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Formação Rápida do Cambriano - Dr. Guy Berthaut

Formação Rápida do Cambriano - Dr. Guy Berthaut

ISSN 0024 4902, Lithology and Mineral Resources, 2011, Vol. 46, No. 1, pp. 60-70. © Pleiades Publishing, Inc., 2011.
Original Russian Text © G. Berthault, A.V. Lalomov, M.A. Tugarova, 2011, published in Litologiya i Poleznye Iskopaemye, 2011, No. 1, pp. 67-79.
Reconstruction of Paleolithodynamic Formation Conditions
of Cambrian-Ordovician Sandstones in the Northwestern
Russian Platform
G. Berthaulta, A. V. Lalomovb, and M. A. Tugarovac
a28 boulevard Thiers, 78250 Meulan, France
e mail: berthault.guy@orange.fr
bInstitute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences,
Staromonetnyi per. 35, Moscow, 119017 Russia
e mail: lalomov@mail.ru
cSt. Petersburg State University, Universitetskaya nab. 7/9, St. Petersburg, 198904 Russia
e mail: tugarova@mail.ru
Received September 16, 2009
Abstract—Analysis of the paleohydrodynamic characteristics of sedimentary environments allowed us to
reconstruct formation conditions of the Cambrian-Ordovician sandstone sequence (COS) in the Leningrad
district. Reconstruction of the paleolithodynamic parameters showed that the real timing of the sequence
(sedimentation duration) is considerably less than the related stratigraphic scale interval. Such a situation is
also encountered in other sedimentary formations. Determination of the real sedimentation rate can affect
the assessment of mineral resources in sedimentary basin.
DOI: 10.1134/S0024490211010020
Lithodynamic processes represent one of the most
question that the drift rate was greater during the stable
important factors in the formation of terrigenous sed
transportation of material (especially in the erosion
imentary sequences. Therefore, the study of pale
phase) than that during the formation of a routine sed
olithodynamics allows us to elucidate formation con
imentary layer. Since it is impossible to establish the
ditions of clastic rocks. Of special interest is the assess
excess value with sufficient reliability in most cases,
ment of quantitative parameters of paleolithodynamic
the drift rate obtained during calculations is minimal.
processes. Such a possibility is provided by recent
(2) Based on the calculated values of the paleodrift
studies in the field of hydraulic engineering, hydrody
namics, and geological engineering, which reveal rela
rate in the facies zone under study the dependence of
sediment load on hydrodynamic characteristics of the
tionships between hydrodynamic characteristics of
depositional environments, parameters of the sedi
environment, and the grain size composition of sedi
ment drift (hereafter, just drift), and textural-struc
ments, one can assess the drift capacity.1 Here, we
tural characteristics of rocks. The established regular
should take into account that such dependences are
ities (with regard to corrections for the solution of a
commonly empirical, each having its own field of
reverse problem) are used in the reconstruction of
application. For instance, the Chezy equation yields
parameters of lithodynamic processes in paleobasins.
the most reliable results for deep drifts with a relatively
The study was carried out in several stages to solve
fine material if the ratio between drift depth and parti
the problem:
cle diameter tends to infinity (Julien, 1995); the Bag
nold equation (Bagnold, 1956) is applicable to a com
(1) Reconstruction of hydrodynamic parameters of
depositional environments based on the grain size
pletely turbulent environment at a great power of
composition and rock textures. Relationships between
drifts; and so on. The validity of choosing a method for
drift rate (scouring velocity and initial precipitation
the reconstruction of lithodynamic parameters of a
rate of sediments of the given size) and grain size char
specific zone in the basin under consideration deter
acteristics of sediments were established based on
mines the accuracy of the results obtained.
experimental and natural observations
(Hjulstrom,
1 The drift capacity means the maximum amount of the material
1935; Grishin, 1982). In paleolithodynamic recon
that can move in a unit of time in the alongshore drift of sedi
structions, one should take into account that the min
ments. The drift power characterizes the real sediment transport
imal drift rate is recorded during settling of the trans
rate. The drift capacity and power coincide for saturated drifts if
ported clastic material on the bottom layer. There is no
the drift is provided with loose material (Morskaya…, 1980).
60
RECONSTRUCTION OF PALEOLITHODYNAMIC FORMATION CONDITIONS
61
Karelian Isthmus
Lake Ladoga
St. Petersburg
Gulf of Finland
Gatchina
Kingisepp
1
2
Fig. 1. Sketch map of the study region. (1) Baltic-Ladoga Glint; (2) location of reference sections.
