THE CLAY MINERALOGY OF THE MOLASSE FORMATIONS OF AZERBAIJAN AND ITS CORREI,ATION WITH PALEOGEOGRAPHY

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1 Clay Minerals (1968) 7, 441. THE CLAY MNERALOGY OF THE MOLASSE FORMATONS OF AZERBAJAN AND TS CORRE,ATON WTH PALEOGEOGRAPHY A. G. SEDOV AND N. V. PASHALY The nstitute o[ Geology o[ the Academy o] Sciences of the Azerbaijan SSR, Baku (Received 11 January 1967) ABSTRACT: An investigation of the clay mineralogy of the molasse formations of Azerbaijan has been carried out. The results have been correlated with climate, the geochemistry of the basin of deposition, the parent rocks and the tectonic stability both in the area of erosion and the zone of accumulation. A knowledge of the relationship between these factors and the clay mineralogy of the sediments may help in the solution of paleogeographic problems. NTRODUCTON The molasse formation is widespread in Azerbaijan and represents the final stage of the development of the alpine geosyncline. t can be separated into upper and and lower divisions on the basis of rock forming conditions, textural and structural features and lithological composition. Stratigraphically the lower group covers the period from the lower Oligocene to the Miocene and the upper group from the beginning of the Pliocene to the top of the Anthropogene. The lower molasse formation is represented mainly by clays interstratified with siltstones and sandstones with subordinate amounts of grits and carbonate rocks, while the upper group is composed of conglomerates and sandstones many metres in thickness and by clays and formations containing organic detritus. The molasse rocks were laid down on the foothills and lower slopes of the Major and Minor Caucasus and of the Talysh Mountains. Genetically they are represented by both marine and continental deposits with occasional lagoonal formations. The clay mineral suites associated with the different tectonic zones and stratigraphical units have not previously been described either in the literature of the U.S.S.R. or abroad. The conditions of formation of these rocks is of considerable importance, not only in Azerbaijan but in other parts of the Soviet Union where they are often associated with deposits of commercial importance such as oil, gas, water, building stones etc.

2 442 A. G. Seidov and N. V. Pashaly Fig. 1 shows diagramatically the correlation between the stratigraphy and clay mineralogy in the different tectonic zones of the lower molasse (A) and upper molasse (B) of Azerbaijan. THE ROCKS OF THE LOWER MOLASSE The rocks of the Maikop suite are typical of the lower molasse, and their clay mineralogy shows considerable variations both in the nature of the clay minerals present and in the relative proportions in which they occur. These differences are a reflection of the nature of the parent rocks, of variations in the conditions of deposition, climatic and tectonic factors and the geochemistry of the basins of sedimentation. The Maikop sediments were derived from the Mesozoic deposits of the southeastern Caucasus,: volcanic rocks of the Lagich mountains, basic and ultrabasic rocks of the northern flank of the Minor Caucasus and of the Talysh mountains, and also from the hypothetical 'Kura Land'. The clay minerals in these sediments are predominantly hydromica along with smaller amounts of kaolinite, montmorillonite and chlorite. Within the Maikop suite four zones containing montmorillonite and hydromica can be distinguished and these can be traced through various tectonic zones of Azerbaijan from foredeeps through the intermontane depressions to the submerged regions of the Major Caucasus under the Caspian Sea. Variations in the mineral composition of these horizons, i.e. in the proportion of hydromica to montmorillonite and in the nature of the accessory clay minerals, proves that the Maikop deposits were laid down in differing facies conditions. The multiple recurrence of horizons containing similar clay mineral associations suggest rhythmic alternations in the sedimentation conditions during Maikop time. The montmorillonite-hydromica association with subordinate amounts of kaolinite, magnesium clay minerals and ferruginous montmorillonite is characteristic of the Lower Maikop deposits of the southern slope of the Kura depression (i.e. the north-western part of the Kirovabad region) and of the foredeep parallel to the range of the Major Caucasus (the Caspian Sea). n other tectonic zones of Azerbaijan (the submerged south-eastern region of the Major Caucasus, the Apsheron peninsula and Kobystan) the proportion of montmorillonite decreases while hydromica with lesser amounts of kaolinite becomes dominant. The zones in which the montmorillonite-hydromica association are dominant and which contain kaolinite in some tectonic regions are indicative of areas of humid climate in which weathering of the primary minerals occasionally reached the extreme (kaolinite) stage of weathering. n the Lower Maikop deposits of the southern slope of the Kura depression magnesium clay minerals occur while these are absent in the foredeep (Caspian) area. This shows that chemical parameters are of considerable importance as well as climatic factors in the Lower Maikop basin. Within the region of the Minor Caucasus and the Talysh mountains volcanic

