VOLUME 21 (4)/August 1991

The Albian Stage in Northwest Bulgaria is of more restricted occurrence compared to the other Lower Cretaceous Stages. It consists of marls interbedded by clayey limestones and by less common sandstones (Sumer Formation) and glauconitic sandstones, siltstones and marls (Malo Peštene Formation). The Albian sediments are of considerable thickness without any hiatuses and condensations in the sections. The deposits contain diverse macrofaunas dominated by ammonites. This paper describes the ammonite and lithological successions in 15 Albian sections. The Albian is considered here to include the L. (L.) tardefurcata Zone from its base up to the S. dispar Zone. The three Albian substages are characterized and new evidence is reported on the Albian ammonite successions in Northwest Bulgaria. It has made it possible to supplement and to further subdivide the zonations proposed by previous authors. Part of the zones where possible, have been subdivided into subzones. The following biostratigraphic units are characterized in ascending order: L. (L.) tardefurcata Zone with L. (L.) tenuicostata Subzone and L. (N.) regularis Subzone; D. mammillatum Zone with P. (I.) steinmanni Subzone; H. dentatus Zone with L. lyelli Subzone and H. spathi Subzone; E. loricatus Zone with A. intermedius Subzone; E. lautus Zone; M. (M.) inflatum Zone with H. orbignyi Subzone, H. varicosum Subzone and C. auritus Subzone; S . dispar Zone. An attempt is made to correlate them with units already established in other parts of the Mediterranean Region.

D. Kozhoukharov. Konstantinovo Metaconglomerate Formation in Sakar Mountain and Marica Zone between Simeonovgrad and Dimitrovgrad, Haskovo District. Konstantinovo Metaconglomerate Formation is introduced for the first time in the Bulgarian lithostratigraphic nomenclature. It is represented by fine to medium in pebble size conglomerates with gne1ss-schist to gneissic ground mass and irregularly ditsributed pieces of pegmatites and aplites, less commonly of quartz-epidote-muscovite schists, microgranites and monomineral grains of quartz and feld spars. The pieces are rounded, lens-shaped or flattened, with ellipsoidal crosssection and dominant dimensions 2 × 3 to 5-7 × 3 × 5 cm, rarely to 15 cm in diameter. The ground mass is formed of coarse-grained mica, quartz and plagioclase. The metamorphic evolution of the rocks of Konstantinovo Formation reflects changes of progressive by regressive stages of metamorphism which define the polymetamorphic evolution of the Precambrian basement. The first regional metamorphism was in amphibolite facies (E 1) and culminated in regional migmatization. It is of general occurrence and defines the general features of the Precambrian rocks from the southern slopes of Sakar Mountain and Marica Zone.