Pelagic Fauna Research Articles

The sub-ice fauna of the Laptev Sea and the adjacent Arctic Ocean in summer 1995

The sub-ice habitat and fauna in the Laptev Sea and the adjacent Arctic Ocean were investigated during the “Polarstern” cruise ARK XI/1 in summer 1995. At the ice-water interface a thin thermo- and halocline developed at many stations due to melting processes. In the lower centi- to decimetres of the ice, an accumulation of organic matter was found (particulate organic carbon: 1.9 mg l−1, chl a: 3.3 μg l−1) that may have provided a food source for the fauna. The water layer directly beneath the ice was inhabited by high numbers of various nauplii (130–23911 ind. m−3), and two ecological groups, the pelagic sub-ice fauna that originates from the surface water plankton, and the sympagic sub-ice fauna that migrates into this boundary layer from the ice interior. The pelagic fauna dominated the sub-ice community both in terms of species number and abundance. Both groups mainly comprised small copepods (e.g. Oithona similis, Oncaea borealis, Pseudocalanus spp., Halectinosoma spp., Tisbe spp.), but foraminifers and pteropods, for example, also occurred regularly. Diversity was generally low. Factors influencing the composition and abundance of the sub-ice fauna were most likely water depth, salinity and sea-ice sediments.

Sequence stratigraphy of the upper jurassic evaporite-carbonate: Sequence at the western area of Wadi Al-Jawf-Marib basin, Yemen

The Upper Jurassic evaporite-carbonate sequence (Sabatayn Formation and its equivalent Madbi Formation or Transition Beds) and its constituent systems tracts have been recognized at the western side of Wadi Al-Jawf-Marib intracratonic rift basin and its adjacent shelf areas. The Sabatayn Formation was subdivided, from the base to the top, into the Shabwa Salt Member, the Layadim Shale Member, the Ayad Gypsum Member and its equivalent M’qah Sandstone Member. The evaporite depositional systems of the lowstand facies (or systems) tract LST began in Middle Kimmeridgian times after the complete drawdown of the shelf area and basin isolation. The effect of ephemeral flooding events of the marine water and/or surface run-off water on the basinal salt pans resulted in the deposition of four or more parasequences (shallowing upward salinas of the Shabwa Salt Member). When the rift was reconnected to the open ocean in Late Kimmeridgian times, the basinal LST and the formerly exposed carbonate sequence of the shelf area were flooded and an onlapping retrogradational transgressive facies tract (TST) represented by the lagoonal-sabkha Layadim Shale and the Ayad Gypsum members and their equivalent shoals, shallow subtidal to intertidal Madbi Formation were established. The continued relative sea-level rise followed by relative sea-level falls due to orogenic movement during Portlandian times resulted in the deposition of aggradational to progradational offlapping strata of early and late highstand facies tract (HST representing part of the Naifa Formation). On the shelf area, the early HST consists of a subtidal ammonitic limestone which changes westwards into a shallow subtidal-intertidal sandy rippled mudstone containing fossil fish remains and varicolored cross bedded sandy mudstone containing plant remains, while the late HST consists of supratidal to sabkha gypsiferous shale and sandstone with gypsum lenses. However, on the shelf slope, a brecciated ammonitic limestone interbedded with calcareous mudstone containing pelagic fauna represent the forced regressive wedge-systems tracts (FRWST).

Paleoecology and Taphonomy of Faunal Assemblages in Gray "Core" (Offshore) Shales in Midcontinent Pennsylvanian Cyclothems

Faunal composition and diversity of macroinvertebrate assemblages in the gray facies of nineteen core shales of Mid-continent Pennsylvanian cyclothems confirm the offshore marine origin previously established for those shales. Diverse assemblages of only calcitic fossils that show grain corrosion at northern Midcontinent localities (Nebraska/Iowa) indicate deposition under oxic to slightly dysoxic conditions within the calcite lysocline but below the aragonite compensation depth for this particular sea. Southward (Kansas), sparser calcitic faunas indicate that core shales there were deposited farther from the northern shoreline in water of greater depth and less bottom oxygen, closer to or locally below the calcite compensation depth. Farther southward near the Ouachita detrital source (Oklahoma), greater influx of sediment permitted rapid burial of shelled organisms to overprint the effects of lysoclines and compensation depths. Both the originally aragonitic molluscs as well as calcitic organisms are preserved in those shales. Fine detrital grain size, radiolarian-bearing nonskeletal phosphate, and pelagic fauna (ammonoids and conodonts) further substantiate an offshore marine origin for the black core shale facies that is surrounded by the gray facies, and which was deposited in a probably deeper, more continually anoxic environment where most benthos was excluded.

