干旱区生态环境知识资源中心

Within the siliciclastic dominated red-bed strata of the Early Devonian Wood Bay Formation, various types of calcareous deposits, characterised by specific sedimentary structures, lithologies, microfacies, carbonate contents and stable isotope signatures, occur at different stratigraphic levels. Pedogenic, nodular calcretes with a thickness from 40 to 150 centimetres, associated with alluvial-plain and overbank sediments, represent palaeosol (calcisols, aridisols) development. The massive nodules show an internal mudstone fabric, calcium carbonate concentrations from 64 to 87 % and carbon and oxygen signatures of -5.3 to -6.4 and -8.5 to -9.9 parts per thousand, respectively. It is likely that they reflect a bio-induced carbonate precipitation in an oxygenated near-surface setting, connected with the evaporation of ground water. Single, up to some decimetres thick beds of calcareous mudstones and marls are characterised by a gradual colour change from reddish-brown to grey-violet upwards, and a meshwork of desiccation cracks at the top. The sediments show calcium carbonate concentrations from 22 to 43 % and carbon and oxygen isotope values ranging from -0.6 to -0.7 parts per thousand and -7.6 to -8.8 parts per thousand, respectively. They are interpreted as deposits of relatively. small, ephemeral lakes, formed by inorganic carbonate precipitation due to the evaporation of surface water. Individual beds and regressive, shallowing-upward sequences, up to a few metres thick, formed of calcareous mudstones to sandstones, marls to calcarenites and argillaceous to sandy limestones represent the depositional products of perennial, low-relief, low-energy lakes. These sediments yield carbonate concentrations from 20 to 81 % and a stable isotope composition of -2.1 to -3.1 parts per thousand for delta(13)C and -9.5 to -10.9 parts per thousand for delta(18)O. They reflect deposition in basinal and littoral lacustrine and palustrine sub-environments, in which carbonate was formed due to inorganic, bio-induced and biotic precipitation.

The different types of calcareous deposits formed under and to semi-arid climatic conditions in various sub-environments of alluvial plains and perennial lakes. Their occurrence at different stratigraphic levels within the Wood Bay Formation can be related to a gradually increasing geomorphologic maturity of the terrestrial basin. The latter was marked by a decrease of the topographic relief and a lowering of sedimentation rates. An increase in landscape stability combined with a general water-level rise led to the recurrent formation of broad lake plains in the centre of the basin.