Micropaleontological study (foraminifera, ostracods) and characterization of the paleoenvironment of middle Cretaceous deposits (Djebel Chemla area, north-eastern Algeria)

The Djebel Chemla section is situated in the north-east of Algeria, close to the town of Tébessa and just a few kilometers from the Tunisian border. This section is composed mainly of a sequence of marl and marly limestone levels, alternating with recurring oyster levels which range in age from Vraconnian to the Early Turonian. Firstly, the micropaleontological study of the content (counting of foraminifera and ostracods, thin sections) allows the subdivision of the section into four biozones, with Rotalipora appenninica, Rotalipora globotruncanoides, Rotalipora cushmani, and Whiteinella archaeocretacea as bio-indicators; and, secondly, it provides the opportunity to specify the paleoenvironment. The supposed depositional environment is rather calm and deep, of an external platform type, with a maximum depth of 200 m. Bottom waters seem to be commonly poorly oxygenated. The end-Cenomanian anoxic crisis was also characterized by the presence of Bahloul facies levels, rich in TOC, in filaments and of globulous planktic foraminifera, including a considerable proportion of Heterohelix. The genus Rotalipora has disappeared a few levels lower down.     

Les événements de la limite Permien–Trias : derniers survivants et/ou premiers re-colonisateurs parmi les ostracodes du Taurus (Sud-Ouest de la Turquie)The events of the Permian–Trias boundary: last survivors and/or first colonisers among the ostracods of the Taurides (southwestern Turkey)

Very few data on Early Triassic marine ostracods are available. In most of cases, the data concern the upper part of the Early Triassic. The Çürük da? section, located in the Western Taurides (Turkey), well stratigraphically constrained by conodonts and foraminifera, has been re-sampled for ostracod study. A significant fauna has been discovered in the Late Permian and the levels of the Earliest Triassic (Hindeodus parvus and Isarcicella isarcica staeschei zones). The ostracods of the basis of the Kokarkuyu Fm. are the oldest Triassic forms ever discovered. The occurrence of Palaeocopes in the earliest Triassic and similarity between the Permian and Induan assemblages suggest that the Lower Triassic Çürük da? ostracods represent a survival assemblage after the Permo-Triassic mass-extinction. To cite this article: S. Crasquin-Soleau et al., C. R. Geoscience 334 (2002) 489–495.     

Holocene evolution of a coastal lagoon, Lake of Tunis, Tunisia

Historical records demonstrate that the Lake of Tunis, Tunisia, was an open bay that became separated from the Mediterranean by an accreting barrier spit, forming a lagoon. Closure of the lagoon was caused by increased sedimentation as a consequence of Roman deforestation of the Medjerda River drainage basin to the north and subsequent accelerated longshore transport. The separation of the lagoon from the Mediterranean was completed in the early 1500s. At present, the 48 km² lagoon averages less than 1 m in depth and reaches eutrophic conditions in the late summer. Consistent with the historical records, the lagoon sedimentary column shows three distinct successive environments of deposition: (1) an arid continental environment; (2) an open marine bay; and (3) the present brackish to hypersaline lagoon. These depositional environments are represented by the lower grey layer, which is less than 0-5 m thick, the middle olive-grey layer, which varies from 1 to 5 m in thickness, and the upper black layer, which is 1 m thick. All of the strata are predominantly silt plus clay, but usually contain at least 10% sand. The lower grey layer consists of pitted quartz sand, with very few abraded, broken molluscan fragments and benthic foraminifera with thick tests. An arid, subaerial depositional environment of latest Pleistocene time best explains these sediment and fossil assemblages. In the middle olive-grey layer, coral, coralline algae, open marine graeses and the dominance of foraminifera over ostracods (expressed as a low percentage of ostracods/ostracods plus foraminifera) attest to a depositional environment of an open marine bay. A sharp increase in the percentage of ostracods/ostracods plus foraminifera and organic carbon from the middle olive-grey layer to the upper black layer signifies a major change in depositional environment. The predominance of ostracods over foraminifera, abundance of gastropods characteristic of eutrophic conditions, high organic carbon content and absence of macro-fossils characteristic of open marine conditions clearly indicate that the upper black layer was deposited in a brackish to hypersaline, eutrophic lagoon which has persisted to the present. Seismic records indicate a karstic bedrock surface underlying the lagoon. The surface is marked by considerable relief, and shows a linear depression which may represent an early sixteenth century ship canal. Heavy metal analyses of total samples in five cores demonstrate that Mn and Fe vary randomly, and are apparently derived from natural sources only. Cd, Cr, Pb, Cu and Zn are typically highest in the uppermost unit, which reflects levels of human contamination since closure of the lagoon.     

