ABSTRACT

Detailed analysis of over 1,000 subsurface Silurian palynology samples from 34 wells has allowed the development of a robust biostratigraphy based on acritarchs, chitinozoans and cryptospores for the Qusaiba Member of the Qalibah Formation, central Saudi Arabia. The new index fossils described herein augment the Arabian Plate Silurian chitinozoan zonation. The high-resolution biostratigraphic zonation consists of nine First Downhole Occurrences (FDOs) from the lower Telychian through Aeronian. In particular, three regionally recognizable palynologic horizons were identified within the lower part of the informally designated Mid-Qusaiba Sandstone (Angochitina hemeri Interval Zone), and above the FDO of Sphaerochitina solutidina. This high level of biostratigraphic resolution provides a framework for the integration of the sedimentology and calibration with global sea level curves, leading to a detailed understanding of the sequence stratigraphic evolution of this part of the Silurian in Saudi Arabia.

Sedimentological core studies identify three Depositional Facies Associations (DFAs) within the Mid-Qusaiba Sandstone interval, including: (1) shelfal deposits (DFA-I) characterized by interbedded hummocky cross-stratified sandstones, graded siltstones and bioturbated mudstones; (2) turbiditic deposits (DFA-II); and (3) an association of heavily contorted and re-sedimented sandstones, siltstones and mudstones (DFA-III) that is considered representative of oversteepened slopes upon the Qusaiba shelf.

Integration of the newly recognized palynostratigraphic horizons and the sedimentological data facilitates an understanding of the sequence stratigraphic evolution of the Mid-Qusaiba Sandstone interval and its immediate precursors. Thus a Maximum Flooding Surface (MFS) is identified from significant palynostratigraphic, as well as sedimentological evidence, and concurs with the MFS identified regionally with the Monograptus convolutus Graptolite Zone. Several mud-prone cyclothems downlap onto the MFS. Each of these is identified by its own palynostratigraphic marker: these mud-prone cyclothems represent the distal parts of a Highstand Systems Tract (HST).

The end of the HST is marked by evidence of a major, episodic drop in relative sea level. Thus, a relationship is identified wherein successive palynostratigraphic marker horizons, newly identified in this study, are partially eroded by the introduction of sandy turbidites (DFA-II). These turbidites arise from storm-induced erosion of gully complexes in the upper submarine slopes that are present as topography upon the Qusaiba shelf. Each of the successive drops in sea level is separated from the next by a minor, subsequent sea level rise, which precludes further submarine erosion and turbidite deposition, and is instead evident in the widespread occurrence of shallow marine (shelfal) muds and sandy tempestites (DFA-I). The lowstand per se is considered to be represented by the most widespread distribution of the DFA-II turbidite deposits, and is associated with the youngest Mid-Qusaiba Sandstone marker horizon identified in this study, namely Rugosphaera agglomerata n.sp. The youngest unit of DFA-II lowstand turbidites is limited in its occurrence to the more proximal parts of the study area, and thus is considered to represent the onset of the succeeding Transgressive Systems Tract (TST).

Of the biostratigraphic indices used for correlation within the Qusaiba Member, Rugosphaera agglomerata and Eupoikilofusa curvata are formally described and two additional important species, Fractoricoronula n.sp. and ?Oppilatala n.sp., are retained in open nomenclature.

INTRODUCTION

An integrated palynological and sedimentological study of the Qusaiba Member of the Qalibah Formation from the subsurface of central Saudi Arabia (Figure 1) was initiated to place the core-based sedimentology in a time stratigraphic framework for sequence stratigraphic interpretations. To accomplish this, nearly continuous sampling of cuttings and core were made from the 34 well penetrations of the Qusaiba Member that were selected for palynological analysis (Figure 1). In the study area, as elsewhere in Saudi Arabia, the Qalibah Formation is bounded above and below by regional unconformities. The Tawil Formation, which ranges from Upper Silurian to Lower Devonian, overlies the Lower Silurian Qalibah, which in turn disconformably overlies the Late Ordovician Sarah Formation in parts of Saudi Arabia. The amount of time represented by this unconformity varies laterally, becoming greater over structural highs (Mahmoud et al., 1992; Aoudeh and Al-Hajri, 1995). The Qalibah Formation is subdivided into a lower more mud prone Qusaiba Member and an upper, coarser clastic Sharawra Member (Mahmoud et al., 1992). The basal part of the Qusaiba contains the diachronous “hot shale” facies that is used to identify the base of the Silurian and is, in many areas, an economically significant hydrocarbon source rock for younger Arabian Plate siliciclastic and carbonate reservoirs (Al-Husseini, 1992; Mahmoud et al., 1992; McGillivray and Husseini, 1992; Jones and Stump, 1999; Lüning et al., 2000). The Ordovician through Devonian lithostratigraphy for the Ghawar field area along with major regional unconformities is shown on Figure 2.

The earlier nomenclatural history of the Silurian System in Saudi Arabia was extensively reviewed by Mahmoud et al. (1992). Janjou et al. (1996) raised the Qalibah Formation to group status and elevated the Qusaiba and Sharawra members to formations based on their mapping of the Al-Qalibah Quadrangle in the northwest of Saudi Arabia. The new nomenclature was also used in the mapping of the adjacent Tabuk Quadrangle (Janjou et al., 1997). This revision has not gained widespread acceptance in the literature (e.g. Sharland et al., 2001). In evaluating the Lower Silurian stratigraphy of central Saudi Arabia, it is apparent that the relationship between the northern and central Silurian succession is not sufficiently well-understood to accept the northern outcrop-based nomenclature in central Saudi Arabia without a detailed regional restudy of the Sharawra Member. Inconsistency in the use of Sharawra in the recent past and the current understanding of the upper contact of the Qusaiba Member indicates that this boundary is not well defined. For example the subsurface reference section for the Sharawra Member as defined by Mahmoud et al. (1992) incorporates the same stratigraphic interval as does the Mid-Qusaiba Sand of Wender et al. (1998). Current stratigraphic practice places the Qusaiba/Sharawra contact well above the Mid-Qusaiba sand (Al-Ruwaili and Miller, 2002; Nicholson et al., 2002). A sequence stratigraphic study of the Qalibah, particularly examining the relationship between the Qusaiba and Sharawra, is needed to evaluate the distribution of the respective shaley and sand-prone facies. Evaluation of these stratigraphic issues is beyond the scope of this paper, which deals only with the Mid-Qusaiba Sandstone and associated deposits in central Saudi Arabia.

The Qalibah Formation is included within the lower part of Megasequence AP3 of Sharland et al. (2001). Their Arabian Plate sequence stratigraphic framework provided the basis for the current work on the Qusaiba Member and the stratigraphic refinements presented below. The Qusaiba was previously implied to represent uninterrupted sedimentation (Mahmoud et al., 1992; Sharland et al., 2001), but Miller and Melvin (2002), and this study, show at that least one unconformity having sequence stratigraphic significance is present within the middle to upper part of the Qusaiba.

For the purposes of this paper our work focused initially on improving the biostratigraphic resolution of the Aeronian part of the Qusaiba Member, which contains a distinctive interval that is commonly between 100 and 200 ft thick, and is significantly more sand-prone than the thick deposits of mudstone that occur immediately below and above it. This sand-prone interval within the Qusaiba has been referred to as the ‘Mid-Qusaiba Sand’ by Wender et al. (1998). Those authors very briefly referred to the unit as comprising a “…crudely-thickening and coarsening-upward sequence and most probably represents a progradational basin floor fan system.” That interpretation was not however substantiated by material data (Wender et al., 1998). In the Ghawar field area and areas to its west and south, the Mid-Qusaiba Sandstone has been penetrated in the subsurface by some 25 wells, and in several it has been cored (Figure 1), although nowhere in its entirety.

The Qusaiba Member palynostratigraphy used here, is based on a succession of papers originating from the 1960s. The earliest illustrations of Qusaiba palynomorphs were from northern Saudi Arabia by Hemer (1965) and McClure (1988a). Subsequently, two cooperative projects involving Saudi Aramco and the Commission Internationale de Microflore du Paléozoïque (Owens et al., 1995; Al-Hajri and Owens, 2000) resulted in significantly improved taxonomic and biostratigraphic understanding of the Silurian succession in Arabia that greatly facilitated the present study. These two projects resulted in a number of papers pertinent to this study: Aoudeh and Al-Hajri (1995), Paris and Al-Hajri (1995), Paris et al. (1995) for chitinozoans; Le Hérissé et al. (1995), Le Hérissé (2000), Al-Ruwaili (2000) for acritarchs, and Steemans et al. (2000) and Wellman et al. (2000) for cryptospores and spores.

Sampling, materials and the techniques used in this study are described in Appendix I (Materials and Methods). Important species and new Arabian Plate Llandovery zonal fossils are described in Appendix II (Systematic Palynology).

BIOSTRATIGRAPHY

The Qusaiba Member of the Qalibah Formation in east central Saudi Arabia ranges in age from Rhuddanian through the late Aeronian/early Telychian stages of the Llandovery Series (Figure 2). The majority of samples used for this study were from cuttings thus the biostratratigraphic events are based on first downhole occurrences (e.g. Figure 3). The biostratigraphic framework used for this study is based on that of Aoudeh and Al-Hajri (1995). They refined the interpreted ages for the upper part of the chitinozoan zonation proposed by Paris and Al-Hajri (1995) and Paris et al. (1995) by incorporating then-new graptolite age control to calibrate local chitinozoan zones with the international graptolite zonation (Aoudeh and Al-Hajri, 1995, their figure 7). Their placement of Angochitina macclurei with turriculatus to sedgwicki zone graptolites provided a critical tie between the two zonations.

In addition to the graptolite age control, the acritarch Beromia rexroadii was discovered from the A. macclurei Zone in Wells 1 and 12. In a core from the upper part of the Qusaiba Member in Well 12, the B. rexroadii First Downhole Occurrence (FDO) occurs 30 ft below that of A. macclurei. This acritarch, which is common in the Lower Sodus Shale, Clinton Group, New York and Lulbegrud Shale Member, Noland Formation, Crab Orchard Group, Kentucky, was originally thought to occur in pre-celloni Zone (upper Telychian) strata (Wood, 1996). However the age of these units is now believed to be in the Aeronian-Telychian boundary interval (Brett et al., 1998; Brett et al., 1990). Futhermore, Beromia rexroadii has been reported from the late Aeronian/early Telychian Tianguá Formation of northern Brazil (Le Hérissé et al., 2001) and in sediments of similar age from the Belgian subsurface (Wauthoz, 1997). Although uncommon in the Qusaiba Member, its presence has provided supporting external age control. The effect of both the graptolite and acritarch evidence is that the top of the A. macclurei Interval Range Zone (sensuParis et al., 1995) is lowered from the upper part of the Telychian to a level in the vicinity of the Telychian/Aeronian boundary [compare Aoudeh and Al-Hajri (1995, their figure 7), to Paris and Al-Hajri (1995, their figure 3) and Paris et al. (1995, their figure 2)].

