ABSTRACT

The architecture of a new alveolinacean, Decastroia razini, from the Cenomanian shallow-water sediments of Socotra Island is described. The shape of the new genus is globular to slightly elongated. The internal structure is characterised by septula and floors that individualise two rows of superposed tubular chamberlets, an upper row of large, cortical chamberlets and a lower row of small, medullar ones. The chamberlets of the same chamber are communicated by preseptal passage.

INTRODUCTION

During mapping of the northwestern region of Socotra Island, under the projects “YOCMAL” and “EUROMARGINS 2007”, numerous specimens of a porcelaneous larger foraminifer belonging to the Alveolinacea were collected. A detailed study of its architectural characteristics has revealed differences between Socotra’s specimens and other Cenomanian genera (see Reichel, 1933, 1937, 1941, 1947; Colalongo, 1963). More details about these genera and their species are given in Schroeder and Neumann (1985) and Calonge et al. (2002). The aim of this paper is to describe this new taxon from Socotra, and to discuss its relationship with the previously published alveolinacean genera and species. The paper is based on three carbonate samples, from which 30 thin sections were prepared. The new foraminifer has been described using more than 200 random sections including axial, equatorial, oblique and tangential sections, which enabled the construction of a three dimensional model.

GEOLOGICAL SETTING

Socotra Island (Republic of Yemen) is located 350 km to the southeast of the Yemeni mainland in the Arabian Sea, at the eastern end of the Gulf of Aden opposite the Horn of Africa (Figure 1). Before the Gulf of Aden rift began in Oligocene time (Lepvrier et al., 2002), Socotra was located adjacent to the Dhofar area in South Oman. Consequently, both areas underwent a similar geological evolution, and the major unconformities within the sedimentary successions are considered equivalent (Samuel et al., 1997).

In Dhofar, the sediments containing alveolinaceans belong to the Dhalqut Formation, which consists of a thick series (200–800 m) of argillaceous marls, marly limestones and limestones with rudists, echinoids and larger foraminifera. Roger et al. (1989, 1994) reported that alveolinaceans occur in two successive intervals: (1) upper part of the Umbaraaf Member in which alveolinaceans, Ovalveolina crassa De Castro and Simplalveolina simplex Reichel, appear together with orbitolines, Orbitolina (Mesorbitolina) aperta (Erman) and O. (Conicorbitolina) conica (D’Archiac); (2) in the Khadrafi and Sarfait members there are no orbitolines and only Praealveolina cretacea Reichel is cited. The ages suggested by these authors for the two intervals are Early and Mid-Late Cenomanian, respectively.

The only sample from Oman containing alveolinaceans for our analysis was collected in the upper part of the Dhalqut Formation during a fieldtrip to the Dhofar area. It shows abundant sections of Cisalveolina fraasi (Gümbel, 1872) associated with agglutinated foraminifera, trocholines, dasycladacean algae and rudist fragments, but no section of Decastroia or Praealveolina has been identified. It is recommended, therefore, that accurate sampling in the Dhofar area should be carried out to confirm the absence or presence of the new genus in this area, and if it occurs, the biostratigraphical relationship with Cisalveolina and Praealveolina species. The Dalqhut Formation of Dhofar is at least partially equivalent to the Natih and Mishrif formations (see Roger et al., 1994), which have been widely studied due to their regional importance as oil reservoirs.

A form with a shell-architecture close to Decastroia was figured by Simmons and Hart (1987) under the name of Praealveolina tenuis Reichel (see their Plate 10.1, fig. 2). According to them this form occurs in the upper parts of the Natih Formation of Mid to Late Cenomanian age.

In the studied area of Socotra Island, the sediments attributed to the Dhalqut Formation are regionally constrained by two unconformities (Figures 2 and 3): at the base an unconformity separates the Dhalqut Formation from the Proterozoic granitic basement, and at the top, the Cenomanian sediments are unconformably overlain by Tertiary deposits of Thanetian age. They consist of about 100 m of mainly siliciclastic deposits that intercalate in the upper part with limestone beds containing rudist fragments and some larger foraminifera (Figure 4). Samples containing the new taxon are located in beds about 40 m below the Tertiary sediments in Jabal Kadarma, in the “Mudstones/Wackestones with porcellaneous foraminifers” interval (Figure 3).

