The Wajid Group is a Palaeozoic siliciclastic succession of southern Saudi Arabia. In the outcrop belt it is ca. 500 m thick, whereas in the subsurface, the thickness increases to more than 4,500 m. The siliciclastic sediments have great reservoir potential for hydrocarbons and for groundwater. Although they represent one of the most important aquifers of the Arabian Peninsula, neither their sedimentologic, lithostratigraphic, nor their reservoir characteristics are satisfactorily known. In this study, a detailed description of lithology and sedimentology is given and the Wajid Group sediments are interpreted in terms of depositional environment and facies architecture. Thirteen lithofacies (LF 1 to LF 13) have been recognised, most of them composed of different subfacies. These lithofacies are grouped into 9 lithofacies associations (LF-A1 through LF-A9). LF-A1 through LF-A3 and LF-A7 represent shallow-marine siliciclastic environments. The remaining lithofacies associations describe periglacial environments of the Hirnantian (Late Ordovician) and Permian Gondwana glaciations. Except for a few pro-glacial fluvial deposits, fluvial successions and aeolian sediments are absent in the outcrops of the Wajid Sandstone.
Five formations are recognised in the Wajid Group: the Dibsiyah, Sanamah, Qalibah, Khusayyayn, and Juwayl formations. They are all separated by major unconformities. The Dibsiyah Formation represents a vast sand-sheet complex with core and margin facies formed under shallow-marine conditions. These marine conditions enabled an abundant fauna to proliferate and leave its traces in the form of Skolithos piperock and Cruziana sp. A late Cambrian to Early Ordovician age is inferred for these deposits from regional considerations. The Sanamah Formation records the Late Ordovician Hirnantian glaciation with coarse sandstones and conglomerates. A variety of glacier-induced sedimentary structures are present. The internal succession is composed of three major sediment packages reflecting three ice advance-retreat cycles. The latest of these cycles is overlain by a few metres of marginal-marine sediments of the Qalibah Formation. The Khusayyayn Formation was deposited probably during Early Devonian times. It also represents a sand-sheet environment characterised by the dominance of mega-scale and giant cross beds and bed sets. A marine depositional environment is assumed from scarce Skolithos sp., and because nearly all indicators of a braided river system are absent. The Juwayl Formation of Permian age was deposited at the interface of the Late Palaeozoic Gondwana ice shield with a large lake that may have covered most of southern Arabia and adjacent areas. Proglacial sandstones and conglomerates were deposited close to the glaciers, whereas fine-grained sediment with dropstones, boulder pavements and a wide spectrum of soft-sediment deformation are characteristic of the lake environment. While the two glacial successions and the Khusayyayn Formation can rather confidently be attributed to the geological time scale, either through seismic correlation or biostratigraphically, the Dibsiyah Formation has not yet been biostratigraphically well dated.
The Wajid Group represents one of the most important groundwater reservoirs of southern Saudi Arabia supplying water to the populated areas east of the Rub’ al-Khali Sand Sea. It is subdivided into two confined coarse-grained siliciclastic aquifers (the Lower Wajid Aquifer System and the Upper Wajid Aquifer System; GTZ/DCo, 2009), which are separated by a major aquitard. Stratigraphically, the Lower Wajid Aquifer System comprises the Dibsiyah and Sanamah formations of Cambrian and Ordovician age; the Upper Wajid Aquifer System is composed of Devonian through Permian units of the Khusayyayn and Juwayl formations. The separating aquitard (GTZ/DCo, 2009) is formed by silty to shaly sediments of the lower Silurian Qalibah Formation. Similarly, these rocks are also now exploration target for hydrocarbons (Moscariello et al., 2009).
Even though these reservoir rocks are of regional importance, their facies architecture and reservoir properties are only poorly known. Although several papers have been published that deal with aspects of the sedimentology and stratigraphic subdivision of the Wajid Group or parts of it (Dabbagh and Rogers, 1983; Kellogg et al., 1986; Alsharhan et al., 1991; Evans et al., 1991; Stump and van der Eem, 1995a, b; Babalola, 1999; Abdulkadir, 2005; Abdulkadir et al., 2006), a comprehensive outline of the lithofacies and architectural framework of the entire group is missing.
In this paper, an integrated concept for lithofacies, lithofacies associations, depositional environments, and stratigraphic architecture for the entire Wajid Group is presented that is intended to establish a framework for future investigations in the Wajid Group on hydrocarbon exploration and groundwater exploitation. The present study is embedded in the hydrogeological investigations of the Wajid Aquifer system, in which a regional groundwater model for the entire Wajid Aquifer system is being developed. The purpose of this study is two-fold: firstly, it is aimed at providing outcrop information about sedimentary facies and their architecture; and secondly at their relation to aquifer properties to better interpret hydraulic properties as determined in pumping tests (e.g. GTZ/DCo, 2009). In this paper, the results of the first part of the study are presented.
The study area is located in the southwestern part of the Kingdom of Saudi Arabia, which during the Palaeozoic occupied a northwestern position of Gondwana (Figure 1) and comprises the entire outcrop belt of the Wajid Group (Figure 2). To the east, the almost horizontal strata are covered by the sands of the Rub’ al-Khali, the Earth’s largest sand desert; to the north and northeast, the strata are overlain by younger Mesozoic deposits. To the west and south, the Wajid Group disappears as a consequence of uplift and erosion following the uplift of the rift shoulders and the corresponding opening of the Red Sea since the Miocene. Outliers on the basement near Abha and west of Najran (Dahran al-Janub; Figure 2) testify to an originally much larger areal distribution of the Wajid sediments. The present outcrop area comprises approximately 44,000 sq km.
This study is based on several field surveys in the outcrop belt of the Wajid Sandstone. Based on the study of the individual type localities used to define the internal lithostratigraphy of the Wajid Group (Kellogg et al., 1986), additional sections were selected and measured in detail. The observations were complemented with field investigations across the entire outcrop belt between Wadi ad-Dawasir in the north and Najran in the south (Figure 2). In addition, a short survey was carried out in the outcrops west of Najran and the outliers on the Arabian Shield.
Originally, all sections had been given their local Bedouin names (see also Keller et al., 2011). However, in order to standardise sections names with respect to other publications (e.g. Kellogg et al., 1986) and the nomenclature used by Saudi Aramco (M. Hulver, personal communication, 2014), sections were renamed and wherever possible, the section name was taken from the geological map of Kellogg et al. (1986). Only where this was not possible, the original Bedouin names were maintained. Table 1 lists the names as used in this paper, as used in Keller et al. (2011), and an exception as used by Melvin and Norton (2013). In Figure 2, the approximate position of the sections have been plotted on a simplified geological map; Figure 3 shows all measured sections in their exact position on a Landsat image derived from Google Earth.
Following Stump and van der Eem (1995a), the Wajid Group lithostratigraphically was subdivided into five formations (Dibsiyah, Sanamah, Qalibah, Khusayyayn, and Juwayl), each of them separated from the underlying unit by a major unconformity. These unconformities are of regional or supra-regional extent and delimit major turnovers in depositional style and setting. In the Dibsiyah and Sanamah formations, continuous sections are present, whereas in the Khusayyayn and Juwayl formations, shorter outcrop successions had to be combined into composite sections to document the stratigraphic architecture of each lithostratigraphic unit. Sedimentary facies were analysed at various hierarchies, including cm- to dm-scale logging of sections, 2-D and 3-D analysis of architectural elements, and facies associations, as well as large-scale photo panels. Along several sections, gamma-ray spectra have been measured to compare the outcrops with their subsurface counterparts (Schönrok, 2011).
The significance of the Wajid Group for water resources management and hydrocarbon exploration results from the dominance of sandstones and conglomeratic sandstones, which mostly are only moderately cemented and hence rather porous and have a good permeability. Correspondingly, only a minor amount of fine-grained siliciclastic sediments is present in the sedimentary succession.
Thirteen lithofacies (LF 1 through LF 13) have been identified in the Wajid Group. The lithofacies description used in this study (LF types) follows the concept of Miall (1978) using grain size, textural maturity, sedimentary structures, bedform dimensions, and bioturbation. However, it is not strictly applied here as it was developed and is used dominantly in fluvial systems.
The description of the internal geometry of the bedforms (Figure 4) follows Reineck and Singh (1980), who have shown that the geometry and thickness of the foresets varies considerably as a function of grain-size modality and current velocity. In the classification of the subaqueous bedforms, especially the large-scale bedforms of the Dibsiyah Formation and the Khusayyayn Formation, the proposals of Ashley (1990) are followed. In the field, horizons, in which the height of a single set of foresets controls the thickness of the bed, were described as micro-scale (5–40 cm), meso-scale (40–75 cm), macro-scale (75–200 cm), and giant (> 200 cm) beds.
A characterization of lithofacies based on sediment petrography alone seems to be too limited. Hence, petrography plus bedforms and their dimensions as first-order descriptors were used to describe the lithofacies. In the Dibsiyah Formation, and to a much lesser extent in the other units, textural changes of the primary sedimentary fabrics through bioturbation are of prime importance. Hence, bioturbation is used as a second-order descriptor and all lithofacies, which have been affected by bioturbation, have been designated a separate lithofacies sub-unit.
Genetically related lithofacies have been combined into lithofacies associations (LF-A1 through LF-A9). The succession of lithofacies within these associations reflects environmental changes and is interpreted in terms of base level change, change in accommodation and relative movements of sea level. These lithofacies associations subdivide the sedimentary succession of the Wajid Sandstone into genetic units. The order of their description basically follows the order of their occurrence from the Dibsiyah Formation towards the Juwayl Formation.
After the terminal Proterozoic Pan-African Orogeny, Gondwana had become stabilised by the early Cambrian (Powell et al., 1993; Rogers, 1996; Pisarevsky et al., 2008). The large continental plate was placed on the southern hemisphere and centred at high latitudes during the Palaeozoic. In North Africa, Arabia, and the adjacent Iranian terranes, a vast depositional platform became established during the early Cambrian after a phase of intensive tropical weathering and peneplanation of the Pan-African Orogen (Sharland et al., 2001; Avigad et al., 2005).
