Print this pageA Triangle Zone at the Eastern Margin of Taranaki Basin, New Zealand
RAIT, GEOFFREY J., and DAVID J. BENNETT
The eastern margin of Taranaki basin is defined by the Taranaki fault, a west-directed thrust that carries Mesozoic greywacke in its hangingwall. Immediately west of this fault Eocene and Oligocene strata are deformed in the Tarata thrust zone, an association of mostly west-directed thrusts that splay from two major detachments, one in Eocene coal measures, the other near top Eocene. Anticlines in the western part of the Tarata thrust zone host producing oilfields. In some of these, including the prolific Waihapa field (tests up to 10,000 bopd), production is from fractures in the uppermost Oligocene Tikorangi Limestone.
Recent seismic mapping in the less explored eastern most part of the thrust zone has revealed a west-dipping panel of uppermost Eocene to Oligocene strata, carried by an east-directed backthrust that is truncated at its eastern end by the Taranaki fault. Material below the backthrust is stacked on west-directed imbricates, some of which also cut the backthrust, and the imbricates and the backthrust have been transported by later movement on the detachments. These structures constitute an abandoned triangle zone, analogous in many respects to the Alberta triangle zones but simpler owing to lower shortening.
This triangle zone is a new play concept in Taranaki basin, and its recognition is aiding in the delineation of new prospects. These include rollovers at Tikorangi Limestone and lower Oligocene levels in the upper panel; anticlines at Tikorangi to upper Eocene levels in the sub-backthrust imbricates; and gentle warps, probably associated with minor reactivation of Eocene growth structures, beneath the lower detachment.