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
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
The few available published studies of Late Cretaceous calcareous nannofossils in the Middle East are limited to biostratigraphic applications. This includes the biostratigraphic study of Coniacian–Maastrichtian chalky and marly sequences of Lebanon by Müller et al. (2010) in which they provided a brief description of the nannofossil markers used for biostratigraphic subdivision as part of a general revision of the Cretaceous and Cenozoic stratigraphy of the country. Other nannofossil biostratigraphic studies include the late Campanian–Maastrichtian upper Mahara group of Yemen (Al-Wosabi and Alaug, 2013), the late Santonian to Maastrichtian Abu Roash and Khoman formations in the northwestern desert of Egypt (Mandur, 2016), the Coniacian to Maastrichtian Themed and Sudr formations of Sinai, Egypt (Faris and Abu Shama, 2006; Farouk and Faris, 2012), and the late Campanian to early Maastrichtian Shiranish Formation of NE Iraq (Farouk et al., 2018). In Iran, Late Cretaceous nannofossil biostratigraphy has been studied by several authors, but the most detailed studies are achieved by Razmjooei et al. (2014), Foroughi et al. (2017) and Razmjooei et al. (2018) for the Gurpi and Abtalkh formations of south and north Iran, respectively, in which nannofossil biozonations were developed for the studied sections. All of these studies mostly focus on solving stratigraphic problems like age constraint and hiatus recognition, whereas very little information is published on assemblage composition, their changes through time or their potential palaeoenvironmental significance.
The Late Cretaceous epoch was one of the warmest periods with a
global sea level that was
The Late Cretaceous is associated with an acme in calcareous nannofossil diversity. The detailed record of Cretaceous nannofossil diversification by Bown et al. (2004) shows a diversity peak after the Turonian that continued until the mid-Maastrichtian before it began to decline toward the K–Pg boundary. The highest diversity is recorded in the Campanian, a time that is also marked by large coccolith size, a characteristic feature that continued until the mid-Maastrichtian. Compared to the Campanian diversification, the Coniacian and Santonian are characterised by low diversity and a low turnover rate (Burnett, 1998). Bown et al. (2004) suggest that cooling within a greenhouse-mode climate system (e.g. Campanian–early Maastrichtian cooler climates) may have stimulated diversification via greater differentiation of the photic zone environment, biogeographic partitioning and increased numbers of endemic taxa at both low and high latitudes. Keller (2008) also related the increased diversity of microfossil and nannofossil groups during the Cretaceous cooling periods to increased weathering, runoff, upwelling and nutrient cycling, while more stable diversity is associated with high sea levels during the warmest Cretaceous intervals (e.g. Turonian–Santonian).
During the Late Cretaceous, deposition of the Fiqa Formation occurred on the northeastern margin of the African–Arabian plate at an equatorial palaeolatitude in the eastern Tethys. The detailed documentation of calcareous nannofossil assemblages at this location thus also fills a gap between the austral and western Tethys provinces. Given the quality of preservation and stratigraphic continuity of nannofossil assemblages from the Fiqa Formation, they will be important for future assessments of Late Cretaceous provincialism. The temporal coverage provided by the Fiqa Formation, from the Coniacian to earliest Maastrichtian, and the quality of preservation of truly tropical calcareous nannofossil assemblages, also has the potential to constrain patterns of tropical diversity. Such data will help to address the question of whether the continued rise in global diversity is primarily driven by the diversification of mid- to high-latitude flora through the Late Cretaceous.
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 excellent preservation within the formation includes some rarely reported and poorly documented Late Cretaceous taxa. The quality of this preservation provides the basis for further application of these assemblages for biostratigraphic and palaeoenvironmental interpretations. It is the aim of this paper to provide a first overview of the taxonomy and broad patterns of calcareous nannofossil assemblage change through the Fiqa Formation. The paper will also provide a key reference point for other studies of Late Cretaceous nannofossil assemblages across the Middle East and other southern Tethyan areas.
In north Oman the carbonate platform that had developed on a passive continental margin during the Early Cretaceous was progressively flooded during Coniacian times, which resulted in the deposition of extensive deep marine shales and marls of the Fiqa Formation in a foreland basin setting (Glennie et al., 1974) referred to as the Aruma Basin. This basin formed as a result of crustal loading associated with over-thrusting and emplacement of oceanic crust known as the Sumail ophiolite and other associated sheets onto the eastern margin of the Arabian Peninsula (Bechennec et al., 1990). This occurred as a result of a major compressional tectonic phase associated with the closure of the Neotethys Ocean (Lippard et al., 1986). The basin was deeper proximal to the thrusted sheets in the north and west of Oman and shallowed toward the southeast as seen from seismic and well data (Forbes et al., 2010). Using the palaeolatitude calculator method of van Hinsbergen et al. (2015), the basin was located at a tropical palaeolatitude during the time of deposition, which is consistent with regional palaeogeographic reconstructions (Fig. 1).
Palaeoceanographic and tectonic reconstruction of Arabia showing the location of the Aruma Basin. Modified from Barrier and Vrielynck (2008) with information from van Hinsbergen et al. (2015).
