Platon Tchoumatchenco

The oldest deep-sea Ophiomorpha and Scolicia and associated trace fossils from the Upper Jurassic–Lower Cretaceous deep-water turbidite deposits of SW Bulgaria

Deep-sea trace fossils of the Nereites ichnofacies occur in the uppermost Kimmeridgian–Berriasian deep-water turbidite deposits of the Kostel Formation in SW Bulgaria. The trace fossil assemblage contains Tithonian Ophiomorpha isp. and Ophiomorpha annulata, which are the oldest deep-sea representatives of this ichnogenus. They indicate much older invasion of Ophiomorpha into deep-sea, which was hitherto dated as to mid-Cretaceous. It is not clear whether the invasion failed or Ophiomorpha survived in deep-sea refuge and became widespread after improving oxygenation of the deep seas after the Cenomanian/Turonian anoxic event. Tithonian Scolicia, produced by irregular echinoids, exhibits a similar distribution pattern.

Thanatocoenoses and biotopes of Lower Jurassic brachiopods in central and western Bulgaria

Abstract Brachiopod biotopes are recognized in the Early Jurassic rocks of Bulgaria. The geographical distribution of these is plotted for each of the stratigraphical zones determined earlier on the basis of brachiopods. The biotopes are related in turn to bathymetry, energy level and bottom type.

Brachiopod thanatocoenoses in the Aalenian, Bajocian and Bathonian of western Bulgaria and their distribution

Abstract The Middle Jurassic deposits in western Bulgaria represent a full sedimentary cycle including a transgression (sandy sediments without brachiopods), shallow-water carbonates (with brachiopods) followed by deep-water deposits (without brachiopods) or by a break in sedimentation. They have been divided into six brachiopod zones: (1) a zone of Acantothiris costata, marking the beginning of carbonate sedimentation in southwestern Bulgaria, with a thanatocoenosis of Rhynchonellida (96%) and Terebratulidina (4%); (2) a zone of A. sentosa with a thanatocoenosis of Rhynchonellida (90%) and Terebratulidina (10%); (3) a zone of Sphaeroidothyris sphaeroidalis with a thanatocoenosis of Terebratulidina (55%) and Rhynchonellida (45%); (4) a zone of A. inflata in southwestern Bulgaria with an assemblage of Rhynchonellida (58%), Terebratulidina (38%) and the first Terebratellidina (4%); in northwestern Bulgaria carbonate deposition begins with a thanatocoenosis of Rhynchonellida (90%) and Terebratulidina (10%); (5) a zone of Wattonithyris wattonensis which in northwestern Bulgaria has an assemblage of Terebratulidina (70%), Terebratellidina (20%), and Rhynchonellida (10%), whereas in southwestern Bulgaria it is represented solely by Terebratulidina (100%); (6) a zone of Acanthothiris spinosa with a thanatocoenosis of Terebratulidina (62%), Rhynchonellida (24%) and Terebratellidina (14%), but this is only represented in northwestern Bulgaria. Data from western Bulgaria show that the highest energy biotopes (related to the beginning of carbonate deposition) were dominated by Rhynchonellida; subsequently, falling energy levels made it possible for Terebratulidina and finally Terebratellidina to populate the sea floor.

On palaeocurrents in central and western Bulgaria in Early Jurassic times

Abstract Fossil orientations and the mode of preservation of brachiopod shells are recorded from the Early Jurassic rocks of central and western Bulgaria. A direct correlation is found between them. Palaeocurrents are shown to have been generally from south to north, with local variations. From the statistical study of the results of orientation measurements of fossils and fossil debris, therr emerge three distinct types of rose diagram, which reflect on the one hand the direction of the palaeocurrent and on the other its intensity, which diminishes as one passes from the first to the third type: 1st type — with one very pronounced principal maximum which is perpendicular to the direction of the palaeocurrent; 2nd type — with one principal maximum and two secondary maxima (the direction of the palaeocurrent is perpendicular to the first and divides the angle between the two secondary maxima); 3rd type — with one poorly defined principal maximum and several secondary maxima. The degree of preservation of the brachiopod shells coincides almost precisely with the three types of diagram. At the localities having the first type of rose diagram, the shells are broken or as single valves; at the localities having the second type of rose diagram, the percentages of shells with single and both valves are about equal, whilst at the localities with the third type the brachiopod shells are almost always with both valves.