Lithodynamic processes represent one of the most important factors in the formation of terrigenous sed imentary sequences. Therefore, the study of pale olithodynamics allows us to elucidate formation con ditions of clastic rocks. Of special interest is the assess ment of quantitative parameters of paleolithodynamic processes. Such a possibility is provided by recent studies in the field of hydraulic engineering, hydrody namics, and geological engineering, which reveal rela tionships between hydrodynamic characteristics of depositional environments, parameters of the sedi ment drift (hereafter, just drift), and textural–struc tural characteristics of rocks. The established regular ities (with regard to corrections for the solution of a reverse problem) are used in the reconstruction of parameters of lithodynamic processes in paleobasins.

The study was carried out in several stages to solve the problem:

(1)Reconstruction of hydrodynamic parameters of depositional environments based on the grain size composition and rock textures. Relationships between drift rate (scouring velocity and initial precipitation rate of sediments of the given size) and grain size char acteristics of sediments were established based on experimental and natural observations (Hjulstrom, 1935; Grishin, 1982). In paleolithodynamic recon structions, one should take into account that the min imal drift rate is recorded during settling of the trans ported clastic material on the bottom layer. There is no

question that the drift rate was greater during the stable transportation of material (especially in the erosion phase) than that during the formation of a routine sed imentary layer. Since it is impossible to establish the excess value with sufficient reliability in most cases, the drift rate obtained during calculations is minimal.

(2)Based on the calculated values of the paleodrift rate in the facies zone under study the dependence of sediment load on hydrodynamic characteristics of the environment, and the grain size composition of sedi

1

ments, one can assess the drift capacity. Here, we should take into account that such dependences are commonly empirical, each having its own field of application. For instance, the Chezy equation yields the most reliable results for deep drifts with a relatively fine material if the ratio between drift depth and parti cle diameter tends to infinity (Julien, 1995); the Bag nold equation (Bagnold, 1956) is applicable to a com pletely turbulent environment at a great power of drifts; and so on. The validity of choosing a method for the reconstruction of lithodynamic parameters of a specific zone in the basin under consideration deter mines the accuracy of the results obtained.

1The drift capacity means the maximum amount of the material that can move in a unit of time in the alongshore drift of sedi ments. The drift power characterizes the real sediment transport rate. The drift capacity and power coincide for saturated drifts if the drift is provided with loose material (Morskaya…, 1980).

S

y

a s

R .

Kingisepp

Izhora R.

Gatchina

To s n a

R .

Luga R .

12

(3)Based on geometric parameters of the forma tion under study (length in two perpendicular direc tions and average thickness), estimates of the drift capacity within the paleofacies zone, the partial ero sion section of this rock complex, and the stability of paleodrift direction, we can assess the real sedimenta tion timing for this formation using the model of “res ervoir sedimentation” (Julien, 1995).

INVESTIGATION OBJECT

The lithodynamic reconstruction was carried out for the sandy part of the Cambrian–Ordovician sequence located in the Leningrad district. First geo logical data on the section were obtained as early as the 19th century. Stratigraphic, paleontological, and lithological results of later investigations (Rukhin, 1939; Ul’st, 1959; and others), as well as information published recently (Geologiya…, 1991; Popov et al., 1989; Dronov and Fedorov, 1995; and others) allowed a substantial lithostratigraphic subdivision of the sec tion, but this statement mainly concerns with the Ordovician clay–carbonate part. The sandy part of the Cambrian–Ordovician sequence remains a compli cated object for stratigraphers and is poorly subdivided into individual layers that could be traced from one exposure to another.

Field works on the study of Cambrian and Ordovi cian rocks in the Leningrad district were carried out in sections considered as reference ones for the region. Most attention was concentrated on exposures in the Tosna and Sablinka river valleys, where the terrigenous sandy sequence between Lower Cambrian “blue clays” and Lower Ordovician black shales of the Pak erorot Horizon is completely exposed. A series of

exposures in the Izhora, Volkhov, and Syas river valleys were also studied (Fig. 1).

