FRESHWATER PALYNOMORPHS IN ACID HYPERSALINE LAKE SEDIMENTS: EVIDENCE OF REWORKING IN LAKE BROWN, SOUTHERN WESTERN AUSTRALIA

One of the dominant features of the landscape of Western Australia is the widespread presence of hundreds of shallow acid hypersaline lakes, some of them hosted by the crystalline rocks of the Yilgarn Craton. Highly dynamic sedimentary processes result in the precipitation, dissolution and reworking of salts (gypsum and halite), iron oxides and other minerals in these lakes. Two 23 meter- and 28 meter-long cores drilled 3.8 kilometers apart in Lake Brown, an acid hypersaline lake located 43 kilometers north of Merredin in southern Western Australia, were studied for their palynological contents in order to understand the lake history. The palynological assemblage consists primarily of Chenopodiaceae, Myrtaceae, Asteraceae, Proteaceae, and Poaceae pollen families, which represent the surrounding modern flora and are adapted to arid conditions. Based on the presence of Myrtaceidites lipsis and abundant Tubulifloridites type pollen, the palynomorph assemblage can be placed within the Myrtaceidites lipsis and Tubulifloridites pleistocenicus pollen zones. Thus, the estimated age for the assemblage is Pliocene to Recent. Another remarkable characteristic of this assemblage is the common occurrence of palynomorphs with affinities to moist conditions and freshwater environments, such as Nothofagus pollen (Nothofagidites emarcidus-heterus), the colonial green alga Botryococcus, and the presence of marine dinoflagellate cysts (e.g., Odontochitina sp.). Earlier studies interpreted the presence of exceptionally well-preserved Nothofagus pollen grains in shallow sediments in nearby Lake Tay as the product of in situ deposition, thereby suggesting that Nothofagus trees survived in refugia sites in Western Australia until Early Pliocene times, despite increasing aridity. However, this study explains their presence due to reworking from Upper Eocene sediments, based on the similarity with taxa observed in the Werillup Formation and the known stratigraphic ranges of selected taxa.