ALBERT

Notes: Thick dolomite-cemented horizons (dolocretes) occur within a fluvial sandstone-mudstone sequence of Late Triassic age in the western part of the Paris Basin, France. Two types of dolomites can be distinguished: (a) nodular dolomitic beds less than a few metres thick, which formed within mottled overbank siltstones and mudstones; and (b) massive dolomite up to 16 m thick, which occurs in coarse grained channel sandstones and conglomerates.The majority of the dolomite consists of a finely crystalline groundmass of dolomicrospar and, less commonly, dolomicrite. Glaebules, irregular spar-filled cracks, spheroidal dolomite, silicification and vuggy porosity are locally abundant in the massive dolomite. In contrast, biologically induced micromorphological features such as rhizocretions and alveolar-septal fabrics were observed in the thin, nodular dolomite beds.The dolomite is near stoichiometric, well ordered and non-ferroan. 18O values range from −7·7 to −0·4%o PDB and 18O values range from −5·1 to + 1·8%0 PDB and no obvious difference in the stable isotopic composition between both types of dolomites was observed. Sr isotope ratios range from 0·7101 to 0·7126 and are invariably higher than the contemporary Triassic sea water.A vadose—pedogenic origin for the thin dolocrete layers is indicated by the occurrence of rhizocretions and other biological structures. Several features, however, argue against a pedogenic origin for the massive carbonates, most notably the absence of biologically induced structures, the occurrence in coarse grained channel (and not overbank) deposits, and the great thickness. These units are thus interpreted as groundwater in origin. Phreatic calcretes of Quaternary age, widespread in inland Australia, are regarded as a modern analogue for the Triassic Paris Basin dolocretes.Petrographic observations argue in favour of primary (proto)dolomite precipitation, although early diagenetic replacement of calcite by (proto)dolomite cannot be ruled out. Strontium and carbon isotope data of early diagenetic dolocrete cements and oxygen isotope data of early diagenetic silica indicate an entirely non-marine, continental origin for the groundwaters. The poorly ordered and non-stoichiometric protodolomite probably underwent stabilization upon further burial resulting in a near-stoichiometric, well ordered dolomite that clearly lacks evidence for pervasive recrystallization.

Notes: Glacial gravels of Late Devensian Dimlington Stadial age (26 000–13 000 years BP) at West Tanfield, North Yorkshire, England, have been cemented by carbonate-rich solutions to produce a strongly indurated calcrete horizon. The low-Mg cements occur as drusy spar, needle fibres, alveolar septal structures, micrite and micropinnacles, indicative of vadose-zone cementation. Some complex pore partition structures attributed to precipitation along meniscus films also occur. These partitions separate air-dominated and water-dominated microenvironments of the vadose zone. The abundance of vadose fabrics shows that the accumulation is not a groundwater calcrete. In addition, much of the carbonate appears to have been precipitated by biological mediation.Carbon and oxygen isotopic data suggest that the carbonate did not form as a result of freezing, as has been suggested for some ‘arctic’soil carbonates. The pollen history of the area since the Devensian suggests that this calcrete precipitated at low temperatures; this contrasts with widely reported occurrences of calcrete in soils of hot arid or semi-arid regions, and suggests that palaeo-calcretes should not be used as absolute palaeoclimatic indicators.The unusual occurrence, albeit localized, of a thick calcrete under a cool and wet climate probably reflects the well-drained nature of the gravels, the abundance of CaCO3 as limestone clasts in the gravel and a high degree of biological activity beneath a forest cover, which created a local environment favouring carbonate precipitation.

Notes: The Miocene alluvial-lacustrine sequences of the Madrid Basin, Spain, formed in highly varied landscapes. The presence of various types of palaeosols allows assessment of the effects of local and external factors on sedimentation, pedogenesis and geomorphological development. In the northern, more arid, tectonically active area, soils were weakly developed in aggrading alluvial fans, dominated by mass flows, reflecting high sedimentation rates. In more distal parts of the fans and in playa lakes calcretes and dolocretes developed; the former were associated with Mg-poor fan sediments while the latter formed on Mg-rich lake clays exposed during minor lake lowstands.The north-east part of the basin had a less arid climate. Alluvial fans in this area were dominated by stream flood deposits, sourced by carbonate terrains. Floodplain and freshwater lake deposits formed in distal areas. The high local supply of calcium carbonate may have contributed to the preferential development of calcretes on the fans. Both the fan and floodplain palaeosols exhibit pedofacies relationships and more mature soils developed in settings more distant from the sediment sources. Palaeosols also developed on pond and lake margin carbonates, and led to the formation of palustrine limestones.The spatial distributions and stratigraphies of palaeosols in the Madrid Basin alluvial fans suggest that soil formation was controlled by local factors. These palaeosols differ from those seen in Quaternary fans, which are characterized by climatically induced periods of stability and instability.

Notes: Laminar calcretes are described from the Lower Carboniferous of South Wales, the Upper Jurassic of southern England and the Upper Jurassic-Lower Cretaceous of northern Spain. They are interpreted as calcified root-mats (horizontal root systems) and are compared with other examples in the geological record and with possible modern analogues. All three occurrences consist of virtually identical, centimetre to decimetre-thick, locally organic carbon-rich, laminar micrites containing up to 50% by volume of millimetre-sized typically calcite-filled, tubular fenestrae set in an irregular but very finely laminated matrix. It is suggested that root-mat calcretes are probably very common in the geological record in peritidal, lacustrine margin and floodplain deposits, but owing to their crudely biogenic microstructure, they more closely resemble cryptalgal laminites than do other laminar calcretes. The recognition of such root-mat calcretes in sedimentary sequences not only provides evidence of subaerial exposure and vegetation cover but can also indicate positions of palaeo-water-tables in certain circumstances.

Notes: Carbonate cementation in the Triassic Otter Sandstone of Budleigh Salterton, SW England, occurs in two distinct forms: large, vertical, concentrically zoned cyclinders, and thin subhorizontal sheets. The former represent rhizocretions of the tap roots of phreatophytic plants which colonized bars and abandoned channels on a large braidplain. The sheets represent cementation around ancient water-tables. The precipitation of the rhizocretions took place, at least in part, during the life of the plants and δ13C and δ18O values support the view that evapo-transpiration induced carbonate precipitation. Palaeosol profiles are rare in the Otter Sandstone, reflecting the geomorphological instability of the braidplain surfaces. In contrast, rhizocretionary and sheet calcretes, which formed several metres below the active depositional surface, are abundant because they had a high preservation potential.