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Quantified Middle to eustatic variations based on Russian Platform : level resolution

DORK SAHAGIAN Department of Geological Sciences and Byrd Polar Research Center, The Ohio State University, Columbus, Ohio 43210 MICHELLE JONES Department of Geological Sciences, The Ohio State University, Columbus, Ohio 43210

ABSTRACT With regard to the first problem, we define eustasy as the relation between the volumes of the ocean basins and total ocean water. A We have constructed a eustatic sea-level curve based on the stra- change in this relationship would affect all ocean-continental bound- tigraphy of the tectonically stable Russian Platform. Sea-level variations aries (shorelines) equally, and be modulated by local epeirogenic and measured against this reference frame were chosen as reliably repre- other tectonic activity. The second problem is more complicated, not senting the long-term relation of global ocean-basin volume and ocean- being subject to simple definition, but is an outgrowth of the first and water volume. In constructing the curve, we bacfcstripped stratigraphic is the primary concern of this paper. Stratigraphic successions evolve data from numerous wells distributed across the Russian Platform and in response to relative sea-level change, a function of both eustasy and then used the present elevation of various stratigraphic horizons to tie the tectonics (Posamentier and others, 1988), and so the stratigraphic resulting curve to present sea level. Most strata observed represent very record can be used to measure sea level. If tectonic events are insig- shallow water deposition (<25 m), and so we were able to estimate nificant, then relative sea-level events will be caused solely by eustatic water-depth variations more reliably than would have been possible in events. Consequently, stratigraphy on demonstrably stable platform a deep-water environment. The stratigraphy of the Russian Platform is areas should provide a reliable indication of eustatic change. riddled with unconformities. This is a result of the ability of even minor Lithofacies distributions in clastic environments (particularly eustatic fluctuations to cause sea level to drop off the platform. These grain size) are controlled largely by the distance of sites of sediment unconformities are important in accurately fixing sea level (0 water deposition from terrestrial sediment sources. One way to observe depth) at various times throughout the late -eariiest . sea-level changes is through their effect on sedimentation patterns The eustatic curve resulting from this study indicates that sea level rose along continental margins (Jervey, 1988; Posamentier and others, by 120 m from the mid-Jurassic (60 m above present) to the mid-Cre- 1988; Posamentier and Vail, 1988). This relationship is affected by sea taceous (180 m above present) and remained at about that level until the level according to the hypsometiy of the relevant continental margin, Tertiary, when it began to drop. The long-term rise was not uniform, which also affects the areal distribution of water depth. In marginal but spasmodic, with many shorter-term eustatic rises and falls. These environments, however, it is difficult to quantify the epeirogenic ac- events had magnitudes of tens of meters over timescales of 1 to 5 m.y. tivity at the level of precision necessary to distinguish tectonic from The causative mechanism for these variations is not clear. The eustatic eustatic controls on facies distributions and depositional sequences. curve resulting from this study can be applied to subsiding basins and Consequently, in order to measure eustasy, it is useful to find a ref- passive margins in order to quantify subsidence history. erence frame such as a stable platform against which stratigraphic and other eustatic signals will accurately reflect the relative volumes of INTRODUCTION ocean water and basins, rather than local tectonic or epeirogenic processes. Many authors noticed long ago (Davis 1896; Johnson, 1919; Bal- Stable continental regions can be found on the basis of flat-lying chin, 1937; Miller, 1939) that sea level has not been invariant and otherwise undeformed strata in North America (Sloan, 1964; throughout Earth history. Long-term sea-level changes (>1 m.y.) Sleep, 1976; Merewether, 1983; Sahagian, 1987), Africa (Sahagian, have been observed qualitatively through their effect on the deposi- 1988), and the Russian Platform (Aleinikov and others, 1980; Sa- tional patterns and shoreline processes of marine fades. The quanti- hagian, 1989). The North American and African regions are limited in fication of sea-level variations, however, has been hampered by two their stratigraphic range, including only some Upper de- fundamental problems (Sahagian and Holland, 1991). The first is a posits. The Russian Platform, however (Fig. 1), owing to its elevation classically unclear definition of the term "sea level," making it difficult in the Mesozoic, has preserved shallow-water deposits ranging in to choose an actual quantity to measure. The second is inaccuracy in from through Upper Cretaceous, and even includes measurement methods which are based on depositional and other sediments in one area. It is thus much more useful for consequences of sea-level changes as observed in various tectonic constructing a truly eustatic sea-level curve than are the other con- environments. tinental regions. It is also more reliable, including a larger stable area

Data Repository item 9319 contains additional material related to this article.

Geological Society of America Bulletin, v. 105, p. 1109-1118, 5 figs., 1 table, August 1993.

