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On palaeocurrents in central and western Bulgaria in Early Jurassic times
Palaeogeography, Palaeoclimatology, Palaeoeeology Elsevier Publishing Company, Amsterdam
Printed in The Netherlands
ON PALAEOCURRENTS IN CENTRAL AND WESTERN BULGARIA IN EARLY JURASSIC TIMES
PLATON TCHOUMATCHENCO
Geological Institute, Bulgarian Academy of Sciences, Sofia (Bulgaria) (Accepted for publication June 5, 1972)
ABSTRACT Tchoumatchenco, P., 1972. On palaeocurrents in central and western Bulgaria in Early Jurassic times. Palaeogeogr., Palaeoclimatol., Palaeoecol., 12: 243-250.
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, there 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. INTRODUCTION C u r r e n t s play an i m p o r t a n t role in t h e life a n d d i s t r i b u t i o n o f t h e m a r i n e fauna, a n d t h e r e c o n s t r u c t i o n o f p a l a e o c u r r e n t s is o f great i m p o r t a n c e in t h e u n d e r s t a n d i n g o f the general palaeoecological p i c t u r e . This is w h y the palaeoecological session o f the P a l a e o n t o l o g i c a l S o c i e t y o f t h e U.S.S.R. r e c o m m e n d t h e use b e i n g m a d e o f all possibilities for statistical s t u d i e s in t h e field o f t h e o r i e n t a t i o n o f shells for the r e c o g n i t i o n o f the p o s i t i o n o f the coastline, o f t h e d i r e c t i o n o f p a l a e o c u r r e n t s and o f the m o v e m e n t o f waves. T h e m a r i n e fauna, in t h e c o u r s e o f its life, takes u p a p o s i t i o n d e t e r m i n e d b y its r e l a t i o n s h i p to t h e c u r r e n t s a c c o r d i n g t o t h e laws o f h y d r o d y n a m i c s . O f t e n b e n t h o n i c o r g a n i s m s are fossilized in this p r i m a r y o r i e n t a t i o n , b u t in the m a j o r i t y o f cases the
244
P. TCHOUMATCHENCO
orientation in which the shells are fossilized is due to the posthumous action of currents. The mode of transport - by saltation or by sliding on the bottom - is the same for shells as for pebbles. Menard and Boucot (1951) have proved that for the transport of a pebble a certain distance it is necessary to have forces ten times greater than for the transport for the same distance of a brachiopod shell of the same size. According to them, the shells of brachiopods are transported with their long axis oriented perpendicularly to the direction of the current without reference to their concave or convex sides being up or down, because after the next saltation the shells change their position. Rasoumikhine (1963, 1964) has proved that pebbles take up an orientation transverse in relation to a current in the centre of a river (rose diagrams have a very accentuated maximum); whilst towards the sides of the river the rose diagram passes into a type possessing one maximum and two secondary diagonal maxima. Finally, at the very edge of the river the rose diagram shows no maxima. According to Lazarenko and Semenov (1967) when pebbles are broken up into groups the predominant orientation is normal to the direction of the current, because they are influenced by rubbing on the floor of the basin and between the pebbles themselves. Kind and Kukharenko (1948) have shown that the transverse position of the greater part of pebbles is characteristic for the central part of a river, whilst when they are farther away from the centre the orientation becomes oblique. Dmitrieva et al. (1962) and Habakov (1964) have noted that pebbles and elongated shells transported by the action of waves, take up an orientation parallel or sub-parallel to the coast. If the shells have an elongated conical form (belemnites, tentaculites, orthoceratides) and undergo transport by the action of waves, almost half of them have their pointed ends oriented in one direction and the other half in the reverse direction; if these shells are transported by the action of currents, the greater part have their pointed ends in the direction of the current. ORIENTATIONANALYSIS I have carried out a great number of orientation measurements in the Lower Jurassic sediments of central and western Bulgaria with the aim of reconstructing the palaeocurrents. I have measured the long axes of brachiopods, of ammonites and of bivalves, but the most useful for these measurements have been the belemnites. The most useful for the measurement of imbrication are the smooth pectens, Cincta, Lobothyris and ammonites. The statistical analysis of the results of orientation measurements have demonstrated that there are three types of rose diagrams with every gradation between them, reflecting on one side the direction of the pataeocurrents, and on the other their strength intensity. The first type of rose diagram has a single principal maximum and a second very feeble maximum perpendicular to the first (Fig. 1). This rose diagram is similar to that Rasoumikhine (1964) has obtained in measuring the orientation of pebbles coming from the centre part of a river, that is to say, the part where the current is strongest. The principal maximum corresponds to the direction perpendicular to the direction of water movement. The direction of imbrication of flat pebbles displays the same character. The
EARLY JURASSIC PALAEOCURRENTS IN BULGARIA
245
N
Fig.l. Rose diagram, I st type.
