Sizing up the sub-Tommotian unconformity in Siberia

Area of Science:

• Geology and Paleontology
• Stratigraphy and Isotope Geochemistry

Background:

• Sedimentary rocks in Siberia offer insights into the Proterozoic-Cambrian boundary.
• This boundary marks a critical period of evolutionary and biogeochemical change.

Purpose of the Study:

• To correlate sedimentary successions in the Anabar region with those in southeastern Siberia.
• To refine the timing of evolutionary events and the duration of the sub-Tommotian unconformity.

Main Methods:

• Petrographic and geochemical screening of sedimentary samples.
• Carbon isotope analysis of rock samples.
• Stratigraphic correlation using fossil assemblages and isotopic data.

Main Results:

• The lower Anabar succession correlates with sub-Tommotian carbonates in southeastern Siberia.
• The upper Anabar succession records an evolutionary interval previously only represented by an unconformity.
• The sub-Tommotian unconformity in southeastern Siberia is estimated to span 3-6 million years.
• Diverse small shelly fossils evolved sequentially before the Tommotian Stage.

Conclusions:

• The Anabar region provides a more complete record of early Cambrian evolution than previously recognized.
• The evolution of small shelly fossils occurred over a longer period than indicated by the Tommotian Stage alone.
• This study refines our understanding of the Proterozoic-Cambrian transition and early Cambrian biostratigraphy.