Ecostratigraphical interpretation of lower Middle Ordovician East Baltic sections based on brachiopods

Abstract

AbstractA detailed ecostratigraphical framework is established for the lower Middle Ordovician Kundan regional stage of the East Baltic area corresponding to theAsaphus expansus,A. ranicepsandA. eichwalditrilobite zones (lower Darriwilian). The study is based on approximately 6200 brachiopods collected bed by bed from limestone sections in northern Estonia (Harku Trench and Saka) and western Russia (Putilovo Quarry, Lava River canyon and Lynna River valley) with, in addition, the first detailed systematic assessment of the Kundan brachiopods of the East Baltic. These sections represent an oblique depth transect some 400 kilometres long, deepening eastwards. Five biofacies associations have been recognized using detrended correspondence and cluster analyses: a shallow-waterLycophoriaassociation, a transitionalGonambonitesassociation and two deeper-water associations, the soft-substrateOrthis callactisand the hard-substrateOrthambonitesassociations. A separate, fifth soft-substrate association is present in the marl beds at the main locality of Putilovo Quarry. The associations reflect a combination of palaeo-water depth and substrate. The biofacies facilitate an ecostratigraphical correlation along the transect, and third and fourth order sea-level curves are reconstructed, reflecting mainly eustasy. The sea-level was relatively low, early in the Kundan, but then rose significantly into theA. ranicepsBiozone. This corroborates the recent discovery of possible small early Darriwilian ice caps on Gondwana.