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as defined by the preserved sediments (Sahagian, 1989; Sahagian and the absence of thick coarse terrigenous sediments. Besides the im- Holland, 1991). This is contrary to the interpretation of some Russian plication that such deep water would require (nonglacial) sea-level investigators who have assumed that all transgressions and regres- changes at possibly unreasonable rates (tens of m/m.y.), the uniform- sions as well as associated facies variations were caused by uplift and ity of sediment thicknesses across the Russian Platform (regardless of subsidence and that by implication, sea level has remained constant lithology or proximity to clastic sources) suggests that deposition was throughout the Mesozoic- (Nalivkin, 1976; Aleinikov and not occurring very far below base level. This is in contrast to results others, 1980). Some have also used present outcrop extent as the full of preliminary analyses of adjacent subsiding basins, where there are region of deposition (Vinogradov, 1968) and have constructed hyp- large variations in thickness, associated with lithology, and interpreted sometric curves on that basis to account for tectonic history, not as lateral variations in (relatively great) water depth. accounting for erosion of the very thin platform sediments. The sea-level curve resulting from the present work has been Although the flat and undeformed nature of the Russian Platform obtained by two complementary approaches (Sahagian and Holland, suggests relative stability, it does not necessarily preclude the possi- 1991). The first is based on the effect of sea-level changes on depo- bility of changes in low-order residual dynamic topographic support sition, and involves backstripping of sediments to quantify the change of continents relative to oceans (Gurnis, 1991). Estimates of plate- in sea level from one to the next. This provides a motion histories (Davis and Solomon, 1981; Gordon and Jurdy, 1986) sea-level curve that is internally consistent and is calibrated and changes in dynamic topography, however, suggest that the Rus- throughout the time represented by preserved sediments, but not sian Platform has not moved significantly with respect to the dynamic beyond. In order to relate the sea-level curve obtained for this time topography (Gurnis, 1991,1992) and that the dynamic topography has interval to present sea level, it is necessary to measure the present not significantly changed since the Jurassic (M. Gurnis, 1993, personal elevation (with respect to present sea level) of various stratigraphic commun.). If, in the future, more refined data and models suggest horizons. This is the second approach which provides a zero on the otherwise, the Russian Platform results will have to be corrected for sea-level elevation scale. this in order to represent the relative volumes of ocean basins and ocean water. STRATIGRAPHIC DATA Eustatic curves which have been compiled in the past (Hallam, 1984, 1991; Haq and others, 1988; Harrison, 1988, 1990) have relied Mesozoic strata are widely distributed on the Russian Platform. on methods of calculating tectonic activity (usually subsidence) in Figure 1 shows some of the wells on the Russian Platform (and ad- order to separate tectonic and eustatic signals reflected in stratigraphic jacent basins) which encounter Mesozoic strata. Preserved strata sequences or paleohypsometric profiles (Greenlee and Moore, 1988). range in age from mid-Jurassic through Upper Cretaceous. In the The errors inherent in these calculations of tectonic Penza region, they extend into the Paleogene. The younger strata are (thermal) subsidence histories (for example, Watts and Steckler, 1979) generally more limited in distribution than the older as a result of have been larger than the total sea-level signal inferred on timescales modern erosion. Alternatively, it would be possible for limited areal of tens of millions of . Thus, these curves have necessarily been distribution to represent differences in depocenter. On the Russian qualitative, and at best, can be used only for short time intervals Platform, however, the uniform distribution of facies within a depo- (<10 m.y.), limited by cumulative errors in subsidence calculations. sitional unit (substage), and the facies relationships between units of By using an epeirogenically stable part of the lithosphere (Russian different ages indicate that the area of preserved strata does not cor- Platform) as a reference frame, this limitation is eliminated because relate with depositional area, but rather with stratigraphic position there has been no discernible tectonic subsidence or uplift. subject to modern erosion in an incoherent drainage . Paleoge- The Russian Platform was at an elevation very close to sea level ographic maps of the Russian Platform have been compiled (Sazonov, throughout most of the Jurassic and Cretaceous. This is indicated by 1962; Vinogradov, 1968; Keller and Predtechenskii, 1974; Mil- the uniformly thin strata and large number of small (subaerial) un- anovsky, 1987) but are generally based on outcrop (and subcrop) conformities observed in the stratigraphy. We thus have a fortuitous extent. On the basis of this study, however, it has become clear that situation that allows us to accurately fix sea level at many closely the extent of marine deposition was considerably greater than that of spaced time intervals (the unconformities), and to reliably estimate preservation, because the preserved facies generally do not vary as the variations between those intervals (shallow-water sediments). Fur- outcrop edge is approached. Paleogene strata, however, are limited in ther, the flat hypsometiy of the Russian Platform would have allowed distribution (Sahagian and Holland, 1991), and therefore, although small changes in sea level to drive extensive lateral transgressions and there is no indication of approach of a depositional zero edge, non- regressions. The distribution of observed lithofacies and biofacies can deposition cannot be ruled out. thus be used as a sensitive indicator of sea-level change and water Many of the preserved strata on the Russian Platform are depth. (In a steeper hypsometry environment, this would not be pos- bounded both above and below by unconformities. These consist of sible.) When the Russian Platform was flooded, there were limited paraconformities and disconformities, but the distinction between the clastic source areas (Urals, Caledonides, and a few small massifs) two is not relevant when constructing a sea-level curve. The uncon- providing sediments to the margins of an extensive and very shallow formities are not seen in the same abundance in strata of the same age epeiric sea. At such times, sands may have been deposited near the in the Caspian and West Siberian basins. This suggests that these shorelines, but a few hundred kilometers into the Russian Platform basins were subsiding while the Russian Platform remained stable and sea, deposition may have been limited to finer-grained , and were therefore more prone to unconformities caused by drops in sea during several time intervals, to pelagic and . Ap- level. (In a subsiding basin, minor or slow eustatic falls will not result parently, the supply of sediments and directed wave and current in a relative sea-level fall, but without subsidence, all eustatic falls are transport mechanisms were insufficient to carry large volumes of reflected as relative falls.) coarse terrigenous sediments to the center of the platform. Thus it The biostratigraphic resolution of the preserved sediments is cannot be assumed that there was deep (>50 m) water on the basis of variable from stage level (several million years) to substage level (few

1110 Geological Society of America Bulletin, August 1993

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Figure 1. Location map of the Russian Platform and adjacent basins showing locations of wells used in the stratigraphie database. Wells are lettered by regional province. M = Moscow (most of the Russian Platform), P — Penza, C = Caspian, S = Central West , Y = Yenisey (northern West Siberia), T = Turgai.

