Properties and development of channel calcretes in a mountain catchment, Tabernas Basin, southeast Spain

Geomorphology 50 (2003) 227 – 250 www.elsevier.com/locate/geomorph

Properties and development of channel calcretes in a mountain catchment, Tabernas Basin, southeast Spain David J. Nash *, Roger F. Smith School of the Environment, University of Brighton, Cockcroft Building, Lewes Road, Brighton, Sussex BN2 4GJ, UK Received 1 September 2000; received in revised form 1 March 2001; accepted 15 July 2002

Abstract Nonpedogenic channel calcretes of up to 3.5 m thick occur in a number of locations across the Tabernas Basin in Almerı´a Province, southeast Spain. They represent the last major phase of late Quaternary calcium carbonate cementation to affect this semiarid drainage basin. Channel calcretes are situated within the basal parts of sequences of < 12-m-thick, poorly sorted, coarse-grained and schist-dominated fluvial gravels that were deposited within confined bedrock channels. Channel calcretes occupy the full basal width of the bedrock channels within which they occur, and mostly crop out near the mouths of tributary valleys to the main Rambla de Tabernas drainage system. Six profiles from five separate channel calcrete outcrops were logged and sampled. All samples were analysed in thin section and under scanning electron microscope (SEM). From these analyses, the macro- and micromorphological properties were identified, and the mode of origin of channel calcrete profiles was assessed. Results indicate that all channel calcretes are massively cemented by calcite and appear uniform at the field profile scale. Microscale analyses reveal that detrital grains within the calcretes are initially coated by micrite, which is overlain by graincoating and pore-filling sparite. A series of broad trends in cement type and micromorphology are recognised within individual field profiles. All profiles exhibit an increasing degree of calcite crystal size in a down-profile direction. Most field profiles predominantly contain micrite in upper parts with an increasing percentage of sparite towards the base. Some profiles are, however, dominated by sparite and show increasing crystal size and occurrence of euhedral crystals towards the profile base. Many profiles also contain evidence of postcalcite cementation diagenesis in the form of dissolution of calcite crystal faces or replacement of the calcite cement by amorphous silica, with alteration mostly occurring in the lower parts of profiles. The trends within the calcrete fabric appear to have developed in conjunction with a fluctuating water table, with the increased crystal size and occurrence of euhedral crystals towards the base of profiles arising from greater duration of wetting in basal zones. Cementation in some field profiles occurred at depths of up to 12 m within the host sediment, well below the zone of capillary rise and pedogenesis, and in the absence of significant organic activity. As such, channel calcretes in the Tabernas Basin may represent an ideal opportunity to observe the influence of groundwater upon cementation of coarse-grained sediments by carbonate. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Channel calcrete; Calcretisation; Groundwater; Tabernas Basin; Southeast Spain

* Corresponding author. E-mail address: [email protected] (D.J. Nash). 0169-555X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 5 5 5 X ( 0 2 ) 0 0 2 1 6 - 7

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1. Introduction Many semiarid and arid parts of the world are characterised by carbonate retention and accumulation within the soil or regolith, which, given sufficient time, may lead to the development of calcareous deposits such as calcretes (Goudie, 1983; Butt, 1992). Calcrete has been defined by Wright and Tucker (1991 p. 1) as ‘‘a near-surface, terrestrial, accumulation of predominantly calcium carbonate, which occurs in a variety of forms from powdery to nodular to highly indurated. It results from the cementation and displacive introduction of calcium carbonate into soil profiles, bedrock and sediments, in areas where vadose and shallow phreatic groundwater becomes saturated with respect to calcium carbonate’’. This definition is probably the most useful general description arising from a quagmire of terminological debate. Calcretes occur in a range of types, but can be broadly grouped into primary pedogenic or nonpedogenic varieties, with secondary detrital or reconstituted calcretes derived from the two primary types (Carlisle, 1983; Table 1). This subdivision is made on the basis that the two categories of primary calcrete have differing modes of genesis. Pedogenic calcretes develop as a result of soil-forming processes, and nonpedogenic calcretes form by a variety of other mechanisms. Not surprisingly, given the nature of this classification, the nonpedogenic calcrete group encompasses the widest range of individual calcrete types and has been further divided into superficial calcretes, gravitational zone calcretes and groundwater calcretes (Carlisle, 1983). Amongst the most widespread varieties of nonpedogenic calcrete are various types of groundwater calcrete associated with either contemporary or

ancient drainage features. These are often subdivided according to their geomorphological setting, but are most frequently referred to as valley or channel calcretes, with both terms used interchangeably (e.g. Carlisle, 1983). Studies to date, however, suggest that valley and channel calcretes are distinct varieties of groundwater calcrete and that the individual terms should only be used in specific instances. Valley calcretes are those which form (or are located) within broad, shallow, drainage courses or valleys, cement valley alluvium; however, they do not necessarily occupy the full width of a drainage course or valley. These have been documented from Australia (e.g. Ellis, 1951; Sanders, 1974; Butt et al., 1977; Mann and Deutscher, 1978; Carlisle et al., 1978; Mann and Horwitz, 1979; Deutscher et al., 1980; Arakel and McConchie, 1982; Carlisle, 1983; Arakel, 1986, 1991; Arakel et al., 1989; Jacobson et al., 1988; Hill et al., 1999; McQueen et al., 1999), Botswana (Shaw and de Vries, 1988; Nash et al., 1994a,b), Morocco (Kaemmerer and Revel, 1991), Namibia (Carlisle et al., 1978; Carlisle, 1983), Saudi Arabia (Miller, 1937), Somalia (Butt, 1992), South Africa (Netterberg, 1969) and the USA (e.g. Reeves, 1983). Valley calcrete masses have been shown to vary considerably in size, with the largest in Australia reaching 10 km in width and 100 km in length (e.g. Arakel and McConchie, 1982; Carlisle, 1983; Arakel, 1986). In contrast, channel calcretes are those which cement sediments within confined channels or exhumed palaeochannels, are of more limited spatial extent and are distinct from valley calcretes in that calcretisation either occurs or occurred across the full channel cross section. These types of calcrete have been described from India (Khadkikar et al., 1998; Rakshit and Sundaram, 1998), Libya (Moseley, 1965), Oman (Stalder, 1975;

Table 1 A genetic classification of calcrete types (after Carlisle, 1983) Calcrete classification

Incorporated calcrete types and mode of formation

Pedogenic calcrete

Caliche; Kunkar; Nari; Petrocalcic horizons. Developed by vertical redistribution of calcium carbonate within a soil profile. Laminar crusts; Case hardening; Gully bed cementation. Formed by surficial transport of calcium carbonate. Gravitational zone calcrete. Formed by downward accumulation of calcium carbonate in irregular permeability channels. Valley (channel) calcrete; Deltaic calcrete; Lake margin calcrete; Alluvial fan calcrete; Cienaga; Fault trace and other groundwater calcretes. Formed by lateral transport of calcium carbonate. Recemented transported calcrete; Calcretes which are brecciated and recemented in situ.

