title: Sizing up the sub-Tommotian unconformity in Siberia.
abstract: Sedimentary rocks in the western Anabar region, northwestern Siberia, preserve an exceptional record of evolution and biogeochemical events near the Proterozoic-Cambrian boundary.  Carbon isotopic data on petrographically and geochemically screened samples collected at 1 to 2 m intervals support correlation of the lower Anabar succession (Staraya Reckha and lower Manykai Formations) with sub-Tommotian carbonates of the Ust'-Yu-doma Formation in southeastern Siberia.  In contrast, the upper Manykai and most of the overlying Medvezhya Formation appear to preserve a sedimentary and paleontological record of an evolutionary important time interval represented in southeastern Siberia only by the sub-Tommotian unconformity.  Correlation of the Anabar section with other northern Siberian successions that contain well-dated volcanic rocks permits the estimate that the sub-Tommotian unconformity in southeastern Siberia spans approximately 3 to 6 m.y. Diverse small shelly fossils (but not archaeocyathans) previously thought to mark the base of the Tommotian Stage evolved sequentially throughout this earlier interval.
authors.affiliation: Botanical Museum, Harvard University, Cambridge, Massachusetts 02138, USA.
authors.forename: A H
authors.initials: AH
authors.lastname: Knoll
authors.forename: A J
authors.initials: AJ
authors.lastname: Kaufman
authors.forename: M A
authors.initials: MA
authors.lastname: Semikhatov
authors.forename: J P
authors.initials: JP
authors.lastname: Grotzinger
authors.forename: W
authors.initials: W
authors.lastname: Adams
mesh_terms: D000818:Animals; D005075:Biological Evolution; D002119:Calcium Carbonate / Q000032:analysis; D002247:Carbon Isotopes; D005580:Fossils; D019015:Geologic Sediments / Q000032:analysis* / Q000737:chemistry; D055593:Geological Phenomena; D005844:Geology; D007448:Invertebrates; D008274:Magnesium / Q000032:analysis; D010163:Paleontology; D012800:Siberia
chemical_list: D002247:Carbon Isotopes; D002119:Calcium Carbonate; D008274:Magnesium; C028042:calcium magnesium carbonate