Sedimentation regime and accommodation space in the Middle Jurassic–Lower Cretaceous on the eastern Russian Plate
S.O. Zorina a,b,*
a Central Scientific Research Institute of Geology of Industrial Minerals, ul. Zimina 4, Kazan, 420097, Russiab Kazan (Volga region) Federal University, ul. Kremlyovskaya 18, Kazan, 420008, Russia
Middle Jurassic–Lower Cretaceous sediments have a wide-spread distribution in the eastern Russian Plate (ERP) (Fig. 1),where they have been the focus of lithostratigraphic, paleon-tological and mineralogical studies for over many yearsbecause they contain such a wide variety of mineral resources.
In the unified stratigraphic charts (Chirva, 1993; Yak-ovleva, 1993), they are subdivided into a succession offormations and sequences, which were identified within eachstructural zone and subzone on the basis of lithological andbiostratigraphic criteria. The composite sections of each zonewere correlated to a succession of regional biostratigraphicunits and subdivisions of the General Stratigraphic Scale.
The revised standard zonation schemes of the BorealJurassic and Lower Cretaceous published in the early 2000s
(Baraboshkin, 2004; Zakharov et al., 2005) provide a robusttool for tying Jurassic and Lower Cretaceous sediments withinthe ERP to the geological time scale (Gradstein et al., 2004)and thus can be used to establish a chronostratigraphicframework for the sedimentary basin that is essential forfurther reconstructions (Zorina, 2008, 2009, 2012b). Thechronostratigraphic chart was constructed from unified cross-sections and added with a detailed sequence stratigraphy ofthe Jurassic and Lower Cretaceous sections in the north-eastern Ulyanovsk–Saratov trough (UST) (Didenko and Zor-ina, 2003a,b).
The chronostratigraphic subdivision of Middle Jurassic–Lower Cretaceous rocks permitted recognition of the alterna-tion between depositional and nondepositional episodes in thehistory of the basin. Tectonic and eustatic cycles werereconstructed on the basis of a lithologic and bathymetricprofile as well as regional eustatic sea-level curves and curveof relative epeirogenic oscillations constructed from it (Zorina,2012a,b).
Russian Geology and Geophysics 55 (2014) 1195–1204
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Although our study revealed strong evidence for linkagetectono-eustatic events and mineral potential of five MiddleJurassic–Lower Cretaceous megasequences (Zorina, 2005,2006), further follow-up studies are still necessary to estimatechanges from a transgressive to a regressive regime as wellas accommodation and sediment supply conditions in thebasin. Such studies are therefore needed to gain an improvedknowledge of the mechanisms governing deposition and stratalarchitecture and are thus essential to fill the gap in ourunderstanding of the basin evolution.
Materials and methods
Inland seas, also called epicontinental basins, are shallowplatform seas that have been periodically connected by straitsto the open oceans. Unlike passive continental margins,epicontinental basins are characterized by complex verticaltectonic movements, which either amplified or attenuated theeffects of eustatic forcing (Haq and Al-Qahtani, 2005). TheJurassic–Cretaceous basin of the Russian Plate is an excellentexample of an epicontinental sea. According to the terminol-ogy of Strakhov (1962), it was a shelf sea with the irregular
sea-bottom topography reflected in the large number of islands(Sazonova and Sazonov, 1967) and shallow water depths(Atlas, 1969). This shelf sea was characterized by the absenceof a continental slope, a key geologic element of oceanicbasins, which favored the formation of offlap stacking patterns(e.g., clinoforms). Instead, sea-level change caused a shorelineto migrate large distances, resulting in widespread depositionof spacious (hundreds of kilometers) facies successions, i.e.,platformal sequences.
The mechanisms responsible for the platformal depositioncan be constrained by the interpretation of the combined effectof eustasy, tectonic “noise”, and the depositional gradient withdifferent amplitude and direction of each process (Catuneanu2002; Catuneanu et al., 1998; Einsele, 2000; Neal and Abreu,2009; Vail et al., 1991; Van Wagoner et al., 1990) (Fig. 2).
