The Neoproterozoic Valdres Group-depositional conditions and stratigraphical developments in the Mellane and Grønsenn-knippa sections, Valdres, Southern Norway

Abstract

The youngest part of the Valdres Group of south-central Norway (mainly the Neoproterozoic Rundemellen unit, Tonian – Cryogenian?) has been sedimentologically and stratigraphically studied and sampled at the Skarvemellen and Rundemellen sections at Mellane (Øystre Slidre) and Grønsennknippa (Vestre Slidre). The goal was to shed light on the depositional conditions and possible stratigraphical correlations to time-equivalent formations in the better-known Hedmark Group to the east. It is hoped that this approach will also help establish an improved basis to understand the related metamorphic Valdres Group units to the west and north of the study area. Depositional environments have been interpreted from sedimentary field data and observations, supported by petrographical, geochemical and mineralogical analyses.

Coarse clastic sedimentary rocks of braided stream/alluvial fan dominates the Valdres Group. The studied succession at Grønsennknippa starts with massive pebbly conglomerates with well-rounded clasts and interbedded sandstone units, unconformably overlying crystalline basement. This package is overlain by sandstones of various alluvial affinity, interrupted only by one conglomerate horizon and one diamictite unit. The latter is possibly correlative to the diamictite recognised in the Valdres Group at Mellane, and may represent a tillite correlatable to the Moelv Tillite (Marinoan glaciation). The Mellane section is more dominated by sandstones and subordinate conglomerates than those at Grønsennknippa. Mineralogical composition and field observations of the Grønsennknippa and Mellane sections indicate relatively short transport distances of the texturally immature sediments. The petrography also reflects a later metamorphic phase (varying greenschist facies), overprinting the original mineralogy.

Comparison between the Valdres Group at Mellane and Grønsennknippa sections indicates that the Grønsennknippa succession exhibits an originally more immature mineralogical and textural signature than those at Mellane. The Grønsennknippa sections are more feldspar-rich and dominated by coarse-grained detritus. This may reflect different positions with respect to the source area/basin margin. In this study, the Valdres Group at Grønsennknippa and Mellane is compared with selected fluvial/alluvial formations of the Hedmark Basin, revealing comparable depositional environments and stratigraphic developments. The occurences of possible tillites at stratigraphical levels comparable to the Moelv Tillite are of particular interest. The petrographical composition and heavy mineral distribution support comparable sediment sources from mainly Sveconorwegian formations in both Valdres and Hedmark basins.