Subtidal Channels Research Articles

Early postlarval fish in the hyperbenthos of the Dutch Delta (south-west Netherlands)

Early (post)larval fish constitute a significant part of the temporary hyperbenthos, i.e. of the fauna living in the lower reaches of the water column close to the substratum. Information on the densities and spatial and temporal variations of these stages, as well as on their lengths at the moment of occurrence, can be an important contribution to the knowledge of their dispersion, migrations and to the identification of possible bottlenecks in their recruitment. Therefore the hyperbenthos of the Voordelta, a shallow coastal zone in front of the Dutch Delta, as well as that of the subtidal channels and the intertidal brackish marsh creeks of the Westerschelde Estuary, was sampled monthly during one year by means of a sledge and a stow net. A total of 37 taxa were recorded from 410 samples. Densities often exceeded 400 ind 1000 m−2 in the Voordelta and 300 ind 1000 m−2 in the Westerschelde, including the tidal marshes of Waarde and Saeftinghe. The dominant taxa were clupeid larvae (predominantly herring Clupea harengus and sprat Sprattus sprattus), Pomatoschistus spp. (a mixture of sand goby P. minutus and Lozano's goby P. lozanoi), common goby Pomatoschistus microps, Syngnathus spp. (probably almost exclusively Nilsson's pipefish S.rostellatus), plaice Pleuronectes platessa, sole Solea solea and flounder Pleuronectes flesus. Most species were found at a larger size in the Westerschelde (and in the tidal marshes) suggesting migration from the shallow coastal area into the estuary. Several species utilize the tidal marshes in the brackish reaches of the Westerschelde. The nursery value of these areas however, is restricted to specific early life history stages of a few species, especially flounder, bass Dicentrarchus labrax and the common goby. The estuary itself seems to function as a nursery for other species like sole and plaice. The hyperbenthic sledge was found to sample specific life history stages of postlarval flatfish (early settlement stages at the onset of asymmetry), which are not easy to sample with other types of sampling gear.

Nagthat Formation: An example of a progradational, tide-dominated Proterozoic succession in Kumaun Lesser Himalaya, India

The Proterozoic Nagthat Formation of the Krol-belt succession, in the Nainital area, is composed mainly of fine- to coarse-grained quartzarenite with a subordinate amount of purple to grey sandstone, siltstone-shale and conglomerate horizons. The association with spilitic lava flows, variable palaeocurrent trends and the restricted lateral extent of the Nagthat Formation within the Krol-belt succession imply an active role for tectonism in the basin of deposition. In the upward coarsening succession of the Nagthat Formation, six major lithofacies have been identified: medium- to coarse-grained gravelly quartzarenite (Lithofacies A), planar cross-bedded, medium-grained quartzarenite (Lithofacies B), horizontally laminated, fine-grained quartzarenite (Lithofacies D), interbedded sandstone-shale (Lithofacies E) and matrix-supported conglomerate (Lithofacies F). The constituent lithofacies are repetitive in nature, forming upward fining unit cycles and interpreted to reflect deposition as upper shore-face, shoals and bars, barrier-beachface, tidal channels (inlets), intertidal–sandflat–mixedflat environments and, occasionally, in the form of gravity flows in subtidal channels. The general upward coarsening succession of the Nagthat Formation represents deposition in a progradational (regressive) barrier island system. The palaeocurrent pattern in the Nagthat Formation is distinctly polymodal and indicates sediment distribution across the roughly NW–SE trending shoreline, in response to a dominating flood tidal current system. The palaeocurrent pattern shows higher variability in the upper shore-face deposits than in the tidalflat domain. A recycled metasedimentary terrain served as the source for the Nagthat Formation, probably supplying the sediments from E, NE and S directions.

The Parasitic Rhizocephalan Barnacle Sacculina Carcini in Crabs of the Forth Estuary, Scotland

In the Forth Estuary, Scotland, parasitic rhizocephalan barnacle Sacculina carcini externae or scars occurred on 64% of Carcinus maenas from the subtidal channel of the middle estuary. These became more frequent downstream, infecting 46·9% in the lower estuary basin. Male and female crabs were infected equally, and infection rates in low-tide samples did not differ from high-tide. The parasites occurred most frequently on crabs of 40–50 mm carapace width (CW) rather than the modal crab size class of 50–60 mm. Downstream, proportionately more smaller than larger crabs bore parasites whereas, further upstream, more larger than smaller crabs bore parasites. There is evidence for the annual appearance of new externae in July-October, especially in the lower estuary basin, followed by their growth and loss after about a year.

