Reappraisal of the age and origin of the Casterton Formation, western Otway Basin, Victoria

• Published in: Australian journal of earth sciences 1997-12, Vol.44 (6), p.819-830
• Main Authors: Mitchell, M. M., Duddy, I. R., O'sullivan, P. B.
• Format: Article
• Language: English
• Subjects: Casterton Formation; fission track analysis; Otway Basin; stratigraphy
• ISSN: 0812-0099; 1440-0952
• DOI: 10.1080/08120099708728357

The Casterton Formation is widely considered to be the basal sedimentary unit of the Otway Basin, deposited during the latest Jurassic to Early Cretaceous in half-graben structures related to rifting between Australia 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 seismic data to be more widespread. Although not formally defined, the unit is currently identified on the basis of the association of a sequence of lithic sandstone with minor mudstone, conglomerate and intercalated basaltic flows in the informal type section in Casterton-1 well. K-Ar radiometric age determinations of a basalt flow from Casterton-1 yield an age range of 153 ± 5 to 120 ± 10 Ma (Late Jurassic to Early Cretaceous). However, the sequence is generally taken to be more restricted in age from latest Jurassic to earliest Neocomian. Some stratigraphers separate the Casterton Formation from the overlying Otway Supergroup with a basin-wide unconformity at the Jurassic/Cretaceous boundary. Fission track and palynological data from the Moyne Falls-1 and Hawkesdale-1 wells, in the Tyrendarra Embayment presented in this paper suggest that the unit, as identified, is as young as Aptian and was deposited synchronously with Otway Supergroup sediments. Apatite chlorine composition, fission track and petrographic analyses suggest provenance from a volcanogenic source similar to that inferred in the overlying Otway Supergroup and no evidence was found to support a significant erosional time break. Therefore, it is apparent that the lithological 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 stratigraphic marker.