Relationship between structural style, overpressures, and modern stress, Baram Delta Province, northwest Borneo

Abstract

The Baram Delta differs from most large (passive margin) deltas because of its location on an active postcollisional margin. Structural style and evolution is influenced both by gravity-driven deformation and regional stress. Key structural features of the delta include (1) late development of the toe fold and thrust belt (latest Miocene-Recent) with respect to 17 Ma recent development of growth faults on the shelf; (2) a narrow active region of extension on the outer shelf upper slope; (3) absence of well-developed shale diapirs; and (4) inner shelf onshore folding, thrusting, and growth fault inversion. Overpressure magnitude, type, distribution, and principal stress orientation and magnitude for the shelfal area were determined from well data. The inner shelf onshore area of inversion displays decreasing vertical stress gradient offshore and pronounced transfer of disequilibrium compaction overpressure from the Setap (shale) Formation into the overlying Belait Formation. This loss of overpressure permitted partial recoupling of once detached deltaic sediments with basement. Evidence for partial recoupling includes decreasing minimum horizontal stress gradient from NE (area of maximum inversion) to SW across the shelf and coast-perpendicular inner shelf Shmax directions appropriate for inversion, but with stress magnitudes insufficient to cause deformation. These observations constrain models to explain why toe fold-thrust belt activity occurred relatively late, while inner shelf folding developed early and is inactive today. During delta progradation disequilibrium compaction overpressures developed in the outer shelf slope area and helped decouple the section from basement late in the delta history, hence compressional deformation shifted further offshore. Conversely onshore inner shelf inversion caused late overpressure loss, resulting in partial recoupling of the deltaic section with basement.