Sedimentary and stratigraphic characteristics of the Maastrichtian Dukamaje formation of southern Iullemmeden Basin: Implications for the paleogeography of the Upper Cretaceous trans-Saharan seaway

Abstract

The Maastrichtian Dukamaje Formation is a fossiliferous, shale- and marl-dominated unit, albeit with contentious paleodepositional environment, of the Rima Group, southern Iullemmeden Basin (northwestern Nigeria). The Basin holds records of the global sea-level highs of the Late Cretaceous - Paleocene. Hence, an improved understanding of the associated sedimentary infill will provide opportunity for an improved reconstruction of paleogeography and ancient stratigraphic sequences. Two different models (a low-energy, hypersaline lagoon and marsh based on predominance of fine grains with gypsum and preserved vertebrate fossils, or high energy/inshore, shallow marine environment supported with Libycoceras ammonite fossil) have been proposed by different workers who studied the same outcrop sections. The discrepancy in these previous models is partly due to inherent depositional complexities and the need to invoke deductive interpretations for epicontinental basins. Here, we used a systematic facies analysis (for the first time and with particular attention to stratigraphic relationships and correlation of some of the outcrop sections) integrated with previous biostratigraphic data in order to reconstruct the paleogeography of the Dukamaje Formation. The objectives are to describe the facies characteristics and provide a well-constrained interpretation for the depositional setting of the Dukamaje Formation. Results: two alternating, low-energy facies associations (marsh and lagoonal and restricted?/nearshore marine) were identified. The marsh and lagoonal facies association is characterised by fossiliferous and gypsiferous shale enriched in arenaceous foraminifera. The restricted marine facies association comprises highly fossiliferous nodular marls. The facies associations form a coastal sabkha facies succession, deposited in a marginal marine environment with likely brackish influence, under a warm tropical climate. The coastal sabkha facies succession is interpreted as a retrogradational system, which suggests that the Dukamaje Formation is a product of marine transgression of a low-lying, embayed, partially restricted, stable coast. By implication, the hypothesised Upper Cretaceous epicontinental sea (“trans-Saharan seaway”) across the North to West Africa was, rather than fully open, restricted in some places. Restrictions were related to presence of embayments in which hypersaline conditions developed due to low circulation. This model is largely consistent with the view of a partially restricted shallow marine for the Upper Cretaceous of the southern Iullemmeden Basin.