If there is a positive amount of accommodation space (A)available and sediment supply (S) is low at δA/δS > 1,depositional trend would be retrogradational (Fig. 2). Instead,if there is a negative amount of accommodation spaceavailable and sediment supply is high at δA/δS < 1, deposi-tional trend would be progradational to degradational (Catun-eanu et al., 1998; Neal and Abreu, 2009; Zorina, 2013).
Fig. 1. Map showing the location of the study area and structural zoning of Jurassic and Lower Cretaceous deposits of the eastern Russian Plate. 1, Lower Cretaceousdeposits; 2, Jurassic deposits; 3, structural zones and their numbers; 4, structural subzones and their numbers; 5, AB profile through composite chronostratigraphicsections of structural zones and subzones. Structural zones after (Yakovleva, 1993): I, Vyatka–Kama depression; II, Moscow syneclise (eastern flank); III, Koverninadepression; IV, Oka–Don depression; V, Murom–Lomov trough; VI, Ulyanovsk–Saratov trough: VI1, Cheboksary Volga region, VI2, northeastern UST (Zorina,2005), VI3, Ulyanosvk–Samara Volga region, VI4, Saratov right bank region, VI5, Saratov Trans-Volga region; VII, Buzuluk depression.
Variations in the accommodation space combined withvariations in the rate of sediment supply were used toinvestigate the role of all three factors controlling the deposi-tion of the Middle Jurassic–Lower Cretaceous sedimentarysequence of the ERP and to identify architectural style.Accommodation space is defined as the product of the areaavailable for sediment to be deposited and the basin depth.The volume of the sequence is defined as the product of itsarea and thickness.
Five megasequences are recognized in the Middle Juras-sic–Lower Cretaceous chronostratigraphic section of the ERPbased on litho-, bio-, and magnetostratigraphic interpretations(Vail et al., 1991). These are the Bajocian–Callovian, Oxfor-dian–Berriasian, Valanginian, Hauterivian–Aptian, and Albianmegasequences (Figs. 3, 4) (Zorina, 2012a), representingsecond-order (3–50 m.y. duration) cycles (Catuneanu, 2002;Vail et al., 1991).
Each megasequence is characterized by the lateral persist-ence of lithology and temporal variability of their architecture(Figs. 3, 4), which is generally caused by local verticaltectonics.
The identified megasequences are composed of noncarbon-ate sandy clays with intercalations of sand layers and clayswith iron-bearing oolites at the top (Bajocian–Callovian);calcareous clays with marl intercalations overlain by blackshales, sandstones, and conglomerates (Oxfordian–Berriasian);phosphate-rich sandstones, conglomerates, and locally carbon-ate-free clays (Valanginian); carbonate-free clays with inter-calations of black shales and sands (Hauterivian–Aptian);noncarbonate sandy clays interbedded with phosphate-richsand layers (Albian).
Each megasequence comprises a set of third-order (se-quence) cycles (Table 1), which represent generalized sectionsof coeval formations and strata identified in different structuralzones and subzones within the ERP (Chirva, 1993; Yakovleva
1993). Detailed lateral and vertical lithologic and stratigraphicinformation on these third-order cycles were used to determinetheir average thickness (m), duration (m.y.), and areal extent(km2), and to estimate sedimentation rate for each cycle(m/m.y.) and the volume of sediment deposited (km3).
Basin accommodation space was quantified using numericvalues for the paleodepth ranges. As suggested in previousstudies, paleobathymetric reconstructions were performed us-ing depth indicator species of benthic foraminifera (Liu et al.,1997; Sharma and Takayanagi, 1982; Wescott et al., 1998).Quantitative paleodepth estimates for the ERP (Figs. 3, 4)were obtained by analyzing microfaunal data from a detailedlithostratigraphic subdivision of the cored section of bore-hole 1 drilled in the northeastern part of the UST (Zorina andStartseva, 2010).
