Jenö Nagy

Environmental significance of foraminiferal morphogroups in Jurassic North sea deltas

Abstract The distribution of morphological groups of foraminifera has been analysed in Toarcian and Bajocian deposits of the northern North Sea and the Yorkshire coast. The deposits are referred to prodeltaic, lower delta front and interdistributary bay environments, and contain mainly or entirely agglutinating assemblages of low to intermediate diversity. A total of 33 agglutinating species are recognized in the studied material, and these are arranged into eleven morphotypes composing seven subgroups and four morphogroups. The categories are distinguished on the basis of main morphological features of the taxa, combined with their life positions and micro habitats as inferred from modern analogues. The prodelta deposits were formed in a normal saline and slightly stagnant depositional setting with foraminiferal faunas composed mainly of surficial and infaunal morphogroups. The lower delta front and interdistributary bay sediments were deposited in shallow hyposaline waters strongly dominated by an epifaunal morphogroup with a partly phytal association. A model of agglutinated morphotype distribution reveals increasing morphotype diversity (and by inference, ecological niche span) from interdistributary bay, through lower delta front, to prodeltaic environments.

Temporal changes in foraminiferal faunas and impact of pollution in Sandebukta, Oslo Fjord

Abstract Sandebukta (Oslo Fjord, Norway) is an inlet with depths up to 70 m, characterised by estuarine water stratification, an influx of pollutants and a mudflat in the inner part. The distribution of its foraminiferal fauna displays essentially an estuarine pattern, but also reveals significant subrecent changes as demonstrated by means of 9 sediment cores supplemented by a larger number of sea bed surface samples. The faunal changes have taken place during the last 100 years and are summarized as follows: (1) Upwards increase in size of the total population shown by the sediment cores. (2) Reduction in the relative frequency of calcareous foraminifera upwards in the cores, so that the recent fauna of the inlet consists mainly of arenaceous species. (3) Alterations in the species composition of populations, most pronounced in deeper waters. Faunal comparisons with outer parts of the fjord system suggest that these changes are, at least partially, induced by pollution originating from human activity. In addition, post mortem destruction of foraminiferal tests is considered as a possibly important modifying factor requiring further study. An important additional reason for the faunal turnover found in the mudflat seems to be successive shallowing during progradation.

Foraminiferal Assemblages as Palaeoenvironmental Bioindicators in Late Jurassic Epicontinental Platforms: Relation with Trophic Conditions

Foraminiferal assemblages from the neritic environment reveal the palaeoecological impact of nutrient types in relation to shore distance and sedimentary setting. Comparatively proximal siliciclastic settings from the Boreal Domain (Brora section, Eastern Scotland) were dominated by inner-shelf primary production in the water column or in sea bottom, while in relatively seawards mixed carbonate-siliciclastic settings from the Western Tethys (Prebetic, Southern Spain), nutrients mainly derived from the inner-shelf source. In both settings, benthic foraminiferal assemblages increased in diversity and proportion of epifauna from eutrophic to oligotrophic conditions. The proximal setting example (Brora Brick Clay Mb.) corresponds to Callovian offshore shelf deposits with a high primary productivity, bottom accumulation of organic matter, and a reduced sedimentation rate for siliciclastics. Eutrophic conditions favoured some infaunal foraminifera. Lately, inner shelf to shoreface transition areas (Fascally Siltstone Mb.), show higher sedimentation rates and turbidity, reducing euphotic-zone range depths and primary production, and then deposits with a lower organic matter content (high-mesotrophic conditions). This determined less agglutinated infaunal foraminifera content and increasing calcitic and aragonitic epifauna, and calcitic opportunists (i.e., Lenticulina). The comparatively distal setting of the Oxfordian example (Prebetic) corresponds to: (i) outer-shelf areas with lower nutrient input (relative oligotrophy) and organic matter accumulation on comparatively firmer substrates (lumpy lithofacies group) showing dominance of calcitic epifaunal foraminifera, and (ii) mid-shelf areas with a higher sedimentation rate and nutrient influx (low-mesotrophic conditions) favouring potentially deep infaunal foraminifers in comparatively unconsolidated and nutrient-rich substrates controlled by instable redox boundary (marl-limestone rhythmite lithofacies).

Foraminiferal morphogroups in dysoxic shelf deposits from the Jurassic of Spitsbergen

Analysis of benthic foraminiferal assemblages was performed in Bathonian to Kimmeridgian deposits through a section covering the lower half of the Agardhfjellet Formation in central Spitsbergen. The section consists mainly of organic-rich shales, which contain low-diversity agglutinated assemblages. In this foraminiferal succession five morphogroups were differentiated according to shell architecture (general shape, mode of coiling and number of chambers), integrated with the supposed microhabitat (epifaunal, shallow infaunal and deep infaunal) and feeding strategy (suspension-feeder, herbivore, bacterivore, etc.). The environmental evolution of the analysed section is interpreted by using the stratigraphic distribution of morphogroups, combined with species diversities and sedimentary data, in a sequence stratigraphic framework. The section comprises two depositional sequences, which demonstrate that species diversity and relative frequency of morphogroups are correlative with transgressive–regressive trends controlling depth and oxygenation of the water column. In both sequences, the maximum flooding interval is characterized by increased organic carbon content, dominance of the epifaunal morphogroups and reduced species diversity: features reflecting the increased degree of stagnation separating the transgressive phase from the regressive phase.

Revised foraminiferal taxa and biostratigraphy of Bathonian to Ryazanian deposits in Spitsbergen

The Bathonian to Ryazanian succession of Central Spitsbergen consists mainly of shales with high to intermediate or- ganic content, and preserves an essentially continuous foraminiferal record composed almost exclusively of agglutinating taxa. The as- semblages are reexamined in three extensively sampled sections, with supplementary material included from other localities in Spitsbergen and the Russian Arctic. The foraminiferal record of Spitsbergen is subdivided into eight interval zones, which are correlated with a Boreal ammonite zonal scheme using a graphic procedure. The age-thickness relationships reveal extremely low average sedimentation rates through the Upper Bathonian-Lower Kimmeridgian and Middle Volgian-Ryazanian intervals, suggesting the possible presence of minor hiatuses. The taxonomical revision of foraminifera was accomplished in the light of comparisons with sample material from Russian occur- rences. The emendation resulted in the proposal of two new genera, Agardhella and Calyptammina, and the following nine new species: Thuramminoides lapilliformis, Saccammina compacta, Scherochorella densiformis, Cribrostomoides subretusus, C. vallatus, Ammoba- culites areniferus, Agardhella placula, Trochammina praerosacea and Gaudryina rostellata.

Main features of foraminiferal distribution reflecting estuarine hydrography in Oslo Fjord

Abstract Foraminiferal assemblages have been analysed in sediment surface samples located along three profiles covering the depth interval 1–120 m in Breidangen, a wide basin forming the middle portion of Oslo Fjord. The study concentrates on the distribution of major faunal features, including suborders and diversities (number of species, Shannon-Wiener index, equitability). In addition, species distribution is recorded and Q-mode factor analysis has distinguished six assemblages. Measurements of hydrographical properties (temperature, salinity, dissolved oxygen) reveal the estuarine stratification of the water column, which is reflected also by the faunal parameters. On the basis of these features, the following subdivision is proposed (with increasing depth): (1) Seasonal layer dominated by calcareous species in the upper, and agglutinated species in the lower part, with low to intermediate diversities; (2) Transitional layer characterized by a maximum in diversity and a reduction in the agglutinated component; (3) Stable deeper water showing high diversity and a further decrease in the agglutinated component. The generally higher frequency of calcareous foraminifera below the seasonal water layer on the eastern side of Breidangen compared to the western side is mainly attributed to the influence of inflowing Skagerrak water. Additionally, it may be a result of increased supply of organic matter to the western side, increasing the oxygen demand in this area.

Micropalaeontological evidence of brackish water conditions during deposition of the Knorringfjellet Formation, Late Triassic-Early Jurassic, Spitsbergen

The Knorringfjellet Formation is a succession of mudstones and sandstones, ranging in age from Norian to Toarcian in western and west-central Spitsbergen, and contains several unconformities with associated hiatuses. Its foraminiferal succession consists almost exclusively of agglutinated taxa, shows extremely low assemblage diversities and a dominance of small-sized species. These faunal features signalize restricted environmental conditions (in contrast to those of a normal marine shelf). Comparisons with ancient and modern analogues suggest that the main restricting factor was hyposaline conditions, to a lesser extent augmented by hypoxia in near-bottom waters. These conditions were caused by high fluvial influx creating gravity stratified water masses. The depositional area was part of an extensive but shallow shelf embayment, which had an open connection to the north to the polar ocean basin. Discontinuities recognized in the foraminiferal succession indicate depositional unconformities within the formation.

Facies Controlled Distribution of Foraminifera in the Jurassic North Sea Basin

Foraminiferal faunas have been analyzed from three localities of the Jurassic North Sea basin: the Statf jord area — Pliensbachian to Bajocian; the Yorkshire coast — Bajocian; north Scotland — Sinemurian, Pliensbachian and Callovian. The various assemblages are defined by distribution of suborders, genera, species and diversities. Based on these parameters and sedimentary f acies data the assemblages are attributed to various marine shelf and deltaic conditions.

Agglutinated Foraminiferal Stratigraphy of Middle Jurassic to Basal Cretaceous Shales, Central Spitsbergen

Foraminiferal distribution data have been used for stratigraphic zonation of a c. 230 m thick sedimentary series comprising the Agardhfjellet and basal Rurikfjellet formations (Callovian to Ryazanian) in central Spitsbergen. The dominant lithologies are organic-rich shales, but siltstones and silty shales with intermediate and low organic content also occur.

New foraminiferal taxa and revised biostratigraphy of Jurassic marginal marine deposits on Anholt, Denmark

Abstract On the north-eastern margin of the Danish Basin (part of the Mesozoic North Sea Basin) the Upper Pliensbachian to Toarcian succession represents the early phase of delta outbuilding, involving marine shelf, distal prodelta and proximal prodelta environments. The marine shelf conditions are characterized by calcareous foraminiferal assemblages dominated by the suborder Lagenina. The prodelta environments are typified by low-diversity agglutinated assemblages concentrated in discrete stratigraphical horizons. In total seventeen foraminiferal species were recognized, the following five of which are described here for the first time: Haplophragmoides propygmaeus, Ammobaculites bivarians, Ammobaculites nanogyrus, Kutsevella spilota, and Bulbobaculites vermiculus. The taxonomic treatment has also lead to a revised biostratigraphic scheme subdividing the foraminiferal succession into six assemblages confined to discrete stratigraphical intervals.