Textural features of Holocene perennial saline lake deposits of the Taoudenni–Agorgott basin, northern Mali

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Sedimentary Geology 127 (1999) 65–84

Textural features of Holocene perennial saline lake deposits of the Taoudenni–Agorgott basin, northern Mali F. Mees * Department of Geology and Soil Science, University of Ghent, Krijgslaan 281 S8, B-9000 Ghent, Belgium Received 25 May 1998; accepted 28 January 1999

Abstract The Holocene salt lake deposits of the Taoudenni–Agorgott basin, northern Mali, mainly consist of sediments with a high glauberite (Na2 Ca(SO4 )2 ) content. The remainder of the deposits largely consists of salt beds with a bloedite (Na2 Mg(SO4 )2 Ð4H2 O), thenardite (Na2 SO4 ) or halite (NaCl) composition. A petrographical study of the deposits demonstrates that they formed in a perennial lake that experienced a gradual decrease in water depth. Textural features of the glauberite-dominated deposits are found to be related to water depth, through the control that this factor exerts on the sensitivity of the lake to changes in water supply and to short-term variations in evaporation rates. In this way, layering — due to variations in glauberite content and crystal size — is inferred to be typical of deposits that formed in shallow water, whereas unstratified deposits are the product of high lake level stages. Halite textures are found to be indicative of the place within the water column where crystal growth occurred (along the lake bottom or higher), which is mainly determined by water depth and partly by evaporation rates. The oldest halite beds are largely unaltered cumulate deposits, whereas the youngest layers developed exclusively through bottom growth. The basal part of one thick halite bed at a level between these two groups of halite layers developed by an alternation of both types of growth, in response to variations in evaporation rates. Variations in mineralogical composition between and within the salt beds that formed during the earliest periods with a higher salinity, up to the first stage with halite formation, record a change in lake water chemistry with time but they are in one instance also determined by an early diagenetic mineral transformation.  1999 Elsevier Science B.V. All rights reserved. Keywords: evaporite deposits; salt lakes; halite; glauberite; bloedite; Taoudenni

1. Introduction The results of petrographical studies of Quaternary, modern or experimentally produced evaporites have greatly contributed to the interpretation of textural features of ancient evaporite deposits. In this way, the mechanisms of halite sedimentaŁ Tel.: C32 (9) 264 4569; Fax: C32 (9) 264 4984; E-mail: [email protected]

tion have been intensively investigated for shallow or ephemeral saline lakes (e.g. Shearman, 1970; Arthurton, 1973; Arakel, 1980, 1988; Ortı´ Cabo et al., 1984; Lowenstein and Hardie, 1985; Logan, 1987; Casas and Lowenstein, 1989; Handford, 1990; Rosen, 1991), which has provided criteria for the recognition of ancient salt deposits that formed under those conditions (e.g. Hardie et al., 1985; Hovorka, 1987; Lowenstein, 1988; Cathro et al., 1992). In contrast to deposits of this type, modern or Quaternary

0037-0738/99/$ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 7 - 0 7 3 8 ( 9 9 ) 0 0 0 1 4 - 7

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evaporite deposits of deeper or perennial lakes have only rarely been the subject of similarly detailed studies (see e.g. Last, 1994), which have only been performed for halite deposits of the Qaidam Basin (Schubel and Lowenstein, 1997) and Death Valley (Li et al., 1996). In this report, textural features are described for Holocene lacustrine deposits that are demonstrated to have formed in a perennial saline lake with an (unquantified) great to very shallow water depth. By presenting a description of the nature of the deposits of a previously seldom studied setting, this study aims to enable a better assessment of the significance of textural features and ultimately to result in a more accurate interpretation of the petrographical characteristics of ancient evaporite deposits in terms of the depositional environment in which they formed. The most abundant salt minerals in the deposits that are described in this paper are halite and glauberite. Halite textures, particularly those of preQuaternary deposits (see references in e.g. Lowenstein, 1988; Casas and Lowenstein, 1989; Handford, 1991), have much more frequently been described than textural features of glauberite deposits. The latter have only been documented for a single basin with recent glauberite accumulations, where they formed exclusively as groundwater precipitates (Arakel and Cohen, 1991), and for the glauberite deposits of two Tertiary basins of the Iberian Peninsula (e.g. Ortı´ Cabo et al., 1979; Menduin˜a et al., 1984; Ordo´n˜ez and Garcı´a del Cura, 1994; Salvany and Ortı´, 1994).

2. Setting The Taoudenni–Agorgott basin, located at 22º400 N 4º000 W in northern Mali (Fig. 1), is a historically important site where halite has been mined since the end of the 16th century. It is located in the lowest, western part of the 100 km long, E–W elongated Taoudenni depression. It represents one of many Quaternary lacustrine basins that developed within this depression, along the contact between

Upper Carboniferous clayey strata and Lower to Upper Carboniferous calcareous formations that are exposed along the gently southward sloping backslope of a cuesta (Fabre, 1991b). The hydrogeological setting of the Quaternary paleolakes of this remote region is poorly documented. The available information about the deposits of the Taoudenni–Agorgott basin is represented by some early descriptions of the strata that are exposed in the mining pits (Monod, 1952, pp. 63–67; Clauzel, 1960, pp. 22–26; Villemur, 1967, p. 138) and by the scientific results of three campaigns that took place during the 1980’s (see e.g. Fabre, 1983; Petit-Maire et al., 1987; Fabre and Petit-Maire, 1988; Petit-Maire, 1991). The published general descriptions of the deposits pertain to only a small number of mining pits, located in the Old and New Agorgott mining areas (Fig. 1). As a result, no information is available on several important aspects, such as the lateral variations in lithofacies and the position of the studied excavations relative to the margins of the basin. Analytical data that have previously been obtained for the deposits comprise pollen data (e.g. Schulz, 1991a,b), radiocarbon dates (e.g. Delibrias et al., 1991; Fabre, 1991a) and some data about the composition of the mineral and organic fractions (Aucour, 1988a,b). Referring to these data and to the macromorphological features of the deposits, the sedimentary record of the Holocene paleolake has been interpreted to provide evidence for an early to middle Holocene humid period, followed, from around 6,500 years BP onwards, by a shift to more arid conditions, leading up to the arid period that continues to the present. The deposits that formed between 6,500 years BP and the beginning of this arid period (around 4,500 years BP) are roughly positioned between the oldest and most recent main halite layers in the studied exposure (layers IX and V; see Fig. 2).

