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Petrographic analyses reveal that five distinct sources have contributed terrigenous sediment to the Gulf of Mexico during the Quaternary: (1) the Apalachicola, (2) the Mississippi, (3) the central Texas rivers, (4) the Rio Grande, and (5) the rivers of northeast Mexico. Sands and silts originating from the Mississippi, Rio Grande, and northeast Mexico have been recognized and mapped from the continental shelf to the abyssal plain. Terrigenous sediments from the other input sources either were so adulterated by adjacent inputs as to be unrecognizable basinward of the shelf (Apalachicola), or were ponded on the shelf and slope by an irregular bottom topography (central Texas rivers, and part of the Mexico input north of Veracruz). In addition to these terrigenous, silicicl stic contributions, the provinces of the Campeche shelf and the Florida shelf contributed nonterrigenous (intrabasinal) carbonates to the Gulf, but only the Campeche contribution can be recognized in the abyssal plain.
The upper 10 m of sediment in the slope, rise, plain, and Mississippi cone is capped with a 20-50 cm layer of Globigerina ooze. Below this ooze the sediments are dominantly argillaceous lutite interstratified with terrigenous (siliciclastic) and nonterrigenous (carbonate) sand and silt interbeds 1 mm-150 cm thick. Other sediment varieties include locally developed conglomerates and calcilutites.
Three mechanisms transported and deposited sediment in the deep areas of the Gulf. (1) Differential pelagic settling is considered responsible for the deposition of much of the clay-size sediment. Settling has produced parallel lamination and parallel bedding which may be recognized in nonbioturbated sequences. (2) Turbidity currents are considered the most important mechanism for the transport of both terrigenous and nonterrigenous sands and silts into and across the abyssal plain. Effects of turbidity currents are best displayed in sand and silt beds thicker than 5 cm. These beds commonly display erosive bases, graded bedding, constant vertical C-D-E sequences of sedimentary structures, and displaced faunas, and are bioturbated only in their uppermost parts. Such sediments show evid nce of slope-controlled deposition. Thickest accumulations (more than 5 cm) are basinward of breaks in slope, and thinner accumulations (less than 5 cm) are typical of more distal and proximal areas. The most significant sources of abyssal turbidites were the Mississippi River and the Campeche shelf. Mississippi-derived turbidites may be traced to the western edge of the abyssal plain; Campeche-derived turbidites extend about 500 km from the base of the Campeche Escarpment to beyond the northern limits of the present abyssal plain. (3) Bottom currents have had little effect on the sands and silts of the deep Gulf. Bottom current influence appears restricted to beds less than 5 cm thick, and has produced sedimentary features including (a) concentrations of tests or heavy minerals in well orted sediments indicating winnowing, (b) micro-cross-lamination demonstrating the existence of ripple marks, (c) burrowing throughout the bed, suggesting a relatively slow rate of sediment deposition, and (d) variable vertical sequences of sedimentary structures and textures indicating a variability of current flow. Evidence indicates that the bottom currents were sporadic, low-flow regime phenomena, local, and of short duration.
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