ABSTRACT

The Wajid Sandstone (Ordovician-Permian) as exposed along the road-cut sections of the Abha and Khamis Mushayt areas in southwestern Saudi Arabia, is a mediun to coarse-grained, mineralogically mature quartz arenite with an average quartz content of over 95%. Monocrystalline quartz is the dominant framework grain followed by polycrystalline quartz, feldspar and micas. The non-opaque heavy mineral assemblage of the sandstone is dominated by zircon, tourmaline and rutile (ZTR). Additional heavy minerals, constituting a very minor fraction of the heavies, include epidote, hornblende, and kyanite. Statistical analysis showed significant correlations between zircon, tourmaline, rutile, epidote and hornblende. Principal component R-mode varimax factor analysis of the heavy mineral distribution data shows two strong associations: (1) tourmaline, zircon, rutile, and (2) epidote and hornblende suggesting several likely provenances including igneous, recycled sedimentary and metamorphic rocks. However, an abundance of the ZTR minerals favors a recycled sedimentary source over other possibilities.

Mineralogical maturity coupled with characteristic heavy mineral associations, consistent north-directed paleoflow evidence, and the tectonic evolutionary history of the region indicate a provenance south of the study area. The most likely provenances of the lower part (Dibsiyah and Khusayyan members) of the Wajid Sandstone are the Neoproterozoic Afif, Abas, Al-Bayda, Al-Mahfid, and Al-Mukalla terranes, and older recycled sediments of the infra-Cambrian Ghabar Group in Yemen to the south. Because Neoproterozic (650-542 Ma) rocks are not widespread in Somalia, Eritrea and Ethiopia, a significant source further to the south is not likely. The dominance of the ultrastable minerals zircon, tourmaline and rutile and apparent absence of metastable, labile minerals in the heavy mineral suite preclude the exposed arc-derived oceanic terrains of the Arabian Shield in the west and north as a significant contributor of the sandstone. An abundance of finer-grained siliciclastic sequences of the same age in the north, is consistent with a northerly transport direction and the existence of a deeper basin (Tabuk Basin?) to the north. The tectonic and depositional model presented in this paper differs from the existing model that envisages sediment transportation and gradual basin filling from west to east during the Paleozoic.

INTRODUCTION

The nature and distribution of heavy minerals are widely used in sedimentological studies of sandstone provenance (Hubert, 1971; Schafer and Dorr, 1997; Uddin and Lundberg, 1998). Heavy minerals are also useful in interpreting source rock geology including mineralogical composition and transport processes (Pettijohn et al., 1987; Da Silva and Vital, 2000). For example, based on the heavy mineral distribution of the upper Tertiary sediments of the Bengal Basin, Uddin and Lundberg (1998) interpreted the unroofing history of the eastern Himalayas and Indo-Burma ranges. Rostovtseva and Shapiro (1998) used heavy mineral suites to determine the provenance of the Paleocene-Eocene clastic rocks of the Komandorsky Islands. On the basis of heavy mineral suites and zircon topology, Schafer and Dorr (1997) identified the provenance of the Saxothuringian Flysch.

The Wajid Sandstone is a major aquifer in Saudi Arabia with a proven groundwater reserve of over 30,000 million cubic meters (Water Atlas of Saudi Arabia, 1984). Several workers, including Evans et al. (1991), indicated that the sandstone extends to the Rub’ Al-Khali Basin, a frontier hydrocarbon exploration region. However, despite its proven and potential economic importance, no detailed sedimentological study (more specifically, its provenance) has so far been done.

