--> Dinoturbation of the Broome Sandstone – Implications for Petroleum Geology

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Dinoturbation of the Broome Sandstone – Implications for Petroleum Geology

Abstract

Dinosaur footprints have been long-recognised in the Early Cretaceous Broome Sandstone in NW Australia. Sporadic coastal outcrop over 250km of this formation is predominantly fine to medium grained sandstone, interpreted to have been deposited in a tide to wave dominated deltaic to coastal setting. Common sedimentary deformation is interpreted as dinoturbation. The observed dinoturbation was produced predominantly by sauropods with footprints up to 1.4m. Individual prints are preserved in plan and section, due to the excellent 3D outcrop caused by modern coastal erosion. The best preserved tracks occur during marine transgression, with footprints being covered in marine mud. Recognisable footprints represent a small proportion of the observed dinoturbation that consists of underprints, overprints, foot withdrawal cylinders, wallows, trample grounds, and generally slumped to homogenised bedding. Persistent trackways record sediment compaction to form synclines and adjacent pierced anticlines. In many outcrops undisturbed sedimentation is rare. Observed dinoturbation morphologies have been applied to the interpretation of deformed beds in some petroleum cores from NW Australia resulting in them being reinterpreted as dinoturbation. From a petroleum perspective, sauropod dinoturbation can connect or homogenise reservoir on a large scale, turning interbedded tidal flat, coastal plain and flood plain strata into an interconnected reservoir. Considering the vast size of dinosaurs and the interpreted large numbers to have trampled the earth compared to average sediment accumulation rates (centimetres per thousand years) it is proposed that dinoturbation in Mesozoic fluvial to deltaic and coastal strata is under-recognised in the rock record and therefore our petroleum reservoirs.