Elsevier

Revue de Micropaléontologie

Volume 60, Issue 3, July–September 2017, Pages 417-432
Revue de Micropaléontologie

ARAMCO-CIMP special volume
Palynological assemblages across the Hercynian unconformity in Western IraqAssemblages palynologiques à travers la discordance hercynienne en Irak occidental

https://doi.org/10.1016/j.revmic.2016.05.005Get rights and content

Abstract

Recent study of samples from borehole KH-5/1 has allowed an assessment of the duration of the hiatus associated with the so-called Hercynian unconformity (also known as the ‘Late Carboniferous unconformity’ or ‘pre-Unayzah unconformity’) in western Iraq. KH-5/1 was drilled as a deep water well and fully cored to TD at 1620 m. The well section spans the unconformity at 670 m depth with the Raha Formation below and the Ga’ara Formation above. The unconformity appears to be associated with non-deposition or erosion of rocks corresponding approximately in age to part of the Serpukhovian and Bashkirian (latest Mississippian to early Pennsylvanian), similar to the duration associated with the same unconformity in well ST-8 situated to the south of KH-5/1 in northern Saudi Arabia. The Ga’ara Formation assemblages above the unconformity in KH-5/1 are similar in character to those described from 4620 to 4200 feet in ST-8. The age of these assemblages in both KH-5/1 and ST-8 is considered in this paper to be Westphalian. The composition of the Ga’ara Formation assemblages in KH-5/1 also shows some similarity to glacigene post-unconformity beds of the 2165 Biozone of the Al Khlata Formation of Oman.

Résumé

Récemment, l’analyse des échantillons du forage KH-5/1 a permis l’évaluation de la durée du hiatus associé à la discordance hercynienne (connue aussi comme la discordance du Carbonifère supérieur) en Irak occidental. Le forage KH-5/1 atteint la profondeur terminale de 1620 m. Il traverse la discordance à une profondeur de 670 m avec la Formation Raha au-dessous et la Formation Ga’ara au-dessus. Vraisemblablement, la discordance est associée à un intervalle de non-dépôt ou d’érosion des roches d’un âge Serpukhovien et Bashkirien (Mississippien supérieur–Pennsylvanien inférieur), de durée équivalente à la discordance reconnue dans le forage ST-8 au nord de L’Arabie saoudite. Les assemblages de la Formation de Ga’ara au-dessus de la discordance dans le forage KH-5/1 sont similaires aux assemblages décrites dans l’intervalle 4620–4200 ft du sondage ST-8. L’étude actuelle propose un âge Westphalien pour les deux assemblages de KH-5/1 et ST-8. La composition des assemblages de la Formation Ga’ara en KH-5/1 montre des similitudes avec des dépôts glaciogéniques, formés après la discordance. Ils correspondent à la Biozone 2165 de La Formation Al Khlata en Oman.

Introduction

Across the Arabian Plate, a stratigraphic gap is present within the Carboniferous. In the southern Arabian Peninsula, the rocks above and below are typically distinguished not only by age but also by their facies, in that beds above the unconformity are glacial in sedimentological character and contain palynomorphs that suggest cold climates, while those below do not show glacial or particular cold climate character palynomorphs. Further north in the Arabian Peninsula this facies distinction is not always so clear. The unconformity is known variously as the pre-Unayzah unconformity (e.g. Al-Husseini, 2004), the Late Carboniferous unconformity (Aqrawi et al., 2010), and the Hercynian unconformity (e.g. Al-Hajri and Owens, 2000; terminology used here). The areal extent of the unconformity and the hiatus it represents are important economically because typical Arabian source and reservoir rocks are often not present because of erosion related to the unconformity.

The duration of the stratigraphic gap associated with the Hercynian unconformity (HU) varies considerably across the Arabian plate. Well sections provide very important evidence as to the biostratigraphy of the unconformity. Northern well sections containing stratigraphy that is useful in defining the unconformity include those in ABSF-29 (offshore Arabian Gulf; Fig. 1) and ST-8 (northern Saudi Arabia). These latter have stratigraphic coverage of both the oldest post-unconformity beds and the youngest pre-unconformity beds, and have been used to most narrowly constrain the age of the event that caused either the non-deposition or erosion associated with the stratigraphic gap. Al-Husseini (2004) used these well sections to suggest an approximate Namurian – early Westphalian (Serpukhovian – Bashkirian; 327–311 Ma; see Fig. 2 for stratigraphic nomenclature) compressional tectonic event.

