Pergamon

1040--6182(95)00007--0

Quaternary International, Vol. 29/30, pp. 49-60, 1995. Copyright © 1995 INQUA/Elsevier Science Ltd

Printed in Great Britain. All fights reserved. 1040-6182/95 $29.00

A PETROLOGICAL AND GEOCHEMICAL STUDY OF THE CALCAREOUS SANDSTONE OF WEST AFRICAN MARINE PLEISTOCENE (CAP DES BICHES,

SENEGAL)

Bachir Diouf,* Pierre Giresse,t Serge Occhietti,~: Christiane Causse§ and Pierre Pichet~: *Ddpartement de Gdologie, Universitd Cheikh Anta Diop, Dakar, S~ndgal

t Laboratoire de S~dimentologie et G~ochimie Marines, CNRS, URA 715, Universitd de Perpignan, France ~.G~otop, Universitd du Quebec ~ Montrdal, Montreal H3C 3P8, Canada

§Laboratoire de G~ologie du Museum National d'Histoire Naturelle, CNRS, URA 723, Paris, France

In Senegal, the marine Pleistocene is represented by several outcrops of calcareous sandstone slabs which underly the recent sandy shoreline south of Dakar. These outcrops show dissolution and recrystallization phenomena increasingly evident as one moves south. They disappear at Toubab Diaiao, i.e. approximately 40 km from Dakar. The outcrop at Cap des Biches has been studied mainly because of the exceptional preservation of some Ostrea calcitic tests which allowed isotopic U/Th measurements. This outcrop of metric thickness was subjected to petrographic and mineralogical examination and to several geochemical analyses (trace elements, stable isotopes, amino acids) which lead to the conclusion that the base of the deposit has been preserved. Two U/Th apparent ages obtained (178 ka and > 240 ka) are older than the only age of the isotopic stage 5c obtained in the area. A post-depositional mobility of U cannot be rejected, and ages may not be reliable. However, a minimal age, considerably older than finite ~4C ages previously published, may be proposed (Eemian Interglacial, i.e. isotopic stage 5). In these environments, geochemical analyses show high Pleistocene sea levels of either warmer and/or more diluted waters than the present levels, i.e. a probable Northern extension of Guinean waters.

INTRODUCTION

The Senegal-Mauritanian Ceno-Mesozoic basin has been partly invaded by several marine transgressions whose shorelines outcrop as aureolas: it is accepted that the progressive uplift of the continent controls decreasing widespread transgressions (H6brard, 1972; Elouard, 1975). From a structural point of view, the shoreline of this basin constitutes a transition between the coast of Morocco which is often strongly epeirogenic (Weisrock, 1980) and that of the Gulf of Guinea which is more stable and does not show uplifted Pleistocene shorelines (Giresse, 1987, 1988). The Senegalese part of the basin presents an already fairly rigid lithosphere which was evidenced in the Senegal valley thanks to 200 km transects of several Holocene geoid surfaces (Faure et al., 1980). The high Pleistocene sea levels in this area are represented by thin slabs of calcareous sandstone between 0 and +1 m south of the Cap Vert Peninsula (Fig. 1) or buried in the Senegal delta (Monteillet, 1986). It is worth noting that the southern limit coincides with a significant ongoing climatic transition which could lead to the disappearance, towards the South, of the more labile Pleistocene calcareous deposits due to the intensification of tropical hydrolysing processes.

On the basis of 14C radiometric measurements alone, the sandstones South of Cap Vert were initially attributed to a high sea level corresponding globally to the relative warming up of isotopic stage 3. These deposits, called Inchirian (Elouard, 1962), would therefore imply an improbable uplifting of the shoreline of 35-40,000 BP whose eustatic level is found between -30 and -45 m according to the curves of different authors (Bloom et al., 1974; Shackleton and Opdyke, 1973; Butzer, 1975; Chappell, 1981; Chappell and Shackleton, 1986). More recent studies (Giresse et al.,

1988; Diouf, 1989) have demonstrated that the 14C radiometric ages were strongly rejuvenated both by recrystallizations or epigeneses developing in certain calcareous bioclasts and by at least three successive cementation stages. Consequently, these deposits can very likely be regarded as shoreline of the last Interglacial high- stands (stage 5). In view of the exceptional diagenetic processes affecting these deposits, a detailed petrographic, mineralogical and geochemical study was necessary to define the factors of the depositional environment and the mechanisms of diagenesis and alteration, and thus to identify the most likely shelly material to allow U/Th radiometric measurements. The outcrop at Cap des Biches seemed the most suitable for this objective, and its detailed study will be compared with that of Pleistocene outcrops at nearby beaches. Unfortunately, the latter provided no abundant shelly material.

METHODS

The sediments and different shells collected were analysed in several steps: observation in thin plates with a scanning electronic microscope, point by point chemical analyses by dispersion of energy (TRACOR system) and dispersion of wavelength (CAMEBAX system). In the last two cases, the bioclast and cement samples were collected with a dentist's drill.

The isotopic measurements of oxygen and carbon were carried out on a single species (usually a single test) after verifying the absence of any recrystallization, and the results were expressed in 813C%o, from the deviation of ~3Cp2C ratios from the PDB standard, after the usual corrections.

