Cyclostratigraphy of the Jurassic rocks in Western Bulgaria

The aim of this research is to check if the method used by Fisher (1991) can gave results, reflecting approximately the orbital cycles in the Milankovitch band. I used the following data: (1) thickness of the study interval – it was necessary to make new redescriptions of some important sections; (2) continuity (in Ma) of the study interval – it is referred to the Time scale used by Hardenbol et al. (1998, Charts 5, 7); (3) speed of sedimentation in Bubnoff units (Ma/meters); (4) number of couplets in the study interval; (5) thickness of a single couplets; (6) continuity (periodicity) of a couplet = average thickness of a couplet divided by the Bubnoff
units. In most of the cases the obtained periodicity of the couplets are closed to or coincide with the theoretical orbital cycles of the Milankovitch band: Eccentricity – El = 98 Ka; E2 =126 Ka (EI, 2 = 98-126 Ka); Obliquity = 41 Ka; Precession – PI = 19 Ka; P2 = 23 Ka (P = 19-23 Ka) (after Fischer, 1991, p.48).
   I studied the limestone/marl couplets in the sections, where existed a good biostratigraphical control, based on ammonites (Sapunov, 1976a, b; 1977a, b; Sapunov in Sapunov et al., 1985, 1988, etc.), brachiopods (Tchoumatchenco, 1977) or calpionellids and precalpionellids (Lakova, 1993, Lakova et al., 1999, etc.). Here are shown the results obtained from the rocks of the Lower Jurassic – sections Berende Izvor, Komsh titsa; Aalenian-Lower Bathonian – section Zhablyano; Bathonian-Lower Callovian – section Koritarska Reka - Dolna Riksa; Callovian-Tithonian – sections Komshtitsa, Gorno Belotintsi, Belogradchik- Railway station Oreshets.