The Palaeozoic sediments of Arabia were deposited on continental crust of a Precambrian shield, and hence in an intracratonic setting, today called the “Arabian Platform”. The hinge zone between the intracratonic (or epicontinental) sedimentary basins on the crust and on that of the passive continental margin, and hence between slow and high subsidence rates, has yet to be documented. During Cambrian time, this segment of the Gondwana margin was in a south-north position, approximately between 40°S and 20°S (Sharland et al., 2001). At that time, a vast siliciclastic epicontinental sea covered the basement of the Gondwanan interior, a configuration that probably persisted into the Ordovician. The palaeo-shoreline of this sea is not known. By the Late Ordovician (Hirnantian), Gondwana had moved into a polar position and Arabia occupied latitudes between approximately 60°S and 40°S (Sharland et al., 2001), well within the reach of the Gondwana ice cap (Vaslet, 1990; Le Heron and Dowdeswell, 2009; Le Heron et al., 2010; Keller et al., 2011).
Tectonically quiet conditions continued during the Silurian and most of the Devonian until the Frasnian (Sharland et al., 2001). On the Arabian Platform, which was in a relatively low latitudinal position, dominantly siliciclastic sedimentation continued under epicontinental conditions. In the study area, they are reflected in the Khusayyayn Formation. During the Late Devonian and the Mississippian (early Carboniferous), the onset of subduction outboard of the Iranian terranes started to affect sedimentation on the former passive margin. Compression and extension through back-arc rifting induced a tensional regime that led to widespread uplift and erosion (Sharland et al., 2001). In the study area, however, these tectonic events are hardly recognised. Concomitantly, Gondwana again moved into a high latitudinal position and during the Pennsylvanian (late Carboniferous) and Permian a major glaciation spread out over much of the continent. The sedimentary products of this glaciation are well known from Arabia and adjacent areas (Helal, 1963, 1964; McClure, 1980; McClure et al., 1988; Alsharhan et al., 1991, 1993; Melvin and Sprague, 2006; Kumpulainen, 2009; Melvin et al., 2010). The co-occurrence of tectonically and glacially induced erosion hampers the interpretation of the nature of depositional settings in these correlative sediments. This is especially true for the Wajid Sandstone, where the unconformity between the pre-Hercynian sediments (Khusayyayn Formation) and the late glacial, post-tectonic deposits (Juwayl Formation) seems to be entirely of erosional origin.
The age of the Wajid Group and its subunits is still only loosely confined. Hardly any biostratigraphically indicative fossil has been found, except for some trilobite tracks, a few fish remains, and a few palynomorphs (McClure, 1980; Besems and Schuurman, 1987; Evans et al., 1991; Forey et al., 1992). Hence, the overall stratigraphic framework is based on the lithologic comparisons with better-dated successions in the northern Kingdom (Stump and van der Eem, 1995a; Al-Laboun, 2000) and the tracking of subsurface seismic surfaces into the outcrops (Evans et al., 1991).
The concepts of the internal lithostratigraphic subdivision of the units within the Wajid Sandstone are still a matter of debate and exemplified by the papers of Kellogg et al. (1986), Alsharhan et al. (1991), Evans et al. (1991), Stump and van der Eem (1995a, b), Al-Laboun (2000) and Al-Husseini (2004). Figure 5 shows the different concepts and the attribution of the units as proposed in this paper, Figure 6 their correlation to Central Saudi Arabia and the subsurface.
The Dibsiyah Formation is a succession of medium-grained to conglomeratic sandstones with few intercalations of finer siliciclastic horizons (Figure 7). Its characteristic feature is the presence of the Skolithos and Cruziana ichnofacies.
Stratigraphic Relationships: The base of the Dibsiyah Formation is always covered by scree from the buttes and mesas in which the formation is exposed. Several wells, drilled into the basement during the investigations of the Wajid Aquifer (GTZ/DCo, 2009), have nowhere encountered a good basal coarse conglomerate. Instead, the basal layers are represented by a thin weathering horizon on top of the basement with angular to subrounded quartz pebbles and feldspar fragments.
About five kilometres WNW of Jabal Dibsiyah, and west of the As-Sulayyil–Tathlith road (Figure 2; approximately around 20°13’28.49”N, 44°9’24.55”E), a few metres of coarse-grained, only slightly lithified sandstones and conglomerates are exposed immediately above the Precambrian basement in a large open plain. The contact, however, is nowhere clearly exposed. The sediments consist of quartz pebble conglomerates much coarser than those of the Dibsiyah sediments proper. Pebble size matches that of clasts in the nearby outcrops of the Sanamah glacial succession. The stratigraphic attribution of these conglomerates is not clear. They may be the basal Cambrian conglomerate (M. Hulver, personal communication, 2009), but likewise Neogene gravel or gravel residuum as mapped by Kellogg et al. (1986). These sediments are here interpreted as Neogene gravel whose clasts are derived from pebble bearing strata of the Sanamah Formation and/or from the Precambrian basement rocks to the west. Consequently, the base of the Wajid Group in the outcrop area seems not to be exposed and the measured sections reflect minimum thicknesses.
The upper boundary of the Dibsiyah Formation with the Sanamah is an erosional unconformity. As sediments of the latter were deposited in valleys of subglacial origin (Keller et al., 2011), the depth of erosion varies strongly. Hitherto, the deepest erosion is observed at Jabal Sanamah, where erosion cuts into the lower part of the Dibsiyah Formation.
Age: No biostratigraphic data are available from the Dibsiyah Formation despite its diverse ichnofauna. Mass occurrences of Skolithos piperock or “Tigillites” as they are known in North Africa and Arabia (Crimes, 1981) are a Cambrian and Ordovician phenomenon (Droser, 1991). Hence, as the Dibsiyah Formation underlies sediments that are confidently correlated to the Hirnantian glaciation and as it contains Cruziana and other trace fossils then it must be of late early Cambrian to Ordovician, pre-Hirnantian age.
Thickness: The preserved thickness of the Dibsiyah Formation is 160 m to 190 m.
No biostratigraphic data are available from the Sanamah Formation, a succession of sandstones and different conglomerates that were deposited in tunnel valleys in the overall context of the Late Ordovician glaciation. The stratigraphic architecture of the deposits reveals three packages of terrestrial, glacially influenced sediments correlated to the Hirnantian Gondwana glaciation (Figure 5; Keller et al., 2011). The glaciologic inventory has recently been described by Keller et al. (2011) and comprises clasts with chatter marks and striations, striations on disconformable surfaces and bedding planes, and fluted surfaces.
Similar sediments, attributed to the Zarqa/Sarah Formation, are now known also from subsurface exploration and outcrop studies in Central Saudi Arabia (Vaslet, 1990; Moscariello et al., 2009; Melvin, 2014). It seems that these sediments formed in a more ice-proximal setting than those of the Sanamah Formation. The correlation of the corresponding strata in southern and Central Saudi Arabia are shown in Figure 6.
Stratigraphic Relationships: The Sanamah Formation unconformably rests on the Dibsiyah Formation. Its attribution to the Late Ordovician is entirely based on the presence of presumed Hirnantian glacial deposits and their seismic correlation to the subsurface (Evans et al., 1991). The basal unconformity represents a laterally linked system of tunnel valleys. They were carved during the first phase of glacier advance. The thickest packages are preserved in the tunnel valleys (Figures 2 and 8), whereas from the intervening areas there is little information on the time-equivalent sediments (Keller et al., 2011). The Sanamah Formation is unconformably overlain by the Khusayyayn Formation. The onlap of the intervening, pre-Khusayyayn, Qalibah Formation as documented in the subsurface (Evans et al., 1991) cannot be observed in the outcrops.
Age: The Sanamah Formation did not yield biostratigraphically indicative fossils. However, its lithology and sedimentology together with its inventory of glacially induced features (see Keller et al., 2011) leave little doubt that these deposits are equivalents of the Hirnantian glacial sediments of North Africa (McClure, 1978; Ghienne and Deynoux, 1998; Ghienne, 2003; Ghienne et al., 2003; Le Heron et al., 2004, 2005, 2009, 2010) and Eritrea and Ethiopia (Bussert and Schrank, 2007; Kumpulainen, 2009).
Thickness: From the erosional geometries at the base of the Sanamah Formation, which locally cuts down into the lower part of the Dibsiyah Formation, Keller et al. (2011) deduced a minimum thickness and thus minimum tunnel valley depth of 140 m.
In the subsurface, known from wells (GTZ/DCo, 2009) and seismic lines (Evans et al., 1991), the Qalibah Formation consists of two members: the lower Qusaiba Member and the upper Sharawra Member. The Qusaiba Member is a succession of dominantly shale with minor siltstones and sandstones; the upper unit of dominantly siltstones and sandstones is called Sharawra Member.
Stratigraphic Relationships: The Qalibah Formation is only exposed in a narrow strip in the south-central part of the study area (Figure 2). There, several metres of calcareous fine-grained siliciclastic sediments unconformably rest on Precambrian basement and hence indicate an episode of regional onlap. These sediments cannot be attributed to either member. The upper contact to the overlying Khusayyayn Formation is very sharp and unconformable.
Age: In the absence of biostratigraphic control and based entirely on seismic correlations (Evans et al., 1991), the outcropping rocks of the Qalibah Formation have been correlated to be the Lower Silurian Qusaiba Member of Central Saudi Arabia.
Thickness: A maximum of 6 to 10 m is preserved in the outcrops of the Qalibah Formation.
Sediments of the Khusayyayn Formation areally dominate the outcrops of the Wajid Group (Figure 2). The Khusayyayn Formation is a rather uniform succession of dominantly coarse sandstones deposited in medium to giant tabular foreset sets (Figure 9).
Stratigraphic Relationships: The base of the Khusayyayn Formation is a regionally developed unconformity. Near Najran, the Khusayyayn Formation directly rests on Precambrian Basement. Near Hima (Figure 2), the basal unconformity can be traced from outcrops, in which the Khusayyayn sandstones successively cut out the Qusaiba Member, directly to outcrops where it rests on the basement. South of Wadi ad-Dawasir, the Khusayyayn rests on different horizons of the Sanamah Formation.
Age: The Khusayyayn Formation yielded some fish remains (Evans et al., 1991), which tentatively have been assigned a lower Middle Devonian age. Evans et al. (1991) also cite Middle Devonian microfloras from a well east of Hima. In general, they assigned an Early Devonian through Mississippian age to the Khusayyayn Formation.
