Holocene peat accumulation in a tropical intermontane mire system, Tasik Bera, West Malaysia: implication for coal formation

Abstract

Modern wetland environments represent the first step in production of peat deposits and are therefore precursors to coal seams. The characteristics of intermontane rheotrophic peat deposits are poorly constrained as modern a nalogues for coals but are critically important for understanding many deposits. Extensive organic rich sediments have developed in a clo sed drainage system in Tasik Bera, Pahang, West Malaysia. This t hick, low- to high-ash , low-sulfur, low-nitrogen peat has accumulated in a dendritic drainage basin since the Mid-Holocene (for at least 4,500 years) and provides a n excellent analogue for many Tertiary coals. Three distinctive ecological environments occur in Tasik Bera that give rise to spatial variation s in peat composition: a) li mnetic environment, dominated by algae and aquatic macrophyt (Utricularia flexuosa) deposits; b) littoral environment, dominated by sedges (Lepironia articulata) and woody shrubs (Pandanus heliocopus); and c) forest swamp dominated by trees and woody shrubs (Eugenia spp., Thoracostyachy um baucanum, Pandanus sp. ). Detailed peat-depth transects and cores in the northern part ofTasik Bera model the vertical and lateral distribution of peat. Peat character is influenced by vegetation, topography, channel geometry and mineral matter input. Kaolinite and minor other clays with occasional laminae offine silt (quartz rich) underlay organic rich deposits. Close to the northern outflow of t h e basin, adjacent the rivers Sungai Jelai and Sungai Bera, sand and sil t form the base of the peat. The Tasik Bera lake system was formed by damming by fluvi al sediments of Sungai Bera, which restricted drainage and led to a high water table and to paludification (initial geogenic accumulation of organic matter) in the basin. Subsequently, due to low r ates of decomposition and n et peat accumulation, the basin progressively under went terres trialization. Arborescent organic rich deposits are widespread throughout the basin. The onset of the mire system in the Tasik Bera basin was dominated by swamp fore st . South ofPos Isk andar, extensive forest swamp in three former tributaries , show highly humified (high CIN-ratio), woody, h emic to sapric peat up to 480 cm thick with intercalations offine sand, sil t and clay deposits towards the base. Open water and littoral vegetation yield fibric , h emic and sapric peat with an average thickness of 250 cm, dominate t h e northern part of Tasik Bera, up to the outflow into Sungai Bera. It is su ggested that the upward vertical succession of decreasing woodiness and increasing lightness in color observed are comparable to features in Tertiary brown coals, i.e . color, texture, plant part compo sition. The tropical freshwater peat deposits at Tasik Bera would yield a low sulfur, low to high ash thin banded stony coal seam of about 30 to 50 cm thick within a later al extent of 250 to 350 km2.