Depositional environment, age and facies of the Middle Triassic Bulog and Rid formations in the Inner Dinarides (Zlatibor Mountain, SW Serbia): evidence for the Anisian break-up of the Neotethys Ocean

Abstract

Below the Middle to lower Upper Jurassic ophiolitic melange and their overlying ophiolite nappes of the Dinaridic Ophiolite Belt in the Zlatibor Mountain area occur olistoliths and slideblocks. These consist of Triassic carbonates and radiolarites of variable age and palaeogeographic provenance. The matrixes of these blocks are late Middle Jurassic radiolarites and clays. The different carbonate rocks were commonly interpreted to derive from the near-by Drina-Ivanjica Unit. In contrast, the radiolarites should represent the original sedimentary cover of the ophiolitic rocks of the Dinaridic Ophiolite Belt, i.e. the obducted oceanic crust of the Neotethys Ocean, originally located far to the east. The carbonate blocks in the Zlatibor (Sirogojno) melange reach several tens to hundreds of metres in size, occasionally even kilometres. Several olistoliths and blocks contain well-preserved parts of the Middle Triassic sedimentary succession. Their stratigraphy and facies evolution allowed the reconstruction of a sedimentary succession originating from the same palaeogeographic provenance of a relatively proximal passive continental margin setting, located originally east of the Drina-Ivanjica Unit. Different red nodular limestones of the Bulog Formation were deposited on top of a drowned Middle Anisian (Pelsonian) shallow-water carbonate ramp; beside condensed sections of red nodular limestones equal-aged thick successions with megabreccias occur, indicating the creation of steep fault escarpments and rapid subsidence. In contrast to this continuous sedimentary succession, Triassic sections of the relatively autochthonous Drina-Ivanjica Unit indicate Late Pelsonian uplift of the Middle Anisian carbonate ramp. After a hiatus (Late Pelsonian to Early Illyrian), deposition of grey cherty limestones with shallow-water debris (newly described as Rid Formation) started in the Middle to Late Illyrian. The focus of this paper is on the age, the depositional environment and the facies characteristics of the Late Anisian hemipelagic successions. Based on this study it is concluded that in the Inner Dinarides domain the Middle Anisian Neotethyan break-up resulted in the generation of a horst-and-graben topography. Blocks were uplifted in a rift shoulder manner and asymmetric basins were formed. Mass flows and slide blocks were mobilized along normal faults of the evolving western passive continental margin of the Neotethys Ocean.