Guerrero-Morelos Platform drowning at the Cenomanian–Turonian boundary, Huitziltepec area, Guerrero State, southern Mexico

Facies successions in three stratigraphic sections (Barranca del Tigre, Axaxacoalco, and Zotoltitlán, Guerrero State, southern Mexico) that comprise middle Cenomanian to lower Turonian rocks of the central part of the Guerrero-Morelos Platform, indicate the drowning of some parts of the platform near the Cenomanian-Turonian boundary. In the western part of the study area (Barranca del Tigre section), Cenomanian shallow-marine limestones (mainly subtidal facies) with abundant benthic fauna (mainly foraminifera) pass upwards to a 7-m-interval of Turonian open-marine nodular limestones with few benthic organisms and then to dark-grey and black laminated pelagic limestones and marls. In the west-central part (Axaxacoalco section), Cenomanian restricted shallow-marine limestones (intertidal to subtidal facies) change abruptly upwards to Turonian black and dark-grey laminated pelagic limestones and marls with only pelagic fauna (calcisphaerulids, planktonic foraminifera, and radiolaria). In the eastern part of the study area (Zotoltitlán section), Cenomanian restricted shallow-marine limestones (mainly intertidal facies) are overlain by 45 m of open-marine limestones showing up-section a rapid decrease in benthic diversity until only calcisphaerulids and echinoderm fragments occur. Overlying these rocks, there are 65 m of nodular shaly limestones in thick strata with renewed benthic fauna, 80 m of shaly and silty limestones intercalated with claystones and some siltstones, and finally, dark grey/black laminated pelagic limestones and marls. The facies successions in the stratigraphic sections suggest the progressive drowning of the Guerrero-Morelos Platform around the Cenomanian-Turonian boundary. We attribute the drowning of some parts of the platform to the occurrence of the Cenomanian-Turonian anoxic event. The impingement of anoxic waters over the platform could produce the drastic reduction of the carbonate producing benthos observed in the stratigraphic sections and therefore a reduction in carbonate accumulation rates. Subsidence and the late Cenomanian-earliest Turonian sea-level rise were then able to drown the platform. This occurred first in the western part, where subtidal conditions dominated and an irreversible drowning occurred, allowing the deposition of organic-rich pelagic sediments over pre-existing shallow-marine carbonates. At the same time, the eastern part, where intertidal conditions dominated, changed to open-marine conditions, shallow first and deep later. Here, a temporal restoration of shallow open-marine carbonate sedimentation resulted, but eventually this region was also drowned, probably by the interplay of terrigenous-clastic supply to the platform and a new impingement of anoxic waters.

Devonian graptolites from southwestern Europe: a review with new data

Lower Devonian graptolite faunas have been recognized in the Normandy and southeastern regions of the Armorican Massif, France; the Pyrenees and Catalonian Coastal Ranges regions and northern Minorca, Balearic Islands, Spain; the southern Hesperian Massif (Ossa Morena Zone) of the Iberian Peninsula; and from southeastern Sardinia, Italy. All but one the of the graptolite faunas collected throughout this large region are from Lochkovian age strata, representing the Monograptus uniformis, Monograptus praehercynicus, and Monograptus hercynicus biozones corresponding to the lower, middle and upper Lochkovian, respectively, and mostly represented by monospecific or low diversity assemblages. Although many individual sections contain representatives of two of the biozones, relatively few reveal all three. A single, poorly preserved faunule, collected in the Ossa Morena region of Spain from strata dated by brachiopods as Pragian–early Emsian may represent the only known graptoloid fauna of post-Lochkovian age. Almost all graptolites have been recovered from condensed successions of black shales and limestone nodules, similar to those of other proto-Tethyan (i.e. outer shelf, with dominantly pelagic faunas) regions such as Thuringia, Bohemia, the Carnic Alps and northwestern Africa. The two exceptions are an occurrence in a shallow-water, coarser clastic sequences at the Carteret locality in Normandy and in deep water turbidites on the island of Minorca. Graptolites are not known from any other thick, shallow water clastic sequences, although whether this is because of paleo environmental exclusion or simply lack of recovery to date is unknown. Other fossil evidence (e.g. chitinozoans), however, indicates continuous marine sedimentation from the Silurian to Devonian. © 1996 John Wiley & Sons, Ltd.