Avigad et al. (2003) investigated the source of the Cambrian sandstones, which are thought to be time equivalent at least with parts of the Wajid Sandstone, in the Elat area of the Levant. Based on the Sensitive High Resolution Ion Microprobe (SHRIMP) zircon age data, they concluded that Neoproterozoic (650-542 Ma) rocks of the Saharan metacraton and southern Afif terrane in Saudi Arabia are possible sources of these zircons. The Cambrian section of Elat and Jordan (Weissbrod and Nachmias, 1986; Amireh, 1991) is rich in the stable heavy mineral assemblage of zircon-rutile-tourmaline and suggests long-distance transport (over 1,000 km, Garfunkel, 1999). SHRIMP data (Avigad et al., 2003) also indicate the Saharan metacraton in the south (Abdelsalam et al., 2002) as a secondary source for the Paleozoic sandstone in Elat and the Arabian Peninsula.

Based on detailed heavy mineral data of the lower part of the Wajid exposure in the Abha-Khamis Mushayt area of the Asir Province, southwestern Saudi Arabia, this study investigates the provenance and regional tectonic settings of this Paleozoic formation.

GEOLOGICAL SETTING

The Paleozoic Wajid Sandstone (type section at Jabal Al-Wajid; 19°06′N; 44°27′E) is exposed for about 300 kilometers stretching from south of Wadi Ad Dawasir to Wadi Ad Habaunah (17°57′N; 44°58′E) along the southeastern edge of the Arabian Shield. The Wajid is up to 900 m thick, and composed of medium to coarse-grained, cross-bedded quartz arenite of possible fluvial to shallow-marine origin (Dabbagh and Rogers, 1983; Kellog et al., 1986; Stump and van der Eem, 1995a, 1995b; and Hussain et al., 1997).

The outcrops studied in this paper are located on the geologic map of the Abha Quadrangle, Sheet 18F, Kingdom of Saudi Arabia (Greenwood, 1985). Here the formation is exposed along the roadside and creek sections of Ahad Rufaidah, Al Soudah, and Al Habalah National Parks in the Abha and Khamis Mushayt areas (17°50′N to 18°15′N; 42°E and 42°53′E) (Figure 1). In the Abha-Khamis Mushayt area, the lower part of the Wajid occurs as erosional outliers within a basement terrain consisting of the Neoproterozoic Baish, Bahah, Jeddah, and Halaban groups, and upper Proterozoic plutonic rocks including gabbro and granite (Brown et al., 1989; Johnson, 2000). Other rocks exposed in the Abha Quadrangle include Tertiary laterite and columnar basalts, sparse Quaternary basalts and alluvium.

The Wajid in the study area, comprises a thick (up to 300 m) sequence of cross-bedded, medium- to coarse-grained, pebbly sandstone (Hussain et al., 1997; Babalola, 1999) overlying the local Precambrian basement. The paleocurrent distribution data suggest a northerly flow and sediment transport direction (Dabbagh and Rogers, 1983; Hussain et al., 2000). Based on overall lithology, more specifically the color and the distribution of ferruginous beds, the lower part of the Wajid is divided into the lower Red Unit and the upper Gray Unit (Figure 2). Babalola (1999) tentatively correlated the Red Unit with the Dibsiyah Member of the Wajid at its type section. Evans et al. (1991) proposed dropping the term Wajid Sandstone in favor of existing Paleozoic nomenclature for the rock formations exposed in the northwest.

MINERAL STUDY

Methodology

This study involved detailed field investigations of a total of 14 outcrops, 11 of which are shown in Figure 3. The three cross-sections from Al-Soudah are not shown in the figure because the locality is about 40 km NW from the other two studied sites, Ahad Rufaidah (OAR) and Al-Habalah (OAH). In addition, the high elevation of the basement and the Wajid Sandstone contact (lowest of 2,695 m) at this locality makes it difficult to include all the outcrops in one diagram. The complete sections are included Babalola (1999). The field investigations included stratigraphic log profiling, measuring paleocurrent orientations implied by cross-bedding and the imbrication of lithoclasts, and the collection of rock samples for laboratory analyses.

