ON THE SECTION OF THE OLDEST NON-METAMORPHIC STRATA AND THE UPPER BOUNDARY OF THE PRECAMBRIAN IN ALTAI

A. F. BELOUSOV

(Presented by Professor, Doctor K. V. Radugin)

I

V. A. Obruchev was the first, 40 years ago, to suppose the presence of pre-Paleozoic strata in the Russian Altai (1915, 1927). He assigned to the Precambrian the gneisses of the Western Sailyugem and Kurai ranges in Southeastern Altai (initially considering them Archean), and to the Proterozoic—the chlorite and quartz slates and tuffaceous sandstones of the Terektin Range in Southeastern Altai, the slates, sandstones, and limestones of the “Chertinskaya suite” of Northern Altai, and later (1935) the siliceous slates, limestones, and tuffogenic sandstones of yet another area along the Katun River in Northern Altai. V. A. Obruchev expressed the opinion that the Proterozoic was widely developed in Gorny Altai. In all the areas he indicated, a Precambrian age for part of the rocks is still accepted by many investigators of Altai.

The section of the oldest strata was studied specifically by K. V. Radugin (1939, 1941, 1948, and others) in Southeastern and Central Altai, and by A. N. Churakov (1941) in Southeastern Altai; they outlined the volume and subdivision of the non-metamorphic Precambrian (Proterozoic), as well as its upper boundary.

Data on the oldest strata of the region and on the boundary between the Proterozoic and the Cambrian are also contained in the works of V. A. Kuznetsov (1937, 1948, 1952, 1954, and others) on the southeastern and other parts of Altai, Yu. A. Kuznetsov (1939) on Central Altai, A. M. Kuzmin (1928) on the extreme northeastern part, and M. K. Vinkman (1948 1—2) on the northern areas. V. P. Nekhoroshev repeatedly touched upon the question of the Cambrian and Precambrian of Gorny Altai (1927, 1932 1—2, 1937, 1947, 1954, and others). In most of his works, except the first and the last (1954), he denied the presence in Altai of the Proterozoic in general, or at least the presence of Precambrian strata sharply separated in the section from the Cambrian.

Recently, extensive material on the geology of the ancient strata of Altai has been obtained through systematic geological surveys and other investigations. The author participated in these works for seven years. The new data have made it possible critically to check and correct the existing stratigraphic schemes of K. V. Radugin, A. N. Churakov, M. K. Vinkman, and others, and also, on more detailed and varied material, to examine the position of the upper boundary of the Precambrian in the region.

II

The nonmetamorphic Precambrian of Altai, distinguished in addition to the presumably Precambrian metamorphic complexes, is represented most fully in a large structural unit—the Katun uplift—which occupies a broad belt in the basin of the Katun River in Northern and Central Altai and continues from there as a narrower belt, under the name of the Kadrin uplift, southeastward in the basin of the Chuya River in Southeastern Altai.

The total thickness of the nonmetamorphic Precambrian in the Katun uplift reaches 11–13 thousand meters. It is represented by three complexes.

The oldest of them is the Sugary series, which was distinguished by the author approximately in the volume of A. N. Churakov’s Yenisei suite in the headwaters of the Kadrin River in Southeastern Altai. The thickness of the series is 3000–4000 m.

In composition and facies character it is a limestone sequence, chiefly pelitogenic, with some participation of algal reef formations. Its section is dominated by marblized limestones—black, dark gray, dark brown, sometimes lightened; at times the limestones are dolomitized. In minor amounts there occur carbonaceous-argillaceous and argillaceous shales of black, gray, and greenish-gray colors, as well as lydite-like siliceous rocks—silicilites (phtanites)—black and veined gray-black. Poorly preserved remains of stromatolitic algae were observed in the limestones.

The Baratal series, overlying the first, is well represented between the Chuya and the headwaters of the Kadrin and along the Chuya River, in the lower course of the Katun River, and also in the Biya uplift of ancient strata, located east of the northern part of the Katun uplift, in the basin of the Biya River in Northeastern Altai. The series includes the carbonate (Yenisei) sequence of K. V. Radugin, which he assigned to the middle Proterozoic, and a small part of the rocks combined by A. N. Churakov into the Kutenbuluk suite. The total thickness of the series is tentatively estimated at 4000–5000 m.

