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Voluminous Lower Pleistocene volcaniclastic deposits from the central and northern Adriatic off-shore (Croatia): where were the edifices?

The Adriatic offshore was perivolcanic area during the Quaternary. Relatively numerous tephra layers and volcaniclastic deposits of Quaternary age were reported throughout the region. Volcanic areas from the Tyrrhenian belt of the central-southern Apennines were supposed as material source region for these layers although a speculation constraining tephra supply from presumed Quaternary volcanoes located east of the Central Adriatic Swell also exists. We focused investigation on voluminous Lower Pleistocene volcaniclastic deposits recognized as marker beds in the middle and northern segment of the Croatian part of Adriatic offshore. Pliocene and Quaternary marine sedimentary succession transgressively overlies strongly tectonized Upper Triassic to Miocene basement. It is predominantly composed of alternating marls and fine grained sandstones. Pliocene sediments are about 30-230 m thick whilst the Pleistocene segment of the succession ranges in depth from several hundred to about 1535 m. Transition from Pliocene to Pleistocene deposits was identified by the first appearance of planktonic foraminiferae Globorotalia truncatulinoides (d’ ORBIGNY) and benthic foraminiferae Hyalinea balthica (SCHROETER). Three Pleistocene marker beds GR, L3 and L2 were recognized on electric logs, and qualified later as volcanoclastics by qualitative interpretation of gamma radiation in about sixty wells drilled in the middle and northern Adriatic. Marker beds L3 and L2 are composed of calcareous smectitic clays (bentonites) and have characteristic individual shapes on GR curve correlative within the area of about 500 km2. The increased natural radioactivity of these marker beds is ascribed to thin (< 2 cm) yellowish severely altered pyroclastics interbedded within marine shale. The oldest marker GR was not cored, other two were available in few drill-hole cores. Clayey core samples taken from the marker intervals were analysed after dissolving in 10% HCl to remove calcite. Zeolitized interliers were analysed in natural state. For mineral analysis samples were suspended and sieved to remove fraction less than 32 µ m. Minerals in this fraction were identified by XRD. Individual minerals and glass shards in the fractions > 32 µ m were concentrated by separation in heavy liquids and by magnetic separation. Concentrated fractions were mixed with epoxy glove and polished for microprobe analysis. Interval 929-931 m from Ksenija-1 well was chosen as the best representative of the younger marker layer L2. The layer is composed of loose calcareous montmorilonite clay containing abundantly fresh glass and volcanic crystals (biotite, sanidine, plagioclase, clinopyroxene, amphibole) and minor eolian particles commonly found in loess deposits (quartz, albite, epidote muscovite, garnet, chlorite). No opaque phases were detected possibly due to eolian fractionation. Close to the bottom of the marker bed a light yellow dense 1 cm thick interlayer occurs composed entirely of volcanic glass and minerals. Glass particles in both units are pumice and compact glass shards showing various morphologies. They are fresh with microprobe totals ranging from 94.07 to 97.44 wt.%. The glass particles are K-rich (K2O = 4.41-6.67 wt% ; K2O/Na2O = 1.7-3.3). In diagrams (Na2O+K2O) against SiO2 and K2O against SiO2 they plot in the field of rhyolite and shoshonitic rhyolithe to high-K calc-alkaline rhyolite, respectively, as two separated groups. Chemical composition of volcanic minerals is consistent with a potassium very rich volcanism of the southern Italian Pliocene-Quaternary volcanic provinces. The older marker layer L3 was sampled in J-15/6 and J-11/2 wells at depth of 915 m and 928 m, respectively. Both layers are composed of dense calcareous montmorilonite clays with a 1.5 cm thick dense ash interlier in the latter. Compared to the marker layer L2, in the selected drill-wells, the marker layer L3 is apparently less voluminous. Volcanic glass was extracted only in the drill-hole J-15/6 beside volcanic crystals which are especially abundant in the zeolitized (phillipsite, analcime) ash interlier. Glass fragments (mostly pumice) are hydrated (microprobe totals 93.01-94.19 wt.%) but not virtually devitrified, possibly being too large (> 80 μ m) to be affected by alterations. In the diagram (Na2O+K2O) against SiO2 the glass fragments plot in the field of phonolite close to the field of trachyte and in the diagram K2O against SiO2 cluster in the silica undersaturated potassic alkaline field (K2O = 8.58-9.86 wt% ; K2O/Na2O = 2.5-3.0). Volcanic crystal population comprises identical minerals as in the rhyolite tephra of the marker layer L2 but shows significantly different chemical compositions. The tephras, primarily deposited on-land, were redeposited into marine environment. Paleoenvinronmental reconstruction based on predominance of planktonic (Globorotalia truncatulinoides (d’ ORBIGNY) versus benthic foraminiferae suggests Early Pleistocene (beginning of Donau-Günz interglacial) deposition in temperate water and different environments. The redeposited tephras from L3 marker interval were found in shallow segment of inner neritic (J-11/2 well) and in outer neritic setting (J-15/6 well). The association of benthic foraminiferae of younger marker L2 in KSE-1 well is very instructive, showing how the depositional environment changes within three meters interval from inner neritic to outer neritic, due to the fast deepening. Bimodal rhyolitic glasses of marker L2 suggest volcanic ashes of two different explosive events redeposited together into the single layer. That is very similar to case history of volcanoclastic turbidite layer vt-7538 on site 974, deposited around 1.8 Ma ago in Tyrrhenian basin. The markers L2 and vt-7538 are nearly identical to PLM-2 rhyolite tephra from the large rhyolitic dome from Palmarola islet, Pontian islands, dated to 1.75 Ma. For the older marker L3 we did not find any correlative Early Pleistocene tephra of the Mediterranean area described so far. Such high-K tephras, involving silica undersaturated phonolite to trachyte pumices, are very similar to younger volcanism in the Roman-Campanian Magmatic Province dated to 1.5 Ma.

Adriatic off-shore; Lower Pleistocene; volcaniclastic deposits; rhyolite and phonolite glasses; Palmarola source volcano(es)

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