Rate Of Sediment Influx Research Articles

Future impacts of climate change on sediment influx rate in hydropower reservoir using SWAT

Climate change causes more frequent and intense rainfall events, leading to severe erosion in the catchment and sediment transferred into rivers and reservoirs. This study focus on long term sediment load in major rivers in Cameron Highlands and prediction of annual sediment inflow into Ringlet Reservoir from 2000 to 2030. Soil Water Assessment Tool (SWAT) is used as the simulation tool, utilising future gridded rainfall 2017 to 2030 under CCSM and future land use 2030. Future annual rainfall is minimum at 1551 mm (in 2030) and maximum at 3150 mm (in 2029). The future projected annual sediment load into Ringlet Reservoir from 2017 to 2030 is averaged at 354,013 m3/year, ranging from 216,981 to 461,886 m3/year. Comparing between the historical period of 2000 to 2016 and future projection (2017–2030), annual sediment load shows an increase of 12 %. To combat the increase sediment yield, catchment management such as erosion control plan, drainage and runoff control must be developed to minimise sediment yield and subsequent effect of high sediment load transport via rivers and drainage network.

Relationships between basin area and sediment yield upstream Shiroro reservoir, north-central Nigeria

Sediment yield and basin area relationship in a watershed is a very crucial factor in Best Management Practices (BMPs) and reservoir sustainability. Therefore, the study was conducted to examines the suspended sediment concentration (SSC) of the four sub-watersheds of Kaduna watershed north-central Nigeria to aid the dam sustainability and BMP within the watershed. Water samples were collected bimonthly for 8 months (March-October 2018) and subjected to laboratory analysis for SSC and turbidity measurements. With the aid of ArcSwat2012, the watershed was delineated into four sub-watersheds, soil and land-use were classified for a proper understanding of the watershed. The findings demonstrated an inverse relationship between the size of the sub-watersheds and SSC. The smallest Dinya sub-watershed (1%) contributes approximately 25% (3781.5mg/l) of the SSC while the largest Kaduna (80%) contributes approximately 26% (3889mg/l), Sarkinpawa (11%) and Gutalu (8%) contributes approximately 26% (3844mg/l) and 23% (3444mg/l) of SSC respectively. The highest average turbidity was recorded in Sarkinpawa sub-watershed, followed by Dinya, Gutalu, and Kaduna. The SSC and turbidity results followed the rainfall pattern while the regression results for the four sub-watersheds shows a good level of linearity between SSC, rainfall and turbidity, Therefore, it is recommended that sediment management practice is to be carried out along sub-watersheds Dinya and Sarkinpawa and Gutalu to reduce the high rate of sediment influx into the reservoir from these smaller sub-watersheds. Also, detail study of the smaller sub-watersheds is highly recommended for a better understanding of the factors responsible for high sediment generation in these sub-watersheds. Keywords : basin area, best management practice, Kaduna watershed, suspended sediment concentration (SSC), Shiroro reservoir, relationship.

Seismic and Sequence Stratigraphic Analysis of Ewan and Oloye Fields (Middle Miocene), Northwestern Niger Delta: Implications for Deltaic Depositional Sequences

