None of units of the general stratigraphic scalebrings about so many controversial reasonings concern-ing the principles of recognition and subdivision, vol-ume, and stratigraphic position than the youngest (Ven-dian) Neoproterozoic strata. Despite the continuousaccumulation of new paleontological, isotopic-geo-chronological, and isotopic-chemostratigraphic data,the Vendian rocks are recognized largely on the basis ofhistorical-geological principle, which is based on theconcept of the most extensive marine transgression inthe Late Precambrian that occupied almost the entireterritory of cratons.
In this connection, reconstruction of the Vendianepoch in the geodynamic evolution of the Mezen Basinsituated in the northeastern EEC and filled with a thickUpper Vendian succession of alumosiliciclastic rocks isespecially important. It is assumed traditionally that theUpper Vendian succession of the Mezen Basin and theentire EEC was formed in a vast shallow-water epicon-tinental basin as the result of transgression induced bypostglacial eustatic rise of sea level and epeirogeneticsubsidence of the craton (Sokolov, 1952, 1997; Keller,
1963; Semikhatov, 1974; Khomentovsky, 1976; Postni-kova, 1977; Bessonova et al., 1980; Aksenov, 1985;Nikishin et al., 1996). It was suggested that the subsid-ence of crystalline basement in the northeastern part ofthe craton and the formation of the Late Vendian basincould be related to the postrift thermal subsidence ofthe lithosphere (Stankovsky et al., 1985; Kostyuchenkoet al., 1999). However, new data on deep structure(Olovyanishnikov et al., 1996; Olovyanishnikov, 1998;
The Neoproterozoic…
, 2004; Aplonov et al., 2006;Kostyuchenko et al., 2006) and radiometric dafing (Geeet al., 2000; Gorokhov et al., 2001) show that the north-eastern EEC margin underwent intense compressionfrom the Timan–Pechora region in the Late Vendian asthe result of collision with a system of island arcs andterrains. Hence, it may be suggested that the Late Ven-dian transgression was related to the geodynamic evo-lution of the adjacent foldbelt (Shatsky, 1952; Keller,1963; Bekker, 1968). However, data on the compositionand location of sediment provenances are insufficient todevelop more reliable paleogeodynamic reconstruc-tions.
Archean and Paleoproterozoic complexes of theBaltic Shield and inner EEC could serve as sediment
Composition of Sediment Provenances and Patterns in Geological History of the Late Vendian Mezen Basin
A. V. Maslov
a
, D. V. Grazhdankin
b
,
c
, V. N. Podkovyrov
d
, Yu. L. Ronkin
a
, and O. P. Lepikhina
a
a
Zavaritsky Institute of Geology and Geochemistry, Uralian Division, Russian Academy of Sciences, Pochtovyi per. 7, Yekaterinburg, 620075 Russia
Institute of Petroleum Geology and Geophysics, Siberian Branch, Russian Academy of Sciences, pr. akademika Koptyuga 3, Novosibirsk, 630090 Russia
c
School of Geological Sciences, University College Dublin, Belfield, Dublin 4, Ireland
d
Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences,nab. Makarova 2, St. Petersburg, 199034 Russia
Received September 17, 2007
Abstract
—Formation conditions of sedimentary successions in the Mezen Basin are considered on the basisof Cr, Th, Sc, Ni, Hf, and REE distribution and model Nd age of the Upper Vendian fine-grained terrigenousrocks. Geochemistry of mudstones and shales of the Lyamitsa, Verkhovka, Zimnie Gory, and Erga formationsin the Belomorian–Kuloi Plateau, as well as the Ust-Pinega and Mezen formations in the Vychegda Trough,does not allow us to consider these stratigraphic units as erosion products of the primitive Archean basement ofthe Baltic Shield or the central segment of the East European Craton (EEC) basement. Taking into account sed-imentological data on the direction of paleoflows in the basin and the model Nd age of the fine-grained terrig-enous rocks, we suggest that the Mezen Basin was filled in the Late Vendian mainly with erosion products ofthe Riphean igneous and metasedimentary complexes of the Timan–Pechora region. These conclusions are con-sistent with the sequence-stratigraphic architecture of sediments in the basin. According to the new model pro-posed, the Late Vendian Mezen Basin was a foredeep formed as a result of subsidence of the northeastern mar-gin of the EEC under the load of overthrusted rock masses of the Timan–Pechora Foldbelt. The clastic materialwas derived from the emerging orogen.
DOI:
10.1134/S002449020803005X
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COMPOSITION OF SEDIMENT PROVENANCES AND PATTERNS 261
provenances for the Late Vendian Mezen Basin, as wellas the Neoproterozoic metasedimentary and igneousrocks of the Timan–Pechora region (Ivanova et al.,1969; Aksenov, 1985; Solontsov et al., 1970; Sochavaet al., 1992; Olovyanishnikov, 1998). However, realcontribution of each of these large provenances to theformation of the Upper Vendian sedimentary succes-sions remains unclear so far.
The composition of pebbles in conglomerates andclastic material of sandstones is traditionally used forreconstruction of the sediment provenance composition(Baturin, 1947; Preobrazhensky and Sarkisyan, 1954;Shvetsov, 1958; Rukhin, 1969;
Grauvakki
, 1972; andothers). The fine-grained clastic rocks (shales, mud-stones, and fine-grained siltstones) were significantlyless involved in this process. At the same time, clayeyrocks have low permeability for postsedimentation flu-ids. They are mixed and homogenized much better thanthe coarse-grained sediments. Therefore, they serverecently as one of the main sources of information onseveral fundamental features of the formation of Pre-cambrian sedimentary successions (Taylor and McLen-nan, 1985;
2003).Precisely the fine-grained terrigenous sedimentaryrocks dominate in the sedimentary succession of theLate Vendian Mezen Basin. Therefore, in order toreconstruct the probable initial composition of rocks inprovenances, we analyzed their geochemical signaturesusing high-precision investigation methods.
GEOLOGY OF THE MEZEN BASIN
The Late Vendian Mezen Basin (~300000 km
2
inarea) is situated at the northeastern EEC marginbetween the eastern slope of the Baltic Shield in thewest and the Kanin–Timan Fold–Thrust Belt (FTB) inthe east (Fig. 1). The Mezen Basin was connected withthe Verkhnekama Basin in the southeast, bounded bythe arch of the Volga–Kama Massif in the south, andadjoined with the intracratonic Moscow Basin in thesouthwest.
Upper Vendian rocks of the Mezen Basin areexposed in coastal cliffs of the White Sea and along riv-ers that downcut deep valleys on the Onega Peninsulaand Belomorian–Kuloi Plateau (Grazhdankin, 2003).In the remaining part of the territory, the sedimentarysuccession was penetrated by numerous deep boreholesand traced along several seismic profiles (Aplonov etal., 2006). In the southeastern White Sea region, UpperVendian rocks are represented by the Lyamitsa,Verkhovka, Zimnie Gory, and Erga formations of theValdai Group (Grazhdankin, 2003). The Padun Forma-tion, which crowns the section of the Valdai Group, hasCambrian age (Grazhdankin and Krayushkin, 2007).Availability of good exposures and dense network ofboreholes with a high core recovery have made it pos-sible to develop for the southeastern White Sea regiona sequence-stratigraphic model of Upper Vendian sedi-
mentary rocks that can be applied to the entire MezenBasin.
SEQUENCE STRATIGRAPHY OF UPPER VENDIAN ROCKS IN THE MEZEN BASIN
The Upper Vendian sedimentary succession of thesoutheastern White Sea region is sandwiched betweenthe underlying subaerial erosion surface of the shelfand the overlying maximum regression surface. Weidentify for the first time four (Agma, Solza, ZimnieGory, and Erga) sequences in the Upper Vendian suc-cession (Fig. 2).
The Agma Sequence comprises the Lyamitsa For-mation and the lower subformation of the VerkhovkaFormation. The distinct lower boundary of thesequence is controlled by a significant lowering of sealevel and, probably, by subaerial erosion of shelf.Flooding of the territory did not leave appreciablesigns, except for a relatively thin (~1 m) gravelstonelayer in the lower part of the Lyamitsa Formation rep-resenting a transgressive systems tract (TST) of theAgma Sequence (Fig. 2). It is overlain by a thick (15–20 m) unit of maroon laminated mudstone. The bottomof this unit serves as a surface of maximum marineflooding of the territory. The thick regressive systemstract (RST) of the Agma Sequence begins from thismudstone unit and consists of cyclic alternation of thelaminated mudstones and mudstone/siltstone coupletswith interbeds of wave-rippled sandstones. Each cycliterepresents a shallowing-upward succession of bedswith the base composed of transgressive laminatedmudstones replaced upsection by a condensed
Timan-Pechora
Plate
UralFoldbelt
Moscow
200
km
1
2 3
56 7
84
Fig. 1.
