Abstract

The Neogene sedimentary section in the subject area exhibits ten distinctive seismic sequences which reflect changing depositional conditions due to the combined effects of eustatic changes, tectonic movements, and the availability of sediment supply.

Sedimentation of the Early Miocene Peutu Formation took place in a period of steady rising sea level and low sediment supply. Over the highs, reefal buildups and subaerial erosion predominated; while in the lows, hemipelagic deposition prevailed. During the Baong deposition in the Middle Miocene, rapid basin subsidence occurred in a period of continuing fall of sea level. The deep marine shales onlapped and eventually buried the highs. Entering the Late Miocene and Pliocene, the Barisan Mountains south of the subject area began emerging during a period of global lowstand of sea level. The resulting regressive sequences represented by the sandy Keutapang and the Lower Seurula Formations were deposited in a relatively shallow marine environment. The Pliocene Upper Seurula and Julu Rayeu Formations are two prograding units deposited in a period of frequent eustatic changes interrupted by four unconformities of possible eustatic origin.

A chronostratigraphic analysis indicates that these formations are associated with unconformities of various time duration. These unconformities are proposed as new subsurface formation boundaries because they are chronostratigraphically meaningful and the intervening sequence represents a succession of strata synchronously deposited in closely related environments.

Sedimentation and structural style are vastly different in areas across a northwesterly trending wrench fault on the west flank of the Arun-Cunda-Peusangan High. To the east, sedimentation is more responsive to eustatic changes. There, the sediments are less disturbed and show well-defined sequence boundaries. To the west, overpressure facilitates thrusting, slumping, and shale swelling. In addition, wrench faults, frequently in the forms of "flower structures" and branching upthrust, cut into the shallow structures, complicating stratigraphic correlations.