Identification of multiple detrital sources for Otway Supergroup sedimentary rocks: Implications for basin models and chronostratigraphic correlations

Abstract

Correlation of apatite chlorine content (wt%) with apatite fission track age (Ma) from Lower Cretaceous Otway Supergroup sediments at present‐day low temperatures, allows identification of two characteristic detrital source regions. Apatites from eroded Palaeozoic basement terrains yield low Cl content (generally < 0.5 wt%) and fission track ages significantly older than the stratigraphic age. By contrast, apatites from contemporaneous volcanogenic detritus record a characteristic spread of chlorine content (0–2.5 wt% Cl, many > 0.5 wt%) and syndepositional fission track ages. Where post‐depositional thermal annealing (> 70°C) has significantly reduced the fission track age, provenance information is preserved in the apatite Cl composition alone. In the Otway Supergroup, evidence for contemporaneous volcanism was found in both the Eumeralla Formation (Albian‐Aptian), and Crayfish Group (Aptian‐Berriasian) in samples located towards the central rift, where less sandy facies dominate. Results suggest that Crayfish Group sediments deposited along the northern margin of the basin were predominantly derived from eroding basement material, while the section located towards the central rift contains a greater proportion of volcanogenic detritus. Evidence from this study suggests that volcanogenic detritus was a distal sediment source throughout the entire early rift phase, prior to the main influx of arc‐related volcanogenic material during deposition of the Eumeralla Formation. As diagenesis of volcanogenic sediments significantly reduces porosity and permeability of the sandstones, reservoir quality and petroleum potential may be significantly reduced in the Crayfish Group in deeper parts of the basin where a greater proportion of volcanogenic detritus is suggested. The results presented here provide important information regarding Lower Cretaceous Otway Basin stratigraphy and clearly indicate that this methodology may have wider application.