Multiple calcrete profiles in the Tabernas Basin,southeast Spain: their origins and geomorphic implications

The paper describes a sequence of Pliocene(? ) to Quaternary age calcretes developed within alluvial fan and fluvial gravels in the Tabernas Basin, Almería Province, southeast Spain. Calcrete profiles are described from sites adjacent to major tributaries of the Rambla de Tabernas. Six distinct calcrete units are identified within the basin. These have variable distributions but have developed in an identifiable evolutionary sequence. Two pairs of calcrete units are widely present across the basin preserving two former land surfaces. Each of the former land surfaces has been planated and subsequently buried by alluvial fan or fluvial gravels. A massive calcrete unit is present at the base of each gravel sequence, immediately in contact with the underlying bedrock, with a less well developed calcrete unit situated at the top of the gravel sequence. The lowest two calcrete units within the basin are more spatially restricted and are confined to the floors and flanks of incised drainage lines. The geochemistry, macro- and micromorphological properties and geomorphological positions of the calcrete units are outlined and, on the basis of this information, their mode of origin identified. Two main modes of calcrete genesis appear to be present: massive calcretes situated in direct contact with bedrock are suggested to have formed by groundwater processes, whilst calcretes situated at the top of gravel sequences are likely to have developed by pedogenic processes. Calcrete genesis is subsequently considered in the context of the reconstruction of the early phases of landscape development, and is suggested to have been controlled by phases of uplift and stability within the Tabernas Basin. © 1998 John Wiley & Sons, Ltd.     

Towards understanding the behaviour of naturally-occurring road construction materials

Summary Naturally-occurring road construction materials generally contain a greater amount of fines, and the fines have a higher plasticity than traditional materials such as a crushed rock aggregates. This makes their behaviour more difficult to understand and predict since soil suction and fabric become important controlling factors. The concepts of suction and fabric are outlined, and their role in controlling the behaviour of road construction materials is illustrated by a fundamental laboratory study of a lateritic gravel and two calcrete gravels. These materials have successfully been used as road base construction materials in low-volume bituminous-surfaced roads in Kenya and Botswana. It is concluded that the presence of fines can be an advantage, since they allow significant suctions to develop and also reduce the permeability.     

Origins of strandline duricrusts around the Makgadikgadi Pans (Botswana Kalahari) as deduced from their chemical and isotope composition

Trace elements together with some O and C isotope analysis were undertaken on duricrust strandline deposits in the palaeo-Makgadikgadi sub-basin (PMSB) to provide insight into palaeo-climatic conditions through the interpretation of calcrete, silcrete–calcrete intergrade and silcrete deposits. Trace element content and relative abundance suggest that the duricrust origins are associated with the long-term weathering of the Karoo Large Igneous Province which underlies the PMSB. This work shows that duricrust origins are related to Ca²⁺ and Si (and associated trace elements) being transported mainly through the groundwater and then subsequently precipitated at different strandline elevations over time. Local groundwater feeding in towards the pan margin and accumulating in near-neutral pan-marginal pools, appears to facilitate Si concentration and permeation of pre-existing calcretes. The silica precipitates as the pH drops when renewed freshwater enters the pools. Hence the inferred palaeo-climatic regime for silcretisation may be similar to that occurring in Botswana at present being dry semi-arid with low seasonal rainfall. In contrast the extensive calcrete precipitation in the strandlines results from abundant Ca²⁺ in adjacent waters which appear to be derived from both local and regional sources. The arrival of Ca²⁺ from regional sources (shown by trace element evidence) infers heavy rainfall in the upper catchment suggesting a major humid event followed by regional drying. Palaeo-climatic inferences suggest the juxtaposition of major humid events interspersed with more normal semi-arid palaeo-climates with an exception obtained from isotope data, of drier and cooler conditions than usual for the region around 80–90 000 years ago. Whereas trace element data can greatly assist in the interpretation of complex deposits such as duricrusts, care should be taken over the use of particular ratios (such as Yb/Gd ratio) which may produce spurious results.     

Morphometry of microstromatolites in calcrete laminar crusts and a fractal model of their growth

The laminar crust, constituting the upper part of calcretes (terrestrial CaCO₃ accumulations inside surficial sediments), is a succession of thin layers of various colors and shapes resembling micro-stromatolites. The crust structure and its diagenetic evolution are similar to stromatolites. A quantitative study of its structure was made using image analysis. Euclidian parameters were calculated to describe lamina shape. Eight hundred and eighty-six laminae were divided into six classes from the flatest forms to columnar shapes. The geometrical relationships between the shapes are interpreted as steps in the growth process of the microstromatolite. A fractal model of laminar crust growth was developed, using the diffusion-limited aggregation model (DLA) and dilation (an operation of mathematical morphology). This model simulates all growth steps observed in thin section and emphasizes the necessity of an interface with the atmosphere to explain the variety of shapes. This growth model supports the theory of a surficial and biogenic origin for certain calcrete laminar crusts.     

'Pseudobreccias' revealed as calcrete mottling and bioturbation in the Late Dinantian of the southern Lake District, UK

Two types of ‘pseudobreccia’, one with grey and the other with brown mottle fabrics, occur in shoaling‐upward cycles of the Urswick Limestone Formation of Asbian (Late Dinantian, Carboniferous) age in the southern Lake District, UK. The grey mottle pseudobreccia occurs in cycle‐base packstones and developed after backfilling and abandonment of Thalassinoides burrow systems. Burrow infills consist of a fine to coarse crystalline microspar that has dull brown to moderate orange colours under cathodoluminescence. Mottling formed when an early diagenetic ‘aerobic decay clock’ operating on buried organic material was stopped, and sediment entered the sulphate reduction zone. This probably occurred during progradation of grainstone shoal facies, after which there was initial exposure to meteoric water. Microspar calcites then formed rapidly as a result of aragonite stabilization. The precipitation of the main meteoric cements and aragonite bioclast dissolution post‐date this stabilisation event. The brown mottle pseudobreccia fabrics are intimately associated with rhizocretions and calcrete, which developed beneath palaeokarstic surfaces capping cycle‐top grainstones and post‐date all depositional fabrics, although they may also follow primary depositional heterogeneities such as burrows. They consist of coarse, inclusion‐rich, microspar calcites that are always very dull to non‐luminescent under cathodoluminescence, sometimes with some thin bright zones. These are interpreted as capillary rise and pedogenic calcrete precipitates. The δ¹⁸O values (?5‰ to ?8‰, PDB) and the δ¹³C values (+2‰ to ?3‰, PDB) of the ‘pseudobreccias’ are lower than the estimated δ¹⁸O values (?3‰ to ?1‰ PDB) and δ¹³C values of (+2‰ to +4‰ PDB) of normal marine calcite precipitated from Late Dinantian sea water, reflecting the influence of meteoric waters and the input of organic carbon.