In terms of tectonics, the sequence under study is located at the northwestern periphery of the Moscow Syneclise that was formed in the terminal Proterozoic. This area was predominated by epeirogenic move ments that governed its regressive–transgressive nature (Geisler, 1956). In the early Paleozoic, a shal lowater sea basin with a high hydrodynamic activity existed within the northwestern Russian Platform. The northern boundary of the basin was governed by the position of the Baltic Shield, which served as a source of clastic material for the sedimentation area. Weath ering crusts have not been established in the Baltic Shield proper, but mineralogical maturity of the clastic material transported to the sedimentation basin (the content of unstable minerals in the heavy fraction of COS does not exceed 10–15%) indicates a deep chemical weathering of rocks in the provenance (Gur vich, 1978).

The sequence is divided into the following three formations from the bottom to top (Fig. 2).

The Middle Cambrian Sablinka Formation (Є2sb). Classic exposures of the formation are located in the Tosna River valley near the Settlement of Ul’yanovka. The Sablinka Formation is composed of light gray, pinkish, yellowish (ferruginized in places), well graded, fine

stones with plastic brownish gray clay interlayers 0.5– 1 cm thick.

The Sablinka Formation is divided into two subfor mations that are similar in the lithological composi tion but different in textures: horizontal parallel ded structures with ripple marks and fine criss lamination predominate in the lower subformation;

unidirectional cross

istic of the upper subformation. The detailed textural analysis of the COS sequence is given in the next sec tion.

The formation extends over the whole Leningrad district east of the Luga River and occurs with erosion on the Lower Cambrian “blue clays.” The erosion boundary is relatively even and downcuttings are wide with gentle slopes. The paleorelief amplitude is several meters. Thickness of the Sablinka Formation increases eastward from 2–3 to 10–13 m.

The Ladoga Formation (Є3ld) occurs with erosion on the Sablinka sandstones. It is represented by yel lowish gray, mediumell graded,

quartzy and quartz–feldspar, and poorly cemented sandstones with Lingula shells along with lenses and isometric spots enriched in ferric oxides.

The lower boundary of the formation is clearly ero sional. Downcuttings of the Ladoga Formation floor (up to 5–10 m wide and 1 m deep) are observed within individual exposures. Downcuttings of the erosional paleorelief include basal pebbley

clay balls encountered in the underlying Sablinka For mation. In the lower part, sand becomes medium grained; cross

encountered. Massive or flat

sandstones (with clay interlayers up to 0.5–1 cm thick) are found higher in the section.

Rocks of the Ladoga Formation are thin: up to 1–

1.2m in the western part of the Leningrad district and up to 3 m in its eastern part.

The Tosna Formation (O1ts) is established through out the whole Leningrad district. It occurs with ero sion on sandstones of the Ladoga Formation and lies with conformity under the Kopor Formation repre sented by black mudstones of the same age. The Tosna Formation is composed of coarse

grained, mainly quartzy, and poorly cemented sand stones with valves of inarticulate brachiopods and detrital material. The trough and cross bedding is characteristic of the rocks. Thickness of the formation varies from 2 to 5 m.

STRUCTURAL ANALYSIS OF ROCKS FROM THE CAMBRIAN–ORDOVICIAN SANDY SEQUENCE AND FACIES–DYNAMIC CONDITIONS OF THEIR FORMATION

Cambrian and Ordovician sandy rocks of the Len ingrad district exhibit sedimentation textures that are interesting and important for understanding the sequence formation—first of all, different types of bedding and ripple marks, as well as inter

tratal erosional surfaces. When studying textures, most attention was concentrated on the shape and spatial position of joints and laminas inside lamina series (if possible, in two perpendicular crossell

as series extension and thickness. Azimuth and dip angles of laminas were also measured. Based on mea surements of the cross bedding, rose diagrams were compiled for each of the distinguished age units: recurrence percentage for crossas

plotted on diagrams. Terminology and classification of bedded structures and ripple marks are given after V.N. Shvanov (1987).

The Sablinka Formation, lower subformation (Є2sb1). Flate

way to cross

flat, parallel, and multidirectional cross bedding is characteristic of the subformation. The upper part of the subformation shows surfaces with ripple marks with the ripples 3.5–4.5 cm high and the spacing between them 20 cm wide (Fig. 3).

According to numerous measurements in rocks exposed in valleys of the Tosna and Lava rivers, dip azimuths of the crosso

opposite directions: westestward and east southeastward (Fig. 4a).

