COMPOSITION AND DEPOSITIONAL ENVIRONMENT OF THE EDIACARAN – CAMBRIAN (?) KURUMSAK FM., BOLSHOI KARATAU, KAZAKHSTAN

Cryogenian-Earliest Cambrian strata in the Bolshoi Karatau of Kazakhstan expose, in contrast to those in the adjacent Malyi Karatau, a deep-water facies dominated by carbonaceous chert and slate (Kurumsak Fm.). Outcrops of this formation and the underlying diamictite (Baykonur Fm.) occur on the flanks of the approx. 40 * 15 km Kurumsak Anticlinorium near Aksumbe, northwestern Bolshoi Karatau, investigated during reconnaissance field work in 2012. All strata are regionally metamorphosed at low grade, veined and folded at outcrop- and map-scale.

Basal diamictites, >100 m thick, consist of boulder beds, gravelly mudstones with polymict and faceted clasts, and some dropstone intervals, overlain by a 2-5 m thick silicified carbonate unit, likely a cap carbonate. Cherts and slates of the overlying Kurumsak Fm. reach approx. 60 m thick. They are black to bluish-grey, thinly bedded and laminated, with little evidence of depositional scouring. Crenulation cleavage due to ptygmatic folding is dense and very well developed. Thin section, SEM and XRF analyses of samples of the Kurumsak Fm. show pure chert (≥ 95 wt-% SiO₂) with very little muscovite, pyrite, organic matter, calcite, and only traces of Al, Fe, Mn, Mg, Ca, and Na. Stratiform V mineralization, approximately mid-section in the Kurumsak Fm., which had been locally/temporally mined, and phosphorite was not observed in outcrop; V (≤ 660 ppm) is bound to muscovite. Only Ba (≤690 ppm), Cr (≤70 ppm), Zr (≤32 ppm), and Cu (≤50 ppm) are significantly enriched, all others occur mostly at the sub-ppm level. REE-patterns are characterized by distinct negative Ce- and Eu-anomalies. Literature documents acritarchs of Early Cambrian age from the overlying Kokbulak carbonates. We found no evidence of major hiatuses in the investigated sections. The age of the Kurumsak Fm. thus remains contentious: Its stratigraphic position above a major Neoproterozoic glacial unit and below Early Cambrian strata, in combination with the absence of well-documented fossils, suggests an Ediacaran age and thus makes the Kurumsak Fm. possibly correlative to the Lijiatuo Fm. of the Yangtze platform in age and depositional setting.