3. Material and methods All available material was collected in the New Agorgott mining area in 1988. It consists of a contin-

Fig. 1. General location of the Taoudenni–Agorgott basin in northern Mali (B, Bamako, G, Gao, T, Timbouctou), geological setting of the basin (after Fabre, 1991a) and location of the sampling area (New Agorgott).

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Fig. 2. Generalized lithological logs of the studied deposits from the New Agorgott area. Depth indications are in meters.

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uous series of oriented samples covering the entire sequence that was exposed in a mining pit, and two cores of the deposits below the base of the exposures at two different locations. One of the cores was taken at the site where the profile was sampled (Core 1); the top of this core corresponds to the base of the exposure. The second core was retrieved from sediments below the floor of another mining pit, at a distance of 1.8 km from the other site (Core 2). No information regarding the position of the top of this second core relative to a level within the exposed sequence is available. The highest bloedite bed, whose base was not reached in Core 1, and the three highest intervals with a low salt content are recognized as correlatable features. The set of thin sections that was prepared covers the entire exposure in a continuous manner. For the cored intervals, this coverage is not as complete. This is also the case for the macromorphological descriptions of the cores, due to the presence of salts that covered their surface upon drying, before the opened cores became available for this study. No information is therefore available for some parts of the cores, and the existence of a correlation between the cores could not be ascertained for several sections. Crystal size is the only feature that was quantified for all individual, millimeter- to centimeter-sized depositional units. Indications of variations of other quantifiable properties such as mineral abundances were used in a consistent manner for the entire sequence. The nature of the constituents other than the sulfate and chloride minerals was recorded but it is not reported in this paper. They consist of a fine-grained detrital silicate fraction and an at least partly endogenic carbonate fraction that consists exclusively of magnesite. The silicate fraction largely consists of clay and only includes an important admixture of silt- to fine sand-sized grains in the upper part of the exposure. These constituents are referred to as ‘clay’ or ‘sandy clay’ throughout the text, irrespective of the magnesite content. XRD analyses were performed for more than a hundred samples, confirming the reported distribution of evaporite minerals that is based on thin section observations.

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4. Results The deposits of the Taoudenni–Agorgott paleolake (Fig. 2) mainly consist of glauberite-rich sediments (Na2 Ca(SO4 )2 ) that are partly unstratified and partly layered due to variations in clay content and crystal size. The deposits also contain a number of halite layers (NaCl) and, in lower parts of the profile, a single thenardite layer (Na2 SO4 ) and two bloedite layers (Na2 Mg(SO4 )2 Ð4H2 O) occur. The sections between these halite and sulfate beds include some intervals with a high clay=glauberite ratio, whose occurrence is restricted to the lower part of the sampled sequence. In all other parts of the intervals that alternate with the mentioned salt beds, glauberite is only largely absent from the deposits in the upper 16 cm of the exposure, where gypsum (CaSO4 Ð2H2 O), bassanite (2CaSO4 ÐH2 O) and anhydrite (CaSO4 ) are the dominant salt minerals (Mees, 1998). A summary of the main textural features and general significance of the different types of deposits is given in Table 1. 4.1. Bloedite and thenardite Both bloedite layers consist of mainly subhedral crystals with a wide range in size (10–250 µm to 0.5–2 cm). A varying amount of glauberite-rich clay occurs between the crystals. In both layers, the bloedite crystals contain euhedral glauberite inclusions in some parts. The glauberite content of the crystals is particularly high within and along sections with a high glauberite-rich clay content. Thenardite only occurs in the higher of the two bloedite beds. Within this layer, the thenardite content increases from the base upward, but thenardite is entirely absent at the top of the bed. Most thenardite occurs as inclusions in the bloedite crystals, in the form of groups of isolated anhedral sections with the same optical orientation (Fig. 3a). In the parts with the highest thenardite content, the deposits also contain some an=subhedral thenardite crystals without associated bloedite and several bloedite–thenardite intergrowths in which thenardite represents the dominant phase. Within these clusters, bloedite partly surrounds the thenardite crystals, which are partly delimited by curved crystal faces. Bloedite also occurs as infillings of cavities within the thenardite crystals, whereby the bloedite crystals display per-

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Table 1 Main petrographical characteristics and general significance of the major types of deposits of the Taoudenni–Agorgott basin Deposits Thenardite=bloedite bloedite layers

thenardite layer

Halite layers XI–VI

layer V

layers IV–I

Main features

General significance

sub=euhedral crystals, partly with glauberite inclusions glauberite-rich clay matrix thenardite within upper layer, as part of intergrowths with bloedite crystals sub=euhedral crystals limited degree of intergrowth no glauberite-rich clay matrix

altered cumulate deposits, with remnants of primary thenardite in upper layer

sub=euhedral crystals, with equant and tabular forms limited degree of intergrowth no fluid inclusions clay intercalations without associated dissolution features along top — crystal intergrowth or clastic fabric along base — large crystal size and=or intergrowth 136–149 cm — alternating layers of small non-intergrown sub= euhedral crystals and larger interlocking sub=anhedral crystals 129–136 cm — similar to layers VI–I 118=119–129 cm — similar to layers XI–VI, with clay infillings of packing voids in part of the interval 99.5–118=119 cm — identical to layers IV–I intergrown an=subhedral crystals fluid inclusions, in banded patterns truncation surfaces near=at top of the layers no cementation features chaotic mudstone halite along base of layer II

