JJEES (2020) 11 (1): 1-11 JJEES ISSN 1995-6681 Jordan Journal of Earth and Environmental Sciences Soft-Sediment Deformation in the Campanian-Maastrichtian Deltaic Deposits of the Afikpo Sub-basin, South-eastern Nigeria: Recognition of Endogenic Trigger Ogechi Ekwenye*, Ayonma Mode, Ifeanyi Oha, Fidelis Onah University of Nigeria, Department of Geology, Nigeria Received 13th June 2019, Accepted 15 August 2019 Abstract In this research, soft-sediment deformation structures are studied and analyzed from the Cretaceous deltaic deposits of the Afikpo Sandstone Member of the Nkporo Formation in Afikpo Sub-basin. These deformation structures are grouped into three morphological and deformational styles which include load structures (load casts, ball-and pillow-structures, flame structures, and pseudonodules), ductile disturbed structures (loop bedding, recumbent folding, and deformed lamination), and brittle deformation (synsedimentary fractures). The facies analysis revealed sedimentary structures that are indicative of storm and wave processes and rapid sedimentation. Though the trace fossils have a low to moderate diversity and abundance, the Skolithos ichnofacies, particularly the Diplocraterion and Skolithos burrows, are relatively frequent. These ichnofossils are indicative of high-energy conditions and rapid sedimentation. The recognition of the deformation processes and their triggers suggests that the soft sediment deformation structures in the study area are not tectonic in origin as they are associated with storm-generated sedimentary structures and structures formed due to rapid sedimentation. Furthermore, the deformational structures are restricted to a single stratigraphic layer, and are not laterally extensive suggesting an endogenic or non-seismic trigger mechanisms such as storm events, overloading, and rapid sedimentation. © 2020 Jordan Journal of Earth and Environmental Sciences. All rights reserved Keywords: Cretaceous deltaic deposit, soft sediment deformation, deformation processes, non-seismic trigger. 1. Introduction Detailed description and interpretation of the soft- Soft sediment deformation structures (SSDS) occur sediment deformation structures (SSDS) in the Afikpo in unconsolidated water-saturated sediments during or Sandstone Member, particularly in the Itigidi region, is not shortly after deposition, and before significant diagenesis yet reported in any published work. This paper records for (Owen et al., 2011). Interest in the study of soft sediment the first time the genesis and implication of soft-sediment deformation structures and their triggering mechanisms have deformation in the Afikpo Sub-basin. Published reports of progressively developed considerably over the past decades detailed soft-sediment deformation structures in Nigerian (Kuenen, 1958; Sanders, 1960; Dott and Howard, 1962; sedimentary basins are mostly limited to the Bima Formation Lowe, 1975; Allen, 1982; Mills, 1983; Owen, 1987; Moretti, of the Northern Benue Trough (Jones, 1962; Benkhelil, 1980, 2000; Rodríguez-Pascua et al., 2000; van Loon, 2009; 1989; Braide, 1992; Guirand and Plaziat, 1993; Samaila et al., Oliveira et al., 2011; Owen and Moretti, 2011; Owen et al., 2006), the Ajali Formation of the Anambra Basin (Ladipo, 2011; Sarkar, et al., 2014; Shanmugam, 2017). Many authors 1988; Obi and Okogbue, 2004; Olabode, 2014), the Central have associated soft-sediment deformation structures with and the Southern Benue Trough (Hoque, 1984; Benkhelil, seismically-induced triggering mechanisms (Mohindra and 1986, 1987; Dim et al., 2016). Detailed facies analysis and Bagati 1996; Moretti, 2000; Rodríguez-Pascua et al., 2000; ichnology of the Campanian-Maastrichtian deposits of the Ettensohn et al., 2002; Samaila et al., 2006; Bhattacharya Afikpo Sandstone Member in the Itigidi region is discussed and Bhattacharya, 2010; Martín-Chivelet et al., 2011; in Mode et al. (2018). This research concentrates on the Lunina and Gladkov, 2016), whereas limited works refer recognition of the soft-sediment deformation structures such deformational structures to non-seismically-induced (SSDS) in the Itigidi area. triggering mechanisms such as storms, waves, tidal surge, floods, overloading, rapid sedimentation, and groundwater In this study, sedimentology and a detailed description movements (Allen, 1982; Owen, 1987; Kerr and Eyles 1991; of the SSDS help establish some genetic relationships Molina et al., 1998; Alfaro et al., 2002; Owen and Moretti, between ordinary sedimentary processes (endogenic 2011; Põldsaar and Ainsaar, 2014). Furthermore, some soft- triggering mechanisms) and the occurrence of deformed sediment deformation structures, such as loop bedding, seem beds. Furthermore, from a regional point of view, this study to