Benthic foraminiferal paleobathymetric estimates for thestudy paleobasin were used to determine water depth rangesof calcareous and agglutinated benthic species in the north-eastern part of the UST during Middle Jurassic–Early Creta-ceous times. Quantitative analysis of Jurassic and EarlyCretaceous benthic foraminifera (total abundance, genus andspecies diversity, appearance of new species, and the numberof agglutinated and calcareous taxa), combined with thepaleoecology of agglutinated and calcareous benthic fora-minifera allowed the estimation of paleobathymetry, andconstruction of the paleobathymetric curve (Zorina and Start-seva, 2010).
Kimmeridgian and Lower Tithonian marls and clays weredeposited in lower neritic to upper bathyal environments(100–250 m). The reconstruction showed a maximum pale-owater depth of about 300 m at the end of the Early Tithonianfollowed by a transgressive shallowing to 100 m and anoxia,which deposited a member of black and bituminous shales.By the end of the Late Tithonian–Berriasian and after a
Fig. 2. Generalized scheme showing the influence of global eustasy, vertical tectonics and sedimentation on stratal architecture (Zorina, 2013), modified afterCatuneanu et al. (1998) and Neal and Abreu (2009).
Valanginian hiatus the depth of the basin was less than 50 m(Sazonova and Sazonov, 1967).
Hauterivian–Middle Aptian clays were deposited in lowerneritic to upper bathyal environments and paleowater depthremained relatively constant with a minimum around 200 m.Bituminous and black shales of the Lower Aptian memberwere deposited in upper bathyal environments up to approxi-mately 250 m water depth.
Middle Albian sediments are clearly deeper than MiddleAptian. The basal Middle Albian beds represent deposition inlower neritic environments, which was followed by a deepen-ing of the basin and a shift to an upper bathyal environment,with a maximum of about 350 m water depth recorded in thelowermost Middle Albian section.
The paleowater depth values were used to calculate accom-modation space (km3) for second- and third-order cycles(Table 1).
Therefore, these quantitative data were used to estimatechanges in the amount of accommodation space betweendifferent cycle phases and compare them with changes in thevolume of a sedimentary sequence. The calculation of theδA/δS ratio for each cycle enabled to define progradation orretrogradation stacking patterns.
Results
A series of main deepening and shallowing episodes inthe study area was identified from the regional sea-level curverecently proposed for this part of the platform (Zorina, 2008,2009, 2012a). In the Middle and late Jurassic, a generaldeepening episode is recorded in the deposition of predomi-nantly clay sequences. In early Bathonian time, a short-livedshallowing event resulted in the accumulation of polymineralsand and sandy clay. Shallowing in late Callovian time gaverise to the development of a regional hiatus. Following anabrupt deepening of the basin in the latest Early Tithonian,which marked the end of an overall deepening trend, therewas an equally abrupt shallowing of sea level. This event ledto the deposition of lithologically contrasting sequences, i.e.,the Lower Tithonian clays capped by the Upper Tithonianblack shales, which in turn are overlain by phosphate-richsandstones and conglomerates.
At the beginning of the Early Cretaceous, continuedshallowing resulted in cessation of deposition and/or completeerosion of older sands and conglomerates in the Late Berri-asian. A minor deepening event in Valanginian times depos-ited a succession of sandstone and phosphatic pebbleconglomerate. The Early Hauterivian is marked by a majorregional hiatus inferred to represent a time of shallowing andnondeposition. The Late Hauterivian–Early Aptian is markedby a minor deepening phase with the establishment of stableconditions, which were followed by a continued shallowingtrend in the Late Aptian and the development of a LateAptian–Early Albian regional hiatus. The Albian sequencesrecord an overall deepening trend.
