Characteristics of selected discontinuity surfaces recorded within the Adriatic-Dinaridic carbonate platform deposits

Površine diskontinuiteta predstavljaju ključne horizonte u sedimentacijskom zapisu, koji su povezani s prekidima sedimentacije, čime predstavljaju hijatuse neovisno o njihovu trajanju. U ovaj ne-interpretativni skupni naziv uključene su različite vrste površina diskontinuiteta, uključujući marinske firmground-e i hardground-e, subaerske površine izlaganja, kao i kompozitne (poligenetske) površine. Površine diskontinuiteta su korisni marker horizonti pri stratigrafskoj korelaciji te se često primjenjuju i u sekvencijskoj stratigrafiji karbonatnih sljedova. Značaj kalkreta i paleokrša, ihnofosila i ihnofacijesa te autigene marinske mineralizacije, pri genetskim interpretacijama površina diskontinuiteta, je vrlo dobro poznat u literaturi. Vrijednosti stabilnih izotopa pedogenih karbonata (kalkreta) su također vrlo vrijedan indikator okoliša meteorske dijageneze, kao i odlično oruđe pri paleokolišnim rekonstrukcijama u kvartarnim sedimentima. Budući da je do sada napravljen vrlo mali broj detaljnih ihnoloških, sedimentoloških, pedo(dija)genetskih, mineraloških i izotopnih analiza površina diskontinuiteta, koje se pojavljuju u mezozojskim do kenozojskim karbonatnim sljedovima Vanjskih Dinarida, postoji potreba za njihovim istraživanjem. Unutar gornjokrednih do paleocenskih karbonatnih naslaga na području središnje Dalmacije (južni dio nekadašnje Jadransko-Dinaridske karbonatne platforme, ADCP), te u donjojurskim naslagama na području jugozapadne Slovenije, zabilježeno je nekoliko vrsta površina diskontinuiteta, uključujući subaerske površine izlaganja, marinske omisijske površine, te kompozitne površine, koje su karakterizirane dobro razvijenim i indikativnim ihnološkim, sedimentološkim, pedo(dija)genetskim, mineraloškim i izotopnim karakteristikama. Regionalni krednopaleogenski (KPg) subaerski nekonformitet, zabilježen u istraživanim formacijama Gornji Humac (konijaksanton; lokalitet Šibenik), Čiovo (gornji kampan; jugozapadni dio otoka Čiovo), te Sumartin (mastrihtpaleocen; zapadni dio otoka Hvara) na području središnje Dalmacije, karakteriziran je biogenim (beta mikrostrukturnim) kalkretama s rizolitima, uključujući in situ Microcodium agregate i tubule korijenja s razvijenom alveolarno-septalnom građom, laminiranim kalkretama i pizoidima (dijelom nastalim kalkretizacijom boksitnih naslaga), kao i tipičnim paleokrškim obilježjima (uključujući in situ i resedimentirane speleoteme). Faza regionalnog subaerskog izlaganja tijekom kasne krede do paleogena, koja je različito trajala na različitim dijelovima ADCP-a, uzrokovana je vrlo intenzivnim kasnokrednim sin-sedimentacijskim tektonskim zbivanjima koji su povezani s kolizijom Jadranske i Euroazijske ploče, što se također očituje i u diferenciranim taložnim okolišima na području ADCP-a tijekom tog razdoblja. Kredno IX paleogenski hijatus je interpretiran kao posljedica dijakronog i različitog izdizanja različitih dijelova platforme(i) kao odgovor na dijakroni forebulging (blago izdizanje predgorja) u fronti približavajućeg dinaridskog orogena. Unutar formacije Gornji Humac (konijaksanton; peritajdalni vapnenci taloženi u unutrašnjosti platforme) na lokalitetu Šibenik zabilježeno je nekoliko površina diskontinuiteta, uključujući marinske firmground-e i kompozitnu površinu. Dok Thalassinoides ihnorod (vjerojatno T. paradoxicus) koji pripada Glossifungites ihnofacijesu karakterizira stadij marinskog firmground-a u oba tipa zabilježenih diskontinuiteta, unutar bioturbacija kompozitne površine je naknadno došlo do razvoja rizogenih laminiranih kalkreta, što upućuje na to da je ova površina prošla kroz naknadni stadij subaerskog izlaganja. Na temelju asocijacije facijesa ispod i iznad dvaju firmground-a zabilježena na lokalitetu Šibenik, proizlazi da su ove površine mogle nastati kao posljedica pada relativne razine mora koji je uzrokovao razdoblje omisije. S druge strane, različite vrste ihnofosila (firmground Thalassinoides bioturbacije i rizogene laminirane kalkrete) zabilježene u kompozitnoj površini upućuju na to da je ova površina prošla kroz nekoliko perioda regresije i transgresije. Unutar formacije Dol (srednji do gornji kampan; padinski taložni okoliši) i nove litostratigrafske jedinice Čiovo (gornji kampan; taložni okoliš vanjske rampe) na jugozapadnom dijelu otoka Čiovo zabilježeno je nekoliko marinskih firmground-a, koji su karakterizirani Thalassinoides bioturbacijama koje pripadaju Glossifungites ihnofacijesu, te s fosfatnom mineralizacijom. Bočno prostiranje donjeg firmground-a koji markira granicu između podinske formacije Dol i krovinske formacije Čiovo veće