Sub-surface Palynostratigraphy of the Palaeozoic of Saudi Arabia
Published:January 01, 2000
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Sa’id Al-Hajri, Bernard Owens, 2000. "Sub-surface Palynostratigraphy of the Palaeozoic of Saudi Arabia", Stratigraphic Palynology of the Palaeozoic of Saudi Arabia, Sa’id Al-Hajri, Bernard Owens
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A Kingdom-wide palynological analysis was carried out on the Palaeozoic section within the framework of the Saudi Aramco-Commission Internationale de Microflore du Paléozoïque (CIMP) Project. Thirty-three biostratigraphic assemblages and zones have been documented as a result of this study. A palynostratigraphic and lithostratigraphic review of the Palaeozoic of Saudi Arabia is provided herein.
Since the early days of hydrocarbon exploration in Saudi Arabia, palynologists have identified a variety of major protists and vegetation types that occur throughout the Palaeozoic sedimentary section of Saudi Arabia. Oftentimes, stratigraphers have worked independently of palaeontologists resulting in different interpretations and age assignments of the Palaeozoic strata of the Kingdom. With the discovery of significant Palaeozoic hydrocarbons by Saudi Aramco in the last decade, it became more urgent to reconcile the two disciplines and to better define the Palaeozoic stratigraphy of Saudi Arabia by integrating the biostratigraphic data with the lithostratigraphy.
This volume presents the results of the second phase of the Saudi Aramco-Commission Internationale de Microflore du Paléozoïque (CIMP) Project. The results of Phase I were published in 1995 in the Review of Palaeobotany and Palynology (Owens et al., 1995). In the second phase, samples from forty-one wells have been investigated (Figures 1 and 2). Within the framework of this phase of the project, 33 biostratigraphic assemblages and biostratigraphic zones have been documented as a result of this intensive study of sub-surface material (Figure 2).
Two regional stratigraphic cross-sections (Figures 3 and 4) provide quick access to the understanding of the regional distribution and complexity of the Palaeozoic stratigraphic units with the exact position of the palynological samples used in this study to qualify the acquired biostratigraphic ages.
The purpose of this paper is twofold. It briefly outlines the Palaeozoic stratigraphy as perceived by Saudi Aramco, and secondly, the paper summarises the palynostratigraphic understanding obtained to this date as a result of the Saudi Aramco-CIMP project, and Saudi Aramco’s in-house data.
The Palaeozoic succession of the Arabian Peninsula comprises several siliciclastic sedimentary packages whose nature of deposition reflects shallow-marine to fluvio-deltaic environments which were influenced by eustatic oscillation and periodic uplifts. These sediments are encountered in the sub-surface of the Arabian Peninsula, and in discontinuous outcrops around the Arabian Shield (Figure 1). Several major coastal onlaps have punctuated a regional northward prograding fluvio-deltaic regime that existed throughout most of the Palaeozoic times. Major periods of diastrophism, and the resultant inter-regional unconformities plus associated fundamental changes in sedimentation define the boundaries of these sedimentary packages and represent major chapters in the geologic history of Saudi Arabia. In the following paragraphs the Palaeozoic palynostratigraphy of the Arabian sedimentary units are briefly discussed in a system-by-system manner.
Sedimentary rocks below the well-known Cambrian stratigraphic units and above the igneous basement have only been documented seismically in the northern part of Saudi Arabia. These sediments appear to be very thick (up to 15,000 feet, B. Naini, personal communication, 1993), however the sediments have not yet been penetrated. The volcano-sedimentary Jubailah Group, on the other hand, crops out on the Arabian Shield, and no biostratigraphic control is available for it. These Infracambrian rocks in northwestern Saudi Arabia have accumulated in a rift basin setting. They seem to be temporal equivalents to the Precambrian (Vendian) Huqf Group of Oman.