Permian ammonoids from Kuala Betis area, Kelantan and their paleogeographic significance

Two new Permian ammonoid bearing localities were recently discovered from Kuala Betis area, Kelantan. At Sungai Peralong, ammonoid cephalopods and some brachiopods were found in thin to moderately bedded tuffaceous siltstone and mudstone. The ammonoid fauna consists of Agathiceras cf. suessi Gemmellaro, Adrianites cf. elegans Gemmellaro, Popanoceras sp., Propinacoceras sp., ?Imitoceras sp. and some small orthoconic ammonoids. At another locality near Sungai Berok, ammonoid fauna was preserved in thin tuffaceous shale interbedded with chert and cherty mudstone. This locality yields Agathiceras cf. suessi Gemmellaro, Adrianites cf. elegans Gemmellaro, Popanoceras sp., Propinacoceras sp. and Hoffmannia cf. hoffmanni (Gemmellaro). The cephalopod assemblages from these localities partly resembles that of the middle Permian Sungai Cheroh cephalopod fauna (Lee, 1978). The Sungai Peralong, Sungai Berok and Sungai Cheroh fauna are all associated with deep water sediment within the Bentung Suture zone. Similarities between these fauna and the Italian Socio fauna and the Basleo fauna of Timor (Lee, 1978) and many other ammonoid occurrences in Europe, North Africa and West Asia (Blendinger et al., 1992) indicated that an oceanic link has been established between these pelagic fauna, perhaps as early as Lower Permian or even earlier.

La limite Cénomanien-Turonien en Tunisie centrale et méridionale: biostratigraphie et paléoenvironnements

The 'Dolomie du Gattar' (part of the Zebbag Formation of central and southern Tunisia) is, for the first time, accurately dated by means of newly-discovered ammonites in underlying and coeval strata. The underlying beds are limestones with a pelagic fauna (ammonites, foraminifers, coccoliths) encompassing three ammonite zones: the Neolobites vibrayeanus Zone (late Cenomanian); the Vascoceras cauvini Zone (end Cenomanian); and the Pseudaspidoceras flexuosum Zone (early Turonian). The Gattar Member is not entirely dolomitic and corresponds to a carbonate platform sequence deposited during a shallowing upwards cycle. It can be divided into a lower part consisting of biogenic limestones rich in rudists ( Durania) and gastropods, and an upper series of limestone and dolostones indicative of tidal flat environments. In the Gafsa Basin this carbonate platform sequence passes laterally into deeper water ammonite-beating facies. From this we deduce that the Gattar Member is earliest Turonian ( flexuosum Zone) in age. The member belongs to a third-order transgressive regressive cycle. The fossiliferous pelagic limestones that mark its base are interpreted as having been deposited during the transgressive phase, the Gattar Member representing the highstand tract. In South Tunisia, the member is topped by an unconformity with evidence of emersion and is interpreted as a sequence boundary. This unconformity is missing in central Tunisia, where uninterrupted marine conditions continued across the Cenomanian-Turonian boundary. As a result, the overlying lower to middle Turonian deposits are diachronous along a north-south axis in Tunisia. The third-order cycle that the Gattar Member encompasses is a major palaeogeographical event on the North African margin and needs to be taken into account in global reconstructions.