The main aim of this works is to define local sequence variability and its implications for deltaic depositional sequences within the studied area. This research work makes use of depth converted three dimensional seismic cube, well logs, core photography descriptions and biostratigraphic reports. Recognition and interpretation of stratal surfaces were based on facies shifts and stacking patterns and reflection terminations derived from the interpretation of seismic data and wireline logs integrated with core photography and biostratigraphic reports. Seismic stratigraphic analysis of well and seismic data suggests that middle Miocene- early Pliocene strata within the studied area were made of six depositional sequences. Sequences are made up aggradational to progradational HST that makes up about 50% of a sequence. TST are thin and widespread while LST are restricted to within WNWESE trending valley fills typically 4 km wide and an average 120 m deep. Outside the LST valleys, transgressive surfaces, maximum flooding surfaces and sequence boundaries are coincident on interfluves. This suggests that during LST times most of the area was periodically subaerially exposed and several incised valleys/canyons were developed. These incisions/canyons acted as a sand feeder that led to the deposition of high amplitude deposits (sandstones) on the slope and within the basin floor setting. Sequences mapped over the study area show local variation associated with growth faults. There is a strong local growth fault control on sequence variability. Further local controls on sequence geometry are shelf instability and slope scars which have a strong geomorphological impact on the area. The depth of incised valleys within the uppermost HST (up to 350 m) is difficult to explain in terms of sea level fall alone because eustatically-driven sea level variations during the Miocene are generally reported to be less than 100 m (330 ft). This suggests that the character of sequence development and depositional facies preserved within the study area depends on relative sea level changes, relative rates of regional structural collapse and sedimentation rate. The occurrence of several incisions in the middle Miocene succession is interpreted as evidence of significant relative sea level fluctuations, and the presence of type-1 sequence boundaries may be the stratigraphic signature of major drops in relative sea level during Miocene and Pliocene. Variability within the depositional sequences is said to be largely controlled by local growth faults, rapid relative sea level changes, basin physiography (shelf edge) and high rate of sediment influx.

Carbonate sedimentation on Pacific coral reefs

The contributions in this special issue represent recent advances in carbonate sediment research in and around the coral reefs of the Pacific regions of Japan, New Caledonia, and Tahiti. The issue contains five articles covering a range of subjects including fundamental research on the use corals and large-sized benthic foraminifers in paleoenvironmental reconstructions, facies analyses of modern off-reef sediments, and isotopic analyses of tricanid shells. The issue opens with a classical sedimentary facies description by Yamano et al. (2015), who examine surface carbonate sediments on the outer shelf and slope around New Caledonia. Facies showed significant variations in composition, from benthos- to plankton-dominated, according to water depth, currents, and local seafloor geomorphology. Their results contribute baseline data for examinations of carbonate sedimentary facies in tropical to subtropical environments. Following the description of general facies patterns, the issue contains further investigations on the use of specific organisms (including corals and benthic foraminifers) in reconstructions of facies and paleoenvironments. Humblet et al. (2015) present identification keys for four widespread genera of fossil corals (Acropora, Isopora, Montipora, and Porites) that are commonly recovered from outcrops and drilled cores. They argue that fossil corals should generally be identified from sectioned material, as coral surfaces are usually obscured by encrusters or sediments, and coral specimens are often broken or eroded. Further work extending this type of investigation to all coral genera would greatly contribute to a better understanding of coral reef development and paleoenvironmental assessments. Hongo and Wirrmann (2015) examine cores from Pleistocene and Holocene barrier reefs in New Caledonia and identify several key species for reef formation and maintenance. They emphasize the need for further research on the restoration and conservation of key species, as some of these species are presently in decline. Fujita and Omori (2015) examine large-sized benthic foraminifers from Recent surface sediments and Pleistocene subsurface deposits obtained during Integrated Ocean Drilling Program (IODP) Expedition 310, off Tahiti. By referring to modern foraminifer assemblages, they show that the Pleistocene sequence off Tahiti was deposited in a fore-reef slope environment. As with corals, the authors propose that an examination of modern assemblages of foraminifers would contribute to a better understanding of facies formation and improved paleoenvironmental reconstructions. In addition to their role in sedimentary facies analysis, carbonate skeletons and shells are reliable recorders of paleoenvironments at high temporal resolutions. Asami et al. (2015) present seasonally resolved records of carbon and oxygen isotopic compositions (δ18O) in the fossil shells of giant clams recovered from archeological sites in Okinawa, Japan. Results demonstrate that the annual mean seawater temperature and salinity at Okinawa were approximately 1–3°C higher and 1–2 lower, respectively, at 4000 yr cal. BP and 1900–1700 yr cal. BP than at present; such data provide insights into patterns of human settlement and modes of life in the past. An understanding of facies formation and the reconstruction of paleoenvironments both require process-based analyses. In addition to the five articles mentioned above, Fujita et al. (2015), whose work is to be included in an extended virtual special issue on this topic, examine processes of carbonate sedimentation in seagrass beds in the Ryukyu Islands, Japan. Based on sedimentation rates estimated from coring and sediment influx rates estimated from sediment traps, they identified a sediment-trapping role of seagrass, and characterized the seagrass beds in the Ryukyu Islands as having high sediment fluxes and extremely low sedimentation rates. Collectively, the special issue presents diverse and insightful analyses of Pacific coral reefs. We thank the authors and reviewers who contributed to this issue. Finally, we dedicate this special issue to the memory of our friend and colleague Guy Cabioch (Fig. 1), who sadly passed away in October 2011. All of the senior or corresponding authors, as well as the guest editors, of this special issue collaborated with him. He led many research cruises and drilling programs, and some of the outcomes of his collaborations appear in this special issue. We sincerely thank Guy and pray from the bottom of our hearts that his soul may rest in peace. Guy Cabioch in a research cruise (previous page) and in the field (this page). The photos are provided by Jean-Michel Boré and Denis Wirrmann, respectively.

