DETERMINATION OF ACTUAL TIME OF SEDIMENTATION OF CAMBRIAN – ORDOVICIAN SANDSTONES OF NORTH-WEST RUSSIAN PLATFORM

 

Guy Berthault – Sedimentologist (France)

Alexander Lalomov - Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry of Russian Academy of Science (IGEM RAS) (lalomov@mail.ru)

 

Presented on General Assembly of  European Geoscience Union (Vienna,  April 16, 2007),

 session SSP-1 (Sedimentology, Stratigraphy, Paleontology)

 

ABSTRACT:

Cambrian – Ordovician sandstones (COS) of St.-Petersburg (SPb) area consists of several formations, which usually are divided by erosion surfaces. Conventional stratigraphy based on paleontological method supposes that the Formations were deposited one after another during 25 million years. The sandstones have obvious evidences of lateral transport of sediments. Taking in account possible interrupts of sedimentation and partial erosion of the sequence and dividing the volume of COS formations by sediment discharge of paleoflow it is possible to estimate real time of sedimentation of COS.

Calculation of total bed sediment load of the paleoflow in applying Einstein procedure indicates that duration of deposition of all COS sequence did not exceeds 3000 years, that is about 0.001% of 25 million years that is assigned to the sequence by stratigraphic chart. Absence of 99.999% of assumed sedimentary sequence is reason for questioning of the conventional chronostratigraphy based of Stenon’s principles that should be revised on the foundation of modern observations and experiments.

 

textures of sediments and Dynamic characteristics of sedimentation environment of cambrian – ordovocian basin

Cambrian – Ordovician sandstones have regional extension (more than 300 km width) and relatively small thickness (30 – 40 m) (fig.1).

 

Fig. 1. Scheme of researched area. Scale approximately 1:2 000 000.

 

The sequence consists of several formations that are presented mainly by fine to medium sandstones with minor siltstones and clays, which usually are divided by erosion surfaces with basal gravel (fig.2). Research of granulometric composition and textures of the sandstones allow determining of dynamic characteristics of sedimentation environment. Detail investigation of structures indicates high dynamic features such as cross-bed series up to 1 m thick (fig. 3), large ripple marks (fig. 4) and inverted lamination (fig. 5).

 

Fig. 2. Generalized stratigraphic column of COS SPb region. Legend: 1- gravel; 2- sandstone with cross-beds; 3- siltstone; 4- clay; 5- shales; 6- fossils.

 

Research of granulometric composition and textures of the sandstones allow determining of dynamic characteristics of sedimentation environment. Detail investigation of structures indicates high dynamic features such as cross-bed series up to 1 m thick (fig. 3), large ripple marks (fig. 4) and inverted lamination (fig. 5).

 

Fig. 3. Unidirectional large-scale cross beds of upper part of Sablino Formation.

 

Fig. 4. Large-scale ripple marks of Sablino Formation.

 

Fig. 5. Inverted lamination as a result of slumping of subaqual dunes.

 

Research of textures such as orientation of cross-beds and ripple marks allow determining direction of the flow. The orientation varies in different formations. Whereas in Lower Sablino, Ladoga and especially Tosno Formation we observe alternate orientation of the cross-bed dips with prevalent direction to east - southeast, in the Upper Member of Sablino Formation we see mostly unidirectional flow. Study of the textures in connection with grain-size analysis in the vertical sequence reveal the sedimentary cycle of deposition that develops from the bottom of Sablino Formation to Tosno (and overlying Koporie shales) as transgressive phase.

Variations of the cross-bed dips in the vertical sequence of COS are illustrated on fig. 6. Rhythmical alternations of the azimuth of the dips in the lateral sandstones of adjacent territories allows proposition of Kulyamin and Smirnov [1973] about tidal nature of these variations and correspondingly about very short (not more than 200 tidal cycles) actual time of deposition of the sequence.    

 

Fig.6. Azimuth’s variation of cross-bedding in Sablino and Tosno strata.

 

 

Comparison of the COS unidirectional cross-bedding structures (fig.7a) with results of experiments in stratification taking place Hydraulic Laboratory of Colorado State University (fig. 7b) [Julien, Lan and Berthault, 1993] indicates on the similar generation of the structures in pulsating water flow.

 

Fig.7. COS unidirectional cross-bedding structures (a) and typical longitudinal view of deposition in experimental flumes (b).

 

 

 

Total bed sediment load calculation in applying Einstein procedure

 

The total bed sediment discharge for unit width qT can be calculated according to Einstein procedure [Julien, 1995] from the sum of the unit bed sediment discharge qb and the unit suspended sediment discharge qs.

qT = qb [1+ I1ln(30h/ds) + I2]                                             

where h – flow depth; ds – sediment size; I1 and I2 integrals that can be solved numerically or with the use of nomographs preparing by Einstein.

