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Geological Society, London, Special Publications; 1987; v. 29; p. 123-136;
DOI: 10.1144/GSL.SP.1987.029.01.11
© 1987 Geological Society of London

Part II Processes

Soft-sediment microfaulting related to compaction within the fluviodeltaic infill of the Soria strike-slip basin (northern Spain)

Michel Guiraud

Géologie Structurale USTL, Pl. Bataillon 34060 Montpellier, France
Department of Geology and Mining, University of Jos, P.M.B. 2084, Jos, Nigeria

Michel Séguret

Géologie Structurale USTL, Pl. Bataillon 34060 Montpellier, France

As much as 8 km of fluvio-deltaic strata accumulated in the Sorio strikeslip basin during the Late Jurassic-Early Cretaceous (Wealdian). The strata were bent in a huge syn-sedimentary syncline related to the development of an extensional half-graben in the basement. The alluvial plain siltstones and clays are cut by a large number of microfaults of a special type in regard to their geometry and their morphoscopic characters. The fault surfaces concave upward dip at 10°–40° to bedding with a strong dispersion of fault plane strikes, providing a characteristic circular pattern. They are closely spaced and cut the material into small bi-pyramidal cone-shaped units with axes normal to bedding.

The fault surfaces are glossy with a metallic glint and a fine striation without crystallization. From electron microscopic observations, this is due to phyllite re-orientation. Along the fault planes the displacement is always dip-slip and slight.

Analysis of fault striation populations by a computer-aided method gives a unique stress tensor responsible for the striation with the maximum compressive stress {sigma}1 normal to the bedding plane along the whole section of the synsedimentary syncline and a R ratio (R = ({sigma}2 – {sigma}3)/({sigma}1 – {sigma}3)) close to 0.

The orientation of the stress tensor, the R = 0, i.e. {sigma}2 = {sigma}3 and the morphological aspect of the fault surfaces similar to shear planes affecting present cohesive soils lead us to conclude that this deformation is related to compaction in cohesive clays contemporaneous with water escape structures in sandstones. This deformation occurs in the evolution of the basin fill from soft sediments to metamorphosed rocks.





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