Rock Magnetic‐Based Cyclic Expression in Late Visean Ramp Carbonates and an Astrochronology for the Late Asbian From Northwest England

Abstract

The late Asbian appears to mark the initial, well‐documented, onset of far‐field glacio‐eustatic changes in equatorial Mississippian strata. This work unravels the nature of cyclicity in upper Asbian shallow marine carbonates, using a combination of petrographic study, rock magnetic proxies and astrochronological testing on samples from the Trowbarrow section, NW England. Rock magnetic data express the content of two types of siliciclastic sources; a marine‐delivered magnetite‐dominated source, and an eolian‐delivered, hematite‐dominated source. The eolian‐sourced material generally peaked during regressive and low‐stand parts of the carbonate rhythms. Astrochronologic testing methods based around the average spectral misfit and TimeOpt methodology show the magnetite abundance proxies are principally carrying the astronomically forced signal. Two likely sedimentation rate models are derived from the five better magnetic proxies using evolutive methods. In addition, a set of three likely major hiatus levels are inferred in the sedimentation rate models, based on testing possible major hiatus scenarios with TimeOpt methods, using eccentricity modulation. From these, using the three best proxies, an average astrochronologic duration for the Trowbarrow section suggests a late Asbian duration of 1976 ± 86 kyr (1σ), and a basal late Asbian age of 334.48 ± 0.35 Ma (2σ). Coupled atmosphere‐ocean models for the late Paleozoic, suggest that lows in short eccentricity correspond to glacials, when inferred delivery of siliciclastic sediment to the carbonate ramp is generally at a maximum. The glacial and lower sea‐level intervals also coincide with maximum delivery of eolian siliciclastics, likely linked to increased aridity and less vegetation cover on adjacent and distal parts of Laurentia.