The Aptian stage, between ∼113 and ∼121 million years ago (Ma), was punctuated by a succession of Oceanic Anoxic Events (OAEs), recording extreme global warmings, dramatic expansions of the ocean's oxygen minimum zones, along with perturbations to the biotic and carbon cycles. However, the chronology of the Aptian stage is poorly constrained, impacting the duration and timing of OAEs. Using a greatly expanded sedimentary composite record (380 m) of key outcropping sections in the Vocontian Basin (SE France) combined with available radiometric dates and correlations to a set of astronomical solutions, we provide a constrained absolute astrochronology of the Aptian stage. The 405 kyr (gVenus–gJupiter) eccentricity astronomical timescale indicates a minimal duration of ∼9.4 Myr for the Aptian stage and an age of 122.6 ± 0.3 Ma for the base of the Aptian, consistent with radioisotope dating. We find a deviation in the periodicity of gMercury–gJupiter eccentricity term in the mid-Aptian stage, at ca. 117.19 ± 0.3 Ma, that we ascribe as an expression of the resonance transition σ = (gMercury – gJupiter) – (sMercury – sVenus), in relation with a strong chaotic orbital motion of Mercury. Such a geological observation is supported by a concomitant resonant transition in the La2004 astronomical model. •The tuned MS data indicates a minimal duration of ∼9.4 Myr for the Aptian stage.•Our revised Aptian duration appears concordant with numerical calibration.•A shift in the gMercury–gJupiter eccentricity term is observed at 117.19 ± 0.3 Ma.•Geological observation is supported by a resonant transition in astronomical model.