Index map of the Upper Vendian sedimentary rocksin the Mezen Basin at the northeastern EEC margin. Thestudied sections: (1) Borehole Agma S18, Syuz’ma andSolza rivers; (2) Zimnie Gory; (3) Borehole Tuchkino 1000;(4) Borehole Kotlas; (5) Borehole Yarensk; (6) BoreholeSeregevo 1; (7) Borehole Storozhevsk 1; (8) Borehole Se-vernaya Keltma 1.
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MASLOV et al.
sequence corresponding to the peak of transgressionand consisting of siltstone/mudstone couplets with car-bonate interbeds. Each cyclite is terminated by a regres-sive member of siltstone/mudstone couplets with inter-beds of wave-rippled sandstones (storm deposits).Some chocolate-brown intervals of laminated mud-stones contain thin interbeds of volcanic tuff. The U–Pbzircon age of tuffs at the base of the Verkhovka Forma-tion is 558
±
1 Ma (Grazhdankin, 2003).The Solza Sequence comprises the upper subforma-
tion of the Verkhovka Formation. The lower boundaryof the sequence represents a maximum regression sur-face (MRS) and coincides with the marine flooding sur-face (MFS). This surface is overlain by alternating vio-let laminated siltstones, which include laterally discon-tinuous fine-grained sandstone beds with sharp erosionbottom and fine parallel and multistoried cross-bedding(casts of fluviomarine channels). The abundance ofsandstones as a matrix of flat-pebble conglomeratesand channel casts indicate a possible proximity to thedistributary system that supplied the clastic material.The remaining part of the sequence is characterized bycyclic structure: the base of each cyclite consists oflaminated siltstone and mud couplets, whereas the topis composed of laterally discontinuous packages of yel-lowish gray sandstones with wave-rippled and fine orcoarse parallel bedding. The beds are locally deformedwith the formation of folds and ball-and-pillow struc-tures. The lateral sucession of the Solza Sequence isinterpreted as a RST mainly related to a periodicalreplacement of relatively quiet settings of submarineplains by fluviomarine settings of sandy shoals.
The Zimnie Gory Sequence comprises sediments ofthe eponymous formation. Its base shows a forcedregression surface and downcutting of the underlyingrocks. The sedimentary succession of the Zimnie GoryFormation occurs only in northern sections of the WhiteSea region. Moreover, the thickness of the formation isreduced over a distance of 30 km from 200 m (BoreholeTorozhma) to 60 m (Zimnie Gory) and completelypinches out in the southwestern direction. The thinningis accompanied by the appearance of several strati-graphic hiatuses with indications of scouring that testi-fies to a substantial role of submarine erosion and prox-imity to the shoreline. Hence, the present-day boundaryof the occurrence of the Zimnie Gory Formation in the
Fig. 2.
Sequence-stratigraphic subdivision of the compositesection of the Upper Vendian sedimentary rocks in thesoutheastern White Sea region. (1) Laminated mudstones,(2) alternating siltstone/mudstone, (3) alternating sand-stone/mudstone, (4) channel facies, (5) alternating sandstones,(6) sandstones with multistoried cross-bedding, (7) sandstoneswith trough-shaped cross-bedding, (8) sandstones with flaserbedding. (TST) transgressive systems tract, (RST) regressivesystems tract, (LSST) lowstand systems tract, (HSST) high-stand systems tract, (SU) subaerial unconformity, (MRS) max-imum regression surface, (MFS) marine flooding surface,(FRS) forced regression surface.
MFS
TST
MFS
SU
1 2 3 4
65 7 8
MFSFRS
MRS
MFS
LSST
TST
RST
555.3 ± 0.3 Ma
558 ± 1 Ma
HSST
TST
MRS
MFS
RST
500
400
300
200
100
RST
erga
form
atio
npad
un f
orm
atio
nzi
mnie
gory
form
atio
nver
khovka
form
atio
nly
amit
sa f
orm
atio
n
600
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COMPOSITION OF SEDIMENT PROVENANCES AND PATTERNS 263
southeastern White Sea region approximately fits theboundary of paleobasin and the limited distribution ofthis formation is caused by sharp reduction of sedimen-tation area. The lateral facies association of the ZimnieGory Formation is interpreted as a lowstand systemstract (LSST). The base of the formation includes apacket of progressively overlapping quartz sandstones,gravelstones, and conglomerates with an MFS as theupper boundary. This surface is overlain by chocolate-brown mudstones with volcanic tuff interbeds. The U–Pb zircon age of tuff is 555.3
±
0.3 Ma (Martin et al.,2000). The middle part of the sequence consists of athick multilayer sandy unit with parallel and multisto-ried cross-bedding of heterogeneous facies. The strati-fication is locally complicated by diverse syngeneticsubmarine slumping that affected thick intervals. TheZimnie Gory Sequence is characterized by progradingstructure and terminated by transgressive silt-stone/mudstone couplets. The subsidence rate at thisstage was likely much higher than the input rate of clas-tic material.
The Erga Sequence corresponds to the Erga Forma-tion and is separated from the underlying LSST by theMRS with related washout and erosion downcuttings(depth as much as 30 m) filled with regularly interstrat-ified mudstones, siltstones, and sandstones character-ized by wave-rippled, fine parallel and gradational bed-ding and diverse submarine slumping (TST). Cyclicityof sedimentary successions and the transgressive char-acter of cyclites indicate the periodical migration offacies belts and suggest that the ongoing subsidence ofthe sedimentary basin was in equilibrium with theintensely input of clastic material. The maximum MFSis drawn at the top of this unit. The overlying variegatedrocks are composed of siltstone/mudstone coupletswith laterally discontinuous medium- and coarse-grained sandstone beds with coarse parallel bedding,multistoried cross-bedding, flat mudstone pebbles, andripple marks. This unit is related to the gradual aggra-dation of the deltaic plain under conditions of starvedbasin and is interpreted as a highstand systems tract(HSST). This interpretation is supported, in particular,by an extreme scarcity of sediments with features ofrepeated erosion or subaerial exposure of the territoryand the absence of tidal flat deposits.
TRACING OF SEQUENCE BOUNDARIES
Regression surfaces established in sections of thesoutheastern White Sea region are traced confidently inboreholes along the regional paleoslope of the MezenBasin, demonstrating the relatively persistent thicknessof lateral facies associations between these surfaces. In thereference section of Borehole Kotlas drilled at the junctionof the Late Vendian Mezen and Moscow basins, surfaces(sequence boundaries) and sedimentary successionsobserved at 2338–1623 m are similar to those in sectionsof the southeastern White Sea region situated 500 km tothe northwest (Fig. 3). The MFS is drawn at the base of
interval 2333.65–2338.45 m composed of brown(locally spotty) mudstones. At the same time, rocks atinterval 2338.45–2034.70 m make up a single lateralfacies association with the regressive systems track ofthe Agma Sequence. In Borehole Kotlas, sandstones atinterval 2034.70–1985.55 m show numerous signs oferosion (scour casts), clusters of flat clay pebbles, andgutter casts) and submarine slumping. The availabledata indicate that the sandstones most probably repre-sent fluviomarine sediments and make up a single lat-eral facies association together with the lower part ofthe Solza Sequence. Analogs of the upper part of theSolza Sequence are recognized confidently at 1985.55–1944.40 m. The wave-rippled sandstones with struc-tures of submarine slumping at 1944.40–1905.90 m areinterpreted as an LSST and correlated with rocks of theZimnie Gory Sequence. Interval 1905.90–1815.00 m inBorehole Kotlas is composed of an interstratification ofmainly yellowish gray mudstones, siltstones, and sand-stones. With respect to a number of attributes, whichcannot be considered here in more detail because ofspace limitation, this unit is comparable with the TSTof the Erga Sequence. The variegated unit (interval1815.00–1623.25 m) of interstratified mudstones, silt-stones, and sandstones with multistoried cross-beddingmakes up a single sequence with the RST of the ErgaSequence. Beginning from a depth of 1623.35 m, sec-tion in Borehole Kotlas contains poorly cemented darkred sandstones comparable with those in the lower partof the Lower Cambrian Padun Formation in the south-
Fig. 3.
Correlation of sequences and sequence boundaries inUpper Vendian sections of the southeastern White Searegion and Borehole Kotlas. Formations: (
lam
) Lyamitsa,(
ver
) Verkhovka, (
zgr
) Zimnie Gory, (
erg
) Erga,(
pad
) Padun. See Fig. 2 for other symbols.
MFS
MRS
MRSFRS
MRS
MRS
1815
1623
1906
1944
2034
2338
pad
erg
2
ver
2
ver
1
lam
zgr
555 Ma
558 Ma
Southeastern WhiteSea region
Borehole Kotlas
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MASLOV et al.
eastern White Sea region (Grazhdankin and Krayush-kin, 2007).
