NAPENews Magazine September 2022 Edition

Sequence Stratigraphy
The Shale Member (Eze-Aku Shale) of
the Eze-Aku Formation is interpreted to be
of shallow marine origin based on
documentation of rich ammonite content
in the shales (Barber, 1957; Reyment,
1965; Murat, 1972; Nwachukwu, 1972;
Umeji, 1984). The widespread nature of
t h i s s h a l e , a s o b s e r v e d i n t h e
study area, suggests a marine incursion.
Initial age dating of the shales using
a m m o n i t e c o n t e n t p l a c e t h e
age of the shales as Turonian (Simpson,
1955). Early works posited that Eze-Aku
Group was deposited during the extensive
marine transgression at the beginning of
the Turonian when the sea invaded the
Benue Trough from the Gulf of Guinea
(Reyment, 1965; Kogbe, 1989). More
recent work by Igwe et al. (2015)dated the
Eze Aku shale facies as Late Cenomanian
to Turonian. Kogbe (1989) suggested that
t h e p e r i o d o f T u r o n i a n m a r i n e
transgression was also characterized
local regression, which is evidenced in the
occurrence of the Amasiri Sandstone.
Igwe and Okoro (2016) divided the Eze-
A k u G r o u p i n t o a t r a n s g r e s s i v e
component, which is the Eze-Aku Shale of
Late Cenomanian to Early Turonian, and
the regressive component, which is the
Amasiri Sandstone of Middle to Late
Turonian age. The range of depositional
environments interpreted for the Amasiri
Sandstone is tied to relative sea-level
changes which affected sedimentation in
the basin. This is used to describe the
sequence stratigraphy of the Amasiri
Sandstone.
Sequence 1
The sedimentary succession of the
Amasiri Sandstone begins with a
transgression which started in the Late
Cenomanian and reached a maximum in
the Early Turonian. This caused a relative
increase in sea level (sea level 1 – sea
level2), and a landward shift in the
shoreline. Shallow offshore sediments
were deposited on the older Abakaliki
Formation of Albian-Cenomanian age.
T h e c o n t a c t b e t w e e n b o t h
lithostratigraphic units marks a candidate
flooding surface (FS-1).The transgression
also brought in sands into the basin. With
the predominantly marine conditions of
the basin, tidal action prevailed and
unidirectional tidal currents reworked the
sands into subtidal sandwaves, which are
which comprises of poor to moderately
sort, medium to coarse-grained sands,
characterized by tabular-planar crossbeds
with erosional surfaces. The offshore
and the subtidal sandwave deposits make
up a candidate transgressive systems
tract (TST-1).
A drop in energy levels causes a change
from sand to shale deposition and the tidal
sandwave deposit is capped by another
offshore shale deposit, though of lesser
thickness compared to the initial one. The
deposition of a relatively lower amount of
dark grey shale over the subtidal
sandwaves suggests the attainment of a
maximum reach of the sea level (sea level
2 – sea level 3). The overlying shale
represent a candidate highstand systems
tract (HST-1) and its lower contact
represents a candidate maximum flooding
surface (MFS-1) which is likely associated
with a global transgressive phase, such as
that which characterized the Early
Turonian. MFS-1 is characterized by
opportunistic Ophiomorpha burrows,
which were most likely produced during
the period of low energy.
Sequence 2
Sequence 2 begins with deposition of
fluvial channel sediments, which are most
likely the initial record of a regressive
phase thatcommenced in the Middle
Turonian. This regression brought about a
drastic relative sea-level fall (sea level 3 –
sea level 4) and basin ward shift in the
shoreline, decreasing accommodation in
the basin. This activated fluvial systems to
flow in the direction of the basin. These
rejuvenated fluvial streams cut channels
into the underlying sediments where
fluvial sediments, which include
conglomerates and trough cross-bedded
sandstones, were deposited. The channel
base in which the sediments occur
probably represents an incised valley, and
a candidate sequence boundary (SB-1)
which underlies a lowstand systems tract.
The fluvial streams are most likely the
source that fed sediments into the basin,
commencing shelf building. This led to the
deposition of coarsening upward
successions of shoreface sediments.
Hence, the fluvial channel as well as the
shoreface deposits make up a candidate
lowstand systems tract (LST-1).
With subsequent sediment input into the
basin, sea level begins to rise relatively
(sea level 4 – sea level 5) and aforeshore
formed above a candidate maximum
flooding surface (MFS-2). The prograding
siliciclastic shoreface deposits are
replaced with a mixture of siliciclastics and
carbonates. This transition signifies a
s i g n i f i c a n t s h i f t f r o m s h o a l i n g
(progradation) of theshoreline to
deepening (retrogradation) and is
characterized by low sediment input and
increase in accommodation. Tidal action
ensues while relative sea level continues
to rise(sea level 5 – sea level 6) and some
sediments are reworked to form tidal sand
ridges, which are characterized by
bidirectional and herringbone crossbedding.
The foreshore and tidal sand
ridge deposits represent the second
candidate highstand systems tract (HST-
2).
A continuation of relative sea level rise
(sea level 6 – sea level 7) leads to another
transgression that deposits offshore
deposits which make up the second
candidate transgressive systems tracts
(TST-2). The contact between the tidal
sand ridges and the offshore deposits
becomes a third candidate flooding
surface (FS-2).
Sequence 3
Sequence 3 begins with a relative drop in
sea level (sea level 7 – sea level 8), which
is just enough to activate fluvial channels
which depositfining-upward successions
of fluvial channel sands (conglomerate
and very coarse sands). However, the
fluvial channels most likely had interaction
with the sea. Evidence of this include mud
streaks, mud-draped crossbedding,
reactivation surfaces and heterolithic
units which overlie the channel sands.
The occurrence of aconglomeratic to very
coarse grained sandstone over offshore
sediments marks the second candidate
sequence boundary (SB-2). The tideinfluenced
fluvial sands represent a
candidate lowstand systems tract (LST-
2). This regressive phase was terminated
by a relative rise in sea level (sea level 8 –
sea level 9) that continued the deposition
of offshore deposits, which most likely
makes up a third candidate transgressive
systems tract (TST-3). The contact of
TST-3 with the underlying sands
represent a third candidate flooding
surface (FS-3). The transgressive
sedimentary package (TST-3) marks the
end of the Amasiri Sandstone deposition.
NAPENEWS AUG/SEPT 2022 33