places where this type of rose diagram are found are situated in the marginal parts of the basin, in the outermost of the present outcrops. These are the following localities (see Fig.4): Techerna reka (No. 23), Makotzevo (No. 19) Kremikovtzi (No. 13) the Ambaritza chalet (No. 26) and also the localities situated on the borders of Vratza island: the Ledenika chalet (No. 18) and the village of Gorno Ozirova (No. l 7). These are the localities where the movements of marine waters have been the strongest. The second type of rose diagram has a principal maximum and two less pronounced maxima diagonal to the first (Fig.2). This type is close to the rose diagram obtained by N
Fig.2. Rose diagram, 2nd type.
Rasoumikhine (1964) in the alluvium situated between the central part of the river and its borders, that is to say, where the principal current forces diminished and the accessory factors are accentuated. The principal maximum has also an orientation perpendicular to the direction of the current: this cuts in two the angle between the two secondary maxima. The localities which have rose diagrams of this type are situated farther from the borders of the sea in relation to those of the first type of rose diagram (see Fig.4). These are the following Localities: the town of Teteven (No. 22), Berende izvor (No. 2), Boukorovtzi (No. 3), Gaganitza (No. 4), Etropole (No. 20) and Milanovo (No. 16).
246
P. TCHOUMATCHENCO
The third type of rose diagram has a poorly marked maximum, a second maximum perpendicular to the first, and two diagonal maxima (Fig.3). This type is similar to the diagram obtained by Rasoumikhine (1964) in the alluvium situated close to the sides of rivers, that is to say, where the principal current is weakest and can only orientate in a stable position a limited number of pebbles. N
Fig.3. Rose diagram, 3rd type. This type of rose diagram is found in localities farthest from the coastline (Fig.4): Gintzi (No.7) and Zimevitza (No.15) or in places where there was calm water near the coast - the locality named Broussove (No.28) in the central Stara planina (Balkan Mountains S.S.). Very often the rose diagrams are of an intermediate type; between the extremes, these reflect the gradual transition in the dynamic regime of the sea. DEGREE OF PRESERVATIONOF THE SHELLS Shells of articulated brachiopods remain joined after the death of the organism. They only become open after long transport; after a very long transport the shells are broken up into pieces. Middlemiss (1962) has figured on a map of the region he has studied, all the outcrops where the brachiopod shells with both valves predominate, the places where there are equal numbers of brachiopods with one or both valves and the places where there are only single or broken valves. I have done the same for the region I have studied (Fig.5). One can see on this map the localities where the shells are for the most part single valves or broken, other localities where the shells are in almost equal abundance as single or paired valves, and the localities where almost all the shells have the two valves still joined. In a valley of the River Tcherna reka (No. 23) there are many shells of Lobothyris subpunctata (Davidson) as single valves one inside the other. The lines which separate the regions with the same degree of shell preservation coincide almost exactly with the localities showing the different kinds of orientation rose diagrams. That is to say, where the rose diagrams are of the first type the brachiopod
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Fig.4. Diagram showing palaeocurrents in the Lower Jurassic basin of central a nd western Bulgaria. 1 = rose diagrams showing the orientation o f the fragments measured ( 'W = 44" indicates the number of specimens measured; " 1 9 " indicates the locality on the diagram where the meas u r emen ts have b een carried out); 2 = scale for the rose diagrams; 3 = dir ec t ion of high-energy palaeocurrents; 4 = direction of medium-energy palaeocurrents; 5 = d i r e c t i o n o f low-energy palaeocurrents; 6 = pr es u med position o f coastline; 7 = Lower Jurassic outcrops.
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Fig.5. Diagram showing the degree of preservation of brachiopod shells. 1 = regions where broken shells and single valves predominate; 2 = regions where the percentages of shells with single and both valves are about equal; 3 = regions where shells with both valves predominate; 4 = limits between the regions of different degrees of shell preservation; 5 = supposed position of the coastline.