A. macclurei and Fractoricoronula n.sp. are the youngest Qusaiba Member zonal fossils in the study area (n.b. Fractoricoronula n.sp. and A. macclurei have coincident FDOs in core from the upper part of Qusaiba Member in Well 12). In northern Saudi Arabia younger Telychian chitinozoan zones are recognized in the Qusaiba (Aoudeh and Al-Hajri, 1995). The two middle-upper Telychian (possibly earliest Wenlock) chitinozoan zones, in ascending stratigraphic order, are the Plectochitina longicornis Total Range Zone and the Bursachitina sp. A Interval Range Zone (Al-Hajri and Paris, 1998; Bursachitina sp. A = Eisenackitina lagenomorpha sensuAoudeh and Al-Hajri, 1995). The range of A. macclurei overlaps with that of P. longicornis (Al-Hajri, 1994, unpublished data). Because the top of the A. macclurei Zone is defined by the appearance of P. longicornis (Aoudeh and Al-Hajri, 1995), it is not possible to recognize A. macclurei Zone in cuttings samples; however, in our study area strata containing P. longicornis have not been recognized and the youngest sediments of the Qusaiba Member contain A. macclurei and Fractoricoronula n.sp. In the Qusaiba Fractoricoronula n.sp. has been identified as Veryhachium cf. checkleyensis and was placed in Le Hérissé et al.’s (1995) Zone 6. Since the age of A. macclurei has been revised, a like revision for Zone 6 is considered necessary.

With the age refinements detailed above, A. hemeri becomes restricted to the Aeronian. Between the FDOs of A. hemeri and S. solutidina, we have added three palynomorph datums, of which two are defined by new species (Figure 3). These are, in descending stratigraphic order, Rugosphaera agglomerata n.sp., Tylotopalla caelamenicutis (persistent) and ?Oppilatala n.sp., which occur regionally in the study area. They are typically associated with muddier intervals within the Mid-Qusaiba Sandstone (Figures 5, 13, 14, and 15) and all occur stratigraphically above the FDO of S. solutidina.

Belonechitina arabiensis was described by Paris and Al-Hajri (1995) and shown on the range chart of Paris et al. (1995) as having a Last Appearance Datum in the A. hemeri Zone. Its stratigraphic significance was noted but it was not included as a zonal index. The FDO of B. arabiensis occurs at or very near the same stratigraphic level as R. agglomerata n.sp. (Figure 3). These two species are nearly interchangeable in their relative position within the zonation. Where present together, they occur either in the same sample or in immediately adjacent samples. The species composition of Mid-Qusaiba Sandstone palynology samples from core between the FDOs of R. agglomerata n.sp. and T. caeleminicutis are shown in the R. agglomerata column in Table 1. These assemblages are numerically dominated by cryptospores and lack indicators of deeper water settings (see Paleoecology of the Mid-Qusaiba Sandstone).

In many central Arabian wells the persistent occurrence of Tylotopalla caelamenicutis generally is within 30 ft or less below the R. agglomerata n.sp. FDO. However, rare, isolated occurrences of T. caelamenicutis have been documented to be present with A. macclurei. The local FDO of T. caelamenicutis (persistent) in Saudi Arabia occurs within the Aeronian (A. hemeri Zone) whereas in Europe T. caelamenicutis has been recorded from early Wenlock sediments (Le Hérissé, 1989). Le Hérissé et al. (1995) included T. caelamenicutis in their zones 4 and 5. The age of Zone 5 was shown to span the Aeronian/Telychian boundary. Based on our work, and the revisions discussed above, the age of this zone should be Aeronian. Palynomorph assemblages that occur in Wells 5 and 6 with T. caelamenicutis (persistent) are given in Table 1. Although these assemblages are similar to those from the R. agglomerata interval, they show a decrease in the diversity of terrestrial components.

In cuttings samples, the FDO of ?Oppilatala n.sp. may occur in the same sample with Sphaerochitina solutidina. However from occurrences in core it becomes clear that ?Oppilatala n.sp. is a slightly younger datum. The assemblages from the ?Oppilatala n.sp. interval have generally more significant marine indicators than those of the superjacent assemblages. It is widely distributed in the southern part of the study area but is removed by erosion toward the north (Figure 15). Palynomorphs present in the ?Oppilatala n.sp. interval of Well 5 are given in Table 1.

In their revised zonation Aoudeh and Al-Hajri (1995, their figure 7) did not include the Angochitina hemeri/Sphaerochitina solutidina Concurrent Range Zone of Paris et al. (1995). The FDO of Sphaerochitina solutidina is a significant and regionally correlative event below the Mid-Qusaiba Sandstone in the study area and we have reinstated it in the zonation used here (Figures 3 and 15). The FDO of S. solutidina follows T. caelamenicutis generally within 60–90 ft, depending on the thickness of intervening sandstones. Combined, these three biostratigraphic events (namely, R. agglomerata n. sp., T. caelamenicutis and ?Oppilatala n. sp.) permit high resolution correlations within the Mid-Qusaiba Sandstone interval nearly at the resolution of 30 ft composite cuttings samples.

Within the Aeronian portion of the Qusaiba Member, the next biostratigraphic datum recognized is the Conochitina alargada/Plectochitina paraguayensis Concurrent Range Zone of Paris et al. (1995). This zone has some graptolite control to constrain its age. Aoudeh and Al-Hajri (1995, p. 149, their figure 7) reported that graptolites of convolutus Zone occur with C. alargada and P. paraguayensis. They also considered that the C. alargada/P. paraguayensis Zone may questionably be extended into the older leptotheca to triangulatus graptolite zones. The newly recognized FDO of Eupoikilofusa curvata n.sp. occurs within the C. alargada/P. paraguayensis Zone (Figure 3). It is a highly distinctive marker species (Plate 3) that occurs in the southern part of the study area, but does not occur in the northern part of Ghawar (Figures 14 and 15).

Qusaiba Member strata assigned to convolutus Zone are very widely distributed both from surface sections in northwestern Saudi Arabia (e.g. El-Khayal, 1985; McClure, 1988b; Aoudeh and Al-Hajri, 1995; see summary in Sharland et al., 2001, p. 153) and from the subsurface in central and southern Arabia (McClure, 1988b). Because of this widespread distribution, Sharland et al. (2001) recognized and defined Maximum Flooding Surface (MFS) S10 at this level. In Miller and Melvin (2004) and this study, we have identified a marked increase in taxonomic diversity that we use as a datum, and which we interpret to reflect this Maximum Flooding Surface (Figures 13, 14 and 15). Although no palynomorphs have extinctions at this level, there is an Occurrence (OCC) of n. sp. aff. Papulogabata at or just above the diversity increase. This form occurs sporadically at various levels throughout the Qusaiba, but in this specific association it has a very widespread geographical distribution.

The base of the Aeronian is defined by the FDO of Ancyrochitina udayanensis (Aoudeh and Al-Hajri, 1995). The Rhuddanian chitinozoan zones are well constrained in terms of graptolite zones (Aoudeh and Al-Hajri, 1995 and Paris et al., 1995).

SEA-LEVELS

Late Ordovician and Early Silurian eustatic sea-level fluctuations were the result of successive continental glaciations of the south polar Gondwanan land mass (Beuf et al., 1971; Grahn and Caputo, 1992; Caputo, 1998; Sutcliffe et al., 2001). The most significant of these, geologically and economically, is the transgression that began in latest Ordovician time and resulted in the widespread deposition of world-class hydrocarbon source rocks (Early Silurian ‘hot shale’ facies) throughout North Africa and the Arabian Plate (Lüning et al., 2000; Carr, 2002). In Saudi Arabia the base of the Qusaiba Member, which is a source rock in many places, is diachronous (Vaslet, 1989; Mahmoud et al., 1992; Aoudeh and Al-Hajri, 1995). Following the initial transgression, two additional Llandovery South American glaciations are known to markedly affect Silurian sea levels (Caputo, 1998; Grahn and Caputo, 1992). These later transgressions, although significant depositionally, have not yet been implicated in source rock formation in Saudi Arabia.

Sharland et al. (2001) recognized and defined MFS S10 in the Aeronian portion of the Qusaiba Member and other coeval Arabian Plate units. This flooding event is associated with the Monograptus convolutus Zone, which is a period of sea level high stand (Loydell, 1998). MFS S10 is the most widely recognizable event in our study area and is also present in the Tabuk area (Rickards and Koren, 1974) and Al-Qasim Province (El-Khayal, 1985). The parasequences defined as “cartographic” units by Janjou et al. (1996) from the Al Qalibah Quadrangle and used in mapping of the Tabuk Quadrangle (Janjou et al., 1997) also reflect fluctuations in Early Silurian sea levels. The graptolite-based biostratigraphy is not sufficiently robust to allow exact dating of the parasequences. The graptolite control in Janjou et al. (1996) implies the upper sandier part of “cartographic” unit 4 and base of unit 5 could possibly be assigned to M. convolutus Zone. Age control no younger than Coronograptus gregarious (s.l.) Zone was noted for the surface sections in the Tabuk Quadrangle and the Tayma Quadrangle (Vaslet et al., 1994). However, Aoudeh and Al-Hajri (1995) reported the occurrence of the Conochitina alargada/Plectochitina paraguayensis Concurrent Range Zone in the subsurface near Tabuk. This occurrence agrees with the graptolite evidence of Rickards and Koren (1974) for the presence of M. convolutus Zone in the area.

Following the post-S10 MFS Highstand deposits, but still within the Aeronian, is a sea-level drop (Figure 4) during which submarine erosion was initiated and deposition of the Mid-Qusaiba Sandstone occurred. This represents a sequence boundary. Lüning et al. (2000, their figure 9) show this as a major sea-level drop in the M. sedgwickii Zone on their world-wide Silurian sea-level chart.

The Aeronian low stand is followed by a transgressive system tract that contains the distinctive acritarch, Fractoricoronula n.sp. This helps characterize the A. macclurei Zone within this event (Al-Ruwaili and Miller, 2002) but the youngest transgressive event (Silurian Eustatic Sea Level Maximum of Lüning et al., 2000) does not appear to be present in our study area.

SEDIMENTOLOGY OF THE MID-QUSAIBA SANDSTONE

To understand the depositional setting of the Mid-Qusaiba Sandstone, over 420 ft of core from five wells in the Ghawar area (Figure 1) were examined and the sedimentology was described and interpreted in detail. This information was then assessed in the context of the new palynostratigraphy described in the preceding part of this study, with a view to better understanding the sequence stratigraphic evolution of this potentially economically important part of the Qusaiba Member.

Two significant Depositional Facies Associations (DFA-I and DFA-II) have been identified from cores of the Mid-Qusaiba Sandstone. Each one is characterized by its own distinguishing set of depositional facies, and is considered to represent a distinctive depositional environment. In addition, a specific Depositional Facies Association (DFA-III) was identified in Well 10 occurring immediately below the Mid-Qusaiba Sandstone: this is considered to be relevant to the overall interpretation of the Mid-Qusaiba Sandstone interval, and is therefore included in this discussion.

Depositional Facies Association I (DFA-I)

This facies association comprises three depositional facies that occur intimately interbedded with each other. It is best illustrated in core from Wells 8, 9 and 10 (Figure 5).

