Palynological and palynofacies analysis of early Silurian shales from borehole CDEG-2a in Dor el Gussa, eastern Murzuq Basin, Libya

Abstract

This work presents palynological and palynofacies analyses from early Silurian Tanezzuft Formation shales recovered from core material of borehole CDEG-2a drilled in Dor el Gussa at the eastern margin of the Murzuq Basin, southern Libya. The material has yielded well-diversified palynomorph and chitinozoan assemblages and organic matter from a depth interval of 37.41 to 50.64 m. The interval above 37.41 m is weathered and barren of palynomorphs. The palynomorph assemblage present from the 37.41-50.64 m interval is early Llandovery (Rhuddanian-early Aeronian) in age and was deposited in a shallow, open marine (but proximal) environment with a minor landward shift between 43.02 and 46.13 m. The oldest chitinozoan assemblage recorded in the studied core samples can be referred to the fragilis regional Biozone. However, because the relative abundance of Spinachitina fragilis is low, it is considered here that the deepest part of the borehole (at 50.64 m) does not penetrate the actual base of the Rhuddanian. Thus, a Rhuddanian-earliest Aeronian age is retained here for the corresponding strata in borehole CDEG-2a. The core includes the boundary between the fragilis and nuayyimensis biozones, which was until now unknown from southern Libya. Furthermore, we describe a new chitinozoan species, Angochitina murzukensis sp. nov. The intervals 37.41-42.36 m and 46.48-50.64 m show a dominance of amorphous organic matter (AOM) Types 1 and 2, suggesting an oil-prone potential, in contrast to interval 43.02-46.13 m, which is dominated by land-derived (vitrinite) Type 3 suggesting a gas-prone potential. The Ordovician-Silurian boundary and therefore the glacial-related Late Ordovician deposits have not been penetrated by the borehole. In all likelihood, this boundary is not far below the bottom of borehole CDEG-2a. (C) 2012 Elsevier B.V. All rights reserved.