AGSO Journal of Australian Geology & Geophysics, 16 (3), 223- 229

Quaternary vadose calcretes revisited Aro V. Arakel l

Vadose calcretisation commences with precipitation of micritic calcite from carbonate-rich meteoric water in the soil moisture zone. Through time, calcrete is subjected to repeated episodes of vadose cementation, dissolution , and brecciation of earlier-formed soil components, giving rise to stratigraphically complex soil profiles. The degree of complexity of individual profiles is largely influenced by the geomorphological setting and hydrological history of the local groundwater systems.

Morphological similarities of calcrete duricrusts form the basis for establishing inventories of textural, compositional and physiog•raphic features for Quaternary calcrete soil profiles and their

Introduction Vadose calcrete is a secondary deposit of mlcntlc and cryptocrystalline calcite, resulting from soil-forming processes within the regolith profile (Arakel & McConchie 1982; Goudie 1983; Milnes & Hutton 1983). It differs in origin from phreatic (or groundwater) calcrete, the latter forming within the aquifer or laterally flowing groundwater zone (Mann & Horwitz 1979; Dixon 1994).

Calcrete duricrusts of Quaternary age are an important component of the regolith landscape in arid and semi-arid regions of Australia. Pedogenic carbonate deposits, petrologi•cally and morphologically similar to Quaternary vadose calcretes, are also common in the geological record. All varieties of calcrete paleosol (relict, buried and exhumed) are found in the geological record (Allen & Wright 1989), and calcrete is particularly abundant in ancient petroleum-bearing alluvial and deltaic sequences.

Vadose calcretes in palaeosols are intrinsically important for indicating subaerial exposure. They are also valuable for deciphering palaeoclimates and palaeogeomorphology and for time resolution. In natural resource assessment, calcretes are used for basin analysis and as indicators of porosity evolution in ancient limestone deposits. Geochemical and geophysical anomalies associated with subsurface calcrete are increasingly being used in mineral and groundwater exploration in regions with significant regolith cover.

The presence of vadose calcrete signifies the influence exerted by pedogenesis on radical modification of original features of the host sediment. Conversely, the absence of pedogenic carbonates from the ancient subaerially exposed sedimentary record may reflect a high rate of sedimentation, low rate of pedogenesis, or the loss of evidence, owing to reworking or by diagenetic overprinting during burial. Another possibility is that evidence may have been missed by the observer. In any case, pedogenic features in calcrete soil profiles offer a powerful tool for identifying and differentiating calcrete paleosols in sedimentary records.

This paper attempts to highlight this point by reviewing the present knowledge of Australian vadose calcretes. It also discusses the significance and application of duricrusts to assessment of natural resources and palaeoenvironmental reconstructions.

Modes of formation and distribution of calcretes Vadose calcretes are an integral part of soil profiles in arid to semi-arid coastal and inland drainage basins of Australia (Fig. I) . Studies of their sedimentary features (Fig. 2) indicate

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evolutionary stages. Such inventories are useful tools for palaeo-en•vironmental reconstruction , although calcrete duricrusts can be either an advantage or an obstacle for mineral exploration, depending on the level of understanding of calcrete and its relationship to the bedrock.

Assessment of the timing of the main phases of carbonate precipitation indicates that calcrete soil profiles can attain maturity within a relatively short time. Problems with establishing a reliable time frame for differentiating Quaternary calcrete duricrusts from older counterparts represent a major challenge for further research.

Contours are median annual rainfall (cm)

400 km ~

Figure 1. Australian basins with major occurrences of vadose calcrete,

that, following initial precIpitation of micritic carbonate in the soil moisture zone, calcrete can undergo repeated episodes of vadose cementation, dissolution and brecciation, owing to fluctuations in local geomorphologic and hydrogeological conditions (Arakel 1982; Arakel & McConchie 1982; Milnes & Hutton 1983; Carlisle et a!. 1978).