Estuarine fronts as conduits for larval transport:hydrodynamics and spatial distribution of Dungeness crab postlarvae

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 164:73-82 (1998) - doi:10.3354/meps164073 Estuarine fronts as conduits for larval transport: hydrodynamics and spatial distribution of Dungeness crab postlarvae David B. Eggleston1,*, David A. Armstrong2, Ward E. Elis1, William S. Patton2 1Department of Marine, Earth and Atmospheric Sciences, North Carolina State University, Raleigh, North Carolina 27695-8202, USA 2University of Washington, School of Fisheries, Box 357980, Seattle, Washington 98195, USA *E-mail: eggleston@ncsu.edu Frontal zones are common hydrographic features in estuaries throughout the world, yet there is no consensus on the importance of fronts to larval transport processes. Frontal circulation may act as a 'barrier' to larval transport since it constrains cross-frontal flows, leading to strong along-frontal flows, which in turn may serve as a type of 'larval conduit' by funneling larvae collected at the front to settlement locations. We examined the larval conduit hypothesis by characterizing hydrodynamic features of apparent frontal structure in Grays Harbor estuary (Washington, USA) and quantifying the effects of frontal structure on the distribution and transport potential of Dungeness crab Cancer magister megalopae during spring-time recruitment pulses. CTD casts made perpendicular to fronts indicated that frontal structure was characteristic of buoyant plume and axial convergent fronts at the bay mouth and at a subtidal channel site within the estuary, respectively. Surface drifters deployed seaward of a front at the bay mouth during early flood tide were advected into the front, demonstrating convergent circulation. Surface drifters remained within buoyant plume fronts, which moved up-estuary during flood tide at 0.4 m s-1, and were then entrained in axial convergent fronts. Drifters moved an average of 10 km up-estuary during a single flood tide; the direction that drifters tracked corresponded to prevailing winds. The mean concentration of megalopae, as measured with neuston nets, was significantly higher in fronts than in adjacent water, 20 to 30 m upstream of the front, irrespective of year or location within the estuary. Paired plankton nets deployed at the bay mouth during flood tide indicated that the mean concentration and flux of megalopae was higher at the surface than at 3 to 4 m depth; this trend was significant for megalopal flux. Variable wind stress during the recruitment period of Dungeness crab, combined with the suggested effect of fronts serving as larval conduits whose direction of transport is influenced by prevailing winds, could significantly influence spatial variation in postlarval supply and subsequent settlement within the estuary. Cancer magister · Drifters · Dungeness crab · Estuarine hydrodynamics · Fronts · Larval transport · Recruitment Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 164. Publication date: April 09, 1998 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1998 Inter-Research.

Foraminiferal associations in the upper Waitemata Harbour, Auckland, New Zealand

Census data on 68 benthic foraminiferal tests in 56 seafloor sediment samples from the upper Waitemata Harbour, Auckland, New Zealand (36° 50' S, 174° 40' E) are analysed by Cluster Analysis. The faunal samples, taken from extreme high tide to 8 m depth, and from strongly brackish to normal marine salinities, are grouped into 7 associations. Characterising species of each association are found by calculating Association Scores for each species, based on its mean abundance, relative abundance, fidelity, persistence, and dominance within each association. The foraminiferal associations are: JE, Jadammina macrescens‐Elphidium excavatum f. clavata – high tidal, sandy mud, in salt meadow and dwarf mangrove swamp, with near normal marine salinity; Tn, Trochammina inflata – around mean high water springs in salt marsh and salt meadow, with variable salinity; M, Miliammina fusca – subtidal channels and intertidal mud banks, mangrove swamp, and salt marsh with reduced salinity; H, Haplophragmoides wilberti – above mean high water, in salt meadow and salt marsh, with slightly reduced salinity; Tr, Trochamminita salsa – intertidal sandy mud banks at head of estuary, with lowest salinity; AM, Ammonia beccarii‐Miliammina fusca – intertidal and shallow subtidal mud fiats and channels, with near normal marine to slightly reduced salinity; A, Ammonia beccarii – intertidal and subtidal (to 8 m+ depth) muddy sand, with near normal marine to slightly reduced salinity. In the cluster analysis dendrogram of samples, the first‐order division produces a three‐way split of: (1) near‐normal marine salinity agglutinated associations (JE,Tn); (2) low‐salinity, agglutinated associations (M,H,Tr); and (3) near‐normal marine salinity, calcareous associations (AM,A). The dominant foraminiferal species are grouped by cluster analysis into five species associations; these correlate closely with the sample associations. There is an overall trend of increasing species diversity from brackish to saline and from intertidal to subtidal. This study supports earlier conclusions that salinity and tidal exposure are the two most influential factors in determining foraminiferal distribution patterns in sheltered tidal harbours and estuaries. A common species in the near‐normal salinity subtidal channels, Siphogenerina striata, may have been introduced into the harbour with foreign shipping.