U/Th measurements were carried out by alpha- spectrometry on unrecrystallized Ostrea tests after

49

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ascertaining the absence of any recrystallization under polarizing microscopy and scanning electron microscopy. Classical separations of U and Th isotopes, as summarized by Lally (1993), were followed. However, a triple spike solution (Causse and Hillaire-Marcel, 1989) was used to check the equilibrium between 232Th and 228Th.

The rate of isoleucine epimerization (D-alloisoleucine/L- isoleucine or Alle/Ile ratio) from total amino acid (free and peptide bound amino acids) was measured by high-pressure liquid chromatography (HPLC), using a Nucleosil ion exchanger resin column. The amino acid concentrations were measured by fluorescence of the derivative combined

with O-phthaldialdehyde, by calculating the area under the peaks of the chromatogram. Since at the same concentration the fluorescent activity of the alloisoleucine derivative is less than that of the isoleucine derivative, a laboratory-equipment correcting factor of the order of 1.25 was used in the calculation of the racemization rates, which measure concentration ratios in this case.

FIELD EVIDENCE AND PREVIOUS STUDIES

South of the Cap Vert peninsula, outcrops of coastal Pleistocene sandstones are usually localized 1-1.5 m above

Study of Calcareous Sandstone in Senegal 51

the present mean sea level where they form 1-2 m thick deposits laid right on top of the Cenozoic bedrock (Cap des Biches, Bargny-Siendou and Toubab-Dialao). We also know the sandstones at Lake Retba and the eolianites at Yoff (Fig. 1).

The sandstone at Cap des Biches, approximately 1 m thick, overlies Ypresian marls whose roof is altered (concretions of calcareous nodules in a grey clay) below the present shoreline. It is a beige shelly sandstone with cross beddings, and the tests are mainly composed of barnacle and mollusc debris (Ostrea, Pecten) which are widely dissolved in the upper two-thirds of the cross-section where they define a millimetre to centimetre porosity. Manganese and/or iron oxides often constitute coatings on cavity walls or linings round the shells. In the upper third of the outcrop, the bioclastic components are practically all dissolved and the presence of Ocypoda crab burrows indicate an upper-beach sedimentation environment. The summit is covered by a thin layer of soil occupied by Opuntia (Fig. 2).

At the top of the beach from Bargny to Siendou, sandstone slabs are fairly often seen which go underground, and up to 100 m behind, this shoreline (Siendou). Here too we have porous shelly sandstones, the porosity of the transported blocks behind the shoreline being particularly high. At Siendou (Fig. 1), the strongly developed cementation gives a distinctly indurated aspect to the rock. At Toubab Dialao, the outcropping slab behind the shoreline is made up of a conglomerate sandstone with big Ostrea shells and laterite debris.

At Lake Retba, the Pleistocene sandstones are found on either side of the former outlet, +1-2 m above the present ocean level and 300-400 m from the coast. They are made up of three superposed 30-40 cm thick slabs of yellow shelly sandstone with parallel strata.

The outcrop at Toundeup-Riya (Yoff) is a predominantly cemented shoreline eolianite approximately 6 m above the

I

100,-

Roots tubule and Ocypoda burrow

Irregular fenestrae with Fe-Mn cutanes or shell v o i d s

Locally preserved i n c l i n e d bedding (spit beach ?)

Abundant whole Ostrea

Fissile marl (lithomarge)

Ypresian

Hard marl

FIG. 2. Schematic cross-section of the Cap des Biches outcrop.

doleritic platform. Only the lower part of the outcrop (60 cm) presents numerous carbonated bioclast nodules indicating marine sedimentation (Lappartient, 1971). Other Pleistocene marine sandstones are observed with the help of drillings several tens of metres under the present sea-bed:

(1) the drilling at Diama, in the substrate of the Senegal delta 10 km from the sea at a depth of 26-30 m, presents a vacuolar sandstone containing sparse shell debris.

(2) at Hann Bay, south of Cap Vert, at between -13 and -17 m, shelly sandstones are found, covered by an erosion surface, and then by loose sand.

Two 14C datings carried out on the shells and cements of Bargny-Minam sandstones (Demoulin and Masse, 1969) indicate ages of 31,000 _+ 1700 BP and 32,500 _+ 2150 BP, but the numerous recrystallizations (Giresse et al., 1988) as well as an altitude which is too high with respect to the reference eustatic curves (Bloom et al., 1974; Shackleton, 1987) cast doubts about these measurements. In contrast, the probability of a high sea level reaching -30 m around 35-40,000 years age, as well as the geometry of the sedimentary particles (Monteillet, 1986), mean that the sub- surface sandstones of the Senegal delta, and possibly at Hann Bay, can be recognized as real remnants of the 'Inchirian' shoreline (intra-Wiirm or Wisconsin).

