Bathymetric change during Early Cretaceous intracratonic marine transgression across the northeastern Eromanga Basin, Australia

Bathymetric change is investigated within the transgressive phase of an ext ensive Early Cretaceous (Aptian-Albian) marine flooding of the Australian c ontinent. Lithofacies and biofacies associations, found in nine outcrop sec tions and four continuously cored bore sections in the northeastern Eromang a Basin, are used to reconstruct depositional environments and to determine palaeobathymetry. Shoreface (c. 0-2 m water depth), transitional (c. <10 m ), upper offshore (c. <50 m), and lower offshore (c. 50-100 m) facies are d istinguished. Curves showing bathymetric changes are constructed for each s tratigraphic section, and are placed in a time framework by biozones and co mpared across the region. Three depositional cycles are tentatively recogni zed during the transgressive phase. The first depositional cycle includes t he Gilbert River Formation and the lower part of the Doncaster Member, and ranged from early Aptian (or late Barremian) to within the late Aptian. Dur ing maximum flooding, water depth was greatest in the southwest of the stud y area. Most sediment accumulation occurred under standstill conditions, an d a fall in relative sea level occurred at the end of the cycle in all area s within the study region. The second cycle, during the later part of the l ate Aptian, includes the upper Doncaster Member and the Jones Valley Member . Most sediment accumulation occurred under fluctuating water depths, but a fall in relative sea level ends the cycle in all areas. The third depositi onal cycle includes the Ranmoor Member, and was represented by a significan t late Early Albian transgressive pulse, recognized in age-equivalent forma tions from widely separated basins in Australia. At maximum flooding, the s ea-floor was deepest in the northeast and shallowest in the southwest of th e study area. The reversal in the direction of the bathymetric gradient bet ween cycle 1 and cycle 3 in the region is probably related to tectonism alo ng the eastern Australian margin. The apparent lack of correspondence betwe en the Australian Albian record and the global sea-level curve suggests tha t continental-wide tectonism was an important influence in determining Aust ralian sea levels during this time. (C) 1999 Academic Press.