Andrew J. Pulham

Biostratigraphic Techniques for Analyzing Benthic Biofacies, Stratigraphic Condensation, and Key Surface Identification, Pliocene and Pleistocene Sediments, Northern Green Canyon and Ewing Bank (Offshore Louisiana), Northern Gulf of Mexico

The northern deep Gulf of Mexico is a geologically complex province consisting of Neogene intraslope minibasins created by sediment loading onto and evacuation of allochthonous salt. Sedimentary fill in the minibasins consists of bathyal turbidite systems with highly variable facies distribution. In this paper, we present three biostratigraphic techniques developed to increase our understanding of the geologic evolution of the northern Green Canyon and Ewing Bank lease areas. The first two techniques address the nature of stratigraphic condensation in minibasins and the relative areal extent of the condensed sections. The third technique uses benthic foraminifers to improve the stratigraphic knowledge of the area. (1) The first technique is concerned with sediment accumulation plots from wells helping to identify possible areas of condensation based on variations in the rate of accumulation. By displaying several wells together, the areal extent of the condensed section can be identified, and the causes for the condensation can be evaluated. (2) The second technique addresses condensation within one minibasin during a 2.5-m.y. interval. Relative abundance values are plotted from a series of wells at selected time surfaces and tied to seismic horizons that are correlated throughout the basin. This allows plotting a series of maps that show how stratigraphic condensation changes throughout a basin. (3) The third technique develops benthic biofacies maps that more accurately reflect the highly irregular paleobathymetry of the slope at the time of deposition. These maps were created by plotting the benthic biofacies of the sediments recovered from a well at the stratigraphic extinction of 29 selected planktonic foraminifer and calcareous nannoplankton species. The maps are integrated with lithostratigraphic and structural palinspastic reconstructions. The resulting maps are interpreted to more accurately reflect benthic biofacies of the continental slope at specific geological times during the Pliocene and Pleistocene.

Deep-water clastic systems in the upper carboniferous (upper mississippian-lower pennsylvanian) shannon basin, western Ireland

The Upper Carboniferous Shannon Basin of western Ireland contains a more-than-2300-m-thick (7540 ft) basin-fill succession, shallowing upward from deep-water to deltaic and incised fluvial deposits. The deep-water basin floor and slope succession is world renowned as an analog for hydrocarbon-bearing deep-water sandstones on several continental margins such as the basins in East and West Africa, South America, the Gulf of Mexico, and not least offshore northwest Europe. The Shannon Basin is frequently visited by both academia and industry for research and training purposes. A series of behind-outcrop research boreholes reveals the subsurface expression of the deep-water rocks and is complemented by seismic-scale cliff exposures. The succession is interpreted as a first-order basin-scale linked sedimentary system. This system can be analyzed using the principles of source-to-sink analysis and leaves the visitor with a complete picture of the basin fill, enhanced by spectacular sedimentological and stratigraphic detail.