RESULTS

Petrography of the Sandstone at Cap des Biches and at Nearby Pleistocene Sandstones

Cap des Biches The deposit is a fine (median between 0.17 and 0.19 mm),

essentially quartz sand in which the biogenic fraction only represents 10--20% (barnacle, mollusc, bryozoan, foraminifer and echinid debris in order of decreasing importance). The presence of cross-bedding associated with the small particle size and with the existence of coastal foraminifers tends to indicate a lower-beach deposit. However, the relative rarity of Foraminifers and the presence of Ocypoda burrows suggest rapid hydrodynamic variations which determine the superposition of successive banks. The fauna includes representatives of the inferior medio-littoral (barnacles and molluscs) environments and the superior infra-littoral environments (echinids in particular); the nature of the species represented indicates the proximity of a rocky coast.

The molluscs are generally badly preserved. Most of them, originally aragonitic, have been dissolved leaving only a cavity delimited by a thin micritic envelope. The cavities have been more or less completely filled by blocky sparite (75-250 ~tm) which enclose allochtonous particles (quartz, foraminifers, spicules) (Fig. 3a). The shells of calcitic origin (Ostrea in particular) are better preserved, especially towards the base, but they are destructured, and even dissolved at the top of the section. The slabs of barnacles which are mainly of calcitic composition are better preserved, but they too show increasing destructuration towards the top of the section where recrystallization of microsparitic calcites can be observed. The chambers are partly filled with micrites (of the first generation), especially

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Study of Calcareous Sandstone in Senegal 53

at the base of the section, and with microsparites (12-80 ~tm). The latter sometimes overlie the isopachous palissade cement which lines the wall of the chambers (Fig. 3b). The bryozoan debris are likewise preserved at the base of the outcrop with a partial or total filling of their zoecia cavities by microsparitic calcite crystals (Fig. 3c). Foraminifers only occur at the base of the outcrop, the walls are often micritized and the chambers filled either by micrite or calcite microsparite. Finally rare echinid debris are found in a destructured state at the base of the section (Fig. 3d).

Calcitic cementation is differentiated both by the nature of its substrate and by its process chronology. Micrites are often observed as an irregular rim around quartz grains, and are also found as intraclastic pockets or as filling of barnacle chambers with, in some cases, a geotropic disposition (Fig. 4a). The cement, in contact with barnacle and bryozoan debris, is palissadic, anisopachous and constituted of crystals approximately 45 ].tm long. It often continues inside the cells, but with shorter crystals (10 p.m). Mosaic microsparites lastly fill in what was left empty in the cells and constitute the most usual sandstone cementation link. Their crystals have rounded, or angular, or sometimes planar contacts and increase in size towards the centre of the pores (up to 90--100 p.m). Finally, in contact with echinid debris, the cement is an epitaxic calcite growing in optic continuity with the bioclast magnesian calcite.

It results an intraparticular porosity especially at the base of the section where the filling of the bioclast voids by the different cements is unfinished. A more extended porosity (Fig. 4b) linked with dissolution develops mainly towards the upper part with millimetre to centimetre pores. The traces of dissolved tests are recognized by their micritic or palissadic envelope.

Comparison with nearby Pleistocene sandstones Buried deposits. Two petrographic characteristics are

common to the drilled samples (Diama, Hann Bay): (1) the existence of micritic relicts around quartz grains or

as clouds in the intergranular cement, (2) the recrystallization as sparry calcite of the completely

dissolved aragonitic bioclasts (molluscs in particular). The micrites formed the initial cement precipitated in a

supersaturated phreatic environment (high levels of mag- nesium). The palissadic cement around the bioclasts appeared next. The inter- and intragranular sparites have lower levels of trace elements than the early micrites, which may indicate a process of cementation in a briny interstitial environment, diluted by the waters percolating from the vadose zone. The deepest levels at Diama seem to have undergone an essentially phreatic diagenesis; indeed, its equigranular microsparitic cement reflects this type of environment (Longman, 1980; Davaud and Strasser, 1984; Froget, 1985).

Outcropping deposits. Outcropping sandstones present three types of cementation:

(1) micrites as rims around quartz grains or as islets trapped in a coarser matrix characterized by high concentrations of trace elements, magnesium in particular. The rims, with their anisopachous disposition, indicate a

vadose cementation; the basal sandstone of the lithified dune at Yoff is the only case of phreatic micritic cement.

(2) anisopachous palissade rims around the bioclasts, here again the sandstone at the base of the dune at Yoff is exceptional in that it does not present this type of cement.

(3) 70-90% of the intragranular cements are constituted by a sparitic to microsparitic mosaic. The low trace-element content suggests a dilution of the productive brines with respect to sea water. The consolidated sandstone on the beach was exposed to the action of vadose waters leading to the dissolution of the tests and the micritic cement and to their recrystallization as coarser calcite. The epitaxic calcite cement on the echinid debris probably also formed during this stage, when vadose-silt material is deposited throughout the porosity.

In a final phase, most of these outcropping sandstones were exposed to the hydrolysis of meteoric waters which completed the dissolution of the aragonitic tests and defined a millimetre to centimetre porosity. It is this process which controlled the precipitation of the sparites and the fossilization of the dune at Yoff.

