Glaciation or not? An analytic review of features of glaciation and
sediment gravity flows: introducing a methodology for field research
Abstract
For more than 150 years, geological features claimed to be evidence for
pre-Pleistocene glaciations have been debated. Advancements in recent
decades, in understanding features generated by glacial and mass flow
processes, are here reviewed. It is timely to make renewed comparisons
and to re-visit the interpretations of data used to support
pre-Pleistocene glaciations. Similarities and differences of Quaternary
glaciogenic and sediment gravity flow features, which are most often
referred to as proxies and evidence of ancient glaciations, are
documented, discussed and closely examined, in order to uncover the
origin of more ancient deposits. It is necessary to use multiple proxies
to develop a correct interpretation of ancient strata. Analyses and
evaluation of data are from a) Quaternary glaciations and glaciers, b)
formations which have been assigned to pre-Pleistocene glaciations, and
c) formations with comparable features associated with mass-flow
deposition (and occasionally tectonics). The aim is not to reinterpret
specific formations and past climate changes, but to enable data to be
evaluated using a broader and more inclusive conceptual framework. To
achieve this goal, detailed descriptions of field evidences are
documented from papers that may suggest different interpretations of
these data. This is not in an intention to present revised
interpretations of these papers, but to collect data and develop a
foundation for enhanced analysis of geologic processes and features.
Regularly occurring features interpreted to be glaciogenic and are
contemporaneous with pre-Pleistocene diamictites which have been
interpreted to be tillites, 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, appearances and documented data from the extent and
thickness of diamictite deposits, environmental and depositional
affinity of fossils in close connection to diamictites, grading and
bedding of diamictites, fabrics, size of erratics, polished and striated
clasts and surfaces (“pavements”), boulder pavements, lineations,
valleys, glaciofluvial deposits, dropstones, laminated sediments,
glaciomarine sediments, periglacial structures, soft sediment tectonics,
and surface microtextures. The analysis of these features provide
detailed documentation that may be used to help identify the origin for
many pre-Pleistocene diamictites. Recent decades of progress in research
relating to glacial and sediment gravity flow processes has 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