Spatial variability in modern tropical peat deposits from Sarawak, Malaysia and Sumatra, Indonesia: analogues for coal

Petrographic and chemical variability in two tropical, low ash, domed peat deposits of similar age and physiographic setting, but different external morphology, were examined as modern analogues for coal. Both occur in microtidal alluvial to deltaic plain settings, support similar arborescent vegetation and have accumulated in the past 5,000 years. One deposit is an extremely convex, mature done, with a surface that rises 10 m above river level; the other is a low-gradient dome, rising only 3 m above river level but with a concave base up to 6 m below. Both deposits have been eroded by adjacent rivers, yet peat in the mature dome has accumulated remote from persistent flooding and exhibits distinctive internal stratigraphy. Decomposed sapric peat floors the deposits and is overlain by increasingly better preserved woody hemic and a cap of coarse fibrous peat derived from less arborescent vegetation. Consistent flooding in the low-gradient dome has maintained conditions for the accumulation of predominantly woody hemic peat, which interfingers with and is overlain by oxidised sapric peat and levee-derived clastic sediments. Peat types in both deposits exhibit distinctive polymodal textures and organo-chemical signatures that correspond to botanical input and degree of decay. Due to the abundance of pre-huminite/vitrinite macerals in all peat types from these deposits, megascopic and microscopic texture is a better discriminator for comparison to ancient coal types. It is suggested that the upward vertical sequence of decreasing woodiness and increasing lightness in colour observed in the mature dome might best be compared to sequences observed in Tertiary brown coals. This 'typical' sequence can be modified or overprinted due to flooding in the burial process. © 1994.