In its composition and facies it is a carbonate-terrigenous-silicilite sequence, marine, with some amount of effusives, chiefly of intermediate composition. More than any other rock type, it contains marblized limestones, partly clayey, siliceous, sandy, having black, bluish-black, dark-gray, and dark-brown colors, sometimes secondarily lightened. Dolomites, dolomitized limestones, and silicilites from light gray to black are present in significant quantities, as are argillaceous, clayey-aleuritic, carbonaceous-argillaceous, siliceous-argillaceous, and calcareous-argillaceous shales—black, gray, sometimes brownish and greenish. In the lower and upper horizons there occur polymictic sandstones of dark-gray and greenish coloration, as well as conglomerates and conglomerate-breccias composed of fragments of sedimentary and sometimes (in the upper horizons) effusive rocks; the fragments in the conglomerates at the base of the sequence reach boulder size. At the top of the series lie flows of plagioclase (including albitophyres) and plagioclase-pyroxene porphyrites, usually of greenstone aspect, spilitized, dirty green in color, rarely reddish; tuffites, effusive conglomerate-breccias, and breccias are associated with the lavas. Stromatolites and osagites have been recorded in the carbonate rocks; from several localities, presumed coelenterates, sponge spicules, and radiolarians are reported.

No sharp unconformity has been recorded between the Sugalin and Baratal series; they lie parallel.

The overlying ancient porphyrite formation is most fully represented in the lower reaches of the Katun and along the Chue River. Its thickness reaches 3000 m.

This is a porphyrite sequence, predominantly of intermediate composition, subaerial. It is dominated by plagioclase and plagioclase-pyroxene porphyrites, partly spilitized. Usually the effusives have a greenstone appearance; sometimes they reveal relics of the diagenetic stage of alteration; their colors are dirty green and sometimes brownish-purple. The lavas are accompanied by tuffs and tuff breccias, tuffites, effusive conglomerate-breccias, and breccias. Dolomites and dolomitized limestones of gray and whitish color occur as separate beds and lenses. There are beds of black, gray, and dark-brown limestones associated with gray and greenish clayey and calcareous-clayey shales, and black and gray siliciliths. Spherical algae and abundant stromatolites occur in the limestones and dolomites; organic formations resembling Newlandia have been noted.

The ancient porphyrite formation lies on the Baratal series parallel to it and without signs of a major hiatus; between them there is a transitional effusive-sedimentary member.

Above lies a complex of reliably Cambrian deposits, whose total thickness in Gorny Altai is no less than 10–11 thousand meters.

In various parts of Gorny Altai, finds of Lower Cambrian faunas are known, showing that rather low horizons of the Lower Cambrian are present here. Unfortunately, rocks with these fossils either have no direct stratigraphic relations with the principal Cambrian and Precambrian formations of Gorny Altai established up to the present time, or these relations have not been reliably clarified.

Recently, along the B. Isha River, west of the village of Choya, within the Biya uplift, trilobites Protolenus, Bergeroniaspis, Bonnia, Bathyuriscidae were found in a gray-colored shale-sandstone sequence with subordinate beds of limestone, together with archaeocyaths redeposited from Ajacicyathus, Coscinocyathus, Loculocyathus, Tumulocyathus, and, together with the archaeocyaths, hyolithids Hyolites and tabulate corals of the genus Bija *). On the basis of the trilobites these formations are comparable with the Ketemen Formation of the fairly well-studied Lower Cambrian of Eastern Siberia.

The gray-colored Lower Cambrian sequence of the B. Isha was provisionally included by K. D. Neshumaeva and others in the Kayanchin Formation of Gorny Altai. This appears to have been confirmed recently in the southern part of the Biya uplift, in the basin of the headwaters of the M. Sumulta River, where the author has established a gray-colored sequence, entirely analogous in composition and appearance, with a thickness of more than 2 km, which, as is characteristic specifically of the Kayanchin Formation, conformably underlies a substantially effusive Cambrian sequence (the Ust-Sema Formation).

Along the Sarasa River, at the Proletarsky state farm near the southwestern margin of the Katun uplift, archaeocyaths have long been known in limestones from a disjunctive block assigned to the Kayanchin Formation, represented by Ajacicyathus camptophragmus (Vol.), Ajac. dwighti (Walc.),

*) In the present work, paleontological determinations and age conclusions have been used, mostly unpublished, by trilobites—O. K. Poletaeva, E. V. Lermontova, L. I. Egorova; by archaeocyaths—P. S. Krasnopeeva, S. V. Chernysheva; by brachiopods—O. K. Poletaeva; and also individual determinations by other persons.