Hydrodynamics and sediment dynamics of The Wash embayment, eastern England

ABSTRACTWater and sediment movement in The Wash embayment has been determined from an extensive set of data, consisting of tidal current readings and suspended sediment concentration measurements. Instantaneous and residual currents in the embayment show a distinct lateral inhomogeneity, whereas vertically the water column is almost homogenous. The central deep water area (30–40 m) of the embayment is dominated by a residual landward water movement, whilst on the margins, the residual movement is seaward. Sediment is supplied predominantly in suspension from the north, through the northern extremity of Boston Deep. Suspended sediment pathways are coincident with the spring tide water movements and the subtidal channels act as the main conduits. Approximately 6·8 × 106 tonnes yr−1 of suspended sediments are supplied to the embayment from offshore areas. Bedload sediment supply is of lesser importance, ∼ 1·4 × 104 tonnes yr−1. Whereas suspended sediment movement appears to be the dominant mode of transport throughout the embayment, bedload transport is important in reforming the sea bed into a variety of bedforms which are particularly well developed on the margins of channels and shoals.

Cyclic Sedimentation of Carbonate and Siliciclastic Deposits on a Late Precambrian Ramp: The Elisabeth Bjerg Formation (Eleonore Bay Supergroup), East Greenland

ABSTRACT The 300 m thick upper Precambrian Elisabeth Bjerg Formation in East Greenland represents a period of intermittent siliciclastic-dominated marine deposition on a carbonate ramp. Four depositional systems are identified within the exposed formation: (1) a siliciclastic ramp system formed by prograding shoreface or delta complexes; (2) a carbonate outer-ramp system comprising rhythmically interbedded carbonate mudstone, subtidal stromatolitic biostromes, calcarenite deposits, and siliciclastic mudstone; (3) a transitional siliciclastic-carbonate system consisting of 0.3-0.6 m thick subtidal rhythmites, created by carbonate mudstone and conglomerates alternating with siliciclastic mudstone; and (4) a carbonate inner-ramp system consisting of intertidal and subtidal channel deposits. On th basis of stacking patterns within the depositional systems, lowstand, transgressive, and highstand systems tracts can be inferred, which are traceable across the entire outcrop area, more than 200 km along and 100 km perpendicular to basin strike. The systems tracts combine to form five 30-100 m thick sequences within the Elisabeth Bjerg Formation and two that extend into the surrounding formations. All of the sequences show the same basic internal architecture, most likely developed in response to third-order glacio-eustatic changes in sea level. The paleogeographic evolution, and the response of the four depositional systems to cyclic changes in sea level, are interpreted using a ramp-to-basin model. During lowstands, sedimentation was dominated by siliciclastic deposition. Stranded lowstand shoreline deposits were formed in the more distal part of the ramp, while the inner part of the ramp was a zone of sediment bypass. During subsequent relative sealevel rise the ramp was flooded and starved of sediment. Early highstands were characterized by carbonate deposition, including the formation of large stromatolite buildups, while siliciclastic deposition was confined to a narrow near-coastal zone. During late highstands decreasing accommodation space resulted in progradation of the siliciclastic deposits, forcing carbonate depos tion basinward.

