Calcrete Deposits Research Articles

Interactions between distal epiclastic and bio‐chemogenic sedimentation at the foothills of a mafic alkaline volcano: The case of the Oligocene Doupovské Hory Volcanic Complex (Czech Republic)

AbstractLate Oligocene (ca 25 Ma) volcano‐sedimentary successions exposed on the western periphery of the Doupovské Hory Volcanic Complex reveal a complex sedimentation history influenced in various ways by decay of the alkali basalt volcanic edifice. Weathering of the volcanic rocks supplied abundant reactants that promoted carbonate precipitation in the peripheral palaeolakes—as evidenced by strongly non‐radiogenic 87Sr/86Sr values (0.7038–0.7041). On the other hand, the sediments of the initial shallow lake became deformed by the bulldozing effect of a debris avalanche. The debris flow and avalanche deposits filled up the original depression, modified the basin morphology and shifted the peripheral lacustrine setting further away from the volcano. At this stage, surface water influx from the surrounding granites conferred a more radiogenic character (87Sr/86Sr values 0.7046–0.7049) to the calcrete deposits. Fossil assemblages as well as limestone textures suggest significant seasonal water‐level fluctuations, possibly reflecting the alternating rainy and dry‐seasons of a prevalently humid Central‐European Late Oligocene climate. The seasonal drying out of the ponds resulted in significant 18O enrichments. Although the ca 0‰ δ13C values might suggest mixing of atmospheric and volcanic CO2 during carbonate precipitation, no active volcanic conduits of relevant age are known in the close vicinity. The lower δ13C values are likely a result of mantle degassing through rift faults, a phenomenon observed in the magmatically extinct Ohře Rift until present. This paper demonstrates that limestones derived from weathered alkaline basalts are characterised by highly non‐radiogenic Sr isotopic ratios (87Sr/86Sr ca 0.704), suggesting a magmatic origin for the Ca within these carbonates. Contrary to the notion of carbonatites being present when highly non‐radiogenic Sr isotopes are found, these results show that Sr isotopes in carbonates formed in alkali basalt‐sourced environments only reveal the source of the Sr (and Ca) ions, not necessarily the presence of carbonatite.

The West Water Formation (Hualapai Plateau, Arizona, USA) as a calcrete-paleosol sequence, and its implications for the Paleogene-Neogene evolution of the southwestern Colorado Plateau

Analyses of stratigraphic sequences within the paleocanyons of the Hualapai Plateau, Arizona, are important because these deposits offer the only evidence for the Paleogene-Neogene geological history of the Grand Canyon area. In this paper, we focus on the origins and paleoenvironmental significance of the West Water Formation, located within the Milkweed and West Water paleocanyons on the Hualapai Plateau. We propose that the supposed “limestone unit” of the West Water Formation at and near its type section is not a limestone; rather, it is a 21m-thick valley calcrete, overprinted by a ~1–2m-thick pedogenic calcrete, and subsequently dolomitized in its upper-to-middle sections, with a superimposed 4m-thick red paleosol. We also propose that this unit is not coeval in age or origin with the Long Point limestone on the Coconino Plateau, and that the presence of a complex calcrete-dolocrete-paleosol alters previous interpretations associated with this unit. Evidence for a calcrete-paleosol origin, beyond the West Water Formation's lack of fossils, includes: its predominantly micritic calcite-palygorskite composition; its textures characteristic of valley and pedogenic calcrete deposits (as exhibited by thin section, SEM, and TEM analyses); and its association with a relatively thick overlying red paleosol that also contains abundant palygorskite. Stable carbon and oxygen isotope values from carbonate cements are also within the range expected of a near-surface calcrete. Carbonate minerals within the calcrete-dolocrete were precipitated at or near the water table in a valley setting due to evaporation and/or CO2 degassing in a semi-arid to arid environment of deposition. High 87Sr/86Sr values within the calcrete were inherited from groundwater infiltrating through Music Mountain Formation arkosic sediments, which were derived from a Precambrian source terrane to the south and southwest. No absolute ages exist for the Music Mountain Formation and West Water Formation in the Milkweed and West Water paleocanyons; these sediments could range in age from the Late Cretaceous to the latest Eocene-earliest Oligocene, and may record the Eocene-Oligocene Transition in the Grand Canyon area.

Fossil Carder Bee's Nest from the Hominin Locality of Taung, South Africa.