cumulate deposits of high lake level stages

Glauberite-bearing deposits layered parts normal grading, in intervals with variable clay content reverse grading, in thin layers without clay matrix sediment inclusions limited occurrence of horizontal alignment of crystals crystal size mainly <500 µm unstratified parts absence of grading and sediment inclusions random orientation of crystals crystal size mainly >500 µm in lower parts — intervals with high clay content, with transitional outer sections

fectly straight crystal faces in some occurrences of incomplete infillings (Fig. 3b). The thenardite layer, which contains only a minor admixture of glauberite-rich clay, consists of large (up to 2.5 cm), lenticular, randomly oriented crystals that are only locally intergrown. Outside the thenardite and bloedite layers, thenardite only occurs in a thin interval along the base of the oldest halite layer (XI). The thenardite crystals are mainly euhedral in the upper part of this interval, but they only

unaltered cumulate deposit

deposits of a transitional period, with long-term variations in water depth and, during a first stage, with short-term variations in evaporation rates

deposits formed by bottom growth, in shallow water

deposits of low lake level stages

deposits of high lake level stages

occur as part of intergrowths with larger bloedite crystals in the basal part. 4.2. Halite All lower halite layers (VI to XI) mainly consist of predominantly sub=euhedral crystals that are intergrown to only a limited extent (Fig. 4a). In most layers, this texture is less well developed along the top, where the deposits either consist of interlock-

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Fig. 3. (a) Euhedral bloedite crystal (b), with thenardite inclusions (t) consisting of isolated sections with the same optical orientation throughout the crystal (upper bloedite layer, Core 1; cross-polarized light [XPL]). (b) Bloedite crystal (b) that is partly delineated by straight crystal faces, as infilling of a cavity in a thenardite crystal (t) (v void) (upper bloedite layer, Core 1; plain polarized light [PPL]).

ing an- to subhedral crystals (layers VI and VII) or of closely packed an=subhedral crystals that are not intergrown (layers VIII and XI) (Fig. 4b). No interval with a different texture occurs along the top of the one halite layer that is separated from the next halite bed by only a thin layer of glauberite-rich clay (halite layer X). The basal part of some halite layers (VIII and XI), as well as the entire lower half of one other layer (VI), are characterized by a high degree of intergrowth. Three halite beds (VII, IX and X) comprise a sharply delimited basal unit that consists of interlocking an=subhedral crystals with a much larger average crystal size (2 mm) than the sub=euhedral crystals of the halite deposits above those units (150 µm) (Fig. 4c; see also Fig. 5a). In one layer (IX), the large halite crystals of this basal unit contain some smaller halite crystals as inclusions. In most layers, the halite deposits mainly consist of equant crystals. All but one of the layers also contain tabular crystals, which occur in high relative amounts in halite beds IX and X (Fig. 4d). The tabular crystals are mainly horizontally aligned, except in a few sections (in layers VII and VIII) that are characterized by a rather large crystal size, with poor size sorting (e.g. 0.5–15 mm in layer VIII), as well as by the co-occurrence of equant and tabular forms (see Fig. 4a). One other feature of halite beds VI to XI is the common occurrence of glauberite-rich clay intercalations, which are all without associated dissolution features. A truncation surface along the top

of the coarse-grained basal unit of layer IX (Fig. 5a) is the only dissolution feature that these older halite beds display. Halite layer V comprises four successive units. The lower unit (136–149 cm depth) consists of alternating layers of large (1–5 mm) interlocking an=subhedral crystals and layers with a smaller grain size (150 µm to 1 mm) that generally consist of loosely stacked sub=euhedral crystals (Fig. 5b); the fine-grained layers only consist of intergrown an=subhedral crystals in a single interval, characterized by a relatively small grain size of the intervening coarser bands (1–2 mm). The second unit (129–136 cm) mainly consists of large (0.5–1 cm) intergrown an=subhedral crystals. These crystals, in contrast to those of all older halite beds, contain a rather high amount of fluid inclusions, whose distribution is often characterized by banded patterns. A third unit (118=119–129 cm) consists of mainly sub=euhedral crystals, with very few fluid inclusions, and has a high porosity that is largely represented by packing voids. A high amount of glauberite-containing clay occurs between the halite crystals in the upper part of the unit and also below a clay intercalation near the middle of the interval. The upper unit (99.5–118=119 cm) consists of large (2–15 mm) an=subhedral crystals, which are intergrown to an important extent throughout the interval (Fig. 5c). The amount of fluid inclusions in this unit increases upward from the base. A horizontal truncation plane, covered by an overgrowth of clear halite with glauberite and

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Fig. 4. (a) Halite deposit, consisting of loosely stacked eu=subhedral crystals, including tabular forms with a random orientation (halite layer VII; PPL). (b) Upper part of a halite layer, consisting of anhedral crystals that are not intergrown to a great extent (clastic texture) (halite layer VIII; PPL). (c) Lower part of a halite layer with a basal unit that consists of large anhedral crystals, covered by a deposit with a smaller crystal size and a mainly euhedral form of the crystals (halite layer X; PPL). (d) Halite deposit with a high relative amount of tabular forms, oriented parallel to the bedding (halite layer X; PPL).

clay inclusions, occurs at 0.5 cm below the top and also below one of the thin clay intercalations within the unit. Halite layers I to IV, with the exception of the lower part of one of these layers (see below), consist

of large (2–10 mm), interlocking, an=subhedral crystals that contain many fluid inclusions. The amount of fluid inclusions generally varies between different parts of the crystals (apart from the typical variations between parallel bands), but no overgrowths of

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Fig. 5. (a) Truncation plane, along the top of the coarse-grained basal unit of a halite layer (halite layer IX; PPL). (b) Contact between layers that respectively consist of large anhedral interlocking crystals and smaller sub=euhedral crystals that are not intergrown to a great extent (halite layer V; PPL). (c) Deposit consisting of large interlocking anhedral crystals with a high but variable amount of fluid inclusions (halite layer V; PPL). (d) Horizontal truncation surface along the top of a halite layer, covered by a clear overgrowth in crystallographical continuity with the halite crystal (with a high amount of fluid inclusions) below that plane (halite layer IV; PPL).

