Some sedimentological and structural studies of the old red sandstone Hitra group, Hitra, Sør-Trøndelag

Abstract

The Old Red Sandstone (O.R.S.) area on Hitra contains an about 1350 m thick, continental succession of conglomerates, sandstones and mudstones, here referred to as the Hitra Group. It has further been divided into 3 formations, from bottom to top the Aune Formation, The Vollan Formation and the Balsnes Formation. These have again been divided into Members A-L.

The group commences with a basal breccia overlain by floodplain mudstones and sandstones. This in turn is overlain by braided river conglomeratic sandstones followed by a cyclic sequence of mudstones and sandstones interpreted as representing floodplain/distal alluvial fan and low sinuosity river deposits. The remaining part of the sequence is an alternation of alluvial fan conglomerates and sandstones, temporary lake mudstones, braided and meandering river sandstones, mudstones and conglomerates and finally a coarse alluvial fan conglomerate.

The sediments were laid down in an area which periodically may have linked up with other O.R.S. areas in the region. Deposition of the Aune and Vollan Formations took place towards the NE, E and SE, while the Balsnes Formation was deposited towards the SE, S and SW. Repeated uplift and rejuvenation of source areas, possibly along marginal faults, resulted in progradation of alluvial fans into more distal parts of the depositional basin, normally characterized by mudstones and sandstones.

Tectonic deformation folded the succession into two, major, ENE-WSW trending synclines with higher order parasitic folds on the limbs. There is a general noncylindrical style of folding with transitions into aberrant folds. This first phase of deformation, which also includes locally strong cleavage development and thrusting, is regarded as of lowermost Upper Devonian age.

Later deformation resulted in N-S trending kink bands and monoclines, E-W trending crenulation cleavage and locally strong faulting. Fault directions are dominantly NE-SW and NW-SE. These structures can probably be related to Permo-Jurassic and later crustal movements. The succession was subjected to low-grade metamorphism during burial and deformation.