The Rashrashiyah Formation of the Sirhan Basin in northern Saudi
Arabia contains diverse assemblages of planktonic foraminifera. We examined
the biostratigraphy, stratigraphic range and preservation of upper Eocene
planktonic foraminifera. Assemblages are well-preserved and diverse, with
40 species and 11 genera. All samples are assigned to the Priabonian
Globigerinatheka semiinvoluta Highest Occurrence Zone (E14), consistent with calcareous nannofossil
biostratigraphy indicating Zone CNE17. Well-preserved planktonic
foraminifera assemblages from the lower part of the upper Eocene are rare
worldwide. Our study provides new insights into the stratigraphic ranges of
many species. We find older (Zone E14) stratigraphic occurrences of several
species of Globoturborotalita previously thought to have evolved in the latest Eocene (Zone
E15, E16) or Oligocene; these include G. barbula, G. cancellata, G. gnaucki, G. pseudopraebulloides, and G. paracancellata. Older stratigraphic
occurrences for Dentoglobigerina taci and Subbotina projecta are also found, and Globigerinatheka kugleri occurs at a younger stratigraphic
level than previously proposed. Our revisions to stratigraphic ranges
indicate that the late Eocene had a higher tropical–subtropical diversity of
planktonic foraminifera than hitherto reported.
Introduction
Planktonic foraminifera are calcareous marine zooplankton. Their abundance
and distinctive morphologies have left a long and valuable marine fossil
record, making them ideal for studies in evolution, climate, and
relationships between diversity and climate change. The diversity of
planktonic foraminifera is high during the middle Eocene (Pearson et al.,
2006; Aze et al., 2011; Fraass et al., 2015), but calcareous and siliceous
zooplankton suffered extinction at the end of the Bartonian associated with
the middle–late Eocene transition (MLET) (Wade, 2004; Kamikuri and Wade,
2012; Wade et al., 2012) and again across the Eocene–Oligocene transition
(EOT) (Wade and Pearson, 2008; Moore and Kamikuri, 2012). However, to date
there have been very few sections with well-preserved lower Priabonian (Zone
E14) planktonic foraminifera assemblages that allow both the wall textures
of specimens to be examined and the full diversity to be documented.
The term Konservat-Lagerstätte (Seilacher, 1970) is used to
characterize exceptional preservation in the fossil record. The Paleogene
sediments of coastal Tanzania contain remarkably well-preserved calcareous
microfossils (Pearson et al., 2001, 2007; Wade and Pearson, 2008), and these
were described as a microfossil Konservat-Lagerstätte by Bown et al. (2008). The development of a calcareous microfossil
Konservat-Lagerstätte appears to be related to clay-rich sediments that
have never been deeply buried (Bown et al., 2008). The clays act as a low-permeability and low-porosity medium, isolating microfossils from chemical
and physical processes of diagenetic alteration. Evidence for exceptional
preservation comes from taxonomic, morphological and geochemical data.
Planktonic foraminifera appear translucent or glassy in reflected light, and
their wall texture is smooth in scanning electron microscope (SEM) images.
These assemblages are probably the nearest approximation to the original
biodiversity, providing a unique snapshot of ancient ecosystems. Planktonic
foraminifera preservation is important to reveal details of wall texture,
imperative for classification and thus phylogeny (e.g., Hemleben and Olsson,
2006). As wall texture underpins genus and higher level taxonomy, only with
high-quality preservation of all the morphological characteristics can
comprehensive assessments of the full diversity be achieved. A lack of
recrystallization and infilling allows delicate features to be observed, for
example intact apertural “teeth” in dentoglobigerinids, the ability to
detect spine holes and therefore distinguish spinose lineages (Pearson and
Wade, 2015; Fayolle and Wade, 2021). It is common to observe a decrease in
the preservation state of buried microfossils. In contrast to “glassy”
preservation, recrystallized specimens appear chalky or white in reflected
light. Diagenesis in planktonic foraminifera can involve overgrowth, changes
in the test crystal structure at the micrometer scale, and/or infilling of the
original test, all of which have significant implications for geochemical
and taxonomic studies.
