The Blackpool shelf intertidal sands are richer in. Cu and Pb trace elements than the subtidal sands, while the latter are relatively enriched in P and. (Cr, Co , Zr ...
PROCEEDINGS OF THE EGYPTIAN ACADEMY OF SCIENCES (VOL. 40, March 1990) ISSN: 1110-0532
Proc. Egypt. Acad, Sci. 40 (X990): 175 - 194
GEOCHEMICAL CHARACTERISTICS OF RECENT SHELF CARBONATE SANDS AND SABKHAS FROM THE CENTRAL RED SEA ARID ENVIRONMENT. BY Ezzat Khedr Aswan Faculty of Science, Aswan, Egypt. Communicated by: Prof. M. EZZ EL-DIN HELMY (Received February 1990)
ABSTRACT P e t r o l o g i c c o n s t i t u e n t s of m o d e r n shelf carbonate sediments and the general t r e n d of c h e m i c a l - e l e m e n t enrichments along the Red Sea coastal a r e a s o u r t h e r n E g y p t are presented in the i n t e r t i d a l and t h e s u b t i d a l zones, a n d d i s c r i m i n a t i v e parameters are proposed. Chemical element interrelationships are studied a n d the b e h a v i o u r of 24 m a j o r and t r a c e e l e m e n t s in f o u r m o d e r n e n v i r o n m e n t s are clustered and interpreted. Intertidal s a b k h a s a r e s i t e s of w e a t h e r i n g of silicate a n d c a r b o n a t e s e d i m e n t s d e r i v e d by means of rain-outbreaks and evaporative sweeping waves. Disentigration of aluminosilicates d u r i n g a n e v a p o r a t i v e p e r i o d followed by a s e a w a r d m i g r a t i o n of c o l l o i d a l silica suggests an answer to the question regarding evolution of d u r i c r u s t s b e n e a t h the s a b k h a s u r f a c e s . Despite the e n r i c h m e n t of m a g n e s i u m i n , s u b t i d a l c a l c a r e o u s s a n d s ( m a r i n e in origin), double carbonate minerals (dolomite) a r e l i k e l y to f o r m in
18 (
10/90)
intertidal sabkhas due to the i n t e r m i t t e n t s u p p l y of r a i n w a t e r to s a b k h a sites (mixing zone model) a n d t h e e x i s t e n c e of m e t a s t a b l e a l u m i n o silicates.
INTRODUCTION Bordering shallow lagoonal water bodies , along a distance of 400 Km between Halaib in Sudan and Marsa-Alam in Egypt (Fig.l), there are two contrasting coastal sedimentary facies, calcareous muddy beach sediments bordering sabkha flats and salt tolerant vegetation; and shelf calcareous sands with saline pan crusts. The sabkha sites occur within the main discharge circus of large drainage systems, (Khedr, 1990), whilst the shelf calcareous sands occur far from the reach of any drainage discharges (Fig.2). In order to specify the behaviour of chemical elements during coastal aggradation of the Red Sea with the intention of erecting some discriminative chemical parameters for the two different types of calcareous sediments, five sabkha sites and extra six calcareous sand sites were carefully chosen (Fig. 1). A working group
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was employed and a total of 22 samples were collected simultaneously along the coastal area from the subtidal and the intcrtidal zones.
SEDIMENTOLOGICAL EVOLUTION OF THE STUDY SHELF: From the coastal plain onto the Miocene based hinterland, the study area gives a lateral sequence of sedimentary facies (67 m thick) that unravel one major cycle of sedimentation (Khedr, and Felesteen 1990). This cycle conforms to the global "Pliocene-Holocene" cycle and indicates a progressive regression of the sea during Late Pliocene (ancient sabkha-reef horizon) and Late Pleistocene (Formation of duricrusts and beach rocks) interrupted by transgressive pulse during Early Pleistocene (coral reef buffls). and succeeded by a steady rise in Holocene sea-level. Evolution of the present shallow lagoons has been commenced by under base-level erosion of stream flowage from the mountain front during Late Pleistocene regression. In the Holocene dry period (Khedr, 1987), a subsequent build-up of fringing reef has brought about the present lagoonal configuration (Fig. 2). The present shore zones are made up mainly of one of the following facies: (1) A carbonate mud facies of back reef marginal tidal sediments, intercalated with distally fluvial carbonate mud (Khedr, 1990). Seasonal repetition of spring tide and inundation of the intertidal and lower supratidal sediments and the consequent slow drawback of evaporative poolwater, crossing the subsurface reefs, increase the amount of evaporite minerals on account of the original carbonate, with continuous displacement of the clastic portion. This association of facies are therefore particulary susceptible to diagenetic changes and give rise to sabkha aggradation. (2) Calcareous sand facies of tide dominated shelves, covered in some parts with a thin saline crust 1 cm thick. No real sabkha sequence is strictly comparable with that found in the carbonate mud facies Nol.
