The Lower Cambrian of the Salair and Gorny Altai (Siberia) revisited

This paper discusses the fossil record, stratigraphy and sedimentary environments of lower Cambrian sequences that crop out in the Salair and Gorny Altai, southern part of western Siberia (Russia). Numerous and well-preserved archaeocyaths and a few small shelly fossils (SSF) have been discovered in carbonate sequences that crop out nearby the Gavrilovka village, Salair. The presence of the Gordonicyathus howelli archaeocyath Zone is established for the first time in this thick sequence of biogenic carbonates, in addition to the Nochoroicyathus mariinskii Zone, which was known previously from the Gavrilovka Formation. Both of these zones are Atdabanian in age. It is likely that accumulation of the Gavrilovka Formation took place in relatively shallow waters, along reef buildups surrounded by lagoons with archaeocyaths. In the Ak-Kaya section of the Gorny Altai, the trilobite species Alacephalus contortus and Poliellaspis rotundata, known previously from the upper Atdabanian, were discovered, only a few meters above horizons bearing lower Botoman trilobites. We may consider that the previously known age range of these two species is slightly longer and should be extended up to the lower Botoman. Alternatively, on the basis of their paleoecological affinity for agitated waters, these trilobites may be interpreted as reworked in lower Botoman strata.