Calcretisation begins with downward percolation of car•bonate-rich soil water through the porosity/permeability zones in near-surface rock or sediment. The development of perme•ability zones in upper parts of the soil profile proceeds concurrently with precipitation of micritic calcite in the lower parts as coatings in and around solution pipes, fracture lines and caverns. Locally, dissolution and reprecipitation of micrite become less important as the initial porosity/permeability zones are progressively 'plugged' by micrite and microsparite cements ('plugged horizon' of Gile et a!. 1966); these processes are then diverted to other parts of the soil horizon. Below the massive calcrete horizon, calcretisation is commonly restricted to formation of calcrete mottles and ramifying sheets. Above the 'plugged' horizon calcretisation continues with development of laminar, breccia, and pisolitic loose soil horizons (Fig. 2).

Two basic modes of micrite 'plugging' in soil profiles have been proposed in the literature (Fig. 3). • One mode relates to micritisation within a calcareous host

224 A.V. ARAKEL

with a relatively deep-seated groundwater setting (e.g. in calcarenite terrains along the coasts of Western Australia; Arake! 1982). In such a setting, the 'plugged' horizon is commonly located above the zone of local groundwater influence (Fig. 3a).

• A second mode of micritic 'plugging' relates to soils associated with non-calcareous hosts in a shallow ground•water setting (i.e. most of the drainages in central Australia; Arakel & McConchie 1982; Jacobson et a1. 1988). Here, micritisation evidently commences in the lower parts of a zone of alternate wetting and drying, which may grade

A

B

c

D

downward into the zone of groundwater fluctuation (Fig. 3b). Consequently, in the majority of calcrete-bearing inland drainage basins, the vadose calcrete grades downward into phreatic calcrete that has precipitated from laterally flowing groundwater solutions (Arakel et al. 1990). A discussion on the interrelationship of vadose calcretes and underlying phreatic calcrete deposits is beyond the scope of this paper.

Figure 2. A-Typical calcrete soil profile from the Perth Basin, comprising the following horizons (in descending order): loose topsoil, pisolitic calcrete, laminar calcrete, and massive calcrete. B-Polished section of a calcrete pisolite, comprising a caIcarenitic nucleus and laminar coatings. C-Calcrete outcrop from the Perth Basin, comprising an upper breccia horizon overlying laminar calcrete and massive calcrete horizons with abundant calcified root moulds. Height of the profile is about 2.2m. D-Magnified rhizocrete from massive calcrete horizon in 'C'. The tubule is about 2.5mm thick.

QUATERNARY VADOSE CALCRETES 225

A Calcareous host, deep-seated groundwater setting (after Arakel , 1982)

Laminar ES3 Calcrete (micrite)

Soil moisture zone

I: I ~VU9S

Micritic 'plug' Massive Breccia

Laminar

Groundwater level

SOLUTION/ PRECIPITATION

PRECIPITATION BRECCIATION COMPLETE PROFILE

Massive

B Non-calcareous host, shallow groundwater setting (after Arakel and McConchie, 1982)

'-.,;."': Runoff

Laminar

Breccia Zone of alternate

wetting and drying Massive Laminar

Groundwater level

(not to scale) 'PLUGGING'

Micritic 'plug'

1 Groundwater fluctuation zone

PRECIPITATION BRECCIATION COMPLETE PROFILE

Massive

16/66

Figure 3. Modes of micritic 'plugging' and development of calcrete soil profiles.

Sedimentary features of the vadose calcretes The key sedimentary features of Australian vadose calcrete deposits are listed in Table 1. These features may be used to establish a set of diagnostic criteria for differentiating vadose calcretes from the host material or other duricrusts.

Structural features , particularly those resulting from karstic dissolution (Fig. 4) and/or silicification of the upper parts of the duricrust mantle (Fig . 5) are common in many Australian inland drainage basins (Arakel et al. 1989). The Quaternary silcretes postdate the main phases of calcretisation and differ significantly from widespread silcrete cappings on buttes and mesas of presumed Tertiary age. Silica (Fig. 5) commonly mimics the textural and morphological features of the host calcrete and is itself impacted by late-stage karstification and collapse of the framework components. The end result is often complex facies relationships within the individual profiles and establishment of new surface morphology and local drainage and groundwater networks (Arakel 1991).

