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

Highlights

• New Early Toarcian d¹³C carbon isotope reference curve identifies discontinuities.

• Ammonite subzones, chrons and d¹³C patterns dated with new ATS age model.

• Karoo-Ferrar magmatism coincides with negative Toarcian Carbon Isotope Excursion.

Abstract

Cyclostratigraphic analyses of Upper Pliensbachian and Lower Toarcian carbon-13 isotope (δ¹³C) 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 carbon-isotope 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, cyclostratigraphic analyses typically underestimate the durations of biozones by failing to account for hiatuses in depositional discontinuities. To account for hiatuses this paper constructs a δ¹³C 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 δ¹³C maximum interval in middle D. tenuicostatum Zone. The T-CIE is characterized by a decreasing δ¹³C trend (c. 0.4 myr falling limb) in D. semicelatum Subzone, a minimum δ¹³C interval (c. 0.4 myr valley) and an increasing δ¹³C 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 ~1.0 myr for the D. tenuicostatum Zone, and ~1.6 myr for the H. serpentinum Zone. Initial volcanism in the Karoo Province correlates with the Pliensbachian/Toarcian boundary at ~183.6 Ma, while its second phase was coeval with the T-CIE. Volcanism in the Ferrar Province correlates with the T-CIE.