Alluvial fan facies in the miocene-pliocene coastal plain sands, Niger Delta, Nigeria

Abstract

A 30 m thick alluvial fan developed in the lower part of the Coastal Plain Sand (Miocene-Pliocene) in southeastern Nigeria comprises four intergradational lithofacies: quartz pebble conglomerate, pebbly sandstone, interstratified pebbly sandstone and parallel to cross-stratified sandstone. The quartz pebble conglomerate facies consists of massive pebble packed beds set in a matrix of coarse to very coarse sand. Unordered poorly developed fabric with rare imbrication is characteristic. Deposition by debris flow on the proximal zone of the fan is interpreted for this facies. The pebbly sandstone facies is a structureless unit composed of pebbles dispersed through a medium- to coarse-grained sand with no significant pebble to pebble contact. Reversed distribution grading as well as cut-and-fill structures are common. Deposition by debris flow within the medial section of the fan is proposed. The interstratified pebbly sandstone and sandstone facies comprises relatively thin alternations of pebbly sandstone and horizontally stratified sandstone beds. Alternate deposition by sheet floods and/or debris flows in the distal part of the fan is suggested. Parallel- and cross-stratified fine to medium sand facies represents sheet flood deposits in the most distal part of the fan. These facies characteristics are similar to arid, semi-arid, and temperate alluvial fans. Fault scarp and/or non-tectonic base-level change may have provided the necessary relief for the formation of the fan. The absence of braided stream deposits associated with the alluvial fan appears to support the earlier contentions ∗ ∗ Le Blanc (1972) and Brown et al. (1973). that alluvial fan and braided stream deposits could be separate in space at any given time.