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Palynology and palynostratigraphy of sub- and surface Triassic, Eastern Adriatic, Croatia: aspects of palaeoclimate, geodynamics and halokinetic facies

Palynological characterization of Triassic deposits from the subsurface of on- and offshore wells and surface sites of the Croatian eastern Adriatic, which belong to the undisturbed part of Adria, resulted in definition of palynological Interval Zones and their palynostratigraphic dating. Triassic deposits of Vlasta-1 exploration well are characterized by eight sporomorph Highest- occurrence Zones (HoZ) suggesting continuous sedimentation of predominantly evaporites during the Ladinian, Carnian, and Norian and predominantly dolomites during the Rhaetian. Oldest dimorphus HoZ was dated as Fassanian- early Longobardian continuously followed by scheuringii HoZ/late Longobardian, magmus HoZ/ Cordevolian, meieri HoZ/Julian, secatus HoZ/Tuvalian, spirabilis HoZ/Lacian, traversei HoZ/ Alaunian-early Sevatian, and germanicus HoZ/late Sevatian-Rhaetian. Eight optional sporomorph Interval Zones were defined in terms of Lowest-occurrence Zone (LoZ) and were meant for palynological zone-to-zone correlation in order to provide a potentially higher resolution of the chronostratigraphy. Oldest dimorphus LoZ was dated as Fassanian-early Longobardian continuously followed by vigens LoZ/late Longobardian-earliest Cordevolian, summus LoZ/Cordevolian-early Julian, maljawkinae LoZ/middle Julian, traversei LoZ/late Julian-Tuvalian-earliest Lacian, rudis LoZ/Lacian-early Alaunian, pseudomassulae LoZ/late Alaunian-Sevatian-early Rhaetian and germanicus LoZ/middle-late Rhaetian. Both germanicus HoZ as well as upper part of pseudomassulae LoZ and germanicus LoZ are associated with the dinoflagellate cyst rhaetica-swabiana Concurrent-range Zone (CrZ) dated as late Sevatian-Rhaetian. The correlation of undisturbed Triassic successions in Vlasta-1 (levelled to the Triassic-Jurassic transition) of the boreholes considered in this work, shows a very good boundary match of palynological zonation, palynostratigraphy and inferred chronostratigraphy together with informal Gamma Ray markers. The correlation, which includes also the diapiric structures, shows that Triassic palynostratigraphy of Vlasta-1 reflects the primary depositional relationships of the Croatian central Adriatic sub- and surface lithostratigraphically contemporaneous deposits before being deformed by halokinetic diapirism. For the first time outside North America, Froelichsporites traversei has been traced throughout its known late Carnian-Norian range being associated with upper meieri HoZ, secatus HoZ, spirabilis HoZ and traversei HoZ (traversei LoZ, rudis LoZ and lower pseudomassulae LoZ). These Interval Zones have shown effectively the potential to serve as interregional palynofloral correlation zones with coevals in SW and E North America and Portugal. Interregional palynostratigraphic correlation of Vlasta-1 with coevals in Northern America, Europe (Alpine Triassic and Germanic Triassic, CEB), circum-Mediterranian and NW Australia revealed a relatively high degree of consistency. Ladinian palynostratigraphy of Vlasta-1 (dimorphus HoZ-scheuringii HoZ) correlates excellently with the Ladinian of Alpine and Germanic Triassic as well as with circum-Mediterranean. The Carnian of Vlasta-1 (magmus HoZ, meieri HoZ and secatus HoZ) is also highly in accordance with palynostratigraphically dated European and circum-Mediterranean Carnian sections, in addition those from Northern America. At present, interregional Norian palynostratigraphy shows some uncertainties due to several factors. Neverthertheless, it has been demonstrated that the Norian in Vlasta-1 (spirabilis HoZ-traversei HoZ) is, from the palynological and sedimentological point of view, represented by continuous deposition within the continuous Ladinian-Rhaetian succession. This allowed a very good correlation with the continuous continental Carnian-Norian deposits of the Newark Basin in Northern America due to F. traversei. So far, the upper part of traversei HoZ may only be tentatively correlated with European, circum- Mediterranean and NW Australian sections because they do not contain F. traversei except very few sections where it occurs within late Carnian-early Norian stratigraphic range. The rhaetica-swabiana CrZ (associated with germanicus HoZ) correlates surprisingly well with the Rhaetian dinocyst record