Gulpuliyul structure: a possible impact structure of Mesoproterozoic age in western Arnhem Land, Northern Territory

Abstract

The Gulpuliyul structure is the eroded remains of a possible impact structure of Mesoproterozoic age, in western Arnhem Land, Northern Territory, on the Arnhem Shelf of the northwestern McArthur Basin. Enigmatic, highly deformed and brecciated strata, within the roughly circular or pentagonal feature about 8.5 km across, contrast with mildly deformed rocks of the surrounding Arnhem Shelf. Shock-metamorphic features have yet to be observed. Other features of the Gulpuliyul Structure are: (i) sharp and faulted outer boundaries; (ii) strata within the structure are younger than adjacent country rocks; i.e., the rocks have been emplaced downwards into the structure; (iii) outcrops display an overall concentric or tangential pattern, the stratigraphy is essentially coherent, and there is an overall younging from the centre outwards; and (iv) strata are commonly overturned by southward-directed thrusting and recumbent folding. It is suggested that the projectile impacted at a shallow angle from the north, to produce a southward-deepening crater about 8.5 km across. The depth of the transient crater was probably between ∼500 – 700 m (minimum) and ∼800 m (maximum). The central uplift probably rebounded only about 300 – 400 m. The present erosion level is thought to lie near the top of the low central uplift, at about or just below the floor of the final crater. The age of the possible impact is Mesoproterozoic (ca 1600 – 1325 Ma); it is most likely to have occurred very early in the Mesoproterozoic (1600 – 1500 Ma).