Facies Svfh: Thin-bedded, hummocky cross-stratified, very fine-grained sandstone

This facies is very common in the Mid-Qusaiba Sandstone. It comprises thin beds of very fine- to fine-grained, micaceous sandstone that range in thickness between 1 and 20 cm (Figure 6a). The beds have very sharp, irregular (scoured) bases that commonly show tool marks (prod marks and groove marks). Internally, they display pronounced lamination that is enhanced by the mica content of the rock: the laminae occur at a very low angle to the basal scoured surface, and are present throughout each bed in laminasets that intersect each other at low angles. Commonly in the higher levels of these beds the laminae display an antiformal, or hummocky aspect (Figure 6b). In many cases these sandstones are graded in the uppermost few centimeters (Figure 6b), with local bioturbation; elsewhere they have sharp upper bed contacts (Figure 6c). Locally, these beds exhibit small vertical burrows (Figure 6a), particularly in the very thin beds (1–2 cm). In one rare, but significant example from Well 8, a 20 cm thick bed of Facies Svfh displays throughout its entire thickness a vertical burrow form that contains well-developed spreite; these appear to be associated with an additional (?later) thin sandy burrow fill that alternates from one side of the burrow to the other, i.e. it appears to spiral its way from the bottom to the top of the burrow (Figure 6d).

The physical structures identified in Facies Svfh are characteristic of hummocky cross-stratification (Harms et al., 1975; Dott and Bourgeouis, 1982), and as such are interpreted as having formed under the effects of waning oscillatory flow by storm waves in a shallow-marine, shelfal setting, between fair weather wave base and storm wave base. Thus, these are ‘event’ beds, and this conclusion is substantiated by the ubiquitous sharp, scoured basal contacts. The character of the vertical burrow described above, and in particular the thin internal spiraled trace, is suggestive of a rapid escape attempt by its occupant, and further testifies to the rapid introduction of sediment under heavy storm conditions.

Facies Zg: Graded siltstone

This is common within DFA-I in the Mid-Qusaiba Sandstone. It consists of thin beds, 1–15 cm thick, of dark gray siltstone that have characteristic sharp irregular bases that in places suggest they may be fluted (Figure 7a). It is not uncommon for the lowermost 2–3 mm of these beds to consist of very fine-grained sandstone (vfL); from this basal unit the sediment grades steadily upwards through silt-sized material into dark gray mudstone of Facies Mb (qv).

These thin, graded siltstones occur in intimate association with the hummocky stratified sandstones of Facies Svfh, and from their sharp bases and graded bedding are similarly thought to be event beds deposited under waning storm conditions (tempestites) in a marine shelf setting.

Facies Mb: Mudstone (with bioturbation)

Facies Mb is a dark gray mudstone that is sub-fissile, with rare laminae of siltstone (< 1 mm thick). It very commonly displays a moderate degree of bioturbation as reflected in faint silty exichnial traces (Figure 7b). The grain size, and laminated and/or bioturbated nature of Facies Mb, as well as the character of the palynomorph assemblages it contains (see later discussion on paleoecology), suggest relatively tranquil, shallow marine (shelfal) conditions. These mudrocks are considered to represent the ‘normal’ or fair-weather conditions that prevailed upon the shelf in Mid-Qusaiba times.

From the foregoing, it can be seen that DFA-I is characteristic of a shallow marine, shelfal depositional setting, wherein ‘normal’, fair-weather deposition of dark gray mudstones (Facies Mb) was periodically interrupted by storm conditions that led to the storm-influenced sediments of Facies Svfh and Zg.Dott and Bourgeouis (1982) have discussed such depositional settings and their associated facies, in particular in relation to the nature of the hummocky cross-stratified sandstone facies. Thus, in certain situations (e.g. the Upper Cretaceous Cape Sebastian Sandstone of the Oregon coast) hummocky cross-stratified sandstone facies can occur in amalgamated units up to 75 m in thickness (cf. Bourgeouis, 1980). Such thick accumulations of hummocky cross-stratified sandstone are generally taken to be indicative of proximal, nearshore to shoreface environments. Where the hummocky stratified sandstones are much thinner, and are more commonly associated with mudstone interbeds, with a significant amount of bioturbation, the deposits are more reasonably assigned to a more distal setting, specifically on the mid- to outer-shelf, and generally above storm wave base (giving due consideration for sediment supply, storm frequency, storm intensity, etc.). Such a setting would be appropriate for the rocks described above from DFA-I in the Mid-Qusaiba Sandstone.

Depositional Facies Association II (DFA-II)

This Depositional Facies Association, as identified in core, consists of four depositional facies. However, in many cases it may comprise only one of those facies (namely Facies Stab), occurring as multi-storey (amalgamated) bedsets (Figure 8a). DFA-II is well represented in core from Wells 5, 6 and 10 (Figure 5).

Facies Stab: Medium-bedded, fine-grained, graded sandstones

This facies is very distinctive, and very different from any of the sandstone facies described above from DFA-I. It comprises medium beds (0.6 m to 1.8 m thick) of fine-grained (fL-fU) micaceous sandstone. Basal contacts are invariably sharp, and commonly loaded (Figure 8b) or erosional (in places with mudclasts) (Figures 8c, d). The lower part of some beds is ‘massive’ to only very faintly planar laminated; in places (e.g. Figure 5, Well 8, levels 30, 34), broken shell debris is observed dispersed throughout this massive unit. Generally, any basal massive sandstone passes upwards into sandstone that is slightly finer-grained, and displays pronounced planar lamination that is enhanced by an abundance of mica (Figure 8a). Locally, this laminated sandstone passes gradationally upwards into a thin horizon of ripple cross-laminated, fine- to very fine-grained sandstone. Elsewhere (e.g. Figure 5, Well 10, levels 73 to 83) the uppermost few centimeters of sandstone beds of Facies Stab display structures reminiscent of small-scale hummocky cross-stratification. Facies Stab may occur as isolated beds, but very commonly it is present as multiply-stacked (amalgamated) bedsets up to 6 m thick (Figure 5, Wells 5 and 6; Figure 8a).

The features of these sandstones, with their sharp bases, sequence of sedimentary structures and overall subtle grain size grading are all suggestive of deposition by gravity-driven turbidity currents, and for the most part can be assigned as Tab or Tabc turbidites, using the universally accepted scheme of Bouma (1962). Those beds in Well 10 that display possible small scale hummocky cross-stratification in their uppermost parts, but are otherwise fully recognizable as ‘turbidite’ deposits may be representative of ‘wave-modified turbidites’, as recognized by Myrow et al. (2002) in the Cambrian of Antarctica. The common tendency for this facies in the Mid-Qusaiba Sandstone to occur in thick amalgamated bedsets suggests that a mechanism existed whereby the turbidity flow was confined: it is reasonable to assume that such a mechanism involved the existence or creation of channels.

Facies Sfd: Fine-grained sandstone with dish structures

This facies was only observed in one location in this study (Figure 5, Well 8, level 43 to 46). It comprises a bed, 90 cm thick, of fine- (fL) to very fine-grained (vfU) sandstone that has a sharp base and is characterized by an abundance of very shallow, concave-upwards, argillaceous laminae that are interpreted as ‘dish structures’ sensuLowe and LoPiccolo (1974). Each ‘dish’ is about 2–3 cm across with a depth of only about 6 mm (Figure 9a). In the uppermost part of the bed, the rock becomes very argillaceous (compositional grading), and the dish-structured sand becomes highly contorted.

The dish structures that distinguish this facies are indicative of significant dewatering, and, when considered along with the sharp basal contact and compositional grading suggest rapid emplacement of a highly fluidized sediment gravity flow.

Facies Svfc: Contorted sandstone

This facies is rare, but persistent within the Mid-Qusaiba Sandstone. It consists of thin intervals (up to 25 cm thick) of very fine- to fine-grained micaceous sandstone (vfU–fL) that are characterized by strongly contorted lamination (Figure 9b). Such beds commonly occur in close association with beds of Facies Stab.

These beds of contorted sandstone were clearly originally deposited very rapidly in a highly fluid and metastable state. It is possible that they were emplaced very rapidly under storm conditions, and that subsequent storm waves impinging heavily on the seafloor caused liquefaction and associated synsedimentary deformation within the beds. However, as noted above, they very commonly occur directly beneath thick beds of Facies Stab (Figure 5, Well 8, levels 25, 29), which suggests that the liquefaction and deformation in Facies Svfc was induced by the rapid emplacement of those thick-bedded sandstones.

Facies Sfr: Ripple cross-laminated sandstone

This facies comprises fine- (fL) to very fine-grained (vfU) micaceous sandstones that are distinguished by the pervasive development of small scale ripple cross-lamination (Figure 9c). These cross-laminations occur in sets of micaceous laminae that are 1–3 cm thick, but which are commonly present in cosets that range from 0.9 to 2.4 m thick (Figure 5, Well 5). In the limited occurrences to date whereby this facies has been observed in core, it is universally seen to occur either immediately below or immediately above representatives of Facies Stab (cf. Figure 5, Wells 5 and 9).

These ripple cross-laminated sandstones were deposited by traction currents in a fairly low energy environment, namely in the lower part of the lower flow regime. The observation that they occur with high repeatability in intimate juxtaposition with sandstones that are considered to have been laid down by turbidity currents in a confined, probably channelized environment, suggests a depositional setting for the rippled sandstones that was closely related to those turbidite-bearing channels. It is likely that whenever the turbidity flows overtopped their confining channels, they experienced an immediate deceleration and loss of competence. Sediment fall-out from these weakened flows led to deposition of the rippled sandstones of Facies Sfr and the creation of submarine levees.

The preceding discussion has identified and elucidated the characteristics of two distinctive Depositional Facies Associations within the Mid-Qusaiba Sandstone: one of these (DFA-I) displays strong affinities to a shallow marine shelfal setting; the other (DFA-II) has the characteristics of a turbidite system, which would generally be attributed to deposition in a setting considerably deeper than shelfal. Furthermore, these two facies associations commonly occur interbedded with each other (Figure 5). In order to fully understand this apparent anomaly, it was decided to examine the sediments that occur immediately below the Mid-Qusaiba Sandstone: such sediments were cored in Well 10 (Figure 5), and are therefore described below as DFA-III.

Depositional Facies Association III (DFA-III)

The sediments of DFA-III comprise three depositional facies, each of which can be considered as forming part of a continuum of depositional events.

Facies Zc: Highly contorted, muddy siltstones

These rocks are gray, extremely heterogeneous muddy and sandy siltstones. They are characterized by wholesale internal disruption, such that no primary depositional fabric can be discerned: instead the rocks display a variety of soft sediment deformation structures, indicative of widespread dewatering and slumping (Figures 10a, b). It can be concluded that these siltstones of Facies Zc were deposited on an extremely unstable and waterlogged substrate, and that as such they were readily susceptible to dewatering, slumping and potential remobilization.

Facies Zah: Mudclast-bearing, sandy siltstones

This facies consists of thin (5–10 cm) beds of argillaceous sandy siltstone. These have sharp, locally erosional bases, and display no evidence of primary sedimentary structures. Instead, they are characterized by a chaotic assortment of contorted balls and streaks of sandstone (Figure 10c) and elongate clasts of mudstone that in many cases display delicate, wispy protuberances (Figures 10c, d). It is clear that these materials contained within Facies Zah are too delicate to have survived transport by normal traction forces. The mudclasts were probably transported as matrix-supported debris within subaqueous debris flows, whereas many of the irregular balls and streaks of sandstone possibly represent injection phenomena. Many of the features of this facies suggest ultimate mobilization of sediment that was initially subjected to instability, injection and slumping.

Facies Svtl: Very thinly bedded, loaded and graded, very fine-grained sandstones

These rocks consist of thinly bedded (3–5 cm) very fine-grained (vfL) sandstones. They have sharp basal contacts that commonly display very strongly developed loading into the underlying sediment (mudstone) (Figures 10e, f). The sandstone above these sharp contacts grades upwards into silty mudstone, displaying very little internal structure. These thinly bedded, graded sandstones were deposited rapidly as thin Tae turbidites upon a thoroughly waterlogged and unstable substrate, as suggested by the degree of loading displayed.