Sequence Stratigraphic Analysis of “XB Field”, Central Swamp Depobelt, Niger Delta Basin, Southern Nigeria

Well logs and biostratigraphic data from six wells in the “XB Field”, central Swamp Depobelt, Niger Delta were integrated to carry out a sequence stratigraphic analysis of depositional systems in the field. The analysis revealed four 3rd order depositional sequences (SEQ1 to 4) bounded by three erosional unconformities interpreted as Sequence Boundaries (SB1 to 3). Transgressive Surfaces of Erosion (TSE1 to 3) that mark the onset of marine flooding and turnarounds from progradational facies to retrogradational facies during sequence build-up were delineated. Three 3rd order Maximum Flooding Surfaces (MFS1, MFS2 and MFS3) characterized by marker shales, high faunal abundance and diversity were also delineated and dated 15.9, 17.4 and 19.4 Ma, respectively. The delineated sequences comprised Lowstand Systems Tracts (progradational packages), Transgressive Systems Tracts (retrogradational packages) and Highstand Systems Tracts (aggradational packages), which reflect depositional systems deposited during different phases of base level changes. The Lowstand Systems Tract (LST) consists of Basin Floor Fans (BFF), Slope Fans and Channel Sands deposited when sea level was low and accommodation space lower than rate of sediment influx. Transgressive Systems Tract (TST) consists of retrogradational marine shales deposited during high relative sea levels and when accommodation space was higher than rate of sediment influx. Highstand Systems Tracts (HST) consisted of shoreface sands displaying mostly aggradational to progradational stacking patterns. The sands of LST and HST show good reservoir qualities while the shales of the TSTs could form potential reservoir seals. The above recognized sequences, were deposited within the Neritic to Bathyal paleoenvironments and are dated mid-Miocene (15.9 - 20.4 Ma) in age.

The effects of a bypass dunefield on the stability of a headland bay beach: A case study

Abstract This paper presents a case study on the modelling of a headland bay beach influenced by sediment input from migrating coastal dunes. The study area is the region around the town of Ingleses on Santa Catarina Island, Santa Catarina state, South-Eastern Brazil. Ingleses has been threatened by nature at two different fronts. At the north-eastern side of the town lies Ingleses Beach, a headland bay beach which seems to be subject to persistent erosion in recent years. On the south-western side the town is threatened by a migrating dunefield, which is encroaching onto houses and infrastructure, but which has not reached the beach in the past 70 years. A second dunefield in the area does not pose a direct threat to the town, but passes it on the eastern side and forms an important influence on the development of the beach. The sediment influx rate from this dunefield to the beach is 10,000 m3/year. To investigate the effect of the dunefields on the morphodynamic system, a numerical model has been created using the software packages SWAN and UNIBEST and the static equilibrium bay shape was determined by the software package MEPBAY. The result of the long-term scenario as predicted by UNIBEST agrees rather well with the static equilibrium bay shape found with MEPBAY. This provides sounds basis for the credibility of both models. Consequently, the predicted evolutional trend of the shoreline seems plausible.

