Sandflat Environment Research Articles

Drainage and environmental evolution across the Permo–Triassic boundary in the south‐east Germanic Basin (north‐east Bavaria)

AbstractThe identification of the factors triggering continental environmental changes at the Permian–Triassic transition is of general interest as they allow a better understanding of the most controversial mass extinction of the Phanerozoic. This study investigates the environmental response to external forcings on continental successions at the south‐east‐margin of the Central European Basin System in Germany. Studies from this area are scarce, and its evolution is poorly understood. This work integrates high‐resolution core‐logging and quantitative sedimentary petrography of Middle Permian–Early Triassic successions recovered from the wells Obernsees‐1 and Lindau‐1. The sedimentological investigation reveals major vertical variations in the depositional settings marked by changes in grain‐size trends and facies associations: (i) the progradation of mass flow/fluvial fans over sandflat environments denotes the Guadalupian–Lopingian; (ii) sabkha to shallow‐marine conditions occur during the uppermost Permian; (iii) the onset of braided fluvial sequences marks the Permian–Triassic boundary; (iv) changes in fluvial style, reflecting increased river sinuosity, and aeolian deposition characterize the remaining Triassic. Environmental changes are accentuated by variations in sediment composition. Proximal deposits (for example, alluvial fan) are lithic arkoses, while more distal associations reveal arkosic composition. Detrital modes point to: (i) a high‐grade metamorphic source feeding the drainage of Lindau‐1 (metamorphic index, MI 441–500); and (ii) a mixed low‐grade and high‐grade metamorphic source (MI 277–395) feeding the Permian of Obernsees‐1, passing upward to high‐grade metamorphic (MI 441–500) and subordinate granitoid sources. Compositional signatures and environmental changes suggest distinct drainage evolutions regulated by upstream climate and tectonic controls. The higher plutonic clast content coeval to the onset of fluvial sedimentation and coarse‐grained influxes reflects local tectonic perturbations attributed to the Early Triassic reactivation of Variscan faults. The evidence of perennial fluvial systems during the lowermost Triassic reflects wetter climate conditions in the upstream catchment area, which contradicts the common view of increased aridity at the Permian–Triassic boundary.

Peesey Swamp—a Last Interglacial (MIS 5e) marine corridor across southern Yorke Peninsula, southern Australia

Peesey Swamp on southern Yorke Peninsula, southern Australia, is a north-northwesterly—south-southeasterly trending lowland depression, approximately 24 km long and 4 to 10 km wide. The richly fossiliferous mollusc and foraminiferal faunal assemblages of the last interglacial Glanville Formation in Peesey Swamp indicate that the paleoenvironment was a low- to medium-energy, sheltered sandflat to shallow-water seaway during the Last Interglacial Maximum (Marine Isotope Substage [MIS] 5e; 128–116 ka). The presence of the fossil bivalve molluscs Katelysia rhytiphora, Chlamys (Equichlamys) bifrons and Fulvia tenuicostata, in life position, indicate that the paleowater depth was ≤4 m in Peesey Swamp. The seaway cut across southern Yorke Peninsula, establishing a marine connection between southern Spencer Gulf and Investigator Strait during deposition of these taxa. Amino acid racemisation dating of the fossil marine molluscs Katelysia sp. and Fulvia tenuicostata confirms that the shelly assemblages at Peesey Swamp are correlative with the last interglacial Glanville Formation, a succession of richly fossiliferous, mixed quartz–skeletal carbonate sands documented from many sites along the southern Australian coastline. This study illustrates the significant changes in the geographical configuration of coastlines that may accompany relatively modest (<5 m) sea-level changes on low-gradient coastal landscapes. KEY POINTS Peesey Swamp on southern Yorke Peninsula was flooded by seawater forming a 4 to 10 km-wide seaway during the Last Interglacial Maximum (MIS 5e). Fossil mollusc and foraminiferal assemblages reveal deposition within shallow subtidal to intertidal sandflat environments indicating that the paleowater depth and height of sea-level in Peesey Swamp was ≤4 m. Amino acid racemisation dating of fossil marine molluscs confirms that the shelly assemblages at Peesey Swamp are of last interglacial (MIS 5e) age. The distribution of the foraminifer Marginopora vertebralis indicates that the eastward flow of the warm-water Leeuwin Current, during the Last Interglacial Maximum, was retarded by the biogeographical barriers of Kangaroo Island and Fleurieu Peninsula.