je od 5 km, što upućuje na regionalnu važnost ovog diskontinuiteta. Do naglog oplićavanja taložnih okoliša na granici između formacija Dol i Čiovo te do nastanka donjeg firmground-a, moglo je doći zbog gornjokampanskog događaja (UCE) koji predstavlja globalni eustatski pad razine mora, koji je također zabilježen i regionalno na južnom dijelu ADCP-a (npr. otok Brač). Unutar donjojurskih dubljevodnih kondenziranih vapnenaca na području Trnovskog Gozda (lokalitet Kovk, jugozapadna Slovenija) (moguće povezanih s maksimumom transgresije tijekom ranog toarcija) zabilježen je firmground karakteriziran Thalassinoides bioturbacijama koje pripadaju Glossifungites ihnofacijesu. Bioklasti bodljikaša i intraklasti madstona, koji predstavljaju komponentu krovinskih bioklastično-intraklastičnih pekstona do grejnstona koji ispunjavanju bioturbacije, zahvaćeni su procesom glaukonitizacije. Glaukonitični minerali, čija je mineralogija dokazana rendgenskom difrakcijom na prahu i X koji predstavljaju dodatan dokaz marinske omisije, mogu biti karakterizirani i zrnatim habitusom, bez mogućnosti prepoznavanja porijekla primarnog supstrata. Negativne 13C (i u manjoj mjeri 18O) vrijednosti biogenih kalkreta, koje markiraju regionalni KPg subaerski nekonformitet na tri analizirana lokaliteta na području središnje Dalmacije (formacija Gornji Humac na lokalitetu Šibenik i formacija Čiovo na jugozapadnom dijelu otoka Čiovo) i istočne Istre (gornjokredni vapnenci na području Labin-Koromačno), značajno se razlikuju od vrijednosti stabilnih izotopa ugljika marinskih vapnenaca domaćina te upućuju na procese zamjene ugljikovih izotopa primarnog CaCO3 sa CO2 koji je nastao respiracijom korijenja i razgradnjom organske tvari u tlu. Postupno povećavanje vrijednosti 13C vapnenaca s povećavanjem dubine ispod površine subaerskog izlaganja (mogući obrazac obrnutog J trenda) na lokalitetu Šibenik također upućuje na subaersko izlaganje ovih gornjokrednih sljedova. Vrijednosti 13C (s rasponom od -13,1 do -8,2 VPDB) i 18O (s rasponom od -10,1 do -6,1 VPDB) svih vrsta KPg kalkreta koje su analizirane u ovom radu su u rasponu vrijednosti kalkreta zabilježenih u literaturi, te su također u rasponu vrijednosti kalkreta koje su zabilježene u recentnim i pleistocenskim subaerski izloženim karbonatnim profilima. Vrijednosti 13C svih vrsta KPg kalkreta, a posebice drugog tipa biogenih kalkreta s tipičnim Microcodium agregatima zabilježenih na lokalitetu Šibenik (13C vrijednosti od -13,1 do -12,3 VPDB), su vrlo blizu ili na donjoj granici apsolutnih vrijednosti pedogenih karbonata koji nastaju u izotopnoj ravnoteži sa CO2 iz tla (-13 do -12 PDB). Ove se vrijednosti očekuju za autigene pedogene karbonate koji su nastali pod isključivim utjecajem C3 zajednice biljaka, te bez očitog unosa izotopno težeg ugljika porijeklom iz primarnih karbonata ili iz atmosferskog CO2. Iako tako niske vrijednosti 13C kalkreta podržavaju interpretaciju da su Microcodium agregati precipitirani pod izravnom biološkom kontrolom unutar tla, otkrivanje preciznih i točnih mehanizama nastanka Microcodium agregata (npr. rizogeni model ili stvaranje pod utjecajem metaboličke aktivnosti mikroba u rizosferi), kao i definiranje njihovog utjecaja na vrijednosti stabilnih izotopa autigenih pedogenih karbonata, zahtijeva daljnja istraživanja. Vrijednosti 13C (od -4,4 do -3,6 VPDB) rizogenih kalkreta, koje su nastale unutar firmground Thalassinoides bioturbacija koji karakteriziraju kompozitnu površinu unutar formacije Gornji Humac na lokalitetu Šibenik, su negativniji od 13C vrijednosti marinskih vapnenaca domaćina te potvrđuju da je kompozitna površina prošla kroz fazu meteorske pedo(dija)geneze. No, vrijednosti 13C kalkreta su manje negativne od vrijednosti koje se očekuju za pedogene karbonate koji su nastali pod prevladavajućim utjecajem C3 zajednice XI biljaka, što upućuje na to da ovdje zabilježene 13C vrijednosti odražavaju kontaminaciju uzoraka s asociranim marinskim karbonatom. Sekundarni karbonati u tlima nastaju u izotopnoj ravnoteži s CO2 u tlu, koji nastaje uglavnom respiracijom korijenja i mikrobijskim raspadom organske tvari u tlu. Potencijalna buduća istraživanja vrijednosti stabilnih izotopa ugljika (13C) i apsolutno datiranje metodom C-14 pedogenih karbonata (prvenstveno rizoliti i kalcificirane stanice korijenja) i asocirane organske tvari (npr. lipida), koje nalazimo u raznim vrstama kvartarnih naslaga na području Hrvatske (npr. sljedovi les-paleotlo u Istri, Kvarneru i u istočnoj Hrvatskoj), mogle bi pridonijeti boljem razumijevanju i nadogradnji paleookolišnih interpretacija u razdoblju kvartara. Ihnološke, sedimentološke, pedo(dija)genetske, mineraloške, te izotopne značajke površina diskontinuiteta