The upper Cenomanian–lower Turonian carbonate platform of the Preafrican Trough,Morocco: Biostratigraphic,paleoecological and paleobiogeographical distribution of ostracods

The upper Cenomanian–lower Turonian paleoenvironments of the Preafrican Trough carbonate platform is characterized by analyzing the structure of the ostracod assemblages and the information provided by other groups, and also by linking together the paleontological and geochemical data (detrital influx-redox-paleoproductivity proxies, δ¹³C curve). Two different domains (eastern and western) can be recognized on the platform during the late Cenomanian, before the onset of the OAE2. The western domain corresponds to a low-energy environment developed on a mid and/or outer ramp with hypoxic waters, low detrital influx and low paleoproductivity. The paleoecological assemblages show limited specific diversity but variable density. The ostracods are opportunistic and unspecialized (r strategists), being associated with Buliminidae, surface and intermediate-water planktonic foraminifera, and fishes. The eastern domain corresponds to an inner ramp and/or peritidal environment with oxic waters, low detrital influx and low paleoproductivity, developed in a higher energy environment with paleoecological assemblages showing high diversity but variable density. The ostracods are more specialized (K strategists), being represented by diverse and constant assemblages associated with diversified benthic foraminifera, calcareous sponges and echinoderms, as well as intermediate- and deep-water planktonic foraminifera. The onset of the OAE2 has no influence on the western ostracod assemblages, but leads to the decline of the ostracod fauna and the disappearance of the deep-water planktonic foraminifera in the eastern domain. During the early Turonian, after the OAE2, the platform becomes an outer ramp with increased paleoproductivity, but is associated with a decrease of taxonomic diversity in hypoxic waters. The ostracods are very sparse and unspecialized, associated with siliceous sponges, Buliminidae, surface-living planktonic foraminifera, fishes and pelagic crinoids. Marine paleobiogeographic communication is relatively easy across the carbonate platforms between the Preafrican Trough and other Moroccan regions, as well as between Morocco and different parts of the South Tethyan and East Atlantic margins belonging to the Cenomanian–Turonian South Tethyan Ostracod Province (STOP). Thirteen new species are described: Cytherella tazzouguertensis n. sp., Bairdiacypris chaabetensis n. sp., Bythocypris amelkisensis n. sp., Pontocypris tadighoustensis n. sp., Procytherura? elongatissima n. sp., Loxoconcha akrabouensis n. sp., Hemiparacytheridea sagittaemucronata n. sp., Rehacythereis errachidiaensis n. sp., Rehacythereis zizensis n. sp., Veenia (Nigeria) tardaensis n. sp., Veenia (Nigeria) mediacostarobusta n. sp., Xestoleberis? preafricanensis n. sp., and Xestoleberis circinatus n. sp.     

Environmental conditions of the Laptev Sea region in the late postglacial time

The comparison between the first results of comprehensive micropaleontological analysis (pollen, spores, foraminifera, and ostracods) and those of radiocarbon dating (AMS¹⁴C) for the sediments of the eastern inner shelf of the Laptev Sea (the core collected from depth of 37 m) indicates that considerable changes in natural conditions in the sea and on land coincide in time and refer to the time period of 1500–1700 years B.P. This period is characterized by changes in microfossils: appearance of thermophilic pollen and planktonic foraminifera and increase in total number of benthic foraminifera and ostracods. Intense warming and humidification of the climate reconstructed for this 200-year period promoted the expansion of large-shrub tundra. Summer air temperatures were lower than that in the peak mid-Holocene climatic optimum by 2°–3°C, but 1°C higher than the present-day temperature. An estuary freshwater basin developed: it was strongly affected by river discharge, but North Atlantic waters also intensely penetrated here in short-term intervals. In general, the studied microfossil complex reflects the relatively stable environmental conditions and decrease in seawater salinity in the eastern part of the Laptev Sea shelf during the last 2300 years.     