million years), depending on stratigraphic as well as geographic po- been described in detail in published and unpublished reports (Sa- sition. The finest resolution is observed in the (Early zonova, 1958; Gerasimov, 1962; Rotenfel'd, 1965; Gerasimov, 1969; Cretaceous), where there are four distinct units in the late Hauterivian Krymgolts, 1972; A. Olferiev, unpub. data). alone. We have divided our data set according to the resolution ob- Outcrops were observed in the field in the area included by well served. The stratigraphic units are not named but are designated as data. Strata were very thin (often only a few meters), but laterally lower , upper Hauterivian (1-4), and so on, as in the Russian continuous and uniform over long distances (several hundred kilo- literature. Biostratigraphic zonation is based primarily on ammonites, meters). This indicates a uniformity of depositional environments at bivalves, forams, and palynomorphs (Gerasimov, 1962,1969; Roten- a given time throughout the Russian Platform. The strata are generally fel'd, 1965; Butkovsky, 1967; Kiymgoltz, 1972). The biostratigraphic unlithified, presumably owing to their lack of overburden since dep- resolution is sufficient to recognize numerous unconformities on the osition. Samples were collected from outcrops at Phosphorite, Lam- basis of missing . ina, Cotelniki, the Volgusha River, and the Varavino River, which are Data were obtained for this study from wells and cores, as well distributed around the Russian Platform throughout the region cov- as correlated outcrops all over the Russian Platform. Mesozoic out- ered by wells denoted "M" north of 54° . Samples were col- crops are numerous and widespread on the Russian Platform, and lected from each stage or substage and analyzed for palynomorphs by individual units are continuous and uniform over long distances, mak- the staff at Exxon Production Research for additional confirmation of ing it straightforward and reliable to correlate between well and nearby biostratigraphic ages. The palynologjcal results are available from the outcrop data. The well data provide lithology, thickness, and elevation GSA Data Repository.1 of depositional units, whereas the outcrops more closely constrain the age and small-scale lateral variability of lithology, as well as providing information regarding sedimentary structures and depositional envi- ronments. Well data were derived from numerous key wells on the 'GSA Data Repository item 9319 is available on request from Documents stable part of the platform, as indicated in Figure 1. These wells have Secretary, GSA, P.O. Box 9140, Boulder, CO 80301.

Geological Society of America Bulletin, August 1993 1111

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TABLE 1. CRITERIA USED IN THIS ANALYSIS FOR ESTIMATING WATER DEPTH AND associated with these environments were (a) shoreface or lagoonal, CHARACTERIZATION OF DEPOSITIONAL ENVIRONMENT 2 m; (b) transitional, 10 m; (c) offshore, 25 m; and (d) deep, >50 m.

