The greatest part of this ridge ("The Cuman ridge" [20J) - situated eastward of 1he sedimentary zone-consisted of intrusive rocks that pre- sented an aureole of ...
REVUE ROUMAINE DE GEOlOGIE, GEOPHYSIQUE ET GEOGRAPHIE S~RIE DE
TIRAGE A PART
TOME 14
2 1970
':DrTlONS
DE
L'ACADeMIE
DE
LA
RepUBLlQUE
SOCIALlSTE
DE ROUMANle
THE PETROGRAPHY OF THE GLAUCONITIC QUARTZARENITES IN THE BLACK SCHISTS NAPPE (THE SECTION OF THE COVASKA VAIJLEY) * BY
DA!\ GRIGORESCU **
l\TRODCCTIOX
In 1951, Filipescu, Draghinda and Mutihac pointed out to the exis tence of three lithological complexes within the "Black Schii:lts Series" in the Cretaceous Fly ch of the Eastern Carpathians on the crosssection of the Co,asna ,alley, in the tectonic unit called the "Black Schists Nappe" (= "Audia Unit"), the age and lithostratigraphical position of which were subsequently determined [l1 j , [12J as follows: 1. the Spherosideritic Complex (V'alanginian-Barremian), 2. the Schistose Complex (Aptian-Lower Albian), and 3. the Siliceous Glauconitic Sandstones Complex (Middle AlbianUpper Albian). The thickness of these three lithological complexes varies from one region to another, because of the tectonic style, in scale-folds, characteristic of the whole Black Schists series. The thickness of these three complexes is of about 600 m. on the section of the Covasna valley, distributed as follows: 230 m - the ~phe rosideritic Complex, 210 m - the Schistose Complex, and 150 m - the Siliceous Glauconitic Sandstones Complex. The frequency of the arenitic banks in the upper complex of the Black Schists Series of this unit is strikingly in contrast with the scarcity of. these banks in the other two complexes; thus it reveals important sedimentogenetical changes during the formation of the Siliceous Glauconitic Sandstones Complex. * Paper presented in the Scientific Session of the Faculty of Geology-Geography, June 1970 ** Faculty of Geology-Geography, Laboratory of Paleontology-Stratigraphy, Buchares t REV. ROU)I GEOL.. GEOPHYS., GEOGR.-SERIE DE GEOLOGIE, 14, 2. 205-216. 1970 (BUCARESTj 5 -
c. 5763
D.
206
2
GRIGORESCU
STnATO~O!\tlCAL CHARACTEIlS Al\D GRAIN-SIZE DISTRIBUTlO~ I~ THE GLALCO~ITE QUARTZARE~ITES
On the section of the Covasna valley the Glauconite Quartzarenites Complex sketches a,n asymmetrical synclinal, its eastern flank being more developed. The lithological terms are grouped in pararhythmical sequences, this being the characteristic of the whole Formation of Black Schists. 2
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• The rhythms have a binary character: the lower part may be formed either of an arenitic term (fine or middle sandstone) or of a siltic one, while the microrudaceous terms can occur very rarely; the upper part js formed of lutaceous terms (argillites, argillaceous schists, often argillosiliceous schists). The relations between the terms of the rhythms were established on the basis of the data furnished by the 45 rhythms very well developed jn the eastern flank of the synclinal. Comparing the cumulated thicknesses of the arenitic or siltic term~ to those of the lutaceous ones, we get the following proportions: - sandstones + silts - 44.60% (sandstones - 36.18%; silts 8.42%); - shales + schists - 55.40 %.