analiziranih u ovom radu, kao i njihove stratigrafske implikacije, mogu se koristiti pri identifikaciji, karakterizaciji i interpretaciji diskontinuiteta prisutnih u karbonatnim naslagama različite starosti.Discontinuity surfaces resulting from breaks in sedimentation and representing hiatuses independent of their duration are key horizons in the sedimentary record. Various types of discontinuity surfaces are included under this non-interpretative term, including marine firmgrounds and hardgrounds, subaerial exposure surfaces, as well as composite (or polygenic) surfaces. Discontinuity surfaces are useful marker horizons for correlation of stratigraphic sections and are highly applicable in carbonate sequence stratigraphy. The significance of substrate-controlled trace fossil suites and calcretes, as well as authigenic marine mineralization, for genetic interpretations of discontinuity surfaces has been emphasized by many researchers. Stable isotopic signatures of pedogenic carbonates (calcretes) also represent a very valuable indicator of meteoric diagenesis, and are commonly used for paleoenvironmental reconstructions in Quaternary deposits. There are only a few records of detailed ichnological, sedimentological, pedo(dia)genetic, mineralogical and isotopic analyses of discontinuity surfaces present in the Mesozoic to Cenozoic carbonate successions of the External Dinarides, which also emphasizes the need for continuing investigation of such surfaces. Several types of discontinuity surfaces, including subaerial exposure, marine omission and composite surfaces, characterized by well-developed and indicative ichnological, sedimentological, pedo(dia)genetic, mineralogical, and isotopic signatures, were recorded within Upper Cretaceous to Paleocene carbonate deposits in central Dalmatia (southern part of the Adriatic-Dinaridic Carbonate Platform, ADCP) and Lower Jurassic deposits in southwestern Slovenia. The regional Cretaceous to Paleogene (KPg) subaerial unconformity, investigated in the Gornji Humac (ConiacianSantonian; Šibenik locality), Čiovo (Upper Campanian, southwestern part of the Island of Čiovo), and Sumartin (MaastrichtianPaleocene; western part of the Island of Hvar) Formations, is characterized by biogenic (beta microfabric) calcretes with rhizoliths, including in situ Micocodium aggregates and root tubules with alveolar-septal structure, laminar calcretes and pisoids (partly formed by bauxite calcretization), as well as typical paleokarst features (including in situ and re-sedimented speleothems). The Late Cretaceous to Paleogene regional emergence phase of variable duration on different parts of the ADCP was caused by very intensive Late Cretaceous syn-sedimentary tectonics related to the collision of Adria with the Eurasian plate. This is reflected in differentiated depositional settings within the ADCP domain during this time period. The KPg hiatus is interpreted as a consequence of diachronous and differential XIII uplifts of various parts of the platform(s) in response to diachronous forebulging in front of the approaching Dinaridic orogen. Several intraformational discontinuity surfaces, including firmground and composite surfaces, were recorded within the Gornji Humac Formation (ConiacianSantonian; intra-platform peritidal limestones) at the Šibenik locality. Thalassinoides boxworks (probably T. paradoxicus) of the Glossifungites ichnofacies characterize the submarine firmground stage in both types of intraformational discontinuities. Rhizogenic laminar calcretes developed subsequently inside burrows of the composite surface and indicate that this surface also experienced subaerial exposure. According to facies associations below and above the two recorded firmgrounds, these surfaces may have formed due to relative sea-level fall, which caused a phase of omission. Different types of trace fossils (firmground Thalassinoides bioturbations and rhizogenic laminar calcretes) recorded in the composite surface indicate that it may have experienced several stages of regression and transgression. Several marine firmgrounds, characterized by Thalassinoides bioturbations of the Glossifungites ichnofacies and phosphatic mineralization, were recorded within the Dol (MiddleUpper Campanian slope deposits) and Čiovo (new lithostratigraphic unit; Upper Campanian outer ramp deposits) Formations, located in the southwestern part of the Island of Čiovo. Lateral extent of the lower firmground (more than 5 km), which marks the boundary between the underlying Dol and the overlying Čiovo Formations, indicates at least regional importance of this firmground. Abrupt shallowing of depositional environments at the boundary between