Calcrete Development Research Articles

Development of calcrete of the Late Cretaceous carbonate platform, northern Jordan, and its tectonic implications

A calcrete horizon, 3–10 m thick, is found exposed in northern Jordan in three localities, Rumman, Marsa’, and Wasfi At Tal forest (WTF). It is situated at the base of the dominantly limestone, Late Cenomanian Hummar Formation, separating it from the underlying marly Fuheis Formation. The calcrete horizon exhibits all the diagnostic features characteristic of calcrete, such as diagenetic pisoids; a mottled or clotted texture; circumgranular cracks around grains in a nodular texture; pendant or microstalactitic cement; meniscus cement; plant roots or rhizogenic structures, both longitudinal and transverse; exfoliation on boulder and cobbles; alagally laminated hard pans; and abundant dissolution of cracks and vugs. Consequently, it represents a subaerial erosional unconformity and type 1 sequence boundary (SB) that was not previously noticed. The Hummar Formation is considered as a third-order sequence with the SB at the base of the calcrete horizon, while the calcrete horizon itself forms the lowstand system tracts (LST). The transgressive systems tracts (TST) involve the lower two third of the Hummar Formation ending with peloidal grainstone representing the maximum flooding surface (MFSs). The rest of the formation is the falling highstand systems tracts (HST). The next SB is not seen due to a thick soil-covered gap. The calcrete horizon formed due to the formation of a paleohigh, in the study area, associated with the compression produced by the initial subduction of the Afro-Arabian Plate under the Eurasian Plate during the Late Cenomanian, slightly earlier than the previously known Turonian subduction and compression.

Late Pleistocene calcretes from Central Anatolia (Lakes Eymir and Mogan, Gölbaşı Basin): Comparison to quaternary calcretes from Turkey

This study is the first to report the radiocarbon ages, mineralogical, morphological, geochemical and stable isotope compositions of calcretes from Neogene sediments around lakes Eymir and Mogan in the Golbasi Basin of Central Anatolia. Morphologically different forms of calcretes in the Golbasi Basin include powdery, nodular, fracture infill, laminar and hardpan types. Calcite is the dominant mineral of calcrete compositions; the diagnostic features of dessication cracks, random fractures, MnO linings and dense sparitic infillings are observed. Chemical analyses show arid conditions with mean annual precipitation of <50 mm. δ13C compositions of the calcretes range from -6.77‰ to -9.32‰ PDB, typical for most pedogenic calcretes, reflecting the development under seasonally arid climates and C3-dominated vegetation cover. δ18O values are between -5.57‰ and - 7.80‰ PDB, indicating the formation from meteoric water in a vadose zone environment. The results suggest that the Middle Pleistocene was arider and warmer, favouring the formation of palygorskite in association with the different forms of calcrete occurrences, whereas the Late Pleistocene was dryer and cooler, supporting the development of calcretes.

Early Syn-Rift Evolution In the West Cameros Basin (Upper Jurassic, NW Iberian Range), Spain

