace. Although the nonmarine trace fossil record probably begins during the Late Ordovician, significant invasion of nonmarine biotopes began close to the Silurian-Devonian transition with the establishment of a mobile arthropod epifauna (Diplichnites ichnoguild) in coastal marine to alluvial plain settings. Additionally, the presence of vertical burrows in Devonian high-energy fluvial deposits reflects the establishment of a stationary, deep suspension-feeding infauna of the Skolithos ichnoguild. The earliest evidence of plant-arthropod interaction occurred close to the Silurian-Devonian boundary, but widespread and varied feeding patterns are known from the Carboniferous. During the Carboniferous, permanent subaqueous lacustrine settings were colonized by a diverse, mobile detritus-feeding epifauna of the Mermia ichnoguild, which reflects a significant paleoenvironmental expansion of trace fossils. Paleozoic ichnologic evidence supports direct routes to the land from marginal marine environments, and migration to lakes from land settings. All nonmarine sedimentary environments were colonized by the Carboniferous, and subsequent patterns indicate an increase in ecospace utilization within already colonized depositional settings. During the Permian, back-filled traces of the Scoyenia ichnoguild record the establishment of a mobile, intermediate-depth, deposit-feeding infauna in alluvial and transitional alluvial-lacustrine sediments. Diversification of land plants and the establishment of ecologically diverse plant communities through time provided new niches to be exploited by arthropods. Nevertheless, most of the evolutionary feeding innovations took place relatively early, during the Late Paleozoic or early Mesozoic. A stationary deep infauna, the Camborygma ichnoguild, was developed in Triassic transitional alluvial-lacustrine deposits. Terrestrial environments hosted the rise of complex social behavioral patterns, as suggested by the probable presence of hymenopteran and isopteran nests in Triassic paleosols. An increase in diversity of trace fossils is detected in Triassic-Jurassic eolian deposits, where the ichnofauna displays more varied behavioral patterns than their Paleozoic counterparts. Also, a mobile, intermediate-depth, deposit-feeding infauna, the Vagorichnus ichnoguild, was established in deep lake environments during the Jurassic. In contrast to Paleozoic permanent subaqueous assemblages typified by surface trails, Jurassic ichnocoenoses are dominated by infaunal burrows. High density of infaunal deposit-feeding traces of the Planolites ichnoguild caused major disruption of lacustrine sedimentary fabrics during the Cretaceous. Most insect mouthpart classes, functional feeding groups, and dietary guilds were established by the end of the Cretaceous. Diversification of modern insects is recorded by the abundance and complexity of structures produced by wasps, bees, dung-beetles, and termites in Cretaceous-Tertiary paleosols. The increase in bioturbation migrated from fluvial and lake-margin settings to permanent subaqueous lacustrine environments through time.

Exploration and Development of the Fluvial Deposits in the Potrerillos Formation, Cuyana Basin, Mendoza, Argentina: ABSTRACT

In the Barrancas area are located the oil fields related to structures developed in the Oriental axis of the Cuyana Basin. The anticlinal axis of the Barraticas oil field has a NNW-SSE strike and dips toward North. The Oriental flank is abruptly bounded by faulting whilst the Occidental one is gently dipping. The Barrancas oil field was developed between the 1940s and 1960s, having both Early Jurassic conglomerates of the Barrancas Formation and Triassic fluvial braided deposits from Las Cabras Formation as the main targets. The review of new plays within the Triassic fluvial cycles belonging to the Potrerillos Formation was encouraged by the success obtained in the recovery of important volumes of oil from the overlying Formations. The application of a sequence stratigraphy approach to the fluvial settings combined with petrophysics data from outcrop studies and a 3D seismic interpretation allowed a new play concept to be conceived. This new play concept will have application in zones far away from the study area, where Potrerillos Formation has exploration interest. A team integrate by geologists, geophysicists, and engineers has produced new opportunities on mature oil fields with up to 300 drilled wells.