In Saudi Arabia, the youngest pre-unconformity beds belong to the Berwath Formation. Al-Husseini (2004) commented on the rarity of this formation in Arabian stratigraphy, pointing out its complete absence from the surface and its rarity in the subsurface, and attributed this to it being the ‘uppermost pre-tectonism’ rock unit and therefore prone to removal by erosion. Al-Husseini (2004) suggested that the event affected extensive north-south trending fault blocks (e.g. the Hawtah and Ghawar structures, Fig. 1), and was expressed in open folds in the Oman Mountains, and may have included further uplift of pre-existing highs such as the Central Arabian Arch and the Haushi-Huqf high in Oman.

Away from the northern Gulf and northern Saudi Arabia, the stratigraphic gap of the HU appears to increase such that post-unconformity beds rest on the Devonian in the Ghawar area (Al-Husseini, 2004). At the Central Arabian Arch, the HU merges with the pre-Khuff unconformity so that the Middle Permian to Triassic Khuff Formation rests on Proterozoic basement (Al-Husseini, 2004). A similar situation pertains in the Oman Mountains (Al-Husseini, 2004).

To the northwest, the strata above the HU rest on progressively older rocks from northern Saudi Arabia into Jordan (Aqrawi et al., 2010), suggesting the likely presence of a very persistent high located around present day Jordan. In the west of Jordan a very large stratigraphic gap representing this trend as well as the onlap of post Carboniferous sequences onto the high is illustrated by spectacular exposures of the Umm Irna Formation (Upper Permian) resting on the Cambrian Umm Ishrin Formation (see Stephenson and Powell, 2013).

To disentangle the effects of the HU from other unconformities, and from the stratigraphic effects of pre-existing highs, accurate dates for the hiatus are required, as a well as an understanding of the areal distribution of the duration of the hiatus. Subcrop maps of the HU are available (e.g. Aqrawi et al., 2010; fig. 3.20) but these are based on very sporadic well sections and uncalibrated seismic interpretation.

Though well sections in ST-8 and ABSF-29 are important because they appear to represent the smallest HU hiatus and therefore offer a chance to date the ‘HU event’, they nevertheless contain only limited core, and the core that has been taken does not cover the pertinent interval. In ST-8, the HU likely occurs within a section of about 400 feet between the (upper) core 13 and core 14. In ABSF-29, the main section of interest falls between cores 12 (upper) and 13, an extent of about 600 feet. Thus, analysis of a combination of cuttings and core has been used to understand the palynological sequence.

In ABSF-29, Clayton et al. (2000) recorded only poorly preserved palynomorphs from cuttings samples of the Berwath Formation above core 13 (Fig. 3), including Aratrisporites saharaensis, Prolycospora rugulosa, Spelaeotriletes arenaceus, Radiizonates genuinus and Vallatisporites agadesi. These assemblages were assigned by Clayton et al. (2000) to the RT Biozone of Visean age, originally erected in Libya (Loboziak and Clayton, 1988). There are no published records of palynology above the section described by Clayton et al. (2000), though Saudi Aramco records (given in Al-Husseini, 2004) describe a palynologically barren interval from 14,820 to 14,660 feet as being assigned to the Unayzah Formation, and an assemblage assigned to Saudi Aramco palynological zone P4 (considered by Al-Husseini (2004) as equivalent to OSPZ2 of Stephenson et al., 2003) at depth 14,665 feet. The reasons for these assignments are not given in Al-Husseini (2004).

In well section ST-8 approximately 1000 km to the northwest of ABSF-29, there is similarly sporadic palynological coverage including core and cuttings. Clayton (1995) reported the MJ Biozone (also established in Libya by Loboziak and Clayton, 1988) in core samples from 4637 and 4642 feet (core 14; Fig. 3), based on the occurrence of monosaccate pollen (the base of the MJ Biozone is marked by the first uphole appearance of monosaccate pollen). Clayton (1995) identified assemblages belonging to the RT zone from samples from 4792, 4950, 4968 and 4980 feet. Al-Husseini (2004) positioned the HU at a point above the upper sample containing the MJ Biozone, i.e. just above 4637 feet. Clayton (1995) also reported a tentative correlation to the RT Biozone of the sequence in the nearby Saudi Arabian well AR’AR-1 from cuttings samples between 4272 and 4436 feet (Fig. 1). The assemblages contained Apiculiretusispora multiseta and Spelaeotriletes triangulus, but lacked P. rugulosa.