3 Clay mineralolgy of molasse formations 443 Kura Depression Southeastern submergence of Major Caucasus Kobyston South North ~ J 8.--~ -!_ll -.ll -=---H --![ = Foredeep Sdtstones S(~ndstones Cong~omer{3~es Mud- breccia []~]] Looms Limestones Abyssar E~ Sholow ~cki~ Logoon [~ S~l(ne Lagoon J~ Hydrom,ca [~ Hydromica 8~ Cloy rains. 5 H y d ~ omica-montrnorillanit e E~ Montmor [llonite- hyd romic( E~ MontmorlHonite ~] KoolinOe {~ HsllDysite [~ Polygorskile "E a, E~ Bei~elhle =~ G~ouconde Mixed-l{3yer m~ns g, o = -'6 Kuro Depression Adjinour.v,l,11111,x g o J Foredeep Southeastern submergence of Molor Cclvcosus,~ ~_~" - '~ i~='~ = ~, - ~ x FG. 1. Variations in lithology, facies and clay mineralogy in molasse sediments of Azerbaijan.

4 444 A. G. Seidov and N. V. Pashaly rocks containing the ferromagnesium minerals pyroxenes and amphiboles are widespread. The breakdown of these minerals and their transformation by diagenesis to magnesium clay minerals, was accelerated in the weakly alkaline regime in the basin of deposition. The significance of the hydrochemical factor is demonstrated by an increase in the hydromica content in an easterly direction and a concomitant reduction in the proportion of montmorillonite, especially the ferruginous variety. A small amount of kaolinite is always present. Some halloysite occurs in the Kobystan area and beidellite in the Apsheron peninsula. The increase in the ratio of hydromica to montmorillonite in an easterly direction accompanies a gradual change from coastal, through shallow to abyssal conditions. However the mixed character of the clay mineral association, containing a predominance of the two main components along with various other minerals (beidellite, palygorskite, halloysite, kaolinite etc.) suggests that the tectonic activity gave rise to accelerated sedimentation which prevented the diagnetic processes reaching equilibrium under the natural conditions of deposition. The clay mineralogy of the Upper Maikop deposits is somewhat different from that of the Lower Maikop. Here there is a much wider development of the montmorillonite-hydromica zone with magnesium clay minerals (in the Kirovabad region of the Talysh mountains) and kaolinite, but the amounts of beidellite and halloysite are greatly decreased. The hydromica minerals of the Upper Maikop contain more authigenic varieties than those of the lower group; and mixed-layer clay mineral structures are also more characteristic. These factors indicate a period of tectonic quiescence in the Upper Maikop period, during which slow sedimentation gave ample time for diagenesis under the physico-chemical conditions prevailing. The increased proportion of montmorillonite and magnesium clay minerals, as well as the presence of mixed-layer components can be associated with the shallowing of the basin of deposition and a change to a more arid climate. ROCKS OF THE UPPER MOLASSE The upper molasse formations and the Anthropogene in particular are widespread in Azerbaijan. As in the case of the Maikop rocks they were deposited in a series of tectonic zones ranging from the foredeeps, the submarine zone of the Major Caucasus and the intermontane area. Stratigraphically they are represented by the Bakinian, Gurgjansky, Khazer and Khvalynsky horizons which vary in origin from continental through lagoonal to marine. The continental deposits are mostly unsorted sandstones and grits while clays predominate in the marine facies, the intermediate lagoonal deposits are characteristically loams. There are more clay beds in the Bakinian and Khvalynsky horizons than in the Khazar. The principal clay minerals of the Anthropogene rocks are hydromica and montmorillonite with subordinate beidellite, palygorskite and chlorite. Kaolinite is rarer than in the Maikop rocks. Glauconite occurs only in the clays of the marine facies. Within each tectonic zone, minor differences in the proportions of the various clay minerals present are frequently observed, but more important differences, involving the nature of the

