A RE-EVALUATION OF THE NUMBER AND MAGNITUDE OF NEOPROTEROZOIC ICE AGES

The Neoproterozoic Ibex Formation, previously considered to represent a basinal facies of platform carbonates in the lower Noonday Dolomite, Great Basin, USA, is shown to rest on the eroded surface of the lower Noonday Dolomite and other older units. At the type section, the basal Ibex Formation consists of polymict conglomerate and laminated mudstone; the upper surface of the mudstone is pierced by large angular clasts of all underlying units, including distinctive lower Noonday tube stromatolites. A finely laminated pink dolostone unit that records negative d¹³C values caps the Ibex conglomerate. We interpret the erosional unconformity upon which the basal Ibex Formation is deposited to be glacioeustatic in origin, the basal conglomerate-pierced mudstone to be glaciogenic, and the overlying dolostone to be a classic cap carbonate. Combined with the record of glacial sediments and cap carbonates from underlying units, the Death Valley succession unambiguously records three discrete Neoproterozoic ice ages in a single continuous section. These new observations provide the lithological and geochemical proof that at least three, and potentially more, global ice ages characterized Neoproterozoic time. As each sustained global glaciation represents a critical environmental hurdle, the number and the magnitude of discrete ice ages is an important constraint on the tempo of metazoan evolution.