the Dol and the Čiovo Fms, together with development of the formational boundary firmground, likely correlate with the regionally recorded (southern part of the ADCP, e.g. Island of Brač) Upper Campanian Event (UCE) that represents a global eustatic fall in sea-level. Firmground, characterized by Thalassinoides bioturbations of the Glossifungites ichnofacies, was recorded within the Lower Jurassic deeper-water condensed limestones (possibly connected with the major Early Toarcian transgressive peak) at the Trnovski Gozd (Kovk locality, southwestern Slovenia). Echinoderm bioclasts, as well as mudstone intraclasts, which represent a component of the overlying bioclastic-intraclastic packstone deposits that infill the firmground burrows, show signs of glauconitization. Glauconitic minerals, which were identified by X-ray powder diffraction analysis and which represent an additional evidence for marine omission, are also characterized with granular habit (with completely overprinted primary substrate). XIV Distinctly negative 13C (and to a lesser extent 18O) signatures of biogenic calcretes marking the regional KPg subaerial unconformity in the three analysed localities from central Dalmatia (Gornji Humac Formation at the Šibenik locality and Čiovo Formation on the Island of Čiovo) and eastern Istria (Upper Cretaceous carbonates at the Labin-Koromačno area) differ significantly from carbon isotopic signatures of primary marine limestones, and reflect carbon isotope exchange of primary marine CaCO3 with CO2 released by root and rhizomicrobial respiration and subsequent precipitation of pedogenic calcrete. A trend of increasing 13C values of the underlying marine limestones with increasing depth from the subaerial unconformity (possible inverted J trend) in the Šibenik locality is also an indication of subaerial exposure of these Upper Cretaceous successions. The 13C values (ranging from -13.1 to -8.2 VPDB) and 18O (from -10.1 to -6.1 VPDB) of all calcrete types analysed here are similar to those reported from calcretes elsewhere, with the 13C values also in the range of calcretes recorded in Recent and Pleistocene subaerially exposed carbonate profiles. The 13C values of all calcrete types, and especially of the second type of biogenic calcretes with typical Microcodium aggregates at the Šibenik locality (13C values of -13.1 to -12.3 VPDB), are very close to or are at the absolute limit values for soil carbonates formed in isotopic equilibrium with soil CO2 (-13 to -12 PDB). These values are expected for authigenic pedogenic carbonates formed exclusively under the influence of C3 plant communities, and without any apparent input of heavier carbon from pre-existing carbonate deposits or from atmospheric CO2. Although such low 13C values of calcretes support the interpretation of Microcodium aggregates as being precipitated under a direct biological control within the soil, the exact formation mechanisms of Microcodium (e.g., rhizogenic, metabolic activity of rhizosphere microbes) and their influence on stable isotope signatures of authigenic pedogenic carbonates require further investigation. The 13C values (-4.4 to -3.6 VPDB) of rhizogenic calcretes formed inside firmground Thalassinoides burrows of the composite surface recorded within the Gornji Humac Formation at the Šibenik locality are more negative than the 13C values of host marine limestones, and confirm that the composite surface went through a phase of meteoric pedo(dia)genesis. However, the overall 13C values of calcretes are more positive than those expected to have formed under the conditions of prevailing C3 plant communities, and are interpreted to reflect contamination with associated marine carbonate. Secondary carbonates in soils and paleosoils may form in isotopic equilibrium with soil CO2, which derives mainly from root respiration and microbial decomposition of soil organic matter. Potential future investigation of stable isotopic carbon signatures (13C), as well as XV radiocarbon dating of pedogenic carbonates (e.g., rhizoliths and calcified root cells) and associated organic matter (e.g., lipids), which can be found in various Quaternary deposits in Croatia (e.g., loess-paleosoil sequences in Istria, Kvarner and in eastern Croatia), may bring to better understanding and improving of paleoenvironmental interpretations of the Quaternary period. Ichnological, sedimentological, pedo(dia)genetic, mineralogical and isotopic signatures of the documented discontinuity surfaces, together with their stratigraphic implications, may be used in identification, characterization, and interpretation of discontinuities from carbonate successions of different stratigraphic age

