James B. Riding

The response of marine phytoplankton and sedimentary organic matter to the early Toarcian (Lower Jurassic) oceanic anoxic event in northern England

Abstract Early Toarcian organic-rich sediments, reflecting the Lower Jurassic oceanic anoxic event, were investigated in the Brown Moor Borehole, North Yorkshire (northern England). Integrated micropalaeontological (calcareous nannofossils and dinoflagellate cysts) and geochemical (rock-eval pyrolysis) analyses reveal a sequence of changes mainly driven by palaeoecological shifts. These changes mainly involve the composition, source and preservation rate of sedimentary organic matter as well as algal population dynamics. A sequence of successive disappearances of individual species during the early Toarcian (Dactylioceras semicelatum and Harpoceras exaratum Ammonite Subzones) affect the composition of the phytoplankton assemblages which become gradually dominated by opportunistic taxa. This range top sequence is followed by the temporary disappearance of both calcareous nannofossils and dinoflagellate cysts (the disappearance event), which are replaced by Tasmanites. An increase in type II kerogen marks this interval. These modifications were related to a gradual water stratification and to the development of a stable pycnocline. These factors controlled the development of an oxygen-minimum zone within the water column and the distribution of nutrients in the surface waters. The disappearance event is coincident with the maximum extent of the oxygen minimum zone. A repopulation event and an increase in type III kerogen mark the end of the phytoplankton crisis in the upper part of the H. exaratum Ammonite Subzone. Preadapted survivors (e.g. Biscutum dubium, Biscutum finchii and Crepidolithus crassus) mainly characterise the repopulated calcareous nannofossil assemblages, whereas a significant turnover affected the dinoflagellate cyst community. The re-established phytoplankton populations are a response to the gradual restoration of mixed marine waters, characterised by meso-eutrophic conditions at the surface and by sustained nutrient regeneration. The integrated calcareous nannofossil and dinoflagellate cyst biostratigraphy provided a reliable biochronological framework for the Brown Moor Borehole succession, allowing correlation with lower Toarcian anoxic strata in central Italy. The integrated micropalaeontological approach indicates some diachroneity of Lower Jurassic Ammonite zones in the Boreal and Tethyan realms.

Campanian–Maastrichtian (Cretaceous) stratigraphy of the James Ross Island area, Antarctica

One of the most important outcrops of uppermost Cretaceous (Campanian–Maastrichtian) sedimentary rocks in the southern high latitudes occurs within the James Ross Island group, northeastern Antarctic Peninsula. These rocks comprise a 1500–2000 m thick sequence of predominantly shallow marine clastic sediments that were deposited within a retro-arc basin. They are virtually undeformed and have yielded prolific invertebrate and vertebrate faunas, and a wide range of plant taxa. Campanian–Maastrichtian strata are contained within two component formations of the Upper Cretaceous-lower Tertiary Marambio Group. The lower Santa Marta Formation is approximately 1100m thick and has three constituent members. Ammonites within the Santa Marta Formation indicate an early to late Campanian age assignment. Dinoflagellates suggest that the lower levels of the formation may range into the Santonian Stage. In the northern James Ross Island to Vega Island region there is a conformable transition into the overlying López de Bertodano Formation. Here the basal Cape Lamb Member is unconformably overlain by the Sandwich Bluff Member. Although there are some lithological and fauna1 discrepancies, which are attributed to marked lateral facies changes across the basin, it is believed that the Cape Lamb Member can be correlated with the lower to middle regions of the 1200 m thick sequence of undifferentiated López de Bertodano Formation lithologies exposed on Seymour Island. Molluscan data indicate a late Campanian–Palaeocene age range for this upper formation. Dinoflagellates again suggest that the lowest beds may be slightly older. The correlation presented provides the basis for a formal subdivision of the Campanian and Maastrichtian stages in the southern high latitudes. It has also revealed two major basin shallowing events (in the late Campanian–early Maastrichtian and latest Maastrichtian, respectively) and the existence of a major fault/fault zone across southeastern James Ross Island. The improved temporal framework will aid late Cretaceous palaeoclimatic and palaeobiological studies in Antarctica.

The trophic structure of the biota of the Peterborough Member, Oxford Clay Formation (Jurassic), UK

The Peterborough Member of the Oxford Clay Formation is organic-rich and contains an abundance of well-preserved vertebrate and invertebrate fossils. A high nutrient input supported a diverse biota. Phytoplankton was exceptionally abundant in the surface water, and formed the basis for an intricate food web in both surface and bottom waters. Top predators include some of the largest known Mesozoic marine reptiles. A giant teleost fish was analogous to modern filter feeding whales and sharks. Benthic faunas depended on organic matter sinking from surface waters, and two parallel food webs may have existed. Trophic partitioning allowed the higher level predators to become diverse, especially the plesiosaurs. Productivity was high in the surface waters, probably high in the mid-water column, and high on the sea floor at times, although benthic diversity may have been reduced due to substrate consistency and/or dysoxia. Bacterial activity within the sediment was also intense. The contribution of organic material to the sea floor was high, but heterotrophic reworking probably reduced the abundance considerably during early diagenesis. A decrease in organic carbon content in the Upper Callovian and Lower Oxfordian parts of the Oxford Clay Formation parallels a decreasing abundance of vertebrate fossils.