Forey et al. (1992) present convincing age control (Lower Devonian: late Pragian–Emsian, equivalent to the upper Tawil/lower Jauf formations) on fossil fish from the Khusayyayn, just west of Najran. Similar fish fossils have been found in the middle and upper Khusayyayn further to the northeast in the Wajid outcrop by M. Hulver (personal communication, 2014).
Thickness: The most continuous succession with 55 m is exposed at Jabal Khusayyayn; however, neither the lower nor the upper boundary is exposed there. The combination and correlation of sections in the northern study area indicate thicknesses of around 150 m. Values of 200 m as reported by Kellogg et al. (1986) and Stump and van der Eem (1995a, b) have not been observed.
The glacial successions of the Juwayl Formation (Figure 10) are known from three different, not interconnected areas. In the northern part of the study area, two NW-trending channels (present-day coordinates) are preserved (Figure 2; Kellogg et al., 1986; Keller et al., 2011), in which Juwayl sediments are found. In addition, in the inter-channel area, there are some outcrops of the Juwayl Formation. To the east, the Juwayl Formation sediments disappear beneath the unconformity at the base of the Upper Permian Khuff Formation. In the southeast, Juwayl Formation sediments are exposed along the Tuwaiq Mountain escarpment (Figure 2) at Bani Khatmah and Khashm Khatmah (McClure, 1980; Melvin and Norton, 2013). The underlying sediments are not known. The succession is there capped by the Jurassic unconformity. Since McClure (1980) it is known that the Juwayl Formation is the product of the Late Palaeozoic Gondwana glaciation. This is supported by new descriptions of the glaciologic inventory and interpretations by Melvin and Sprague (2006), Melvin et al. (2010), and Keller et al. (2011). Melvin and Norton (2013) recently described the glacial successions of Jabal Khurb al-Ahmar (Figure 2; see also Table 1) and the Bani Khashm/Khashm Khatmah area (Figure 2) in detail and give a correlation to subsurface successions in other parts of Saudi Arabia.
The outcrops west of Najran (Dahran al-Janub; Figure 2) show a succession of fluvial sediments on top of undoubted Khusayyayn strata. These fluvial deposits with their sharp basal surface have been described by Dabbagh and Rogers (1983); however they were not able to assign these rocks to a lithostratigraphic unit. Alsharhan et al. (1991) compared these rocks with those of the Juwayl Formation at Bani Khatmah and concluded that they might correspond to this formation. Based on their lithologic characteristics, the siliciclastic sediments west of Najran, distinctly finer than those of the Khusayyayn Formation and show a fining-upward motif similar to other Juwayl Formation successions, are here also attributed to the Juwayl Formation.
Stratigraphic Relationships: The Juwayl Formation rests unconformably on the Khusayyayn Formation. The unconformity has different expressions in the Wajid outcrop belt. In the north, the unconformity is the result of subglacial erosion (tunnel valleys), whilst in the other areas it seems to be a paraconformity.
Age: From the Juwayl Formation, microflora data are available (McClure, 1980; Besems and Schuurmann, 1987). They constrain the depositional age of the Juwayl Formation to the early Permian (Sakmarian; see also Stephenson and Filatoff, 2000). According to these authors, the stratigraphic gap between the Juwayl Formation beneath and the Khuff Formation above may comprise as much as the interval from the Artinskian/early Kazanian through much of the Tatarian (Figure 5).
Melvin and Norton (2013, and references therein) give a comprehensive summary on the age control of the Juwayl Formation in the subsurface (Unayzah Formation A-C with Unayzah A being the youngest unit) and the correlation to the outcrop belt of the Wajid Group, especially the Bani Khashm/Khashm Khatmah area (Figure 2). Unayzah C and B are of late Carboniferous to early Permian (Sakmarian) age. Unayzah C and B are correlated with the lower and upper Juwayl Formation (Figure 6). Notwithstanding, a lower and upper Juwayl subdivision in the outcrop belt has never been demonstrated. Following this age assignment, the stratigraphic gap between the Juwayl Formation and the overlying Khuff Formation hence would comprise most of the Sakmarian through the middle Permian (Capitanian). Sediments correlative to the Unayzah C are apparently absent in the Wajid Group (Figure 6).
Thickness: The composite section for the northern area including the channel fills is approximately 125 m thick. A similar thickness was determined for the Bani Khatmah area with its lake facies by McClure (1980), whereas Hadley and Schmidt (1975) report thicknesses between 50 m and 135 m from that area.
LITHOFACIES OF THE WAJID GROUP
LF 1: Shale
Pure shale successions (LF 1) are very rare in the sedimentary succession of the Wajid Group. Individual packages of more than 20 cm thickness have only been found in the Qalibah Formation, where they are grey to beige, and in the Juwayl Formation (Figure 11a), where they are often entirely silicified.
Shales are a typical product of low-energy environments (e.g. lakes) or slack water in the nearshore marine environment, where settling out of suspension is the dominant depositional process.
LF 2: Shale and Siltstone
Intimate alternations of shale and siltstone represent LF 2. The sediments are variegated and locally form successions of several metres thickness. In the Juwayl Formation, shale and siltstone laminae form characteristic dark-light couplets with a thickness of a few mm (Figure 11b). Isolated basement boulders with glacial striations have locally been observed in these deposits. In several sections, these sediments are strongly silicified.
The shales and siltstones were deposited in low-energy environments as evidenced by the absence of any kind of current induced structures. The rhythmic alternation of light silty and dark shaly laminae is characteristic of varve sediments, typical of glacial lake environments (Einsele, 2000). This is corroborated by the scattered glacial basement clasts here interpreted as dropstones. Some of the individual layers may have originated from distal turbidity currents, which are not uncommon in lakes (Einsele, 2000).
LF 3: Siltstone
Thick massive siltstones (LF 3.1) without clay-grade component have only been observed in the Sanamah and Juwayl formations, where these brown and beige sediments attain several 10s of centimetres thickness. Siltstones of LF 3.2 and LF 3.3 are usually thin-bedded, show horizontal lamination and ripples and are of reddish to brown and beige colour (Figures 11c, d). In LF 3.3 (Dibsiyah Formation), distinguished by the presence of bioturbation, in many but not all beds of the primary sedimentary structures have been obliterated by burrowing.
The massive siltstones of LF 3.1 represent mass-flow deposition as shown by the absence of current induced structures. The siltstones likely were deposited through distal turbidity currents that originated close to the mouth of a glacially fed river. Sedimentary structures indicate that the siltstones of LF 3.2 and LF 3.3 were formed from bed load transport under lower flow regime conditions. Marine conditions are interpreted from the presence of Skolithos sp. and Rosselia sp. Where these deposits are associated with 2-D or 3-D dunes (LF 9 through 11), they likely are the product of the vast submarine interdune areas where current velocities are distinctly lower than in the dune fields themselves (Ashley, 1990).
LF 4: Siltstone to Fine Sandstone
Siltstones and sandstones form thin- to medium-bedded horizons of intensively red or locally yellow colours. Internal stratification is almost absent; locally, low-angle cross bedding and ripple-drift cross bedding have been observed. The sediments were deposited in micro-scale or meso-scale bedforms. They show strong and large-scale soft-sediment deformation associated with shearing and thrusting within discrete horizons. Structures include truncated folds, diapiric, dome-like, and large-scale ball-and-pillow structures.
The sediments record low-energy conditions with an alternation or a combination of bedload transport and fall out of suspension. Nevertheless, a large amount of sediment was provided, which was rapidly deposited and subsequently subject to dewatering. As these deposits are found in the Sanamah and Juwayl formations associated with glacial shearing and striations and are interpreted as ice-distal products. Decreasing energy of the rivers draining the glaciers led to the deposition of coarse sediment close to the glacial front, but further downstream only silts and sands were delivered. Locally, these sediments may even represent deposits of a delta in a glacial lake. Deformation of the sediment, especially when associated with shearing and thrusting, is interpreted to result from subsequent glacier advance (Le Heron et al., 2005; Keller et al., 2011).
LF 5: Sandstone, Ripple-drift Cross Bedding
Fine- to medium-grained sandstones in dominantly micro-scale beds with cross bedding represent LF 5. The sediment is moderately sorted to well sorted. Whereas in many horizons, the cross bedding originated from unidirectional flow, in others bidirectional cross bedding, often symmetrical, was observed. From 3-D outcrops it becomes apparent that the latter are wave ripples (Figure 11e).
The sediment is the product of a moderate- to low-energy environment, in which bedload transport was the main agent. Bedforms indicate unidirectional and bidirectional transport. Unidirectional flow is not characteristic of a specific environment; hence, these sediments may be of marine or may be of fluvial origin. They can only be interpreted in the context of facies associations described below. The bidirectional bedforms are likely to represent tidal influence although well-developed mud flasers or mud drapes are absent that ideally would separate the wave ripples.
LF 6: Sandstone, Flaser Bedding
LF 6 is composed of fine- to medium-grained sandstones, moderately to well sorted, with well-developed flaser bedding (Figure 11f). The sediments are generally present in micro-scale beds and may be stacked to bedsets of more than one metre thickness.
The sandstones were deposited in moderate- to low-energy environments with dominantly bedload transport. Slack-water periods are documented through the fall-out of mud, which gave rise to the flaser texture of the sediment. Although flaser bedding may occur in ephemeral stream deposits (Martin, 2000), it is dominantly a shallow-marine structure (Reineck and Singh, 1980).
LF 7: Sandstone, Horizontal to Low-angle Cross Bedding
LF 7 is made up of fine- to coarse-grained sandstones in meso-scale to macro-scale beds. All of these deposits are horizontally bedded or show low-angle cross bedding (Figure 4). As there are distinct differences in the grain size, LF 7.1 represents fine- to medium-grained, moderately sorted sandstones, whereas LF 7.2 is composed of almost unimodal white coarse sand (Figure 11g). LF 7.3 is poorly sorted medium to coarse sand, which served as a substrate for burrowers, who effectively destroyed many but not all of the primary sedimentary structures. All these sediments show beige to intensively red colours. The individual laminae locally are graded. Sometimes the lamination is accentuated by heavy mineral stringers.
In coarse-grained sediments, horizontal lamination is mainly produced under high-energy, upper flow regime conditions (Paola et al., 1989; Cheel, 1984). The principal mechanisms are high-frequency scouring and subsequent filling and low-frequency migration of bedforms. These bedforms have a height of several grains and amplitudes of up to 100 cm; they are superimposed to form the laminae. In flume studies (Paola et al., 1989; Cheel, 1984), these processes were able to produce graded laminae and to produce heavy-mineral segregations as observed in LF 7.
