Features of the lithological structure of the Lower Cambrian Pestrotsvet Formation near the Oi-Muran reef massif (Yakutia)

Abstract

Cambrian sections of the Lena Pillars National Park and adjacent areas in the south-east of the Siberian platform have been the subject of paleontological-stratigraphic, lithological, facies, paleogeographic and structural-tectonic studies. In this article we provide a detailed description of the Lower Cambrian Pestrotsvet Formation in the off-reef facies near the Oi-Muran reef massif. The description is given by the coastal outcrops of the Lena River above Sinsk village: 27 “Bachyk” (left bank), 43 and 44 (right bank). The exposed sediments in these outcrops belong to the off-reef shallow part of the open normal saline sedimentary basin adjacent to the Oi-Muran fragment (reef array) of the grand West Yakut Barrier Reef. We conducted a detailed description of the typical part of the section to identify the specifics of sediment accumulation in the Pestrotsvet Formation. A thin alternation of limestone laminae (with thickness varying from 3–5 to 10–20, less often 30–40 cm) was established. They are characterized by a light gray clean, pinkish-gray color, without visible impurities, and burgundy clay with abundant admixtures of ferruginous pigments. The individual intervals of the section are characterized by laminae of maroon marlstone. The rhythmic shift in the section of rocks of different compositions is the result of frequent, however, according geological standards, short-term transgressions and regressions. The origin of some laminae of comparatively pure gray and pinkish-gray limestones that contain abundant remains of cyanobacteria and algae is related to the rock-forming activity of these microorganisms. The materials presented in this article contribute to studies on the off-reef facies of the West Yakut Barrier Reef, one of the oldest reefs on Earth, which played a major role in the emergence of reef biota.