Low-ash peat deposits from a dendritic, intermontane basin in the tropics:: a new model for good quality coals

Many good quality, thick, low-ash Carboniferous, Cretaceous and Tertiary coal deposits have dulling-upward sequences that have been interpreted to be of ombrotrophic, lowland, coastal origin. Previous studies of potential modern analogues for thick, dulling-upward coal sequences have focused on tropical, ombrotrophic coastal peats. However, most modern coastal ombrotrophic deposits would yield all bright coal seams. New evidence from peat deposits of the southern intermontane Tasek Bera Basin, Western Malaysia, reveals that thick, low-ash, low-sulfur peat may originate in narrow tributary valleys with moderately steep flank gradients. These deposits may be favorable precursors to dulling-upward coals, in that they contain high wood and low-ash content at depth and medium wood and slightly increasing ash content in the upper parts. Peat accumulation in the Tasek Bera Basin began in Mid-Holocene time. During the past 4500 years, 500 cm of low-ash (< 5 wt.%), low-sulfur (< 0.4 wt.%) woody peat was deposited in narrow (400-700 m), elongate (3 km) tributary valleys of the southern Tasek Bera swamp system. Before onset of peat accumulation, a thick (> 40 cm) white, fluvial silty clay with wood fragments was deposited. The clay-rich sediments are overlain by limnic black mud, sandy clay and dark-brown mud-rich peat up to 70 cm thick. The ash content of the mud-rich peat is as high as 31 wt.%. The high-ash peat is overlain by low-ash, sapric to hemic peat up to 5 m thick. The basal 3 m of the peat deposits contain abundant logs and large woody fragments, whereas the top 2 m have less hard wood, but more arborescent fragments of palmae, such as roots from Pandanus atrocarpus. The acrotelm (top 50 cm) has a low-to-medium ash content (< 10 wt.%). With burial, the deposits of southern Tasek Bera Basin would yield a dendritic sediment pattern of sandstone, shale, carbonaceous shale and a dulling-upward, vitrain-rich coal, overlain by carbonaceous shale. Because of the dendritic nature of the Tasek Bera Basin, coal seams would most likely have a similar pattern as the Carboniferous humic coal deposits of the Black Warrior Basin in Alabama (USA). However, the good quality coals with dulling-upward characteristics would be concentrated in the southern tributaries of the basin. (C) 2001 Elsevier Science B.V. All rights reserved.