Ajac cf. patulus (Born.), Coscinocyathus torgaschinensis (Vol.), Cosc. homentovskii (Vol.), Thalamocyathus gerassimovensis Krasn.

P. S. Krasnopeeva preliminarily considers that the archaeocyathid fauna of the Sarasy River consists of forms characteristic of the Kameshki and Gavrilovka archaeocyathid complexes.

The Kayanchin Formation occupies a quite clear position in the section; it crops out in the central and northern parts of the Katun uplift near the village of Elanda and the settlement of Ust-Sema. Here it has a thickness of up to 1050 m. In general it may be characterized as a marine limestone-terrigenous formation with effusive elements. It is dominated by marbled limestones of gray and white color, strongly reefed, partly clayey and sandy. Clayey, siliceous-clayey, calcareous-clayey shales and polymict sandstones of gray-green and gray colors are present, as well as conglomerates and conglomerate breccias, including boulder and block varieties (in the basal horizons). Sheets of spilitized diabase and porphyry lavas occur, with which tuffites and effusive conglomerate breccias are associated.

In the limestones of the upper half of the formation section, trilobites are known: Bergeroniellus, Cobboldia, Chondragraulos, Edelsteinaspis, Erbia, Granularia, Kooteniella, Pagetia, Weymonthia; brachiopods, pteropods Hyolites, algae, and numerous finds of archaeocyathids close to the Obruchev (polyscyathid) complex of the Kuznetsk Alatau—representatives of the genera Archaeocyathus (Spirocyathus), Clathrocyathus, Erbocyathus, Ethmophyllum, Retecyathus, Tegerocyathus, as well as some forms known in the Upper Kameshki complex of the Eastern Sayan—from Ajacicyathus, Thalamocyathus.

The age of the formation, according to trilobites, among which there are protolenids (Bergeroniellus), is determined as within the upper third of the Lower Cambrian.

A major unconformity is established at the base of the formation. It lies transgressively on the eroded surface of rocks of the older Porphyry and Baratal formations. In places, indistinct angular unconformities have been recorded on its lower side. Its basal conglomerates include fragments of various rocks from both underlying formations, with relict fracture microtectonics.

Along the Katun River, in contrast to the Biya uplift, the Kayanchin Formation is represented by relatively thin geanticlinal facies and possibly has a smaller stratigraphic volume than in the Biya uplift, where even older horizons with Lower Cambrian fauna than those known so far are probable within it.

Above lies a sequence distinguished in the Katun uplift under the name of the Ust-Sema Formation. Its total thickness locally apparently exceeds 3000–3500 m.

In terms of lithologic-facies type, this is an effusive-sedimentary sequence, marine, partly with products of subaerial eruptions. The formation is composed predominantly of lava rocks of diagenetic and partly greenschist aspect, together with their derivatives. Among the lava rocks, which are mainly basic in composition, there are augite porphyries (chiefly in the lower parts), pyroxene-plagioclase and plagioclase porphyries of gray, green, and violet-red colors. Tuffs and tuff breccias occur. Graywacke tuffites, effusive breccias, and conglomerate breccias are abundantly represented; less common are products of more complete sedimentary sorting—normal polymict sandstones, clayey, siliceous-clayey, and marly shales of green and violet-red tones, marbled

limestones. Upward through the section of the suite, the role of clastic products of the erosion of effusives increases.

In the lower horizons of the suite in the Katun uplift, scant remains of poorly preserved trilobites of the Lower Cambrian family Eodiscidae, etc., brachiopods, and archaeocyaths have been found in primary and redeposited occurrence. The latter are represented mainly by the same forms as in the upper part of the Kayancha suite, showing only slight signs of rejuvenation.

The Ust-Sema suite in the near-Katun part of the Katun uplift overlies the Kayancha suite without signs of significant unconformity. Similar in composition, thick—approximately 2500–4000 m—substantially effusive accumulations are developed in the southern part of the Biya uplift and farther east, in the area of Lake Teletskoye (the Priteletskoye uplift), as well as along the southwestern margin of the Uimen depression (according to data of A. A. Zenkova, K. D. Neshumaeva, V. M. Sennikov, the author, and others).

