Climatic Controls on Depositional Setting and Alluvial Architecture, Doba Basin, Chad

Abstract

Doba Basin, Chad, is situated within the Central African rift system and contains up to 13 km (8 mi) of Cretaceous alluvial and lacustrine deposits. The complex climatic and tectonic evolution of the region, spanning the early to late Cretaceous is uniquely recorded in the stratigraphic architecture. In this chapter, we present a model of alluvial and lacustrine fill, which invokes high-frequency climatic fluctuations superimposed on longer term climatic cycles and variable accommodation produced by extensional tectonic processes. Upper Cretaceous (Cenomanian–Coniacian) Doba Basin stratal successions are interpreted to represent alluvial depositional systems that systematically varied along their longitudinal depositional profile, ultimately terminating in a lake, analogous to modern Lake Chad. Three distinct facies belts characterize the alluvial succession and one facies belt describes the lacustrine succession. Structurally controlled, sediment entry points fed upstream (updip) alluvial facies are composed of amalgamated midchannel bars that developed within multiple coeval low-sinuosity channels. These channels are stacked vertically and laterally, forming thick and aerially extensive channel complexes. Medial (middip) alluvial facies consist of semiamalgamated bars. Channel complexes in medial depositional regions are thinner and less persistent than upstream channel complexes. Distal (downdip) fluvial-facies belts are characterized by a more heterolithic style of sedimentary bedding, suggesting that deposition occurred during fluctuating slack water and relatively low-energy discharge conditions. Distal channel complexes are thin and more laterally extensive than those of the medial regions. The fluvial system terminates in shallow ephemeral to perennial ponds and lakes, building small coalesced terminal splay complexes. Channel complexes that accumulated within the three alluvial facies belts are overlain by mudstone-prone flood-plain deposits, both of which combine to form alluvial depositional sequences. Amalgamated channel complex elements constitute the sand-prone reservoir intervals and consist of vertically and laterally stacked channel-fill elements. Channel complex elements are bounded below by regional unconformities and above by abandonment surfaces. Amalgamated channel complexes are overlain by a mudstone-prone interval dominated by flood-plain deposits containing isolated channels and are referred to as nonamalgamated channel complex elements. The mudstone-prone intervals are bounded at their top by regional unconformities. Age constraints for alluvial depositional sequences indicate that they accumulate during time spans of 200 to 500 k.y. Depositional sequences cluster vertically and laterally to form an alluvial sequence-set element. Three types of sequence-set elements are defined and include amalgamated, semiamalgamated, and nonamalgamated sequence sets. Sequence-set elements form the building blocks of an alluvial composite sequence, which is bounded above and below by regional unconformities. Composite sequences in the Upper Cretaceous strata of Doba Basin, Chad, range from 1 to 3 m.y. The alluvial and lacustrine depositional facies architecture and sequence-stratigraphic framework of the upper Cretaceous strata, Doba Basin, Chad, are attributed to high-frequency climatic fluctuations of 200 to 500 k.y. duration superimposed on longer term climatic cycles of 1 to 3 m.y. and variable accommodation that occurred during late-stage extensional processes. The stratal architectures of depositional sequences and composite sequences are interpreted to be controlled by an extrinsic forcing mechanism of climatic cycles that influenced water discharge and sediment transport. Humid periods correspond to times of erosion, transport, and deposition of coarse clastic sediment. These stratigraphic intervals are dominated by fluvial confined-flow sandstone elements and are commonly bounded above and below by unconfined-flow mudstone facies that also formed during humid climatic conditions. Arid periods correlate to intervals of low coarse clastic supply to the basin and widespread deposition of flood-plain unconfined-flow mudstone deposits. The cyclic variations of discharge and sediment flux that occur at 200 to 500 k.y. correspond to the accumulation of alluvial depositional sequences and those of the 1- to 3-m.y. cycle relate to the deposition of alluvial composite sequences. Understanding the complex interplay of the long-term structural evolution of the Doba Basin and climatic fluctuations recorded within the Cretaceous strata is essential in the accurate prediction of reservoir and seal distribution and continuity. This understanding has proven critical in developing an effective depletion strategy for the three field areas of southern Chad.