REGIONAL GEOGRAPHY
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
|