Cenomanian–Turonian Oceanic Anoxic Event (OAE2) Imprint on the Northwestern part of the Adriatic Carbonate Platform and a Coeval Intra-Platform Basin (Istria and Premuda Island, Croatia)

The Cenomanian–Turonian boundary (CTB) on the intra-Tethyan Adriatic Carbonate Platform (AdCP) is generally characterised by a transition between microbially laminated and/or bioclastic limestones to calcisphere-rich massive limestone with bioturbated intervals, organic-rich interbeds, firmgrounds, as well as neptunian dikes, carbonate turbidites, tempestites and slumped structures. Compilation of the results from two study sites in the northwestern part of the AdCP and from previous research (on Istria Peninsula and islands in the Adriatic Sea in Croatia) provides a more complete overview of geological events and paleoenvironmental conditions that transformed the formerly contiguous shallow-marine environments during this time period. For the first time, a comparison between protected inner-platform area (Barban section) and a coeval intra-platform basin (Premuda Island section) during the CTB was made. This study utilized a combination of litho-, bio-, and microfacies studies with SEM, EDS, TOC, δ13C and δ18O stable isotope analyses. The stratigraphic successions start with shallow-marine carbonate deposits of the Milna Formation that is conformably overlain by the drowned-platform deposits of the Sveti Duh Formation on the platform and by the Veli Rat Formation in the contemporaneously developed intraplatform basin. These deposits are in turn overlain by the Gornji Humac Formation, which represents re-establishment of shallow-marine depositional systems on the AdCP, whereas the deeper water environment persisted in the intra-platform basin until the Santonian. Despite diagenetic modifications of shallow-marine carbonate deposits, the results of TOC and stable isotope analyses indicate the influence of global Oceanic Anoxic Event 2 (OAE2). Combination of local and regional synsedimentary tectonics and global Late Cretaceous sea-level changes accompanied by anoxic and hypoxic conditions, extinction of numerous benthic foraminifera, diversification and expansion of planktonic foraminifera and calcareous dinoflagellates, provide new insights into the character of the CTB interval in this part of the Tethyan realm

