REGIONAL STRATIGRAPHY OF SOUTH EASTERN NIGERIA

Regionally, the mapped area is within the Southern end of the Benue trough. The oldest sedimentary rocks in Nigeria is in this trough and they are of lower Cretaceous age. The cretaceous stratigraphic record of the Southern Benue trough is represented by sediments deposited by three main marine depositional cycles: Albian-Cenomanian; Turonian-Santonian and campano-Maastrichtian. (Reyment, 1965, Ofoegbu, 1985). Shortly after the break up of African and South America, a continental condition favourable for the deposition of fluvio-deltaic sediment which occurred in the Albian age. The aptian sediment is presented by the Ogoja sandstones.

            The first marine transgression in this trough occurred in the middle Albian age with the deposition of the Asu river group sediments with type locality along the banks of Asu River (Reyment, 1965). The sediments consist of rather poorly bedded sandy shales with sandstone and sandy –limestone lenses.

            The regressive phase of the first marine transgression led to the deposition of the Cenomanian sediments. The beds of this age are located in the Southeastern part of Nigeria, around Calabar. These beds have been assigned as the Odukpani Formation (Reyment, 1965). It consist of arkosic sandstones, limestones and alternating limestones and shales which became gradually more predominantly shally in its uppermost parts (Reyment, 1965).

            The Turonian is overlain by sediments of Coniacian age. The Coniacian is represented by the Awgu Formation (Reyment, 1965). The formation consists of bluish grey, well bedded shales. The Santonian age was heralded by a tectonic event. This tectonic epirogenic event led to the uplift, folding and widespread erosion of the pre-santonian sediment in the trough.

            The Campano-Maastrichtian experienced the third sedimentary cycle in this trough. It started with the deposition of the Nkporo formation (Nkporo shales). It consist of dark shales and mudstone with occasional thin beds of sandy shales and sandstones. A regressive phase of this transgression occurred in middle Maastrichtian with the deposition of the mamu formation and subsequent deposition of Ajali sandstones. A minor transgression occurred in late Maastrichtian with the deposition of Nsukka formation. This stratigraphy is represented in Table 1 below.

Table 1: Regional Stratigraphic Sequence of South Eastern part of Nigeria. (Modified from Reyment (1965), Murat (1972)

AGE	FORMATION	SEDIMENTARY CYCLE
Pliocene	Benin Formation	Niger-Delta basin (Third tectonic phase)
Pleistocene	Ogwash-Asaba Foramtion
Eocene	Ameki Formation
Paleocene	Imo Shale
Maastrichtian	Nsukka  Formation Ajali Sandstone Mamu Formation	Anambra-Afikpo Basin (second sedimentary cycle).
Campanian	Nkporo/Enugu Shale (including Afikpo Sandstone and Owerri Sandstones).
Santonian Coniacian	Awgu Shale	Abakaliki-Benue Basin (first sedimentary cycle).
Turanian Cenomanian Albian	Ezeaku Formation Odukpani Formation Asu-River Group