Environment Criteria Water depth These depths are less than those corresponding to similar depositional environments on continental shelves, as discussed above. Terrestrial , plants, soils, bauxite, fluvial deposits -2 The depositional history of the Russian Platform as inferred from Shoreface Beach, , mudcracks, evaporites, 2 Lagoon Muds, fossils 2 the preserved stratigraphy is summarized as follows. Only a few ex- Reef Fossils 2 Transition Zone Storm beds, fossils 10 amples of fossils are indicated (mostly and bivalves), but Offshore Fossils 25 Deep Fossils ' 50 more complete lists can be found in the published literature (for ex- ample, Gerasimov, 1962,1969; Kiymgoltz, 1972). During much of the Note: the most reliable environmental data were derived from outcrops, where sedimentary , relatively deep water (perhaps 25 m or more) shales were structures and abundant megafossils are available (Sahagian and Holland, 1991). Depositional units are easily correlated in the subsurface short distances to wells and cores between outcrops. being deposited. Bathonian and sediments are preserved only in the northern portion of the Russian Platform. Examples of species preserved in Bathonian strata on the Russian Platform include macrocephalus Schloth, and Pseudokosmoceras sp. In general, lithofacies distributions and faunas (benthonic forams During the early Callovian, there was a transition to the deposition of and bivalves) can be used to infer depositional environment. The silt and fine sands, indicating a general regression. There was a hiatus relationship between clastic grain size and water depth has been ob- between early and middle Callovian, after which deposition contin- served on modern continental shelves (Howard and Reineck, 1981) ued, starting with oolites at the base of middle Callovian and fining and has been applied to paleo-depositional environment. We assume upward to deep-water shales. Callovian species include Macrocepha- that grain size is controlled by distance from clastic source as well as lites macrocephalus Schloth, elatmae Nik. and C. modi- energy in the depositional environment. The hypsometry of the Rus- olare Luid. deposition was widespread on the Russian sian Platform during the Mesozoic was apparently much closer to Platform. Most wells analyzed in this study encountered black horizontal than present (or past) continental shelves. Consequently, of early, middle, and late Oxfordian age. The shale was absent only the relation between water depth (depositional environment) and dis- in wells located in the northwestern corner of the Russian Platform tance from clastic source was very different from that of modern (wells M307, M331, M9, and M13; see Fig. 1), presumably due to continental shelves, such that distal facies such as shales and carbon- subsequent erosion. Oxfordian faunas include cordatum ates could be deposited in shallow water (<50 m). In our analysis Sow., C. vertebrate Sow., and Euraspidoceras perarmatum Sow. (Table 1), we have used a relationship between lithology and water During time, similar shale deposition continued with no depth very different from that appropriate for continental margins. unconformity recognized between Oxfordian and Kimmeridgian sed- Many strata appear to have been deposited as sands in 0- to 10-m iments. The main difference between Kimmeridgian and Oxfordian depth. In general, clastic sources were far from sites of preserved stratigraphy is that the Oxfordian appears throughout the Russian deposition (500-1,000 km) due to flat hypsometry. Only during rela- Platform, but the Kimmeridgian is present only in northern sections. tively short intervals when rising or sinking sea level quickly swept the We attribute this to the greater susceptibility of the overlying Kim- shoreline across the Russian Platform were proximal facies deposited meridgian to erosion during a subsequent lower Volgian erosional (sands, and so on). The rates of deposition at these times, however, event. The Kimmeridgian is represented by stephanoides may have been much greater than that during sea-level highstands, Trd., and Desmosphinctes pralairei. leading to a disproportionate fraction of sands. The lower Volgian is absent from all wells and outcrops. The During times of highstands, the only clastic sources in the vicinity middle Volgian and two subunits of upper Volgian are observed over of the Russian Platform were the Urals, the Caledonides (Scandina- a broad area of the Russian Platform. The middle Volgian is repre- via), and the Bohemian and other massifs to the southwest (Ziegler, sented by medium and coarse sand (with phosphorite) bounded both 1982). During times of slightly lower sea level, when these primary above and below by subaerial unconformities and includes fossils of sources were still some distance from the shoreline on the Russian (Zarajskites) scythicus Visch., V. zarajskensis Mich., and Cy- Platform, the dominant clastic source may have been redeposition of lindroteuthis magnifiquenstedti Rouil. Upper Volgian deposition in- previously deposited material (Hein and others, 1991; Hilbrecht, volved thick white fine sand with ammonites, bivalves, gastropods, and 1989). This recycling of sediment may have contributed to the trans- flora, such as Kashpuites julgens Trd., and subditus Trd. port of sands and their broad distribution across the Russian Platform, Very little deposition or preservation took place in despite the lack of nearby primary clastic sources (Caledonides, time. The few wells that encountered Berriasian strata revealed a thin Urals). An example of direct evidence for recycling can be found in (0.9-1.4 m) layer of medium and coarse sand. Outcrops to the south- the Novouzensk borehole (to the southeast of the Russian Platform) east of Moscow include a very thin layer of lithified phosphorite where ammonites are found in upper Hauterivian strata deposited during early Berriasian time. The Valanginian is also pre- (Rotenfel'd, 1965). Because of the tectonic stability of the platform and served only locally. Thus a majority of the well data indicate a large the apparent slow rate of deposition, we interpret transgression and subaerial unconformity between the upper Volgian and lower Hau- regression to be generally "forced" by eustasy (Posamentier and terivian (145-135 Ma) (Harland and others, 1990). The Valanginian others, 1990,1992) rather than caused by tectonics or progradation. faunas include rjasanensis Venez., costata Geras, Depositional environments were assessed based on lithology and and limaciforme Geras. faunal distributions. In general, medium sand or coarser was taken to The Hauterivian was a stage of relatively thick deposition of the represent a shoreface facies, fine sand and silt were considered tran- Russian Platform; it is divided into three substages. The upper Hau- sitional, and shale was considered offshore or lagoonal, depending on terivian is further divided into four subunits. Lower, middle, and upper fauna, gradations, and character of adjacent beds. was consid- Hauterivian 3 and 4 strata each overlie unconformities with ered deep water (or at least far from clastic sources). The depths shallow-water sands. Upper Hauterivian 1 and 2 are missing on the

