Sedimentological controls on palynomorph preservation, Triassic red-bed facies, UK Central North Sea and West Midlands
Development of Middle Triassic red-bed plays is commonly hampered by a
lack of understanding of the stratigraphic relationships between reservoir
sandstones. This inadequacy reflects poor palynological recoveries and a general
deficiency in understanding the controls on palynological preservation. The
sedimentology and palynology of Triassic red-bed facies, from the UK North Sea
and onshore analogues, are studied to determine the sedimentary controls on
palynological preservation and to investigate whether palynology is useful in these
facies, where other stratigraphic techniques do not always provide unique
The Skagerrak Formation (Quadrants 22, 29 and 30) typically comprises
ephemeral channel and sheet-flood deposits in the north, but includes sediments
deposited in perennially wet, alluvial plain and lacustrine settings in the south.
Further north (Quadrant 210), the Cormorant Formation comprises dry alluvial
deposits. Onshore, the Bromsgrove Sandstone Formation is characterised by
ephemeral channel deposits that pass progressively upwards into
tidally-influenced, fluvial and estuarine deposits; these are partly comparable with
sediments in the Tarporley Siltstone Formation.
Palynological analysis reveals that, in the absence of palynomorphs,
palynodebris and absolute organic concentration can distinguish between
preservational regimes, and thus environment. Palynological preservation
demonstrates a correlation with facies deposited in perennially wet, alluvial plain,
lacustrine and tidally influenced settings. Organic assemblages distinguish
between members in the Bromsgrove Sandstone Formation, and can subdivide
members on palaeoenvironmental criteria, which is of local value in correlation.
Palynological assemblages are mostly lacking where ephemeral depositional
processes were dominant. The assemblages demonstrate a close relationship
with sedimentary facies, their associations, and sediment colour, but the oxidation
potential of pore fluids, during and soon after deposition, is an overriding control
on organic preservation. These relationships are all beneficial for targeting
sediments for further palynological analysis.