Abstract

Intensive, modern exploration of the North Sumatra Basin has emphasized the need for revision and refinement of former rock stratigraphic concepts and nomenclature, particularly of the transgressive pre-Baong sediments. Early Tertiary sedimentation began in isolated sub-basins; mica quartz breccias, quartz conglomerates, and coarse-grained mica sandstones (Parapat Formation) were deposited as fluvial fans along the edges of the basins. These were overlain and interfingered basin-wards with dark carbonaceous shales and marls of euxinic environment (Bampo Formation). In Early Miocene, subsidence slowed and became more uniform throughout the basin, resulting in wide-spread carbonate-arenaceous deposition. Arenaceous sediments with some interbedded carbonates (Belumai Formation) of shallow marine environment were deposited in the central and south-southeastern region, while limestone, marls and shales. (Peutu Formation) of more openmarine environment were deposited in the northwestern region. The type of sediments deposited continued in part, however, to reflect local pre-Tertiary topography and block faulting. These Lower Miocene formations are in turn overlain by the basin-wide, lithologically uniform shales of the Baong Formation, marking the maximum Tertiary transgression of the North Sumatra Basin. The formation names in the parentheses above are reaffirmed, or proposed, as terms designating formations.