The thickness of the Upper Vendian sedimentarysuccession in the Mezen Basin increases across thepaleoslope from Borehole Kotlas toward the Kanin–Timan Fold–Thrust Belt. In the southeastern segmentof this basin (Vychegda Trough), the Upper Vendiancomprises a thick (more than 1300 m) succession ofcomplex mudstone/siltstone couplets (with sandstoneinterbeds) recovered by deep parametric boreholes(Olovyanishnikov, 1998). Low core recovery in theboreholes hampers the elaboration of the detailed strati-graphic scheme and facies–genetic investigation. Themost distinct sequence boundary is represented by theMRS at the bottom of the Erga Sequence traced confi-dently in borehole sections throughout the basin owingto the sharp facies change. This boundary divides theUpper Vendian sedimentary rocks of the VychegdaTrough into two units traditionally recognized as theUst-Pinega and Mezen formations. The lower unit(Borehole Seregovo, interval 2000–1550 m; BoreholeStorozhevsk, interval 2502–2070 m) is composed ofdark gray and greenish gray rocks, whereas the upperunit (Borehole Seregovo, interval 1550–1140 m; Bore-hole Storozhevsk, interval 2070–1266 m) is composedof variegated (greenish gray, yellowish gray, red-brown, and light gray) rocks (Fig. 4). In terms of lithol-ogy, the upper (variegated) unit is correlated with inter-val 1906–1623 m in Borehole Kotlas, which, in turn, iscomparable with the Erga Formation in the southeast-ern White Sea region. All these units taken togethermake up a single lateral facies association (ErgaSequence) bounded from below and above by the MRS.
In terms of volume suggested in our paper, the ErgaSequence corresponds to the Kotlin Stratohorizon,which represents a facies association (a part of sedi-mentary basin) bounded from below and above by sur-faces of the abrupt migration of facies (facies unconfor-mities). The thickness of the Kotlin facies association inthe Mezen Basin progressively increases from 280 m inthe southwest (Borehole Kotlas) to 800 m in the north-east (Borehole Storozhevsk), whereas the thickness of
the underlying Redkino facies association increases inthis direction insignificantly.
Sequence boundaries recognized in our work aretraced in boreholes in Upper Vendian sedimentary suc-cessions of the intracratonic basin in the Moscow Basin(Fig. 5). Here, The Upper Vendian succession is subdi-vided into the Pletenevka, Gavrilov-Yam, Nepeitsino,Makar’ev, and Lyubim formations (
Stratigrafiches-kaya
…, 1996). The Reshma Formation correlated withthe Padun Formation in the southeastern White Searegion is most likely a Lower Cambrian unit.
The sedimentary succession of the Pletenevka andGavrilov-Yam formations, as well as the lower subfor-mation of the Nepeitsino Formation in the MoscowBasin, is composed of cyclic alternation of laminatedmudstones (with volcanic tuff interbeds) and silt-stone/mudstone couplets (with rare sandstone inter-beds). Taken together, these rocks make up a single lat-eral facies association with the Agma Sequence of theMezen Basin. The thickness of this unit increases from40–50 m in the western part of the basin to 300 m in thenortheast. The overlying upper subformation of theNepeitsino Formation (interstratified sandstones, silt-stones, and mudstones) reaches 90 m in thickness andcorrelates with interval 2034–1944 m in Borehole Kot-las. Thus, the upper subformation of the NepeitsinoFormation makes up a single lateral facies associationwith the Solza Sequence. This unit is absent in the westand northwest of the Moscow Basin. Its maximumthickness is established in the northeastern area at theconjugation with the Mezen Basin. This unit is sepa-rated from the underlying rocks by an MRS.
The Zimnie Gory Sequence in the Moscow Basin isrepresented by the Makar’ev Formation confined to asmall northeastern area of the basin (eastern part of theYaroslavl Depression). Here, the Makar’ev Formation(200 m thick) is composed of alternating laminatedmudstones and siltstones along with quartz sandstones.This formation is unknown in the remaining part of theMoscow Basin.
Different levels of the Nepeitsino and Makar’ev for-mations are transgressively overlain by variegatedrocks of the Lyubim Formation, which is correlated
Southeastern WhiteSea region
540
385
1623
1906
2338
2000
1550
1136 1266
2070
2505
Kotlas Seregovo Storozhevsk
Fig. 4.
Position of the facies unconformity separating the Upper Vendian Redkino (I) and Kotlin (II) sequences along (southeasternWhite Sea region–Borehole Kotlas) and across (Borehole Kotlas–Borehole Storozhevsk) the regional paleoslope of the MezenBasin.
II
I
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COMPOSITION OF SEDIMENT PROVENANCES AND PATTERNS 265
with interval 1906–1623 m in Borehole Kotlas and withthe Erga Formation in the southeastern White Searegion. All these stratigraphic units make up a singleTST of the Erga Sequence. The base of this sequence inthe Mezen Basin includes a TST (Fig. 2), the thicknessof which attains 90 m in Borehole Kotlas (interval1906–1815 m). However, its reliable analogs are miss-ing in the Moscow Basin. It may be suggested that theMRS and MFS, which bound the TST of the ErgaSequence from below and above, respectively, mergeinto a single MRS at the bottom of the Lyubim Forma-tion in the intracratonic Moscow Basin.
The lower part of the Lyubim Formation at thesouthern, southwestern, and northwestern slopes of theMoscow Basin contain packages of sandstones of highmineralogical and textural maturity (Kirsanov, 1968),which most likely represent products of the extensivesediment reworking by bottom currents on a vast sandyshoal during the limited supply of a new clastic mate-rial. It is important to note that persistent conglomeratecovers, indications of subaerial exposure, or redepos-ited products of weathering have not been detectedhere. Regional hiatus established between the Redkinoand Kotlin stratohorizons during the compilation ofstratigraphic scheme for Vendian rocks in the MoscowBasin (
Stratigraficheskaya
…, 1996) is apparentlyrelated to subaqueous reworking during the lowstandrather than subaerial exposure and erosion in pre-Lyubim time. It may be suggested that the intracatonicbasin of the Moscow Basin was rather isolated and sit-uated beyond the sediment dispersal zone in the lateNepeitsino–Makar’ev time. Quartz sandstone packagesat the base of the Lyubim Formation are condensed sed-iments genetically related to the Solza and Zimnie Gorysequences of the Mezen Basin. These sediments wereformed as products of the extensive sediment rework-
ing by bottom currents under conditions of deficiencyin clastic material.
ARCHITECTURE OF THE LATE VENDIAN SEDIMENTARY SUCCESSION
IN THE MEZEN BASIN
Tracing of sequence boundaries and lateral faciesassociations in boreholes and along seismic profiles hasshown that sediments were almost continuously depos-ited during the entire Late Vendian in the Mezen Basinand only during two episodes of the highest sealevelstanding in the Moscow Basin (Fig. 6). The first epi-sode corresponds to the Agma time (Redkino transgres-sion); the second one, to the Erga time (Kotlin trans-gression). In contrast to the Moscow Basin, subsidenceof the Mezen Basin proceeded during the entire UpperVendian and was compensated by the input of clasticmaterial: sedimentation was not interrupted here evenin the Zimnie Gory time characterized by insufficientinput of clastic material to the main EEC territory.
The Upper Vendian sedimentary succession in theMezen Basin shows a boundary of drastic change in theprogradational sedimentation trend that coincides withthe Agma/Solza sequence boundary. The AgmaSequence is mainly composed of shallow-water marinesediments, whereas all other sequences consist of fluvi-omarine sediments (Grazhdankin, 2003). In the AgmaSequence, the most proximal sediments are confined toa narrow (no wider than 100 km) zone on the easternslope of the Baltic Shield characterized by thickeningof progradation wedges in the regressive part of cyclites(Grazhdankin, 2003). Distal facies of the AgmaSequence are related to the junction of the basin withthe Timan–Kanin FTB (Fig. 6).
Nevel Redkino Lyubim Kotlas
637
790
833
250 km
1365
1576
1714
2444
2690
2762
2819
2928
1623
1906
1944
2034
2338
lub
mak
nep
2
yam-nep
1
Fig. 5.
Tracing of boundaries in Upper Vendian rocks of the Moscow Basin. Formations: (
yam
) Gavrilov-Yam, (
nep
) Nepeitsino,(
mak
) Makar’ev, (
lub
) Lyubim.
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MASLOV et al.
