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ABSTRACT: Characteristics of inversion structures associated with the Baram Delta, NW Borneo

Morley, Chris , University Brunei Darussalam, Bandar Seri Begawan, Brunei

The Miocene-Recent history of delta evolution in Brunei Darussalam and adjacent regions of Sarawak and Sabah is closely linked with episodic phases of compression, as are other large deltas around Borneo. Inversion anticlines have two main trends N-S and NE-SW. Onshore the N-S trend is dominant and may reflect structures following underlying basement trends, or alignment approximately perpendicular to the regional compression direction. Offshore the anticlines more commonly have a NE-SW orientation. The NE-SW trends appear to be aligned along the sites of old extensional counter-regional faults. The counter regional faults show little evidence for reversal of motion. More likely is reactive mobile shale diapirs associated with the faults served to localize the anticlines. In outcrop and from seismic reflection data growth of the anticlines was episodic but lasted for considerable periods of the Miocene and Pliocene. Associated features include rotation of early normal faults and shale dykes, shifts in the location of crestal grabens with time, thinning and onlap of sediments onto growing structures, an influence by structure on sedimentary facies distribution and the creation of angular unconformities. Commonly there is little reactivation of deltaic normal faults, however the Miri anticline displays good outcrop examples of normal faults reactivated with reverse fault motions and low-angle thrusts. Phases of inversion may have enhanced overpressured conditions, which upon the cessation of compression led to an episode of enhanced fluid (including hydrocarbon) migration around the basin.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia