Tidal Flat Sequence Research Articles

Ordovician deep dolomite reservoirs in the intracratonic Ordos Basin, China: Depositional model and Diagenetic evolution

Recent natural gas discoveries indicate that non-karstification-dominated reservoirs exist in the intracratonic Ordos Basin. This study examines the sedimentological and geochemical characteristics needed to clarify the depositional model and diagenetic evolution process of this newly discovered reservoir type. The depositional environment of the dolomite reservoir can be characterized as a tidal flat that grew from the Central Paleo-uplift to the eastern depression by cyclic progradation on an epeiric platform. A tidal flat sequence can extend laterally as a progradational wedge in each cycle of sea level fluctuation. The sheet-shaped peritidal shoal facies associations patched on the wedge represent potential dolomite reservoirs and can be recognized by the presence of doloarenite that has been altered into a vaguely relict grained-texture by diagenesis. Although continuing destructive diagenesis has led to reservoir densification, burial dolomitization and burial dissolution with facies selectivity have tended to occur in peritidal shoal facies associations, thus improving the quality of the dolomite reservoirs. These models provide new insights for targeting deep dolomite hydrocarbon reservoirs in intracratonic basins.

Lithostratigraphy and depositional environments of the upper cretaceous deposits in the central Taurides (S Turkey)

The study area is located in the Central Taurides (southern Turkey), which is bounded by the Kirkkavak fault to the west and Ecemis fault to the east. The sequences are studied in detail based on measured sections composed of the rocks deposited during the Cenomanian–Maastrichtian and located within different tectonic units previously described in the Taurides. The study materials include 217 thin section data from seven Cenomanian–Maastrichtian sequences of outcropping in different parts of the Central Taurides. The sediments deposited during the Cenomanian–Maastrichtian period in the Central Taurides are subdivided into eight units based on their lithological, paleontological, and textural properties. The lower boundaries of the upper Santonian and Campanian are unconformable contacts. The Upper Cretaceous sequence starts with the middle Cenomanian and represents a continuation of the Lower Cretaceous tidal flat and shelf lagoon sequence. Upper Turonian–Coniacian sediments are not observed due to the eustatic sea level drop. The second main transgression period of the Upper Cretaceous platform took place in the Santonian. This unit is represented by limestones composed of wackestones/packstones containing benthic foraminifera and rudist fragments, which are deposited in tidal flats and subtidal environments. The late Campanian starts with a transgression, and the environment transformed transitions into slope facies from inner platform facies, as a result of the thrust of ophiolitic rocks. In the following period, slope front and basin plain environments were dominant due to the increasing slope. Slumped pelagic limestones were deposited on the slope. Planktonic foraminiferal pelagic limestones were unconformably deposited on plaque limestone in the slope front environment depending on the increase in slope gradient and local faulting. As a result of decreasing tectonic activity, the sediments were deposited onto a stable basin plain. They were initially fed from the nearby carbonate platform and then by siliciclastic turbidites derived from the thrusted ophiolitic rocks. In this study, the lithostratigraphic properties of the Cenomanian–Maastrichtian units outcropping in various parts of the Central Taurides are described. The sedimentary deposits described here suggest different basinal conditions in the region.

Optical dating of Holocene tidal deposits from the southwestern coast of the South Yellow Sea using different grain-size quartz fractions