While the microfossil Lagerstätten preservation from Tanzania has led to
important insights into the Eocene calcareous microfossil preservation (Bown
et al., 2008), paleoecology and diversity (Wade and Pearson, 2008; Dunkley
Jones et al., 2008), and paleoclimates (Pearson et al., 2001, 2007, 2008,
2009; Lear et al., 2008), Tanzania is not unique in offering such
exceptional preservation. Other sites exist, though their records are not
always as continuous (e.g., Alabama, Miller et al., 2008; Ocean Drilling
Program Site 647, Firth et al., 2013; Integrated Ocean Drilling Program
Expedition 342, Norris et al., 2014). Paleogene sites that contain
excellently preserved microfossils provide significant insights into
planktonic foraminiferal taxonomy and evolutionary snapshots into planktonic
foraminiferal history. High-resolution SEM analysis of well-preserved
planktonic foraminifera can reveal primary wall fabrics that have not
previously been observed. Detailed taxonomic studies are critical to
understanding the phylogeny and evolution of planktonic foraminifera through
the late Eocene. The Tanzania cores, however, do not have recovery in the
lower part of the upper Eocene (Priabonian), as such planktonic foraminifera
taxonomic work focused on the middle Eocene (Pearson et al., 2006) and EOT
(Pearson and Wade, 2015). Here we present new biostratigraphic results and
taxonomic insights from well-preserved material from Saudi Arabia.
The Paleogene Planktonic Foraminifera Working Group described 14 new
taxa as part of the Atlas of Oligocene Planktonic Foraminifera (Wade et al., 2018a). These included four new species
described from the late Eocene and early Oligocene: Globigerina archaeobulloides Hemleben and Olsson,
Globoturborotalita paracancellata Olsson and Hemleben, Globoturborotalita pseudopraebulloides Olsson and Hemleben, and Subbotina projecta Olsson, Pearson and Wade.
However, the early stratigraphic ranges of these species were not well
constrained, requiring examination of well-preserved upper Eocene sediments
to determine if these species were present. Our new study section with
well-preserved planktonic foraminifera from Saudi Arabia allows us to
document the presence or absence of these newly described species and
provide constraints to the stratigraphic ranges.
The calcareous nannofossil biostratigraphy from the upper Eocene
Rashrashiyah Formation in northern Saudi Arabia was recently published in
Aljahdali et al. (2020). Here we present a pilot investigation of the
planktonic foraminifera results for the same section. Whilst our sample set
is limited, we present 96 stacked light microscope and SEM images of the
diverse late Eocene assemblages. Our taxonomic investigations utilize the
Atlas of Oligocene Planktonic Foraminifera (Wade et al., 2018a), and we find several species recently described in that
volume but not previously recorded outside the Oligocene. Planktonic
foraminifera results are integrated with previous calcareous nannofossil
records, and the implications for planktonic foraminiferal stratigraphic
ranges are discussed.
Materials and methodsGeological setting
The Sirhan (also known as the Azraq-Sirhan) Basin is a northwest–southeast-oriented, regional antisynclinal structure, located in the northern
Arabian Peninsula (Fig. 1). It is part of the Syrian Arc System (Guiraud et
al., 2001) where subsidence in the early Paleogene Tethyan seafloor resulted
in deposition of carbonate and mixed sediments (Al-Rawi, 2014). The
Rashrashiyah Formation crops out in the eastern flank of the Sirhan Basin
(31.46∘ N, 37.36∘ E) and consists of upper Eocene chalks, claystone
and limestone (Meissner et al., 1990; Halawani, 2001; Aljahdali et al.,
2020) (Figs. 2 and 3). We investigated the uppermost 10 m of the
Rashrashiyah Formation. Five samples were obtained, with a resolution of one
sample per meter, and assigned the prefix A–E (Figs. 2 and 3, Table 1). To
provide stratigraphic constraints, an additional sample (Sample F) was taken
close to the upper contact boundary between Rashrashiyah and Sirhan
formations. The sampling is the same as in Aljahdali et al. (2020) for
calcareous nannofossils.
