Lacustrine laminated sediments (laminites) present in Late Miocene formations of the Hõ Âjar Basin, SE Spain, display well developed loop bedding, a structure consisting of bundles of laminae that are sharply constricted at intervals, giving a morphology of loops or links of a chain. The laminite sequences, which are interbedded with turbidite marlstones, were accumulated on the bottom of a permanently strati®ed lake developed in a rapidly subsiding basin limited by 010°and 105°normal faults. As deduced from both macro-and microdeformational analyses, the basin evolved under an extensional stress ®eld throughout the Late Miocene. Four main types of loops, simple and complex loops with subcategories, have been recognized within the laminite sequence. Simple loops of type 1 show the best de®nite pattern, quite similar to`pinch and swell structures', a type of boudinage typical of stretching of alternating beds where the competence contrast is not strongly marked. The remaining loop types display contortion and occasional breakage of laminae (microfaulted edges) indicative of microdeformation near the boundary between the ductile-brittle deformational ®elds. The distribution of the various loop types across the laminite sequence re¯ects an interplay between progressive lithi®cation of the laminites as sedimentation progressed and tectonic stresses which affected the sediment sequence. Accordingly, a mechanism of deformation under an extensional stress ®eld, ultimately related to the creep movement of the main basin faults which resulted in successive seismic shocks of low magnitude, is proposed to explain the formation of loop bedding in the laminites.

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