3
GLAUCONITIC QUARTZARENITES IN THE BLACK SCHISTS NAPPE
207
Sandstones occur in banks with massive structure their thickness varying from 4 to 145 em. The study of the thin sections of the samples collected from the different levels of 24 sandstone banks pointed out, in most cases, to the presence of the internal structure, but the separations between the different levels characterized by a certain grain-size distribution are poor. The multiple graded bedding having a twofold (very rarely threefold) l'epetition of the level with a relatively coarse granulometry is characteristic of the banks with thicknesses of more than 50 cm. The banks thinner than 50 cm generally present the type of "continuous graded bedding with poor separations between different granulometrical classes" (Ksiazkiewicz, 1954). The separations are sometimes so poor that only the comparative study of the lower and of the upper part of the banks reveals perceptible granulometrical differences. The joints - occurring as thin levels, with thickness between a few mm and 2 cm, b3come narrow of thick through their direction. These joints are to be found only as part of the banks with thicknesses of more than 50 cm, being placed at the upper part of a graded bedding sequence connected to them by a new similar sequence in the same bank. Sole marks are represented especially by load casts and flute casts, more rarely by drag marks (groove and striation casts). The study of the grain-size distribution of the quartzarenites led us to the following conclusions: - the fine arenitic fraction (0.1-0.2 mm) and the siltic one « 0.1 mm) are the prevailing fractions, usually representing more than 70%. Contents poorer in elements belonging to these granulometrical classes are indicated by some relatively coarse arenites situated in the middle part of the complex. - the percentage of the grains> 0.5 mm usually presents values comprised between 0.35-2.24% at the lower levels, while the upper ones are practically devoid of such grains. Oontents > 10 % are to be found in some banks situated at the lower and middle parts of the complex. - the detrital grains from the bank do not present an orientation, occurring randomly in the mass of the matrix; the arenitic levels in which the grains manifest a tendency towards a parallel disposing of their long axis with the bedding are to be found very seldom. - no direct connection between the lower level granulometry and the thickness of the respective bed can be established; some beds of 4-5 cm thickness include in their lower level a higher percentage of grains > 0.5 mm than some other beds much thicker. The coefficient of asymmetry (Sk) showing the relations between the coarse admixtures and the fine ones, indicates in most cases, values < 1 (0.81-0.96); the values > 1 (1.03-1.22) are not characteristic. The coefficient of sorting (So) varies within the limits "well sorted" and "modp.rate sorted" (in Fuechtbauer's scq,le) presenting vn.}ues com-
208
D.
GRlGORESCU
1
prised between 1.26 and 1.61 ; the levels situated in the upper part of the banks or of the graded sequences present better coefficients of sorting.
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Fig. 3. - Grain size distribu· tion in a bank of continuous graded bedding with poor se· paration between the granu· lometrical classes.
5
GLA UCONITIC QUARTZARENlTES IN THE BLACK SCHISTS NAPPE
209
TUE l'ETHOGflAPHII:.\L Cll.\H.\CTERS OF THE GL\UI:O.\ITE n• I '.UrrZ.\HE:'.IlTES A, THE EiOICL\SIIC FRACTIO.·
This fraction is dominated by the frequency of the quartz grains that - together with the metaquartzite fragments - usually represents about 95% of the total amount of the epiclastic components. In this way, the majority of the sandstones corresponds to the "quartzarenite" type (in McBride's classification), a small number of relatively coarse sandstones belonging to the "subarkose" arenites. As regards the roundness degree, most of the quartz grains belong to the categories "subangular" and "angular". The coefficient P indicating the roundness degree varies between 12.6 and 31.8. The grains > 0.5 mm usually indicate good roundness degrees, corresponding to tt, e classes "rounded" and "well rounded". au~r!z,Quartziie
and Cher!: Quarb:;;renite
Whic Arkose
Fel dspdthic Litharenite •
,
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Fig. -I. - :\lc. Bride's diagram showing the petrographical composition of the arenites in the Glauconite Quartzarenite Complex (Covasna Valley).
The study of some different levels belonging to 8 arenitic banks taking into account 100-150 grains on certain directions indicates the preponderance of the quartz grains with undulatory extinction. The ratio between the quartz grains with undulatory extinction and those with nonundulatory extinction is greater as regards the fine Bandstones (69% quartz with undulatory extinction, 31 % quartz with
210
D.