All of the facies described above that characterize DFA-III are similar in that they all show evidence of having been deposited on an unstable and waterlogged substrate, and involved processes that ranged from slumping and injection to full remobilization of sediment as debris flows and turbidites.

It is commonly accepted that the Qusaiba Member of the Qalibah Formation in Saudi Arabia was laid down upon an extensive, gently inclined depositional ramp, and that any ‘shelf-slope break’ would have been located far to the northeast in Paleo-Tethys (Nicholson et al., 2002). Certainly, none of the palynological evidence gleaned from the present study suggests a depositional setting any deeper than shelfal; in that regard, the deepest part of the shelf can be identified palynologically with the sediments assigned to DFA-III. The evidence for slumping and remobilization that characterizes the sediments of DFA-III is strongly suggestive of a significant depositional slope, and so it must be concluded that the ‘Qusaiba Ramp’ was not of itself devoid of topographic relief, i.e. it had a variable gradient that resulted in it being locally over-steepened.

There remains the anomaly that there occur stratigraphically above the over-steepened slope (identified by DFA-III in this study; see Figure 5, Well 10) two interstratified Facies Associations, one of which (DFA-I) displays normal shelfal affinities (and therefore to be expected), and one of which (DFA-II) displays the features of turbidite (gravity flow) sedimentation, which might be expected more to occur at the base of any slope. In order to evaluate fully the nature of these facies relationships, and hence the depositional setting and evolution of the Mid-Qusaiba Sandstone, the distribution of these various sediments must be considered further, in the context of the paleoecology and stratigraphy revealed by the new biostratigraphy described in the preceding part of this paper.

PALEOECOLOGY OF THE MID-QUSAIBA SANDSTONE

In a succession modified by Aeronian South American glacially induced eustatic sea-level changes (Grahn and Caputo, 1992; Caputo, 1998), facies changes will reflect changes in water depth. Because shelly benthos are largely absent from the cores examined, palynomorphs are used to make paleoecologic interpretations. Several deepening and shallowing events are shown by diversity trends for the Aeronian part of the succession (Figures 13, 14, and 15). On these charts, higher diversity (species richness) generally means bathymetrically deeper settings. Along with fluctuations in diversity the assemblage composition is also critical to the interpretation of the depositional setting (e.g. dominance of terrestrial vs. marine assemblage composition). Beck and Strother (2001) have pointed out that deeper marine assemblages contain an increased percentage of acanthomorphic and other acritarchs and increased number of Chitinozoa with a concomitant decrease in the diversity and number of terrestrially-derived palynomorphs. These trends are present in the Qusaiba Member (Table 2). Increasing chitinozoan diversity and abundance with increasing water depth was also recognized by Laufeld (1979) from the Silurian of Gotland. Progressively shallower assemblages, conversely, show an increase in the abundances of terrestrially-derived palynofossils (Beck and Strother, 2001). Their interpretations are supported by a body of research on the distribution of benthic communities for the Silurian section they studied. The Qusaiba Member palynomorph assemblages show similar environmental trends to those described by Beck and Strother (1997, 2001) and Laufeld (1979).

Thus the palynomorph assemblages recovered from mudstones of DFA-I indicate, as does the sedimentology, that these sediments were deposited in a shelfal setting. The assemblages are unquestionably marine in origin, but the marine components (acritarchs and chitinozoans) are not diverse and are numerically subordinate to the terrestrial palynomorphs (Table 2; Figure 11a). Wells 6, 8, 9, and 10 yielded counts for DFA-I mudstones containing either R. agglomerata n.sp. or T. caelamenicutis (persistent) that are all remarkably consistent; total terrestrial components are considered to be allochthonous whereas the chitinozoans are indigenous. Both morphologic groups reflect the relative shallow-water marine depositional setting.

In contrast to the relatively constant composition of the DFA-I assemblages, the total terrestrial and chitinozoan components in DFA-III varied greatly in a relatively short stratigraphic interval (terrestrial from 25–71% and chitinozoans from 60–20%). This variability reflects the compositional difference between a sample containing the deeper-water marine indigenous angochitinid chitinozoan-dominant assemblage (Table 2, Well 10, level 10; Figure 11b) and a sample with an allochthonous terrestrial input (Table 2, Well 10, level 6) within DFA-III.

What is significant about this DFA-III data set is that the variability of the chitinozoan/total terrestrial percentages shows that it is possible to demonstrate that the indigenous element of deeper water palynomorph assemblages has a recognizable signature, in this case an abundance of chitinozoans vs. total terrestrial. Also, when compared to the composition of DFA-I it shows that the assemblages are depositionally deeper (Table 1). If the Mid-Qusaiba Sandstones were transported and deposited in a deeper water environment, than recognized in DFA III, the dominantly cryptospore assemblages would become diluted with palynomorphs indicative of deeper water environments and acritarchs would become more prominent in the samples (Beck and Strother, 2001). Because acritarchs are not numerically important components of the assemblage and cryptospores are significant, the depositional setting of the Mid-Qusaiba Sandstone is considered shelfal.

STRATIGRAPHIC ARCHITECTURE OF THE MID-QUSAIBA SANDSTONE

Stratigraphic sections have been constructed that display the correlation of the new palynological markers identified and described above, and which are associated most closely with, and immediately below, the Mid-Qusaiba Sandstone interval, namely Rugosphaera agglomerata n.sp., Tylotopalla caelamenicutis (persistent), ?Oppilatala n.sp and Sphaerochitina solutidina. The datum that has been selected for each of the stratigraphic sections is a palynomorph diversity spike that occurs either immediately below, or is directly associated with, a specific occurrence of the palynomorph n.sp. aff. Papulogabata. Studies concurrent with the present work (Miller and Melvin, 2004) have confirmed that this particular occurrence of n.sp. aff. Papulogabata and its associated diversity ‘spike’ has the most widespread geographical distribution of all the significant palynologically identified events adopted for stratigraphic usage in this part of the Silurian of the Arabian Plate. The palynological diversity ‘spike’ per se represents fully open marine conditions. In a number of wells, and specifically in Well 10 in the context of this study, n.sp. aff. Papulogabata and its associated diversity ‘spike’ have been identified from core samples (as opposed to 30 ft ditch cuttings). This diversity spike also occurs with M. convolutus (Bolin, 1982, our Figure 15) and with Conochitina alargada, which has been associated with convolutus Zone graptolites (Verniers et al., 1995). Well 10 is the convolutus-bearing subsurface well shown in McClure (1988b, his figure 2).

In Well 10, there occurs in very close proximity to the high diversity event in the core a thin (< 10 cm) but highly distinctive horizon of ‘cone-in-cone’ calcite mineralization (Figure 12). Similar such distinctive horizons were recognized by MacKenzie (1972) from the Spore-bearing Member (now Bluefish Member) of the Hare Indian Formation in northern Canada. He considered they formed as a result of very early diagenesis, very close to the sediment/water interface. Al-Aasm et al. (1992) postulated that metastable skeletal body fossils, which accumulate during periods of low sedimentation, sourced the Bluefish fibrous calcite. They also indicated that the formation of the reconstituted fibrous calcite beds occurred at relatively shallow burial depths, from 10s to hundreds of meters with over pressuring. The Hare Indian Formation cone-in-cone horizon occurs in the lower organic-rich Bluefish Member, which contains pelagic fossils including representatives of the microphytoplankton genus, Leiosphaeridia (MacKenzie, 1974). The Qusaiba cone-in-cone horizon documented here is also encased in organic-rich, black, laminated mudstones and is interpreted as an indication of minimum clastic input (Figure 12). That is to say, it formed in a location furthest from the source of sediment, and/or in the deepest water setting (Miller and Melvin, 2004).

The coincident occurrence of these factors, namely the geographically widespread occurrence of a palynological high diversity event associated with the OCC of n.sp. aff. Papulogabata, and the proximity of indicators of deep water far removed from a source of clastic input, is interpreted as indicating that the diversity ‘spike’ represents the most likely candidate within this part of the Qusaiba for a Maximum Flooding Horizon. This inference is substantiated by the concurrence of the n.sp. aff. Papulogabata horizon with the Monograptus convolutus Graptolite Zone. The convolutus Zone has been identified on a global scale as representing a time of high sea level (Loydell, 1998). Sharland et al. (2001, p.153) consider the Silurian maximum flooding event (MFS S10) on the Arabian Plate to occur in the M. convolutus Zone. This is corroborated by the additional biostratigraphic and sedimentological information presented here.

Figure 13 is the stratigraphic section A-A′ identified on Figure 1 that links Wells 1, 2 and 3. The significant attribute of this diagram is the subparallelism of a number of palynomorph First Downhole Occurrences. Thickening in Well 3 can be explained by the presence of significant additional thicknesses of sandstone between Sphaerochitina solutidina and Tylotopalla caelamenicutis (persistent). The base of this sand apparently erodes strata containing ?Oppilatala n.sp. in Wells 2 and 3. This sub-parallelism of FDOs of Fractoricoronula n.sp., the three Mid-Qusaiba Sandstone palynomorph marker horizons and Eupoikilofusa curvata n.sp., is interpreted as indicative that this stratigraphic section A-A′ is oriented more or less parallel to Qusaiba Member depositional strike, namely NW-SE.

Figure 14 is the stratigraphic section B-B′ identified in Figure 1 that links Wells 4, 1 and 5. The orientation of this section, between Wells 1 and 4 in particular, is at a high angle to that represented by section A-A′ (Figure 1), and can therefore be taken as more nearly representative of a depositional dip section. Significantly, the marker horizon E. curvata n.sp. downlaps on to the n.sp. aff. Papulogabata horizon/MFS; above this, S. solutidina, the lowest marker associated with the Mid-Qusaiba Sandstone, appears to parallel this downlapping trend. N.sp. aff. Papulogabata has been interpreted above to represent a horizon at or just above Maximum Flooding Surface, and so the downlapping orientation of the sediments that overlie it, not surprisingly represents the clinothems of the Highstand Systems Tract that overlies MFS S10.

Of equal importance to this discussion of Figure 14 is the observation that the FDOs represented by R. agglomerata n.sp., T. caelamenicutis and ?Oppilatala n.sp. are not present at Well 4 (n.b. the first FDO recognized is that of S. solutidina). The orientation of section B-B′ places Well 4 in the most updip part of the depositional slope at the time; thus, the absence of these markers must be related to either non-deposition upon a paleo-high, or more probably an erosional event associated with the introduction of the Mid-Qusaiba Sandstone. Surfaces such as the erosional event within the Qusaiba mentioned above can not be resolved without biostratigraphic control and/or cores. Erosional surfaces have not been recognized by other means. There is no core from this interval within Well 4, but examination of the wireline logs (Figure 14) shows that the basal contact of the Mid-Qusaiba Sandstone in that well is extremely sharp, and occurs abruptly above the S. solutidina FDO. Based on this evidence, it is likely that the base of the Mid-Qusaiba Sandstone represents an unconformity of some significance that, in the location of Well 4, removed the equivalent stratigraphic thickness of the three marker horizons that overlie the S. solutidina FDO horizon. Furthermore, the identification of an unconformity overlying sediments attributed to a Highstand Systems Tract, and introducing sandstones over mudstones, encourages the conclusion that this unconformity represents a Sequence Boundary, and thus identifies a significant drop in relative sea level in Mid-Qusaiba times. The nature of such a reduction in relative sea level can be further elucidated by referring to wells wherein there is access to core material, and which are oriented at a high angle to depositional strike. Such a situation is represented in Figure 15.

