The Bigganjárgga Tillite re-visited: a stratigraphic and sedimentological study

Abstract

The Neoproterozoic “tillite”-bearing sequences of the Smalfjord Formation in Varanger, northern Norway have retrieved considerable attention in geological literature since first described and discovered over a century ago. Much of the attention relates to the so-called Reusch’s Moraine also known as the Bigganjárgga Tillite, which rests directly on top of a striated sandstone surface pointing to glacial scouring and a glacial origin (i.e. a tillite). Correlations of similar deposits from the same era elsewhere in the world have resulted in theories of a world-spanning glaciation, the “snowball Earth”-theory, where great amounts of the Earth was covered by snow and ice. However, the theory is criticized by many, and better understanding of diamictite deposits in general have caused several re-evaluations (globally) of these presumed tillites, which apparently can be easily confused with various coarse-grained gravity flow deposits (i.e. debrites and slump deposits).

In this thesis, the classical type locality for the Varanger Ice Age at Bigganjárgga in Varangerbotn, eastern Finnmark, has been scrutinized with one question in mind, is the Bigganjárgga Tillite really a tillite or may it represent something else? In order to answer this question, a detailed facies analysis is combined with a digital outcrop model and thin section analysis. From the facies analysis, four facies associations are recognized: FA 1 – thin-bedded sandstone (representing fluvial deposits), FA 2 – thick-bedded diamictite (representing tillite or debrite deposits), FA 3 – thin-bedded sandstone (representing turbidites) and FA 4 – thin-bedded diamictite (representing fine-grained debrite or slump deposit). The appearance of rip-up mud clasts and several associated negative imprints within FA 1 suggest that the unit most likely was consolidated before erosion of the Bigganjárgga Unconformity (a part of the regional Varangerfjorden Unconformity) and deposition of the Bigganjárgga Diamictite (FA 2) took place. A confirmation to this was given by stylolite observations in the field and imbricated grains (with irregular grain-grain contacts), and quartz cementation in the petrographic analysis consistent with pressure solution occurring at a considerable burial depth (c. 2 km in sedimentary basins with normal geothermal gradient). Thus, it seems likely that the debated striations associated to the Bigganjárgga Unconformity developed in consolidated material, implying glacial abrasion.

The Bigganjárgga Unconformity at the outcrop is seen in relation to the extensive regional Varangerfjorden Unconformity where clear evidences for glacial erosion have been reported in previous studies. Therefore, it is assumed that the investigated unconformity was made by glacial erosion that occurred at regional scale. The Bigganjárgga Diamictite (FA 2) itself is seen in relation to the surrounding units below and above. Due to the appearance of FA 3 (turbidite sandstones) and FA 4 above (fine-grained gravity flow deposits) the diamictite, a regional relative sea level rise is assumed eventually drowning and flooding the diamictite body. The preserved evidence surrounding the Bigganjárgga Diamictite that caused the glacial origin assumptions are the cemented sandstone and the regional unconformity below with striations correlative to nearby glacial direction features, the burial by fine-grained turbidites and fine-grained gravity flow deposits with randomly clasts up to cobble size units above the diamictite associated with subsidence and ice rafted debris.