Extraordinary flat surfaces of the Sub-Cambrian peneplain around the shores of LakeVänern nearKinnekulle
Thesub-Cambrian peneplain is an ancient, extremely flat, erosion surface (peneplain) that has been exhumed and exposed by erosion from underCambrianstrata over large swathes ofFennoscandia. Eastward, where this peneplain dips below Cambrian and other LowerPaleozoic cover rocks. Theexposed parts of this peneplain are extraordinarily flat with relief of less than 20 m. The overlying cover rocks demonstrate that the peneplain was flooded byshallow seas during the Early Paleozoic.[1]Being the oldest identifiable peneplain in its area the Sub-Cambrian peneplain qualifies as a primary peneplain.[2][3]
The surface was first identified byArvid Högbom in a 1910 publication, withSten Rudberg publishing the first extensive map in 1954. This mapping has been improved upon byKarna Lidmar-Bergström since the 1980s.[4][5]
The Sub-Cambrian peneplain extends as an almost continuous belt along the eastern coast of Sweden for some 700 km (430 miles) from north to south.[6] Near Stockholm andHudiksvall the peneplain is densely dissected by joint valleys and at theHigh Coast is the Sub-Cambrian peneplain is both highly uplifted and eroded.[7][8][9] More inland the peneplain can be traced at the crestal region of theSouth Swedish Dome where it is dissected by joint valleys.[7][8] The Sub-Cambrian peneplain in the crestal region of the South Swedish Dome is the highest step in apiedmonttreppen system seen inSmåland.[5] In southern Sweden the peneplain surfaces tilt away from the crest of South Swedish Dome, to the northwest inVästergötland, to the northeast inÖstergötland and to the east in eastern Småland.[2] At this last region the sub-Cambrian peneplain is truncated to the west by a well defined and prominentscarp that separates it from theSouth Småland peneplain to the west.[5][A]
Much of theHardangervidda plateau in the Norway is believed to be an uplifted part of the peneplain
In theCentral Swedish lowland the peneplain extends further west being 450 km (280 miles) wide from west to east.[6] Immediately east and south of lakeVänern the peneplain tilts west and north respectively. This is reflected in that the southeastern part of the lake is very shallow but gets progressively deeper towards the northwest.[11] InBohuslän, at the northern end of theSwedish West Coast, there is some uncertainty over whether the hilltops areremnants of the peneplain.[12] A similar situation occurs in centralHalland.[5] Further west, parts of thePaleic surface in Norway have been interpreted to be part of the peneplain that has beentectonically uplifted and is apparently disrupted by NNE–SSW trendingfaults. Near the 1,100-metre (3,600 ft) highHardangervidda plateau in Norway is the Sub-Cambrian peneplain which has been uplifted at least 1,000 metres (3,300 ft),[13] albeit Hardangervidda itself is part of a much younger peneplain formed in theMiocene epoch.[14]
In northwestern Finland theOstrobothnian Plain is a continuation of the peneplain.[15] To the east the Sub-Cambrian peneplain continues as anunconformity beneath theEast European Platform.[6][16][B] On a grand-scale the peneplain is not completely flat as it has beendeformed. This deformation is anisostatic response to erosion and the load ofPhanerozoic sediments that rests above much of the peneplain.[16] The peneplain is characterized by a general lack ofinselbergs.[2][6] One exception to this is the islandBlå Jungfrun in theBaltic Sea which is an ancient inselberg formed inPrecambrian time andburied insandstone after its formation. Blå Jungfrun remained buried untilerosion of the East European Platform freed it in geologically recent times.[17] Further southeast a series of buried inselbergs on top the peneplain have been identified throughseismic reflection inLithuania.[18]
Interpretations ofJotnian sandstone imply that much of the Baltic Shield have had faint relief since theMesoproterozoic,[19][20] but noexhumed peneplain from this period has been preserved.[7][C] The low relief terrain on which the Jotnian sandstone deposited was disturbed by theSveconorwegian orogeny in western Sweden about 1,000 million years ago and then begun to erode again into a terrain of subdued relief.[5]
The peneplain formed after 600 million years ago but prior to theCambriantransgression. The basement rocks forming the peneplain surface were exhumed from depths where the temperature was in excess of 100 °C (212 °F) prior to the formation of peneplain.[8]Karna Lidmar-Bergström and co-workers assume the peneplain formed through acycle of erosion with a preceding brief valley phase and that it grades down to a formersea level.[1][6] Due to the absence of land vegetation in Precambrian timessheet wash is thought to have been an important process of erosion leading to the formation of extensivepediments. Sheet wash would also have hindered the formation of deep weathering profiles. Indeed, at the places the substrate of the Sub-Cambrian peneplain iskaolinized it never exceeds a few meters in depth.[12] In Norway'sFinnmark the peneplain is roughly contemporaneous with the formation ofkaolinite,smectite andillite up to 15 metres (49 feet) below the surface'scontact with marine sedimentary rock of Cambrian age.[21]
Limestonestacks ofByrum's raukar inÖland laterally close to the exhumed parts of the peneplain and also close to the buried parts of the peneplain.
The flatness of the peneplain meant that during the Cambriantransgression very large areas were swiftly flooded forming large and shallowinland seas in changing configurations. The new relief formed on top of Cambrian sediments smoothed out irregularities in the peneplain.[22] Early Cambriansandstones overlying the peneplain in southern Norway,Scania andBornholm have likely never been recycled. This means the parent rocks of the sandstone were eroded and the sediment strongly reworked and weathered reachingsedimentary maturity with no other in-between step or hiatus.[23] The source areas for these sandstones are local rocks from theTransscandinavian Igneous Belt or theSveconorwegian andGothianorogens.[23]
