Ordovician–Silurian boundary strata of the Indian Himalaya: Record of the latest Ordovician Boda event

Integrated analysis of sedimentological, stratigraphic, paleontological, and chemostratigraphic data of the Ordovician–Silurian Pin Formation exposed in the Parahio Valley in north India reveals the nature and history of depositional systems in the Himalayan region at that time. Previous faunal data indicate that much of the unit is Katian in age, and that the Ordovician–Silurian boundary could exist near the base of its uppermost member, although no definitive Silurian fossils were found. We recovered a lower Silurian ozarkodinid fauna within the upper member that includes Ozarkodina sp. cf. hassi, Oulodus sp., Panderodus unicostatus, and Distamodus sp., the latter of which is a lower Silurian index fossil, thus confirming a Silurian, specifically Llandovery, age for the uppermost Pin Formation. Here we present the first high-resolution (<0.5 m spacing) δ13Ccarb profile for rocks of this age in the northern Himalaya. In addition, our profile is potentially the highest-resolution δ13Ccarb curve to date from any location through the latest Katian Stage of the Ordovician. Numerous late Katian published curves exist, but they have variable values and geometries, in part due to low-resolution sampling and diagenetic problems. Our Gondwanan Pin Formation curve is nearly identical to one from the western U.S., indicating that the curves likely represent a remarkably faithful representation of the isotopic signature of latest Ordovician to earliest Silurian seawater during a transition into a major icehouse episode. Our data include a long-term positive excursion that is capped by a conspicuous positive spike, recorded within a thin calcareous-algae-rich shale unit. Directly overlying strata show a very abrupt negative shift (>1.5‰) and then further negative drift of ∼1.3‰. Our correlations to other sections globally indicate that the pattern represents the Paroveja ( = BC-14, Elkhorn) positive excursion, the last major excursion prior to the Katian–Hirnantian boundary interval. Further correlations indicate that the Pin Formation also records an excursion in that boundary interval, herein named the KaH excursion ( = lower HICE), which is a record of initial Hirnantian global cooling. The HICE itself is removed under an unconformity at the base or top of a sandstone unit. More generally, our data cover a late Katian faunal and climatic shift, known as the Boda event, which is generally interpreted to represent a pre-Hirnantian warming interval. The large positive Paroveja excursion in our data is a signal of part of the event, and possibly a mid-Boda cooling episode. Overall, our data provide a definitive isotopic curve for the upper Katian to lower Hirnantian and the resulting intercontinental chemostratigraphic framework of a biostratigraphically contentious interval exposes previous miscorrelations of isotopic excursions and provides a standard for chronostratigraphic studies of this critical interval in Earth history.