The Hammerhead Delta—deepwater fold-thrust belt, Bight Basin, Australia: 2D kinematic and geomechanical reconstructions

Abstract

The Hammerhead Delta—deepwater fold-thrust belt is located in the Ceduna Sub-basin of the Bight Basin, offshore southern Australia. It is a short lived gravity gliding system, Late Santonian-Maastrichtian in age. It exhibits a distinctive spoon shape in cross-section and detaches on a master horizon above Santonian marine shales of the Tiger Supersequence. Here, we have interpreted a large seismic dataset—including the recently acquired regional two-dimensional seismic dataset provided by Ion Geophysical—to constrain the regional structural geometry of the Hammerhead Delta—deepwater fold-thrust belt. Two structural restorations were completed to quantify the amount of extension and shortening in the system. These restorations were: a two-dimensional kinematic restoration, using 2D MOVE; and a two-dimensional geomechanical restoration, using Dynel 2D. By comparing results from the two techniques we demonstrate that the amount of observed extension in the delta top is nearly balanced by the shortening in the delta toe. The near balance (< 2 % excess extension) of the system is a unique result. Other passive margin systems demonstrate larger amounts of extension compared to shortening, due to the regional-scale pro-gradational nature of the systems. These results suggest that the balanced geometry of the Hammerhead Delta—deepwater fold-thrust belt is consistent with either a sudden decrease in sediment supply during the upper Maastrichtian, resulting in a cessation of prograding fault activity, or a loss of extension to the underlying Cenomanian growth faults or some combination thereof. Thus, the system failed to develop into an extensive passive margin delta—deepwater fold-thrust belt.