Abstract Alluvial systems are sensitive sedimentary systems that can preserve useful information about tectonics, drainage area (lithology and size), and climate. However, research on this subject over the last three decades has made it evident that the interaction of these three factors and their control over the alluvial architecture and facies are complex. The stratigraphic and sedimentological analysis of the earliest syn-rift sedimentary record (Tithonian) in the West Cameros Basin has confirmed that sedimentation during the beginning of extension in this area took place in a series of halfgraben basins in which three different types of fan-shaped alluvial systems were deposited: two relatively large systems, specifically, a “highly channelized alluvial fan system” and a “distributive fluvial fan system changing downstream to an axial tributary fluvial system,” were the exception; and small “poorly channelized alluvial fan systems” were the norm. A semiarid climate is inferred during this earliest syn-rift sedimentary record from the steady and extensive calcrete development. This semiarid climate should play an important role both in how coarser sediment discharges to the rift halfgraben basins occurred and in the dominant transverse organization of the alluvial depositional systems. However, the climate and source-area lithology can be considered steady factors, so the main factors considered as the cause of these sedimentary and architectural heterogeneities are the tectonics and drainage-area sizes, and specifically, the different behavior of each extensional structure in terms of the rate of tectonic uplift and subsidence. This indicates how fault activity and drainage-area sizes during the earliest stages of a rift system can generate high degrees of heterogeneity in the architecture of alluvial basins and how this heterogeneity is recorded by the alluvial systems in the form of differences in their facies association and architecture. Therefore, this work concludes that both tectonics and catchment sizes exerted primary control on the sedimentary characteristics of associated alluvial systems in basins generated from this type of extensional setting, and care must be taken when making climatic and tectonic interpretations using the sedimentary record of these alluvial systems because many changes that traditionally have been attributed to climate variability can be explained solely by the differential behavior of each extensional structure and/or changes in its associated drainage area.

Coupled micromorphological and stable isotope analysis of Quaternary calcrete development

Pedogenic calcretes are widespread in arid and semi-arid regions. Using calcrete profiles from four river terraces of the Rio Alias in southeast Spain, this study explores the potential of using detailed micromorphological and stable isotopic analysis to more fully understand the impacts of Quaternary environmental change on calcrete development. The four profiles increase in carbonate complexity with progressive age, reflecting calcretisation over multiple glacial–interglacial cycles since MIS 9 (c. 300ka). Calcrete profiles contain a mixture of Alpha (non-biogenic) and Beta (biogenic) microfabrics. Alpha fabrics have higher δ13C and δ18O values. The profiles contain a range of crystal textures, but there is little difference between the δ13C and δ18O values of spar, microspar, and micrite cements. Strong positive covariance between δ13C and δ18O suggests that both isotopes are responding to the same environmental parameter, which is inferred to be relative aridity. The study reveals that the detailed co-analysis of calcrete micromorphology and stable isotope signatures can allow patterns of calcrete formation to be placed into a wider palaeoclimatic context. This demonstrates the potential of this technique to more reliably constrain the palaeoenvironmental significance of secondary carbonates in dryland settings where other proxy records may be poorly preserved.

Deciphering the history of hydrologic and climatic changes on carbonate lowstand surfaces: calcrete and organic-matter/evaporite facies association on a palimpsest Middle Jurassic landscape from Portugal

The unusual occurrence of calcretes and prominent organic matter in the Middle Jurassic (Lower Bathonian, Serra de Aire Formation) of the Lusitanian Basin of western Portugal (Western Iberian Margin) revealed a complex palimpsest exposure record, here interpreted as reflecting hydrological changes caused by phases of emergence and immersion. It serves as a potential model for understanding stratigraphic development at lowstand surfaces in carbonate successions. The exposure-dominated facies association grades upwards into peritidal and lagoonal limestones, and the interval is assigned to the regressive peak of a Transgressive-Regressive Facies Cycle (2nd order) of the thick Middle Jurassic carbonate ramp succession.The Galinha Quarry, Fátima region, NE of Lisbon, a type section for this lowstand assemblage, exhibits varied calcretes, with black-clasts, interbedded with, and grading into: organic-rich marly/clayey seams and lenses, locally with carbonate nodules; carbonates with evaporite traces; microbial laminites; black-clast and fenestral limestones; some lithofacies are dolomitized. The palynofacies contains phytoclasts associated with less refractory, more prone to degradation components, which suggests natural combustion/pyrolysis (wild fires). The lowstand surface represents a low relief landscape with small depressions/ponds bordering a more distal marginal-littoral setting; the partly subaerial and partly subaqueous settings were subjected to lengthy exposure and to fluctuating, very shallow water bodies and water table. Coeval climatic regime was a seasonally dry/wet one, with dry/semi-arid phases dominating over the sub-humid, as shown by the combined record of intense calcrete development, rhizogenic structures, microbial mats, brecciation, desiccation, evaporites and wild fire evidence. However, sea level rise caused changes to shallow, sea-water influenced restricted lagoonal-peritidal settings.Comparisons and differences with modern and ancient coast marginal carbonates are made to provide a guide to the variability in such lowstand deposystems in the stratigraphic record.