Figure 15 displays the stratigraphic section B′-B″ (Figure 1) which is oriented SSE-NNE along the length of the Ghawar structure, and, significantly at a high angle to the depositional strike as determined from Section A-A (Figure 13). It includes seven wells, of which five contain a significant amount of core through the Mid-Qusaiba Sandstone (Figure 5 for core descriptions). The Datum for this stratigraphic section is the diversity ‘spike’ associated with n.sp. aff. Papulogabata (see above). The four significant Mid-Qusaiba palynostratigraphic marker horizons that were identified earlier in this report are shown, as are the depositional facies associations that occur between them, and whose sedimentological characteristics were also described in a previous section (also see Figure 5). It is clear from Figure 15 that the Mid-Qusaiba Sandstone is not a simple entity, and that its distribution in terms of gross facies, time and space is in fact quite complex.

The palynostratigraphic marker that occurs closest to the base of the Mid-Qusaiba Sandstone interval is ?Oppilatala n.sp., as was observed in Wells 4 and 1 (Figure 14). This marker is present in Wells 5 through 8, but is not recorded in Wells 9 and 10 (i.e. furthest down the paleoslope). Instead, in the latter two wells, a thin sandstone is encountered having turbiditic affinities (based on core in Well 10).

Figure 13: Strike cross-section A–A′. See Figure 1 for the line of section and text for discussion.

The depositional attributes of such deposits would suggest that the ?Oppilatala n.sp. marker had been present, but was subsequently removed by the erosive nature of these gravity flow sandstones. These sandstones represent the lowermost deposits of the Mid-Qusaiba Sandstone in Figure 15.

The next important identifiable marker is T. caelamenicutis (persistent). This is shown in Figure 15 to be essentially correlatable along the length of the section. More significantly, at the northern (more distal) end of the section its presence in Well 10 is recorded only within a thick interval dominated by Facies Stab sandstones. It is very likely that in that location this palynomarker was reworked by the incoming turbiditic sandstones, which are therefore interpreted as having scoured down into and below the original depositional level of T. caelamenicutis-bearing strata. It should be noted that this substantial body of turbiditic sandstones (approximately 13 m based on log and core data) is nonetheless limited in occurrence to Well 10 in this line of section. This is confirmed in Well 11 (distal to Well 10), where this species of Tylotopalla reappears within a mudstone (Figure 15).

The highest palynostratigraphic marker associated with the Mid-Qusaiba Sandstone is R. agglomerata n.sp. It is shown in Figure 15 that this marker is absent in three wells, namely Wells 5, 6 and 11. In Wells 5 and 6 it appears to have been eroded out of the section by the lowermost bed of a thick package of Facies Stab turbiditic sandstones. Indeed, it seems possible to correlate this sandstone package along the length of the section to Well 11 (Figure 15). In that well, the R. agglomerata n.sp. marker is again absent, removed as a result of erosion by the turbiditic sandstones.

This package of sandstone merits further discussion. In Well 6 (Figures 5 and 15), core studies reveal that it comprises at least two units of amalgamated turbiditic sandstones (Facies Stab) that are separated by a thin dark gray mudstone. In Well 5 (Figures 5 and 15), the sandstone package is again shown in core to comprise at least two units of amalgamated turbidites, but in this case they are separated by an interval of Facies Sfr ripple-laminated sandstones that most likely represent levee deposits lateral to a turbidite-filled channel. We suggest that these levee deposits are laterally equivalent in time to the thin mudstone identified above that separates the two amalgamated turbidite units of Well 6. In Well 8 (Figures 5 and 15), two units of turbidite sandstones are once more identified, separated by an interval of mudstones and thin shelfal sandstones (DFA-I). Thus, it appears that, immediately following deposition of the R. agglomerata n.sp. horizon, there occurred at least two significant events of clastic turbidite deposition. The earlier of these was widespread in occurrence, extending from Well 5 northwards to Well 11. In at least two recorded locations (Wells 6 and 11) it removed the R. agglomerata n.sp. horizon by erosion. Elsewhere, the thickness of mudstones that lies between this earlier turbidite unit and the underlying R. agglomerata n.sp. horizon varies significantly (Figure 15). These thickness variations, as well as the actual absence of the R. agglomerata n.sp. horizon in places, are interpreted as representing varying degrees of erosional downcutting (channeling) by the turbidite sandstones. It is uncertain if turbidite sands were responsible for the stripping of the R. agglomerata n.sp. to S. solutidina interval in Well 4 (Figure 14). The later of the uppermost two turbidite depositional events identified above appears to have been of more limited extent, and confined to more paleo-proximal locations (Figure 15).

DISCUSSION

From the palynological and sedimentological data presented above, a number of conclusions can be drawn about the Mid-Qusaiba Sandstone in east central Saudi Arabia. Significant new palynological data illustrate how this stratigraphic unit was deposited in water depths no deeper than open marine shelf. This is substantiated, in large part, by sedimentological studies of core material, and the identification of widespread deposits showing shelfal affinities (DFA-I). However, these sediments occur interstratified with, in places, thick intervals of turbiditic sandstones (DFA-II). Both facies associations overlie sediments of DFA-III, which palynology and sedimentology both indicate to represent the deepest water environment examined in this study, namely over-steepened slopes upon the Mid-Qusaiba shelf.

When the palynological and sedimentological data are integrated in a stratigraphic context (Figure 15), it is clear that each of the new palynological markers identified with the Mid-Qusaiba Sandstone was intimately associated with a significant event characterized by sediment gravity flow deposition.

Those events are manifest in an irregular geographical distribution of coarse-grained sediment suggestive of significant channeling. The gravity flow deposits of DFA-II nowhere appear to be overlain DFA-III deposits; rather, the available evidence suggests that over-steepened slope deposits, where identifiable occur stratigraphically beneath the DFA-I / DFA-II package that characterizes the Mid-Qusaiba Sandstone sensu stricto.

Earlier discussion of the stratigraphic section represented in Figure 14 noted the absence of section represented by R. agglomerata n.sp. through ?Oppilatala n.sp. in the most updip well (Well 4), and suggested that this could be related to removal by erosion associated with a significant drop in relative sea level in Mid-Qusaiba Sandstone times. Certainly, significant drops in relative sea level are commonly associated with turbidite-type deposition in a sequence stratigraphic context (Posamentier and Vail, 1988). However the most common scenario in that regard is for the formation of a basin floor fan resulting from sediment bypass of the shelf when relative sea level falls below the shelf edge. The abundance of shelfal deposits within the Mid-Qusaiba Sandstone demonstrates that any drop in relative sea level that led to the deposition of these sandstones did not proceed so far as to fall beneath the shelf edge, if indeed such a feature existed. Furthermore, the interstratified nature of DFA-I with DFA-II is considered significant.

Recent discussion of the Eocene of Spitsbergen by Plink-Björklund et al. (2001) is of relevance in this regard. Those authors identified inter alia sand-prone clinothems that developed on Eocene shelf margins of a small foreland basin when sea level at times fell to or below the shelf edge. Of those sand-prone clinothems, some had their sand budget partitioned mainly out on to the basin floor (basin floor fans) (Type 1 clinothems), but most trapped their sand on the slope only (Type 2 clinothems) (Plink-Björklund et al., 2001). It should be noted that these authors calculated water depths in the Eocene foreland basin, based on the dimensions of their clinothems, to be in the range of 100–200 m (Plink-Björklund et al., 2001).

In the latter (more common) examples, the Type 2 clinothems are visible as down-slope-thinning wedges that have readily identifiable segments to them. Thus, lower-slope to base-of-slope segments are dominated by lobes consisting of broad, shallow channels and sheet-like turbidites that become heterolithic and muddy out on the basin floor. The middle-slope segment of these Eocene clinothem complexes is dominated by narrow channels (chutes) that feed downslope to progradational chute-mouth lobes. Chutes contain ungraded and laminated sandstone beds up to 3 m thick, whereas the chute-mouth lobes show alternations of thinner, ungraded to laminated or rippled sandstones, and become muddier and more heterolithic downslope. The predominant Facies Stab sandstones that characterize DFA-II seem to bear great similarity to chute turbidite sandstones. Shelf-edge to upper-slope segments of the clinothems are dominated by upward-coarsening and -thickening sediments that include cross-bedded, coarse-grained sandstones and wave-rippled sandstones indicating delta distributary channels and wave-influenced mouth bars respectively (Plink-Björklund et al., 2001). Such shallow-water sediments have not been recorded anywhere in the Mid-Qusaiba Sandstone in this study.

Swift et al. (1987) investigated in outcrop the stratigraphic relationships of the distal parts of the Kenilworth Member of the Cretaceous Blackhawk Formation in Utah. They identified along the length of the outcrop zone an erosional surface that occurs in the Mancos Shale at some distance beneath the Kenilworth Member. That horizon has been identified biostratigraphically by Fouch et al. (1983) as the Desmoscaphites bassleri Zone, which is the open marine equivalent of the Emery Sandstone Member of the Mancos Shale (Swift et al., 1987). This surface is associated with a limited number of lithofacies, including sandstones that are present as lenticular channel fills, and which in many cases appear massive. These lenses are seen on a surface that is also characterized by numerous shale-filled channels. Elsewhere, convex-upward sandstone lenses are observed resting on an erosional surface “…in which both sandstone-filled and shale-filled gullies appear” (Swift et al., 1987). Those authors interpreted the channeled surface as a submarine erosion surface, and noted that “…the observed small-scale channels resemble the dendritic gully systems seen on modern shelf edges and upper slopes (Kelling and Stanley, 1976)” (Swift et al., 1987). This conclusion was compounded by their observation that there was a “…total lack of subaerial valleys cut in the … surface, and the lack of prograding shoreface sequences under it clearly indicate that the … shoreline never got as far as the area of the study; erosion … must have occurred in a submarine setting” (Swift et al., 1987).

These observations and the resulting conclusions can be directly applied to the stratigraphic setting of the Mid-Qusaiba Sandstone. The lowermost representatives of the Mid-Qusaiba Sandstone in this study occur just above the Sphaerochitina solutidina FDO (Figure 15). The underlying sediments are mudstone clinothems of the preceding Highstand Systems Tract, as was discussed earlier (Figure 14). They contain no nearshore facies such as shoreface sequences, and so it is concluded that the basal erosion surface of the Mid-Qusaiba Sandstone as illustrated in this study was created in an entirely open marine setting.

The Mid-Qusaiba Sandstone itself comprises two Depositional Facies Associations that have been described earlier. Specifically, there appears to be a close relationship between each of the newly identified palynomarkers and the erosional surfaces that define the base of DFA-II wherever it occurs (Figure 15). Thus, the oldest occurrences of DFA-II are identified fairly distally (in Wells 9 and 10), where they erode out the strata containing ?Oppilatala n.sp. The next observed occurrence of DFA-II sandstones is younger, and (in the context of the section presented) very limited in its distribution, predominantly in Well 10 where it is thickly developed and appears to erode into the T. caelamenicutis (persistent) horizon. The most widespread development of DFA-II is the youngest, and this is seen to extend along the length of the section shown (Figure 15), where it displays thickening and thinning above, and local erosion into, the R. agglomerata n.sp. palynomarker horizon, as was discussed above. These relationships are of crucial importance to the sequence stratigraphic development of the Mid-Qusaiba Sandstone.