Evidence for Internal Zone unroofing from foreland basin sediments, Betic Cordillera, SE Spain

Tertiary sediments at the boundary of the Internal and External Zones in the eastern Betic Cordillera record evidence of compressive deformation in the Internal Zone and its subsequent exhumation. Sediments on the north side of the Sierra Espun̄ associated with the structurally highest unit of the Internal Zone, the Malaguide Complex, are interpreted as a foreland or piggyback basin which formed in response to the deforming Internal Zone. Basin evolution has been reconstructed using an integrated analysis of sedimentary facies, petrography, heavy minerals and structural data. The basin fill contains Eocene shallow-marine carbonate shelf/ramp sediments, early Oligocene fan conglomerates and coralgal reefs transitional to probable slope deposits of the late Oligocene, and a late Oligocene to mid-Miocene deep-water sequence with pelagic faunas. The provenance analyses illustrate an unroofing history of the Internal Zone orogenic edifice. Locally uplifted Mesozoic to Eocene carbonates at the thrust front were the source of lower Oligocene conglomerates. Clastic detritus supplied to the late Oligocene to Aquitanian basinal sediments was mainly derived from the Permo-Triassic and Palaeozoic basement of the Malaguide Complex. The presence of small amounts of first-cycle metamorphic minerals indicates the start of exhumation of the deeper metamorphic Alpujarride Complex. From the Burdigalian onwards a metamorphic terrane dominated the source area of the deep-water sediments, and by the Langhian high pressure minerals from the deepest parts of the Internal Zone orogenic edifice appear in the sedimentary sequence. Rapid exhumation recorded in the early Miocene sediments may be the result of extensional exhumation of the Internal Zone metamorphic complexes.

La formation bahloul en tunisie centro-septentrionale: variations latérales, nouvelle datation et nouvelle interprétation en terme de stratigraphie séquentielle

From various geological traverses made in North-central Tunisia and northern Tunisia the Bahloul Formation (Burollet, 1956) is considered to be variable in thickness and facies. This formation is characterized essentially by its lithology and structure, consisting of alternating laminated and darkish micritic limestones interbedded with dark foliated marls which are rich in organic matter (mean T.O.C. 2.66%). At the type locality (Oued Bahloul), the Bahloul limestones are wackestones rich in pelagic fauna (Ammonites, Calcisphaerulidae, Rotalipora, Whiteinella, Dicarinella, etc). This formation overlies a massive 2 m thick limestone unit; the base of which is eroded, rich in phosphate and quartz grains. The same unit consists of wackestone-packstone carbonate rich in Calcisphaerulidae, with filaments and benthonic debris at the top of the formation. Thus, the base of this unit which covers the Fahdene Formation corresponds to an unconformity, shown by a rapid change in the depositional regime. The base of the overlying Kef Formation is also marked by an abrupt change in the depositional regime. Microfauna and microfacies analyses cofirm that the Bahloul Formation, at its type locality, began in the late Cenomanian. This is confirmed by Ammonites ( Calycoceras naviculare Mantell, with Eucalycoceras gothicum Kossmat, Mantelliceras villei Coquand) and ended in the early to middle Turonian. The richness and the pelagic nature of the fauna prove that the Bahloul Formation sediments were deposed in a continental-slope environment during a transgressive interval within the UZ-25 sequence (Vail et al., 1987).

CARNIAN CONODONTS FROM UPPER TRIASSIC STRATA OF TAMARIN SECTION (SAN CASSIANO FM., DOLOMITES, ITALY)

A Late Triassic conodont fauna is described and illustrated for the first time from the San Cassiano Formation of the eastern Dolomites (Tamarin). The co-occurrence of Metapolygnathus auriformis , M. carnicus , M. baloghi , M. polygnathiformis , Gladigondolella tethydis , Gl. arcuata and Gl. malayensis malayensis indicates a Late Julian age. The fauna corresponds to the auriformis Zone (Austriacum Zone according to ammonoid zones). This pelagic conodont fauna, characteristic of relatively deep water, shows strong faunal affinities with coeval faunas from European Tethyan regions(mainly Greece, Turkey, NE Hungary, and Alps) and northern Kumaun region of Tethys Himalaya.