Aminostratigraphy and sea-level history of the Pleistocene Bridgewater Formation, Mount Gambier region, southern Australia

A geochronological framework based on amino acid racemisation (AAR) and constrained by previously reported optically stimulated luminescence (OSL) ages is presented for the evolution and paleosea-level record of the Pleistocene Bridgewater Formation of the Mount Gambier region, of southern Australia. Within the study area, the Bridgewater Formation is represented by late early Pleistocene [Marine Isotope Stage (MIS) 23 at 933 ka] to Holocene barrier shoreline successions deposited during sea-level highstands. Regional monotonic uplift (0.13 mm yr–1) and pervasive calcrete development during the Pleistocene have preserved the sequence of calcarenite (mixed quartz-skeletal carbonate sand) shoreline complexes from denudation. AAR analyses confirm that the barriers generally increase in age landwards and correlate with sea-level highstands associated with interglacials as defined by the marine oxygen isotope record. AAR analyses on the benthic foraminifer Elphidium crispum have proved more reliable than the whole-rock method in extending the age range of AAR dating of these relict shoreline successions. Paleosea-levels from the coastal plain are as follows: MIS 7, –9 ± 2 m; MIS 9, 4 ± 1 m; and a minimum sea-level of 2 ± 2 m is derived for MIS 11. Paleosea-level could not be determined for MIS 15, 19 or 23 as diagnostic sea-level indicators were not identified within these sedimentary successions. Dismal Range, dated at 933 ± 145 ka (MIS 23), represents a correlative feature to the East Naracoorte Range but is some 25 km seaward of the Kanawinka Fault compared with the same barrier at Naracoorte. Mingbool Range (788 ± 18 ka) is of similar age to the West Naracoorte Range (MIS 19) and formed as an arcuate shoreline complex that became attached to the higher relief of the area represented by the Mount Burr Volcanic Province. The higher topographical relief resulted from crustal doming of the Oligo-Miocene Gambier Limestone caused by the intrusion of magma associated with the volcanic province. The AAR age of 788 ± 118 ka for Mingbool Range indicates that the Mount Burr volcanics predate the deposition of this shoreline complex.

Health and social factors associated with alternate routes of administration of prescription opioids among young adult multidrug users

The unusual occurrence of calcretes and prominent organic matter in the Middle Jurassic (Lower Bathonian, Serra de Aire Formation) of the Lusitanian Basin of western Portugal (Western Iberian Margin) revealed a complex palimpsest exposure record, here interpreted as reflecting hydrological changes caused by phases of emergence and immersion. It serves as a potential model for understanding stratigraphic development at lowstand surfaces in carbonate successions. The exposure-dominated facies association grades upwards into peritidal and lagoonal limestones, and the interval is assigned to the regressive peak of a Transgressive-Regressive Facies Cycle (2nd order) of the thick Middle Jurassic carbonate ramp succession.The Galinha Quarry, Fátima region, NE of Lisbon, a type section for this lowstand assemblage, exhibits varied calcretes, with black-clasts, interbedded with, and grading into: organic-rich marly/clayey seams and lenses, locally with carbonate nodules; carbonates with evaporite traces; microbial laminites; black-clast and fenestral limestones; some lithofacies are dolomitized. The palynofacies contains phytoclasts associated with less refractory, more prone to degradation components, which suggests natural combustion/pyrolysis (wild fires). The lowstand surface represents a low relief landscape with small depressions/ponds bordering a more distal marginal-littoral setting; the partly subaerial and partly subaqueous settings were subjected to lengthy exposure and to fluctuating, very shallow water bodies and water table. Coeval climatic regime was a seasonally dry/wet one, with dry/semi-arid phases dominating over the sub-humid, as shown by the combined record of intense calcrete development, rhizogenic structures, microbial mats, brecciation, desiccation, evaporites and wild fire evidence. However, sea level rise caused changes to shallow, sea-water influenced restricted lagoonal-peritidal settings.Comparisons and differences with modern and ancient coast marginal carbonates are made to provide a guide to the variability in such lowstand deposystems in the stratigraphic record.