Petrochronological study of chloritoid schist from Medvednica Mountain (Zagorje Mid-Transdanubian zone, Croatia)

The metamorphic conditions and evolution of the Palaeozoic-Mesozoic metamorphic complex of Medvednica Mountain (Zagorje-Mid-Transdanubian zone, Croatia) are still a matter of debate. The results of the investigation of five samples of metapelitic schists with the mineral association of quartz, white mica and chlorite are presented. The studied schists are part of the continental margin of Adria and were metamorphosed under upper greenschist- to amphibolite-facies conditions. The focus of this study is a sample representing the highest metamorphic grade that additionally contains chloritoid blasts. Pressure-temperature pseudosection modelling together with classical geothermobarometric calculations yielded peak metamorphic conditions of 0.94 ± 0.05 GPa and 550 ± 20 °C for chloritoid schist. Monazite in-situ U-Th-total Pb electron microprobe dating indicates two metamorphic events at 167 ± 2 Ma and 143 ± 2 Ma, which are interpreted as the time of monazite growth during two distinct metamorphic phases. The formation of the chloritoid paragenesis is related to the older event (around 167 Ma) and linked with the Middle Jurassic subduction-accretion processes of Neotethys-derived ophiolitic lithologies. The younger metamorphic event (around 143 Ma) is related to the obduction of ophiolites onto the continental margin of Adria

A time-space window between Eocene karst bauxite genesis and the first molasse deposition in the Dinaric Foreland Basin in the North Dalmatia, Croatia

Karst bauxite deposits in the North Dalmatian piggyback basin (NDPGB) are a part of the Mediterranean bauxite belt, which is the largest European bauxite deposit zone; however, there is a general lack of information regarding the genesis, age, and precursor of the bauxite deposits in this region. In this study, we combined detrital zircon U–Pb geochronology with compositional, mineralogical, and morphological data from four bauxite locations in the NDPGB to provide a new palaeogeographical and palaeoenvironmental evolution model for the Lutetian–Rupelian timeframe of the NDPGB. The Eocene climatic conditions began with the Palaeocene–Eocene Thermal Maximum event (∼56 Ma), followed by the Early Eocene Climatic Optimum (∼49 Ma) and Middle Eocene Climatic Optimum (∼40 Ma), and were completed as a cooling trend culminating around the Eocene/Oligocene boundary (∼34 Ma), with a shift towards an icehouse climate. These events were coeval with the continuous drift of the African continent towards Eurasia and the subsequent closure of the western part of the former Neo-Tethys Ocean associated with massive volcanic activity. Based on the bauxite deposits of the NDPGB, Early Eocene limestones formed in the last phase of the long-lasting Adriatic Carbonate Platform. The Middle Eocene orogenic activity resulted in an elevation in this area. High average temperatures, accelerated hydrological cycles and precipitation, and intensive continental weathering with increased volcanic carbon input resulted in favourable conditions for the development of karst bauxites at this time. Further Upper Eocene tectonic deformation of the NDPGB area resulted in the development of bauxite traps and enabled redeposition of the initial bauxite material. Subsequently, the bauxite deposits were covered with clastic carbonate molasse derived from the intensive erosion of the young Dinaric orogeny. The implications of this study are as follows. First, it provides new information on the timing of bauxitisation in the area by providing the first radiometric zircon geochronology, which refined and restricted the time window for bauxite formation in this region. Additionally, our results provide a new perspective on the possibility of aeolian precursors in karst bauxite formation and provide new constraints on the first tectonic marks of the initial Dinaric orogeny

A time-space window between Eocene karst bauxite genesis and the first molasse deposition in the Dinaric Foreland Basin in the North Dalmatia, Croatia

