Megaspores and associated palynofloras of Middle Jurassic fluvio-deltaic sequences in North Yorkshire and the northern North Sea: a biofacies-based approach to palaeoenvironmental analysis and modelling

Abstract

Outcrops of the Early Bajocian, Gristhorpe Member (Ravenscar Group) in North Yorkshire have been subjected to a multi-disciplinary analysis which establishes a key reference section at Yons Nab. Eight micro-biofaces types (MBs) are defined based on counts of transitional marine microfaunas, megaspores and plant-derived debris. Associated palynofloral associations (PAs) are used to qualify the micro-biofacies defined to provide an integrated model for interpreting salinity-freshwater trends associated with deltaic progradation and retreat. Megaspore and small-spore trends within the delta-plain are shown to have been closely linked to peat-mires forming in topographic lows, with variations across the delta-plain attributable to the presence of the semi-permanent and seasonal water tables. A reconstruction of the floodplain-peat-mire depositional setting is proposed where the composition of host vegetation (e.g. ferns, lycopsids) varied in relation to groundwater levels. Biostratigraphic correlation within the Gristhorpe Member is attempted using the base of 'established' delta-plain deposition (megaspore dominant MBs/ spore-dominant PAs) and the top and base agglutinated foraminifera-dominant MBs (brackish). Using these criteria the Gristhorpe Member is modelled in terms of a southward progradation of delta-plain facies into a bay-fill sequence, which persisted at Yons Nab. With the delta-plain established, the MB/PA trends and sedimentology suggest a northward shift towards the higher delta-plain, with seasonal standing water only. The MB/PA model was tested on core samples from the coeval lower Brent Group from northern North Sea well 34/10-B-12 (Gullfaks Field); MB/PA trends are shown to compare closely with those of the Gristhorpe Member at Yons Nab, with the same megaspore/small-spore associations developed within similar lithofacies. On this basis transitional marine, bay-fill and delta-plain deposition are defined in the cored section. These comparative data suggest that the dispersal of megaspore and miospore host floras occurred effectively across the Mid North Sea High, and that they colonized the same niche habitats associated with bay-fill and delta-plain development.

Lycopsid megaspores from the Upper Triassic of Svalbard and their relationship to the floras and palaeoenvironments of northern Pangaea

“…Megaspore extraction follows standard volumetric micropalaeontological techniques, which have previously been successful for the extraction of mesofossils, as well as for microfaunal components such as foraminifera and ostracods (see Morris et al . ; Morris & Batten ). Analysis involved quantitative counts of megaspores, other mesofossils and (charcoalified and non‐charcoalified) plant debris.…”

“…Analytical methods and the criteria for defining micro‐biofacies follows the approach applied by Morris & Batten () in their palaeoenvironmental evaluation of the Middle Jurassic fluvio‐deltaic deposits of North Yorkshire, UK, and the Viking Graben, North Sea. Micro‐biofacies (MB) types were assigned based on megaspore species composition, abundance and diversity, and linked to the presence of cuticles, and the abundance of charcoal and other types of plant debris.…”

Megaspores from the Lower Jurassic (Pliensbachian) Rotzo Formation (Monti Lessini, northern Italy) and their palaeoenvironmental implications

“…The distinct imperfect reticulum and the fact that the reticulum tends to disappear, leading to a more tuberculate structure, permits to assign our specimens to Horstisporites harrisii. Distribution: This species is well known from the Jurassic Yorkshire sediments (Morris and Batten 2016), where one of the most important Jurassic floras of Europe comes from (Harris 1961(Harris , 1964(Harris , 1969(Harris , 1979Harris et al 1974). The species is also well distributed in the Triassic of France, Rhaetian to Jurassic of North America, Europe and Asia and in the Cretaceous of Germany and North America (Gry 1969;Bergad 1974;Arjang 1975;Marcinkiewicz 1980;Taugourdeau-Lantz 1983, 1984Yang 1986;Kovach and Batten 1989;Batten and Kovach 1990;Feist-Burkhardt et al 2009).…”

Dinosaur-plant interactions within a Middle Jurassic ecosystem—palynology of the Burniston Bay dinosaur footprint locality, Yorkshire, UK

“…Megaspores are also relatively common (e.g. Morris and Batten 2016) and most, if not all of the megaspores described from the Yorkshire Middle Jurassic originate from lycopsids. Their relatively high diversity (summarised in in association with lycophyte microspores indicate that lycophytes were more abundant and diverse than the macrofossil record would suggest.…”