(3) Based on geometric parameters of the forma
exposures in the Izhora, Volkhov, and Syas river valleys
tion under study (length in two perpendicular direc
were also studied (Fig. 1).
tions and average thickness), estimates of the drift
In terms of tectonics, the sequence under study is
capacity within the paleofacies zone, the partial ero
located at the northwestern periphery of the Moscow
sion section of this rock complex, and the stability of
Syneclise that was formed in the terminal Proterozoic.
paleodrift direction, we can assess the real sedimenta
This area was predominated by epeirogenic move
tion timing for this formation using the model of “res
ments that governed its regressive-transgressive
ervoir sedimentation” (Julien, 1995).
nature (Geisler, 1956). In the early Paleozoic, a shal
low water sea basin with a high hydrodynamic activity
existed within the northwestern Russian Platform. The
INVESTIGATION OBJECT
northern boundary of the basin was governed by the
position of the Baltic Shield, which served as a source
The lithodynamic reconstruction was carried out
of clastic material for the sedimentation area. Weath
for the sandy part of the Cambrian-Ordovician
ering crusts have not been established in the Baltic
sequence located in the Leningrad district. First geo
Shield proper, but mineralogical maturity of the clastic
logical data on the section were obtained as early as the
material transported to the sedimentation basin (the
19th century. Stratigraphic, paleontological, and
content of unstable minerals in the heavy fraction of
lithological results of later investigations
(Rukhin,
COS does not exceed 10-15%) indicates a deep
1939; Ul’st, 1959; and others), as well as information
chemical weathering of rocks in the provenance (Gur
published recently (Geologiya…, 1991; Popov et al.,
vich, 1978).
1989; Dronov and Fedorov, 1995; and others) allowed
a substantial lithostratigraphic subdivision of the sec
The sequence is divided into the following three
tion, but this statement mainly concerns with the
formations from the bottom to top (Fig. 2).
Ordovician clay-carbonate part. The sandy part of the
The Middle Cambrian Sablinka Formation (Є2sb).
Cambrian-Ordovician sequence remains a compli
Classic exposures of the formation are located in the
cated object for stratigraphers and is poorly subdivided
Tosna River valley near the Settlement of Ul’yanovka.
into individual layers that could be traced from one
The Sablinka Formation is composed of light gray,
exposure to another.
pinkish, yellowish
(ferruginized in places), well
graded, fine grained, poorly cemented quartzy sand
Field works on the study of Cambrian and Ordovi
stones with plastic brownish gray clay interlayers 0.5-
cian rocks in the Leningrad district were carried out in
1 cm thick.
sections considered as reference ones for the region.
Most attention was concentrated on exposures in the
The Sablinka Formation is divided into two subfor
Tosna and Sablinka river valleys, where the terrigenous
mations that are similar in the lithological composi
sandy sequence between Lower Cambrian
“blue
tion but different in textures: horizontal parallel bed
clays” and Lower Ordovician black shales of the Pak
ded structures with ripple marks and fine criss cross
erorot Horizon is completely exposed. A series of
lamination predominate in the lower subformation;
LITHOLOGY AND MINERAL RESOURCES Vol. 46
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62
BERTHAULT et al.
W - SW
E - NE
Lamoshka
Izhora
Tosno
Volkhov
Lava
Syas
O1kp
O1kp
O1ts
O1ts
Є3ld
Є2sb2
Є3ld
Є1si
Є2sb1
1 m
50 km
1
2
3
4
5
6
7
8
9
Fig. 2. Section of Cambrian-Ordovician sandstones in the Leningrad district.
(1) Pebble; (2) coarse to medium grained sand; (3) fine grained sand; (4) clay; (5) shale; (6) shell detritus; (7) unidirectional
cross bedded series; (8) criss cross bedding; (9) ripple marks. (Sb1) Sablinka Formation, lower subformation; (Sb2) Sablinka For
mation, upper subformation; (Ld) Ladoga Formation; (Ts) Tosna Formation.
unidirectional cross bedded structures are character
clay balls encountered in the underlying Sablinka For
istic of the upper subformation. The detailed textural
mation. In the lower part, sand becomes medium
analysis of the COS sequence is given in the next sec
grained; cross bedded structures and ripple marks are
tion.
encountered. Massive or flat bedded fine grained
The formation extends over the whole Leningrad
sandstones (with clay interlayers up to 0.5-1 cm thick)
district east of the Luga River and occurs with erosion
are found higher in the section.
on the Lower Cambrian “blue clays.” The erosion
boundary is relatively even and downcuttings are wide
Rocks of the Ladoga Formation are thin: up to 1-
with gentle slopes. The paleorelief amplitude is several
1.2 m in the western part of the Leningrad district and
meters. Thickness of the Sablinka Formation
up to 3 m in its eastern part.
increases eastward from 2-3 to 10-13 m.