At many localities, archaeocyaths and algae have been collected from limestone lenses and reefs in this complex; a few trilobites and brachiopods are also known. According to the available determinations, archaeocyaths from some localities may be assigned to the late Kameshkov and Obruchev complexes; the effusive-sedimentary accumulations containing them may in part prove to be synchronous with the Kayancha suite. From the majority of localities, however, the archaeocyaths are correlated with the Sanashtykgol complex of the Western Sayan, analogues of which are not yet known in the Ust-Sema suite of the Katun River. For determining the age of the substantially effusive Cambrian formation under consideration, finds of fauna in the south of the Biya uplift along the B. Isha River, in the vicinity of the village of Choya, are especially interesting. Here a sequence of conglomerates, interbedded with sandstones, clayey and marly shales, and limestones, with a thickness on the order of 1000 m, is exposed. In its general structural position, this sequence most probably belongs to the upper parts of the substantially effusive formation (it should be noted that in the Ust-Sema suite of the Katun River, for example, near the settlement of Ust-Sema, the upper parts of the section are also composed mainly of conglomerates). In the limestones and marly shales, trilobites Serrodiscus and Calodiscus (known in the Lower Cambrian) have been found together with Conocoryphe (widespread in the Middle Cambrian), as well as Poliellina, Pagetia, and poorly preserved remains resembling Corynexochidae; and in limestones and limestone fragments of the near-reef breccias and conglomerates there are fossils corresponding to the fauna of the Sanashtykgol limestones of the Western Sayan—brachiopods Bagenovia and others together with archaeocyaths of Ajacicyathus, Annulocyathus, Claruscyathus, Erbocyathus, Ethmophyllum, Tercyathus, and others. The age of the Sanashtykgol horizon, as is known, is disputed; it is referred either to the uppermost Lower Cambrian (N. S. Zaitsev and N. V. Pokrovskaya, 1950; I. T. Zhuravleva, 1954), or to the lower part of the Middle Cambrian (A. G. Sivov, 1953; P. S. Krasnopeeva, 1947, 1954).

In the conglomerates of the substantially coarse-clastic sequence under consideration, pebbles of pink alaskite granites and other granitoids occur. Fragments of granitoids and basic intrusive rocks have also been noted in the accumulations of the Katun uplift, analogous in their composition and position in the section to the sequence with the fauna of the Sanashtykgol type. These accumulations compose horizons through which the Ust-Sema suite is lithologically gradually replaced by the overlying substantially sedimentary suite (see below).

Granitoid fragments, including those from deep levels of erosion of intrusions, also occur in large quantities in the conglomerates

younger Cambrian strata in the south of the Biya salient and, in enormous abundance in this same area, are contained in the thick conglomerates of the upper Tremadoc series with trilobites, which transgressively and with a very large hiatus rests on different horizons of the Cambrian.

In composition, the intrusive rocks of the clasts of all these conglomerates correspond to some part of the bodies of essentially granodiorite–trondhjemite composition, including very large ones, which in the northeastern part of Gorny Altai cut through Lower Cambrian effusive-sedimentary accumulations older than the strata with the Sanashtykgol fauna.

Thus, in Altai, as also in the adjacent Western Sayan (A. G. Sivov, 1953), a major Cambrian intrusion is established, with an age older than the upper part of the Lower Cambrian or the boundary between the Lower and Middle Cambrian.

At the base of the strata of Sanashtykgol age, at least in the Biya salient, there is probably an unconformity, similar to that inferred for the northern slope of the Western Sayan (A. G. Sivov, 1953, et al.).

Higher in the section of the Altai Cambrian there follow almost exclusively sedimentary strata.

As already noted, in the near-Katun belt of Gorny Altai the Ust-Sema is overlain by an essentially sedimentary suite. It is a marine sequence with characteristic siliceous-jasper elements and a small quantity of effusives. It is composed mainly of shales—argillaceous, calcareous, and siliceous-argillaceous (jaspers), siltstones and polymictic, essentially quartz–feldspar sandstones of green and violet-red colors. Present are marbleized limestones and siltstones of various colors, conglomerates and conglomerate-breccias with clasts of sedimentary rocks. Lava sheets of diagenetic and partly greenstone (spilite) aspect occur, having an andesitic and andesite-basaltic composition, as well as tuffites and effusive conglomerates. Flysch-like rhythmicity is often manifested in the section of the suite.