Geochemical Analysis of the Sandstones at Cap des Biches

Trace elements X-ray analysis of all preserved and determinable bioclasts

indicates that they are mostly constituted of non-magnesian calcite. The intensification gradient of the diagenetic processes from the base to the top of the outcrop is verified in the chemical composition of the three reference levels. Trace elements (strontium, sodium) are best preserved in the base bioclasts than in the destructured summit bioclasts which, in contrast, often have a higher magnesium content. The sodium concentration is especially clear in the foraminifer tests at the base of the outcrop. One exception is the barnacle debris at the roof which retains high strontium and sodium concentrations (Table 1).

Similarly, the calcititic cement shows no trace of strontium but has a fairly homogeneous magnesium content, higher than that of the bioclasts. A fair amount of sodium is found in the micrites (first generation of cement), but it tends to disappear in the sparites, especially at the summit. Generally speaking, the cements at the summit of the outcrop have lost most of their trace elements (Mg, Sr, Na and Fe). The micrites are more conservative than the sparites, except for the intragranular sparites at the median level where the Na/Ca x 102 molar ratio reaches 0.31 (Table 2). Iron and sodium appear closely linked in most cases.

Stable isotope composition The isotopic values (Table 3) of the Pecten and Ostrea

samples gathered at the base of the section are assumed to represent the initial compositions of these shells at the time of their biogenetic crystallization. They give an indication of the isotopic composition of the sea water in which the shells formed. They are slightly less than those found today in the oyster Gryphea gasar and in barnacles. If we ignore any possible fractionation of the isotopes linked to the meta- bolism of different species, the isotopic deficit observed can

54 B. Diouf et al.

FIG. 4. (a) Calcit ic cementat ion in contact with a quartz grain: micrit ic rim, mc, then more or less palissade microsparite, mp, then sparite. (b) Linking phase of calcitic microsparite, mp, towards the top of the section, the preserved calcitic mollusc debris, mo, is associated with

high porosity.

TABLE 1. Trace element concentration of determinable bioclasts at various levels in the Cap des Biches cross-section

TABLE 2. Trace element concentration of cement calcites at various levels in the Cap des Biches cross-section

Mg2+/Ca2+ Sre+/Ca 2+ Na+/Ca 2+ Fe2+/Ca 2+ × 10- 2 × 10- 2 × 10- 2 × 10- 2

MgZ+/Ca2 ~ Sr2./CaZ + Na+/Ca2+ Fe2+/Ca2 ÷ Intergranular CB2 1.9 - - 0.16 0.35 CB4 1.52 - - 1.18 0.047

× 10 -2 × 10- 2 x 10- 2 × 10-2 micrite CB6 1.69 - - 0.94 0.13

Calcit ic CB2 1.4 0.89 0.85 0.098 Intergranular CB2 1.8 CB4 1.3 - - 0.32 0.19 CB4 1.74

Molluscs CB6 1.57 - - 0.25 0.05 spa_rite CB6 1.5

CB2 1.7 0.0071 0.77 0.087 Intragranular CB4 2.84 Barnacles CB4 1.44 - - 0.61 0.098 micrite CB6 2.03

CB6 1.25 0.044 1.01 0.11

Foraminifera CB2 1.2 0.010 3.53 0.35 Intragranular CB2 1.8 CB4 1.75 CB6 1.45 - - 0.28 0.057 sparite CB6 1.67

0.083 0.066 0.31 0.045

0.069 0.027

0.72 4.3 0.52 4.05

0.05 0.022 0.75 0.10 0.19 0.025

Study of Calcareous Sandstone in Senegal 55

TABLE 3. Isotopic values of shells and cement calcites from horizons of the Cap des Biches cross-section; some comparative or reference values are given on the right

Spadtic Micritic Ypresian Ostrea Pecten Balanus cement cement marls

~/%o ~sO 13C lsO 13C ~sO 13C JsO 13C 180 ~3C ~sO 13C

CB6 -3.86 -0.9 -4.12 -1.04 -5.66 -4.96 100

9O

80

70

60

50

40

30 cm

20

10

CB7 -3.28 -0.67 -3.77 -0.9

-3.79 -1.3

CB2 -1 .64 0.50 -3.85 -1.2 -3.62 -1.22

CB10 -1.42 0.01 -3.85 -0.8

-3.83 -1.22

-2.55 -1.7

Calcareous nodules in colluvium

Calcareous crust on the

-2.72 -1.05 rock bench

0.05 -5.7 Gryphea gasar

Present 0.06 0.37 barnacles

be attributed to two factors. The sea water in which Pecten and Ostrea at Cap des Biches developed was apparently either more diluted or warmer than present-day sea water, or both. The hypothesis of a palaeotemperature higher than the present one can be attributed to several distinct variables: climatic optimum, atmospheric or oceanic conditions favourable to a warming up of intertidal waters: conditions which are met today deep inside the Gulf of Guinea, off the coast of Cameroon. The isotopic composition of an Ostrea