A total of 44 samples were processed for heavy mineral analysis. The samples were disaggregated in water and wet-sieved with a 63µ-mesh. The residues were air-dried and sieved to collect size fractions ranging from 2-3ϕ (250-125µ) for heavy mineral separation using tribromoethane (Bromoform ®; specific gravity 2.89). Because size fractions ranging from 250 to 125µ best represent the heavy mineral content of a sandstone sample (Carver, 1971; Uddin and Lundberg, 1998), these fractions were collected for thin-section petrography. The separated heavy minerals were weighed and calculated in terms of weight percentage of each sample. The collected heavies (both opaque and non-opaque) were later mounted on glass slides for microscopic study. An average of 200 translucent detrital grains were identified from each slide following the line counting, and percentage of different heavy minerals were determined following the method outlined by Mange and Maurer (1992).

Framework Mineralogy

Thin-section petrography shows that the lower part of the Wajid Sandstone at the study area is dominantly a pebbly, coarse- to medium-grained, occasionally silty to fine-grained, poor to moderately well-sorted, quartz arenite with average quartz content in excess of 95% by volume (Figure 4). The roundness of the quartz grains varies from angular to subrounded. While most of the quartz grains are monocrystalline, subordinate amounts of polycrystalline quartz grains were also recognized. The identified monocrystalline grains show straight to undulose extinction. Most of the grains are unstrained. The polycrystalline grains are equant to subequant in shape, and often occur as composite grains that are sutured together. Inclusions of zircon, tourmaline, rutile, feldspar, hornblende, biotite, muscovite, and fluid are also common in the quartz grains.

A few grains of highly degraded microcline and plagioclase feldspar were recognized. Micas, represented mainly by muscovite, range from 0 to 5% of the sediments. The percentage of the accessory minerals (zircon, tourmaline, rutile, epidote, and hornblende) is low (up to 1%). Calcite, often replaced by iron oxides, is the dominant cement type. The amount of matrix (mostly kaolinite and chert particles) is low, ranging from 2 to 6% and averaging between 2 and 3%.

Heavy Minerals Occurrence and Quantification

The heavy mineral content in the Wajid is low, ranging from 0.01 to 4.3% in the Red Unit, and 0.2 to 0.7% in the Gray Unit (HAP Series in Table 1). The distribution of the opaque minerals (mainly authigenic hematite and goethite) ranges from 13 to 99% of the Red Unit and 20.5 to 76.2% in the Gray Unit (Table 1) in the heavy mineral fractions. Zircon, tourmaline and rutile dominate the non-opaque fraction of the heavy mineral suite. Other heavy minerals identified in the non-opaque fraction include epidote, hornblende and kyanite.

The zircon-tourmaline-rutile (ZTR) index, which indicates mineralogical maturity of sediments (Hubert, 1962; Stattegger, 1986; Moral-Cardona et al., 1996), accounts for 74.2 to 100% and 78.1 to 99.4% of the non-opaque suites in the Red and Gray units, respectively (Table 1). A high ZTR index usually characterizes sediments of recycled origin. However, selective dissolution and leaching of metastable minerals may also lead to high ZTR values.

Heavy Minerals Description

This section describes the microscopic characteristics of the common non-opaque heavy minerals identified in the Wajid Sandstone.

Tourmaline

The tourmaline grains observed are generally subhedral, fragmented, lath-shaped with pyramidal to rounded terminations. A few grains are partially rounded to well-rounded and show overgrowth (Plate 1: a and f). The grains are mainly reddish-brown to pinkish, with some pale yellow to bluish-green in color. They exhibit high birefringence, parallel extinction and strong pleochroism. External striations and inclusions including some rod-like, needle to lath-shaped crystals and fluid inclusions were observed in few grains (Plate 1: b, c and i).

Zircon

The zircon grains are colorless to pale yellow with characteristic adamantine luster, high birefringence, relief and parallel extinction. The observed grains are generally rounded to euhedral crystals with either rounded or pyramidal ends (Plate 2: a, c, d, e, f and g). Fluid inclusions and needle to lath-shaped crystals are common (Plates 2: a, c, g and h). Evidence of stress was also observed in some of the grains.

