The depths of the Early Paleozoic sedimentary basins of the Paleoasian Ocean: lithofacies and bioindicator estimates

Abstract

Estimating the paleobasin depth is one of the most difficult problems in studying the evolution of sedimentation. Considerable depth variations in ocean and shelf paleobasins are due not only to their origin but also to the evolution of their large constituents owing to the differentiation of the dissected bottom. The Early Paleozoic volcanic-sedimentary and sedimentary rocks of Gorny Altai have been studied from this standpoint. They formed in the paleoceanic rises of the Early Cambrian Kuznetsk–Altai island arc, in the Late Cambrian–Early Ordovician Altai segment of the Paleoasian Ocean, and in the outer zone of the Late Ordovician Altai shelf basin. Geologic, geochemical, and lithofacies data permitted expert estimates of the absolute vertical depths of individual paleobasin fragments with siliceous sedimentation, calibrated at the final stage by bioindicator methods. The upper parts of the paleoceanic rises in the Early Cambrian Kuznetsk–Altai island arc are presumed to occur at depths of 300–400 m; those of the paleoceanic rises in the Late Cambrian–Early Ordovician Altai segment of the Paleoasian Ocean, at depths of 500–1200 m; and the outer edge of the outer zone of the Late Ordovician Altai shelf basin, at depths of 150–500 m.

Hirnantian Isotope Carbon Excursion in Gorny Altai, southwestern Siberia

“…Ordovician-Silurian strata in Gorny Altai are represented mainly by shelf successions, with some deposits of oceanic setting known from the Lower Ordovician. The Ordovician and Silurian sedimentary successions of Gorny Altai consist mainly of the rhythmic alteration of siliciclastic and carbonate rocks with rare volcanic intercalations (Sennikov et al 2008(Sennikov et al , 2011(Sennikov et al , 2014.…”

“…The late Katian-early Hirnantian sediments in the western and central parts of the Gorny Altai basin belong to the Tekhten Formation. This is a complex unit, 150-700 m thick, of predominantly carbonate composition where reef facies, siliciclastic-carbonate interreef facies and siliciclastic back-reef basin facies are represented in different parts of the basin (Sennikov et al 2008(Sennikov et al , 2011. The reef facies with massive algal bioherms might have formed in an up to 700 km long and 3-5 km wide distinctive facies belt on the shelf edge (Sennikov et al 2008).…”