Seismostratigraphic inversion: Appraisal, ambiguity, and uncertainty

Geologic process models predict the geometry of geologic strata and their petrophysical properties, based on mathematical models of geological processes that affect the formation and evolution of geologic strata. Such processes include erosion, sediment transport, and deposition. The resulting forward model is typically nonlinear. Given observations and a misfit measure, one may attempt inversion of these models to estimate process parameters that yield compatible predictions. For seismostratigraphic inversion, seismic data are used as observations. We tested such an algorithm in a prograding-delta environment to examine the effect of using different seismic attributes as observations and, thus, different choices of misfit measures. The first measure,based on the degree of parallelism between seismic reflectors and modeled geologic strata, demonstrated a trade-off between geologic time and the sediment-influx rate used to parameterize the model. A second misfit measure used unwrapped seismic instantaneous phase as a crude proxy to relative geologic time, which regularized the model parameters. Then last, we combined the two measures to take advantage of their individual characteristics. For most of these inversion experiments, we obtained results that capture the geometry of the geologic strata as observed on the seismic data. With the exception of the depositional time-rate trade-off, where the same strata can be obtained in a shorter geologic interval when rates are increased, we found the inversion to be surprisingly stable, with a unique cluster of acceptable parameters, despite the nonlinearity of the geologic forward model.

Quantitative seismostratigraphic inversion of a prograding delta from seismic data

Quantitative stratigraphy predicts the geometry of geologic strata and their petrophysical properties based on mathematical models of geological processes and parameters which affect the formation and evolution of geologic strata. Such processes include erosion, sediment transport, and deposition. Quantitative stratigraphic inversion estimates the process parameters from a priori data by systematic modeling until the predictions are compatible with the observations. In the case of quantitative seismostratigraphic inversion, the a priori data are predominantly of the seismic kind. To evaluate feasibility, we designed a preliminary algorithm which we applied in a prograding delta environment. We found that by fixing the sediment influx rate, subaerial transport rate, and the initial topography, we were able to reproduce the basic features of the delta with only three to five parameters which included subaqueous transport rate and its depth-dependent decay rate, base level, and sediment source location and spatial extent.

Implications for Neoarchaean ocean chemistry from primary carbonate mineralogy of the Campbellrand‐Malmani Platform, South Africa

AbstractThe precipitation of calcite and aragonite as encrustations directly on the seafloor was an important platform‐building process during deposition of the 2560–2520 Ma Campbellrand‐Malmani carbonate platform, South Africa. Aragonite fans and fibrous coatings are common in unrestricted, shallow subtidal to intertidal facies. They are also present in restricted facies, but are absent from deep subtidal facies. Decimetre‐thick fibrous calcite encrustations are present to abundant in all depositional environments except the deepest slope and basinal facies. The proportion of the rock composed of carbonate that precipitated as encrustations or in primary voids ranges from 0% to > 65% depending on the facies. Subtidal facies commonly contain 20–35%in situ precipitated carbonate, demonstrating that Neoarchaean sea water was supersaturated with respect to aragonite, carbonate crystal growth rates were rapid compared with sediment influx rates, and the dynamics of carbonate precipitation were different from those in younger carbonate platforms. The abundance of aragonite pseudomorphs suggests that sea‐water pH was neutral to alkaline, whereas the paucity of micrite suggests the presence of inhibitors to calcite and aragonite nucleation in the mixed zone of the oceans.