A microfaunal and sedimentary record of environmental change within the late Holocene sediments of Boca do Rio (Algarve,Portugal)

The foraminifera and ostracods observed in a late Holocene sedimentary sequence within a fluvial valley at Boca do Rio in the coastal zone of the western Algarve, Portugal, reveal a general, though not smooth transition from marine to fluvial conditions. The relative influence of these two environments appears to depend on the degree of permeability of the barrier system at the coast. Optically stimulated luminescence dates and palaeoecological information obtained from the sequence suggest that barrier formation may be related to changes in climate and/or patterns of ocean current circulation. An unusual deposit rich in sand and gravel found within the otherwise mud-dominated sequence has been dated at AD 1801 ± 76 years. This deposit contains foraminifera and ostracods which indicate marine conditions, and which contrast markedly with the brackish-water, estuarine assemblages found in the mud deposits. The rapid transition in the foraminifera and ostracod assemblages indicates a short-lived coastal flooding, which may represent the tsunami associated with the Lisbon earthquake of AD 1755. The variations in the foraminifera and ostracod assemblages also suggest subsidence during the earthquake, with uplift having occurred in the period since then.     

Distribution of foraminifera and ostracods in the Onkaparinga Estuary,South Australia

Analysis of sediment and aquatic vegetation samples collected along the Onkaparinga estuary, South Australia, revealed the distribution patterns of benthic and epi-benthic micro-organisms, specifically foraminifera and ostracods. The total assemblage provided an assemblage ‘snap shot,’ contemporaneous with the environmental conditions at the time of sampling, as well as seasonally and time-averaged distribution information. Species richness is low in the upper reaches of the estuary and favours species with a wide salinity tolerance. Observed species richness increases seawards. Species richness and abundance within species is greater where there is conspicuous aquatic vegetation. Thus, species richness and distribution appear to be related to the proximity to the sea and the provision of diverse habitable substrate. Foraminiferal species, Ammonia beccarii and Elphidium excavatum, and ostracods belonging to the genera Paracypria, Xestolebris and Leptocythere dominate the total assemblage. These species are characteristic of euryhaline conditions. The mutual maximum occurrences of Reophax barwonensis, Ammobaculites barwonensis and Trochammina inflata signify mid-estuarine conditions. In the lower regions of the estuary, the appearance of shallow marine species of foraminifera such as Elphidium crispum and Quinqueloculina poeyana, and ostracods Bairdoppilata sp., Hemicytherura spp. and Paranesidea spp. signifies interactions with the adjacent inner shelf coastal waters. Throughout the entire length of the estuary, the presence of vegetal substrate created a strong numerical bias towards live occurrences of the ostracods Paracypria sp. and Xestolebris cedunaensis. These species are potentially useful as proxies for paleoenvironmental interpretations of estuarine sediments.     

Calcareous nannofossils from the Santa Marta Formation (Upper Cretaceous), northern James Ross Island,Antarctic Peninsula

This study reports on the most stratigraphically extensive nannofloras yet recovered from the Lachman Crags Member of the Santa Marta Formation, James Ross Island, Antarctic Peninsula. The productive samples are dated as early Campanian. These ages are in accord with those provided by ammonites, foraminifera, ostracods and radiolarians from the same locality. The consistent and relatively abundant presence of Gephyrobiscutum diabolum throughout the productive part of the section, a species that has previously only been documented from the Falkland Plateau, extends its geographic distribution to higher latitudes, at least to the Antarctic Peninsula area.     

Integrated Rhaetian foraminiferal and conodont biostratigraphy from the Slovenian Basin, eastern Southern Alps

Conodonts and re-deposited shallow-water benthic foraminifera co-occur in the Upper Norian-Rhaetian Slatnik Formation of the Slovenian Basin (Southern Alps). The Mt. Kobla and Mt. Slatnik sections were investigated in order to give the first direct correlation between these two groups. The Norian-Rhaetian boundary (NRB) is placed at the First Appearance Datum of the conodont Misikella posthernsteini and is approached with the First Occurrence of the foraminifer Involutina turgida. An association of Trocholina turris with duostominids is a good marker for the Rhaetian. Careful revision of literature combined with our data culminated in reambulation of stratigraphical ranges for several foraminiferal species. The foraminiferal Triassic-Jurassic boundary (TJB) can be defined with the Last Occurrence of Galeanella tollmanni, ??Sigmoilina?? schaeferae or duostominids. The absence of the uppermost Rhaetian Misikella ultima conodont zone and the close coincidence of foraminiferal and conodont TJB could point to the presence of hiati in the upper part of the Slatnik Formation. Consequently, the exact pattern of disappearance of Late Triassic foraminifera at the boundary could not be observed. It is clear, however, that the foraminiferal assemblage was decimated for reef-dwelling foraminifera, duostominids and most of the platform-dwelling foraminifera. Biotic changes at the TJB are best explained as a result of a biocalcification crisis.     

