Paralic Environments Research Articles

Evolution of a trench‐slope basin within the Cascadia subduction margin: the Neogene Humboldt Basin, California

ABSTRACTThe Neogene Humboldt (Eel River) Basin is located along the north‐eastern margin of the Pacific Ocean within the Cascadia subduction zone. This sedimentary basin originated near the base of the accretionary prism in post‐Eocene time. Subduction processes since that time have elevated strata in the south‐eastern portion of the basin above sea level. High‐resolution chronostratigraphic data from the onshore portion of the Humboldt Basin enable correlation of time‐equivalent lithofacies across the palaeomargin, reconstruction of slope‐basin evolution, and preliminary delineation of climatic and tectonic influence on lithological variation. Emergent basin fill is divided into five lithofacies which clearly document shoaling of the inner trench slope from deep‐water environments in early Miocene time to paralic environments in Pleistocene time. The oldest strata consist of hemipelagic mudstones and minor debris‐flow breccias deposited in a deep‐water setting during elevated sea level. These strata are overlain by glauconite‐rich, fine‐grained turbidites which heralded an increasing influx of terrigenous detritus. Water depths shoaled earlier in the eastern basin area as the palaeoshoreline prograded seaward. Turbidite deposition ceased in the eastern basin area at about 2‐2 Ma, whereas 22 km to the west, turbidite deposition continued until about 1‐8 Ma. Lithofacies at the western study site change abruptly across a middle Pleistocene unconformity from outer shelf to paralic deposits. In the east, a more complete Pleistocene section records transition from outer to inner shelf, beach and fluvial environments.The Humboldt Basin lithofacies sequence is overprinted by eustatic control of sediment source. Comparison of sediment character with palaeoceanographic conditions indicates dominance of hemipelagic facies during periods of elevated sea level in the middle Miocene and early Pliocene when depocentres were isolated from terrigenous sediment. Glauconite‐rich facies were mobilized from an upper slope setting following these periods of elevated sea level and redeposited in a deep‐marine environment. Pleistocene shoreline lithofacies display glacio‐esutatic control of depositional environment by recording several cycles of nearshore to fluvial progressions.General models of accretionary prism behaviour and trench‐slope basin evolution are compatible with the overall coarsening‐upward lithofacies sequence filling the Humboldt Basin. Early structural barriers precluded deposition of terrigenous material except from locally derived debris flows; subsequent shoaling and burial of deactivated thrust‐folds enabled turbidity flows to reach the basin floor.However, late‐stage tectonism apparently controlled the onset of coarse‐grained deposition in this sequence. Significant sand‐rich turbidite deposition began in the middle Pliocene, synchronous with tectonic uplift of the southern basin margin. Conversely, cessation of turbidite deposition in the eastern basin area in latest Pliocene time was synchronous with growth of anticlinal structures which again blocked widespread dispersal of turbidity flows. This middle Pliocene to Holocene period of crustal shortening is synchronous with continued reduction in spreading rate along the southern Juan de Fuca ridge, and probably reflects partial coupling between the subducting lithosphere and the overlying accretionary prism.

Can the confinement index (calculated on the basis of foraminiferal populations) be used in the study of coastal evolution during the quaternary?

Abstract One of the major present preoccupations of ecologists is to foresee the effect of the predicted sea level change on the coastline, and therefore on the biosphere. Prospective studies first require a thorough knowledge of present conditions and of past evolution. Paralic environments are very sensitive to coastline variations, and provide one of the most sensitive tools for studying sea level change. One of their main characteristics is the degree of marine influence, named confinement, that depends on climatic conditions and on the coastline morphology and evolution. However, these environments are under the influence of complex interacting parameters, and their study requires a set of complementary tools. A confinement index ( Ic ), calculated on the basis of the main species of foraminifers, has been worked out in the paralic environments of west Africa. It provides a numerical value that can be calculated rapidly, and gives an immediate objective indication of slight changes in the environment, with a tendency toward more restricted ( Ic increases) or less restricted ( Ic decreases) conditions. This tendency might be in relation with sea level change. Thus, Ic is proposed as a preliminary tool for quantifying changes in the paralic ecosystems. Used together with a set of complementary indicators (sedimentology, geochemistry, and so on) it can be used for the detection of sea level change.

