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.