Lower Jurassic (Hettangian–Pliensbachian) microfossil biostratigraphy of the Ballinlea-1 well,Rathlin Basin,Northern Ireland,United Kingdom

The thickest section of Early Jurassic strata known from onshore Ireland (total Jurassic thickness 566 m) is reported from the Ballinlea-1 well (Rathlin Basin) situated on the north coast of Northern Ireland. A biostratigraphical and palaeoenvironmental assessment is presented for this section largely based on calcareous benthic microfossils (foraminifera and ostracods). The Early Jurassic Waterloo Mudstone Formation (Lias Group) of Northern Ireland has previously received little micropalaeontological attention, therefore this work provides an opportunity to enhance palaeogeographic and palaeoenvironmental understanding for the Early Jurassic of the province, and this paper illustrates the key microfossil taxa of this age from Ireland for the first time. The records, based on ditch-cuttings samples, demonstrate a stratigraphical range from Hettangian to Early Pliensbachian, consistent with other wells and boreholes in this basin. The assemblage compositions are comparable to those elsewhere in the European boreal Atlantic realm. Hettangian to earliest Sinemurian microfossil assemblages are generally of low diversity and are numerically dominated by metacopid ostracods with occasional influxes of foraminifera. Gradually, foraminiferal abundance (often dominated by species of the Lagenida) come to exceed those of the ostracods in the Early Sinemurian reaching their greatest diversity in the Late Sinemurian. The sediments are considered to represent an inner to mid-shelf environment throughout while the record thickness for this region indicates ongoing syn-sedimentary fault movement along the basin margins within this period.     

Microfossil evidence for salinity events in the Holocene Coorong Lagoon,South Australia

Nearly 6 m of uncompacted muddy sediment was recovered from the floor of the northern Coorong Lagoon in the core Coorong #5. Radiocarbon analyses of molluscan shells indicate that sedimentation at the core site commenced before 6830 ± 90 yr cal BP, and the presence of Pinus pollen confirms a modern age for the uppermost 0.5 m. Microfossils extracted from the core sediment samples, 2 cm slices at 10 cm intervals, included the foraminifera Ammonia sp., Elphidium excavatum and Elphidium gunteri; the ostracods Osticythere baragwanathi and Leptocythere lacustris; and charophyte oogonia. Shell fragments of the estuarine bivalve Spisula (Notospisula) trigonella in the lowermost 0.7 m of the core are evidence that these sediments were subject to some marine influence, but the absence of foraminifera and ostracods from this same interval indicates that at the core site salinity was not sufficient to support populations of these organisms. Thus, prior to 6830 ± 90 yr cal BP the Younghusband Peninsula was in place, in part isolating the northern lagoon from the Southern Ocean. The initial recorded salinity event is signified by abundant Ammonia sp. at a core depth of 5.2 m. The duration of this event was relatively brief; foraminifera were mostly absent in the immediately overlying 2 m, representing ca 700 yr of sedimentation. This observation is attributed to substantial inflow of freshwater from the River Murray. In the upper 3.0 m, Ammonia sp. was present in most core samples indicating that for most of the past 6000 years the Coorong Lagoon has been sufficiently saline to support a continuing population of this species. At a core depth of 1.3 m, the sediment sample yielded >2000 tests of Ammonia sp., and they were accompanied by maximum pre-modern numbers of E. excavatum, O. baragwanathi and oogonia. Taken together, these data signify the maximum pre-modern salinity event recorded in the core sediments, probably correlating in time with regional drought conditions at ca 3500 yr BP. Elphidium gunteri is confined to the modern sediments where it is abundant and accompanied by equally large numbers of Ammonia sp., E. excavatum, O. baragwanathi and L. lacustris. These data collectively indicate water conditions that are significantly changed from those that prevailed in the Coorong Lagoon for most of the Holocene.     

