STRUCTURAL AND STRATIGRAPHIC CONTROLS ON THE DEVELOPMENT OF JOINTS WITHIN THE OLIANA ANTICLINE, SOUTHERN PYRENEES, SPAIN

This investigation focuses on determining the controls on jointing within Eocene - Oligocene marine marls and fluvial and alluvial continental synorogenic units on the southeastern flank of the Oliana anticline. The proximal correlates of these rocks were broken into four units and dated using magnetostratigraphy by Burbank et al. (1992). Unit 1 (~37 Ma) is characterized by sandstone (57%) with subordinate fine-grained (Fm) lithofacies (38%) and minor conglomerate (5%). Unit 2 (~36 Ma) is typified by sandstone (59%), with subordinate conglomerate (26%) and fine-grained lithofacies (15%). Unit 3 (35-34 Ma) and Unit 4 (<34 Ma) are similar. Both are characterized by a dominance of sandstone (70%), with subordinate fine-grained lithofacies (21%) and minor conglomerate (9%). The general structural dip of beds within the 1.4 km thickness of these units range from 65?, 155? (dip, dip direction) in Unit 1 to horizontal in Unit 4, and reflect their deposition at different stages of fold development. Several generations of elastico-frictional deformation affected this region, as evidenced by the development of bedding-perpendicular joints. Joint density and size are controlled by the mechanical properties of the jointed rocks; the ratio of bed thickness to joint spacing is typically ~1:1.When unplunged and unfolded with bedding, the joints in each unit reveal one main set oriented NW-SE with associated cross joints. The angle between these joints is 75?-90? in the sandstones and conglomerates and 25?-45? in siltstones and mudstones. The joints are interpreted to be controlled by local stresses due to folding with the specific orientation of the cross joints controlled by the relative competence of the jointed units.