Carbonate facies evolution, bio- and carbon isotope stratigraphy through the Ludfordian (Silurian) on the shelf of Baltica (Lithuania)

A 95-metre-long subsurface stratigraphical interval of the late Ludlow (Silurian) representing shelf deposits of the Baltica palaeocontinent (Lithuania) was investigated in terms of bio-, carbon isotope stratigraphy as well as litho- and microfacies. The conodont biostratigraphic data confirm the Ludfordian age (Kockelella variabilis Zone to Jeppssonia crispa Zone) of the succession. The deposits recorded a positive stable carbon isotope δ13Ccarb excursion referred to as the Mid-Ludfordian Carbon Isotope Excursion beginning in the uppermost Polygnathoides siluricus Zone and fully developed throughout the P. latialata-Jeppssonia snajdri Interval Biozone. The lithologically variable succession reflects a general shallowing-upward trend encompassing a broad range of different depositional environments and palaeowater depths. In vertical succession, deep-shelf argillaceous-marly sediments gradually pass into shallow-water carbonates, including reef-related lithologies, and extend into peritidal environments. Such a transition is indicative of prolonged progradational conditions. Furthermore, the sedimentological record provides evidence for short-term fluctuations in sea level that could be of glacio-eustatic origin. Microbial carbonates occur in significant abundance as oncoids within beds, which recorded the highest amplitude of δ 13Ccarb, reaching a maximum value of +7.5‰.