Constraining the duration of the southern Gondwana Irati-Whitehill Sea through cyclostratigraphy and its relation with deep-time astronomical solutions

Abstract

The search for orbital cycles preserved in the sedimentary record is at the forefront of the refinement of the Geologic Time Scale. Semi-enclosed to restricted environments and epicontinental seas are suited for this type of analysis because they are very responsive to the climate conditions around the basin depocenter. This study presents the first astronomical time scale for the late Paleozoic Irati-Whitehill Sea (southern Gondwana – Paraná basin) using downhole logging of HV-44-RS. Applying orbital tuning and statistical null hypothesis techniques over the natural gamma-ray, we built two floating astronomical time scales for the same record and anchored them on a new UPb (bentonite) numerical age of 280.8 ± 1.4 Ma. Our results place the Irati Formation between 283.68 and 280.50 ± 2.98 (orbital tuning) and 284.27–280.47 ± 1.41 Ma (null hypothesis), giving the vast epicontinental Irati-Whitehill Sea its most precise minimum duration so far of 3.16 ± 1.02–3.80 ± 0.1 Myr during the late Cisuralian. Spectral analysis suggests a depositional pattern strongly modulated by long (405-kyr) and short (∼ 100-kyr) eccentricity cycles. As an exercise, we compare one of the produced astronomical age models and recently calculated eccentricity solutions that reach the late Paleozoic interval (ZB20d). Such comparison shows a remarkable correlation in variability and amplitude with this eccentricity solution. Astronomical forcing is also related to black shale accumulation with periods of high organic carbon lasting ∼405 kyr and occurring every two long eccentricity cycles after maximum regressive surface, demonstrating a predictive pattern behind these late Cisuralian regional anoxia. Our approach indicates that the Irati Formation and other correlative southern Gondwana basins are a hotspot for cyclostratigraphy still underexplored. Furthermore, we contribute to verifying astronomical solutions in deep geological time.