Molar tooth structures in calcareous nodules, early Neoproterozoic Burovaya Formation, Turukhansk region, Siberia

Abstract

Molar tooth structures are abundant in large (1–2 m diameter) carbonate nodules within fine-grained, subtidal carbonates of the early Neoproterozoic (lower Upper Riphean) Burovaya Formation along the Sukhaya Tunguska River, Turukhansk Uplift, northwestern Siberia. Although molar tooth structures are regionally abundant in this unit, here they occur only within the nodules. Stable isotopic compositions of molar-tooth-filling dolomicrospar cements and of thinly bedded dolomicrite within and surrounding the nodules are indistinguishable from one another. The carbon isotopic compositions (mean δ 13C=+2.8‰ PDB±0.4) reflect mean average oceanic surface water composition during their formation; the light oxygen isotopic compositions (mean δ 18O=−6.4‰ PDB±2.2) are generally similar to those of other little-altered Meso- to Neoproterozoic limestones and dolostones. These molar tooth structures have no features that would support a tectonic origin; they more likely formed through bacterial processes. Carbonate cement filling of these voids occurred soon after their formation, but the mechanism responsible for this carbonate precipitation is currently uncertain. Local restriction of molar tooth structures to early diagenetic nodules suggests that penecontemporaneous lithification was required for the formation, or at least preservation, of these widespread Mesoproterozoic to Neoproterozoic features.