Nonpedogenic superficial calcrete Nonpedogenic gravitational zone calcrete Nonpedogenic groundwater calcrete Detrital and reconstituted calcrete

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Maizels, 1988, 1990a,b) and Spain (Nash and Smith, 1998). Despite the breadth of geographical coverage, research into both valley and channel calcretes has almost exclusively concentrated upon calcrete mineralogy and the macroscale morphological properties of profiles. At the profile scale, valley and channel calcretes are commonly massive, nodular or brecciated and do not exhibit the profile organisation typical of pedogenic calcretes of similar thickness (Wright and Tucker, 1991). Phreatic calcium carbonate cementation can also occur to considerable depths below the land surface with valley and channel calcretes recognised as forming at greater depths than other calcrete types. For example, Australian valley calcretes reach tens of metres thick (Arakel et al., 1989; Carlisle, 1983), and calcium carbonate and dolomite cementation of channel gravels in Oman extends to depths of over 200 m (Maizels, 1988, 1990a,b). The emphasis placed upon macromorphological properties is typical of the majority of research into nonpedogenic calcrete types, with relatively few studies describing their detailed petrological character-

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istics or considering their origins. This lack of understanding has led Wright and Tucker (1991, p. 10) to suggest that ‘‘much more work is needed to define the ‘groundwater calcrete facies’ and to devise criteria for its recognition’’, and the same is true for the entire range of nonpedogenic calcrete types. However, at this point in time, it may be difficult to provide a general definition of a nonpedogenic calcrete facies, given that such a definition would need to embrace a wide variety of calcrete types, which may have developed by a number of different mechanisms. It may, instead, be preferable to initially increase our understanding of individual varieties of nonpedogenic calcrete. This paper is intended as a step towards this aim by expanding our knowledge of one variety of groundwater calcrete, namely channel calcrete. This study focuses upon calcretes that have developed at the base of coarse and relatively uniform gravel deposits situated within bedrock channels in the Tabernas Basin, an intermontane drainage basin within the Betic Cordillera of Almerı´a Province, southeast Spain (Nash and Smith, 1998). The parent sediment, depth of formation and simple, uniform

Fig. 1. View of calcrete units within the vicinity of the Rambla de Reinelo Iban˜ez looking in a northeasterly direction from 37j01.91VN, 002j25.39VW towards Tabernas. The person is standing on the channel calcrete body (labelled C), where profiles S6a (TAB 96/11) and S6c (TAB 97/1) were analysed. The two major calcium carbonate-cemented planation surfaces within the basin are also visible, with the uppermost planated surface (labelled A and capped by pedogenic calcrete S2 and groundwater calcrete S1; Fig. 2) to the top left of view, and a remnant of the lower planated surface (labelled B and capped by pedogenic calcrete S4 and groundwater calcrete S3) to the top right of view.

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topographic situation in which calcium carbonate cementation has taken place provide an ideal setting for the characterisation of channel calcrete properties. Channel calcretes in the Tabernas Basin have developed well below the zone of capillary rise and are mostly unaffected by pedogenic processes. This is in contrast to many other complex valley and channel calcretes described within the literature that are often difficult to interpret because their genesis involved a combination of pedogenic and nonpedogenic processes. This paper builds upon the general account of calcretes in the Tabernas Basin by Nash and Smith (1998) by focusing solely upon the macro- and micromorphological properties of channel calcretes. On the basis of these results, an explanation for channel calcrete origin is put forward.

2. Geological and environmental setting The Tabernas Basin, which is bounded by the Betic Sierra de los Filabres to the north, the Sierra Alhamilla to the south and the Sorbas Basin to the east, has a mean annual precipitation of 245 mm (Eliaz and Ruiz, 1977). The mountain ranges to the north and south are dominated by Precambrian to Triassic mica schists

and other high-grade metamorphic rocks of the Nevado –Filabrides complex. In the northern Sierra Alhamilla, those rocks are partly overlain by nappes of Palaeozoic to Triassic low-grade metamorphic and Triassic sedimentary rocks of the Alpujarride complex (Weijermars et al., 1985). The Tabernas Basin developed as a result of movements of the African and European plates from the Jurassic to the Miocene as well as subsequent neotectonic activity. Marine basin sedimentation began following Miocene uplift of the Sierra de los Filabres and later uplift of the Sierra Alhamilla. Pre-Tortonian conglomerates, Tortonian and Messinian marls, turbidites, conglomerates, marine-mud and sandstone laminites subsequently infilled the basin (Weijermars et al., 1985). Transgression, subsequent regression and erosion during the Pliocene culminated in the deposition of continental barranco-type deposits and debris flows across the western part of the Tabernas Basin (Postma, 1984a,b). A simplified geological map of the basin is provided by Nash and Smith (1998). A suite of calcretes of Pliocene to Quaternary age (Fig. 1) developed within these subsequent fluvial and alluvial fan sediments, the general characteristics of which have been described by Nash and Smith (1998, 1999). Six distinct calcrete units can be seen within

Fig. 2. Schematic diagram of pedogenic, groundwater and channel calcrete relationships within the Tabernas Basin.

D.J. Nash, R.F. Smith / Geomorphology 50 (2003) 227–250

Fig. 3. Locations of sampled channel calcrete profiles within the Tabernas Basin.