Horizontal lamination can form in a variety of environments and under different energetic conditions. In a marine environment (bioturbation), such beds develop from unidirectional flow under upper flow regime conditions (Harms et al., 1982). Sand is transported in discrete thin sheets along the bed and net sand input into the system (aggradation). Similar structures of Early Palaeozoic age in Laurentia (Jordan Sandstone; Runkel, 1994; Byers and Dott, 1995) have been interpreted as beach swash lamination.
LF 8: Sandstone, Fine to Medium, Tabular Cross Bedding
LF 8 is composed of fine, medium, and coarse sandstones deposited in beds a few tens of centimetres (micro-scale) to almost two metres (macro-scale) thick. The bounding surfaces of the foreset sets are planar and commonly almost horizontal (Figure 11h). All geometries from purely angular (Figure 4; Figures 12a, b) to increasingly curved with a tangential basal contact of the foresets (Figure 12c) to sigmoidal geometries (Figure 12d) have been observed and partly used to distinguish between the subfacies.
LF 8.1.2 is fine- to medium-grained sandstone, yellowish to red, deposited in micro- to meso-scale beds. The foresets are mm-thick and moderately to strongly curved (LF 8.1.2).
LF 8.2 consists of locally pebbly, medium- and coarse-grained sandstones in meso-scale to macro-scale beds (Figure 12c). LF 8.2.1 has micro-scale to meso-scale foreset and cosets locally contains abundant feldspar fragments, and, in the Dibsiyah Formation, is frequently burrowed. Individual foresets are mm-thick to several mm thick and dominantly moderately curved. LF 8.2.2 shows medium to thick foresets that locally are 15 cm thick (Figure 12f). The foresets are of the angular type (Figure 4). Where preserved, the sediments of LF 8.2.2 that have synsedimentarily been deformed into large recumbent folds (Figure 12g) or are slumped (Figure 12h), have been designated LF 8.2.3.
LF 8.3 (LF 8.3.1) is represented by medium and coarse sandstones, also with medium to thick foresets (Figure 13a). In contrast to LF 8.2, however, the individual foresets are thick and often graded from basal pebble laminae into sand of coarse or medium grain size (Figure 13b). Similar to LF 8.2, feldspar clasts are abundant in some horizons. LF 8.3.2 is distinguished on the presence of abundant burrows of Skolithos sp. (Figure 13c) and related species and of Cruziana sp. (Figure 13d) and only observed in the Dibsiyah Formation.
LF 8.4 consists of individual foresets which are graded. In addition, the entire set of laminae is graded with coarser laminae at the base and finer laminae towards the top.
LF 8.5 is medium- to coarse-grained, graded sandstone.
Throughout the Wajid Group, sets and cosets (Figure 4) of micro-scale, meso-scale, and macro-scale LF 8 sediments have been observed. In addition, giant sets with a height > 200 cm are present in the Khusayyayn Formation.
The tabular sandstone beds (Figure 4) originated in moderate- to high-energy environments. Tabular sandstones are the typical product of migrating large 2-D dunes (Ashley, 1990) under unidirectional flow conditions with bedload transport. In LF 8.1, the shape of the foresets indicates an increase of fine-grained material that settles out of suspension from the angular type to the curved variety. Sigmoidal structures are typical of full preservation of the ripples (Reineck and Singh, 1980). In LF 8.2, the angular foresets again indicate negligible presence of fine-grained material, which in turn is favourable for the formation of slumps and of recumbent folds because of the reduction of cohesion.
LF 8.3 and LF 8.4 are characterised by graded foresets. Their often tangential basal contact and their sometimes diffuse internal lamination indicate the presence of finer-grained material. It is here inferred that these phenomena indicate fallout of suspended material during unsteady flow conditions. Although not exclusively a tidal phenomenon, these conditions with the resulting graded foresets are typical of tidal environments (Dalrymple and Rhodes, 1995).
LF 9: Sandstone, Macro-scale 2-D Trough Cross Bedding
Although in general rather similar, LF 9 and LF 10 are distinguished because of the distinct grain-size association of coarse sand and abundant quartz pebbles in LF 10. Quartz pebbles are absent in LF 9 and the sorting in general is better. The typical feature of these deposits is their depositional geometry in trough cross beds with planar bounding surfaces (Figure 4). Individual beds extend over relatively wide distances in transport direction, so that they are usually several times wider than high. In general, sediments have been deposited in meso-scale and macro-scale bedforms. Micro-scale units are present only to a minor extent.
LF 9.1 is moderately sorted and of medium to coarse composition.
LF 9.2 is sandstone deposited in 2-D trough cross beds, well sorted, either within the medium sand fraction or in the coarse sand fraction. Quartz grains are moderately rounded to well rounded.
LF 9.3 is similar to LF 9.1; however, bioturbation is prominent.
In all subfacies, siltstone intraclasts have been frequently observed at the base of the scour fills. Similar to LF 8, sandstones of LF 9 were deposited under medium- to high-energy conditions. As pointed out by Ashley (1990), 2-D trough cross-bedded sandstones result from the migration of 3-D dunes (Figure 4). In comparison with their 2-D counterparts, transport energy must be at least slightly higher to produce 3-D geometries. Mudstone intraclasts suggest the presence of low-energy environments in the vicinity (inter-dune environments), where mud settled down and was reworked during the return to higher energy conditions during the next dune advance. No indicators of tidal influence are recognised; hence, the main agent in sediment transport and dune formation were most likely waves.
LF 10: Sandstone to Pebbly Sandstone, Macro-scale 2-D Trough Cross Bedding
LF 10 is mainly present in macro-scale bedforms and to lesser extends in meso-scale forms. LF 10.1 is conglomeratic sandstone, deposited in 2-D trough cross beds (Figure 13e). Rounding of the quartz grains varies between subangular to moderately rounded; sorting is moderate to poor.
LF 10.2 is represented by graded medium and coarse sandstones. In LF 10.2.1, the entire bed of coarse to medium or fine sand is graded. LF 10.2.2 shows medium to thick foresets, in which the individual foresets are graded from basal pebble laminae into coarse and rarely medium sand.
LF 10.3 is a rare type of sediment in the Wajid Group and only present close to the contact between the Sanamah Formation and the underlying Dibsiyah Formation. It consists of inversely graded conglomeratic sandstones with large tabular clasts of grey siltstone (Figure 13f).
LF 10.4 and LF 10.5 preserve the record of moderate to intensive bioturbation. Whereas the pebbly or conglomeratic sandstones of LF 10.4 show the activity of trilobites (Cruziana tracks; Figure 13d), sands of LF 10.5 were occupied by vertical burrowers whose activities are recorded as Skolithos sp. and related forms (Figure 13c).
The hydrodynamic interpretation for LF 10 is the same as for LF 9. However, the absence of quartz pebbles in LF 9 indicates an important shift in environmental conditions in the depositional system from proximal to distal settings, which was beyond the transport capacity for quartz pebbles. The reasons for this shift will be discussed subsequently.
LF 11: Sandstone to Pebbly Sandstone, 3-D Trough Cross Beds
Medium, but dominantly coarse sandstones with varying amounts of pebbles with 3-D trough cross bedding (Figure 13g) constitute LF 11.1. Locally in the Sanamah Formation, individual foresets are up to 25 cm thick and graded. Sorting is poor. Pebbles are mainly of vein quartz origin and preserve glacial striations and chatter marks.
LF 11.2 is distinguished on the presence of graded foresets, often several mm to rarely 15 mm thick, in which a basal pebble layer successively grades into fine to medium sand (Figure 13h). The 3-D troughs are a few tens of metres wide and up to a few metres deep. Vertically, stacks up to four 3-D troughs were found.
Soft sediment deformation is common in these deposits. Small-scale load casts, dome-like structures, large-scale ball-and-pillow structures and truncated folds have been observed. These sandstones represent high-energy deposits of a (glacio-) fluvial environment, in which channels were frequently fluctuating and transported large amounts of sediment. LF 11.2 was deposited more episodically, probably as a response to thawing-freezing (seasonal) events. The channel systems with their grain-size distribution are typical of braided rivers. They originated beneath the glacier (striations) and transported the subglacially eroded material to the plains in front of the glacier or ice shield.
LF 12: Sandstone, Massive
LF 12.1 is massive, often structureless sandstone with pebbles, cobbles or boulders. Blocks floating in the matrix with sizes up to several metres have been observed, which consist of the same material as the matrix they are associated with. Locally, unusual erosional features with nearly vertical walls are present that represent channels. These channels are filled with the same sand that constitutes the remainder of the massive sandstones. Locally, strongly concave internal cross bedding is visible. In other places, lobe structures are observed, where sediment was creeping a few metres down the local relief, even into the channels but then became frozen in place. Individual packages of these yellowish to brownish unstructured sediments are up to 30 m thick (Figure 14a).
The massive rocks of LF 12.1 interfinger with the 3-D trough cross-bedded sandstones of LF 11. Apparently, they developed from large-scale synsedimentary liquefaction of LF 11. Hyperconcentrated flows transported large blocks of coarse sandstone and caused oversteepened and vertical channel walls (Figure 14b), and lobe structures. The massive facies results from temporal liquefaction during short pulses of deglaciation or even temperature rise in summer. As the transport processes were highly viscous and honey-like and as it results from temporary thawing, this unique facies association has been termed “Sorbet” facies by Keller et al. (2011).
LF 12.2 is yellow to beige and grey medium to coarse massive sandstone with a varying amount of pebbles and/or cobbles. These sandstones and conglomeratic sandstones were deposited in medium to thick horizons without apparent bedding. Only locally, a weak large-scale cross-stratification can be observed. In general, the clasts are mainly randomly distributed but locally they are concentrated in patches. Some of the sandstone beds have an erosional base; others were deposited in broad channels some 10 m wide. Locally, slumping disrupted the beds and generated floating intraclasts.
