New occurrence of rocky-coast facies near Ronov nad Doubravou (Kolín lithofacial development, Bohemian Cretaceous Basin)
Published online: 2022-05-04 A rocky-coast facies in the Bohemian Cretaceous Basin
(BCB) are developed mainly in the Kolín lithofacial area at its southern
margin. Ronov – the newly described locality – is situated south
of Ronov nad Doubravou, on the right bank of the Doubrava river
near the St. Kříž church. The outcrop located in a ditch was partially
excavated in order to study the character and extent of Cretaceous
sediments (Beds No. 1–3; see Figs. 1–3). Bed 1 is composed of red clay with yellow streaks, representing
lateritically weathered crystalline bedrock – likely amphibolites
and garnet migmatites (Fig. 2). Overlying bed 2 consists of yellow
to gray-green clay; in the lower part, the layer contains fragments
of fossils that occur in the overlying layer, and sandy “intraclasts”
interpreted as bioturbations. The most conspicuous bed 3 (25 cm in
thickness) is represented by calcareous, coarse-grained sandstone
to conglomerate with glauconite and abundant fauna (mainly
fragmentary; Figs. 4 and 5). This type of rock in the wider area
(between Ronov nad Doubravou and Žleby) was reported by Sejček
(1982) who described three occurrences as scree or fragments on
the surface but has not found any rock outcrops in situ despite
excavation work. The individual layers were sampled for the study of foraminifers,
calcareous nannoplankton and macrofauna. Shells of Rhynchostreon
suborbiculatum, which are always disarticulated, often fragmented
and bioturbated (Entobia exogyrarum), represent the most prominent
element of macrofauna. In addition, Rastellum diluvianum and
Amphidonte halitoidea, unspecified large colonies of bryozoans, spines
of the echinoids Cidaris vesiculosa, complete shells of brachiopods
Cyclothyris aff. difformis and tiny teeth of sharks Otodus rarely
occur. Calcareous nannofossils were found only in bed 3. Isolated
small, mostly difficult to identify fragments belong to long-range
stratigraphic taxa (Fig. 6) and may indicate an interval from the
uppermost Cenomanian to the lowermost Turonian. Similar poor
nannofossil assemblages are known also in other rocky-coast facies
of the BCB. The refining indicator of stratigraphic classification is thus
macrofauna. Similar communities with Rhynchostreon suborbiculatum
and echinoderms are known from Upper Cenomanian and Lower
Turonian in the area of Heřmanův Městec, Čáslav and especially
Kutná Hora. However, brachiopods Cyclothyris aff. difformis and
oyster Amphidonte halitoidea recorded from Ronov are typical
representatives of Upper Cenomanian “rocky-coast” faunas, whereas
typically Lower Turonian elements such as the oyster Exogyra
reticulata, Exogyra sigmoidea, or the spines of the Cidaris sorigneti
are missing in the studied community. The study of macrofossil and nannofossil communities shows
that the Upper Cretaceous rocks, represented by the calcareous
glauconitic sandstones to conglomerates in Ronov locality, are most
likely of the Upper Cenomanian age and represent the Korycany
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