Abstract
Abstract. This study presents the first detailed calcareous nannofossil assemblage data from the Late Cretaceous succession of the subsurface Aruma Basin, north Oman. The taxonomic description and documentation of assemblage composition are based on extensive quantitative analysis of ditch cuttings and side-wall samples from eight hydrocarbon exploration wells across north Oman. The samples studied from those wells cover the Coniacian to earliest Maastrichtian deep marine shales and marls of the subsurface Fiqa Formation. These fine-grained siliciclastic deposits often yield moderately to well-preserved nannofossil assemblages, especially in the Campanian intervals. Consequently, diverse assemblages have been recorded from the Fiqa Formation, with a total diversity of ∼200 species, including two new species, Staurolithites ormae sp. nov. and Chiastozygus fahudensis sp. nov., which are illustrated and described. Extensive imaging of this diversity is provided here, as are stratigraphic distributions of the main components from a key reference well in north Oman, W-4. Poorly described groups such as Staurolithites are closely investigated and their utility for stratigraphic applications is highlighted. Relative abundances of nannofossil taxa with strong palaeoenvironmental preferences have revealed new insights into the palaeo-productivity of the basin. High-fertility species like Discorhabdus ignotus, Biscutum constans and Zeugrhabdotus erectus show substantial variations in abundance throughout the Fiqa Formation, reflecting long-term shifts in the productivity conditions of the basin. This is supported by abundance patterns of Watznaueria barnesiae and Micula staurophora that show a broadly inverse correlation with the high-fertility species. The Fiqa Formation represents a key regional seal unit for the Cretaceous hydrocarbon reservoirs of Oman, as well as being a productive unit elsewhere in the Arabian Peninsula. Beyond the Aruma Basin of Oman, this study will provide a key reference point for future biostratigraphic or palaeoenvironmental analyses of the Late Cretaceous calcareous nannofossil assemblages across the Middle East and other southern Tethyan areas.
Highlights
The Fiqa Formation sensu Hughes Clark (1988) and Forbes et al (2010) in the subsurface of northern Oman provides a unique window into truly pelagic, Late Cretaceous ecosystems of the Arabian Peninsula
This paper aims to address a number of the questions outlined above by providing the first detailed study of calcareous nannofossil assemblages through the Late Cretaceous Fiqa Formation
The identification of UC14 is based on the presence of Bukryaster hayi and base Broinsonia parca parca at 111 m; that of UC12–13 is based on the observation of base Broinsonia parca parca to top Lithastrinus septenarius at 1280 m
Summary
The Fiqa Formation sensu Hughes Clark (1988) and Forbes et al (2010) in the subsurface of northern Oman provides a unique window into truly pelagic, Late Cretaceous ecosystems of the Arabian Peninsula. Deposited through ∼ 15 Ma, from the Coniacian to earliest Maastrichtian, it consists of very thick sequences of marls and shales, interpreted as representing deposition in a deep marine setting within a foreland basin developed in the north of Oman (Glennie et al, 1974). These sediments often yield well-preserved and abundant pelagic microfossils, including calcareous nannofossils, that have great potential for environmental, evolutionary and biostratigraphic studies. The Fiqa Formation and its equivalents across the Arabian Peninsula are key regional seals for Early to middle Cretaceous hydrocarbon reservoirs. Past unpublished industry studies have touched on these applications, but generally have focused on stratigraphically or spatially limited well-to-well correlations
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