Rutile

Rutiles occur as prismatic, rounded to euhedral crystals (Plate 3: a, b and c). The grains are yellow to deep red-brown in color, and show characteristic oblique cross-striation patterns (Plate 3: a and b).

Hornblende

Hornblendes are represented by platy grains with hacksaw edges. These grains show characteristic oblique extinction, strong pleochroism, yellowish to greenish coloration and two sets of cleavages (Plate 3: d, e and f). Some hornblende grains identified show signs of corrosion and dissolution (Babalola, 1999).

Epidote

Epidotes are angular to subangular, pale yellow to pale green (Plate 3: g and h), showing one set of cleavages and parallel extinction. The characteristic weak pleochroism distinguishes epidote from tourmaline grains in the samples.

Kyanite

Kyanite is a rare mineral in the Wajid. It is colorless to light grey and platy to bladed in shape (Plate 3: i). It shows low birefringence and high relief.

FACTOR ANALYSIS

Multivariate statistical techniques including cluster, factor and discriminant analyses are widely used in various geological investigations including provenance studies (Imbrie and van Andel, 1964; Mezzadri and Saccani, 1989). The factor analysis technique statistically cross-matches mineral proportions among all the samples. It produces a number of factors or mineral associations that refine the provenance or depositional provinces as initially determined from the individual minerals (Wong, 2002). A straightforward approach to factor analysis and its application in heavy mineral data is provided in Imbrie and van Andel (1964).

In the present study the R-mode principal componnent varimax rotated method was used to extract factors from the heavy mineral distribution data. The Q-mode factor analysis technique was used to complement the results derived by the R-mode method. Correlation matrices were used to determine the association of heavy minerals in different factors.

Correlation Matrix

Based on R-mode factor analysis, several heavy minerals in the Wajid Sandstone (including zircon, tourmaline, rutile, epidote and hornblende) can be correlated (Table 2). In the Red Unit, tourmaline correlates positively with most of the minerals except kyanite. The correlation coefficients (r2) range from 0.44 with rutile to 0.64 for zircon. The r2 values are 0.81 between rutile and zircon, and 0.75 between rutile and epidote. In the Gray Unit, zircon correlates positively with tourmaline and rutile with r2 values of 0.57 and 0.35, respectively. The correlations between rutile and tourmaline, and epidote and tourmaline are weak. However, a strong and significant (r2=0.84). correlation was observed between epidote and hornblende.

Correlations based on Q-mode factor analysis (Table 3) show high correlation coefficient among several samples from both the Red and Gray units. The r2 values are generally greater than 0.6, with average values ranging from 0.8 to 0.9.

R-Mode Factor Analysis and Interpretation

The principal component varimax R-mode factor analysis showed that two factors account for 75% of the total variance in the Red Unit (Table 4). In contrast, a total of three factors account for 84% of the total variance in the Gray Unit (Table 4). In the Red Unit, tourmaline, zircon, rutile, epidote and hornblende show strong association in Factor 1 with high positive factor loadings ranging from 0.75 to 0.90. The factor is characterized by strong positive loadings for tourmaline (0.73), zircon (0.91) and kyanite (0.74) in the Gray Unit. The association of tourmaline, zircon, rutile, epidote and hornblende, as determined by the factor analysis, indicates several different provenances including, igneous, recycled sedimentary, and metamorphic rocks. However, an abundance of the ZTR minerals in this suite should favor a recycled sedimentary source over other sources, unless the metastable minerals are lost from the sediments either by weathering or diagenesis. The strong positive loadings for tourmaline, zircon and kyanite as determined in the Gray Unit, may indicate either a metamorphic or recycled sedimentary source. However, considering the fact that kyanite is not a common mineral in the Wajid Sandstone, a high-grade metamorphic terrain is not a likely source.