A baited remote underwater video system (BRUVS) assessment of elasmobranch diversity and abundance on the eastern Caicos Bank (Turks and Caicos Islands); an environment in transition

The present study was undertaken to assess the diversity and abundance of elasmobranch fishes in coral reef and sand flat environments on the eastern Caicos Bank, with a view to informing marine spatial planning as the island of South Caicos and its environs transition to a tourism-based economy. Using baited remote underwater video systems (BRUVS), the nurse shark Ginglymostoma cirratum, Caribbean reef shark Carcharhinus perezi, spotted eagle ray Aetobatus narinari, southern stingray Hypanus americanus, lemon shark Negaprion brevirostris, tiger shark Galeocerdo cuvier, blacknose shark Carcharhinus acronotus, and great hammerhead shark Sphyrna mokarran were observed to use these waters, with G. cirratum and C. perezi being particularly abundant. Species diversity and overall abundance was greater in the reef environment than on the sand flats, but G. cirratum was equally abundant in both environments. Furthermore, even reef-associated species such as C. perezi were occasionally encountered on the flats a considerable distance from the reef. This indicates that although marine conservation efforts in the Turks and Caicos Islands should continue to focus on coral reef areas, less dramatic environments such as sand flats should not be ignored.

Stratigraphic evolution of the northern part of the Cretaceous Neungju Basin, South Korea

Several nonmarine basins were formed in SW Korea under an extensional or transtensional tectonic regime during the Cretaceous. The Neungju Basin is one of these basins, filled by alluvial to lacustrine deposits interbedded with thick, amalgamated tuff beds. Facies analysis in the northern part of the basin shows that the basin fill is composed of four facies associations, representing (1) alluvial fan, (2) alluvial plain, (3) sandflat, and (4) marginal playa lake environments. The basin fill can be subdivided into four stratigraphic units by interbedded thick amalgamated tuff beds. The lowermost unit (unit I) is composed of alluvial plain deposits. The lack of any vertical depositional trend indicates a balance between basin subsidence and sediment supply. The middle unit (units II and III) show regional variations. The extensive lake formed in the central to northern parts of the basin, which are later partly filled by proximal sediments from surrounding areas. In contrast, only alluvial plain to sandflat environments occurred in the southern part. The uppermost unit (unit IV) shows a transition from alluvial fan to sandflat, comprising a fining-upward trend that reflects a gradual decrease in sediment supply and slope-gradient during denudation of uplifted source areas. The variations in depositional patterns were possibly caused by the relationship between volcanism-driven sediment supply and basin subsidence. Basin subsidence may have been sufficient to compensate the volcanism-driven sediment supply prior to the deposition of units II and III, resulting in the conformable deposition of lake sediments over thick amalgamated tuff beds. In contrast, volcanism-driven sediment supply may have reduced the generation of accommodation prior to the deposition of unit IV, resulting in the deposition of conglomerates over thick tuff beds. The traditional depositional model composed of syn-eruption aggradation and inter-eruption degradation may not sufficiently explain the development of sedimentary succession, in which the balance between volcanism-driven sediment supply versus basin subsidence coeval to volcanism is variable.

Middle Ordovician acritarchs and problematic organic-walled microfossils from the Saq-Hanadir transitional beds in the QSIM-801 well, Saudi Arabia

