Integrated bio- and cyclostratigraphy of Middle Triassic (Anisian) ramp deposits, NW Bulgaria

Abstract

A cyclostratigraphic interpretation of peritidal to shallow-marine ramp deposits of the early Middle Triassic (Anisian) Opletnya Member exposed in outcrops along the Iskar River gorge, NW Bulgaria, is presented. Based on facies trends and bounding surfaces, depositional sequences of several orders can be identified. New biostratigraphic data provide a time frame of the studied succession with placement of the boundaries of the Anisian substages and show that the Aegean (early Anisian) substage lasted about 1.6 Myr. In the corresponding interval in the two studied sections, 80 elementary sequences are counted. Five elementary sequences compose a small-scale sequence. The prominent cyclic pattern of the Opletnya Member can thus be interpreted in terms of Milankovitch cyclicity: elementary sequences represent the precession (20-kyr) cycle and small-scale sequences the short eccentricity (100-kyr) cycle in the Milankovitch frequency band. Medium-scale sequences are defined based on lithology but only in two cases can be attributed to the long eccentricity cycle of 405 kyr. The transgressive-regressive facies trends within the sequences of all scales imply that they were controlled by sea-level changes, and that these were in tune with the climate changes induced by the orbital cycles. However, the complexity of facies and sedimentary structures seen in the Opletnya Member also implies that additional factors such as lateral migration of sediment bodies across the ramp were active. In addition, three major sequence boundaries have been identified in the studied sections, which can be correlated with the boundaries Ol4, An1, and An2 of the Tethyan realm.

Irish-type Zn-Pb deposits around the world

“…The mineralization at Sedmochislenitsi is restricted to the Svidol Formation and the lowermost 130-140m of the Mogila Formation (effectively the Opeletnya Member) (Rentzsch, 1963;Ajdanlijsky et al, 2019). At Kalaminata, south-east of Medna-Plakanitsa, minor Zn-Pb mineralization is seen at similar stratigraphic levels (Figures 4,7 & 8).…”

“…The mineralization at Sedmochislenitsi is entirely hosted within the Anisian-aged Svidol and Mogila Formations. The Anisian Stage of the Triassic has been described by Ajdanlijsky et al, (2019) as being a crucial time interval in Earth's history to understand carbonate platform reorganization and the incipient break-up of the supercontinent Pangaea.…”

“…Other common features of the Opletnya Member are erosional surfaces locally showing channel morphology which, in many cases, have channel flanks that are steep to almost vertical, indicating a relatively advanced degree of early lithification of the sediment into which the channel was cut. (Ajdanlijsky et al, 2019).…”

“…Figure 4: Stratigraphic column of the Triassic sequence at Sedmochislenitsi and position of the mineralization (in blue). (Modified afterAjdanlijsky et al 2019) …”

The Sedmochislenitsi Zn-Pb-Cu-Ag deposit, NW Bulgaria: could it be Irish-type? A review of the evidence

“…The mineralization at Sedmochislenitsi is entirely hosted within the Anisian-aged Svidol and Mogila Formations. The Anisian Stage of the Triassic has been described by Ajdanlijsky et al, (2019) as being a crucial time interval in Earth's history to understand carbonate platform reorganization and the incipient break-up of the supercontinent Pangaea.…”