GRIGORESCU
6
nonundulatory extinction) and smaller (57% and respectively 43%) as regards the sandstones with a relatively higher granulometry. The feldspars currently represent < 3 % of the total epiclastic fraction (frequently 0.62-1.30%). Higher percentages up to 7.08% are found in the composition of some relatively coarse quartzarenites. The potash feldspars (orthoclase, very rarely microcline) and the plagioclase acids (oligoclase 10-20% An., albite 5-10% An.) are prevailing. The potash feldspars are mainly subjected to the process of argillization, while the plagioclases are frequently more or less calcitizated. The calcitization progresses on cleavages sometimes affecting the whole • gram. Micas are rarely found in the lower levels, but they occur more frequently within the fine arenitic and siltic levels in the upper part of the banks. The muscovite ~heets of 1-2 mm are prevailing while the chlorite and especially the biotite ones are to be found quite rarely. Often the muscovite sheets mould the outline of some detrital grains, probably because of the folding pressure (Fig. 7 A). Heavy minemls do not represent usually more than 1 %; as a rule they stand for 0.12-0.23% of the thin section surface. The heavy-mineral association occurring in the quartzarenites is formed of zircon, epidote, brown tourmaline, magnetite and pyrite. The following minerals are added in subordonate quantities: apatite, zoisite, clinozoisite, staurolite, rutile and garnet. The heavy minerals are strongly rounded, their crystallographic outlines being practically absent. Lithic fragments. The lithic fragments are represented both by elements generated by the erosion of the internal ridges and by the material released as a result of the intraformational processes of erosion (= intraclastic fragments). According to their frequency, the elements of the first category are the following: - metaquartzites, - microsparites and biosparitel< including algal fragments (i.e. Cadosina sp.), - muscovitie micaschists, - chloritoschists, - chlorito-muscovitic micaschists, - quartzo-feldspathic fragments with albite phenocrysts. These lithic fragments generally are smaller than 1.5 mm and they have a rounded shape. The intraclastic fragments are represented by: - pelletoidal and pseudo-oolitic micrites of 0.2-0.4 mm in size; - bituminous and siliceous argillites, angular shaped ranging from 0.8 to 2 mm in size.
7
GLAUCONITIC QUARTZARENITES IN THE BLACK SCHISTS NAP PE
211
B. THE ORGANIC FRACTION
The organic fraction is represented by the following two main conRtituents : - the bituminous organic matter disseminated within the argillaceous matrix (the C. org. contents varying between 0.04 and 0.96 % [14 J). - the tests and skeleton fragments which usually represent less than 0.3% of the surface of the thin sections. The following groups of organisms are represented by: Dasycladacean (Acicularia sp.) and Protococcacean algae (Globochaete); foraminifera - especially benthonic ones (Ammodiscinae, Haplophragmoidininae, Lituolinae, Textulariidae, Vernenilininae, Lenticulinidae) - rarely planktonic forms (Hedbergellinae); radiolaria (Sphaeroidea); siliceous sponges (both megasclaere and microsclaere), hexacorals (very rarely); bryozoans; brachiopodes; mollusca (lamellibranches and gastropodes); echinoderms (echinoid plates and spines, crinoid ossicles). Of all these the sponge spicules stand for the most important organic constituent, playing an important lithogenetic part in the formation of the siliceous cement of some quartzarenites. The sponge spicules are concentrated at certain levels situated in the upper part of some arenitic banks, within some narrow levels of arenaceous spiculites or even spiculites. The spicules are preserved in microcrystalline chalcedony, fibrous chalcedony, and sometimes in microcrystalline quartz. In sandstones the spicules are often completely calcitised, while in the spiculite levels they are partially replaced by calcite rhombohedrons sometimes disposed in the aggregate. The fragments of echinoderms as well as those of mollusca, brachiopods and bryozoans are to be found almost exclusively in the constitution of the quartzarenites belonging to the lower and middle parts of the complex, where they may represent up to 5 % of the surface of the thin sections.