Direction of the progradational trend abruptlychanges to the opposite one in the Solza, Zimnie Gory,and Erga sequences. The genetic composition of sedi-ments and their structure suggest the following conclu-sion: since the Solza time, sediments were deposited ina setting resembling subaqueous extension of deltasand shoreface in the wave-agitated and more distalzones, whereas the predominant subaerial part of deltaswas situated in the northeastern Timan–Kanin FTBarea. This is supported by the distinct unimodal distri-bution of paleocurrent directions established from mea-surements of the dip of cross laminas and the strike ofvarious erosion features, such as scour, gutter, andchannel casts (Grazhdankin and Bronnikov, 1996;Grazhdankin 2003, 2004; Grazhdankin et al., 2005).The proximal sedimentation setting was localizedalong the northeastern margin of the Mezen Basin at itsjunction with the Timan–Kanin FTB, while the distalsetting was situated on the eastern slope of the BalticShield; i.e., the proximal and distal facies were relatedto two large sediment provenances with principally dif-ferent compositions and ages of rock complexes.Therefore, it is necessary to carry out a special discus-sion concerning the composition of rocks in each sedi-ment provenance and its contribution to the Late Ven-dian sedimentary succession in the Mezen Basin.
MATERIALS AND METHODS
The composition of rocks in provenances of theMezen Basin was examined in more than 90 mudstonesamples taken by A.V. Sochava, D.V. Borkhvardt, andM.B. Gnilovskaya from cores of Borehole Tuchkino1000 drilled in the 1980s on the Belomorian–Kuloi Pla-
teau by Expedition no. 17 of the Nevskgeologiya Indus-trial-Geological Association. In addition, we sampledthe Upper Vendian mudstones and clays in outcrops andcores from boreholes that recovered the Upper Vendianrocks on the Onega Peninsula and coastal cliffs of Zim-nie Gory: at the bottom of the Lyamitsa Formation(Borehole Amga S18), in the middle part of theVerkhovka Formation (outcrops along the Syuz’ma andSolza rivers), and in the lower and upper units of theZimnie Gory Formation (outcrops in Zimnie Gory). Westudied Upper Vendian rocks of the Vychegda Troughin cores from boreholes North Keltma 1, Storozhevsk 1,Seregovo 1, and Yarensk (collection of V.G. Olovyan-ishnikov). We also used analytical data on the bulkchemical composition of mudstones from BoreholeTuchkino 1000 taken from the PRECSED database ofchemical analyses of the Precambrian sedimentaryrocks compiled at the Institute of Precambrian Geologyand Geochronology, St. Petersburg. Contents of traceelements (14 REEs, Li, Be, Sc, Ti, Cr, Ni, V, Co, Cu,Zn, Ga, Rb, Sr, Y, Zr, Nb, Mo, Sb, Cs, Ba, Hf, Ta, Tl,Pb, Th, and U) were determined by the ICP-MS methodat the Institute of Geology and Geochemistry, Yekater-inburg (O.P. Lepikhina and O.Yu. Popova, analysts).
The samples (~50 mg in weight) were treated in amixture of hydrofluoric and nitric acids (5: 1) at 150
°
Cin Teflon autoclaves up to the point of complete decom-position. After boiling down, the samples were trans-ferred to 3% nitric acid solution (coefficient of the ini-tial sample dilution is ~103) and investigated on an Ele-ment-2 ICP/HR-MS analyzer. Indium (
115
In) was usedas an internal standard; Basalt Columbia River samplesBCR-1 and BCR-2 (USGS), as external standards. Theexternal standard was measured consecutively after
NEMoscowSW Minsk
Moscow Basin(intracratonic basin)
Mezen Basin(foreland basin)
4
3
2
1
Fig. 6.
Sequence-stratigraphic architecture of the Mezen and Moscow basins in the Late Vendian. Sequences: (1) Agma, (2) Solza,(3) Zimnie Gory, (4) Erga. Arrows indicate directions of progradation trends.
LITHOLOGY AND MINERAL RESOURCES
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COMPOSITION OF SEDIMENT PROVENANCES AND PATTERNS 267
each 5–10 samples in order to check fluctuations ofdevice parameters. The analytical accuracy varied from3% for normal concentrations to 20–50 rel % for lowconcentrations close to the detection limits. Reproduc-ibility was checked by the analysis of randomlyselected duplicate samples.
The Sr and Nd isotopic compositions were deter-mined with isotope dilution on a Finnigan MAT-262mass spectrometer. All chemical procedures were per-formed in a clean room with the forced atmospheric airsupply through HEPA filters using Teflon and quartzlaboratory glasswares and specially purified reagents(Ronkin et al., 2005). Preparation of samples for themeasurement of Sm and Nd concentrations and theirisotopic compositions included the following proce-dures: (1) decomposition of samples; (2) separation ofREEs; and (3) separation of Sm and Nd.
The samples were decomposed in a mixture ofhydrofluoric and hydrochloric acids in Teflon auto-claves. The charge with a certain amount of
150
Sm +
149
Nd spike (based on the optimal spike (0.15 g)/Nd(1
μ
g) proportion) was kept at a temperature of 150–180
°
C until its complete decomposition. The reactionmixture was dried and treated with 10 N HCl to destroyfluorides. After repeated decomposition and evapora-tion, the dry residue was dissolved in 2 ml of 2.3 N HCland centrifuged. The obtained transparent solution wastransferred to the first chromatographic column withcationite (Dowex
50
×
8 200–400 mesh), where theREEs were separated from the major constituents of thesample by the stepwise elution with 2.3 N and 3.9 NHCl. Further, the eluate fraction with Nd, Sm, otherREEs, and traces of some other elements was boileddown and dissolved in 0.1 N HCl (0.6 ml). Sm and Ndwere separated in the second chromatographic columnwith di-(2-ethylhexyl) orthophosphoric acid applied onpolytrifluorochloroethylene (KEL-F).
The gradient elution with 0.1 N and 0.3 N HCl wasapplied for the more efficient separation of Nd and Smfrom traces of alkali earth elements. The procedureblank was commonly below 0.3 ng for Nd and 0.2 ngfor Sm. The Sm and Nd isotopic compositions in
150
Sm +
149
Nd spike and samples were measured on aFinnigan MAT-262 multicollector mass spectrometer instatic regime.
Rhenium filaments preliminarily annealed in anexhaust unit to eliminate admixtures were used as ionsources. The Nd isotope ratios were normalized to
146
Nd/
144
Nd = 0.7219. The internal statistics of massspectrometry provided a high precision of measure-ment results (>0.002 rel % for
143
Nd/
144
Nd). Reproduc-ibility of the results obtained was estimated from paral-lel measurements of the La Jolla standard (California,United States). Accuracy of determination of theSm/Nd ratio was estimated at 0.2–0.5% from the long-term reproducibility of BCR-1 and BCR-2 measure-ments. Parameters of evolution diagrams were calcu-lated using special regression softwares (Ludwig,
1992). The decay constant 6.54
×
10
–12
yr
–1
(Steiger andJäger, 1977) was used in age calculations.
LITHOCHEMISTRY OF THE FINE-GRAINED TERRIGENOUS ROCKS
In terms of chemical composition, the majority ofsamples studied correspond to chlorite–smectite–hydromica clays. Some samples correspond to hydrom-ica clays with abundant feldspar dissemination(Fig. 7a). The (Na
2
O + K
2
O) content ranges from 4.5 to7.0 wt %. According to (Yudovich and Ketris, 2000),the hydrolyzate module, HM = (Al
2
O
3
+ TiO
2
+Fe
2
O
3
+ FeO + MnO)/SiO
2
corresponds to normosial-lites (Fig. 7b). In the TiO
2
–TM diagram (Yudovich,1981), data points of the Upper Vendian mudstonesfrom the southeastern White Sea region overlap thefields of hydromica and smectite clays. In terms of themain lithochemical characteristics, the Upper Vendianmudstones from the Mezen Basin do not differ frommost Riphean and Vendian fine-grained terrigenousrocks from the central and eastern parts of the EEC(Sochava et al., 1992). In the log(SiO
2
/Al
2
O
3
)–log(Fe
2
O
3
/K
2O) diagram (Herron, 1988), the over-whelming majority of data points of the Upper Vendianmudstones from the southeastern White Sea region islocalized in the shale field, whereas the remaining datapoints are located in the wacke field (Fig. 7c). Hence,samples analyzed in our work are sufficiently homoge-neous, and geochemical signatures of mudstones maybe used, with certain reservations, for reconstruction ofthe composition and age of sediment provenances.
In the K2O/Na2O–SiO2/Al2O3 diagram (Fig. 8a),most data points of the Upper Vendian fine-grainedclastic rocks of the Mezen Basin1 fit the values charac-teristic of bulk compositions of representative samplesof clayey rocks (Ronov and Khlebnikova, 1961; Petti-john, 1978). In our opinion, like the PAAS-normalizedcontents of some trace elements in the Verkhovka andErga formations (Figs. 8b, 8c), this similarity indicatesthe lack of significant postsedimentary redistribution ofmajor and minor elements in the Upper Vendian clayeyrocks of the Mezen Basin.