Factor 2 is characterized by weak loadings for rutile, zircon and hornblende in the Red Unit. In contrast, epidote and hornblende are strongly loaded in the Gray Unit with factor loadings of 0.95 and 0.96, respectively. Briggs (1965) interpreted the association for epidote, hornblende and sphene as an indicator of a plagioclase amphibolite-rich source. Mezzadri and Saccani (1989) noted high factor loadings for epidote in sediments derived from a metasedimentary source terrain. Similarly, in one of the factor-analysis delineated provinces in the Gulf of the Farallones, Wong (2001a) used an abundance of hornblende as an indicator of a granitic source.

Factor 3, which accounts for 20.6% total variance and 1.24 eigenvalue, comprises exclusively the Gray Unit samples. Ultrastable heavy minerals, zircon and rutile with factor loadings of 0.98 and 0.38, respectively, characterize this factor.

Q-Mode Factor Analysis and Interpretation

Q-mode factor analysis shows (Table 5) that a total of three factors account for 95% of the total variance in the Red Unit while two factors account for 91.52% of the total variance in the Gray Unit. The factors were identified based on factor loadings of 0.6 and higher values.

Factor 1 is dominated by the high ZTR samples from both Red and Grey Units from Ahad Rufaidah (AR), Al Habalah (HA), Al Soudah areas. An abundance of ZTR minerals should indicate a recycled sedimentary source or mineralogical maturity. Factor 2 includes samples from the Red and Gray units from all three localities (Ahad Rufaidah, Al Habalah and Al Soudah), and like Factor 1, shows an abundance of ultrastable ZTR minerals. However, compared to the distribution in the Red Unit of Factor 1, the distribution of tourmaline (61.3%) and epidote (9.02%) in Factor 2 is high indicating a greater influence of metamorphic source for this factor. Tourmaline (57%) and rutile (9.45%) content of the Gray Unit is also higher than its counterpart in Factor 1. The association of these minerals with zircon is consistent with the interpretation of a recycled sedimentray source. However, mineralogical maturity resulting from selective depletion of metastable and unstable minerals by weathering at source or diagenesis, may also explain high ZTR content of these sediments.

Factor 3 is characteristized by an association of zircon (38.75%), rutile (17.9%), epidote (11.6%) and hornblende (3.5%) and is largely represented by the Red Unit samples from Al Soudah area. The heavy mineral association of Factor 3 may derive from many different source terranes including igneous, recycled sedimentary, or metamorphic rock suite.

PROVENANCE DISCUSSION

The characteristics of different heavy minerals in the Wajid suggest a number of sources including both acid and basic igneous, low-grade metamorphic, and recycled sedimentary rocks (Table 6). The dominant types of zircons are pale yellow, euhedral grains showing abundant inclusions. Such zircons are common in felsic to intermediate granitoid, and recycled sedimentary rocks (Schafer and Dorr, 1997; Ehrmann and Polozek, 1999; von Eynatten and Gaupp, 1999). The presence of a few glossy crystals of zircon indicates a metamorphic source (Schafer and Dorr, 1997). The tourmaline grains are dominantly reddish-brown to pink, inclusion-rich, idiomorphic, and hypidiomorphic to fragmentary forms. These types of tourmaline are typical of felsic granitic to pegmatitic sources (von Eynatten and Gaupp, 1999). The occurrences of subrounded to rounded zircon and tourmaline grains in sediments may indicate either a sedimentary source or reworking of the Proterozoic basement metasediments (Uddin and Lundberg, 1998; Ehrmann and Polozek, 1999). With the exception of the strongly alkaline suites such as alkaline ultramafics, alkaline granitoids, and hydrothermal granitic pegmatites with restricted occurrences of Nb-rich rutiles (Tollo and Haggerty, 1987; Vlassopoulos et al., 1993; Preston et al., 2002), rutile is very rare in igneous rocks. It is, however, common in regionally metamorphosed pelites and metabasites (Frost and Lindsley, 1991; Force, 1980). The common occurrence of this mineral in the Wajid indicates a strongly alkaline igneous or metasediment source.

The association of zircon, tourmaline and rutile as observed in the Wajid indicates a felsic to intermediate provenance in a stable cratonic setting (Uddin and Lundberg, 1998). Zulfa (1985) and Burnett and Quirk (2001), however, suggested that the presence of ultrastable heavy minerals (zircon, tourmaline and rutile) should indicate a recyled sedimentary source. Preston et al. (2002) interpreted a heavy mineral suite consisting of zircon, tourmaline, apatite and rutile as a likely metasediment or granitic source. Similarly, Morton et al. (1992) ascribed a heavy mineral suite dominated by zircon, apatite and tourmaline to pre-existing sedimentary source rocks, and suggested that the presence of common, relatively unabraded zircon (like seen in the Wajid) may indicate an intermediate to felsic volcanic source.

The occurrence of epidotes and kyanite in the Wajid Sandstone suggests a metamorphic source. Epidotes, though also found in igneous rocks, occur mostly in rocks from the greenschist, epidote-amphibolite facies and high-pressure metamorphic phases (Deer et al., 1982; Ehrmann and Polozek, 1999; Asiedu et al., 2000). The green hornblende identified in this study is known to be an ubiquitous component of both metamorphic and igneous rocks (Ehrmann and Polozek, 1999), indicating a predominantly metamorphic or igneous source rock. The strong positive correlation (r2 = 0.84) between epidote and hornblende in the Gray Unit suggests derivation from similar sources.

Factor analysis suggests a strong association between several ultrastable minerals including zircon, tourmaline and rutile. Such an association is interpreted as a recycled sedimentary source in a stable cratonic setting (Uddin and Lundberg, 1998; Ehrmann and Polozek, 1999). Mature passive continental margins are characterized by a high percentage of zircon, tourmaline, and epidote as well as other less common minerals of metamorphic and felsic intrusive origin (Nechaev and Isphording, 1993; von Eynatten and Gaupp, 1999).

TECTONIC AND DEPOSITIONAL SETTING

During the late Proterozoic to late Paleozoic, the northern part of the African Plate and the Arabian Plate formed a part of the broad, continental shelf of Gondwanaland (Al-Laboun, 1990; Beydoun, 1991; McGillivray and Husseini, 1992; Stump and van der Eem, 1995a, 1995b; Wender et al., 1998; Johnson, 2000; Konert et al., 2001). The Arabian Plate lay between latitudes 30oS and 60oS during the Paleozoic (Konert et al., 2001; McGillivary and Husseini, 1992; Beydoun, 1991) indicating that the region had a mainly temperate climate with intermittent exposure to tropical conditions in the Middle Cambrian, Devonian and Late Permian. These paleolatitudes, along with glaciation during the Late Carboniferous and Early Permian favored siliciclastic sedimentation.

Based on the Paleozoic regional tectonic settings, a depositional model (Figure 5) is proposed for the Wajid Sandstone and its age equivalent sequences (Saq and Qasim formations) in the north. This model is similar with the slope and deep-water progradational setting of the Silurian Qalibah Formation from south to north and northeast (Mahmoud et al., 1992; Rahmani, 2003). Because the Central Arabian Arch developed in the late Paleozoic (Beydoun, 1991), the model suggests progradation of the sediments from the south and gradual filling of a basin (intrashelf depression, Konert et al., 2001) to the north. The thicker, basinal sequences occupied the subsurface of the north and northeastern Arabian Peninsula.