The data obtained allow us to establish the genetic type of textures. Linearity and, as a rule, parallelism of joint series, shape of laminas, bedding pattern in per pendicular sections, and narrow rays of rose diagrams directly indicate the generation of these textures due to the migration of rectilinear transverse sand ridges under the influence of bottom currents. Moreover, inclined joints suggest the migration of ridges during a pulsating input of the material (Kutyrev, 1968), whereas symmetrical ripple marks formed in the wave agitation zone indicate the shallowater nature of the basin (Frolov, 1992).

Flater part of the sub

formation suggest that this part of the section was accumulated under relatively deepater conditions below the wave agitation zone (without bottom cur rents) during the settling of sediments from suspension delivered from the adjacent shallower regions of the shelf. Sedimentation conditions changed during dep osition of the upper part: cross

opposite dip directions of oblique lamina and ripple marks indicate that the textures formed in shallow water, hydrodynamically active marine conditions in the wave agitation zone with periodic bottom (most

Fig. 3. Ripple marks in sediments of the Sablinka Forma tion.

probably, tidal) currents. Each tide or ebb cycle formed its own ridge system, which partially or com pletely destroyed earlier ridges and buried them as crosseen

the flow direction and the inclination of cross

series is ambiguous (Kutyrev, 1968), the rose diagram of cross bedding can approximately reflect the clastic material transport in the paleobasin. For the studied sequence characterized by two opposite directions of material transport, the resultant component is directed eastward and indicates an alternating (inter tidal?) regime during the alongshore eastward drift of

2

sediments .

The Sablinka Formation, upper subformation (Є2sb2). The upper part of the Sablinka Formation shows asymmetrical ripple marks with the following parame ters: 30–50 cm long, 3–6 cm high, ripple indexes varying from 6–7 to 10, and gentle/steep slope ratio ranges within 1–3.

Relatively thick and chiefly extended unidirec tional (predominantly to the east) cross

impart a specific appearance to the member (Fig. 5). Thickness of the series is 25–35 cm and length is no less than 10 m. Joints are straight and subhorizontal.

Deformed and overturned cross

of the synsedimentary syngenetic nature appear in sandstones in the western part of the Leningrad district (Fig. 6). This fact most likely indicates the destruction of sand ridges during increase of the flow rate above the critical value possible for their existence (Reineck and Singh, 1978). The general eastern direction of cross bedded series inclination is retained within the entire domain of the Sablinka Subformation.

2The alongshore drift of sediments means a resultant unidirec tional alongshore transport of sediments over a long time inter val. The drift of sediments may proceed both under the influence of wave energy and diverse currents (for instance, wind

tidal) (Morskaya…, 1980).

The nature of textures suggests that the sequence was formed in a stable hydrodynamic regime under the influence of mainly unidirectional long

with intensity decreasing from west to east. The east ward drift direction substantially dominated (Fig. 4b).

The Ladoga Formation (Є3ld) occurs with hiatus on the Sablinka sandstones with basal pebble beds (clay balls) at the base. They are overlain by the cross

ded sandstones with a series of small thickness (15–20 cm) and length (1–1.5 m). The cross bedding is flat, crisscross, and multidirectional. Laminas are empha sized by linguloid shells. Symmetrical ripple marks (probably waveeloped at the top of

cross

It is apparent that the basal layer of the Ladoga For mation was deposited under conditions of the suprac ritical erosional rate of the flow. Then, the sediments of the Ladoga Formation were mainly deposited in less active hydrodynamic conditions (probably related to deepening of the basin) under the influence of differ ently oriented waves and tidal currents. Azimuths of cross

latitudianal migration of sand material during the result ant alongshore eastward drift of sediments (Fig. 4c).

The Tosna Formation (O1ts). The trough and criss cross

either on the basal cross

thick) or without them above the contact with the Ladoga Formation, represents the main textural

parameter determining the appearance of the Tosna Formation. This bedding type was attributed in litera ture to the migration of crescent sand ridges along the bottom, which are formed under the influence of a strong but mainly turbulent flow (Kutyrev, 1968; Shvanov, 1987). The height of paleoridges is likely comparable with the thickness of cross

and varies from 8–9 to 20 cm.