Sedimentary rocks of the Cambrian system are divided into three stratigraphic units: Siq, Burj, and the lower part of the Saq Formation. The fluvial sandstones of the Siq Formation and the marine Burj carbonates have only been encountered in northwestern Arabia where they have yielded Early and Middle Cambrian acritarchs from the Jordanian sub-surface (Keegan et al., 1990).
The overlaying Saq Formation is divided into two members at outcrop, the Risha and Sajir members, which probably once covered most or all of Arabia. In the present study, the siltstones and shales from the lowermost part of the Saq Formation and the upper part of the Burj Formation in the Khursaniyah-81 (KRSN-81) well (M.D. Mahmoud, M.I. Al-Husseini and A.A. Al-Laboun, Saudi Aramco unpublished report, 1988) yielded Early to Middle Cambrian acritarchs (Molyneux and Al-Hajri, this volume). This assemblage is characterised by the occurrence of Annulum squamaceum (Volkova) Martin 1983.
Saq Formation deposition started in the Middle Cambrian and ended in the early Llanvirn. An Arenig to early Llanvirn age is documented in palynological samples from the uppermost part of the Saq Formation of Khursaniyah-81 (Molyneux and Al-Hajri, this volume). Commencing in the middle Llanvirn, several major progradational packages deposited the shelf mudstones and shales of the Hanadir, Kahfah, Ra’an, and Quwarah members of the Qasim Formation.
A late Llanvirn-early Llandeilo (late Abereiddian) age has been documented in Ain Dar-196 (ANDR-196) from the Hanadir Member, where the chitinozoans Lincohitina pissotensis Paris, 1981, and Laufeldochitina clavata (Jenkins, 1967) are the most distinguishing features of the palynological assemblages (Paris et al., this volume). Also from the same well, late Caradoc chitinozoan assemblages 3B and 3A belonging respectively to Tanuchitina fistulosa (Taugourdeau and de Jekhowsky 1960), and Fungochitina fungiformis (Eisenack 1931), were recovered from the Ra’an Member. Undiagnostic Belonechitina species were recovered from the Quwarah Member of Ain Dar-196. More characteristic chitinozoan biozones (Armoricochitina nigerica (Bouche (1965), and Ancyrochitina merga (Jenkins 1970)) from the Quwarah Member that suggest an early to middle Ashgill age (Owens et al., 1995) have previously been recovered from sub-surface material of northwestern Saudi Arabia.
A coarse sandstone facies of the Qasim Formation occurs in central and southern Saudi Arabia. This facies has been designated the “Abu-Jifan Group” (T. Stump, unpublished Saudi Aramco Proprietary Reports, 1993; Connally and Wiltse, 1995). While the age relationships for the members of the Qasim Formation have been clearly documented, the chronostratigraphy of the so-called “Abu-Jifan Group” is poorly understood. Initial results from this project indicate that the age of this group equates with that of the lower Qasim Formation or the upper Saq Formation (Molyneux and Al-Hajri, this volume). This facies is believed to be a lateral correlative to the exposed Dibsiyah Formation of southwestern Saudi Arabia.
Towards the end of the Ordovician, sea-level fluctuation due to glaciation in Northern Gondwana resulted in the deposition of irregular and scattered pods of sediment throughout Arabia, named the Sarah Formation. The sandy nature of this unit makes it difficult to date, and only a broad Ashgill age was assigned to this unit in Abu Jifan-25 (ABJF-25), and Mazalij-1 (MZLJ-1) wells, based on poor recovery of palynomorphs (Molyneux and Al-Hajri, this volume). On the other hand, cryptospores recovered from the Sarah Formation of Hawiyah-151 (HWYH-151) suggest an Early Silurian age (Wellman et al., this volume). The Sarah Formation was assigned a late Ashgill (Hirnantian) age at the Jalamid area of northern Saudi Arabia (Al-Hajri et al., 1999).