The co-genesis of silica and carbonate species appears to be favoured by the presence of mildly alkaline pore solutions with high Mg:Ca ratios (Arakel et al. 1986b). Values of isotopic analysis for 13C and 180 in analysed calcrete profiles

are consistent with the precipitation of dissolved carbonate in groundwater, which probably reflects the loss of CO2 (Chen & Polach 1986; Jacobson et al. 1988). Significant fluctuations in CO2, particularly during intense evapotranspiration, is known to result in pH variation and extension of the carbonate-silica precipitation front into the soil-moisture zone (cf. Carlisle et al. 1978).

Time frame for calcrete profile development Because of low average sedimentation rates in terrestrial settings, soil formation is expected to have a 'residence time' in excess of tens of thousands of years in the upper part of the weathering profile. The residence time will depend on the sedimentation rate and the thickness of the zone of active soil formation.

Vadose calcretes from the Karinga Creek drainage system in the Northern Territory, dated by 14C techniques, extend back to 20,000 years BP, a time corresponding to the start of aeolian reworking of central Australian playa deposits. The coexistence of superimposed calcareous soil profiles and karst dissolutional features in the Karinga Creek palaeodrainage channel suggests episodes of minor climatic relaxation. ESR

226 A.V. ARAKEL

Table 1. Key sedimentary features of vadose calcretes.

Feature

Biogenic features

Colouration and discolouration

Destratification

Boundary/contact relationships

Granulometric/mineralogical gradients

Chemical gradients

Clay mineralogy

Structural features and internal fabrics

Silicification

Indicators I Causes

Rootlets. root moulds and casts, rhizocretions, terrestrial faunal evidence, coprolites and traces of palaeosol microbes.

Reddening and mottling due to changes in soil redox and diagenesis.

Due to pedoturbation, mass-volume changes, soil creep, etc.

Superimposed by erosional contacts.

Indicating both physical and chemical transformations.

Including trace elements and stable isotopic values.

Indicating past changes, soil moisture regime, and past wetting and drying cycles.

Pseudo-anticlinal structures, erosional and exokarst features.

Diagenetic replacement of calcareous material by opaline and quartzose silica.

ages between 22 000 and 27 000 years BP were obtained for vadose calcretes in the nearby Curtin Springs area (Chen et al. 1988, 1990). Although these ESR ages should be considered with caution, because they are based on estimates of dose rates rather than direct measurement, overall they corroborate the occurrence of a major phase of regional groundwater

discharge. On the basis of moisture requirements for carbonate leaching, an arid to semi-arid climate has been proposed for most of the calcareous mantle development in this region.

The ESR ages of 34 00-075 000 years BP for the Curtin Springs phreatic calcretes, in drill cores at depths to 12.5 m below surface, probably date the main phase of groundwa•ter-related calcretisation. The older ESR ages obtained from mottled calcrete lenses at depths of 16-17 m relate to earlier episodes of calcretisation.

A

B

Overall, the available evidence indicates that, although chemical precipitation and alteration processes have been con•tinuous to the present time, the near-surface genesis of vadose calcrete probably took place within a remarkably short time.

Calcrete soil landscape features in regolith terrains It is apparent that the interaction between climatic, hydrologic and geomorphic processes exerts an overriding influence on the mode of occurrence and distribution of calcrete duricrusts in arid and semi-arid regions of Australia. The morphological similarities and abundance of Quaternary calcrete deposits in

c Figure 4. Karst features in Quaternary calcrete outcrops of the Amadeus Basin. A-Large karst calcrete clasts showing internal layering features and coated with laminar calcrete. B-Ero•sion/bevelling followed by deposition of pisolitic calcrete. C-En•closure of ferruginous pebbles in micritic coating of a calcrete clast (see Arakel 1992 for further details).