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the central Tabernas Basin including pedogenic and groundwater calcretes (Fig. 2). Two pairs of calcrete units are present at high levels across the basin, preserving two former land surfaces that were cut across Tortonian and Messinian bedrock. Both of the former land surfaces were planated and subsequently buried by alluvial fan or fluvial gravels that are dominantly schistose in the north of the basin, but lithologically more variable to the south. A massive groundwater calcrete unit is present at the base of each gravel sequence, immediately in contact with the underlying bedrock. A less well-developed pedogenic calcrete unit is situated at the top of the gravel sequence. These groundwater and pedogenic units are rarely seen in contact apart from in locations where the gravel sequence is thin such as at the basin margins. Calcrete genesis seems to have been controlled by phases of uplift and stability interacting with phases of gravel input, perhaps resulting from climatic change. Also present are localised calcrete units developed within gravels associated with small alluvial fans. The channel calcretes shown schematically at the base of drainage lines in Fig. 2 represent the youngest phase of substantial calcium carbonate

cementation, which has been followed by relatively recent incision of the main Rambla de Tabernas drainage system. The main carbonate sources for the development of pedogenic calcretes within the basin are likely to be surface wash and atmospheric inputs, whereas weathered Tortonian and Messinian bedrock is identified as the major source for groundwater calcrete formation including the channel calcretes described in this study.

3. Methodology Channel calcretes were examined in six profiles from five channel calcrete outcrops in representative areas on both sides of the Tabernas Basin (Fig. 3). These were the exposures within the Rambla del Bu´ho to the northwest of Tabernas (Profile N6) and those in southern tributaries of the Rambla de Tabernas (profiles S6a to e). Precise locations of all sample profiles are given in Table 2. At each site, calcretes were logged and sampled, with the combined calcrete matrix and cement colour determined using a Munsell Soil Colour chart. Petrological and micromorpholog-

Table 2 Characteristics of channel calcrete profiles from the Rambla del Bu´ho and the area south of the Rambla de Tabernas Calcrete profile number

Profile location

Profile code

Profile thickness (m)

Munsell colour range

Profile summary

N6

37j04.38VN 002j25.58VW

TAB 96/6

1.20 – 2.00

2.5Y 6/2

S6a

37j01.91VN 002j25.39VW

TAB 96/11

1.40

10YR 3/2

S6b

37j01.16VN 002j25.60VW

TAB 96/12

3.50

10YR 4/2 – 2.5Y 4/1

S6c

37j01.91VN 002j25.39VW 37j01.97VN 002j25.35VW

TAB 97/1

2.00

10YR 3/2

Uniform brownish grey, well-cemented calcrete in fluvial gravel situated in a broad incised channel, with the calcrete in direct contact with underlying bedrock. Any sediment, which may have overlain the calcrete, has been eroded. Strongly cemented calcrete exposed in a 3.5-m-high waterfall within a bedrock channel. Abrupt contact with bedrock at base and sides of channel. Calcrete has developed in the basal 2 m of 12 – 14 m thickness of weakly cemented sediment deposited within the incised bedrock channel. Strongly cemented calcrete with weathered upper surface developed at the base of sediments deposited within an incised channel and in contact with underlying bedrock. Description as for Calcrete S6a.

TAB 97/2

0.60

10YR 4/2 – 2.5Y 4/1

37j02.42VN 002j23.10VW

TAB 97/3

>1.70 (base not seen)

10YR 4/2

S6d

S6e

Highly cemented calcrete exposed at the top of a 6-m-high waterfall, with base of profile in abrupt contact with bedrock. The relationship between the calcrete and the adjacent and overlying sediment is unclear, but appears to be a product of postcalcretisation cutting and filling. Strongly cemented calcrete forming a low knickpoint in the valley floor. Base of profile not exposed.

Table 3 Point count data for channel calcrete samples from the Tabernas Basin (to nearest 0.5%, based on 400 point counts) TAB 96/6

Sample no.

1

2

Depth (cm)

0

50

Quartz clasts Lithic clasts Calcrete clasts Heavy minerals Bioclasts Total clasts Primary pore space Grain-coating micrite Pore-filling micrite Pore-filling microsparite and sparite Total cement + primary pore space

17.0

TAB 96/11 TAB 96/12 3

1.5

1

120 0 0.5

2.5

2

1

100

0

4.0

2

TAB 97/1 3

1

100 220 30 2.0

0

1.5

0.5

TAB 97/2

2

3

4

60

90

110 130 150 170 200 0

3.0

5.5

4.0

5

7.5

6

3.5

7

2.5

8

2.0

1

3.0

TAB 97/3

2

3

4

1

2

3

4

20

40

60

0

20

40

60

1.5

2.5

4.0

9.5

0.5

1.0

3.5

5

6

80

100 120 150

4.0

2.0

7

3.5

8

4.0

49.5 53.0 68.0 57.5 61.0 63.0 53.5 46.5 52.0 53.0 56.5 46.5 29.5 44.0 44.0 59.5 42.5 35.0 39.0 42.0 51.0 50.5 45.5 41.5 49.0 54.5 46.0 36.5 0

0

0

1.0

0

0

0

0

0

4.5

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

2.0

2.5

1.5

1.5

1.5

1.0

3.5

1.5

0

0

4.0

2.0

3.5

1.5

0.5

4.5

1.5

3.0

2.5

1.5

0

2.5

0

0.5

2.0

2.0

1.5

3.5

0.5 0 0 0 0 0 0 0 1.5 0 1.0 2.0 2.5 0.5 0 0 2.5 0 0 0 0 0 0 0.5 1.0 1.5 0 1.0 69.0 57.0 70.0 61.5 66.5 66.0 57.0 49.5 54.0 60.5 67.0 54.5 43.0 49.5 47.0 66.0 49.5 39.5 44.0 47.5 60.5 52.5 46.5 46.0 56.0 59.5 51.0 45.0 0 3.5 1.0 4.5 3.0 5.0 7.0 1.5 0 0 1.0 3.0 3.5 1.5 2.0 1.5 4.5 0 4.0 1.5 3.5 7.5 5.5 7.0 3.0 5.5 2.0 2.0 0

0

0

0

4.0

7.0 33.0 27.0 29.0 34.0 38.0 45.0 35.5 25.5 38.0 40.0 33.5 37.5 17.5 27.5 48.0 38.0 51.0 12.5 17.5 20.0 20.0 10.0 10.5 12.5 25.0

