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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.
Highlights
Fluvio-deltaic sections of the Middle Jurassic, Ravenscar Group have been the subject of numerous studies since the turn of the twentieth century, with the publication of papers and monographs focused on its well-preserved plant fossils, including those of A.C. Seward (1900), T.G. Halle (1913), H
New data obtained from two field sections at Yons Nab and Cloughton Wyke provide an integrated micro-biofacies model which is applied to a cored section of similar age through part of the Broom and Ness formations in the Gullfaks Field, Viking Graben, northern North Sea
The oldest samples are from the Yons Nab Beds, a basal quasi-marine part of the Gristhorpe Member considered by some authors to represent a ‘regressive phase’ of the Millepore Bed (Bate 1959; Hemingway 1974)
Summary
Journal of Micropalaeontology doi:10.1144/jmpaleo2015-017 | Vol 35 | 2016 | pp. 151–172. Megaspores and associated palynofloras of Middle Jurassic fluviodeltaic sequences in North Yorkshire and the northern North Sea: a biofacies-based approach to palaeoenvironmental analysis and modelling
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