The stratigraphy, depositional environments and periods of deformation of the Wajid outcrop belt, southwestern Saudi Arabia

The Wajid outcrop belt in southwestern Saudi Arabia is an area of over 22,000 square kilometers where strata of Middle(?) Cambrian through to Jurassic crop out. The Early Palaeozoic sediments in southwestern Arabia have been placed in the Wajid sandstone. This paper elevates this unit to group status and recognizes its former members as formations. The remaining Palaeozoic successions are placed in the Qalibah and Khuff formations. The Middle(?) Cambrian through to Early(?) Ordovician Dibsiyah Formation, Wajid Group, correlates with the Saq sandstone of central and northern Saudi Arabia. The ‘lower’ Dibsiyah buries a topography developed on the Arabian shield during the Late Precambrian Pan-African and/or the late Early Cambrian periods of uplift. The late Early Cambrian period of deformation is documented in southwestern Saudi Arabia and adjacent Oman by an angular unconformity which separates the Dibsiyah Formation correlative from the underlying Late Precambrian-Early Cambrian Lower Haima Group. The ‘lower’ Dibsiyah was deposited in a northward prograding fluvial apron. The ‘upper’ Dibsiyah was deposited in intertidal to shallow subtidal marine settings during a period of coastal onlap. The Late(?) Ordovician Sanamah Formation overlies the Dibsiyah Formation in angular and/or erosional unconformity. This unconformity was developed during a period of regional uplift which began near the close of the Middle Ordovician. The Sanamah Formation was deposited in broad valley complexes excavated in fluvial settings. The Early Silurian Qusaiba Member of the Qalibah Formation unconformably overlies the Sanamah. The Qalibah Formation was deposited in shallow marine to mid-shelf settings during a major period of coastal onlap. The angular unconformity to disconformity, which separates the Qalibah Formation from underlying units, resulted from diastrophism of Late Ordovician to earliest Silurian age. The Early Devonian to Middle(?) Carboniferous Khusayyayn Formation unconformably overlies the Qalibah Formation. The Khusayyayn Formation was deposited in shallow marine, lagoonal, eolian and fluvio-deltaic settings. The unconformity separating the Khusayyayn Formation from the Qalibah Formation documents a period of regional diastrophism of late Early Silurian to late Early Devonian age. Unconformably overlying the Khusayyayn is the Middle Carboniferous through Early Permian Juwayl Formation. This unit was deposited in broad valley complexes excavated in fluvial settings. The base Juwayl unconformity was produced by the Hercynian orogeny. The Permian Khuff Formation unconformably overlies the Juwayl and the former deposited in shallow marine settings. The base Khuff Formation unconformity resulted from a period of major coastal onlap. The Early Triassic Sudair shale succeeds the Khuff and the former was deposited during a period of coastal offlap in lagoonal to coastal plain settings. Near the end of the Early Triassic the Arabian Peninsula underwent another period of tectonism. In central and southwest Saudi Arabia this event is reflected by the erosion of the Sudair shale and older units from the higher portions of reactivated fault blocks and the syndepositional thickening of the Jilh Formation on the down thrown sides of these fault blocks.

Plant and mammal diversity in the Paleocene to early Eocene of the Bighorn Basin

Abundant plant and vertebrate fossils have been recovered from fluvial sediments deposited in the Bighorn Basin, Wyoming, during the first 13 m.y. of the Tertiary. Here we outline and discuss changes in the composition and diversity of floras and faunas during this period, which includes the recovery of terrestrial ecosystems from the K/T boundary extinctions, and later, during the Paleocene-Eocene transition, the greatest global warming of the Cenozoic. Floral diversity has been studied at three levels of spatial resolution: sub-local (at individual collecting sites), local (along a single bed or stratigraphic horizon), and basin-wide (regional). Sub-local diversity shows a moderate increase from the early to late Paleocene, followed by a decrease across the Paleocene/Eocene boundary, then an increase into the later early Eocene. Local heterogeneity was lower in Paleocene backswamp floras, although distinct groups of species dominated in different local fluvial settings such as backswamps and alluvial ridges. Heterogeneity of backswamp forests increased by about 65% from the early to late Wasatchian (early Eocene). The number of plant species inferred from the Bighorn Basin dataset rose gradually from the Puercan to an early Clarkforkian peak of about 40 species, declined sharply to about 25 species by the Clarkforkian/Wasatchian boundary, then rose through the Wasatchian to about 50 species. A regional analysis of mammalian genera shows high turnover and a rapidly increasing number of genera within a million years of the K/T boundary (10–50 genera), a slight decline to 40 genera by the early Clarkforkian, then an increase from 40 to 75 genera by the late Wasatchian. Our analyses found no major extinctions in mammals during the Paleocene and early Eocene in the Bighorn Basin, but a one-third decrease in the number of plant species at about the Paleocene/Eocene boundary. Rates of taxonomic turnover were much higher for mammals than plants. The diversity trends for plants and mammals show little congruence, implying that the two groups responded in a very different manner to post K/T extinction opportunities. There is also little congruence between plant diversity levels and change in mean annual temperature (MAT) as inferred from foliar physiognomy.