The southern part of the Arabian Peninsula is characterized by the presence of several terranes including Asir, Afif, Abas, Al-Bayda, Al-Mahfid, and Al-Mukalla (Windley et al., 1996; Johnson, 2000). The Afif terrane is interpreted as an Andean-type continental margin, and is characterized by monotonous orthogenesis intercalated with arc-type pillow basalts, andesite, rhyolites, plutonic granitoid and gabbroic suites (Table 7). According to Windley et al. (1996), the Asir and the Afif terranes of the southern and eastern parts of the Arabian Shield in Saudi Arabia, extend all the way to Yemen. As the Afif terrane covers most of the region south of the study area, it should be the most significant source for the Wajid Sandstone. Johnson and Stewart (1996) questioned the extension of the Afif terrane to Yemen. This uncertainty raises the alternative possibility that the Wajid Sandstone was sourced by other continental terranes in the south including the Abas and Al-Mahfid terranes in Yemen (Figure 6). The distribution of Neoproterozic exposures (650-542 Ma, Avigad et al., 2003) is limited in Somalia, Eritrea, Djibouti and Ethiopia and other countries, implying a significant source further to the south is not likely.

Based on trace element data, Hussain et al. (2000) indicated that part of the Wajid may have been derived from an unknown island arc and supracrustal provenance. This interpretation seems unlikely unless chemical weathering at the source profoundly modified the mineralogical composition; heavy minerals representing such an island arc setting are either absent or very poorly represented in the Wajid. Rather, the abundance of rounded quartz grains, and mineralogical maturity (indicated by high ZTR index) of the Wajid implies an older, recycled sedimentary source. The infra-Cambrian Ghabar Group of Yemen includes several thick intervals of pebbly, coarse-grained sandstone (Figure 7). These older sediments are a more likely source of the Wajid.

DISCUSSION AND CONCLUSIONS

The strong association of tourmaline, zircon, rutile, epidote and hornblende in the Wajid, as determined by statistical analysis suggests several different provenances, including igneous, recycled sedimentary and metamorphic rocks. However, an abunadance of the ZTR minerals favors an intermediate or acidic igneous and or recycled sediment over metamorphic source. The nature and distribution of heavy minerals, coupled with regional tectonic settings and distribution of different tectonic terranes in the region, indicate the Neoproterozoic Afif, Abas, Al-Bayda, Al-Mahfid, and Al-Mukalla terranes in Yemen in the south, as the ultimate source of the framework mineralogy of the Wajid. In addition, the thick intervals of pebbly, coarse-grained sandstone in the infra-Cambrian Ghabar Group of Yemen may also be a source.

The ZTR index of the heavy minerals is high, confirming the mineralogical maturity of the sediments (Stattegger, 1986; Moral-Cardona et al., 1996) and loss of metastable heavies. The high ZTR index may also be attributed to:

The chemical index of alteration (CIA), which is a measure of degree of weathering (Nesbitt and Young, 1996, 1982), also suggests extensive chemical weathering of heavy minerals in the Wajid (Hussain et al., 2000). Morton and Hallsworth (1999) noted an abundance of tourmaline, zircon, rutile and apatite in the Devonian Old Red Sandstone from Europe. According to them, a close association of these ultrastable heavy minerals in the sediments, indicates extensive chemical weathering and elimination of metastable ferromagnesian minerals. Apatite may be absent in sediments as a result of dissolution by low pH meteoric water (Morton, 1985; Morton and Hallsworth, 1999, 1994). Thus the apparent absence of apatite in the Wajid may also indicate various degrees of weathering and diagenesis of the heavy mineral distribution. The high percentage of opaque minerals (mainly hematite and goethite) and ZTR index, in addition to indicating selective decomposition of metastable and labile minerals, may also suggest a high degree of hydraulic fractionation (Vital and Guedez, 2000).

The textural characteristics (grain size, angularity, etc.), mineralogical maturity, consistent paleocurrent flow direction to the north, and the abundance of the finer-grained siliciclastics to the north, favor sediment transport from the south to the north, and basin fill by gradual progradation. Northerly paleocurrent orientations of the lower Paleozoic Saq and Qasim formations in northwest Saudi Arabia, coupled with recent zircon SHRIMP isotope data of the Paleozoic sandstone from Elat area support this conclusion. These characteristics, and the tectonic evolution of the Arabian Peninsula, do not favor the present-day exposed Arabian Shield as a significant contributor to the Wajid Sandstone. The tectonic and sedimentation model presented here differs from earlier models, and suggests the need for additional studies to investigate the source of other Paleozoic formations in the region.