Map showing location of the studied section and geological
contexts of the Rashrashiyah Formation in the north-western part of Saudi
Arabia. (a) Geological map of the Qurayyat area showing the extension of
the Rashrashiyah Formation around the Qurayyat Regional Airport (modified
after Aljahdali et al., 2020, and Wallace et al., 1994). (b) Location of
the Rashrashiyah Formation exposures in north-western Saudi Arabia (map
produced using Ocean Drilling Stratigraphic Network https://www.odsn.de/, last access: 17 August 2021).
Photograph of the sampled levels of the locally exposed
Rashrashiyah Formation section near the Saudi–Jordanian boarders. Letters
A–F represent sampling levels (photo courtesy of the Saudi Geological
Survey).
Lithology of the studied Rashrashiyah Formation section. Letters
A–F represent sample levels.
Preservation and relative abundances of planktonic foraminifera
species in the studied part of the Rashrashiyah Formation. * Berggren and
Pearson (2005).
For planktonic foraminiferal analysis, samples were washed over a 63 µm
sieve and oven-dried at <40∘C. The dried sample was separated
into > 425, 355–425, 250–355, 150–250 and 63–150 µm size
fractions, and each size fraction was examined under the light microscope.
Following the taxonomy of Pearson et al. (2006), Pearson and Wade (2015), and
Wade et al. (2018a), the abundance of each species was semi-qualitatively
assessed as follows: A – abundant; C – common; F – few; R – rare (Table 1).
Preservation was assessed as VG (very good), where specimens are glassy
under the light microscope with no infilling; G (good), where specimens
are semi-translucent and with no infilling; M (moderate), where specimens
are recrystallized and the test walls are opaque, and VP (very poor),
where specimens are fragmented, opaque and infilled.
Selected specimens were picked for z-stacked light microscope and SEM
imaging. A z-stacking light microscope was used to take images in three
views (umbilical, edge and spiral view). For SEM imaging specimens were
placed on stubs using double-sided tape. Each stub was coated in an argon
and gold atmosphere using a sputter coater. The illustrated specimens (Figs. 4–9) are deposited in the Natural History Museum, London, UK (NHMUK PM PF
75192–75251).
Samples A–E contain abundant and diverse assemblages of planktonic
foraminifera, with a total of 40 species and 11 genera (Table 1). The
assemblages are characteristic of tropical–subtropical pelagic settings,
with common and abundant species of Globigerinatheka, Globoturborotalita, small (<250µm)
Acarinina, Dentoglobigerina, Subbotina, Turborotalia, Hantkenina and Pseudohastigerina. Globigerina officinalis, Globorotaloides quadrocameratus, and Turborotalita quinqueloba are also present (Table 1, Figs. 4–10). Sample F was barren.
Preservation in Samples A to E ranges from very good to moderate. There is
some iron staining throughout. We use the zonal scheme of Berggren and
Pearson (2005) and Wade et al. (2011). The absence of the large muricate
taxa and the presence of Globigerinatheka semiinvoluta in Samples C, D and E indicate that the section
can be assigned to the planktonic foraminifera Globigerinatheka semiinvoluta HOZ (Zone E14; Berggren and
Pearson, 2005; Wade et al., 2011).
Stratigraphic section, planktonic foraminifera biostratigraphy
and range chart of species recorded in the Rashrashiyah Formation.
Taxa with earlier stratigraphic ranges than previously reported are in red;
later stratigraphic occurrences are in blue. Solid line indicates species
was found in the associated sample, and dashed lines are used when the species was not found in the sample but assumed to continue based on evidence
from elsewhere (e.g., Pearson et al., 2006; Wade et al., 2018a). Legend for
lithologies as in Fig. 3.
