Sedimentary Patterns of Microfaunas in Northern Gulf of Mexico1

Abstract

The planktonic foraminiferal fauna in the Gulf of Mexico is composed of mixed mid- and low-latitude species; the mid-latitude forms may be relict from the last glacial stage. Planktonic Foraminifera are characteristic of offshore water; they are rare on the inner continental shelf in this area; apparent depth zonation in the continental shelf sediments is due to frequency distributions. The Holocene planktonic assemblage is underlain by a mid-latitude assemblage of glacial age. The benthonic faunas of the marginal marine environments, including marsh, lagoon, and beach, are delineated and are related to the known general characteristics of the environments. Distinctive faunas associated with deltas are fluvial marine, found in the Mississippi estuary, and deltaic marine or deltaic lagoonal. The best-known patterns of Foraminifera in the open ocean are associated with depth. Faunas of the following environments can be easily recognized--turbulent zone, 0-10 fathoms (0-18 m); inner continental shelf, 0-30 fathoms (0-55 m); outer continental shelf, 30-60 fathoms (55-110 m); upper continental slope, 60-500 fathoms (110-910 m); and lower continental slope and deep sea, deeper than 500 fathoms (910 m). It is not possible to evaluate single ecologic factors causing these patterns, but they can be discussed in general terms. Range of variation in the environmental features is an important general concept. Ecologically distinctive nearshore water masses are formed by varying amounts of runoff. The continental shelf in the Gulf of Mexico is overlain by the seasonal layer; the permanent thermocline water is on the upper continental slope; and the deep water is below 500 fathoms (910 m). Marginal marine biofacies of Ostracoda in the Rockport and Mississippi Delta areas have patterns similar to the foraminiferal biofacies. Rates of deposition based upon live-total populations of Foraminifera correlate well with depth changes in the eastern Mississippi Delta. Low sedimentation rates are indicated for the lower portion of the Rockport area lagoons and high sedimentation rates for Mississippi Sound and Mobile Bay. The northeastern Gulf of Mexico continental shelf appears to be receiving little sediment; rates are high near the Mississippi Delta; and appreciable deposition is occurring only on the inner part of the northwest shelf. The amount of Holocene sedimentation on the continental slope and in the deep sea is based on thickness of the beds containing modern planktonic foraminiferal fauna. In the northwest there is more than 150 centimeters of Holocene sediment on the upper slope, with a decrease to about 100 centimeters at depths greater than 1,000 fathoms (1,829 m). In the central area few cores penetrated the modern fauna and there is more than 200 centimeters of Holocene sediment at depths less than 1,000 fathoms (1,829 m), but sediment in the deep basin is not significantly thicker than that in the west. There is relatively thin Holocene sediment on the upper continental slope in the eastern sector; thicker deposits are found on the lower slope and basin.