CLAY MINERAL CLUES TO ESTUARINE SEDIMENT PROVENANCE

The clay mineral composition of estuarine sediments reflects tectonic and climatic conditions that exist in the source areas, modified by differential transportation and diagenesis. Rangoon River (Burma) and Selangor River (Malaysia) illustrate extremes that may be compared with mid-latitude estuaries elsewhere.

Rangoon River is an estuarine distributary of the Irrawaddy River whose headwaters drain the Himalayas; the main stream then flows within an inter-arc trough adjacent to a Cenozoic volcanic arc. The river carries well-defined muscovite, chlorite, quartz, and feldspar above Mandalay. Below Mandalay and above the delta, the Cenozoic volcanics add significant amounts of fine-grained smectite. The estuary is macro-tidal. During the dry season, high sediment concentrations and multi-layered suspensions are found and the net transport is landward. Smectite is most abundant in the concentrated suspensions, while muscovite and chlorite dominate in the low concentration suspensions during average and neap tidal flows. This leads to preferential concentration of smectite in estuarine deposits. High discharge during the monsoon often flushes much of the accumulated sediment load seaward.

Selangor River drains granitic and metamorphic terrain of Paleozoic age. The deep, tropically weathered soils contain clays rich in kaolinite and gibbsite (and little mica or feldspar). These clay minerals become the most abundant components of the estuarine sediments. The Selangor is a mesotidal estuary where multi-layered suspensions have not been detected. Little differentiation in mineral composition exists between the suspended and deposited sediments. The clay minerals reflect their origin in a mature tropical landscape. The highest freshwater discharges rarely push the 0 o/oo isohaline beyond the mouth, so the sediment load is trapped within the estuary.

The clay mineral composition of estuarine sediment in mid-latitude and temperate estuaries is usually intermediate in composition to those above, and it reflects intermediate tectonic and climatic conditions in their source areas.