31.0 35.5 22.0

0

0

0

3.5

0

0

0

0

4.5 11.0

0

1.0

0.5

3.5

3.5

3.0

0

0.5

2.0

1.0

1.5

0.5

0

0

4.5 13.0 13.5 12.5 13.5 17.0 12.5 14.0

0

0

12.0

7.0

2.0

4.0

4.0

1.0

0

1.0

11.5 13.5 26.0 23.0 26.5 23.0 34.5 27.0

D.J. Nash, R.F. Smith / Geomorphology 50 (2003) 227–250

Sample site

31.0 43.0 30.0 38.5 33.5 34.0 43.0 50.5 46.0 39.5 33.0 45.5 57.0 50.5 53.0 34.0 50.5 60.5 56.0 52.7 39.5 47.5 53.5 54.0 44.0 40.5 49.0 55.0

233

234

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ical analyses were carried out on subsamples. A total of 56 petrographic thin sections (two thin sections per sample from each profile) were examined using a binocular petrological microscope, with 400 point counts per thin section made to quantify the percentage of host grains, carbonate types and pore space (Table 3). Carbonate types were classified as micrite ( < 2 Am size), microsparite and sparite (>2 Am size), and identified as either grain coating or pore filling. There was no evidence of significant secondary porosity within samples; hence, only primary pore space is indicated in Table 3. Additionally, 56 palladium-coated calcrete fragments (two fragments per sample from each field profile) were analysed using a Jeol-type scanning electron microscope (SEM), on either a 5- or 10-kV setting, fitted with an Oxford Instruments ISIS energy-dispersive spectrometer.

4. Macroscale calcrete characteristics 4.1. Geomorphic setting of channel calcretes Channel calcretes crop out in a number of currently active stream channels across the central Tabernas

Basin (Fig. 3). The calcretes were formed at the base of gravel deposits situated within and infilling bedrock channels (Fig. 2) that are graded towards an axial drainage system with a higher base level than that of the present streams. Incision appears to have predated the ponding which resulted in the formation of a lake in the central part of the valley (see Harvey et al., this volume), and the channel fills within which the calcretes occur are likely to be contemporaneous with this lake. Recent incision of the Rambla de Tabernas drainage system due to base-level lowering has caused the modern channels to erode in a headward direction, leading to incision through the preexisting gravel deposits and the exposure of calcretes within presentday channel floors. Channel calcretes do not crop out in all tributaries; however, where they are exhumed, some occupy large areas of the streambed. Because the infilled bedrock channels were graded to a former higher base level, the calcretes lining the base of these channels have a gentler gradient than the present stream systems. As a result, the calcretes act as local geological controls upon stream incision, creating a stepped channel longitudinal profile. The nature of this longitudinal profile allows access to calcrete profiles that are exposed within channels either at pronounced

Fig. 4. View of profile S6b showing the control exerted by the 3.5-m-thick channel calcrete upon the stream longitudinal profile.

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Fig. 5. Channel calcrete (to left of person) within the Rambla de Reinelo Iban˜ez containing calcrete profiles S6a and S6c, showing (A) the relationship between the 2-m-thick channel calcrete body and overlying 10 m of uncemented fluvial gravel, and (B) a close-up of sampled profile S6c (TAB 97/1) (hammer length = 25 cm).

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Fig. 6. Channel calcrete profile S6d (with person on top) showing (A) the position of the calcrete outcrop within a tributary of the Rambla de Tabernas where it creates a pronounced knickpoint, and (B) a close-up of the sampled profile (TAB 97/2) (camera lens cap width = 7 cm).

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knickpoints and waterfalls (up to 6 m high; Fig. 4) or in gentle steps in the channel bed (1.5 to 2 m high) where the stream has eroded through the calcrete mass. 4.2. Field profile characteristics The characteristics of individual channel calcrete profiles are documented in Table 2, with only general properties described here. In all sampled profiles, the calcrete mass is situated at the base of a sequence of poorly sorted flood sediments that appear to have been deposited within confined, deeply incised (up to 25 m) bedrock channels as the end-product of a cut and fill cycle. Calcrete S6a is a typical example, in which a calcrete body occurs in the basal 2 m of approximately 12 m of weakly cemented to uncemented gravels (Fig. 5A, B). The sediments within and above the calcrete body exhibit a broadly similar grain size distribution, with no clear break in sed-

237

imentation that might suggest burial of a preexisting calcrete. The channel calcretes are invariably massive and structureless, with the host sediments exhibiting limited evidence of sorting or bedding (Fig. 6A, B). All profiles are extremely well cemented throughout, with little variation in the degree of induration, although the uppermost parts of many are more friable. This may be because the calcretes were originally more weakly cemented in their uppermost sections, possibly within the vadose zone, but may also be a product of exposure to wetting and drying processes in the stream bed. The calcretes typically exhibit a broad lens shape, reflecting the shape of the bedrock channel within which the host gravel sequences were deposited. It is not possible to make generalisations about the shape of the upper boundary of the calcrete body owing to erosion of the upper surface by fluvial activity following removal of the overlying gravels, but it is likely that it was relatively flat.

Table 4 Petrographical characteristics of channel calcrete profiles Calcrete section

