Foraminifera and arcellaceans ("thecamoebians") were examined from 73 surface samples collected to represent four vegetation zones (I-IV) that have been defined in the Mississippi Delta Plain. Previous studies of benthonic foraminifera had not differentiated these environments. Saline marshes (I) are characterized by a vertical zonation typical of most marshes, with a variety of estuarine species in the low marsh and trochamminids in the upper marsh. Brackish marshes (II) are dominated by a completely different set of species, all agglutinated, but the vertical zonation is weak, probably because of the low vertical gradient. Emergent freshwater or intermediate marshes (III) are dominated by arcellaceans but there are significant numbers of foraminifera, indicating episodic marine incursions. The freshwater floating marsh zone (IV) has only arcellaceans. A previous study of arcellaceans living in the lacustrine environment below the floating marsh reveals a different arcellacean fauna than that observed above it. These data suggest that marsh foraminifera have potential application as sea-level indicators in the delta region and that fossil remains of these protozoans can be used to differentiate coastal vegetation zones in the fossil record.

The Holocene Mississippi Delta System consists of six cyclic deltaic packages. Associated peat deposits are planar and eutrophic, and accumulate in areas abandoned by deltaic sedimentation. Peats that formed in large-scale interlobe basins are the subject of this paper. True peats (>75% organic matter) average 81.7% organic matter on a dry-weight basis. Controls on quality and lateral distribution of peat deposits are: botanical parent material, the balance between subsidence and accretion rates, detrital clastic influx and marine inundation. Botanical parent material, inferred from 13 C signatures and microtome sections, is always freshwater, originating either in forested swamps, or under herbaceous (floating) mats. Sphagnum spp. is essentially absent. In order for peat to accumulate, rates of accretion and subsidence must balance. Long-term subsidence rates in the study area increase from about 0.10 to 0.30 cm a -1 over a distance of 40 km from upper to lower delta plain. Subsurface accretion rates for true peats at a depth of 2-3 m, however, average about 0.05 cm a -1 . Renewed coarse detrital clastic influx terminates peat accumulation. The largest quantities of true peat are found together with relatively large amounts of organic-poor clastic sediments (mostly clays). Delta lobe abandonment and consequent coastal erosion causes salt water intrusion into freshwater environments. Resulting saline marsh sediments have maximum organic matter contents of 35%. In a transgressive setting, these may overlie older high-quality (low-ash) freshwater peats. Leaching experiments reveal the presence of water-soluble salts in both transgressed and fresh peats. Leaching reduces ash values by 1/5 to 1/3, suggesting that such losses may occur during early diagenesis. It is possible that some commercial coal seams originated in analogous interlobe environments, where leaching and possibly silica mobilization reduced the originally high ash contents to acceptable levels.

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