Répartition des foraminifères benthiques dans la zone sud-ouest du système estuarien-lagunaire d'Iguape-Cananéia (Brésil)

One hundred species of benthic foraminifera were found in the study area (25oS - 48oW). The surface sediment of this lagoon contains from 0 to 3,000 tests per 50 cm3 and up to 32 species. The foraminifera fauna of the lagoonal system of Iguape-Cananeia is the richest among those described in other paralic environments of Brazil. The richness of foraminifera fauna can result from the strong marine influence in the Baia de Trapande. The foraminifera distribution in the study area is characterized by the occurrence of marine assemblages nearby the mouth, with a gradual increase in agglutinating species, such as,Ammotiwn salsum and Gaudryina exilis. In the confined areas, with low salinity, the assemblages are oligospecific and Milammina earlandi Is dominant. The abundance of Pararotalia in the Mar de Cananeia shows that this environment is less restricted than Mar de Cubatao, where the genus is absent. The distribution of foraminifera species shown to be also influenced by urbain sewages.

Late Holocene erosional shoreface retreat within a silicilastic-to-carbonate transition zone, east central Florida, USA

ABSTRACT This study has reconstructed the late Holocene evolution of a section of the North American Atlantic coast barrier-island system in a siliciclastic-to-carbonate transition zone. A transgressive stratigraphy, analogous to that recognized beneath the siliciclastic barriers of the embayed Atlantic and Gulf coasts, was identified in the study area and generated by erosional shoreface retreat of the barrier island during Holocene sea-level rise. Vibracores from the backbarrier revealed the preservation of a thin (< 3.5 m) Holocene sediment succession consisting of muddy skeletal sand overlain by intercalations of clean skeletal sand and muddy skeletal sand capped by fibrous peat. In the foreshore, the fibrous peat is compressed and abruptly overlain by coarse shell hash grading upward into skeletal sand. The entire Holocene section rests unconformably upon a thin (< 0.25 m) quartz sand and featureless, gently seaward-dipping Pleistocene limestone surface. Sedimentologic, paleontologic, stratigraphic, and radiocarbon data suggest that this sequence is transgressive and was generated during erosional shoreface retreat of a wave-dominated, microtidal barrier-island system. The basal, muddy skeletal sand is interpreted to have been deposited in a shallow (3 m) backbarrier lagoon. These sediments were buried during overwash events that transported skeletal sand across the barrier and into the lagoon. Repeated overwash generated a shallowing-upward sequence capped by organic-rich tidal-wetland sediment. Radiocarbon dates suggest that wetland sedimentation started between 1200 and 1960 yr B.P. Landward retreat of the barrier is indicated by the foreshore unconformity (ravinement surface) that now truncates the fibrous peat. The preservation potential of the entire Holocene paralic section is probably low, given the lack of significant antecedent topographic relief and relatively slow rate of sea-level rise. The preservation potential should increase seaward, however, because the older paralic environments would have been subjected to faster sea-level rise.

Dynamics of continental and paralic sedimentation in Africa - Quaternary models

The tectonic, morphoclimatic, eustatic and biological factors controlling the dynamics of continental and marginal littoral deposition have been recorded and preserved as sedimentary and diagenetic palaeoenvironmental signatures. Those which are reviewed in this paper relate to the Phanerozoic eon and indicate the particular importance of Africa in the study of sedimentation in endoreic basins, rifts and paralic environments. The geographical extent and geomorphology of the continent mean that a wide range of zonal climates are represented, from equatorial to mediterranean, that are not affected unduly by relief. They are marked, however, by strong contrasts between dry and wet seasons which can sometimes be represented by discontinuities in the sediments. The record of climatic zones, together with evidence of climate-induced diagenesis, allows a reconstruction of the latitudinal drift of the African plate after the break-up of Pangea at the end of the Jurassic.

