Occurrences of elaterate pollen from the Lower Cretaceous of Ghana: Implications for biostratigraphy and palaeoclimatology

Elaterate pollen with elater-like protruberances including Elaterocolpites castelaini , Elaterosporites klaszii , E. protensus , E. verrucatus , Elateropollenites jardinei , Galaeocornea causea , G. clavis , Sofrepites legouxae , have been recovered from the 1S-3AX well in the offshore Tano Basin. The assemblage has been interpreted as Albian - Cenomanian age, and is indicative of an arid to semi-arid palaeoclimatic conditions for these periods in the Tano Basin. Similar species have been interpreted as Albian – Cenomanian in other localities within the Africa-South America (ASA) province and thus allows for a palynostratigraphic correlation with these localities in the ASA province.


INTRODUCTION
A total of 39 cutting samples have been obtained and analysed from the offshore Tano 1S-3AX well in the Tano Basin (Fig. 1). The Mesozoic to tertiary rocks of the basin occur on the eastern side of a crescent-shaped basin which is located along the coast of the Atlantic Ocean. The basin extends to the southeastern corners of Cote I'voire and continues into the Gulf of Guinea. The Tano Basin began its tectonic-sedimentary life as an extensional rift basin modified by wrench tectonism. This rifting was initiated by complex movements due to the separation of the continents of South America and Africa. This was most likely initiated in the Barremian and Aptian times. It is thought that movement along a series of transform faults including faults in the Romanche Fault Zone during this continental separation led to the development of the large rift basin in the Tano area (Davies, 1989). As a result of these movements, by Aptian -early Albian time, a large rift basin had developed in the Tano Basin area. This was followed in middle -late Albian times by widespread deposition of shallow marine sandstones and shales with minor limestone in the area. General evidence suggests that final separation on the continents took place in latest Albian (Davies 1989). It is speculated that, a thermal anomaly with subsequent uplift occurred at the margin of the newly created African and Brazilian continental plates in the Tano area. This uplift occurred in late Albian time and may be the plate tectonic model for the development of the Tano structural trend. This paper presents the occurrences of elater-bearing species including Elaterocolpites castelaini, Elaterosporites klaszii, E. protensus, E. verrucatus, Elateropollenites jardinei, Galaeocornea causea, G. clavis, Sofrepites legouxae, in the Tano basin and also provides updated information on their occurrence in some parts of the Africa-South America (ASA) province (Fig. 2).   Remarks: Specimen has elliptical to subspherical central body with a strongly convex distal face, thick exine and ornamented with spines (5-7 µ long, 2.5 µ wide at base). It also bears 3 pairs of U-shaped cylindrical appendages of almost equal lengths.