Stratigraphy, depositional history and environments of deposition of Cretaceous through Tertiary strata in the Lamu Basin, southeast Kenya and implications for reservoirs for hydrocarbon exploration

The Lamu Basin, which is characterized by an extensional tectonic style, is the failed arm of a tri-radial rift system and possesses tectonic and stratigraphic elements in the post-rift series that are analogous to those of the west coast of Africa. Development of the southern part of the basin as a passive margin, is closely related to considerations of the pre-drift position of Madagascar and formation of the Indian Ocean basin during Mesozoic time. Cretaceous and Tertiary strata in the basin comprise an eastward-thickening gross succession of sediments on which eustatic sea-level fluctuations and a sequence of unconformities related to pulses of transgressive and regressive depositional trends, are superimposed. Recognition of these trends has provided the basis for classification of the strata into megasequences representing distinct provinces with regard to time stratigraphy, sedimentation, tectonics, depositional environments and hydrocarbon potential. Megasequence I (Karoo Group and Jurassic) includes strata of the Permian through Jurassic, the discussion of which is beyond the scope of this paper. Megasequence II (Sabaki Group) includes strata of the Cretaceous and Early Paleocene, deposited in tide-influenced shelf and marine settings. These are, in ascending order: the Ewaso Sands, the Walu Shale, the Hagarso Limestone, the Freretown Limestone and the Kofia Sands. Megasequence III (Tana Group) includes strata of the Eocene through Oligocene, which were deposited in fluvial, deltaic, and restricted-shelf settings. These are, in ascending order: the Kipini Formation, the Pate Limestone, the Linderina Limestone, the Dodori Limestone and the Barren Beds Formation. Megasequence IV (Coastal Group) includes strata of the Miocene through Pliocene, which were deposited in restricted-shelf, middle- to outer-shelf, deep-marine and fluvial settings. These are, in ascending order: the Baratumu Formation, the Lamu Reefs, the Simba Shales and the Marafa Formation. Determination and description of these lithostratigraphic units, their distribution and palaeohistory have been imperative for predicting reservoirs for hydrocarbon exploration.

Cores from the Miocene Castor Creek Member of the Fleming Formation, Fort Polk, Louisiana: Relationship to the Outcropping Miocene Terrestrial Vertebrate Fossil-Bearing Beds: ABSTRACT

ABSTRACT A series of clays and silty clays with moderately developed paleosols were recovered from three 15 m cores drilled in the Castor Creek member or the Fleming Formation in western Louisiana. These cores were taken to provide stratigraphic control for the outcropping conglomeratic beds bearing small Miocene terrestrial mammal teeth. These fine-grained core sediments and outcropping conglomeratic units were deposited in a fluvial setting. The conglomeratic units were not penetrated by coring indicating that they are very discontinuous, localized deposits. The sedimentology and distribution of these deposits and a thin interval of poorly sorted, cross-bedded sandstone found in one core, suggest that these coarse deposits formed at the bases of minor channels or gullies. The fine-grained core sediments are floodplain deposits. These thick sequences of massive gray clays with paleosol features such as mottles, roots, root casts, carbonate nodules and slickensides reflect deposition in moderate to well-drained, seasonally wet floodplain backswamps. Intervals of relatively unaltered parallel and wavy laminated silts and clays may represent crevasse-splay development or lacustrine delta-fill. Moderately developed soil profiles, containing A, B, and C horizons, characterize these fine-grained sediments. Mottle coloring indicating oxidation and possible water table fluctuations is prevalent throughout the paleosols. Roots and large root burrows which characterize A horizons are abundant. Stage II carbonate development is common in most B and some C horizons. Slickensides, resulting from the alternating wetting and drying of soils, are common. In general, these paleosol features support the interpretation of a moderately-drained, seasonally wet fluvial setting.