Benthic foraminifera used as indicators of a gradient of marine influence in paralic environments of Western Africa

A scale of confinement in the recent paralic environments of western Africa can be proposed based on the distribution of foraminiferal assemblages. It is thus possible to describe the gradual evolution from an oceanic environment, where marine species are dominant, to a continental environment characterized by the disappearance of foraminifers and the development of thecamoebians. The peculiar environments of athalassic salt lakes are not considered here. This scale, based on four characteristic assemblages and extended to 90 species can be applied to recent or fossil paralic ecosystems. However, it must be used with caution as its significance may change with hydrological properties.

Recent foraminiferal assemblages and their distribution relative to environmental stress in the paralic environments of West Africa (Cape Timiris to Ebrie Lagoon)

Other| July 01, 1990 Recent foraminiferal assemblages and their distribution relative to environmental stress in the paralic environments of West Africa (Cape Timiris to Ebrie Lagoon) Jean-Pierre Debenay Jean-Pierre Debenay Search for other works by this author on: GSW Google Scholar Author and Article Information Jean-Pierre Debenay Publisher: Cushman Foundation for Foraminiferal Research First Online: 03 Mar 2017 Online ISSN: 1943-264X Print ISSN: 0096-1191 GeoRef, Copyright 2004, American Geological Institute. Journal of Foraminiferal Research (1990) 20 (3): 267–282. https://doi.org/10.2113/gsjfr.20.3.267 Article history First Online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Jean-Pierre Debenay; Recent foraminiferal assemblages and their distribution relative to environmental stress in the paralic environments of West Africa (Cape Timiris to Ebrie Lagoon). Journal of Foraminiferal Research 1990;; 20 (3): 267–282. doi: https://doi.org/10.2113/gsjfr.20.3.267 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyJournal of Foraminiferal Research Search Advanced Search This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Structure and origin of terpenoid hydrocarbons in some German coals

Abstract Hydrocarbon extracts of coal samples from major coal fields in West Germany have been investigated by gas chromatography-mass spectrometry (GC/MS). The samples are of Carboniferous and Tertiary age and derive from limnic and paralic environments, the Saar District, the Ruhr Area and the Lower Rhine Basin. Differences in facies, maturity and age reflected in the distribution patterns of the extracted alkylcyclohexanes. An unexpected occurrence of cyclic diterpenoid hydrocarbons in some samples of Carboniferous age has been found. The tetracyclic compounds mainly show phyllocladane type structures and can serve as source indicators. Hydrocarbons obtained from chemical degradation of the macromolecular material of the samples also revealed phyllocladane-type compounds which are normally associated to resins of modern and extinct familes of conifers. Origin and structural variations of this class of hydrocarbons are discussed in terms of maturity and palaeobotanical implications.

Retrogradational Fluvio-Paralic Coal-Forming Environments, South Island, New Zealand: ABSTRACT

The West Coast Region of South Island, New Zealand, records Paleogene history of peneplanation, fluvio-paralic aggradation, and marine transgression. At Buller coalfield, Eocene Brunner coal measures rest unconformably on Paleozoic to Cretaceous basement rocks and interfinger upward with Eocene, marine Kaiata Formation. During Eocene, rising sea level controlled the architecture of fluvio-paralic deposits and strongly influenced the origin and properties of Brunner coals. The Brunner coal measures contain as much as 40-ft thick, high to low volatile bituminous coal beds with 0.5-16% ash and 1-9% sulfur that is greatest in the upper coal beds. The Brunner can be divided informally into lower and upper coal measures. The lower coal measures are dominated by pebble conglomerates and multierosional, trough-crossbedded conglomeratic sandstones; the upper coal measures by stacked, multilateral, trough crossbedded, granular sandstones with subordinate siltstones and mudstones. Bioturbated sandstones with brackish-marine Ophiomorpha-like trace fossils are common in the upper coal measures. The lower coal measures were deposited in exhumed paleovalleys of the peneplain surface. These valleys, as deep as 50 ft, were initially aggraded by gravelly braided streams. Raised bogs formed in abandoned braid valleys and extended onto surrounding basement highs. The upper coal measures were deposited in sandy bedload, meanderingmore » streams with paralic wave-reworked deltas, and attached barriers. Peat bogs in the paralic environments accumulated thin coals, whereas bogs that developed on abandoned meander ridges formed thick, lenticular coals. Sea-level rise resulted in inundation introducing sulfur into the peat and resulted in high sinuosity fluvial architecture. Gradual incursion of brackish-marine waters favored bioturbation of paralic sediments along retrograding coastlines.« less

