THE MEDITERRANEAN LOWER CRETACEOUS

1.2. Accepted Stratigraphie Schemes
1.2.1. General Remarks
The model of the stratigraphie classification of the Lower Cretaceous in Bulgaria,
which was elaborated in the previous stages, is based on the general inherent trends
in the development of this series in the Mediterranean Region and on the results
of the studies carried out so far on the lithological and faunistic successions in natural
localities and in the boreholes.
The model of the stratigraphy of the Lower Cretaceous, accepted in the present
monograph, reveals some differences with respect to the statute, volume and boundaries
of some stages and substages. Tn this book the Berriasian is assumed to be a
separate stage at the base of the Cretaceous, unlike some authors who consider it
to be part of the Valanginian (Димитрова, 1967). The boundary between
the Hauterivian and Barremian is marked between the ammonite zones of Pseudothurmannia
angulicostata and Crioceratites emerici, unlike Breskovski (1975)
who includes the beds with Pseudothurmannia at the base of the Barremian. The
Clansayesian is accepted to be the uppermost substage of the Aptian (Ни ко л о в,
1970), unlike D i m i t г о v а (Димитрова, 1967) who places it at the base
of the Albian.
Lower Cretaceous sediments are divided into stratigraphie units, differentiated
in three classification systems: (a) lithostratigraphic (groups, formations, members);
(b) biostratigraphic (mainly ammonite oppel-zones); (c) chronostratigraphic (stages,
substages).
1.2.2. Bio- and Chronostratigraphic Units
The Lower Cretaceous comprises the following stages: Berriasian, Valanginian'
Hauterivian, Barremian, Aptian and Albian. All stages are from the standard International
Stratigraphie Scale (Tabic 14).
1.2.3. Lithostratigraphic Units Related to the Lower Cretaceous in Bulgaria
Until the beginning of the 1960's, the stratigraphie classification of Lower Cretaceous
sediments in Bulgaria was represented only by means of chronostratigraphic units.
The wide regional studies and especially the investigation of borehole materials
permitted the elaboration of a very comprehensive scheme of the official lithostratigraphic
units as a primary matrix of all stratigraphie evidence.
Lower Cretaceous in Bulgaria is connected with the following lithostratigraphic
units: Glozene Formation, Slivnica Formation, Brestnica Formation, Kostel Formation,
Hânevci Formation, Zlatarica Formation, Cerni Osâm Formation, Tica
Formation, Salas Formation, Kaspican Formation, Kamcija Formation, Gorna
Orjahovica Formation, Mramoren Formation, Razgrad Formation, Vraca Urgonian
Group, Lovec Urgonian Group, Trambes Formation, Sumer Formation,
Svistov Formation, Ruse Formation, Elesnica Formation, Malo Pestene Formation,
Roman Formation, Simeonovo Formation and Spasovo Formation (Figs. 4—15).
These are the formal lithostratigraphic units with which the Lower Cretaceous
is connected over most of its distribution. In addition to them, there are some informal
lithostratigraphic units which participate in the structure of the Lower Cretaceous:
terrigenous formation (Carpathian type) revealed near the town of Kula;
Urgonian limestones (undivided) in the Western Fore-Balkan and in the Central
Fore-Balkan (Straza — Vitite Steni); Paraurgonian formation — developed around
the Urgonian complexes in Southwestern Bulgaria and in the Fore-Balkan; marl
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