Facies and petrophysical signature of the Tournaisian/Viséan (Lower Carboniferous) sea-level cycle in carbonate ramp to basinal settings of the Wales-Brabant massif, British Isles

We studied the relationships between stratigraphic distribution of outcrop spectral gamma-ray, magnetic susceptibility and carbonate facies stacking patterns across the regionally significant transgressive–regressive cycle at the Tournaisian/Viséan boundary (Tn/V, early Carboniferous) in southern Great Britain and Ireland (South Wales, North Staffordshire and Dublin Basin). The Tn/V boundary coincides with a prominent climatic pulse connected with the Late Paleozoic glaciation of Gondwana. The aim was to correlate the gamma-ray and magnetic susceptibility log patterns in carbonate ramp- and basin settings and discuss the global/regional nature and magnitude of this transgressive–regressive cycle. A robust ramp-to-basin correlation was produced based on the log patterns, facies stacking patterns and foraminifer biostratigraphy. The concentrations of K and Th, the “clay” gamma-ray values and, partly, magnetic susceptibility are dependent on facies and show systematic changes along the inferred bathymetric profile from inner ramp to outer ramp and basin. A model of carbonate productivity-driven dilution of fine-grained siliciclastics in CaCO3 as the major control on the petrophysical patterns is discussed. The cleaning-up and cleaning-down petrophysical trends are related to down-dip and up-dip shifts of the carbonate factory with changing relative sea level. In middle-to-outer ramp and basin settings, this generates petrophysical trends just opposite to Paleozoic carbonate shelves where peaks in magnetic susceptibility are known to be associated with peak regressions. A distinct, late Tournaisian to early Viséan regressive-to-transgressive cycle with a prominent sequence boundary located close to the Tn/V stage boundary can be seen in the sections. Glacioeustatic origin of the sequence boundary is inferred from its correlation with Tn/V boundary sections from Europe, carbon isotope data from South China and the glacial deposits in the southern hemisphere mentioned by previous authors.

► Gamma-ray signal is used as a proxy of primary carbonate production and dispersal.
► Tournaisian–Viséan boundary coincides with glacioeustatic sea-level fall.
► Lower Carboniferous glacieustatuc fluctuations show hierarchical patterns.
► Magnetic susceptibility has low correlation potential in deep-water ramp settings.