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clear halite are recognized. A horizontal truncation surface occurs at or near the top of all four layers. Except in the youngest layer, these surfaces are again covered by thin overgrowths of clear halite, which are in crystallographical continuity with the halite crystals below the truncation planes (Fig. 5d). In all four layers, the halite crystals are intergrown to a very high degree. The porosity of layer IV is greater than that of the other layers, due to the presence of a number of large voids that are delineated by crystal faces with rounded edges. Similar voids or voids with smooth curved sides are nearly absent from the higher layers. Thin planar voids that separate neighboring crystals occur exclusively in the upper halite bed. The lower part of layer II has an entirely different texture than the one that was previously described for layers I to IV. It consists of large sub=euhedral crystals that are embedded in the glauberite-rich clay of a sharply delimited unit with an inclined lower boundary that is parallel to the sides of the underlying glauberite layer (Fig. 6a). 4.3. Glauberite One important textural aspect of the glauberiterich deposits is the presence or absence of layering, on a millimeter to centimeter scale, due to variations in the relative amounts of (sandy) clay and glauberite and=or to variations in the size of the glauberite crystals. The layered intervals are characterized by regular bedding throughout the exposure. Within the lowest intervals with predominantly unstratified deposits, several sections are characterized both by a low glauberite content and by a large size of the glauberite crystals (1–5 mm). In the central part of these sections, the deposits are laminated, due to (large) variations in magnesite content, and contain only few or no glauberite crystals. Outside these central parts, the deposits are not laminated and do not contain any relics of a laminated structure; they also have a higher glauberite content, which often increases away from the center. Some of these transitional zones are characterized by a gradual decrease in crystal size in the same direction. In all intervals with unstratified deposits, the size of a large portion of the glauberite crystals is greater than 500 µm. In the layered intervals, the average size of the crystals is most commonly below

that value; this size limit is regularly exceeded, but never throughout several successive layers. Within the layered intervals, the layers with an important admixture of detrital silt to fine sand are commonly characterized by a low glauberite content and a large crystal size (0.5–1 mm). At various levels, the deposits contain intervals with normal grading (Fig. 6b,c) or reverse grading (Fig. 6d). Their occurrence is strictly confined to the layered sections, with the exception of the mentioned occurrences in the intervals with a low glauberite content. The graded intervals are more common in the 16–99.5 cm unit than in all other parts of the deposits. Within this unit, all but one of the intervals with reverse grading are thin fine-grained glauberite layers without any clay, whose crystal size typically increases from 25–50 µm to 150 µm. In some other fine-grained layers of this type, the size of the crystals is smaller in the lower part of the layer than in higher parts, without displaying a gradual trend. Most of the intervals with normal grading in the 16–99.5 cm unit, which are 1 to 3 cm thick, have a high clay content that commonly varies between successive horizontal bands (see Fig. 6c). They typically show a decrease in crystal size from about 1–2 mm to 50 µm. Several intervals with normal grading are covered by pure, fine-grained glauberite layers, which commonly display an upward increase in crystal size. All intervals with normal grading, or the associated fine-grained glauberite layers, are covered by a (sandy) clay layer without any glauberite or with only some large glauberite crystals (500 µm). The mentioned features that characterize most occurrences of both types of grading in the 16–99.5 cm interval are more rare in lower parts of the deposits. Throughout the studied sequence, the glauberite deposits mainly consist of randomly oriented crystals. A preferentially horizontal orientation is only observed for a limited number of intervals, whose occurrence is mainly confined to the layered sections. All intervals of this type at levels below halite layer VI and part of those at levels between halite layers V and VI contain either no or only a minor admixture of clay (Fig. 7a). All other occurrences are characterized by a high (sandy) clay content (Fig. 7b). A horizontal alignment of the glauberite crystals is also a feature of the thin intercalations that most halite layers and one bloedite bed contain.

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Fig. 6. (a) Chaotic mudstone halite (halite layer II; PPL). (b) Interval with normal grading, covered by a fine-grained glauberite layer (39.5–41.5 cm depth interval; PPL). (c) Same feature as in (b), with variations in clay content between horizontal layers without concomitant changes in crystal size (42–45 cm depth interval; PPL). (d) Fine-grained glauberite layer with reverse grading (21.5–22 cm depth interval; XPL).