The tidal flat deposit provides ideal sedimentary records for paleoenvironmental studies. Reliable chronology is crucial to utilize this archive for deciphering the history of environmental changes. In this study, we applied optically stimulated luminescence (OSL) dating method to a Holocene tidal flat sequence using both coarse-grained (CG, 90–200μm) and fine-grained (FG, 4–11μm) quartz extracts from a sedimentary core (YZ07) in western coast of the South Yellow Sea. The luminescence characteristics of the two grain-size fractions were investigated and then their resulting OSL ages were systematically compared. The results suggested that most tidal flat deposits are well bleached and their FG quartz ages are generally consistent with CG quartz ages, while some samples have CG ages underestimated compared with FG, likely resulted from the K-feldspar contamination for CG quartz. Hence, we applied post-IR OSL dating and pulsed OSL dating techniques; they could overcome the problems caused by feldspar contamination, and yielded identical dates as FG OSL ages. All OSL ages are generally in stratigraphic order; in contrast, the 14C ages are much more disorder and characterized with severe inversions. Finally, the age framework of the tidal flat sequence under this study was constructed based on the 30 OSL ages and one acceptable radiocarbon age. According to the age-depth model, three main periods of sedimentation-rate (SR) variation were identified. These SR changes are probably associated with sea-level rise/fall history, and the depocenter landward/seaward movement as well as the transition of depositional process within the Holocene delta initiation. The depositional environment changes were also reflected in sedimentological features of the tidal flat deposits in our study area.

The Heuksan mud belt on the tide-dominated shelf of Korea: a supply-driven depositional system?

A 20–50-km-wide, approx. 200-km-long and 50-m-thick mud deposit around Heuksan Island, southwest Korea, extends from the mouth of the Geum River to the shelf off Jeju Island. The deposit formed in association with postglacial sea-level rise. However, the substantial thickness and rapid seaward progradation of the deposit over a period of only about 3–4 thousand years, as postulated by previous investigators, are difficult to reconcile with the rapid sea-level rise during the early to mid-Holocene. To resolve this discrepancy, the origin and stratigraphic evolution of the Heuksan mud belt were reexamined using high-resolution seismic profiles and three drill cores penetrating down to the basement. A sedimentary facies analysis with AMS 14C age control facilitated reconstruction of the postglacial evolution of the deposit. The seismic profiles reveal that the Heuksan mud deposit can be divided into three depositional events clearly defined by distinct spatiotemporal bounding surfaces. The lower unit contains retrograding tidal flat sequences formed during an earlier transgression commencing at ca.16.0 ka and terminating at 11.6 ka. The middle unit consists of rapidly prograding mud clinoforms deposited from 11–6.5 ka. This unit extends 80 km onto the distal shelf, and is characterized by sedimentation rates of 5.0 cm/year. From 6.5–6 ka, progradation was interrupted for a 500 year period during which the deposit retreated by 35 km due to a decrease in the sedimentation rate to 0.18 cm/year. The abrupt stratigraphic break was evidently triggered by a sudden change in sediment supply, resulting in rapid retreat of the unit despite continuing local sea-level rise. Renewed sedimentation commenced at about 6.0 ka, being initially characterized by clinoform progradation, followed by autoretreat from about 2.5 ka onward due to a decreasing sediment supply. The Heuksan mud belt may thus represent an example of a shelf depositional system driven predominantly by the rate of sediment supply, rather than by sea-level rise.

Magnetic and diffuse reflectance spectroscopic characterization of iron oxides in the tidal flat sequence from the coastal plain of Jiangsu Province, China

In this paper, we present the results of research on a coastal lowland Holocene tidal flat sequence in order to explore the variations in magnetic mineralogy during the process of tidal flat sedimentation and subsequent land formation. Three cores (∼6 m in length) were collected from the coastal plain in Jiangsu Province, China, and investigated with magnetic measurements, diffuse reflectance spectroscopy (DRS) and free iron oxide (Fed) analyses. The tidal flat sequence shows a fining-upward trend, and the top ∼2 m of each core with redoximorphic feature was interpreted to be a salt marsh facies in origin. Unmixing of isothermal remanent magnetization acquisition curves identify magnetite and maghemite as well as high-coercivity hematite and goethite, with the latter iron oxides confirmed by DRS analysis. Enrichment of goethite, hematite and maghemite occurs in the salt marsh deposits in comparison to the lower intertidal and subtidal deposits, with goethite being the dominant iron oxide. The changes in hard isothermal remanent magnetization acquired in field above 100 mT field (HIRM100) and the S–100 ratio largely reflect the presence of medium-coercivity maghemite in absolute content and relative proportions, respectively. The approach used for iron oxide characterization may offer an efficient diagnostic tool for recognizing sediments or soils subjected to redox condition oscillation elsewhere and contribute to studies of iron cycling in (palaeo-)environmental researches.

