Significant New Biostratigraphic Horizons in the Qusaiba Member of the Silurian Qalibah Formation of Central Saudi Arabia, and T
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GeoArabia, Vol. 10, No. 1, 2005 Gulf PetroLink, Bahrain Significant new biostratigraphic horizons in the Qusaiba Member of the Silurian Qalibah Formation of central Saudi Arabia, and their sedimentologic expression in a sequence stratigraphic context Merrell A. Miller and John Melvin 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. 49 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/10/1/49/4564720/miller.pdf by guest on 27 September 2021 Miller and Melvin Qusaiba Sequence Stratigraphy, Saudi Arabia 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, ������ Caspian ������ ���������� Sea ������� � ������ ������ ����� ������ ���� � ����� Med Sea ���� ����� ����� ���� ������ ������ Arabian Gulf �������� �� ������� ������� ����� �� ����� ������� ��� ��������� ����� ������ �������� ���� �� ���� ������������ ������ ����������� ������ Red ������� ����� Sea ����� � � ������� Arabian Sea ������� ��������� ����� �������� Gulf of Aden ������ ������ � ���������� ������� ��������� ����� ����� ������������ � � ������ ����� ������ ������� ����� ��������� � ������� ������� ���������� � ���� � ������ ������ ����� ������� ������� ������ ��������� �� ���� ����� ����� ������� ���� ������ ������ ������ ����������� ������� �� ������ �� ������� ������ ������� � � �������� ������ �������� ����� ������� ������� � � �� �� ������������� Figure 1: Location map of east central Saudi Arabia showing the wells studied with outlines of major oil (green) and gas (red) fields. Numbered wells are mentioned in the text and systematic palynology (Appendix II). Cross-sections A–A’ (wells 1, 2 and 3), B–B’ (wells 4, 1 and 5) and B’–B” (wells 5, 6, 7, 8, 9, 10 and 11) are shown in Figures 13, 14 and 15, respectively. The Mid-Qusaiba Sandstone was cored in Wells 5, 6, 8, 9 and 10 (Figure 5 shows the graphic core logs). 50 51 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/10/1/49/4564720/miller.pdf by guest on 27 September 2021 Miller and Melvin Qusaiba Sequence Stratigraphy, Saudi Arabia ������� ������� ������ ������� �� ��������� ��������� ������ ����� ��� ����� � � � ��� ����� ��� ��������� ������� ��� �� ������ ��������������� ��� ���������� �������� ������� ������������ �������� ��� �������� ��������� ������� ������� �������� ���������� ������ ������������ ���� ���������� ����������� ��� ���������� ����� ���� ����� ������� ����� ����� ���� ���������� ����������� ������ ������ ������� Figure 2: Generalized lithostratigraphy for east central Saudi Arabia (after Wender et al., 1998, figure 3). PZSU = pre-Zarqa/Sarah unconformity, PTU = pre-Tawil unconformity, PUU = pre- Unayzah Unconformity, PKU = pre-Khuff unconformity. Absolute ages for the Llandovery are from Gradstein and Ogg (1996). Ordovician stage names are currently being revised by the Ordovician Subcommission. 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 50 51 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/10/1/49/4564720/miller.pdf by guest on 27 September 2021 Miller and Melvin Qusaiba Sequence Stratigraphy, Saudi Arabia 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