Depositional and Stratigraphic Architecture of a Sand-Rich Turbidite System: The Upper Carboniferous Ross Sandstone, Western Ireland

Elliott, T., University of Liverpool, England

Abstract

The Ross Sandstone turbidite system is a 380 m thick, sand-rich system deposited in an Upper Carboniferous intracratonic basin during ca. 500,000 years. The turbidites are fine-grained and thin-bedded (30 cm maximum), though amalalgamation into thicker un its is common. Depositional elements include: low net:gross sheet-like turbidites deposited at the distal fringe of the system; high net:gross sheet-like turbidites deposited as lobes and channel wings; low and high sinuousity turbidite channels; megaflute erosion surfaces and mud-filled gullies; and fine-grained slumps and debris flows. The stratigraphic architecture of the system is dominated by the effects of glacially forced sea level fluctuations that created a series of condensed sections that sub-d ivide the system into sequences several tens of metres in average thickness and of 50-65k years duration. Condensed sections are represented by organic-rich mudstones containing faunal peak horizons. Sequence boundaries are identified just above condensed sections by an abrupt return of sand-rich turbidite facies or slump/debris flow complexes. In overall terms the system has a net aggradational to progradational trend and is overlain by a slope system. Lower sequences are dominated by sheet-like, unconfi ned turbidites whereas sequences in the middle to upper part of the system are dominated by turbidite channels. Trends in sequences record fluctuations in sediment supply forced by variations in sea level.