In the upper part of the exposure, above the highest main halite layer (V), the glauberite crystals commonly contain (sandy) clay inclusions. This feature is much less common in lower parts of the sequence, where its occurrence is restricted to the layered intervals. Within the 16–99.5 cm interval, the amount

of included material generally varies with the (sandy) clay content of the layer in which they occur. The nature of the inclusions varies with the nature of the material between the crystals, in a way that they are identical with regard to grain size, color and magnesite content. Above the 51.5–57.5 cm depth interval, the

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Fig. 7. (a) Glauberite layer, with hardly any clay, consisting of horizontally aligned crystals (Core 1, 42–42.5 cm depth interval; PPL). (b) Clay layer with a preferentially horizontal orientation of the glauberite crystals (56 cm depth; PPL). (c) Glauberite crystals in which the occurrence of clay inclusions is confined to the center (39.5 cm depth; PPL). (d) Glauberite occurrence with clay as inclusion throughout the crystals (63.5–65 cm depth interval; PPL).

occurrence of inclusions is nearly always restricted to the central part of the crystals (Fig. 7c). This interval also marks a level where major changes occur in the nature of the (sandy) clay matrix, which has a coarser grain size, a higher magnesite content and a reddish color above the transitional zone. In this upper part of the profile, both the absolute and relative dimensions of the parts of the crystals with and without inclusions are independent of crystal size, which is also true for the graded intervals. In lower parts, a confinement of the inclusions to the central part of the crystals is only rarely observed (Fig. 7d). The deposits along the base of the salt beds generally consist of fine-grained glauberite layers without any admixture of clay or with only a low clay content. Some of these glauberite layers display

normal or reverse grading. The nature of the units that cover the salt beds is more varied, both with regard to clay content — which is high along the top of halite layers VI to VIII — and crystal size.

5. Discussion 5.1. Bloedite and thenardite As accumulations of subhedral crystals, albeit with some glauberite and clay in between, the bloedite beds have the general appearance of nearly unaltered cumulate deposits. The primary deposits have however been strongly affected by diagenetic processes. Diagenetic growth, rather than syndepositional bot-

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tom growth, is for instance documented by the presence of glauberite inclusions in part of the bloedite crystals. A partly postdepositional nucleation and growth of bloedite is also suggested by the very poor size sorting of the bloedite crystals, which distinguishes the bloedite beds from cumulate deposits in other parts of the profile. A final expression of the diagenetic processes that affected the deposits is the relationship between thenardite and bloedite in the highest of the two bloedite layers. Thenardite certainly constitutes the pre-existing phase, which was subsequently dissolved and which now mainly occurs as remnants that are enclosed by bloedite crystals. This is documented by the wide ranges in the degree of dissolution of the thenardite crystals and in the degree of development of bloedite overgrowths. Phase relations in the Na–Mg–SO4 –Cl system (Harvie and Weare, 1980) show that, as long as halite saturation is not reached, a backreaction of thenardite to bloedite does not take place during the progressive evaporative concentration of brines. The observed relationship between thenardite and bloedite must therefore be the result of a later interaction with less concentrated brines, during the event that ended the period of evaporite sedimentation that the salt bed records. The observed upward increase in thenardite content in the lower part of this bloedite layer is the reverse of the expected trend in case of a purely diagenetic origin with a downward advance of diagenesis. This trend therefore seems to be controlled by a change in the relative amounts of bloedite and thenardite in the primary deposits. The thenardite layer that formed at a more recent stage, consisting of rather loosely stacked crystals, is a largely unaltered cumulate deposit. The thenardite crystals along the base of the oldest halite layer are also primary precipitates, which are partly unmodified and partly affected by a dissolution that was accompanied by bloedite formation. The primary deposits of the first stage with a higher salinity than the periods with glauberite formation consisted of bloedite and those of later stages consecutively consisted of bloedite–thenardite, thenardite, thenardite–halite and halite. The gradual change in lake water composition that these variations record is also registered for two of these stages, by an increase of the thenardite=bloedite ratio within the lower part of the second salt bed and

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by a thenardite–halite succession in the fourth. The inferred changes in brine composition certainly included the enrichment of chloride as a conservative element during the earlier part of the lake’s recorded history, before halite saturation was reached. The decrease in relative magnesium concentration in the lake water in the course of that period can be explained by a decrease in lake level that may already have affected the lake during this stage. Both the change in environmental conditions that caused this decline and the resulting decrease in surface area can contribute to an increase in degree of evaporative concentration of the inflow waters before they reach the lake. This increase would have been accompanied by an increased selective removal of ion species by the precipitation of minerals with a relatively low solubility, including magnesium-bearing carbonates. 5.2. Halite 5.2.1. Halite layers XI to VI Layers VI to XI are cumulate deposits that have undergone only minor syn- or postdepositional changes. Deposits of this type are formed by the settling of crystals that formed at the air–brine interface, in a lake with a sufficiently great depth to preclude the occurrence of bottom growth. The restriction of bottom growth to shallow waters is quite well established, as indications of this type of growth have only rarely been reported for deep lakes (e.g. Beyth, 1980; Last, 1993). According to one estimate (Smoot and Lowenstein, 1991, p. 207), depths of less than 1 or 2 m are required for the occurrence of bottom growth. The texture of the cumulate deposits of the Taoudenni–Agorgott basin is rather variable in some respects, but the differences in hydrological conditions between successive periods of halite sedimentation that these variations must record can not be deduced. For some characteristics, such as the relative amounts of equant and tabular forms, the factors that determined the nature of the halite textures can not even be identified. The high degree of intergrowth in the upper part of some layers seem to be related to the dilution accompanying the rise in lake level that ended the periods of halite sedimentation, which initiated the dissolution=reprecipitation of halite along the lake bed. The texture of the intervals at the top of two