Core samples from the QSIM-801 water well, drilled in central Saudi Arabia, cover a 93-foot interval spanning the transition between the Sajir Member of the Saq Formation, that consists mainly of sandstones of tidal sand flat environments, and the Hanadir Member of the Qasim Formation, characterized by argillaceous graptolitic mudstones, corresponding to a tidal delta front. The samples contain abundant, exceptionally well-preserved and diverse palynomorphs, which include cryptospores, acritarchs and chitinozoans, other problematic organic-walled microfossils as well as other organic particles such as cuticle-like fragments. The studied interval is biostratigraphically well constrained by the presence of chitinozoans of the formosa and pissotensis Zones of late-early to late Darriwilian age (Middle Ordovician) in the uppermost Saq Formation and Hanadir Member. The biostratigraphic age of the Sajir Member considered to span the Dapingian–Darriwilian boundary, is re-discussed based on the results herein. The uppermost part of the Sajir Member yielded the ichnofossil, Phycodes fusiforme. Acritarch assemblages from the Sajir Member of the Saq Formation are poorly diversified and dominated by sphaeromorphs. More diverse assemblages of acritarchs, associated with enigmatic forms, occur in the Hanadir Member of the Qasim Formation. The contact between the two formations and the transition between the palynomorph assemblages are sharp, suggesting a stratigraphic hiatus. A quantitative analysis allows us to discuss the paleoenvironmental changes and possibly climatic changes associated with an hypothesis of ice house conditions during this period. Among the diagnostic acritarch taxa observed are Frankea breviuscula, F. longiuscula, Baltisphaeridium ternatum, Dasydorus cirritus, Dicrodiacrodium ancoriforme, Poikilofusa ciliaris, Pterospermopsis colbathii and Uncinisphaera fusticula. These are associated with other typical forms known to range across the Lower–Middle Ordovician boundary, such as Aremoricanium rigaudae, Aureotesta clathrata, Barakella fortunata, B. rara, Baltisphaeridium klabavense, Glaucotesta latiramosa and Striatotheca spp. Galeate and peteinoid acritarchs are also well represented, as well as tiny forms of ultraplanctonic size. Three new species of acritarchs are proposed: Frankea longiuscula var. darriwilense var. nov, Micrhystridium regulum sp. nov, and Tyrannus proteus sp. nov. Repeated occurrences throughout the section of cryptospores, problematic microfossils such as organic filaments, cuticle-like tissues, striated and pigmented leiospheres frequently in clusters, are interpreted to reflect recurrent terrestrial and freshwater inputs in the depositional environment. Single-specimen, high-resolution analyses using Confocal Laser Scanning Microscopy on the enigmatic form Tyrannus proteus sp. nov. show fluorescence emission spectra and microstructural properties significantly different from those of typical marine acritarchs from the same levels.

Sedimentology of the Essaouira Basin (Meskala Field) in context of regional sediment distribution patterns during upper Triassic pluvial events

Upper Triassic continental clastics (TAGI: Trias Argilo-Greseux Inferieur) in the Essaouira Basin are largely restricted to the subsurface, which has limited analysis of the depositional environments and led to speculation on potential provenance of the fluvial systems. Facies analysis of core from the Meskala Field onshore Essaouira Basin is compared with tentatively time-equivalent deposits exposed in extensive outcrops in the Argana Valley, to propose a process orientated model for local versus regional sediment distribution patterns in the continuously evolving Moroccan Atlantic rift during Carnian to Norian times. The study aims to unravel the climatic overprint and improve the understanding of paleo-climatic variations along the Moroccan Atlantic margin to previously recognised Upper Triassic pluvial events.In the Essaouira Basin, four facies associations representing a progressive evolution from proximal to distal facies belts in a continental rift were established. Early ephemeral braided river systems are succeeded by a wet aeolian sandflat environment with a strong arid climatic overprint (FA1). This is followed by the onset of perennial fluvial deposits with extensive floodplain fines (FA2), accompanied by a distinct shift in fluvial style, suggesting increase in discharge and related humidity, either locally or in the catchment area. The fluvial facies transitions to a shallow lacustrine or playa lake delta environment (FA3), which exhibits cyclical abandonment. The delta is progressively overlain by a terminal playa with extensive, mottled mudstones (FA4), interpreted to present a return from cyclical humid-arid conditions to prevailing aridity in the basin.In terms of regional distribution and sediment source provenance, paleocurrent data from Carnian to Norian deposits (T5 to T8 member) in the Argana Valley suggest paleoflow focused towards the S and SW, not directed towards the Meskala area in the NW as previously suggested. A major depo-centre for fluvial sediments is instead located in the southern Argana Valley, possibly the Souss Basin. To effectively source the reservoir sandstones found in the Meskala Field, a more local provenance area has hence to be envisaged. Despite this, the direct comparison of the genetic evolution of sedimentary sequences in the Argana Valley and Essaouira Basin shows a similar progression from dominantly arid ephemeral depositional environments to humid perennial sedimentation, returning to prominent arid conditions. This suggests climatic control in both regions, where an enhanced humid signal drives perennial fluvial flow in otherwise arid dominated sequences. On a regional scale, this is suggested to record the impact of strong Triassic pluvial events previously recognised in other basins along the Central Atlantic margin during the Carnian to Norian periods.