Owens and Turner (1995) studied 9 cuttings samples from the well section in ST-8 above core 14 (depth interval 4620 to 4200 feet). They described assemblages contaminated by downhole caved specimens of Permian palynomorphs, but also what they considered as in situ palynomorphs such as Laevigatosporites spp. and Thymospora spp. Several taxa, originally described from the Carboniferous coal belts of Euramerica were identified and illustrated including Vestispora fenestrata and Reticulatisporites polygonalis, suggesting a relatively well constrained age of early to mid Asturian (OT Biozone of Clayton et al., 1977). No quantitative data were given by Owens and Turner (1995), only brief details and a range chart, so it is difficult to determine the full character of the assemblages. Owens and Turner (1995) regarded the sequence as equivalent to the Al Khlata Formation, the ‘post-HU’ glacigene formation of Oman. Al-Husseini (2004) figured this relationship (2004; fig. 9) by updating the correlation to the Unayzah Formation (the approximate equivalent of the Al Khlata Formation in Saudi Arabia). Since the work of Owens and Turner (1995), there have been no detailed published accounts of the palynology of the Unayzah Formation equivalent interval 4620 to 4200 feet, apart from personal communications from J. Filatoff included in Al-Husseini (2004). These include an attribution of:

  • interval 4425–4400 feet to Saudi Aramco C2 Biozone (in part equivalent to OSPZ1 of Stephenson et al., 2003, according to Al-Husseini, 2004);

  • interval 4360–4345 feet to Saudi Aramco P3 Biozone (equivalent to OSPZ3-4 of Stephenson et al., 2003, according to Al-Husseini, 2004)

The ‘Al Khlata Formation equivalent’ section from 4620 to 4200 feet in ST-8 dated by Owens and Turner (1995) as early to mid Asturian is in conflict with the Saudi Aramco P3 age suggested for the interval 4360–4345 feet, which translates to a broadly Permian age. Al-Husseini (2004) illustrated this conflict in his figure 9. The conflict hinges around whether Euramerican taxa described by Owens and Turner (1995) such as V. fenestrata and R. polygonalis are in situ. With this in mind for the present study, three of the cuttings slides 4370–4430, 4470–4550 and 4520–4550 feet studied by Owens and Turner (1995) were re-examined for this study. The full quantitative results are shown on Fig. 4.

The assemblages from 4370 to 4550 feet are dominated by bisaccate and monosaccate pollen and species of the spongeous zonate spore genus Lundbladispora (e.g. L. braziliensis). S. triangulus, Punctatosporites granifer and Densoisporites truswellii are also common. Some taxa such as the Mid-Late Permian alga Reduviasporonites chalastus (Foster et al., 2002) are likely caved from higher levels as are many bisaccate pollen taxa. A number of taxa of clear Euramerican aspect occur including V. fenestrata and R. polygonalis, but these are very rare. This re-examination of the slides only shows therefore that an attribution to the early to mid Asturian is reasonable based on the slides. It does not resolve the possibility of laboratory contamination (and therefore misassignment to Asturian) or of misassignment of assemblages based on caved specimens (to Saudi Aramco P3 Biozone, for example).

Recent study of a comprehensive suite of core samples from borehole KH-5/1 in western Iraq has allowed a reassessment of the time period represented by the rocks above and below the HU and the character of in situ assemblages in the northern Arabian Plate. KH-5/1 (Fig. 1) was drilled as a deep water well and fully cored to TD at 1620 m. The well section spans the HU with the Raha Formation below and the Ga’ara Formation above. The boundary between the two formations is picked at 670 m by Al-Hadidy (2007). Al-Hadidy (2007, p. 108) reports that the boundary ‘…appears to represent a major hiatus in the middle Carboniferous and a change from a marine setting in the early Carboniferous (Visean-Serpukhovian) to a continental setting in the late Carboniferous…’. Aqrawi et al. (2010) also identified the boundary as an unconformity. The Raha Formation (formerly known as the Suffi Formation) does not crop out in Iraq and is defined in KH-5/1 where according to Al-Hadidy (2007) it is 184 m thick, and consists of black limnic claystones and shales with thin cross-bedded sandstones and fossiliferous limestone beds. Plant fossil-rich shale is also present as well as argillaceous coal horizons (Aqrawi et al., 2010). The overlying Ga’ara Formation consists of 5 units (Al-Hadidy, 2007) including mixed mudrock – sandstone coarsening – up and fining-up sequences, and thick mudrock sequences (Fig. 5). The sandstone units are commonly cross-bedded (Jassim and Goff, 2006).