The Buxton-Norlim Limeworks southwest of Taung, South Africa, is renowned for the discovery of the first Australopithecus africanus fossil, the ‘Taung Child’. The hominin was recovered from a distinctive pink calcrete that contains an abundance of invertebrate ichnofauna belonging to the Coprinisphaera ichnofacies. Here we describe the first fossil bee’s nest, attributed to the ichnogenus Celliforma, from the Plio-Pleistocene of Africa. Petrographic examination of a cell lining revealed the preservation of an intricate organic matrix lined with the calcitic casts of numerous plant trichomes–a nesting behaviour unique to the modern-day carder bees (Anthidiini). The presence of Celliforma considered alongside several other recorded ichnofossils can be indicative of a dry, savannah environment, in agreement with recent work on the palaeoenvironment of Plio-Pleistocene southern Africa. Moreover, the occurrence of ground-nesting bees provides further evidence that the pink calcrete deposits are of pedogenic origin, rather than speleogenic origin as has previously been assumed. This study demonstrates the potential value of insect trace fossils as palaeoenvironmental indicators.

Pedogenetic Calcretes In Early Syn-Rift Alluvial Systems (Upper Jurassic, West Cameros Basin), Northern Spain

Exceptional pedogenetic calcretes occur in the earliest syn-rift sedimentary record (Tithonian) of the west Cameros Basin. These calcretes show different types of pedogenetic facies (nodular, massive, laminar, and brecciated–pisolithic) and microfabrics: brecciated intraclasts, carbonate nodules, coated clasts and grains, etched quartz grains, rhizocretions, pedotubules and fragmented alveolar–septal structures. In addition, calcrete fragments have been observed in the conglomerate and sandy channelized deposits that form the alluvial systems, indicating the reworking of previous calcrete deposits. Calcrete facies appear organized in several sequences developed over different architectural elements in three different types of alluvial systems. The type of calcrete sequence is different depending on the architectural arrangement of each alluvial-fan system. Laminar (L (MN)–S) and brecciated–pisolithic (BP (L)–S) calcrete sequences are predominant in the alluvial sedimentary record of poorly channelized alluvial systems. These calcretes are thick, morphologically complex, and laterally continuous, due to the formation of long-term stable subaerial surfaces favored by the intermittent behavior of the alluvial systems. Massive-nodular (M (N)–S) and laminar (L (MN)–S) sequences are predominant in highly channelized alluvial systems and develop only locally in interchannel or floodplain areas where sedimentation stops temporally. Calcrete sequences in the distributive fluvial fan systems can be considered as poorly developed, being massive-nodular (M (N)–S) and nodular (N–S). After analyzing the location of these calcrete sequences in the studied record and their genesis and environmental significance, their variations can be related to different autocyclic factors such as sedimentation rate, channel network organization, texture of the host sediment, and water-table influence. Also three main allocyclic factors such as climate, tectonics, and drainage area controlled the carbonate sedimentation and therefore the development of the calcretes.

Three new subterranean diving beetles (Coleoptera: Dytiscidae) from the Yeelirrie groundwater calcretes, Western Australia, and their distribution between several calcrete deposits including a potential mine site

Three new subterranean diving beetles (Coleoptera: Dytiscidae) from the Yeelirrie groundwater calcretes, Western Australia, and their distribution between several calcrete deposits including a potential mine site

Mineralogy and origin of surficial uranium deposits hosted in travertine and calcrete from central Jordan