The section of the suite is well represented in the area of the well-known Oroktoy marble deposit, where its thickness is estimated at approximately 3000 m.

A thick sequence of rhythmically bedded green-gray tuffites and quartz–feldspar sandstones, gravelites, conglomerates, argillaceous and siliceous shales with marbleized limestones, substantially overlying the effusive Cambrian accumulations, was recently distinguished by V. M. Sennikov, the author, and others along the northeastern margin of the Katun salient.

Organic remains are not known in this suite (the Kaim or Oroktoy suite) within the Katun salient. By its position in the section it should be assigned to the lower half of the Middle Cambrian.

In the Biya salient, on the right bank of the B. Isha River, in a similar sedimentary sequence, which is in unclear structural relations with other Cambrian strata and was described by K. D. Neshumaeva and others, a trilobite fauna of the lower half of the Middle Cambrian has recently been found (Granularia, Pagetia, Oryctocephalina, Erbiidae). The sequence is composed of gray, gray-green, and brown-violet argillaceous and marly shales and polymictic sandstones with separate beds of limestones, conglomerates with pebbles of diverse rocks, among which occur gray, medium-grained granodiorites or trondhjemites, pink alaskite granites, and greenish-gray fine-grained diorites.

In the south of the Biya salient, A. A. Zenkova, K. D. Neshumaeva, O. K. Poletaeva, the author, and others have recently studied approximately

a one-and-a-half-kilometer-thick series of shallow-water marine deposits—gray, green, and lilac clay shales, sandstones, greenish-gray marls, and oolitic limestones. In various parts of this suite trilobites and brachiopods have been collected. Among the trilobites identified are Acrocephalites, Acrocephalops, Anomocare, Agnostus, Clavagnostus, Diplagnostus, Grandagnostus, Phaidagnostus, Leiopyge, Liostracus, Loganellus, Olenoides, Orloviella, Papyriaspis, Prohedinia, Solenopleura. The age of the fauna falls within the second half of the Middle Cambrian, corresponding to the Paradoxides forchhammeri zone.

The suite rests, with a break, directly upon the sequence with the Sanashtykol fauna.

In one small area in the middle part of the Katun salient there is exposed a variegated conglomerate–sandstone–shale suite with impure limestones, similar to the one just described and approximately coeval with it, which the author calls the Chemal suite. Its exposed thickness is 550 m. It lies transgressively, with angular unconformity, on the rocks of the ancient porphyrite suite and is dated by trilobites, together with which brachiopods occur. The trilobites (Acrocephalites, Agnostus, Anomocare, Metanomocare, Orlovia, Peronopsis, Solenopleura) belong to the Paradoxides forchhammeri zone, being somewhat younger than the Middle Cambrian fauna from the Biya salient.

The Cambrian section of Altai south of the Biya salient is supplemented by an Upper Cambrian suite, which was studied by A. A. Zenkova, Zh. D. Nikolskaya, O. K. Poletaeva, K. D. Neshumaeva, and others. This suite along the Kul’bich River has a thickness of several hundred meters, consists of variegated conglomerates, sandstones, marls, and limestones, and contains a trilobite fauna (Glyptagnostus, Conokephalina, and representatives of Asaphidae, Hlaenidae, and Leiostogidae). According to some data, the suite rests directly on the eroded surface of granitoid-intrusion rocks, containing fragments of the latter in its conglomerates.

III

All geologists of Altai now agree in recognizing, or allowing for, a Precambrian age at least for part of the oldest, predominantly carbonate, sedimentary formations. At present, for Altai, it is not the presence of the Precambrian that may provoke discussion, but only the position of its upper boundary.

If we turn to the history of the identification and substantiation of the boundary between the Cambrian and the Precambrian in this region and in other regions of the Sayan–Altai folded area, as well as in many other areas, we can see that one or several of the following criteria were used: 1) paleontological evidence; 2) the vertical extent of the column beneath paleontologically characterized Cambrian beds; 3) correlation, by direct geological criteria, of the most ancient sequences of the region with Precambrian complexes of classic sections; 4) the presence of unconformities associated with dislocational, tectono-magmatic, and orogenic phases; 5) differences in the degree and intensity of metamorphism and dislocation; 6) differences in the primary composition and structure of complexes assigned to the Cambrian and to the Precambrian.

