Autor: Ana Kamenski, MSc.
The Aiza research area covers over 650 km2 of the northern Adriatic offshore, a common Adriatic foreland of the older Dinarides on the NE, and the younger Apennines on the SW. This paper’s main objective is to reconstruct the evolution of the carbonate platform and the geometry of its margin in the area. It also aims to define the structural-tectonic setting of the research area, with a particular focus on investigating the possible continuation of inferred transverse structures (oriented obliquely or perpendicularly to the platform margin and the orogenic belts).
Figure 1 (a) Location, paleoenvironmental map after Grandić et al. (2010). and tectonic map after Korbar (2009) of the study area, including the position of the Aiza research area, locations of the two exploration wells, Susak more-1 and Alessandra-1, situated offshore Kvarner and southern Istria. Contours delineate the shorelines of the mainland and the Kvarner islands. A dashed-dotted line indicates position of the 2D regional seismic profile CROP-M16. Inset shows map’s location in the Northern Adriatic area and the thrust fronts of the surrounding orogenic belts. (b) The frame of the Aiza 3D Block depicts placement of the seismic sections, including inlines, crosslines, composite lines, and 2D seismic lines. These sections have been selected to provide a clear and representative reflection of the subsurface relationships within the exploration area covered by seismic data.
High-quality 3D reflection seismic data were used to investigate the area’s Mesozoic to Cenozoic tectono-stratigraphic evolution. Four main seismo-stratigraphical horizons were recognized: Base of Carbonate Platform (BCP), Top of Carbonate Platform (TCP), Messinian Erosional Surface (MES), and a Plio-Quaternary horizon (PlQh), as well as the dominant faults. The results depict the geological setting and tectonic evolution of the area.
Figure 2 Stratigraphic correlation of Alessandra-1 and Susak more-1 wells with main prominent horizons, adopted from Špelić et al. (2021). These horizons include BCP (Base Carbonate Platform), TCP (Top Carbonate Platform), MES (Messinian Erosional Surface), and PlQh (Plio-Quaternary horizon). Well positions are depicted in relation to the investigated Aiza Block and the 3D seismic area available for this research.
Figure 3 (a) Location of seismic profiles. (b) Seismic profile IL 9435 with interpreted horizons tentatively calibrated using data from the remote Susak more-1 well. The position of the carbonate platform margin is indicated by the red solid line, extrapolated by the red dashed line. (c) Seismic section XL 2069 illustrates the platform escarpment on both the NW and SE sides. (d) Seismic section IL 10330 reveals the SE slope of the platform, along with the presence of extensional faults (short red solid lines) and indications (red dashed lines) of strike-slip faulting intersecting the profile.
Figure 4 Horizon mapping on seismic section XL 2844: (a) Location of seismic profile; (b) Uninterpreted classic seismic view with highlighted position of four main horizons (yellow arrows); (c) Seismic attribute Cosine of phase view with the interpreted BCP horizon; (d) Seismic attribute 3D edge enhancement view with the interpreted PlQh and TCP horizons.
Figure 5 Structural-stratigraphic interpretation of seismic sections IL 9910 and XL 2844: (a) On seismic section IL 9910, the major structural features include a pronounced platform margin marked by a steep slope (indicated by the thick red line on the left) and a relatively simple intra-platform basin (represented by the two thick red lines on the right). Thin red lines depict extensional faults. Abbreviations used: Adratic Basin (AB), Adriatic Carbonate Platform (ACP), and Tethyan Mega Platform (TMP). (b) Seismic section XL 2844 reveals a more complex intra-platform basin within a releasing bend (listric fault, thick red line on the left) and a restraining bend (strike-slip fault, middle thick red line). Some faults within the hybrid flower structure exhibit signs of reactivation (double arrows). Additionally, preserved anticlinal structures formed as a result of fault reactivation with an opposite character are indicated by black arrows. Multiple reflectors are marked by “m”. (c) Location of the seismic sections is shown for reference.
Figure 6 (a) Uninterpreted and (b) Interpreted XL 1544 seismic section. This section depicts chronostatigraphic Plio-Quaternary sequences of the study area. The grey rectangles represent cropped areas displayed in (c) and the black arrows highlight the presence of gas chimneys. (c) Seismic sequences, denoted as SEQ1 to SEQ7, are described here along with their defining characteristics. (d) Inset displays the location of the section for reference.
