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Application of Rift Sequence Stratigraphy in Seismic Interpretation of the Lower and Middle Miocene Rocks in Eastern Part of the Drava Depression

Formation of the Lower and Middle Miocene rocks in Eastern part of the Drava Depression, as well as in the entire North Croatian Basin, is associated with rifting processes and sedimentary infill of the depocenters created during the syn-rift tectonic phase. Surface outcrops of Lower Miocene succession consist of alluvial sandstones and conglomerates, while Middle Miocene sediments are composed of Lithothamnion limestone’s to marls, indicating sedimentation in shallow marine environment. Core samples from exploration wells differ in lithology - from rockfall breccia and debris conglomerates to both shallow and deeper marine sediments of various lithology. Syn-rift sedimentation usually comprises Miocene volcanic rocks confirmed both in on outcrops and from the well data. Seismic 3D data and interpreted seismic facies suggest that identified lithologies of Lower and Middle Miocene could be spaciously linked. Rift sequence stratigraphic approach (Prosser, 1993) defines tectono-stratigraphic units, which are associated with cogenetic fault activity during the syn-rift phase. The syn-rift tectonic phase units are primarily detected by tilting of reflexes in the hanging wall units and by other seismic facies characteristic that are specific for the syn-rift sedimentation. This is how the pre-rift, syn-rift and post-rift, main tectono- stratigraphic units were interpreted and mapped in this study. In thicker portions of Lower and Middle Miocene successions, the rift sequence architecture can also be studied by interpretation of smaller units - system tracts. Interpreted seismic data pinpoint to cogenetic formation of depocenters as a result of extensional reactivation of the pre-Neogene WNW- ESE striking faulted structures that happened during Lower and Middle Miocene. Syn-rift depocenters were formed either directly in the hanging wall of these reactivated faults or along newly formed normal faults that were structurally predefined by the older pre-Neogene faulted structures. Pre-rift units are usually overlain with initial rift system tract, i.e., the basal seismic facies defined on seismic sections. The initial rift system tract can be distinguished in the thickest portions of the Lower and Middle Miocene strata as onlaping or downlaping on the pre-rift units. Main syn-rift system tract is characterized by: 1) significant vertical offsets along normal faults, 2) tilting of the hanging wall blocks, and 3) seismic facies of marginal fan-aprons. In marginal areas of the syn-rift depocenters only a portion of the main syn-rift system tract can present entire syn-rift succession. In the same time, the well data indicate that initial and main syn-rift system tracts comprise of the entire Lower Miocene and part of the Middle Miocene sediments. Immediate post-rift system tract is characterized by cessation of normal fault activity, onlap on the syn-rift sediments and marked paleo-structures as well as by continuous seismic facies. Immediate post-rift system tract is composed of the upper portion of the Middle Miocene together with the lowermost Pannonian sediments. In the study area, the late post-rift system tract represents the Lake Pannon infill, excluding its lowermost portion. This study suggests that the rift sequence seismo- stratigraphic analyses provides the basis for delineation of prinicipal tectono-stratigraphic units, which can give an additional information on the evolution of the rift basin in Drava Depression. In regions with significant thickness of these rock units in the deep subsurface, this methodology can be used for mapping of the Lower and Middle Miocene rock units in more detail. In this way achieved results can also facilitate the spatial correlation of the Lower and Middle Miocene rocks regionally.

Drava depression, Miocene, syn-rift, rift sequnce stratigraphy, seismic interpretation

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