Comparative organic petrology of interlayered sandstones, siltstones, mudstones and coals in the Upper Carboniferous Ruhr basin, Northwest Germany, and their thermal history and methane generation

In the coal-mining Ruhr-area, Upper Carboniferous rocks (ca. 4000 m) consist of interlayered sandstones, siltstones, mudstones and coals. They were deposited in a tropical, paralic environment where alternating fluvial sedimentation, occasional marine ingressions, and swamp growth resulted in an irregular cyclic succession. The total sedimentary package contains on an average 6 Vol.-% of organic matter. About 70 Vol.-% of the organic matter occurs in coal seams, the rest as dispersed organic matter in clastic rocks. The organic matter is autochthonous in the coals and allochthonous in associated sandstones and siltstones. It consists of about 70% vitrinite, 20% inertinite, and 10% liptinite. The overall maceral group composition is the same for coals and dispersed organic matter. This surprising similarity is caused by a nearly exclusive input of land-plant derived organic matter to swamps and fluvial systems and a similar degree of preservation. Highest average liptinite contents (% of total macerals) were found in unrooted mudstones, highest average inertinite contents in coarse-grained siltstones and highest average vitrinite percentages in sandstones.

Sequence biologique en milieu paradeltaique, exemple de la lettenkohle

A particular kind of sedimentation took place in the western Vosges during the Lettenkohle (late Trias). Following the formation of coastal lagoons, a wide delta spreading from the Germanic basin, deposited fine arenites with marine and fresh-water elements. Within the fluvial beds, some sequences appear which present calcified Lingula phospharenites, under silty argillites containing Estheria (Conchostraca). Superposition of a marine influenced ecosystem, with reduced population, by a fresh-water one, sequentially repeated at the end of a megasequence, indicates that the fluvial environment was going to take progressively the place of the paralic environment.

Cold-climate, fluvial to paralic coal-forming environments in the Permian Collinsville Coal Measures, Bowen Basin, Australia

The Middle Permian Collinsville Coal Measures of the northern Bowen Basin illustrate a range of cold to cold-temperate, coal-forming environments. Cold climate is indicated by Glossopteris flora in the coal measures, and by restricted marine fauna dominated by brachiopods and bryozoa in correlative marine sequences of the Back Creek Group which contains also abundant lonestones (dropstones). Sedimentation was characterised by an overall transgression, interrupted by local fluvial and coastal progradation in a shallow, epicontinental sea during a relatively quiescent tectonic period. Six sedimentary environments are represented: fluvial, fluvio-paralic, barrier-strandplain, back-barrier, tidal flat and open marine. The basal coal formed from peat of swamps of abandoned areas of gravelly braided streams, and is massive, dull, and with high ash (20%), low sulphur (1%) contents. Overlying coals developed from peats formed in fluvio-paralic and paralic environments, and thicker seams are generally brighter, with low to moderate ash (8–17%) and moderate to high total sulphur (1–6%) contents. Seams associated with fluvial influence show splits and high ash yield, while seams associated with coastal deposits show high sulphur levels (up to 21%). In contrast to reported models of coal-forming environments, no clearly defined deltaic or inter-distributary bay-fill sequences were identified in the area studied. Rather, vast freshwater wetlands backed low-gradient, progradational coasts locally having bars and barriers. The barriers were not prerequisites for substantial peat accumulation, although may have locally assisted peatland development by raising the profile of coastal equilibrium.

