REAPPRAISAL OF THE AGE AND ORIGIN OF THE CASTERTON FORMATION, WESTERNOTWAY BASIN, VICTORIA

The Casterton Formation is widely considered to be the basal sedimenta ry unit of the Otway Basin, deposited during the latest Jurassic to Ea rly Cretaceous in half-graben structures related to rifting between Au stralia and Antarctica. The unit has been intersected in a handful of onshore wells as a relatively thin (13-535 m) basal unit of the Otway Supergroup overlying Palaeozoic basement, but is interpreted from seis mic data to be: more widespread. Although not formally defined, the un it is currently identified on the basis of the association of a sequen ce of lithic sandstone with minor mudstone, conglomerate and intercala ted basaltic flows in the informal type section in Casterton-1 well. K -Ar radiometric age determinations of a basalt flow from Casterton-1 y ield an age range of 153 +/- 5 to 120 +/- 10 Ma (Late Jurassic to Earl y Cretaceous). However, the sequence is generally taken to be more res tricted in age from latest Jurassic to earliest Neocomian. Some strati graphers separate the Casterton Formation from the overlying Otway Sup ergroup with a basin-wide unconformity at the Jurassic/Cretaceous boun dary. Fission track and palynological data from the Moyne Falls-1 and Hawkesdale-1 wells, in the Tyrendarra Embayment presented in this pape r suggest that the unit, as identified, is as young as Aptian and was deposited synchronously with Otway Supergroup sediments. Apatite chlor ine composition, fission track and petrographic analyses suggest prove nance from a volcanogenic source similar to that inferred in the overl ying Otway Supergroup and no evidence was found to support a significa nt erosional time break. Therefore, it is apparent that the lithologic al association defining the Casterton Formation was not unique in the evolution of the Otway Basin fill, and the definition of the Casterton Formation requires revision if the unit is to be a useful stratigraph ic marker.