The Siberian Platform is an ancient piece of the continental crust that was an island continent in Cambrian time. Three distinct facies belts comprise the Early Cambrian and early Middle Cambrian of the Siberian Platform (Fig. 1): a western belt of restricted-circulation, evaporitic carbonate sediments (Turukhansk-lrkutsk-Olekma facies region), areef-shoal transitional belt of archaeocyathan-microbial facies (Anabar-Sinian facies region), and an open-marine eastern and northern belt (Judoma-Olenek facies region), (Savitsky and Astashkin, 1979). In the northeast of the Siberian Platform the best sections comprising the Lower and Middle Cambrian boundary deposits are located near the Kharaulakh Mountains (Lazarenko and Repina, 1983), on the Olenek Uplift (Savitsky et al., 1972) and in outcrops along the Nekekit and Boroulakh rivers (Savitsky and Shaloanov, 1972). The Kharaulakh Mountains and Olenek Uplift sections are assigned to the Anabar-Sinian facies region, and sections along the Nekekit and Boroulakh rivers are assigned to the Judoma-Olenek facies region (Fig. 1). Inasmuch as across the Lower and Middle Cambrian of the northeastern platform there existed various facies conditions (shallow to deep shelf), the trilobite complexes from here also differ.
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