from England, Alpine facies, Germanic Triassic (CEB) and NW Australia. In the part of Adria of the central Adriatic Sea, no evidence of an intra-Carnian “pluvial phase”, i.e. “humid event” characterized by hygrophytic palynological assemblages and a shift to siliciclastic sediments, could be provided. The record from Vlasta-1 and other sub- and surface sites in the central Adriatic Sea indicate that in this part of the westernmost Neotethys realm stable (semi-)arid conditions prevailed throughout the evaporitic Carnian with predominantly xerophytic palynoflora lacking “humid pulses” as reported from N America, Europe or elsewhere. In particular, the palynological record of the Cordevolian magmus HoZ and Julian meieri HoZ in Vlasta-1 (represented by ̴500 m thick subaquatic halite succession) as well as secatus HoZ (anhydrite-dolomitic succession), suggests (semi-)arid climate. Similarly, throughout the Norian-Rhaetian no evidence of an (semi-)arid-humid shift could be provided. The Norian in Vlasta-1 and Vlatka-1 and at other sub- and surface sites of the study area is characterized by predominantly evaporite- dolomitic facies with predominantly xerophytic palynoflora. The Rhaetian, although represented by dolomitic facies, contains Corollina spp. dominated assemblages suggesting even harsher, warmer (semi)-arid climate which is substantiated by the occurrences of anhydrite at different stratigraphic levels within the Rhaetian. Sedimentologically, evidence of any increased siliciclastic input within the Italian Burano facies (Carnian-Norian membro evaporitico and Rhaetian membro dolomitico) could not be provided. The lack of any increased siliciclastic input into the Carnian-Norian-Rhaetian successions may reflect the “intra-oceanic” position of Adria in the westernmost part of the Neotethys. However, it is the predominantly xerophytic palynoflora that indicates the (semi-) arid climate, probably in a zonal pattern during that period as indicated by interregional correlation of the palynological record at lower palaeolatitudes including Adria. While the Triassic of Vlasta-1 is represented by continuous Ladinian-Rhaetian palynofloral record, in Rovinj-1, Pula-1, Susak more-1 and Maja-1 the palynofloras of scheuringii HoZ, magmus HoZ, meieri HoZ, secatus HoZ and spirabilis HoZ (upper part of dimorphus LoZ, vigens LoZ, maljawkinae LoZ, traversei LoZ and lower part of rudis LoZ) are missing. These relationships suggest a Longobardian-Lacian hiatus and the onset of Alaunian-early Sevatian palynoflora of traversei HoZ (upper rudis LoZ and lower-middle pseudomassulae LoZ) continuously followed by germanicus HoZ (upper pseudomassulae LoZ and germanicus LoZ) and rhaetica-swabiana CrZ, respectively, which was recognized in all other boreholes of the study area, except in subsurface of Kornati more-1 and Vis-1 as well as surface of Vis (Komiža) and Palagruža Islands due to halokinetic diapirism. Palynostratigraphically dated tuff bearing Ladinian sub- and surface sediments regularly contain a variety of reworked Permian-early Anisian palynomorphs as well as variable quantities of tuff and/or tuffite. These relationships are interpreted in terms of depositional conditions created on Adria during the Ladinian aborted rifting phase, i.e. syn- rift uplift and syn-rift subsiding and opening that resulted in exposure to erosion of older sediments and accompanying volcanic activity, respectively. Middle Triassic subcrustal doming and subsiding created on Adria different depositional settings resulting at places with hiatuses within the Longobardian-Lacian range and at other places in continuous deposition of thick series of halite and other types of evaporites during that period. In Kornati more- 1 diapiric structure, the origin of tuff and/or tuffite, associated with chaotically distributed evaporites palynostratgraphically dated within Carnian-Norian range, is likely related to the middle-early late Carnian volcanic activity in the eastern Mediterranean which agrees with occurrences of tuff within undisturbed late Carnian evaporitic successions of Vlasta-1. Once the subsidences were filled up with various types of evaporites during the late Longobardian-Lacian hiatus, the onset of Alaunian (middle Norian) evaporite-dolomitic sedimentation spread all over the study area and lasted up to late Norian (early Sevatian) which marked the final crustal stabilisation of Adria. The transition early-late Sevatian was marked by transgression