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other layers, consisting of closely packed an- to subhedral crystals that are not intergrown, is, on the other hand, of a clastic origin. The texture of the intervals with randomly oriented tabular crystals is by contrast not related to a physical disturbance but to the unevenness of the surface on which the crystals settled. The lack of any differences in texture between the upper and main parts of layer X is compatible with the limited thickness of the interval that separates it from the next halite bed, as both features indicate that the magnitude of the change in the nature of the lake during the event that ended this period of halite sedimentation was relatively small. Because the coarse-grained, massive basal units of layers VII, IX and X are succeeded by unmodified cumulate deposits, the texture of those units is interpreted to have developed syndepositionally. Because these basal layers display features that are indicative of bottom growth, their formation during the early stages of these periods of halite sedimentation is unexpected. The partial dissolution of halite crystals along the top of the unit in one layer indicates that a homogenization of the water column by a turn-over event took place after its development, before the onset of the ensuing period of cumulate deposition. This could relate the formation of the basal unit to a gradual increase in salinity of the initially undersaturated waters along the lake bottom by the dissolution of crystals that formed at the air–brine interface (see e.g. Schmalz, 1969; Hardie et al., 1978). The rise in salinity along the lake bed ultimately led to halite saturation being reached, resulting in bottom growth of halite. Under these conditions, settling crystals could be preserved, as recorded by the presence of halite inclusions in some crystals. The fact that homogenization is accompanied by a decrease in salinity implies that most of the water column — between the saturated bottom and surface layers — was undersaturated during these periods of bottom growth. If similar hydrological conditions prevailed at the onset of every period of halite deposition, the low porosity in the lower part of the other layers, which do not display a prominent difference in crystal size between the basal and main parts of the halite beds, can also be due to syndepositional processes of a similar nature. The absence of dissolution features that are associated with clay intercalations shows that a supply

of detrital material to the lake bed was not accompanied by a freshening of the entire water mass. Dilute inflow waters only spread out over the denser brine of the lake in thin sheets, allowing part of their sediment load to reach the center of the basin (Sonnenfeld and Hudec, 1985). Both the absence of these features and the absence of truncation surfaces in halite layers XI to VI (except for the one occurrence that was mentioned before) are partly controlled by the relatively great depth of the lake during this early period. 5.2.2. Halite layer V The texture of the layered lower unit of layer V has previously never been recorded for modern or ancient halite deposits. It developed by an alternation of periods of halite sedimentation through the settling of crystals that formed at the air–brine interface and periods of bottom growth, producing alternating layers of fine-grained cumulate deposits and larger interlocking crystals. The former were deposited during periods with high evaporation rates and resulting high rates of halite sedimentation, whereas bottom growth only occurred during the intervening periods with lower evaporation rates (see e.g. Lowenstein and Hardie, 1985, p. 636; Hovorka, 1987, pp. 1033– 1034). The fact that bottom growth did occur, if only periodically, infers that the lake was shallow at this time. The nature of this interval also illustrates very clearly that the texture of deposits that consist of interlocking an=subhedral crystals can be syndepositional in origin. In the one interval of this basal unit where variations in crystal size are not accompanied by changes in fabric, these variations do not appear to be related to changes in the manner of halite sedimentation. They mainly reflect changes in the size of crystals that settled on the lake floor, as determined by the length of the period of crystal growth along the lake surface, which depends on the frequency of the disturbance of this plane by wind. Syndepositional overgrowths clearly did develop, as shown by the mosaic-like fabric, but the repeated, abrupt changes in crystal size are regarded as evidence for a sedimentary control of crystal size. The nature of the second and third units of halite layer V shows that the former developed during a period that was dominated by halite sedimentation through bottom growth, whereas the latter again con-

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sists of a cumulate deposit that did not undergo any major syndepositional changes. Variations in evaporation rates can account for short-term changes in the nature of the mechanism of halite deposition, but textural differences between thick intervals must reflect a more persistent change in hydrological conditions. In this way, the first of these intervals corresponds to a period with lower lake levels, which is also suggested by the presence of fluid inclusions in sets of parallel bands (see discussion for the most recent halite deposits). The two occurrences of an influx of silicaclastic material that produced the infillings of packing voids in the 118=119–129 cm interval were again not accompanied by a dilution that led to the dissolution of halite. The texture of the upper unit of halite layer V is identical to that of layers I to IV and can therefore be considered to have formed under similar conditions, which are discussed below. 5.2.3. Halite layers IV to I The texture of halite layers IV to I and the upper part of layer V is typical of halite beds that formed by competitive bottom growth, which suggests that the depth of the lake was less than about 2 m. Shallow water depths are also indicated by the occurrence of banded patterns due to variations in the amount of fluid inclusions. This type of variations between bands that are perpendicular to the direction of growth is commonly attributed to changes in growth rates, because more inclusions are entrapped during faster growth (e.g. Sorby, 1858, p. 460; Roedder, 1984). In this way, the occurrence of these patterns in crystals that formed by bottom growth indicates a shallow water depth, because only under those conditions can short-term variations in ambient conditions affect the properties of brines that are in contact with the growing crystals. The upward increase in the amount of fluid inclusions within the upper unit of halite layer V, as well as their nearabsence in all older halite deposits, indicate that not only the development of banded patterns but also the appearance of fluid inclusions in itself are characteristic of low lake level stages. Their presence documents faster growth rates during those stages, which, unlike the conditions that favor the development of banding, are not necessarily the direct result of a shallower depth and a smaller volume.

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The truncation surfaces that occur at or near the top of the halite layers represent another type of feature that is indicative of shallow waters. Their occurrence implies a small total volume of the brine, which can be readily diluted to a concentration below halite saturation by a freshwater influx. The syntaxial overgrowths of clear halite, with inclusions of glauberite-bearing clay, that cover these surfaces, partly formed prior to the ensuing period of sedimentation and partly during the early stages of those periods, by mainly displacive and partly incorporative growth. Apart from these horizontal truncation surfaces, the deposits do not display any prominent dissolution features. Dissolution is only recorded by the thin planar voids between the constituent crystals of the upper layer, which is indicative of shallower conditions for this final period of halite sedimentation, and also by the rounding of crystal edges. In addition, cementation features are completely lacking. The texture of the deposits is therefore of an almost entirely primary origin, unmodified by the processes that affect halite deposits in ephemeral lakes, as described by Lowenstein and Hardie (1985). Their texture is quite similar to that of the perennial saline lake deposits of the Qaidam Basin, which have been inferred to have formed at depths of 2.2 to 3 m (Schubel and Lowenstein, 1997). The lower part of halite layer II consists of chaotic mudstone halite, as defined by Handford (1981). The development of this texture has been attributed to the displacive growth of halite crystals in the vadose or phreatic zones, below an exposed surface (Smith, 1971; Gornitz and Schreiber, 1981; Handford, 1982; Cathro et al., 1992), to an introduction of silicaclastic material into dissolution cavities that developed during a period of subaerial exposure (Hovorka, 1987) and to the dissolution and reprecipitation of halite in an ephemeral lake (Handford, 1982). None of these models seems to apply here, as several features, such as the restriction of the occurrence of the halite crystals to a sharply delimited depositional unit with an inclined base, point to a syndepositional origin in the strictest sense. 5.3. Glauberite Several features of the deposits relate the formation of the layered and unstratified intervals to