From the bottom to top, bedded structures vary from cross

ding passes into the cross, flat, parallel or alternate bedding with an upsection thinning of cross

series up to the appearance of small obscure cross bedded structures.

The rose diagram of lamina dip in sandstones of the Tosna Formation demonstrates two cross directions of the sediment transport: the main sublatitudinal dip (Fig. 4d) with the prevailing eastward direction and the additional submeridional dip with the prevailing southestward direction.

We can assume that sands of the Tosna Formation were formed under the influence of an intense turbu lent flow grading with time into the temperate laminar one. The alternate sediment migration proceeded under conditions of the basic eastward transport of the material.

Hence, the studied Cambrian–Ordovician terrige nous sequence shows a regular increase in the hydro dynamic activity during sedimentation within the Sablinka Formation from its bottom to top and a suc cessive decrease in the activity during deposition of the Ladoga and Tosna formations. In general, the inten sity of hydrodynamic processes decreased eastward in the area, probably, due to an increase in the paleobasin depth.

Table 1 demonstrates average values of grain size characteristics of the studied sediments for the distin guished Middle Cambrian–Lower Ordovician forma tions in the Leningrad district. Analysis of grain size parameters of sediments along the strike suggests that they are mainly marked by decrease in size and increase in the degree of grading (σ) and structural maturity (excess) from west to east.

Table 1. Grain size parameters of the main stratigraphic units

Fig. 6. Deformed cross

rocks of the Sablinka Formation.

The sequences in the section are generally charac terized by the cyclic nature of variation in grain size characteristics during small fluctuations of these parameters, with amplitude increasing to the top of the section.

CALCULATION OF DRIFT PARAMETERS

Many formulas have been proposed for the calcula tion of drift parameters over the last fifty years. How ever, no universal method has been elaborated so far, and each of the available equations has its own sphere of application. Standing out amidst several calculation models are some basic ones, which pretend to be com plex and universal, and their simplified versions that are less refined and oriented to the solution of partic ular problems with a simpler mathematical apparatus.

In the proposed methods, the drift capacity is cal culated based on grain size characteristics of sedi ments and parameters of depositional environments. Parameters of the environment for paleohydrody namic reconstructions can be established with some

constraints determined by the solution of a reverse problem: calculation based on grain size characteris tics of sediments under study reflects hydrodynamic characteristics of the flow at the sedimentation stage, flow intensity at the sediment transport stage being probably higher.

The Einstein method (Einstein, 1950) is one of the basic methods in geoengineering lithodynamic calcu lations. The method is applicable for calculation of the total discharge of sediment load (tractional and sus pended). Its application is constrained by the predom inance of bed load transported by traction and salta tion over the suspended load, as well as a considerable width of water channel relative to its depth, where the hydraulic radius of the channel (Rh) equal to the cross section area/“wet perimeter” length (width plus dou ble depth) ratio is nearly equal to the channel depth. These peculiarities of the Einstein method suggest that the error of its application is minimal for bottom cur rents in a shallow sea basin composed of sandy mate rial.

The specific total sediment discharge per flow width unit qt can be calculated according to the Ein stein method as the total discharge of bed load qb and suspended qs load that can be expressed by the equa tion:

h

<0.01 mm (in total, about 450 samples) were averaged and grouped for the further treatment in three grain size classes, each representing no less than 19% of the total material volume (0.45–0.22, 0.22–0.11, 0.11– 0.055 mm). We also calculated other necessary param eters (average size of particles in the class; settling velocity for particles of this size; and percentiles d16, d35, d50, d65, d84) (Table 2).

The hydraulic size in Table 2 was calculated by the formula:

where G is the specific weight of particles; g is the free fall acceleration; ds is the diameter of sediment parti cles, CD is the drag coefficient related to the Reynolds

number for ball(Rep) CD = 24/Rep (Julien, 1995).

The calculation is made for each distinguished grain size class, and the obtained results are summed up.

A detailed description of the Einstein method for practical calculations is given in (Julien, 1995). Results of an analogous calculation made for the COS of the Leningrad district allowed us to determine the specific capacity of drift for each of four studied sequences (Table 3).

where Vi is the unit vector of the dip of cross

series, Σ|V+i – V–i| is the sum of absolute values of vec tor differences for opposite directions, and ΣVi is the sum of values of all rose diagram vectors. For symmet rical distribution, Каs = 0; for unidirectional distribu tion, Каs = 1. The calculated coefficients of asymmetry for the studied sequences are presented in Table 3.

