VOLUME 47 (1)/May 2018

No abstract is available for this article.

Fossil sea lilies of the genus Isselicrinus Rovereto, 1914 were found near the village of Raven, Momchilgrad Municipality, in the Eastern Rhodopes in 1978. They were recovered from the rocks of the Raven complex. The latter is composed of rhyolite pyroclastics with layers of reef limestones. With the Ar/Ar method, the pyroclastics of thе Raven complex have been dated as 31.19–30.97 Ma (early Oligocene). The presence of reef limestones suggests shallow coastal waters with normal salinity and a warm climate, in which Isselicrinus lived. These were the outskirts of a tropical basin, inhabited by faunas, which today have their descendants in the tropical areas.

The origin of dolomitized burrows (Cruziana ichnofacies) in limestones and dolomitic layers in limestone-dolostone ribbon rocks was studied on the basis of petrography, X-ray diffractometry, geochemical data and isotope signatures of the dolomites. Selective dolomitization of the burrows with a local source of Mg occurred in a near-surface setting from non-evaporitic solutions with low Mg/Ca ratio. The low-temperature precipitation of non-stoichiometric (with Ca excess) dolomite was microbially mediated and controlled by bacterial sulphate reduction. The carbon for dolomite formation was largely derived from seawater and/or dissolution of precursor carbonate sediments, and partly derived from the decomposition of organic matter. Selective dolomitization of primarily argillaceous carbonate layers in the ribbon rocks was mainly associated with an autochthonous source of Mg. Clay mineral transformations (i.e., illitization of smectite) at intermediate burial depths supplied Mg ions for the formation of Ca-rich (locally ferroan) dolomite. However, differential diagenesis of the originally clay-rich and clay-poor layers may also have favoured early dolomite precipitation, whereupon contemporaneous seawater was the primary Mg source during shallow burial. The obtained results show that various factors control the formation of fine-grained, non-stoichiometric dolomite with macroscopically recognizable fabric-selective character of replacement.