Population dynamics and secondary production of gastropods on a sheltered beach in south-eastern Brazil: a comparison between an herbivore and a scavenger

A greater amount of organic matter and the higher productivity in sheltered environments, such as muddy beaches, can be attributed to the presence of bacteria and plants, which provide more food in these environments than in exposed sandy beaches. However, the influence of this productivity on the population biology of different trophic levels has not been adequately assessed in sheltered environments. We examined the variation of population dynamics and secondary production of two intertidal gastropods, the herbivorous Cerithium atratum and the scavenger Nassarius vibex, in a sandflat environment. Monthly samplings were conducted at Flexeiras beach according to a systematic design. The highest population abundances were observed in spring for both species. The herbivorous species had higher abundance, individual sizes, growth performance, mean biomass, secondary production somatic, slower growth acceleration (K) and longer life spans than the scavenger species. Similar mortality was observed between both gastropods and could be explained by biological (i.e. predation) and physical (e.g. organic material and sediment type) factors. The life-history traits of the gastropod species responded to trophic levels. Herbivores had higher abundance, growth and production efficiency than did scavengers. However, environmental factors, such as temperature and food quality and availability, also affected these species.

The role of detrital anhydrite in diagenesis of aeolian sandstones (Upper Rotliegend, The Netherlands): Implications for reservoir-quality prediction

The Rotliegend (Upper Permian) reservoir interval in the Southern Permian Basin (SPB) contains low-permeability streaks corresponding to anhydrite-cemented intervals. An integrated study was conducted using core, cuttings, thin sections and well-log data from a gas exploration well and two geothermal wells that target the zone of interest. This study aims at understanding the origin and nature of these low-permeability streaks, as well as their impact on reservoir properties, and to establish a predictive model of their spatial distribution. High-resolution XRF core-scanning analysis allowed to extrapolate spot observations in thin sections to the entire core. Diagenetic history includes grain rearrangement and anhydrite, haematitic clay coatings, dolomite rims, quartz overgrowths, kaolinite and second-generation carbonates as cementing phases. Coupling of all data reveals the detrital origin of the anhydrite/gypsum grains which were deposited together with the coarse-grained sand fraction in an aeolian sandflat environment. Such partially or completely dissolved grains acted as local sources of anhydrite cement and as nuclei for precipitation, explaining its preferential occurrence in coarse-grained laminae. Thick gypscretes in the vicinity likely supplied the anhydrite/gypsum grains. A conceptual model is proposed, including the location of nearby gypscretes and the prevailing west-southwest aeolian transport direction on the southern rim of the SPB.

Foraminiferal record of the postglacial (Holocene) marine transgression and subsequent regression, northern Spencer Gulf, South Australia

Core SG120 recovered 3.65 m of Quaternary sediment from a northern, shallow-water environment of Spencer Gulf, a marine embayment into the southern continental margin of Australia. Previous investigations had revealed that the upper interval 0 – 148 cm is Holocene marine bioclastic sediment, and that the lower Late Pleistocene interval 250 – 365 cm, with its carbonate palaeosol, had a similar marine origin. However, the age and origin of the interval 148 – 250 cm remained subject to ambiguous interpretation. Re-examination of core SG120, employing detailed foraminiferal analysis, has revealed that this middle unit records the earliest sedimentation associated with the postglacial marine transgression into the northern gulf. These basal Holocene sediments, which incorporated broken, corroded and carbonate-encrusted tests from the underlying palaeosol, together with tests of more pristine appearance, were deposited in a shallow-water, seagrass sandflat environment similar to those in coastal settings of the modern gulf. The lithological change at 148 cm has therefore been reinterpreted as a facies change related to increasing water depth. Radiocarbon analyses of fossil molluscs support this interpretation and reveal that marine transgression, at the site of SG120, was initiated prior to 8600 y cal BP. Selected species of foraminifers (Nubecularia lucifuga, Massilina milletti, Peneroplis planatus, Discorbis dimidiatus, Elphidium crispum and E. macelliforme) together reveal a consistent record of the final stages of the transgression with maximum water depth indicated at a core depth of 90 cm. Subsequent regression, which has been attributed to the combined effects of hydroisostatic uplift and sediment aggradation, is equally recorded by the foraminiferal assemblages.

