Abstract

Moreton Bay, near Brisbane, Australia, is a large back-barrier lagoon system opening in the north to the Pacific Ocean. The bay is protected by large, drowned Pleistocene dune-island barriers in the east (mainly relict), but in the south, the bay narrows to a complex tidal channel system protected by an active barrier island system. The islands are separated by an active tidal delta systems, and several incised estuarine rivers feed into the bay from the mainland. The bay represents a mixed wave and tidal-dominated system, that includes substantial Quaternary barrier island, lagoonal and estuarine sediments.

Continuous seismic reflection profiling in the southern Moreton Bay shows a series of incised valleys that traversed a coastal plain towards paleo-shorelines lying to the east of the present barrier island system. The incised-valleys, some of which have been drilled on the coastal plain adjacent to the modern Logan River, are attributed to the fluvial down-cutting associated with glacio-eustatic lowstand intervals of the late Quaternary. These valleys cut older Quaternary sediments and typically rest on bedrock. In the southern area, these incised valleys (up to 35 m deep and up to 3 km between interfluves) probably represent the bedrock-incised paleo-Logan River system. Previous work and remote sensing data show that the position of these valleys are controlled by structural lineaments in the pre-Quaternary basement.

The high resolution seismic profiles show that most of the sedimentation is confined to the incised valleys, and four-phases of sedimentary fill can be recognized and related to distinct phases of relative sea-level and sedimentary processes. The significance to petroleum exploration involving models of incised valley systems dissecting back-barrier settings is briefly discussed.