DiscussionPreservation
Four criteria were presented by Pearson and Burgess (2008) for
distinguishing foraminifera tests that are not significantly recrystallized:
(1) tests should be glassy or translucent in reflected light; (2) ultrafine
features such as spines (if initially possessed) should survive; (3) smooth
parts of the test such as the apertural lips, sutures, outer surface (in
some species) and inner surface (in most species) should appear smooth at
the submicron scale in high-resolution SEM images; and (4) in cross section
the submicron microgranular texture of the wall (if originally possessed)
should be clear when the test is broken.
Test preservation is very good in Sample A, specimens are glassy beneath the
light microscope and spines are preserved on Globigerinatheka, satisfying the criteria for
excellent preservation of Pearson and Burgess (2008). In Samples B and D,
the preservation is good, with specimens translucent under the light
microscope. Moderate preservation is recorded for Samples C and E, with some
infilling, encrustation and fragmentation (Figs. 4–9). Preservation is
superior to other sections in this region (e.g., Ramadan et al., 2021).
Biostratigraphy
The planktonic foraminiferal assemblage indicates that all samples are of
Priabonian age. No Morozovelloides or large Acarinina were found (except a singular dwarfed A. rohri in
Sample A), indicating that all samples are above the middle–late Eocene
turnover (MLET; Kamikuri and Wade, 2012; Wade et al., 2012). The extinction
of M. crassatus marks the base of the Globigerinatheka semiinvoluta HOZ Zone (Zone E14) (Berggren and Pearson, 2005;
Wade et al., 2011). The top (T) M. crassatus occurs within Chron C17n.3n and is
calibrated to 38.073 ± 0.005 Ma at the Varignano section in Italy
(Luciani et al., 2020) based on the Pälike et al. (2006)
magnetochronology. This converts to 38.0 Ma on the Westerhold et al. (2014)
magnetochronology, which is the current standard used in the most recent
edition of the Geologic Time Scale (GTS2020; Speijer et al., 2020). The
entire section studied is therefore younger than 38 Ma.
We found a singular dwarfed specimen of Acarinina rohri in Sample A (Table 1). In the
western North Atlantic (ODP Site 1052) a reduction in specimen size of
Morozovelloides crassatus from 500 to 350 µm was recorded during the MLET, due to photosymbiont
bleaching and environmental stress (Wade et al., 2008; Wade and Olsson,
2009). Small <250µm specimens of Acarinina consisting of A. collactea, A. echinata and A. medizzai are
found in all samples (Fig. 4). Small acarininids have been shown to range
beyond the MLET and into the Oligocene (Berggren et al., 2006; Wade and
Hernitz Kucenjak, 2018; Luciani et al., 2020), though they are not
consistently present in late Eocene samples from Jordan (Farouk et al.,
2015).
Globigerinatheka is common, with specimens of G. barri, G. index, G. korotkovi, G. kugleri, G. mexicana, G. semiinvoluta, and G. tropicalis (Premoli Silva et al., 2006).
Globigerinatheka semiinvoluta is present in samples C, D and E but absent from samples A and B. The Base (B) G. semiinvoluta is a secondary bioevent within Zone E14 but was previously used as a
primary bioevent to mark the base of Zone P15 (Berggren et al., 1995). It
has been calibrated to Chron C17n in several sections including ODP Site
1052 (Wade, 2004; Wade et al., 2012), Varignano (Luciani et al., 2020) and
Alano (Agnini et al., 2021). At the Varignano section, B G. semiinvoluta is calibrated to
Chron C17n.2n with an age of 37.665 ± 0.006 Ma (timescale of Pälike
et al., 2006) (Luciani et al., 2020), which converts to 37.54 Ma on
Westerhold et al. (2014) magnetochronology (Fig. 11). The lack of G. semiinvoluta in
Samples A and B raises the possibility that these samples could be from the
short stratigraphic interval between the T M. crassatus and the B G. semiinvoluta. This interval was
designated the Turborotalia pseudoampliapertura Zone by Haggag (1990) and has been recognized in several
sections worldwide (Canudo and Molina, 1992; Wade, 2004; Agnini et al.,
2011; Strougo et al., 2013). The B G. semiinvoluta may be diachronous, as the duration of
the interval between T M. crassatus and B G. semiinvoluta is only 20 kyr at ODP Site 1052 (Wade, 2004;
Wade et al., 2012), 330 kyr at Alano and 408 kyr at the Varignano section
(Luciani et al., 2020). Note, however, the discrepancy could be due to the
criteria used to separate G. semiinvoluta from its ancestor G. mexicana (Premoli Silva et al., 2006).
