Sr-isotope stratigraphy (Sr-87/Sr-86) of the lowermost Toarcian of Peniche, Portugal, and its relation to ammonite zonations

摘要

At Peniche, Portugal, the GSSP for the base of the Toarcian and in a Submediterranean province, a value of 0.707 073 +/- 0.000 002 (2s. e., n = 13) is determined for Sr-87/Sr-86 at the base of the Toarcian. In Yorkshire, UK, a Subboreal province, the base of the Toarcian has an Sr-87/Sr-86 value of 0.707 072 +/- 0.000 002 (2s. e., n = 23). The similarity of the values shows that, within analytical uncertainty, these levels are exactly equivalent in time. At Peniche, the base of the Serpentinum (Falciferum/Levisoni) Chronozone has an Sr-87/Sr-86 value of 0.707 096 +/- 0.000 003 (2.s.e). The value for this boundary in Yorkshire is 0.707 094 +/- 0.000 002 (2s. e., n = 27). The similarity of the values shows that, within analytical uncertainty, these levels are exactly equivalent in time. The same ammonite successions occur through the lowermost Toarcian successions at Peniche and in Yorkshire. Between Peniche and Yorkshire, there is therefore an exact coincidence, within measurement uncertainty, between the Lower Toarcian, Tethyan Realm, Tenuicostatum (Polymorphum) [ammonite] Chronozone of Peniche and the Lower Toarcian, Boreal Realm, Tenuicostatum [ammonite] Chronozone of Yorkshire. The Sr-87/Sr-86 profile through the Tenuicostatum Subchronozone at Peniche reveals large variations in sedimentation rate, with lowest rates in a condensed basal 3 m of the zone, highest rates in mid-zone, with rates decreasing upward through the upper half of the zone. Apportioning time in proportion to Sr-87/Sr-86 shows that around half of Tenuicostatum time is condensed into the basal 3 m of the section.