Stable isotopic (δ13C and δ18O) signatures of biogenic calcretes marking discontinuity surfaces: a case study from Upper Cretaceous carbonates of central Dalmatia and eastern Istria, Croatia

Biogenic calcretes associated with a regional Cretaceous to Paleogene subaerial unconformity and an intraformational composite (polygenic) surface in Upper Cretaceous intra-platform peritidal successions in central Dalmatia and eastern Istria, Croatia (Adriatic-Dinaridic Carbonate Platform), were analyzed for their δ13C and δ18O signatures in order to provide insight into the conditions of subaerial exposure and calcrete development. The distinctly negative δ13C signatures of biogenic calcretes marking the regional subaerial unconformity differ considerably from the δ13C values of the host marine limestones. This indicates carbon isotope exchange of primary marine CaCO3 with CO2 released by root and rhizomicrobial respiration and subsequent precipitation of pedogenic calcrete. The range of δ13C (from −13.1 to −8.2 ‰ Vienna PeeDee Belemnite standard, VPDB) and δ18O (from −10.1 to −6.1 ‰ VPDB) values of calcretes are similar to those reported from calcretes elsewhere, and the δ13C values of biogenic calcretes with typical Microcodium aggregates (−13.1 to −12.3 ‰ VPDB) at the Sibenik locality are very close to, or at the lower limit of, values for soil carbonates formed in isotopic equilibrium with soil CO2. These values are expected for authigenic pedogenic carbonates formed under the influence of C3 plant communities, without influence from heavier carbon from pre-existing carbonate and lack of input of atmospheric CO2. Such low δ13C values support the interpretation of Microcodium aggregates as being precipitated under a direct biological control within the soil, although the relationship between formation mechanisms and stable isotope signatures of Microcodium needs further investigation. The δ13C values (−4.4 to −3.6 ‰ VPDB) of rhizogenic calcretes formed inside firmground Thalassinoides burrows of the composite surface at the Sibenik locality are more negative than the δ13C values of the host marine limestones, which confirms that the composite surface went through a phase of meteoric pedo(dia)genesis. However, the overall δ13C values of calcretes are less negative than expected, which might reflect contamination from associated primary marine carbonate. This study represents the first detailed stable isotope investigation of calcretes from carbonate successions of the External Dinarides, and the results may be applied to discontinuities present in other shallow-water carbonate rock successions.

Calcretes and palustrine carbonates in the Oligo-Miocene clastic–carbonate unit of the Farafra Oasis, Western Desert, Egypt: Their origin and paleoenvironmental significance

The origin and paleoenvironmental significance of the calcretes and palustrine carbonates in the Oligo-Miocene clastic–carbonate unit that outcrops at the eastern sector of the Farafra Depression (Western Desert, Egypt) have been discussed based on field and petrographic investigations. The calcretes–palustrine carbonates assemblage occurs above a siliciclastic/distal alluvial-floodplain facies. The calcretes represent the transition from the underlying siliciclastic/distal alluvial-floodplain facies to the overlying palustrine carbonates. The calcrete-host rocks are muddy sandstones and sandy mudrocks.This study reveals the occurrence of groundwater calcretes with an upward gradational maturity pattern, ranging from incipient to nodular and to massive calcretes. The calcretes micromorphological analysis suggests that they were originated in vadose and phreatic diagenetic environments by groundwater through evaporation, degassing with no biological activity. The palustrine carbonates are also recognized above the calcrete horizons. They occur in the form of micritic limestones displaying different features that indicate their modifications during pedogenesis and subaerial exposure. These features include clotted-peloidal texture, fenestral fabric, mottling, pseudo-brecciation, desiccation cracks, pseudomicrokarst, root traces and silicification of the lime mud. The calcretes–palustrine carbonates assemblage records a progressive decrease in the terrigenous supply and a continuous rise of the groundwater table associated with local subsidence in a semi-arid to sub-humid climate. A model is suggested for the development of calcretes and palustrine carbonates in the study area.

