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Detrital peat formation in the tropical Mahakam River delta, Kalimantan, eastern Borneo: Sedimentation, plant composition, and geochemistry

By
Robert A. Gastaldo
Robert A. Gastaldo
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George P. Allen
George P. Allen
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Alain Y. Huc
Alain Y. Huc
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Published:
January 01, 1993

The Holocene Mahakam River delta, Kalimantan, Indonesia, is a complex fluvial-and tidal-influenced regime prograding into the Makassar Strait. Three plant communities are segregated and controlled by salinity and edaphic conditions. Salt-tolerant mangrove taxa pioneer the lower delta plain tidal flats and are quickly replaced by Nypa palms. These palms comprise the vast majority of dense swamps in the delta plain. A more diverse hardwood forest replaces Nypa swamps when sediments have accumulated to subaerial heights where better drainage is possible. The interior of the island supports well-established primary tropical forest. Autochthonous peat deposits have not been identified in the region.

Fluvial distributary channels are the principal conduits through which plant parts originating in these communities are transported to the delta front. Plant parts that reach the delta front may remain resident at the sediment-water interface until they are reworked into accumulations, as thick as 2.5 m, onlapping the interdistributary lower delta plain tidal flats. These allochthonous peat bodies are composed of fragmented canopy detritus from various sources, but mainly from dicotyledonous angiosperms. Plant parts include leaves, cuticles, wood fragments, petiole parts (both dicotyledonous angiosperm and monocotyledonous Nypa), damar (dipterocarp resins), fruits, and seeds. Deposits occur as high-tide beach ridges. Peat beach ridges alternate with tidal mud flats as the headlands aggrade into the Makassar Strait.

Geochemical analyses have been conducted on the peat recovered in bulk samples and from core. These include 14C dating, Total Organic Content (TOC), bulk sulfur content, Rock-Eval pyrolysis, Py-GC and Py-GCMS. Peat is Holocene (Recent) in origin, with 14C dates of bulk samples as old as 1050 yr B.P. Damars recovered from the beaches are also Recent in origin (2645 ± 215 yr B.P.; 930 ± 205 yr B.P.). TOC ranges from 27.5%–39.4% with accompanying hydrogen indice (HI) varying from 250–450. Sulfur ranges from 0.65%–2.75%. H/C ratios average 1.43, and O/C ratios average 0.44. The occurrence of relatively high values of the HI are probably due to the incorporation of pieces of Recent damar which exhibit HI as high as 1,130.

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GSA Special Papers

Modern and Ancient Coal-Forming Environments

James C. Cobb
James C. Cobb
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C. Blaine Cecil
C. Blaine Cecil
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Geological Society of America
Volume
286
ISBN print:
9780813722863
Publication date:
January 01, 1993

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