Application of Sequence Stratigraphy to the Prioritization of Incremental Growth Opportunities in Mature Reservoirs: An Example from Frio Fluvial-Deltaic Sandstones, T-C-B Field, South Texas: ABSTRACT

ABSTRACT The U. S. Department of Energy has identified mature fluvial-deltaic reservoirs as being the highest priority reservoir type for near-term domestic reserve-growth potential. Detailed characterization studies to locate the estimated 15 billion barrels of mobile oil remaining in these reservoirs must focus first on those reservoirs with the greatest potential if they are to be economically successful. In quick-look analyses that estimate reserve-growth potential, stratigraphic heterogeneity is the most difficult factor to evaluate. Through detailed study of Frio Formation upper delta-plain fluvial reservoirs in T-C-B field, south Texas, and by using outcrop observations from the Ferron Sandstone, central Utah, a model has been developed that relates stratigraphic heterogeneity to rate of accommodation during deposition, a feature that varies predictably throughout a depositional cycle. Upper delta-plain fluvial channelbelts deposited at the base of a depositional cycle, during periods of low accommodation, are narrow, few in number, and relatively internally homogeneous. As a result, they are not effectively contacted by rigid well patterns but can drain large reservoir volumes through individual completions. Because they are commonly isolated within floodplain mudstones, they possess the potential for stratigraphically trapped accumulations away from the structural crest. In contrast, channelbelts deposited at the top of a depositional cycle, during periods of high accommodation, are broad and internally heterogeneous. Despite appearing laterally continuous, they are extensively compartmented by high volumes of low-permeability siltstones and shales, as well as mudclast-rich lag deposits that drape channel boundaries and limit fluid flow at channel-on-channel contacts. AU other factors being equal, these channelbelt types possess the greatest reserve-growth potential because past completions have only contacted small reservoir volumes. This accommodation-based model for reservoir heterogeneity in upper delta-plain fluvial settings represents the first in a new generation of reservoir models. Similar models developed for reservoirs deposited in the spectrum of depositional settings will improve the effectiveness and increase the use of detailed reservoir characterization studies, resulting in the improved identification and recovery of oil and gas remaining in mature reservoirs throughout the United States.

Paleodepositional setting of the Late Cretaceous Pautût Flora from West Greenland as determined by sedimentological and plant taphonomical data

Preservation of the Santonian/Campanian Pautût Flora in West Greenland is mainly the result of a high energy river system that flowed across a well vegetated delta plain to carry and deposit plant parts and terrestrial clastic sediments into a low energy marine setting. A diverse flora is represented mainly in three depositional settings: (1) a dominant angiosperm vegetation preserved with fern and a few conifer species is found within backswamp lakes possibly near rivers; (2) conifer organs and a sparse angiosperm leaf association occur in nearshore marine deltaic deposits dominated by fluvial sedimentation; and (3) a unique Pautût angiosperm vegetational assemblage with a few conifer and cycad species preserved in a brackish to fully marine bay setting. Fossil plant material preserved in the backswamp lakes and fluvial settings probably were deribed mostly from local sources indicating that angiosperms were able, to a degree, to invade into the backswamp Arctic habitats considered to have been dominated by conifers. However, these backswamp habitats probably were short-lived oxbow lakes with a successional angiosperm vegetation. Plant material in the fully marine setting is allochthonous, very diverse, and probably derived mostly from the upper delta palin. The observed differences within the fossil plant assemblages deposited in the marine setting, those containing mostly conifer parts occur in nearshore marine environments as compared to assemblages with mostly angiosperm leaves in more distal marine environments, are used to interpret sorting of different plant parts due to individual taphonomical characters. This means that the heavier conifer cones in the nearshore marine setting sank sooner than the lighter leaves, which were dispersed further out to sea. Strengthening the argument for sorting is that both types of plant parts had a final resting position due to subaqueous transport. The unique angiosperm leaf assemblage found in the fully marine setting is used to support the interpretation that the upper delta plain levees, or extrabasinal sites, were inhabited by a vegetational community distinct from that near the backswamp areas of the lower delta plain. There are a few angiosperm species in common between the distal and nearshore marine and the backswamp lake settings. These species are found througout the Pautût area and exemplify a riparian element which was more prone to enter a depositional regime. Nearshore habitats were inhabited by non-woody plants.

Fluviomarine Sequences in the Lower Cretaceous of the West Netherlands Basin

The Lower Cretaceous of the West Netherlands Basin contains fluvial deposits of the Delfland Group, overlain by shallow marine sediments of the Rijnland Group. Sequence stratigraphy was applied to both succession, to study the nature of the transgression and to identify possible reservoir-seal pairs. A total of six depositional sequences has been identified using extensive new biostratigraphy and sedimentology. Three of the sequences contain fluvial sediments only, two are mixed fluvial and marine, and one is fully marine. The typical thickness per sequence is 200-400 m. Characteristic for the fluvial sequences is an overall fining-upward pattern. The sequence boundaries have been correlated on logs and on regional seismic. Each of the main depositional settings displays a characteristic seismic facies, which has been used to reconstruct the sedimentological facies distribution. The results demonstrate that the source direction of the Delfland Group was predominantly southeast, controlled by the tectonic grain. Strong thickness variations within the lowermost sequence indicate syndepositional rifting, which confined the main channel systems. The major intervening flood-plain shales correspond to base-level highstands, which implies they can be correlated regionally and may act as intraformational seals. The marine transgression entered the basin from the north during the late Valanginian. Bymore » the early Barremian, it covered the entire basin. Retrograding barrier sands from the transgressive systems tract are the best reservoirs within the marine succession. In both fluvial and marine settings, the use of sequence stratigraphy has enabled recognition of several types stratigraphic traps.« less