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EARLY JURASSIC PALAEOCURRENTS IN BULGARIA
249
shells are predominantly of the first type - the brachiopod shells are predominantly separated valves and/or broken; where the shells are equally separated and joined, the rose diagrams are of the second type and finally where the valves are for the most part joined the areas correspond with those of the rose diagrams of the third type.
CHARACTER OFTHEPALAEOCURRENTS In the region of the marine basin which covered central and western Bulgaria during Early Jurassic times, one can show active movement of the water generally from south to north. The rose diagrams are of the first type or transitional towards the second type in that the brachiopod shells are separated and fragmented, or one finds at the same time shells equally separated or joined. These water movements are preserved on a wide front and we must think that they were produced by waves or surf. In the more internal parts of the basin this general movement from south to north is differentiated into local currents. The rose diagrams are of the second type whilst the shell preservation is also of the second type - equal quantities of separated and joined valves. In the Etropole area (No. 20) and at Teteven (No. 20) the direction of the general palaeocurrent from south to north changes towards northeast and around the village of Lessidren (No. 25) towards the east. This change in direction is due to the presence of the Island of Vratza. The coastline of this island has given birth to a little local palaeocurrent around the village of Glojene (No. 21) coming from the north to the south. In the region of Sofia the general direction from south to north changes towards the northwest around the village of Boukorovtzi (No. 3) and to the northeast around Komstitza (No. 1) reaching also to the village of Zimevitza (No. 15). In the area between the villages of Gintzi (No. 7) and Zimevitza (No. 15) the strength of this palaeocurrent has significantly diminished - the rose diagrams are of the third type and the shells of the brachiopods are almost all with both valves joined. The Island of Vratza records local palaeocurrent_s also in the area between the chalet of Ledenika (No. 18) and the village of Gorno Ozirovo (No. 17), where a palaeocurrent comes from the southeast towards northwest around the hamlet of Ledenika (No. 18) (and changes towards the north-northwest near Gorno Ozirovo, so that around the village of Milanovo (No. 16) a palaeocurrent comes from north to south). These palaeocurrents were of considerable energy - the rose diagrams were of the first type and the shell preservation are of the second type, half with both valves, the other half with single valves. From the direction of Belogradchik came a palaeocurrent from northwest to southeast (near the village of Beli reel (No. 5) which changed around the village of Gaganitza (No. 4) towards the south-southeast. The energy of this palaeocurrent diminished progressively; near Beli mel (No. 5) the rose diagram is intermediate between the first and the second type whilst around Gaganitza (No. 4) the rose diagram is of the second type.
250
P. TCHOUMATCHENCO
In the western parts of the region there was a strong palaeocurrent demonstrated by rose diagrams of the first type and by broken shells. It came from the east (around the village of Jabliano, No. 12), changed in direction towards the northwest around the village of Staniovtzi (No. 10) to the neighbourhood of Slivnitza where it went to the northeast and joined around Kalotina (No. 2) with the main palaeocurrent coming from the southeast.