LF 12.3 is structureless fine to medium sandstone only observed in the Sanamah Formation. It is mainly red but also yellow to beige, and in contrast to the other sandstones of LF 12, at least moderately sorted. LF 12.2 and LF 12.3 are present in both glacially influenced units of the Wajid Group, the Sanamah and Juwayl formations. Sediment was provided in large quantities and transported under high-energy conditions. The absence of internal textures points to deposition from hyperconcentrated flows. In modern glacial environments, these facies are typical of sandur flats in front of ice sheets or glaciers (Maizels, 1997; Russell et al., 2006). The better sorting and rounding in LF 12.3, which is present in the Sanamah Formation only, may indicate greater distances to the (subglacial) source area or, more likely, recycling of older sands.
LF 12.4 is a rather frequent deposit in the upper part of the Dibsiyah Formation. It consists of coarse-grained, locally pebbly sandstones, in which any original depositional feature has been entirely obliterated by the burrowing activity of Skolithos sp. and related animals (Figure 14c). LF 12.4 is actually an end member of the weakly to strongly bioturbated coarse sandstones of LF 9 and LF 10 in the Dibsiyah Formation; hence, the original bedforms most probably were 2-D and 3-D dunes as is observed in the overlying and underlying beds (Figure 14d).
LF 13: Conglomerate
LF 13.1 (clast-supported) contains abundant pebbles and cobbles; sorting is poor. The mainly reddish “matrix” is composed of medium to coarse sand (Figure 14e). The conglomerate is mainly monomictic to oligomictic with moderately rounded to well-rounded clasts of vein-quartz origin (Figures 14e, f).
In LF 13.1.1, the pebbles are of low sphericity and have a rather long a-axis. In the Sanamah Formation, big slabs of grey silty sandstones were apparently eroded from the underlying Dibsiyah Formation. The thickness of the individual conglomerate horizons varies between 20 cm and 3 m (Figure 14f).
LF 13.1.2 is gravelly to pebbly quartz pebble conglomerate with subordinate coarse sand. The grains are moderately rounded to well-rounded clasts of high sphericity. In addition, siltstone clasts and shale intraclasts have been observed. Individual layers are up to 20 cm thick but stacks of these sediments may accumulate to 200 cm thickness. Sedimentary structures include angular foresets in planar cross beds and rare sigmoidal foresets.
LF 13.2 is matrix-supported conglomerates. Monomictic matrix-supported conglomerates (LF 13.2.1) are dominant in the Sanamah Formation. They are poorly sorted and have a sandy to silty matrix, in which subangular to well-rounded clasts of vein-quartz origin are found. Some of these clasts show chatter marks and striations, undoubtedly of glacial origin. In addition, slabs of the clast-supported conglomerates (LF 13.1) are present in some horizons.
Polymictic matrix-supported conglomerates with a sandy to silty matrix represent LF 13.2.2 (Figure 14g); they are found both in the Sanamah Formation and the Juwayl Formation. These sediments form mostly poorly sorted structureless horizons with beds up to 6 m thick, in which pebbles and boulders are irregularly distributed within the matrix. Sometimes however, they are concentrated in nests and boulder pavements.
Polymictic conglomerates with a shaly matrix (LF 13.2.3) are only observed in the Juwayl Formation. They are in general present as well bedded horizons, in which the clasts are irregularly distributed and often show impact structures (Figure 14h).
In the Sanamah Formation, the clast spectrum of LF 13.2.2 is relatively restricted with vein quartz, feldspar clasts, pebbles and boulders from the basement. In addition, clasts from the underlying strata of the Dibsiyah Formation have been identified. In the Juwayl Formation, magmatic, metamorphic and sedimentary rocks are represented in the clast spectrum. Many of the clasts of LF 13.2 show glacial striations, the distinct shape of glacially transported clasts, and chatter marks.
LITHOFACIES ASSOCIATIONS AND DEPOSITIONAL ENVIRONMENTS
Standing alone, several of the lithofacies described above are non-diagnostic for a specific depositional environment. However, when grouped into lithofacies associations, these associations characterise well-defined depositional environments. Nine lithofacies associations have been recognised in the sediments of the Wajid Group (LF-A1 through LF-A9). Some of the associations (LF-A4 through LF-A6 and LF-A9) have recently been documented by Keller et al. (2011). Table 1 lists the lithofacies associations (LF-A) and the constituting lithofacies (LF) in addition to their occurrence in the formations of the Wajid Group.
LF-A1: Conglomeratic 3-D Dunes Association (Dibsiyah Formation)
This lithofacies association is composed of quartz pebble conglomerates (LF 13.1.2) and a variety of (pebbly) sandstones deposited with 2-D trough cross beds of LF 10.1, 10.2, and 10.3. In addition, siltstone layers (LF 3.2, 3.3) are present in often laterally discontinuous beds, because they have been eroded and truncated by the currents depositing the overlying sandstones. Consequently, the former presence of fine-grained material locally is not documented in the sedimentary succession itself, but by their fragments in coarser sediments, into which they were incorporated through erosion and redeposition.
The contacts between the beds are often sharp and show reactivation surfaces. Bed amalgamation is common. Beds and bedsets are of macro- and meso-scale, only locally micro-scale beds have been observed. Bioturbation is present throughout this association, mainly represented by Skolithos sp., Rosselia sp. and Monocraterion sp.; other unidentified tubes are common. Cruziana sp. and other trilobite tracks have also been found.
LF-A1 records high-energy, marine conditions dominated by the presence of 3-D dunes (Figure 4). 3-D dunes with their relatively simple internal structure are the result of strong unidirectional flows (Ashley, 1990). On shelves or epicontinental platforms, storms, geostrophic currents, or strong tidal currents are the main transport agents. No evidence of storm deposition or tidal influence has been encountered in the Dibsiyah Formation, so that strong asymmetrical wave-induced currents were responsible for sediment transport. Strong unidirectional flow is corroborated by steeply inclined foresets with angles > 25°. Bidirectional currents regularly produce slip faces smaller than 20° (Flemming, 1982; Harms et al., 1982).
2-D dunes and 3-D dunes are relatively stable phenomena when compared to small-scale bedforms (ripples). They may be quasi-stationary for months or years (Bokuniewicz et al., 1977; Allen and Collinson, 1979; Allen, 1980); hence in the interdune areas, which are protected from the currents, low-energy depositional pools are present, in which fine-grained material (clay or silt) can be deposited. This is documented by the truncated siltstone horizons (Figure 15a). Scouring in front of the large-scale bedforms during lateral migration then leads to erosion of the interdune sediment and its incorporation into the dunes as intraclasts.
LF-A2: 2-D–3-D Dunes Complex Association (Dibsiyah and Khusayyayn Formations)
Although a rather monotonous succession, it is the most spectacular and most easily recognised association in the outcrop belt of the Wajid Group because of the giant and thick foresets of the tabular cross beds so typical of the Khusayyayn Formation (Figure 15b).
The association comprises sediments with tabular cross laminae (LF 8; Figures 12b and 15b). In addition, sediments with 2-D trough cross bedding (LF 9, LF 10) are present and a few horizons with parallel bedding (LF 7) have been observed. The bounding surfaces are always sharp and in many cases show signs of erosion and reactivation. Locally, bed sets show typical patterns of herringbone cross-stratification (Figure 12b). Sedimentary structures (Figures 12g, h) include slumping, ball-and-pillow structures, and large-scale overturned beds. The presence of ball-and-pillow structures is remarkable, given the coarse sandy nature of the sediment. Apparently, subtle changes in the texture between underlying and overlying beds were sufficient to permit these processes to occur.
In the Dibsiyah Formation, the amount of vertical burrows in the beds varies considerably; in the Khusayyayn Formation only a few Skolithos burrows have been observed.
Similar to LF-A1, this association records deposition under high-energy, marine conditions. The sediments are the product of strong unidirectional currents responsible for the formation of 2-D dunes and 3-D dunes. However, the co-occurrence of 2-D dunes (tabular sandstone bodies) and 3-D dunes (trough cross-bedded sandstones) indicates fluctuations in the transport energy in the environment (Ashley, 1990) and an overall decrease when compared to LF-A1. This is corroborated by the drastic decrease in the amount of conglomeratic layers and pebbles in the sediment. The association records an alternation of tide-dominated episodes and wave-dominated times.
LF-A3: 2-D Dunes-piperock Association (Dibsiyah Formation)
The most characteristic feature of this association are horizons, locally 7 m thick, that are entirely burrowed by Skolithos sp. and related organisms (“Tigillites” or “piperock”; Figure 14c). No primary structure is found in these massive rocks. Some of these very thick horizons show undulating surfaces and as these horizons weather out as positive forms, some of them appear as “reef-like” mound structures. Internally, tubes close to 100 cm long (!) have been observed. They alternate regularly with thick beds of tabularly cross-bedded sandstones (Figures 14d and 15d). The lower and upper contacts of the horizons are always sharp.
In general, the physical conditions of sediment transport and the depositional environment are similar to LF-A2. In contrast to LF-A2, however, there is no more quartz-pebble conglomerate in the succession and pebbles are almost absent from the sediment. In comparison to LF-A2, this association is the product of a more episodic sediment accumulation. Almost all dunes show reactivation or erosional surfaces separating well-preserved layers without bioturbation from the massive piperock that is totally burrowed.
LF-A4: Glacio-fluvial Conglomerate Association (Sanamah and Juwayl Formations)
In both formations, conglomeratic associations are present, which originated from glacio-fluvial processes. The association in the Sanamah Formation is dominated by conglomerates and coarse sandstones, silt or clay are absent. In contrast, in the Juwayl Formation fine and medium sandstones and locally some siltstones are present. Hence this association is subdivided into LF-A4a: massive to coarse glacio-fluvial conglomerate association (Sanamah Formation) and LF-A4b: diverse glacio-fluvial conglomerate association (Juwayl Formation).
LF-A4a is dominated by conglomerates deposited in bedded or massive units. The association is composed of LF 11.1, LF 12.2, LF 13.1, and LF 13.2.1. Sediments of LF 13.2.2 (polymictic matrix-supported conglomerate) are present but rare. Within this facies association, almost all contacts between beds are erosional (Figures 13g and 14b). The depth of the scours and channels varies from centimetres to several tens of metres. The largest erosional phenomena are the basal depositional surfaces, i.e. the base of the tunnel valleys.
In LF-A4b, massive and conglomeratic rocks of LF 10, LF 11.1, LF 12.4, and LF 13.2.2 alternate with finer-grained siliciclastic deposits of LF 7.1 and LF 8.2. In addition, massive siltstones (LF 3.1) are locally present. Massive sandstone laterally passes into 3-D trough cross-bedded sandstones and the base of the coarse units is mostly erosional. Channels are several metres wide and are rarely more than one metre deep.
