Journal of Conference Abstracts

The Response of Carbonate Accumulation to Tectonic Subsidence and Eustacy in a Jurassic Platform of the Pelagonian Zone of the Hellenides

Rudolph Scherreiks (rudolph.scherreiks@iaag.geo.uni-muenchen.de)¹, Oskar Ebli² & Felix Schlagintweit

¹ Geologische Staatssammlung, Luisenstr. 37, 80333 Munich, Germany

² Institute of Palaeontology,University of Munich, Richard-Wagnerstr.2, 80333 Munich, Germany

The stratigraphic section, located north-east of the Xiron massif in northern Evvoia, Greece, consists of a generally deepening upward facies succession, composed of peritidal facies at the bottom, intercalations of reefal rudstones and siliceous carbonate mudstones in the mid-section, and of neritic debris carbonates, radiolarites, and greywackes towards the top. This facies succession and minor recurring shallowing upward cycles indicate that carbonate accumulation responded to an interplay of tectonic subsidence and oscillating sea-level (Scherreiks, 1998). The section culminates with a tectonically emplaced mélange and ophiolite nappes (Baumgartner & Bernoulli, 1976). Facies distributions and thicknesses indicate that the platform deepened and was starved towards the direction from which the ophiolite was emplaced.

* The early platform environments were characterised by tidal flats, back barrier lagoons, barrier-beaches, and fore-slope environments, documented respectively by dolomitic stromatolites and pisolites, exhibiting vadose diagenetic features; bioturbated dolo-mudstones and gastropod floatstones; cross-bedded oolites and imbricate molluscan rudstones; and hummocky mudstones. Regional comparisons indicate an age-range, for this part of the section, of Upper Triassic to about the Late Lower Jurassic. Carbonate accumulation kept pace with hundreds of metres of platform subsidence, while maintaining shallow peritidal environments, during a period of plate spreading which began with the break-up of the super-continent Pangaea. Metre-scale shallowing upward cycles, each culminating in vadose facies, show that 3rd-5th order eustacy superimposed the 2nd order Zuni cycle of general Global sea-level rise (Plint et al.,1992).

*· The mid-section consists of shallowing parasequences of nodular chert-spiculite mudstones and oolitic grainstones, and an intercalated reefal rudstone higher up in the section. The change from peritidal facies to these parasequences of neritic and deeper water facies signals an increased rate of tectonic subsidence with superimposed cyclic eustacy, which apparently was also favourable for the establishment of coral/hydrozoan reefs during the Kimmeridgian and Tithonian. The stepped up platform subsidence is interpreted to have begun with intra-oceanic plate convergence, dated by Spray & Roddick (1980) at 180-170 Ma, the typical metamorphic age of the amphibolite components in the mélanges at the base of the Eohellenic ophiolite nappes. The tectonically induced subsidence and the increased eustatic sea-level rise of the Upper Jurassic, although conducive for reef accommodation initially, inevitably led to their drowning and burial beneath sponge-spiculite muddy facies.

*· The upper part of the section begins with a succession of neritic and reefal debris-limestones which interdigitate with spiculites and radiolarites, followed by a succession of radiolarites with intercalated turbiditic greywackes towards the top. This part of the section documents rapid tectonic subsidence and the final collapse of the platform below the concomitant CCD prior to the tectonic emplacement of the Eohellenic ophiolite nappe during about the latest Tithonian (Baumgartner & Bernoulli, 1976; Jacobshagen et al., 1976; Robertson, 1991).

The results of this study have been encouraging in giving some insight into the changing modes of carbonate accumulation in a plate tectonic scenario with superimposed eustacy.

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Robertson AHF, Geol Mag, 128, 2-41, (1991).

Scherreiks R, Terra Nostra, 98/1, 72-73, (1998).

Spray JG, Roddick JC, Contr Mineral Petrol, 72, 43-55, (1980).

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