Ice-rafted dropstones in “postglacial” Cryogenian cap carbonates

Abstract

AbstractDropstones of ice-rafted origin are typically cited as key cold-climate evidence in Cryogenian strata and, according to conventional wisdom, should not occur in postglacial, warm-water carbonates. In Namibia, the Chuos Formation (early Cryogenian) contains abundant dropstone-bearing intervals and striated clasts. It is capped by the Rasthof Formation, composed of laminites in its lower portion and microbial carbonates above. These laminites are locally found to contain pebble- and granule-sized lonestones in abundance. At the Omutirapo outcrop, meter-thick floatstone beds occur at the flanks of a Chuos paleovalley and are readily interpreted as mass-flow deposits. At Rasthof Farm, however, the clasts warp, deflect, and penetrate hundreds of carbonate laminations at both the outcrop and thin-section scale. We propose that these are dropstones, and we infer an ice-rafting mechanism. Evidence for vestigial glaciation concomitant with cap carbonate deposition thus merits a reappraisal of the depositional conditions of cap carbonates and their paleoclimatic significance.