Triassic palaeosols in the upper Narrabeen group of new South Wales. part I: Features of the palaeosols

Abstract

The sea‐cliffs north of Sydney expose a complex of Triassic palaeosols, pedoliths, and sedimentary rocks.

The most obvious and diagnostic features of the palaeosols are fossil roots in place and markedly leached or reddened, relatively massive, clay‐rich strata. Associated coaly layers and fossil plant remains in adjacent sediments show varying degrees of decomposition. The A horizons of some of these palaeosols have been silicified by plant opal and contain abundant insect, earthworm, and larger animal burrows, cradle knolls, and basket podzols. Many of the palaeosols have well‐preserved peds and their upper horizons slake more readily in water than their lower horizons. Their B horizons may consist of extensive layers of siderite nodules or red claystone with tubular grey mottles around old root channels.

Less mature palaeosols show some relict sedimentary bedding and ripple marks within their profiles. More mature palaeosols, which appear massive in the field, may show anomalous grainsize variation in thin section, remaining from sedimentary bedding.

Soils can be eroded and deposited as pedoliths. Conglomerates of palaeosol clay pebbles and siderite nodules are easy to recognize as pedoliths. Finer‐grained pedoliths have the distinctive mineralogy and some of the small structures, but not the larger structures and field relations, of palaeosols.

Siderite crystals and nodules developed in the gleyed organic and A horizons of some of the clayey palaeosols shortly after they were covered by sediment and subsided below base level. With further compaction and dewatering, the ferric‐oxide minerals became redder by inverting to hematite, mineralized joints developed in some massive B horizons, some peds were accentuated by slickensides, and some root channels and coal cleat were filled with copper minerals.