Predicting sediment flux from fold and thrust belts

The rate of sediment influx to a basin exerts a first‐order control on stratal architecture. Despite its importance, however, little is known about how sediment flux varies as a function of morphotectonic processes in the source terrain, such as fold and thrust growth, variations in bedrock lithology, drainage pattern changes and temporary sediment storage in intermontane basins. In this study, these factors are explored with a mathematical model of topographic evolution which couples fluvial erosion with fold and thrust kinematics. The model is calibrated by comparing predicted topographic relief with relief measured from a DEM of the Central Zagros Mountains fold belt. The sediment‐flux curve produced by the Zagros fold belt simulation shows a delay between the onset of uplift and the ensuing sediment flux response. This delay is a combination of the natural response time of the geomorphic system and a time lag associated with filling, and then subsequently uplifting and re‐eroding, the proximal part of the basin. Because deformation typically propagates toward the foreland, the latter time lag may be common to many ancient foreland basins. Model results further suggest that the response time of the bedrock fluvial system is a function of rock resistance, of the width of the region subject to uplift and erosion, and, assuming a nonlinear dependence of fluvial erosion upon channel gradient, of uplift rate. The geomorphic response time for the calibrated Zagros model is on the order of a few million years, which is commensurate with, or somewhat larger than, typical recurrence intervals for episodes of thrusting. However, model experiments also highlight the potential for significant variations in both geomorphic response time and in sediment flux as a function of varying rock resistance. Given a reasonable erodibility contrast between resistant and erodible lithologies, model sediment flux curves show significant sediment flux variations that are related solely to changes in rock resistance as the outcrop pattern changes. An additional control on sediment flux to a basin is drainage diversion in response to folding or thrusting, which can produce major shifts in the location and magnitude of sediment source points. Finally, these models illustrate the potential for a significant mismatch between tectonic events and sediment influx to a basin in cases where sediment is temporarily ponded in an intermontane basin and later remobilized.

Controls on accumulation of a volcaniclastic fan, Ruapehu composite volcano, New Zealand

The Whangaehu fan is the youngest sedimentary component on the eastern ring plain surrounding Ruapehu volcano. Fan history comprises constructional (830–200 years bp) and dissectional (<200 years bp) phases. The constructional phase includes four aggradational periods associated with both syneruptive and inter-eruptive behavior. All four aggradational periods began when deposition by large lahars changed flow conditions on the fan from channelized to unchannelized. Subsequent behavior was a function of the rate of sediment influx to the fan. The rate of sediment influx, in turn, was controlled by frequency and magnitude of volcanic eruptions, short-term climate change, and the amount of sediment stored on the volcano flanks. Fanwide aggradation occurred when rates of sediment influx and deposition on the fan were high enough to maintain unchannelized flow conditions on the fan surface. Maintenance of an undissected surface required sedimentation from frequent and large lahars that prevented major dissection between events. These conditions were best met during major eruptive episodes when high frequency and magnitude eruptions blanketed the volcano flanks with tephra and rates of lahar initiation were high. During major eruptive episodes, volcanism is the primary control on sedimentation. Climatic variations do not influence sediment accumulation. Local aggradation occurred when lahars were too small to maintain unchannelized flow across the entire fan. In this case, only the major channel system received much sediment following the deposition from the initial lahar. This localized aggradation occurred if (1) the sediment reservoir on the flank was large enough for floods to bulk into debris flows and (2) sedimentation events were frequent enough to maintain sediment supply to only some parts of the fan. These conditions were met during both minor eruptive and inter-eruptive episodes. In both cases, a large sediment reservoir remained on the volcano flanks from previous major eruptive intervals. Periods of increased storm activity produced floods that bulked to relatively small debris flows. When the sediment reservoir was depleted, the fan entered the present dissectional phase. Syneruptive and noneruptive lahars are mostly channelized and sediment bypasses the fan. Fan deposits are rapidly reworked. This is the present case at Ruapehu, even though the volcano is in a minor eruptive episode and the climate favors generation of intense storm floods.

The Lower Carboniferous Stainmore Basin, N. England: extensional basin tectonics and sedimentation

Recently released industry seismic data image a 6 km thick Dinantian succession in the Stainmore Basin, and are used to interpret deposition in response to extensional subsidence. The basin is bounded to the north by the Lunedale–Wigglesworth–Butterknowle en echelon suite of normal faults. An early (?Courceyan–?Arundian) syn-rift sequence more than 4 km thick includes footwall-derived clastic fans. A basinal terrigenous sequence deposited through the Arundian, Holkerian and early Asbian contrasts with coeval carbonates on the Ravenstonedale Shelf and the Askrigg Block. Deposition continued in the basin whilst the Alston block and northern parts of the basin were emergent during early Asbian times. A late Asbian relative base level rise and the possible cessation of active rifting allowed both basin and structural highs to be inundated by shallow marine carbonates. Rhythmic ‘Yoredale’ sedimentation followed. Dinantian depositional patterns reflect the interplay between active extensional faulting, a suspected local inversion event, changes in sediment influx rates, sea level variations and differential rates of compaction between basement highs and the basin. However, error margins on the available biostratigraphy and timing of active faulting prevent the detailed separation of these variables and their impact on sedimentation in the Dinantian.