5 Clay mineralogy of molasse formations 445 clay minerals present, are frequently seen between adjacent tectonic zones. Within the lower half of the Khazar horizon there is a considerable variation in clay mineralogy, and, in fact, the lower half of each stratigraphic horizon appears to be similar in this respect. This distribution of the clay mineral suites is in accordance with the variation in the tectonic activity which showed a maximum at the lower part of each of the periods Khazar, Bakinian and Khvalynsky. The lower Khazar maximum was the most pronounced. The accelerated rate of sedimentation of detrital material derived from the Major and Minor Caucasus and deposited in the Kura depression, gave rise to very mixed clay mineralogy in marine, shallow-water and coasted facies. Hydromica predominate_s in the marine Paleocaspian region where all other components are reduced. However, in the deep-water facies of the Bakinian archipelago there are more erratic variations in the mineralogy due to erosion of insular projections and submarine elevations, and to outbursts of mud volcanos. n the submerged zone of the Major Caucasus (Apsheron peninsula) there is a sporadic occurrence of montmorillonite and beidellite along with the predominant hydromica. n the foredeep areas, i.e. the Caspian and Lenkoran regions, the clay mineralogy of the deposits is simple, as the detrital material comes predominantly from one of two sources, i.e. either the Major Caucasus or the Talysh mountains. n the Caspian region the clay mineralogy is mainly hydromica, while in the Lenkoran area it is a hydromica-montmorillonite mixture with a small amount of kaolinite. This difference in clay mineralogy, arising from variations in the parent rocks, has persisted after burial of the sediments. This indicates that the rate of accumulation of detritus was so rapid that diagenetic changes which would bring the clay mineralogy into equilibrium with the physico-chemical conditions in the basin of deposition, could not take place. The appearance of authigenic kaolinite in the upper part of the Khvalynsky deposits of the Lenkoran region indicates a slackening in the rate of sedimentation and a consequent increase in diagenesis. The presence of magnesium-bearing clay minerals in more horizons of the Anthropogene of the Kurinsky area shows the importance of the chemical factor in the sedimentation basin. These magnesium clay minerals were derived from the pyroxenes and amphiboles from the volcanic rocks of the Minor Caucasus. Although volcanic rocks also occur in the Talysh area, magnesium clay minerals are not observed in the anthropogene clay of the Lenkoran area. The absence of magnesian minerals here may have resulted from the influence of a considerable supply of river water to the Lenkoran basin reducing the salinity of the sea-water and so inhibiting the formation of clay minerals containing magnesium. Two regions of coastal lagoonal deposits can be recognized in the Kura depression. These range in age from Bakinian to Khazar. The deposits along the northern slope of the depression are dominated by the clay mineral hydromica with smaller amounts of montmorillonite and kaolinite. The sediments in the axial region again have hydromica as the principal clay mineral, but also contain palygorskite and a higher proportion of montmorillonite. The differences in the clay mineralogy of

6 446 A. G. Seidov and N. V. Pashaly these two areas of sedimentation can be correlated with their drainage conditions. The paleorivers from the Major Caucasus drained into the northern slope of the Kura depression maintaining the comparative freshness of the water. The Minor Caucasus with a relatively drier climate, and a proportion of basic igneous rocks, drained into the axial part of the Kura depression, which therefore had a higher salinity. The clay mineralogy of molasse deposits in the geosynclinal area of Azerbaijan was controlled by a number of factors which were operative at the time of deposition; the mineralogy and physiography of the country rocks being eroded, the rate of accumulation of sediments and the geochemistry of the basins of sedimentation which together controlled the extent and degree of diagenetic alteration, and the climate prevailing in the area. An understanding of these processes should make possible the study of conditions obtaining in the geological past from a knowledge of the clay mineralogy of sedimentary rocks. REFERENCES KOSSOVSKAYA A.G. & S~TOV A.D. (1959) Factors determining the mineralogy of clay rocks in platform and geosynclinal regions, in Data on the Geology Mineralogy and Utilization of Clays in the U.S.S.R. zd. Akad. Nauk. S.S.S.R. Moscow. PASHALY N.V. (1964) Lithology o[ the Quaternery Deposits of Eastern Azerbai]an. zd. Akad. Nauk. Azerb. S.S.R. Baku. SEDOV A.G. (1962) Lithology o] the Maikop suite in Azerbaijan. zd. Akad. Nauk. Azerb. S.S.R. Baku. SEDOV A.G. (1967) Some problems relating to the mineralogy of sediments, Clay Miner. Bull. 5, 329. STRAKUOV N.M. (1964) Principles o] the Theory o[ Lithogenesis. zd. Akad. Nauk. S.S.S.R., Moscow. SULTANOV A.D. & GORN V.A. (1963) Productive Strata o/ Ore Deposits in the Southern Caspian Basin. zd. Akad. Nauk. Azerb. S.S.R.

Sediment and Sedimentary rock

Sediment and Sedimentary rock Sediment: An accumulation of loose mineral grains, such as boulders, pebbles, sand, silt or mud, which are not cemented together. Mechanical and chemical weathering produces