Karst bauxite deposits in the North Dalmatian piggyback basin (NDPGB) are a part of the Mediterranean bauxite belt, which is the largest European bauxite deposit zone; however, there is a general lack of information regarding the genesis, age, and precursor of the bauxite deposits in this region. In this study, we combined detrital zircon U–Pb geochronology with compositional, mineralogical, and morphological data from four bauxite locations in the NDPGB to provide a new palaeogeographical and palaeoenvironmental evolution model for the Lutetian–Rupelian timeframe of the NDPGB. The Eocene climatic conditions began with the Palaeocene–Eocene Thermal Maximum event (∼56 Ma), followed by the Early Eocene Climatic Optimum (∼49 Ma) and Middle Eocene Climatic Optimum (∼40 Ma), and were completed as a cooling trend culminating around the Eocene/Oligocene boundary (∼34 Ma), with a shift towards an icehouse climate. These events were coeval with the continuous drift of the African continent towards Eurasia and the subsequent closure of the western part of the former Neo-Tethys Ocean associated with massive volcanic activity. Based on the bauxite deposits of the NDPGB, Early Eocene limestones formed in the last phase of the long-lasting Adriatic Carbonate Platform. The Middle Eocene orogenic activity resulted in an elevation in this area. High average temperatures, accelerated hydrological cycles and precipitation, and intensive continental weathering with increased volcanic carbon input resulted in favourable conditions for the development of karst bauxites at this time. Further Upper Eocene tectonic deformation of the NDPGB area resulted in the development of bauxite traps and enabled redeposition of the initial bauxite material. Subsequently, the bauxite deposits were covered with clastic carbonate molasse derived from the intensive erosion of the young Dinaric orogeny. The implications of this study are as follows. First, it provides new information on the timing of bauxitisation in the area by providing the first radiometric zircon geochronology, which refined and restricted the time window for bauxite formation in this region. Additionally, our results provide a new perspective on the possibility of aeolian precursors in karst bauxite formation and provide new constraints on the first tectonic marks of the initial Dinaric orogeny

Stable Isotopic (δ\u3csup\u3e13\u3c/sup\u3eC and δ\u3csup\u3e18\u3c/sup\u3eO) Signatures of Biogenic Calcretes Marking Discontinuity Surfaces: A Case Study from Upper Cretaceous Carbonates of Central Dalmatia and Eastern Istria, Croatia

Biogenic calcretes associated with a regional Cretaceous to Paleogene subaerial unconformity and an intraformational composite (polygenic) surface in Upper Cretaceous intra-platform peritidal successions in central Dalmatia and eastern Istria, Croatia (Adriatic-Dinaridic Carbonate Platform), were analyzed for their δ13C and δ18O signatures in order to provide insight into the conditions of subaerial exposure and calcrete development. The distinctly negative δ13C signatures of biogenic calcretes marking the regional subaerial unconformity differ considerably from the δ13C values of the host marine limestones. This indicates carbon isotope exchange of primary marine CaCO3 with CO2 released by root and rhizomicrobial respiration and subsequent precipitation of pedogenic calcrete. The range of δ13C (from -13.1 to -8.2 ‰ Vienna PeeDee Belemnite standard, VPDB) and δ18O (from -10.1 to -6.1 ‰ VPDB) values of calcretes are similar to those reported from calcretes elsewhere, and the δ13C values of biogenic calcretes with typical Microcodium aggregates (-13.1 to -12.3 ‰ VPDB) at the Šibenik locality are very close to, or at the lower limit of, values for soil carbonates formed in isotopic equilibrium with soil CO2. These values are expected for authigenic pedogenic carbonates formed under the influence of C3 plant communities, without influence from heavier carbon from pre-existing carbonate and lack of input of atmospheric CO2. Such low δ13C values support the interpretation of Microcodium aggregates as being precipitated under a direct biological control within the soil, although the relationship between formation mechanisms and stable isotope signatures of Microcodium needs further investigation. The δ13C values (-4.4 to -3.6 ‰ VPDB) of rhizogenic calcretes formed inside firmground Thalassinoides burrows of the composite surface at the Šibenik locality are more negative than the δ13C values of the host marine limestones, which confirms that the composite surface went through a phase of meteoric pedo(dia)genesis. However, the overall δ13C values of calcretes are less negative than expected, which might reflect contamination from associated primary marine carbonate. This study represents the first detailed stable isotope investigation of calcretes from carbonate successions of the External Dinarides, and the results may be applied to discontinuities present in other shallow-water carbonate rock successions

