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1 2006

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Jurassic radiolarians from reworked manganese nodules in an oceanic mélange, Argolis Peninsula (Greece)

Chiari M., Baumgartner P., Bernoulli D., Bortolotti V., Marcucci M., Photiades A., Principi G.

Volumina Jurassica

2006

Vol. 4, no. 1

40-40

In the eastern Argolis area, the lowermost paleotectonic unit, the Didhimi-Trapezona Basal Series includes Upper Triassic to middle Liassic shallow-water carbonates (Pantokrator Limestone) which formed the western Pelagonian margin bordering an oceanic domain. Drowning of the margin is documented by condensed pelagic limestones of Toarcian to Middle Jurassic age, followed up-section by Upper Jurassic radiolarian cherts that grade into siliceous mudstones with interbedded sandstones rich in volcanic fragments (Dhimaina Fm) documenting the erosion of ophiolites. Up-section the Potami Fm, includes, besides clasts of boninitic affinity, olistholiths of Middle Jurassic siliceous mudstones and shales. The sedimentary structures in the Potami Formation document gravitational emplacement from a nearby source, i.e., from the ophiolites that became obducted onto the Pelagonian margin in Late Jurassic time. During Tertiary, this ophiolite nappe and its basement were involved again in thrusting and nappe formation. Along the road from Angelokastron to Sofiko, 2 km east of Angelokaston a small quarry exposes pervasively sheared, dark reddish-brown, radiolarian-bearing cherty shales occurring in the footwall of a thrust bringing them into contact with the Pantokrator Limestone. Across the shales, manganese nodules, 5 to 15 cm in diameter and with a nucleus of radiolarian chert, are irregularly scattered. Several manganese nodules and several chert samples from the siliceous shale matrix were collected for radiolarian biochronological analyses. One nucleus of a manganese nodule yielded a well-preserved Liassic radiolarian assemblage with Pseudoheliodiscus alpinus Kozur & Mostler, Thurstonia magnaglobus Whalen & Carter, Paronaella ravanensis Whalen & Carter and Praehexasaturnalis poultoni Whalen & Carter. This assemblage indicates a Hettangian age. In contrast, several chert nodules from the siliceous shale matrix, yielded Middle Jurassic radiolarian assemblages. Sample GR 62F from a red chert, collected in the siliceous matrix, yielded: Palinandromeda sognoensis Baumgartner and Parahsuum(?) natorense El Kadiri; this assemblage indicates an Early-Middle Aalenian to Early-Middle Bajocian age (UAZ. 1-3). Sample GR 61 from a red chert, collected in the siliceous matrix, yielded: Striatojaponicapsa plicarum ssp. A , Tricolocapsa(?) fusiformis Yao with Eucyrtidiellum unumaense dentatum Baumgartner, Stylocapsa oblongula Kocher and Zhamoidellum ventricosum Dumitrica; this assemblage indicates a Middle Bathonian age (UAZ. 6). We suggest that the Liassic manganese nodules with their cherty cores, certainly more consolidated than their original matrix, became exhumed on the slope of an intra-oceanic accretionary wedge, were redeposited at its toe and embedded in a Middle Jurassic siliceous mudstone matrix, probably on the floor of the Early-Middle Jurassic Vardar Ocean. During the westward advancement of the east-directed Vardar subduction, the mudstones were incorporated into the prism and were finally emplaced as olistholiths on the Pelagonian margin during the Kimmeridgian, shortly before the Vardar obduction onto the Pelagonian margin.