Sedimentary ore deposits

Summary Until the last two decades the magmato-hydrothermal theories for the origin of ores were considered to be predominant. The change in viewpoint over 20 years or so towards a general acceptance of various syngenetic and epigenetic concepts in ore genesis is outlined in the opening section of this review. In the following section the concentration of metals particularly Al, Ni and to some extent Au in the weathering crust is described. The ways in which metals are concentrated by physical sedimentation are outlined, with particular attention to gold and tin placer deposits and the importance of concepts such as geomorphic thresholds, complex response, and episodic erosion is emphasized. For these deposits it is shown that following the release of elements from bedrock sources they are cycled through the sedimentary system. Essentially, elements move from proximal to distal fluvial sites and from stratigraphically lower to higher positions. Resultant orebodies trend from high grade/low tonnage to lower grade/higher tonnage in a cycle. Chemical sediments, and particularly ironstones are then discussed. Oolitic ironstones occur in two major phases in the Phanerozoic and although their formation in nearshore sheltered sites is becoming established the precise mode of formation, whether primary or diagenetic, is still uncertain. The mainly Precambrian ‘banded iron formations’ pose the problems of the source and transport mechanism for such vast volumes of iron-bearing sediment. Various hypotheses are discussed and a palaeoenvironmental approach adopted. The possibility is emphasized that the unroofing of mafic Archaean basement in the early Proterozoic may well have been a critical factor in their formation and time distribution. Ore deposits of lead, zinc, copper and uranium are found in continental or marine sediments. The metals are derived from bedrock, pass through the weathering crust and are then incorporated into the sedimentary prism as ‘fluid’ or detrital oxide complexes. Migration takes place in interstratal solutions and they are finally precipitated as ores in the diagenetic environment. A broad 2-fold environmental division can be recognized, with some ores forming in fluviatile, lacustrine or paralic environments (Cu, U and to a lesser extent Pb, Zn) and secondly those in nearshore to offshore basin or platform sites (Pb, Zn). The movement of metallic fluids out of basinal shales and into basin margin facies, often up growth faults, is described and a model is outlined to account for lead-zinc deposits associated with carbonates and black shales. The formation of copper and uranium deposits in continental facies associations and above sub-redbed unconformities, the origin of uranium-rich conglomerates in the Proterozoic, the formation of sediment-hosted copper deposits and the copper-gold-uranium association in sedimentary rocks are all discussed in terms of a ‘fluid unroofing’ model. In the final section it is emphasized that we need to refine ‘fluid unroofing’ models, and have a better understanding of the way metal is cycled through the sedimentary — diagnetic environment. The importance of sophisticated facies analysis, the relevance of ‘maturity’ with regard to metals as well as oils, and the significance of biological mediation in ore formation are all emphasized. Finally the critical importance of the weathering horizon in establishing the potential metal fertility of any sedimentary basin is noted.

Le confinement, paramètre essentiel de la dynamique biologique du domaine paralique

Recent studies on present lagoonal faunas show that, contrary to accepted opinions, salinity is not the main ecological parameter in paralic environments. Location of species and quantitative biological gradients (biomass, density, ...) in paralic populations essentially depend on an original parameter which could be expressed, in each point, as the lapse of time necessary to renew the marine elements at that point. It is called confinement (with regard to the open sea), on the analogy of the usual meaning of this word. In any given point of a paralic basin, the geochemistry (salinity, chemical composition) depends not only on confinement but also on the water balance of this basin : positive or negative it would induce lower or higher salinities than sea-water. Hence, the biological zonation of paralic middles do not fit the geocnemical and sedimentological zonations. The existence of strictly paralic species, the great stability of lagoonal populations due to the wide changeableness of abiotic parameters and a common biological dynamism contrast with the geochemical diversity of the paralic middles. Hence, they should be no longer considered as dependencies of the marine or continental realms neither as intermediate middles : they form together a genuine paralic realm apart from both traditionnal realms of hydrobiosphere. This is reinforced by the sedimentological features of paralic areas : high sedimentation rates, importance of biogenic phases. Such a point of view beeing accepted, the classifications of paralic middles according to salinities must be abandonned, the ecological importance of osmoregulation must be reconsidered, the biological indicators of paleosalinities must be suspected. On the whole, the role played by the paralic realm in the biosphere and lithosphere history seems more important than usually assessed. At least, the confinement could be a guide for the economical exploitation of paralic middles.

