GeoArabia, Vol. 9, No. 1, 2004 Gulf PetroLink, Bahrain Arabian Plate Sequence Stratigraphy – revisions to SP2 Peter R. Sharland, David M. Casey, Roger B. Davies, Michael D. Simmons and Owen E. Sutcliffe INTRODUCTION The purpose of this note is to present revisions to the SP2 scheme resulting either from significant errors in SP2, or from newly published data that challenges SP2, or from newly published data identifying new maximum flooding surfaces. The publication of Arabian Plate Sequence Stratigraphy (Sharland et al., 2001), commonly referred to as SP2, provided a unifying stratigraphic interpretation of the Arabian Plate within a modern sequence stratigraphic framework. In 2002 revisions to the stratigraphic positions of some SP2 Cretaceous maximum flooding surfaces (MFS), and some resulting new interpretations, were presented by Davies et al. (2002). New interpretations of mixed carbonate-clastic systems presented by these authors have application to other parts of Middle East stratigraphy. Since 2001, many excellent new papers have been published, and oral presentations made, containing new data and/or interpretations (e.g. Al-Eidan et al., 2001; Brew et al., 2001; Konert et al., 2001; Ziegler, 2001; Al-Suwaidi and Aziz, 2002; Nehlig et al., 2002; van Buchem et al., 2002; Boote and Mou, 2003; Price and Fell, 2003; Stephenson et al., 2003 - to name but a few). The interpretations presented here are based on this new post-SP2 literature, as well as re-interpretations of older literature in the light of this new work. Following discussions with Gulf PetroLink, the Ne�ex SP2 authors have been encouraged to provide a summary chronostratigraphic up-date based on this new literature for GEO 2004 of the SP2 interpretation, with particular emphasis on (1) revisions to the published geological timescale; (2) changes to SP2 megasequence boundaries (position and dating); (3) changes to SP2 maximum flooding surfaces (position and dating); (4) the identification and dating of any new TMS and/or MFS; and (5) any proposed changes to the SP2 sequence stratigraphic nomenclature. This GeoArabia Stratigraphic Note and the accompanying two chronostratigraphic chart enclosures (Enclosure 1 Mesozoic and Cenozoic, and Enclosure 2 Palaeozoic and Precambrian), are intended to update those in SP2. These new charts have been extended into Jordan (in the north) and southwest Oman and Yemen (in the south). Significant changes to MFS are made in the Neogene, Palaeogene and Permo-Triassic sections. Geological Timescale The a�ached chronostratigraphic chart enclosures utilise the Phanerozoic timescale of Gradstein and Ogg (1996) (G&O ‘96). Over the last few years there have been potential revisions to parts of the G&O ’96 timescale, offering the potential to merge the up-dated parts with the unchanged parts resulting in a ‘hybrid’ timescale (see, for example, the more recent revisions to the Permian timescale illustrated by Stephenson et al. (2003) versus G&O ’96). However, the G&O ’96 timescale is still the most up- to-date single chart for the entire Phanerozoic. Most importantly, the G&O ’96 timescale was used in SP2, and therefore allows an easy comparison between the enclosed updated charts and those in SP2. All absolute ages given herein thus relate to G&O ’96. We are aware that a new geological timescale, ratified by the International Commission on Stratigraphy, will be published in 2004, most likely to coincide with the International Geological Congress meeting 199 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/9/1/199/4566762/sharlnd.pdf by guest on 02 October 2021 Sharland et al. Stratigraphic Note 2003 in Florence in July 2004. This new timescale is expected to cover the entire Phanerozoic, and will thus potentially replace the G&O ’96 timescale used here and in SP2. The only change to the G&O ‘96 timescale we utilise here is a slight revision of the age of the Precambrian-Cambrian boundary from 545 Ma to 542 Ma. This change been made on the basis of some well-constrained dates from Namibia that are associated with rocks of the basal Cambrian Phycodes pedum Zone (Grotzinger et al., 1995) and, more recently, new radiometric dates from ashes associated with Late Precambrian – Cambrian faunal turnover in the Ara Group of Oman (Amthor et al., 2003; Al-Husseini et al., 2003). Megasequence Boundary Revisions The boundaries of the 11 Arabian Plate (AP) tectono-stratigraphic megasequences (TMS, see SP2) are plate-wide unconformity surfaces that record major changes in accommodation space, resulting from plate-scale tectonic events. Literature published since 2001 allows the re-definition of 2 of these unconformity surfaces: Top AP5 = revised from near base Wordian (255 Ma) to latest Kungurian (257 Ma); Top AP4 = revised from late Stephanian (295 Ma) to earliest Westphalian (315 Ma). Top AP5 Revision: On the basis of new data within Stephenson et al. (2003) and Angiolini et al. (2003) the age of this TMS boundary is