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Geology and landforms of Two Peoples Bay Nature Reserve, Western Australia

Ian Tyler , Phillip Playford, Katy Evans

Abstract

Context. The underlying geology strongly influences the landforms, the composition of soils, and the hydrogeology of Two Peoples Bay Nature Reserve. Methods. The Reserve and its surrounds were mapped by P. E. Playford in 1982. In 2024, I. M. Tyler and K. Evans carried out field visits to review that mapping, viewed available aerial photography and geophysical imagery, and reviewed published research. Key results. The main rock types consist of ca. 1290 Ma granitic gneiss and a ca. 1180 Ma granite intrusion that formed during Part I and Part II respectively of the Mesoproterozoic AlbanyFraser Orogeny. They are part of a Meso- to Neoproterozoic plate amalgamation forming the Rodinian Supercontinent, which put the AlbanyFraser Orogen at the heart of east Gondwana for ~1.0 billion years. At 165 Ma the Australian plate began to break out, and from 83 Ma the Eucla Basin opened from the west, following the plate suture within the orogen. Cenozoic landforms and surficial deposits record a history of northward plate movement, tectonic tilting and uplift, changing climate and apparent sea-level change, colliding with SE Asia at around 23 Ma. The limestone, sandstone and siltstone of the silica-rich, middle to upper Eocene (5034 Ma) shallow marine Plantagenet Group was deposited in a warm, marine embayment with granite monadnocks, including Maardjitup Gurlin/Mount Gardner, forming islands. The uplifted Eocene sea floor was lateritized in the Miocene and is now covered by stabilised Holocene eolian sand dunes, along with peaty swamps and lakes. This contrasts with the prominent granite outcrops, which are infilled with Pleistocene coastal ‘limestone’ eolianite deposits, overlain by Holocene residual quartz sands. Conclusions. The influence of plate tectonics on the geological evolution of an area is profound, from the origin and composition of the oldest rocks to the present landscape and the youngest surficial deposits. Plate movement controls tectonic setting, and the cycle of continental rifting, ‘drifting’ and plate collision produces apparent medium- to long-term sea level rises and falls, and climate change as the Australian Plate separates from India and Antarctica and collides with southeast Asia. Implications. Understanding the geological history of an area, not just the distribution of rock types and their composition, is essential to understanding landscape evolution and the natural history of the flora and fauna that inhabit it

PC24084  Accepted 11 November 2024

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