Spatial and temporal variability in the Holocene sea-level record of the South Australian coastline

Studies of past sea levels based on radiocarbon-dated field samples suffer variously from compilation of information from disparate locations and the imprecise nature of the dated indicators. Geographic segregation and systematic elimination of poor samples from time–depth data sets leads to improved interpretation of Holocene sea-level history. An example is presented from Southern Australia, where this is achieved through selection of higher-resolution palaeoenvironmental indicators, separation of transgressive from regressive populations, and geographic regionalisation of data. In Southern Australia, fossil sea-level indicators preserved in prograding coastal plain settings include seagrass, sandflat, mangrove, samphire and chenier ridge organo–sedimentary remains. These provide sea-level time–depth data points with a variety of elevational and dating errors. Preserved organic remains at the transition from Posidonia seagrass to intertidal sandflat environment, and from sandflat to Avicennia mangrove environment provide the most precise local data for tracing sea-level change. Time–depth plots of 233 dated sea-level indicators from South Australia generate a broad sea-level envelope tracing the Southern Australian transgression from 10,000 to ca. 6000 radiocarbon years BP, followed by a more or less consistent level to the present. Finer details of sea-level behaviour are only apparent after systematic selection, separation and regionalisation of the data. The data indicate a very rapid sea-level rise in the early Holocene, at about 16 mm/year, reaching present levels at 6400 years BP. This was followed by regionally variable regression and emergence of the land of 1–3 m, a process that continues to the present. The systematic increase from 1 to 3 m in the elevation of the 6400-year BP highstand with distance away from the shelf margin is consistent with a hydro-isostatic origin for the emergence as predicted by geophysical models.

Geochemistry of waters and brines from the Salinas Grandes basin, Córdoba, Argentina. II. Gypsum dissolution-calcite precipitation, and brine evolution

Rivers and streams originating in the surrounding mountainous area are the major sources of salt in the Salinas Grandes basin (Cordoba, Argentina). These rivers infiltrate when they reach the sandflat or in the fringes of the mudflat, feeding springs which often form shallow lakes. Presently, the lakes are distant from the playa edge, thus allowing inflow waters to dissolve ancient (Pleistocene?) evaporite beds. In the sandflat environment, two dominant types of water have been recognized (SO42−-Cl−-HCO3−-Na+, and Cl−-SO42−-HCO32−-Na+), both considered as original members of the brine in the saline complex. Two main sources of solutes were distinguished, one related to the waters supplied by the southern sector and another to waters of the eastern sector. As a result of the chemical evolution in the playa environment, all brines belong to the neutral type (Cl−-SO42−-Na+). Following Hardie and Eugster's (1970) model, waters from the southern sector should evolve towards an alkaline brine (Cl−-SO42−-HCO3−-Na+), whereas those from to the eastern sector should evolve towards a neutral one (Cl−-SO42−-Na+). A computer simulation was carried out to model the chemical evolution of source waters. The results obtained by this methodology showed the same dichotomy (alkaline vs. neutral) established by Hardie and Eugster's (1970) model. The deficit in alkalinity could not be explained by any of the mechanisms published until now. Gypsum dissolution is the most likely mechanism which accounts for the chemical evolution of the waters investigated. When such a process is included in the computations, the Ca2+ supplied by gypsum beds generates an increase in the ion activity product (aCa+2·aCO32−) and produces a significant change in the 2Ca+2/(2CO32−+HCO3−) ratio, switching from values less than 1 to values greater than 1. This process determines the precipitation of calcite, and leads to a decrease in alkalinity, which in turn would explain the existence of a neutral brine in the saline complex. An intermediate salinity brine was detected in the mudflat, which, according to the model (Hardie and Eugster, 1970), should evolve towards a SO42−-free neutral brine (Cl−-Na+-Ca2+). The absence of this type of brine may be explained through mixing processes.

