Character and Distribution of Hybrid Event Beds in the Ross Formation, Western Ireland — New Constraints from Behind-Outcrop Coring

Distributitive deepwater systems can comprise stacked hybrid event beds in addition to conventional turbidites. Hybrid event beds record turbulence dampening in gravity currents close to their runout limit, particularly where the flows are strongly depletive, longitudinally fractionated and where they have acquired clay via erosion up-dip. Hybrid flows deposit both clean sand from the remaining turbulent part of the flow, and clay-prone sand reflecting the onset of transitional behavior in the rear and margins of the flow. Consequently reservoir and non- or marginal-reservoir sandstones (including linked debrites) are interleaved at bed scale.

The Carboniferous Ross Formation (c. 500 m thick) together with the underlying Clare Shale represents the early Deepwater part of the progradational fill to the Clare Basin. Much of the Ross formed as stacked distributitive systems dominated by sheet elements with subordinate channels, with phases of fan building interrupted by high-frequency condensed sections. A program of behind-outcrop coring targeting the Ross Formation is currently underway. Drilling to date has acquired >670 m of split core and associated wireline logs from both the upper and lower Ross and importantly spanning the unexposed onset of the system on Loop Head at two sites. The split cores are important as they reveal details of the finer grained bed tops that can be difficult to inspect in weathered and cleaved surface exposures. Hybrid event beds are identified as important bed types at many levels within the Ross including (1) distal mud-prone event beds (fluid muds?) which dominate the upper part of the Clare Shale before the arrival of significant sand into the basin (2) outsized (m-scale) and relatively coarse-grained beds comprised dewatered and banded sandstones with argillaceous caps recording the first arrival of sand - the ‘Cosheen beds’ seen in the Ballybunion outcrops and (3) bundles of thinner hybrid beds in thickening-upward cycles locally associated with channels or resting on bypass surfaces. Like subsurface systems elsewhere, hybrid bed-dominated packages alternate with those composed of conventional turbidites. Hybrid beds in the outer Ross record rare larger scale and longer run-out events that erosionally modified the inner fan together with the distal runout facies of normal scale events (both turbulent and with suppressed turbulence). Thickening-upward cycles may record the down-fan extension of distributitive channels.
 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California