NAPENews Magazine September 2022 Edition

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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 NAPENEWS AUG/SEPT <strong>2022</strong> 24
obtained by sedimentological logging include rock type and textures, sedimentary structures, nature of bedding and contacts, attitudes and thicknesses of bedding, lateral extents of outcrops, as well as ichnofossils occurrence, size, degree of b i o t u r b a t i o n , e t h o l o g i c a l diversity. The data obtained were then subjected to facies analysis. T h i s a n a l y s i s i n v o l v e d identification of lithofacies, which w e r e u s e d t o i n t e r p r e t p a l a e o d e p o s i t i o n a l environments based on their a s s o c i a t i o n s . T h e s e p a l a e o d e p o s i t i o n a l e n v i r o n m e n t s w e r e t h e n reconstructed according to their order of occurrence in relation to relative sea-level changes. These reconstructions, that showed which environments are products of transgression and regression, were then used in interpreting the sequence stratigraphy of the Amasiri Sandstone Member. Figure 1: Geologic setting of the study area. (A) is a map of Nigeria showing the sedimentary basins and basement complex (modified after Obaje, 2009); (B) is a geologic map of the study area. RESULTS LITHOFACIES ANALYSIS Facies Descriptions: T w e l v e ( 1 2 ) f a c i e s a n d s i x t e e n ( 1 6 ) s u b - f a c i e s w e r e i d e n t i f i e d f r o m t h e s t u d i e d o u t c r o p s e c t i o n s i n the study area (Table 1). A summary of the facies well as their descriptions and their interpretations is thus presented (Table 1). Table 1: Summary of description and interpretation of lithofacies in the Amasiri Sandstone, Southern Benue Trough, southeastern Nigeria. Lithofacies and Facies code Conglomerate facies (G) Sub-facies G1: Horizontally stratified conglomerate G2: Planar crossstratified conglomerate G3: massive conglomerate Textures Clast-supported with coarse-grained sand matrix; poorly sorted; angular to subrounded clasts; monomictic; clast size ranges from granule to cobble. Clast-supported, very coarse-grained sand matrix with little clay; monomictic, poorly sorted, angular to rounded clasts Clast-supported, coarsegrained sand matrix with fair amount of clay; polyomictic, poorly sorted, angular to subrounded clasts Sedimentary structures Horizontally bedding Planar crossstratification Structureless, mud flasers Ichnology Not observed Not observed Not observed Depositional process Longitudinal bars, channel lag deposit, sieve deposit Linguoid bars Sediment-gravity flows or debris-falls in an environment with fluctuations in flow strength, which permits the transportation of sands in ripples followed by low energy periods where mud drapes the ripples (Boggs, 2001). NAPENEWS AUG/SEPT <strong>2022</strong> 25
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