Nature and Causes of Transgressive and Regressive Cycles in a Lower Paleozoic Quartzose Sheet Sand: ABSTRACT

The stratigraphic section in the interior of North America contains a series of flower Paleozoic quartzose sheet sandstones, and their classic representative is the St. Peter Sandstone. The St. Peter was deposited during a middle Ordovician transgression, but it is far thicker than recent transgressive shelf sands. Thus it has been hypothesized that the formation was deposited during repeated transgressions and regressions of the continental interior. A newly excavated section of the St. Peter Sandstone in the Twin Cities basin of Minnesota supports this hypothesis and bears evidence for the nature and causes of transgressions and regressions during its accumulation. The section exposes two intervals of fine to very fine-grained, thick-bedded, massive, bioturbated sandstone which were deposited during transgression of shelf environments. They are separated by an interval of medium to fine-grained sandstone with multiple mutually truncating scours, tabular and trough cross-beds (including herringbones) with shale drapes on their foresets, wavy ripples, shale rip-up clasts, and minor bioturbation traces, which were deposited in a tide-dominated coastal sand-flat environment which prograded onto the open shelf. This regressive deposit is laterally discontinuous over a distance of 4 km, which suggests that the transgressions and regressions in the Twin Cities basin were causedmore » by variations in the rate of sediment supply.« less

Sedimentary and diagenetic controls on red-bed ore genesis; the middle Tertiary San Bartolo copper deposit, Antofagasta Province, Chile

The middle Tertiary Pacencia Group of Antofagasta Province, northern Chile, consists of over 2 km of continental sediments. These include alluvial fan, playa, and lacustrine facies, representing deposition in an arid, closed basin. Copper mineralization is hosted by sheet and ribbon sandstone bodies, which were deposited in a high-energy playa-marginal sandflat environment. Ore minerals occur dominantly as a matrix to these lithic arkoses, replacing earlier carbonate and sulfate cements.The main ore minerals are native copper, cuprite, and atacamite, although there is an along-strike passage into copper sulfide cements. In a structurally complex zone, copper-iron and copper sulfides were deposited in crosscutting fractures, commonly replacing evaporite veins. Primary chalcopyrite and bornite exhibit progressive oxidation to anilite, digenite, and djurleite. Chalcocite is not found. Much of this material has oxidized further to covellite of normal and blaubleibender varieties. Galena and native silver are present in uneconomic amounts. Atacamite has a complex paragenesis, representing, at least locally, the earliest copper-bearing phase.Provenance studies of the host sediments and lead isotope data for the sulfides indicate that both the sandstones and the metals were derived from a calc-alkaline volcano-plutonic complex to the east of the basin, an area now obscured by the recent volcanic Andes. Sulfur isotope results for copper sulfides show very light delta 34 S values (-33ppm), indicative of biogenic sulfate reduction under conditions of unrestricted sulfate supply.

Eocene Marine to Nonmarine (Deltaic) Deposits, Lower Piru Creek, Los Angeles and Ventura Counties, California: ABSTRACT