and a shift from supratidal-lagoon-intertidal facies with evaporite-dolomitic deposition to shallow subtidal marine facies with dolomitic deposition spreading all over the study area which lasted throughout the Rhaetian. Adria remained relatively undisturbed up to recent times as substantiated by continuously deposited Alaunian-Rhaetian successions conformingly overlain by an almost uniformly thick series of Jurassic-Cretaceous carbonate successions of the Adriatic Carbonate Platform (AdCP), which was recognized in the whole study area (including the Italian areas of the Apulian Carbonate Platform and the deep sea sediments of the Adriatic Basin) except those parts disturbed by halokinetic diapirism. It is herein considered that since the times of Ladinian aborted rifting, Adria gradually stabilized up to middle Norian and remained relatively undisturbed up to recent times because it was out of the range of compressional tectonics delineated by the outmost Dinaric frontal thrust to the NE and Apenninic frontal thrust to the SW, respectively. As opposed to former stratigraphic position of the halokinetic facies within Permian-Early Triassic range, on the basis of the herein presented palynostratigraphic dating it is for the first time documented that the age of the halokinetic facies in the central Adriatic, responsible for creating of many salt swell, salt dome and diapiric structures, is within the Ladinian-Carnian range. Along with undisturbed Triassic successions in Vlasta-1, within which almost all of the Ladinian-middle Carnian evaporites are represented by at least 700 m thick halite deposits, the suggested age of the halokinetic facies is substantiated also by the palynological record from halite and other types of evaporites as well as evaporite- carbonate-siliciclastic-tuffitic deposits within the diapiric structures of both sub- (Kornati more-1, Vis-1) and surface (Vis and Palagruža Islands) of the Croatian central Adriatic. In the area around Kornati-more-1, the halokinetic facies was likely developed within the Carnian range because within the diapiric structure no evidence of older palynoflora could be provided. At present, evidence of Permian and/or Early Triassic fragments of evaporites within the diapiric structures, which contain only sporomorphs within that range, could not be provided in the sub- and surface deposits of the study area. All known surface occurrences of basalt- andesite and associated volcanic rocks in the Croatian central Adriatic are related to the diapiric structures because the effusions were syngenetic with the Ladinian part of the halokinetic facies. In the later dynamics, the halokinetic facies set in motion and uplifted to the surface also basalt-andesite boulders ; “volcanic” islets Brusnik and Jabuka as well as andesite boulders and volcanic agglomerates within the chaotic volcanogenic-sedimentary complex of Komiža field (Vis Island) and the uppermost part of Vis-1 borehole. In areas lacking halite sedimentation syngenetic with the Ladinian extrusions, e.g. Rovinj-1 where the extrusive volcanics are associated with and overlain by the siliciclastic-carbonate- tuffitic horizon palynostratigraphically dated as Ladinian, the magmatic rocks remained in their primary subsurface position conformably overlain by a thick series of upper late Triassic-Cretaceous carbonates of the AdCP. In the central and south Adriatic, there are currently no drilling data (both Croatian and Italian side) on the primary position of Ladinian extrusions. Given that each sample (both sub- and surface) of the tuff/ tuffite bearing halite-evaporite-carbonate- siliciclastic deposits (lower part of the Ladinian-middle Carnian halokinetic facies) contained palynoflora allowing palynostratigraphic dating as Ladinian, it is herein considered that the magmatic activity is related to the Ladinian magmatic zone outlined as Vis (Komiža) – Rovinj-1 magmatic zone. The still active diapirism suggests that the main source of seismicity in the central Adriatic is due to halokinetic tectonics. According to the data presented herein, Adria microplate remained a relatively stable, undisturbed palaeogeographic unit during its long history lacking complex tectono-structural elements of the surrounding circum-Adriatic frontal thrust belts.

Palynology, palynostratigraphy, Triassic, palaeoclimate, geodynamics, halokinetic facies, eastern Adriatic, Croatia

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