Section code

No. of samples

Micromorphological characteristics identified in thin-section and under SEM

N6

TAB 96/6

3

S6a

TAB 96/11

2

S6b

TAB 96/12

3

S6c

TAB 97/1

8

S6d

TAB 97/2

4

S6e

TAB 97/3

8

Host sediment is predominantly lithic clasts (49.5% to 68.0%) with minor quartz grains, shell fragments and heavy minerals. Cement is dominated by sparite which occurs mostly as a pore-filling cement with minor quantities of micrite towards void centres. Lower parts of the profile have well-developed rhombohedral calcite crystals with, in places, evidence of crystal dissolution. Host sediment is predominantly lithic clasts (57.5% to 61.0%) with minor quartz fragments and heavy minerals. Micrite is present as a grain-coating and void-filling cement. Some pore spaces are infilled by sparite or pseudosparitic cements in lower parts of the profile. Host sediment is predominantly lithic clasts (46.5% to 63.0%) with minor quartz fragments and heavy minerals. Micrite is present as both as a grain-coating (visible under SEM but not in thin-section) and void-filling cement. Some pore spaces are filled with sparite, with the percentage sparite increasing towards the base of the profile. Partially calcified filaments are present in the uppermost sample and appear to have replaced organic structures. Host sediment is predominantly lithic clasts (29.5% to 59.5%) with minor quartz fragments, heavy minerals, diatoms, gastropod shells and fragments of older calcretes. Micrite is present as a grain-coating cement both in thin-section and under SEM, as well as occurring as a pore-filling cement and sediment. Sparite is present mostly as a late-stage pore-filling cement, with increasing amounts of sparite present towards the base of the profile. Calcified root hairs (up to 10 Am diameter and 140 Am length) are present throughout the profile. Host sediment is predominantly lithic clasts (35.0% to 42.5%) with minor quartz fragments and heavy minerals. Cement is dominated by pore-filling and grain-coating micrite, with sparite present as a late stage pore-filling cement in the upper three quarters of the profile. Many clasts in the upper parts of the profile exhibit grain fretting, whilst the lowest parts contain minor patches of amorphous silica cement. Host sediment is predominantly lithic clasts (36.5% to 54.5%) with minor quartz fragments, heavy minerals and gastropod shell fragments. Cement is variable, with both micrite and sparite occurring as grain-coatings and void-fills, with increasing amounts of sparite towards the base of the profile. Some samples show two distinct phases of cementation, with micrite occurring as an initial grain-coating overlain by sparite towards the centre of voids. Micrite also infills some small voids.

238 D.J. Nash, R.F. Smith / Geomorphology 50 (2003) 227–250 Fig. 7. Photomicrographs of channel calcretes (cross-polarised light, scale bar 1 mm length) showing cement characteristics: (A) Schist fragments overlain by grain-coating micrite (labelled M) with subsequent micritic layer—sample TAB 96/12/1; (B) Schist fragments cemented by grain-coating micrite with later stage sparite and microsparite (labelled S) infilling voids—sample TAB 96/12/3; (C) Grain-coating and void-filling micrite within schist gravel—sample TAB 96/11/1; (D) Spherulitic structures occurring as a grain-coating and void-fill within schist gravel, with localised etching of schist grains—sample TAB 96/12/3; (E) Sparite and microsparite cement from the upper part of profile N6, with localised etching of schist grains—sample TAB 96/6/2; (F) Sparite cement from the lower part of profile N6, with localised etching of schist grains—sample TAB 96/12/3.

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239

Fig. 7 (continued).

The dimensions of the calcrete outcrops are variable, but are partly dependent upon the shape of the channel within which they developed. Width is most clearly related to the dimensions of the bedrock channel, with the widest outcrop (containing calcrete profiles S6a and c) present within the Rambla de Reinelo Iban˜ez where calcrete occupies the entire 20m channel cross section. Thicknesses vary from 0.6 m in the case of calcrete S6d to approximately 3.5 m for calcrete S6b. This is mainly controlled by the channel shape that determines the typical lens-like cross sectional shape of the calcrete. Minor thickness variations

are also produced within individual calcrete profiles where the base of the original bedrock channel undulates. Channel bed variability influences the thickness of the sedimentary infill and hence the thickness of the calcrete developed at the base of the infill. Similar thickness variations caused by irregularities in the underlying bedrock surface are apparent in older groundwater calcretes exposed elsewhere in the basin (Nash and Smith, 1998). However, thickness differences between individual calcretes may also reflect the water table depth at the time of cementation, as will be discussed below. All calcrete masses are located at the

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distal ends of channels close to the main valley axis presently occupied by the Rambla de Tabernas. It is not possible to ascertain the total length of any calcrete mass on the basis of surface exposures because they are partially buried by gravel deposits at the base of many present-day streams. However, the channel calcrete body containing sampled profiles S6a and S6c within the Rambla de Reinelo Iban˜ez can be traced intermittently for over 300 m along the channel bed.

5. Calcrete micromorphology 5.1. Calcrete fabrics Mean point count data for each channel calcrete sample are given in Table 3 (based on 400 counts per thin section), together with a general description of the micromorphological properties of samples from each field profile in Table 4. The host sediment in all samples consists of poorly sorted subangular schist clasts, quartz grains and various heavy minerals together with minor amounts of comminuted detrital diatom and gastropod remains present in some samples. Two samples from separate field profiles (calcretes S6a and c) sampled within the same channel calcrete body within the Rambla de Reinelo Iban˜ez also contain small reworked fragments of preexisting calcrete from higher elevations within the basin, some of which exhibit glaebular structures and may be of a pedogenic origin. However, there appears to be little significant variation in host sediment characteristics despite differences in the metamorphic rock types present on the north and south side of the basin (Weijermars et al., 1985). The calcretes mainly exhibit clast-supported fabrics although some thin sections indicate a floating fabric. The percentage of clasts present within individual thin sections ranges from 39.5% to 70.0%. All samples are well cemented, with primary pore space in the range 0% to 7.5%. All samples exhibit predominantly alpha-

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fabrics (Wright, 1990), similar to the K-fabric of Gile et al. (1966), with varying proportions of micrite, microsparite and sparite, dependent upon position within the profile. None of the examined samples contain pedogenic structures such as glaebules (except where fragments of older calcretes form part of the host sediment), and there is only limited evidence for expansive growth during cementation. Some thin sections contain indications of grain expansion, grain explosion or minor etching of host sediments, but the majority of evidence suggests that calcretisation occurred at depth within the gravel body, most probably in a nonpedogenic setting. There are, however, biogenic features (described below) within profiles TAB 97/1 and the uppermost parts of TAB 96/12 that may suggest limited organic activity during calcrete formation. In most sections, micrite appears to have been the earliest deposit and occurs as a coating of small randomly oriented calcite crystals on host sediment grains. It is difficult to determine whether the micrite was deposited as a sediment or precipitated as a cement. However, as the coatings are crystalline and do not appear to incorporate additional minerals, it is most likely that precipitation was the primary mechanism for their emplacement. Grain-coating micrite is visible in every sample under the SEM, varying in thickness from incomplete or partial coverage of individual grains up to complete grain coatings of approximately 50 Am thick (Figs. 7A and 8A). Pseudosparitic grain-coating mosaics are also present in some samples (e.g. TAB 96/6/2 from the middle part of calcrete N6; Fig. 8B). Grain-coating micrite is commonly overlain by either a microsparite or sparite cement (Figs. 7B and 8C, D). In rare cases where samples contain neither microsparite nor sparite, micrite may fill the pore spaces as either a cement or late-stage sediment as well as coating grains (Fig. 7C). In some samples, spherulitic structures are also present within the cement (Fig. 7D). Sparite is most prevalent away from grain interfaces and towards the centre of pores or voids, and where present either