Secondary uranium encrustations are hosted in thick travertine and calcrete deposits of Pleistocene–Recent age in central Jordan. The central Jordan varicolored marble and travertine are equivalent to the active metamorphic area in Maqarin, north Jordan. More than 100 samples were collected from the outcrops of the varicolored marble, travertine, calcrete, and the yellow uranium encrustations. The secondary yellow encrustations are mainly composed of uranyl vanadate complexes. Tyuyamunite Ca(UO2)2V25+O8·3(H2O)–strelkinite Na2(UO2)2V2O8·6(H2O) solid solution series are the major components and their composition reflects changes in the Ca/Na ratio in solution. Potentially, new vanadium free calcium uranate phases (restricted to the varicolored marble) were identified with CaO:UO3 ratios different from the known mineral vorlanite (CaU6+)O4.Carbon and oxygen isotope data from calcite in the varicolored marble are characterized by Rayleigh-type enrichment in light isotopes associated with release of 13C and 18O enriched CO2 by high temperature decarbonation during combustion of the bituminous marl. Stable isotope results from uranium hosted travertine and calcrete varieties exhibit a wide range in isotopic values, between decarbonated and normal sedimentary carbonate rocks. The depleted δ13C and δ18O values in the travertine are related to the kinetic reaction of atmospheric CO2 with hyperalkaline Ca(OH)2 water. The gradual enrichment of δ13C and δ18O values in the calcrete towards equilibrium with the surrounding environment is related to continuous evaporation during seasonal dry periods.Uranium mineralization in central Jordan resulted from the interplay of tectonic, climatic, hydrologic, and depositional events. The large distribution of surficial uranium occurrences hosted in travertine and calcrete deposits is related to the artesian ascending groundwater that formed extensive lakes along NNW–SSE trending depressions. Fresh groundwater moved upward through the highly fractured phosphate, bituminous marl and varicolored marble to form unusual highly alkaline water (hydroxide–sulfate type) enriched with sensitive redox elements among which were U and V.

Palustrine carbonates and pedogenic calcretes in the Çal basin of SW Anatolia: Implications for the Plio-Pleistocene regional climatic pattern in the eastern Mediterranean

Abstract Sedimentologic and stable isotopic studies in the Cal basin-fill provide new insights on the late Neogene–Quaternary climatic pattern in the eastern Mediterranean. Long-term subsidence and terrestrial sedimentation in the basin beginning in the late Miocene are recorded in alluvial fan and fluvial deposits overlain by Pliocene palustrine carbonates. An erosional unconformity separates the Pleistocene siliciclastic and calcrete deposits above as a single sedimentary unit resting on top of floodplain fines and restricted to the basin center. The stable isotopic analysis of the palustrine carbonates and pedogenic calcretes displays a wide range of values (− 7.98

Development and lithogenesis of the palustrine and calcrete deposits of the Dibdibba Alluvial Fan, Kuwait

A model is proposed for the development of the Quaternary palustrine carbonate–calcrete association, which occurs as hard crust capping low hills at a distal flood plain of Al Dibdibba alluvial fan located at southwestern Kuwait. Field occurrence, detailed petrographic investigation and geochemical analysis revealed that a single cycle of groundwater calcrete with vertical gradational maturity pattern was developed. This represents a continuous sedimentological cycle during which flood sheet conditions prevailed with intermittent periods of humid and arid conditions. Subsequently, calcitic micrite was continuously precipitated from small, shallow, local, isolated and short lived ponds fringed by freshwater marshes with abundant charophyte meadows. The latter were developed as a result of flooding scattered depressions by groundwater supersaturated with respect to calcite due to rise of groundwater table. The deposition of two facies of carbonate muds, namely; biomicrite and pelintraclasts skeletal micrites was followed by a drought phase which witnessed desiccation of the fresh water ponds and significant drop in groundwater level. A sequence of pedogenic and diagenetic processes acted on the deposited carbonate muds are manifested by: (a) desiccation cracks, (b) micrite neomorphism, (c) infilling of root burrows and some cracks by aeolian siliciclastics, (d) karstification, (e) marmorization, (f) calcretization of root burrow infill and development of pseudo-rhizocrete, (g) calcite cementation and mineral authigenesis, and (h) silcretization. These processes are responsible for the development of hard palustrine carbonate crust. At the advent of aridity, the whole system of Al Dibdibba alluvial fan was subjected to deflation. This resulted in reversing the paleotopography of the hard crusted palustrine depressions into carbonate capped domal hills.

Mudflat/distal fan and shallow lake sedimentation (upper Vallesian–Turolian) in the Tianshui Basin, Central China: Evidence against the late Miocene eolian loess

