Glaciation-induced features or sediment gravity flows——An analytic review

For more than 150 years, geologic characteristics claimed to be evidence for pre-Pleistocene glaciations have been debated. Advancements in recent decades, in understanding features generated by mainly glacial and mass flow processes, are here reviewed. Detailed studies of data offered in support of prePleistocene glaciations have led to revisions that involve environments of mass movements. Similarities and differences between Quaternary glaciogenic and mass movement features are examined, to provide a more systematic methodology for analysing the origins of more ancient deposits. Analyses and evaluation of data are from a) Quaternary glaciogenic sediments, b) formations which have been assigned to pre-Pleistocene glaciations, and c) formations with comparable features associated with mass movements(and occasionally tectonics). Multiple proxies are assembled to develop correct interpretations of ancient strata. The aim is not per se to reinterpret specific formations and past climate changes, but to enable data to be evaluated using a broader and more inclusive conceptual framework.Regularly occurring pre-Pleistocene features interpreted to be glaciogenic, have often been shown to have few or no Quaternary glaciogenic equivalents. These same features commonly form by sediment gravity flows or other non-glacial processes, which may have led to misinterpretations of ancient deposits. These features include, for example, environmental affinity of fossils, grading, bedding, fabrics, size and appearance of erratics, polished and striated clasts and surfaces(“pavements”), dropstones, and surface microtextures.Recent decades of progress in research relating to glacial and sediment gravity flow processes have resulted in proposals by geologists, based on more detailed field data, more often of an origin by mass movements and tectonism than glaciation.The most coherent data of this review, i.e., appearances of features produced by glaciation, sediment gravity flows and a few other geological processes, are summarized in a Diamict Origin Table.

Neoproterozoic sedimentary facies and glacial periods in the southwest of Tarim Block

Based on the analysis of sedimentary facies and chemical index of alteration(CIA) in Nanhua and Ediacaran Systems in the southwest of Tarim Block,some features of glacial records in Neoproterozoic become more clear.Six sedimentary facies have been divided in the study area,including alluvial fan facies,lacustrine facies,glacial facies,littoral facies,neritic facies,and lagoonal facies,showing that this area underwent a process from continent to marine,with mainly littoral and neritic sedimentation.Two cold events have been recognized by analysis of CIA values in the study area,called Bolong and Yutang glaciation,respectively.They present as thick-layer tillite deposition in the Bolong Formation and thin-layer tillite deposition in the Yutang Formation,respectively.The Bolong glacial period in the study area can be correlated to the Yulmeinak glacial period in Aksu area,Tereeken glacial period in Qurugtagh area,and the Nantuo glacial period in South China,which is equivalent to the universally acknowledged Marinoan glacial period.The Yutang glacial period can be correlated to the Hankalchough glacial period in Qurugtagh,which is equivalent to Gaskers glaciation in Newfoundland.