DEPOSITIONAL ENVIRONMENTS AND HYDROCARBON POTENTIAL OF THE EVERGREEN FORMATION, ATP 145P, SURAT BASIN, QUEENSLAND

Hydrocarbon discoveries in the Jurassic Evergreen Formation along the western shelf of the Surat Basin lie in a northwest-trending structurally controlled belt adjacent to Bridge-operated ATP 145P. Shows and commercial accumulations are generally associated with specific sand/shale ratios, which influence source, reservoir, and seal effectiveness. Proximity to basement and early faults are other favourable factors.Four distinct log facies were calibrated against cuttings and core and interpreted genetically. Small, stream-dominated alluvial-fan systems extended from low escarpments in the west, but were reworked along their basinward margins prior to being overstepped by aggradational, onlapping distal sediments. Prominent east-northeast-trending belts occupied by mixed-load fluvial channel systems terminated along a temporary shoreline of coalescing wave-dominated deltas. This deltaic-shore-zone sand belt was truncated by fluvial-distributary channels during the ensuing regression to another still-stand some 25 km farther east. Shoreline positions fluctuated locally in response to competition between rate of sediment influx and the gradually rising base level; but the net effect was regional transgression, with reworked shorezone/nearshore sheetsands grading up into silty shales of the shallow epeiric platform.On the basis of a petrographic study the highest source potential for the Evergreen Formation in ATP 145P is around Spring Grove 1 well. A band of high source potential lies parallel to the deltaic shoreline, in a generally north-south direction. The highest source potential of the Triassic Moolayember Formation lies around Spring Grove 1 and in a north-south trending arc through Roswin 1 and Sirrah 4. The source potential of the combined Triassic Snake Creek Member, Showgrounds and (Permian) Kianga formations is highest around Roswin 1. In the area of Roswin 1 all formations studied show a high potential as source rocks for hydrocarbons. Based on the available data, the Triassic Moolayember Formation appears to be the only formation common to all producing wells with a high source potential in each well.Zones of thick, porous sands are not very prospective for hydrocarbons in the study area, possibly because of ineffective seal. True shales are rare in the Evergreen Formation. The dominant fine-grained lithology is silt-stone, which by itself provides a leaky or dynamic seal at best. Thin, reworked delta mouth bar and splay sands with thick seals may offer the best reservoir objectives. Known hydrocarbons generally occur in structural-strati-graphic traps with the structural component caused by compaction and subsequent draping over basement topography, and the stratigraphic component arising from the lenticularity of the fluvio-deltaic sand bodies.

Quartz sand grain shape and other criteria used to distinguish glacial and non‐glacial events in a marine core from Frobisher Bay, Baffin Island, N.W.T., Canada

ABSTRACTAnalyses of quartz sand grain shape, sediment influx rates and foraminifera define glacial and non‐glacial episodes in a 9.69 m core from Frobisher Bay, Arctic Canada. Five radiocarbon dates on organic matter provide a preliminary core chronology, with a basal date of 11,910 yr BP. Quartz sand grain morphology is measured for samples at seven core levels using: (1) Fourier shape analysis; (2) percentage of grain surface conchoidally fractured. Samples at 2.0 and 7.5 m are most fractured and have Fourier roughness coefficients similar to particles sampled directly from glacier ice. These two samples probably represent glacial events in the core. Major intervals of non‐carbonate sand influx occur at 9.0–4.5 m and 3.5–1.5 m, separated by several thousand years of slower sedimentation. Detrital carbonate influx rates are relatively high prior to 4.3 m, then decline rapidly indicating a shift in sediment provenance from limestones flooring Frobisher Bay to rafting from far‐travelled icebergs. Bio‐ and lithostratigraphic analysis allows definition of five core units: (1) an environment similar to today below 8.5 m; (2) glacial conditions from 8.5–6.8 m, associated with ice proximal to the core site; (3) ameliorating conditions from 6.8 to 3.2 m; (4) cooler conditions from 3.2 to 0.5 m, probably related to increased iceberg flux rather than neoglacial advances of nearby ice caps; (5) an environment similar to today from 0.5 m. Sand grains sampled at 7.5 and 2.0 m, whose shapes indicate they are derived from glacier ice, lie within the cooler or glacial units defined from foraminiferal analysis. This indicates that quantitative measurement of particle surface morphology can provide useful environmental information in studies of marine cores.