New data on microbiota of the Middle Volgian substage in the Loino Section (Kirov oblast)

The calcareous nannoplankton was established for the first time in Upper Volgian deposits of the Loino Section (Kirov oblast). Here, single coccolites of representatives of genus Watznaueria of wide stratigraphic and geographic distributions dominate. Representatives of genus Zuegrhabdotus are less common, whereas species of genus Polypodorhabdus are rare. The taxonomic composition of the calcareous nannoplankton assemblage does not allow us to distinguish the nannoplankton zone in the Loino Section, but it gives us an idea about its distribution at the Volgian Stage in the Russian Plate. As corresponding microfauna, foraminifers, assigned to the Lenticulina infravolgaensis–Saracenaria pravoslavlevi Zone, and ostracods were identified to substantiate the age of host deposits. The ostracod assemblage is assigned to the Macrodentina (Polydentina) subtriangularis Beds established on the Volga River left bank in deposits corresponding to the ammonite zones Panderi and Virgatus assigned to the middle Volgian substage. On the basis of ostracods, the warm-water well-aerating environment of the upper sublittoral (as deep as 50 m) was reconstructed.     

Oxygen and carbon isotopes in Jordanian phosphorites and associated fossils

Stable isotopes have proven to be efficient tools for paleoenvironmental analysis and interpretation of paleotemperature. Oxygen and carbon isotopes were analyzed in carbonate flourapatite (francolite), oyster shells, tests of foraminifera and ostracods from the Phosphorite Unit throughout Jordan.Isotopic analysis showed δ¹⁸O to be enriched in authigenic francolite in Upper Cretaceous in NW Jordan, indicating lower temperatures, a deeper depositional environment and lower salinity than Central Jordan. In Central Jordan, the local basin of Hafira shows enrichment of δ¹⁸O indicating a deeper depositional environment than shallower highs in Mutarammil and Wadi El-Hasa. The δ¹³C shows that the depositional environment was oxic to suboxic and may have reached the suboxic to anoxic interface in the deeper environment in NW Jordan.δ¹⁸O values in tests of foraminifera and ostracods are similar to δ¹⁸O values of authigenic phosphate, which is enriched in NW Jordan, indicating lower temperature, lower salinity and a deeper environment than Central Jordan. In Central Jordan, δ¹³C shows more depletion in the Sultani section due to land derived organic carbon (food web supply) carried by terrestrial water draining to the sea.The δ¹⁸O in oyster shells show an upward enrichment in the Wadi El-Hasa section, which indicate an increase of intense upwelling, enrichment of nutrients, development of productivity and growth of oyster buildups. Meanwhile, Hafira shows enrichment of δ¹⁸O and lower temperature, in agreement with foraminifera and ostracods. The two samples of oysters from SE Jordan, although affected by diagenesis, show heavier oxygen to the north, indicating a deeper water environment and lower salinity in the same basin.     

Oxygen level,primary productivity,and water turbulence during the OAE2 interval of Zagros Basin (SW Iran): Benthic foraminiferal variations in the carbonate microfacies

Cenomanian/Turonian boundary (upper Sarvak Formation) benthic foraminiferal assemblages were analyzed to reconstruct oxygen level, primary productivity, and water turbulence in the Izeh Zone, Zagros Basin. The interplay between environmental perturbations during the Oceanic Anoxic Event 2 (OAE2) and regional tectonic activities in the Zagros Basin resulted in formation of various benthic foraminiferal assemblages in the study section. The OAE2 interval at the region of study starts with extinction of rotaliporids at the onset of δ¹³C positive excursion (peak “a”), which is associated with population of infaunal benthic foraminifera (especially Bolivina alata). The following interval at the onset of Whiteinella archaeocretacea Biozone is characterized by the total absence of benthic taxa and dominance of planoheterohelicids (“Heterohelix shift”) in the black shale strata, indicating expansion of oxygen minimum zone and unhospitable conditions for both benthic and planktic foraminifera. The upper part of OAE2 interval (including δ¹³C peaks “b” and “c”) coincides with harbinger of Neo-Tethys closure in the Arabian Plate, causing a compressional tectonic regime, and creation of uplifted terrains in the basin. The relative sea level started to locally fall in this succession, which was accompanied by a better ventilation of seafloor, lower TOC contents, and reappearance of benthic foraminifera.     

