Climatic Controls on Alluvial Architecture, Doba Basin, Chad

Penny E. Patterson¹, Clive R. Jones², and Raymond Skelly^2
1 ExxonMobil Upstream Research Company, Houston, TX
² ExxonMobil Exploration Company, Houston, TX

Doba Basin, Chad, situated within the Central African rift system, contains up to 9 km of Cretaceous alluvial and lacustrine deposits. An integrated climate and tectonic model is proposed for development of the alluvial architecture and invokes superimposed high-frequency climatic fluctuations on a longer-term climatic cycle and variable accommodation produced by extensional processes.

Upper Cretaceous Doba Basin strata are interpreted to represent fluvial deposits that systematically varied along their depositional profile. Three distinct channel-complex architectures characterize the alluvial succession. Structurally controlled sediment entry points fed up-dip fluvial facies that are comprised of low-sinuosity braid channels that amalgamate to form thick and aerially extensive channel complexes. Mid-dip fluvial facies consist of low-sinuosity single-thread channels that form thinner and less laterally persistent channel complexes. Down-dip fluvial facies are characterized by heterolithic channel-fill and splay deposits that stack to form thin, moderately extensive distributive channel complexes.

In vertical succession, channel-complex elements are overlain by floodplain-dominated intervals containing isolated channel complexes and rare lacustrine mudstones. Within a sequence-stratigraphic context, sand-prone channel complexes are interpreted as lowstand systems tracts, whereas mudstone-prone floodplain intervals are interpreted as transgressive systems tracts. Examination of pedogenically modified strata reveals that climatic fluctuations governed development of depositional sequences. Moist temperate climatic conditions, recorded by vertisol development and occurrence of lacustrine mudstones, prevailed during degradation of the alluvial landscape and continued through fluvial aggradation. Subsequently, drier conditions prevailed as evidenced by alfisol development, coinciding with accumulation of thick floodplain mudstone intervals. Similarly, larger-scale composite sequences formed during episodes of longer-term climatic variation, from moist tropical to dry temperate conditions.