Einstein introduces several corrections factors accounting for hydraulically smooth boundaries, sediment transport by size fractions, hiding factors, grain resistance instead of total resistance, velocity and pressure corrections. This method is expected to work best when the bedload constitutes the most significant portion of the total load. According to analysis of textures and grain size analysis of the sediments we could to conclude that the condition is available for COS.

Einstein’s bed sediment discharge function gives the rate at which flow of any magnitude in a given channel transports the individual sediment sizes found in the bed material. Therefore equation (1) could be presented as:

qT  =    Σ iTqTi                                               (2)

where iT is content of a fraction of total bed material; qTi – discharge of i-fraction.

All necessary data were obtained from the field and laboratory investigations.

 

Calculation of the time of deposition    

Total bed sediment load calculation in applying Einstein procedure as a single parameter do not allows determining possible deposition time for all sequence of Cambrian-Ordovician sandstones of St.-Peterburg region. Einstein procedure is suitable for steady unidirectional flow. Real flow was not steady neither direction not intensity. Analysis of textures result to conclusion of alternate mode of the flow, therefore we should take this fact into account. Also, it is necessary to consider partial erosion of strata on the border of the Formations.

Inasmuch as specific sediment discharge is calculated, knowledge of full thickness of the Formation taking into consideration of partial erosion, resulting vector of transport and extension of the Formations in the direction of the flow let us possibility to calculate duration of deposition of the Formation.

Taking into account of variation of the flow direction  

Orientation of cross-beds indicates that during the deposition of COS the flow was orientated mainly from west – northwest to east – southeast. At the same time, the flow was not steady: we see variations of the orientation within the sequence that is evidence of changing of direction of the flow in time. Therefore for calculation of the duration of deposition we should use resulting vector of the flow (sum of the vectors). The resulting vector is taken into account by coefficient of effectiveness of the flow (Cf) that is equal 0.8 for Upper Member of Sablino Formation; 0.5 for Lower Member of Sablino Formation; 0.4 for Ladoga Formation and 0.3 for Tosno Formation.

Taking into account of partial erosion of strata

Detail analysis of structure of COS and erosion surfaces on extended territory shows that boundaries between Formations have mostly plane character without deep erosion troughs. Most obvious and pronounced erosion surface within COS sequence is in the bottom of Ladoga Formation. It has shallow erosion pockets with pebbles of clay. Others contacts between Formation have not so obvious evidences of erosion. In our calculation we should take into account erosion of some parts of the sequence. For this purpose we take into calculation not average thickness but sum of maximal thicknesses of the Formation containing COS.

Whereas multiplying erosion and deposition should result to increasing of concentration of heavy minerals in the sand sediments up to industrial important placers [Placer deposits … , 1997], we have possibility to estimate rate of erosion and re-deposition on the base of content of heavy minerals. Relatively significant concentration of heavy minerals we see only in Ladoga Formation (Table 2) that confirm proposal that the Formation has close relation to erosion processes. Ladoga sandstones have concentration of heavy minerals about three times more that in underlying deposits of Sablino Formation. Therefore it is possible to suppose that initial thickness of Ladoga Formation was about three times more that the final one: light minerals (mostly, quarts) were removed by the flow that result to increasing of content of heavy fraction in three times correspondingly. Thus, we put into calculation thickness of Ladoga Formation reformed by coefficient of erosion Ce = 3. For other Formations Ce = 1.

Resulting calculation

Using, specific sediment discharge per unit width according to Einstein procedure (qT); maximal thickness of the Formation (Hmax,) multiplying by coefficient of erosion (Ce); extension of the Formations in direction of the flow (L) and coefficient of effectiveness of the flow (Cf) we can obtain duration of deposition of the Formations (Td) by formula (3): 

Results of the calculation are shown in the Table.

Table. Result of calculation duration of deposition time for Cambrian – Ordovician sandstones of St.-Petersburg region in applying Einstein procedure.

 

Formation

(Member)

qT

m3/day x m

Cf

L

Hmax

Duration of deposition (Td) (years)

Sb1

38.62

0.5

100

8

114

Sb2

8.52

0.8

80

4

129

Ld

5.11

0.4

100

9*

1205

Ts

3.72

0.3

120

5

1471

TOTAL

26

2919

Definitions: qT, Specific sediment discharge per unit width according to Einstein procedure; Hmax, maximal thickness of the Formation, m; L, extension of the Formation in direction of the flow, km; Cf, coefficient of effectiveness of the flow. 

* value reformed by coefficient of erosion.

Relative error of the calculation is too large for exact determining of the duration of sedimentation because of approximate estimation of some parameters such as the channel slope, direction of the flow, extension of Formation and coefficient of effectiveness, therefore we can definitely determine only order of the value of the duration. Thus we can suppose that according to Einstein procedure with our additions, the duration of deposition of all sequence of Cambrian-Ordovician Sandstones of SPb region did not exceeds 3000 years, that is about 0.001% of 25 million years that is assigned to the sequence by stratigraphic chart.

