Studies of the pre-miocene sedimentary cover of the Troodos Massif, Cyprus.
2015-11-19T09:02:55Z (GMT) by
Resting on the Cretaceous pillow lavas of the Troodos Massif, interpreted as an uplifted slice of ridge-generated ocean floor, there is a thick sequence of predominantly pelagic sediments, ranging in age from Campanian to Mid- Tertiary. The earliest pelagic sediments are umbers: iron- and manganese-rich mudstones comparable with the basal sediments of the East Pacific Rise. Umber deposits occur in hollows underlain by thin lava breccias, also by deeper zones of intensely veined and fragmented pillow lavas. Occasional thicker umber deposits are located in elongate fault-controlled depressions: within and above thick lava breccias, restricted to near the south Troodos Arakapas transform fault; and also in depressions inherited from, earlier emplacement of stratiform cupriferous sulphides. Umbers originated in the Campanian due to the waning stages of Troodos ridge volcanism. The sub-umber veining and brecciation was produced by late- stage hydrothermal activity, associated initially with a brief episode of violent volcanism, then with more quiescent discharge of submarine thermal springs from a geothermal system. Large volumes of dilute metalliferous brines were released into open marine waters whereupon umbers were rapidly precipitated. In contrast to umbers, the ochres are ferruginous, but manganese-depleted sediments, which are also associated with massive sulphide orebodies located at depth in the Troodos pillow lavas. Ochres were deposited in less oxidizing seawater, than umbers, within fault-bounded hollows; they formed by a combination of in situ submarine oxidation, erosion of sulphides and sub-adjacent lavas, and, as precipitates from submarine thermal springs. Overall, extrusion of pillow lavas, sulphide emplacement, deposition of umbers and ochres are all interpreted as related events in the evolution of the Troodos ocean ridge. Overlying radiolarites contain well preserved Radiolaria in which silica diagenesis is comparable to that of cherts encountered by the Deep Sea Drilling Project. In wholly autochthonous successions, Maastrichtian chalks follow, and show evidence of solution, indicating deposition close to the carbonate compensation depth. All these sediments are thin (tens of metres). Palaeocene and Lower Eocene chalks are much thicker (to 300 m) but very variable. They contain abundant cherts, some of which replace turbiditic beds of foraminiferal origin. Later chalks, and other sediments record, shallowing waters, a major Miocene emergence, followed by peneplanation, before renewed uplift in the Pleistocene. The uplift history of the Troodos Massif has important implications for emplacement of ocean floor at high crustal levels. In the south-west of Cyprus, a thick sequence of volcano- genic clay and sandstone occurs above the umbers and radio-larites, and beneath allochthonous nappes and melange. These include serpentinite masses, derived from Troodos- type oceanic basement, and the Mamonia rocks; sediments and volcanics of Triassic to Jurassic age, of continental shelf to pelagic facies. In general, ocean floor generation was followed by pelagic, non-calcareous sedimentation; then by intense calc-alkaline volcanism, indicative of an arc-trench gap environment; then by quiet pelagic chalk deposition. The overall tectonic setting points to subduction of Troodos ocean floor in a north-dipping subduction zone until this was finally ended by collision of the Troodos ocean ridge with the southern continental margin of the Turkish Plate. Geotectonic implications are discussed.