Based on the presence of clasts of undoubtedly glacial origin (chatter marks, striated clasts) and the succession of facies in the Sanamah Formation, this association was interpreted to be a proglacial outwash fan in front of the glacier with its sediment provided by strong subglacial melt-water streams (Keller et al., 2011; facies S1). These streams deposited their load subaqueously within the channels that had previously been carved. Thickness and geometry of these fans indicate ample accommodation. The alternation of water-lain sediment transported by high-energy melt water and of gravitationally induced mass-flow deposits indicates strong turbulence in the discharging melt water and discontinuous sediment supply. This is corroborated by the poor sorting and the massive bedding, often without any recognisable internal structure.
A similar scenario can be developed for the Juwayl Formation. There, however, the presence of finer-grained sandstones and some siltstones indicate that the melt-water streams were not as strongly laterally confined as in the Sanamah Formation. Instead, between individual glacier outlets, sandur flats, braided-river remnants, and probably small lakes were present that added to the more diverse lithologic inventory. As discussed by Keller et al. (2011), these differences between Ordovician and Permian deposits may simply be due to different ice dynamics and the distance to the glacier front with the Permian deposits recording a more ice-distal setting.
LF-A5: Massive to 3-D Cross-bedded Sandstone Association (Sanamah and Juwayl Formations)
It is composed of massive sandstones of LF 12 together with pebbly, 3-D trough cross-bedded sandstones of LF 11.1. In the Juwayl Formation, these facies interfinger with well bedded sandstones and siltstones (LF 4).
These sediment packages are organised in irregular channel-like features cutting deeply into the underlying rocks. The most prominent feature of this association is its arrangement in large clinoforms up to 50 m high, which can be traced over several hundreds of metres (Keller et al., 2011).
Sediment was provided in large quantities and transported by high-energy subglacial melt waters (Keller et al., 2011, facies S2). Individual pulses of melt-water flow are recorded as thick graded layers as observed in the Jabal Masqarah section (Figure 7). Catastrophic melt-water outbursts may have deposited some of the massive sandstones that resemble the subaerial jökulhlaups of modern environments on Iceland (Maizels, 1991, 1993, 1997). The absence in other beds of internal bedding features points to hyperconcentrated mass-flow deposition. These facies are typical of sandur flats in proglacial settings downstream of proximal fluvial conglomerates of LF-A5 (Maizels, 1997; Russell et al., 2006).
As the sediments in the Juwayl Formation are in general slightly finer, they represent a more distal glacio-fluvial setting than those of the Sanamah Formation. Large-scale deformation structures observed in these sediments are typical of glacial shearing of not yet fully lithified sediment (Figure 15c).
Locally in the Sanamah Formation, this facies association is organised in large-scale clinoforms, several metres thick and hundreds of metres wide. They are frequently observed in Gilbert-type deltas that may have developed within the water-filled channels or in ice-dammed lakes in front of the glaciers.
LF-A6: Siltstones to Fine-grained Sandstone Association (Sanamah and Juwayl Formations)
This association is a rapidly alternating succession of shales (LF 1), shale-siltstone alternations, (LF 2) siltstones (LF 3.1, LF 3.2), siltstones to fine sandstones (LF 4) and occasionally some sandstones of LF 5 (Figure 15e). Individual lithofacies units are arranged in centimetre to a few decimetre-thick depositional units. Together they form stacks several metres to few tens of metres thick.
A decrease in transport energy and less turbulent flow conditions when compared to LF-A5 are documented by the overall finer grain size, scarceness of mass-flow deposits and less frequent erosional surfaces. Deposition occurred partly from falling out of suspension but dominantly from bedload transport. The few massive siltstones and sandstones were deposited from mass flows. In this facies association, the entire suite of lithologies originated in subaqueous environments of a glaciofluvial plain. The increase in fine-grained material towards the top of the succession (Figure 15f) indicates increasing distance to the retreating glaciers.
Sediments of this facies association record repeated events of soft sediment deformation (Figure 15g). This was probably caused by deformation of unfrozen sediments during a short-lived glacial surge during late stages of deglaciation (Le Heron et al., 2005).
LF-A7: Calcareous Fine-grained Siliciclastic Association (Qalibah Formation)
The sediments of this lithofacies association have not been described in detail in the section on lithofacies, because this succession is rarely more than 10 m thick and the individual beds are rarely more than a few centimetres thick.
Several outcrops have been visited, in which a thin succession of fine-grained calcareous siliciclastic sediments is present (Figure 15h). It consists of reddish calcareous siltstones with shell fragments (brachiopods?) and locally intensive bioturbation, of green shales, and calcareous white sandstones. Intercalations of white crinkly calcareous layers, which resemble microbial mats, are present. A few evaporite-dissolution microbreccias have also been observed.
The facies association records a restricted-marine environment, probably slightly hypersaline, as indicated by a monospecific fauna, microbial mats, and some structures that can be attributed to haloturbation and evaporite dissolution.
LF-A8: Shale-Siltstone Association (Juwayl Formation)
This association is composed of thick packages of relatively pure clay shales in alternation with massive (LF 3.1) and laminated (LF 3.2) siltstones. Micro-scale bedforms dominate (Figure 16a). Locally, white fine-grained sandstones with low-angle tabular cross beds are present (LF 8.1.1) in micro-scale to meso-scale units.
The sediments record low-energy depositional conditions. Whereas the shales were deposited in low-energy environments, the siltstones record both mass-flow deposition (LF 3.1) and current activity in the lower flow regime. The massive siltstones likely are distal turbidites. The white sandstones are difficult to interpret. Evans et al. (1991) and Melvin and Sprague (2006) interpreted similar deposits in Central Saudi Arabia as aeolian deposits; they may similarly reflect shallow-marine deposits, in which tidal activity accounts for the sorting and rounding of the sediment.
This facies association represents a succession deposited during a relative rise in water level. This could have been a rise in the level of the glacial lake that existed in much of southern Arabia, or it was sea-level rise during advanced deglaciation. No unequivocal evidence is present to support either possibility. Vestiges of glacial processes are no longer visible in this succession, which terminates the Wajid Sandstone Group in the Bani Ruhayyah section, and which is erosionally overlain by the Khuff carbonate rocks.
LF-A9: Diamictic Siltstone and Shale Association (Juwayl Formation)
This is fundamentally a fine-grained sediment association (LF 1, LF 2, and LF 3.1). In addition, and most important for the environmental interpretation, is the presence of LF 13.2.3 (polymictic conglomerates in a clay or silt matrix). This succession is tens of metres thick (Figure 16b) and shows large-scale soft-sediment deformation structures (folding, thrusting, dragging, water escape; Figures 15c, 15e).
The varve deposits of LF 2 indicate that this facies association was deposited in a glacial lake. The fine-grained material was provided by subglacial melt waters draining into the lake, where fractionation created buoyant plumes of suspended sediment. Settling out of suspension is then reflected in the mm-thick laminae of light and dark particles usually attributed to seasonal variations of supply in sediment and organic matter (Einsele, 2000). Boulder pavements or pebble carpets and outsized boulders record the presence of icebergs. Drift of icebergs across the lake is reflected in deformation structures caused by impact of the keels onto the lake bottom.
The basal succession of the Dibsiyah Formation is characterised by 2-D trough cross-bedded conglomeratic sandstones and sandstones (LF-A1; Figure 7). The coarse material was delivered through a braided-river/braid-plain system. The sedimentary structures with 3-D dunes and abundant conglomeratic layers testify to very-high-energy conditions. The presence of Skolithos sp. and Cruziana in several localities indicates that all these sediments were deposited under marine conditions. This succession represents a high-energy, near-shore marine environment with tidal channels. Near-shore deposits and their transition into a hypothetical braid plain at the lower end of the braided river system must have been located cratonward farther south and west.
Up-section (Figure 7), LF-A2 with its increasing amount of bioturbation of the Skolithos-type indicates that the environment became even more favourable for early Palaeozoic marine life. The increasing 2-D dune structures and the decrease of conglomerates and pebbles in the succession indicate that the sediments were deposited in slightly deeper and less energetic environments. This is interpreted to reflect a relative sea-level rise and increasing distances to the braided-river/braid-plain supply system. This relative sea-level rise and the concomitant migration of the shoreline are attributed to the global overall sea-level rise during the Cambrian and Early Ordovician (Ross and Ross, 1988, 1995; Miller et al., 2005).
The third unit is an alternation of tabular cross-bedded sandstones and massive “Tigillites” (LF-A3; Figure 7). Almost all bedding contacts reflect discontinuous sedimentation and erosion. The third unit was deposited under starved conditions in accommodation-controlled regime with episodic sediment accumulation and redistribution. A prospering fauna took advantage of these conditions. At this time, sea-level rise continued and the concomitant retrogradation further diminished sediment supply.
Probably the most important feature of the entire succession is the absence of small-scale cycles or parasequences. Only in the lowermost part (LF-A1; Figure 7) are a few successions present, in which a fining-upward or a thinning-upward pattern has been observed. Hence, there are no small-scale stacking patterns that would allow the recognition of progradation, aggradation, or retrogradation of the system.
Together, however, the three lithofacies associations of the Dibsiyah Formation (Figure 7) represent parts of a vast retrogradational sand-sheet complex (Stride et al., 1982). Nearest to the shore, large compound dunes form the core of the complex. In the Dibsiyah Formation, this core is represented by LF-A1 and LF-A2. Farther offshore, smaller dunes follow, in the Dibsiyah Formation LF-A2 and LF-A3. The zone of small ripples with increasing fine-grained material still farther offshore is not preserved in the Wajid Group.
The sedimentary succession of the Dibsiyah Formation mirrors one major retrogradational event from the core of a marine sand-sheet complex at the base towards its outer parts in the upper part of the succession.
The basal unit (LF-A4a; Figure 8) represents the proximal proglacial conglomeratic channel fill (Figure 14e) of the initial glacier advance-retreat cycle. The entire succession of LF-A4a with its numerous erosional surfaces testifies to small-scale, probably seasonal changes of sediment supply, deposition and erosion. Sediments probably were deposited partly in a subaqueous, or partly in a subaerial setting. The former is indicated by depositional lobes dipping down-valley, and probably filling standing-water bodies in the valleys during the glacial retreat. The latter corresponds to sandur flats and the massive sandstones of the jökulhlaups.
