Palaeoecology of syn-rift topography: A Late Jurassic footwall island on the Josephine Ridge, Central Graben, North Sea

• Integrated palynological, geophysical, petrophysical and sedimentological study of the Upper Jurassic section of the Josephine Ridge area of the Central Graben, UK Continental Shelf, which investigated palaeoecologies of the Jade and Judy horsts, the tops of which are not preserved.
• Samples from mudstones adjacent to the Jade Horst are rich in dinoflagellate cysts; samples adjacent to the Judy Horst show a rich assemblage of terrestrial miospores, which decline in abundance moving away from the high.
• Ecological investigations reveal the Judy Horst was subaerially exposed during the Late Jurassic, forming a low lying footwall island.
• This provides a new methodology for investigating tectonically induced topography, where palaeo-highs are not preserved.
• Implications for sediment source and distribution in syn-rift basins are presented.

Understanding rift topography is essential for determining source areas, sediment pathways, and the type of sediment delivered to a rift basin; factors essential for interpreting petroleum systems in ancient rifts. Here we investigate Upper Jurassic sediments from the Josephine Ridge region of the Central Graben, North Sea, by integrating geophysical, palynological, petrophysical and sedimentological datasets to analyse the palaeoenvironments of the Jade and Judy horsts, the tops of which are not preserved. Interpretation of geophysical and petrophysical data together with core descriptions shows study wells to step progressively away from the Josephine Ridge into adjacent grabens. One hundred and five palynological samples from six wells range from the Oxfordian to the Lower Tithonian, spanning the syn-rift period of the Central Graben. Samples from the adjacent grabens and the Jade Horst are rich in dinoflagellate cysts and possess < 20% terrestrial palynomorphs. Samples from the Judy Horst contain a wide range of terrestrial palynomorphs, dominated by lycopsid, fern and moss spores, representing c.50% of the recovered palynomorphs. Correspondence analysis of the assemblages implies that Jade did not possess a terrestrial ecosystem; Judy samples define seven groupings of related miospores, all interpreted to represent very low lying, relatively early successional type environments. This implies subaerial exposure of the Judy Horst during the Late Jurassic, which is interpreted to have formed an isolated, low relief, footwall crest island. This study provides a new methodology for investigating rift topography, particularly in cases where the tops of horsts were subsequently removed by erosion. The Judy Island would have separated the Central Graben into its eastern and western arms earlier than previously predicted, in the Late Oxfordian, with consequences for distribution of shallow and deep-marine reservoir quality sediments.