Fig. 8. Scanning electron micrographs showing channel calcrete cement characteristics: (A) Grain-coating micrite and microsparite overlying a quartz fragment—sample TAB 97/2/4 from a depth of 0.6 m within calcrete S6d; (B) Pseudosparitic grain-coating cement overlying schist fragments—sample TAB 96/6/2 from a depth of 0.5 m within calcrete N6; (C) Section through grain-coating cement overlying schist fragments showing the relationship between the thin initial micrite coating (labelled M) and subsequently precipitated sparite cement (labelled S)—sample TAB 96/6/2 from a depth of 0.6 m within calcrete S6d; (D) Close-up view of same sample showing the micrite/sparite contact (labelled E) and evidence of localised etching of the detrital grain.

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partially or completely fills pore spaces. Cross sections through grain coatings are rarely visible under the SEM; however, where they do occur, the relationship between micrite and sparite is clearly successional with little evidence that sparite has developed by neomorphism of preexisting micrite (Figs. 8C, D and 9A). The maximum thickness of sparite overlying micrite seen within the samples is approximately 120 Am (Fig. 9B). There are also cases where sparite appears to directly cement grains (Fig. 9A). This may be the result of neomorphism, but there is no direct evidence to confirm this suggestion. Furthermore, some samples from profile N6 (Table 4) also show evidence of later deposition of micrite on top of sparite, which may suggest two origins for micrite within the calcrete; an early cement and later sediment or cement (Fig. 9A, D). Consistent variations in the distribution of sparite and micrite are recognised within five of the six sampled profiles (calcretes S6a to S6e). In these profiles, the amount of sparite shows a clear increase with depth towards the base of the calcrete outcrop (for example, calcrete S6c containing 1% sparite at the top of the profile and 13.5% at the base; Fig. 7E, F). This increase in sparite is usually, but not always, accompanied by a decrease in the percentage of micrite present. In the remaining profile (calcrete N6), there is comparatively little micrite present, and the cement is dominated by sparite. In this profile there appears to be a down-profile change in the degree of crystal development within the sparite cement, as opposed to the increase in percentage of sparite seen in other profiles, with larger rhombohedral calcite crystals towards the base (Fig. 9C, D). 5.2. Biogenic features Another characteristic of the sampled profiles is the variable amount of biogenic features resulting from the calcification of organic material within the calcretes. Some calcretes contain calcified organic remains that appear to have been present within the host sediment prior to cementation. For example, isolated fragments of calcified root hairs are present in many samples from profile TAB 97/1 (Fig. 10A, B). Individual root hair fragments are up to 10 Am in length, but do not appear to be attached to individual clasts and do not show any preferred orientation. This may suggest that

they were present within the original host sediment and have simply become calcified, although it is possible that fragmentation occurred during the cementation process. Other secondary calcium carbonate features are more indicative of the role of organic processes in calcrete formation. Nest-like mats of calcite filaments are present in the uppermost sample from profile TAB 96/12 and appear to have replaced preexisting organic structures, such as fungal hyphae, originally attached to surrounding clasts (Fig. 10C, D). The presence of such organic structures may indicate that cementation in this profile (S6c) occurred at shallower depths than at other localities. However, it is also possible that the calcified organic materials are remnants of root structures from deep-rooting species, tapping groundwater located in the base of channel sediments. 5.3. Postcarbonate cementation diagenetic alteration Some samples also contain localised evidence of postcementation diagenetic alteration that can be recognised under the SEM. Alteration is most obvious in samples containing high proportions of sparite cement and usually takes the form of dissolution of calcite crystal edges and faces. The most extensive evidence of crystal dissolution is usually found towards the base of profiles (e.g. sample 3 from profile TAB 96/6; Fig. 11A, B). The other main form of diagenetic alteration is represented by the presence of localised patches of amorphous silica cement towards the base of profiles. This is most prominent in calcrete S6d (sample TAB 97/2/4; Fig. 11C, D), where silica appears to have replaced other cementing agents within the calcite matrix. SEM analysis of this sample shows evidence of localised dissolution of silicate minerals within schist grains that may have provided a source of silica. The presence of both calcite dissolution and silica precipitation features is suggestive that the pH of pore water solutions was < 9.0, as that is the level below which calcite solubility rapidly increases and silica solubility significantly decreases (Nash and Shaw, 1998). The fact that evidence of alteration is mostly found in the lowermost parts of profiles may also indicate that alteration only occurred at times when the water table within the calcrete body was low. It is, however, difficult to ascertain the difference in relative time between calcrete cementation and diagenetic alteration.

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Fig. 9. Scanning electron micrographs showing channel calcrete cement characteristics: (A) Section through rare sparite grain-coating cement developed upon a schist fragment, overlain by micrite cement—sample TAB 97/3/7 from a depth of 1.20 m within calcrete S6e; (B) Section through a grain-coating cement which partially infills a void between three schist fragments—sample TAB 96/12/3 from a depth of 2.20 m within calcrete S6b; (C) Well-developed rhombohedral calcite grain-coating sparite cement—sample TAB 96/6/3 from a depth of 1.20 m within calcrete N6; (D) Close-up view of same sample showing micrite and clay minerals coating rhombohedral calcite.

244 D.J. Nash, R.F. Smith / Geomorphology 50 (2003) 227–250 Fig. 10. Scanning electron micrographs of biogenic features within channel calcrete cements: (A) and (B) Calcified root hairs within sample TAB 97/1/4 from a depth of 1.10 m in calcrete profile S6c; (C) and (D) Nest-like mat of calcite filaments which appear to have partially calcified preexisting organic structures within sample TAB 96/12/1 from close to the surface of calcrete profile S6b.

