Early Jurassic palaeoenvironments in the Surat Basin, Australia - marine incursion into eastern Gondwana

Interpretations of palaeodepositional environments are important for reconstructing Earth history. Only a few maps showing the Jurassic depositional environments in eastern Australia currently exist. Consequently, a detailed understanding of the setting of Australia in Gondwana is lacking. Core, wireline logs, two‐dimensional and three‐dimensional seismic from the Precipice Sandstone and Evergreen Formation in the Surat Basin have been used to construct maps showing the evolution of depositional environments through the Early Jurassic. The results indicate the succession consists of three third‐order sequences (Sequence 1 to Sequence 3) that were controlled by eustatic sea level. The lowstand systems tract in Sequence 1 comprises braidplain deposits, confined to a fairway that parallels the basin centre. The strata were initially deposited in two sub‐basins, with rivers flowing in different orientations in each sub‐basin. The transgressive systems tract of Sequence 1 to lowstand systems tract of Sequence 3 is dominated by fluvio–deltaic systems infilling a single merged basin centre. Finally, the transgressive and highstand systems tracts of Sequence 3 show nearshore environments depositing sediment into a shallow marine basin. In the youngest part of this interval, ironstone shoals are the most conspicuous facies, the thickness and number of which increase towards the north and east. This study interprets a corridor to the open ocean through the Clarence–Moreton Basin, or the Carpentaria and Papuan basins, evidence of which has been eroded. These results challenge a commonly held view that eastern Australia was not influenced by eustasy, and propose a more dynamic palaeogeographic setting comprising a mixture of fluvial, deltaic and shallow marine sedimentary environments. This work can be used to unravel the stratigraphic relationships between Mesozoic eastern Australian basins, or in other basins globally as an analogue for understanding the complex interplay of paralic depositional systems in data poor areas

Surface morphology of quartz grains from tropical soils and its significance for assessing soil weathering

The surface morphology of quartz grains can indicate the degree of weathering of soil material. We have compared two methods of assessing the relative weathering of soils on the basis of differences in the surface morphology of quartz sand grains in a catena of soils in Rwanda. One method is based on the presence or absence of surface features indicative of weathering or freshness, while the other uses the frequency and size of dissolution etch pits. A ranking of relative weathering could be obtained using the first technique for the slightly and somewhat weathered soils but not for the weathered soils. On the other hand, weathering trends and differences between the horizons studied were detected in weathered soils using the second method. The introduction of more specific definitions of the weathering classes used in the latter method leads to clear improvement of the inter-observer reproducibility of the weathering classification. The surface features on the quartz grains suggest that the soil at the summit is less weathered than the other soils of the sequence. Quartz grains from the well-drained soils on the slopes, which are subjected to more intense leaching and thus to stronger chemical weathering, have more triangular etch pits and chatter marks. In the imperfectly drained soils in the valley bottom quartz grains are less etched because dissolution is inhibited by the oversaturation in silica of the drainage waters