Late Silurian trace fossils from the Melbourne Formation,Studley Park,Victoria, southeastern Australia

An ichnoassemblage of 10 ichnospecies is described for the first time from the Late Silurian Melbourne Formation at Studley Park, Victoria, southeastern Australia. The ichnofauna is preserved in a typical deep-water turbidite succession of alternating thin- to thick-bedded sandstone and thin- to medium-bedded mudrocks. Trace fossils observed within the study site have been assigned to three main ichnofacies. Ichnofacies 1 is best developed on the linguoid-rippled upper surface of thin sandstone beds and includes Laevicyclus, Aulichnites, Nereites, Helminthoidichnites, small Chondrites and possible Zoophycos. Ichnofacies 2 is very similar to Ichnofacies 1 in ichnospecies composition but instead contains large forms of Chondrites together with other thin burrow types usually poorly preserved and in very low abundance compared with Ichnofacies 1. Ichnofacies 3 is preserved mainly as casts on the underside of medium- to thick-bedded turbiditic sandstones, and has a very low diversity, with Planolites being the most common trace. A detailed analysis of the ichnofabrics and tiering structures of these ichnofacies suggest that Ichnofacies 1 and 3 represent ‘simple tiering’, in contrast to Ichnofacies 2, which is more characteristic of ‘complex tiering’. Despite the differences in ichnospecies composition and ichnofabrics between the three recognized ichnofacies, the collective ichnoassemblage from the study site can be assigned confidently to the Nereites ichnofacies and is, therefore, interpreted to have formed in a distal submarine fan environment of lower bathyal to abyssal depth. Further, it is possible to recognize two main subenvironments within this deep-sea setting to account for the differences between the ichnofacies. Ichnofacies 1 and 2 are interpreted to represent a typical Nereites ichnofacies located on a level basin floor subenvironment of relatively low energy conditions at the distal end of a submarine fan deposit. In comparison, Ichnofacies 3 is dominated by Planolites with rare other facies-crossing trace fossil forms, and lacks Nereites. It is, therefore, best interpreted as representing a relatively high-energy environment, possibly a distributary channel near the distal end of the submarine fan system.     

Pliocene sand injectites from a submarine lobe fringe during hydrocarbon migration and salt diapirism: a seismic example from the Lower Congo Basin

Large‐scale conical and saucer‐shaped sand injectites have been identified in the Upper Miocene sediments of the Lower Congo Basin. These structures are evidenced on the 3D high‐resolution seismic data at about 600 ms TWT (two‐way traveltime) beneath the seabed. The conical and saucer‐shaped anomalies range from 20 to 80 m in height, 50 to 300 m in diameter, and 10 to 20 ms TWT in thickness. They are located within a sedimentary interval of about 100 m in thickness and are aligned over 20 km in dip direction (NE‐SW), above the NW margin of an underlying Upper Miocene submarine fan. We have interpreted the conical and saucer‐shaped anomalies as upward‐emplaced sand injectites sourced from the Upper Miocene fan because of their discordant character, the postsedimentary uplifting of the sediments overlying the cones and saucer‐shaped bodies, the alignment with the lateral fringe of the Upper Miocene submarine fan, and the geological context. Sand injection dates from the Miocene–Pliocene transition (approximately 5.3 Ma). The prerequisite overpressure to the sand injection process may be due to the buoyancy effect of hydrocarbons accumulated in the margins of the fan. Additionally, overpressure could have been enhanced by the lateral transfer of fluids operating in the inclined margins of the lobe. The short duration of sand injection and the presence of many sandstone intrusions suggested that the process of injection was triggered by an event, likely due to a nearby fault displacement related to diapiric movements. This is the first time that sand injectites of seismic scale have been described from the Lower Congo Basin. The localized nature of these injectites has led to a change in the migration path of fluids through the sedimentary cover. Consequently, the sand intrusions are both evidence and vectors of fluid migration within the basin fill.