In many past attempts to establish and substantiate the boundary between the Cambrian and the Precambrian in various regions of the Sayan–Altai, a serious shortcoming lay in an insufficiently developed, and often one-sided, methodological approach to solving the problem. In the setting of the very complex geological development and structure of the oldest folded

complexes; this predetermined the insufficient persuasiveness of many schemes and conclusions and their instability.

Under such complex conditions of investigation, in order to obtain the most correct, reliable, and convincing results, it is especially important to adhere to a comprehensive geological analysis and a many-sided substantiation of the boundary under consideration. This may guard against hasty and incorrect conclusions and make it possible to find and substantiate in the section a truly essential, qualitatively important stratigraphic and historical boundary. At the same time, it is evidently necessary to introduce into the method some additional criteria as well.

Let us turn to the Altai section.

The eleven- to thirteen-kilometer thickness of non-metamorphic strata, older than the formations with Lower Cambrian fauna, inevitably compels one to assume the presence among them of Precambrian strata.

Up to the present time there have been no at all reliable finds of Lower Paleozoic fossils in any of the three oldest suites of the section described—the Sugara, Baratal, and ancient porphyrite suites—despite the fact that these strata have been subjected to systematic mapping, including detailed mapping, with paleontological searches. In these suites, just as in analogous strata of Gornaya Shoria and other regions of the Sayan-Altai, there are fossils known from the classic sections of the Precambrian. Unfortunately, in Altai these remains have not yet been specially studied, and there are no reliable paleontological determinations.

In the thick sedimentary accumulations of the two lowest series, a number of important compositional features make it possible to see analogues of known Precambrian marine strata of the Southern Urals, China, and regions of Precambrian platforms that occupy a similar position in the section. What is meant is the essentially carbonate composition of the strata, the abundance of siliceous sediments and magnesian rocks, including marine pelitogenic dolomites, the abundant organic carbonaceousness of the rocks, etc. These features, with the exception of the first, also apply to one degree or another to the sedimentary elements of the ancient porphyrite suite, which, as was indicated, is stratigraphically closely joined with the Baratal series.

Present data for the region as a whole show that the rocks of both the three oldest strata and the lower and lowermost parts of the Middle Cambrian have, in general, the same extreme degree of dislocation, almost everywhere occurring steeply, on their heads, or overturned. But the metamorphism of the sedimentary rocks of the lower complex generally reveals a somewhat higher intensity, which is especially expressed in the widely developed, considerable secondary redistribution of carbonate, siliceous, and carbonaceous matter. The effusive rocks of the Baratal and ancient porphyrite strata are characterized by an almost ubiquitous and generally significantly deeper greenschist alteration than the Lower Cambrian ones.

Of the Lower Cambrian strata, direct relationships with the more ancient suites, as already stated, are clearly established in the Katun uplift for the Kayancha suite, which lies with a major structural-denudational unconformity.

In the central part of the Katun uplift along the Katun River and on its right bank (the Chemal River), it is possible to establish a sharply different strike of the folds: northwestern for the Kayancha, Ust-Sema, and Kaim suites, and east-northeastern for the Baratal and ancient porphyrite suites. This discordance, manifested over an area of several hundred square kilometers, is somewhat masked by partial inheritance, as well as by partial reworking by the younger Cambrian-

by folds of the ancient folding of northeastern strike. The difference in the trends of the Cambrian folds from those of the more ancient suites was also noted in the northern part of the Katun uplift by M. K. Vinkman (1948-2).

A major unconformity with the oldest complex, if one takes into account the observed manifestations of sharp discordance of the folds, will apparently be characteristic of the entire Lower Cambrian present in the region.

The totality of the facts set forth leads us to the conclusion that the three oldest normal sequences of Altai should be assigned to the Precambrian.

This conclusion is confirmed by a comparison of the two delimited parts of the section in terms of the composition and structure of their sedimentary rocks and formations, which reflect the specific features of the geological conditions under which these complexes were formed. A comparative analysis of the principal features of the composition and rhythmicity of the rocks and formations, as we have been able to ascertain, provides important evidence for establishing the natural boundary between the Cambrian and the Precambrian on the basis of local material.

