Megabeds in Istrian Flysch as markers of synsedimentary tectonics within the Dinaric foredeep (Croatia)

Istrian Flysch was deposited during the Eocene in the Dinaric foredeep and is composed of hemipelagic marls and various gravity flow deposits. The latter are predominantly 5-40 cm thick turbidites, developed mostly as laminated and cross-rippled sandstone beds (Tb-e, Tc-e and Td-e Bouma sequences). In addition to the turbidites, there are deposits characterized by a significant thickness, occasionally more than 10 m, described as complex (bipartite) megabeds. The megabeds are composed of debrites in the lower part (Division I), and high-density turbidites in the upper part (Division II). The distinct clast composition of each megabed indicates that the lithoclasts were derived from tectonically active slopes and fault scarps along which collapses of the different parts of the Cretaceous to Palaeogene neritic carbonate succession, that underlie the Flysch, occurred. The Division II deposits are well cemented, normally graded calcirudite/calcarenites composed mostly of orthophragminids, nummulitids, and red algae, originating from outer ramp environments. Redeposited marl, observed in the matrix of the debrites and as intraclasts in some megabeds, implies that the collapses along the synsedimentary fault scarps and steep slopes also occurred within the foredeep itself, during the rapid tectono-sedimentary evolution of the Dinaric foreland basin

Maastrichtian to Palaeocene and Eocene pelagic carbonates on the island of Svetac (central Adriatic, Croatia)

Maastrichtian to Palaeocene pelagic carbonates on the central Adriatic island of Svetac (Sveti Andrija) are the only outcrops reported to date that document pelagic deposition during the Cretaceous–Palaeogene (K–Pg) transition within the Adriatic Basin. An approximately 3 m thick succession at the Smokvica locality contains a rich and diverse planktonic foraminiferal assemblage which allows dating of the succession and the recognition of some biostratigraphic zones. The lower part of the Smokvica section consists of 1.5 m thick pelagic biomicrite characterized by the abundance of late Maastrichtian planktonic foraminifera that indicate the Abathomphalus mayaroensis Zone. An intercalation of intraclastic floatstone <0.5 m in thickness occurring on top of Maastrichtian pelagic biomicrite is characterized by unsorted pelagic intraclasts floating within the pelagic matrix, and both components only contain Maastrichtian planktonic foraminifera.The floatstone is overlain by another <0.5 m thick intraclastic-bioclastic floatstone characterized by rounded pelagic intraclasts (plasticlasts) containing Maastrichtian planktonic foraminifera floating in the pelagic matrix. The matrix of the later floatstone contains Maastrichtian and Palaeocene planktonic foraminifera mixed together. Thus, the transition from the Maastrichtian to the Palaeocene is not continuous but is characterized by amalgamated debrites that are related to at least two separated re-depositional events within the basin. The overlying pure pelagic biomicrite is rich in planktonic foraminifera that indicates the Palaeocene P3 Zone. A few metres apart, after the covered interval, there are also Eocene pelagic biomicrites with planktonic foraminifera which indicate the Eocene E9 Zone, characterized by the co-appearance of benthic foraminifera (Discocyclina) floating within the pelagic matrix. It is assumed that a Maastrichtian opening of the deep-water environment connected to the Adriatic Basin within the former Adriatic Carbonate Platform west of the island of Vis could be related to a re-activation of an inherited transverse fault zone.</p

The Zagreb (Croatia) M5.5 Earthquake on 22 March 2020

On 22 March 2020, Zagreb was struck by an M5.5 earthquake that had been expected for more than 100 years and revealed all the failures in the construction of residential buildings in the Croatian capital, especially those built in the first half of the 20th century. Because of that, extensive seismological, geological, geodetic and structural engineering surveys were conducted immediately after the main shock. This study provides descriptions of damage, specifying the building performances and their correlation with the local soil characteristics, i.e., seismic motion amplification. Co-seismic vertical ground displacement was estimated, and the most affected area is identified according to Sentinel-1 interferometric wide-swath data. Finally, preliminary 3D structural modeling of the earthquake sequence was performed, and two major faults were modeled using inverse distance weight (IDW) interpolation of the grouped hypocenters. The first-order assessment of seismic amplification (due to site conditions) in the Zagreb area for the M5.5 earthquake shows that ground motions of approximately 0.16–0.19 g were amplified at least twice. The observed co-seismic deformation (based on Sentinel-1A IW SLC images) implies an approximately 3 cm uplift of the epicentral area that covers approximately 20 km2. Based on the preliminary spatial and temporal analyses of the Zagreb 2020 earthquake sequence, the main shock and the first aftershocks evidently occurred in the subsurface of the Medvednica Mountains along a deep-seated southeast-dipping thrust fault, recognized as the primary (master) fault. The co-seismic rupture propagated along the thrust towards northwest during the first half-hour of the earthquake sequence, which can be clearly seen from the time-lapse visualization. The preliminary results strongly support one of the debated models of the active tectonic setting of the Medvednica Mountains and will contribute to a better assessment of the seismic hazard for the wider Zagreb area