Relationships between modules, such as TM(TiO2/Al2O3), FM (FeO + Fe2O3 + MnO)/(Al2O3 +TiO2), SPM (Na2O + K2O/Al2O3, and HM, make it pos-sible to reconstruct the initial nature of fine alumosilici-clastic rocks (Yudovich and Ketris, 2000). Mudstonesof the Lyamitsa Formation (n = 9) reveal positive TM–FM correlation (r = 0.74) and negative SPM–HM cor-
1 Mudstones from the upper part of the Erga Formation (BoreholeTuchkino 1000, interval 200–290 m) with K2O/Na2O >> 10.0 isthe only exception. Only these samples may be regarded, with acertain degree of conventionality, as rocks affected by potassicmetasomatism. Our previous data (Grazhdankin et al., 2005) alsodo not confirm the substantial postsedimentary alteration of theUpper Vendian clayey rocks of the Mezen Basin (Malov, 2003,2004).
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MASLOV et al.
relation (r = –0.53). However, the K2O/Al2O3 ratio inthese mudstones varies from 0.14 to 0.25, most likelyindicating a substantial contribution of the lithogeniccomponent (Cox et al., 1995). Mudstones of theVerkhovka Formation (n = 29) are characterized by aweak positive TM–FM correlation (r = 0.29) and a dis-tinct negative SPM–HM correlation (r = –0.56). TheK2O/Al2O3 ratio in these mudstones is low (medianvalue is 0.22). In terms of TM–FM (–0.40, –0.70) andSPM–HM (0.05, –0.50) relationships and theK2O/Al2O3 ratio (0.26 and 0.28, respectively, the fine-grained terrigenous rocks of the Zimnie Gory (n = 7)and Erga (n = 22) formations are most likely lithogenicproducts.
The median K2O/Na2O value is 2.16 in mudstonesof the Lyamitsa, 2.31 in the fine-grained terrigenousrocks of the Verkhovka Formation, 5.14 in rocks of theZimnie Gory Formation, and 12.61 in mudstones andclays of the Erga formations. This fact indicatesinvolvement of sufficiently mature (in terms ofgeochemistry) blocks of the continental crust in theLate Vendian erosional processes (Engel et al., 1974).
DISTRIBUTION OF Th, Sc, Cr, Ni, Hf, AND REE IN THE FINE-GRAINED TERRIGENOUS ROCKS
The median Th, Sc, Cr, Ni, and Hf contents in theUpper Vendian mudstones of the southeastern WhiteSea region are 13.8, 17.4, 80.8, 40.3 and 4.9 ppm,respectively. In the southeastern Mezen Basin, the cor-responding contents are 11.4, 13.0, 78.8, 39.9, and5.4 ppm, respectively. Table 1 shows maximal and min-imal contents of these elements. It should be noted thatthe highest Cr content in mudstones of the VychegdaTrough reaches 100 ppm, whereas the lowest value isonly 18.5 ppm. In the southeastern White Sea region,the Cr content more than 100 ppm was detected only in6 samples among 47 analyzed mudstones. The lowestCr content here is 36 ppm. The median Cr and Ni con-tents are rather low and atypical of erosion products ofthe Archean primitive protoliths. In the Ni–Cr diagram(Taylor and McLennan, 1985), data points of the UpperVendian mudstones from the Mezen Basin fall withinthe field of post-Archean fine-grained terrigenousrocks.
In the Co/Hf–Ce/Cr diagram (Fig. 9a), data points ofthe Upper Vendian mudstones from the Mezen Basinare localized close to model geochemical objects, such
0.80.60.40.20SPM
(a)100
10–1
10–2
10–3
Erga Formation
Zimnie Gory Formation
Verkhovka Formation
Lyamitsa Formation
FM
86420Na2O + K2O
(b)0.8HM
0.6
0.4
0.2
Supersiallite
Normosiallite
Hyposiallite
2.0
1.5
1.0
0.5
0
–0.5
–1.02.52.01.51.00.50
log(SiO2/Al2O3)
log(Fe2O3/K2O)
(c)
III
II
I
VI
IV
Fe-shale
Fe-sandstone
Sublitha-renite
QuartzareniteSubarkoseArkose
Wacke
Shale
Fig. 7. Data points of Upper Vendian mudstones from theBelomorian–Kuloi Plateau plotted on the (a) SPM–FM,(b) (Na2O + K2O)–HM, and (c) log(SiO2/Al2O3)–log(Fe2O3/K2O) diagrams. Fields of clayey rocks in theSPM–FM diagram (Fig. 7a): (I) kaolinite-dominated,(II) smectite-dominated with kaolinite and hydromicaadmixtures, (III) chlorite-dominated with Fe-hydromicaadmixture, (IV) chlorite–hydromica, (V) chlorite–smectite–hydromica, (VI) hydromica with significant feldspar dis-semination.
V
LITHOLOGY AND MINERAL RESOURCES Vol. 43 No. 3 2008
COMPOSITION OF SEDIMENT PROVENANCES AND PATTERNS 269
101
100
10210110010–1
SiO2/Al2O3
K2O/Na2O
Erga Formation
Zimnie Gory Formation
Verkhovka Formation
Lyamitsa Formation
101
100
10–1
(a)
(b)
101
100
10–1
(c)
Sc V Cr Co Ni Y La Ce Sm Eu Gd Tb Dy Ho Yb Lu Hf Th
Rock/PAAS
10
15
64
12
11
1
3
2
13
9
5
87
Fig. 8. Data points of mudstones and shales from various Upper Vendian formations of the Belomorian–Kuloi Plateau plotted onthe (a) K2O/Na2O–SiO2/Al2O3 diagram and PAAS-normalized contents of some trace elements in the fine-grained clastic rocksfrom the (b) Erga and (c) Verkhovka formations.Data points in panel (a): (1, 2) Precambrian slates; (3) Cambrian slates; (4) varved mudstone of the Huron Supergroup; (5) averageshale, after Clarke; (6) composite sample of 27 Mesozoic and Cenozoic mudstones; (7) composite sample of 57 Paleozoic mud-stones; (8) composite sample of 29 Paleozoic slates, 1 Mesozoic mudstone, and 6 Precambrian mudstones; (9) composite sampleof 33 Precambrian slates; (10) composite sample of 235 mud specimens from the Mississippi River delta; (11, 12) late glacial varvedclays; (13) Precambrian mudstones, after (Pettijohn, 1977); (14) marine clays; (15) continental clays of cold and moderately coldclimate, after (Ronov and Khlebnikova, 1961).
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LITHOLOGY AND MINERAL RESOURCES Vol. 43 No. 3 2008
MASLOV et al.
Table 1. Median, minimal, and maximal contents of some trace elements (ppm) in Upper Vendian mudstones of the Belo-morian–Kuloi Plateau and Vychegda Trough
Element
Belomorian–Kuloi Plateau Vychegda Trough
median minimumcontent
maximumcontent median minimum
contentmaximum
content
Sc 17.42 6.07 26.57 13.01 1.67 20.08
Cr 80.81 35.98 146.09 78.77 18.55 100.00
Ni 40.28 0.18 66.90 39.87 5.15 48.19
Ce 95.23 49.83 122.69 75.34 16.95 110.43
Hf 4.86 3.00 9.45 5.38 0.59 7.87
Th 13.83 2.85 19.97 11.36 1.92 19.85
as Proterozoic cratonic shales (Condie, 1993) and thepost-Archean average Australian shale (PAAS) (Taylorand McLennan, 1985). The Ce/Cr ratio, one of the mostsensitive indicator of existence of erosion products ofthe primitive Archean protoliths (Maslov, 2007), is0.75–0.80 or more in many analyzed samples of theUpper Vendian mudstones and only 0.12 in the Archeancratonic shales (Condie, 1993). The Co/Hf ratio is usu-ally less than 5 in the Upper Vendian mudstones of the
Mezen Basin and more than 11 in the average Archeanmudstone (Taylor and McLennan, 1985). In addition,about 60% of data points of mudstones plotted on theTh–La diagram are localized close to the PAAS. Hence,the Archean protoliths probably did not play a signifi-cant role in the formation of Late Vendian sedimentarysuccessions in the Mezen Basin. The fine alumosilici-clastic material delivered to the sedimentary basin wassignificantly mature and most likely represented ero-sion product of post-Archean rocks.
The median total REE content is 210 ppm (range81–273 ppm) in the Upper Vendian mudstones of thesoutheastern White Sea region and 180 ppm (46–263 ppm) in fine-grained terrigenous rocks of theVychegda Trough. Hence, a rather mature continentalcrust dominated in the Late Vendian in the drainagesystems that surrounded the Mezen Basin.