Identification and significance of aeolian deposits within the dominantly fluvial Sherwood Sandstone Group of the East Irish Sea Basin UK

Abstract The Triassic Sherwood Sandstone Group sediments of the East Irish Sea Basin are over 4000 ft thick and comprise medium- to coarse-grained sandstones and rare thin mudstones. Facies models developed during production drilling on the South Morecambe Field show that deposition occurred in a braided fluvial setting with minor aeolian episodes. The major facies associations present are: (A) major channel fill; (B) ephemeral channel fill; (C) non-channelized sheetflood deposits; (D) and (E) non-reservoir fines (abandonment and playa respectively); (F) aeolian dune and sand-sheet. Volumetrically, the major channel (A) and sheetflood (C) facies dominate, and the alternation of these facies associations is correlatable across the field. The aeolian sandstones form units varying in thickness from a few grain diameters up to 3 m thick. The sand-sheet and dune deposits can be correlated over considerable distances and show pressure communication on RFT logs. Although the aeolian sandstones comprise only 5–10% of the reservoir they have very high porosities and permeabilities (up to 30% and 10 darcies) and make disproportionate contributions to flow into the wellbore. The depositional criteria used to differentiate between aeolian and fluvial deposition in the South Morecambe Field are pinstripe lamination, good sorting and lack of rounded clay clasts, but these criteria are not definitive. As a result of the dissolution of an early diagenetic cement, aeolian sandstones have very high porosities compared to fluvial sandstones. This high porosity is reflected in high sonic transit times and allows aeolian sandstones to be identified tentatively in uncored wells by use of sonic logs.

Lithostratigraphy and deep-water setting of the upper Nanaimo Group (Upper Cretaceous), outer Gulf Islands of southwestern British Columbia

The Upper Cretaceous Nanaimo Group consists of extensive shaly units and lenticular bodies of sandstone and conglomerate deposited in neritic to bathyal marine paleoenvironments. Single sandstone and conglomerate lenses cannot be traced throughout the basin, but instead tend to concentrate at particular stratigraphic levels, defining crudely sheet-like composite units of coarse-grained rocks. Many of the lenticular coarse-grained units rest in major channels, as deep as about 550 m. Sand and gravel were generally transported to the west and northwest by sediment gravity flows through these channels into the basin centre. Recognition of widespread channeling is critical to the definition of formal lithostratigraphic units (four formations and two informal members are defined here for the upper Nanaimo Group).The top of the Nanaimo Group is probably truncated by a major unconformity with local relief of at least 2 km. The effect of the unconformity can only be seen by mapping the orientation of offshore ridges in the Strait of Georgia, which can be traced into the sandstones and conglomerates above the inferred unconformity on Tumbo and Cabbage islands. Large scale cross-bedding, unidirectional paleocurrents, and lack of evidence for marine conditions favour a subaerial, fluvial setting for deposition of these rocks, which correlate to the lithologically similar Paleogene Chuckanut Formation on the Sucia Islands.

Paleobiology of early angiosperms: evidence from sedimentological associations in the Early Cretaceous Potomac Group of the eastern U.S.A.

