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Enigmatic Brushy, Cherry & Bell Canyon Formations (Permian), Marathon Foreland Basin, USA: Poor Outcrop Analogs for Deep-Sea-Turbidite

Higgs, Roger1

1Geoclastica Ltd, Oxford, United Kingdom.

The Brushy Canyon Fm, popular as an analog for deep-sea turbidite oilfields, is reinterpreted here as lake shelf deposits, based on wave ripples, HCS, SCS, lack of in situ marine fossils and scarcity of ichnofossils. In the N, the Brushy onlaps a subaerial (karst) unconformity, reflecting uplift whereby a former marine gulf (Lr San Andres ramp atop Victorio platform, beside a slope & "basin", beside Val Verde deep-sea flysch trough) shrank to form a fresh-brackish lake (shelf & trough). Like the Black Sea and Lake Maracaibo, the lake was saltier during glacioeustatically-tied highstands (ocean-wedge entry over Hovey spillpoint) than lowstands (brimful). Most Brushy sand beds are f/vf, massive, ungraded and up to 40cm thick (amalgamated <10m), suggesting river-fed turbidity (hyperpycnal) flows too slow for traction, lasting for weeks (equatorial monsoon). Beds with HCS are storm-modified hyperpycnites. Background varved(?) carbonaceous silt is interpreted as lofting rhythmite, spread by lake circulation (wind drift). Rivers crossed the San Andres outcrop in caves: narrowings jammed logs/leaves (thus scarce in Brushy); deep fissures trapped bedload (>fs); cave walls supplied (buoyant) reworked fusulinids in Brushy sands. Rivers ended at drowned gorges (rias) and underflowed the lake, carving shallow (m) channels, each feeding a hyperpycnite lobe. Lake falls and rises made channels reincise (stack) or backfill. Silt-draped scours reflect lowstand storm erosion (waves+drift). Ria fill includes wall-derived conglomerate. Between rias, receding cliffs left a wave-cut platform onlapped by shelf silt. Eventually, the previous "basin-margin" slope disappeared (onlapped by Brushy). Younger lake-shelf facies (Cherry-Bell) interfinger landward with Goat-Capitan carbonates reciprocally (arid highstand marine ramp; humid lowstand lake shelf clastics). "Forereef" clinoforms are artifacts of Cherry-Bell preferential compaction (c. 50%; accommodating Castile shallow-lake evaporites). Caves and gorges that fed the Cherry-Bell contain Capitan "breccia". The Brushy-Cherry-Bell (BCB) are poor analogs for deep-sea turbidites, whose dissimilar processes (surge-type flows; no waves) produce fan lobes differing from lake-shelf lobes in area, heterogeneity and grain size, and channels that are leveed and sinuous. The lake model is crucial for BCB exploration and development, e.g. sustained slow flows are prone to Coriolis veering (left or right, depending on paleoequator position).

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