Dinoflagellate cyst stratigraphy of the Kimmeridge Clay (Upper Jurassic) from the Dorset coast, Southern England

Abstract A palynological investigation of the Upper Jurassic Kimmeridge Clay in its type area (Dorset, southern England) identified twenty‐five stratigraphically useful dinoflagellate cyst datums (range tops and bases). These have enabled the Kimmeridgian dinoflagellate cyst zonation of Woollam and Riding (1983) to be substantially refined. Eleven subzones are established within Woollam and Ridings Gonyaulacysta jurassica ‐ Scriniodinium crystallinum (Gj/Sc), Scriniodinium luridum (SI) and Glossodinium dimorphum ‐ Dingodinium tuberosum (Gd/Dt) zones, which span part of the Oxfordian and the entire Kimmeridgian stages. The indices of these zones, and the overlying Ctenidodinium culmulum ‐ Ctenidodinium panneum (Cc/Cp) Zone, are amended and their definitions revised. The dinoflagellate cyst floras are broadly comparable to others from northern Europe, but show certain differences with coeval Tethyan assemblages. Two new skolochorate dinoflagellate cyst species are described, Oligosphaeridium patulum sp. no...

Toarcian to Bathonian (Jurassic) palynology of the Inner Hebrides, Northwest Scotland

Abstract A palynological investigation of the Toarcian to Bathonian Raasay Ironstone Formation, Bearreraig Sandstone Formation and Great Estuarine Group of the Inner Hebrides has provided an important Scottish reference section, with many stratigraphically useful dinoflagellate cyst and miospore datums. The Raasay Ironstone Formation and the Dun Caan Shales Member of the Bearreraig Sandstone Formation dinoflagellate cyst assemblages are dominated by Nannoceratopsis with the Parvocysta suite present in lower proportions. The latter suite appears to be consistent with a species diversity gradient, increasing from England northwards to Spitsbergen. Representatives of the earliest sexiform gonyaulacacean dinoflagellate cysts have their inceptions in the early Bajocian Bearreraig Sandstone Formation at its type section; possible Carboniferous spores were also recovered from this formation. The youngest member of the Bearreraig Sandstone Formation, the Garantiana Clay Member, yielded Acanthaulax crispa and othe...

Revised Pre-Devensian glacial stratigraphy in Norfolk, England, based on mapping and till provenance

Mapping combined with till provenance studies have resulted in a re-appraisal of the pre-Devensian glacial stratigraphy of Norfolk, England. The traditional model invoked two formations, a North Sea Drift Formation (NSDF) overlain by a Lowestoft Formation, formed by co-existing icesheets originating in Scandinavia and Northern Britain respectively. The NSDF included three diamictons, the First, Second and Third Cromer tills. The Briton’s Lane Sands and Gravels were considered to overlie the Lowestoft Formation. However, our work has shown this stratigraphy to be untenable, and we propose a model of several glaciations instead of co-existing ice-sheets. In our revised stratigraphy, the oldest formation, the Happisburgh Formation (including the Happisburgh or First Cromer Till) includes massive, sandy tills derived from northern Britain. The overlying Lowestoft Formation, including the Second Cromer (Walcott) Till is confirmed as derived from the west, introducing much Jurassic material as well as Chalk. The Sheringham Cliffs Formation includes both brown sandy tills (the Third Cromer Till) and ‘marly drift’, in a variety of tectonic relationships, and derived from the north and NNW. Finally the Briton’s Lane Formation is the only formation to include Scandinavian erratics. Dating of the four formations is at varying levels of confidence, with the Lowestoft Formation most confidently confirmed as MIS 12. The Happisburgh Formation is believed to represent an earlier glaciation, and MIS 16 is proposed. The Sheringham Cliffs Formation is tentatively believed to date from MIS 10, and the Briton’s Lane Formation is assigned to MIS 6.