C. THE GLAUCONITE QUARTZARENITE MATRIX
An intimate association between the argillaceous matrix (partially primary) and the secondary siliceous cement represented by the cryptocrystalline silica is characteristic of the glauconite quartzarenite banks thicker than 15 cm. In most cases, the microscopical estimates indicate the preponderance of the siliceous cement over the argillaceous matrix in the lower part of the banks, while in the upper part the argiLlaceous material is prevailing, sometimes blurring the presence of the siliceous cement.
212
D.
GRlGORESCU
8
The argillo-siliceous matrix * generally has a basal character. The planimetrical studies point to 15.20-33.46% matrix in the lower levelR, while in the upper ones the matrix can surpass even 50%. The matrix is dark-coloured and according to various authors this is due either to the hydI'otroilite [lJ or to the organic substance [HJ, both of them disseminated finely in its mass. In contact with the argillo-siliceous matrix, the outlines of t,he quartz grains are frequently diffused, a phenomenon which is usually limited to certain parts of the grain boundaries, more rarely affecting its whole outline (Fig. 5 D). Such phenomena were interpreted by Oayeux [8J as being the result of the secondary increases. The secondary overgrowths around the quartz grains are quite rarely to be found. The pre:>ence of this type of matrix seems to be connected with the diagenetic changes undergone by the primary argillaceous material, a process in which an important quantity of silica is released. In the argillo-siliceous matrix, isolated zones of chalcedony or microcrystalline quartz aggregates appear disseminated quite frequently, probably as a result of the recrystallizations of the cryptocrystalline silica in the cement. Quite often the matrix is replaced in a more or less advanced degree by microgranular calcite. The cement of chemical and biochemical precipitation practically devoid of argillaceous material is mainly represented by opal, partially replaced by fibrous chalcedony or microcrystalline quartz. Interesting processes of the silica differentiation took place in the voids of the rock mass: there can be noticed very spectacular transitions from opal of the cement to fibrous chalcedony and authigenic quartz, the last one having a zoned aspect determined by successive stages of growth (Fig. 6 A). The overgTowths developed on the detritic grains boundaries are very much developed (Fig. 5 A). This type of cement is quite rarely to be found, characterizing some beds, less than 15 cm. thickness, with relatively coarse granulometry, in comparison with the type of argillo-siliceous matrix. The cement occupies a relatively scanty space. The sponge spicules occur frequently, that is why, at least partially, the silica may be considered of organic • • orlgm. D. THE ALTHIGENIC CmJPONENTS
In the constitution of the quartzarenites the following categories of minerals prove an authigenic origin: silica (opal, chalcedony, quartz) feld pars, phyllosilicates (muscovite, glauconite, leptochlorite, argillaceous minerals), carbonates (calcite, dolomite, siderite), sulphates (barite), • The term "argillo-siliceolls matrix" is uscd for the fluency of thc expression, ill orJer to cJf'scribe this type of matrix.