The Tianshui Basin in central China contains a thick sedimentary sequence (~ 1400 m) of continental deposits, Aragonian to Villafranchian (Miocene-Pliocene) in age. Intense Himalayan movements around the Paleogene/Miocene boundary triggered the uplift of mountain ranges around the Tianshui Basin, providing the deposition site for continental sediments. The sedimentary infill of the basin consists of four stratigraphic units (I to IV). This paper focuses on Unit II. Most of the accommodation space was occupied by Unit I, so during the sedimentation of Unit II, the morphology of the basin was relatively flat, promoting the development of wide distal fan/mudflat areas and wide shallow lakes. Deposits include: red mudstones, pedogenic and groundwater calcretes, reworked calcrete deposits, sheet-floods, fluvial channels, rippled sandstones/siltstones, ooidal/peloidal packstones, palustrine limestones, bioturbated marls and intraclastic limestones/marls. The characteristics and organization of the deposits indicate the gradual transition from alluvial to lake environments. Within the distal fan/mudflat, the deposition of reworked calcrete clasts as one of the most striking facies of the basin may be attributed to substantial recycling of calcrete levels and red clays. At the lake margins, the presence of ooids and palustrine limestones suggests the possibility of ramp-like margins within different energy settings. The origin of some of the deposits of the basin's QA-I section is under discussion, and their consideration as eolian has been recently proposed. However, we believe this possibility is precluded by the characteristics of the deposits. Our proposal has important implications for revising the interpretation of Miocene paleoclimatic conditions in central Asia.

Occurrence of cyclic palustrine and calcrete deposits within the lower Pliocene Hagul formation, East Cairo district, Egypt

A depositional model of the lower Pliocene Hagul formation, which is exposed in the East Cairo district (Egypt), is proposed with more than 10 depositional cycles recognized. Field occurrence, detailed petrographic investigation and geochemical analysis revealed that the sediments within each cycle are the result of three sequential sedimentological processes: (1) alluvial sedimentation, (2) calcretization, and (3) precipitation of palustrine carbonate. It was concluded that Hagul formation has been deposited within the distal part of an alluvial plain during three successive climatic conditions: a humid climate during which alluvial sediments were deposited, a semi-arid climate with episodic precipitation which was favorable for pedogenic calcrete development, and a sub-humid climate during which groundwater level was gradually elevated and groundwater calcrete accumulated. Rising groundwater level continued until shallow wetlands covered the area and palustrine limestone was precipitated. Variations in the thickness and the nature of the host sediment, calcrete and palustrine limestone cycle suggest that each of the sedimentation processes varied from cycle to cycle.

Short-range endemism in hypogean environments: the pseudoscorpion genera Tyrannochthonius and Lagynochthonius (Pseudoscorpiones: Chthoniidae) in the semiarid zone of Western Australia

We studied a variety of chthoniid pseudoscorpions collected from the semiarid zone of Western Australia. Five new species of Tyrannochthonius Chamberlin, 1929 and three new species of Lagynochthonius Beier, 1951 are named and described from the Pilbara and Gasgoyne regions, and surrounding areas such as Barrow Island. Tyrannochthonius basme, sp. nov. and Lagynochthonius asema, sp. nov. from pisolitic mesas near Pannawonica, T. garthhumphreysi, sp. nov. from limestone karst on Barrow Island, T. souchomalus, sp. nov. from calcrete deposits near Cue, T. billhumphreysi, sp. nov. and L. polydentatus, sp. nov. from a calcrete deposit on Sturt Meadows Station and L. leemouldi, sp. nov. from calcrete near Marble Bar are all considered to represent hypogean species as all exhibit typical troglomorphic adaptations including total loss of eyes and attenuated appendages. New records are provided for T. brooksi Harvey and T. butleri Harvey from Cape Range peninsula. A new epigean species, T. aridus, sp. nov., was found on Barrow Island and the Pilbara mainland. Two further putative new species based upon nymphal specimens from subterranean environments are described but not named owing to the lack of adult specimens. Although the epigean species T. aridus, sp. nov. is relatively widespread, all of the subterranean species are thought to represent short-range endemic species as they have been found at very few locations, all of which occur in localised habitats such as limestone or within mesa formations. Tyrannochthonius chamorro Chamberlin, 1947 from Guam is transferred to the genus Lagynochthonius, creating the new combination Lagynochthonius chamorro (Chamberlin 1947).