Very brief details of the palynology of three samples (772, 754.6 and 735 m) from the upper part of the Raha Formation in KH-5/1 (unpublished MSc thesis by Al Moula, 2002, in Al-Hadidy, 2007, figure 25) record amongst others P. rugulosa, Spelaeotriletes balteatus, Crassispora kosankei, Crassispora trychera, Vallatisporites ciliaris, and species of Retusotriletes, Punctatisporites, Indotriradites, and Colatisporites. These were assigned to the Visean.

Nader et al. (1994) studied four samples from the lower part of the Ga’ara Formation in KH-5/1. They did not differentiate the samples and gave no quantitative details but recorded a diverse assemblage including Raistrickia aculeata, Raistrickia saetosa, S. arenaceus, Striangulus, Vallatisporites verrucosus, V. vallatus, Potonieisporites novicus, Plicatipollenites malabarensis, as well as species of Kraeuselisporites and Lundbladispora. They assigned the assemblage to the Stephanian. Elsewhere in western Iraq, Nader et al. (1993) recorded assemblages from the upper part of the Ga’ara Formation in shallow wells BH100/B/85, BHTI9 and BHUB8A (Fig. 1), which they regarded as Early Permian in age. Kalibová-Kaiserová (1989) recorded assemblages containing Thymospora opaqua, Marsupipollenites sinuosus and Lueckisporites from the Ga’ara Formation in K 5-2 well at Rutba, 50 km south of KH-5/1 in western Iraq.

For this study, 38 core samples were taken between 796 m and 309 m in KH-5/1. The preparation of strew mounts for palynological analysis involved well-established procedures of crushing followed by hydrochloric and hydrofluoric acid treatments (Wood et al., 1996). Post-hydrofluoric acid organic residues were oxidized with Schulze's Solution and dilute nitric acid. All specimens are held in the collection of the British Geological Survey, Keyworth, Nottingham NG12 5GG, UK. Samples yielded diverse and abundant palynomorphs generally between 796 and 744 m and between 665 and 486 m. Other parts of the section were barren of palynomorphs, or nearly so.

The aim of the study was to document assemblages from the Raha and Ga’ara formations, concentrating particularly on the contact between the two (the assumed position of the HU), and to correlate the sequences with better known sequences in Saudi Arabia, and particularly glaciogenic sequences in the southern part of the Arabian Peninsula in Oman.

Section snippets

Palynological details

Two broad assemblages occur between 796 and 486 m (Fig. 5, Fig. 6, Fig. 7, Fig. 8, Fig. 9, Fig. 10, Fig. 11, Fig. 12, Fig. 13, Fig. 14, Table 1). The lowest (between 796 and 739 m) here called ‘Assemblage A’ consists of dominantly spongeous zonate spores mainly assignable to the genus Lunbladispora, species of Vallatisporites, and Spelaeotriletes (mainly S. triangulus); also present are Schopfipollenites ellipsoides, A. multiseta, Rugospora sp. and Retusotriletes incohatus. Two specimens of

Comparison and correlation

Assemblage A is similar to the RT Biozone assemblages recorded from ABSF-29 by Clayton et al. (2000) in that it contains common V. verrucosus, S. arenaceus and R. incohatus. It is also similar to the RT Biozone of the sequence in Saudi Arabian well AR’AR-1 between 4272 and 4436 feet in that it contains A. multiseta (Clayton, 1995), and to the RT Biozone sequence in ST-8 (Clayton, 1995) in containing S. ellipsoides. The base of the succeeding MJ Biozone was fixed by Loboziak and Clayton (1988)

Conclusions

Palynology indicates that well KH-5/1 likely contains a hiatus associated with the Hercynian unconformity associated with non-deposition or erosion of rocks corresponding approximately in age to part of the Serpukhovian and Bashkirian, similar to the duration associated with the same unconformity in well ST-8 situated to the south of KH-5/1 in northern Saudi Arabia. However, it is also possible that the palynologically barren section (of maximum thickness 74 m) between but not including 739 and

Disclosure of interest

The authors declare that they have no competing interest.

Acknowledgment

Michael H. Stephenson publishes with the permission of the Executive Director, British Geological Survey (NERC). The author would like to thank the reviewers Dr D. McLean and Prof. G. Clayton, as well as Drs N. Hooker and P. Breuer, for helpful comments.

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