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periods with, respectively, a shallow and greater depth of the lake. The existence of this relationship between glauberite texture and water depth is mainly deduced from the nature of the textural features of the glauberite deposits. It is however also substantiated by the distribution of the layered and unstratified deposits in relation to variations in petrographical characteristics of the halite layers and to the distribution of intervals with a low glauberite content. The laminated central parts of the layers with a low glauberite content in the 341C cm interval must have formed over periods of several years, partly with lake water concentrations below glauberite saturation for considerable lengths of time. In this way, these sections are different from the (sandy) clay intercalations in higher parts of the profile, which formed during singular events. The preservation of a laminated structure is more directly related to periods with high lake levels, as these stages are more likely to be characterized by the lack of a postdepositional physical disturbance or bioturbation. In this way, the absence of lamination in the lower and upper parts of these intervals, which can not be attributed to the growth of glauberite crystals, is indicative of lower lake levels. The lake also had a higher salinity during these early and late stages, in view of the higher glauberite content. The gradual changes in crystal size and glauberite content in some of these transitional zones record the gradual nature of the changes in hydrological conditions and also confirm that their current texture developed syndepositionally. The textural features of the layered intervals, particularly the occurrence of grading and the presence of inclusions, are more clearly related to water depth than those that characterize the unstratified deposits. The significance of the two mentioned features is discussed in the next few paragraphs. The results of experimental studies (Edinger, 1973) demonstrate that changes in crystal size can be related to changes in degree of supersaturation. This factor has been invoked to explain some of the few recorded occurrences of grading in chemical deposits (Kelts and Hsu¨, 1978; Magee, 1991). Reverse grading has also been related to an increase in salinity with time (Ogniben, 1955) and to the observation that the crystals that develop first in crystal growth experiments are smaller than those that form at a

later stage (Garrison et al., 1978). Normal grading has been described as a clastic texture for one basin (Hardie and Eugster, 1971). Its occurrence has also been related to a decrease of the length of time that crystals can develop, combined with an increase in the concentration of other dissolved salts (Watson, 1985). The possibility of a clastic origin can be excluded for grading in the studied deposits, e.g. because of the euhedral form of the glauberite crystals and the lack of concomitant gradual changes in the nature of the silicate fraction. The intervals with normal grading are therefore considered to record a progressive increase in degree of supersaturation, in accordance with the available experimental evidence. During the formation of these intervals, an inflow of water was maintained, as demonstrated by the high clay content of these sections. An increase in the degree of supersaturation implies that the removal of ions by glauberite formation did not keep pace with the increase of their concentration while the brines were being subjected to evaporation. The formation of the fine-grained glauberite layers that cover many of the graded intervals represents a final stage of glauberite deposition, after the supply of inflow waters came to an end. The reverse grading that characterizes several of the fine-grained glauberite layers is due to a decrease in degree of supersaturation through glauberite formation. The variations in clay content between successive horizontal layers within the intervals with normal grading, without concomitant changes in crystal size or deviations from a gradual trend, are again due to a difference in density between the inflow waters and the dense brine of the shallow lake. An influx of dilute waters did result in a freshening of the entire water column at other times. Such occasions are for instance recorded by the layers with a lower glauberite content and a larger crystal size that cover the intervals with normal grading (or the associated fine-grained glauberite layers), which mark the events that ended the cycle to which each of these intervals correspond. Sediment inclusions are very common in crystals that develop from interstitial brines within a sediment matrix, to the extent that their presence is often considered to be indicative of subsurface growth in studies of ancient evaporite deposits (e.g. Ortı´ Cabo et al., 1979, and Salvany and Ortı´, 1994,

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for the Spanish Miocene glauberite deposits). In the Taoudenni–Agorgott basin, however, the deposits in which this feature developed formed subaqueously. This is demonstrated by the finely layered structure of the deposits, with changes in crystal size and glauberite content, and by the general absence of features that are indicative of crystal growth during periods of subaerial exposure. The repeated occurrence of desiccation has been suggested by some authors (e.g. Aucour, 1988b; Øxnevad, 1991), based on the observation of features interpreted as desiccation cracks. Thin section observations have however shown that these small planar voids can not be interpreted as syngenetic mudcracks, both because sediment infillings are always lacking and because the cracks also developed within clay lenses that are not associated with horizontal surfaces. Because the inclusions partly consist of detrital silt to fine sand, they also did not develop during crystal growth within the water column. The crystals also can not have formed at levels far below the brine–sediment interface, e.g. in view of the occurrence of grading which closely records gradual changes in lake water composition. It must therefore be concluded that the crystals mainly formed near the top of the lake bed. Only the crystals that constitute the pure fine-grained glauberite layers may have nucleated and grown at the lake surface. Because the crystals whose variations in size record gradual changes in the composition of the brine developed through bottom growth, the lake must have been shallow and well mixed during this lake stage. Because smaller amounts of sediment are incorporated as growth rates decrease (Kastner, 1970), the inclusion-free outer part of the crystals in the upper part of the profile is inferred to have developed during a later stage of slow displacive growth. The nature of the occurrence of these overgrowths in graded intervals demonstrates that they do not represent late-diagenetic formations. As synsedimentary features, in the strictest sense, they most probably developed upon cooling of the brines, which lowers the solubility of glauberite. The required variations in temperature can be assumed to represent differences between day and night temperatures, as a strong seasonality cannot be expected to have occurred in this region or to be able to produce the observed patterns, which demand short-term varia-