The Silurian system is made up of pro-delta mudstones of the Qusaiba Member, the delta front siltstones and sandstones of the Sharawra Member, and the basal sandstones of the braided stream deposits of the Tawil Formation. These sediments accumulated in rapidly subsiding basins which were inundated by high organic productivity seas. Analysis of the Qusaiba and Sharawra members palynofacies, gamma-ray log cycles and the inferred orbital cyclicities suggest time durations of 7.1 and 2.1 million years for the two units, respectively (Marshall, this volume).
The organically rich shales of the Qalibah Formation reflect the abundant and diverse flora and fauna preserved in these deposits. Researchers continue to document new diagnostic species from this formation (Al-Hajri and Paris, 1988; Al-Ruwaili, this volume; Paris and Al-Hajri, 1995; Steemans et al., this volume). In this volume, Al-Ruwaili describes eight new stratigraphically important acritarch species and Le Hérissé documents seven acritarch assemblages which characterise the Qusaiba and Sharawra members of the Qalibah Formation. Moreover, the Imperfectotriletes spp.-Laeovolancis divellomedia spore Assemblage Biozone suggests a late Aeronian to early Telychian age for the lower and middle parts of the Qusaiba Member in Nuayyim-2 (NYYM-2) (Steemans et al., this volume), and Hawiyah-151 wells. On the other hand, the upper part of the Qusaiba Member in Hawiyah-151 is characterised by the late Telychian to early Homerian chulus-nanus spore Assemblage Biozone (Wellman et al., this volume).
The Wenlock brevicosta-verrucatus spore Assemblage Biozone was recovered from the Sharawra Member in Hawiyah-151 well (Wellman et al., this volume). These results are consistent with those based on chitinozoans from the Sharawra Member of northwestern Saudi Arabia (Al-Hajri and Paris, 1998). No palynomorphs have been recovered from the Tawil Formation in the present study, however, previous investigations suggest a Ludlow to Early Devonian age for this unit (Al-Hajri and Paris, 1998; Owens et al., 1995).
The Devonian System of Saudi Arabia is represented by the upper part of the Tawil Formation, the Jauf and the Jubah formations. A regional disconformity separates these units from the underlying sediments. The Jauf Formation changes facies on a regional scale where its depositional environment is interpreted to be marine in northwestern Saudi Arabia and marginal-marine to continental in the central and southern regions.
In Phase I of the Saudi Aramco-CIMP Project a Lochkovian to Pragian age was proposed for the Tawil Formation (Owens et al., 1995), and an Emsian to Frasnian age was assigned to the Jauf and Jubah formations (Owens et al., 1995). The ages of the Jubah Formation have been confirmed in the current phase from well S-462 (Loboziak, this volume). Furthermore, the latest Famennian Retispora lepidophyta Biozone and the earliest Tournasian Indotriradites explanatus Assemblage were documented respectively in Harmaliyah-51 (HRML-51), and Abu Safah-29 (ABSF-29) wells, and therefore marking this the youngest recorded Jubah Formation in Arabia (Al-Hajri et al., 1999; Clayton et al., this volume).
The Carboniferous System of Saudi Arabia is represented by the sporadic occurrence of the Berwath Formation and the lower Al Khlata Formation equivalent of Oman. The lower part of the Juwayl Formation is of a suspected Late Carboniferous age. A Visean to early Namurian age (RT and MJ zones) has been documented for the Berwath Formation in Abu Safah-29 (ABSF-29), ST-8, Ar’ar-1, and Haradh-601 (HRDH-601) (Clayton et al., this volume; Owens et al., this volume). The Berwath Formation is separated from the overlaying Al Khlata Formation equivalent by the Hercynian Unconformity where a significant hiatus is marked in ST-8 (Owens et al., 1995). A Westphalian-D to Stephanian Al Khlata equivalent interval was identified immediately above the Namurian Berwath Formation in the aforementioned well. Other Late Carboniferous assemblages were recovered from the Al Khlata Formation in the Omani Amal-6, and Amal-9 wells.