REFERENCES Ager, D. V., 1963. Principles of Palaeoecology. McGraw-Hill,New York, N.Y., 371 pp. Aibulatov, N. A., 1966. Vdolberegovoe peremechtenie nanossov otmelich beregov. Nauka, Moscow, 160 pp. (in Russian). Baskom, V., 1966. Volni i pliagi. Dynamika morskoi poverkhnostL Gidrometeorologitsheskoe izdatelstvo, Leningrad, 278 pp. (in Russian). Dmitrieva, E. V., Erchova, G. I. and Orechnikova, E. K., 1962. Atlas textur i structur ossadochnikh gornikh porod, A. V. Habakov (Editor), Vol. L Oblomochnie i glinistie porodi. Gosgeoltechizdat, Moscow, 578 pp. (in Russian). Gilbert, G. K., 1914. The transportation of d~bris by running water. U.S. Geol. Surv., Prof. Pap., 86:263 pp. Glen, J. W., Douner, J. J. and West, R. C., 1957. On the mechanism by which stones in till become oriented. Am. J. Sci., 255: 194-205. Habakov, A. V., 1964. Uporiadotshenoe zaleganie rakovin endoceratitov v arenigskih sloiah kupda v Pribaltike i nekotorie problemi dinamitsheskoi paleogeographii ordovika. Bull. Mosk. O-va lzpit. Prir., Otd. Geol., 39 (5): 48-76 (in Russian). Kind, N. V. and Kukharenko, A. A., 1948. K voprosu ob orientirovki galek v retchnom potoke. Zap. Vses. Mineral. O-vo, 1 : 90-96 (in Russian). Lazarenko, A. A. and Semenov, E. V., 1967. Mekhanism obrazovania orientirovki galek v otlojeniah potokov. Dokl. Akad. Nauk U.S.S.R., 6 (172): 1401-1404 (in Russian). Markovskii, B. P., 1966. Metodi biofacialnogo analisa. Nauka, Moscow, 178 pp. (in Russian). Menard, H. W. and Boucot, A. J., 1951. Experiments on the movement of shells by water. Am. J. Sci., 249: 131-151. Middlemiss, F. A., 1962. Brachiopod ecology and Lower Greensland paleogeography. Paleontology, 5: 253-267. Rasoumikhine, N. V., 1963. Experimentalnie isledovania orientirovki galek na modeli retshnogo rusla. Vestn. Leningr. Gos. Univ., Ser. GeoL Geogr., 24 (4): 77-85 (in Russian). Rasoumikhine, N. V., 1964. Orientirovka galek v ruslo nekotorih rek Urala. Vestn. Leningr. Gos. Univ., set. Geol. Geog., 18 (3): 98-104 (in Russian). Raukas, A., 1962. Regularities in the distribution of pebbles in the tills of Estonia. Izv. Akad. Nauk Est. S.S.R., Set. Fiz. Mat. Tekh. Nauk, 2:140-153 (in Russian with English summary). Sapunov, I. G., Tchoumatchenco, P. V. and Shopov, V. L., 1971. Concerning certain features of the palaeogeography of the Teteven area in the Early Jurassic. Bull. Geol. Inst., Ser. Stratigr. Lithol., 20:33-62 (in Bulgarian with Russian and English summaries). Sarkissian, S. G. and Klimova, L. T., 1955. Orientirovka galek i methodi ikh izutshenia dlia paleogeographytsbeskih postroenil. Izd. Akad. Nauk U.S.S.R., Moscow, 165 pp. (in Russian).
Printed in The Netherlands
ON PALAEOCURRENTS IN CENTRAL AND WESTERN BULGARIA IN EARLY JURASSIC TIMES
PLATON TCHOUMATCHENCO
Geological Institute, Bulgarian Academy of Sciences, Sofia (Bulgaria) (Accepted for publication June 5, 1972)
ABSTRACT Tchoumatchenco, P., 1972. On palaeocurrents in central and western Bulgaria in Early Jurassic times. Palaeogeogr., Palaeoclimatol., Palaeoecol., 12: 243-250.
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, there 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. INTRODUCTION C u r r e n t s play an i m p o r t a n t role in t h e life a n d d i s t r i b u t i o n o f t h e m a r i n e fauna, a n d t h e r e c o n s t r u c t i o n o f p a l a e o c u r r e n t s is o f great i m p o r t a n c e in t h e u n d e r s t a n d i n g o f the general palaeoecological p i c t u r e . This is w h y the palaeoecological session o f the P a l a e o n t o l o g i c a l S o c i e t y o f t h e U.S.S.R. r e c o m m e n d t h e use b e i n g m a d e o f all possibilities for statistical s t u d i e s in t h e field o f t h e o r i e n t a t i o n o f shells for the r e c o g n i t i o n o f the p o s i t i o n o f the coastline, o f t h e d i r e c t i o n o f p a l a e o c u r r e n t s and o f the m o v e m e n t o f waves. T h e m a r i n e fauna, in t h e c o u r s e o f its life, takes u p a p o s i t i o n d e t e r m i n e d b y its r e l a t i o n s h i p to t h e c u r r e n t s a c c o r d i n g t o t h e laws o f h y d r o d y n a m i c s . O f t e n b e n t h o n i c o r g a n i s m s are fossilized in this p r i m a r y o r i e n t a t i o n , b u t in the m a j o r i t y o f cases the
244
P. TCHOUMATCHENCO
