Basal sediments of the western Great Artesian Basin record the change from a continental to marine depositional environment during the early Cretaceous. The sequence consists of the Algebuckina Sandstone, the Cadna-Owie Formation and the Mount Anna Sandstone Member. These units are overlain by the uniformly marine Bulldog Shale. The Algebuckina Sandstone consists of a lower succession of poorly sorted kaolinitic sandstones with typical angular current bedding, and an upper succession of well-sorted, concavely current bedded sandstones. The Cadna-Owie Formation rests disconformably on the Algebuckina Sandstone, and consists of micaceous and carbonaceous siltstone, Fontainbleau type and oolitic sandstones, and lenticular boulder beds. The well sorted Mount Anna Sandstone Member occurs within the upper part of the Cadna-owie Formation, and is characterised by an abundance of red porphyry pebbles. Isopachs of these basal units indicate that their distribution pattern was controlled largely by fossil topographic features. The lower kaolinitic beds of the Algebuckina Sandstone were deposited on a stable, senile land surface under low gradient fluviatile conditions, with episodic sheet floods. A seasonally arid climate is suggested by the presence of dreikanters. The upper Algebuckina Sandstone is thought to have formed under conditions of depositional regression and, as indicated by the fossil flora, in a moist, subtropical climate. The Cadna-owie Formation represents the sedimentary record of the Lower Cretaceous marine transgression. The sediments of this formation, with the exception of the Mount Anna Sandstone Member, were deposited under shallow water, marginal marine conditions, and a number of specialised marginal environments are documented by a variety of lithologies which compose this stratigraphic interval. The Mount Anna Sandstone Member was deposited in a fluviatile-deltaic environment, and the abundance of red porphyry pebbles contained within this sedimentary unit demonstrates a major uplift of the porphyry provenance area at the south-western basin margin in early Cretaceous time, resulting in the formation of the Gawler Range Massif. The existence of a previously postulated Lower Cretaceous glaciation is refuted. Correlation of these basal beds with equivalent stratigraphic units recognised in the deeper parts and along the southern and north-western margins of the Great Artesian Basin are discussed.