Post-impact event bed (tsunamite) at the Cretaceous-Palaeogene boundary deposited on a distal carbonate platform interior

We show crucial evidence for the Cretaceous–Palaeogene (K–Pg) boundary event recorded within a rare succession deposited in an inner-platform lagoon on top of a Mesozoic, tropical, intra-oceanic (western Tethys) Adriatic carbonate platform, which is exposed at Likva cove on the island of Brač (Croatia). The last terminal Maastrichtian fossils appear within a distinct 10–12 cm thick event bed that is characterized by soft-sediment bioturbation and rare shocked-quartz grains, and is interpreted as a distal tsunamite. Directly overlying this is a 2 cm thick reddish-brown clayey mudstone containing planktonic foraminifera typical of the basal Danian, and with elevated platinum-group elements in chondritic proportions indicating a clear link to the Chicxulub asteroid impact. These results strongly support the first discovery of a “potential” K–Pg boundary tsunamite on the neighboring island of Hvar, and these two complementary sections represent probably the most complete record of the event among known distal shallow-marine successions

Geologic reconnaissance of the island of Velika Palagruža (central Adriatic, Croatia)

Velika Palagruža (Pelagosa) is the largest island of the Palagruža archipelago (central Adriatic Sea, Croatia). Despite its minute size the island bears a certain geological interest being the only exposed piece of land in the central part (Mid-Adriatic ridge) of the common Adriatic foreland of the Apenninic and the Dinaridic orogenic domains. The litho-, bio-, and chemostratigraphic (strontium and sulphur isotopes) characteristics of the sedimentary units, along with tectono-structural and geomorphic characteristics of the island, are described in this paper. The oldest Žalo unit is composed of highly deformed siliciclastics containing gypsum, and carbonates of Middle Triassic (Ladinian) age. This unit represents a transitional fl uvial-to-shallow marine, occasionally evaporitic environment, typical of the Middle Triassic rifting phase of the Adriatic microplate. Soft and strongly deformed Žalo unit deposits are found along a probably still active, WNW&ndash;ESE striking, subvertical, oblique-slip fault that crosses the entire length of the island. The Žalo unit is probably in diapiric contact with the Lanterna unit, poorly defi ned as Late Triassic, and characterized by dolomite with chert and dolomite breccia, presumably deposited in a transitional platform-to-basin environment of an evolving Adriatic basin. The Lanterna unit deposits are capped by Miocene biocalcarenites of the Salamandrija unit over an almost perpendicular discordance, possibly representing an unconformity, suggesting that an early deformational phase preceded a Miocene marine transgression. Talus, landslide deposits, and humic soil make up the cover of the bedrock sedimentary succession, and they represent the ultimate phase of emersion of the island, which probably occurred during Pliocene(?) to Quaternary times. An active neotectonic regime of the central Adriatic is evidenced by present-day seismicity, while recent uplifting of the island is shown by the presence of remnants of pebbly palaeobeach deposits, marine (erosional) straths, and cyanobacterial supratidal encrustations (pelagosite) currently observed at various elevations above mean sea level. </span

Tectonic Influence on Speleogenesis of Sea Caves on Biševo Island (UNESCO Global Geopark Vis Archipelago, Adriatic Sea, Croatia)

A geological and speleological investigation was conducted in the famous Blue Cave (Modra špilja) and the Monk Seal Cave (Medvidina špilja) on Biševo Island (Croatia) to promote the island’s geoheritage through the new Visitor Centre. The island is mainly composed of Cretaceous to Paleogene neritic carbonates, which form the bedrock, whereas parts of the island are covered with thin Quaternary sediments. The caves are of small dimensions and a simple layout, composed of the main channel and few shorter side channels, all positioned in the tidal zone. Thus, the caves are semi-submerged sea caves located along the coastline. The Blue Cave and the Monk Seal Cave developed within the bedrock limestones and dolostones, respectively, within a zone of left-lateral NNE–SSW striking strike-slip faults that belong to the Biševo fault system. Conjugated discontinuities within the carbonate bedrock indicate a specific strike-slip tectonic regime. Additionally, the host rocks were probably also deformed and fractured during the rise of salt diapirs that characterise this part of the Adriatic foreland. Tectonic and bedding discontinuities form the fragments of the host rock, that combined with the impacts of the strong southern waves, significantly influenced the genesis of the caves

Estimation of the High-Frequency Attenuation Parameter Kappa for the Zagreb (Croatia) Seismic Stations

