The stress systems in mudflows during deposition, as revealed by the fabric of some Carboniferous pebbly mudstones in Spain

Abstract

In the Cantabrian Mountains (northern Spain) the Upper Carboniferous contains various formations with a typically syntectonic character; one of them is the Westphalian Prioro Formation which crops our in the Tejerina Syncline (province of León). This formation mainly consists of fine-grained turbidites (laminites), but intercalations of pebbly mudstones are present. In 11 exposures the pebble fabric has been determined. In most cases the orientation of 50 pebbles was measured, in two cases this was possible only for 25 pebbles: it appears that these restricted numbers do not influence the overall picture. Another observation is that the orientation of the pebbles is not influenced by the degree of flatness, if the flatness value is 1.25 at least. The fabrics of the pebbly mudstones are pictured in contour diagrams: it appears that various patterns can be found. but in relative frequencies which are unexpected. The explanation for this phenomenon is that the tail of the mudflow should be considered as built up by a large number of individual small tongues (which behave like separate frontal lobes), moving more or less independently.