NAPENews Magazine September 2022 Edition

characterized by a monospecific suite of
Ophiomorphaburrows which occur
predominantly as endichnial traces, most
likely created by infaunal species. The
presence of trace fossils belonging to the
Skolithosichnofacies indicates normal
marine water deposition (Pemberton et al.,
1992; Gingras et al., 2002). The lowdiversity
ichnofossils assemblage is
suggestive of environmental duress typical
of marginal marine/brackish-water settings
(Pemberton et al., 1992; MacEachern and
Pemberton, 1994; Rossetti and Santos
Júnior, 2004; Buatois et al., 1999; Gingras
and MacEachern, 2012). The dominance of
infaunal traces suggest a survival
adaptation of organisms the seek refuge in
the deep-infaunal habitats due to rapid and
extreme salinity fluctuations at the sediment
surface (Sanders et al., 1965; Rhoads,
1975; Buatois et al., 1999). Low diversity,
monospecific and infaunalichno fossil suites
have been recorded for marginal marine
sequences by Hakes (1976), Archer and
Maples (1984), and Martin (1993).
DISCUSSIONS
Palaeogeography
The facies interpretation shows evidence
that the sediments of the Amasiri Sandstone
are of a shallow marine depositional setting,
with local evidences of fluvial processes.
The initial occurrence of the offshore
deposits suggests a marine incursion
(Figure 6A). Associated with the shales are
extensive sand bodies, which are
interpreted as subtidal sandwaves (Figure
6B). These sand bodies are over 20m thick
and are characterized by large-scale
crossbedding, with forsets dipping at angles
of up to 24. Unidirectional paleocurrents as
well as reactivation surfaces suggest a tidal
influence on the crossbeds. These largescale
cross beds are interpreted to betidal
sand waves. Despite the high energy
associated with the tidal sand waves,
evidence of energy fluctuations is
evidenced in the occurrence of gradations
within the forsets, and the occurrence of
sparsehorizontal Thalassinoides and
Paleophycus burrows that dominate some
intervals. Depositional energy must have
slowed enough for more offshore shale to be
deposited over the sandwaves (Figure 6C).
Locally in the Akpohaarea, a fluvial setting
becomes prevalent, depositing channelized
sandstone bodies (Figure 6D) which are
characterized by large scale trough crossbeds.Shelf
building commences and
shoreface sediments were deposited
( F i g u r e 6 E ) . T h e s u c c e s s i o n i s
characterized by muddy and intensely
bioturbated sandstones at the base and
grades up into moderately sorted
sandstones which are characterized by
Figure 5: Tide-influenced fluvial channel facies associations. (A) is a representative log of
the Tide-influenced fluvial channel facies association. (B) is an outcrop image of
tide-influenced fluvial channel deposits at Afikpo.
Ophiomorpha burrows and bivalve shells.
The siliciclastic sediments are replaced by a
combination of siliciclastic and carbonate
facies, indicating a change to a shallow
mixed siliciclastic-carbonate shelf setting
( F i g u r e 6 F ) . T h e s e d e p o s i t s a r e
characterized by intensely-bioturbated
sandstones and fossili ferrous limestones.
The deposition of calcareous facies is
suggestive of sediment starvation.
Continuous slow sedimentation of clastics,
shoaling and warming of the sea water
probably increase deposition of limestone
(Dill et al., 1997) in the basin. The
precipitation of carbonate in the basin was
induced when the sea water became more
alkaline. Aerobic conditions, shallow depth
and warm temperature of the environment
most likely encouraged thriving of benthic
organisms such asbivalves. The favourable
environmental conditions are evidenced in
the intense bioturbation and large sizes of
bioturbation structures in the sandstones.
The fossili ferrous nature of the limestone
suggest that the foreshore setting was
prone to catastrophic events such as
storms, which most have caused mass
mortality of the organisms, whose shells
were then incorporated into the limestones
(Nwajide, 2013).While the foreshore may
have indicated an imminent return to marine
conditions, the shallow nature ofthe sea
must have allowed a reworking of sediments
by tides, which produced the sand ridges
(Figure 6G). These sandbodies are
characterized by their elongated ridge-like
form and are oriented parallel to paleo flow
direction indicated by their characteristic bidirectional
cross-bedding. The presence of
a monospecific suite of Ophiomorpha
burrows, indicates fluctuating energy
conditions. The transition from tidereworked
sands to offshore facies, indicates
a return to marine conditions. The lowenergy
energy conditions favoured the
deposition of the thick pile of shallow
offshore shales which seal the sand bodies
(Figure 6H).Depositional conditions
changed again from marine to brackish
water setting as observed in succeeding
tide-influenced fluvial channel deposits
(Figure 6I). Reactivation of fluvial channels
probably led to the deposition of massive
coarse to medium-grained sandstone and
c o n g l o m e r a t e . H o w e v e r , a n o p e n
interaction with thesea, allowed influence of
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