Preservation potential of individual couplet and deposition rates on mudflats in the Changjiang Estuary

Non-barrier mudflats in the Changjiang Estuary is examined to obtain preservation potentials and deposition rates over different time scales. The preservation potential of couplet is ca. 46.6% within daily scale and decreases as the time-interval extending, with 9.2% on a semi-lunar scale and 3.7% on a seasonal scale. In the long term, only 1.6% of daily deposition can potentially be preserved in the tidal flat sequence and other 98.4% have been eroded and leave sedimentary discordances, discernable or un-discernable, in the mudflat deposits. A negative log-linear relationship is examined between sedimentation rates and the time spans. The length of sedimentary hiatuses on the mudflat deposition were estimated with considerable uncertainty, but it can provide from an actualistic base some further insights into the nature and origin of ancient mudflat deposits with emphasis on the sedimentary discordances.

Tidal-Flat Sequence controlled by Holocene Sea-Level Rise in Gomso Bay, West Coast of Korea

Intertidal deposits of Gomso Bay, western coast of Korea (eastern Yellow Sea) consist of a basement, pre-Holocene oxidized unit, and a Holocene tidal unit in ascending order. The pre-Holocene oxidized unit, a yellowish stiff mud belonging to the last stadial (or glacial) stage before 12000yr B.P., contains plant roots, plant fragments, and vertical and horizontal microfractures indicating soil-formation. However, it contains no marine shells or microfossils. The Holocene tidal unit is composed of lower mud facies (upper-flat mud) and upper sand/muddy sand facies (middle-to lower-flat sand), and generally shows a coarsening-upward, retrograding trend, thus reflecting a Holocene relative sea-level rise.14C age/depth plot for dated samples (peats and shells) shows that the relative sea-level of 7000yr B.P. stood at about −6·5m from the present mean sea level, and the relative sea-levels of 4000yr B.P. and 2000yr B.P. stood at −3m and −2·5m, respectively.

Marine transgression and regression in Miocene sequences of northern Pegu (Bago) Yoma, Central Myanmar

Neogene strata of the northern part of the Pegu (Bago) Yoma Range, Central Myanmar, contain a series of shallow marine clastic sediments with stratigraphic ages ranging from the Early to Late Miocene. The studied succession (around 750 m thick) is composed of three major stratigraphic units deposited during a major regression and four major transgressive cycles in the Early to Late Miocene. The transgressive deposits consist of elongate sand-bars and broad sand-sheets that pass headward into mixed-flats of tidal environments. Marine flooding in transgressive deposits is associated with coquina beds and allochthonous coral-bearing sandy limestone bands. Major marine regressions are associated with lowstand progradation of thick estuary point-bars passing up into upper sand-flat sand bodies encased within the tidal flat sequences and lower shoreface deposits with local unconformities. The succession initially formed in a large scale incised-valley system, and was later interrupted by two major marine transgressions in the generally regressive or basinward-stepping stratigraphic sequences. Successive sandbodies were formed during a sea-level lowstand and early stage of the subsequent relative rise of sea level in a tide-dominated estuary system in the eastern part of the Central Myanmar Tertiary Basin during Early to Late Miocene times.

Origin of Zn-Pb-Ag Sulfide Mineralization within Upper Proterozoic Phosphate-Rich Carbonate Strata, Irecê Basin, Bahia, Brazil