Wind effects on prey availability: How northward migrating waders use brackish and hypersaline lagoons in the sivash, Ukraine

Large numbers of waders migrating northward in spring use the Sivash, a large system of shallow, brackish and hypersaline lagoons in the Black Sea and Azov Sea region (Ukraine). The bottoms of these lagoons are often uncovered by the wind. Hence, for waders the time and space available for feeding depend on wind conditions. In hypersaline lagoons the benthic and pelagic fauna was very poor, consisting mainly of chironomid larvae (0.19 g AFDM·m −2) and brine shrimps Artemia salina, respectively. Brine shrimp abundance was correlated with salinity, wind force, wind direction and water depth. Dunlin Calidris alpina and curlew sandpiper Calidris ferruginea were the only species feeding on brine shrimp. As brine shrimp densities are higher in deeper water, smaller waders such as broad-billed sandpipers Limicola falcinellus are too short-legged to reach exploitable densities of brine shrimp. In brackish lagoons the benthic and pelagic fauna was rich, consisting of polychaetes, bivalves, gastropods, chironomid larvae, isopods and amphipods (8.9 to 30.5 g AFDM·m −2), but there were no brine shrimps. Prey biomass increased with the distance from the coast, being highest on the site that was most frequently inundated. Dunlin, broad-billed sandpiper and grey plover Pluvialis squatarola were the most abundant birds in the brackish lagoon. Due to the effects of wind-tides only a small area was usually available as a feeding site. Gammarus insensibilis was the alternative prey resource in the water layer, and their density varied with wind direction in the same way as brine shrimp. Curlew sandpipers and dunlins in the hypersaline lagoons and broad-billed sandpipers in the brackish lagoons often changed feeding sites, probably following the variation in prey availability. Only because of the large size and variety of lagoons are waders in the Sivash always able to find good feeding sites.

Sedimentology of middle ordovician carbonates in the Argentine precordillera: evidence of regional relative sea-level changes

The Las Aguaditas Formation in the Argentine Precordillera of San Juan is the only Ordovician carbonate sequence deposited on a slope. Spiculites, mudstones and calcisiltites represent the autochthonous sediments, characterized by a fine lamination, rare fossils and their dark colour. The pelagic fauna consists of rare radiolarians/calcispheres, trinucleid trilobites, graptolites and conodonts. The latter are typical of an open marine environment and proved a Llanvirn—Llandeilo age for the Las Aguaditas Formation.

Nature of environmental variability experienced by benthic and pelagic animals in the St. Lawrence Estuary, Canada

We compared the short-term variability in salinity, temperature and concentration of suspended particulate matter experienced by benthic and pelagic animals in the middle St. Lawrence Estuary on both horizontal and vertical axes. In addition, we examined the effect of 3 patterns of vertical distribution (no vertical migrations, die1 and tidal vertical migrations) on the environmental variability experienced by pelagic animals. Results demonstrated that the nature of environmental variability is different for the 2 types of animal. For benthic animals, environmental variability is experienced on the horizontal axis, whereas the vertical axis is more important for pelagic animals. The magnitude and frequency of abiotic fluctuations experienced by benthic animals depend on the environmental factors that control the longitudinal position of the estuarine gradient, whereas for pelagic animals they mostly depend on the animals' behaviour on the vertical axis. Consequently. favourable habitats for benthic and pelagic animals in the estuary are different. The most upstream section of the estuary is the most variable for benthic animals because the tides of greatest range generate the highest abiotic fluctuations on the horizontal axis. In contrast, the same section is the most abiotically stable for pelagic animals because tides of highest energy generate the strongest vertical mixing. The highest abiotic fluctuations experienced by these animals are in the most vertically stratified waters downstream in the estuary. As a result, minimum numbers of species of benthic and pelagic animals are reached at different locations in the estuary corresponding to the areas which are the most abiotically variable for each type of animal. We propose that well-mixed parts of estuaries should be considered as abiotically stable habitats for the pelagic fauna, whereas vertically stratified areas should be considered as abiotically variable except in the case where vertical rnigratlons are not necessary for the retention of animals.

