Origin, distribution, and degradation of sedimentary organic matter in a modern tropical deltaic system (Mahakam Delta, Borneo, Indonesia).

摘要

The modern, tidally influenced Mahakam Delta progrades onto the eastern Borneo continental shelf. The processes controlling the occurrence and preservation of sedimentary organic matter in a tropical deltaic setting are unknown. Understanding these processes has direct application in explaining the unusual origin of oil and gas generated from terrestrial plant debris (rather than typically from algal remains) in many tropical Miocene basins.; Water salinity and dissolved oxygen, and sediment pH, Eh, and TOC were measured. Organic matter was extracted by acid digestion from 200 surface sediment samples collected from the Mahakam River, the delta, and the continental shelf. Seventeen types of organic particles were identified and point counted using transmitted light and fluorescence microscopy. Their relative proportions have characterized the different deltaic and shelf depositional environments.; Most of the sedimentary organic matter is derived from the delta plain vegetation and is highly dominated by phytoclasts (plant debris). These phytoclasts originated from soft-plant tissues (e.g., leaves, petioles) composed of polysaccharides (cellulose, hemicellulose), and are preserved chemically and structurally in subaquatic deltaic sediments. But, although polysaccharides are not prone to preservation in sediments, they survive due to climatic and hydrodynamic conditions. The tropical vegetation produces vast quantities of leaves year-round. Tidal actions periodically move leaves from the delta plain forest floor into tidal and distributary channels, where strong ebb-tide currents carry them offshore. Because of this large input of leaves and petioles into subaquatic sediments, unique geochemical conditions have developed where organic matter is in excess concentration compared to the oxidants utilized by microorganisms. Consequently, polysaccharides are preserved in abundance in tidal channel, fluvial distributary, subtidal platform, delta front, and proximal prodelta sediments. Because preserved phytoclasts have bright fluorescent properties, these deposits are considered to be potentially oil-prone source rocks. Therefore, cellulosic debris derived from the deltaic vegetation would be the major contributor of Tertiary humic oils and gas.; For the first time a delta has been examined as a system to understand the origin, transport, distribution, and degradation of its sediment organic content. An innovative, holistic approach was successfully applied, integrating field parameters to the study of sedimentary organic matter.