The detailed analysis of erosional surfaces shows that erosional boundaries within the studied Cam brian–Ordovician sequence can be divided into two types. Erosional interlayer surfaces inside formations are discontinuous and nonpersistent along the strike. Such textures are determined by the turbulent nature and local pulsation of drift velocities (Berthault, 2002).They exert no substantial influence on the total thickness of the sequence.

Taking into account peculiarities of erosion con tacts between formations, one can infer that sheet ero sion essentially dominated over riverbed (deep erosion. Under these conditions, baselevel of the ero sion of sequences under study is not always reliably established. Therefore, in order to get a more correct value of the primary volume, we take into account the maximal revealed thickness of the sequence (Hmax) assuming that the primary thickness of sediments and, correspondingly, the formation volume could be greater.

Using the calculated value specific capacity of drift (qt), coefficient of asymmetry for the drift (Каs), length of the sequence in the direction drift direction (L) (about 200 km in the segment accessible for study),

and the maximal established thickness of the sequence

(Hmax), the sedimentation duration for the COS sequence in the Leningrad district (ts) can be calcu

lated by the formula:

Table 3. Parameters of the formation of Cambrian–Ordovi cian sandstones in the Leningrad district based on the Einstein method (1950) and Julien model of “reservoir filling” (1995)

Note: (q1) Specific capacity of drift (sediment discharge) per drift width unit (calculation based on the Einstein method); (Kas) asymmetry coefficient for rose diagram of cross bedding; (L) reliably established length of the studied sequence within the study region; (Hmax) max imal thickness of the sequence; (ts) sedimentation time based on formula (3); (Sb1) Sablinka Formation, lower subformation; (Sb2) Sablinka Formation, upper sub formation; (Ld) Ladoga Formation; (Ts) Tosna For mation.

The calculation results are presented in Table 3. The relative error of parameters involved in the cal

culation can be rather high. In some cases, the relative error of primary parameters is extremely hard to esti mate. Therefore, we can state with confidence only the order of the value under calculation.

Values of the specific capacity of drift obtained for different COS units confirm the inference based on the suggesting a cyclic regressive–transgressive struc ture of the sequence. Such a similarity of the results obtained by independent methods indicates the real assessment of sedimentation parameters for the pale obasin.

RELATIONSHIP BETWEEN

SEDIMENTOLOGICAL

AND STRATIGRAPHIC DATA

Thus, we observe a situation when the sedimenta tion duration substantially differs from the duration of stratigraphic time interval (hereafter, stratigraphic duration) correlated to the sequence under study, which varies from 20 to 30 Ma according to different assessments.

To determine the time of hiatuses (sediment

rewashing), we use the following formula (Romanovskii, 1977):

where V is the sedimentation rate, k is the coefficient including the thinning of primarily formed layers (cor rection for compaction), H is the maximal thickness of rocks within the distinguished stratigraphic unit, T is the unit duration (Ma), and T* is the total time of hia tuses, and p is the measure considering the intensity of interlayer washouts during the sequence formation. Then, the hiatus time can be calculated by the for mula:

Substituting in formula (8) the values T = 25 Ma, V = 26 × 10–4 m/yr, and k = 1.2 (the average compac tion value for sands is taken to be 20%), we reckon p = 1 (intralayer washouts are of the local nature) and thick ness is 26 m. Thence, the time corresponding to hia tuses for COS sedimentation makes up:

T* = 25 × 106 yr – 1.2 × 26 m/26 × 10–4 m/yr

=24.988 × 106 yr.

Thus, the calculated real time of formation (sedi mentation duration) corresponds to about 0.05% of the stratigraphic age of this sequence. It should be noted that the sedimentation duration based on the Einstein method is of the conservative nature. If we proceed from sedimentation characteristics of sedi ments, the duration obtained for their formation appears to be extremely low in the geological scale. Based on the analysis of intertidal cycles, Kulyamin and Smirnov (1973) showed that the “pure” sedimen tation time for similar COS in the Baltic region is esti

mated at approximately 170 paleodays (133 for the Middle–Upper Cambrian Sablinka sandstones and 40 for the Lower Ordovician Pakerort sandstones). The above authors write: “The values obtained are shock ing” (Kulyamin and Smirnov, 1973, p. 699). They attribute such results to an infinitesimal preservation of sediments in analogous sections with respect to the stratigraphic time range.

