ROSS AND BUDE FORMATIONS (CARBONIFEROUS, IRELAND AND ENGLAND): REINTERPRETED AS LAKE-SHELF TURBIDITES
R. Higgs*
*Geoclastica Ltd Sedimentology Consulting, 16 Norham End, Norham Road, Oxford OX2 6SG, UK
email: rogerhiggs@geoclastica.com
The Ross Formation (Namurian, Ireland) and the near-identical Bude Formation (Westphalian, England), both amply described in the literature, are used by oil companies as deep-sea-fan reservoir analogues. However, the Ross Formation is reinterpreted here, like the Bude Formation in recent publications, to be composed of river-fed turbidites deposited on the wave-influenced northern shelf of a Variscan foreland-basin lake, which also had a southern flysch trough.
Key features of these formations are: (i) two classes of thin (< 0.4m) sandstone "event bed" in shale comprising (a) structureless turbidite-like beds, and (b) rippled beds with combined-flow ripples and/or hummocky cross-stratification, neither structure having previously been reported from the Ross Formation; (ii) "trademark" tabular packets (1-10m) of amalgamated event beds, which interfinger laterally with mudstones; (iii) sharp packet bases and tops; (iv) rare sinuous channel fills; and (v) rare thick (1-10m) shale units, each containing a thin (cm-dm) fossiliferous band.
The fossil bands are interpreted here as maximum flooding surfaces, reflecting glacioeustatic marine incursions over the lake spill point (sill), forcing the lake to rise and to turn marine or strongly brackish; these bands define Galloway-type depositional sequences 50-100m thick. During eustatic falls, the lake was forced down to sill level, where it perched and turned fresh (desalination). Sandstone packets are attributed to the falling-stage and lowstand systems tracts, each packet representing a higher-order lowstand systems tract. Packets are interpreted as tongue shaped, supplied by river-fed underflows. Packet bases (sharp) represent the storm-wave -graded equilibrium shelf profile, glacioeustatically forced to its lowstand position. On this erosion surface were deposited underflow turbidites produced by floods in the catchment. Occasional catastrophic storms on the lake shaved these turbidites and interfingering fair-weather muds back down to the equilibrium level, leaving behind a subsidence-accommodated increment whose surface was sculpted by storm wind- and wave currents, forming hummocks, combined-flow ripples and erosional megaflutes. Whenever a river-fed underflow accompanied one of these storms, the resulting highly erosive combined flow carved a sinuous channel on the wave-sculpted equilibrium surface. Sandstone-shale intervals separating the sandstone packets are interpreted as transgressive- and highstand systems tracts. They contain both turbidites and wave-modified turbidites (rippled beds), deposited on the out-of-equilibrium drowned shelf.
A gradual rotation in sole-mark direction with time in both formations is attributed to a reversal of Coriolis deflection as the plate drifted north across the equator, causing underflows to be deflected along-shelf geostrophically to flow first NEwards and then SWwards on an inferred SE-facing shelf.
The lack of evidence for emergence of the Ross and Bude Formations, in spite of the great thicknesses (460m and 1,290m, respectively) of these shallow-water deposits, is attributed to regulation of minimum water depth firstly by the lake sill blocking eustatically-forced exposure, and secondly by storm grading, preventing emergence by sedimentation.