Figure 7 Acquired tectonic configuration based on the interpretation of seismic sections. The fault polygons are represented in a 3D view from three different perspectives: (a) From the south; (b) From the southwest; and (c) From the top. Time-slice maps have been selected at six distinct depths: (d) −3000 ms, (e) −2500 ms, (f) −2000 ms, (g) −1500 ms, (h) -1000 ms, and (i) -500 ms. The sharp transitions from high to low amplitudes on these maps represent significant variations in elevation. Red lines delineate the primary faults within the study area, while black lines indicate the platform’s margin and white lines delineate the edges of the intra-platform basin.
Figure 8 Time-structural maps for the: (a) BCP; (b) TCP; (c) MES; and (d) PlQh horizons. Isochron intervals are set at 25 ms TWT for (a), (b), and (c), while (d) has a 10 ms TWT isochron interval. The grey outlines represent all seismic sections referenced in this paper.
Figure 9 The 3D model illustrates the configuration of platform carbonates within the Aiza exploration Block. The upper image of TCP surface depicts the Adriatic carbonate platform margin and the intra-platform basin, while the lower image of the BCP surface depicts the dome-shaped geometry of the basement. To enhance visibility, a vertical exaggeration factor of five has been applied to the Z-scale. The contour interval is 25 ms TWT.
Figure 10 (a) Location of the sections, with highlighted interpretation of the carbonate platform margin (thick black line) and the major Kvarner Fault System marked by red line; (b) Seismic section CROP-M16, with a black rectangle indicating the Aiza area and a yellow rectangle indicating the apparent structure previously recognized by Del Ben (2002); (c) Composite section CL-1; (d) Composite section CL-2. Both sections include interpreted regional horizons and the Kvarner Fault System; the Z-scale in these figures has a vertical exaggeration factor of five.
Figure 11 Schematic cross-section of the Dinaric Foreland Basin system from the late Cretaceous to Palaeogene (modified after Korbar (2009) and DeCelles & Giles (1996)), with the proposed position of the Aiza area (red rectangle). Not to scale.
Figure 12 Geological map of the Istria and Kvarner region (adopted and modified after Špelić et al. (2021) and references therein), integrated with findings from Grandić et al. (2013). This map encompasses the significant structural insights obtained by this research, such as the delineation of the Mesozoic Adriatic carbonate platform margin and the interpretation of major faults, including their extrapolation towards the fault system identified by Špelić et al. (2021).
A long-lasting (Jurassic to Cretaceous) stable NW-SE striking platform margin evolved probably along the inherited Triassic normal fault. The marginal belt of the platform was affected during the Late Cretaceous to Palaeogene by extension and opening of the intra-platform basin, probably on the southern limb of the then developing Dinaric forebulge. The transverse fault system (Kvarner fault) was probably reactivated as a strike-slip zone during the late Miocene tectonic reorganization. The area was tilted to the SW during the Pliocene, in the distal foreland of the progressively northward propagating Northern Apennines. Sub-horizontal late Quaternary cover of Dinaric and Apenninic structures could imply active subsidence of the foreland in between nowadays sub-vertically exhuming neighboring orogenic belts.
References:
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Del Ben, A. Interpretation of the CROP M-16 seismic section in the Central Adriatic Sea. Mem. Soc. Geol. Ital. 2002, 57, 327–333.
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Kamenski, A.; Korbar, T. Platform-to-Basin Evolution of a Tectonically Indistinct Part of a Multiple Foreland—Analysis of a 3D Seismic Block in the Northern Adriatic Sea (Croatian Offshore). Geosciences 2023, 13, 323. https://doi.org/10.3390/geosciences13110323.
Korbar, T. Orogenic evolution of the External Dinarides in the NE Adriatic region: A model constrained by tectonostratigraphy of Upper Cretaceous to Paleogene carbonates. Earth-Sci. Rev. 2009, 96, 296–312.
Špelić, M.; del Ben, A.; Petrinjak, K. Structural setting and geodynamics of the Kvarner area (Northern Adriatic). Mar. Petrol. Geol. 2021, 125, 104857.
Ana Kamenski, MSc., PhD student/scholarship holder of the Faculty of Mining, Geology and Petroleum Engineering, University of Zagreb, and Senior Expert Associate at the Department of Geology of the Croatian Geological Survey.
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