The Unayzah and Khuff formations, and possibly the upper part of the Juwayl Formation make up the Permian System of Saudi Arabia. The oldest Permian assemblage was recovered from Jufarah-1 (JFRH-1) where a Sakmarian (P3 zone) age was documented in a grey shale interval in the middle part of the Unayzah red siltstones. A younger assemblage (?Artinskian-Kungurian, P2 zone) was recovered from the Unayzah Formation of Hilwah-3 (HLWH-3) well (Stephenson and Filatoff, 2000a, this volume). The Unayzah Formation is separated from the basal Khuff clastics by a significant hiatus where a Tatarian age (P1 zone) has been documented for the latter in many different wells such as Dilam-1 (DILM-1), and Haradh-51 (Stephenson and Filatoff, 2000b, this volume). This finding confirms earlier reports on the Pre-Khuff Unconformity (Senalp and Al-Duaiji, 1995). The carbonates of the Khuff Formation were the result of a major transgression and found to be overstepping older formations in many localities. These carbonates bracket the Permian-Triassic boundary which is often marked by a marine biostratigraphic event reflected by the occurrence of abundant acritarchs such as Veryhachium valensii and Micrhystridium species.
Thirty-three palynological assemblages and zones have been documented from forty-one Palaeozoic wells in Saudi Arabia.
Middle Cambrian to early Llanvirn ages have been documented from the Saq Formation.
A late Abereiddian age was documented from the Hanadir Member of the Qasim Formation of eastern Saudi Arabia.
A late Caradoc age was assigned to the Ra’an Member of the Qasim Formation in Ain Dar-196.
The “Abu-Jifan Group” is believed to be a heterogeneous stratigraphic package composed of different proximal extremities of the Saq, Qasim, and the Sarah formations.
Cryptospores and miospores are of biostratigraphic value in the Qusaiba Member.
The Sharawra Member is characterised by the Wenlock brevicosta-verrucatus spore Assemblage Biozone.
Eifelian to Tournaisian spore assemblages have been recovered from the Jubah Formation.
The Berwath Formation is of Visean to early Namurian age, and the lower Al Khlata equivalent in Saudi Arabia is of Late Carboniferous age.
The Unayzah Formation is characterised by Early Permian (Sakmarian-Kungurian) spore and pollen assemblages.
The basal Khuff clastics are separated from the Unayzah Formation by a significant hiatus. The basal Khuff clastics are of Tatarian age.
The Permo-Triassic boundary occurs within the Khuff carbonates.
We are very grateful to Moujahed Al-Husseini, Abdulaziz Al-Duaiji, John Filatoff, Mansour Al-Ruwaili, Riyadh Rahmani, and Muhittin Senalp, for their valuable advice, criticisms and review of this manuscript. We also acknowledge with gratitude the Saudi Arabian Ministry of Petroleum and Mineral Resources and the Saudi Arabian Oil Company for permission to publish this article.
ABOUT THE EDITORS
Sa’id Al-Hajri is Supervisor of the Regional Mapping and Special Projects Unit of the Saudi Arabian Oil Company. He holds a BSc in Geology from the University of Petroleum and Minerals, and an MSc in Geosciences from Penn State University. Sa’id is professionally interested in the Early Palaeozoic palynology and stratigraphy of northern Gondwana. He is a member of CIMP, AASP, BMS, and DGS, and has published several papers on geological and palynological subjects.
Bernard Owens is an Upper Palaeozoic Palynologist who has recently retired after 33 years with the British Geological Survey in Nottingham, UK, where he was also Manager of the Basin Analysis and Stratigraphy Group. During the past 18 years he has managed major research projects involving teams of European palynologists in both Libya and the Arabian Peninsula. He has recently been elected for a second time as President of the Commission Internationale de Microflore du Paléozoïque and is currently working with the University of Sheffield where he is Director of the Centre for Palynological Studies.