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Russian Platform, having been either not deposited (low sea level), or PROCEDURE FOR SEA-LEVEL CURVE CONSTRUCTION eroded before upper Hauterivian 3. The middle and upper Hauterivian 3 both exhibit a fining-upward trend to shale within a few meters, Data Compilation interpreted as major transgressive deposition. The upper Hauterivian 4 is a deep-water black shale with silt at the base. The upper Hau- Well data were compiled in tabular form for each stratigraphic terivian 3 is interpreted to have been deposited in a water depth of unit and included location (locality and lat./long.), well identification 10 m; the upper Hauterivian 4, in a water depth of 25 m. Hauterivian number, elevation of top of stratigraphic horizon, depth to base of species include Leopoldia biassalensis Kar., Speetoniceras versicolor horizon, thickness, lithology, depositional environment, criteria for Trd.', and S. inversus M. Pavl. determining depositional environment, and age (stage or substage). The lithology of strata as indicated by wells Well data were derived from the published literature as well as from throughout the Moscow and Penza regions, as well as the northern unpublished Soviet reports and well descriptions at the Centregeolo- Caspian basin, suggests a coarsening of sediments toward the north- gia, Moscow, and are available from the GSA Data Repository. west. The lower Barremian is represented by shales; the upper Bar- remian, by alternating silts and fine sands. This regressive trend con- Backstripping tinued over the entire Russian Platform, eventually leading to exposure and a subaerial unconformity between Barremian and Apt- Backstripping relates eustatic changes to sedimentation (stratal ian. The Barremian is represented by Simbriskites decheni Roem., S. thicknesses) and changes in water depth (based on depositional en- progrediens, and 5. umbonatus Lah. The lower Aptian is divided into vironments). Backstripping yields important eustatic or tectonic in- two subunits (1. Aptian 1,1. Aptian 2). Lower Aptian 1 is composed formation because the stratigraphy and of sedimentary mostly of silt at the base and then grading into a medium sand, which basins is a function of four factors and their mutual interactions: continues through the lower Aptian 2. No unconformity is observed tectonic subsidence, sedimentation rate, water depth, and eustasy. between 1. Aptian 1 and 1. Aptian 2. Lower Aptian thicknesses are These are in turn related through the backstripping equation (Steckler fairly large in comparison with the average thickness of other strata for and others, 1988). the Russian Platform. These thick sands (15-50 m) are not interpreted As the data in this study were derived from an area determined to represent prograding delta fronts, but rather widespread near-shore to be epeirogenically stable (Sahagian, 1989), there was no need for a sand-sheets deposits because they are uniform in thickness tectonic subsidence term which is normally the dominant source of throughout the Russian Platform. Lower Aptian fossils include De- error in backstripping calculations. It was, in fact, this aspect of the shayesites deshayesi Leym, and Aconeceras trautscholdi Sinz. The analysis which attracted us to use the Russian Platform as a sea-level upper Aptian and lower have only rarely been encountered in reference frame in this investigation. Our simple Airy backstripping wells, but where observed are represented by relatively deep water routine accounted for compaction and loading of sediments and water shales and silts, suggesting that they are missing elsewhere as a result (Angevine and others, 1990; Watts, 1988; Sahagian, in press). Because of subsequent erosion. Upper Aptian and lower Albian species in- thicknesses of individual depositional units were relatively constant clude Hoplitesdentatus Sow.,//, interruptusBrag., andH. splendens across the stable part of the Russian Platform, a distance greater than Sow. The lithology of the middle and upper Albian over most of the the lithospheric flexural wavelength, Airy isostatic response to litho- Russian Platform suggests that water depth increased from 10 to 25 m spheric loading was maintained. Each horizon was assigned a sand- between the middle and upper Albian. stone, shale, or compaction curve on the basis of its pre- Lower (fine and medium) sands are found in only dominant sedimentary character (Sahagian and Holland, 1991). two of the wells on the Russian Platform, and there is no upper Because of the thinness of most units (usually 1 to 10 m, and rarely as Cenomanian preserved at all. The lower Cenomanian is represented much as 40 m), errors resulting from compaction and isostatic cor- by conica Sow,, asper Lam, and P. orbicularis Sow. rection are minimized. The relatively thicker section as- The lower Turanian is preserved in only one well and is represented sumed is considered to be unaffected by the thin Mesozoic-Cenozoic by a , indicating relatively deep water with species of overburden. labiatus Schloth. and corbovis Forb. We interpret this to mean that there was a mid- erosional event (rather than non- Unconformities deposition during the upper Cenomanian-lower Turonian) leading to an upper Cenomanian-lower Turonian unconformity throughout The well data indicate a large number of unconformities much of the Russian Platform. This interpretation is supported by throughout the stratigraphic section. These include missing biozones additional stratigraphic data from West Siberia, as discussed below. and sometimes missing stages. This indicates that the variations in sea There was no upper Turonian or lower encountered in any level responsible for the unconformities were greater than the maxi- of the wells on the Russian Platform, but a transgressive event took mum water depths during deposition of the underlying units. The place during the , causing water depth to increase from 2 m presence of subaerial unconformities fixes water depth to 0 m, pro- to 25 m (as indicated by transition from shoreface sands to offshore viding numerous limits on water-depth estimation of adjacent beds. In facies). Santonian faunas include propinqua Mob., B. this sense, the unconformities are as important in the construction of praecursor Stoll., and verus fragilis Arkh. a sea-level curve as the preserved strata which they bound. We have (and younger) strata are best represented at the southeast edge of the assumed that all observed unconformities were subaerial (as opposed Russian Platform in the Penza region. Campanian and to being caused by transgressive erosion, or variations in wavebase, strata around Penza consist of chert, chalk, marl, and shale. Some for instance). This assumption may introduce a source of error into the species present in the Campanian are Gonioteuthis quadrata Blv. and analysis, and it is discussed below. Belemnitella praecursor Stoll. Maastrichtian species include Belem- Two types of unconformities were considered in this analysis: nella lanceolata Schloth. and B. desnensis Jel. erosional, and merely nondepositional. Both types are taken to rep-

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niques on well data from the Caspian and West Siberian basins pro- vides information during the Russian Platform unconformities. The subsiding basin data are useful only for short timescales (short relative to subsidence rates), but are well suited to "fill in the gaps" left by unconformities in the Russian Platform sections.

RESULTS

The sea-level curves resulting from the individual wells used in this investigation are presented in Figure 2, in which the various results are superposed. The curves superpose with only small variations. The individual error bars for each horizon at each well due primarily to errors in water depth estimation of ±2, 5, and 10 m for shoreface, transitional, and offshore facies, respectively. The variations between curves in Figure 2, in addition to the individual errors inherent in each well have been combined to produce the sea-level band illustrated in 60 80 100 120 140 160 Figure 3. At unconformities, the width of the band is zero. This is AGE (Ma) because it is impossible to estimate the magnitude of error during unconformities, and not because there is no error. Sea level may have Figure 2. Sea-level curves compiled from several individual wells on been slightly higher if the unconformity was caused by transgressive the Russian Platform. Error bars for each point were placed primarily or other submarine erosion, or may have been any amount lower if according to water depth, for example, 2 ± 2 m, 10 ± 5 m, 25 ± 10 m. caused by subaerial exposure. These represent large fractional errors, but because of the very shallow depths, have small absolute magnitudes, as discussed in the text. Al- though generally very shallow (interpreted usually < 25 m), water depth represents the largest error source. Additional error sources include potential correlation errors (manifest in unit thickness errors) and dif- ferential compaction. Horizontal lines indicate m^jor unconformities in individual well sections. Throughout the Russian Platform, unconfor- mities are found between almost every depositional unit. This has im- portant implications for the construction of a calibrated sea-level curve. The unconformities represent missing stratigraphie intervals within which there is no sea-level record. They are, however, very important in fixingwate r depth to zero at the end of each subaerial unconformity. As some unconformities may represent significant erosion, the base of unconformities cannot be used to indicate zero water depth at the age of the underlying strata. Continuous stratigraphie data from adjacent sub- siding basins, however, can be used to provide sea-level information during the Russian Platform unconformities, as discussed in the text. The high degree of agreement between curves generated from stratig- raphy separated by nearly 1,000 km suggests that the earlier interpre- tation (Sahagian and Holland, 1991) of Russian Platform stability is indeed valid.