shell, taken half-way up section at Cap des Biches is of the same order of magnitude, ~lso = -3.3 to -4 and ~ 1 3 C ~- - 0.76 to- l .31, as that of Balanus samples taken all over the section and of the micritic and sparitic cements (Table 3). It shows a significant isotopic impoverishment, linked to the recrystallization process. The 180 impoverishment in the micritic cement is slightly more marked towards the top. These compositions reflect the equilibrium conditions of the water and the CO2 dissolved in it at the time of secondary recrystallizations (approximately 50% of the analysed volume). The impoverishment observed in a tropical context excludes recrystallization in a confined marginal environment. It results either from an increase in the equilibrium temperature at the time of recrystallization, in the absence of evaporation (Hoers, 1987) or, more probably, from an isotopic dilution of the water in equilibrium by the circulation of meteoric and continental water (Beier, 1985, 1987). These two components indicate an immersion linked to and followed by the presence of flesh water. The dissolution of shells towards the top, the impoverishment in lsO more marked towards the top in the sparitic cement and the washing out of strontium favour the hypothesis of dissolution and recrystallization in the presence of vadose water. The estimated isotopic composition of rainwater in the area should be of the order o f - 3 to -4%dSMOW, i.e. the equivalent of approximately -3%dPDB.

The micritic and sparitic cements have a similar isotopic composition, which indicates that the geochemical crystallization conditions of the early micrite and of the later microsparites and sparites were almost identical. This interpretation favours a very long period of continental recrystallization. The moderate amount of 13C (approximately -1.2) indicates a relatively low biogenetic CO2 contribution during recrystallization. Moreover, the average composition of this second batch of samples is between that of the present calcareous crust of the platform visible at the limit of marine immersion (8~80 = -2.72 and 8~3C = -1.05) to which it is relatively close, and that of the calcareous nodules found in the colluvial deposits juxtaposed to the beachrock (8180 =-5.66 and ~13C =-4.96). The negative values of 13C in the calcareous nodules of the colluvions imply a high biogenetic activity in the continental environment.

The beachrock at Cap des Biches seems to result from the cementation of a former thanatocenotic shelly deposit situated in the lower intertidal zone. The cementation began at the start of exundation. It is composed of an early micritic phase, followed by a microsparitic and sparitic phase. It is associated with the dissolution of aragonitic shells which increases towards the top. The recrystallization as calcite occurred in equilibrium with water which was low in 180, similar in composition to rainwater, and which had a small proportion of organogenetic CO2.

Aminochronology Because of the absence of a regional amino-

chronobiological scale, the epimerization rates in shells at Cap des Biches are compared with two groups of Holocene samples, dated with 14C, taken at Pointe de Fann, South of Cap Vert and at NGallou. Laboratory analysis was carried

56 B. Diouf et al.

out on collections of one to seven specimens by species and by localization.

Amino acid dating is based on a natural reaction: the transformation of L-shaped amino acids, the only form produced by living organisms, into o-shaped amino acids after the death of the organisms (Hare and Abelson, 1968). The two forms of one chiral amino acid are called enantiomers. This racemization is expressed as D/L ratios. The reaction is called epimerization for amino acids with two chiral carbons, especially isoleucine (Alle/Ile ratios) and the amino acid forms are called diastereoisomers. The reaction depends mainly on the effective heat received by the fossilized organisms, i.e. it increases with temperature and time until it reaches an equilibrium mixture (D/L = 1 for eniantiomers and Alle/Ile = 1.3 for diastereoisomers) (Kvenvolden, 1975; Shroeder and Bada, 1976; Wehmiller, 1982; Rutter et al., 1985). The rate of racemization may be characterized by considerable variations between genus (Wehmiller, 1984; Miller and Mangerud, 1985; Hearty etal., 1986; Miller and Bringham-Grette, 1989). With aminochronology, it is thus possible to compare the relative ages of two samples of the same species with the same thermal history or to calculate an absolute age when the thermal history is reconstituted and when chronological references independent of the method are available (Rutter et al., 1985; McCoy, 1987).

The Holocene samples which were measured confirm the very high rate of epimerization in a tropical climate (Dakar mean temperature around 25°C): average Alle/Ile of 0.154 + 0.55 for Patella safiana dated at 1910 + 100 BP (1-403) at Pointe de Fann and 0.263 for Ostrea dated 2700 + 100 BP

(UQ-1484) at NGallou. The high ratio for Ostrea confirm that this genus, with a very porous shell, has a high rate of epimerization (Kimber and Griffin, 1987). In contrast, Patella, with a dense shell, is a suitable material for aminochronological analysis (Bowen and Sykes, 1985; Occhietti et al., 1983). The Patella epimerization rate is roughly two and half the rate measured in Atlantic Morocco where the mean temperature is 18.5*C (Occhietti et al., 1993) and can be considered as representative of the Dakar region.

At Cap des Biches, the Alle/lle ratios of four different species show a disparity between the species and a very high intraspecific disparity for Balanus (Table 4). In our opinion, the relevant values for the age of the fossils in the littoral sandstone are higher ratios: ratios higher than 1.00 and close to the diastereoisomeric equilibium for two unidentified fragments, 0.909 for Pecten and 1.144 for Balanus. This opinion is based on the idea developed by Murray-Wallace (1993) on low values of racemization in the total acid hydrolisate with respect to age. As rates of racemization vary according to the location of amino acids within peptide chains (Mitterer and Kriausakul, 1984), free amino acids and amino acids bound in lower-molecular weight peptides are selectively removed from fossils during in situ leaching. As a consequence, the measured ratios for the total amino acids are 'disproportionately weighted by the less extensively racemized amino acids bound in higher-molecular weight polypeptides' (Murray-Wallace, 1993, p. 22). This effect could be applied to the lower ratios from Balanus which might also be related to another causes. The fossilized shells in the littoral sandstone may have been reworked from palaeoshorelines of different ages before being accumulated

