Can Differences in Hinterland Provenance Influence Strike-Parallel Variations in the Structural Style in Fold and Thrust Belts? An Example from NW Borneo

Cullen, Andrew¹; De Vera, Jose²
¹Chesapeake Energy, Oklahoma City, OK.
²Shell E&P, Rijswijk, Netherlands.

The NW Borneo deepwater fold-thrust belt (NWBTB) deforms Miocene to Recent deepwater clastic sediments of an under-filled foreland basin that overlies rifted continental crust of the Dangerous Grounds. The NWBTB is a hybrid system driven by both tectonic shortening and gravity-driven extension on the shelf. The relative percentage of tectonic shortening increases along strike, SW to NE. Four structural domains are defined on the basis of the dip direction of major faults on the shelf, deepwater fold geometry, spacing of thrusts, the number of detachment levels, and the depth an "ultimate" detachment in the Middle Miocene Setap shale.

D1: High-amplitude, symmetric detached folds; thrust faulting is minimal. The shelf is dominated by down-to-basin growth faults of Baram Delta.

D2: Asymmetric thrust-generated folds terminate into tear faults and lateral ramps; multiple detachment levels are present and triangle zones are poorly developed. Large counter-regional growth faults dominate the shelf.

D3: Similar to Domain 2 (the array of thrust-generated structures includes well-developed triangle zones). On the shelf, down-to-the-basin, normal faults prevail.

D4: Strongly imbricated, poorly imaged, multiple thrusts are draped by gently folded Mio-Pliocene pelagic shale; extension on the shelf is minimal.

Such contrasts in structural style can be attributed to multiple factors, including strength of and depth to the basal detachment, reactivated structures in the underlying rift, localized pulses of deformation in the hinterland, and the rheology of the deformed material. The large amplitude folds of D1 are absent from the other domains. D1 is characterized by the vertical stacking of alternating parallel-concordant and chaotic seismic facies that we interpret as pelagic and debris flows, respectively. The folds have thickened, imbricated, cores; consistent with a mechanically-weak, low viscosity contrast lithology, such as over-pressured shale. Paleo-drainage analysis indicates the Baram Delta was the major sediment delivery system for D1; whereas the other domains were supplied by multiple rivers of the Champion system. The D1 hinterland is dominated by Oligo-Miocene shale. In contrast, the sand-rich Crocker Formation was the main provenance for the Champion-system. We favor interpreting Domain 1 as a shale-dominated part of the NWBTB that offers an elegant example of how hinterland provenance can influence structural style in an active fold belt.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012