revised from near base Wordian (255 Ma) to latest Kungurian (257 Ma) (see Stephenson et al., 2003 for an illustration of current Perman timescales). The P20 MFS establishes the Khuff carbonate (and equivalents) across the new passive margin across much of Arabia. This revision is caused by recognition that MFS P20 is clearly Wordian (= Kazanian) in age (see below). In Oman, Stephenson et al. (2003) show that the oldest rocks within the P20 TST above the top AP5 unconformity (comprising the Upper Gharif Member) lie within part of the poorly calibrated OPZ5 Biozone. Although this biozone is poorly chronostratigraphically-calibrated, it is dated as Roadian- Wordian, which is also supported by the occurrence of plant megafossils (Broutin et al., 1995). Correlative P20 TST rocks include the basal Khuff clastics of Saudi Arabia and its equivalents. Therefore, in most places, the oldest sediments above the top AP5 unconformity are Roadian in age, and probably slightly older (i.e. latest Kungurian at 257 Ma) in (undrilled off-structure) subsiding depocentres. Top AP4 Revision: Top AP4 was incorrectly located in SP2 and is revised from late Stephanian (295 Ma) to earliest Westphalian (315 Ma) because the climax of the Hercynian Orogeny associated with this TMS boundary is shown to be around 315-330 Ma by Konert et al. (2001). These authors regard the Visean–Namurian Berwath Formation of Saudi Arabia as ‘syn-Hercynian’ (i.e. it potentially onlaps Hercynian structures and may be partly deformed by them), which places a limitation on the age of this TMS boundary. Maximum Flooding Surface Revisions Arabian Plate Sequence Stratigraphy identified 63 MFS using a ‘decimal’ nomenclature (see SP2 and below). Two new MFS are recognised here, making 65 in total. One of the key criteria for recognising MFS in the SP2 scheme was the fact that they could be biostratigraphically calibrated in terms of age at their Reference Section (see SP2), and were regionally extensive and thus correlatable over wide areas. Additional, probable higher frequency, surfaces (e.g. those in the Umm Er Radhuma Formation) were not given decimal MFS status due to the lack (at that time) of age-significant data, even though they were very likely to be regionally extensive surfaces. As far as possible, we feel that age-significant data should be provided to support MFS proposals – without age-diagnostic data surfaces cannot be placed on a chronostratigraphic chart. 200 201 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/9/1/199/4566762/sharlnd.pdf by guest on 02 October 2021 Sharland et al. Stratigraphic Note 2003 Recent literature shows that the MFS identified in SP2 have stood the test of time relatively well, although much new work in the region is now seeing the light of day. Due to the publication of new data, or re-evaluation of existing literature (especially van Bellen et al., 1959 – the Iraq Stratigraphic Lexicon), we now revise 8 MFS and identify 2 new MFS as follows: MFS Ng40: new reference section and age; MFS Ng30: newly defined MFS, reference section and age; MFS Ng20: unchanged reference section, revised age (was MFS Ng30 in SP2); MFS Ng10: new reference section and age (was previously MFS Ng20 in SP2); MFS Pg50: new reference section and age (was previously Ng10 in SP2); MFS Pg40: newly defined MFS, reference section and age; MFS Tr10: new geological age, reference section remains the same; MFS P40: new geological age, reference section remains the same; MFS P30: new geological age, reference section remains the same; MFS Cm10: new geological age, reference section remains the same. Oligocene-Miocene Revisions (MFS Pg40 to Ng40): Literature covering the Oligocene – Miocene succession of the Middle East, particularly the elongate NW-trending Zagros foreland region, contains conflicting data, age and correlation interpretations (see for example the differences of opinion between Jones and Racey (1994) and Goff et al. (1995). For this succession in SP2 we incorrectly placed most emphasis on data within Motiei (1993) from southern Iran. Re-evaluation of data in the Iraq Stratigraphic Lexicon (van Bellen et al., 1959) shows that the be�er and more complete stratigraphic and age data exists in that work and should have been used in SP2. The extensive revisions herein to the Oligocene – Miocene MFS thus result from moving from Motiei (1993) to van Bellen et al. (1959) as the key source of published data. In SP2, four MFS (Pg30, Ng10, Ng20 and Ng30) were recognised in the Oligocene–mid-Miocene succession of the Middle East based on Motiei (1993). However, van Bellen et al. (1959) recognise five (not four) cycles of carbonate progradation and retreat separated by widespread unconformities (see Figure 1, redrawn a�er van Bellen et al., 1959). This figure indicates that additional MFS are present above those recognised in SP2, and that all of the Oligo-Miocene SP2 MFS thus need revision with regard to their age and stratigraphic placement.