Based on the sedimentation analysis of the COS from the Leningrad district, “pure sedimentation time for Lower Paleozoic sands can be estimated at 100– 200 yr. The paradox is that geological time of the Sablinka sequence formation amounts to 10–20 Ma” (Tugarova et al., 2001, p. 89). The authors explain this paradox by the rewashing of sediments in shallow water marine conditions with active lithodynamics, where processes of accumulation and seafloor erosion occur side by side and replace one another depending on parameters of storms and currents.

Such a situation is not unique. S.V. Mayen wrote: “Due to a wide development of concealed hiatuses…, only a negligible (0.01–0.001%) share of total sedi mentation time is commonly documented” (Mayen, 1989, p. 24).

Since relationship between erosion and transport parameters of the drift is exponential, the main vol ume of geological work (erosion–transfer–deposi tion) under intense hydrodynamic conditions is accomplished during activation and is far in excess of geological work performed under stable conditions. For instance, all erosional work and the most part of accumulation in alluvial channels take place during flood and at its recession (Chalov, 2008). The coastline deformation during a year is mainly governed by two or three most intense storms (Rukovodstvo…, 1975). Major hydrodynamic events in paleobasins related (presumably) to megatsunami caused by tectonic pro cesses can play a crucial role in the deposition of the lower (marine) molasse, which terminates the com plete sedimentological evolution of deep ocean trenches (Lalomov, 2007). On continental slopes with intense dynamic processes, such as landslides or large scale turbid flows, thick sedimentary sequences can be deposited instantly from the geological standpoint.

All these objects are characterized by a sharp inconsistency between the stratigraphic duration pre scribed to this sediment complex and the real time of sedimentation. Along with elements formed under intense (sometimes catastrophic) sedimentation con ditions, which make up the main part of the section, the rock complexes include (to be more exact, must include) evidence of long

the most part of deposited sediments. The evidence is not always present in the explicit form, and this state ment is valid not only for terrigenous rocks. As S.I. Romanovskii writes, “…even a monotonous lime stone sequence includes concealed breaks (diastems), which account for much of the time responsible for the section formation. However, since there is no possibil

ity to get even rough estimates of the hiatus duration, geologists have to ignore this issue. …In oceans, a con siderable part of time falls on hiatuses…. Erosion can

not be considered here as the main cause of section incompleteness, although other causes cannot also be pointed out exactly. Marine geologists have found a fortunate avoidance of this complicated problem and designated the hiatus as the period of nondeposition of sediments. Thus, the geological record … fixes short activation intervals separated by essentially longer inter vals of inactivity” (Romanovskii, 1988, pp. 22, 23).

The relationship between such notions as “sedi mentation rate,” “sediment deposition rate,” and “section increment rate” is the subject of wide specu lation in the geological literature at present (Romanovskii, 1977, 1988; Lithogeodinamika…, 1998; Baikov and Sedletskii, 2001; and others), and this is related not only to pure scientific interest. For many mineral resources of the sedimentary genesis, the opti mal relationship between sedimentation rate and sec tion increment rate is the governing factor for their formation. For instance, titanium–zircon placers rep resent a product of the enrichment of mineralogically mature sandy sediments under conditions of stable lithodynamic processes of moderate intensity (Patyk Kara et al., 2004). It is relatively fast (by geological standards) sedimentation of the COS that probably is responsible for the following fact: commercial Ti–Zr placers have not been revealed in the region so far despite the concurrence of many favorable factors (availability of the source of heavy ore minerals in igneous and metamorphic rocks of the Baltic Shield, presence of intermediate collectors of Ti–Zr minerals in the Late Precambrian and Early Cambrian sedi mentary complexes, and mineralogical maturity of the COS in the northwestern Russian Platform).

The sedimentation rate has a direct influence on the formation of mineral resources at the stage of sed imentation. This shows up in the process of placer for mation and, most probably, chemogenic sediments of the sedimentation series. Therefore, the knowledge of the real sedimentation rate is important not only for lithology and sedimentology, but also for the study of processes responsible for the formation of sedimentary mineral resources.

