Facies analysis, depositional environments and paleoclimate of the Cretaceous Bima Formation in the Gongola Sub – Basin, Northern Benue Trough, NE Nigeria

Abstract

Facies analysis of the Cretaceous Bima Formation in the Gongola Sub –basin of the Northern Benue Trough northeastern Nigeria indicated that the Lower Bima Member is composed of alluvial fan and braided river facies associations. The alluvial fan depositional environment dominantly consists of debris flow facies that commonly occur as matrix supported conglomerate. This facies is locally associated with grain supported conglomerate and mudstone facies, representing sieve channel and mud flow deposits respectively, and these deposits may account for the proximal alluvial fan region of the Lower Bima Member. The distal fan facies were represented by gravel-bed braided river system of probably Scot – type model. This grade into sandy braided river systems with well developed floodplains facies, forming probably at the lowermost portion of the alluvial fan depositional gradient, where it inter-fingers with basinal facies. In the Middle Bima Member, the facies architecture is dominantly suggestive of deep perennial sand-bed braided river system with thickly developed amalgamated trough crossbedded sandstone facies fining to mudstone. Couplets of shallow channels are also locally common, attesting to the varying topography of the basin. The Upper Bima Member is characterized by shallow perennial sand-bed braided river system composed of successive succession of planar and trough crossbedded sandstone facies associations, and shallower channels of the flashy ephemeral sheetflood sand – bed river systems defined by interbedded succession of small scale trough crossbedded sandstone facies and parallel laminated sandstone facies. The overall stacking pattern of the facies succession of the Bima Formation in the Gongola Sub – basin is generally thinning and fining upwards cycles, indicating scarp retreat and deposition in a relatively passive margin setting. Dominance of kaolinite in the clay mineral fraction of the Bima Formation points to predominance of humid sub – tropical to tropical climatic conditions. This favors pedogenic activities which are manifested in the several occurrences of paleosols. Pronounced periods of arid climatic conditions are also notable from the subordinate smectite mineralization. Chlorite mineralization at some localities is indicative of elevation of the provenance area, and this is synonymous with deposition of the Bima Formation, because of its syn – depositional tectonics. The absences of lacustrine shales in the syn – rift stratigraphic architecture of the Bima Formation indicates that the lower Cretaceous petroleum system that are common in the West and Central African Rift basins are generally barren in the Gongola Sub – basin of the Northern Benue Trough.