Devonian Chitinozoa and Acritarcha from Exploratory Oil Wells on the Shelf and Coastal Region of Ghana, West Africa

Sixty-eight species of Chitinozoa belonging to 15 genera, and 37 species of Acritarcha belonging to 17 genera from Devonian sediments of exploratory oil wells on the shelf and coastal region of Ghana, West Africa, are figured and described. Twenty species and three genera of the Chitinozoa, and twelve species and two genera of the Acritarcha are considered new, but formal names have not been proposed yet.

The Ghanaian chitinozoan and acritarch assemblages have several elements in common with those from the Devonian of Europe, North Africa, North America, and especially with an upper Lower Devonian (Emsian) to a lower Upper Devonian (Frasnian) chitinozoan and acritarch assemblages from Brazil, South America.

Four tentative chitinozoan assemblage zones have been proposed for the marine section of the Devonian sediments observed from the wells as follows, in descending order: “Zone D”: Angochitina mourai Assemblage Zone; “Zone C”: Angochitina devonica–Angochitina (Ramochitina) ramofi Assemblage Zone; “Zone B”: Cladochitina varsipinosa Assemblage Zone; “Zone A”: Ancyrochitina sp. 6 Assemblage Zone.

Microfossil evidence indicates that a marine transgression from the westerly direction, with respect to the present magnetic pole position, occurred in Ghana during the late Early Devonian (Emsian) time. The sea was very shallow, of an epicontinental platform type and for a long period of time periodic fluctuations in water level ensued. Marine regression probably started during the early Late Devonian (Frasnian) and was slow. In the west, deltaic conditions probably followed the marine regression. Continental deposition followed the complete withdrawal of the Devonian Sea which was in turn followed by brackish water conditions.

This study indicates a single and probably a continuous sedimentation cycle during the Devonian period in Ghana. It appears that a restricted stratigraphic interval in the Middle Shale Formation, or Clay Shale, of the Accraian Series outcropping along the east coast of Ghana corresponds to the lower section of the marine sediments observed from the wells, and a restricted stratigraphic interval in the Takoradi Shale of the Sekondi Series outcropping along the west coast of Ghana corresponds to the upper section of the marine sediments and lower part of the overlying continental material. The Clay Shale carries macrofossils assigning an Early Devonian or Middle Devonian age to the Accraian Series and the Takoradi macrofossils, as well as plant fragments, have been used to date the Sekondi Series as Late Devonian or Early Carboniferous age. Several units in the Accraian Series and the Sekondi Series are probably synchronous and their lithologic dissimilarities are assumed to be reflections of local variations in the supply of clastic materials.

The four tentative chitinozoan assemblage zones proposed by the writer for the marine Devonian sediments in Ghana correlate closely, both in microfossil content and diagnostic species ranges of the zones, with similar zones proposed by Lange (1967) for Devonian sediments in the Parana Basin, Brazil, South America. Thus, this study reinforces the evidence of a common geologic setting for both sides of the Atlantic Ocean off West Africa and South America during the Devonian period.