Thus, following the creation of accommodation space with the MFS S10, a Highstand Systems Tract of mud-prone clinothems was established, these being recognized by their various palynomarker horizons. Very shortly after the establishment of the ?Oppilatala n.sp. marker and its equivalent clinothem, relative sea level started to drop. It fell rapidly to depths that were within storm wave base, and this led to the periodic deposition of storm-derived sandstones and siltstones of DFA-I (Figure 16a). Continuing fall of relative sea level allowed the storms to attack the irregular submarine topography of the Mid-Qusaiba sea floor and to erode out gullies (or chutes) in the uppermost parts of the over steepened slopes. Pre-existing storm-derived sands were remobilized, and funneled into the gullies as gravity flows (Figure 16b).

Plink-Björklund et al. (2001) have noted how chutes/gullies in these settings are small, probably too small ultimately to confine the turbidity currents. The turbidity currents thus overflow the banks of their chutes and disperse. This dispersal increases the basal area of the gravity flow, thereby increasing friction and causing the turbidity currents to decelerate i.e. to effect deposition. As a result of this deceleration and deposition, the turbidites wane and die out before they reach the basin floor (Plink-Björklund et al., 2001). Consequent on this set of circumstances, it can be inferred that the gravity flows at no time acquire the ability to entrain sediment from the substrate, and hence they at no time develop the energy associated with the “ignited” stable state at which they are self-sustained, highly erosive, and competent to scour out submarine canyons into the slope (Parker, 1982). In the absence of submarine canyon development, the system is deprived of a mechanism for significant sediment bypass to the basin floor, and hence it is extremely unlikely that basin floor fans will be developed. This analysis thus refutes the proposition of Wender et al. (1998) that the Mid-Qusaiba Sand represents a progradational basin floor fan system.

Following the initial drop in relative sea level that led to the gullying and erosion of the Oppilatala n.sp. marker at Wells 9 and 10, there followed a slight rise in relative sea level that cut off the conditions for gullying temporarily, and deposition of the Mid-Qusaiba Sandstone was limited to DFA-I tempestites. However, the fall in relative sea level resumed shortly after deposition of the T. caelamenicutis (persistent) FDO, and it appears that it fell to an even lower level than previously. This is evident in the thick development of DFA-II turbidite sandstones in the relatively distal setting of Well 10, where the sandstones eroded out the T. caelamenicutis (persistent) marker. Once again however, following this episode of turbidite sedimentation, relative sea level rose slightly, inhibiting gully formation and deposition, and only DFA-I tempestites were deposited across the Mid-Qusaiba shelf until shortly after R. agglomerata n.sp. was deposited. Then, a drop in relative sea level led to widespread gullying and associated turbidite deposition: amalgamation of turbidite flow units prevailed within the gullies, until eventually these were overtopped, and thinner turbidite sheets were distributed across the Mid-Qusaiba inter-gully shelf/slopes, as well as localized levees of rippled sand more proximal to the gully margins. This widespread depositional episode was the first of two characterized by turbidite deposition: the second followed very closely upon the first, with another thick pile of amalgamated gravity flow sandstones. However, the deposits of this latest event were more restricted in their distribution, and appear to have been limited to more proximal areas of the shelf, at least within the context of the section shown in Figure 15.

The above discussion demonstrates that the drop in relative sea level that is associated with the onset of deposition of the Mid-Qusaiba Sandstone, and which has been ascribed to the formation of a significant sequence boundary across the Arabian Plate (Sharland et al., 2001), was episodic in nature, and can be identified as a composite of at least three step-wise drops in relative sea level, each of which was followed by a slight sea level rise. The phase of turbidite deposition that occurred just after R. agglomerata n.sp. deposition, and which was the most widespread in terms of its identification in the current study, most likely represents the ‘true’ lowstand deposits of the Mid-Qusaiba ‘lowstand event’. Those sandstones were very rapidly superceded by a fourth and final phase of turbidite deposition. These youngest sediments were restricted in their geographical distribution to the more southern (paleoproximal) part of the study area (Figure 15). They are thus considered to represent the last vestiges of the Mid-Qusaiba drop in relative sea level, and herald the onset of the subsequent transgressive systems tract.

In proposing these ideas regarding the sequence stratigraphy of the Mid-Qusaiba Sandstone, we are of course fully cognizant of the limitations to the arguments, not least the very great spacing between wells, and the very limited availability of core material, without which the crucial identification of Depositional Facies Associations cannot be made. Nonetheless, we believe that this study, by integrating newly recognized biostratigraphic marker horizons in a high-resolution context with detailed sedimentology, successfully expands our understanding of a potentially important subsurface reservoir unit.

CONCLUSIONS

Recognition of new biostratigraphically significant palynomorphs integrated with the Arabian Plate chitinozoan zonation and calibrated with selected graptolites and acritarchs allows the creation of a highly refined zonation based on First Downhole Occurrences. This approach utilizes cuttings samples and creates a robust data set with which to build a biostratigraphy for sequence stratigraphic interpretation. Within the early Telychian and Aeronian a total of nine significant biostratigraphic events (FDOs) have been recognized. These regionally wide-spread correlative ‘horizons’ follow each other in short order, especially in the vicinity of the informally defined Mid-Qusaiba Sandstone, where biostratigraphic resolution is at the sampling interval (e.g. at 30 ft). This refined zonation allows the recognition of a history of submarine erosional events (unconformities at the base of the sand bodies), here-to-fore unparalleled in the study of the Arabian Silurian.

The glacio-eustatic sea level drawdown in the Aeronian is globally recognized (Loydell, 1998). The biostratigraphic control used here allows correlation with the Llandovery sea-level curves and gives a foundation for the sequence stratigraphy developed here.

Sedimentological interpretation of core material from across the study area in east central Saudi Arabia has identified three significant Depositional Facies Associations within the Mid-Qusaiba Sandstone. These include a shelfal facies association (DFA-I) that is characterized by thinly interbedded hummocky-stratified sandstones, siltstones and bioturbated mudstones; a turbiditic sandstone facies association (DFA-II), and an association of heavily contorted and re-sedimented sandstones, siltstones and mudstones (DFA-III) that is considered representative of over steepened slopes upon the Qusaiba shelf. DFA-I and DFA-II occur interstratified with each other, and in all observable cases are present only stratigraphically above DFA-III.

Stratigraphic integration of the newly recognized palynostratigraphic horizons and the sedimentological data elucidates the sequence stratigraphic evolution of the middle part (Aeronian) of the Qusaiba Member of the Qalibah Formation in east central Saudi Arabia. Thus, a significant (maximum) flooding surface (MFS) is identified with the mutual concurrence of: (a) the very wide occurrence (but not FDO) of n.sp. aff. Papulogabata; (b) a universally associated palynological diversity spike; and (c) in places the occurrence of indicators of starved sedimentation conditions. This MFS occurs within a sea-level high stand that is globally identified with the M. convolutus Zone. Overlying, and downlapping on to the n.sp. aff. Papulogabata/MFS 10 flooding surface there occurs a number of mud-prone clinothems each of which is identified by its own palynostratigraphic marker e.g. Eupoikilofusa curvata n.sp., Sphaerochitina solutidina. These mud-prone clinothems represent the distal parts of a Highstand Systems Tract (HST).

The end of the HST is marked by evidence for a major, step-wise drop in relative sea level. That evidence is manifest in the close relationship whereby successive palynostratigraphic marker horizons, newly identified in this study, are partially eroded by storm waves and the introduction of sandy turbidites (DFA-II). Those turbidity flow deposits arise from the erosion of gully complexes in the upper submarine slopes that are present as topography upon the Qusaiba shelf. The creation of the gullies takes place as a result of the lowered sea level bringing the sea floor within the erosive regime of storm wave base. The lowstand per se is represented by the most widespread distribution of DFA-II turbidite deposits, and associated with erosion of the youngest Mid-Qusaiba Sandstone marker horizon identified in this study, namely R. agglomerata n.sp. That erosion surface might therefore be considered the true Sequence Boundary of the Mid-Qusaiba lowstand event. Each of these successive drops in sea level is separated from the next by a minor, subsequent sea level rise, evident in the widespread occurrence of shallow marine muds and tempestites (DFA-I). The youngest unit of DFA-II lowstand shelfal turbidite deposits is limited in its occurrence to the more proximal parts of the study area, and thus is considered to represent the onset of the succeeding Transgressive Systems Tract (TST).

ACKNOWLEDGMENTS

We acknowledge the Saudi Arabian Ministry of Petroleum and Mineral Resources and the Saudi Arabian Oil Company (Saudi Aramco) for granting permission to present and publish this paper. We also extend our appreciation to the conveners of GEO2002 for inviting us to publish this work in GeoArabia. We thank Mansour Al-Ruwaili and Dr. John Filatoff of Saudi Aramco’s Geological Technical Services Division (GLTSD) and SA′id Al-Hajri (Saudi Aramco, Central Area Exploration) for enlightening discussions on the Silurian palynology and stratigraphy of Saudi Arabia. Dr. Muhittin Senalp (Saudi Aramco, Geology Technology Team) graciously shared his knowledge on the stratigraphy of the Qusaiba and its nomenclatural history. We thank Gene Cousart and Vic Tegelan (Saudi Aramco Graphics Design Unit, Cartographic Imaging Division) for drafting our figures. David Bacchus (GLTSD) kindly helped with obtaining logs and data for creation of cross-sections. Kathleen Haughney, Saudi Aramco Exploration & Producing Information Center, is thanked for obtaining literature and checking references. Dr. Rob Fensome (Canadian Geological Survey) graciously provided opinions on the necessary locality information required to establish new species. Mohsen Al-Eid and Mubarak Al-Dossary (GLTSD) processed many of the palynology samples used. Ali Al-Zahrani and Hadi Al-Uraij were instrumental in arranging for cores to be laid out at GLTSD’s Core Facility. The final design and drafting of graphics by Gulf PetroLink is acknowledged. Finally we thank the two anonymous GeoArabia reviewers for their constructive comments and suggestions. Seonaid Macdonald is acknowledged for helping edit the final page proofs.

APPENDIX I: Material and Methods

The palynological data for this Mid-Qusaiba Sandstone study were extracted from a database of over one thousand core and cuttings samples from 34 Silurian wells in Ghawar field and areas to the west and south (Figure 1). The Mid-Qusaiba Sandstone was absent by erosion in areas to the south and southwest and in two cases well penetrations did not reach the Mid-Qusaiba Sandstone. In most cases palynomorph occurrence data were obtained from composite cuttings samples processed at regular 30 ft intervals (for discussion of cuttings sampling see O’Neill et al., 1999); however in rare cases sample intervals as great as 50 ft or as small as 10 ft were used. Cores have been adjusted to match log depths but cuttings samples were retained at driller’s depths. Four hundred and twenty feet of core from five wells were described in the course of this study. Detailed core logs were compiled from examination of 2.5- to 4-inch cores (Figure 5).

Palynology sample processing for both core and cuttings samples consisted of hydrochloric acid treatment followed by hydrofluoric acid. Neutralized residues were then sieved through a 20 μm screen followed by heavy liquid separation of organic residues from minerals in 2.0 sp. gr. zinc bromide. Residues were sieved a second time following heavy liquid separation. Generally one unoxidized slide and two nitric acid oxidized slides were prepared for each sample. Residues were mounted on coverslips with polyvinyl alcohol and glued to labeled slides with Petroepoxy®.