orientation in which the shells are fossilized is due to the posthumous action of currents. The mode of transport - by saltation or by sliding on the bottom - is the same for shells as for pebbles. Menard and Boucot (1951) have proved that for the transport of a pebble a certain distance it is necessary to have forces ten times greater than for the transport for the same distance of a brachiopod shell of the same size. According to them, the shells of brachiopods are transported with their long axis oriented perpendicularly to the direction of the current without reference to their concave or convex sides being up or down, because after the next saltation the shells change their position. Rasoumikhine (1963, 1964) has proved that pebbles take up an orientation transverse in relation to a current in the centre of a river (rose diagrams have a very accentuated maximum); whilst towards the sides of the river the rose diagram passes into a type possessing one maximum and two secondary diagonal maxima. Finally, at the very edge of the river the rose diagram shows no maxima. According to Lazarenko and Semenov (1967) when pebbles are broken up into groups the predominant orientation is normal to the direction of the current, because they are influenced by rubbing on the floor of the basin and between the pebbles themselves. Kind and Kukharenko (1948) have shown that the transverse position of the greater part of pebbles is characteristic for the central part of a river, whilst when they are farther away from the centre the orientation becomes oblique. Dmitrieva et al. (1962) and Habakov (1964) have noted that pebbles and elongated shells transported by the action of waves, take up an orientation parallel or sub-parallel to the coast. If the shells have an elongated conical form (belemnites, tentaculites, orthoceratides) and undergo transport by the action of waves, almost half of them have their pointed ends oriented in one direction and the other half in the reverse direction; if these shells are transported by the action of currents, the greater part have their pointed ends in the direction of the current. ORIENTATIONANALYSIS I have carried out a great number of orientation measurements in the Lower Jurassic sediments of central and western Bulgaria with the aim of reconstructing the palaeocurrents. I have measured the long axes of brachiopods, of ammonites and of bivalves, but the most useful for these measurements have been the belemnites. The most useful for the measurement of imbrication are the smooth pectens, Cincta, Lobothyris and ammonites. The statistical analysis of the results of orientation measurements have demonstrated that there are three types of rose diagrams with every gradation between them, reflecting on one side the direction of the pataeocurrents, and on the other their strength intensity. The first type of rose diagram has a single principal maximum and a second very feeble maximum perpendicular to the first (Fig. 1). This rose diagram is similar to that Rasoumikhine (1964) has obtained in measuring the orientation of pebbles coming from the centre part of a river, that is to say, the part where the current is strongest. The principal maximum corresponds to the direction perpendicular to the direction of water movement. The direction of imbrication of flat pebbles displays the same character. The
EARLY JURASSIC PALAEOCURRENTS IN BULGARIA
245
N
Fig.l. Rose diagram, I st type.
places where this type of rose diagram are found are situated in the marginal parts of the basin, in the outermost of the present outcrops. These are the following localities (see Fig.4): Techerna reka (No. 23), Makotzevo (No. 19) Kremikovtzi (No. 13) the Ambaritza chalet (No. 26) and also the localities situated on the borders of Vratza island: the Ledenika chalet (No. 18) and the village of Gorno Ozirova (No. l 7). These are the localities where the movements of marine waters have been the strongest. The second type of rose diagram has a principal maximum and two less pronounced maxima diagonal to the first (Fig.2). This type is close to the rose diagram obtained by N
Fig.2. Rose diagram, 2nd type.
Rasoumikhine (1964) in the alluvium situated between the central part of the river and its borders, that is to say, where the principal current forces diminished and the accessory factors are accentuated. The principal maximum has also an orientation perpendicular to the direction of the current: this cuts in two the angle between the two secondary maxima. The localities which have rose diagrams of this type are situated farther from the borders of the sea in relation to those of the first type of rose diagram (see Fig.4). These are the following Localities: the town of Teteven (No. 22), Berende izvor (No. 2), Boukorovtzi (No. 3), Gaganitza (No. 4), Etropole (No. 20) and Milanovo (No. 16).