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tions. The base of the zone with sediment inclusions in the center of the crystals roughly coincides with a level where changes in the nature of the (sandy) clay matrix record a drop in lake level (Mees, 1996). The development of this pattern of sediment inclusions therefore appears to be favored in a lake with a shallow depth. Because the effect of variations in ambient temperatures on lake bottom temperatures will be more pronounced in shallow lakes, this observation is compatible with a temperature control on the development of the overgrowths. The uniformity of the unstratified deposits reflects an absence of a response to short-term variations in environmental conditions that are clearly recorded by the layered intervals. This lower sensitivity is related to the greater volume and resulting greater depth of the lake, of which the brine composition is less readily affected by an influx of freshwater or by changes in evaporation rates. The unstratified sections also lack the typical textural features indicative of shallow water depths that characterize the layered deposits. The large crystal size of the unstratified deposits indicates the absence of periods with a high degree of supersaturation. The place within the water column where nucleation and crystal growth occurred can not be deduced from the textural features of the unstratified deposits. Only the formation of the few intervals with horizontally oriented glauberite crystals can be related to the settling of crystals that formed at the air–brine interface. This is not the case for the preferentially horizontally oriented crystals (with clay inclusions) in layered intervals; the alignment of these crystals is probably related to the compaction of the clay layers in which they occur (F. Ortı´, pers. commun.), although the evenness of the surface on which they formed may also be a factor. During the development of those parts of the unstratified deposits with crystals that formed at the lake surface, the changes in degree of supersaturation that determine the variations in crystal size are only required to have affected the upper part of the water column, instead of the entire volume of the lake (in contrast to the stages of intrasediment bottom growth when the graded intervals of the layered sections were formed). When the variations in the nature of the glauberite-bearing deposits are discussed in terms of the lake level history of the basin, the formation

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of halite layers VI, VII and XI can be said to have been ended abruptly by a sharp rise in lake level, followed by a gradual decline, till halite saturation was again reached. The other sections that separate the halite layers are both much thinner and continuously layered, which shows that the shorter interruptions were predictably not accompanied by an important rise in lake level. The inferred abrupt ending of halite sedimentation is also expressed at a smaller scale, by the high clay content of the units that overlie halite layers VI and VII. The occurrence of fine-grained glauberite layers along the base of all halite beds records the high rates of evaporative concentration that eventually led to halite saturation being reached.

6. Conclusions The petrological study of the Holocene deposits of the Taoudenni–Agorgott basin demonstrates that they were formed in a perennial saline lake with a great to very shallow depth (whose absolute values can not be deduced from the reported observations for a single sampling site). The profile does not include any deposits that formed in an ephemeral lake or a mudflat environment during the final stages of lacustrine sedimentation, which is only due to a non-preservation of the most recent deposits, as demonstrated by the study of the surface layer of the exposure (Mees, 1998). The present study deals with the subrecent, largely unaltered, deposits of a type for which hardly any petrographical descriptions are available. It therefore provides some new information regarding the significance of certain textural features, which may contribute to the interpretation of petrographical characteristics of ancient evaporite deposits. Some of the conclusions concerning the conditions in which some commonly reported textural features can develop are that, in perennial lakes with an active evaporite sedimentation, layered and unstratified deposits can respectively form during low and high lake level stages; normal and reverse grading are explained by referring to variations in degree of supersaturation; sediment inclusions can develop during crystal growth at the brine– sediment interface in a shallow perennial lake; clear overgrowths can be syndepositional features, instead

of being the result of an early- or late-diagenetic postdepositional accretion; a horizontal alignment of tabular crystals is much less common than could be expected, which is partly due to the dependence of its occurrence on the evenness of the surface on which the crystals have settled; fluid inclusions are largely absent in cumulate deposits and their abundance in deposits that developed by bottom growth increases with decreasing water depth; deposits that form during early stages of periods of halite sedimentation that are mainly characterized by cumulate deposition often display features that are indicative of bottom growth; the termination of periods of cumulate deposition of halite are often characterized by cementation and the development of clastic textures; chaotic mudstone halite can be a syndepositional formation. Some of these conclusions concern textural features whose significance for this particular site is found to be different from the interpretation that they almost invariably receive in studies of ancient evaporite deposits. This shows that simple textural criteria can only be used with great care. An analysis of vertical variations for individual sites, combining observations for several textural features, represents the most reliable approach. This study does however also offer an illustration of the importance of textural studies of evaporite deposits as a tool for the recognition of depositional environments. This relevance is illustrated by demonstrating that, although changes in mineralogical composition provide a record of a long-term change in chemical composition of the lake water and more short-term variations in salinity, only variations in the nature of textural features can be used as lake level indicators. This is the case both because water depth determines the place within the water column where crystal growth occurs and because the volume of the brine determines the effect of freshwater influxes and changes in evaporation rates on the concentration of the brine, thereby exerting a control on factors that ultimately determine the texture of the deposits.

Acknowledgements The members of the 1988 expedition team, led by Dr. Nicole Petit-Maire (University of Aix-Mar-

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seille II), are thanked for granting permission to study the material that they collected. I am particularly indebted to Dr. Erhard Schulz (University of Wu¨rzburg) for arranging access to the samples and for sharing his knowledge of the site. The author was research assistant with the Belgian National Fund for Scientific Research during part of the period when this study was conducted.

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