Lately, we proposed a hypothesis that the ancestral angiosperm was a diminutive, rhizomatous to scrambling perennial herb with small simple flowers. Our phylogenetic studies suggest that arborescent to shrubby magnolialians with large, multiparted, complex flowers are derived, rather ancestral as commonly thought. We suggest that the early angiosperm, due to its rhizomatous habit, would have been able to survive in more ephemeral habitats. A test of this idea would be to examine the sedimentological context of early angiosperms.Despite a number of reports of early angiosperm megafossils from Barremian to middle Albian (mid- to late Early Cretaceous) age sediments, only the Potomac Group of the eastern U.S.A. has provided a stratigraphic sequence of early angiosperm diversification. Furthermore, study of this sequence has also linked observations on floristic assemblages to the lithofacies in which they occur, leading Doyle and Hickey to report an ecological expansion of early angiosperms from relatively near-channel environments to a much broader range of flood plain sites during the Barremian/Aptin to latest Albian/early Cenomanian interval represented by Potomac Group deposition. From this those authors inferred that the early angiosperms were “riparian weeds” that grew on moderately unstable, near-channel sites subject to periodic flooding and episodes of alluviation.We recently expanded on these inferences by examining the sedimentology of Dutch Gap, an early angiosperm site at the earliest level of the Potomac Group (Barremain/Aptian-Pollen Zone I) near Richmond, VA. Sediments here appear to represent the meander belt of a fluvial setting. A series of multi-storied channel units at the site are filled with arkosic, medium to coarse, sub-angular sand with pebble- to boulder-size clasts of gneiss, claystone and mudstone concentrated at channel bases, reactivation surfaces, and on lateral accretion surfaces. These channels are cut into inferred levee and splay deposits that consist of thin interbeds of sand and silt alternating with dark grey, micaceous mudstone to claystone. This sequence fines in an inferred distal direction from the channels and the mudstone/claystone interbeds thicken at the expense of the coarser units until they coalesce into relatively thick, dark grey clay beds with only thin silty laminae. These beds are thought to represent floodbasin deposits.The megafossil plants recovered from our studies at Dutch Gap exhibit a clear pattern of association with each other and with these lithofacies. Angiosperms (Celastrophyllum, Rogersia, and an unnamed new form) together with ferns are found in relatively silty interbeds that lie in what appear to be distal levee settings, while the backswamp was dominated by the bennittitalean Dioonites buchianus. Conifers of diverse kinds are dominant only in pinkish, silty clay clasts that are inferred to have been transported from drier flood-plain environments.This study provides added evidence of an association between early angiosperms and moderately unstable, channel-margin sites and, in addition, calls attention to the potential that detailed sedimentological investigations have of providing important evidence on early angiosperm paleoecology.

Stratigraphy and Petroleum Potential of the Mississippian Chainman Shale and Diamond Peak Formation, East-Central Nevada

An eastward-thinning and shallowing-upward Middle and Late Mississippian group of sedimentary rocks accumulated east of the Antler orogenic highland. The Chainman Shale consists of fine-grained siliciclastic and carbonate rocks deposited in deep-water and slope environments; it ranges from 400 ft thick in the east to 2,400 ft thick in the west, where it interfingers with the Diamond Peak Formation. Fine- to coarse-grained siliciclastic and carbonate rocks up to 5,100 ft thick comprise the Diamond Peak Formation and were deposited in submarine fan, fluvial, shoreline, and deltaic settings. Antler basin, up to 600 ft of medium-grained siliciclastic strata, the Scotty Wash Quartzite, represent deposition in shelf and deltaic environments. The Chainman, Diamond Peak, Scotty Wash, and Pennsylvanian Ely Limestone comprise two unconformity-bounded stratigraphic sequences. Sequence 1 strata were deposited in an asymmetric basin formed by subsidence due to thrust loading, which controlled sedimentation. Sequence 2 strata were deposited in a symmetric, shallow to emergent setting where variation in subsidence and sediment supply controlled sedimentation. Mississippian sandstone reservoir quality is a function of depositional environment, with maximum porosity occurring in Diamond Peak shoreline strata (sequence 2) in the central Antler basin. Organic carbon and maturation data suggest the Chainman Shale is presentlymore » generating hydrocarbons capable of sourcing structural-, stratigraphic-, and unconformity-controlled traps in east-central Nevada.« less

Cambrian Paleogeography of the Great Basin

The Cambrian Period encompasses an interval from about 570 Ma to about 505 Ma. Rock sequences, aggregating more than 4 km thick, were originally deposited as clastic and carbonate sediments in fluvial, nearshore, and marine settings along the western Cordilleran passive margin, which was located 10-15{degree} north of the equator. One of the more easily studies areas within the Cordillera is the Great Basin province in the western United States, where Cambrian strata are well exposed within at least 75 block faulted mountain ranges. The Lower Cambrian of the Great Basin was dominated by fluvial and nearshore marine siliciclastics deposited across a broad passive margin. Although shallow marine carbonates were generally restricted to the southeastern Great Basin, a regionally extensive carbonate platform development near the end of the Early Cambrian. The last major influx of cratonally derived clastics completely covered the carbonate platform at the onset of the Middle Cambrian. The carbonate platform reestablished itself in a more cratonward position throughout the rest of the Cambrian, although complex facies patterns resulted from environmental shifts, periodic siliciclastic input, and several drowning events. This resulted in highly variable lithologic mosaics, which may partially account for the over one hundred formational designations currentlymore » in use for the Cambrian of the Great Basin.« less