resent times of relatively low sea level and thus surface exposure. Nondepositional unconformities are interpreted to be the result of AGE (Ma) sea-level drops which exposed recently deposited sediments, without significant erosion. Thus the depositional record left by the previous Figure 3. Eustatic sea-level curve based on the stable Russian Plat- period of higher sea level was preserved until the next transgression form. As this is a quantified curve relative to present sea level, error in and further deposition. This type of unconformity is interpreted when calculations and data sources must be represented. Error bounds are shoreface sands occur both below (regressive) and above (transgres- represented by the width of the band, and stem from uncertainties in sive) the unconformity. Erosional unconformities are interpreted water depth as well as minor differences between curves in Figure 2. A when deeper-water lithofacies occur directly below an unconformity. large unconformity is indicated by dashed lines. Sea level may have In this case, it is important not to infer that sea level was low during dropped significantly below the level indicated by any unconformity, but the entire time of missing strata in the unconformity. it is impossible, based solely on the Russian Platform, to determine by Although times of low sea level resulted in many unconformities how much. Continuous stratigraphic data from the Yenisey region on the Russian Platform, there are adjacent subsiding basins which (northern West Siberia) were used to fill another (Cenomanian- underwent more continuous deposition during thermal subsidence Turonian) unconformity on the Russian Platform between 89 and 95 Ma after a rifting or stretching event. Using the same backstripping tech- (see Fig. 5).

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200 Bathonian-Berriasian, but there is a systematic offset in the Hauteriv- PENZA RESULTS ian-Barremian. [The limited preliminary data available during early 175 H parts of this study (Sahagian and Holland, 1991) did not reveal this feature.] The magnitude of the offset is 25 m and suggests that either •150 the main part of the platform uplifted by 25 m during the Hauterivian- Hauterivian-Barremian Barremian, or that the Penza region subsided. As the Penza region is event 125 - immediately adjacent to the subsiding Caspian depression (separated by a normal fault along the Volga River), its stability is suspect. Further, the well data indicate that throughout the Penza region, there 100 - is a slight increase in the magnitude of this subsidence toward the U< J Caspian depression. Consequently, we cannot use the Penza data tn 75 from the Hauterivian-Barremian. Further, we cannot relate the pre- Hauterivian sea level to post-Barremian sea level. This allows data 50 from the two time intervals on either side of the Hauterivian-Bar- remian to be used only for characterizing sea-level events within the 25 intervals. The fact that such an offset is resolvable in our results suggests that the sea-level curve can be used to quantify epeirogenic history in subsiding basins or passive margins as well.

u m The position of the sea-level curves based on the Russian Plat- u CD form can be fixed relative to present sea level, if the Russian Platform ~T I T T l'I1 — I I 60 70 80 90 100110120130140150160170 did not experience any epeirogeny as a unit between the Cretaceous and the present as discussed above. The present elevation of various AGE (Ma) strata from the stable part of the Russian Platform were used to fix the Figure 4. Same as Figure 3, but based on the wells in the Penza position of the eustatic curve derived by backstripping (Fig. 3). Our region. Note that Figures 3 and 4 indicate similar sea-level histories from results show that sea level was as low as 60 m above present in the 125 to 65 Ma and from 160 to 135 Ma, but from 135 to 125 Ma, there Middle Jurassic and rose to as much as 150 m above present by the is an offset of about 25 m. This indicates that either the Penza region Albian and again in the Maastrichtian. Throughout most of the world, subsided during this time interval, or the Moscow region uplifted. We hypsometric and sequence stratigraphic data indicate a Cenomani- suggest the former on the basis of Penza's proximity to the subsiding an-lower Turonian transgression (Hancock and Kauffman, 1979; Rey- Caspian basin. The ability of our analysis to resolve epeirogenic motions ment and Dingle, 1987; Sahagian, 1987, 1988; Haq and others, 1988; of this small magnitude suggests that Figure 3 can be applied to basin and Harrison, 1988). On the Russian Platform, any lower Turonian strata passive-margin stratigraphic data for the purpose of quantifying sub- deposited were apparently eroded away during a middle Turonian sidence history. eustatic drop, discussed below. Comparison with well data from the West Siberian region indi- cates a close correlation of sea-level events and provides information The sea-level curve indicates a long-term rise in sea level of 120 regarding sea-level minima during the times within unconformities on m between the mid-Jurassic and the end of the Cretaceous, with rapid the Platform. The West Siberian data cannot be used to reliably quan- sea-level-rise events in the Hauterivian, upper Albian, and late Ce- tify an independent sea-level curve because of basin subsidence in the nomanian-early Turonian. The magnitude of any sea-level fall which region, but they are useful on short timescales (less than a few million results in a subaerial unconformity is unconstrained. On our sea-level years) for interpolation purposes during Russian Platform uncon- curve, we indicate a minimum sea-level fall equal to the maximum formities, because the strata are well correlated with those on the water depth of the underlying unit. The sea-level drop may have been Russian Platform (Hoedemaeker, 1987; Zakharov and others, 1991). much greater and in many erosional cases probably was; however, Preliminary results from the merging of stratigraphically continuous those magnitudes cannot be quantified on the bases of the Russian West Siberian data with the Russian Platform indicate a rise during the Platform stratigraphy. Such estimates can be obtained from strati- upper Cenomanian and lower Turonian of an additional 30 m (Fig. 5). graphic data from adjacent subsiding regions such as the Caspian This would make a sea-level peak in the lower Turonian of 180 m depression and the West Siberian Lowlands, where the deposition above present (Fig. 3). was continuous throughout the Platform unconformities. In these areas, accurate water-depth estimates are difficult for times of high sea DISCUSSION level because subsidence led to deep water. During the Platform un- conformities, however, lower sea level led to shallow water, which Error Analysis could be more accurately estimated on the basis of facies and faunas. All of the wells from the Russian Platform indicate the same The Russian Platform may be the most reliable frame of reference sea-level history with respect to long-term trends as well as major for quantifying sea-level variations throughout the time interval mid- short-term events (Fig. 3). The wells from the Penza province, at the Jurassic to Tertiary. Like any system of measurement, it is not perfect, southeast margin of the Russian Platform (denoted by "P" in Fig. 1) as indicated by the finite width of the sea-level band in Figure 3. As also show the same sea-level history (Fig. 4); however, the two sets such, the sources of error must be identified, and the magnitudes of of data do not superpose with complete correlation. The sea-level errors must be quantified. As this work is based on stratigraphy, there curves from individual wells from Penza match those from the rest of is a potential for error in dating and correlating strata based on the platform for the time period Aptian-Paleogene, and they match for macrofauna and microfauna. Errors in placement of biostratigraphic

