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The AAPG/Datapages Combined Publications Database
Paleoenvironmental studies using benthic foraminifers and total fauna can be used to identify displaced shallow-water sands. A productive sand in Eugene Island field, which has a high resistivity but suppressed spontaneous-potential, was conventionally cored to determine reservoir characteristics and environment of deposition.
Grain-size analysis shows a composition of very fine sand with a large silt and clay component. Studies of sand-size distribution throughout the 53-ft core did not reveal graded bedding, thus excluding turbidity currents as a depositional mechanism. Analysis of the benthic fauna within the sand unit indicates that the sands and thin-bedded shales were originally deposited on the inner to middle shelf. The occurrence of bathyal shale above and below the productive unit suggests that the shallow-water sands were transported basinward into a slope environment.
Regional paleobathymetric maps indicate that there was a progradation of the shelf edge during deposition of the sand unit. This evidence, along with the fine-grained character of the sands, suggests that a deltaic complex was developing updip of the field.
The depositional environment is very similar to that described by J. M. Coleman and others near the modern Mississippi River Delta. The processes that are moving shallow-water sands across the shelf, stimulating mass movement and shelf-ridge slumping, were also active around ancient deltas.
Based on the modern analog, it is interpreted that the field sand is part of a debris flow initiated by shelf-edge failure. The geometry of the sand unit also supports this hypothesis.
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