Abstract--A geochemical study on the Albian and Turonian shales from the southern Benue trough of Nigeria was carried out to establish their geochemical ...
Journalof African EarthSciences,Vol. 6, No. 5, pp. 633-641, 1987
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Major and trace element geochemistry of Aibian and Turonian shales from the Southern Benue trough, Nigeria L. C. AMAJOR Department of Geology, University of Port Harcourt, Port Harcourt, Nigeria
(Received for publication 5 September 1986) Abstract--A geochemical study on the Albian and Turonian shales from the southern Benue trough of Nigeria was carried out to establish their geochemical fingerprints. The results show that these shale units are geochemically dissimilar both in major element oxides, trace and some rare earth elements. The element pairs which differentiate them best are MgO/Fe203, MgO/MnO, CaO/P2Os, CaO/Sr, K20/Rb, Zr/P2Os, Pb/Th, Sr/Rb and Zr/Y. The Albian shales are chemically fairly similar to the North American Albian Mowry shale (sediments of shallow restricted inland seas) whereas the Turonian shales compare favourably well with the average normal shales. Changes in source rock characteristics, paleo-depositional environments and diagenesis are collectively thought to have caused the variation in geochemistry of these shales. The paleogeographic significance of the chemical variation is discussed.
INTRODUCTION THE STRATIGRAPHICevolution of the southern Benue trough begins with the Asu River Group (mid Albian) followed unconformably by the Turonian Ezeaku Formation. This is conformably succeeded by the Coniacian Awgu Shale. A Santonian angular unconformity separates these units from post-Santonian sedimentary successions (Fig. 1). The pre-Santonian lithostratigraphic units are essentially shales with localized development of sandstones and limestones, and all are intruded by dolerite sills and dykes. The mid-Albian Asu River Group sediments were deposited by the first marine transgression in the trough. This shallow sea, thought to be the northern extension of the early South Atlantic ocean, shoaled around the middle Benue (Fig. 1). Some workers in the area believe that pyroclastic volcanism was associated with the sediments (Mineral Survey of Southern Nigeria Reports 1905-1909 Reyment 1965). In contrast, the Turonian Sea is believed to have inundated the whole trough as a result of a merger between the Northern Tethys and the South Atlantic seas (Furon 1963, Reyment 1965). Furthermore, (Reyment 1972, Reyment and Tait 1972) suggested that more normal marine conditions in the South Atlantic were first established in Turonian times as a result of the free merging of the South Atlantic with the North Atlantic and Tethys seas at this time. This latitude of variation in the depositional environments of these rocks, if correct, should be reflected in the sediments, at least geochemically, as they are believed to have a common provenance. Most importantly, the lithologic similarity of both stratigraphic units make it difficult to identify their boundary, the Cenomanian hiatus, in the field. This problem is more acute in the southern Benue trough where both the Asu River Group and Ezeaku Formation are mostly shales. The problem is amplified by the rarity and/or paucity of forams in the older unit and in places in the Ezeaku Formation (Petters
1977). Furthermore, the mineralogy and geochemistry of these rocks have, hitherto, not been characterized. The purpose of the present paper is to document the geochemical characteristics of the Albian Asu River Group shales and the Turonian Ezeaku shales. It is hoped that this study will clarify aspects of the origin of these sediments, provide geochemical criteria for distinguishing and correlating both units in the absence of fossils and bring to light some new areas of needed research. The region chosen for the study is the southern Benue trough (Fig. 1).
GEOLOGICAL SETTING The Benue trough is an elongated Cretaceous rift which trends northeast for about 800 km with a width which varies from 80 to 90 km. The lithic fill is more than 5000 m in thickness (Nwachukwu 1972). The initial marine transgression in the trough occurred in mid-Albian times. A sequence of shales, limestones and sandstones, 2000 m thick, constituting the Asu River Group, was deposited. Shales dominate the southern part of the trough and these grade northwards into platform carbonates and strandline sands. A poorly known Cenomanian tectonic event restricted marine sedimentation to the extreme southeast of the trough (Calabar flank). A succession of arkosic sandstones, shales, limestones and sandstones, typical of a transgressive shore line deposit, was laid down and known as the Odukpani Formation. These rest unconformably on basement rocks. This stratigraphic break is difficult to pick in the field. Shales, limestones and sandstones comprising the Ezeaku and Awgu Formations were deposited by the extensive Turonian-Coniacian marine transgression. The Ezeaku Formation (Turonian) is a 100 m-thick sequence of shales with localized development of limestones and sandstones. In the study area, these rocks rest
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L . C . AMAJOR
LEGEND I.
Precambrion
Basement Comptex
2. ALbion Asu River Group 3. Cenomanian Odukpani 4.
Turanian
Ezeaku Fm
5.
Coniacian
Awgu Sh.
7. Momu
Fm
Fm
~-
8.
Ajoli
Sst
~o
9.
Nsukka Fm
I0. PaLeocene