be specifically related with seismic trigger mechanisms describes and interprets the occurrence of soft-sediment (Calvo et al., 1998; Rodríguez-Pascua et al., 2000; Martín- deformation structures in the Afikpo Sandstone deposits of Chivelet et al., 2011). the Afikpo Sub-basin. * Corresponding author e-mail: [email protected] 2 Ekwenye et al. / JJEES (2020) 11 (1): 1-11 2. Geological Setting et al., (1982); Benkhelil (1989) suggested a magmatic The Afikpo sub-basin commonly referred to as Afikpo activity during the opening and filling of the Benue Trough Syncline is located in south-eastern Nigeria, and its origin which led to the deposition of the Abakaliki pyroclastics. is linked to the formation of the Benue Trough. The Benue However, Hoque (1984) argued that the pyroclastics are Trough originated from the break-up of the Gondwana post-Santonian in age. Sedimentation in the Southern Benue supercontinent which resulted in the separation of the Trough (Abakaliki Basin) commenced in the Aptian-Albian African plate and South-American plate during the Late times, and was halted due to the Santonian thermo-tectonism Cretaceous (Reyment, 1965; Murat, 1972; Nwachukwu, (Figure 2). This Santonian compressional phase resulted in 1972; Olade, 1975; Kogbe, 1976; Petters, 1978; Wright, 1981; the folding, faulting and uplifting of the Abakaliki Basin to Benkhelil, 1982, 1989; Hoque and Nwajide, 1984; Maurin form an anticlinorium; displacing depocentres westward and et al., 1986). The trough is part of a large West and Central eastward to form the Anambra Basin and Afikpo Sub-basin African Rift System (WCARS) (Genik, 1992) which resulted (Simpson, 1954; Benkhelil, 1986, 1989; Okoro et al., 2012a). from the crustal stretching of the African plate consequent Sedimentation commenced in the Afikpo Sub-basin during upon the break-up of the Afro-American plate (Figure 1). the Campanian to Early Maastrichtian with the Nkporo The rift structure is generated by sinistral displacements Shale and Afikpo Sandstone Members (Simpson, 1954; along a pre-existing zone of northeast-southwest trending Reyment, 1965) which formed the Nkporo Formation; this is transcurrent fault (Benkhelil, 1982, 1989; Maurin et al., followed by the Maastrichtian Coal Measures which include 1986; Popoff et al., 1986). The Benue Trough occurred as the Mamu, Ajali, and Nsukka formations, with the Nsukka an elongate intracratonic structure over 1,000 km long Formation terminating sedimentation in the basin (Nwajide, and up to 250 km at its widest part. Olade (1979); Popoff 2005). Figure 1. Tectonic map of the West and Central African Rift Systems (WCARS), showing the Benue Trough, the Anambra Basin, and the Afikpo Sub-basin (modified after Oha et al., 2016; Mode et al., 2018). Figure 2. Tectonostratigraphy of the Abakiliki-Benue Trough and the Afikpo Sub-basin (Modified and redrawn from Okoro et al., 2012a; Nwajide, 2013; Ekwenye et al., 2015). Note: SST stands for Sandstone; FM stands for Formation Ekwenye et al. / JJEES (2020) 11 (1): 1-11 3 Figure 3. Geologic map of study area showing the major outcrop locations within Itigidi and environs. 3. Sedimentology Diplocraterion, Skolithos and Teichichnus. The bioturbated Several works have been carried out to investigate the sandstone facies which tops the distal prodeltaic deposits is depositional environments of the Nkporo Formation in intensely bioturbated consisting of Skolithos, Cruziana, and the Afikpo Sub-basin, most of the studies are done on a Zoophycus ichnofacies. regional scale (Simpson, 1954; Reyment, 1965; Banerjee, A coarsening upward succession of sandy and muddy 1981; Benkhelil, 1986; Mode, 1991; Petters and Edet, 1996; heteroliths, followed by stacks of cross-bedded and Odigi, 2011; Okoro et al., 2012a, b), and there are limited data horizontally-bedded coarsening upward sandstone and on the Itigidi-Ediba regions (Benkhelil, 1986; Mode et al., hummocky cross-stratified sandstone, dominates the 2018). The study area is underlain by the Afikpo Sandstone proximal delta front (FA 4). FA 4 exhibits relatively fewer Member of the Nkporo Formation (Figure 3). Mode et al., soft sediment deformation structures such as load casts 2018 described the sedimentary units in the Itigidi region and and flame structures which occur locally within the sandy grouped them into six facies associations (FA 1–6) depicting heterolithic and muddy heterolithic units. The load casts a prograding deltaic strata deposited on the shelf, prodelta, show low-diversity ichnofossils consisting of Skolithos, distal delta front, proximal delta front, distributary mouth Trichichnus burrows. bar, and bay-fill sub-environments. Abridged sedimentology The bioturbation intensity is low to moderate. Cruziana of the study area is integrated