The lateral extent of Jurassic and Lower Cretaceoussequences within the ERP is 300,000 km2, but it varied intime between 0 (Early Hauterivian) to maximum values(Barremian). Quantitative data about lateral extent of thesedeposits coupled with numeric values of paleodepths wereused to estimate both temporal and spatial changes in accom-modation space.
The trangressive–regressive cyclicity is defined by vari-ations in areal extent of Middle Jurassic–Lower Cretaceoussedimentary sequences and the bounding relationships be-tween underlying and overlying strata (Figs. 3, 4). Therefore,third-order cycles are correlated with the regional sea-levelcurve.
It is shown that all megasequences, except for the Albian,were deposited during the transgressive phase. The LateCallovian, Late Tithonian–Berriasian, and Early Hauterivianregressions were short-lived and intense, as indicated byepisodes of nondeposition across the entire ERP. Duringdeposition of the Albian megasequence, the transgression hada shorter duration than the regression, suggesting a quite stabletectonoeustatic framework, and hence sedimentation regime inthe region.
Third-order transgressive–regressive cycles provide detailsabout the spatial and temporal relationships between Jurassicmegasequences. For example, the Oxfordian–Berriasian mega-sequence is superimposed with three third-order cycles. Duringthe early Cretaceous, second- and third-order cycles werealmost synchronous. The transition from unstable environ-ments and frequent transgressive–regressive changes in theJurassic to more quiescent environments during the Cretaceouswas probably caused by the stabilization of vertical tectonicmovements.
Of particular interest is the Early Aptian strata containingbituminous and black shales, which represents a regionalrecord of a global oceanic anoxic event OAE-1a (Gavrilov etal., 2002; Zorina, 2009). The results of present study do notconfirm the conclusions of some previous studies (Gavrilov etal., 2002) that these anoxic layers were formed by coastalerosion during rapid marine transgression following the ces-sation of a regressive cycle. The bituminous shales weredeposited at the end of the long transgressive phase of asecond-order cycle. A sudden regressive phase and minorshallowing of the basin occurred immediately after thedeposition of these black shale layers (Fig. 4). The reconstruc-tions showed that anoxia was resulted from rapidly changingglobal paleoenvironmental conditions and a biotic crisis (Read,1998) rather than the abrupt change in the transgressive–re-gressive regime.
Sedimentation parameters. The Middle Jurassic–LowerCretaceous units were deposited at rather constant sedimenta-tion rates with several short-term peaks. The Late Bajocian–Callovian clays, sandy clays, and sands were deposited at25–33 m/m.y., while the Oxfordian–Early Callovain clays andmarls were deposited at a rate of up to 5–15 m/m.y. In theearliest Late Tithonian, at the onset of bituminous and blackshale deposition, the study area was characterized by thefour-fold increase of the sedimentation rate (from 8 m/m.y. to
33 m/m.y.). The overlying Upper Tithonian–Berriasian andValanginian sand and conglomerate units were deposited atextremely slow rates (0.8 and 1.6 m/m.y., respectively). TheLate Hauterivian–Aptian clays were deposited under quietconditions at a rate of up to 11–17 m/m.y, which decreasedto 8 m/m.y. in Late Albian clays.
Variations in the volume of sediment correlate well withchanges in the rate of sediment supply, i.e., an increase in thevolume of sediment is accompanied by higher sedimentationrates and vice versa. However, no such trend is apparent inthree intervals of the section, which exhibit the lack of a directrelationship between these two parameters. The deposition ofKimmeridgian–Tithonian clays and marls was characterizedby an increase in the volume of sediment and a decrease inthe rate of sediment supply. In contrast, Upper Tithonianbituminous and black shales and Lower–Middle Albian claysand sands were deposited at higher sedimentation rates underconditions of low sediment supply.
Accommodation space. A comparison of quantitativeestimates of accommodation space and sediment supplyallowed the following conclusions to be drawn. The MiddleJurassic–Lower Cretaceous sequences of the ERP contain nointervals in which the amount of accommodation space is lessthan the amount of sediment being supplied for these se-quences, which could result in the infill of the basin bysediments. In contrast to the West Siberian basin, theseconditions in the ERP were not conducive to creation ofclinoforms.
