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Late Eocene megabed with glass sphaerules and microtectites in eastern Adriatic flysch (Croatia)

Glass sphaerules occur at 4 levels in Late Eocene marls of Central Dalmatian (Eastern Adriatic) flysch basin. The best studied, and with most sphaerules recovered is a composite turbidite 2-7, which is the youngest in the sequence and belongs to Turborotalia cerroazulensis-hantkenina spp zone. The 2-7 bed comprises impact-generated debris in its marl part, which is represented by perfectly clear (or just slightly dimmed) glass sphaerules 91 to 530 μ across ; teardrop transparent microtectites 1225 μ long, and fragments of vesicular glass 450 to 1000 μ across. Some sphaerules and microtectites comprise fluid inclusions with diameter up to 41 μ . Composition of the sphaerules (28, 9 % SiO2, 17, 22 % CaO, 7, 84 % MgO, 1, 17 % SO3, 41, 01 CO2) indicates possible sedimentary target. The 2-7 composite turbidite is 14 m thick amalgamated unit, made of at least 7 normally-graded units each of which displays abundant dish and pillar water-escape structures. The arenite part of the bed is capped by 5, 7 m thick massive marl. This marl is overlain by 18, 5 m thick marl which is composed of alternating laminated and massive units, without any arenite interbeds. Impactogenic debris occurs in lower massive marl, and exclusively in laminated portions of the overlying marl. The massive marl interbeded with the laminated one, is practically sterile, both with respect to impactogenic debris and fauna. The 2-7 megabed was interpreted as a surging flow deposit. The arenite part of the megabed was deposited from turbulent flows, hence they represent true turbidites. Individual flows carried identical debris what documents the same source, and the deposition was very rapid as documented by water-escape structures. Individual flows followed closely one another, because no mud caps were deposited between, but instead, erosion created amalgamation of the beds. Since the flows had identical source (made primarily of biogenic debris) and followed one another in very close succession, we are inclined to interpret the whole arenite bed as essentially a single-event deposit, created by waning surging turbidity current. Capping massive impactite-bearing mud was deposited from ponded turbidity current tail, whereas the impactite-bearing laminated marls probably deposited from low concentration turbidity currents which originated by slope failures and redeposited some of previously deposited marls. Interbeded massive marls were probably deposited from turbidity current tails.

flysch; megabeds; impactite; sphaerules; Eocene

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