GLA UCONITIC QUARTZARENlTES IN THE BLACK SCHISTS NAPPE
9
213
sulphides (pyrite), iron oxides and hydroxides (hematite, goethite, lepidochrocite, limonite). In this paper we shall insist only on the glauconite, that presents a particular sedimentological interest, characterizing the neutral hydrochemical environment. The glauconite has a constant development within the arenitic banks. Its frequency indicated by the planimetric studies varies between 0.86% and 15.24%. It occurs more frequently at the fine arenitic levels with developed argillo-siliceous matrix and more rarely at the relatively coarse levels where it reaches minimal values. The types of occurrence of the glauconite in these quartzarenites are the following : - isolated grains, 0.2-0.4 mm in size, with a more or less regular outline (subspherica.l or elliptical) or frequently, coalescent grains with lobate aspect. - disseminated pigment into the matrix, sometimes concentrated under the form of greenish clouds without a distinct outline (Fig. 6 D). - thin cou,ting around the detritic grains (F ig. 6 B, 0). - cement developed in limited portions that includes fine detl'itic grams. Quite seldom the glauconite is found on the cleavages of the feldspars or it is found diagenesing sponge spicules. The studied glauconite presents colours that vary between greenemerald and dark-green almost black. The dark colour is characteristic of the glauconite in banks with a higher degi'ee of alteration. The grains are devoid of pleochroism. In crossed nicoh; the cryptocrystalline structure is very distinct, a fact that causes ~a characteristic aggregate polarization. Quite frequently, the glauconite grains are partially replaced by pigmentary pyrite, by small isometric crystals of pyrite (0.03-0.05 mm in size), and also by calcite or siderite rhombohedrons. The replacement of the glauconite by calcite is sometimes very advanced, glauconite relicts occurring as residual network inside the large calcite crystals. The alteration of the glauconite consists mainly in its replacing by limonite. The alteration begins along the boundaries of the grains and progresses towards their centres alongside the radial cracks. Besides the autochthonous glauconite, there can be found in a subordinate quantity, well rounded glauconite grains which present a peripherical dark thin zone of alteration that suggests the idea of an intraformational transport that had preceded its inclusion into the mass of the sediment. Of all the other authigenic minerals a characteristic development is presented by the pyrite that appears as small anhedral geains, as a pigment disseminated into the matrix, or more rarely as cement developed in limited zones. •
"214
D.
GRIGORESCU
1(}
We attribute an authigenic origin to the fine muscovite flakes developed alongside the boundaries of some quartz grains, sometimes penetrated by it. Such sheets are relatively frequent in the qnartzarenites with a developed argillace01ls matrix. Of the authigenic calbonates, the Ihombohedrons of dolomite (0.2 -0.5 mm in size) are preferentially dtveloped in the quartzarenites with siliceous cements, (Fig. 7 D), while the calcite appears generally as replacing the argillaceous matrix and the sponge spicules. The authigenic feldspars (orthoclase) and the barite as euhedral cl·ystals are very rarely found, occurring together at some quartzarenitic Jeveli' with argillo-siliceous matrix. (Fig. 7 B, C). SEUI ~I K\TO GE.\'ETI tAl. C:O.\'(;LUSlO.\'S
A. THE SOURCE AREA AND THE DEPOS IT ION OF THE SEDIMENTARY ;\lATERIAL.
The frequency of the fine arenitic and siltic grains in the quartzalcnites as well as the high percentages of the matrix point to the existence of a source area relatively far from the sedimentary zone of the quartzarenites. The measurings of the directional structures indicated paleocurrents orientated NNW-SSE for these arenites of the Audia Unit [9J. On the basis of these data the existence of a western ridge was presumed, comtituted of epizonal and mesozonal metamorphic rocks which are submerged most of the time [2J. It may be presumed that this ridge represented the permanent source area that worked as such all the time during the sedimentogenesis of the Black Schists Series in this tectonic unit. Besides these observations that refer exclusively to the arenites, in order to have a more complete image of the source area, we want to point out once again to the existence within the same lithological complex of some microbreccias and breccias including numerous elements (1.5 mm2 cm in f-lize) of intrusiye feldspar rocks and quartzfeldspar rocks (monzonite, syenogranite and granodiorite) much more rarely elements of effusive rockf-l with the same chemical composition (latite, riodacite). It is known that this material was supplied by a ridge of the northern Dobrudja origin. The greatest part of this ridge ("The Cuman ridge" [20J) - situated eastward of 1he sedimentary zone-consisted of intrusive rocks that presented an aureole of comagmatic effusive rocks, not so much developed, if we take into account the scarcity of such fragments in comparison with the fragments of intrusive rocks. The evidences of the existence of this source are particularly characteristic of the upper complex of the Black Schists. The periodical
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