Middle to Upper Eocene stratigraphic nomenclature and deposition in the Eucla Basin

Abstract

The Eucla Basin has the largest onshore extent of Cenozoic marine sediments anywhere in the world. The sediments provide a record of the evolving marine environments of the Southern Ocean and the terrestrial hinterland of the Australian continent. However, owing to its size and remoteness, the Eucla Basin is comparatively understudied. This is exacerbated by the scattered and often deeply weathered nature of the outcrops along the margins of the basin, and the inaccessibility of exposures in the basin centre, except in cliffs and caves. The extent and isolation of the Eucla Basin over two states has resulted in conflicting and overlapping stratigraphic nomenclature, especially of the marginal sediments. Therefore, we propose rationalising the nomenclature of the Eocene rocks in the region based on three guiding principles: the use of consistent terminology across the region; the recognition of the Importance of allostratigraphy in defining stratigraphic architecture, In particular two 3rd-order cycles correlated with the Tortachilla and Tuketja transgressions; and continuity with past usage wherever possible, with a minimum of new terminology. We propose eight major changes to the existing nomenclature: (1) abandoning the term Bremer Basin for the marine and marginal marine to non-marine Eocene sediments that infill palaeovalleys and form a veneer across crystalline basement In southwest Western Australia and including these sediments in the margin of the Eucla Basin; a similar situation exists in the east, where the Eocene sediments that have been included in the Polda Basin are likewise a marginal extension of the Eucla Basin; (ii) Introducing the term Maralinga Formation for all Middle Eocene non-marine to marginal marine sediments, including those previously included in the lower part of the Pidinga Formation in South Australia, and North Royal Formation for similar sediments in Western Australia: these replace the previous informal usage of lower Pidinga and lower Werillup Formation, respectively; (iii) restricting Hampton Sandstone to its original usage for a calcareous marine sand underlying the Wilson Bluff Limestone; (iv) raising the Paling Member of the Wilson Bluff Limestone to formation status; (v) using Pidinga Formation for all Upper Eocene carbonaceous sediments on the margins of the Eucla Basin in South Australia, and Werillup Formation for all such sediments in Western Australia, including the marginal palaeovalleys; terms such as Wollubar Sandstone in the palaeovalleys of the Yilgarn Craton, and Poelpena and Wanilla Formations in the Eocene part of the former Polda Basin should be abandoned; (vi) using the term Pallinup Formation for all Upper Eocene spicule-rich sediments along the western margin of the Eucla Basin; (vii) recognising the formation status of the Upper Eocene spicular marine sediments in the eastern Eucla Basin that were formerly termed the Khasta Member of the Hampton Sandstone and the Bring Member of the Pidinga Formation, abandoning the term Bring Member, and including those rocks, and similar sediments of the Poelpena Formation in the Polda Basin, In the new Khasta Formation; and (viii) abandoning the term Toolinna Limestone previously applied to Upper Eocene grainstone along the western margin of the Eucla Basin as it is a facies of the Wilson Bluff Limestone, whereas the grainstone at the type locality at Toolinna Cove is in fact Abrakurrie Limestone and is indistinguishable from the rest of that formation. We believe this rationalisation emphasises the unity of stratigraphy across much of southern Australia and, thus, will facilitate research on the Eucla Basin as a whole.