In mudstones, correlation links between La and Yb,on the one hand, and Al2O3, K2O, TiO2, and P2O5, onthe other hand, show specific patterns for each UpperVendian lithostratigraphic level (Table 2). Therefore, itis difficult to make any definite conclusions. For exam-ple, mudstones of the Lyamitsa Formation reveal a sig-nificant positive correlation of La and Yb with Al2O3,TiO2, and P2O5, whereas correlation with K2O isabsent. In these mudstones, REEs are associated notonly with the phosphate phase and Ti-bearing minerals,but also with the clay fraction. In mudstones of theVerkhovka Formation, La has a positive correlationwith Al2O3 and K2O (r = 0.30 and 0.56, respectively),whereas Yb lacks such a correlation. These elementsare not also correlated with oxides of Ti and P. In clayeyrocks of the Erga level, La lacks significant correlationwith oxides of Al, Ti, and P; coefficient of correlationbetween La and K2O is only 0.15; and Yb does not cor-relate with any of the above-mentioned oxides. In addi-tion, La and Yb in mudstones of the entire Upper Ven-dian succession do not demonstrate any significant pos-itive correlation with oxides of Al, K, Ti, and P.
In the Eu/Eu*–Th/Sc diagram (Fig. 9b), data pointsof the Upper Vendian mudstones from the Mezen Basinoccupy a transitional position between the Archean and
Table 2. Coefficients of correlation of La and Yb with ma-jor oxides in Upper Vendian lithostratigraphic subdivisionsof the Belomorian–Kuloi Plateau and in the sedimentary suc-cession as a whole
Elementand oxide
Formation TotalsuccessionErga Verkhovka Lyamitsa
La–0.47 0.30 0.54 0.04
Al2O3
La0.15 0.56 –0.41 0.12
K2O
La–0.28 0.16 0.47 –0.19
TiO2
La–0.43 –0.19 0.66 –0.17
P2O5
Yb–0.53 –0.23 0.28 –0.14
Al2O3
Yb–0.11 0.16 –0.54 0.14
K2O
Yb–0.07 –0.42 0.20 –0.30
TiO2
Yb–0.55 0.14 0.74 –0.14
P2O5
Number of samples 16 22 6 44
LITHOLOGY AND MINERAL RESOURCES Vol. 43 No. 3 2008
COMPOSITION OF SEDIMENT PROVENANCES AND PATTERNS 271
2.5
2.0
1.5
1.0
0.5
0
(a)
Mudstones of the Belomorian–Kuloi PlateauMudstones of the Vychegda Trough
20161284Co/Hf
Ce/Cr
101
100
10–1
10–2
1.21.00.80.60.4
(b)
Th/Sc
1.2
1.0
0.8
0.6
0.43.02.52.01.51.00.50
Eu/Eu*
GdN/YbN
Eu/Eu*
Post-Archeancratonic sediments
AR-G
AR-TTG
PR-TTG
PR-F
AR-F
PR-B
AR-B
1
2
3
(c)
PR-G
Fig. 9. Data points of Upper Vendian mudstones from thenorthwestern and southeastern parts of the Mezen Basinplotted on the (a) Co/Hf–Ce/Cr, (b) Eu/Eu*–Th/Sc, and(c) GdN/YbN–Eu/Eu* diagrams. (1) Archean cratonicshales, after (Condie, 1993); (2) Proterozoic cratonic shale,after (Condie, 1993); (3) post-Archean average Australianshale, after (Taylor and McLennan, 1985).
Proterozoic granitoids, on the one hand, and silicic vol-canics of the same ages, on the other hand. A rather lowLaN/YbN ratio in the analyzed mudstone samples indi-cates the absence of Archean tonalite–trondhjemitegneisses in the sediment provenances.
In the Eu/Eu*–GdN/YbN diagram (Fig. 9c), only4 samples (~6%) among 62 analyzed are depleted inHREE. Under certain conditions, this fact could sug-gest the erosion of primitive Archean rocks (Taylor andMcLennan, 1985, 1995; Panahi and Young, 1977; andothers). Other data points are located in the field ofpost-Archean cratonic sediments.
The chondrite-normalized REE patterns of mud-stones from the Mezen Basin confirm that the maturepost-Archean rocks dominated in provenances. Theprominent negative Eu anomaly (Fig. 10) indicates asubstantial role of intracrustal fractionation in feedingprovinces (Taylor and McLennan, 1985). Low LaN/YbN
values (7.82–11.26) testify to the occurrence of post-Archean igneous rocks in provenances (Condie andWronkiewicz, 1990). Relatively flat REE patterns areinherent to erosion products of basic rocks (Taylor andMcLennan, 1995) (Table 3). According to (Belyakovaand Stepanenko, 1991; Pease et al, 2004), the REE pat-terns of mudstones from the Mezen Basin (based onmedian REE contents in particular lithostratigraphicunits) are close to the REE patterns of granitoids fromthe Timan–Pechora region penetrated by boreholesIzhma 1–Izhma 3; Pechora 2; and Bol’shaya Zemlya 1(Fig. 11).
Vertical (in coordinates of conditional time) varia-tions of LaN/YbN, LREE/HREE, Eu/Eu*, and GdN/YbN
in the Upper Vendian mudstones were established onlyfor the southeastern White Sea region. The LaN/YbN
ratio varies from 5.5 to 12.2. The highest values(11.2 to 12.2 in 7 samples and 7.9–9.6 in 2 samples)were noted in the Verkhovka mudstones. Dispersion ofthe LaN/YbN ratio is minimal in the Verkhovka mud-stones and markedly higher in the Erga and Lyamitsaformations (e.g., 5.5–11.2 in mudstones of theLyamitsa Formation). The LREE/HREE ratio in Bore-hole Tuchkino 1000 usually varies from 8 to 10. How-ever, this ratio increases to 12–15 in some samples fromthe lower part of this formation, the Zimnie Gory For-mation, and the base of the upper subformation of theErga Formation. The enrichment in LREE is character-istic of the post-Archean fine-grained sedimentaryrocks (Taylor and McLennan, 1985, 1995; and others).The Eu/Eu* ratio commonly remains stable in the ver-
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LITHOLOGY AND MINERAL RESOURCES Vol. 43 No. 3 2008
COMPOSITION OF SEDIMENT PROVENANCES AND PATTERNS 273
tical direction (median 0.65, standard deviation 0.04)and is slightly higher (median 0.66, standard deviation0.06) only in the fine-grained terrigenous rocks of theLyamitsa level. Mudstones from the Zimnie Gory For-mation (one sample from Borehole Tuchkino 1000 andtwo samples from outcrop in Zimnie Gory) are charac-terized by Eu/Eu* = 0.55–0.61. In the lower one-thirdof the Erga Formation, two samples yielded Eu/Eu* =0.69–0.70. The GdN/YbN ratio does not show any vari-ations in the section and commonly equals 1.40–1.95,which is typical of the post-Archean shales (Taylor andMcLennan, 1985). All these facts suggest the absenceof any cardinal changes in the input of fine alumosilici-clastic material during the entire Late Vendian and,consequently, the stability of sedimentation systemsand clastic material delivery from different sources.
MODEL ND AGE OF THE FINE-GRAINED TERRIGENOUS ROCKS
Estimation of the model Nd age of fine-grained ter-rigenous rocks from the sufficiently thick sedimentarysuccessions is one of the most sensitive tools for thereconstruction of rock age in provenances (O’Nions etal., 1983; Frost and O’Nions, 1984; Miller and
O’Nions, 1984, 1985; Miller et al., 1986; Burwash etal., 1988; Dia et al., 1990; Li and McCulloch, 1990;Kovalenko et al., 1999; Kovach et al., 1999, 2000; Pod-kovyrov, 2001; Podkovyrov et al., 2002). This approachis based on the following concept: volume of the conti-nental crust extracted from the mantle at a certainmoment is characterized by certain chemical and isoto-pic parameters. Their knowledge makes it possible tocalculate the inferred timing of differentiation of com-ponents from the mantle. It is assumed that Sm and Nddo not undergo isotope fractionation during the forma-
103
102
101
100
LaCe
PrNd
SmEu
GdTb
DyHo
ErTm
YbLu
(a)
Erga Formation
Zimnie Gory Formation
Verkhovka Formation
Lyamitsa Formation
Ust-pinega Formation
Mezen Formation
LaCe
PrNd
SmEu
GdTb
DyHo
ErTm
YbLu
(b)
Borehole Izhma 1
Borehole Izhma 2
Borehole Izhma 3
Borehole Pechora 1
Borehole Pechora 2
Borehole Bol’shaya Zemlya 1
Borehole Bol’shaya Zemlya 2
Fig. 11. Chondrite-normalized REE patterns of Upper Vendian rocks (median values): (a) mudstones of Belomorian–Kuloi Plateauand Vychegda Trough; (b) granitoids of the Timan–Pechora region.