PREVIOUS WORK Because of the extreme complexity of the modern
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interacting processes and environments during the Holocene (Tucker, 1985), no previous geochemical work has been done on the modern shelf calcareous sands and sabkhas of the Red Sea. However there are relatively few published analyses of modern evaporite materials in other localities (Moore, 1960; Dean and Tung, 1974; Gavish, 1974). Variation in concentration of K, Na, Ca. and Mg with density of water in different localities was the goal of these studies. However, Shearman, 1966 proposed a hypothetical schematic diagram for trend of mineral and brine diagenesis in carbonate and non-carbonate sabkhas. Moreover, Dean (1978) summarized some of the published and unpublished chemical analyses of the more common evaporite minerals. In an attempt to solve the geologic problems Dean (1978) discussed several examples in which several trace and minor elements can be used. An account of invertebrate skeletal composition is given by Horowitz and Potter (1971) and the physical chemistry of sea water and carbonate reaction is given by Berner (1971), Broecker (1974) , Bathurst (1975), and Krauskopf (1979). On a petrologic basis Lowenstein and Hardie (1985) gave some characteristics of the Holocene ephemeral saline pan deposits. In their recent work Humphrey (1987 & 1988), Burns and Rossinsky (1989) presented the geochemical signatures of Pleistocene dolomites of Brabados, West Indies. Consequently a sedimentological model has recently been erected explaining the formation of modem coastal sabkhas at the Red Sea marginal area (Khedr 1990). The model proposes that sabkhaization is intimately interwoven with back reef marginal tidal sediments intercalated with calcareous mud of distally fluvial type. Moreover Khedr (ibid) has reported on petrologic constituents of five logs, each of 60cm depth, along the subtidal-intertidal^ zone of ABU-GHUSSON (Fl(i.l). He (IBID) concluded that forams, algae, and gypsum contents are higher in the lower intertidal zone than in those of the subtidal zone. In contrast, values of microgastropods and carbonate grains are higher in the subtidal zone. Salinity of the near shore lagoonal water was also reported along
Geochemical Characteristics
marginal area (400 Km) and indicated a slight increase southward.
BASIS OF THE ANALYSES The gravel sized grains were picked out from the samples and the remainder of every sample was subjected to a simple petrologic study (Table 1) and detail XRF analyses for all major elements and some trace elements. SO 4, CO 2, and H2O plus organics were also determined. A total of 25 determination of chemical composition for each sample was caried out. A sum of 550 values was obtained and listed in four tables (No 2 to 5 ) representing four different marginal marine environments of deposition, namely, intertidal calcareous sands (I. cal), subtidal calcareous sands (S. cal), intertidal sabkhas (I. sbk) and subtidal sabkhas (S.sbk). Ten additional shelf sand samples from five sites along the coastal area of Blackpool (England) were also included in the present work and the mean values of their chemical analyses were included in Tables No 2 and 3. In order to assess the enrichment trend of elements perpendicular to the beach line, normalized mean values of elemental composition were calculated individually for calcareous sands and sabkhas and schematically plotted in Fig (3). Ratios of Ca/Mg , Na/Mg, Na/K, Sr/Ca, K/Rb, and SO/CO3 were calculated and their logarithmic values were mutually plotted against each other Fig (4). Clustering analyses were carried' out for correlation coefficient values of major and trace elements of the four environments of deposition with the intertion of throwing some lights on the element-interrelationship (figs 5 & 6).
RESULTS AND DISCUSSION (1) PETROLOGY Following Thorp's (1936) tabulation of the percentage distribution of constituents of modern lime-bearing sediments, Table (1) lists the average of the counted constituents of the Red Sea samples.
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The Red Sea calcareous sand samples show relatively highter values of foraminiferas, mineral grains (mostly quartz), oolites, and evaporites than those of the lime-bearing sediments of Florida and the Bahamas. On the other hand, the Red Sea calcareous mud (sabkha) samples are richer in coral fragments, silt, caliche (calcrete), and evaporites than the Florida and the Bahamas samples.