CONCLUSIONS

Thus, the application of lithodynamic geoengi neering calculations for assessing the sedimentation duration of the sandy portion in the COS in the Len ingrad district showed that these sandstones were formed instantaneously from the geological stand point, and the sedimentation duration of the sequence does not exceed 0.05% of its stratigraphic age interval. This work has confirmed ideas of former researchers about a rather fast formation of the sequence and pre sents the quantitative assessment of sedimentation.

Conditions, under which the sedimentation time essentially differs from the stratigraphic one, are char acteristic not only for the shallowater platform ter rigenous formations (e.g., the COS of the northwest ern Russian Platform), but also a series of other sedi mentary formations. Therefore, the traditional method of calculating the sedimentation rate by sub division of the sequence thickness into the duration of the comparable stratigraphic scale interval can yield a fortiori understated value.

Since the sedimentation rate has a direct influence on the formation of sedimentary mineral resources of the sedimentogenic series (placers and partially chemogenic ores), the real sedimentation rate should be taken into account in the study of sedimentary ore genesis.

ACKNOWLEDGMENTS

We are grateful to M.V. Platonov (Faculty of Geol ogy, St. Petersburg State University) for the assistance in field works.

This work was supported by the Guy Berthault Foundation (France) and the Russian Foundation for Basic Research (project no. 09

REFERENCES

Bagnold, R.A., Flow of Cohesionless Grains in Fluid, Phi los. Trans. R. Soc. London, 1956, no. 954, pp. 235–297.

Baikov, A.A. and Sedletskii, V.I., Superhigh Rates of Terrig enous Sedimentation on the Continental Block in the Phanerozoic, in Problemy litologii, geokhimii i osadochnogo rudogeneza (Problems of Lithology, Geochemistry, and Sedimentary Ore Genesis), Moscow: Nauka, 2001, pp. 93– 108.

Berthault, G., Analysis of the Main Principles of Stratigra phy Based on Experimental Data, Litol. Polezn. Iskop., 2002, no. 5, pp. 509–515 [Lithol. Miner. Resour. (Engl. Transl.), 2002, no. 5, pp. 485–497].

Botvinkina, L.N., Metodicheskoe rukovodstvo po izucheniyu sloistosti (Manual for the Study of Bedding), Moscow: Nauka, 1965.

Chalov, R.S., Ruslovedenie: teoriya, geografiya, praktika

(Study of Channels: Theory, Geography, and Practice),

Moscow: LKI, 2008.

Dronov, A.V. and Fedorov, P.V., Carboniferous Ordovician in the Vicinity of St. Petersburg: Stratigraphy of Yellow and Frozen Sediments, Vestn. St. Petersb. Univ., Ser. Geol. Geogr., 1995, issue 2, no. 14, pp. 9–16.

Einstein, H.A., The Bed Load Function for Sediment Transport in Open Channel Flow, Technical Bulletin No 1026, Washington, DC: U.S. Dept. Agric., Soil Cons. Serv., 1950, pp. 1–78.

Frolov, V.T., Litologiya (Lithology), Moscow: Mosk. Gos. Univ., 1992.

Geisler, A.N., New Data on the Lower Paleozoic Stratigra phy and Tectonics in the Northwestern Russian Platform, in Materialy po geologii evropeiskoi territorii SSSR (Materi

als on the Geology of the European Part of the USSR), Moscow: Gosgeoltekhizdat, 1956, pp. 174–184.

Geologiya i geomorfologiya Baltiiskogo morya. Svodnaya ob"yasnitel’naya zapiska k geologicheskim kartam M 1:500 000 (Geology and Geomorphology of the Baltic Sea: Summary Explanatory Note to Geological Maps of Scale 1: 500 000) Grigyalis, A.A., Ed., Leningrad: Nedra, 1991.

Grishin, N.N., Mekhanika pridonnykh nanosov (Mechanics of Nearm Debris), Moscow: Nauka, 1982.

Gurvich, S.I., Zakonomernosti razmeshcheniya redko metal’nykh i olovonosnykh rossypei (Regularities in the Dis tribution of Rare Metal Stanniferous Placers), Moscow: Nedra, 1978.