Comparative study of the hyperbenthos of three European estuaries

The hyperbenthic fauna of the subtidal channels of the Eems (N. Netherlands), Westerschelde (S.W. Netherlands), and Gironde (S.W. France) estuaries was sampled within a 15-day period in summer 1991. In each estuary, quantitative samples were taken at regularly spaced stations covering the entire salinity gradient from marine conditions at the mouth to nearly freshwater conditions upstream. The diversity of the samples and the distribution of the species along the main estuarine gradients were assessed. Hyperbenthic communities were identified using different multivariate statistical techniques. The species composition and the density and biomass of the dominant species of each community were compared among communities.

A cretaceous ‐ early tertiary Macrotidal estuarine‐fluvial succession: Puponga coal measures in Whanganui inlet, onshore Pakawau sub‐basin, Northwest Nelson, New Zealand

Abstract Detailed lithofacies analysis of the Puponga Coal Measures Member along the western shore of the Whanganui Inlet, northwest Nelson, is based on five measured sections i hat incorporate parts of the interfingering and underlying North Cape Formation and overlying Farewell Formation. Two contrasting depositional systems, lithofacies Association A and B, are recognised on the basis of sedimentary structures, textures, paleocurrent data, and the local presence of coal and dinoflagellates. Association A1–3, North Cape Formation, which includes the Puponga Coal Measures Member (Haumurian ‐ Lower Teurian), represents a progradational sequence of estuarine, fluvial, floodplain, and mire deposits. Sub‐environments include: high energy subtidal channels, upper‐flow regime subtidal to intertidal sand shoals or flats, intertidal mixed sand/mud and mud flats, salt marsh, mires, fluvial distributary or crevasse channels and levees, and open/closed bays. The overall setting is a macrotidal estuary over which mi...

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.

Sporolithon ptychoides Heydrich and S. episporum (Howe) Dawson: two crustose coralline red algae (Corallinales, Sporolithaceae) in South Africa

The crustose coralline red alga, Sporolithon ptychoides Heydrich, is recorded from tide pools, subtidal surge channels, and on rock substrate on the southernmost coral reefs in the western Indian Ocean, at Sodwana Bay National Park, Natal Province, South Africa. Sporolithon episporum (Howe) Dawson is reported from large tide pools, surge channels, and shallow subtidal rocks subjected to sand scour and inundation in the same area, as well as from the south-western Cape Province. The material of S. episporum is compared with the type specimen, and the latter is described and illustrated. The two species are distinguished on the basis of cell connections and the burying of sporangia. Sporolithon ptychoides has both cell fusions and secondary pit connections, and sporangia buried in distinct rows. Sporolithon episporum has secondary pits but not cell fusions, and generally sheds old sporangia.

Lime-mud layers in high-energy tidal channels: A record of hurricane deposition

Research Article| July 01, 1993 Lime-mud layers in high-energy tidal channels: A record of hurricane deposition Eugene A. Shinn; Eugene A. Shinn 1U.S. Geological Survey Coastal Center, 600 4th Street South, St. Petersburg, Florida 33701 Florida 33701 Search for other works by this author on: GSW Google Scholar Randolph P. Steinen; Randolph P. Steinen 2Department of Geology, University of Connecticut, Storrs, Connecticut 06268 Search for other works by this author on: GSW Google Scholar Robert F. Dill; Robert F. Dill 3GeoMarine Inc., San Diego, California 92106 Search for other works by this author on: GSW Google Scholar Richard Major Richard Major 4Bureau of Economic Geology, University of Texas, Austin, Texas 78713 Search for other works by this author on: GSW Google Scholar Geology (1993) 21 (7): 603–606. https://doi.org/10.1130/0091-7613(1993)021<0603:LMLIHE>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Eugene A. Shinn, Randolph P. Steinen, Robert F. Dill, Richard Major; Lime-mud layers in high-energy tidal channels: A record of hurricane deposition. Geology 1993;; 21 (7): 603–606. doi: https://doi.org/10.1130/0091-7613(1993)021<0603:LMLIHE>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract During or immediately following the transit of Hurricane Andrew (August 23-24, 1992) across the northern part of the Great Bahama Bank, thin laminated beds of carbonate mud were deposited in high-energy subtidal channels (4 m depth) through the ooid shoals of south Cat Cay and Joulters Cays. During our reconnaissance seven weeks later, we observed lime-mud beds exposed in the troughs of submarine oolite dunes and ripples. The mud layers were underlain and locally covered by ooid sand. The mud beds were lenticular and up to 5 cm thick. Their bases cast the underlying rippled surface. The layers were composed of soft silt- and sand-sized pellets and peloids and in some areas contained freshly preserved Thalassia blades and other organic debris along planes of lamination. The beds had a gelatinous consistency and locally had been penetrated by burrowers and plants. Layers of lime mud had also settled on bioturbated, plant-stabilized flats and in lagoonal settings but were quickly reworked and made unrecognizable by the burrowing of organisms. Thicker, more cohesive (and therefore older) mud beds and angular mud fragments associated with ooids from Joulters Cays have similar characteristics but lack fresh plant fragments. We infer that these older beds were similarly deposited and thus record the passage of previous hurricanes or tropical storms. Storm layers are preserved within channel sediments because migrating ooids prevent attack by the burrowing activity off organisms. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Observations on shelf and subtidal channel flow: Implications of sediment dispersal seaward of the Keum River estuary, Korea

