Ofioliti

Volume 21, Issue No. 1, 1996

ACTIVITY IN WESTERN TETHYS: EVIDENCE FROM THE KALUR CHERTS OF THE MIRDITA OPHIOLITES (NORTHERN ALBANIA).

I.W. Aiello*

* Dipartimento di Scienze della Terra, Universitˆ di Firenze, Italy

Key-words: Kalur Cherts, radiolarites, bottom-currents, clay-minerals, Mirdita,Jurassic, Tethys. Albania.

ABSTRACT

The Middle Jurassic Kalur Cherts of the Mirdita region in northern Albania (for the first time formally defined) provide new information about the mechanisms that control the rhythmicity of bedding in cherts and the post-depositional reworking of radiolarian-rich sediments. The Kalur Cherts were deposited in two different tectonic settings: Western Mid Oceanic Ridge (MOR) type ophiolites and Eastern Supra-Subduction (SSZ) type ophiolites. In this paper we present the stratigraphic and sedimentologic analysis of seven chert section overlying the Western ophiolite belt in the Central Mirdita area. Depending on their geographic position, chert sections have been grouped in southern type (ST) and northern type (NT). In both (ST) and (NT) Cherts the lower part of the formation is dominated by the alternation between well laminated, red, radiolarian chert beds and paper-thin siliceous claystones. Lamination is mainly the product of bottom-current activity that winnowed the radiolarian oozes shortly after their deposition, while the rhythmicity of chert bedding can be explained by cyclic variations of radiolarian productivity. The distribuition of clay-species in Kalur Cherts suggests a different source for clays in (ST) cherts and clays in (NT) cherts during the deposition of the lower part. In the upper part of the Kalur Cherts, the claystone interbeds are thicker, and several chert beds are redeposited as fine-grained turbidite. Moreover, the rhythmic bedding seems mainly to be the product of terrigenous dilution cycles.