Paleosols in distal alluvial sequences and their formation mechanisms: Insights from a high-resolution record in a foreland basin during early Miocene (SE France)

Abstract

The stratigraphic distribution of paleosols provides a framework to investigate the controls on distal alluvial fan architecture. This study focused on paleosol-bound alluvial cycles (PACs) from the southern Digne-Valensole foreland Basin (SE France, early Miocene). We investigated the duration of the paleosol formation and identified the controlling mechanisms on their temporal and spatial distribution. Paleosols formed primarily from fine-grained sediments (65% shale, 16% lime mud). We identified three pedotypes: Calcisols from reddening throughout the weathering profile, Bw horizon overlying Bk horizon; vertic Calcisols with similar features including slickensides; and gleyic Calcisols showing saturated conditions superimposed on well-drained Calcisol features. Duration of Calcisol formation, estimated from the time span of aggrading PACs, was compatible with those from Quaternary Calcisols: median 11 to 29 kyr for maturity Stage II, > 90% of the dataset); range 20–58 kyr for maturity Stage III-IV. The shortest PACs time spans (< 10 kyr) indicate episodic sedimentation in the distal floodplain. Spatially or temporally variable sediment input in the intermediate floodplain prevented the development of paleosols at short time scales, resulting in longer PACs time spans (40 to 70 kyr) in the range of astronomical-related climate cycles. Autogenic processes also controlled the PACs stacking pattern as illustrated by stacked meter scale PACs formed during periods of relative sea-lowering indicative of a regular down-dip slope limiting river entrenchment; or during a period of high base-level in areas of low sediment supply suggesting complete diversion of the flow laterally from the previous course. The age-constrained PACs enabled to document on high-resolution time scales (a few to 100 s of kyr) the interactions between two components of the alluvial architecture, the episodic aggradation and the pedogenic processes that are difficult to determine in fossil alluvial successions. • Paleosol-bound alluvial cycles studied at 10 4 to 10 5 timescales over 3.5 Myr. • Durations of Aquitanian Calcisol formation were comparable to Quaternary ones. • Episodic floodplain aggradation is documented for timescale shorter than 10 kyr. • The potential of paleosol maturity in evaluating climate fluctuations is discussed. • Autogenic controls were identified from stacking pattern of Calcisols.