Cenomanian–Turonian oceanic anoxic event (OAE2) imprint on the northwestern part of the Adriatic Carbonate Platform and a coeval intra-platform basin (Istria and Premuda Island, Croatia)

The Cenomanian–Turonian boundary (CTB) on the intra-Tethyan Adriatic Carbonate Platform (AdCP) is generally characterised by a transition between microbially laminated and/or bioclastic limestones to calcisphere-rich massive limestone with bioturbated intervals, organic-rich interbeds, firmgrounds, as well as neptunian dikes, carbonate turbidites, tempestites and slumped structures. Compilation of the results from two study sites in the northwestern part of the AdCP and from previous research (on Istria Peninsula and islands in the Adriatic Sea in Croatia) provides a more complete overview of geological events and paleoenvironmental conditions that transformed the formerly contiguous shallow-marine environments during this time period. For the first time, a comparison between protected inner-platform area (Barban section) and a coeval intra-platform basin (Premuda Island section) during the CTB was made. This study utilized a combination of litho-, bio-, and microfacies studies with SEM, EDS, TOC, δ13C and δ18O stable isotope analyses. The stratigraphic successions start with shallow-marine carbonate deposits of the Milna Formation that is conformably overlain by the drowned-platform deposits of the Sveti Duh Formation on the platform and by the Veli Rat Formation in the contemporaneously developed intraplatform basin. These deposits are in turn overlain by the Gornji Humac Formation, which represents re-establishment of shallow-marine depositional systems on the AdCP, whereas the deeper water environment persisted in the intra-platform basin until the Santonian. Despite diagenetic modifications of shallow-marine carbonate deposits, the results of TOC and stable isotope analyses indicate the influence of global Oceanic Anoxic Event 2 (OAE2). Combination of local and regional synsedimentary tectonics and global Late Cretaceous sea-level changes accompanied by anoxic and hypoxic conditions, extinction of numerous benthic foraminifera, diversification and expansion of planktonic foraminifera and calcareous dinoflagellates, provide new insights into the character of the CTB interval in this part of the Tethyan realm

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

Miocene syn-rift evolution of the North Croatian Basin (Carpathian–Pannonian Region): new constraints from Mts. Kalnik and Požeška gora volcaniclastic record with regional implications

Mts. Kalnik and Požeška gora volcaniclastic sequences hold valuable information concerning the Miocene syn-rift evolution of the North Croatian Basin, and the evolution of the Carpathian–Pannonian Region and the Central Paratethys. We present volcanological, high-precision geochronological, and compositional data of volcanic glass to constrain their tephrochronology, magmatic provenance, and timing of the initial Central Paratethys flooding of the North Croatian Basin. Based on CA-ID-TIMS U–Pb zircon ages (18.060 ± 0.023 Ma for Mt. Kalnik and 15.345 ± 0.020 Ma for Mt. Požeška gora) and coeval 40Ar/39Ar sanidine ages (18.14 ± 0.38 Ma and 18.25 ± 0.38 Ma for Mt. Kalnik and 15.34 ± 0.32 Ma and 15.43 ± 0.32 Ma for Mt. Požeška gora), Mt. Kalnik rhyolitic massive ignimbrites and Mt. Požeška gora rhyolitic primary volcaniclastic turbidites are coeval with Carpathian–Pannonian Region Miocene post-collisional silicic volcanism, which was caused by lithospheric thinning of the Pannonian Basin. Their affiliation to Carpathian–Pannonian Region magmatic activity is supported by their subduction-related geochemical signatures. Although Mts. Kalnik and Požeška gora volcaniclastics are coeval with the Bükkalja Volcanic Field Csv-2 rhyolitic ignimbrites, North Alpine Foreland Basin, Styrian Basin, Vienna Basin, and Dinaride Lake System bentonites and volcaniclastic deposits, reliable tephrochronological interpretations based on comparison of volcanic glass geochemical composition are not possible due to a lack of data and/or methodological discrepancies. Our new high-precision geochronology data prove that the initial Middle Miocene (Badenian) marine flooding of parts of the North Croatian Basin occurred at least ~ 0.35 Ma (during the NN4 Zone) before the generally accepted ~ 15 Ma maximum flooding age at the basin scale, calibrating the timing of the onset of the widespread “mid-Langhian” Central Paratethys flooding