Although much of the fine-grained sediment in the south-eastern Yellow Sea is attributed to discharge from the Keum River, dispersal of these sediments by tide or wind has never been determined by direct current measurements. In this paper we provide in situ observations of currents in two dispersal systems immediately seaward of the Keum River entrance. One of these is a subtidal channel system that defines flow from the shallow subtidal region seaward to the shelf, and the other is the inner shelf mudstream where mixing is maintained primarily by tides. Subtidal channels have moderate current speeds but low suspended sediment concentrations. Instantaneous current speeds during spring tide were of the order 100 cm s −1 and suspended sediment concentrations were typically less than 10 mg l −1 at the surface and 40 mg l −1 at the bottom. Under fair weather conditions, subtidal channels play a negligible role in offshore sediment dispersal since sediment concentrations during flood appear to equal or exceed those during ebb. Instantaneous currents at two stations on the shelf were of the same order as those in subtidal channels, but displayed net flow to the south at speeds of 7·8 and 3·8 cm s −1 . We speculate that subtidal channels provide the conduits for transporting sediments directly offshore during winter monsoons. On an annual time scale, one model for sedimentation would be low sediment influx from rivers but high influx from tidal flats to the coast during winter, when north winds generate large waves in the Yellow Sea, and the reverse in summer, when winds from the south produce little wave activity. Coastal muds would thus be eroded in winter and replenished by high river discharge in summer. Further, it is reasonable to assume that residual flow to the south would strengthen in winter as the monsoon season progressed. Thus the introduction of resuspended tidal flat sediments would coincide with the best developed dispersal system.

Conditions of the peracarid populations of subtidal communities in Northern Brittany ten years after the Amoco Cadiz oil spill

Peracarid populations were greatly reduced in 1978 by oil from the Amoco Cadiz. Ten years after the spill, a benthic survey was conducted in the soft-bottom infralittoral communities of the bays of Morlaix and Lannion and the Aber Wrac'h channel to study the state of recovery of peracarid populations. Living in isolated populations in fine sand and muddy sand communities with low potential for immigration, the recolonization and the reconstitution of these perturbed populations was expected to be slow. The amphipod populations from the subtidal channel of Aber Wrac'h, which were initially the most affected by the oil spill were in the least advanced state of recovery. Some species present in abundance before the oil spill were not rediscovered. Nevertheless, ten years after the oil spill, most of the populations had completely recovered.

Sedimentologic Approaches to Paleobathymetry, with Applications to the Merced Formation of Central California