The Irece Basin is a relatively small late Proterozoic basin within the São Francisco craton of northeast-central Brazil. Stratabound Zn-Pb-Ag sulfide and phosphate-rich units occur within a 50-m-thick tidal-flat sequence of dolomitic limestone and cherty dolostone within the lower Salitre Formation of the Una Group. Phosphate is concentrated in stromatolitic structures; metallic mineralization is represented by stratiform masses and disseminations of sulfide minerals in a tidal-flat sequence that has relict evaporitic structures. Zn-Pb-Ag sulfide concentrations in the Irece area are associated with parallel laminated, silty dolostone with shallow-water sedimentary structures, including laminar stromatolites, mud cracks, tepee structures, collapse breccias, length-slow quartz nodules, and pseudomorphs after evaporitic sulfates. Sulfide minerals include pyrite, sphalerite, galena, marcasite, jordanite, tetrahedrite, and covellite, in order of decreasing abundance. Nodular and bladed sulfide forms are pseudomorphous after evaporitic sulfate minerals. Replacement textures, such as corroded grain boundaries between sulfide minerals, are common. Sulfur-isotope analysis indicates relatively uniform heavy δ34S values. Barite has a δ34S range from +25 to +31‰, CDT. Pyrite, sphalerite, galena, and jordanite-representative of a variety of textural types-have a δ34S range of +19 to + 23‰; δ34S values for Irece barite probably represent original late Proterozoic sea-water sulfate values. The consistently heavy δ34S values of the Irece sulfides likely are the result of thermochemical reduction of a limited evaporitic sulfate source by organic matter that is abundant in the Salitre carbonates. Major Zn-Pb-Ag concentrations appear to be an overprint on the earlier iron sulfide-rich zone. Limited fluid-inclusion data suggest precipitation in the range of 140 to 200°C from formation waters with salinities ranging from 3 to 12 wt%. Deep fluid circulation may have been promoted by reactivation of basement faults originally related to the Irece Basin's development. Petrographic and fluid-inclusion data suggest that sulfide mineralization may have occurred within a mixing zone of metal-bearing, high-temperature basinal brines with meteoric water. Metallic mineralization scavenged sulfur from pre-existing sulfides or from direct reduction of evaporitic sulfate minerals. The metal and phosphate concentrations in the Irece Basin possess some similiar geologic characteristics that result from a common host-rock depositional environment. However, existing information indicates that the two deposit types are not directly related genetically, although the origin of both is related to the organic-rich nature of the host sequence.

A depositional and diagenetic model for the Eocene Sulcis coal basin of SW Sardinia

Abstract Detailed sedimentological, mineralogical and petrographical analysis of closely spaced cored boreholes has enabled the development of a revised depositional model for the early Eocene coal-bearing Produttivo Formation of the Sulcis Basin. The deposition of autochthonous-hypautochthonous palustrine-lacustrine coals and associated carbonates was interrupted episodically by sedimentation of allochthonous lithocalcirudites and lithocalcarenites. The latter clastics display characteristic upward shoaling tidal flat sequences related to marine incursions. This interpretation is in contrast to the previously accepted fluvial origin of the detrital episodes. The coarse basal transgressive lag deposits consist of various carbonate intraclasts, dolosparite, grains consisting of calcite cement and euhedral-subhedral evaporite-bearing quartz grains. Combined stable isotope and cathodoluminiscence analysis has revealed a complex diagenetic history for the clastic deposits. The potential extrabasinal or intrabasinal provenance of the clasts, in particular the origin of the evaporite relicts, is discussed. The subtropical-tropical coastal marshes of the Florida Everglades (USA) are proposed as a possible modern analogue for the subbituminous Sulcis coals.

The Silty Beds: a tidal flat sequence at the junction of the Lower Greensand and Gault (Albian, Lower Cretaceous) of Bedfordshire, England

The Silty Beds are a Lower Albian tidal flat deposit with sediments of the intertidal and subtidal zone. They are interpreted as the deposit of a sheltered shore, near an estuary mouth, with low wave energy and low tidal range. The Silty Beds are a well-bedded series of sands, silts and clays with minor ironstone and intermittent carbonate lenses occurring towards the top of the unit. These beds include carbonaceous material, pyritized, glauconitic and other trace fossils as well as glauconite-rich sands. Some beds are highly bioturbated, while others show only moderate burrowing. Where there is no burrowing activity, detailed sedimentary structure has been preserved, such as fine parallel-lamination, cross-stratification, fine ripple lamination and abundant flaser bedding. The environment of deposition is interpreted as tidal sand and mud flats with intermittent freshwater and brackish lagoons. Ironstone development was probably early diagenetic. The top of the succession culminates in emergence and erosion of the topmost ironstone. Cavities, thus formed, were subsequently infilled with a bioclastic limestone (the Shenley Limestone) during the transgression leading into the deeper water marine clays of the Gault.

