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AAPG Bulletin, V.
Multiscale stratigraphic analysis of a structurally confined submarine fan: Carboniferous Ross Sandstone, Ireland
David R. Pyles1
1Chevron Center of Research Excellence, Department of Geology and Geological Engineering, Colorado School of Mines, Golden, Colorado; [email protected]
An integrated, multiscale analysis of the Ross Sandstone documents important evolutionary patterns of a structurally confined submarine fan. Through time, the following are evident: (1) depocenters of high-frequency stratigraphic cycles stack vertically, (2) the depositional area increased, (3) regional paleocurrent patterns define a fanlike pattern, (4) the Ross Sandstone correlates in the landward direction to a bypass surface, (5) the Ross Sandstone inherited the basin physiography from the preceding Visean carbonates, and (6) the Ross Sandstone is ponded in the Carboniferous Shannon Basin. These observations demonstrate that the submarine fan filled an actively subsiding, structurally confined basin in an aggradational pattern.
A high-resolution geologic map and a cross section of the Ross Sandstone at Loop Head Peninsula depict the location and stratigraphic architecture of every exposure of the Ross Sandstone. These data reveal that (1) lobes are the dominant architectural element (by area) at all stratigraphic positions in the Ross Sandstone; (2) although channels are the most commonly studied features in the Ross Sandstone, they occupy 7% or less of the total cross sectional area of the formation; (3) channels increase upward at the expense of lobes; (4) slumps increase upward; and (5) percent sandstone decreases upward mostly because of an increase in slumps. These trends define an increasing architectural diversity upward within the Ross Sandstone.
Observations in the Ross Sandstone coupled with regional patterns for younger formations in the shallowing-upward, basin-fill succession are used to introduce a unifying model for the stratigraphic evolution of the Namurian strata in the basin. The model proposes that each formation in the shallowing-upward succession reflects its own depositional system related to an evolving landscape, and each was sourced from a different direction.
This study quantifies vertical trends in stratigraphic features and analyzes them in a statistical manner to test the significance of external controls on local stacking patterns. The statistical analysis reveals that temporal changes in regional conditions, including sediment supply, source area, and basin shape, are statistically and empirically related to local changes in stratigraphy, including architectural element associations and percent sandstone. These relationships suggest that fans with a larger area have more architectural diversity and heterogeneity than their smaller counterparts. Regardless of fan area, lobes are the dominant architectural element. Also, source area is related to the type of architectural element present and percent sandstone. Lobes and their genetically related channels were sourced from the south, whereas slumps and heterolithic strata are mostly sourced from the west.
Physical similarities, including climate, eustasy, size and shape of the basin, thickness of stratigraphic cycles, regional stacking patterns, subsidence, percent sandstone, and architectural elements between the Ross Sandstone and the ponded phase of the northern Gulf of Mexico minibasin-fill successions, imply that the Ross Sandstone is an excellent outcrop analog for these successions.
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