DISCUSSIONS AND CONCLUSIONS
The elater-bearing pollen have been attributed to the Albian -Cenomanian Elaterate Province of Africa South America (ASA). This concept was introduced by Herngreen (1975) and has been also referred to as Northern Gondwana Province (Brenner, 1976) and Elaterosporties phytoprovince (Srivastava, 1981).
Their unique morphology with protuberances characterise this otherwise heterogeneous group of pollen grains with and short stratigraphic range. They appeared in the lower Albian sediments of the low latitude region, diversified, became numerically important in the upper Albian -Cenomanian and died out at the end of Cenomanian, which saw rapid diversification and rise to dominance of the angiosperms (Herngreen et al. 1996;Vallati, 2013). The elater pollen have attracted the attention of palynologists and has contributed to their application in palynostratigraphy and paleobiogeography (Schrank, 2001). According to Vallati (2013), the morphological characteristics of these grains are unknown from extant pollen grains and up to now in situ specimens have not found. A probable ephedroid affinity for the elaterates has been proposed (Shrank, 2001; Dino et al.1999;Crane, 1988). The ranges of other associated palynomorphs in the assemblage point to an Albian -Cenomanian age.
The distribution of the characteristic elements of this province paralleled the palaeolatitude and the axis of the Elaterate Province approximates the palaeoequator (Herngreen, 1998;Dino et al., 1999). The province is recognized in ASA region, Middle East, (Fig 3) and recently Bahamas islands, China and Papua New Guinea (Herngreen, 1996(Herngreen, , 1998Dueǹas-Jimenez, 1990). According to Herngreen (1998), their presence suggests that climate was the main controlling factor of the geographical extent of the province. This phytogeographic belt is characterized by the presence of elater-bearing species and related forms which are restricted to the province, variety of polyplicate forms (ephedroids, etc), absence of bi and trisaccate gymnosperous pollen, scarcity of fern spores and the presence of angiospermous pollen.
The presence of elaterate pollen have been interpreted as indicating arid -semi arid palaeoclimatic conditions (Herngreen and Dueǹas-Jimenez, 1990;Herngreen et al., 1996;Schrank, 2001). But because of the unknown botanical affinities, and taking into account wall stratification and ultrastructural compartibility of the elaterates (Sofrepites, Elateroplicites, Elaterosporites) with that of Ephedripites and other Cretaceous polyplicates, Dino et al., (1999) suggested that elaterates and polyplicates are botanically related. Dino et al., (1990) thus intimated that palaeoclimatic factor controlling the distribution of the elater pollen may be obtained from knowledge of associated vegetation and the types of environment they inhabited. They also opined that associated plant fossil provided a perspective of the vegetation and the enclosing sediments reflect the interaction of physiographic/biotic processes in the region.
The associated plant fossils (Classopollis, ephedroids) with the elaterates in our materials are similar to that of Dino et al., (1990) and Schrank (2001), which suggests that a hot-arid to semi-arid climatic condition must have prevailed during deposition of the Albian -Cenomanian sediments in the Tano Basin. Mahmoud and Deaf (2007), Dino et al., (1999) and Schrank (2001) have reported that Afropollis and Elaterate pollen parent plants flourished in humid coastal plains. The presence therefore of the Afropollis pollen in the samples investigated here suggests that similar conditions prevailed in Ghana during the Albian -Cenomanian time.
The continental breakup, drift and initiation of new oceans worldwide were experienced during the Cretaceous period. The opening of the South Atlantic Ocean, which separated South America from Africa, was initiated by crustal thinning and thermodynamic uplifts during the Late Jurassic. According to Dino et al., (1999), which is reiterated by Abubakar et al., (2006), the elater-bearing plants were found in dry zones under warm climates, and that their diversification and abundance were the response to climatic changes associated with the opening and enlargement of the northern parts of the South Atlantic ocean in the latest Aptian to early Albian, which came to a close during the late Albian -Cenomanian time. Dino et al., (1990) also opined that, the widening and deepening of the South Atlantic Ocean at the close of Cenomanian, resulting in temperature drop, culminated in the disappearance of the elaterate pollen from the stratigraphic record.
Contrary to the Brazilian and Equadorian basins, where Dino et al., (1990) have reported that the earliest appearance and highest diversity and frequency levels of the elaterates seem to coincide with the transgressive event during the late Albian -Cenomanaian time, the elaterates in this study, reached their maximum diversity and frequency levels in fluvial and lacustrine continental facies. This observation supports Abubakar's (2006) suggestion that the appearance and subsequent abundance of the elaterates is evolutionary and palaeoclimatic in nature and that the palaeoclimatic influence may not necessarily be related to the opening of the northern part of the southern Atlantic Ocean, as suggested by Dino et al., (1990).
Overlying the Cenomanian fluvial/lacustrine facies are the limestones which is as a result of deposition in open marine waters which is devoid of the elater pollen. This supports the suggestion of Dino et al., (1990) that the elater pollen disappeared as a result of climatic fluctuations or perturbations during the onset of deeper and more marine conditions as the South American Atlantic Ocean widened at the close of the Cenomanian.