TABLE 4. Epimerization measurements in shells at Cap des Biches compared with two series of Holocene samples (Pointe de Fann and NGallou)

Age

Patella safiana Ostrea Balanus Thais haemastoma Pecten n.i. mollusc Lab. number

Site UQA AIle/lle Alle/Ile Alle/Ile Alle/Ile Alle/Ile Alle/Ile

178 ka > 240 ka

1910 ± 100 1-403

2700± 100 UQ-1484

Cap des Biches

Pointe de Fann

NGaUou

1399 1400 1401 1404 1402 1403

511-1 511-2

514-1 514-2 514-3 514-4 514-5 1397 1398

513-1 513-2 512-1 512-2

778-1 778-2 778-3 778-4 778-5

0.138 0.143 0.079 0.125 0.142 0.251 0.199

0.715

0.26 0.241 0.26 0.312 0.242

1.144 0.398 0.82 0.55

0.141 0.139

0.172 0.177

0.919 1.074 1.088

Study of Calcareous Sandstone in Senegal 57

and fossilized. This hypothesis is not supported by field data, as this region is highly active pedogenetically and tectonically stable. The hypothesis of micro-organism fossilization in recrystallized calcite, during the different phases of dissolution-recrystallization, especially of calcicolous micro-organisms in a tropical climate, corresponds more closely to the partly tubular structure of Balanus and to the history of the littoral sandstone. In our opinion, the relevant values for the age of the fossils in the littoral sandstone are the highest ratios: ratios higher than 1, i.e. the racemic mixture for two unidentified fragments, 0.909 for Pecten and 1.144 for Balanus. In New Guinea (mean temperature 27°C) and on the Great Barrier Reef in Australia (mean temperature 26°C), Hearty and Aharon (1988) calibrated the epimerization rates of Tridacna gigas and T. maxima by 14C and U/Th datings on corals. They obtained a variation of the epimerization rate in three phases: a rapid phase of 0.07/ka during the first 8000 years, an intermediate phase up to 60 ka during which the rate is approximately 0.006/ka, and a phase which ends up with a quasi-equilibrium of 1.30 around 185 ka.

The Holocene Ostrea samples from Senegal have an Alle/Ile ratio of roughly the same range as the Tridacna rates obtained by Hearty and Aharon (1988): 0.243 at 25°C for a 14C age of 2700 BP compared to 0.24, 0.33 and 0.21 at 26°C for, respectively, 1100, 2200 and 3600 BP. This would mean that Tridacna ratios can be cautiously used as comparative values. At Cap des Biches, the relevant ratios comprised between 0.7 and 1.144 would correspond to the Tridacna ratios indicative of a time span from the Hearty and Aharon phase 2-phase 3 transition to the beginning of phase 3, i.e. in absolute ages between roughly 35 ka and 130 ka. If the temperature factor is taken into account, the Cap des Biches ratios would be increased by 20 to 40% and the time range would be changed to older ages, comprised roughly between 50 ka and > 185 ka, over the age of epimerization equilibrium. Therefore, the absolute age of Patella and Balanus at Cap des Biches can be estimated as being of the order of the last interglacial age in its widest sense (stage 5), approximately 85,000-130,000 year as none of the ratios from the Patella and Balanus shells exposed under tropical temperature has reached the epimerization equilibrium.

U/Th Data and Chronological Hypothesis Three Ostrea fragment samples were tentatively dated by

the U/Th disequilibrium method. Two of them (CB 1 and CB2) were taken at the base of the Quaternary sediments section at Cap des Biches. The third (N'Diael) was collected

on the east side of the Senegal delta, at the same altitude, close to present-day sea level. Analytical results are reported in Table 5.

The three apparent ages calculated are different and, except for the third one, are considerably older than expected. Despite good evidence of geochemical system closure from mineralogical and stable isotope data, we have to question the two terms of the U/Th system.

All thorium spectra show a weak 232Th activity. This indication of unsupported 23°Th is without significant consequence to the calculated apparent age, at least for two samples. A classical value, equal to one, for the initial 23°Wh/232Th activity ratio would give apparent ages of > 227 ka, 175 ka and 88 ka, respectively, to samples CB 1, CB2 and N'Diael, instead of > 240 ka, 178 ka and 103 ka for uncorrected values, that represents a negligible difference, lower than 16 (26 for N'Diael), between the two calculations.