QUATERNARY VADOSE CALCRETES 227

VOID FILLING

Chalcedonic

Micrite ~ Vadose zone

~ Phreatic zone

MASSIVE TO

NODULAR CALCRETE

~~-r Cryptocrystalline ~ ~sl"ca

Fibrous

Groundwater fluctuation

zone

calcite

~ Cryptocrystalline

:JJJ!P'''~ ~ooq

·0 ~ ~ . ·0 Fibrous calcite

Standing water level

' ., ' "

: ..... : .. . :.'

MASSIVE CALCRETE

.~

VOID FILLING

Micrite

Opaline silica

~o~ spherulites

o ~ ·0----- Chalcedonic ~ silica groundmass

Coarse calcite

\ I Microcrystalline

16/69

Figure S. Key petrofabric features related to silicification of vadose calcretes (after Arakel et al. 1989).

the Australian regolith indicate that the pedogenic processes act as a continuum within the upper few metres of the landscape (Arakel et al. 1990). These processes evidently give rise to a number of intrinsic petrofabrics that are common to vadose calcretes in most profiles studied (Arakel & McConchie 1982; Wright 1986). Calcrete morphologies and associated fabrics can be used to assess causative stages in hydrologic-geomor•phic interactions. From these stages, time frames for devel•opment of calcareous soil landscape units within a regolith terrain may be defined.

The key morphologic features that can confidently be attributed to vadose calcretes in a regolith terrain include: • Multiple soil profiles resulting from physical redistribution

of calcrete fragments and formation of complex soil profiles, which may incorporate two or more genetically distinct 'intra-formational ' pisolitic calcrete horizons (Fig. 6; Arakel 1986a).

• Exokarstfeatures produced at landscape level by continuous development and destruction of secondary porosity/perme•ability zones in topographically elevated areas followed by soil creep and redistribution of calcrete duricrust components on the lower surface mantle (Fig. 4; Arakel 1991).

• Groundwater discharge outlets developed in areas with well-established networks of karst solution pipes and cavities in calcrete/silcrete duricrusts. The karst features act as effective conduits for the discharge of regional shallow groundwater, particularly following major rainfall (Arakel & Cohen 1991).

• Relief inversion brought about by differential erosion of calcareous material and overlying soil or alluvial layers. Calcrete initially deposited in topographic lows becomes elevated to its present high topographic position by virtue of progressive removal of more easily eroded adjacent material. Within the time span indicated for calcrete soil profile

development, the morphologic features listed above are largely inter-linked and often time-transgressive. In this context, karstic features in calcrete duricrusts have an important influence on the shallow groundwater recharge-discharge pattern and the geomorphological evolution of many Australian regional groundwater discharge zones (Jacobson & Arakel 1986; Arakel 1991). The influence of variation in groundwater discharge patterns is shown by bevelling processes and their impacts on the evolution of playa lakes and associated land forms in arid regions (Bowler 1986).

Vadose calcretes and resource evaluation Anomalous concentration of specific minerals through pedo•genesis is not uncommon in arid zone calcrete duricrusts. However, many near-surface mineral concentrations are not strictly pedogenic in origin, as they extend to the shallow phreatic zone via the capillary fringe. Thus, successful application of geochemical and geophysical methods of exploration in calcareous regolith terrains will require an understanding of the origin of components of a calcrete landform and their relationships with the bedrock and local groundwater system. To some extent, the level of understanding of calcrete can be judged from the importance given to it by an explorationist as either an advantage or an obstacle to mineral exploration (cf. Joyce & Mazzucchelli 1978).

