NAPENews Magazine September 2022 Edition

Sandy
heterolith
facies (Hs)
Muddy
heterolith
facies (Hm)
Shale facies
(Fsc)
Fosiliferrous
limestone (Lf)
Dolerite
facies (D)
Millimetre to centimetre
scale intercalations of
dominant fine-grained
sands, with silt and mud
Millimetre scale
intercalations of dominant
silt/mud, with very finegrained
sand
Grey coloured shale;
locally mottled
Limestone made up of
mineralized brachiopod
shells.
Medium-grained
melanocratic igneous rock
with vitreous lustre.
Horizontal
laminations
Horizontal
laminations
Horizontal
laminations
Abundant whole
and fragmented
shells of
brachiopods
Igneous sill
Not observed
Moderately bioturbated
by Planolites
and Paleophycus
Not observed
Not observed
Not observed
Lateral accreting point bars
Low or waning energy flow
condition
Low energy and oxygendepleted
conditions
Deposition in a low-energy
environment with poor
winnowing activity ; most
likely interrupted by
catastrophic event such as a
storm
Emplacement during tectonic
activity that uplifted the
southern Benue Trough
Facies Associations
Facies Association 1 (FA 1): Shallow
offshore
FA 1 is comprised of shale (Fsc) and
sandy heterolith facies (Hs) (Figure 2A).
Exposures of FA 1 occur at Amenu,
Akpoha, Aba-omege, Ozara-ukwu and
Afikpo. Thickness of exposures range
from 4m to over 50m. FA 1 occurs as the
basal and topmost parts of the Amasiri
Sandstone.
Facies Association 2 (FA 2): Subtidal
sandwave
FA 2 is characterized by large scale planar
cross-bedded sandstone facies (Sp4),
with subordinate planar crossbedded
sandstone facies (Sp1), horizontal
laminated sandstones facies (Sh1 and
Sh3), and structureless sandstone facies
(Sm1 and Sm2) (Figure 2B). FA 2 is
exposed at Akpoha and extends to Abbaomege.
Exposures of FA 2 at both
locations show that it overlies offshore
deposits (FA 1). Generally coarse-grained
in texture, the sandstone is poorly to
moderately-sorted. Outcrops of the
sandstone occur as extensive sheets of
sand, with tabular geometry. Estimated
total thickness of the outcrop is about
28m. the large-scale crossbeds that
characterize the outcrop are interpreted
as tidal sandwaves (Allen, 1980; Stride et
al., 1982). Their forests dip at angles
ranging from 4 to 24°. The forsets show a
gradation in sand size from coarse to fine,
indicating decreasing peak current
strength (Stride et al., 1982). The crossbeds
are unidirectional, indicating a
south-west paleo flow that ranges
between 232 and 236 . Tidal sandwaves
are produced during the lower flow regime
at near-surface mean spring tidal current
-1
speeds of more than 65cms (Stride et al.,
1982; Johnson and Baldwin, 1996). A tidal
origin is attributed to the sandstone due to
the presence of mud drapes on the crossbeds
and reactivation surfaces. A similar
interpretation has been assigned to the
lowermost sandstone member of the Imo
Shale (Ekwenye et al., 2014).
Facies Association 3 (FA 3): Fluvial
channel
FA 3 comprises of conglomerate facies
(G1 and G2), trough crossbedded
sandstone facies (St), horizontal
laminated sandstone facies (Sh1 and
Sh3), structureless sandstone facies
(Sm1 and Sm2), and planar crossbedded
sandstone facies (Sp1) (Figure 3A). FA 3
is exposed within the premises of the
Government Technical College, Akpoha.
Estimated thickness of the outcrop is
about 11m. A fining-upward motif is
observed, starting with conglomerate
facies at the base, through trough
crossbedded sandstone facies, to
horizontal laminated sandstone facies
and structureless sandstone facies (Sh3,
Sm1 and Sm2). Different parts of the
outcrop correspond to different fluvial subenvironments.
G1 and G2 correspond to
the base of the channel. They are
interpreted as longitudinal and linguoid
bars respectively (Miall, 1977; 1978;
1981). The conglomerate facies are
stacked into multistory sheets, which are
likely products of laterally mobile highenergy
bedload streams Collinson (1996).
Paleo flow indicators in G2 show a net
northeast paleocurrent direction.
St, Sh3, Sm2 and Sp1 make up the
channel sandbody. This exposure is
about 2m thick, and is dominated by upper
part of the lower flow regime (Miall, 1977;
1978). This crossbedding is similar to that
of Singh (1972) who referred to them as
“channel-fill cross-bedding”. He opined
that trough crossbedding of this scale are
a result of channels cutting across a levee
during high-water periods of the river.
Sh1, Sh3, Sm1, and Sm2 make up the
floodplain deposits. An initial facies
succession shows a coarsening upward
motif interpreted to be a result of the
formation of levees, which are banks of
sediments fed by the deposition of coarser
sediments at the channel edge by the
rapidly flowing river (Nichols, 2009).
However, this subsequently changes to a
fining upward succession, which may
signify the dissipation of current velocity
and energy as the flood leaves the
confines of the channel and spread out
(Nichols, 2009), or the abandonment of a
crevasse-splay which is formed by the
breaching of a levee (Collinson, 1996).
NAPENEWS AUG/SEPT 2022 27