The question of uranium content stability is more difficult to solve, and is critical for each individual sample of our very small population. Large differences between 23sU content (0.12, 0.33 and 0.73 ppm) for these three samples could indicate, but do not demonstrate, postdepositional addition or leaching of this radioactive element. In fact, a different U content may exist at the origin, reflecting local variations in U in a coastal environment or specific variations of uranium fixation on different organo-mineral structures of various shells (Causse et al., 1993). It is noticeable that unrecrystallized corals also show variable U content, from 2 to 4 ppm, as reported in a recent compilation by Burnett and Veeh (1992). In agreement with the U-conservation hypothesis, we note the very good homogeneity of 234U/238U activity ratios. Because of preferential mobility of 234U during leaching process (Moreira-Nordemann, 1980; Ghaleb etal., 1990), it is hazardous to assume that secondary mobility of uranium in that environment would result in such close values, identical in value taking into account analytical uncertainty (_+ 16). This value is classical for marine molluscs from beach deposits (Hillaire-Marcel et al., 1986; Cause et al., 1993) and does not suggest any post- depositional alteration of U content or isotopic composition. If U leaching was responsible for the measured variation of U content from 0.73 to 0.12 ppm, one could expect measurable variation for 234U/238U ratios, even by alpha spectrometry. Likewise, the two samples from Cap des Biches reveal practically the same 228Th excess, corresponding to a recent (a few decades) fixation of Z28Ra, that could be related to a recent U addition to the calcitic

TABLE 5. Isotopic values and apparent U/Th ages; errors related to counting statistics are presented with a confidence range of one standard deviation;* this value does not take into account the presence of 232Th which is not negligible in this case. With a 23°Th/232Th activity ratio of 1 when the uranium was

fixed, the corrected age would be 89 ka (+11/-10)

Site and 238U 234Uf238U 23°Th]232Th 230Th/234U

number Lab sample Age field ppm Activity ratios ka

Cap des Biches CB 1 L-0329 0.124 ± 0.004 1.244 ± 0.045 15.0 ± 1.6 1.097 ± 0.040 > 240 CB 2 L-0330 0.726 ± 0.022 1.244 ± 0.038 38.1 ± 5.6 0.841 ± 0.030 178 + 19/-16 Dial N-0268 0330 ± 0.010 1.240 ± 0.043 4.9 ± 0.3 0.630 ± 0.030 103" + 11/-10

58 B. Diouf et al.

shells (giving too young apparent ages). Moreover, if secondary variations of U were responsible for variations in apparent ages, a larger quantity of uranium would be associated with a younger age, This is not the case, and no correlation can be found between apparent ages and U content.

However, the best argument for closure of the U/Th system, i.e. consistency of results for a lot of samples, cannot be seriously invoked for the limited number of samples available for our study. Then, if the age of isotopic stage 5, and probably the substage 5a, is a reasonable hypothesis for the N'Diael oyster sample, older ages (isotopic stage 7?) cannot be rejected for the other two samples.

DISCUSSION AND CONCLUSIONS

in the resulting alteration. The very negative values of ~80 in the barnacle extragranular and intragranular sparitic cements mainly reflect the dilution of interstitial waters by the meteoric flows, while the relatively low values of ~13C show the limited contribution of biogenic CO 2 during recrystallization. These conditions prevailed from the very first stages of cementation.

Diagenesis, at the scale of the outcrops, follows a gradient from the base to the top. The carbon isotopic ratios show the dominant marine influence at the base, the underlying sandstones have more negative values, but are still very different from those of the pedogenetic calcareous nodules of the neighbouring Holocene colluvia: the secondary cementations have primarily formed from the dissolved aragonitic carbonates.

From a Petrographic Point of View Almost no aragonitic bioclasts have been preserved in the

Pleistocene, and their recrystallization as calcite is almost always total. The more stable calcitic tests such as Ostrea, Balanus and bryozoans in particular, have retained their original mineralogy, but have suffered a more or less elaborate destructuration of their shell and have undergone a secondary cementation, which is systematic in the case of barnacles, bryozoans and foraminifers and localized in the case of oysters and calcareous algae. Only a few Ostrea tests are free from any recrystallization. The cements result from a generalized sparitic recrystallization, and only a few vestiges remain of earlier micritic or palissadic cementation. The pores which were spared by this cementation are occupied by vadose-silt with calcite microsparite of continental origin (Dunham, 1962).

From a Geochemical Point of View The geochemistry of the trace elements in the sandstones

illustrates the stages of a fairly long diagenetic history first in more or less confined brackish interstitial waters under the influence of evaporation and sometimes, under the control of aggressive continental fresh water. Thus, the calcitic tests, the sole survivors, are depleted in trace elements in comparison both with the horizons preserved at the base of the outcrops and with the present-day and Holocene reference tests analysed (Diouf, 1989) or known in the literature (Milliman, 1974). This trend of trace element deficits indicates a diagenesis progressing in an open geochemical system.

The isotopic information at Cap des Biches underlines the major role of briny water in this diagenesis and of fresh water

Applications to Radiometric Measurements Senegalese Pleistocene sandstones have so far been dated

on total rock using radiocarbon. These measurements obviously have no meaning, since the cement dating usually gives an order of magnitude of the most recent lithogenetic stage rather than of the initial accumulation; also, bioclast dating can give different answers depending on the stability of the mineral and its degree of recrystallization. Even in the case of the calcitic grypheas at Bargny-Siendou (Demoulin and Masse, 1967), the destruction of the tests and the recrystallization which can affect them cast doubt on the age of 32,500 BP attributed to them. The same goes, afortiori, for the age of 31,400 BP obtained from a mixture of tests and cements for beach sandstones at Bargny (Elouard, 1967): several generations of cements of different ages and origins have been recognized and the significance of the bioclast intragranular calcitizations is not known. Among the different ages already obtained (Table 6), it is perhaps only the 'infinite' ages that indicate non-recrystallized material.