The Tosna Formation (O1ts) is established through
The Ladoga Formation (Є3ld) occurs with erosion
out the whole Leningrad district. It occurs with ero
on the Sablinka sandstones. It is represented by yel
sion on sandstones of the Ladoga Formation and lies
lowish gray, medium to fine grained, well graded,
with conformity under the Kopor Formation repre
quartzy and quartz-feldspar, and poorly cemented
sented by black mudstones of the same age. The Tosna
sandstones with Lingula shells along with lenses and
isometric spots enriched in ferric oxides.
Formation is composed of coarse to medium
grained, mainly quartzy, and poorly cemented sand
The lower boundary of the formation is clearly ero
sional. Downcuttings of the Ladoga Formation floor
stones with valves of inarticulate brachiopods and
(up to 5-10 m wide and 1 m deep) are observed within
detrital material. The trough and cross bedding is
individual exposures. Downcuttings of the erosional
characteristic of the rocks. Thickness of the formation
paleorelief include basal pebble beds of brownish gray
varies from 2 to 5 m.
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RECONSTRUCTION OF PALEOLITHODYNAMIC FORMATION CONDITIONS
63
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 and intras
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 cross sections), as well
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
Fig. 3. Ripple marks in sediments of the Sablinka Forma
compiled for each of the distinguished age units:
tion.
recurrence percentage for cross bedded series was
plotted on diagrams. Terminology and classification of
probably, tidal) currents. Each tide or ebb cycle
bedded structures and ripple marks are given after
formed its own ridge system, which partially or com
V.N. Shvanov (1987).
pletely destroyed earlier ridges and buried them as
The Sablinka Formation, lower subformation (Є2sb1).
cross bedded series. Although relationship between
Flat bedded structures distinguished at its base give
the flow direction and the inclination of cross bedded
way to cross bedding higher in the section. In general,
series is ambiguous (Kutyrev, 1968), the rose diagram
flat, parallel, and multidirectional cross bedding is
of cross bedding can approximately reflect the clastic
characteristic of the subformation. The upper part of
material transport in the paleobasin. For the studied
the subformation shows surfaces with ripple marks
sequence characterized by two opposite directions of
with the ripples 3.5-4.5 cm high and the spacing
material transport, the resultant component is
between them 20 cm wide (Fig. 3).
directed eastward and indicates an alternating (inter
According to numerous measurements in rocks
tidal?) regime during the alongshore eastward drift of
exposed in valleys of the Tosna and Lava rivers, dip
sediments2.
azimuths of the cross bedded laminas exhibit two
opposite directions: west northwestward and east
The Sablinka Formation, upper subformation (Є2sb2).
southeastward (Fig. 4a).
The upper part of the Sablinka Formation shows
The data obtained allow us to establish the genetic
asymmetrical ripple marks with the following parame
type of textures. Linearity and, as a rule, parallelism of
ters:
30-50 cm long, 3-6 cm high, ripple indexes
joint series, shape of laminas, bedding pattern in per
varying from 6-7 to 10, and gentle/steep slope ratio
pendicular sections, and narrow rays of rose diagrams
ranges within 1-3.
directly indicate the generation of these textures due to
Relatively thick and chiefly extended unidirec
the migration of rectilinear transverse sand ridges
tional (predominantly to the east) cross bedded series
under the influence of bottom currents. Moreover,
impart a specific appearance to the member (Fig. 5).
inclined joints suggest the migration of ridges during a
Thickness of the series is 25-35 cm and length is no
pulsating input of the material
(Kutyrev,
1968),
less than 10 m. Joints are straight and subhorizontal.
whereas symmetrical ripple marks formed in the wave
agitation zone indicate the shallow water nature of the
Deformed and overturned cross bedded structures
basin (Frolov, 1992).