Brief Communication: Beyond the South African cave paradigm—Australopithecus africanus from Plio–Pleistocene paleosol deposits at Taung

Following the discovery of the "Taung Child" (Australopithecus africanus) in 1924 in the Buxton-Norlim Limeworks near Taung, the fossil-bearing deposits associated with the Dart and Hrdlička pinnacles have been interpreted as the mined remnants of cave sediments that formed within the Plio-Pleistocene Thabaseek Tufa: either as a younger cave-fill or as contemporaneous carapace caves. When combined with the Plio-Pleistocene dolomitic cave deposits from the "Cradle of Humankind," a rather restricted view emerges that South African early hominins derived from cave deposits, whereas those of east and central Africa are derived from fluvio-lacustrine and paleosol deposits. We undertook a sedimentological and paleomagnetic analysis of the pink-colored deposit (PCS) from which the "Taung Child" is purported to have derived and demonstrate that it is a calcrete, a carbonate-rich pedogenic sediment, which formed on the paleo-land surface. The deposit extends 100 s of meters laterally beyond the Dart and Hrdlička Pinnacles where it is interbedded with the Thabaseek Tufa, indicating multiple episodes of calcrete development and tufa growth. The presence of in situ rhizoconcretions and insect trace fossils (Celliforma sp. and Coprinisphaera sp.) and the distinctive carbonate microfabric confirm that the pink deposit is a pedogenic calcrete, not a calcified cave sediment. Paleomagnetic and stratigraphic evidence indicates that a second, reversed polarity, fossil-bearing deposit (YRSS) is a younger fissure-fill formed within a solutional cavity of the normal polarity tufa and pink calcrete (PCS). These observations have implications for the dating, environment, and taphonomy of the site, and increase the likelihood of future fossil discoveries within the Buxton-Norlim Limeworks.

Multi-proxy evidences of Mid-Pleistocene dry climates observed on calcretes in Central Turkey

Palaeosols are very important in reconstructing palaeoclimate studies, since they are accepted as useful climate markers and as potential providers of proxy data. The study of the palaeosols of the Plio-Pleistocene rock sequences around Ankara revealed evidence for a reconstruction of the Quaternary palaeoclimate of the region. The study area is located in Bala, south-east of Ankara, which contains Middle Pleistocene red palaeosols with powdery to nodular calcrete developments, alternating with channel deposits. During this warm and arid period in the Pleistocene, the limited water available in the soil led to the accumulation of low magnesian carbonates, forming calcretes. The clay fractions of the samples were X-rayed using air-drying, ethylene-glycol solvation and heating treatments. The clay mineral contents were determined as smectite, kaolinite, illite and chlorite. Palygorskite was also identified. In the relative abundances of clay minerals, smectite, the most abundant clay mineral, is depleted towards the upper part of the section while the amount of palygorskite increases. Since palygorskite is the only pedogenic mineral formed during the Pleistocene, its presence can be accepted as evidence of the dominant arid climatic conditions. The isotopic composition of carbonates in the Bala section exhibits a slightly wider range in \delta^{13}C composition from -5.98%o to -9.22%o and a narrower range in \delta^{18}O composition from -7.19%o to -8.66%o. The carbon isotope values clearly imply that arid to semiarid flora C4 is dominant, with C3:C4 mixed vegetation. This study suggests that the Middle Pleistocene is the time of the Mid-Brunhes Event when the dominantly warm climatic temperatures led to the development of calcretes in Bala, Ankara, as with all Quaternary Mediterranean-type calcretes.