New U/Th datings of Pleistocene oysters, collected at an altitude close to the present sea level, assumed to belong to high sea level time, corresponding to isotope stage 5, yield two, among three, apparent ages considerably older than previously estimated. However, it is not possible to ignore these chronological data. The third one could be related to isotope stage 5a or 5c. Unfortunately, so poor a collection of available samples do not allow one to establish more settled conclusions.

Palaeogeographic and Structural Application The first three U/Th datings of the Senegal Pleistocene

marine shoreline all concern oyster tests collected between 0

TABLE 6. Radiometric ages of marine deposits from the Senegalese coast from previous studies: Demoulin and Masse (1967) and Elouard (1969)

Dated deposits Heights related to the OIGN Localizations Ages ~4C BP

Ostrea -10.2 m Mouit 16.000 Mixing of shells and cements from a beach-rock 0 m Bargny 31.400 Cardium - 13 m Mouit~Gandiol 31.300 Gryphea gasar 0/+2 m Bargny-Siendou 32.500 Partly dissolved Arca -16 m Mouit~Gandiol 34.300 Beach-rock -12 m Diama I > 33.700

Study of Calcareous Sandstone in Senegal 59

and +1 m. The ages (103 ka, 178 ka and > 240 ka) are too different and too few in number to indicate the exact age of these coastal sandstones. However, the 103 ka age with the least unfavourable geochemical parameters indicates an interglacial Eemian sea level sensu lato; isotopic stage 5e would be the most probable (125 ka) as the previous 5a and 5c were lower (Kaufman, 1986). Ages near 200 ka are also well known for a shoreline altitude close to the present one (Gaven et al., 1979; Gascoyne and Harmon, 1992).

These datings would indicate the absence of significant positive epeirogenic movements on this Senegalese coast; this conclusion, at the Pleistocene scale, agrees with that of a relatively rigid Senegalese margin at the Holocene scale (Faure et al., 1980). Consequently, the 'Inchirian' marine deposits dated at around 35,000 BP cannot have been raised from a shore at -30/-45 m, elevation suggested by various authors (Bloom et al., 1974; Shackleton and Opdike, 1973; Butzer, 1975; Chappell, 1982; Chappell and Shackleton, 1986) to an altitude close to or higher than the present sea level.

The 'real' Inchirian in this zone and in the regions to the South must be deeper, in the buried horizons of the Senegal delta (Diama) or in the submarine zone, between - 10 and-30 m below the present sea level (Dakar-marine). The sandstones drilled at Mouit and Diama in the delta of the Senegal River have 14C ages of approximately 31,000 and > 34,000 years; they locally occur offshore as beds at -20 to -30 m where the wave action prevents their burial. The sandstones situated between -17 and -25 m on the Mauritanian platform (Einsele et al., 1977) or those between -16.5 and -21 m in the Mboro region (Pinson-Mouillot, 1980) must also be linked to this episode. At Thiaroye, the sediments lying over the Dakar-marine sandstones contain shelly material dated at 4280 _+ 160 BP (tests of Pitaria

tumens in life position, at -12 m IGN; Elouard et al., 1977); they postdate the underlying Inchirian sandstones. In the intertropical region of the Ivory Coast (Fredoux, 1977), Nigeria (Sowunmi, 1981) and Congo (Malounguila et al.,

1986), this medio-Wtirmian shore is also observed between -25 and -40 m and has been dated several times by the ~4C method applied to mangrove peat.

In contrast, in the neighbouring Mauritanian basin, positive epeirogenic movements may have affected the deposits of the Pleistocene high sea levels (Inchirian or Ai'oujian). Thus, the 'Inchirian' high level is observed at an altitude of less than 2 m near the present shoreline, but it reaches +7 m more than 7 km inland. This change of elevation may imply a rocking movement linked to continental flexure (Faure and Elouard, 1967), but which, even here, would be insufficient to uplift shoreline features which formed 30 m below mean sea level 35,000 BP.

If it is accepted that the deposits at Cap des Biches were deposited during isotopic stage 5 sensu lato, it is possible to envisage a marine shoreline environment at that time defined by waters both warmer and more diluted than at present, i.e. a more northern extension of the Guinean waters.

Finally, the importance of postsedimentary meteoric actions on these Pleistocene carbonate sandstones, and in particular the progression of dissolutions, is sufficient to

explain the disappearance of such outcrops from Toubab Diallao and their generalized absence on the intertropical shore of the Gulf of Guinea (s.1.) down to Angola.

ACKNOWLEDGEMENTS

The isotopic measurements on oxygen and carbon were carded out by Gilles St-Jean at the Department of Geology of the University of Ottawa (Canada). The epimerization rates of shells were measured at the Laboratory of Aminochronology by Louise Cournoyer and U/Th datings were achieved at the Laboratory of Geology of Quaternary (CNRS, Marseille). We also thank the reviewers, P. Kindler and M. Bernat, who contributed substantially to the final manuscript.

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