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Patterns, processes and models – an analytical review of current ambiguous interpretations of the evidence for pre-Pleistocene glaciations

Molén Mats O.

Geologos

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2023

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Vol. 29, No. 3

139--166

EN

Models (paradigms) and former interpretations have often been presupposed when conducting field research. In the 19th century diamictites were for the first time interpreted to have originated from ancient glaciations. These interpretations have to a large part prevailed in the geological community, although there has been much progress in the areas of sedimentology, glaciology and physical geography. The present work is an effort to find criteria which most clearly discriminate between geological features produced by different processes, mainly glaciation and mass flow, the latter predominantly sediment gravity flows. Geological features which have been interpreted to have formed by glaciation throughout pre-Pleistocene Earth history are compared to similar-appearing geological features formed by mass flow and tectonics, so as to uncover variations in the appearance between features resulting from these different processes. The starting point for this comparison is documentation of the appearance of Quaternary products of erosion and deposition, in order to discern the origin of older formations. It is shown that the appearance and origin of pavements, dropstones, valleys, small-scale landforms, surface microtextures and most other geological features may in some cases be equivocal, but in others the details are indicative of the process which generated the feature. Detailed geological field data which have been compiled by geologists from outcrops of pre-Pleistocene strata, more often than is considered in most papers, commonly point to a mass flow origin, mainly a sediment gravity flow origin, rather than a glaciogenic or- igin. A process of multiple working hypotheses or interpretations is therefore advocated, based mainly on a comparison of the appearance of features formed by different geological processes documented from different research disciplines. Instead of starting with current interpretations or models, this multiple working hypothesis or methodology helps to avoid confirmation bias and jumping to conclusions.

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The origin of upper Precambrian diamictites, northern Norway: a case study applicable to diamictites in general

Molén M. O.

Geologos

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2017

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Vol. 23, No. 3

163--181

EN

Upper Precambrian diamictites in Varangerfjorden (northern Norway) have been examined for evidence of origin, whether glaciogenic, gravity flow or polygenetic. Studies of geomorphology, sedimentology and surface microtextures on quartz sand grains are integrated to provide multiple pieces of evidence for the geological agents responsible for the origin of the diamictites. The documented sedimentary and erosional structures, formerly interpreted in a glaciogenic context (e.g., diamict structure, pavements and striations) have been reanalysed. Field and laboratory data demonstrate that, contrary to conclusions reached in many earlier studies, the diamictites and adjacent deposits did not originate from glaciogenic processes. Evidence from macrostructures may occasionally be equivocal or can be interpreted as representing reworked, glacially derived material. Evidence from surface microtextures, from outcrops which are believed to exhibit the most unequivocal signs for glaciation, display no imprint at all of glaciogenic processes, and a multicyclical origin of the deposits can be demonstrated. The geological context implies (and no geological data contradict this) an origin by gravity flows, possibly in a submarine fan environment. This reinterpretation of the diamictites in northern Norway may imply that the palaeoclimatological hypothesis of a deep frozen earth during parts of the Neoproterozoic has to be revised.

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The Kapp Lyell diamictites (Upper Proterozoic) at Bellsund, Spitsbergen: rock-sequence, sedimentological features, palaeoenvironment

Birkenmajer K.

Studia Geologica Polonica

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2010

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Vol. 133

7-50

EN

The Upper Proterozoic meta-diamictites (Kapp Lyell Formation) exposed south of Bellsund, central Spitsbergen, believed by many authors to represent the Vendian (Varangian) glacial tillites, have been investigated. Character of their relictic sedimentary features does not confirm, however, its glacial origin. The diamictites formed mainly as debris-flow and slope-creep terrestrial covers derived from frost-shattered steep rocky margins of a predominantly fresh-water depositional basin. The deposition took place under seasonally differentiated moderate-cool climatic conditions comparable to the modern Subarctic periglacial zone, however outside the reach of glaciers and ice-sheet. Uncommon finer-grade varved deposits containing haphazardly distributed clasts (drop- stones) probably formed in shallow fresh-water lakes that froze during winter; the clasts were subsequently dispersed during summer in the lakes as ice-rafted dropstones. Shallow erosional channels infilled with sandstone within the diamictite complex, might be evidences for braided river systems that were active during summer. Rare, well-rounded cannonball-size cobble/boulder beds represent a fossil high-energy, probably marine beach.Uncommon thin limestone intercalations, found in a predominantly coarse-clastic suite, might suggest occasional incursions of a shallow sea.

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The Kapp Lyell Diamictite (Late Proterozoic), Bellsund, Spitsbergen: Sedimentological Evidence for its Non-Glacial Origin

Birkenmajer K.

Bulletin of the Polish Academy of Sciences. Earth Sciences

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2003

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Vol. 51, nr 1

65-78

EN

The Kapp Lyell diamictite complex south of Bellsund, Spitsbergen, believed so far to represent tillites of the Late Proterozoic (Vendian) glaciation, was investigated by the present author in 2002. It was found that sedimentological evidence for glacial erosion and/or deposition active during formation of the diamictite, is missing from the whole Kapp Lyell metasedimentary pile. Diamictites, the predominant rocks of this pile, represent terrestrial debris-flow and slope-creep covers derived from frost-shattered rocky margins of depositional basin. They consist mainly of dolostone, limestone and quartzite clasts. Greenstone clasts and lenses, and granitoid clasts, appear in stratigraphically higher part of the succession (in green diamictite). This clast distribution resembles that known from metaconglomerates (diamictites) of the Slyngfjellet Formation (base of Upper Proterozoic) in Wedel Jarlsberg Land, suggesting a close age correlation between the two. Uncommon finer-grade varved deposits with mainly dolostone and quartzite dropstones probably formed in restricted fresh-water reservoirs that froze during winter. During summer, drifting winter ice provided a mechanism for dispersion of clasts (ice-rafted dropstones) from lake beach/coast to water reservoir. Shallow erosional channels filled with sandstone, which occur in the diamictites, might be an evidence for ephemeral braided river systems that were active during summer. There are also some evidences for proximity of a high-energy, probably marine, beach. The Late Proterozoic Kapp Lyell diamictite complex was deposited mainly as talus aprons in a wide valley rimmed by rocky ridges, under seasonally differentiated, moderate-cool climate, comparable with the modern Subarctic periglacial zone, however outside reach of glaciers or ice-sheet. Frost-shattering during winter was probably the main factor of disintegration of source rocks.