Table 3. Median ratios of chondrite-normalized REE con-tents in Upper Vendian mudstones of the Belomorian–KuloiPlateau and Vychegda Trough
Formation LaN/YbN LaN/SmN GdN/YbN Eu/Eu*
Erga 9.01 3.55 1.64 0.64
Zimnie Gory 9.94 4.58 1.42 0.60
Verkhovka 11.26 4.29 1.71 0.64
Lyamitsa 7.82 3.25 1.68 0.67
Mezen 8.17 3.42 1.48 0.69
Ust-Pinega 8.62 3.28 1.75 0.68
274
LITHOLOGY AND MINERAL RESOURCES Vol. 43 No. 3 2008
MASLOV et al.
tion of sedimentary rocks. Therefore, parameters of theSm–Nd isotopic system in the fine-grained clastic rocksallow us to judge about the model age of large continen-tal segments (provenances) (Miller and O’Nions, 1984;Dia et al., 1990; Kovalenko et al., 1999; Kovach et al.,2000; Kotov, 2003).
We calculated the model Nd age (TDM) for 10 sam-ples of the Upper Vendian mudstones and 2 samples ofthe Padun silty mudstones (Table 4, Fig. 12). This set ofsamples characterizes the Borehole Chidviya 770 sec-tion and several outcrops, including the outcrop in Zim-nie Gory. The model Nd age of mudstones from thelower part of the Lyamitsa Formation is 1.53 Ga. Thefine-grained terrigenous rocks of the Verkhovka Forma-tion have the model age of 1.73–1.71 Ga. Approxi-mately the same value was obtained for mudstones ofthe Erga Formation, whereas clayey rocks of the Zim-nie Gory Formation have a model age of 1.57–1.53 Ga.The model Nd age of silty mudstones of the Padun For-mation is 1.66–1.64 Ga.
DISCUSSION
Lithochemistry of the Upper Vendian mudstones inthe Mezen Basin is distinguished by relatively highcontents of lithophile elements, probably, related toeither multiple redeposition (recycling) or localizationof sediment provenances in a humid paleoclimatic
belt in the second half of the Late Vendian (Grazh-dankin et al., 2005). Another distinctive feature, highK2O/Na2O ratio, suggests the involvement of geochem-ically mature blocks of the continental crust in the LateVendian erosional processes. In terms of otherlithochemical characteristics, the Upper Vendian mud-stones of the Mezen Basin do not differ much from themajority of other Riphean–Vendian fine-grained terrig-enous rocks in the central and eastern regions of theEEC. Samples analyzed in our work are sufficientlyhomogeneous for the estimation of composition andage of sediment provenances.
The Late Vendian Mezen sedimentary basin wassurrounded by contrasting petrographic provinces: theBaltic Shield (Kola–Karelian Geoblock) in the west,inner regions of the EEC in the south, and the Timan–Pechora Plate in the north and east. Each of these prov-inces could serve as sediment provenance (Aksenov,1985; Olovyanishnikov, 1998; Puchkov, 1997, 2000;Olovyanishnikov et al., 2000; Roberts and Siedlecka,2002).
Archean and Proterozoic rocks are exposed now inthe Kola–Karelian Geoblock of the Baltic Shield(Dokembriiskaya…, 1988; Obshchyaya…, 2002; Ran-nii…, 2005; Slabunov et al., 2006). The Archean rocks(~85% of the entire territory of this geoblock) are com-posed of tonalite–trondhjemite gneisses, granites, mig-matites, granulites, tholeiitic basalts, metamorphosed
Table 4. Model Nd age of Upper Vendian mudstones and Lower Cambrian Padun Formation in the Mezen Basin
Note: (*) Conditional stratigraphic age accepted for the calculation of εNd(T); (**) uncertainty is not greater than ±0.5% (±2σ); (***) cal-culated after (DePaolo, 1981). The concentration and isotopic composition of Sm and Nd were determined with isotopic dilutionusing a mixed (149Sm + 150Nd) spike and acidic decomposition of starting material (Ronkin et al., 2005). Reproducibility ofthe measured 143Nd/144Nd and 147Sm/144Nd ratios was monitored with La Jolla and BRC-2 standards. Location of the analyzedsamples: (grd1) Borehole Agma S18, depth 205 m; (grd2) 12 m above the foot of a large outcrop of the Verkhovka Formation nearthe Suzma Settlement; (pm21) Borehole Chidviya 770, depth 553.7 m; (pm17) the same borehole, depth 523.5 m; (pm13) the sameborehole, depth 470 m; (grd5) outcrop in Zimnie Gory, 15 m above the bottom of the Zimnie Gory Formation; (grd6) outcrop inZimnie Gory, 64 m above the bottom of the Zimnie Gory Formation; (pm12) Borehole Chidviya 770, depth 413.5 m; (pm10)the same borehole, depth 320 m; (pm7) the same borehole, depth 248 m; (pm5) the same borehole, depth 151 m; (pm1) the sameborehole, depth 74.3 m.
TDM***
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ferrobasalts, metadacites, metarhyolites, gabbroan-orthosites, and alkali granites. The Paleoproterozoicrocks (~10% of the territory) consist of diverse rocks(products of weathering, terrigenous rocks with con-glomerates, tillites, molasses, black shales, stroma-tolitic dolomites, and volcanic rocks in association withstratiform metapyroxenites, granophyres, intrusivegabbroic rocks, and ophiolitic gabbros. The Ripheanrocks are represented by conglomerates, arkosic sand-stones, and shales (Turii Formation, Rybachii Group,and others), as well as gabbronorites, charnockites, gra-nodiorites, alkali granites, and granosyenites(Dokembriiskaya…, 1988).
Igneous associations and supracrustal complexes ofthe main lithotectonic elements of the Baltic Shield arecharacterized by the following TDM values (Ga): KeretComplex 2.80, Central Belomorian Belt 2.84, ChupaBelt 3.00–2.83, Kola Complex 3.06, West PuolankaComplex 3.23–2.83, Voche-Lambina Belt 2.76 and2.81–2.87, Keivy Slate Belt 2.81, Ranua Terrane 3.48,tonalites of North Finland 3.20–3.50, tonalitic gneissesin the vicinity of the Kola Superdeep 2.85–2.95, andKolvitsa Belt 2.83–2.82 (Slabunov et al., 2006).
In terms of composition and structure, the centralEEC segment was most likely similar with the BalticShield in the Late Vendian (Bogdanova, 1986; Geolog-icheskaya…, 1996).
The basement of the Timan–Pechora region is com-posed of the Meso- and Neoproterozoic metasedimen-tary and igneous rocks (Belyakova and Stepanenko,1991; Gee et al., 2000, The Neoproterozoic…, 2004).The basement is exposed in the Kanin–Timan Range.The Izhma Plate and the Neoproterozoic accretionarycomplex situated on the northeastern side (Pechora,Bol’shaya Zemlya, and Varandei–Adz’va collisionalzones) are the major tectonic units of the Timan–Pechora region. The Izhma zone is composed of theRiphean metapsammitic and metapelitic rocks that arecrosscut by granites dated at 550–560 Ma. Volcaniclas-tic rocks, gabbros and diorites dated at 565 Ma domi-nate in the Pechora zone. Granites are subordinate here.In the Bol’shaya Zemlya zone, metavolcanic andmetavolcaniclastic rocks are associated with rhyolites,rhyolite porphyres, granophyres, and granitoid intru-sions varying from 618 to 567 Ma in age. The positivegravity and magnetic anomalies inherent to this zonemost likely indicate the occurrence of numerous maficintrusions in the basement. According to the geophysi-cal data, ultramafic rock massifs are widespread in theVarandei–Adz’va zone. They are relicts of the pre-Uraloceanic crust spatially associated with the Neoprotero-zoic (670 Ma) ophiolites Engane-Pe and island-arcassociations of the Marun-Keu Complex.
The results obtained allowed us to estimate roughlythe contribution of each probable sediment provenanceto the deposition of fine alumosiliciclastic material inthe Upper Vendian Mezen Basin.