Water level and atmospheric humidity history of Lake Ichkeul (northern Tunisia) during the last 3000 years

Changes in paleoecology and climate of northern Tunisia during the last 3000 years were reconstructed based on the flood history interpreted from a 172-cm sediment core of Lake Ichkeul (NW Tunisia). Seven wet/dry episodes were identified based on biological (ostracods, foraminifera and mollusks) and biotic indices (H and E index, species richness and abundance). These proxy-based environmental changes were supported by correspondence analyses (CAs) and ecophenotypic responses of the brackish taxon Cyprideis torosa in addition to a grain-size study. Two dry episodes were identified in the lower (EP1) and upper (EP7) parts of the core. These were marked by the dominance of the brackish ostracod C. torosa. High water salinity was indicated by the presence of the brackish ostracod Loxoconcha elliptica, the foraminifer Ammonia sp. and lagoonal mollusks. The occurrence of the freshwater ostracods Ilyocypris sp., Herpetocypris sp., Dawinula stevensoni and Limnocythere inopinata was associated with high species richness and ecophenotypic changes of C. torosa valves indicating lower salinity during three major wet episodes (EP2, EP4, and EP6). The ecological and environmental changes, occurring between 3050 and 50 cal. year BP, are most likely linked to an increase of fluvial inputs which are also recognized in several other Mediterranean lakes. The humid episodes were interrupted by two saline periods (EP3 and EP5) during which freshwater ostracod assemblages declined, diversity indices dropped to the lowest values, and fine-grained sediments became dominant. The top of the core is characterized by the remarkable dominance of L. elliptica coupled with C. torosa, the absence of freshwater ostracods, and the changes in grain-size sediment parameters which are most likely the result of anthropogenic activities. The changes in hydrochemistry and sedimentology were attributed to the deepening of the Bizerte navigation canal, main wadis damming, and Tinja sluice construction which mostly occurred in the XX^th century.     

Coniacian-Santonian Planktic Stratigraphy in central Tunisia

In this study, we describe a new stratigraphy of three exposed sections in central Tunisia, integrating Coniacian and Santonian planktic foraminifera and calcareous nannoplankton, supported by ammonite and inoceramid bioevents. In the three sections, the Coniacian/Santonian (C/S) boundary lies slightly above the lowest occurrence (LO) of the calcareous nannofossil Lucianorhabdus cayeuxii, which marks nannofossil Zone CC16 and matches well with the LO of the planktic foraminifera Dicarinella asymetrica. It also lies ∼4–7 m below the LO of the inoceramid Platyceramus cycloides and the ammonite Texanites (Texanites) sp. Comparing these marker C/S bioevents with the global stratotype section, the Olazagutia section (Spain) shows that the stratigraphic range of the bioevents are variable. This observation must be taken into consideration when making regional chronostratigraphic correlations.     

Ecotopic and ontogenetic trophic variation in mojarras (Pisces: Gerreidae)

Seven species of mojarras (Gerreidae) were collected from three stations in the Indian River lagoon near the Sebastian Inlet, Florida over an eighteen month period to study spatial and temporal variations in occurrence and feeding habits. Two major habitats common to this area were represented in the study; a sandy beach and a seagrass flat. Of the 2,899 gerreids collected,Eucinostomus gula represented 67% and was found primarily in theHalodule seagrass site. The next most abundant species,E. argenteus (11%),E. jonesi andE. lefroyi (each 10%), were found most frequently at the inlet site.Diapterus auratus, E. melanopterus, andGerres cinereus were captured infrequently and were considered incidental. Prey items found included amphipods, bivalves, copepods, fish eggs, foraminifera, isopods, nematodes, ostracods, polychaetes and crustacean parts. Significant amounts of sand and unidentifiable amorphous debris were also included in the gut content analysis. Little interspecific variation in diet was found among the four major species examined. The general shift seen from a diet composed primarily of copepods and other crustaceans to one of polychaetes suggests an intraspecific ontogenetic progression. Spatial variations in diet composition were also indicated between the two major habitats. Feeding analyses showed gerreids to be diurnal feeders with the greatest consumption (total gut content weight) occurring at dusk.     