Hjulstrom, F., The Morphological Activity of Rivers as Illustrated by River Fyris, Bull. Geol. Inst. Uppsala, 1935, no. 25, pp. 89–122.

Julien, P., Erosion and Sedimentation, Cambridge: Cam bridge University Press, 1995.

Kulyamin, L.L. and Smirnov, L.S., Intertidal Cycles of Sed imentation in Cambrian–Ordovician Sands of the Baltic Region, Dokl. Akad. Nauk SSSR, Ser. Geol., 1973, vol. 212, no. 1

Kutyrev, E.I., Usloviya obrazovaniya i interpretatsiya kosoi sloichatosti (Formation Conditions and Interpretation of Oblique Bedding), Leningrad: Nedra, 1968.

Lalomov, A.V., Reconstruction of Paleohydrodynamic For mation Conditions of Upper Jurassic Conglomerates in the Crimean Peninsula, Litol. Polezn. Iskop., 2007, no. 3, pp. 298–311 [Lithol. Miner. Resour. (Engl. Transl.), 2007, no. 3, pp. 275–285].

Litogeodinamika i minerageniya osadochnykh basseinov (Lithogeodynamics and Minerageny of Sedimentary Basins), St. Petersburg: VSEGEI, 1998, p. 480.

Mayen, S.V., Vvedenie v teoriyu stratigrafii (Introduction to the Theory of Stratigraphy), Moscow: Nauka, 1989.

Morskaya geomorfologiya. Terminologicheskii spravochnik (Marine Geomorphology: Terminological Handbook), Moscow: Mysl’, 1980, p. 280.

Patykva, N.V., and Bardeeva, E.G.,

History of the Formation of Titanium–Zirconium Sands in the Central Deposit in the European Part of Russia, Litol.

Polezn. Iskop., 2004, no. 6, pp. 451–465 [Lithol. Miner. Resour. (Engl. Transl.), 2004, no. 6, pp. 441–453].

Popov, L.E., Khazanovich, K.K., Borovko, N.G., et al., Opornye razrezy i stratigrafiya kembro

tonosnoi obolovoi tolshchi na severo

formy (Reference Sections and Stratigraphy of the Cam brian–Ordovician Phosphoritevo Sequence

in the Northwestern Russian Platform), Leningrad: Nauka, 1989.

Reineck, H.E. and Singh, I.B., Depositional Sedimentary Environments, Berlin: Blackwell Sci. Publ., 1978. Trans lated under the title Obstanovki terrigennogo osadkonako pleniya, Moscow: Nedra, 1981.

Romanovskii, S.I., Sedimentologicheskie osnovy Litologii, (Sedimentological Principles of Lithology), L.: Nedra, 1977, 408 p.

Romanovskii, S.I., Fizicheskaya sedimentologiya (Physical Sedimentology), Leningrad: Nedra, 1988.

Rukhin, L.B., Cambrian–Silurian Sandy Sequence of the Leningrad District, Uchen. Zap. LGU, Ser. Geol.ochv. Nauk, 1939, issue 4, no. 11. pp. 89–101.

Rukovodstvo po metodam issledovaniya i rascheta peremesh cheniya nanosov i dinamike beregov pri inzhenernykh izys kaniyakh (Manual for the Study and Calculation of Dislo cations of Debris and Dynamics of Coasts during Engineer ing Surveys), Moscow: Gidrometeoizdat, 1975, p. 239.

Shvanov, V.N., Petrografiya peschanykh porod (komponent nyi sostav, sistematika i opisanie mineral’nykh vidov) (Petrography of Sandy Rocks: Composition, Systematics, and Description of Mineral Species), Leningrad: Nedra, 1987.

Tugarova, M.A., Platonov, M.V., and Sergeeva, E.I., Litho dynamic Characteristics of Terrigenous Sedimentation of the Cambrian–Lower Ordovician Sequence in the Lenin grad District, in Istoricheskaya geologiya i evolyutsionnaya geografiya (Historical Geology and Evolutionary Geogra phy), St. Petersburg: NOU Amadeus, 2001, pp. 81–91.

Ulst, R.J., Lower Paleozoic and Silurian Sediments of the Baltic Region and Content of Dispersed Organic Matter Therein , Riga: AN Latv. SSR, 1959.