Influence of Basin History on Reservoir Quality of Sandstones: Upper Cretaceous of Northern Mexico: ABSTRACT

Upper Cretaceous sandstones of northern Mexico for a distance of 200 km (125 mi) south of the Rio Grande have similar mineral composition but differ markedly in reservoir quality in the north versus the south depending on the post-depositional history of the basins in which the sandstones were deposited. The sandstones are composed largely of detritus from volcanic and intrusive igneous rocks and were deposited in paralic and fluvial environments. Sandstones in the north were never buried more than 1,000 m (3,300 ft) and were subjected to slow, gentle, basinward down-warping. They underwent a complex diagenetic history of cementation by chlorite, quartz, calcite, and kaolinite, and the development of modest secondary porosity; and they form hydrocarbon reservoir rocks of moderate quality (^phgr = 5 to 15%, k = 5 to 300 md). Sandstones to the south were buried rapidly by 1,000 to 4,00 m (3,300 to 13,000 ft) of younger strata and immediately thereafter underwent strong compressional folding and local thrust faulting during the Laramide orogeny. The sandstones lost from 20 to 35% porosity by compaction and the remainder of the porosity by cementation with calcite. They are tight, did not develop secondary porosity, and have no sho s of hydrocarbons. During slow subsidence of sandstone-shale sequences in the north, the shale and associated organic matter underwent normal maturation events. Shale water was expelled in stages, organic matter evolved to produce liquid and gaseous hydrocarbons, and acid formation water was generated. The associated sandstones underwent a diagenetic sequence, including the development of secondary porosity from the acid formation water, that is typical of many sedimentary basins. To the south, Laramide compressive forces caused strong compaction and premature de-watering of the Upper Cretaceous shales. Most water present in the shales was expelled prior to maturation of hydrocarbons. Thus, compaction was rapid and severe and there was no opportunity for the development of typical formation waters that ight have developed secondary porosity. Close to the Sierra Madre front, rapid and early expulsion of water produced a strong fracture cleavage in shale and siltstone. End_of_Article - Last_Page 510------------

LAND-PLANT SOURCE ROCKS FOR OIL AND THEIR SIGNIFICANCE IN AUSTRALIAN BASINS

Many Australian oils are rich in paraffin waxes which are derived from the remains of terrestrial plants. Although the land-plant contribution to oils, particularly those found in a paralic or deltaic environment, is well established, opinion is divided on the ability of non-marine coaly sediments to generate and expel commercial quantities of oil. It appears that some coal measure sequences have generated mainly gas whilst others are the source of large oil accumulations. The composition of coals deposited in Australia has varied through geological time as a result of differences in climate, geological setting, depositional environment and stage of floral evolution. Consequently, most Australian pre-Jurassic coal measure sequences are deficient in exinite macerals and are therefore mainly gas-prone. In contrast, Jurassic to Tertiary coal-rich sequences often contain abundant exinite and may have substantial potential to generate oil in commercial quantities, as demonstrated by the well-known Gippsland Basin (Bass Strait) oilfields.A similar trend is observed worldwide, where, despite the extraordinary global abundance of Late Palaeozoic coals, only minor amounts of crude oil of land-plant origin are known to be associated with them. However, there appears to be a close relationship between the occurrence of waxy, land-plant-derived crudes and coaly sediments of Cretaceous and Tertiary age. This is thought to be a result of the dominance of conifers in swamp floras of these periods, together with the evolution of the angiosperms (flowering plants) in the Late Cretaceous.