246
P. TCHOUMATCHENCO
The third type of rose diagram has a poorly marked maximum, a second maximum perpendicular to the first, and two diagonal maxima (Fig.3). This type is similar to the diagram obtained by Rasoumikhine (1964) in the alluvium situated close to the sides of rivers, that is to say, where the principal current is weakest and can only orientate in a stable position a limited number of pebbles. N
Fig.3. Rose diagram, 3rd type. This type of rose diagram is found in localities farthest from the coastline (Fig.4): Gintzi (No.7) and Zimevitza (No.15) or in places where there was calm water near the coast - the locality named Broussove (No.28) in the central Stara planina (Balkan Mountains S.S.). Very often the rose diagrams are of an intermediate type; between the extremes, these reflect the gradual transition in the dynamic regime of the sea. DEGREE OF PRESERVATIONOF THE SHELLS Shells of articulated brachiopods remain joined after the death of the organism. They only become open after long transport; after a very long transport the shells are broken up into pieces. Middlemiss (1962) has figured on a map of the region he has studied, all the outcrops where the brachiopod shells with both valves predominate, the places where there are equal numbers of brachiopods with one or both valves and the places where there are only single or broken valves. I have done the same for the region I have studied (Fig.5). One can see on this map the localities where the shells are for the most part single valves or broken, other localities where the shells are in almost equal abundance as single or paired valves, and the localities where almost all the shells have the two valves still joined. In a valley of the River Tcherna reka (No. 23) there are many shells of Lobothyris subpunctata (Davidson) as single valves one inside the other. The lines which separate the regions with the same degree of shell preservation coincide almost exactly with the localities showing the different kinds of orientation rose diagrams. That is to say, where the rose diagrams are of the first type the brachiopod
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Fig.4. Diagram showing palaeocurrents in the Lower Jurassic basin of central a nd western Bulgaria. 1 = rose diagrams showing the orientation o f the fragments measured ( 'W = 44" indicates the number of specimens measured; " 1 9 " indicates the locality on the diagram where the meas u r emen ts have b een carried out); 2 = scale for the rose diagrams; 3 = dir ec t ion of high-energy palaeocurrents; 4 = direction of medium-energy palaeocurrents; 5 = d i r e c t i o n o f low-energy palaeocurrents; 6 = pr es u med position o f coastline; 7 = Lower Jurassic outcrops.
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Fig.5. Diagram showing the degree of preservation of brachiopod shells. 1 = regions where broken shells and single valves predominate; 2 = regions where the percentages of shells with single and both valves are about equal; 3 = regions where shells with both valves predominate; 4 = limits between the regions of different degrees of shell preservation; 5 = supposed position of the coastline.
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EARLY JURASSIC PALAEOCURRENTS IN BULGARIA
249
shells are predominantly of the first type - the brachiopod shells are predominantly separated valves and/or broken; where the shells are equally separated and joined, the rose diagrams are of the second type and finally where the valves are for the most part joined the areas correspond with those of the rose diagrams of the third type.
CHARACTER OFTHEPALAEOCURRENTS In the region of the marine basin which covered central and western Bulgaria during Early Jurassic times, one can show active movement of the water generally from south to north. The rose diagrams are of the first type or transitional towards the second type in that the brachiopod shells are separated and fragmented, or one finds at the same time shells equally separated or joined. These water movements are preserved on a wide front and we must think that they were produced by waves or surf. In the more internal parts of the basin this general movement from south to north is differentiated into local currents. The rose diagrams are of the second type whilst the shell preservation is also of the second type - equal quantities of separated and joined valves. In the Etropole area (No. 20) and at Teteven (No. 20) the direction of the general palaeocurrent from south to north changes towards northeast and around the village of Lessidren (No. 25) towards the east. This change in direction is due to the presence of the Island of Vratza. The coastline of this island has given birth to a little local palaeocurrent around the village of Glojene (No. 21) coming from the north to the south. In the region of Sofia the general direction from south to north changes towards the northwest around the village of Boukorovtzi (No. 3) and to the northeast around Komstitza (No. 1) reaching also to the village of Zimevitza (No. 15). In the area between the villages of Gintzi (No. 7) and Zimevitza (No. 15) the strength of this palaeocurrent has significantly diminished - the rose diagrams are of the third type and the shells of the brachiopods are almost all with both valves joined. The Island of Vratza records local palaeocurrent_s also in the area between the chalet of Ledenika (No. 18) and the village of Gorno Ozirovo (No. 17), where a palaeocurrent comes from the southeast towards northwest around the hamlet of Ledenika (No. 18) (and changes towards the north-northwest near Gorno Ozirovo, so that around the village of Milanovo (No. 16) a palaeocurrent comes from north to south). These palaeocurrents were of considerable energy - the rose diagrams were of the first type and the shell preservation are of the second type, half with both valves, the other half with single valves. From the direction of Belogradchik came a palaeocurrent from northwest to southeast (near the village of Beli reel (No. 5) which changed around the village of Gaganitza (No. 4) towards the south-southeast. The energy of this palaeocurrent diminished progressively; near Beli mel (No. 5) the rose diagram is intermediate between the first and the second type whilst around Gaganitza (No. 4) the rose diagram is of the second type.
