Pre-Late Devonian unconformity in the Kłodzko area excavated: a record of Eo-Variscan metamorphism and exhumation in the Sudetes

The results of excavation works aimed at exposing the pre-Late Devonian unconformity in the vicinity of Klodzko (Middle Sudetes, NE Bohemian Massif) are reported. The unconformity, first described by Bederke in 1924, provides important constraints on the timing of the exhumation of metamorphic complexes in the Sudetes. However, despite its importance, the unconformity is nowhere exposed at present (with the possible exception of the gabbro blocks at one locality - Mt.Wapnica in Dzikowiec), and has been inaccessible for direct observation for decades. Therefore, new excavation works were conceived and done to confirm the unconformity's existence and to describe details of the contact between the metamorphic basement and the Devonian sedimentary cover. Two localities, at Łączna and Gologlowy, were selected for the excavation, based on detailed mapping and an EM31 conductivity survey. In both localities, four trenches, 2.5-3 m deep and up to 24 m long, were dug across the expected contact zone. Along the trenches in both sites the unconformity was excavated. At each site, the metamorphic rocks are in primary, sedimentary contact with the overlying basal sedimentary breccias and conglomerates. There is no evidence of tectonic disturbance at the contact. This angular unconformity must have formed during a relatively narrow time interval of c. 10-15 Ma, between the early Givetian and late Frasnian or Famennian. This timing is constrained by the late Frasnian?- to Famennian age of the limestones directly overlying the basal conglomerates and by the recently revised early Givetian age of a coralline fauna from the metamorphosed limestones of the Klodzko Metamorphic Unit at Maly Bozkow. The existence of this unconformity implies that at the turn of the Middle and Late Devonian times, freshly deformed and metamorphosed rocks were exhumed and onlapped by sediments of the Bardo sequence, which, eventually, became folded during latest Visean/Namurian times.