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180 essarily less than the resulting total variation between the component Cenomanian-Turonian curves in Figure 2. Water-depth estimation is more accurate for shallow water where 170 - Sea Level the variation of fades with water depth is more pronounced. For shoreface and lagoonal environments, the assigned depth is 2 m with an error of an additional 2 m. For transitional environments, the water depth is 10 ± 5 m; for offshore environments, the depth is 25 ± 10 m. Although these errors represent large percentages, their absolute mag- nitudes are relatively small due to the shallow-water depths involved. The observation that the curves in Figure 2 superpose with close correlation suggests that our errors are not underestimated, as well as supporting the previous interpretation regarding the stability of the platform throughout the time of deposition (Sahagian, 1989; Sahagian and Holland, 1991). In addition to error associated with backstripping, there is error —i 1 1 1 1 1 1 1— introduced by minor epeirogeny and/or differential compaction of 85 86 87 88 89 90 91 92 93 94 95 sediments. This is more difficult to quantify a priori, but it is apparently AGE (Ma) small because the curves obtained from individual wells superpose to within the width of the sea-level band in Figure 3. This does not Figure 5. Enlargement of the Cenomanian-Turonian Russian Plat- preclude epeirogeny of the entire Russian Platform en masse, but this form unconformity as filled in by more continuous stratigraphic data is unlikely on the basis of mantle dynamics, as discussed above. from West Siberia (Yenisey river region). The West Siberian basin has thick sediments as a result of thermal subsidence. The Russian Platform Comparison with Other Sea-Level Curves unconformity was filled by subtracting the slope of the Siberian relative sea-level curve (10 m/m.y.) from the Siberian data for the time interval The sea-level curve resulting from this study differs in method of Cenomanian-Turonian from the apparent sea-level curve generated compilation as well as form from those derived by other methods. The from West Siberia. This removed the long-term slope but left the short- most widely cited curves to date are those of Haq and others (1987), term structure of the curve intact. After the endpoints matched those of which were based primarily on sequence stratigraphic analysis of the Russian Platform curve unconformity (at 94 and 88.5 Ma), the two passive-margin data. The long-term curve drawn on the peaks of the data sets were merged, and a continuous curve was generated. Note the short-term curve of Haq and others (1988) indicates a sea-level rise of profound sea-level drop between 90 and 89 Ma. This is interpreted to be 240 m from the Bathonian to Turanian. This is significantly greater the cause of the erosional unconformity on the Russian Platform. than the net rise for that period resulting from the present study and is attributed to cumulative sequence thickness and compaction error, boundaries could lead to erroneous relative thicknesses of adjacent in addition to the long-term subsidence of the passive margins upon units. In this case, however, shortages in one unit would be made up which the Haq and others' (1988) curve was based. On a shorter in the next, and so there would be no long-term effect. As the strat- timescale, there are qualitative similarities between Figure 3 and the igraphic sections of the Russian Platform have been very well studied curve of Haq and others (1988) in that the major transgressive events by Russian (and other) (for example, Sazonov, 1958; of the Hauterivian, Albian, and Cenomanian-Turonian are observed Nalivkin, 1973; Vinogradov, 1968; Zakharov and Bogomolov, 1984; in both. Note that the timescale is different (Fig. 3 is based on Harland, Hoedemaeker, 1987; Mesezhnikov, 1984; Sachs, 1976,1979; Gerasi- 1990), and that the relative magnitudes of the events do not agree. We mov, 1962,1969), this source of error is considered minor at the level cannot estimate the sea-level minima represented by unconformities of stage and substage resolution of the eustatic curve in Figure 3. in Figure 3, and there is no indication of error estimates in the curve Potential sources of error in the backstripping analysis include meas- of Haq and others (1988) to help to assess the significance of the urements of thickness, identification of lithology (for compaction), and disparity. The Albian peak of Haq and others (1988), however, is 60 water-depth estimation. Of these, the only significant error arises from m higher than the Hauterivian peak, and the Cenomanian-Turonian is water-depth estimation. Lithology and thickness are relatively well 10 m higher than the Albian. In Figure 3, the Albian is 20 m higher than established, and even large fractional errors would have a small effect the Hauterivian, and the Cenomanian-Turonian is 35 m higher than the on the results because the strata are very thin on the Russian Platform Albian (excluding error bars). The "short-term" curve of Haq and (a few meters per stage). There is, however, an additional source of others (1988) includes many minor events which were transparent to error in thickness measurement arising from erosion. Thicknesses of our analysis with stage and substage resolution. We were able, how- contemporaneous strata across the platform are slightly variable pri- ever, to obtain finer resolution in the Cenomanian-Turonian on the marily due to differences in the extent of erosion during unconformi- basis of merging Russian Platform stratigraphic data with West Sibe- ties. These uncertainties can lead to errors in individual sea-level- rian data (Fig. 5). In this interval, the two curves are remarkably event magnitudes. For instance, if a ravinement surface or similar in the four sea-level peaks (Fig. 5) between the major sea-level transgressive surface of erosion were produced, then later filled with lows in the mid-Cenomanian and mid-Turonian. The relative magni- sediments, it would lead to an underestimate of the magnitude of the tudes of these events, however, are slightly different (Fig. 5; Haq and sea-level rise and fall associated with the partially eroded unit. This others, 1988). would be counteracted by the deposition of additional ravinement- Harrison and others (1983) and Harrison (1990) constructed a filling sediments in the next unit, however. This source of error is sea-level curve based on hypsometry, and inferred a sea-level rise of difficult to estimate independently, but it can be constrained as nec- 80 ± 70 m for the same time interval (Bathonian-Turonian). Another