Cenomanian events in the deep western Basque Basin: the Leioa section

Analysis of the microfaunas (foraminifera, ostracods) and the stable isotope values (δ¹³C, δ¹⁸O) of the Leioa section, as representative of the deep Basque Basin, has allowed us to propose a detailed palaeoenvironmental reconstruction of this region during the Cenomanian, as well as to register global chronostratigraphic reference levels to facilitate interregional correlations. During the Cenomanian, part of the basin, the Plentzia Trough, was occupied by intermediate water masses, as deduced by the relative percentages of planktonic (Rotalipora) and benthonic foraminifera. A noticeable change is observed at the middle-late Cenomanian transition: the replacement of the dominance of keeled (rota-liporids) by incipiently-keeled (dicarinellids, praeglobotruncanids) planktonic foraminifera, indicative of the new influence of the upper intermediate waters. The temporary effect of shallow waters is deduced in one interval of the latest early Cenomanian and two more of the middle Cenomanian, as indicated by the dominance of globular planktonic foraminifera (hedbergellids). These water masses were moderately to strongly hypoxic (<4 to <2 ml/l of disolved oxygen) after the ostracod platycopid signal and benthonic foraminiferal hypoxic indicators. The dysaerobia may have been particulary strong (almost anoxia?) during part of the middle Cenomanian. Micronutrient availability was also restricted during several intervals of the middle Cenomanian, as indicated by the sudden decrease in the species diversity of the calcitic benthonics during the period when increased trends of the δ¹³C isotopic signals are observed. The combination of both hypoxia and nutrient depletion produced drastic changes in the microfaunal assemblages, with emigrations and local extinctions, showing benthonic perturbations from the time indicated by the base of theRotalipora reicheliZone onwards. From the beginning until the end of the middle Cenomanian, eleven of these perturbations are recorded as regional bioevents, using as bioevent-markers, intervals where microfauna was absent (including benthic-free intervals, B-FI; benthonic calcitic-free intervals, BC-FI; and ostracod-free intervals, O-FI). These changes led to the renewal of the microfaunas; benthonic foraminifera renewed their specific stocks during the early to early middle Cenomanian, with planktonic foraminifera and ostracods undergoing renewal at the end of the middle Cenomanian. Isotope values of δ¹⁸O and δ¹³C are consistent with the palaeoenvironmental changes detected by the microfaunas; their maximum and minimum shifts coincide with the bioevents. The double-peaked positive shift of δ¹³C for the mid-Cenomanian of northwest Europe (Jenkynset al., 1994; Paulet al., 1994a) has been recognized in this series of the Basque Basin. The palaeoenvironmental perturbations deduced in the Cenomanian of the Leioa section are attributed essentially to palaeoceanographic changes, where intermediate water masses profoundly influenced the planktonic and benthonic ecosystems. The influence of other local causes, such as volcanic activity at that time, or tectonics between the Iberian and European plates, are more difficult to prove. Several of the bioevents defined in the middle Cenomanian of this basin could probably be global in nature, and thus may be useful for establishing interregional correlations.     

Stratigraphy and paleobathymetric interpretation of the Cretaceous Karai shale formation of Uttatur Group, TamilNadu, India

The Karai shale Formation of the Uttatur Group is exposed in a bad land area at the western margin of the Cauvery Basin. This shale has been investigated based on foraminiferal fauna and clay minerals. The foraminiferal assemblages obtained contain predominantly calcareous benthic foraminifera, rare planktic and arenaceous foraminifera. The planktic foraminiferal index taxa Planomalina buxtorfi, Rotalipora reicheli, Praeglobotruncana stephani, and Hedbergella portsdownensis suggest the late Albian to middle Turonian age. The benthic assemblage dominated by Lenticulina, Gavelinella, Osangularia and Quadrimorphina, suggests an outer neritic (100–200 m) environment. The clay mineral content dominated by kaolinite-illite-montmorillonite indicates that the Karai shale was formed from weathering of igneous rocks.     

Clypeorbis? ultima n. sp. from the uppermost Maastrichtian of Austria: The youngest representative of the Clypeorbinae Sigal, 1952 (calcareous benthic foraminifera)?

A second possible species of the genus Clypeorbis Douvillé [type-species C. mammillata (Schlumberger)] is described from the topmost Maastrichtian of the Kambühel Formation of the Northern Calcareous Alps, Austria. This comparatively small-sized, asymmetric calcareous benthic foraminifer is characterized by its rather large biloculine embryonic apparatus and a reduced ventral umbo. It occurs in a mixed calcilithic–bioclastic littoral facies together with orbitoids, rotaliids, Siderolites, and other mainly calcareous benthic foraminifera as well as red algae. The stratigraphy is constrained by means of associated planktonic foraminifera, indicating the latest Maastrichtian age (CF-3 hariaensis zone). Clypeorbis? ultima n. sp. might represent the youngest representative of the Clypeorbinae Sigal that became extinct at the Cretaceous–Paleogene boundary.