Subunit IIIA consists of clayey nannofossil chalk to nannofossil-bearing organic clayey siltstone. Subunit IIIB consists of organic-bearing silty claystone and organic clayey siltstone. These two units are dark gray to black. Dinoflagellates are rare but spores and pollen are abundant, and the abundant dispersed organic matter is dominantly from land plants. Geochemical and micropaleontological evidence suggests a presence of brackish conditions in part, with normal marine salinities at other times.
Calcareous microfossils are sporadic, rarer downward, and moderately to poorly preserved.
Agglutinating benthic foraminifers are sporadic. The environment of deposition was reducing, shelfal to bathyal marine, and protected from currents and waves. Evidence from palynology suggests a subtropical to temperate climate, with a terrestrial plant assemblage containing abundant ferns. Calcareous nannofossils are represented by a warm-water assemblage containing warmer water elements than those previously found elsewhere at equivalent latitudes in the Southern Ocean. Plate reconstructions suggest that the Kerguelen Plateau may have been shielding the Australo-Antarctic Gulf from cold water from the west so that the only water entering the gulf came from warmer areas north of western Australia.