The comparable accommodation space and sediment supplywere estimated for the Upper Bathonian–Middle Calloviansandy clay (29 and 24 km3, respectively) and Upper Hau-terivian–Lower Aptian clay strata (64 and 51 km3), whichindicate the absence of characteristic grain-size trends and thepresence of an aggradational sequence stacking pattern. Foreach of the remaining sequences, accommodation space isestimated to be 1.4 times (Upper Albian clays) to 11.4 times(Upper Oxfodian clays) higher than sediment supply.
It was shown that changes in marine accommodation spaceavailable for sediment to be deposited can be directly equatedto changes in sediment supply for the sequences, i.e., anincrease in accommodation space was accompanied by anincrease in sediment supply and, vice versa, a decrease inaccommodation space was accompanied by a decrease insediment supply. The only exception is the Middle Albianinterval, which records a sudden (more than a five-fold)increase in the amount of accommodation space with decreas-ing sediment supply.
Discussion
Vertical variation of δA/δS. The ratio between changes inaccommodation space and sediment supply (δA/δS) wasoriginally used to classify progradational–retrogradationalstacking patterns typical of passive margins (Catuneanu et al.,1998; Neal and Abreu, 2009). In our study we applied this
ratio to distinguish sedimentary sequences deposited in aspecific epicontinental environment (Figs. 3, 4).
All of the Middle Jurassic–Lower Cretaceous megase-quences are characterized by δA/δS values greater than 1,which are indicative of a retrogradation stacking pattern. Thehighest δA/δS value (12.5) was reported from the Valanginianmegasequence. In the Late Aptian interval the estimatedδA/δS value was less than 1, which is consistent with aregressive phase and an upward increase in sand content ofthe Albian shale sequence (progradation stacking).
The calculations also show that δA/δS values are higherthan 1 and vary greatly in all third-order cycles. It isnoteworthy that during early Late Tithonian time, the onset ofbituminous shale deposition is marked by δA/δS = 0.3, whichpoints toward a shallowing event inferred from microfaunalevidence. Another shallowing interval is identified in the LateAlbian (δA/δS = 0.4). The highest value of δA/δS (85) isreported from a shot-term regressive episode during theMiddle Albian. This abrupt increase in δA relative to δS duringmarine regression can be explained by a period of lowterrigenous supply.
As noted above, a distinctive feature of the evolution ofthe Middle Jurassic–Lower Cretaceous sedimentary basin isthat accommodation space exceeded sediment supply. There-fore, even for each of the time intervals represented by adistinct progradational stacking pattern (Lower Bathoniansands overlying Bajocian clays; Upper Tithonian–Berriasiansands and phosphorite-rich conglomerates overlying UpperTithonian black shales), the value of δA/δS is >1. Based ontheoretic modeling, this should result in a retrogradationalstacking pattern typical of a deepening event.
The difference between stacking patterns in individual timeintervals and the estimated values of δA/δS relates to theadditional mechanism by which sequences may form andwhich are not accounted for by the applied formula. Onepossible mechanism is the presence of stratigraphic breaks inthe Bathonian and Late Tithonian–Berriasian, which wereaccompanied by erosion and reworking of sand strata. Under-estimation of the volume of sediment supply may cause a shiftof the relationship toward the creation of accommodationspace. This can be exemplified most clearly by the UpperTithonian–Berriasian sequence with δA/δS of 4.3, which istypical of retrogradation stacking patterns. These results allowthe following conclusion to be drawn. The disagreementbetween the sequence lithologic architecture and the estimatedvalues of the accommodation–sedimentation ratio suggests thepresence of unidentified hiatuses during which sedimentarystrata were eroded. The estimated values greater than 1 implythe more intense erosion.
