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The AAPG/Datapages Combined Publications Database

GCAGS Transactions

Abstract


Gulf Coast Association of Geological Societies Transactions
Vol. 36 (1986), Pages 541-551

Distal Shelf and Upper Slope Sediments Deposited During Rising Sea Level, North-Central Gulf of Mexico

Harry H. Roberts, (1) Indra B. Singh, (2) James M. Coleman (1)

ABSTRACT

Sediments of Louisiana's outer continental shelf and upper slope are generally fine grained but highly variable with regard to internal structures and inclusions. They are spread over diverse shelf-to-slope topography resulting from rapid localized sedimentation, mass movement, salt tectonics, and sediment redistribution by marine agents. This variability has made development of concepts of sedimentation and typical depositional sequences difficult.

Analyses of numerous recent sediment cores in conjunction with high-resolution seismic have provided a data base for describing sediments of the distal shelf/upper slope deposited during the last major rise in sea level. Major contrasts in sediment properties occur in two basic depositional settings: (1) opposite a prograding delta lobe and (2) beyond the reach of abundant sediment input from the Mississippi River.

Sediments of the distal shelf/upper slope seaward of the active Balize delta lobe are comprised of wedges of mudflow deposits separated by thin, finely laminated units (commonly < 2 m thick) of more slowly deposited sediments containing calcareous microfauna tests, bioturbation features, and early diagenetic products. Mudflow sediments are generally remolded, commonly contain gas-related features, flow structures, and soft-sediment deformation plus inclined bedding. Bioturbation features and diagenetic products are generally missing, and calcareous microfauna tests are rare. Mudflows are acoustically amorphous, while thin interlobe deposits, which separate successive mudflow lobes, are usually strong nonparallel reflectors. Mudflow units thin downslope, so that thin-graded beds of terrigenous silts to clays alternate with thin hemipelagic units, creating tightly spaced parallel to subparallel high-resolution seismic reflectors.

Areas of low terrigenous sediment input are composed of hemipelagic deposits containing abundant calcareous microfauna tests, bioturbation features, macroscopic shell debris, and diagenetic products. Unlike their rapidly deposited counterparts, these sediments are devoid of gas-related features and contain much more evidence of resedimentation resulting from resuspension/deposition by marine processes. Graded beds commonly contain abundant redeposited calcareous components, and debris flow deposits are composed of deep- rather than shallow-water sediments. Closely spaced parallel reflectors constitute the typical seismic signature. However, recent salt tectonics has imposed complicated structure on the area.

Delta switching and compaction/subsidence cause a single rising sea-level cycle to contain stacked sequences of these two major lithofacies associations. On the distal shelf, composite mudflow deposits are commonly one to two orders of magnitude thicker than their time-equivalent hemipelagic counterparts. These sedimentary cycles thin downslope, but remain distinguishable.


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