Thus the bioevent may be isochronous, but independent workers have not been
unified in the discrimination of the first true G.semiinvoluta morphospecies.
Section log of the upper Eocene Rashrashiyah Formation with
planktonic foraminifera and calcareous nannofossil zonal schemes and
bioevents. Drawing of G. semiinvoluta from Wade (2004). Legend for lithologies as in Fig. 3.
T Planorotalites capdevilensis is slightly older than B G. semiinvoluta in the Italian sections of Alano (Agnini et
al., 2011) and Varignano (Luciani et al., 2020). Wade (2004) found a longer
range of P. capdevilensis from ODP Site 1052, western North Atlantic. P. capdevilensis was not found in this
study of the Rashrashiyah Formation.
The characteristics of the assemblage are very consistent with stratigraphic
equivalent sections studied in the Adriatic, Egypt and Armenia (Wade et al.,
2012; Strougo et al., 2013; Cotton et al., 2017; Ramadan et al., 2021;
Salama et al., 2021). However, despite the high diversity, no Catapsydrax are present.
This contrasts Adriatic cores where Catapsydrax increase in abundance at the base
Priabonian (Wade et al., 2012). We record a high diversity of species
belonging to Globoturborotalita (Figs. 8, 9 and 10), and G. pseudopraebulloides is unusually abundant (Fig. 9).
Stratigraphic ranges
The stratigraphic distribution of the recorded species is shown in Fig. 10. The stratigraphic ranges of many of the non-marker species are different
to published schemes (e.g., Pearson et al., 2006; Aze et al., 2011; Wade et
al., 2018a). Many species thought to have evolved in Zone E16 or Zone O1 are
found at this section in Zone E14, suggesting their stratigraphic range
needs revision. For example, Globoturborotalita barbula, Globoturborotalita pseudopraebulloides, Globoturborotalita paracancellata, Globoturborotalita cancellata, Globoturborotalita gnaucki and Subbotina projecta (Figs. 5, 8 and 9) were all thought to
evolve between upper Eocene Zone E15 and lower Oligocene Zone O2
(Spezzaferri et al., 2018; Wade et al., 2018b, c). However, they are all
present in Zone E14, with some species, for instance, Globoturborotalita pseudopraebulloides in high abundance (Fig. 9). The
discrepancy between first occurrence in the published literature and this
study is most likely due to the excellent preservation of this section
coupled with the lack of recent taxonomic investigations on the late Eocene.
Within the combined nannofossil and planktonic foraminifera assemblages, we
do not see any evidence for reworking in these sediments as an explanation
for the lower occurrences of certain species compared to their previously
published stratigraphic ranges.
Dentoglobigerina is a diverse genus that evolved in the middle Eocene. We find and
illustrate (Fig. 5.6 and 5.7) two specimens that we could not confidently
place in any of the previously described species: we refer to these as
Dentoglobigerina sp. 1 and Dentoglobigerina sp. 2, pending further investigations (see Appendix A for
taxonomic notes). Many of the stratigraphic ranges presented in Pearson et
al. (2006) have already been extended to older levels by Wade et al. (2018a)
and Fayolle and Wade (2021). Dentoglobigerina eotripartita is present from the base sample (Sample A),
confirming the stratigraphic range suggested in Wade et al. (2018a).
Dentoglobigerina taci was thought to be confined to the Eocene–Oligocene boundary interval (Zone E16 to Zone O1) (Pearson and Wade, 2015; Wade et al., 2018a). Here we find
Dentoglobigerina taci in Zone E14 (Figs. 5 and 10), extending its evolution to several million
years earlier. We find a singular rare occurrence of D. tripartita in Sample C (Fig. 5.5).