Stratigraphic Reference Section for Georges Bank Basin--Depositional Model for New England Passive Margin

A multichannel seismic reflection profile (U.S. Geological Survey line 19), calibrated with the COST G-1, COST G-2, and Shell Mohican I-100 wells, and seismic-sequence analysis shows that the chronostratigraphic and lithostratigraphic units and depositional history of the Georges Bank basin are similar to those of the Scotian basin. Carbonate rocks of the Iroquois and Abenaki Formations, as much as 16,000 ft (4,800 m) thick, dominated the eastern half of the Georges Bank basin during the Jurassic. As much as 7,500 ft (2,300 m) of the coeval terrigenous clastic deposits of the Mohican, Mohawk, and Mic Mac Formations accumulated updip (westward) in sublittoral, paralic, and nonmarine environments. Siliciclastic deposition, as much as 6,000 ft (1,800 m), dominated the entire basin throughout the Cretaceous and Cenozoic, and it was punctuated briefly by carbonate deposition during the Hauterivian and Paleogene. Tentative correlation between the Georges Bank basin sequences and those of the adjacent, deep North American basin suggests that the deep-sea facies were strongly influenced by depositional events on the shelf. Deposition in both areas has been sensitive to changes in sea level and to paleoclimatic cycles.

Mannville Group of Lloydminster Heavy Oil Fields, Canada--Depositional Overview: ABSTRACT

A regional subsurface study of 200 cores and several thousand well logs has led to an interpretation of the depositional history for the Lloydminster area of western Canada. Major geologic controls on deposition were tectonism in western North America, a series of northwest-trending ridges formed of Paleozoic carbonates found in the central plains, salt collapse within the depositional basin, and eustatic changes associated with Boreal and Gulfian seas. On the basis of genetic units, the Mannville Group has been divided into three units: lower, middle, and upper. The lower unit is dominantly a siliceous, fine to medium-grained sandstone which is found in the lows between the northwest-trending ridges of Paleozoic carbonates. The unit is up to 60 m thick with the dominant sedimentary structures being high-angle (30°) cross-stratification. The middle unit consists mainly of upward-coarsening, very fine to fine-grained, quartzose sheet sandstones, 6 to 10 m thick, associated with a restricted marine microflora and microfauna. A typical sequence begins with a basal bioturbated shaly siltstone and proceeds upward through wavy lenticular sandstones in silty shales, wave-rippled sandstones with mud drapes, wavy-bedded sandstones, and low-angle (0 to 10°) cross-laminated sandstones. This sequence is commonly capped by a coal or gray carbonaceous shale. Although sheetlike in appearance, the lateral continuity of the sandstones is commonly broken by shale or sand-filled channels. The upper unit is comprised of thick (up to 35 m) lenticular channel fills of cross-bedded, lithofeldspathic or quartzose sandstones which grade laterally into interbedded deposits of current-rippled sheet sandstones, siltstones, mudstones, and coals. The vertical sequence of environments is interpreted to be: post-Paleozoic incision followed by valley-fill deposition of an aggrading north-flowing fluvial system (lower unit), transgression followed by regressive sedimentation in wave-dominated paralic environments (middle unit), and continued regressive deposition within an areally extensive, north-flowing anastomosed fluvial network (upper unit). Mannville deposition in the Lloydminster area was terminated by a major transgression which deposited the widespread marine shales of the Colorado Group. End_of_Article - Last_Page 620------------

Paleogeographic Setting and Depositional Environments of Santa Margarita Formation, Ventura County, California: ABSTRACT

