Coupled tectonic and sedimentary evolution in asymmetric extensional basins: The Konjic Basin in the Dinaride Lakes System of Bosnia and Herzegovina

Abstract

The evolution of sedimentary basins can be derived by studying the link between fault kinematics and sedimentation. Sedimentation in asymmetric extensional basins is controlled by the accommodation space formed in the hanging wall of normal faults and by the variability of a source area controlled by their coeval footwall exhumation. One very good place to study this relationship is the External Dinarides of Bosnia and Herzegovina, where a large number of Miocene asymmetric extensional basins have formed by the reactivation of an inherited Cretaceous – Eocene nappe stack. These Early-Middle Miocene basins formed as endemic and endorheic lakes and are generally known as the Dinaride Lakes System (DLS). In the DLS, one very good place to analyse the link between the activation of normal faults and the associated sedimentation is the Konjic Basin in the central part of the External Dinarides, which contains continuous and well-preserved outcrops exposing both the normal faults and their associated sedimentary facies. In this basin, we have performed a detailed kinematic and sedimentological mapping that allowed a correlation between normal faulting events and the associated depositional facies. The results show that normal faults have been created in response to a first extensional stage oriented NE-SW that was followed by a second extensional stage oriented NNE-SSW. Sedimentological field observations have allowed the definition of facies units that were subsequently grouped in facies associations. These facies association are characterized by fining and coarsening upward patterns, interpreted to be formed in response to second-order progradation-retrogradation (P-R) cycles. The correlation with the moments of normal fault activation shows a causal link between faulting and slope instability. Activation of normal faults triggers avalanching of coarse material towards the deeper parts of the basin, resulting in alternating successions of alluvial-fluvial and deltaic deposits. These sediments record downslope flow transformations from debris flow to subaqueous turbidity currents, which create the observed fining and coarsening upward patterns. This study indicated at least six tectonically induced second-order P-R cycles that formed in response to the activation of clear sets of normal faults. Grouping the second-order P-R cycles allows the definition of a lower order P-R cycle, associated with the opening and subsequent filling-up of the entire basin. Our results indicate that the Middle Miocene basin evolution of the Konjic Basin was controlled by large-scale exhumation in the footwall. Previous studies on DLS basins in the External Dinarides show similar characteristics for this Early-Middle Miocene period of extension, generally thought to be related to the back-arc opening of the Pannonian Basin. In these DLS basins footwall exhumation is created by extension and is accommodated by detachments branching into inherited weak zones. The footwall uplift that controlled the sedimentation of the Konjic Basin development is accommodated by an in opposite directed normal detachment when compared with the larger scale Buscovača detachment that controls the evolution of the larger Sarajevo-Zenica Basin situated northwards. On the overall, our research contributes to a better understanding of the mechanisms controlling the sedimentary response to extensional basin formation and the genetic link with footwall exhumation in asymmetric intra-montane basins.