Overlying this basal package is a succession of massive sandstones (LF-A5; Figure 8), which were deposited across an erosional surface with abundant striations. These striations were formed during a readvance of the ice sheet. The sediment is typical of sandur flats in front of glaciers or ice sheets. As this LF was often deposited in large-scale clinoforms, which fill the higher parts of the valleys, they are interpreted as Gilbert-type deltas that prograded from sandur flats into drowned valleys during glacial recession. Praeg (2003) described similar processes and architectures from Pleistocene tunnel valleys.
The third glacial advance failed to carve a major surface. Instead, following the basal coarser-grained sediments, the remainder of the succession (LF-A6; Figure 8) is characteristic of large sediment supply and concomitant increase of accommodation. The fining up of the sediments and the increasing distance of the depositional environment to the sediment source probably reflects the last pulses of the waning of the ice sheet. The increase in accommodation corresponds to the concomitant rise of the base level at the end of the Hirnantian glaciation.
The outcrops of the Qalibah Formation (Figures 2, 15h, 16a) do not permit a detailed discussion of its facies architecture. Their marginal-marine character leaves doubt whether they represent the Qusaiba Member, which is part of the widespread Lower Silurian transgression recognised over much of Arabia and North Africa (Lüning et al., 2000, 2005) or whether they correspond to the Sharawra Member, a more coarse-grained succession deposited during the ensuing transgression.
Previous authors (Dabbagh and Rogers, 1983; Evans et al., 1991; Stump and van der Eem, 1995a) proposed a fluvial to aeolian depositional environment for the Khusayyayn Formation. This is most unlikely given the very uniform distribution of mega-scale and giant cross beds (Figure 16c), and the grain size and sorting, which would be very atypical for an aeolian system. In addition, no other lithology or architectural element of a braided river (or a meandering river) system has been identified. Especially the absence of 3-D trough cross beds and fluvial channel stacks is hardly compatible with a fluvial interpretation.
Skolithos burrows are good marine indicators; hence it is likely that all the bedforms of the Khusayyayn Formation are of marine origin. Individual beds are traceable over hundred metres in the outcrop and so are bedsets. Giant dunes near Hima are traceable over several hundreds of metres, and the few lithofacies types described from the formation are observed in all outcrops from Najran to the northern study area (Figure 9). Hence, a high-energy, shallow-marine depositional environment similar to that of the Dibsiyah Formation is most likely for the Khusayyayn Formation. The tremendous amount of medium and coarse sand distributed over much of the outcrop belt, but also recorded from the subsurface (GTZ/DCo, 2009), formed a vast sand-sheet complex. This complex required a well-developed distribution system, probably in the form of braided-river/braid-plain systems. No traces of this system are preserved in the Khusayyayn Formation, but it must have been active to the west and south of the Wajid outcrop belt.
The outcrops near Abha and Khamis Mushayit have been partly attributed to the Khusayyayn Formation by Babalola (1999), which from their lithology and sedimentology seems very reasonable. Farther east towards Najran, Dabbagh and Rogers (1983) described similar deposits. All authors presented detailed cross-bed measurements, which uniformly indicate an overall southern provenance of the deposits. Although it cannot conclusively be proven, there seem to be more vestiges of 3-D dunes in the south than farther north supporting the model of a southern provenance of the sediments This observation was also made by Dabbagh and Rogers (1983). The core of the sand sheet, dominated by 3-D structures, was in the south, and the outer parts with dominance of 2-D structures are present farther north.
Although no continuous succession is present, the reconstruction of the approximate palaeogeographic positions of the sections measured permits a vertical reconstruction of facies shifts. At Jabal Khusayyayn (Figures 2, 3 and 9), a package of macro-scale and giant bedforms successively passes into meso-scale and a few micro-scale bedforms. Overlying is a succession with well-developed herringbone cross-stratification in meso-scale beds. Opposing currents of similar intensity produced these structures and the loss of accommodation led to thinner beds and bedsets. This succession within LF-A2 is interpreted to record a relative fall of sea level from subtidal into intertidal environments.
Sections representing a stratigraphic higher interval with the contact to the Juwayl Formation (Jabal Abood; Figures 2, 3 and 9) record the return to deeper conditions and the development of meso-scale and macro-scale bedforms.
Characteristic for the upper part of the Khusayyayn Formation are meso-scale and macro-scale bedforms with abundant slumping. Repeatedly, dewatering and concomitant deformation led to overturned folds several metres “thick” (see also Dirner, 2007).
Mainly fine-grained siliciclastic sediments have been preserved in the Bani Khatmah area (Figure 2). From these deposits, a glacial lake can be reconstructed and together with the observations by Kruck and Thiele (1983) and Pollastro (2003) they permit the reconstruction of a vast glacial lake over much of southern Arabia. Similar deposits are known from Ethiopia (Bussert and Schrank, 2007) and from other areas of adjacent Gondwana fragments (Torsvik and Cocks, 2011). Whether these individual lake deposits each represent an independent depositional basin (Torsvik and Cocks, 2011) or whether they were interconnected to form a single giant glacial lake (Pollastro, 2003; Keller et al., 2011) remains open for discussion.
In the northern area, the basal deposits of the Juwayl Formation fill two major glacially cut channels or valleys. Whether they represent tunnel valleys is not clear. Sediments belong to LF-A5 (massive to 3-D cross-bedded sandstones) and mostly represent the “Sorbet” facies (Figure 14a). Locally, they form packages more than 30 m thick. Near the base of this unit and locally covering the flanks of the valleys, are varve deposits (LF 2) either as huge blocks in the Sorbet facies or as strongly deformed relicted beds above undeformed Khusayyayn strata. Apparently, the glacial lake that mainly extended to the south into Yemen and Oman was present also in this area and covered the post-Khusayyayn erosional surface. At Jabal Blehan (Figures 2, 3 and 10), the initial lake sediments (varves of LF-A9) were later reworked by the mass flows of the “Sorbet” facies (LF-A5). The glacial lake was filled up and driven back during glacier advance. A vast fluvio-glacial plain with the corresponding sediments developed. Maximum glacial advance probably coincides with the transition from LF-A5 to the overlying LF-A6 with siltstones and fine-grained sandstones (unit J3 of Keller et al., 2011). The latter represents a more ice-distal succession deposited during the retreat of the glacier or the ice sheet.
A somewhat different succession is present at Jabal Sa’eb (Figures 2, 3 and 10), where glacio-fluvial conglomerates are present (LF-A4b), which is overlain by shales and siltstones (LF-A8). Within the conglomerates of LF-A4b, granitic boulders and other basement components are present indicating a source area on the Arabian Shield to the west. The overlying fine-grained siliciclastic deposits (unit J3 of Keller et al., 2011) reflect the rise of the lake level and its expansion towards the north. Similar to Jabal Blehan, the lower unit represents deposits of a glacier advance, whereas the overlying fine-grained siliciclastic rocks mirror a glacial retreat.
If the individual outcrop areas of the Juwayl Formation are considered, it seems that in the north depositional environments are preserved that were closer to the ice or glacier margin. Sediments of LF-A4b or LF-A5 are absent in the south, where mainly distal glacio-fluvial and lake sediments are preserved. A lithologic correlation between the areas could not be established. However, the transition from glacier advance (cold phase) to glacier retreat (warm phase) postulated in the northern sections might be reflected in the southern sections near Bani Khatmah. There, the lower part of the succession shows dominantly bedded sandstones and siltstones with few dropstones and little or absent deformation. Above a marked surface with a well-developed boulder pavement, dropstones increase suddenly and this part of the succession was affected by grounding icebergs (Keller et al., 2011).
If this interpretation is correct, then the upper part of the northern sections and their southern correlatives reflect one retreat of the Gondwana ice sheet and the expansion of the glacial lake at the expense of more ice-proximal environments.
STANDARD COMPOSITE WAJID LITHOLOG
In this paper, a refined stratigraphic log (Figure 18) for the Wajid Group is presented. This log is composed of individual sections that all are described in this paper (Figures 7 to 10) and that are combined in a composite log. Given the presence of tunnel valleys in the glacial deposits, combining sections from known palaeogeographic positions (Figure 17) leads to a more realistic approach to distribution and thicknesses of the individual formations and of the entire Wajid Group and to its stratigraphic architecture.
The new master log (Figure 18) starts with the sediments of the Dibsiyah Formation, in which three lithofacies associations have been recognised (LF-A1 to LF-A3). The standard section for this part of the Wajid Group at Jabal Dibsiyah (Figure 7) is ca. 180 m. The lower part of the section with LF-A1 varies between 25 m and 50 m thickness in the different outcrops (Figure 7). For LF-A2, thicknesses between 40 m and 50 m have been measured. The upper unit (LF-A3) has a minimum thickness of 90 m as measured at Jabal Dibsiyah, but it might have been considerably more.
Above the glacially induced unconformity at the base of the Sanamah Formation, LF-A4 represents the basal sediments of the valley fills (Figure 8). The reference section was chosen at Jabal Sanamah, where the most complete succession of Sanamah sediments is preserved. The basal channel fill is approximately 25 m thick (Figure 8). It is erosionally overlain by massive to cross-bedded sandstones of LF-A5. At Jabal Masqarah, 70 m of this facies association are preserved beneath the Khusayyayn unconformity (Figure 8). Yet another unconformity separates this succession from the overlying rocks of LF-A6. From Jabal Sanamah, a minimum thickness of approximately 130 m has been taken for the master log (Figure 18).
The most important information for hydrogeology and the characterisation of reservoir properties in the outcrop belt is that the Sanamah Formation has only been observed within the tunnel valleys (Figure 17). If the Sanamah Formation ever was deposited outside the valleys, i.e. on the valley shoulders and hence on top of the Dibsiyah Formation, it must have been erosionally removed prior to the deposition of the Khusayyayn Formation. Consequently, as the Dibsiyah is up to 190 m thick and as the Sanamah Formation is only present within valleys in the Dibsiyah Formation and does not overtop it, the thickness of the package Dibsiyah-Sanamah is also only about 190–200 m.