SYSTEMATIC DESCRIPTION

Decastroia new genus

Derivation of Name: In honour of Piero de Castro who studied Cenomanian alveolinaceans.

Type Species:Decastroia razini n. sp.

Diagnosis: Globular or subglobular to elongated shells with porcelaneous walls. The successive chambers are planispirally and involutedly coiled. The apertural face is covered by two superposed rows of apertures extending from pole-to-pole. The apertures of the upper row have a large diametre, while those located in the lower row have a small diametre (Figure 5). The apertural axes are aligned in subsequent chambers.

Chamber lumina are subdivided into two rows of tubular chamberlets by means of septula and floors following the typical pattern of the alveolinaceans (Hamaoui, 1984; Vicedo et al., 2009). The chamberlets of the upper, peripherical row (cortical chamberlets) are large, while those of the lower row (medullar chamberlets) are much smaller. The two rows of chamberlets remain constant in both equatorial and polar regions, for the entire ontogeny. Sporadically, further pole-ward additional rows of medullar chamberlets may appear. Septula are aligned from one chamber to the next and they are interrupted by the preseptal passage. All chamberlets of the same chamber are connected by the preseptal space, which is wider in the upper part of the chamber (level of the peripheral, cortical chamberlets).

Differential Diagnosis:Decastroia differs from CisalveolinaReichel, 1941 (type species: C. fallaxReichel, 1941) because this genus has a single, slit-like aperture extending from pole-to-pole, one row of chamberlets, septula alternating from one chamber to the next, and pre- and post-septal passages. OvalveolinaReichel, 1937 (type species: O. ovum d’Orbigny, 1850) resembles Decastroia in having continuous septula, but the former shows only one row of chamberlets. MultispirinaReichel, 1947 (type species M. iraniensisReichel, 1947) differs from the other genera by having a typical multiple spire.

The genus PraealveolinaReichel, 1933 (type species: P. cretacea d’Archiac, 1835) has been defined by continuously arranged septula, a single preseptal passage running parallel to the septum and supplementary chamberlets appearing below the main chamberlets in the polar regions. These characteristics make this genus similar to Decastroia. Nevertheless, comparative studies carried out with material from the type locality (Îlle Madame, southwest France) reveals that the supplementary chamberlets of P. cretacea appear less regularly in the equatorial region of the shell than in Decastroia. Otherwise, the genus SellialveolinaColalongo, 1963 (type species: S. viallii Colalongo) differs from Decastroia in terms of its axially compressed morphology and the presence of several rows of medullar chamberlets developed in the central part of the chambers. Moreover, the preseptal space in Sellialveolina shows a regular wideness (Vicedo et al., 2011).

Decastroia razinin. sp. (Figures 6 to 8)

Derivation of Name: In honour of Philippe Razin who studied the geology of Socotra Island.

Holotype: PUAB-82016 (Figure 6.9). Deposited in the micropaleontological collection of Departament de Geologia, Universitat Autònoma de Barcelona, Spain.

Paratypes: PUAB-82017 (Figure 6.10) and PUAB-82018 (Figure 7.4).

Type Locality: Jabal Kadarma, Socotra Island (Figures 1 and 2).

Type Level: Cenomanian.

Associated Fauna:Rotorbinella mesogeensis (Tronchetti) and Trocholina sp.

Diagnosis: Globular or subglobular to slightly elongated, non-dimorphic morphotype. The maximum axial diametre is about 2.4 mm. The megalospheric generation starts with a proloculus of 120–220 μm followed by a flexostyle of about half a whorl. Chambers following the flexostyle are planispirally arranged, 7–9 whorls compose an adult shell. Cortical chamberlets appear in the first whorl, while medullar ones appear in the third to fourth whorl. In the axial section the shape of the former has a circular to subrectangular outline, while the latter always have a circular morphology. The number of cortical chamberlets (counted in a quarter of an axial section) in the fourth whorl varies from 9 to 12.

