Heterogeneous brittle deformation in the Devonian carbonate rocks of the Pillara Range, Canning Basin: Implications for the structural evolution of the Lennard Shelf

Abstract

Deformation documented in the Upper Devonian carbonate rocks of the Lennard Shelf shows a complex history involving discrete synsedimentary structures and faults. The reefal carbonate successions of the Lennard Shelf were deposited along the northeastern margin of the extensional Fitzroy Trough in the Canning Basin. Extensional deformation on the regional scale is concentrated along normal faults trending northwest parallel to the shelf margins, and in structural corridors that are oblique to the strike of the shelf. In the Pillara Range‐Limestone Billy Hills region, a corridor of northeast‐trending deformation is associated with dextral offset of the reef margins, and is interpreted to represent an accommodation zone, comprising individual en echelon sinistral oblique‐slip faults. Synsedimentary deformation controlled reef and platform facies distribution, and caused fracturing with extensive development of neptunian dykes. Later structures, which post‐date marine, radiaxial fibrous cements, are spatially associated with the earlier synsedimentary structures, and are interpreted to represent reactivation of these structures. Meso‐scale brittle and brittle‐ductile structures of this later phase include faults associated with extensive fault breccias, matrices of sparry calcite and ore sulfides, tectonic stylolites, and en echelon tension gashes filled with spar calcite. Importantly, this deformation is brittle and discrete, such that the wall rocks adjoining the deformation structures are undeformed, preserving complete and unstrained fossils and primary sedimentary structures and textures. The geometry and kinematics of these structures are consistent with sinistral oblique‐slip movement on a regional scale along northeast‐trending basement‐hosted faults. The northeast‐trending faults are contained in a northeast‐trending zone that accommodated offset of the regionally extensive basin‐bounding, normal, listric faults of the Fitzroy Trough. Within this zone there was reorientation of the regional stress field, in which σ1 (maximum compressive stress) was initially vertical, to one in which σ1 was locally subhorizontal and broadly north‐northwest to south‐southeast trending. Deformation commenced in the late Givetian, during reef development, and continued through to at least the Early Carboniferous. The reactivation appears to have been most pronounced in the very Late Devonian or Early Carboniferous, and all deformation had ceased prior to deposition of the Permo‐Carboniferous rocks in the study area. Post‐Permo‐Carboniferous deformation occurred in the Fitzroy Trough, but left the Lennard Shelf unaffected.