Palynomorph checklists were derived from the examination of an unoxidized slide and one oxidized slide per sample. The occurrences of acritarchs, chitinozoans and cryptospores were used to identify correlative horizons based on the First Downhole Occurrence (FDO) of regionally occurring taxa. The exception to this is the Occurrence (OCC) of n.sp. aff. Papulogabata, which occurs sporadically through out the Silurian section but occurs consistently at one level. FDOs and OCCs were plotted as cuttings sample tops. The term persistent, as it is applied to the occurrence of T. caelamenicutis, refers to its FDO and the regular appearance of specimens in successive downhole samples. Counts of 300 grains were made for selected core samples (mudstones only) for paleoenvironmental interpretations (Table 2). Samples adjacent to those selected for counts were scanned for anomalous trends; if obviously different from the counted control samples they also were counted. Palynomorph occurrence data were loaded into StrataBugs® software and cross-sections were initially prepared using Oil Field Data Management® software (ODM). Diversity data, at cutting interval bases, were imported into ODM directly from StrataBugs® and plotted against the gamma logs (Figures 1315). Core samples were plotted at log depth; the composite cuttings samples were plotted at driller’s depths. The diversity as shown here is essentially species richness (total number of species or taxonomic categories (e.g., arthropod cuticle, graptolite fragments, etc.) recorded).

Type specimens for the new species described in Appendix II are housed in the Micropalaeontology Division, Palaeontology Department, The Natural History Museum, London, UK under the designation PM, FA or FM. Illustrated specimens are retained in the collections of Saudi Aramco. All palynomorph taxa mentioned in the text are listed with authorship in Appendix III.

APPENDIX II: Systematic Palynology

AlgaeIncertae Sedis

Group Acritarcha Evitt 1964

BeromiaVavrdová 1986 emend. Wood 1996

Type species: Beromia clipeataVavrdová 1986, p. 354, plate 3, figures 12.

Beromia rexroadiiWood 1996

Plate 1, Figure 2

1996 Beromia rexroadii Wood, p. 182–185; plate 1, figures 112, plate 2, figures 110, plate 3, figures 16. 2001 Beromia rexroadiiWood 1996 – Le Hérissé et al., plate 1, figure 3.

Remarks: Beromia rexroadii occurs infrequently in the Qusaiba Member; however it is significant biostratigraphically in that it confirms the age of the A. macclurei Zone as being late Aeronian to early Telychian. It occurs too rarely to be used routinely for biostratigraphy. B. rexroadii occurs 30 ft below the FDO of A. macclurei in core from the upper part of the Qusaiba Member in Well 12. Wood (1996) recorded the occurrence of B. rexroadii as being older than the celloni Conodont Zone in Eastern North America. Brett et al. (1990) and Brett et al. (1998) restricted the ages of the B. rexroadii-bearing Lower Sodus Shale of New York and Lulbegrud Shale Member of the Noland Formation of Kentucky to the Aeronian/Telychian boundary interval. B. rexroadii has also been recovered from the Tianguá Formation of northern Brazil (Le Hérissé et al., 2001) and core samples from Belgium (Wauthoz, 1997); both spanning the Aeronian/Telychian boundary.

GenusEupoikilofusaCramer 1970

Type species: Eupoikilofusa striatifera (Cramer) Cramer 1970, p. 85–86.

Eupoikilofusa curvatan.sp.

Plate 3, Figures 15

Derivation of name: curvata for the curved vesicle axis.

Holotype:Plate 3, Figure 1, Well 13, core depth 8,805–8,807.5 ft, slide 71–2, W60, PM FA 160.

Paratypes:Plate 3, Figure 3, Well 13, core depth 8,807.5-8810 ft, slide 72–2, N51 3, PM FA 161; Plate 3, Figure 4, Well 13, core depth 8,805–8,807.5 ft, slide 71–2, R44, PM FA 162.

Type locality and type stratum: Well 13, core 11, depth 8,805–8,807.5 ft, Qusaiba Member of the Qalibah Formation, Llandovery.

Material: Twenty specimens in various states of preservation from the Aeronian and Rhuddanian portion of the Qusaiba Member.

Description: Concavo-convex leiofusid with acuminate polar processes. Central body laterally compressed, reniform, with 8–10 striae per surface, striae subparallel with the vesicle sides near the middle of the vesicle but converging toward the poles, lined by up to two rows of granulate or papillate ornament per striation side. Ornament < 0.25 μm in diameter; < 0.5 μm high. Ornamented striae end at the bases of the gently tapering, polar processes; processes unornamented. Interiors of processes and central body communicate freely. Vesicle wall <0.5 μm thick. No regularly occurring excystment structure noted.

Measurements: Total length: 90 to 127 μm (length strongly dependent on compression and orientation), holotype: 127 μm; central body length: 47(55)65 μm, holotype: 65 μm; central body width: 20(25)29 μm, holotype: 28 μm; polar process length: 25(34)55 μm, holotype: 32–35 μm. 10 specimens measured.

Comparison: Dactylofusa saudiarabica differs from E. curvata n.sp. by having a spiral vesicle and presence of short “denticules” on the ridges, which may extend on to the polar processes (Cramer and Díez, 1972, p. 160, plate 35, figure 52). The ornament present on the striae of E. curvata n.sp. varies from granulate to papillate, in some cases becoming finer toward the center of the concave side. Both Baiomeniscus and Eupoikilofusa rochesterensis have similar ornament distribution on vesicle ridges but lack the pronounced polar processes of E. curvata n.sp. Both D. saudiarabica and E. rochesterensis have been reported from the Lower Silurian of Saudi Arabia (Le Hérissé, 2000).

GenusFractoricoronulaColbath 1979

Type species: Fractoricoronula cubitaliaColbath 1979, p. 15, plate 5, figures 12. Fractoricoronulan.sp.

Plate 1, Figure 1

Description: Vesicle triangular, unilayered, with processes arising at the apices. Process interiors separated from vesicle interiors by thick basal plugs. Vesicle wall and processes laevigate. No excystment structure noted.

Measurements: Central body diameter: 31 μm. Process length > 28 μm (broken process). One specimen measured.

Remarks: A full description of this Fractoricoronula species, its stratigraphic significance and distribution in eastern Saudi Arabia is in preparation by Miller and Al-Ruwaili. Fractoricoronula n.sp. has been recorded from Aeronian to Telychian sediments from the subsurface of Libya by Hill and Richardson (1985) as ?Dateriocradus monterrosae, the Llandovery of Saudi Arabia (Le Hérissé et al., 1995; Le Hérissé, 2000) and the Tianguá Formation of northern Brazil (Le Hérissé et al., 2001) as Veryhachium cf. checkleyense.

GenusPapulogabataPlayfordinPlayford and Dring 1981

Type species: Papulogabata annulata Playford inPlayford and Dring 1981, p. 50, plate 13, figures 16, text-figures 9a,b.

n.sp. aff.Papulogabata

Plate 2, Figure 4

Description: Vesicle circular in outline, presumably hemispherical when inflated enclosing a cavity. Ornamented surface divided into a single polar field surrounded by six to eight equatorial fields. Fields psilate, separated by rounded thickenings or folds of the vesicle wall up to 3 μm wide. Vesicle wall 1 μm thick. Excystment possibly by circular pylome in the polar field ca. 13 μm in diameter. Surface opposite that bearing the pylome has a centrally located thickening.

Measurements: Vesicle diameter 24 to 29 μm, four specimens measured.

Remarks: This form has been attributed to Clypeolus tortugaides by Le Hérissé (2000, plate 9, figures e, f), with which it has overall morphological similarities. However, it differs in several specific and important features. Firstly this new form appears to have an enclosed vesicle. C. tortugaides, in contrast, is a circular, shield-like species that typically occurs as disparate hemicysts (Miller et al., 1997, plate 1, figures 1b, 3). Secondly, the arrangement of fields is more regular than that of C. tortugaides, especially in that there is only one polar field. Within this polar field there appears to be a circular pylome. If C. tortugaides enclosed a central cavity, two hemicysts are necessary and the excystment would have been by complete equatorial rupture. Papulogabata annulata shows some similar morphological characteristics to the new species described here; a pylome on one surface and the opposite having a centrally located, circular thickened area. A specimen of P. annulata (Playford and Dring, 1981, plate 13, figure 3) is very similar to the specimens here informally attributed to that genus from the Qusaiba Member in that weakly developed equatorial fields are present (e.g. “…slightly scalloped margin…” of Playford and Dring, 1981). Scanning electron microscopy is necessary to confirm the morphological observations described here before a new taxon can be established or assignment to Papulogabata confirmed.

GenusOppilatalaLoeblich and Wicander 1976

Type species:Oppilatala vulgarisLoeblich and Wicander 1976, p. 20, plate 6, figures 1113. ?Oppilatalan.sp.

Plate 1, Figures 35

Description: Acanthomorphic acritarch with spherical vesicle, circular to subcircular when compressed. Eight to nine hollow, thin-walled processes (< 0.25 μm thick), which do not communicate with the vesicle interior, are evenly distributed on the vesicle. Cylindrical processes, distinctly granulate, heteromorphic, and distally acuminate, branched or manate. Ornament of fine ridges radially arrayed on the vesicle at the base of the processes. Vesicle wall ~ 1.0 μm thick. The vesicle surface is irregularly covered with granules (0.5 to 2 μm wide and 1.0 to 2.0 μm high). Smaller granulate ornament is present randomly on the vesicle but larger granules are concentrated at the margin of the excystment structure where they are denser and may even be in the form of pila. Excystment by partial equatorial or near equatorial split, possibly circinate.

Measurements: Total diameter: 51–55 μm, vesicle diameter: 26–28 μm, process length: 13–17 μm, process width at base: 2–4 μm. Process number 8–9. Four specimens measured.

Remarks: ?Oppilatala n.sp. is retained in open nomenclature because an insufficient number of well-preserved, complete specimens were recovered and the ambiguous morphology of the excystment structure. It is only provisionally assigned to the genus Oppilatala because it has neither distinct basal plugs in the proximal parts of the processes nor the type of branched processes typical for that genus. The delicate, thin-walled processes are easily stripped from the central body; this is common in areas of elevated thermal maturities. In these cases specimens of ?Oppilatala n.sp. can easily be confused with species of Lophosphaeridium. The presence of the concentrated variably sized granules surrounding the excystment structure is a feature that allows its recognition, as are the thickenings of the wall at the site of process attachment. ?Oppilatala n.sp. differs from all described species of Oppilatala by its very thin processes. The present material is not sufficiently well preserved to identify the exact type of excystment structure. It appears to be in an equatorial or near-equatorial position. The absence of hemicysts or distinct breaks in the equatorial plane suggests the possibility of a circinate type of excystment (Miller, 1987).

GenusTylotopallaLoeblich 1970

Type species: Tylotopalla digitiferaLoeblich 1970, p. 738–739, Figures 33 D-F.

Tylotopalla caelamenicutisLoeblich 1970

Plate 1, figures 79.