ACKNOWLEDGEMENT

The suggestions and constructive criticisms made by AbdulAziz A. Al-Laboun, and Peter Johnson of the USGS Mission in Riyadh are sincerely appreciated. The authors acknowledge the logistic and computing facilities provided by King Fahd University of Petroleum and Minerals (KFUPM) to carry out the research. Thanks to Jowaher Raza for lending his expertise in drafting the figures. Thanks are also due to David M. Rohr of Sul Ross State University, Texas, and Gabor Korvin of the Earth Sciences Department at KFUPM for their critical review of the manuscript. Constructive criticism and feedback from two anonymous reviewers helped to further improve and clarify the manuscript. The current study is part of a major research project (LGP 283) funded by King AbdulAziz City for Science and Technology (KACST). The authors also acknowledge a Grant received from the American Association of Petroleum Geologists (AAPG) to conduct the research. The authors thank two anonymous reviewers and GeoArabia’s editors for their useful comments. The design and drafting of the final graphics was by Gulf PetroLink.

ABOUT THE AUTHORS

Mahbub Hussain holds an MSc in Geology from Acadia University, Nova Scotia, Canada, and a PhD in Geosciences from the University of Texas at Dallas (1986). Prior to joining King Fahd University of Petroleum & Minerals (KFUPM) in 1996, Mahbub taught at several universities in the USA and Canada, including Sul Ross State University in Texas. While at Sul Ross, Mahbub worked closely with the Bureau of Economic Geology at the University of Texas at Austin, and several oil companies (Arco, Exxon, and Texaco) on various aspects of the petroleum geology of the Permian Basin, west Texas. In recent years he has been involved in projects dealing with reservoir characterization, basin analysis, biomarker applications in source-bed study and oil-oil correlations, and provenance study of Paleozoic clastic sequences in the Wajid and Tabuk Basins in Saudi Arabia. Mahbub is a member of the AAPG, GSA and Dhahran Geosciences Society.

mahbub@kfupm.edu.sa

Lameed O. Babalola earned an MS degree in Geology from King Fahd University of Petroleum & Minerals (KFUPM), Dhahran, in 1999. He also received an MSc in Applied Geology (Micropaleontology) in 1992, and a BSc (Honors) in Geology in 1985 from Obafemi Awolowo University and the University of Ilorin, respectively, in Nigeria. Lameed began his professional career as a trainee Geologist with Gulf Oil Company (now ChevronTexaco) Nigeria Limited. Between 1991 and 1997, he worked as a Palynostratigrapher with several geological consultancy outfits in Nigeria. He joined the Center for Petroleum and Minerals (CPM) at the Research Institute of KFUPM in 2001 as a Scientist where he participated in a number of research projects involving carbonate core description, petrophysics and reservoir characterization, and occurrences of solid hydrocarbons in carbonate reservoirs. Lameed is currently a PhD student at the Earth Sciences Department at Carleton University, Canada. Lameed is a member of the Dhahran Geological Society, the Nigerian Association of Petroleum Explorationists and the Nigerian Mining and Geosciences Society.

lameed@yahoo.com

Mustafa M. Hariri obtained a PhD from the South Dakota School of Mines and Technology in 1995. His PhD research focused on lineament and fracture studies and the application of GIS. Before joining King Fahd University of Petroleum & Minerals (KFUPM) in 1989 as Lecturer, he obtained an MSc in Economic Geology from King Abdulaziz University, while working as a Mineral Exploration Geologist with the Deputy Ministry for Mineral Resources. Mustafa’s current research interest is in the field of structural geology, particularly fractures studies, lineament interpretation and remote sensing, and GIS applications. He is presently Chairman of the Earth Sciences Department at KFUPM and is also a Board Member of the Saudi Geological Survey and the Research Institute at KFUPM.

mmhariri@kfupm.edu.sa