Globigerinathekids are abundant in the samples. Premoli Silva et al. (2006)
state that the extinction of G. barri occurs towards the end of Zone E14. Here we
find G. barri in Samples A, B and C (Fig. 6), but not in the younger part of the
section, potentially constraining the extinction of G. barri to early Zone E14. Our
extinction horizon for G. barri is consistent with results from coeval Egyptian
sections (Strougo et al., 2013). In Premoli Silva et al. (2006) the
extinction of Globigerinatheka kugleri is given as Zone E13, but we find this species ranging higher
(Zone E14).
Globoturborotalita is a long-ranging genus, from the Eocene to the present. Many species have
been described, though their ranges, in general, are poorly constrained.
Globoturborotalita barbula was previously only known from the Eocene–Oligocene boundary interval
(Pearson and Wade, 2015; Spezzaferri et al., 2018). We find specimens in
Samples A and B, suggesting this species evolved earlier than previously
thought (Pearson and Wade, 2015; Spezzaferri et al., 2018). The
stratigraphic range of G. cancellata is not well constrained, and until Spezzaferri et
al. (2018) it had not been recorded outside of the Oligocene Globorotalia opima opima Zone, from
which it was described. Spezzaferri et al. (2018) found and illustrated
specimens from Zone O1 and suggested a questionable range from Zone E16 to
Zone O5. We find and illustrate specimens from Zone E14 (Fig. 8), indicating
that this species has a much longer stratigraphic range than previously
suggested. G. paracancellata was described from the upper Oligocene, with recorded specimens
occurring from the upper Eocene Zone E16 (Spezzaferri et al., 2018). Here we
extend the stratigraphic range of G. paracancellata with specimens illustrated from Zone E14
(Fig. 9). G. gnaucki is abundant in Samples B, C and D (Table 1, Fig. 8). This species
was thought to evolve in Zone E15 (Spezzaferri et al., 2018), but this study
suggests it evolved in Zone E14 or older (Fig. 10). We find specimens that
we refer to as Globoturborotalita cf. G. labiacrassata (Fig. 9). These have a high-arched umbilical aperture
and lobate profile but lack the thick rim boarding the aperture that is
characteristic of this species. Our specimens are in Zone E14 and thus much
older than the first appearance of G. labiacrassata (Zone O2) suggested by Spezzaferri et
al. (2018). Further investigations are required to determine if these forms
are a new species.
In Pearson et al. (2006) the evolution of Pseudohastigerina naguewichiensis from P. micra occurs at the base of Zone
E15. We find and illustrate P. naguewichiensis in Zone E14 (Fig. 4). Our earlier evolution of
this species is in agreement with Cotton et al. (2017), though an older
range (Zone E13) is suggested by Strougo et al. (2013). The oldest
previously recorded specimens of Subbotina projecta are from upper Eocene Zone E16. Here we
document and illustrate specimens from Zone E14 (Table 1, Figs. 5 and 10).
The occurrence of T. quinqueloba in Sample D (Figs. 8 and 10) confirms the stratigraphic
range recorded in Pearson and Kučera (2018).
Despite the relatively good preservation and the high diversity of species
recorded from the Rashrashiyah Formation (Table 1, Figs. 4–10), there are
some species that were expected to be present, based on previous range chart
compilations (e.g., Pearson et al., 2006; Aze et al., 2011), that were not
found. These include Paragloborotalia griffinoides and P. nana, Chiloguembelinaototara, C. cubensis, and species of the genus Catapsydrax. However, we
note that these species are recorded in upper Eocene sections from Egypt
(Ramadan et al., 2021). We suspect that the absence of these taxa in the
Rashrashiyah Formation is due to the environment, which is indicative of
warm and oligotrophic conditions.
The amendment of planktonic foraminifera stratigraphic ranges has
ramifications for the phylogenies and tropical–subtropical diversity charts.