Reliability of the calculation is confirmed by independent sedimentological methods based on modern analogues of the strata that provide the estimation even shorter than Einstein procedure. Based on the analysis of tidal cycles, Kulyamin and Smirnov established that the pure deposition period of Cambrian – Ordovician sandstones in the Baltic region is approximately 170 days, including a mere of 130 days for Middle – Upper Cambrian Sablino sandstones and 40 days for Lower Ordovician Pakerort sandstones (analogue of Tosno Formation) [Kulyamin and Smirnov, 1973]. Study of analogous sediments in the Leningrad district also showed that the pure deposition time of Lower Paleozoic sands is estimated at 100 – 200 yr., while stratigraphic duration of the Sablino Formation is 10 – 15  Ma [Tugarova et al., 2001].

The effect of large difference between sedimentological and stratigraphical age of strata is observed in many natural objects not only in researched region but also over the world. They explain this effect by erosion of significant part of the sediments in geological sequences and by the multiple rewashing of sediments in shallow-water zones with an active lithodynamics characterized by alternations of sedimentation and seafloor abrasion depending on the force of gales and currents. Meyen believed that only a tiny share (0.01- 0.001%) of the total time of sedimentation in shallow-water zones is usually documented because of the abundance of latent hiatuses [Meyen, 1989].

We took into account alternation of direction of the flow by coefficient of effectiveness of the flow that allow eliminating influence of multiplying migration of the sediments in the point of observation. Also, we consider partial erosion of the strata. As we took into calculation maximal values of the parameters, so the calculation gives us maximal duration; real time of deposition could be less.

Accumulation of thick fine grained layers of clay with significant content of organic carbon that are evidence of long interrupts of sedimentation (stagnation) are not observed in the COS sequence. In the Sablino Formation, which maximal thickness is 15 ì, total thickness of clay interlayers vary from few centimetres (in the valley of river Tosna) up to few tens of centimeters (in the valley Lava). Content of organic carbon is about 0.1 – 0.3%; the content is invariable in the vertical sequence that is evidence of lack of stagnation periods.

 

conclusion

Detail research of Cambrian – Ordovician Sandstones of St.-Peterburg region shows that the sequence is result of single deposition cycle that develops from Sablino Formation to Tosno (and overlying Koporie shales) as transgressive phase according to changing of paleohydraulic conditions. Inner erosion surfaces were result of variations of intensity and competence of the flow rather than long interrupt of sedimentation and erosion of strata in subaerial conditions. During all period of deposition resulting vector of paleoflow had prevail direction from west-northwest to east-southeast.

Range of erosion processes was moderate: though we cannot to determine initial thickness of the Formations certainly, indirect indicators show that the initial thickness of three main Formations (Sablino, Ladoga and Tosno) did not exceeds 26 m.

Calculation of total bed sediment load of the paleoflow in applying Einstein procedure indicates that specific sediment discharge per unit width had variations from approximately 40 m3/day per unit width in Lower Member of Sablino Formation to about 4 m3/day/m in Tosno Formation. Applying of the discharge to available volume of the Formations allows to suppose that duration of deposition of all sequence of Cambrian-Ordovician Sandstones of SPb region did not exceeds 3000 years, that is about 0.001% of 25 million years that is assigned to the sequence by stratigraphic chart. Neither interrupts of sedimentation (stagnation) nor erosion of part of the strata could explain this lack of coincidence. Absence of 99.999% of assumed sedimentary sequence is reason for questioning of the conventional chronostratigraphy based of Stenon’s principles that should be revised on the foundation of modern observations and experiments [Berthault, 2004, 2006].

ASKNOWLEDGEMENT

We are thanks to researchers of Lithological Department of Geological Faculty of St.-Peterburg State University Dr. Marina Tugarova and Dr.Mikhail Platonov for taking part in the field and laboratory investigations.

 

References

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Julien, P.Y., Lan,Y., and Berthault, G., Experiments on Stratification of Heterogeneous Sand Mixtures, Bull. Soc. Geol. France, 1993, vol. 164, no. 5, pp. 649–660.

Kulyamin, L.L. and Smirnov, L.S. Tidal Sedimentation Cycles in Cambrian-Ordovician Sands of the Baltic Region. Dokl. Akad. Nauk SSSR, Ser. Geol., 1973, vol. 212, no. 1–3, pp. 696 – 699.

Meyen, S.V. Introduction to the theory of stratigraphy. Moscow, Nauka Publishers. 1989.

Tugarova, M.A., Platonov, M.V., and Sergeeva, E.I. Lithodynamic Characteristics of Terrigenous Sedimentation in the Cambrian-Late Ordovician Sequence of the Leningrad District. Historical Geology and Evolutionary Geography, St-. Peterburg: NOU Amadeus, 2001, pp. 81-91.