A 790-m thick vertical section of lower middle Eocene sandstone represents a regressive deltaic sequence that grades upward from delta front through lower delta plain into upper delta plain. The sandstone crops out as a narrow strip of overturned east-west strata and south-plunging open folds within the Topatopa Mountains, 16 km north of the town of Piru, California. The sandstone gradationally overlies prodelta/transition-zone siltstone of uppermost Capay stage (early Eocene) and underlies, with a 15° angular unconformity, Eocene to Oligocene nonmarine boulder conglomerate of the Sespe Formation. The lower 95 m of the sandstone is characterized by an upward-coarsening sequence, fine at the base to medium at the top, laminated to bioturbated sandstone. Bioturbation is as high as 75% with Ophiomorpha burrows common. Shallow marine mollusks diagnostic of the Domengine stage (late early through early middle Eocene) occur within lenses of medium sandstone. These rocks were deposited in a shoreface environment on a delta front. Above the delta-front deposits are about 100 m of coarse sandstone. Structureless and bioturbated sandstone is dominant, but herringbone cross-bedding, planar cross-bedding 10-150 cm high, planar lamination, and scour-and-fill structures are common. Structures are interpreted as megaripples, sand waves, and channels in a tidal sand-flat environment. Coal lenses higher in the section are interpreted to be marsh deposits. Bioturbation can be up to 80% and Ostrea occurs locally. This tidal flat differs from most cited modern examples because it lacks a mud component or associated mud-flat deposits. The tidal sand-flat and marsh deposits were formed within a lower delta plain. Overlying these rocks is a 205-m interval of interfingering tidal-flat and braided-river deposits. The remaining 390 m is comprised mostly of a sequence of fining-upward cycles overlain by an interval of structureless sandstone. Fining-upward sequences consist of trough cross-bedded, very coarse sandstone at the base grading into red and green mudstone at the top. Environmental interpretations are diagonal bars for the sandstone and abandoned channels for the mudstone. The structureless sandstone locally contains scour-and-fill structures and gravel lenses. No fossils were found. This uppermost 390-m thick section was deposited by a braided-river system within an upper delta plain. About 1.5 km to the west, these braided-river deposits thin to about 140 m, and the interval of interfingering tidal-flat and braided-river deposits thickens to 380 m. The shoreface, tidal-flat, and braided-river deposits within these Eocene beds are similar to those found on the modern Frazier River deltaic system in Vancouver, British Columbia. End_of_Article - Last_Page 483------------

Santa Ana River: An example of a sandy braided floodplain system showing sediment source area imprintation and selective sediment modification

The Santa Ana River floodplain can be divided into five geomorphic environments recognized by both morphology and sedimentary characteristics: braided channels, bars, vegetated islands, sand flats and floodplain terraces. Braided channels which vary in width from 30 to 80 m and are bordered by sand flats, flow round vegetated islands and emergent mid-channel bars. Deep braided channels have linguoid dume bedforms which are reworked during falling flood stages. If channel bars emerge in early spring, vegetation is rapidly established and they may become vegetated islands. Bars emerging in minor channels and in areas of increased channel width may develop into extensive sand flats. Away from the channel margins, sand flats are subject to wind erosion as the water table declines in summer and fall. This sediment is deposited in sand sheets and vegetated dunes. These distal sand flat environments are characterized by a well-sorted, medium sand-sized sediment population and are separated from negatively skewed, moderately sorted coarse channel sands by a mixed sediment population along channel margins. The drainage basin discharge is dominated by the regional Mediterranean climate. The coarse channel sediment reflects the large temporal variations in drainage discharge and the resulting greater variability and competence to transport sediment. Major decreases in channel width during both seasonal summer drought and prolonged periodic drought exposes large areas of the floodplain to selective wind winnowing of fluvially derived sediment. This process operates on a small scale today, but during arid conditions in the late Wisconsin interglacial interval (prior to 22,000 yrs B.P.) extensive sand flats along the Santa Ana River, distal alluvial fans, and sediment from minor tributaries were the sediment source for a major riverine dune field adjacent to the floodplain. This dune field was stabilized during the late Wisconsin glacial interval. Subsequent channel entrenchment has isolated this dune field from its source region. Santa Ana River sediments originate from two distinct geologic provinces: the San Gabriel-San Bernardino granitic and metamorphic complex to the north and the granitic Perris Block to the south. Settling-tube analysis of sediments from tributary streams draining the Perris Block shows a distinctive bimodal distribution characterized by minimal sediment in the very coarse sand size (−1 to 0 phi size) inherited from the granitic grus parent material. This is imprinted onto all sediment from this source. Transportation results in sediment erosion; sediments are better sorted and finer grained with increasing distance from their source. The sediment in these intermittent tributary streams frequently represents a single sedimentary event (minor flash flood), and all the sediment, including the fines, is left behind. This produces a negatively skewed sediment distribution when the fines are added to an existing deposit within the limits of a predetermined size range. Older alluvium is strongly negatively skewed and may indicate in-situ weathering. Coarse channel sands from the Santa Ana River maintain this characteristic sediment distribution, but selective deposition and winnowing by the wind on the sand flats creates a distinctive floodplain sub-environment. Analysis of these deposits and material from tributary streams illustrates that a particle population is dependent upon grain size distribution at the initial source. Sediment populations are modified by subsequent erosion, selective grain-size transport deposition, and total deposition and mixing of several source areas.