(2) ENRICHMENT OF ELEMENTALCOMPOSITION PERPENDICULAR TO THE BEACH LINE. In order to achieve this purpose weighted mean values of every element in the 22 samples were calculated. The mean values of the four marginal Red Sea zones together with the marginal Atlantic Ocean zones (Blackpool) were normalize relative to the weighted mean, listed in Table (6) and portrayed in Fig (3). Comparison between the normalized mean values, led to the following deductions can be delineated. - The intertidal calcareous sands are richer in Mg, Ca, P, CO,, S0 4 , and (Rb, Pb trace elements) than the subtidal calcareous sands. The latter are therefore enriched in Si, Ti, Fe, Mn, K, and (Cr, Co, Ni, Zn, Y, Zr, Nb trace elements). Fig. (3B). - The intertidal sabkha sediments are enriched in Mn, Ca, P, S0 3 , and (Cr, Co, Cu, Zn, Rb, Nb, Pb trace elements) relative to the subtidal sabkhas. However, the latter are relatively enriched in Si, Ti, Fe, K, C 0 3 , and (Ni, Sr trace elements). - The Blackpool shelf intertidal sands are richer in Cu and Pb trace elements than the subtidal sands, while the latter are relatively enriched in P and (Cr, Co , Zr trace elements). However, all other normalized values of elements in the intertidal and subitdal zones are of the same order of magnitude. Collectively, the normalized values of elemental composition in Blackpool shelf sands are under the weighted mean values, excepting for Si and Rb in the two shelf zones, Pb in the intertidal zone and Zr in the subtidal zone. To address the question as to which elements are more concentrated in the four types of the
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marginal Red Sea sediments, the highest normalized value (Table 6) of Mg, C 0 2 ; and (Rb, Ba trace elements) are noticed in the intertidal calcareous sands. The subtidal calcareous sands are however characterized by the highest values of Ti, Al, Fe, K, and the trace elements Cr, Co, Ni, Zn, Y, Zr and Nb . On the other hand, the intertidal sabkha sediments exhibit die highest normalized mean value of Mn, Na, P, S 0 3 and the trace elements Pb , whilst that of Sr is noticed in the
subtidal sabkhas.
3- DISCRIMINATIVE PARAMETERS In order to erect a geochemical model that can be used for discrimination between the four types of Recent Sediments at issue and for the purpose of further correlation, ternary diagrams were plotted Fig (4A) and different elemental ratios were calculated and their logarithmic values were plotted againest each other and are provided in
t
• •
Calcareous sand. Calcareous mud (sabkhas;. Fig. 1 : Location map.
Proc. Egypt. Acad. Sci. 40 (1990)
Geochemical Characteristics
Fig. (4B). The ternary diagram I in Fig (4A) indicates that Mg is more intensively incorporated in calcareous sands than in sabkhas with two indicator regions for each sediment. The CCS contents show more concentrations in the intertidal calcareous sands which may safely be attributed to formation of double carbonate minerals (dolomite). Diagram II in Fig (4A) clearly defines two regions of intertidal sabkhas and intertidal carbonate sands. Tables (2 to 5 ) show median values and ranges of element-ratios. The Ca/Mg rations of the four sediments are of the same order excepting the subtidal calcareous sands where the value (6.70) is about half the more magnitude. However, the Ca
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content seems to be more than ten-fold the Mg content in the intertidal and subtidal sabkha zones. Sr/Ca Na/Mg and Na/K ratios of the sabkha samples are generally higher than those of the calcareous sand samples with two-fold order of magnitude. The intertidal sabkha samples are characterized by the highest median value of SO3/CO2 ratios (1.34), followed in decreasing order, by the subtidal calcareous sand (0.12). The upper limit of the calculated range of Ca/Mg, Na/Mg; Na/K. and S 0 3 / C 0 2 ratios Ts present in intertidal sabkhas. That of Sr/Ca ratios is present in subtidal calcareous sand, whilst that of K/Rb is
MAIN
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Supratidal i o r o Intartldal. tana CALCAREOUS
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° C * » • '"III c„ Fig. 2 : (A) Schematic distribution of facies and morphological elements of the Red Sea coastal area and the hinterland. No fixed scale applies (B&C) , General cross-sections through the calcareous sand and the calcareous mud beaches (modified after Khedr, 1990).
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SHELF CALCAREOUS SAND (RED SEA) INXERTIDAL ZONE
SUBTIDAL ZONE
Ca, Mg, P,
co 2 , so 3
SHELF CALCAREOUS MUD (SABKHA, RED SEA) INXERTIDAL ZONE
SUBTIDAL ZONE
C SHELF QUARTZ-SAND (BLACKPOOL, ENGLAND) INTERTIDAL ZONE
SUBTIDAL ZONE
Fig. 3 : Enrichment trend of chemical elements vertical to the beach line in, (A) , shelf calcareous sands; (B) sabkha beaches; and (C), the Blackpool shelf-quartz-sands. Note: elements between brackets are the traces.
Proc. Egypt. Acad. Sci. 40 (1990)
Geochemical Characteristics
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present in subtidal sabkhas. On the other hand, the lower limit of Ca/Mg and Na/Mg, Na/K, Ca/Sr, and SO3/CO2 ratios are present in intertidal calcareous sands, whilst t h a t o f K / R b ratio is present in intertidal sabkha sample. Plotting the logarithmic values of each of the ratios (Na/K) agamst (SOyCO,) and (Ca/Mg) against (S(VC0 2 ), Fig. (4B), can be used for further correlation purposes between shelf SO3 calcareous samples from other localities.
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