MICRODEFORMATION OF LACUSTRINE LAMINITE SEQUENCES FROM LATE MIOCENE FORMATIONS OF SE SPAIN - AN INTERPRETATION OF LOOP BEDDING

Lacustrine laminated sediments (laminites) present in Late Miocene for mations of the Hijar Basin, SE Spain, display well developed loop bedd ing, a structure consisting of bundles of laminae that are sharply con stricted at intervals, giving a morphology of loops or links of a chai n. The laminite sequences, which are interbedded with turbidite marlst ones, were accumulated on the bottom of a permanently stratified lake developed in a rapidly subsiding basin limited by 010 degrees and 105 degrees normal faults. As deduced from both macro- and microdeformatio nal analyses, the basin evolved under an extensional stress field thro ughout the Late Miocene. Four main types of loops, simple and complex loops with subcategories, have been recognized within the laminite seq uence. Simple loops of type 1 show the best definite pattern, quite si milar to 'pinch and swell structures', a type of boudinage typical of stretching of alternating beds where the competence contrast is not st rongly marked. The remaining loop types display contortion and occasio nal breakage of laminae (microfaulted edges) indicative of microdeform ation near the boundary between the ductile-brittle deformational fiel ds. The distribution of the various loop types across the laminite seq uence reflects an interplay between progressive lithification of the l aminites as sedimentation progressed and tectonic stresses which affec ted the sediment sequence. Accordingly, a mechanism of deformation und er an extensional stress field, ultimately related to the creep moveme nt of the main basin faults which resulted in successive seismic shock s of low magnitude, is proposed to explain the formation of loop beddi ng in the laminites.