Specimens of the microspheric generation, less abundant than those of the megalospheric generation, start with a small, poorly preserved glomerulus. The adult specimens reach a maximum axial diametre of around 2.5 mm with 15–16 planispiral whorls. Cortical chamberlets appear in the first to second whorl. Medullar chamberlets appear in the seventh to eighth whorl (Figure 9).

Occurrence: at present, all the studied specimens are from Jabal Kadarma. Nevertheless, sections similar to Decastroia have also been recognized in the sediments from the Cenomanian of the Natih Formation (Oman Mountains, northern Oman; personal communication, Esmeralda Caus). Moreover, some pictures referred to the genera Praealveolina or Pseudedomia in regional studies (e.g. Simmons and Hart, 1987; Smith et al., 1990) should be probably placed in the new genus. Most of these figures are not centred sections, and the details about their architecture cannot be properly observed. Further studies in the Cenomanian sediments of the Middle East are needed to constrain its stratigraphical position.

CONCLUSIONS

The new alveolinacean Decastroia razini n. gen, n. sp. shows: (1) Globular to slightly elongated shells with two rows of superposed apertures of large and small diametre, respectively. (2) Septula and floors subdivide the chamber lumina into two rows of tubular chamberlets: an upper, peripherical row with large cortical chamberlets and a lower one with small medullar chamberlets. (3) The septula are aligned from one chamber to the next, and near the septum they are interrupted by the preseptal passage. According to the characteristics described above, the new genus has intermediate characteristics between Praealveolina and Sellialveolina.

ACKNOWLEDGMENTS

We would like to thank P. Razin (Institut EGID Bordeaux 3, France) and C. Robin (Université de Rennes, France) for their geological contributions. Lukas Hottinger (Museum of Natural History of Basel, Switzerland) and Esmeralda Caus (Universitat Autònoma de Barcelona, Spain) are also gratefully acknowledged for their very useful suggestions on alveolinacean foraminifera. This paper is a contribution to the projects CGL2009-08371 and CGL2008-00809 (coordinator E. Caus) from the Spanish Ministry of Science and Innovation and to the projects YOCMAL and GDR “Marges”-Enregistrement tectono-sedimentaire de l’evolution geodynamique des marges conjuguees du Golfe d’Aden (coordinator Sylvie Leroy) from the French Agence Nationale de la Recherche. The authors thank the two anonymous reviewers for their suggestions and GeoArabia Designer, Nestor “Niño” Buhay IV, for designing the paper for press.

ABOUT THE AUTHORS

Vicent Vicedo studied Geology at the Universitat Autònoma of Barcelona (U.A.B.), where he received his BSc (2004) and PhD (2008) diplomas with honours. His PhD project involved a study of the shell-architecture of Upper Cretaceous rhapydioninids (larger foraminifera) of the Caribbean and Tethyan domains. Vicent worked on several funded projects on micropaleontology and biostratigraphy, integrating fieldwork, studies of microfacies and modeling of larger foraminifera. Currently, he is member of the group of micropaleontology of the U.A.B.

vicent.vicedo@campus.uab.es

Josep Serra-Kiel is Professor Emeritus of the Faculty of Geology at the University of Barcelona, Spain. He is a specialist in larger foraminifera and biostratigraphy of the Paleogene. Josep was Head of the Department of Stratigraphy, Paleontology and Marine Geosciences for several years and the coordinator in developing the Shallow Benthic Zones of Paleogene in the frame of the I.C.G.P. 286 Early Paleogene Benthos and 393 Neritic events at the Middle-Upper Eocene boundary. He participated in the fieldwork and biostratigraphy of the Paleogene sediments of the Zagros and Oman Mountains, Dhofar, Yemen and Socotra under the projects Middle East Basins Evolution and GDR Marges. Josep collaborated with the PEMEX Company on the biostratigraphic study of the Eocene – Lower Miocene interval of the Yucatán Peninsula.

josepserra@ub.edu