Geochemical and C, O, Sr, and U-series isotopic evidence for the meteoric origin of calcrete at Solitario Wash, Crater Flat, Nevada, USA

Calcite-rich soils (calcrete) in alluvium and colluvium at Solitario Wash, Crater Flat, Nevada, USA, contain pedogenic calcite and opaline silica similar to soils present elsewhere in the semi-arid southwestern United States. Nevertheless, a ground-water discharge origin for the Solitario Wash soil deposits was proposed in a series of publications proposing elevation-dependent variations of carbon and oxygen isotopes in calcrete samples. Discharge of ground water in the past would raise the possibility of future flooding in the unsaturated zone at Yucca Mountain, Nevada, site of a proposed high-level nuclear waste repository. New geochemical and carbon, oxygen, strontium, and uranium-series isotopic data disprove the presence of systematic elevation-isotopic composition relations, which are the main justification given for a proposed ground-water discharge origin of the calcrete deposits at Solitario Wash. Values of δ13C (−4.1 to −7.8 per mil [‰]), δ18O (23.8–17.2‰), 87Sr/86Sr (0.71270–0.71146), and initial 234U/238U activity ratios of about 1.6 in the new calcrete samples are within ranges previously observed in pedogenic carbonate deposits at Yucca Mountain and are incompatible with a ground-water origin for the calcrete. Variations in carbon and oxygen isotopes in Solitario Wash calcrete likely are caused by pedogenic deposition from meteoric water under varying Quaternary climatic conditions over hundreds of thousands of years.

Characteristics, evolution and distribution of Quaternary channel calcretes, southern Jordan

AbstractCemented and calcretized conglomerates of Quaternary age are found within ancient river channels exposed at various heights (between +1 and +125 m) above the current bed of the Wadi Dana in southern Jordan. In cross‐section, the calcrete deposits are typically lensoid in shape and are found infilling palaeochannels cut into the bedrock. The fossilized channel sediments preserve evidence of past river conditions, sediment loads, source areas, phases of river down‐cutting and sediment accretion, as well as post‐depositional alteration processes.Samples have been analysed in thin section and under the scanning electron microscope in order to determine the calcrete micromorphology, cement types and crystal sizes. The results show that the river deposits have undergone significant displacement and replacement of the detrital grains by CaCO3, and this has resulted in the cementation in places of over 4 m of river material. The cements display an alpha‐type fabric with no evidence of biogenic activity. The nature of the cement fabric within and between deposits differs as a function of two key variables: (i) the specific location within the channel and (ii) height above the modern wadi floor. Firstly, in many of the deposits cement crystal size varies, with sparite dominating at depth within impermeable palaeowadi channels, whereas on the flanks of the channels and near the top of the channels micrite is the main type of cement. These variations are thought to be a result of differences in the length of residence time of calcium carbonate‐rich fluids in different parts of the channel and the spatially varying influence of evaporation. Secondly, distinct changes in cement fabric can be seen in channels of different ages with a reduction in the amount of micrite and an increase in the amount of secondary recrystallized pore‐filling spar over time. Copyright © 2004 John Wiley & Sons, Ltd.

Reworked calcretes: their significance in the reconstruction of alluvial sequences (Permian and Triassic, Minorca, Balearic Islands, Spain)

The Permian and Triassic of Minorca (Balearic Islands) consists of a 670-m-thick, red, alluvial succession that includes in situ calcrete profiles and reworked calcrete material. In the Permian succession, the calcretes vary from laminar forms developed on the Carboniferous basement to weakly developed nodular calcretes in fluvial sediments. The palaeosols in the Triassic are mostly dolomitic, and the profiles reach up to Stage III of soil development (Spec. Pap.-Geol. Surv. Am. 203, (1995) 1). The clasts, formed through reworking of the palaeosol profiles, are about 0.5–10 cm across and include mosaics of calcite/dolomite crystals, brecciated clasts, rhizolith fragments, and aggregates of clay and/or silt. These clasts appear in three different types of deposits. Type 1 corresponds to lenticular bodies that fill small scour surfaces, and consists only of intraformational conglomerates. These deposits are interpreted as ephemeral channels and sheet-floods that represent the interfluvial drainage systems that captured only the precipitation falling on the alluvial plain. Type 2 includes sand dune 3-D bodies with flat bottoms and convex tops. These bodies are about 20 cm high and 2 m wide, and were formed by floodwaters that flowed down the levees of the major streams. Type 3 channel deposits contain reworked calcretes and extrabasinal clasts, which overlie erosive surfaces and are found in layers within cross-bedded sandstones and conglomerates. These are interpreted as channel-floor lag deposits of major channels that entered from distant uplands and drained the alluvial plain. Variations in the aggradation rates of the floodplain resulted in five different infill stages. In the lowstand to early transgressive interval, as in stages I (P1) and IV (B1), the fluvial deposits filled palaeovalleys; calcretes and reworked calcrete deposits were of difficult formation (apart from terraces) and preservation. Accommodation space was at its greatest in the transgressive, stages II (P2) and V (B2). This caused the greatest aggradation of the floodplains, which are formed of thick sequences of fine-grained sediments, isolated meandering channels, weakly developed calcretes (compound) and reworked calcrete deposits, mostly of types 1 and 2. The density of channels notably increased in stage III (P3), highstand interval, because of the reduction of accommodation space, this could favour the formation of composite or even cumulative palaeosols, but of difficult preservation. Reworked calcrete deposits are mostly of type 3, but types 1 and 2 are also recognised. The reworked calcrete deposits are an important part of the Permian and Triassic fluvial sediments and their occurrence and characteristics are important in order to interpret the infill of terrestrial basins and the construction of floodplains.

Origin and palaeo‐environments of calcareous sediments in the Moshaweng dry valley, southeast Botswana

AbstractQuaternary sedimentation in the Moshaweng dry valley of southeastern Botswana is evaluated on the basis of geomorphological evolution and sedimentological analyses. Stratigraphic evidence reveals an upper surface (1095 m) containing abundant sil‐calcrete, an intermediate surface (1085 m) in which sil‐calcrete underlies nodular calcrete and lower (1075 m) surface in which sil‐calcrete and nodular calcrete are interbedded. This subdivision is reflected in the geochemical composition of the sediments which show an overall trend of decreasing SiO2 content (and increasing CaCO3 content) with depth from the highest to the lowest surface levels. The calcretes and sil‐calcretes represent modifications of pre‐existing detrital Kalahari Group sand and basal Kalahari pebbles which thinned over a Karoo bedrock high. Modification took place during wet periods when abundant Ca++‐rich groundwater flowed along the structurally aligned valley system. With the onset of drier conditions, water table fluctuations led to the precipitation of nodular calcretes in the phreatic layer to a depth of about 20 m. A major geochemical change resulted in the preferential silicification of the nodular calcrete deposits. Conditions for silica mobilization may be related to drying‐induced salinity and in situ geochemical differentiation brought about by pebble dissociation towards the top of the sediment pile. As calcretization and valley formation progressed to lower levels, silica release took place on a diminishing scale. Thermoluminescence dating infers a mid‐Pleistocene age for sil‐calcrete formation suggesting that valley evolution and original calcrete precipitation are much older. Late stage dissolution of CaCO3 from pre‐existing surface calcretes or sil‐calcretes led to the formation of pedogenic case‐hardened deposits during a time of reduced flow through the Moshaweng system possibly during the upper or late Pleistocene. Copyright © 2002 John Wiley & Sons, Ltd.

Silicification of calcrete in palaeodrainage basins of the Australian arid zone

Quaternary calcrete deposits in palaeodrainage basins of the arid zone of Western and central Australia have developed near the surface within a remarkably short span of time. Silicification of the calcrete, involving the filling of voids and replacement of calcite by opaline and quartzose silica, has taken place in both the vadose and phreatic hydrological zones as a late diagenetic event. Diagenetic changes in the textures of silicified calcrete commonly correspond with silica-phase transformation and reflect replacement of carbonate through dissolution and reprecipitation. Silicified calcrete horizons in ancient terrestrial (lacustrine and groundwater) carbonate deposits may be related to the migration history of the silica in palaeohydrological systems. The presence of silica as thin grain coatings, veins and pore and cavity linings is a manifestation of the intricate balance between the solution of host calcrete and precipitation of silica under prevailing arid/semi-arid hydrological conditions. Such cogenesis, although spatially highly variable, has an important bearing on preservation and redistribution of economic mineral ores, and on the occurrence of fresh groundwater aquifers in the internal drainage basins of Australia.

Silicification of calcrete in palaeodrainage basins of the Australian arid zone

Quaternary calcrete deposits in palaeodrainage basins of the arid zone of Western and central Australia have developed near the surface within a remarkably short span of time. Silicification of the calcrete, involving the filling of voids and replacement of calcite by opaline and quartzose silica, has taken place in both the vadose and phreatic hydrological zones as a late diagenetic event. Diagenetic changes in the textures of silicified calcrete commonly correspond with silica-phase transformation and reflect replacement of carbonate through dissolution and reprecipitation. Silicified calcrete horizons in ancient terrestrial (lacustrine and groundwater) carbonate deposits may be related to the migration history of the silica in palaeohydrological systems. The presence of silica as thin grain coatings, veins and pore and cavity linings is a manifestation of the intricate balance between the solution of host calcrete and precipitation of silica under prevailing arid/semi-arid hydrological conditions. Such cogenesis, although spatially highly variable, has an important bearing on preservation and redistribution of economic mineral ores, and on the occurrence of fresh groundwater aquifers in the internal drainage basins of Australia.