Based on analytical data in (Pease et al., 2004), wecalculated model Nd ages of intrusive rocks (syn- andposttectonic granites) in the Timanide folded basement.The TDM age is 1.80–1.48 Ga for granitic rocks of theIzhma zone and 0.8–1.0 Ga for intrusive rocks of thePechora zone. Both Early Riphean (TDM ~ 1.5 Ga) andLate Riphean (TDM ~ 0.98–1.0 Ga) granitioids aredeveloped in the Bol’shaya Zemlya zone. Within therelatively small error limits, the model Nd age of theUpper Vendian mudstones from the Belomorian–KuloiPlateau (1.75–1.53 Ga) is comparable with the oldermodel ages of granitoids in the Timan–Pechora region(1.8–1.5 Ga). Hence, the overwhelming majority of theUpper Vendian mudstones could be related to the ero-sion of exhumed complexes of the folded basement ofthe Timan–Pechora region. Fine alumosiliciclasticrocks could also be derived from younger (1.0–0.8 Ga)granitoids. However, their contribution could not sig-nificantly exceed 5%. If the fine alumosiliciclasticmaterial was supplied to the Mezen Basin from the Bal-tic Shield (median model Nd age ~2.9 Ga), its contribu-tion could not also exceed 5%, given that only oldergranitoids were eroded in the Timan–Pechora region.Finally, the model Nd ages calculated for the Upper
1800170016001500Age, Ma
1000
800
600
400
200
0Depth, m
Padun Formation
Erga FormationZimnie Gory Formation
Verkhovka Formation
Lyamitsa Formation
Fig. 12. Variation of model Nd age of Upper Vendian andLower Cambrian mudstones in the northwestern MezenBasin.
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Vendian sedimentary rocks of the southeastern WhiteSea region do not rule out the participation of youngergranitoids of the Timan–Pechora region and theArchean crystalline complexes of the Baltic Shield insediment provenances (their proportion is 1 : 2). How-ever, this suggestion is not supported by geochemicalsignatures of the Upper Vendian fine-grained terrige-nous rocks in the Mezen Basin. For example, they arecharacterized by the following features: median Cr andNi contents are low (atypical of Archean sources);Co/Hf, Ce/Cr, Eu/Eu*, and Th/Sc ratios are close tothose in the PAAS; the LaN/YbN ratio is low (atypical ofthe Archean TTG association); rocks with low HREEcontent are subordinate; and chondrite-normalizedREE patterns are similar to those in most post-Archeanfine-grained alumosiliciclastic rocks.
Thus, our data on the chemistry of mudstones andtheir model Nd age indicate that the Mezen Basin waslargely filled with erosion products of igneous andmetasedimentary complexes of the Timan–Pechoraregion in the second half of the Late Vendian (since560 Ma ago). This conclusion is confirmed indepen-dently by the results of sedimentological study (Grazh-dankin, 2003, 2004) and the data on sequence-strati-graphic analysis. For example, the thickest of lateralfacies associations (settings with the maximum subsid-ence rate) are confined to the proximal areas of thebasin located near the Timan–Pechora region. The rela-tively reduced condensed sections (settings with theminimum subsidence rate) occur in the distal intracra-tonic zone. Thus, architecture of the Late VendianMezen Basin differs from that of epicontinental andpassive marginal basins, where intensity of subsidenceincreases toward the distal zone. In the Mezen Basin,the subsidence rate increased toward the proximalzone. This is characteristic of foreland basins (Posa-mentier and Allen, 1993).
As a rule, foreland basins represent intensely sub-siding structural units that migrate toward the under-thrusting plate. Moreover, their sedimentary successionnear the suture is deformed into the monovergent fold–thrust packages of low foothills and is involved in ero-sional processes. However, the Upper Vendian sedi-mentary wedge of the Mezen Basin is distinguished bya relatively simple geologic structure and geometry ofsequences. Most likely, the Upper Vendian sedimentaryrocks were deposited in the distal zone of a foredeepthat was not affected by deformation. The forelandcharacter of the Mezen Basin implies the following sce-nario: (i) the sedimentation zone was produced by sub-sidence of the northeastern EEC margin under the loadof masses of the overthrusted Timan–Pechora Foldbelt;(ii) the clastic material was supplied from the risingfoldbelt.
In accordance with the resolution of the Interdepart-mental Stratigraphic Committee of Russia concerningthe General Stratigraphic Scale of the Precambrian innorthern Eurasia, the lower boundary of the Upper Ven-
dian is drawn at the bottom of the Redkino stratohori-zon based on the principle of “distinctness of therespective historical-geological boundary related to thepostglacial Valdai transgression” (Postanovleniya…,1992, p. 32). The position of this boundary was basedon the onset of a new stage in the geological evolutionof stratotype locality (in fact, the onset of sedimenta-tion) rather than the timing of a paleobiologic event. Bydefinition, the lower boundary of the Upper Vendian istied up to the unconformity at the base of sedimentarysuccession. However, the results obtained in our workshow that the unconformity is related to flooding of theterritory due to the load of masses of the overthrustedTiman–Pechora Foldbelt rather than to postglacialtransgression. Although the progradational trend of theAgma Sequence is directed away from the eastern slopeof the Baltic Shield toward the Timan–Pechora region,our data on the composition and age of sediment prov-enances at the earliest stage of the “Redkino transgres-sion” do not reveal a substantial role of the EEC base-ment as a source of clastic material. The progradationaltrend of the Agma Sequence established in our worklikely reflects a higher degree of reworking and sequen-tial sorting of sediments by waves in the shallow-waterzone. In this connection, it is necessary to search and fixup a new lower chronostratigraphic boundary of theUpper Vendian, because its current understanding isinconsistent with the real historical-geological situationin the EEC and inadequate for tracing this boundarybeyond the stratotype locality.
Since 560 Ma ago, the foreland character and com-position of sediment provenances of the Mezen Basinwere not subjected to substantial changes and wereinherited by the Early Cambrian (Lontova) evolutionstage. In particular, this inference is supported by thefacies–genetic patterns of sandstones (Grazhdankinand Krayushkin, 2007), the relatively young model Ndage of silty mudstones of the Padun Formation (1.66–1.64 Ga), and TDM values (~1.7–1.6 Ga) obtained forblue clays in northern Estonia (Gorokhov et al., 2007).However, peneplanation of sediment provenancesshould have been completed by the Middle Cambrian,as suggested by the spreading of the Upper Cambrian–Tremadocian fauna-containing sediments of the BalticBasin over the Mezen Basin and the Timan–Pechoraregion, as well as the formation of persistent biogeo-graphic links between these regions and the BalticBasin (Popov and Gorjansky, 1994; Moczydlowskaet al., 2004).
CONCLUSIONS
The results obtained in our work allow us to formu-late the following principal statements.
The Upper Vendian tuffaceous and alumosiliciclas-tic rocks of the Mezen Basin make up a sedimentarywedge. The wedge attains the maximum thickness of1300 m on the northeastern EEC margin (along its junc-tion with the Timan–Pechora region) and gradually
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pinches out to southwest. The sedimentary successionof the Mezen Basin is divided into four (Agma, Solza,Zimnie Gory, and Erga) sequences. The proximal faciesof the Agma, Solza, and Zimnie Gory sequences areconfined to the Timanide FTB with settings character-ized by the maximum subsidence rate.
In terms of composition and geochemistry, theUpper Vendian fine-grained terrigenous rocks of theMezen Basin do not look like erosion products of theprimitive Archean sources. Th, Sc, Cr, Ni, Hf, and REEpatterns in mudstones testify to the predominance ofhighly mature post-Archean rocks in the paleodrainagebasins. Moreover, REE patterns in the mudstones aresimilar to the chondrite-normalized REE patterns ingranitoids of the Timan–Pechora region.
The highly uniform model Nd age obtained formudstones in the entire Upper Vendian succession alsotestifies to a single or similar provenance of clasticmaterial for the Mezen Basin during the entire LateVendian. The model Nd age of the Upper Vendian fine-grained terrigenous rocks in the Mezen Basin (1.76–1.53 Ga) is sufficiently close to the model ages of gran-itoids in the Timanide folded basement. The models ofmixing suggest that the Baltic Shield and the innerprovinces of the EEC played an insignificant role assources of the Upper Vendian fine-grained alumosilici-clastic material for the Mezen Basin.
The results of sequence-stratigraphic study of theUpper Vendian sedimentary rocks and analysis of thedistribution of REEs and some HSFEs in the UpperVendian mudstones are consistent with the followingconcept: the northeastern EEC margin underwent astrong compression from the Timan–Pechora Foldbeltin the Late Vendian due to the collision with island arcsand terrains. These data also suggest that the collisiondue to the load of overthrusted rock masses provokedthe subsidence of the craton margin and formation ofthe Mezen Basin. In this process, the Timan–PechoraFoldbelt served as the main sediment provenance.Thus, the collision occurred not in the Early Cambrianbut slightly earlier in the Late Vendian (~560 Ma ago),as was indicated by Bekker (1968). The lower boundaryof the Upper Vendian (Redkino Stratohorizon) is mostlikely related to the flooding of the territory due to load-ing of the craton margin by rock masses of the over-thrusted Timan–Pechora Foldbelt rather than to thepostglacial transgression.
ACKNOWLEDGMENTS
This work was supported by the Russian Foundationfor Basic Research (project no. 06-05-64223) and thePresidium of the Russian Academy of Sciences (pro-gram no. 18). D.V. Grazhdankin acknowledges supportof the Irish Research Council for Science, Engineeringand Technology.
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