Shallow Mud Diapirs Over a Buried Delta Toe Thrust Anticline, Tarakan Basin, Borneo

Pieter Van Rensbergen
Ghent University, Gent, Belgium

The Late Miocene Aster anticline is the most landward and oldest anticline of a deep water fold and thrust belt in the Tarakan Basin, offshore northeast Borneo. Its NE-SW orientation is slightly oblique to the main NNE-SSW trend of the toe thrust anticlines. Its internal structure is obscured by chaotic reflections. The anticline is uncomformably overlain by thick Plio-Pleistocene prograding slope sediments. The crest of the anticline occurs at a subsurface depth of about 1 sec in a water depth of about 1100 m – 1300 m. Pervasive sediment deformation and localised diapirism occurs in the Plio-Pleistocene overburden sediments. Where the diapirs reach up to the sea floor, large slumps or mud flows occur.

Aster anticline is not a deep-seated diapir. Mud diapirs are limited to the Plio-Pleistocene overburden and mainly occur over two semi-circular zones that consist of stacked slides that jumble up the stratigraphy over the crest of the anticlines. Over these deformation zones 15 diapiric structures of only 200 m-300 m in diameter are observed. They seem to originate from the deformed Pliocene sediments and pierce a 0.3 s thick undeformed younger sequence.

The deformation structures in the Pliocene overburden provide information about the tectonic activity and help to understand the origin and structure of Aster anticline. Diapirs are interpreted to be caused by fluid injection along active segments of steep reverse faults into the Pliocene overburden and probably mark the strike of the active faults.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005