Abstract Calcic pedocomplexes in the Siluro-Devonian Old Red Sandstone (ORS) of the Anglo-Welsh Basin (UK) have traditionally been interpreted as paleosols developed in dryland depositional environments. Their recognition has been used to indicate a range of controls, including climate, landscape stability, sedimentation rate, soil residence time, and proximity to alluvial channels (the pedofacies concept). A study of the Devonian Ridgeway Conglomerate Formation (RCF) in Pembrokeshire, southwest Wales, has, however challenged some of these notions, recognizing that many calcretes were not developed in soil horizons. The RCF was deposited as part of a dryland alluvial fan and axial fluvial valley complex. Regionally, structural blocks and basins were defined by a series of extensional faults, with the RCF being deposited in a half-graben as a hanging-wall alluvial fan. The RCF is heterolithic, comprising conglomerates, sandstones, and mudstones that reflect differences in processes, suggesting sheetfloods, low-relief lateral accretion, and cohesive debris flows across the alluvial fan. Pedogenic calcretes are common in all areas of the fan and axial fluvial zone. In mudstone and sandstone-grade lithofacies they comprise common horizonated nodules and subhorizontal crystallaria sheets in association with pedogenic indicators such as drab haloes, desiccation cracks, and ped textures. Also observed are both horizontal and vertical root traces, some of which have been the focus of micrite nodule growth (rhizogenic calcretes). Wedge-shaped peds are absent. Pedogenic profiles display upward-increasing percentages of nodules, and may be capped by blocky, massive calcrete and laminated micrite that developed in small ponded areas. In gravel-grade lithofacies, the pedogenic expression is different, and comprises carbonate-coated clasts with pendant and pore-occluding calcrete fabrics. Pedogenic calcretes are best developed in proximal areas of the fan, possibly on terraces adjacent to fan-channel entrenchment zones (the pedofacies concept). Proximal fan areas may also have had increased soil residence times due to reduced sedimentation rates compared to distal fan and axial fluvial valley zones. In distal fan and axial alluvial zones, thin layerbound micritic groundwater calcretes are common, typically being sharp based with upper surfaces comprising vertical and cylindrical nodules that possibly developed in the capillary-fringe zone. Inclined heterolithic bedsets, the deposits of laterally accreted ephemeral channels also commonly contain layer-bound micritic calcretes, again interpreted as having a groundwater origin. Lake-margin calcretes comprising centimeter-thick, laminated micrite, represent possible calcretized matgrounds in fan-toe, ephemeral ponds. The identification of common non-pedogenic calcretes in the RCF begs the question: how much of the ORS calcretes are similarly non-pedogenic in nature? Our analysis may act as a cautionary check for subsurface work where carbonate horizons in alluvial suites are being modelled solely in accordance with the pedofacies concept.

Abstract The stable isotopic composition of pedogenic carbonate forms in equilibrium with environmental parameters and, thus, records palaeoenvironmental signals. The aims of this study are to synthesize available data on the stable isotopic composition of Quaternary pedogenic carbonates in calcareous parent materials of Iran and to decipher paleoenvironmental implications of the isotopic data for the country. Isotopic composition and microfabric of pedogenic carbonates in 18 pedons in both gravelly (calcareous alluvium in central Iran) and non-gravelly deposits (calcareous loess in northeastern Iran) have been investigated. The results indicate that in limestone-derived soils of central Iran in situ weathering of calcareous pebbles is a major source of Ca for genesis of the carbonates, and carbonate features consist of micritic calcite crystals. In the loessic soils of northeastern Iran, pedogenic carbonates show a dominance of nodule morphology and are classified as orthic nodules. Microfabric analysis reveals that most of the carbonates have not been altered by diagenetic processes, especially the Holocene carbonates, and are suitable for isotopic study and palaeoreconstructions. In limestone-derived soils within the arid region of central Iran, the δ 18 O and the δ 13 C values of carbonates indicate their enrichment due to the effects of evaporative water loss, a decline in plant density and the entrance of atmospheric CO 2 into the soils. In semi-arid ecosystems of central and northeastern Iran, most of the Holocene carbonates have formed in equilibrium with the ambient environment and are suitable for palaeoenvironmental reconstructions. The combination of carbon and oxygen isotopic data demonstrates the dominant role of climate in determining the δ 13 C values of carbonates. There is a strong relationship between the δ 13 C values of carbonates and rainfall, and between O isotopes and aridity indices. Stable isotope patterns in Holocene soils appear to provide data for models that can then be used to interpret the many localities where Pleistocene-aged soils and associated carbonate exist.

Short-term sediment accumulation directly on basalt flows without the damming of drainage is not well studied. Can lakes form on basalt surfaces during times of volcanic quiescence between outpourings? Large faulted downthrown blocks of the lower Jurassic Kalkrand basalt within the Hardap Recreational Resort in central Namibia provided depressions for sediment accumulation during two periods of volcanic quiescence. The sedimentary infill was preserved as two sedimentary interlayers from 6 to 8 m in thickness between lava flow units. These sedimentary rocks, formerly interpreted as dominantly lacustrine deposits, now can be recognized as alluvial fill because of the presence of pedogenic mud aggregates. Sedimentation processes filling these fault-bound depressions included sheetflooding and grain flow, archived in successions containing sandstone, siltstone, and mudrock. Large cracks and fractures on the basalt surfaces, filled with quartz sandstone, showed that the regional water table was below the basalt flow surface, making the formation of lakes impossible, especially because of the permeability and porosity of such flows in general. No pattern or cyclicity during sediment deposition was found and paleocurrent measurements showed random sedimentary influx directions, probably related to the lava field surface topography. Autochthonous weathering of basalts added to the allochthonous drainage input of quartz-rich sand and silt that produced sandstone and siltstone containing smectitic clays. Most of these clay-rich siltstones and sandstones contained mud laminae within traction-load sedimentary structures, providing evidence for the presence of pedogenic mud aggregates within these sheetflood deposits. Geothermal groundwaters circulating through the basalt and the sediments, mixing with meteoric waters, produced Ca-rich diagenetic waters with an elevated temperature to precipitate calcite cement and dendritic crystals. This work demonstrates that characteristics of mud deposition can provide important clues in determining the true depositional paleoenvironment of these Jurassic sedimentary interlayers between basalt flows.