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The Gwna melange of north Wales is a sequence of pebbly mudstones and broken formations interstratified with undisturbed volcaniclastic units. The individual clasts, generally lying in a matrix of arenaceous mudstone, locally range up to a kilometer in outcrop. The clasts include graywackes, arenites, quartzites, limestones, basalts, cherts, and volcaniclastics.
The melange has been frequently cited as the "type" melange indicative of a tectonic origin. Recent mapping, petrography, and sedimentologic analysis have produced evidence for a sedimentary (olistostromal) origin for most of the melange. Sedimentary structures present include clastic dikes, resedimentation and soft-sediment injection features, and dewatering "cleavages" in various stages of development. The upper contact of the melange is at least partly a sedimentary-erosional contact with overlying undisturbed beds of composition similar to that of the melange. Certain larger clasts (up to 50 m) of graywacke exhibit oriented tectonic kink folds. These large clasts are in sedimentary contact with the melange, and may be explained best as undisturbed sedimentary-slide units (olistoli hs) that were folded by later regional tectonic deformation. Smaller clasts within the melange responded to these regional forces mainly by continued movement, or shearing, within their ductile matrices.
This olistostromal sequence possibly represents an unstable continental margin associated with a previously postulated late Precambrian subduction zone.
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