Rate of Dissolution of Carbonate Sediments by Microboring Organisms, Davies Reef, Australia

ABSTRACT Skeletal carbonate sediments on the lagoon floor of Davies Reef (Great Barrier Reef, Australia) are subject to intense attack by microboring organisms, principally algae. The rate at which this microboring process dissolves carbonate was measured experimentally. Fresh, unbored, molluscan sand samples placed in the shallow lagoon (5 m water depth) experienced a weight loss of about 0.9 g/30 g sample (or 3%) in one year due to dissolution of carbonate by microborers (revealed by direct measurement of weight change and point-count analysis). This figure may be converted to about 350 g CaCO3 dissolved/m2 lagoon floor/year, which is equivalent to between 18 and 30% of the sediment influx rate to this lagoon, averaged over the past 9,000 years. Consideration of the exp rimental design suggests that the experiment underestimated the true rate of microboring in lagoon floor sediments. We conclude that the extent of microboring in carbonate sands may provide information on the rate of sediment deposition, and that dissolution of carbonate sediments by microborers is a significant factor in whole-reef CaCO3 budgets.

Rapid Sedimentation in Low-Energy, Microtidal Estuary: ABSTRACT

The St. Lucie River is an elongate, forked estuary formed during the Holocene rise in sea level and located on the southeast coast of Florida. It is 15 km (9 mi) long from its mouth to the end of each fork, and is a maximum of 3 km (2 mi) wide. Mean depth is about 3 m (10 ft). Although seasonal variation does exist, the combination of freshwater runoff and microtidal conditions produces a well-mixed, nearby freshwater estuarine system. Extensive subbottom profiling and vibracoring have provided data for interpretation of the Holocene history of sediment accumulation in the estuary. Well-developed paleochannels and thickness of the surficial mud layer can be delineated by seismic profiling and confirmed by cores. Analysis of the distribution of 137Cs shows that the rate of sediment accumulation in the estuary over the past 3 decades has approached 1 cm/year. Comparison of bathymetry between the late 19th century and the present shows a similar rate of accumulation. Agriculture and residential development has been pronounced in the drainage basin, beginning in the 1920s. It is likely that this activity has produced a great increase in the rate of sediment influx to the estuary. The configuration of the estuary in combination with inefficient tidal flushing has permitted much of this sediment to accumulate. End_of_Article - Last_Page 469------------

Late Pleistocene History of Tongue of the Ocean, Bahamas

The late Pleistocene history of Tongue of the Ocean, Bahamas, is delineated by study of six piston cores, 6 to 10 m in length, from the axial region of this deep marine embayment. Quantitative biostratigraphic analysis of four of the cores indicates the history of the last glacial cycle. One of these cores contains an unconformity between late Pleistocene and middle Eocene sediments. Paleoecologic analysis of planktonic foraminiferal thanato-coenoses indicates paleotemperature fluctuations of 3° to 4°C between interglacial and glacial stages, and a trend toward lower paleosalinity from the interglacial to postglacial stages. Both the microfauna and sediments in Tongue of the Ocean reflect the influence of late Pleistocene paleoclimates. Sediment in Tongue of the Ocean is derived primarily from the adjacent Bahamian banks; the sediment history, therefore, has been controlled mainly by eustatic variations in sea level. A low stand of sea level during the glacial stage effectively shut down the shallow-water carbonate production on the banks and resulted in a lower rate of sediment influx. A lower rate of production of planktonic foraminiferal tests during the glacial stage also contributed to this reduced sedimentation rate. The sedimentary record of Tongue of the Ocean is just the opposite of that found on most deep continental margins where glacial stages produced higher rates of sedimentation than interglacial stages. Most of the sediment in Tongue of the Ocean was winnowed from the banks, probably mainly during storms, and was deposited as a pelagic “snow.” Turbidites may not be as important in the axial region as previously reported. The carbonate mineralogy of Tongue of the Ocean sediment reflects the mineralogic complexity of organisms on the adjacent banks and possibly diagenetic alteration of carbonate sediment on the sea floor.