Polygenetic development and paleoenvironmental implications of a Pleistocene calcrete at Tongoy, central northern Chile

Polygenetic development and paleoenvironmental implications of a Pleistocene calcrete at Tongoy, central northern Chile

Development of calcrete and clinoforms during emergence and flooding of the Late Cenomanian carbonate platform, Jordan

The Late Cenomanian Hummar Formation was studied in three sections in north and central Jordan, at Aameriyya, northeast of Na’ur and the Wadi Haur areas. The base in the Aameriyya area is marked by a subaerial unconformity overlain by a calcrete and a paleokarstic horizon, separating the underlying Fuheis Formation marl from the overlying Hummar Formation limestone. The emergent Aameriyya area is interpreted to have been a paleohigh, as a response to tectonism, and a basin and swell topography is invoked for the Late Cenomanian carbonate platform in this region. The Hummar Formation is believed to form one complete depositional sequence; the calcrete-karst represents a lowstand systems tract, the overlying 2-m massive rudstone/floatstone represents the transgressive systems tracts (TST), and the cortoid grainstone/packstone with clinoforms the highstand systems tracts. The topmost miliolid limestone is probably the late highstand topset of the sequence, followed upwards by the TST of the Shueib Formation marl of the next sequence. The sequence boundary at the upper contact of the Hummar Formation can be correlated regionally whereas the sequence boundary at its base with subaerial exposure has not been reported elsewhere in Jordan, the Negev, or Sinai.

Identification of an anastomosing river system in the Early Cretaceous Khorat Basin, northeastern Thailand, using stratigraphy and paleosols

The Phu Kradung and Phra Wihan formations of the Early Cretaceous Khorat Group were investigated to characterize the depositional system and paleoclimate, based on stratigraphy, paleosol features, paleosol profiles, and reworked deposits. The sediments of both formations are classified into 12 facies on the basis of their primary depositional attributes, such as bedding, grain size, texture, and sedimentary structures. Furthermore, these facies are grouped into eight architectural elements based on stratigraphic relationships and outcrop-scale macroforms. The proportions of each architectural element indicate that the depositional system was that of an anastomosing river. The anastomosing river system in the studied section is characterized by a high rate of vertical floodplain aggradation and the development of a relatively stable crevasse channel. Paleosols are developed mainly in floodplain deposits, and are characterized by the development of calcretes. A semi-arid to sub-humid paleoclimate is deduced from the occurrence of the calcretes. In addition, their microstructures suggest that these calcretes were formed under several different conditions, with micritic carbonate precipitating under relatively arid climates, and alveolar septal structures under semi-arid to sub-humid conditions. Using the thicknesses of the calcrete horizons, we conclude that the difference in the amount of precipitation between the wet and dry periods was highly variable, without any order, during deposition of the studied section. The thicknesses of intervals between successive calcrete horizons indicate that the inflow of fine materials occurred randomly across the floodplain. Partly superimposed, multiple calcrete horizons were developed during periods of low sediment supply. A random deposition of floodplain sediments appears to characterize the anastomosing river system of the Khorat Basin.

Development of a Pleistocene calcrete over a sequence of marine terraces at Tongoy (north-central Chile) and its paleoenvironmental implications

