Composition and grain size of an eocene coal bed in southeastern Kalimantan, Indonesia

Abstract

A study of an Eocene coal in southeastern Kalimantan , Indonesia shows a relationship between megascopically determined coal types and microscopically determined kinds and sizes of organic components. Microscopic examination of small (3 × 3 × 3 cm), uncrushed, chemically eteched block samples revealed that the coal was composed of plant parts and tissues set in a matrix of both finegrained particulate and amorphous material. The material identified as plant parts consists of stems and roots with secondary growth, leaves, and one unknown plant structure. However, most components with cellular features cannot be identified as particular organs and are designated only as woody tissue and categorized by cell wall preservation, that is, well, moderately, or poorly preserved. The particulate matrix is composed of cell wall fragments, cell fillings, resins, spores, algae and unidentifiable fluorescing fragments. Fungal remains are also present within the matrix and are the only oxidized material represented within the coal. The amorphous matrix consists of unstructured (at × 400) humic gels and bitumen. Size measurement of the organic constituents shows that each particulate component possesses its own size distribution that approaches normality when transformed to a - log 2 (Ø) scale. The size distribution of most of the plant parts is nearly symmetrical around a mean of 3–4 φ, whereas means of the particulate matrix components are in the 8–9 φ size range. Differences in proportions of the plant parts and matrix components determine the character of the megascopic coal types. Bright banded coal types contain the greatest proportion of well preserved plant parts; whereas the bright nonbanded and dull, steel-gray coal types contain fewer well preserved plant parts and consist mostly of plants parts with poor cell wall preservation. Hence, the megascopically recognizable coal types appear to reflect differences in particle size arising from the degree of preservation of the plant material. Concentration of decay-resistant resin and cell fillings in the non-banded and dull coal types is believed to be a result of the loss of intact plant material. An absence of larger (> 2 mm) plant material in the Eocene coal contrasts with the proportions indicated by data from a Miocene lignite and a Holocene peat, both from Kalimantan. In the latter two deposits logs and roots > 2 mm thick comprise as much as 10–15%. The paucity of large plant material in the Eocene coal is a function of the original peat-forming vegetation. The Eocene coal formed from palms and ferns which are easily degraded through microbial activity. In contrast, the lignite and peat deposits accumulated from woody angiosperms that are relatively more resistant to decay. However, all three deposits have similar size distributions among the smaller (< 2 mm) organic components. As in the Eocene coal, plant parts in the Holocene and Miocene have mode4s in the 2–3 φ range and particulate matrices in the 8–9 φ range. These data indicate that the processes of plant degradation may follow similar pathways regardless of origin. Large plant parts are first broken down to 2–3 φ and this may represent a “stable” size. Further degradation reduces plant parts to finger grained matrix material in the 8–10 φ range.