How to Determine the Origin of Potential Cryogenian Neoproterozoic Dropstones From Death Valley

A diamictite dichotomy: Glacial conveyor belts and olistostromes in the Neoproterozoic of Death Valley, California, USA

Authors:

Le Heron et al

Abstract:

Multiple intercalations of glacially derived and slope-derived diamictites testify to the drawbacks of correlating Neoproterozoic diamictites more widely, but shed new light on the close interrelationship of these processes in the Cryogenian world. In the Neoproterozoic of Death Valley, California (USA), rifting of Rodinia occurred concomitantly with a major glacial event that deposited the Kingston Peak Formation. A new sedimentologic investigation of this formation in the Silurian Hills demonstrates, for the first time, that some diamictites are ultimately of glacial origin. Abundant dropstone textures occur in interstratified heterolithic deposits, with clasts of identical composition (gneiss, schist, granite, metabasite, quartzite) to those of boulder-bearing diamictites suggesting a common source (the glacial conveyor belt). In stark contrast, megaclast-bearing diamictites, yielding clasts of carbonate and siliciclastic preglacial strata as much as 100 m across, are interpreted as olistostromes. The occurrence of syn-sedimentary faults within the succession allows glacial versus slope-derived material to be distinguished for the first time.

Glacial dropstones in the western Tethys during the late Aptian–early Albian cold snap

Glacial dropstones in the western Tethys during the late Aptian–early Albian cold snap: Palaeoclimate and palaeogeographic implications for the mid-Cretaceous

Authors:

Rodríguez-López et al

Abstract:

The late Jurassic–early Cretaceous is commonly considered the only cold climatic interval in Earth history without any direct evidence of polar ice. A newly discovered dropstone-bearing interval from the subtropical Iberian Basin (western Tethys) is described and provides evidence of contemporaneous polar glaciation. This interval is correlated laterally for 4.8 km and contains a boulder and two cobble-sized quartzite dropstones that are encased in mid-Cretaceous fissile black shales and fine-grained sandstones. Based on previously published dimensions of similar large clasts, only glacial dropstones and impact ejecta blocks reach the dimensions of the boulder-sized dropstone reported from Iberia. The dropstones show morphological features compatible with glacial transport and abrasion in a subglacial setting which closely resembles the features observed in recent glacial boulders exposed near the snouts of glaciers in Iceland. These Late Aptian dropstones from Spain correlate with many other similar erratics in the northern and southern palaeohemispheres, and suggest that ice sheets formed around the palaeo-North Pole during certain periods of the early Cretaceous. Our results and associated evidence such as the occurrence of glendonites, tillites, moderate- to high-amplitude sea-level oscillations worldwide, minimum pCO2 concentrations, variation in calcareous nannofossil assemblages from low and high latitudes and isotopic excursions suggest that during the mid-Cretaceous there were periods of ice growth and decay that influenced the palaeotemperature, palaeoecology and sedimentology of the marine realm. The new data from Iberia are supported by recent results from Arctic Canada that indicate cool shelves and a mid-Cretaceous cold snap that developed for ~ 6 Myr between 118 and 112 Ma. The late Aptian dropstones reported in eastern Iberia were likely transported from high northern latitudes towards subtropical ones in the western Tethys by an extreme iceberg drift similar to those occurring at the present day in the Atlantic Ocean. Icebergs released from a northern fringing ice sheet may have travelled southwards through the Greenland–Norwegian Seaway.

Evidence of Iceberg Dropstones From Carboniferous/Permian Tibet

Features, provenance, and tectonic significance of Carboniferous-Permian glacial marine diamictites in the Southern Qiangtang–Baoshan block, Tibetan Plateau

Authors:

Fan et al

Abstract:

In this study, we conducted profile measurements, gravel composition analyses, and U-Pb dating on detrital zircons from a representative glacial marine diamictite in the Gangmaco–Dabure area of the Southern Qiangtang–Baoshan block, Tibetan Plateau. We conclude that the diamictite was formed in a glacial marine environment from the outer edge of the continental shelf to the continental slope and deep sea, in what is now the Southern Qiangtang–Baoshan block. Four distinct glacial-interglacial cycles were identified in the diamictite, which record a minimum of four stages of Gondwana glaciation in the area of the Southern Qiangtang–Baoshan block. Combined with regional geological information, we also conclude that during the Carboniferous-Permian, sediments containing the glacial marine diamictite derived from Gondwana, in the region extending from India to the Tethys Himalaya area, Lhasa and Southern Qiangtang–Baoshan blocks, recorded the transition from continental, neritic to abyssal environments. Gravel assemblages and U-Pb dating of detrital zircons in the glacial marine diamictite indicate that the provenance of the diamictite was Indian Gondwana. We infer that during the Late Paleozoic, the northern margin of the Indian Gondwana continued to be influenced by the Early Palaeozoic tectonic set-up, when Indian Gondwana was under an erosional regime, and the Tethys Himalaya area, Lhasa and Southern Qiangtang–Baoshan blocks were deposited on a passive continental margin.