of the synsedimentary syngenetic nature appear in
Flat bedded structures in the lower part of the sub
sandstones in the western part of the Leningrad district
formation suggest that this part of the section was
(Fig. 6). This fact most likely indicates the destruction
accumulated under relatively deep water conditions
of sand ridges during increase of the flow rate above the
below the wave agitation zone (without bottom cur
critical value possible for their existence (Reineck and
rents) during the settling of sediments from suspension
Singh, 1978). The general eastern direction of cross
delivered from the adjacent shallower regions of the
bedded series inclination is retained within the entire
shelf. Sedimentation conditions changed during dep
domain of the Sablinka Subformation.
osition of the upper part: cross bedded series with
2 The alongshore drift of sediments means a resultant unidirec
opposite dip directions of oblique lamina and ripple
tional alongshore transport of sediments over a long time inter
marks indicate that the textures formed in shallow
val. The drift of sediments may proceed both under the influence
water, hydrodynamically active marine conditions in
of wave energy and diverse currents (for instance, wind borne or
the wave agitation zone with periodic bottom (most
tidal) (Morskaya…, 1980).
LITHOLOGY AND MINERAL RESOURCES Vol. 46
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64
BERTHAULT et al.
(a)
(b)
0
0
180
180
(c)
(d)
0
0
180
180
Fig. 4. Rose diagrams of cross bedding directions in the Cambrian-Ordovician sandstone sequence in the Leningrad district:
(a) Sablinka Formation, lower subformation; (b) Sablinka Formation, upper subformation; (c) Ladoga Formation; (d) Tosna
Formation.
The nature of textures suggests that the sequence
The Ladoga Formation (Є3ld) occurs with hiatus on
was formed in a stable hydrodynamic regime under the
the Sablinka sandstones with basal pebble beds (clay
balls) at the base. They are overlain by the cross bed
influence of mainly unidirectional long term drift,
ded sandstones with a series of small thickness (15-20
with intensity decreasing from west to east. The east
cm) and length (1-1.5 m). The cross bedding is flat,
ward drift direction substantially dominated (Fig. 4b).
crisscross, and multidirectional. Laminas are empha
sized by linguloid shells. Symmetrical ripple marks
(probably wave related) are developed at the top of
cross bedded sandstones.
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 bedded lamina dip indicate the alternating sub
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 bedded horizon (1-1.2 m thick), which occurs
either on the basal cross bedded sandstones (about 20 cm
Fig. 5. Unidirectional cross bedded series in sandstones of
thick) or without them above the contact with the
the Sablinka Formation.
Ladoga Formation, represents the main textural
LITHOLOGY AND MINERAL RESOURCES Vol. 46
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RECONSTRUCTION OF PALEOLITHODYNAMIC FORMATION CONDITIONS
65
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 bedded series
and varies from 8-9 to 20 cm.
From the bottom to top, bedded structures vary
from cross bedded to trough bedded; the trough bed
ding passes into the cross, flat, parallel or alternate
bedding with an upsection thinning of cross bedded
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
Fig. 6. Deformed cross bedded sedimentary structures in
rocks of the Sablinka Formation.
the additional submeridional dip with the prevailing
south southwestward direction.
We can assume that sands of the Tosna Formation
The sequences in the section are generally charac
were formed under the influence of an intense turbu
terized by the cyclic nature of variation in grain size
lent flow grading with time into the temperate laminar
characteristics during small fluctuations of these
one. The alternate sediment migration proceeded
parameters, with amplitude increasing to the top of
under conditions of the basic eastward transport of the
the section.
material.
Hence, the studied Cambrian-Ordovician terrige
nous sequence shows a regular increase in the hydro
CALCULATION OF DRIFT PARAMETERS
dynamic activity during sedimentation within the
Many formulas have been proposed for the calcula
Sablinka Formation from its bottom to top and a suc
tion of drift parameters over the last fifty years. How
cessive decrease in the activity during deposition of the
Ladoga and Tosna formations. In general, the inten
ever, no universal method has been elaborated so far,
and each of the available equations has its own sphere
sity of hydrodynamic processes decreased eastward in
of application. Standing out amidst several calculation
the area, probably, due to an increase in the paleobasin
models are some basic ones, which pretend to be com
depth.
