Abstract

Kilometre-scale synsedimentary faults associated with well-preserved sedimentary successions on both hanging-wall and footwall sides are rarely seen in outcrop. However, one such example is found in a Middle Miocene deltaic sand–shale sequence exposed along the Jerudong Anticline in onshore Brunei Darussalam. Integration of detailed outcrop information with regional geophysical subsurface data documents that: (1) initiation of the Jalan Tutong growth fault was related to delta-front instability and failure; (2) sediment loading under shallow-marine conditions maintained syndepositional faulting; (3) growth-fault abandonment coincided with a significant basinward shift of deposition and subaerial exposure. High-resolution analysis of facies characteristics, stratal expansion and throw development within the studied growth sequence indicates that differential fault movement strongly influenced facies composition, sedimentary geometry and bedding characteristics: sandstone-poor shelf to lower-shoreface successions developed during periods of rapidly increasing fault movement, whereas sandstone-rich, amalgamated upper-shoreface deposits preferentially formed during intervals of constant or decreasing throw development. The data and interpretations presented may help to focus attention on the complex interplay between tectonics and sedimentation associated with the growth and decay of deltaic faults by providing for the first time an outcrop-based view on a feature documented in similar scale only by geophysical subsurface data.

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