This excursion guide describes the glacigenic rocks of the Smalfjord and Mortensnes Formations, and parts of the intervening Nyborg Formation, at the base of the Vestertana Group in the Tanafjord-Varangerfjord region of East Finnmark, North Norway. These are agreed to be either of glacial (sensu lato) origin or significant to our understanding of Neoproterozoic glacial processes and events. The Smalfjord and Mortensnes Formations have been equated with the Marinoan (636.3 ± 4.9 Ma and 635.5 ± 1.2 Ma) and Gaskiers (584-582 Ma) glaciations, respectively. The rocks, which are superbly exposed, display a wide range of sedimentary lithologies/facies; lodgment, banded, deformation, flow, and melt-out diamictites; as well as glaciomarine, proglacial, and fluvioglacial sediments. Diamictites derived from carbonate units showing the Trezona and Wonoka negative δ13C anomalies are seen.
Glaciotectonic and/or soft-sediment structures described include folds, normal and thrust faults (imbricate fans), flanking structures, shear-sense criteria (sigma clasts), striations (with paleoloess), lobate-cuspate contacts, downward intrusion of fluidized sediments (with bridge structures), pro-and subglacial channels, nested channels, graded beds, possible iceberg dump structures, ghost clasts, dropstones and lonestones, allochthonous rafts of substrate sediments, ball-and-pillow structures, convolute laminations (pseudonodules), delta foresets, ice-crystal molds, sandstone dikes, and a possible kettle hole. The complex geometry of the sub-Smalfjord unconformity onto older Neoproterozoic sediments and the Archean Baltic Shield is documented at several scales. Marinoan cap dolostones (Maieberg negative δ13C anomaly), with typical sheet cracking (and “pseudostromatolites”) and atypical clastic inter-beds, are described from the base of the Nyborg Formation at several localities.
An unconformity that might be “hiding” the Sturtian glaciation and dolomitebearing rocks lying to the north of the Trollfjorden-Komagelva Fault on Varanger-halvøya, thought to be correlatives of the 810 Ma and younger Bitter Springs negative δ13C event, are also described.
Compared to many other areas with Neoproterozoic glacial sequences, the area is easily accessible and has a wide range of modern facilities.
Figures & Tables
Neoproterozoic Glacial and Associated Facies in the Tanafjord-Varangerfjord Area, Finnmark, North Norway
The Neoproterozoic glacial deposits of north Norway (snowball Earth Marinoan-aged Smalfjord and the younger Gaskiers-aged Mortensnes Formations) are superbly documented and illustrated in this comprehensive eight-day field guide. Lodgement, banded, deformation, flow, and melt-out diamictites derived from gneissic, clastic, and dolomitic sources are described, as well as glaciomarine, proglacial, and fluvioglacial sediments. Outcrops showing glaciotectonic folds, faults, flanking structures, shear-sense criteria (sigma-clasts), fluidized sediments, pro- and subglacial channels, iceberg dump structures, ghost clasts, dropstones, ice-crystal molds, and a ?kettle hole are included. The classic glacial striations at Oaibaccanjar'ga are described in detail. Marinoan cap dolostones overlying the Smalford Formation are included. In comparison to other Neoproterozoic glacial successions, this area is easily accessible by airplane and/or car, and most outcrops occur along the roadside or can be easily reached by small boat. This volume includes information about accommodations and small boat rental, and a fascinating summary of the natural and local history.