plex and universal, and their simplified versions that
Table 1 demonstrates average values of grain size
are less refined and oriented to the solution of partic
characteristics of the studied sediments for the distin
ular problems with a simpler mathematical apparatus.
guished Middle Cambrian-Lower Ordovician forma
tions in the Leningrad district. Analysis of grain size
In the proposed methods, the drift capacity is cal
parameters of sediments along the strike suggests that
culated based on grain size characteristics of sedi
they are mainly marked by decrease in size and
ments and parameters of depositional environments.
increase in the degree of grading (σ) and structural
Parameters of the environment for paleohydrody
maturity (excess) from west to east.
namic reconstructions can be established with some
Table 1. Grain size parameters of the main stratigraphic units
Sablinka Formation (Є2sb)
Ladoga Formation (Є3ld)
Tosna Formation (O1ts)
west
center
east
west
center
east
west
center
east
Ma, mm
0.28
0.18
0.16
0.13
0.23
0.12
0.30
0.26
0.21
σ, mm
0.56
0.61
0.62
0.41
0.59
0.48
0.57
0.53
0.64
As
2.22
1.5
1.76
1.12
1.9
1.35
2.25
1.9
1.58
Ex
10.9
9.6
12.8
4.4
5.4
6.2
17.5
15.3
21.5
Hr (entropy)
0.65
0.59
0.54
0.72
0.61
0.64
0.61
0.64
0.56
Note: Data on grain sizing of clay interlayers were not taken into account. (Ma) Arithmetic mean for grain size, (σ) standard deviation, (As)
asymmetry of distribution, (Ex) excess, (Hr) relative entropy of distribution.
LITHOLOGY AND MINERAL RESOURCES Vol. 46
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66
BERTHAULT et al.
constraints determined by the solution of a reverse
<0.01 mm (in total, about 450 samples) were averaged
problem: calculation based on grain size characteris
and grouped for the further treatment in three grain
tics of sediments under study reflects hydrodynamic
size classes, each representing no less than 19% of the
characteristics of the flow at the sedimentation stage,
total material volume (0.45-0.22, 0.22-0.11, 0.11-
flow intensity at the sediment transport stage being
0.055 mm). We also calculated other necessary param
probably higher.
eters (average size of particles in the class; settling
The Einstein method (Einstein, 1950) is one of the
velocity for particles of this size; and percentiles d16,
basic methods in geoengineering lithodynamic calcu
d35d50d65d84) (Table 2).
lations. The method is applicable for calculation of the
The hydraulic size in Table 2 was calculated by the
total discharge of sediment load (tractional and sus
formula:
pended). Its application is constrained by the predom
w = (4(- 1)gds/3CD)0.5,
(4)
inance of bed load transported by traction and salta
tion over the suspended load, as well as a considerable
where G is the specific weight of particles; g is the free
width of water channel relative to its depth, where the
fall acceleration; ds is the diameter of sediment parti
hydraulic radius of the channel (Rh) equal to the cross
cles, CD is the drag coefficient related to the Reynolds
section area/“wet perimeter” length (width plus dou
number for ball shaped particles (RepCD = 24/Rep
ble depth) ratio is nearly equal to the channel depth.
(Julien, 1995).
These peculiarities of the Einstein method suggest that
The calculation is made for each distinguished grain
the error of its application is minimal for bottom cur
size class, and the obtained results are summed up.
rents in a shallow sea basin composed of sandy mate
rial.
A detailed description of the Einstein method for
The specific total sediment discharge per flow
practical calculations is given in
(Julien,
1995).
width unit qt can be calculated according to the Ein
Results of an analogous calculation made for the COS
stein method as the total discharge of bed load qb and
of the Leningrad district allowed us to determine the
suspended qs load that can be expressed by the equa
specific capacity of drift for each of four studied
tion:
sequences (Table 3).
h
qt
=
qb
+
Cvxdz,
(1)
CALCULATION OF SEDIMENTATION
DURATION IN THE SEQUENCE
0
UNDER STUDY
where h is the flow depth; С is the suspended load con
centration; vx is the horizontal component of the
Parameter of the specific capacity of drift is insuffi
velocity in the flow direction (x); z is the vertical coor
cient for calculating the sedimentation duration for
dinate.
the sequence under study, since this parameter in the
pure state is applicable only in the case of unidirec
Omitting complicated mathematical transforma
tional and temporally stable drift. In actual practice,
tions presented in the monograph Erosion and Sedi
parameters of drifts are changeable with time and
mentation (Julien, 1995), we obtain the equation:
space. The structural analysis of sediments presented
qt = qb[1 + I1ln(30h/ds) + I2],
(2)
above suggests periodic changes in the drift direction
where ds is the medium size of suspended load, and
and variations in its intensity that are manifested as
two integrals I1 and I2 have a numerical solution or can
inter
and intraformation erosion boundaries
be calculated using nomograms elaborated by Ein
(increase in drift intensity) and clay interlayers
stein.