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Fig. 11. Scanning electron micrographs of postcementation diagenetic alteration features within channel calcrete cements: (A) and (B) Dissolution features on rhombohedral calcite grain-coating sparite cement—sample TAB 96/6/3 from a depth of 1.20 m within calcrete N6; (C) Amorphous silica cement replacing grain-coating calcite from a depth of 0.60 m within sample TAB 97/2/4 from profile S6d; (D) Close-up view of silica cement (labelled S) from the same sample clearly showing the conchoidal fracture on the cement surface.

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6. Discussion 6.1. Mode of channel calcrete formation The precise mechanisms by which nonpedogenic valley and channel calcretes develop are not yet fully understood, although they are considered to form by different processes than many pedogenic calcretes. The key difference is that formation occurs as a result of carbonate-rich subsurface waters flowing laterally through alluvial material (Carlisle, 1983; Wright and Tucker, 1991). This contrasts strongly with most pedogenic calcretes in which vertical transfers of carbonate within a soil profile are the most important carbonate source. Development of channel calcretes in the Tabernas Basin appears to conform to this general distinction. Formation is closely controlled by along-channel movement of calcium bicarbonate-charged subsurface water, with weathered Messinian and Tortonian bedrock providing a key carbonate source (Nash and Smith, 1998). Lateral groundwater flow alone, however, cannot account for many of the macro- and microscale characteristics of calcretes within the study area. The observed distribution and general stratigraphy of calcite cement types suggests that cementation was also strongly influenced by vertical fluctuations in the height of the water table over time. This suggestion is made on the basis that calcite crystal size increases towards the base of all profiles, together with an increase in the occurrence of euhedral crystals. Calcite crystal size and morphology in calcrete is controlled by a number of factors, of which one of the most important is the duration of the wetting phase that strongly affects the rate of calcite precipitation (Wright and Tucker, 1991). The observed down-profile gradient from micrite- to sparite-dominated would suggest that basal parts of the host sediments were wetter for longer periods than upper parts. This would be expected where gravels are in contact with relatively impermeable underlying bedrock. Crystal size is also controlled by other factors such as the host sediment texture and chemistry (e.g. Wieder and Yaalon, 1982), the presence of clay, organic and sesquioxide coatings on grains (e.g. Ducloux et al., 1984) and the presence of organisms (e.g. Goudie, 1996). However, given that the host sediment shows little sedimentological or mineralogical variability

(Nash and Smith, 1998), it would appear that particle size, organisation and chemistry do not exert a major influence upon the distribution of micrite and sparite. The alpha-fabric observed in the majority of samples is also indicative of the minimal influence of biota in carbonate precipitation (Wright, 1990), but this is to be expected given that channel calcretes have developed in the basal sections of gravel sequences of up to 12 m thick. The uppermost parts of some profiles contain localised evidence of biogenic activity that may be indicative of early stages of pedogenesis under a less thick gravel cover, but could equally be remnants of roots from deep-rooting plant species. However, as the gravel that would have originally overlain the calcrete has been removed from these profiles, the depth at which cementation took place is difficult to assess. Observations from the majority of samples suggest that micrite was initially precipitated as a cement onto host sediment surfaces, with only small crystals developing as a result of interactions of the calcite crystal surfaces with the host minerals (Ducloux et al., 1984; Wright and Tucker, 1991). The initial micrite grain coating was subsequently overlain by sparite, with possible evidence of neomorphism in some thin sections. Longer periods of crystal growth took place in moisture-rich basal parts of profiles allowing the development of larger sparite crystals, as opposed to micrite-dominated upper sections that experienced wetting and drying as the water table fluctuated. Given that all profiles are relatively uniformly cemented, it would appear that either groundwater levels maintained an overall average depth during the period of cementation, or that the locus of cementation migrated gradually upwards as calcite precipitation progressively blocked pore spaces in lower parts of the profile. However, there is no clear micromorphological evidence to confirm either suggestion. It has been demonstrated that carbonate precipitation in nonpedogenic calcretes generally occurs towards the basal section of the vadose zone, immediately above the upper surface of the water table as well as within the phreatic zone. Precipitation can occur due to a number of mechanisms, of which evaporation or evapotranspiration, biological agency, the degassing of CO2, direct precipitation from saturated groundwater, and the common ion effect are most significant (Goudie, 1983). On the basis of the

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thickness of gravels overlying the majority of calcrete bodies, it is likely that channel calcrete cementation took place at a considerable distance below the zone of capillary rise and, therefore, mostly in the absence of biological activity. This would suggest that evaporation, evapotranspiration and organic agency are unlikely to be major factors influencing calcrete formation, although the upper parts of some profiles contain limited evidence for biological activity in the form of calcified organic remains. The common ion effect could have been a significant depositional mechanism, but this is only likely to have occurred in areas where side valleys join the main valley axis, and then only if Ca/Mg sulphates or chlorides were present in sufficient volumes to mix with calcium bicarbonate-bearing water moving down tributary valleys (Carlisle, 1983). This leaves CO2 degassing associated with a fluctuating water table and direct precipitation from saturated groundwater as the most important general mechanisms driving calcium carbonate precipitation. Water-table fluctuations may have occurred due to variations in the level of the late Quaternary lake which occupied the central part of the main valley (see Harvey et al., this volume), especially at the southern sites which are at an elevation near that of the lake itself. Degassing could, in theory, occur at the range of depths at which channel calcretes in the Tabernas Basin are found and would be an important mechanism by which CO2 loss could occur through a porous, uncemented gravel deposit. Calcite precipitation is also known to occur in areas where alluvial permeability decreases, where the alluvial cover overlying basement bedrock is relatively thin (i.e. where bedrock ‘‘highs’’ occur) and subsurface water is brought towards the surface (Wright and Tucker, 1991), but it is unclear how important these controls might be in the Tabernas Basin. 6.2. Comparison with other valley and channel calcretes Water table fluctuations have been recognised as exerting an important influence on the formation of nonpedogenic groundwater calcretes in a number of studies including those of valley calcretes developed within valley alluvium in Western Australia (e.g. Arakel, 1986; Jacobson et al., 1988; Arakel et al., 1989) and Oman (Stalder, 1975; Maizels, 1988,

