Palaeosols in the upper Narrabeen group of New South Wales as evidence of Early Triassic palaeoenvironments without exact modern analogues*

Abstract

Early Triassic palaeosols in the sea cliffs north of Sydney are evidence of a cool‐temperate, seasonal, humid palaeoclimate. As is typical in such climates, the palaeosols include Ultisols, Inceptisols and Entisols. These ancient soils supported a variety of broadleaf, deciduous and needle‐leaf forests, as well as oligotrophic heaths, coastal marshes and vegetation early in ecological succession to colonise disturbed ground. Their fauna of earthworms, crayfish, insects, amphibians and reptiles is now known from a variety of trace fossils. Although many of these palaeosols show evidence of waterlogging and include ganisters, sphaerosiderite and siderite nodules typical of coal measures palaeosols, no coal has been found in this sequence. Other palaeosols are strongly oxidised and were well‐drained, and yet were copiously rooted and presumably densely forested. Thus, upland environments were well vegetated by Early Triassic time, if not earlier. The parent material of these ancient soils included fertile volcanic sands and oligotrophic quartz sands. Many of the palaeosols represent times for formation of only hundreds to thousands of years but some formed over tens to hundreds of thousands of years and represent times of very slow sediment accumulation. Several aspects of Early Triassic palaeosols can now be seen as peculiar compared with soils of today. None of the palaeosols are thought to have been Spodosols now common in humid‐temperate regions, perhaps because strongly podsolising plants had not yet evolved. A global lack of coal, even in humid wetlands represented by these palaeosols, is an outstanding anomaly of the Early Triassic that has been called the ‘coal gap’. The profound chemical weathering of some of these palaeosols is comparable to that of mid‐latitude soils (24–38°), and is anomalous compared with soils now at the high latitudes (65–70°) postulated for the Sydney Basin during the Early Triassic. The coal gap and anomalous polar warmth may be legacies of the Permian‐Triassic life crisis and ensuing CO2 greenhouse.