Ediacaria in the Siberian hypostratotype of the Riphean

Abstract

The study of stratigraphy and paleontology of the Riphean/Vendian boundary strata is fundamental to decoding the transitional (Neoproterozoic) stage in the evolution of the biosphere. During this stage, the Proterozoic-style biota with limited morphological diversity, small size of individual organisms, lack of biogeographic zonation, and low rates of evolutionary turnover was replaced by the diverse Phanerozoic-style biota, with bioprovinciality and high dynamics of macroevolutionary processes [1]. The study of the transitional period invokes the analysis of the most representative Upper Proterozoic sections along the periphery of the Siberian Craton. Here, the uppermost Riphean was separated into the Baikalian Complex [2], which was proposed as a Regional Stage [3] or, later, as an Erathem of the General Stratigraphic Chart for the Precambrian of Northern Eurasia and equivalent to the Cryogenian of the International Stratigraphic Chart for the Precambrian [4]. Ediacaran fossils recently discovered in the Ui Group suggest that the biosphere shift commenced during the Baikalian. In 2005, large isolated outcrops of light- and yellowish gray quartzitic and arkosic sandstones, light gray siltstones and greenish gray mudstones were studied in the middle reaches of the Maya River on a segment between the mouths of the Malyi Kandyk and UlakhanKrestyakh Creeks (eastern slope of the Aldan Shield) (Fig. 1). The sections comprise the stratotype of the Kandyk Formation of the Ui Group, Upper Riphean [5, 6]. The total thickness of the succession exceeds 300 m (Fig. 1). The upper part of the sequence can be traced in large-blocky Felsenmeere, screes, and isolated outcrops at the mouth of the Yudoma River, where it is overlain by gray flaggy dolostones and oncolitic limestones of the Yudoma Group of Vendian age [2, 6, 7]. The studied sedimentary succession of the Kandyk Formation consists of two large depositional systems. The first depositional system, up to 100 m in thickness (Fig. 1, Member 1), has a distinctive complexly rhythmic stratification pattern composed of alternating sheets of wavy-bedded sandstones (0.1‐0.4 m), intervals of siltstone‐mudstone couplets, and sheets of wavy-laminated limestones (up to 0.7 m). The second depositional system, over 200 m thick (Fig. 1, members 2‐4), has a relatively simple structure: thick (up to 70 m) packages of thin- and wavy-laminated and thick-bedded and cross-bedded sandstones are interstratified with thick (up to 65 m) intervals of siltstone‐mudstone couplets.