The most essential characteristic features of the composition and structure of the sedimentary parts of the lower complex are: a predominantly carbonate composition; abundant development of dolomites, including marine pelitogenic dolomites, and dolomitized limestones; widespread occurrence of purely siliceous sediments; a relatively poor composition of clastic rocks, practically limited only to products of erosion of rocks of this same complex; abundant organic carbonaceousness of the rocks, with which their usually dark and subdued colors are associated; often abundant primary pyritization of the sediments; a generally coarse rhythm of bedding, and weak stratification of thick carbonate sequences and members.

The sediments of the Cambrian complex are characterized chiefly by a terrigenous composition; among the carbonate rocks, dolomites and dolomitized limestones are uncharacteristic, occurring only rarely in the Kayancha suite; purely siliceous sediments are likewise little characteristic (the exceptions are some horizons of the Kaima suite); the composition of clastic material in the terrigenous rocks is rather rich and varied, beginning with the lower horizons; carbonaceousness in the sedimentary rocks is observed rarely, and they usually have light and saturated colors; the rhythm of bedding is, in general, finer and more varied than in the lower complex.

These characteristic features reflect very substantial differences in the geotectonic, paleogeographic, and geochemical setting of accumulation of the compared parts of the section.

In considering the boundary between the Altai Cambrian and Precambrian, attention may also be drawn to the succession and rhythm of the change in types of bedded formations in the normal stratigraphic section.

Thus, the Lower Cambrian and the lower part of the Middle Cambrian deposits, widely distributed in the region, document strong subsidence, with accumulation at first of terrigenous-limestone, then effusive-sedimentary, and then terrigenous sequences with limestones. Subsidence was complicated by temporary uplifts with erosion. The sharply shallow-water and molasse deposits of the Upper Cambrian, separated from the adjacent sequences by major breaks, characterize an increase in uplift.

The Lower Cambrian limestone-terrigenous deposits and the effusive-sedimentary deposits with abundantly represented basic lavas correspond to known formations of the very beginning of major geosynclinal cycles. It is appropriate to note that in the Priteletskii region they include bodies of hyperbasites. The large granodiorite intrusion of the south

of the Biya uplift and the Priteletskii area marks the beginning of the middle stage (reversal) of the geosynclinal cycle.

This circumstance, together with the already noted discordance of the folds of the reliably Cambrian strata, on the one hand, and of the more ancient sequences, on the other, argues for the geotectonic independence of the Cambrian complex, documenting a separate geotectonic cycle.

Thus, a whole set of criteria makes it possible to distinguish in Altai three oldest non-metamorphic sequences within the Precambrian—the Sugarin, Baratal, and ancient porphyrite sequences. This agrees with the views of a number of West Siberian geologists regarding the volume of the non-metamorphic Precambrian of other districts of the Sayano-Altai region (the Kuznetsk Alatau and Gornaya Shoriya, the Eastern Sayan, etc.).

Until recently, the age of the non-metamorphic Precambrian suites of Altai was considered by almost all investigators to be Middle Proterozoic, and the presence of the Upper Proterozoic here was denied. Recently V. A. Kuznetsov (1952, 1954), taking into account the analogy of these sequences with the known Riphean and Sinian complexes, as well as the generally close structural connection between the Precambrian and the Cambrian in the Sayano-Altai region, has assigned them to the Upper Cambrian. The author also tended toward the same view, assigning the ancient porphyrite suite to the upper horizons of the Precambrian. However, if the fact of the sharp discordance between the folds of the Precambrian and Cambrian strata is taken into account, it may be assumed that in Gornyi Altai, beneath the unconformably overlying Cambrian, a number of elements not only of the Lower Cambrian but also of the upper Precambrian are absent from the section.

LITERATURE

Vinkman M. K.—Stratigraphy of the oldest deposits of the northeastern part of Gornyi Altai. Doklady Akademii nauk SSSR, new ser., LXI, No. 1, 1948 (1).

Vinkman M. K.—Structure and stratigraphy of the ancient sequences of the Katun anticlinal mountainous uplift in Northern Altai. Trudy Gorno-geol. in-ta Zap.-Sib. filiala Akad. nauk SSSR, issue 2, 1948 (2).