Sedimentologic evidence for paleobathymetty, like paleontologic evidence, is largely indirect in that it mostly reflects processes and conditions that are themselves partly depth-dependent. Sedimentologic approaches can be a useful adjunct to paleontology for interpreting paleodepths, particularly in relatively shallow water. Pleistocene deposits of the Merced Formation on the San Francisco Peninsula, California, provide an excellent opportunity to demonstrate the methods, value, and difficulties of the sedimentologic approach. Distinction between subaerial and subaqueous deposition can be drawn on the basis of eolian structure, paleosols, and other sedimentary features. The spacing of symmetric ripples relative to their grain size and lenticular laminae of heavy minerals may be useful for identifying deposition in very shallow water. The boundary between intertidal and subtidal deposits serves to establish ancient sea-level positions. The boundary is reflected by the upward transitionsfrom cross-bedded sand (nearshore facies) to planar-bedded sand (foreshore facies) in open-coast deposits, and from laminated sand andlor mud (subtidal channel bank facies) to bioturbated sand andlor mud (intertidal flat facies) in the deposits of coastal embayments. Where the physical setting (i.e., wave energy, sediment input, etc.) can be reasonably reconstructed, paleodepths in shelffacies are indicated by the maximum depth of cross-bedding and gravel deposits generated by the effects of shoaling waves, the contact between shelf sand and mud, and the deepest occurrence of shell lags. In Pleistocene shelf deposits of the Merced Formation on the San Francisco Peninsula, California, these depths are placed at 10 + 5 m, 60 + 10 m, and 100 + 20 m, respectively, based on their inferred depth on the modern central California shelf. At greater depths the sedimentologic approach loses resolution and reliability. Interpretations based on the distribution of turbidites, evidence of anoxic conditions, and calcium carbonate compensation depths and lysoclines are probably most useful for establishing paleodepth in a relative sense only. Integration of the sedimentologic and paleontologic approaches carries a high potential for improving our ability to determine paleobathymetry. INTRODUCTION

Use of Salt-Marsh Peat Reefs by Small Juvenile Lobsters on Cape Cod, Massachusetts

The habitats utilized by small juvenile (<40 mm carapace length, CL) lobsters (Homarus americanus) are poorly known. We discovered and studied an undescribed juvenile lobster habitat in Nauset Marsh, Cape Cod. Juvenile lobsters (X=26.7 mm carapace length, 6 to 72 mm, n=38) were collected from suction samples primarily 0144 01 in “peat reef” habitats during the period from August 1985 through October 1986. The reefs consisted of large blocks ofSpartina alterniflora peat that had separated from the marsh surface and fallen into adjacent subtidal marsh channels. The smallest lobsters (6 to 7 mm CL) were collected from peat reefs in October 1985, and April and July 1986. In these habitats, juvenile lobster density averaged 2.5 individuals m−2 (range 0–5.7) in suction samples. Peat reef habitats occur in other salt marshes in the northeastern United States and may be an important nursery habitat for small juvenile lobsters.

Sedimentary environments and facies of the subtropical mgeni estuary, Southeast Africa

AbstractThe Mgeni Estuary is situated on the subtropical, mainly microtidal Natal coast. Modern sedimentary environments in the estuary comprise two groups. Barrier‐associated environments include inlet channel, inlet beachface, tidal delta, washover fans, transverse intertidal bars and aeolian dunes. Estuarine environments include subtidal channels, interidal bars, back‐barrier lagoon, tidal creek, tidal creek side‐attached bars, creek mouth bar, mangrove fringe and supratidal mudflats. Each sedimentary facies is described in terms of grain‐size, sedimentary structures, and sedimentary processes. The distinctive flora and fauna play an important role in facies recognition. Vertical sequences produced by infilling of the estuary and subsequent coastal erosion are discussed. The facies are considered sufficiently distinct to warrant recognition in the geological record.

Fluvial, tidal and storm sedimentation in the Chilhowee Group (Lower Cambrian), northeastern Tennessee, U.S.A.