Another important conclusion can be made from the fullcorrespondence between the actual and estimated values(Fig. 2): if δA/δS < 1 implies a shallowing succession andδA/δS > 1 implies a deepening succession, such successionsshould be interpreted to represent continuous sedimentation.
Progradational–retrogradational cycles. The prograda-tion–retrogradation cycling scale was constructed on thebasis of grain-size variations through the section and the
δA/δS values calculated for second- and third-order cycles(Figs. 3, 4).
All megasequences comprise both retrogradational andprogradational components, except the Valanginian megase-quence, in which the progradational component is eroded.Such architectural similarity relates to the classic mechanismsby which these sequences may form: deepening duringtransgression or shallowing during regression. However, thisconclusion is only valid for the most general trends of thebasin evolution.
Not all third-order cycles, which constitute megasequences,exhibit the above trend. For example, the Lower Bathonianinterval of the Bajocian–Callovian megasequence displays atransgressive shallowing resulted in superimposition of sandsand sandy clays on the Upper Bajocian clays. The Middle–Upper Oxfordian and Upper Kimmeridgian–Lower Tithonianclays and marls were deposited during a regressive phase,which was accompanied by deepening of the basin. Anotherexcellent example is the deposition of Upper Tithonianbituminous and combustible shales during a transgressiveshallowing. Such stepwise regressive–transgressive deepen-ing during the Oxfordian–Early Tithonian and transgressive–regressive shallowing during the Late Tithonian–Berriasianwere probably caused by short-term manifestations of localtectonic “noise”, and depositional hiatuses accompanied by theerosion of missing elements in the structure of third-ordercycles.
The Lower Cretaceous succession exhibits no mismatchbetween transgressive–regressive and retrogradational–progra-dational cycling, which provides another supporting evidencefor a quiet tectonoeustatic and sedimentation regime duringthe Early Cretaceous compared to that of Middle–Late Jurassictimes.
Conclusions
1. A distinctive feature of the evolution of the MiddleJurassic–Lower Cretaceous sedimentary basin is the excess ofaccommodation space over sediment supply. It was revealedthe absence of intervals in which the amount of accommoda-tion space is less than the amount of sediment being suppliedfor these sequences, which could result in the infill of thebasin by sediments. In contrast to the West Siberian basin,these conditions in the ERP were not conducive to creationof clinoforms.
2. The difference between stacking patterns in individualtime intervals and the estimated values of δA/δS may beindicative of the presence of stratigraphic breaks in theBathonian and Late Tithonian–Berriasian, which were accom-panied by erosion and reworking of sand strata.
3. The architectural similarity in almost all of the MiddleJurassic–Lower Cretaceous megasequences relates to the sametectonoeustatic mechanisms by which these sequences mayform: deepening during transgression or shallowing duringregression. The stepwise regressive–transgressive deepeningduring the Oxfordian–Early Tithonian and transgressive–re-
gressive shallowing during the Late Tithonian–Berriasian wereprobably caused by short-term manifestations of local tectonic“noise”, and depositional hiatuses accompanied by the erosionof missing elements in the structure of third-order cycles.
4. The Lower Cretaceous succession demonstrates nodiscrepancy between transgressive–regressive and retrograda-tional–progradational cycling, which provides another support-ing evidence for a quiet tectonoeustatic and sedimentationregime during the Early Cretaceous compared to that ofMiddle–Late Jurassic times.
Acknowledgements. The author would like to thankV.P. Devyatov and an anonymous reviewer for valuablesuggestions and comments which helped to improve themanuscript.
This study was supported by the UNESCO and the Inter-national Union of Geological Sciences Program (UNESCO-IUGS-IGCP Project no. 609) “Climate-environmental de-teriorations during greenhouse phases: Causes and conse-quences of short-term Cretaceous sea-level changes”. Thework was funded by the subsidy of the Russian Governmentto support the Program of competitive growth of KazanFederal University among world class academic centers anduniversities.
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