On the stratigraphic evolution of a structurally confined submarine fan, Carboniferous Ross Sandstone, western Ireland

The Carboniferous Ross Sandstone of western Ireland is an ancient submarine fan that filled an actively subsiding, structurally confined basin. Basin-scale stacking patterns in the Ross Sandstone are aggradational and sedimentation was focused in the basin-center position. Locally at the center of the basin, the Ross Sandstone contains systematic upward changes including: increasing paleocurrent diversity, increasing channel-form bodies, increasing chaotic-contorted bodies, and decreasing sandstone. These systematic upward changes were caused by temporal changes in basin-scale (boundary) conditions including: increasing sediment supply, increasing depositional area, increasing frequency of earthquakes, increasing steepness of the proximal slope, decreasing accommodation, and increasing contribution from a western provenance.; Medial submarine-fan strata in the Ross Sandstone contain channel-form bodies with highly erosional, master surface margins; channel-form bodies with weakly erosional, serrate margins; and lobe-form bodies juxtaposed to one another. Previous conceptual submarine fan models place these different architectural elements in distinctly different parts of the submarine fan. The juxtaposition of these apparently unrelated architectural elements in medial submarine-fan strata is explained by the three-level hierarchical stacking if submarine channels and their basinward depositional lobes.; There is a relationship among basin-margin morphology, basin-scale stacking patterns, and local attributes of submarine-fan strata. This relationship was established by comparing and contrasting two ancient submarine-fan systems that represent end members in the basin-margin morphology continuum. The aggradational basin-scale stacking patterns in the Carboniferous Ross Sandstone are related to an out-of-grade basin-margin morphology, whereas the progradational basin-scale stacking patterns of the Cretaceous Lewis Shale are related to a graded basin-margin morphology. Out-of-grade basin margins contain canyons on the proximal margin, submarine-fan strata are located at the depocenter of each fourth order stratigraphic cycle, depositional length of the submarine fan for each stratigraphic cycle increases upwards, and the submarine fan contains a relatively high proportion of chaotic-contorted bodies and lobe-form bodies. Graded basin margins do not contain submarine canyons, submarine-fan strata are located basinward of the depocenter in fourth-order stratigraphic cycles, depositional length of submarine-fan strata in fourth-order stratigraphic cycles remains uniform, and the submarine fan contains a low proportion of chaotic-contorted bodies and lobe-form bodies.