(decrease in drift intensity) that should be taken into
account in calculations.
The function suggested by Einstein for the calcula
tion of drift capacity takes into account the relation
Orientation of the cross bedding indicates a peri
ship between different grain size classes of sediment in
odic change in the drift direction in all of the studied
flows of different intensities. On this basis, the equa
sequences, with the ESE direction generally being the
tion (1) can be presented as:
predominant one. With such a drift regime, the input
of material to a unit cell of the active layer and incre
qt = Σitqti,
(3)
ment of the section thickness are determined by the
where it is the content of i grain size class in sediment;
difference in opposite vectors of material transport rel
qti is the specific discharge of i grain size class.
ative to the general hydrodynamic energy in the unit
Gathering of necessary information about bottom
cell (the sum of all vectors).
sediments of a paleobasin is the first step in the method
For assessing the total drift efficiency based on the
application. We distinguished four spatially stable sed
rose diagram of cross bedding directions, we have pro
imentary complexes: the lower and upper subforma
posed the coefficient of asymmetry (Каs) calculated by
tions of the Sablinka Formation, as well as the Ladoga
the formula:
and Tosna formations. The results of the grain size anal
ysis for 19 size classes within the range from >2 mm to
Каs = |V+i - V-i|/ΣVi,
(5)
LITHOLOGY AND MINERAL RESOURCES Vol. 46
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RECONSTRUCTION OF PALEOLITHODYNAMIC FORMATION CONDITIONS
67
Table 2. Grain size characteristics of Cambrian-Ordovician sandstones
Grain size, mm
Grain size composition in time units, %
Hydraulic size (fall ve
locity in water), w, mm/s
Fractions
average (d
Sb1
Sb2
Ld
Ts
3)
>0.45
0.64
2.52
3.87
7.12
0.45-0.22
0.34
21.97
40.21
24.08
36.88
42
0.22-0.11
0.17
49.02
28.48
31.87
44.21
19
0.11-0.055
0.08
22.47
24.34
32.97
9.44
5
<0.055
5.90
4.44
7.21
2.35
Percentile
d16
0.082
0.088
0.070
0.106
d35
0.112
0.112
0.095
0.150
d50
0.134
0.170
0.117
0.190
d65
0.168
0.217
0.162
0.220
d84
0.220
0.250
0.250
0.280
Note: (Sb1) Sablinka Formation, lower subformation; (Sb2) Sablinka Formation, upper subformation; (Ld) Ladoga Formation;
(Ts) Tosna Formation. Percentiles d
16d35, etc. denote the particle size (mm), relative to which 16, 35, etc. % of particles
have smaller sizes.
where Vi is the unit vector of the dip of cross bedded
and the maximal established thickness of the sequence
series, Σ|V+i - V-i| is the sum of absolute values of vec
(Hmax), the sedimentation duration for the COS
tor differences for opposite directions, and ΣVi is the
sequence in the Leningrad district (ts) can be calcu
sum of values of all rose diagram vectors. For symmet
lated by the formula:
rical distribution, Каs = 0; for unidirectional distribu
ts = (HmaxL)/(qtKаs).
(6)
tion, Каs = 1. The calculated coefficients of asymmetry
for the studied sequences are presented in Table 3.
The detailed analysis of erosional surfaces shows
Table 3. Parameters of the formation of Cambrian-Ordovi
that erosional boundaries within the studied Cam
cian sandstones in the Leningrad district based on the Einstein
brian-Ordovician sequence can be divided into two
method (1950) and Julien model of “reservoir filling” (1995)
types. Erosional interlayer surfaces inside formations
are discontinuous and nonpersistent along the strike.
Studied
q1,m2/day
Каs
Lkm
Hmaxm
ts, yr
sequence
Such textures are determined by the turbulent nature
and local pulsation of drift velocities
(Berthault,
2002).They exert no substantial influence on the total
Sb1
4.7
0.34
200
8
2755
thickness of the sequence.