Three-dimensional antidunes as HCS mimics in a fluvial sandstone; the Pennsylvanian South Bar Formation near Sydney, Nova Scotia

ABSTRACT An array of three-dimensional bedforms is present on bedding planes within a 20-35-cm-thick medium-grained sandstone unit in the braided-fluvial South Bar Formation (Pennsylvanian), near Sydney, Nova Scotia. The unit is near the top of a 4.7-m-thick channel sandstone and is attributed to a late stage of fill prior to the abrupt abandonment of the channel. The bedforms comprise radially symmetric mounds and intervening curvilinear hollows, and they contain concordant strata. They closely resemble hummocks and swales but are identified as three-dimensional antidunes based on their fluvial setting and the ubiquitous presence of well-defined current lineation, oriented parallel to unidirectional paleocurrent indicators within the channel fill. The antidunes secreted vertically in paleoflo s with mean depth and velocity of about 0.12 m and 1.0 m/s. Antidune bedforms are rare in the stratigraphic record, because such conditions are seldom maintained long enough to accrete a preservable set of strata. Three-dimensional antidunes such as these could easily be mistaken for hummocks and swales and the environment misidentified as storm-influenced if the nature and relationships of current lineation were not observed, or if incomplete exposure did not permit recognition of shallow-water origin.

Spherulitic Pedogenic Carbonate Nodules from the Lower Tuscaloosa; A Tool to Be Used in Paleogeochemical Exploration for Hydrocarbons?: ABSTRACT

Geochemical exploration for hydrocarbons consists primarily of sampling soils and surface sediments for anomalous concentrations of a variety of materials resulting from vertical migration of hydrocarbon-contaminated subsurface waters. In addition to traces of light hydrocarbons, the anomalous materials include gases such as radon and other contaminants from oil, as well as substances concentrated by the physico-chemical properties of the vertically migrating waters. Iron concentrations are high because of the reduced condition of these waters, calcium, magnesium, and bicarbonate ions because of the acidity of the waters. Many authors have commented on the association of spherulitic aggregates of siderite in fossil soils and sedimentary facies such as levees and overbank splays in fluvial settings. Researchers have suggested that iron may have been derived by erosion from areas of lateritic weathering and transported into the area as coatings on clay minerals or complexed with organic polymers. It has been reported and illustrated that abundant dolomite spherulites in surface profiles of outcropping sediments of a variety of ages and mineral compositions from Kuwait, all from areas of surface hydrocarbon seeps or over known subsurface accumulations. Many of the spherulites are nucleated around hydrocarbon droplets, and the significance of the distribution is clear. Itmore » is hypothesizes that the dolomite spherulites are precipitated by the action of bacteria feeding on hydrocarbons, an idea supported by the work of Lalou, which demonstrated that spheroids are common products of bacterial carbonate precipitation. The authors suggest that some, at least, spherosiderites may also be related to vertically migrating oilfield fluids. Iron, calcium, and magnesium may all be supplied by these waters, while the bacterial action apparently required for spherulitic morphologies to form in the pedologic setting may result from hydrocarbons in the same rising waters.« less

Facies Architecture in a Fluvial-Deltaic Sequence, Upper Crooked Fork Group (Pennsylvanian), Tennessee and Kentucky: ABSTRACT

The upper Crooked Fork Group on the Cumberland plateau in northeastern Tennessee and southeastern Kentucky includes the Wartburg Sandstone (Corbin of Kentucky) and surrounding fine-grained strata of the Breathitt Formation below the Poplar Creek Coal. Detailed study of outcrops employs lateral profiles and architectural element analysis. Examination of bounding surfaces within and between elements indicates a sixfold hierarchy based on areal extent and geometry. The Wartburg Sandstone consists largely of three-dimensional macroform bar complexes. Foreset macroform elements predominate, with minor lateral accretion elements present and no apparent internal cyclicity. The channelized base of the Wartburg, predominance of large-scale planar and trough cross-stratification, low paleocurrent variance, and presence of soft-sediment deformation elements and massive sandstone elements with large mudstone intraclasts suggest a low-sinuosity fluvial setting. Strata overlying the Wartburg display fine-grained lateral accretion elements and associated overbank-fines. This suggests an upward change in fluvial style in the upper Crooked Fork Group from a predominantly coarse-grained low-sinuosity setting to a fine-grained high-sinuosity setting. The abundance of downstream and lateral accretion elements, plant debris, rooting, and thin discontinuous coals; paucity of burrows and bioturbation; and presence of thin marine horizons combined with the facies architecture suggest a channelized depositional setting with some marinemore » influence, indicative of fluvial or fluvial-dominated deltaic deposits.« less