Transgression of an estuarine channel and tidal flat complex: the Lower Triassic of Barles, Alpes de Haute Provence, France

ABSTRACTThe Lower Triassic succession of Barles, Alpes de Haute Provence, France, comprises an unconformable quartz arenite sand body of 90m thickness. The succession may be informally divided into (i) lower channellized cross‐bedded member overlain by (ii) an upper fining upward member. The lower member comprises vertically stacked, subtidal channel units separated into five major sand bodies by thin developments of fine grained channel margin and shoal deposits. Subtidal channel fill deposits are dominated by varying scales of cross bedding. These scales vary systematically from the base to the top of the member, with large scale planar sets dominating the lowest channel sand body (sand body 1), medium scale planar and trough cross bedding characterizing sand bodies 2‐4, the largest scale planar sets in the highest sand body (sand body 5). This upward change in cross bedding scale is concomitant with a decrease in both the relief of major channel sand body erosion surfaces, and the proportion of preserved interchannel shoal deposits. The succeeding fining upward member comprises small scale tidal channel units overlain by channel shoal and tidal flat deposits. Tidal flat sequences are characterized by parallel laminated, wave and current rippled sandstones separated by bioturbated, fine grained siltstones and mudstones.The vertical variation in facies of the Lower Triassic succession suggests two main periods of deposition. The lower member is considered to preserve successively more seaward components of a transgressive estuarine complex. The overlying upper member records the seaward progradation of tidal channel, shoal and tidal flat environments.The unconformity bounded nature of the lower member, combined with its systematic variation in facies, suggests it may represent an incised valley‐estuarine fill developed in response to an early Triassic relative sea level fall and subsequent rise. Succeeding tidal channel and tidal flat deposits forming the upper fining upward member reflect a change in sediment supply and/or rate of relative sea level rise comparable with a progradational shoreline. It is unclear whether this final depositional episode represents a period of highstand progradation or a later lowstand shoreline system developed following a further period of relative sea level fall and rise.

Storm-deposited sandstones (tempestites) and related ichnofossils of the Late Ordovician Georgian Bay Formation, southern Ontario, Canada

The Late Ordovician Geogian Bay Formation of southern Ontario, Canada, comprises up to 250 m of grey to blue–grey shales interbedded with highly fossiliferous calcareous sandstones. These strata were deposited in equatorial paleolatitudes after 448 Ma in a shallow foreland basin created by overthrusting along the eastern margin of North America (the Taconic orogeny). The Georgian Bay Formation comprises the middle part of an upward-shallowing progradational sequence from deep-water transgressive shales of the underlying Whitby Formation to muddy tidal-flat sequences of the overlying Queenston Formation. Exposures in brickyard and river cuts near Toronto, and northwards along a narrow outcrop belt along the foot of the Niagara Escarpment, show laterally extensive (100 m+), sharp-based sheets of sandstone up to 1 m thick, with gutter casts and washed-out (hypichnial) trace fossils (dominantly Planolites and Paleophycus) on their lower bedding surfaces. Detailed examination of sandstone beds in outcrop and in three boreholes that penetrate the formation shows that the beds are composed internally of a basal fossil hash layer overlain by flat, hummocky, and wave-rippled divisions. Bed tops show a variety of wave-ripple forms and are heavily bioturbated (dominantly Bifungites, Conostichus, Diplocraterion, Didymaulichnus, Teichichnus). Sandstone sheets are interpreted as storm deposits (tempestites) resulting from tropical storms (hurricanes) transporting fine-grained suspended sediment from a delta plain onto a muddy shelf to the west.