Aggregations of myctophid fishes with other pelagic fauna

Direct underwater observations of myctophid fishes were conducted to 630 m in submarine canyons along the outer margin of the northeast United States continental shelf with the Johnson-Sea-Link II submersible. Data revealed that myctophids occurred in aggregations of other pelagic fauna (e.g., ctenophores, sergestids, amphipods) when densities of the former were low but the latter were high. We suggest that myctophids selectively occur with other pelagic fauna to reduce the probability of direct contact with predators and to take advantage of aggregate search abilities for common prey taxa.

Silurian Myodocopes: Pioneer pelagic ostracods and the chronology of an ecological shift

Abstract. Analysis of all relevant palaeontological and global geological data strongly supports the notion that representatives of Silurian myodocope ostracods had pelagic lifestyles and habitats and that they may well be, within the Ostracoda, pioneer colonisers of such environments. Morphological evidence (from fossil and Recent myodocopes) combined with facies distributional and concomitant faunal evidence (from the Silurian of, for example, Britain, France, Czechoslovakia, Sardinia, Australia and China) endorses the idea that myodocope ostracods may have undergone a benthic to pelagic ecological shift during mid Silurian times.Lower Silurian myodocopes lived, with benthic associates, on well oxygenated shelves. Upper Silurian ostracods lived, typically with low diversity, largely pelagic faunas in outer shelf topographic lows or off-shelf basin slopes, and are characteristically associated with deposits which are in part suggestive of lowered oxygen levels or even anoxic conditions. A pre-adaptation for swimming may have allowed Silurian myodocopes to respond to environmental forcing (negative oxygen levels; positive trophic and nutrient incentives; rises in sea levels) by migrating, through time, up the water column.

Interactions with the Benthos Alter Pelagic Food Web Structure in Coastal Waters

Results from studies in experimental enclosures containing both water column and benthic communities show that the benthos has an important effect on the structure and productivity of pelagic food webs whether the system is nutrient enriched or nutrient limited. Research over a 10-yr period in 13-m3 mesocosms showed that changes in the pelagic food web were correlated with different sediment communities and with the presence or absence of a benthos. The abundance of copepods was inversely correlated with numbers of macrofauna. At both low and high nutrient levels, systems without benthos had greatly enhanced numbers of carnivorous holozooplankton including ctenophores, medusans, chaetognaths, and fish. Our observations indicate that the presence of the benthos shortens the pelagic food web.and inhibits the response of pelagic fauna to nutrient enrichment in well-mixed coastal waters. The strength of benthic–pelagic coupling, which is controlled by the amount of turbulence in the water column, may be more important to food web structure than the rate of nutrient supply and could determine which subsystem responds to eutrophication.

The Dilemma of Gaps in Carbonate Stratigraphic Sequences: A Case History from the Jurassic of the Venetian Alps, Italy

The Lower Ammonitico Rosso (L.A.R.) of the western Venetian Alps is a thin, red nodular limestone that overlies a thick Late Triassic to Middle Jurassic (Aalenian) shallow-water platform sequence. It is thought to represent a Middle Jurassic (Bajocian) drowning event whereby the Trento Platform became a submerged plateau. The L.A.R. is problematic in that it (1) is rich in ammonites and other pelagic fauna; (2) contains stromatolites, oncolites, and shallow-water sedimentary structures; and (3) directly overlies platform sediments that contain complex brecciated fabrics filled by cement and Bajocian-age sediment. These seemingly contradictory features have generated much debate as to the sequence stratigraphy of the Trento Platform. New evidence suggests that the L.A.R. was deposited in a shallow-water environment following a period of subaerial exposure. In an east-west transect from the platform edge to platform interior, a clear transition is exhibited from high- and low-energy open-marine facies to restricted lagoonal facies. High-energy open-marine facies include well-sorted skeletal- and peloidal-rich sands possessing low-angle to planar cross stratification and thin, fairly sorted ammonite- and belemnite-rich gravels. Sands include both shallow-water and pelagic fauna; gravels commonly contain ripup clasts from underlying sediments. Low-energy open-marine facies consist of buff colored wackestones and packstones that containmore » ammonites, belemnites, pelagic bivalves, and peloids. With respect to paleogeography, the abrupt transition from open-marine facies at the platform edge to lagoonal facies in the platform interior indicates that thrombolites, stromatolites, and oncolites accumulated in a shallow restricted environment.« less

