Geophysical evaluation of the enigmatic Bedout basement high, offshore northwestern Australia

Abstract

The Bedout High in the Roebuck Basin (formerly offshore Canning Basin) on the northwest shelf of Australia is an unusual structure, which has been controversially interpreted as an end-Permian impact structure similar in size to the K–T boundary Chicxulub Crater. We present a geophysical perspective of the associated debate, based on deep seismic reflection, refraction and well data. The basement and crust in the Roebuck Basin display a number of features that distinguish them from other basins along the northwest Australian margin, including major crustal thinning and the presence of a thick layer of interpreted magmatic underplating. The Bedout High consists of two separate highs separated by a Paleozoic fault, and is associated with a Moho uplift of 7–8 km, and is about 40–50 km wide. The normal fault separating the two highs trends NNW–SSE, roughly paralleling a Paleozoic fault system associated with rifting in the Canning Basin and terminating below the interpreted top-Permian reflection. There are no circular, symmetric fault zones bounding the proposed annular trough, and the distinct difference in seismic character normally associated with impact breccias versus layered sediments above are not expressed in deep multichannel seismic data. The end-Permian horizon exhibits little topography, with well-layered units both below and above. The area around the Bedout High stands out as an area of low velocity basement: 5400–5600 m/s compared to 5800–6000 m/s for other nearby basement areas located in a similar depth range, but known complex impact sites are not characterized by a unique seismic basement velocity signature. Both seismic velocity analysis, revealing a thick underplated layer in the lower crust, and thermal modelling based on data from well La Grange-1 and basalts drilled on top of the Bedout High, are consistent with rifting above anomalously hot mantle. The available geophysical and geological data are compatible with an interpretation of the Bedout structure as a basement high formed by two consecutive Paleozoic and Mesozoic episodes of rifting roughly orthogonal to each other, associated with basin formation east and west of the Bedout High, but fail nearly all unequivocal criteria for impact crater recognition.

Introduction

The Bedout High, a basement high in the Roebuck Basin on the Northwest Australian Margin (Fig. 1), has been interpreted as an end-Permian impact structure [1], [2]. This has resulted in a vigorous debate, expressed in three comments [3], [4], [5] and replies [6], [7], [8] in Science focused on whether or not the Bedout High was caused by an extraterrestrial impact, mainly based on petrological arguments. In the impact scenario, the Bedout structure is interpreted by Becker et al. [2] to be similar in size to the Chicxulub crater, about 200 km in diameter, and the Bedout High is thought to represent the central uplift of the impact crater [6]. In this paper we present an analysis of available seismic reflection, refraction and well data associated with the Bedout structure, and discuss the likelihood of it being an impact structure.

The Bedout High is located in the Roebuck (formerly part of the offshore Canning) Basin, one of the largest sedimentary basins in Western Australia, bounded to the west by the Argo Abyssal Plain (Fig. 1). The Bedout High is flanked by two sub-basins of the Roebuck Basin, the Bedout Sub-Basin to the southeast, and the Rowley Sub-Basin to the northwest (Fig. 2). A Paleozoic west-northwest trending extensional structural grain defines major tectonic units in the Canning Basin [9]. These structures are well visible in marine gravity anomalies, especially on and around the Broome Platform and within and west of the Rowley Sub-Basin (Fig. 2). A set of northeast trending structures was imposed on the older structural grain during Mesozoic rifting and continental breakup, as expressed by the orientation of the Rowley Sub-Basin itself (Fig. 2). A major tectonic event has been interpreted as extensive uplift, faulting and volcanism in Late Permian/Early Triassic time in the Roebuck Basin, known as the “Bedout Movement” [10]. Gorter [1] interpreted the “Bedout Movement” producing the Bedout High through a bolide impact. However, Colwell and Stagg [10] pointed out, that the Bedout Movement is mainly defined in the onshore part of the Canning Basin (Fig. 1), where it represents a widespread tectonic event and thus is not confined to the vicinity of the Bedout High.

Continental breakup occurred at the seaward edge of the Roebuck Basin at about 155 Ma; the oldest identifiable magnetic anomaly along the margin is anomaly M25A, aged 154.5 Ma [11], in accordance with ODP drilling results [12], based on the Gradstein et al. [13] timescale (Fig. 1). The geometry of the rifted margin is unusual in that it does not follow the main NE–SW structural grain. Instead the Argo Abyssal Plain margin is indented between the Exmouth Plateau to the south and the Scott Plateau to the north, with the apex of the indentation corresponding to the narrowest portion of the margin, pointing towards the Roebuck Basin (Fig. 1).

The margin is characterized by an up to 18 km deep Paleozoic/Mesozoic trough, as expressed on the deep seismic reflection lines 120-1 (Fig. 2, Fig. 3, for profile location) and 120-4 (Fig. 4) [14]. The Paleozoic (Ordovician(?)–Permian) section is ∼0.8 s (two way travel time—TWT) thick, but considerably thinned to about ∼0.2 s TWT over the Bedout High. The Triassic–Jurassic sequence is highly variable in thickness. On the seaward portion of line 120-1, the Triassic–Jurassic section forms a seaward-thickening wedge reaching a maximum thickness of ∼3.75 s TWT adjacent to the lower continental slope. At the seaward flank of the section, Middle Triassic folds and Late Triassic faults affect the ∼1.9 s TWT thick Triassic section. The post-Jurassic sedimentary section includes < 1 km of Cretaceous section overlain by Tertiary section that reaches a maximum thickness of ∼1.5 km beneath the upper continental slope. The boundary between continental and oceanic crust (COB) is well defined, situated on the seaward side of a deep-seated tilted block of continental crust at the edge of the Argo Abyssal Plain (Fig. 2). The lower continental slope is effectively the basin edge, and is steep (15–20°) and controlled by faults [14]. Erosion of the Jurassic section landward of the fault indicates significant uplift of the basin flank at breakup. In the following we will review evidence for and against the impact hypothesis based on two deep seismic reflection lines crossing the Bedout High, basement topography maps based on a grid of seismic lines [15], a seismic refraction profile coincident with line 120-1 and industry well data in the area.