The subsurface Fiqa Formation is part of the Late Cretaceous Aruma Group of Oman and is widespread throughout Oman, reaching a thickness of more than 1 km in the northwest (Boote et al., 1990). It is separated into lower shale-dominated facies like the Shargi Member and upper carbonate-dominated facies like the Arada Member (Forbes et al., 2010) (Fig. 2b). The formation is thicker and predominantly pelagic shale in the north, thinning and changing into shallower marine facies to the south and southeast of Oman (Forbes et al., 2010) (Fig. 2c). In the northern Oman mountains (known as the Al-Hajar Mountains), the time equivalent lithostratigraphic units to the subsurface Fiqa Formation are the local, nearshore facies mapped as the Muti Formation, Juweiza Formation and Qahlah Formation (Glennie et al., 1974). These formations were deposited at the northern margins of the basin in close proximity to the advancing thrust sheets (Robertson, 1987). The Juweiza and Qahlah formations are also referred to the subsurface clastic-dominated intervals proximal to the Oman mountains (Fig. 2c). The Fiqa Formation is chronostratigraphically correlated with the Gurpi Formation southeast of the Zagros Mountains of Iran, the Aruma Formation in Kuwait and Saudi Arabia, and the Aruma Formation and the Fiqa Formation of the United Arab Emirates (Ziegler, 2001) (Fig. 2a).
Some of the first published foraminiferal biostratigraphy work assigned an age of Santonian to Campanian to the Fiqa Formation (e.g. Glennie et al., 1974). The micropalaeontological facies have since been extensively studied for exploration purposes, with the most recent age assignment of late Coniacian to late Campanian based on the local planktonic foraminiferal zonation schemes of Sikkema (1991), revised by Osterloff et al. (2001) and Packer (2002). Micropalaeontological analysis from northern Oman by Packer (2002) indicates a neritic environment developed during the deposition of the Fiqa Formation within the early stages of foreland basin development. It deepened into upper to middle bathyal settings with rapid rates of sediment deposition during most of the Santonian to middle Campanian. These deep marine conditions were interrupted by several episodes of shallowing of sea level. During most of the late Campanian to Maastrichtian, sea level continued to fall with a generally high-energy, shallow marine environment developing over most of north and interior Oman.
The study is based on the quantitative analysis of 230 subsurface samples
provided by the Petroleum Development Oman (PDO) from the Late Cretaceous Fiqa
Formation. Samples included ditch cuttings and side-wall cores from nine
hydrocarbon exploration wells from north and central Oman (Fig. 2a). Samples
were prepared as simple smear slides following the standard technique of Bown
and Young (1998). The cascading count technique of Styzen (1997) is followed
using 100 fields of view. Species observation and description are based on
light microscopy in cross-polarised and phase-contrast light using a Zeiss
Scope.A1 at
Based on calcareous nannofossil biostratigraphy (Zainab Al Rawahi and
Tom Dunkley Jones,
unpublished data), the studied Fiqa
successions span the late Coniacian to late Campanian nannofossil Zones UC11
to UC16 and possibly the earliest Maastrichtian UC17 nannofossil zone of
the Burnett (1998) scheme for the Tethyan realm (UC
The recovered succession within W-4 is dated to lie within the late Coniacian
to early Campanian Zones UC11 to UC14 (Fig. 4). The identification of UC14 is
based on the presence of
Due to the susceptibility of coccoliths to dissolution and overgrowth, the
preservation of nannofossil in the study wells can be variable across
lithologies and with different burial depths. Quantifying the quality of
preservation through a succession is important as variations may cause
preservational bias on the resulting assemblage data (Thierstein, 1980; Roth,
1984). To describe the preservation, a general preservation scale has been
applied here, which is outlined below.
VG: very good – no evidence of dissolution and/or recrystallisation, no
alteration of primary morphological characteristics, and specimens
identifiable to the species level. G: good – little or no evidence of dissolution and/or recrystallisation;
primary morphological characteristics unaltered or only slightly altered;
specimens identifiable to the species level. M: moderate – specimens exhibit some etching and/or recrystallisation;
primary morphological characteristics somewhat altered; however, most
specimens identifiable to the species level. P: poor – specimens were severely etched or overgrown; primary morphological
characteristics largely destroyed; fragmentation has occurred; specimens
often could not be identified at the species and/or generic level.
Based on a visual inspection of etching and overgrowth the preservation is very
good to good in the soft, grey shale layers of the Shargi Member and in the
soft, grey marl layers of the Arada Member. The quality of preservation
decreases to moderate as the amount of coarser clastic input increases
up-hole in well sections, for example in the silty and sandy grey shale
horizons of the Shargi Member. Intervals of highly compacted shale within the
lower Shargi Member are sometimes present and have moderate to poorly
preserved nannofossil assemblages with indications of dissolution and
overgrowth. A further reduction of preservation is recorded in the carbonate-rich, white chalk and argillaceous limestone layers of the Arada Member,
where nannofossil recovery is very poor.
