Abstract
AbstractAggradation and fluvial incision controlled by downstream base‐level changes at timescales of 10 to 500 kyr is incorporated in classic sequence stratigraphic models. However, upstream climate control on sediment supply and discharge variability causes fluvial incision and aggradation as well. Orbital forcing often regulates climate change at 10 to 500 kyr timescales while tectonic processes such as flexural (un)loading exert a dominant control at timescales longer than 500 kyr. It remains challenging to attribute fluvial incision and aggradation to upstream or downstream processes or disentangle allogenic from autogenic forcing, because time control is mostly limited in fluvial successions. The Palaeocene outcrops of the fluvial Lebo Shale Member in north‐eastern Montana (Williston Basin, USA) constitute an exception. This study uses a distinctive tephra layer and two geomagnetic polarity reversals to create a 15 km long chronostratigraphic framework based on the correlation of twelve sections. Three aggradation–incision sequences are identified with durations of approximately 400 kyr, suggesting a relation with long‐eccentricity. This age control further reveals that incision occurred during the approach of – or during – a 405 kyr long‐eccentricity minimum. A long‐term relaxation of the hydrological cycle related to such an orbital phasing potentially exerts an upstream climate control on river incision. Upstream, an expanding vegetation cover is expected because of an increasingly constant moisture supply to source areas. Entrapping by vegetation led to a significantly reduced sediment supply relative to discharge, especially at times of low evapotranspiration. Hence, high discharges resulted in incision. This study assesses the long‐eccentricity regulated climate control on fluvial aggradation and incision in a new aggradation–incision sequence model.
Highlights
Many fluvial successions are built by aggradation intermittently interrupted by degradation, operating at 10 kyr to 10 Myr timescales
Age control and time-stratigraphic correlations in a ca 15 km long stratigraphic fence panel of the fluvial Lebo Shale Member in north-eastern Montana reveal the presence of ten 100 kyr scale coal–clastic successions (CCS) and three 400 kyr scale aggradation–incision sequences (AIS)
This implies that the second fluvial incision phase, i.e. the incision of AIS-2, corresponds to the transition from long-eccentricity maximum to minimum or to the minimum itself (Fig. S2)
Summary
Many fluvial successions are built by aggradation intermittently interrupted by degradation (incision), operating at 10 kyr to 10 Myr timescales. Aggradation occurs when sediment supply is significantly higher than maximum bedload transport rate, and incision occurs when sediment supply is significantly lower than bedload transport rate Both aggradation and incision are controlled by internal (autogenic) and/or external (allogenic) factors on sediment supply (Mackin, 1948; Leopold & Bull, 1979; Blum & To€rnqvist, 2000; Catuneanu, 2006; Holbrook et al, 2006; Miall, 2014). On 10 kyr to 10 Myr timescales, fluvial incision and aggradation are attributed to an interplay of allogenic controls: geomorphic base-level (i.e. sea-level, lake-level, or a drainage network trunk channel axis), climate and tectonics Base-level change remains a major cause for fluvial incision and aggradation in sequence stratigraphic models In addition to downstream control by aquifereustasy, orbital-forced climate changes in the hinterland (i.e. precipitation, weathering and vegetation), that influence sediment supply and discharge, exert an important upstream control on sedimentation at the basin-scale
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