1970 Tylotopalla caelamenicutis Loeblich, p. 738; figure 33 A-C.

2000 Tylotopalla caelamenicutisLoeblich 1970 – Le Hérissé, plate 7, figure g.

Description: Acritarch with spherical vesicle, circular to subcircular when compressed. Vesicle covered with short conical, apparently solid, striate processes with acuminate, or more rarely notched, terminations. The striae cause the processes to appear stellate in cross-section. Vesicle wall 1.0 μm thick, and granulate. No excystment structure recognized.

Measurements: Vesicle diameter: 25(28)33 μm, process height: 1 – 2 μm, process width at base: ca. 2.0 μm. Process number up to 25 per side. Eleven specimens measured.

Remarks: The compact nature of this acritarch allows its recognition in most thermal conditions. The highest consistent stratigraphic occurrence of T. caelamenicutis (persistent) in eastern Saudi Arabia is within the Angochitina hemeri Interval Range Zone, although very rare isolated occurrences have been noted in younger Silurian section. It has been reported from northern Europe as occurring as high as Wenlock (Le Hérissé, 1989). The relatively small size of these specimens favor T. caelamenicutis rather than T. deerlijkianum.

Cryptospore Alete Monads

GenusRugosphaeraStrother and Traverse 1979

Type species: Rugosphaera tuscarorensisStrother and Traverse 1979, p. 6, plate 1, figures 13. Rugosphaera agglomeratan.sp.

Plate 1, Figure 6; Plate 2, Figures 13, 59

Origin of name: agglomerata for the occurrence of numerous specimens in clusters.

Material: Several hundred specimens from the Mid-Qusaiba Sandstone and older parts of the Qusaiba Member.

Holotype:Plate 2, Figure 3, Well 10, core depth 13,126.8 ft, slide 48,845–1, M43, PM FM 1843.

Paratypes:Plate 1, Figure 6, Well 10, core depth 13,126.8 ft, slide 48,845–1, U46 4, PM FM 1841; Plate 2, Figure 1, Well 1, cuttings depth 13,840–13,870 ft, slide 47,630–1, S56 1, PM FM 1842; Plate 2, Figure 5, Well 1, cuttings depth 13,840–13,870 ft, slide 47,630–1, N51, PM FM 1844; Plate 2, Figure 6, Well 1, cuttings depth 13,840–13,870 ft, slide 47,630–1, M54, PM FM 1845; Plate 2, Figure 8, Well 10, core depth 13,126.8 ft, slide 48,845–1, M50 3, PM FM 1846; Wellman et al. (2000), Well 8, core depth 13,753 ft, slide 1, K53 1, FM 1,642, plate 2, figure g; Wellman et al. (2000), Well 8, core depth 13,273.6 ft, slide 1, L54 1, FM 1,644, plate 2, figure j.

Type locality and type stratum: Well 10, core depth 13,126.8 ft, Qusaiba Member of the Qalibah Formation, Llandovery.

Description: Circular alete cryptospores that occur as monads, dyads (?) and in monospecific clusters. Contact area between cryptospores may be thickened; there is no communication. Ornament consists of closely spaced rounded anastomosing sinuous rugulae. Width of rugula 0.5 to 1.0 μm; height < 0.5 μm. Rugulae ~0.5 μm apart. Wall unilayered.

Measurements: Diameter: 18(23)29 μm, holotype: 24 μm. 55 specimens measured.

Remarks: Rugosphaera agglomerata was first illustrated and described in open nomenclature by Wellman et al. (2000) as Rugosphaera sp. A from the Mid-Quasiba Sandstone of the HWYH-151 well. R. agglomerata n.sp. occurs as monads (Wellman et al., 2000, plate 2, figure g), as apparently permanent dyads (Plate 2, Figures 8, 9), and monospecific aggregates of numerous (up to 13 individual monads) cryptospores (Plate 1, Figure 6; Plate 2, Figures 1, 2, 5, 6).

Comparison: R. agglomerata n.sp. differs from R. tuscarorensis in that the muri are denser and more regular. R. cereba is larger and apparently has two wall layers (Miller and Eames, 1982, p. 249). The paired specimens of R. agglomerata are superficially similar to those of the bilayered species Segestrespora (Pseudodyadospora) rugosa, however in the latter a thickened contact area (ring), with or without a crosswall, equally or subequally divides the dyad (Johnson, 1985, p. 337). This feature is at the maximum width of the dyads. In R. agglomerata the contact is not the full width of the specimens nor are individuals in communication with each other. This mode of occurrence shown by R. agglomerata is also present in R. tuscarorensis. R. tuscarorensis occurs in dyads (Steemans et al., 2000, plate 3, figure m) and the thickening present at the junction of paired specimens is also clearly shown by the two upper specimens of R. tuscarorensis illustrated in plate 3, figure l (Steemans et al., 2000). They also (op cit., p. 102) reported R. tuscarorensis as occurring in monospecific clusters of up to 20 specimens. Because of the morphologic similarity with R. tuscarorensis, the type species of the genus, our new species is placed in Rugosphaera.

APPENDIX III: List of palynomorph taxa mentioned in the text.

All palynomorph genera and species mentioned in the running text or Table 1 are alphabetically listed below by genera and species within genera under their respective morphologic categories.

Acritarchs and Prasinophytes

BaiomeniscusLoeblich 1970

Beromia clipeataVavrdová 1986

Beromia rexroadiiWood 1996

Cymatiosphaera densisepta?Miller and Eames 1982

Clypeolus tortugaides (Cramer) Miller, Playford and Le Hérrisé 1997

Dactylofusa saudiarabiae Cramer and Díez 1972

?Dateriocradus monterrosae (Cramer) Pöthé de Baldis 1981 sensuHill et al. (1985)

Dictyotidium spp.

Diexallophasis remota (Deunff) emend. Playford 1977

Eupoikilofusa curvata n.sp.

Eupoikilofusa rochesterensisCramer 1970

ex Eisenack, Cramer and Díez 1976

Eupoikilofusa striatifera (Cramer) Cramer 1970

Fractoricoronula cubitaliaColbath 1979

Fractoricoronula n.sp.

Leiosphaeridia spp.

Lophosphaeridium spp.

Melikeropalla sp. 1 sensu Le Hérrisé (2000)

Micrhystridium spp.

Multiplicisphaeridium spp.

n.sp. aff. Papulogabata

Papulogabata annulata Playford inPlayford and Dring 1981

Onondagella n.sp.

OppilatalaLoeblich and Wicander 1976

?Oppilatala n.sp.

Oppilatala sp.

Oppilatala vulgarisLoeblich and Wicander 1976

TylotopallaLoeblich 1970

Tylotopalla caelamenicutisLoeblich 1970

Tylotopalla digitiferaLoeblich 1970

Tylotopalla deerlijkianum (Martin) Martin 1978

Veryhachium cf. checkleyense Dorning 1981 sensuLe Hérissé et al. 1995

Veryhachium europaeum Stockmans and Willière 1960

Veryhachium trispinosum (Eisenack) Stockmans and Willière 1962

Visbysphaera microspinosa (Eisenack) Lister 1970

Visbysphaera spp.

Chitinozoans

Ancyrochitina sp.

Ancyrochitina udaynanensisParis and Al-Hajri 1995

Angochitina hemeriParis and Al-Hajri 1995

Angochitina macclureiParis and Al-Hajri 1995

Angochitina sp.

Belonechitina arabiensisParis and Al-Hajri 1995

Belonechitina spp.

Bursachitina sp. A sensuAl-Hajri and Paris (1998)

Calpichitina densa (Eisenack 1962)

Conochitina alargada Cramer 1967

Conochitina edjelensis? Taugourdeau 1963

Conochitina elongata Taugourdeau 1963

Conochitina sp.

Cyathochitina caputoi da Costa 1971

Eisenackitina lagenomorpha sensu Aouled and Al-Hajri (1995)

Plectochitina longicornis (Taugourdeau and de Jekhowsky 1960)

Plectochitina paraguayensis Wood and Miller 1991

Plectochitina spp.

Pterochitina deichaii Taugourdeau 1963

Sphaerochitina solutidina Paris 1988

Cryptospores/Spores

Abditusdyadus laevigatus Wellman and Richardson 1966

Ambitisporites avitutus/dilutus

Ambitispories sp.

Archaeozonotriletes chulus (Cramer) Richardson and Lister 1969

Dyadospora murusattenuataStrother and Traverse 1979 emend. Burgess and Richardson 1991 Dyadospora murusdensaStrother and Traverse 1979 emend Burgess and Richardson 1991 Hispanaediscus? species 2 sensu Steemans, Higgs and Wellman (2000)

Hispanaediscus? species 3 sensu Steemans, Higgs and Wellman (2000)

Hispanaediscus spp.

Imperfectotriletes patinatusSteemans, Higgs and Wellman 2000

Imperfectotriletes vavrdovaeSteemans, Higgs and Wellman 2000

Imperfectotriletes spp.

Laevolancis divellomedia (Chibrikova) Burgess and Richardson 1991

Laevolancis divellomedia/plicata Pseudodyadospora laevigataJohnson 1985Pseudodyadospora petasus Wellman and Richardson 1993

Rimosotetras problematica Burgess 1991

Rugosphaera cerebraMiller and Eames 1982

Rugosphaera agglomerata n.sp.

Rugosphaera sp. A sensu Wellman, Higgs and Steemans (2000)

Rugosphaera tuscarorensisStrother and Traverse 1979

Segestrespora (Pseudodyadospora) rugosa (Johnson) Burgess 1991

Tetrahedraletes medinensisStrother and Traverse 1979 emend. Wellman and Richardson 1993 Velatitetras laevigata Burgess 1991

Miscellaneous Palynomorphs

Plant Fragments

Laevitubulus sp.

Porcatitubulus sp.

Tortotubus protuberansJohnson 1985

Zooclasts and Algae

Arpylorus sp.

Moyeria cabottii (Cramer) Miller and Eames 1982

ABOUT THE AUTHORS

Merrell Miller is a Palynologist with Saudi Aramco’s Geological Technical Services Division in Dhahran. He received a Masters degree from Ohio State University in 1976 and since then has gained experience in industrial palynology working with Texaco and Amoco. From 1978 to 1991, Merrell worked extensively on Early Paleozoic palynology for Amoco Research including North African and Arabian Plate biostratigraphy and source rocks. He was in Amoco’s international technical service organization from 1992 until 1999. Merrell is a member of the AASP, The Micropalaeontological Society, CIMP, Dhahran Geoscience Society, the Paleontological Society, and SEPM. His publications include paleoenvironmental, taxonomic and biostratigraphic topics.

merrell.miller@aramco.com

John Melvin is Team Leader for Pre-Khuff Special Studies with Saudi Aramco’s Gas Fields Characterization Division. His primary technical responsibilities lie in the clastic sedimentology and reservoir stratigraphy of the Unayzah and Basal Khuff Clastic reservoirs. Prior to holding this position, he worked on the sequence stratigraphy of the Upper Ordovician and Silurian rocks of Saudi Arabia in Aramco’s Geological R&D Division. He obtained his PhD from the University of Edinburgh in Scotland, and then spent over 20 years with BP employed as an Applied Sedimentologist. There he was involved in a large number of exploration and development reservoir studies in the North Sea and Alaska. He then spent 6 years as a Consulting Sedimentologist and Stratigrapher, successfully concluding reservoir studies in Egypt, Libya, Colombia and the North Sea, before joining Saudi Aramco in 2001. John has published several articles on applied sedimentology, and is a member of the AAPG, Dhahran Geoscience Society, IAS and PESGB.

john.melvin@aramco.com