In the well-preserved nannofossil assemblages of the Campanian and late
Santonian intervals, the assemblages are characterised by the common presence
of coccospheres (e.g. Plate 7, fig. 37; Plate 8, fig. 10, Plate 11,
fig. 36) and
consistent occurrence of holococcoliths such as
Across the study wells, highly diverse assemblages (
Nannofossil abundance and diversity in the Fiqa Fm. compared to Ca wt % and palaeo-depth, W-4.
In general, the overall average diversity is around 40 species per sample, which changes only slightly depending on nannofossil preservation and the loss of smaller and solution-sensitive taxa. This might reflect a general stability in the depositional environment. Even though the species richness does not change dramatically, the assemblages are continuously fluctuating in composition, which is not reflected in the nearly homogeneous lithology of the formation. The main trends of the assemblage fluctuations are discussed below.
Abundance distribution patterns of the main components in the nannofossil assemblages, W-4.
Throughout the Fiqa Formation in the study wells, calcareous nannofossil
assemblages are characterised by high abundances of the placolith coccoliths
Characteristic features of the late Campanian to early Maastrichtian
intervals of the Fiqa Formation that are not encountered in W-4 but in W-7
and W-8 include the more frequent occurrence of
The overall assemblage composition and characteristics are similar to the
Late Cretaceous Tanzanian assemblages studied by Lees (2007). This includes
the identification of several of the new heterococcolith species described
from the Tanzanian Drilling Project materials like
There is one exceptional interval during which this group becomes very common in the
few latest Campanian to early Maastrichtian samples from W-8, for which specimens
show great variation in coccolith size and rim width (e.g. Plate 1,
figs. 5–6). Hence, the morphometric subdivision of Varol (1989) and
Thibault (2010) could be potentially applied only in these intervals. These
samples are the only ones in which
The main factor used to differentiate between the axial-cross species is the
size.
Characteristic features of the main species of
Through the Fiqa Formation, nannofossil assemblages show some distinct variations in species compositions. These variations are recorded in a lithology that is almost homogeneous. In this paper we focus on these nannofossil assemblage changes across the Fiqa Formation recovered from well W-4. In this section, the most distinct abundance changes are discussed and compared to previous studies of Late Cretaceous nannofossil environmental preferences and responses to environmental change. The new data presented here provide important information on the palaeoenvironments of the Aruma Basin and help constrain the biogeographic distribution of key Cretaceous species.
Here we investigate the abundances, within the Fiqa Formation, of Late
Cretaceous taxa with known strong latitudinal controls on their biogeographic
distributions (Fig. 6). Species that dominate nannofossil assemblages of the
Fiqa Formation (Sect. 4.4.1) are generally typical of tropical Late
Cretaceous Tethyan assemblages (e.g. Wagreich, 1992; Lees, 2007). For
instance, relative abundances of
Distribution of selected
Variation in nannofossil abundances and assemblage composition often reflects
changes in the palaeoenvironmental conditions of ocean surface waters like
nutrient supply, detrital input and surface water salinity (Mutterlose et
al., 2005). One of the strongest assemblage changes recorded in W-4 is an
interval of very low
Changing abundance patterns of proxy marker taxa in the W-4, integrated with Ca% and palaeoenvironment.
The most widely used taxa with high surface water fertility are
The calcareous nannofossil assemblages from the Late Cretaceous Aruma Basin
are well-preserved, abundant and diverse, allowing for a detailed and improved
taxonomic description of some poorly documented and/or described Cretaceous species
and the identification of two new species.
Two new heterococcolith species,
Plate 4, figs. 9–16.
The name derivation is after the Al Arma Mountains (pronounced as “Al Ormah”), where the type section of the Aruma Group–Formation was first described; “Ormah” is also the true Arabic pronunciation of Aruma.
This is a small species (3.5–4.5
This new species is distinguished from other
Plate 4, fig. 9 (fig. 10 same specimen).
Plate 4, fig. 11 (fig. 12 same specimen).
W-6, NW Oman.
UC12-13
W-4, W-6, W-7, late Santonian to late Campanian
Plate 4, figs. 39–41.
The name derivation is after the Fahud oil field (W-4), from which the species was first identified.
This is a medium-sized (5–7
This new species is distinguished from other
Plate 4, fig. 39 (fig. 40 same specimen).
Plate 4, fig. 41.
W-3, N Oman.
UC14
W-3, W-4, W-6, W-7, W-8; late Coniacian to late Campanian.
Full data for the study are currently inaccessible as they
are protected by a Petroleum Development Oman (PDO) and Ministry of Oil and Gas
of Oman (MOG) confidentiality agreement with the authors. The published study
data are released by PDO and MOG approval to be published in the
The supplement related to this article is available online at:
Data collection, sample analyses and interpretation have been undertaken by ZAR under the supervision of TDJ. ZAR wrote the paper with guidance from TDJ.
The authors declare that they have no conflict of interest.
Thank you to PDO for sponsoring the study and providing the data and to MOG for their permission to publish this study. Thanks to Peter Osterloff (Shell UK) and Stephen Packer (Millennia SC Ltd) for their comments on the paper. Jackie Lees is thanked for her comments on the Late Cretaceous species identification. Edited by: Emanuela Mattioli Reviewed by: Nicolas Thibault, Silvia Gardin, and Christian Linnert