250
P. TCHOUMATCHENCO
In the western parts of the region there was a strong palaeocurrent demonstrated by rose diagrams of the first type and by broken shells. It came from the east (around the village of Jabliano, No. 12), changed in direction towards the northwest around the village of Staniovtzi (No. 10) to the neighbourhood of Slivnitza where it went to the northeast and joined around Kalotina (No. 2) with the main palaeocurrent coming from the southeast.
REFERENCES Ager, D. V., 1963. Principles of Palaeoecology. McGraw-Hill,New York, N.Y., 371 pp. Aibulatov, N. A., 1966. Vdolberegovoe peremechtenie nanossov otmelich beregov. Nauka, Moscow, 160 pp. (in Russian). Baskom, V., 1966. Volni i pliagi. Dynamika morskoi poverkhnostL Gidrometeorologitsheskoe izdatelstvo, Leningrad, 278 pp. (in Russian). Dmitrieva, E. V., Erchova, G. I. and Orechnikova, E. K., 1962. Atlas textur i structur ossadochnikh gornikh porod, A. V. Habakov (Editor), Vol. L Oblomochnie i glinistie porodi. Gosgeoltechizdat, Moscow, 578 pp. (in Russian). Gilbert, G. K., 1914. The transportation of d~bris by running water. U.S. Geol. Surv., Prof. Pap., 86:263 pp. Glen, J. W., Douner, J. J. and West, R. C., 1957. On the mechanism by which stones in till become oriented. Am. J. Sci., 255: 194-205. Habakov, A. V., 1964. Uporiadotshenoe zaleganie rakovin endoceratitov v arenigskih sloiah kupda v Pribaltike i nekotorie problemi dinamitsheskoi paleogeographii ordovika. Bull. Mosk. O-va lzpit. Prir., Otd. Geol., 39 (5): 48-76 (in Russian). Kind, N. V. and Kukharenko, A. A., 1948. K voprosu ob orientirovki galek v retchnom potoke. Zap. Vses. Mineral. O-vo, 1 : 90-96 (in Russian). Lazarenko, A. A. and Semenov, E. V., 1967. Mekhanism obrazovania orientirovki galek v otlojeniah potokov. Dokl. Akad. Nauk U.S.S.R., 6 (172): 1401-1404 (in Russian). Markovskii, B. P., 1966. Metodi biofacialnogo analisa. Nauka, Moscow, 178 pp. (in Russian). Menard, H. W. and Boucot, A. J., 1951. Experiments on the movement of shells by water. Am. J. Sci., 249: 131-151. Middlemiss, F. A., 1962. Brachiopod ecology and Lower Greensland paleogeography. Paleontology, 5: 253-267. Rasoumikhine, N. V., 1963. Experimentalnie isledovania orientirovki galek na modeli retshnogo rusla. Vestn. Leningr. Gos. Univ., Ser. GeoL Geogr., 24 (4): 77-85 (in Russian). Rasoumikhine, N. V., 1964. Orientirovka galek v ruslo nekotorih rek Urala. Vestn. Leningr. Gos. Univ., set. Geol. Geog., 18 (3): 98-104 (in Russian). Raukas, A., 1962. Regularities in the distribution of pebbles in the tills of Estonia. Izv. Akad. Nauk Est. S.S.R., Set. Fiz. Mat. Tekh. Nauk, 2:140-153 (in Russian with English summary). Sapunov, I. G., Tchoumatchenco, P. V. and Shopov, V. L., 1971. Concerning certain features of the palaeogeography of the Teteven area in the Early Jurassic. Bull. Geol. Inst., Ser. Stratigr. Lithol., 20:33-62 (in Bulgarian with Russian and English summaries). Sarkissian, S. G. and Klimova, L. T., 1955. Orientirovka galek i methodi ikh izutshenia dlia paleogeographytsbeskih postroenil. Izd. Akad. Nauk U.S.S.R., Moscow, 165 pp. (in Russian).