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1990a,b). A typical Australian valley calcrete profile consists of a 2-m basal zone of calcrete mottles or nodules (near the base of the zone of water table fluctuation), which passes upward into a 3-m-thick massive groundwater calcrete that is brecciated and/or partially silicified in its upper sections, and may be subdivided into a lower ‘porcellaneous’ and upper ‘earthy’ zone (Carlisle, 1983). This, in turn, passes into a massive vadose or capillary-rise pedogenic calcrete above the zone of water table fluctuation that may be up to 6 m in thickness. Calcretes are commonly densely crystalline and dominated by micritic to microsparitic calcite groundmasses, and their upper surfaces may also exhibit laminar or rhizoconcretionary zones where the calcrete extends into the capillary-rise zone (Semeniuk and Meagher, 1981). Initial formation is suggested to occur within the phreatic zone, leading to the development of pods or domes of calcrete within the channel alluvium (Mann and Horwitz, 1979). These may, in time, coalesce, deform and brecciate due to expansion during the course of carbonate cementation and may form surface mounds. The process continues with older carbonate lifted above the water table by younger carbonate precipitated underneath being eroded and reworked by subaerial processes. Silicification of the calcrete may occur during this time due to changing groundwater chemistry and surface weathering products percolating through the profile. By this process, calcretisation gradually extends across the drainage line and extends up valley away from the initial site of cementation. Channel calcretes from Tabernas appear to be different from Australian valley calcretes, both at a macro- and micromorphological scale and in terms of their likely mode of development. Whilst groundwater fluctuations have been a significant control on cement distribution at the profile scale, changes in the height of the water table in the Tabernas channel calcretes appear to be considerably smaller (ca. 2 m) when compared with Australian examples (>5 m fluctuation). Given the depth at which most Tabernas channel calcretes have developed, it would appear that water table fluctuations took place well below the zone of potential capillary rise. The coarse nature of the parent sediment would also be likely to inhibit any upward capillary movement. This may partly explain the absence of a pedogenic calcrete at the upper surface

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of the gravel sequence, although this is more likely due to the fact that the host gravel deposition and subsequent basal cementation occurred relatively recently within the development of the Tabernas Basin (Nash and Smith, 1998). The depth at which cementation took place may also have contributed to the absence of laminar and rhizoconcretionary zones in samples from the Tabernas Basin as well as the lack of displacive micrite growth in pore spaces when compared to Australian valley calcretes. However, remains of fungal hyphae in uppermost parts of calcrete S6b suggest that cementation may have taken place closer to the surface in this profile. The closest analogy to the calcretes described in this study in terms of cement type and, possibly, mode of development is that of channel calcretes that developed within river deposits and exhumed palaeochannels in Oman. In the Oman mountains, Stalder (1975) described recent channel deposits cemented by both micrite and sparite, with the micritic cement suggested to have developed under vadose conditions, whereas the sparry component developed beneath the water table. Many of these calcretes exhibit rhythmic cementation patterns that Stalder (1975) attributed to formation in association with a fluctuating water table. The palaeochannel calcretes (Maizels, 1988, 1990a,b) are cemented by both ‘clear sparry calcite’ encrusting host sediments and calcite with an ‘opaque, clayey appearance’, but the interpretation of their cements is somewhat different from those described by Stalder (1975). The two different cement types are suggested to reflect differences in environmental conditions at the time of cementation, with the former developing due to the replacement of host sediments under more humid conditions with higher rates of evapotranspiration, whilst the opaque cements represent less replacement and contain a higher percentage of original grains (Maizels, 1990b). However, it should be noted that there is no evidence of extensive replacement or displacement during the process of calcium carbonate cementation in the samples described within this study.

7. Conclusions Separating pedogenic and groundwater influences upon calcrete formation is often problematic, partic-

ularly in situations where groundwater fluctuations have taken place in close proximity to the zone of near-surface pedogenesis, which results in the development of fused profiles. This study of channel calcretes developed within back-filled bedrock channels in the Tabernas Basin, however, offers an opportunity to view the impact of groundwater upon the cementation process, largely in isolation from surface influences. The channel calcretes described here appear to have developed at depths of up to 12 m below the surface, probably due to CO2 degassing or direct precipitation of calcite from bicarbonate-rich groundwater. Carbonate accumulation appears to have taken place mostly as a result of water table fluctuations occurring at depth within coarse-grained host sediments as there is only limited evidence to suggest either significant organic agency or a pedogenic influence upon formation. As such, the trends identified within this study may be diagnostic of a nonpedogenic channel calcrete facies for calcretes developed within coarse-grained deposits. Channel calcrete profiles in the Tabernas Basin reach thicknesses of 3.5 m and are typically highly indurated and uniformly massive in character. However, despite their consistent macromorphology, all profiles show clear trends in micromorphology with evidence of increasing organisation of calcium carbonate crystal development with depth. This trend takes two forms. Profiles dominated by a sparite cement show an increase in both crystal size and the amount of euhedral crystals present towards their base, whereas profiles dominated by micrite in their upper parts trend towards a microsparite- or sparite-dominated cement at lower levels. This appears to reflect the fact that lower parts of profiles, in contact with relatively impermeable underlying bedrock, remained saturated for longer periods compared with upper zones. The dominance of micrite or sparite within a profile appears to be unrelated to particle size and host sediment chemistry, but is more closely dependent upon the presumed degree and duration of saturation of the host material. The channel calcretes represent the last phase of major calcium carbonate cementation within the tributary valleys of the Tabernas Basin and are of late Quaternary age, forming within sediments which are broadly contemporaneous with the presence of the lake which occupied the centre of the main valley

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after ca. 150ka (Harvey et al., this volume). With maximum thicknesses of at least 3.5 m, they demonstrate the potential for substantial and relatively rapid carbonate accumulation in semiarid southwest Europe, given a suitable carbonate source.

Acknowledgements This research was funded by the Earth and Environmental Science Research Unit of the University of Brighton. Thin section preparation was undertaken by John Bollard, whilst Mike Helias provided assistance with SEM analyses. Figures were drawn by Sue Rowlands and Hazel Lintott of the Cartographic Unit, University of Sussex. The manuscript was greatly improved by correspondence with Dr. Steve Hill of CRC LEME, University of Canberra. The authors would like to express special thanks to Lindy Walsh of Cortijo Urra Field Centre, Sorbas, for her hospitality during the course of fieldwork.

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