Roger Higgs1
(1) Geoclastica Ltd Sedimentology Consulting, Oxford, England
ABSTRACT: Temporal Swing in Carboniferous Lacustrine Turbidite Paleocurrents, Crackington-Bude and Ross-Gull Island Formations, England and Ireland: Record of Continental Drift across Equator (Coriolis Switch)?
The upper Crackington and Bude Formations are fine grained, weakly channeled lacustrine turbidites, of disputed shelf versus fan bathymetry, punctuated by marine bands and 'slumps' (probably seismites essentially in situ). Sole marks show a reversal of mean paleocurrent direction over time, toward the NE, then SE, then SW. A possible explanation is a switch in Coriolis force, consistent with paleocontinental reconstructions which show SW England drifting north across the equator at that time (Westphalian; e.g. 5 cm/yr drift over assumed 10 my Westphalian A-B timespan would total 500 km, i.e. 5 deg latitude). Thus, Crackington-Bude turbidity currents, derived from a hypothetical SE-facing basin margin, would have been deflected initially leftward by Coriolis force (southern hemisphere), producing shore-parallel geostrophic currents flowing NE. Later, at the equator, where deflection is zero, currents flowed 'downhill' (SE). Finally, rightward deflection produced shore-parallel currents flowing SW.
Comparisons with geostrophic storm flows on modern and ancient shelves are appealing, especially as the Crackington-Bude contain HCS (rare) and abundant near-symmetrical ripples. Furthermore, shelf turbidity currents would deflect readily because they are slow (dm/sec) and sustained (days), fed by river floods or storm-wind-driven downwelling, unlike slump-generated turbidity currents on a slope or fan (m/sec+; hours). Flow-constraining levees must have been essentially absent.
Paleocurrents in near-identical facies (Ross-Gull Island Formations, Namurian) of western Ireland similarly swing 90 degrees, from NE to SE, interpretable as a NE-oriented shelf transiting the equator slightly earlier (Ross is 2 deg N of Bude).
Both the Bude and Ross have been dubiously used as outcrop models of deep-sea-turbidite petroleum reservoirs.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado