Groundwater Calcretes Research Articles

Calcretes in semi-arid alluvial systems: formative pathways and sinks

Late Quaternary deposits in Gujarat, western India show an abundant development of calcretes. Three major sinks of carbonate in the alluvial deposits are recognized: (1) groundwater calcretes, (2) pedogenic calcretes, and (3) calcrete conglomerates. Groundwater calcretes originate from carbonate-saturated waters travelling preferentially along stratification planes. Pedogenic calcretes form through soil-forming processes typically in extra-channel areas. Calcrete conglomerates occur as ribbons, sheets and lenses due to the reworking of both pedogenic as well as groundwater calcretes. As a result a pathway of calcretization develops that has the route: groundwater calcrete to pedogenic calcrete to calcrete-conglomerate. The formation of pedogenic calcretes over sediments containing groundwater calcretes demonstrates that (1) apart from aeolian dust, river waters are also a major source of carbonate, and (2) pedogenic carbonates may attain large sizes at accelerated rates due to the presence of pre-existing groundwater calcretes. Consequently, the maturity of a soil may be overestimated if determined by following established morphogenetic sequences.

Palaeodrainage at Uluru-Kata Tjuta National Park and implications for water resources.

The stakeholders in the atypical rangeland area of Uluyu-Kata Tjufa National Park in the Northern Territory are the Aboriginal owners, hundreds of thousands of tourists, and vulnerable desert wildlife species. Sustainable water resources and maintenance of the integrity of the unique geological, ecological and cultural environment at Uluyu are major issues being addressed by the trustees of the Park. The Dune Plains area between Uluyu (Ayers Rock) and Kata Tjula (the Olgas) is the setting for a deep palaeovalley. The flat, dune-covered landscape of the present-day Dune Plains area provides no indication of the existence of the underlying palaeovalley. Integrated datasets including aeromagnetic and airborne gamma-ray spectrometric imagery, processed Landsat Thematic Mapper imagery, a digital elevation model, and water-bore logs, have enabled reconstruction of the Dune Plains palaeoriver valley. The datasets reveal a heterogeneous basement topography of domes and basins with 100 m of vertical relief (a buried 'mini-Kata Tjuia') at depth beneath the Dune Plains area, between the rock monoliths. The ancient valley became completely infilled with lake, river and wind-borne sediments during the Tertiary and Quaternary. Major faults traversing the basement beneath the area influenced early erosion and valley initiation. The palaeovalley was originally a closed valley which later evolved to a through-flowing river that fed Lake Amadeus to the north. The Dune Plains palaeovalley is now the setting for a compound bedrock- Cainozoic-sediment aquifer system which is the major source of water supply for the inhabitants and tourists of Uluyu-Kata Tjufa National Park. Major landscape units making up the Uluyu area include groundwater calcrete, sheetwash slopes of red earth, sandplains and dunefields, and modern ephemeral alluvial braidplains that are constrained to corridors afforded by swale networks. The sheetwash unit forms gently sloping aprons around outcrops and supports banded mulga shrubland. During rainfall, surface run-off acts as a 'sheetflow recharge mechanism'. This mechanism maximises water conservation allowing survival of the mulga groves and associated ecosystems. During major rainfall events, the sheetflow processes augment replenishment of the aquifer system at the base of the slopes. The palaeodrainage configuration and hydrodynamics of both the surface environment and the subsurface aquifer system are complex. The methodology used to reconstruct the Dune Plains palaeodrainage and associated aquifer system, and the hydrodynamic processes described for the Uluru area are applicable to widespread rangeland areas elsewhere across the continent. Keywords: Uluyu, Kata Tjula, Dune Plains, palaeovalley, palaeodrainage, hydrology, groundwater, aquifer, sheetwash, red earths, calcrete, hydrodynamics, mulga (Acacia aneura), run-off, run-on, recharge.

Calcretes at sequence boundaries in upper carboniferous cyclothems of the Sydney Basin, Atlantic Canada

Nodular and associated groundwater calcretes in coal-bearing cyclothems of the Sydney Basin mark the boundary between underlying marginal-marine and overlying alluvial deposits. They are inferred to represent lowstand surfaces (sequence boundaries). The host sediments include both sand/siltstones and palustrine limestones. Microfabrics of calcretes with detrital hosts show evidence of replacement, displacement and shrinkage (alpha fabrics), suggesting that the calcretes formed under relatively arid conditions. In contrast, microfabrics of nodular calcretes with limestone hosts exhibit predominantly beta fabrics, with root-induced brecciation and subordinate shrinkage fabrics. The calcretes and host limestones are inferred to have formed in topographic lows. Thick alluvial sediments with red calcic vertisols overlie the calcretes, and formed within the transgressive systems tract under conditions of abundant sediment supply. Coals and associated hydromorphic palaeosols lie near the transgressive maximum and in the highstand systems tract. The presence of calcareous palaeosols on lowstand surfaces suggests that the lowstands were times of relative climatic aridity.

Dynamic model of the genesis of calcretes replacing silicate rocks in semi-arid regions

In both pedogenic and groundwater calcretes, calcium carbonate precipitates in voids, or displacing other grains, or replacing underlying parent silicates. Replacement textures are widespread in pedogenic calcrete. Many calcretes also contain magnesium layer silicates and minor chert. We present a reaction-transport model that accounts for the genesis of replacement in calcretes and for their mineralogy. Replacement is difficult to account for geochemically because it requires simultaneous removal of large amounts of silicates and import of also large amounts of CaCO 3. In the model the genesis of replacement is directly related to seasonally alternating dry-wet climates and to appropriate groundwater (or circulating soil water) compositions. In a dry season, water evaporation causes CaCO 3 and sepiolite (or attapulgite) to precipitate. If groundwater contains enough Mg 2+, sepiolite precipitation by the chemical-divide mechanism depletes SiO 2(aq), resulting in the dissolution of parent silicates. In the following wet season, sepiolite dissolves fast, and silica and cations are flushed away by rainwater, making room for CaCO 3 precipitation in the next dry season. As climate cycles repeat, CaCO 3 is accumulated and silicates are removed. The sepiolite (or attapulgite, or Mg-smectite) serves as a temporary storage of silica between seasons. If the groundwater contains too little aqueous Mg then the model predicts growth of calcium carbonate without removing silicates, thus producing void filling and or displacive textures instead of replacement. The model consists of a set of nonlinear partial differential equations taking account of mass conservation, dispersion, advection, rainwater infiltration, evaporation, and the kinetics of mineral reactions. The hydrodynamics of unsaturated media is applied in determining water flow in calcrete profiles. Wet/dry seasonal changes are incorporated by alternating the upper boundary conditions. The model successfully produces the mineral and textural zonation observed in many calcretes (namely, at the bottom of the profile the parent rock is first replaced by sepiolite, only part of which is in turn replaced by calcite, whereas at the top of the calcrete the sepiolite is itself completely replaced by the calcite, which appears to “directly” replace the parent rock). The model can produce calcrete near the Earth's surface well above the water table. Low P CO 2 , intensive evaporation, and long dry seasons all are predicted to produce thicker calcretes. Calcretes constitute an effective geochemical tool that, via replacement, removes parent silicate rocks and shapes the landscape of semi-arid countries. The model herein provides a mechanism that accounts for the efficiency of replacement in removing silicates.

A Holocene calcrete from North Yorkshire, England: implications for interpreting palaeoclimates using calcretes

ABSTRACTGlacial 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.

Alluvial Paleosols and Pedofacies Sequences in the Permian Lower Beaufort of the Southwestern Karoo Basin, South Africa

ABSTRACT Three types of paleosol profiles are described as representative of the range of soil types that developed on the ancient alluvial plains of the southwestern Karoo Basin during the Late Permian. Continuous 3-dimensional exposures of the Lower Beaufort (Adelaide Subgroup) strata allow detailed sedimentological, taphonomic, and paleopedological observation of thick floodplain sequences that contain numerous stacked paleosol profiles. Various floodplain strata contain characteristic paleosol profiles that are regulated by the periodicity and competence of overbank flooding, both of which decrease with increasing distance from the contributing channel. Paleosols developed in levee or channel-bank deposits are immature calcic Entisols displaying evidence of early carbonate translocation but without well-developed horizonation. Proximal floodbasin paleosols, at the foot of the meanderbelt slope, are mature calcic Vertisols with more fully developed profiles containing zones of both carbonate and clay illuviation as well as some groundwater calcretes. Slickensided stress argillans in the B-horizons are indicative of multiple cycles of wetting and drying. Distal floodbasin paleosols are dominated by hydromorphic and evaporitic processes as a result of their axial position in the floodbasin. Sheet-like networks of mudcrack casts composed of calcretized aeolian sand and silt suggest that dust was a major source of calcium carbonate to the e soils. The paleosols are interpreted as having formed under warm to hot, semi-arid climatic conditions with a strongly seasonal rainfall of 500 to 800 mm per annum and a mean annual moisture deficiency. The geomorphic setting was similar to the Holocene Indo-Gangetic alluvial plain. Vertebrate burrow casts in the proximal floodbasin facies indicate a minimum depth to watertable of 150 cm. The distribution of rooted horizons reflects dense vegetation flanking major drainage channels and sparse cover in the distal floodbasin. A simple stratigraphic sequence of once laterally continuous paleosols is termed a pedofacies. Stacked sequences of pedofacies within approximately 20-50 m thick, and 100-250 m thick stratigraphic successions reflect second- and first-order sedimentary cycles respectively. Both scales of cyclicity are attributed to autocyclically controlled avulsions of differentially aggrading channels in the first instance and of complete drainage networks in the second.

Origins of some Early Carboniferous calcrete fabrics revealed by cathodoluminescence: implications for interpreting the sites of calcrete formation

Calcretes from the Lower Carboniferous marginal marine limestones of South Wales, Germany and Belgium exhibit complex calcite microspar fabrics with rhombic, irregular and spherical crystals in variable mosaics. While many of these fabrics resemble classic recrystallization microspars, having irregular crystal forms and non-planar crystal boundaries, cathodoluminescence microscopy reveals that they result from multiple phases of precipitation and dissolution. No evidence for recrystallization by crystal boundary migration or replacement during the transformation of less stable calcite minerals (aragonite or high-magnesian calcite) is seen. Identical microspar fabrics have been widely described in Quaternary calcareous soils, which may also be primary fabrics and not due to recrystallization or mineral transformation. Cathodoluminescence provides a useful tool for interpreting calcrete fabrics and, in the case of these Carboniferous forms, indicates multiple phases of carbonate saturation and leaching reflecting the prominent wetting and drying phases evidenced by associated vertic soils horizons. The complex cycles of precipitation and dissolution are characteristic of pedogenic calcretes, and the recognition of such features using cathodoluminescence may provide a means of differentiating pedogenic from groundwater calcretes.

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.

Holocene climate history of coastal southwestern Australia using calcrete as an indicator

Sheets of groundwater calcrete in the stratigraphic sequences of beach/dune units deposited in the interval mid Holocene-present, record a history of variable Holocene climate. Studies of Holocene coastal-plain areas of the Perth Basin spanning a gradient of climate from humid to semi-arid show that calcrete today is clearly forming as a sheet just above the water table in the zone of capillary rise in humid environments, as thin, isolated lenses under distinct outliers of vegetation zones in subhumid environments, and not at all in semi-arid zones. However, such calcrete sheets are not widespread earlier in the Holocene at levels where former water table positions can be inferred at the various localities. Evidently, apart from small isolated lenses, calcrete sheets did not develop at most localities earlier in the Holocene. The very distinct sealevel history, and the corollary, a distinct water table history, of Holocene coastal plain sequences enable a reconstruction of calcrete history because each water table level should have a distinct calcrete sheet if it was developed. The evidence shows a mid to late Holocene period of relatively more arid conditions and an extension of more humid climates only in the past 2800 14C yrs.

Distribution of Calcrete in Holocene Coastal Sands in Relationship to Climate, Southwestern Australia

Seven study areas located over 320 km of coastal plain in southwestern Australia show the distribution of groundwater and vadose calcrete within Holocene coastal sands (age <7,100 yr BP,) over a regional climate gradient. The calcrete exhibits a range of structures that include rhizoconcretionary, mottled, massive, and laminar types. Groundwater calcrete, composed of mottled, massive, and laminar types, occurs as a thin sheet in the zone of capillary rise just above the water table. Vadose calcrete is predominantly rhizoconcretionary. At all sites where it is well developed the groundwater and vadose calcretes are closely associated with vegetation. Calcrete is well developed in southern areas that are humid. Further north, as the rainfall decreases and evaporation increases, calcrete is less common until it is absent or rare at the semiarid northern limit of the study region. Its limit of occurrence is correlated with a region whose rainfall is ca 800 mm/annum, evaporation is ca 1,900 mm/annum, and mean summer temperature is 23 degrees C. This study shows, therefore, that a trend toward a drier climate may not be conducive to the development of calcrete, although such a view prevails in the literature.

Groundwater calcrete deposits in Australia some observations from Western Australia

Abstract Groundwater calcretes are a non‐pedogenic form of calcrete occurring in broad fossil drainage systems. In Australia, they only occur north of about latitude 30°S and their formation requires arid conditions with very high potential evaporation as well as the periodic recharge of groundwater systems. They are linear, tabular limestone bodies, averaging about 10 m thick, occurring at or close to the surface and forming gentle mounds. The most extensive are located in the centre of the zone of distribution, where the climatic conditions are optimal, and are of the order of 100 km by 10 km. Some groundwater calcretes are fossil and strongly dissected, others are modern and still in process of formation. A model of carbonate precepitation below the water table, i.e. in the phreatic zone, resulting in continuous calcrete growth, accounts for their morphology, their textures and the apparently inverted stratigraphy suggested by carbon‐14 dating. In areas of maximum growth, small anticlines develop which have structures engendered by both shear and concentric folding due to upward growth pressure.