Sb2
8.5
0.63
200
4
409
Taking into account peculiarities of erosion con
tacts between formations, one can infer that sheet ero
Ld
5.1
0.49
200
3
656
sion essentially dominated over riverbed (deep sea)
erosion. Under these conditions, baselevel of the ero
Ts
3.7
0.47
200
4
1565
sion of sequences under study is not always reliably
Total:
26
5384
established. Therefore, in order to get a more correct
value of the primary volume, we take into account the
Note: (q1) Specific capacity of drift (sediment discharge) per
maximal revealed thickness of the sequence (Hmax)
drift width unit
(calculation based on the Einstein
assuming that the primary thickness of sediments and,
method); (Kas) asymmetry coefficient for rose diagram
correspondingly, the formation volume could be
of cross bedding; (L) reliably established length of the
greater.
studied sequence within the study region; (Hmax) max
imal thickness of the sequence; (ts) sedimentation time
Using the calculated value specific capacity of drift
based on formula (3); (Sb1) Sablinka Formation, lower
(qt), coefficient of asymmetry for the drift (Каs), length
subformation; (Sb2) Sablinka Formation, upper sub
formation; (Ld) Ladoga Formation; (Ts) Tosna For
of the sequence in the direction drift direction (L)
mation.
(about 200 km in the segment accessible for study),
LITHOLOGY AND MINERAL RESOURCES Vol. 46
No. 1
2011
68
BERTHAULT et al.
The calculation results are presented in Table 3.
mated at approximately 170 paleodays (133 for the
The relative error of parameters involved in the cal
Middle-Upper Cambrian Sablinka sandstones and 40
culation can be rather high. In some cases, the relative
for the Lower Ordovician Pakerort sandstones). The
error of primary parameters is extremely hard to esti
above authors write: “The values obtained are shock
mate. Therefore, we can state with confidence only the
ing” (Kulyamin and Smirnov, 1973, p. 699). They
order of the value under calculation.
attribute such results to an infinitesimal preservation
of sediments in analogous sections with respect to the
Values of the specific capacity of drift obtained for
stratigraphic time range.
different COS units confirm the inference based on
the suggesting a cyclic regressive-transgressive struc
Based on the sedimentation analysis of the COS
ture of the sequence. Such a similarity of the results
from the Leningrad district, “pure sedimentation time
obtained by independent methods indicates the real
for Lower Paleozoic sands can be estimated at 100-
assessment of sedimentation parameters for the pale
200 yr. The paradox is that geological time of the
obasin.
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
RELATIONSHIP BETWEEN
water marine conditions with active lithodynamics,
SEDIMENTOLOGICAL
where processes of accumulation and seafloor erosion
AND STRATIGRAPHIC DATA
occur side by side and replace one another depending
on parameters of storms and currents.
Thus, we observe a situation when the sedimenta
tion duration substantially differs from the duration of
Such a situation is not unique. S.V. Mayen wrote:
stratigraphic time interval
(hereafter, stratigraphic
“Due to a wide development of concealed hiatuses…,
duration) correlated to the sequence under study,
only a negligible (0.01-0.001%) share of total sedi
which varies from 20 to 30 Ma according to different
mentation time is commonly documented” (Mayen,
assessments.
1989, p. 24).
To determine the time of hiatuses
(sediment
Since relationship between erosion and transport
rewashing), we use the following formula
parameters of the drift is exponential, the main vol
(Romanovskii, 1977):
ume of geological work (erosion-transfer-deposi
tion) under intense hydrodynamic conditions is
V = kH/(T - T*)p,
(7)
accomplished during activation and is far in excess of
where V is the sedimentation rate, k is the coefficient
geological work performed under stable conditions.
including the thinning of primarily formed layers (cor
For instance, all erosional work and the most part of
rection for compaction), H is the maximal thickness of
accumulation in alluvial channels take place during
rocks within the distinguished stratigraphic unit, T is
flood and at its recession (Chalov, 2008). The coastline
the unit duration (Ma), and Tis the total time of hia
deformation during a year is mainly governed by two or
tuses, and p is the measure considering the intensity of
three most intense storms
(Rukovodstvo…,
1975).
interlayer washouts during the sequence formation.