The Santa Margarita Formation in Ventura County, California, represents a transgressive-regressive sequence of rock deposited in a late Miocene shallow marine and paralic environment. A relatively steady low rate of subsidence with an initially low rate of sedimentation produced the transgressive phase. Increased rate of sedimentation produced a seaward progradational sequence or regressive phase. Rocks that represent the transgressive sequence are herein divided into two facies: (1) facies A, a basal conglomerate zone and thick beds of bioturbated, locally fossiliferous, fine-grained sandstone with some large-scale cross bedding; and (2) facies B, laminated mudstone, phosphatic mudstone, and pelletic phosphate. Rocks that represent the regressive sequence conformably overlie facies B and are herein divided into three facies: (1) facies C, medium to very coarse-grained fossiliferous sandstone characterized by small to large-scale cross bedding; (2) facies D, thin to thick beds of laminated, nodular, and honeycomb gypsum and intercalated claystone, and lenticular-bedded and interlaminated mudstone and very fine-grained sandstone; (3) facies E, medium to thick-bedded mudstone and medium to very coarse-grained sandstone characterized by cross bedding. Facies A represents marine deposition in a high-wave-energy, shoreface environment. Facies B represents deposition in a low-wave-energy, embayed, inner-shelf environment. Facies C was deposited in a high-wave-energy, east-west-trending, shoreface environment consisting of barriers, bars, End_Page 1000------------------------------ tidal channels, and shoals. Facies D was deposited behind the barrier complex in hypersaline lagoon, evaporitic intertidal-flat, and supratidal-falt environments. Facies E represents deposition on a prograding fluvial plain. A modern analog for the late Miocene barrier-lagoon-tidal flat complex exposed in Sespe Creek area is the coastal zone between San Felipe, Baja California, and the present mouth of the Colorado River. End_of_Article - Last_Page 1001------------

Bottom Morphology and Shallow Subbottom Sedimentary Structures on U.S. Continental Shelf Between Cape Hatteras, North Carolina, and Jacksonville, Florida: ABSTRACT

During the period July 1978 to 1980, a survey of the U.S. South Atlantic shelf was made to determine the occurrence and distribution of biologically productive bottom areas and bottom and shallow subbottom geologic features of potential hazard to petroleum exploration and production. Methods of data acquisition included the use of high-resolution seismic systems, sidescan sonar, towed television, and minisub. Shelf sediments are largely composed of 2 to 5 m of Quaternary sands and silty clays with mixtures of shell that were deposited in fluvial, paralic, and shallow-marine environments. Greater sediment thicknesses (up to 40 m) are present in buried river channels, tidal inlets, and cut-and-fill structures associated with meandering tidal streams. Surficial sediments exhibit a wide range of bed-form type and maturity. Ripples of less than 0.5 m wavelength and megaripples of 0.5 to 1 m wavelength are common across the shelf. Crest orientation is generally north-south. Algal growth along the crests of many of the smaller bed forms as well as abundant bioturbation and tracks indicate bottom stability except during storms. Sand waves up to 100 m long are common, particularly within the 25-m isobath and are present in discrete fields 3 km or more wide with north-south crest orientation. They are less common, less sharply defined offshore and usually underlie smaller bed forms of different orientation. These features may have resulted from infrequent major storms and, because of rising sea level, may now be below the effect of most s orm-generated waves. Although the mobility of these large bed forms is inferred by the presence of buried reefs within some of the sand bodies, the time and rate of movement are unknown. Many areas of the shelf exhibit irregular, low-relief bed forms that appear to be in advance stages of deterioration from intense bioturbation. Several distinct, acoustcally hard reflectors present in the shallow subbottom over much of the shelf represent indurated strata which on outcrop provide substrate for the development of reef and hardground communities. These temperate bioherms are primarily populated by sponges and octocorals and, on the basis of topographic expression, are classified as low-relief hardgrounds (> 0.5 m), moderate-relief reefs (up to 2 m), and high-relief reefs (up to 15 m). Low-relief hardgrounds are the most widely distributed of the three types but are the least productive in terms of biomass. Moderate- and high-relief reefs occur most commonly off North and South Carolina and along most of the shelf break. Substrate ranges from Pleistocene to Miocene in age and locally contains significant perc ntages of phosphate grains. The total area of shelf occupied by reefs and hardgrounds is estimated to be less than 10%. Outcrop patterns reflect both local bottom processes and regional structural framework. In the middle to outer shelf off the Georgia coast, a relatively narrow zone of large channels and numerous cut-and-fill structures occur in Quaternary deposits just landward of a shallow buried Miocene high. These features are very similar to sedimentary structures associated with present-day barrier islands, tidal inlets, and estuaries and strongly suggest a sea-level stillstand near the end of the last Pleistocene regression or early in the present transgression. End_of_Article - Last_Page 1663------------