NEOPROTEROZOIC AND EARLY CAMBRIAN STRATIGRAPHY AND δ13C RECORD OF LAKE KHUBSUGUL REGION, NORTHERN MONGOLIA

Neoproterozoic and Early Cambrian clastic, carbonate, and phosphatic sediments form the backbone of the Horidal Saridag Mountains, west of Lake Khubsugul, northern Mongolia. The Neoproterozoic Darkhat Group contains two diamictite units separated by < 230 m of deep-water carbonate rhythmites and debris flows. The overlying Khubsugul Group contains < 1000 m of limestones and dolostones with reworked phosphorite near the base. Fossils have not been recovered in this study.

Bed-penetrating dropstones and a diverse clast assemblage suggest that both diamictites are of a glacial origin. Carbon-isotope values for the inter-glacial sediments are between +2 and +6 permil. Cm-scale barite fans are present in the < 2 m thick dolostone that overlies the upper diamictite. Within this thin, putative cap carbonate, carbon-isotope values are also depleted (~-4 permil). Taken together, this evidence is consistent with a Strurtian age for the lower diamictite and a Marinoan age for the upper diamictite.

The Khubsugul Group directly overlies the barite-bearing carbonate and begins with silts, deep-water rhythmites, debris flows, and phosphorite beds, which shallow up to thrombolites, grainstones, and bioturbated muds. A prominent flooding surface ~250 m above the base of the Khubsugul Group divides the succession into lower and upper portions. Detailed carbon isotope stratigraphy of the upper Khubsugul correlates well with the Tommotian, Atdabanian, and Botomian δ¹³C records from the Anti-Atlas of Morocco and with Siberian records. U-Pb dates in Morocco provide an absolute time scale for the Early Cambrian and constrain sedimentation rates in the upper Khubsugul to an average of 80 m/Myr. The condensed, lower Khubsugul Group likely spans some portion of the Ediacaran Period.