The range charts presented in Pearson et al. (2006), Wade et al. (2018a),
incorporated into Time-Scale Creator (Fordham et al., 2018), and the Mikrotax
online portal (Huber et al., 2016) will require revision. Many species
evolved earlier than previously thought, particularly within the
Globoturborotalita genus. The extensions of the range of Pseudohastigerinanaguewichiensis and Turborotalita quinqueloba were already suggested in
Cotton et al. (2017) and confirmed here. Our study implies that the late
Eocene tropical–subtropical diversity is higher than previously suggested in
compilations (Ezard et al., 2011; Fraass et al., 2015; Lowery et al., 2020).
Many of the species found in this study of the late Eocene extend through
the Eocene–Oligocene transition and into the Oligocene, suggesting that the
rate of turnover at the Eocene–Oligocene transition is not as large as
previously thought and requires re-investigation.
Integrated calcareous biostratigraphy
The calcareous nannofossil assemblages were studied by Aljahdali et al. (2020) indicating that the section corresponds to Zone CNE17 of Agnini et
al. (2014) and Zone NP18 of Martini (1971). The section is Priabonian age
(upper Eocene), as indicated by the presence of Chiasmolithus oamaruensis. The T C. grandis between Samples B
and C allows identification of the base Zone CP15 of Okada and Bukry (1980).
The base common (Bc) of Cribrocentrum erbae also occurs between Samples B and C. Throughout the
section C. erbae increases in abundance consist with Zone CNE17 of Agnini et al. (2014). A single specimen of Isthmolithus recurvus was documented in Sample E (Aljahdali et al.,
2020); however, this is not used in the biostratigraphic interpretation. The
planktonic foraminifera biostratigraphy is consistent with the calcareous
nannofossil biostratigraphy and can also be compared to other
integrated calcareous biostratigraphic studies (e.g., Strougo et al., 2013;
Farouk et al., 2015; Cotton et al., 2017; Luciani et al., 2020; Agnini et
al., 2021).
The base of the Priabonian Global Stratotype Section and Point (GSSP) was
recently defined by Agnini et al. (2021) at the Alano di Piave section
(north-eastern Italy). The Bartonian–Priabonian boundary is placed at the
prominent 14–16 cm crystal tuff layer known as the “Tiziano bed” at 63.57 m.
With our current 1 m sampling resolution and the three bioevents between
Samples B and C, we are unable to confidently calculate sedimentation rates,
but it would appear that the base of the section is either at, or very close
to, the Bartonian–Priabonian boundary.
Conclusions
The planktonic foraminiferal biostratigraphy, integrated with calcareous
nannofossil biostratigraphy, provides a robust stratigraphic framework for
the Rashrashiyah Formation, indicating that the section is Priabonian (upper
Eocene) in age. Planktonic foraminifera assemblages are diverse and
extremely well-preserved. Our study reveals differing stratigraphic ranges
to the ones established in the literature. Higher-resolution sampling will
allow the horizons for Base G. semiinvoluta, Top C. grandis and Base Common C. erbae to be differentiated.
Our study implies that the late Eocene tropical–subtropical diversity is
likely to be higher than previously suggested in data compilations.
