Hierarchical Stacking of Submarine Channels and Their Depositional Lobes in Distributary Landscapes, Carboniferous Ross Sandstone, Western Ireland

David Pyles
Bureau of Economic Geology, Austin, TX

The Carboniferous Ross Sandstone of western Ireland contains some of the world's best exposures of submarine channels and lobes. A detailed study of the Ross Sandstone at Loop Head Peninsula was carried out to address the vertical stacking of these architectural elements.

Locally at Loop Head Peninsula, channel-form bodies increase upward from 3 to 7% (by cross-sectional area). Although the proportion of channel-form bodies increases upwards, lobe-form bodies remain the dominant architectural element in the Ross Sandstone. A regional cross section demonstrates that the upward increase in proportion of channel-form bodies does not reflect changes in distance from the shelf edge or changes in the physiographic position. Rather the basin-scale stacking patterns are aggradational and strata at Loop Head Peninsula always record medial basin-floor strata.

Regardless of stratigraphic position, outcrops of the Ross Sandstone at Loop Head Peninsula contain the same architectural association. That association is (1) highly erosional channel-form bodies with master erosional surfaces (2) weakly erosional channel-form bodies with serrate margins and (3) lobe-form bodies. The juxtaposition of highly erosional and depositional architectural elements is interpreted to result from a hierarchical stacking of submarine channels and their basinward lobes on an aggrading distributary landform. The hierarchy includes three levels: individual channel-lobe systems (6th order), distributary channel-lobe systems (5th order), and distributary channel-lobe complexes (4th order). The external shape of each system is the same in plan view; however, size and time span of existence increases by the hierarchical level.