Growth of the Bengal Deep-Sea Fan and Denudation in the Himalayas

A geological and geophysical survey in 1968 has shown that the Bengal Deep-Sea Fan is almost 3000 km long, and 1000 km wide. We estimate that it may exceed 12 km in thickness. The sediments of the fan have been transported by turbidity currents from the Ganges-Brahmaputra River delta, through the “Swatch of No Ground” submarine canyon and into an extensive, complex, meandering, and braided net of fan valleys. Present rate of sediment influx suggests a regional rate of denudation in the Himalayan source area of over 70 cm/10 3 years. The sediment section in reflection profiles of the fan has been subdivided into three units separated by prominent unconformities. Volumes of the upper two units compared with the sediment influx rate extrapolated into the past suggest that the unconformities may be late Miocene and earliest Pleistocene. These times correspond to periods of orogeny in the Himalayas and suggest contemporaneity between plate-edge orogeny and mid-plate tectonic activity.

Cyclic Sedimentary Sequences in Frontier Formation (Upper Cretaceous), Casper Arch Area, Wyoming, and Some Stratigraphic and Possible Paleoenvironmental Implications: ABSTRACT

The Frontier Formation is generally about 1,000 ft. thick in the Casper arch area and consists of a series End_Page 2035------------------------------ of asymmetric, upward-coarsening, depositional sequences referred to here as Four major cycles called sandstone zones by others, have been studied in exposures and mapped in the subsurface in a preliminary manner. Thinner, less conspicuous cycles occur within each major cycle. Where typically developed, a cycle begins with relatively non-sandy gray shale at the base and becomes progressively more sandy upward, terminating where the cycle is thickest with a conspicuous body of fine- to medium-grained, submature to mature, chert-rich orthoquartzite having a relatively sharp upper boundary. Chert-rich pebble conglomerate of uncertain origin occurs in places within the sandier parts of the cycles. Detailed correlations of bentonitic layers reveal that the major cycles pinch and swell laterally in a complex but generally systematic manner, displaying two- to four-fold thickness variations. The relatively thick parts of each major cycle generally lie on flanks of the underlying cycle. Proceeding upward through the sequence, axes of maximum sandstone development are offset progressively eastward away from the central Casper arch. The uppermost cycle also is relatively thick and sandy on the arch. Upward coarsening within the cycles and the complex, but orderly, shifting pattern of the depositional axes of the major cycles suggest that the rate of sediment influx exceeded the rate of subsidence in the area during deposition of the Frontier. The thickness variations are not most plausibly explained by differential compaction or by differential subsidence, at least in the lower three major cycles. Thickness trends suggest that the major source of sand was north and northwest of the Casper arch area. Although the cycles appear to be essentially regressive in nature, sand deposition under transgressive conditions is suggested in places by more glauconitic and (or) calcareous upper parts of the sandstone bodies. West of the Casper arch, coaly beds and channel-like conglomerate lenses directly overlie sandstone bodies in cycles that otherwise are similar. Definite evidence of non-marine deposition has not been observed in the Casper arch study area and general observations suggest that the sequence is essentially marine. As reasoned from stratigraphic observations, the general slope at the top of the lowermost major cycle did not exceed ½° and the maximum depth of water in the area was about 100 ft. Although understanding of the depositional agents must await additional studies specifically aimed at this objective, some analogies are drawn between gross characteristics of the cycles and typical features of deltaic deposits along modern shorelines. More realistic concepts of the internal geometry and genesis of sandstone and shale complexes could be of considerable aid to the petroleum geologist in explaining and predicting oil and gas occurrences. End_of_Article - Last_Page 2036------------