ABSTRACTThe Lower Cambrian Chilhowee Group of northeastern Tennessee consists of the Unicoi, Hampton and Erwin Formations, and is divided into four facies. The conglomerate facies occurs only within the lower 200 m of measured section (the Unicoi Formation) and consists of fine‐grained to pebbly quartz wacke with rare thin beds of laminated siltstone. Low‐angle to horizontally laminated, fine‐grained sandstone with laminae and lenses of granules and pebbles represents upper flow‐regime, overbank deposition within a braided stream system that was close to a coastline. Medium‐scale, planar‐tabular cross‐stratified conglomerate in which megaripple bedforms are preserved is interpreted as representing deposition in interbar pools of braided channels, as flood stage waned and larger bedforms ceased to migrate. Large‐scale, planar‐tabular cross‐stratified conglomerate beds represent migration of large transverse bars within a broad braided stream channel during high flood stage.The sandstone facies occurs throughout the Chilhowee Group, and is therefore interbedded with all other facies. It consists of mainly medium‐ to very coarse‐grained, subarkosic to arkosic arenite. Thinly interbedded, laminated siltstone and sandstone, which may exhibit wavy or lenticular bedding, represents deposition during slack water periods between ebb and flood tides. Large‐scale planar‐tabular and trough cross‐stratification reflects deposition within the deepest areas of subtidal channels, whereas medium‐scale cross‐stratification represents deposition in shallower water on shoals separating channels. Fining‐ and thinning‐upward sequences most likely resulted from the longshore migration of channels and shoals.The hummocky facies occurs only in the Erwin Formation and consists of horizontally laminated to hummocky stratified, fine‐grained arkosic to subarkosic arenite interbedded with equal amounts of bioturbated mudstone. It represents deposition between storm and fairweather wave‐base by combined‐flow storm currents.The quartz arenite facies is characterized by an absence of fine‐grained units and lithologically consists of a super‐mature, medium‐ to coarse‐grained quartz arenite. Large‐scale planar‐tabular cross‐stratification and abundant low‐angle cross‐stratification with rare symmetrical ripples (lower quartz arenite facies) occurs interbedded with the braided fluvial conglomerate facies, and was deposited within either a ridge‐and‐runnel system or a system of nearshore bars. Large‐scale, planar‐tabular cross‐stratification (upper quartz arenite facies), which forms the top of two 40 m‐thick coarsening‐upward sequences of the type: hummocky faciessandstone faciesquartz arenite facies, probably represents deposition on sand ridges that formed on a sand‐starved shelf as transgression caused the detachment and reworking of shoreface channel‐shoal couplets.Palaeocurrent data for the Chilhowee Group are unimodal but widely dispersed from 0° to 180°, and exhibit a minor mode to the west. The data are interpreted to reflect the influence of longshore, tidal and storm currents. The ichnofossil assemblage changes upsection from one characterized only by Paleophycus to a Skolithos ichnofacies and finally to a Cruziana ichnofacies. The facies sequence, biogenic and palaeocurrent data reflect the interaction through time of (I) non‐marine and marine processes; and (2) transgression coupled with shoreline progradation. The Chilhowee Group represents an overall deepening from terrestrial deposition to a marine shoreface that experienced both longshore and tidal currents, and finally to a storm shelf environment that periodically shoaled upward.

Facies and Paleoenvironments of Campanian Basal Belly River Sands, Southern Alberta, Canada: A Case Study Based on Multidisciplined Approach Using Both Conventional Sedimentology and Palynofacies Synthesis: ABSTRACT

Basal Belly River sands are not a single, continuous body but are composed of many separate bodies of sand occupying slightly different stratigraphic positions. The shape of the Belly River Formation, its internal geometry, and its lithology fit well with the clastic wedge association of deltaic sediments. The textural and mineralogical immaturity of most of the Belly River sediments indicates rapid deposition, also characteristic of deltaic deposits. A detailed sedimentologic and palynofacies investigation of over 2930 ft of core from several wells in southern Alberta has led to the recognition of several distinct lithofacies. The lithofacies of the subaqueous part of the delta are (1) nearshore marine bar and interbar marine facies, (2) prodelta marine shales and siltstones, and (3) distal and proximal delta front sand facies. The subaerial part of the delta and interdeltaic shoreline sediments consists of 15 lithofacies that can be grouped as follows: estuarine distributary channel; barrier bar; barrier face and back barrier tidal channel; subtidal sand and shale-sand channels and flats; peat swamp; transgressive tidal sand ridges; and upper delta plain/alluvial meandering rivers and overbank complexes. Best reservoir developments are found in barrier bar, transgressive tidal sand ridges, subtidal channels, and point bar facies. The more » Basal Belly River sands of prograding deltaic origin experienced episodes of modification from an early river-dominated delta to a tidally dominated one later. This is evident from the paleogeography, geometry, continuity, and internal characteristics of the Basal Belly River sand bodies. « less

Spatial variability and variations in bedload transport direction in a subtidal channel as indicated by sonographs

AbstractThree sonographs of a part of a subtidal channel, located in the Oosterschelde, are described and interpreted. The sonographs are used to determine the asymmetry of the bedforms. Current profiles and directions are used to explain differences in dimensions and characteristics of the bedforms. The relations between current velocity parameters and bedform characteristics in intertidal zones do not apply to subtidal forms. The division in the depth‐velocity diagrams between the types of 2d and 3d bedforms of Dalrymple et al. (1978) is not valid.