NAPENews Magazine September 2022 Edition

Depositional Evolution and Sequence Stratigraphy of the
Amasiri Sandstone Member of the Eze-Aku Formation,
Southern Benue Trough
Ogechi C. Ekwenye, AyonmaW. Mode, and Ogechukwu A. Moghalu*
Department of Geology, University of Nigeria, Nsukka, Nigeria
Peer Reviewed Technical Article
ABSTRACT
Integrated lithofacies and sequence stratigraphic analyses of the Late Cenomanian to Turonian Amasiri Sandstone Member of the Eze-
Aku Formation in the Southern Benue Trough, was carried out, to decipher the depositional evolution and establish the sequence
stratigraphic framework. Results from facies analysis reveals 16 facies, which occur in associations that are characteristic of a
predominantly shallow marine environment. The interpreted sub-environments include shallow offshore, subtidal sandwave, shorefaceforeshore,
tidal sandridges, fluvial channel, and tide-influenced fluvial channel settings. Parasequence delineation of the facies
associations reveal retrograding, aggrading, and prograding packages which were deposited during relative sea-level rise and fall. Three
depositional sequences which document the depositional evolution of the Amasiri Sandstone Member from the Late Cenomanian to
Turonian, were recognized based on the parasequence stacking patterns. Sequence 1 is an incomplete sequence, comprising of a
transgressive systems tract, consisting of subtidal sandwave encased in shallow offshore sediments. Sequence 2 is a more complete
sequence that comprises of fluvial channel and shoreface sediments as the lowstand systems tract, foreshore and tidal sand ridge
sediments as the highstand systems tract, and offshore sediments as the transgressive systems tract. The third sequence is an also an
incomplete one with tide-influenced fluvial channel and offshore sediments as the lowstand and transgressive systems tracts respectively.
Key words: Amasiri Sandstone, palaeodepositional environments, sequence stratigraphy, relative sea level changes.
INTRODUCTION
The Amasiri Sandstone, a member of the
Eze-Aku Formation in the Southern Benue
Trough (Murat, 1972; Nwachukwu, 1972),
comprises of mud, sandstone, heterolith
and limestone facies, whose characteristics
indicate a range of palaeoenvironmental
conditions. Previous research on the
sandstone member has focused on its
sedimentology with the aim of interpreting
the palaeodepositional environment(s)
(Banerjee, 1980;Amajor, 1987; Dim et al.,
2016; Okoro and Igwe, 2014). The
depositional evolution of the member in
relation to relative sea-level change is
however, relatively less studied. In the
absence of this, the sequence stratigraphy
of the lithostratigraphic unit is not known.
Deposited in a predominantly shallow
marine setting, the facies of the Amasiri
Sandstone Member are grouped into facies
a s s o c i a t i o n s t h a t i n d i c a t e
palaeoenvironments that prevailed during
periods of transgression and regression. In
this study, the lithofacies identified within the
Amasiri Sandstone are subjected to
analysis with the aim of interpreting the
evolving palaeoenvironment(s) that
prevailed as the lithostratigraphic unit was
being deposited. These environments are
then linked to eustatic changes and used to
interpret the sequence stratigraphy of the
lithostratigraphic unit.
GEOLOGIC SETTING
The formation and subsequent deposition in
the Benue Troughis regarded as Abakaliki-
Benue tectonic phase (Murat, 1972), which
is the first of three tectonic phases that
controlled the evolution of the southern
Nigeria sedimentary basins and subsequent
deposition within the basins. The evolution
of the Benue Trough began with the spread
of the South Atlantic in the Late Jurassic.
This sea-floor spread allowed for the
separation of the Africa and South American
plates around a triple junction. With the
complete spread of two arms of the triple
junction, the third arm failed to spread. This
failed arm was a transcurrent fault zone that
runs through the African Plate, forming the
West and Central African Rift System, which
opened up a series wrench basins on the
continent; one of which is the Benue Trough
(Genik, 1993). The Benue Trough is a linear
NE-SW trending basin that extends from the
southern part of the Nigerian sector (Figure
1) into Cameroun, extending over 1000km.
The opening of the Benue Trough is
controlled by a N60°E fault system that
passes along the axis of the trough (Maurin
et al., 1986). Upon creation of the basin,
deposition began with fanglomerates, which
deposited by activated river streams that
trended into the basin. The first marine
sediments, were deposited during the
Albian. A transgression of the Gulf of Guinea
deposited the Asu River Group in the
southern and central Benue Trough. A
retreat of the South Atlantic from its far
reaches in the trough allowed for deposition
of the marginal marine Mfamosing
Formation in the Calabar Flank by the
Cenomanian (Akpan, 1990). The Turonian
saw the deposition of the Eze-Aku
Formation (Figure 1) during a second
transgressive phase. This phase also saw
the subsequent deposition of the Awgu
Formation during the Coniacian. However,
the termination of this transgressive phase
is evidenced in the deposition of regressive
Agbani and Ogugu Sandstone members.
Following these in the Santonian was the
folding and inversion of southern Benue
Trough. This lead to the formation of the
Abakaliki Anticlinorium and the beginning of
the second tectonic phase (Anambra-Benin
phase). With the uplift of the southern
Benue Trough, the depo-centre shifted to
the northwest and southeast of the
anticlinorium, where the resulting Anambra
Basin and Afikpo Sub-basins were created
respectively. Subsequent deposition began
in the basins with the Nkporo Group during
the Campanian. This was succeeded by the
Mamu, Ajali and Nsukka formations, whose
ages range from Maastrichtian to Danian.
METHODOLOGY
The area of focus covers the Amenu,
Akpoha, Ibi, Ozara-ukwu, Amasiri and
Afikpo areas, defined by latitudes N 5° 50'
00'' and N 6° 00' 00''; and longitudes E 7°51'
00'' and E 8° 00' 00'' (Figure 1). The study
was carried out using detailed field studies
within the study area. Field mapping
involved locating and sedimentological
logging of outcrop sections. The data
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