The city of Zagreb (Croatian capital) is situated in the contact area of three major regional tectonic units: the SE Alps, NW Dinarides, and Tisza Unit in the southwestern margin of the Pannonian Basin. The Zagreb seismic zone encompasses the Medvednica Mountains and the city of Zagreb with its surrounding areas, which was struck by the strongest instrumentally recorded earthquake (M5.5) on 22 March 2020. The objective of this contribution is the estimation of the high-frequency attenuation spectral parameter kappa (&kappa;) and its local site-specific component for the Zagreb (Croatia) seismic stations to which we were particularly encouraged after the scale of the damage after the Zagreb 2020 earthquake. We tested linear dependence of &kappa; with epicentral distance using traditional linear least square regression, linear regression for data with errors, and constrained model at close distances to estimate near-site attenuation (&kappa;0). Regression-estimated site kappa values at zero-distance are within the range of the uncertainty (&plusmn;1 standard deviation) with constrained &kappa;0 value as well within the range of existing global &kappa;0 and VS30 (shear wave velocity in the top 30 m) values. Spatial distribution of &kappa; within the Zagreb seismic zone shows that &kappa; is not isotropic and high-frequency attenuation anisotropy is probably affected by local and regional geological variability, regional active faults and a complex tectonic structure in each direction

Response of a Carbonate Platform to the Cenomanian-Turonian Drowning and OAE 2: A Case Study from the Adriatic Platform (Dalmatia, Croatia)

Global perturbations during the Cenomanian–Turonian boundary (CTB) interval and the Oceanic Anoxic Event 2 (OAE 2) represent one of the most extensively studied past environmental changes. To explore the response of various carbonate-platform depositional systems to such major environmental perturbations, strata of the intra-Tethyan Adriatic carbonate platform (sensu stricto) from the island of Bracˇ (Adriatic Sea, Croatia) provide excellent exposures and a previously well-established Upper Cretaceous lithostratigraphic framework. Within this context, this study integrated lithostratigraphy, biostratigraphy, and chemostratigraphy to describe a drowned-platform succession underlain and overlain by peritidal carbonates. Carbon-isotope stratigraphy of this succession revealed a shift towards positive d13C values that reached +4 to +5% VPDB, and represent the CTB interval excursion plateau. We observed variations in thickness of the drowned-platform successions and explained them by three superimposed mechanisms: (1) diachronous drowning of platform relief; (2) intra-platform redeposition of parts of the successions by various mass-gravity transport processes (indicating enhanced instability due to increasing accommodation space related to the late Cenomanian platform drowning and synsedimentary tectonics); and (3) migration of major carbonate factories during the recovery of shallow-platform environments. The results indicate that the CTB interval event caused unusual increase in accommodation space on the carbonate platform enabling open-marine influence and synsedimentary redeposition. However, widespread organic-rich black shales reported from coeval strata of other regions have not been documented from the platform-top successions to date, and were probably accumulated in deeper (anoxic) settings below the rising sea level

Discontinuity Surfaces in Upper Cretaceous to Paleogene Carbonates of Central Dalmatia (Croatia): Glossifungites Ichnofacies, Biogenic Calcretes, and Stratigraphic Implications

Substrate-controlled ichnofacies and biogenic calcretes represent key features for identification and interpretation of discontinuities in the carbonate rock record, which are of great significance for stratigraphic interpretations and correlations. Intraformational firmground and composite surfaces, as well as a regional Cretaceous to Paleogene (K-Pg) subaerial unconformity, developed in Upper Cretaceous to Paleogene intra-platform peritidal successions in central Dalmatia, Croatia (Adriatic-Dinaridic Carbonate Platform, ADCP), were analyzed for their trace fossil and subaerial exposure features. Thalassinoides (probably T. paradoxicus) box-work burrow systems of the substrate-controlled Glossifungites ichnofacies characterize the two documented firmgrounds and one composite (polygenic) surface. Rhizogenic laminar calcretes developed subsequently inside burrows of the composite surface through diagenetic overprint of marine sediment that passively infilled the burrows. While the formation of the two firmgrounds was probably caused by cessation of precipitation and/or deposition of calcium carbonate due to relative sea-level fall, the recorded trace fossils associated with the composite surface indicate that this surface developed through both submarine firmground and subaerial exposure stages probably caused by several episodes of regression and transgression, and exemplifies the general complexity of hiatal surfaces in shallow-marine carbonate successions. The regional K-Pg subaerial unconformity is characterized by biogenic (beta microfabric) calcretes with rhizoliths including Microcodium aggregates, root tubules, as well as alveolar-septal structures. Laminar calcretes and pisoids, together with in situ and resedimented speleothems, and bauxitic deposits, were also recorded. The unconformity developed due to formation of a forebulge in front of the approaching Dinaridic orogen. Ichnological and subaerial exposure features, together with stratigraphic implications derived from the analyzed discontinuities, serve as examples that can be applied to discontinuities present in carbonate successions elsewhere