A late early Albian marine transgressive pulse over northeastern Australia, precursor to epeiric basin anoxia: Foraminiferal evidence

Abstract

A major transgressive pulse occurred during the late early Albian in the western Papuan Basin, and totally changed the character of foraminiferal faunas in the basin, from impoverished agglutinated-dominated assemblages to diverse calcareous assemblages. A similar change is reflected in coeval deposits of other basins on the northeastern margin of the Australian continent (Laura Basin and northern Carpentaria Basin), and to a lesser extent in the intracratonic Eromanga and Surat Basins. This pulse preceded the condensed section of the Toolebuc Formation in the Eromanga Basin (of late middle Albian to early late Albian age), and succeeded deposition of the first radiolarian-rich units (late Aptian) in all basins. A rapid marine regression took place in the western Papuan Basin immediately after the latest Albian (95 Ma), and reflected the final retreat of marine conditions in the intracratonic basins to the south. Foraminiferal biofacies indicate that the late early Albian transgressive pulse resulted in peak sea levels in all basins; between 200 m and 500 m deep along the northern edge of the Cretaceous continent (northern Papuan Basin), decreasing to about 150 m in the northern Carpentaria Basin and the Laura Basin, and to 50–100 m in the southeastern Carpentaria Basin, northeastern Eromanga Basin, and southwestern Eromanga Basin, and to around 50 m in the Surat Basin. The change in bathymetry during the transgressive pulse was greatest in the northern marginal basins (possibly a deepening of over 100 m), and decreased to the southeast (about 10–20 m) where increased sand deposition occurred. Although the transgressive pulse bought slightly more saline and more oxygenated bottom water far into the basin, a barrier still existed in bottom waters across the central Carpentaria Basin between slightly brackish, dysaerobic conditions to the south and normal marine conditions to the north. After peak sea level was reached, bottom-water circulation within the southern Carpentaria and Eromanga sea decreased and this was accompanied by a decrease in sediment influx. During the late middle Albian through early late Albian anoxic conditions existed in central parts of the epeiric basins and a condensed section of black shales represented by the Toolebuc Formation was deposited. The late early Albian sea-level rise was partly controlled by eustatism, but may also have been influenced in different areas by varying amounts of continental uplift and subsidence associated with tectonism in the volcanic arc along the eastern Australian margin. The transgressive pulse broadly coincides with Albian transgressions reported widely from passive margins and epeiric basins elsewhere, but precise correlation is difficult. The consistently high sea level during the middle and early late Albian and the regression at the end of the Albian are out-of-phase with events on other continents and probably resulted from increasing tectonism along the eastern continental margin.