Late Cretaceous stratigraphy and sedimentology of Cape Lamb, Vega Island, Antarctica

Abstract An important new section for the latest Cretaceous marine sedimentary record is described from Cape Lamb, Vega Island, Antarctica. Some 480 m thick, it has been divided into three lithostratigraphic units of member status. Both micro- and macrofossil evidence indicate that members A and B are of late Campanian to early and mid-Maastrichtian age; the latter unit is unconformably overlain by the late Maastrichtian Member C. Definition of the lithostratigraphic units will aid local correlations within the James Ross Island area. Partial correlation with the major Cretaceous-Tertiary boundary reference section on Seymour Island has been achieved using both ammonite and dinoflagellate cyst taxa. It is apparent that there are major lithofacies differences between the two localities, which are thought to reflect a proximal to distal depth gradient into the depositional basin. The macrofauna has been grouped into two broad associations: a lower one based on the ammonite Gunnarites and an upper one based on Maorites . It is hoped that this sequence may form the basis of a formal kossmaticeratid ammonite zonation of the latest Cretaceous of the southern Gondwana margins. The sedimentology of members A and B reflects deposition in a shelf setting and records a transgressive-regressive pulse. Member C was deposited in shelf, shoreface and marginal marine environments, and shows evidence of both coeval arc volcanism and localized basin uplift.

Miocene glaciomarine sedimentation in the northern Antarctic Peninsula region: the stratigraphy and sedimentology of the Hobbs Glacier Formation, James Ross Island

The onshore record of Cenozoic glaciation in the Antarctic Peninsula region is limited to a number of isolated localities on Alexander Island, the South Shetland Islands and in the James Ross Island area. In the James Ross Island area, Late Cretaceous sedimentary rocks are unconformably overlain by a unit of diamictites and tuffs, which occur at the base of the James Ross Island Volcanic Group. These rocks are here defined as the Hobbs Glacier Formation, and on the basis of palynological studies are assigned to a Miocene (?late Miocene) age. The diamictites are interpreted as representing glaciomarine sedimentation close to the grounding line of either a floating ice shelf or a grounded tidewater glacier in a marine basin. Provenance studies indicate that the glacier was flowing from the Antarctic Peninsula towards the southeast. Volcanic tuffs conformably overlie the diamictites and are interpreted as representing deposition in a periglacial delta front setting in either a marine or non-marine basin, away from direct glacial influence. The Hobbs Glacier Formation and overlying James Ross Island Volcanic Group help to enhance our understanding of the Neogene glacial chronology of West Antarctica.

Upper Oligocene to Lowermost Miocene Strata of King George Island, South Shetland Islands, Antarctica: Stratigraphy, Facies Analysis, and Implications for the Glacial History of the Antarctic Peninsula

The Cape Melville Formation (CMF), exposed on southeastern King George Island, South Shetland Islands, provides rare evidence of extensive earliest Miocene glaciation in the Antarctic Peninsula region. The formation records the presence of regional marine-based grounded ice on the continental shelf. It overlies disconformably the upper Oligocene Destruction Bay Formation, which consists of sandstones recording nonglacial shallow marine conditions. Four units have been identified within the approximately 150 m thickness of the CMF. The basal unit (A) consists of coarse glacigenic debris-flow facies interbedded with glaciomarine mudstone and sandstone. The overlying unit (B) is mainly fine-grained. This succession may represent relatively ice-proximal deposition followed by glacial retreat and/or relative sea-level rise. The upper CMF (units C and D) was deposited in an ice-distal marine environment, with intermittent input of coarse glacigenic debris, mainly from ice rafting. Thin beds of pelagic carbonate ooze within unit C indicate periods of low terrigenous sediment input and high productivity. Lithologically diverse glacigenic gravel clasts (mainly ice-rafted debris) in the CMF had a wide regional source area, suggesting that ice cover was widespread regionally and included calving ice margins. For a small proportion of clasts the nearest known source is the mountains fringing the southern Weddell Sea. Such clasts were presumably transported north in debris-laden icebergs by a strong, cold Weddell Sea surface current. A temperate glacial setting is tentatively inferred from the CMF. Palynological results confirm and enhance the paleoenvironmental interpretation from the sedimentology, and include the first early Miocene dinoflagellate cyst assemblages recorded on the Antarctic Peninsula. This reappraisal of the glacial record from the CMF provides valuable constraints on the Antarctic cryosphere and regional paleoenvironments in the mid-Cenozoic.

Dinoflagellate cysts from the type area of the bathonian stage (middle jurassic; Southwest England)

Abstract The Bathonian sequence of the type area, around Bath, southwest England, yields diverse and well-preserved organic-walled microplankton floras dominated by ctenidodinioid dinoflagellate cysts, principally Ctenidodinium combazii Dupin 1968, and Dichadogonyaulax sellwoodii Sarjeant 1975 together with lesser numbers of Korystocysta spp. The stratigraphical distribution of dinoflagellate cysts within the Bathonian indicates few biostratigraphical events likely to be of correlative value. Comparison with other areas suggests that the geographical distribution of the dominant species reflects the supposed salinity levels of Bathonian surface waters, C. combazii being less tolerant of reduced salinity than D. sellwoodii or Korystocysta spp.