In the outcrop belt, only some 6 m to 10 m of the Qalibah Formation are preserved and shown in the master log (Figure 16). No section has been measured here, but the outcrops at Hima are chosen for the Wajid master log (LF-A7). There, it onlaps the basement and in turn is overlain unconformably by the Khusayyayn Formation.
In the composite log, the Khusayyayn Formation is mainly represented by the type section, where some 55 m of a sand-sheet complex crop out (LF-A2; Figure 9) attributed to the lower part of the succession. The unconformity with the Juwayl Formation is exposed at Jabal Abood (Figures 2, 3 and 9). 50 m of strongly deformed sediments of the upper Khussayyayn Formation are unconformably overlain by massive sandstones (“Sorbet facies”, LF 12; LF-A5) of the Juwayl Formation. These 50 m have been incorporated into the standard composite log. In Figure 9, two possible correlations of this section to the type section at Jabal Khusayyayn are shown. If alternative “A” is correct then the entire thickness of the Wajid Group is some 20 m less than given in the master log.
At Jabal Khurb al-Ahmar (Figures 2, 3 and 10), the type section of the “Sorbet facies”, ca. 50 m of these facies are exposed. Unfortunately, neither the lower nor the upper contact is exposed. Hence, only a minimum thickness of 50 m has been incorporated into the master log, which continues with the strata exposed at Jabal Blehan (Figures 2, 3 and 10), where Sorbet facies (LF-A5) pass into LF-A6 with siltstones and fine sandstones (ca. 30 m).
A similar succession, although represented through LF-A4b and LF-A8, is present at Jabal Sa’eb (Figures 2, 10 and 16d). The succession of events between the deposition of these beds (ca. 50 m) and the overlying Khuff Formation remains speculative.
The Wajid Group of southern Saudi Arabia can be divided into five formations: Dibsiyah Formation, Sanamah Formation, Qalibah Formation, Khusayyayn Formation and Juwayl Formation. Thirteen lithofacies have been distinguished (LF 1 through LF 13), which cover the entire spectrum of siliciclastic grain-size classifications. Shales and siltstones are relatively rare in the succession, whereas sandstones, especially medium-grained to coarse-grained sandstones abound. Conglomerates are locally abundant in the Sanamah Formation and in the Juwayl Formation. These 13 lithofacies have been combined in 9 lithofacies associations (LF-A1 through LF-A9).
LF-A1 through LF-A3 describe large-scale submarine sand-sheet complexes. These complexes are present in the Dibsiyah Formation and the Khusayyayn Formation. The sedimentologic and ichnofaunal characteristics of both units indicate marine depositional environments throughout. A fluvial environment for the basal Dibsiyah Formation and the Khusayyayn Formation as interpreted by Evans et al. (1991) and Stump and van der Eem (1995a, b) is unlikely.
LF-A4 through LF-A6 are the product of glacial environments. In the Sanamah Formation, this was the Hirnantian glaciation, in the Juwayl Formation it was the Permian part of the Late Palaeozoic Gondwana glaciation. In combination, all three facies associations record proximal to distal glacial environments. The succession of lithofacies and facies associations permits the recognition of three glacier advance – retreat cycles in the Sanamah Formation and of two in the Juwayl Formation. Whereas in the Sanamah Formation only deposits of subglacial tunnel valleys and the adjacent proglacial environments are preserved, in the Juwayl Formation additionally remnants of a large glacial lake are preserved. LF-A8 and LF-A9 of the Juwayl Formation are dominantly fine-grained associations. A few conglomeratic layers testify to the proximity of glaciers or an ice sheet from where the coarse components were provided by fluvial processes or as fallout from icebergs. Together these two associations record a rise in water level during late stages of the glaciation, be it a large glacial lake or be it the regional deglacial transgression. This glacial lake was present over much of the southern Arabian Peninsula and might have extended into Africa.
The few metres of sediment preserved of the Qalibah Formation do not permit a definite assignment to a facies belt. However, fine-grained calcareous sediments with stromatolitic structures point to a restricted-marine environment. Whether these sediments belong to the Qusaiba Member or whether they belong to the Sharawra Member remains open to discussion.
The sedimentary processes, starved shallow-marine and terrestrial-glaciogenic, and the relationship between preserved and non-preserved time in the sediments indicate that throughout the Palaeozoic, southern Saudi Arabia was located in an epicratonal setting, in which tectonic subsidence and relative sea-level changes exerted only minor control.
The stratigraphic architecture and the spatial relations among the formations show that the Wajid Group is much thinner in the outcrop belt than hitherto assumed. A combined thickness of around 400 m has been documented; however it cannot be excluded that locally 500 m to 600 m have been preserved.
This paper is part of the PhD thesis of H.F. Al-Ajmi as part of the aquifer study of the Wajid Sandstone, carried out on behalf of the (Saudi Arabian) Ministry of Water and Electricity (MoWE) by GTZ/DCo (now GIZ/DCo) in cooperation with the Technical University of Darmstadt (Germany). We would like to thank the Ministry of Water and Electricity and its representatives, Abdullah Al-Husayen, Ali Al-Tukhais and Mohammad Al-Saud, for their unlimited support. We would like to thank R. Rausch and J. Döhler (GTZ/DCo) for the logistic support and their interest in the progress of the study. Thanks also to all personnel of GTZ/DCo, especially H. Dirks, N. Michelsen, J. Karpiel, D. Schönrok, A. Al Noor, who participated in field work and gave other support. Thanks also to H. Bock and A. Kallioras for their discussions in the field. Special thanks to our colleagues, especially C. Schüth, and the technicians at TU Darmstadt, for their help and stimulation and the lasting interest in the progress of the work. We are grateful to The National Agricultural Development Company (NADEC), which provided free accommodation for all personnel during fieldwork. Thanks go to J. Moreau and S. Lüning for their review of an earlier version of the manuscript and many suggestions to improve it. We would like to thank especially the two reviewers of GeoArabia, Owen Sutcliffe (Neftex) and Mike Hulver (Saudi Aramco), for their thoughtful reviews and many comments that helped to improve the manuscript. GeoArabia’s Assistant Editor Kathy Breining is thanked for proofreading the manuscript and Production Co-manager Arnold Egdane is thanked for designing the paper for press.
ABOUT THE AUTHORS
Hussain Fahad Al-Ajmi is Head of the Research Department and Advisor to the Deputy Minister of Water Affairs in the Ministry of Water and Electricity, Riyadh, Saudi Arabia. He holds a BSc in Geology from King Saud University in Riyadh, and his MSc from King Saud University was on sedimentology and stratigraphy of Palaeozoic rocks in the northern Kingdom. Hussain’s doctoral research focused on sedimentology, stratigraphy and reservoir quality of the Palaeozoic Wajid Sandstone in SW Saudi Arabia. He received his PhD from the Technical University of Darmstadt, Germany. Hussain’s present activity is to establish a group of scientists in the ministry to develop new projects in groundwater research and management.
Martin Keller is extracurricular Professor of Geology at Erlangen University and Director of Water Affairs at Babeltower Engineering Consultants, Al Khobar, Kingdom of Saudi Arabia. His Diploma thesis (1983) and PhD thesis (1987) were on interactions between sedimentation and tectonics in the Lower Devonian of northern Spain working at the Technische Universität Darmstadt, Germany. The interactions between basin-scale sedimentation, expressed in the sequence-stratigraphic architecture of the basin fill, and the driving (plate-) tectonic processes subsequently became the main focus of his research after joining Erlangen University. He was involved in major research projects on the sedimentary, structural and plate tectonic evolution of the Argentine Precordillera and its presumably Laurentian provenance; the evolution of the southwestern margin of Laurentia in Utah, Nevada and California during Neoproterozoic and early Paleozoic time; and the interactions between the onset of the Variscan Orogenesis in northern Spain and sequence-stratigraphic architecture of the passive-margin and foreland-basin fills. In 2008, he was invited to supervise fieldwork for Hussain Al-Ajmi’s doctoral thesis on the Wajid aquifer and subsequently joined GIZ/DCo as Chief Geologist and later managed the Rub’ al-Khali water resources project. He was responsible for the geology and basin analysis of the Wajid aquifer, the Biyadh-Wasia-Aruma aquifer and the aquifers in the Rub’ al-Khali. Since 2013 he is responsible for the water projects of Babeltower Engineering Consultants. Martin is member of several international and national societies.
Matthias Hinderer is a Full Professor of Applied Sedimentary Geology at the Technische Universität Darmstadt, Germany. He holds a Diploma in Geology and Paleontology of the University of Tübingen, Germany. His PhD was dedicated to the hydrogeology of Triassic sandstone aquifers in the Black Forest. During this time Matthias started to characterize sandstone aquifers in terms of their reservoir properties and he learned to quantify chemical fluxes. The quantification of sediment and chemical fluxes from local to global scales became his major subject of research during his postdoc career. In 2001, he was appointed as Professor in Darmstadt and started several projects in clastic sedimentary geology with specific focus on glacial and fluvial systems. These studies included also various outcrop analogue and reservoir studies. In 2007 he was invited by GIZ/Dornier Consulting and the Ministry of Water and Electricity to work on the sedimentology of regional aquifers in the Kingdom of Saudi Arabia. In this frame, he supervised the PhD thesis of Hussain Al-Ajmi dealing with the sedimentary facies of the Wajid Sandstone. A major task was the study of Palaeozoic glacial deposits which Matthias could compare with their younger counterparts in the Pleistocene forelands around the Alps, where he worked for more than 10 years. Matthias is a member of SEPM, AAPG, GSA, and national geological societies. Since 2009, he is speaker of the central European Section of SEPM.
Claudio Miro Filomena is a Production Geologist at Shell Global Solutions International in Rijswijk, The Netherlands. He holds a Diploma in Geology from the University of Tübingen, Germany. In 2008, he completed his diploma thesis on the sedimentology of the Lower Wajid Group, contributing to the Wajid aquifer study of GIZ/Dornier Consulting and the Ministry of Water and Electricity of the Kingdom of Saudi Arabia. During his PhD and post-Doc research at the GeoZentrum Nordbayern, Erlangen, Germany, he applied ultrasonic measurements to sandstones in various outcrop and laboratory studies, comprising high-resolution characterization of Permian-Triassic sandstone reservoirs and the evaluation of natural building stone quality. Claudio is particularly interested in research projects combining the fields of sedimentology and petrophysics. He is a member of AAPG, IAS, SEPM, and national geological societies in Germany and The Netherlands.