Investigation of sensitive signals due to gamma-ray irradiation of chemical precipitates. A feasibility study for ESR dating of gypsum, phosphate and calcrete deposits

In order to assess the application of ESR to dating of authigenic chemical precipitates (including gypsum, phosphate and calcrete deposits), a wide variety of samples of different age, purity, mineralogy and geographical distribution were utilized. Observations of a number of radiation-induced signals indicate that the accumulated dose (AD) derived from individual signals may vary by one or two orders of magnitude. Thus the selection of an “appropriate” signal of the ESR spectrum is critically important in ESR dating. For this reason, the first stage of our investigation involved the irradiation of selected samples by a 60Co γ-ray source, with a known absorbed dose (100 or 400 Gy), to check the reliability of AD values obtained from the routine method. The results enable us to select a suitable signal for dating. A comparison of ESR and 14C ages of duplicate pedogenic calcrete from late Pleistocene sediment indicates that a valid method for the assessments of annual dose rate is of paramount importance. Based on our current experience, ESR appears to be a feasible technique for dating chemical precipitates such as gypsum, phosphate and calcrete deposits.

Composition and origin of some calcrete deposits in south Western Desert of Egypt

The examined calcretes outcropping at southwestern Egypt are mainly developed on the Cretaceous Nubian sandstones. They are morphologically classified as hardpan type, and geologically described as valley calcretes. Twelve calcrete and six Nubian samples were petrographically, mineralogically and chemically examined. Their heavy mineral constituents were also studied. Data obtained showed that our calcretes consist mainly of low Mg-calcite and quartz with minor quantities of feldspar and gypsum. The principal clay mineral identified in the clay fractions are kaolinite and palygorskite, associated with minor amounts of illite/smectite mixed layer, illite and smectite. These clay minerals are partly detrital inherited from the original sediments and partly authigenic developed during the calcrete formation. Comparisons with the chemistry of both Egyptian coastal and world calcretes show that the studied calcrete is low in CaO, MgO and P 2O 5 and is highly enriched in SiO 2, Al 2O 3, K 2O and TiO 2 contents. The examined trace elements are largely detrital in origin and their distribution is mainly controlled by the contents of quartz, clays, carbonate and iron oxides.

The central Australian groundwater discharge zone: Evolution of associated calcrete and gypcrete deposits

The groundwater discharge zone of central Australia is marked by a chain of playas extending 500 km from Lake Hopkins in Western Australia through Lake Neale, Lake Amadeus and numerous smaller playas to the Finke River in the Northern Territory. This great valley is devoid of surface drainage, and the playas and their associated landforms and chemical sediments are a result of groundwater transmission, and discharge from a large regional flow system. The valley contains extensive groundwater calcrete deposits, which are commonly silicified. The playas contain gypsum and glauberite deposits resulting from the evaporative concentration of discharging groundwater to 250 g/1total dissolved solids. Thin gypcrete crusts have developed on the playa margins and islands as a result of induration of bordering gypsum dunes. Diagenetic changes in the calcrete and gypcrete are caused by dissolution and reprecipitation through groundwater movement. Calcrete and gypcrete samples from playas near Curtin Springs, NT, dated by electron spin resonance (ESR) — calibrated by 14C dates on calcrete and the uranium content of gypcrete — are mostly in the range 8000–16 000 years BP, and are evidence of groundwater discharge conditions similar to those of the present day. Calcrete in the vadose zone, above the present water‐table, gives ESR dates in the range 22 000–27 000 years BP suggesting episodes of high intensity rainfall at that time. Phreatic calcrete, below the water‐table, with ages in the range 34 000–75 000 years BP, provides evidence of older episodes of calcretization. The ESR and 14C dates for the chemical sediments suggest that this groundwater flow system has been an important feature in the region for much of the Late Quaternary. Although there are problems with the interpretation of ESR dating for chemical sediments with diagenetic alteration, the technique shows promise as a dating tool.