The effect of depositional environment on coal distribution and quality parameters in a portion of the highveld coalfield, South Africa

Eight successive lithofacies associations are identified and described; they are each laterally continuous and represent a certain depositional phase throughout the study area. These are related to sedimentary processes, from which a three-dimensional paleoenvironmental model is derived. Subsequently, coal distribution and coal quality characteristics are linked to the depositional features of the model. The identified depositional phasescommenced with subglacial, glaciofluvial and glaciolacustrine settings, with associated Gilbert-type deltas. These are followed by meandering and minor braided fluvial settings, characterized by laterally and vertically highly variable lithofacies, in which the main coal-bearing strata were formed. Coal distribution and quality parameters (ash and volatile matter content, calorific value) are closely related to paleoenvironmental characteristics. Major changes of coal distribution and quality parameters are mainly associated with active and abandoned channels and these parameters are less variable in floodplain settings.

Micromorphological and SEM analysis of cretaceous-paleogene petrosols from Eastern Montana and western North Dakota

Microscopy is used to distinguish pedogenic features from those associated with primary sedimentary fabric and non-pedogenic diagenesis in a suite of 65 million year old sediments. Sand-to-clay-sized material was deposited in meandering river systems over a broad coastal alluvial plain, and pedogenesis occurred in all lithologies. A variety of petrosol (lithified soil) horizons which exhibit both syndepositional bedding and pedogenic and non-pedogenic diagenetic features remains today. Features attributable to the in-situ interaction between root and soil provide the most convincing evidence of pedogenesis. In addition, features such as illuvial clay and iron-oxide accumulations around voids, which are commonly but not exclusively associated with soil environments provide accessory evidence of pedogenesis. Stress-related void and ground-mass birefringence fabrics are evident in the petrosols and some purely sedimentary deposits, and thus cannot be used as diagnostic indicators of pedogenesis. In this fluvial setting, erosion and transport of soil material occurred, which is indicated by the presence of pedorelicts in petrosol horizons and purely sedimentary deposits. Detailed study both by petrographic and by scanning electron microscope elucidates pedogenic indicators not visible in hand samples. Such tools are useful in resolving ambiguities between pedogenesis and non-pedogenic diagenesis.

Paleosols spanning the Cretaceous-Paleogene transition, eastern Montana and western North Dakota

Paleosols occur in exposures of the latest Cretaceous Hell Creek and Paleocene Tullock (= Ludlow) Formations in Montana and western North Dakota. These units are composed mainly of interbedded siltstones and sandstones of meandering fluvial origin. The paleosols indicate that changes in ancient soil development occurred concomitantly with the better-known faunal transition. In order of decreasing abundance, silty soils, sandy soils, organic soils, and volcanic soils are preserved. Pedogenic features present in these soils include roots, microscopic segregations of amorphous material, birefringence fabric, and soil structures. The features suggest that throughout the Cretaceous-Paleogene transition pedogenic processes in the region produced immature profiles, an observation consistent with the unstable, fluvial setting in which the ancient soils formed. Gleization was a dominant process in this setting, and podzolization modified some sandy soils. The association of the features enables recognition of O, A/E, Btg, Bhs, Bg, BC, Cg, and C horizons. During middle Hell Creek time, soils formed in a poorly drained setting that was only stable enough to permit incipient pedogenesis. The bulk of the pedogenesis occurred in levee and flood-plain deposits; soils also occurred on point-bar and crevasse-splay deposits. In topographically depressed regions, organic accumulations formed with minimal soil development. Matted plant debris is the product of this environment. Gley features and segregations of iron oxides around voids suggest fluctuation of the water table. By latest Hell Creek time, the mean level of the water table rose, and in the lowest Tullock (Ludlow) exposures, extensive ponded deposits are preserved. Vegetation accumulated at a rate sufficient for coal formation. The amount of fluctuation apparently was reduced, and pedogenesis was further inhibited, as indicated by the virtual absence of illuviated clays in the sediments.