The importance of the Norte Chico region in north-central Chile has long been recognized for the paleoclimates recorded in its soils. This area lies in an extreme climate gradient between the hyper-arid Atacama Desert in the north and a Mediterranean climate in the south, which has made it very sensitive to past climate changes. Nevertheless, few paleoclimate studies have been undertaken in the region, and these were mostly concentrated on the Holocene. We studied Pleistocene climate changes recorded in soils that formed over a series of marine terraces near Tongoy about 60 km south of La Serena. The calcrete and soil development took place on four marine terraces associated with Marine Isotope Stages MIS 11, MIS 7e, MIS 5e, and MIS 1. The different types of calcretes that developed on the three oldest terraces containing calcareous material indicate that they developed during different periods, and that climatic conditions favorable for the development of these soils existed in the area at least from MIS 11 (412 ka) until post-MIS 5e (125 ka). The calcrete horizons show well-defined development stages recording cyclic climate changes varying between arid and more humid during the late Pleistocene. These climate changes recorded in the Tongoy soils are reflected by sedimentological, geomorphological and pedogenic processes. Climate cycles have only been recorded previously for the post-MIS 5e stage in the area, this study being the first to include climate variations reaching MIS 11.

Evidence of past environmental conditions during the evolution of a calcretised Wadi System in Southern Jordan using stable isotopes

A stratigraphically and temporally ordered sequence of channel calcretes preserved along the Wadi Dana, Southern Jordan, records the Quaternary evolution of the formation and infilling of rock-cut channels and their subsequent incision in a tectonically subsiding basin. It is currently unknown under what palaeoenvironmental conditions these non-pedogenic calcretes formed. Stable isotope analyses have been used to investigate whether any past topographical, hydrological, vegetational, diagenetic, and/or temporal signatures can be identified from the channel calcretes. The results of this research indicate that channel calcrete development is influenced by altitudinal variation (affecting vegetation and hydrology) within the landscape as well as location within individual wadi channels (which has an affect on diagenetic processes). The lack of calcretisation of a terrace that is between +1 and 1.5m above the modern wadi floor supports the idea that the environment is currently too arid for calcrete to develop. Thus the presence of older Wadi Dana channel duricrusts suggests wetter conditions when they formed. The δ18O data of the calcrete implies cooler conditions than today, as there is little evidence of a strong evaporative effect. Any temporal control is limited and mainly a function of stratigraphic position with some numeric dating.

Calcretes, fluviolacustrine sediments and subsidence patterns in Permo‐Triassic salt‐walled minibasins of the south Urals, Russia

AbstractThe south Uralian foreland basin forms part of the giant, yet sparsely documented, PreCaspian salt tectonic province. The basin can potentially add much to the understanding of fluviolacustrine sedimentation within salt‐walled minibasins, where the literature has been highly reliant on only a few examples (such as the Paradox Basin of Utah). This paper describes the Late Permian terrestrial fill of the Kul’chumovo salt minibasin near Orenburg in the south Urals in which sediments were deposited in a range of channel, overbank and lacustrine environments. Palaeomagnetic stratigraphy shows that, during the Late Permian, the basin had a relatively slow and uniform subsidence pattern with widespread pedogenesis and calcrete development. Angular unconformities or halokinetic sequence boundaries cannot be recognized within the relatively fine‐grained fill, and stratigraphic and spatial variations in facies are therefore critical to understanding the subsidence history of the salt minibasin. Coarse‐grained channel belts show evidence for lateral relocation within the minibasin while the development of a thick stack of calcrete hardpans indicates that opposing parts of the minibasin became largely inactive for prolonged periods (possibly in the order of one million years). The regular vertical stacking of calcrete hardpans within floodplain mudstones provides further evidence that halokinetic minibasin growth is inherently episodic and cyclical.

Preservation of beach ridges due to pedogenic calcrete development in the Tongoy palaeobay, North-Central Chile