Origin of Sulfide and Phosphate Deposits in Upper Proterozoic Carbonate Strata, Irece Basin, Bahia, Brazil

Carbonate strata of the Una Group represent late Proterozoic platform sedimentation in the Irece basin of north-central Brazil. Stratabound sulfide- and phosphate-rich units occur within a 50-m thick tidal flat sequence of dolomitic limestone and cherty dolomite. Three types of primary phosphate concentrations are present: columnar stromatolitic, laminar stromatolitic, and intraclastic. Resedimented phosphate clasts and phosphatic units interbedded with non phosphatic dolomites suggest early diagenetic replacement of algal carbonate units. Local stratabound Zn-Pb-Ag sulfide concentrations at the Tres Irmas prospect occur within silty dolomite with shallow water sedimentary structures and local disturbed laminae, synsedimentary faults, and breccias. Sulfide minerals include pyrite, sphalerite, galena, marcasite, jordanite, tetrahedrite, and covellite. Pyrite crystal aggregates commonly show bladed forms. Nodular aggregates of length-slow quartz are locally associated with sulfides. Sulfur isotope analyses indicate relatively uniform heavy {delta}{sup 34}S values. Barite shows a {delta}{sup 34}S range from +25.2 to +29.6{per thousand}, CDT. Pyrite and sphalerite representative of a variety of textural types have a {delta}{sup 34}S range of +20.2 to +22.6{per thousand}. Late Proterozoic evaporite sulfates show a wide range of {delta}{sup 34} S values from about +10 to +28{per thousand}. Thus, the {delta}{sup 34}S values for Irece barite could reflect original seawater sulfate values.more » However, the relatively heavy {delta}{sup 34}S values of the associated sulfides suggests that the original seawater sulfate was modified by bacterial sulfate reduction processes in shallow sea floor sediments. Textural and {delta}{sup 34}S evidence suggests that a later stage of metallic mineralization scavenged sulfur from preexisting sulfides or from direct reduction of evaporitic sulfate minerals.« less

Microfossils (foraminifera, Ostracoda) from an Eemian (last Interglacial) tidal flat sequence in south-west Denmark

The north-west European coastal plain was indented by several shallow bays and expanses of tidal flat sedimentation during the Eemian Interglacial. One of these bays, at Højer, south-west Denmark, has been the subject of intensive research and several lines of evidence indicate an Eemian age. This paper discusses the foraminifera and Ostracoda present in these deposits. Three phases of accumulation are recognized: an initial brackish tidal flat grading into a more open tidal flat environment; a phase of shallow, subtidal deposition (Turritella-Ton) when water depths may have attained 15–20 m; and a final phase of shallowing conditions reflecting a fall in sea-level. The warm climate which prevailed during the Eemian enabled the migration of certain species of more southerly affinity into the North Sea. These include the foraminifera Elphidium lidoense, Elphidium translucens, Quinqueloculina aspera, and Quinqueloculina seminulum var. Jugosa, and the Ostracoda Callistocythere cf. praecincta, Carinocythereis whitei, Semicytherura arcachonensis, and Aurila aff. A. prasini. None of these taxa are present in the Holocene faunas of the North Sea and only two are known from Holsteinian Interglacial deposits. These taxa are, therefore, considered as index fossils for the Eemian in this region.

Sedimentary Textures of Recent Belizean Peritidal Dolomite

ABSTRACT Recent surface and buried dolomite crusts and massive dolomite beds on Ambergris Cay, Belize have formed on topographically low-relief, peritidal flats whereon the hydrologic system is characterized by a dominant phreatic diagenetic overprint. As a consequence of their low subaerial exposure index, such dolomites and associated undolomitized surface sediments lack many of the textural and microfabric features that are considered diagnostic of the peritidal environment. The progressive dolomitization of precursor sandy, fossiliferous sediments (wackestones to packstones) is accompanied by a textural change to dolowackestones with allochem ghosts, and a porosity system that resembles, but is distinct from, syndepositional fenestral fabrics formed in unconsolidated or partly consolidated peritidal sediments. The microcrystalline dolomite cements in these rocks are predominantly of phreatic aspect, and only rare examples of possible vadose diagenetic fabrics are observed. These texturally anomalous peritidal dolomites, characterized by a markedly subtidal appearance, may have fossil counterparts in many ancient carbonate tidal-flat sequences.

