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Accretion and Origin of Organic Horizons in Mississippi Delta

Elisabeth C. Kosters, Gail L. Chmura, Randolph A. McBride

Barataria basin is a large-scale interdistributary basin of the Mississippi delta, measuring about 150 km from its apex to the Gulf of Mexico. The basin developed about 2,500 years ago as a result of the interplay of different deltaic distributary systems. Our data base consists of about one hundred 7.5-cm-diameter, 3 to 10-m deep vibracores, and about fifty 15-cm-diameter, 50-cm-deep hand-held cores. Lithology, moisture/ash data, x-ray radiographs, pollen counts, and carbon isotope information are some of the parameters used to aid in the interpretation of the sedimentary and ecological origin of the different organic horizons.

Organic facies are (incipient) organic-poor marsh (5-35% organic matter by dry weight), organic-rich marsh (35-75% organic matter), and true peat (75% organic matter). Organic facies occur as more or less distinct horizons; contacts between organic and detrital clastic strata are generally sharp. Each horizon of a certain quality range is thought to be the result of somewhat similar original conditions, such as botanical parent material, salinity regime, detrital clastic influx, and subsidence rates.

True peats are probably all of a freshwater swamp or marsh origin, and most peats and organic-rich horizons are thought to have accumulated in situ in swamps and marshes, although lack of rooting on x-ray radiographs indicates that some of the peat may have accumulated semi-autochthonously beneath floating marshes. Both above and below ground biomass contribute to subsurface organic horizons, but their relative contributions need to be established. Carbon isotope data help establish the botanical parent material of organic horizons. Pollen spectra dominated by fern taxa indicate that greatest carbon accumulation may occur during initial stages of fresh marsh development.

Intercalating clays, containing large amounts of detrital organic remnants, may represent shallow subaqueous areas close to an eroding marshline, where eroded marsh material becomes incorporated into the sediments after the passage of storms or fronts.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.