Facies Patterns and Depositional Models of Permian Sabkha Complex--Red Cave Formation, Texas Panhandle: ABSTRACT

The Red Cave Formation (Permian, Leonard Series) in the Texas Panhandle consists of cyclic, red-bed clastic and carbonate-evaporite members that were deposited in an extensive coastal sabkha, desert wadi plain, and a carbonate inner shelf which bordered the northern Midland basin. Evaporite members were deposited in carbonate-evaporite crustal sabkhas and clastic members were deposited in mud-rich coastal to continental sabkhas. North to south, red-bed wadi-plain facies pass into coastal sabkha facies and inner-shelf dolomite facies. In a Randall County core, vertical sequences commonly include slightly fossiliferous, faintly laminated to burrowed dolomitic mudstone and pellet wackestone overlain by cross-laminated oolitic or pellet packstone to grainstone, followed by algal-laminated dolomitic mudstone and nodular anhydrite in dolomite matrix. A progradational carbonate shoreline is inferred, with supratidal or sabkha evaporite to intertidal algal-mat and sand-flat environments passing seaward into a shallow, muddy subtidal inner shelf. Mud-rich sabkha sequences culminate with red to green mudstone and anhydrite above shoreline carbonates. Carbonate and evaporite facies pinch out generally toward the northwe t and northeast into wadi-plain red beds. These facies include ripple-drift cross-laminated siltstone and sandstone deposited in braided fluvial channels, adhesion-rippled siltstone, and red to green mudstone deposited in mud-flat and interchannel environments. Desiccation features, intraclasts, root zones, and paleosol horizons attest to subaerial exposure and probable nonmarine conditions. Large-scale cyclicity of red-bed clastic and carbonate-evaporite members probably was controlled by the relative supply or fluctuating input of clastics to sabkhas by way of fluvial systems rather than by absolute sea-level changes. Partial modern analogs to Red Cave sabkha depositional models are the coastal mud flats and alluvial fans in the northwestern Gulf of California, tidal flats and an ephemeral stream delta (Wooramel delta) in Gladstone Embayment, Shark Bay, Australia, and the Trucial Coast sabkhas in the Persian Gulf. Each setting has certain facets that are remarkably similar to intepreted paleoenvironments and lithofacies of the Red Cave Formation. End_of_Article - Last_Page 761------------

Early Precambrian Tidal Sedimentation in the Pongola Supergroup of South Africa

ABSTRACT Sedimentary sequences which resemble the regressive intertidalite model of Klein (1971, 1972) occur associated with volcanic rocks in the 3,000 m.y. Pongola Supergroup. Two inliers in northern Natal which were selected for the present study include prominent orthoquartzitic sandstones with thinner siltstones and mudstones. The arenite member at the base of the Wit-Mfolozi section is attributed mainly to shallow marine deposition. The upper 5 m display features which are suggestive of a low tidal sandflat environment. These include reactivation structures, herringbone cross-stratification, and ripple forms indicative of decreasing water depth and possible subaerial exposure. Above this is the alternating arenaceous-argillaceous member with a distinctive association of shrinkage cracks, mudclast breccia, and double-crested, flat-topped and interference ripples. Similar features occur in modern mid-tidal flat deposits. A high tidal mudflat environment is envisaged for the uppermost unit, the argillite member. A hematitic (jaspilite) bed is locally present at the top of the sequence and suggests the possibility of biological liberation of oxygen in periodically flooded depressions in the inner mudflat. Three similar upward-fining sequences are present in the Mpongoza inlier. Paleotidal ranges of between 12 and 25 m are inferred. The maximum figure is considerably in excess of any present-day tidal amplitudes, and lends support to the hypothesis of an early Precambrian origin for the earth moon system.