Weathering of a gypsum-calcareous mudstone under semi-arid environment at Tabernas, SE Spain: laboratory and field-based experimental approaches

The weathering of a Late Miocene gypsum-calcareous mudstone outcropping in large badland areas of SE Spain, under a semi-arid Mediterranean climate, w as studied by means of two experimental approaches. Field and laboratory ex periments were carried out to reproduce, though in accelerated form. some o f the weathering conditions of the consolidated mudstone. In the laboratory , three sequences of 5, 10 and 20 wetting-drying cycles were produced on un disturbed blocks of fresh mudstone samples. At the end of the three sequenc es, samples were analysed for their micromorphology, elemental and soluble salt chemistry, and total mineralogy. Unweathered dry samples, as blanks. a nd permanently wet samples were also analysed. In the field, two small plot s of freshly exposed mudstone were monitored over 3 years for their respons e to natural weathering in terms of morphological changes and sediment outp ut. The porosity was increased by a few wetting-drying cycles, as assessed by significant increases in water absorption capacity of the mudstone. A co mbination of three factors is responsible for mudstone weathering: repeated cycles of wetting-drying, the presence of geologically-induced cracks and fissures, and dissolution-crystallisation of relatively soluble minerals. g ypsum being the most abundant within this category. A few wetting-drying cy cles were sufficient to reveal ion migration (specially Na+ Ca++, Mg++, SO4 -, HCO- and Cl-) within the mudstone, explaining mineral dissolution. In th e field, surface weathering rates from 0.7 to 8 mm year(-1) were measured. Weathering rates were found to be proportional to the number of rainfall ev ents during the sampling periods, confirming what was found in laboratory c onditions, namely. that the number of wetting-drying cycles has the greates t influence on weathering. These weathering rates might he considered as th e probable range of incision rates under present semi-arid conditions. (C) 2001 Elsevier Science B.V. All rights reserved.