Multicyclic History of the Northern India Continental Margin (Northwestern Himalaya) (1)

The geologic evolution of northern India is best recorded in the stratigraphic succession of the Zanskar Range (northwestern Himalaya), which represents the most complete transect through this ancient continental margin. Sedimentary history began in the late Proterozoic, and recorded a late Pan-African orogenic event around the Cambrian-Ordovician boundary, when the Gondwana supercontinent was eventually assembled. The following long period of epicontinental deposition in shallow seas linked to palaeo-Tethys lasted until the Early Permian, when a neo-Tethyan rift began to open between paleo-India and the Cimmerian microcontinents. Neo-Tethyan history can be subdivided into two sedimentary megasequences, both recording a major tectonic and magmatic event in the lower part. The first one began with breakup in the Late Permian and lasted until the end of the Jurassic. The second one started in the Early Cretaceous with the final detachment of India from Gondwana and the opening of the Indian Ocean, and ended with the India-Eurasia collision in the Early Eocene. The two megasequences can be in turn subdivided into six transgressive/regressive supersequences bounded by tectonically enhanced unconformities. Basal sandstone units of Early Permian, Late Permian, Norian, Callovian, Early Cretaceous, and Paleocene age are invariably associated with oolitic ironstones or reworked glauco-phosphorites, and mark the transgre sive part of each supersequence. Next, condensed nodular carbonates or shales with pelagic fauna are typically overlain by thick shallowing-upward marly units capped by regressive platformal carbonates. The six tectono-eustatic supercycles reflect successive rifting episodes which punctuated the progressive separation of India from the rest of Gondwana, and document the combination of plate/microplate reorganizations and eustatic, climatic, and oceanographic changes in the Tethyan domain. After the onset of collision between India and Asia close to the Paleocene/Eocene boundary, obduction of the remnants of the neo-Tethys ocean floor onto the Indian margin began, and the latter underwent multiphase deformation with fold-thrust shortening followed by heating and extension. After the main metamorphic event, ophiolitic nappes were re-thrusted and finally emplaced with their sedimentary sole on top of the passive-margin succession.

Food limitation and benthic regulation of populations of the copepod Acartia hudsonica Pinhey in nutrient‐limited and nutrient‐enriched systems

Evidence that population size of copepods in coastal marine systems can be both resource and predator limited was obtained from experimental enclosures with and without the sediment community in place. The novel finding was that meroplankton and attached benthos were apparently effective competitors and predators of planktonic copepods in both nutrient‐limited and nutrient‐enriched systems. During the spring period of increase Acartia hudsonica, as well as other abundant species of copepods, reached considerably higher numbers in four mesocosms without sediments than in four other mesocosms with the normal sediment community in place regardless of rate of nutrient input to the mesocosms. Measurements of population parameters of A. hudsonica indicated equivalent rates of reproduction, growth, and recruitment in nutrient‐enriched treatments with and without sediments. Suppression of population growth in the presence of sediments was therefore not due to food limitation but, by inference, to higher rates of mortality associated with high abundances of potential predators of benthic origin. In unenriched control mesocosms, resource competition with the benthos may also have contributed to low population densities in tanks with sediments (reproductive rates were more food limited in enclosures with sediment that in those without sediment). These results demonstrate important interactions between benthic and pelagic fauna that have often been overlooked but nevertheless may be important in regulating dynamics of pelagic copepod populations in shallow, well‐mixed waters.

A laminated hemipelagic facies from the Wenlock and Ludlow of the Welsh Basin

A hemipelagic fades from the Silurian succession in Wales consists of finely laminated, dominantly silt-grade sediment. It can be interbedded with sand- or mud-grade turbidites, or can occur as an independent facies. Its occurrence and character was controlled mainly by low bottom water oxicity. The pelagic fauna of the laminated hemipelagite supports this view. Many of the silt-grade particles have an aggregate structure and probably represent the faecal pellets of oceanic zooplankton. The fine lamination is thought to have formed by alternations of planktonic blooms and increased discharges of silt into the basin. By comparison with modern examples this alternation was likely to have been annual or seasonal.