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Famous "Fine-Grained Turbidite" Outcrops (Ireland, South Africa, USA): Reinterpreted as Lake-Shelf Hyperpycnites, Unsuitable as Deep-Sea Reservoir Analogs

Higgs, Roger1

1Geoclastica Ltd, Oxford, United Kingdom.

Four lookalike formations (Ross, Skoorsteenberg, Laingsburg, Brushy Canyon) are popular as analogs for Cenozoic deep-sea-turbidite reservoirs on passive margins (e.g. Brazil, Africa, GoM), despite their non-passive setting and historically controversial depo-environment. A 5th lookalike is the Bude Fm (UK), disputed longer. These 5 "fine-grained" or "Bude-type" turbidite outcrops share 10 traits: (1) Carbo-Permian; (2) intra-Pangea foreland basins; (3) adjacent deep-water trough (Variscan, Val Verde, inferred Swartberg); (4) background mud/siltstone, enclosing proximal channels (<1km wide) and distal lobes (km wide), both comprising mostly vf/f, thin (<40cm), amalgamated (<10m), massive, ungraded sand beds; (5) interspersed vf/f beds (<40cm) with wave evidence (HCS, SCS, wave ripples, combined-flow tool marks); (6) indigenous marine fossils, if any, confined to a few thin (cm-dm) shale bands; (7) Mermia-like ichnofacies; (8) common, undulating, mud-draped scours; (9) paleocurrent evidence for flow deviation; (10) rare pseudo-slumps (gradational bases). The interpreted environment was a storm-influenced shelf (<150m?) in "sea-level lakes" (cf. Black Sea shelf). Pseudo-slumps confirm low gradient (no travel) and may reflect storm-wave loading or earthquakes. Lake level and salinity were tied to glacioeustasy: highstands were more brackish (ocean-wedge entry along spillway) than lowstands (lake brimful). Sand fineness, massiveness and non-grading suggest hyperpycnal flows that were steady, depletive, sustained (weeks; rainy/meltwater season) and too slow for traction (<25cm/sec). At lowstand, inner-shelf channels fed mid-shelf hyperpycnite lobes. Lake rise or fall caused channel backfilling (distal burial by inner lobe) or reincision (channel stacking), respectively. Flow slowness and longevity favored Coriolis veering (curved lobes?). Channel sidelap indicates thin (m) flows. Laminated carbonaceous silt represents loftite spread by wind drift. Beds with HCS etc are storm-modified hyperpycnites. Mud-draped scours reflect lowstand storm erosion (waves + wind drift + hyperpycnal flow), preventing shelf emergence (eroded surplus accommodated in adjacent trough). Thus, these "fine-grained turbidites" are poor analogs for deep-sea turbidites, whose dissimilar processes (surge-type flows faster & briefer; no waves) produce fan lobes differing from lake-shelf lobes in volume, curvature, heterogeneity and grain size, and channels that are leveed and more sinuous.

AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil