Upper Delta Plain

LEVEES and CHANNEL MARGINS are sites that are biased towards the degradation of aerial plant parts (Scheihing and Pfefferkorn, 1984 - Orinoco Delta, Venezuela ; Kosters and others, 1986 - Mississippi River Delta, Louisiana; Gastaldo, 1986, 1989 - Mobile-Tensaw River Delta, Alabama ; Gastaldo and Huc, 1992 - Mahakam River Delta, Kalimantan, Indonesia ; Gastaldo and Staub, 1996 - Rajang River Delta, Sarawak, East Malaysia).

Upper delta ALLUVIAL PLAIN FORESTED SWAMPS are characteristic of both the temperate and tropical sites. The density and diversity of plant communities in these regimes are different and reflected, in part, by their taphoassemblages.

Mobile-Tensaw Taxodium Swamp

Fluctuations in subsidence rates or breaching of the levee may result in the development of shallow lakes.

Channel Systems

DISTRIBUTARY CHANNELS are the main conduits through which plant macrodetritus is moved into and through the delta. Channel depths may exceed 30 m and it is difficult to recover channel bottom samples from these sites.

Orinoco Leaf Litters in Channel Margins

To date, information about plant part incorporation has been obtained from channel margins and subaqueous lateral channel bars.

Delta Vibracore of Lateral Channel Bar

The ABANDONMENT of any channel provides a quiet water depositional site in which clastic influx may be cyclical (tidal-influence) or intermittent (fluvial flooding). See: Biostratinomic Processes in Lakes.

The TERMINUS of an active channel, either in the delta front or within an interdistributary bay, is a site where the coarser load is deposited typically in MOUTH BARS.

Lower Delta Plain

INTERDISTRIBUTARY BAYS develop between river channels as they prograde seaward. In fluvial-dominated deltas these differ in mode of origin and sedimentological characteristics than those in tidally-influenced zones.

Crevasse splay; 
Mobile-Tensaw River delta, AL Identifiable plant assemblages occur when a levee, surrounding the interdistributary bay, has been breached and CREVASSE SPLAY sediments accumulate.

Differences in Temperate Zones

The seasonality of temperate regions prohibits exclusively tropical vegetation from inhabiting low-lying coastal zone. This is due to occasional freezing.

Differences in Tropical Zones

In contrast, lower delta plains in the tropics are dominated by monotypic or low diversity stands of palms and/or mangrove taxa. Over 70% of the world's coastlines are vegetated by mangroves (salt-tolerant plants that include trees [e.g., Rhizophora and Avicennia], palms [Nipa], and ferns [Acrostichum]).

Channel vibracore in Nipa Swamp

In the Mahakam, monotypic swamps of Nipa fructicans occur throughout the lower delta plain; There is a narrow seaward fringe of mangrove trees.

In the Rajang, a peat-accumulating deltaic setting, the lower delta plain is vegetated by a dipterocarp-angiosperm dominated forest, growing on an organic (peat) substrate. In general, mangroves fringe channel margins that are influenced by tides, bringing brackish waters inland.

TIDAL FLATS are composed of fine-grained siliciclastics.

Mahakam River 
Peat Beach In areas away from active fluvial processes, phytoclasts that had been transported to the delta front may be reworked onto the tidal flats by wave action (Gastaldo and others, 1993). These accumulations may exceed 2 m in thickness, forming BEACHES along the coast line.

©Copyright 1997 by Robert A. Gastaldo. All rights reserved. No part of these lecture notes may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission from the author.