Major hydrodynamic events in paleobasins related
Then, the hiatus time can be calculated by the for
(presumably) to megatsunami caused by tectonic pro
mula:
cesses can play a crucial role in the deposition of the
lower (marine) molasse, which terminates the com
T* = T - kH/(Vp).
(8)
plete sedimentological evolution of deep ocean
Substituting in formula (8) the values T = 25 Ma,
trenches (Lalomov, 2007). On continental slopes with
V = 26 × 10-4 m/yr, and k = 1.2 (the average compac
intense dynamic processes, such as landslides or large
tion value for sands is taken to be 20%), we reckon p = 1
scale turbid flows, thick sedimentary sequences can be
(intralayer washouts are of the local nature) and thick
deposited instantly from the geological standpoint.
ness is 26 m. Thence, the time corresponding to hia
All these objects are characterized by a sharp
tuses for COS sedimentation makes up:
inconsistency between the stratigraphic duration pre
T* = 25 × 106 yr - 1.2 × 26 m/26 × 10-4 m/yr
scribed to this sediment complex and the real time of
24.988 × 106 yr.
sedimentation. Along with elements formed under
Thus, the calculated real time of formation (sedi
intense (sometimes catastrophic) sedimentation con
mentation duration) corresponds to about 0.05% of
ditions, which make up the main part of the section,
the stratigraphic age of this sequence. It should be
the rock complexes include (to be more exact, must
noted that the sedimentation duration based on the
include) evidence of long term hiatuses or erosion of
Einstein method is of the conservative nature. If we
the most part of deposited sediments. The evidence is
proceed from sedimentation characteristics of sedi
not always present in the explicit form, and this state
ments, the duration obtained for their formation
ment is valid not only for terrigenous rocks. As
appears to be extremely low in the geological scale.
S.I. Romanovskii writes, “…even a monotonous lime
Based on the analysis of intertidal cycles, Kulyamin
stone sequence includes concealed breaks (diastems),
and Smirnov (1973) showed that the “pure” sedimen
which account for much of the time responsible for the
tation time for similar COS in the Baltic region is esti
section formation. However, since there is no possibil
LITHOLOGY AND MINERAL RESOURCES Vol. 46
2011
No. 1
RECONSTRUCTION OF PALEOLITHODYNAMIC FORMATION CONDITIONS
69
ity to get even rough estimates of the hiatus duration,
Conditions, under which the sedimentation time
geologists have to ignore this issue. …In oceans, a con
essentially differs from the stratigraphic one, are char
siderable part of time falls on hiatuses…. Erosion can
acteristic not only for the shallow water platform ter
not be considered here as the main cause of section
rigenous formations (e.g., the COS of the northwest
incompleteness, although other causes cannot also be
ern Russian Platform), but also a series of other sedi
pointed out exactly. Marine geologists have found a
mentary formations. Therefore, the traditional
fortunate avoidance of this complicated problem and
method of calculating the sedimentation rate by sub
designated the hiatus as the period of nondeposition of
division of the sequence thickness into the duration of
sediments. Thus, the geological record … fixes short
the comparable stratigraphic scale interval can yield a
activation intervals separated by essentially longer inter
fortiori understated value.
vals of inactivity” (Romanovskii, 1988, pp. 22, 23).
Since the sedimentation rate has a direct influence
The relationship between such notions as “sedi
on the formation of sedimentary mineral resources of
the sedimentogenic series
(placers and partially
mentation rate,”
“sediment deposition rate,” and
chemogenic ores), the real sedimentation rate should
“section increment rate” is the subject of wide specu
lation in the geological literature at present
be taken into account in the study of sedimentary ore
(Romanovskii, 1977, 1988; Lithogeodinamika…, 1998;
genesis.
Baikov and Sedletskii, 2001; and others), and this is
related not only to pure scientific interest. For many
ACKNOWLEDGMENTS
mineral resources of the sedimentary genesis, the opti
mal relationship between sedimentation rate and sec
We are grateful to M.V. Platonov (Faculty of Geol
tion increment rate is the governing factor for their
ogy, St. Petersburg State University) for the assistance
formation. For instance, titanium-zircon placers rep
in field works.
resent a product of the enrichment of mineralogically
This work was supported by the Guy Berthault
mature sandy sediments under conditions of stable
Foundation (France) and the Russian Foundation for
lithodynamic processes of moderate intensity (Patyk
Basic Research (project no. 09 05 00268 a).
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
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LITHOLOGY AND MINERAL RESOURCES V