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sea-level curve based on hypsometry (Hallam, 1977,1984) suggests a Eustatic sea level rose by 120 m between the mid-Jurassic and 200-m rise for the same interval. We attribute these disparate values in an irregular trend punctuated by many short-term and the large standard errors associated with the hypsometric tech- (1 to 5 m.y.) sea-level drops. Although these drops usually resulted in nique to differential epeirogeny and tectonics of active and passive unconformities on the platform and hence do not provide a quantifi- margins, erosion and paleogeographic error, and the assumption that cation of the magnitudes of sea-level falls, information during these each continent's paleohypsometry was identical to present hypsom- times of missing strata may be provided by stratigraphic data from the etry even in the presence of evident epeirogenic deformation (Sa- subsiding Caspian and West Siberian basins. The eustatic curve re- hagian, 1987, 1988). sulting from this study can be fixed relative to present sea level by measuring the present elevation of various stratigraphic horizons on Mechanisms of Eustatic Change the Russian Platform. On this basis, sea level rose from a low of 60 m above present in the mid-Jurassic to a high of 180 m in the Late The establishment of the history of eustatic sea level as illustrated Cretaceous. in Figures 3 and 5 provides insights regarding the mechanisms which The long-term eustatic trend established in this study could have may have been responsible for the eustatic variations at various time- been caused by a number of mechanisms. Our results have identified scales (Harrison, 1990; Sahagian and Watts, 1991). The 120-m sea- eustatic variations of significant amplitude (several meters) on short level rise at the longest timescale (70 m.y.) does not provide a strong timescales (<2 m.y.). These would correspond to the third- and high- constraint on possible mechanisms. er-order cycles inferred by sequence stratigraphic analyses. Although On shorter timescales, more important constraints are provided. confirming that such eustatic variations exist, on the basis of this This study suggests that during the early Hauterivian, there was a study, we can provide no causative mechanism for these rapid eustatic rise of at least 30 m in a timespan of only 1 m.y. This indicates variations. a rate of sea-level rise of at least 30 m/m.y. The eustatic rise throughout The eustatic curve obtained in this study can be applied to passive the entire Hauterivian was at least 50 m over 3 m.y. (Harland and margins and other subsiding basins for the purpose of quantifying others, 1990), making a rate of at least 17 m/m.y. These are faster than thermal subsidence history more precisely than previously possible. the rate considered possible by variations in average ocean age (mid- This application may shed light on basin processes at a level of detail ocean-ridge spreading) (Pitman, 1978; Kominz, 1984; Heller and An- that has been previously unattainable due to the inability to separate gevine, 1985; Angevine and others, 1987) and has a magnitude greater eustatic from tectonic influences on stratigraphic sequences. than that considered reasonable for submarine volcanism and the development of seamounts, and so on (Harrison, 1988, 1990). The ACKNOWLEDGMENTS mechanism for this and other large-magnitude, short-timescale eustatic variations during nonglacial times (Barron and others, 1981) The authors are grateful to V. Zakharov, A. Beisel, and J. Bogo- is yet to be definitively explained. One possibility may be climatic molov for their guidance, collaboration, and hospitality at the Russian forcing of variable water storage in lakes and ground water, which may Academy of Sciences, Novosibirsk; and to A. Zakharov and A. 01- provide as much as 8 m of sea-level variation (Jacobs and Sahagian, feriev for logistical support, field guidance, and access to unpublished 1993) but is still insufficient to account for the entire magnitude of the stratigraphic data at Centregeologia, Moscow. Thanks also go to Mar- observed eustatic variations. tin Farley for palynological analyses. Thoughtful reviews were pro- vided by M. Posamentier, W. Devlin, and D. . Application to Passive Margins

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Posamentier, H., and Vail, P., 1988, Eustatic controls on clastic sedimentation II—Sequence and systems tract models: Society of Economic Paleontologists and Mineralogists Special Publication 42, MANUSCRIPT RECEIVED BY THE SOCIETY JULY 13,1992 p. 125-154. REVISED MANUSCRIPT RECEIVED JANUARY 11,1993 Posamentier, H., Jervey, M., and Vail, P., 1988, Eustatic controls on clastic sedimentation I—Conceptual MANUSCRIPT ACCEPTED JANUARY 21,1993

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