Zhuravleva I. T.—Archaeocyathids of the Siberian Platform and their significance for Cambrian stratigraphy in Siberia. In: Problems of the Geology of Asia, vol. I, Publishing House of the Academy of Sciences of the USSR, 1954.

Zaitsev N. S. and Pokrovskaya N. V.—On the structure of adjacent parts of the Western Sayan and Tuva. Izv. AN SSSR, geological series, No. 6, 1950.

Krasnopeeva P. S.—The principal archaeocyathid complexes of Western Siberia. Vestnik Zap.-Sib. geol. upr., No. 2, 1947.

Krasnopeeva P. S.—Materials on the paleontology and stratigraphy of the archaeocyathids of Western Siberia. Doklady AN SSSR, new ser., vol. XCIX, No. 4, 1954.

Kuznetsov V. A.—Mercury deposits of Oirotia. Collection Oirotia, SOPS, Academy of Sciences of the USSR, 1937.

Kuznetsov V. A.—The Precambrian in Gornyi Altai. Trudy Gorno-geol. in-ta Zap.-Sib. filiala AN SSSR, issue 2, 1948.

Kuznetsov V. A.—The principal stages of geotectonic development of the south of the Altai-Sayan mountainous region. Trudy Gorno-geol. in-ta, Zap.-Sib. filiala AN SSSR, issue 12, 1952.

Kuznetsov V. A.—Geotectonic regionalization of the Altai-Sayan folded region. In: Problems of the Geology of Asia, vol. I, Publishing House of the Academy of Sciences of the USSR, 1954.

Kuznetsov Yu. A.—Geological structure of the central part of Gornyi Altai. Mat. po geol. Zap. Sibiri, issue 41, 1939.

Kuzmin A. M.—Materials on the stratigraphy and tectonics of the Kuznetsk Alatau, Salair, and the Kuznetsk Basin. Izv. Sib. otd. geol. komiteta, vol. VII, issue 2, 1928.

Nekhoroshev V. P.—Cambrian and Precambrian in Gornyi Altai. Vestnik geol. komiteta, No. 1, 1927.

Nekhoroshev V. P.—Materials for the geology of Gornyi Altai. Trudy Vsesoyuzn. geol.-razved. obedineniya, issue 177, 1932 (1).

Nekhoroshev V. P.—Geological sketch of Altai. Essays on the Geology of Siberia, 1932 (2).

Nekhoroshev V. P.—Tectonics of Altai. Proceedings of the XVII Session of the International Geological Congress, vol. II, 1937.

Nekhoroshev V. P.—On the Precambrian and on the peculiarities of the stratigraphy of Western Siberia. Sov. geol., collection 27, 1947.

Nekhoroshev V. P.—Features of the geological structure of Altai and its southwestern periphery. In: Problems of the Geology of Asia, vol. 1, Publishing House of the Academy of Sciences of the USSR, 1954.

Obruchev V. A.—Altai Etudes. On the tectonics of Russian Altai. Zemlevedenie, book 3, 1915.

Obruchev V. A.—Geological Survey of Siberia. Gosizdat, 1927.

Obruchev V. A.—Geology of Siberia, vol. 1, Academy of Sciences of the USSR, 1935.

Radugin K. V.—The Precambrian of the Altai–Sayan mountain region. Stratigraphy of the USSR, vol. I, 1939.

Radugin K. V.—Geological sketch of the Chemal sheet of Gorny Altai. Transactions of the Oirot Complex Expedition, SOPS, Academy of Sciences of the USSR, collection Gorny Altai, Geology, vol. I, 1941.

Radugin K. V.—Results and tasks of the study of the ancient Cambrian and Precambrian of Western Siberia. Transactions of the Mining-Geological Institute of the West Siberian Branch of the Academy of Sciences of the USSR, issue 2, 1948.

Sivov A. G.—Lower Cambrian of the Western Sayan. Bulletin of the Tomsk Polytechnic Institute, vol. 74, 1953.

Churakov A. N.—Geological structure of the eastern part of the Aigulak and the western margin of the Kurai ranges. Transactions of the Oirot Complex Expedition, SOPS, Academy of Sciences of the USSR, collection Gorny Altai, Geology, vol. I, 1941.

Corrections and Noticed Misprints

In S. A. Stroitelev’s article “Study of the Crystallization of Epsomite and Mirabilite,” for technical reasons braces have been replaced by square brackets, for example: [100] instead of {100}, etc.