Abstract: Coal Environments

James M. Coleman

Organic materials are buried as peat deposits in coastal plains and deltaic settings that span the climatic regions of the globe. Upon burial, diagenesis, and passage of geologic time, peats are converted to coals and lignites, and an understanding of the environments of deposition under which they accumulate is critical to exploit efficiently existing coal reserves. Floristic composition, peat geochemistry, lateral and vertical geometry of peats, and the relations of the peat deposits to nonorganic sediments are functions of depositional processes such as climate, tectonics of the depositional basin, sediment yield and dispersal patterns, and riverine and nearshore coastal dynamics. Depositional models are produced in which organic-rich deposits form a significant part o the sediments and display distinct vertical and lateral interfingering relations with the coastal-plain deposits.

In Arctic settings (North Slope, Alaska), peats are composed primarily of sedges, grasses, and mosses and form thick deposits which cap channel and beach sands and interfinger laterally with organic-rich lagoon and lacustrine silts and clays. Supratidal sediments, evaporites, and sandy-channel deposits interfinger with erratically distributed woody -peat layers in the arid-deltaic setting (northwest Australia). The organic-rich deposits accumulate in saline mangrove environments, and often a wide variety of early diagenetic products is formed in underlying deposits. In temperate climates (Mississippi River delta), peat deposits vary in thickness and display a wide range of floristic composition and organic content. Freshwater peats in the alluvial valley are normally thick (2 to 5 m), woody in nature, and display low lateral continuity. Pyrite and vivianite are common inclusions in the peat, and iron and calcium carbonate are produced diagenetically in the underlying clays. Brackish and saline coastal peats are normally thin (< 2 m), but each bed has wide lateral continuity. Marshy, nonwoody components contribute significantly to total organic content, and early diagenetic products are common within and beneath the peat layers. In the tropical Klang River delta, thick, widespread peat deposits (5 to 15 m) overlie lower deltaic silty and clayey tidal-flat deposits, and the peats interfinger laterally with river-channel sands. Early cementation of the peats is common; excessive leaching of the underlying clay deposits also is common. Thick fresh- and saline-water peats accumulate in a large (60,000 sq km) coastal plain of Surinam and the Guianas, along the northern coast of South America. In many instances peat deposits exceed 40 m in thickness and interfinger locally with sandy-beach deposits. Organic-bound sands (humates) are abundant, and diagenetic components are common both within the peats and in adjacent deposits.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA