Controls on the genesis, sedimentary architecture, and preservation potential of dryland alluvial successions in stable continental interiors: insights from the incising Modder River, South Africa
Interpretations of the allogenic and autogenic controls on dryland alluvial successions are commonly hampered by incomplete knowledge of the sedimentology of modern dryland river floodplains, in part because of the limited subsurface sediment exposures available in typically stable or aggrading riverine settings. Along many valleys in the South African interior, however, river incision and associated formation of “dongas” (gullies and badland-type settings) provides extensive exposures of Cenozoic alluvial successions, which enable assessment of the controls on their genesis, sedimentary architecture, and preservation potential. This paper focuses on the incised Modder River at Erfkroon, Free State, located ∼ 1480 km (river distance) inland from the Atlantic Ocean. At Erfkroon, numerous dongas have formed in an ∼ 15-m-thick alluvial succession deposited within a narrow (< 500 m) valley carved predominantly in erodible, fine-grained sedimentary rocks (“shales”). Facies associations include channel deposits that vary in texture from sandy gravel to silty sand, and overbank deposits consisting predominantly of sandy mud to muddy sand that have varying degrees of pedogenic overprinting. Younger deposits are stacked upon, crosscut, or onlap older deposits, indicating a complex history of cut and fill. Changing assemblages of associated fossil fauna and archaeological artifacts, luminescence ages, and paleosol characteristics demonstrate net sediment accumulation over at least the last 42 ka under humid to arid climatic conditions. By contrast, the present-day situation of deep channel and donga incision into bedrock appears to represent a new phase of sediment evacuation and valley deepening. Comparison with other incising dryland rivers in the South African interior suggests that: 1) while phases of cut and fill are driven largely by climatic fluctuations, major phases of incision into bedrock are controlled by breaching of downstream resistant rock barriers; 2) long-term sediment preservation is limited in narrow valleys that are subject to extended periods of base-level stability followed by episodic, major incisional phases, but is greater along broader valleys where rivers are undergoing lateral migration and more progressive long-term incision. Study of incising rivers such as the Modder can yield important insights into the controls on alluvial successions in stable continental interiors beyond the range of sea-level changes, which may help with the interpretation of inland alluvial valleys preserved in the geological record.