Systematic sampling and analysis of calcareous soil particles in the surface veneer is becoming a common practice for detecting anomalous levels of elements that can become concentrated in the soil horizons through a variety of dispersion processes (cf. Lintern et al. 1992). Sampling of calcrete pebbles may be a useful approach in metal exploration in arid zone terrains because of low leaching susceptibility of calcareous soil particles. However, caution is needed in the interpretation of results from discriminant analysis of calcareous soil surveys. This applies particularly to extrapolation of the results from

228 A. V. ARAKEL

,I"

':' :::tJi.·' .. t ~. .~'~ ""'::-'

Figure 6. Multiple calcrete soil profile from the Perth Basin.

calcrete pebbles that represent only part of the source rock mass. Results from selective sampling of surface calcrete pebbles need to be interpreted in terms of 3D variables and heterogeneous processes. Soil geochemical surveys need to be conducted in conjunction with mapping of landscape units, so that the relationships between bedrock, overlying materials, and landforms are established before geochemical results are interpreted. An additional consideration is the negligible value of calcrete pebble mineralogy represented in each size population in the non-skeletal fraction. Because of extensive modification of host particles during soil profile development, pebble size of the sample medium is irrelevant

Opportunities for research Despite the widespread occurrence of calcrete duricrusts and their potential use for successful resource exploration and/or palaeoenvironmental reconstruction, the vadose calcretes re•main one of the least understood features of the Australian regolith . Because of limitations with field information and the incongruency of field data from past studies, the discrimi•nation of Quaternary duricrusts from their older counterparts is at present difficult This is particularly the case for Tertiary(?)landscapes, where soil horizons appear to incorporate genetically different authigenic carbonates and carbonate host rocks. The presence of 'intraformational' carbonate pebbles and nodules complicates age determination for stratigraphic correlation and palaeoclimatic inference (cL Callen et a1. 1983). Identification and genetic discrimination of carbonate

A

1·'~iI~~~:;~lrllllll~PJ··I.tJrl

B

...... :.:-~

c

I· : . .':.'::. ·1 Calcrete (micrite)

(not to scale) 16/67

Figure 7. Stages involved in relief inversion in calcrete soil terrains.

components before the determination of carbonate ages is, therefore, an important research challenge.

Another important research frontier is the assessment of variation in calcrete morphology and mineralogy along a major climatic gradient, to determine how climate and vegetation influence biogenic versus abiogenic calcretes. For example, there is a nearly five-fold increase in precipitation with corresponding changes in vegetation types from central Australia to Adelaide. Comparison of calcretes along such a regional transect would provide an excellent opportunity for the assessment of evolutionary features of calcareous landforms in different climatic zones.

Conclusions • Vadose calcretes initially form by mlcnte precIpItation

within the soil moisture zone; the thickness and lateral extent of the resultant calcrete soil profiles depend on local hydrology (e.g. groundwater depth), physiographic setting, and textural characteristics of the host material.

• Stratigraphically complex calcrete soil profiles result from multi-stage redistribution of calcrete components in the soil landscape in a relatively short time.

• Quaternary calcretes with similar textural, compositional and physiographic features can be grouped together ac•cording to their common morphological features. Such classification can aid assessment of the evolutionary stages and the time frame for the main phases of carbonate precipitation. The key processes controlling morphological development of vadose calcretes include relief inversion, multiple soil profile development, silicification, and kar•stification.

• A reasonable understanding of the origin of calcrete components and their relationships with the bedrock is needed for meaningful application of geochemical and geophysical signatures of calcareous soils to mineral exploration programs. Concurrent soil sampling and map•ping of associated landscape units are needed for a meaningful outcome from a discriminant analysis of

calcareous soil survey results. • Although a major formative phase of vadose calcrete in

Australian regolith landscapes appears to coincide with the onset of aridity in the Quaternary, there are problems in establishing a reliable time frame for differentiating these calcareous duricrusts from those that may have formed during relatively wetter intervals in the Quaternary or earlier. These offer a major challenge for future research.

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Milnes, A. R. & Hutton, J .T. , 1983. Calcretes in Australia. In: Soils , an Australian viewpoint. CSIRO Melbourne & Academic Press , London, 928 pp.

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