Abstract

The Belumai Formation Limestone Member has been studied in the field and petrographically in three river sections near Bohorok in North Sumatra. Distinct lithotypes were recognised and are described and their environmental significance discussed.

In the Bohorok and Landak river sections very shallow open marine shelf conglomerates and limestones accumulated initially on a local topographic high and these were overlain by deeper shelf limestones. In the Berkail river section similar deeper shelf limestones were deposited throughout the period except for shales and marls of the continental slope which grade into the overlying Baong formation. In the Bohorok river a condensed sequence and erosion surface indicate local emergence but this was not recorded elsewhere in the area studied.

No reefal facies limestones were discovered in the area in spite of the shallow, agitated water and good oceanic circulation and reasons for this absence are suggested. Work in progress on outlying wells in the region is expected to shed further light on the depositional environments.