Calibrating the Early Toarcian (Early Jurassic) with stratigraphic black holes (SBH)

Cyclostratigraphic analyses of Upper Pliensbachian and Lower Toarcian carbon-13 isotope (delta C-13) data, together with radiometric dating, are used to calibrate biozones and magnetic chrons in the Astronomical Time Scale (ATS). In turn, the ATS is used to date sea-level and climate cycles in relation to the Early Toarcian carbonisotope excursion (T-CIE) and the Karoo-Ferrar large Igneous Provinces. The resulting chronology however is insufficiently accurate to determine if these global-scale events are causally related. In particular, cydostratigraphic analyses typically underestimate the durations of biozones by failing to account for hiatuses in depositional discontinuities. To account for hiatuses this paper constructs a delta C-13 reference curve consisting of correlative segments from several localities and dates them with ammonite zones and subzones. By comparing the reference curve to those from numerous localities, four major discontinuity-prone intervals were identified and named 'stratigraphic black holes' (SBH). SBH 1 occurs in the Late Pliensbachian P. spinatum Zone. Early Toarcian SBH 2 occurs in a delta C-13 maximum interval in middle D. tenuicostatum Zone. The T-CIE is characterized by a decreasing delta C-13 trend (c. 0.4 myr falling limb) in D. semicelattim Subzone, a minimum delta C-13 interval (c. 0.4 myr valley) and an increasing delta C-13 trend (c. 0.4 myr rising limb) in the E. elegantulum Subzone. SBH 3 occurs at base T-CIE rising limb and SBH 4 near its top or above it in a c. 0.4 myr, post-T-CIE plateau in upper E. elegantulum Subzone. Comparisons to published floating chronologies resulted in an Early Toarcian timescale with similar to 1.0 myr for the D. tenuicostatum Zone, and similar to 1.6 myr for the H. serpentinum Zone. Initial volcanism in the Karoo Province correlates with the Pliensbachianfroarcian boundary at similar to 183.6 Ma, while its second phase was coeval with the T-CIE. Volcanism in the Ferrar Province correlates with the T-CIE. (C) 2020 The Authors. Published by Elsevier B.V. on behalf of International Association for Gondwana Research.