At the Tongoy palaeobay in north-central Chile, a series of beach ridges developed during seaward progradation that took place after the MIS 11 sea-level highstand (412 ka). The microrelief left by this succession of beach ridges has been well preserved from erosion due to the development of a calcrete (petrocalcic horizons), which was resistant to the chemical and physical weathering that characterized the area during humid phases of the late Pleistocene and middle Holocene. Macro- and micro-morphological analysis shows that the calcrete is of pedogenic origin and formed during two stages: in the first stage a massive (beta) calcrete developed, followed during the second stage by a laminar (alpha) calcrete. Each event in the development of the calcrete was intimately related to the foregoing process, mainly due to changes in the permeability of the profile horizons. During the first stages of development, organisms played an important role in the precipitation of calcite, which made the calcrete less permeable and favored the accumulation of ponded water during the wet season. As a result of this increased humidity, calcium carbonate with a laminar structure was precipitated. The development of the calcrete was also intimately associated with the evolution of the drainage network, which is characterized by a trellis pattern of tributaries parallel to the beach ridges. This study demonstrates the importance of soil genesis in the geomorphological evolution of landscapes.

A ∼200 ka record of climatic change and dune activity in the Thar Desert, India

An 18.4 m excavated dune section in the Thar Desert of India with a chronology based on 12 TL ages and a basal age of ∼190 ka has preserved 12 cycles of dune accretion, soil formation, calcrete development, and subsequent erosion, together with the presence of stone artefacts ranging in age from Lower Palaeolithic to Mesolithic, coeval with more humid climatic interludes. Phases of soil development and carbonate precipitation were relatively wet and phases of dune accretion relatively dry, so that there were 12 significant moist intervals separated by 11 drier intervals during the past ∼190 ka. The calculated time interval between successive phases of dune sand accumulation ranged from 22.2 ka to 15.8 ka, with a mean of 19.0 ka. These values are consistent with a precessional influence on dune activity and on the associated onset of early monsoonal activity in this region. Carbon isotopes measured on organic matter within the sand profiles show consistent values close to −21.6 ± 1‰, pointing to deposition during a transitional climatic regime characterized by a change from open C3 grassland to C4 woodland or forest.

Carbon and oxygen isotope evidence for high-frequency (10 4–10 5 yr) and My-scale glacio-eustasy in Middle Pennsylvanian cyclic carbonates (Gray Mesa Formation), central New Mexico

We combine cyclo- and sequence stratigraphy along with whole rock δ 13C and conodont apatite δ 18O analysis to document high-frequency (10 4–10 5 yr) and My-scale sea-level changes for the Middle Pennsylvanian (Desmoinesian or Moscovian) Gray Mesa Formation of central New Mexico. Approximately 75 subtidal cycles (1–8 m) are grouped into 4 1/2 My-scale depositional sequences (40–80 m). About 50% of the cycles show evidence of prolonged subaerial exposure at cycle tops with the development of calcretes, diagenetic mottling, and regolith intraclasts. High-resolution δ 13C analysis of whole rock limestones across nine of the cycles indicates that the cycle tops were diagenetically altered by isotopically light, meteoric fluids during sea-level fall and lowstand. These δ 13C trends support the interpretation that high-frequency sea-level changes were responsible for cycle development. Conodont apatite δ 18O values from sampled cycles indicate that the high-frequency sea-level changes were driven by glacio-eustasy combined with changes in surface seawater temperature (SST). δ 18O values from conodont apatite, spanning parts of three depositional sequences indicate that My-scale glacio-eustasy and/or SST changes controlled sequence development. δ 18O shifts indicate that the magnitudes of 10 4–10 5 yr glacio-eustasy were between ∼ 55 and 170 + m combined with tropical SST changes of ∼ 1.5°–6 °C. Calculated My-scale glacio-eustatic oscillations were between ∼ 60 and 140 m with SST changes of < 3.5 °C. The most plausible driver for the My-scale paleoclimate changes is long-period obliquity (∼ 1.2 My) variations. These calculated high-frequency, glacio-eustatic values are similar or greater than Pleistocene values, and lie within the range estimated for other Middle Pennsylvanian successions using a variety of independent eustatic proxies. The similarity in range of magnitudes between high-frequency and My-scale sea-level changes combined with the large differences in magnitudes between individual high-frequency sea-level oscillations helps explain the lack of systematic cycle-stacking patterns within these Pennsylvanian icehouse sequences.