Tidal-Flat Sedimentation from Hurricane Kate, Caicos Platform, British West Indies

ABSTRACT Hurricane-generated, thinly bedded grainstones are the dominant style of stratification on the carbonate tidal-flat complex on Caicos Platform, British West Indies. This is in dramatic contrast to the winter-storm-generated millimeter-thick laminae which dominate stratification on the northwest Andros tidal flats and have become the general criteria for recognizing ancient carbonate tidal-flat sequences. Hurricane Kate, which passed directly across the Caicos tidal flats on 18 November 1985, provided an opportunity both to document the character and distribution of a specific storm-sediment layer and to evaluate the role of hurricanes in molding the geometry of the tidal-flat system. Though moderate in strength and producing only incomplete flooding, Hurricane Kate deposited a layer of fine to very fine peloidal grainstone 0.5-2 cm in thickness across significant portions of the extremely broad, low-relief shore and channel levees and the inland algal marsh. These storm layers are interbedded with organic-rich layers which represent prolonged periods of growth of Scytonema algal mat between hurricanes. Recognition that centimeter-thick grainstone layers dominate entire tidal-flat complexes necessitates both a thorough modification of criteria for defining ancient carbonate tidal-flat deposits and a reevaluation of supposedly subtidal, thinly bedded limestones.

Facies Relationships and Depositional Environments of Lower Tuscaloosa Formation Reservoir Sandstones in McComb and Little Creek Field Areas, Southwest Mississippi: ABSTRACT

Facies Relationships and Depositional Environments of Lower Tuscaloosa Formation Reservoir Sandstones in McComb and Little Creek Field Areas, Southwest Mississippi: ABSTRACT

Niton Field: An Estuarine Sandstone Reservoir: ABSTRACT

The Niton field is a classic stratigraphic trap caused by the updip migration of hydrocarbons along a gentle southwesterly regional tilt (50 ft/mi, 9.5 m/km) into reservoir sandstones of the Basal Quartz Formation. These reservoir sandstones were deposited during the Late Jurassic to Early Cretaceous transgression over an unconformable surface on Jurassic sediments. As sea level rose, the topography on the unconformity gave rise to a number of tide-dominant estuarine valleys, 3 to 5 mi (5 to 8 km) wide at their mouths. Two major factors related to this depositional setting exerted the greatest control on the distribution of Basal Quartz reservoir potential sandstones in the Niton field. (1) The topography of the unconformity governed the lateral extent of estuarine sand bodies deposited during the transgression. Maximum thickness of the sand bodies occurs either (a) along the long axes of paleovalleys, or (b) at entrances to the paleovalleys. (2) Diagenetic patterns of cementing were related to the original environments of deposition. The occurrence of swamps on the topographically high areas generated acidic ground waters which leached carbonate and silica from underlying sediments. This leached material was reprecipitated as silica cements in sediments overlying topographic highs and as calcite ce ents in tidal-flat sequences of the estuary fringes. These cements reduced porosities and permeabilities sufficiently to produce updip and capping seals to reservoir sandstones. Open marine sandstones deposited at the mouths of estuaries were only lightly cemented and, therefore, became the primary reservoir sandstones. By comparing the depositional setting of the Niton area with modern analogs along Holocene transgressive coastlines, it is apparent that similar stratigraphically controlled sandstone bodies should exist along depositional strike in other drowned estuarine river valleys. Thus, by comprehending the depositional and diagenetic setting of the Basal Quartz Formation, the energy explorationist has a predictive tool that can be used to discover new areas of reservoir potential sandstones. End_of_Article - Last_Page 583------------

Depositional Model of Sand-Dominated Mesotidal Estuary: ABSTRACT

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