THE MEDITERRANEAN LOWER CRETACEOUS

eds formed during the Early Cretaceous, in which there is a chain of ammonite
successions from the Berriasian (Pseudosubplanites grandis Zone) to the Albian
(Stoliczkaia dispar Zone) included at the top. The Lower Cretaceous includes marine,
lacustrine-swamp and continental sediments.
2.1. Lower Boundary
One of the most debatable problems in Mesozoic stratigraphy concerns the boundary
between the Jurassic and the Cretaceous Systems, i. e. the lower boundary of the
Lower Cretaceous. Over the past twenty years this problem has been discussed at
several specialized symposia (Lyon, 1963; Lyon—Neuchâtel, 1973; Moscow,
1975; Sofia—Elena, 1977; Munich, 1982). According to a number of authors, the
boundary between the Jurassic and the Cretaceous Systems should pass along the
boundary between the ammonite zones Paraulacosphinctes transitorius (Malbosiceras
chaperi Subzone in Southeastern France) and Pseudosubplanites grandis.
Гп this way the Berriasian is at the basis of the Lower Cretaceous. According to other
researchers, the Jurassic System should include the Berriasian as well, while the Cretaceous
should start from the Valanginian Stage.
This problem will not be discussed in detail here, because there is abundant relevant
literature. Only some essential points will be outlined.
At the end of the Jurassic the progressive differentiation both in the palaeogeogiaphic
environment and in the faunistic provinces became very clear (U h-
1 i g, 1911; A г к e 1 I, 1956). This objective picture of clearly manifested provincialism
at the end of the Jurassic and at the beginning of the Cretaceous Period
is reflected in the definitions of the stages during this interval, resulting in numerous
difficulties in the correlation and synchronization of the sediments from the different
provinces. Therefore, there exist two parallel stratigraphie classifications of
the boundary beds between the two systems in the Tethys and Boreal Realms (A rk
e 1 1, 1956). The problem is further complicated also by the circumstance that
in a number of regions there exist continental formations at the Jurassic-Cretaceous
(Purbeckian-Wealdian) boundary.
On the basis of the evolution of the ammonite faunas and of some intrinsic
trends in the geological development of the Tethys (A г к e 1 1, 1956; Nikolov,
1967; 1982; S a p u n о v, 1977; H a 1 1 a m, 1969; Age r, 1981), the boundary
between the Paraulacosphinctes transitorius Zone (M. chaperi Subzone) below
and the Pseudosubplanites grandis Zone above is assumed to be the boundary between
the Jurassic and the Cretaceous Systems, i. e. the lower boundary of the Lower
Cretaceous.
2.2. Upper Boundary
The upper boundary of the Lower Cretaceous coincides with the boundary between
the standard ammonite zones of Stoliczkaia dispar below and Mantelliceras mantelli
above.
2.3. Radiometric Data
Numerous analyses of radiometric data about the Early Cretaceous have been published
over the past 25 years (K u 1 p, 1961 ; С a s e y, 1964; Nachev, Lilov,
1975; Van H i n t e, 1976; H a r 1 a n d et al., 1982, and others).
К u 1 p (1961) elaborated a radiometric scale of the Cretaceous, which was subsequently
specified by a number of authors.
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