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An Early Jurassic braided river system from Mehadia, South Carpathians, Romania

Kędzior A., Popa M. E.

Geological Quarterly

2018

Vol. 62, No. 2

415--432

The Lower Jurassic (Hettangian–Sinemurian) continental deposits of the Mehadia area (Caraş–Severin County) belong to the Presacina Basin, Upper Danubian Units of the South Carpathians, Romania. This un-named succession is exposed along large areas within the Greaţca Quarry where deposits of a complex braided river system can be seen. Nine clastic facies are identified within the Mehadia section and grouped into two facies associations reflecting two main sub-environments. (A) a fine-grained association, up to 6.5 m thick, composed of greyish black and dark grey massive and horizontally laminated mudstones interbedded with fine- to medium-grained sandstones showing ripple lamination, tabular low-angle large-scale cross-bedding and in some places massive sandstones. Within this association coarsening and fining-upward sequences are visible, while subordinate pensymmetrical-like sequences were observed. The sediments of the fine-grained association are interpreted as having been deposited along an alluvial plain from flood waters. The sandstone interbeds are interpreted as crevasse channel-fills and the thinner sandstone bodies as proximal crevasse splays. (B) a coarse-grained facies association represented by sandstones and conglomerates, both clast- and matrix-supported, from granule to pebble clast sizes. Almost all basal members of this association are underlain by erosional surfaces, while internal erosional surfaces occur at some levels. The coarse-grained facies association is interpreted as having been deposited within active river channels of a sandy braided river system. The occurrence of numerous erosional surfaces, the recurrence of the facies within this association (channel bodies), as well as the abundant mudstone and sandstone intraclasts all point to frequent conditions of erosion (including redeposition) of previously deposited sediments. The relatively low fine-grained sediment content (<25%) can be explained by two factors: (1) a creation rate for accommodation space was not enough for the accumulation of a large amount of overbank deposits; (2) a lack of larger quantities of fine-grained material being transported in the river system. Both factors may have operated independently or interacted.

Upper Cretaceous volcanoclastic deposits from the Haţeg basin, south Carpathians (Romania): K-Ar ages and intrabasinal correlation

Bojar A.-V., Halas S., Bojar H.-P., Grigorescu D., Vasile S.

Geochronometria

2011

Vol. 38, no. 2

182-188

In order to constrain the age of the Upper Cretaceous continental Densuş-Ciula Formation from the Haţeg basin, South Carpathians, and correlate it with the other continental unit that occurs in the region, the Sânpetru Formation, we separated and dated by the K-Ar method biotites and amphi-boles from volcanoclastic deposits. The mineral phases analysed are from two tuff layers and volcanic bombs cropping out near Rachitova village. Two tuff layers from the Densuş-Ciula Formation give early Maastrichtian ages of 69.8š1.3 and 71.3š1.6 Ma, respectively. The ages determined for the tuff layers constrain the age of deposition for the Densuş-Ciula Formation and enable further correlations with the available palaeomagnetic data from the deposits occurring along the Sibişel Valley that be-long to the Sânpetru Formation. The volcanic bombs collected near to Răchitova village are andesites and dacites. The age determined by K-Ar method on hornblende separated from a volcanic bomb is 82.7š1.5 Ma, which is older than the underlying Campanian marine deposits in turbidite facies. This suggests that the volcanic bombs were re-deposited during the early Maastrichtian. Thus, the volcan-ics found at Răchitova have at least two origins: one type is related to an explosive synsedimentary volcanic activity, and the other type is represented by older andesitic/dacitic bombs, which most probably originate from a volcanic centre situated in the Haţeg region.