Taxonomic list of species in this study
Acarinina collactea (Finlay).Acarinina echinata (Bolli), Fig. 4.1.Acarinina medizzai (Toumarkine and Bolli), Fig. 4.2–4.6.Acarinina rohri (Brönnimann and Bermúdez).Dentoglobigerina eotripartita Pearson, Wade, and Olsson, Fig. 5.1.Dentoglobigerina galavisi (Bermúdez), Fig. 5.2.Dentoglobigerina pseudovenezuelana (Blow and Banner).Dentoglobigerina taci Pearson and Wade, Fig. 5.3 and 5.4.Dentoglobigerina tripartita (Koch), Fig. 5.5.Dentoglobigerina sp. 1, Fig. 5.7. This specimen has a dentoglobigerinid wall texture, a compressed final chamber and pronounced tooth.Dentoglobigerina sp. 2, Fig. 5.6. This specimen has incised umbilical sutures and a compressed final chamber. In umbilical view this specimen bears a close morphological resemblance to the drawing of Globigerinatheka index by Postuma (1971). However, due to the wall texture and lack of supplementary apertures, we consider this specimen to belong within Dentoglobigerina, pending further investigations.Globigerina officinalis Subbotina.Globigerinatheka barri Brönnimann, Fig. 6.3 and 6.4.Globigerinatheka index (Finlay), Fig. 6.5 and 6.6.Globigerinatheka korotkovi (Keller).Globigerinatheka kugleri (Bolli, Loeblich, and Tappan), Fig. 6.7 and 6.8. Globigerinatheka mexicana (Cushman), Fig. 6.9 and 6.10.Globigerinatheka semiinvoluta (Keijzer), Fig. 7.4 and 7.5.Globigerinatheka tropicalis (Blow and Banner), Fig. 7.1–7.3.Globorotaloides quadrocameratus Olsson, Pearson, and Huber, Fig. 7.6.Globoturborotalita barbula Pearson and Wade, Fig. 8.1 and 8.2.Globoturborotalita cancellata (Pessagno), Fig. 8.3–8.5.Globoturborotalita gnaucki (Blow and Banner), Fig. 8.6.Globoturborotalita cf. G. labiacrassata (Jenkins), Fig. 9.1 and 9.2. These specimens have a high-arched umbilical aperture and lobate profile but lack the thick rim boarding the aperture that is characteristic of G. labiacrassata.Globoturborotalita ouachitaensis (Howe and Wallace), Fig. 8.7 and 8.8.Globoturborotalita paracancellata Olsson and Hemleben, Fig. 9.3 and 9.4.Globoturborotalita pseudopraebulloides Olsson and Hemleben, Fig. 9.5–9.9.Hantkenina alabamensis Cushman.Hantkenina primitiva Cushman and Jarvis, Fig. 6.2.Pseudohastigerina micra (Cole), Fig. 4.9–4.12.Pseudohastigerina naguewichiensis (Myatliuk), Fig. 4.7 and 4.8.Subbotina corpulenta (Subbotina), Fig. 6.1.Subbotina linaperta (Finlay).Subbotina projecta Olsson, Pearson, and Wade, Fig. 5.8.Subbotina utilisindex Jenkins and Orr, Fig. 5.9 and 5.10.Subbotina yeguanensis (Weinzierl and Applin).Turborotalia ampliapertura (Bolli), Fig. 7.7.Turborotalia cerroazulensis (Cole), Fig. 7.8–7.10.Turborotalia cunialensis (Toumarkine and Bolli).Turborotalia increbescens (Bandy).Turborotalia pomeroli (Toumarkine and Bolli).Turborotalita quinqueloba (Natland), Fig. 8.9.
Data availability
The data generated in this study are included within the paper and in Table 1. Imaged specimens are deposited at the Natural History Museum, London, UK.
Author contributions
BW conducted the analyses and wrote the paper. MA conceived the project and
the prepared lithographic logs. MA, YM, AM, SA and IZ conducted fieldwork,
sampled the studied section and provided comments to manuscript text and
figures.
Competing interests
The authors declare that they have no conflict of interest.
Disclaimer
Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Acknowledgements
We are extremely grateful to Natalie Cheng, who conducted the light and SEM
imaging, prepared the plates and assisted in taxonomic discussion, and to
Marcin Latas, who assisted with sample preparation and the map. Comments on
an earlier version of the manuscript were provided by Paul Pearson, Florent Fayolle and Alessio Fabbrini. We thank the CEO of the Saudi Geological
Survey Eng. Abdullah M. Al-Shamrani and vice president Saleh Al-Sefry, Nasser Aljahdali and Wadee Kashghari for supporting fieldwork, permission to use
SGS labs and equipment for this work. We thank Helen Coxall and an anonymous reviewer for their constructive
suggestions. This paper was edited by Kirsty Edgar, who provided additional
insights and comments that improved the manuscript.
Financial support
Bridget S. Wade was supported by UK Natural Environment Research Council (NERC) reference
number NE/G014817.
Review statement
This paper was edited by Kirsty Edgar and reviewed by Helen Coxall and one anonymous referee.
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