DISSERTATIONES GEOLOGICAE UNIVERSITATIS TARTUENSIS 27
DISSERTATIONES GEOLOGICAE UNIVERSITATIS TARTUENSIS 27
KATI TÄNAVSUU-MILKEVICIENE Transgressive to regressive turnaround
in the Middle Devonian Baltic Basin
This dissertation was accepted for the commencement of the degree of Doctor of Philosophy in Geology at the University of Tartu on August 28, 2009 by the Scientific Council of the Institute of Ecology and Earth Sciences, University of Tartu.
Supervisors: Prof. Kalle Kirsimäe, University of Tartu; Dr. Leho Ainsaar, University of Tartu
Opponent: Dr. Risto Kumpulainen, University of Stockholm, Sweden Commencement: Room 1019, Ravila 14a, Tartu, on 18th of December 2009 at
12:15.
Publication of this thesis is granted by the Institute of Ecology and Earth Sciences, University of Tartu.
ISSN 1406–2658
ISBN 978–9949–19–277–9 (trükis) ISBN 978–9949–19–278–6 (PDF)
Autoriõigus Kati Tänavsuu-Milkeviciene, 2009 Tartu Ülikooli Kirjastus
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CONTENTS
LIST OF ORIGINAL PUBLICATIONS ... 6
1. INTRODUCTION ... 7
2. GEOLOGICAL SETTING ... 9
3. STRATIGRAPHY ... 11
4. METHODS ... 13
5. RESULTS AND DISCUSSION ... 14
5.1 Carbonate-rich facies associations (1) ... 14
5.2 Mixed facies associations (2) ... 18
5.3 Siliciclastic-rich facies associations (3) ... 20
5.4 Basin evolution during the Narva and Aruküla times ... 245
5.5 Causes of transgressive to regressive turnaround ... 27
6. CONCLUSIONS ... 30
ACKNOWLEDGEMENTS ... 31
REFERENCES ... 32
SUMMARY IN ESTONIAN ... 38
PAPERS I–III ... 41
LIST OF ORIGINAL PUBLICATIONS
The thesis is based on the following papers, in the text referred to by their Roman numerals. The papers are reprinted by kind permission of the publishers.
I Tänavsuu-Milkeviciene, K., Plink-Björklund, P. and Kirsimäe, K. (2008) Synsedimentary brecciation in the Eifelian (Middle Devonian) Baltic Basin: sudden catastrophe or diagenetic collapse? Terra Nova, 20, 449–
454.
II Tänavsuu-Milkeviciene, K., Plink-Björklund, P., Kirsimäe, K. and Ainsaar, L. (2009) Coeval versus reciprocal mixed carbonate-siliciclastic deposition, Middle Devonian Baltic Basin, Eastern Europe: implications from the regional tectonic development. Sedimentology, 56, 1250–1274.
III Tänavsuu-Milkeviciene, K. and Plink-Björklund, P. (2009) Recognizing tide-dominated versus tide-influenced deltas: Middle Devonian strata of the Baltic Basin. Journal of Sedimentary Research, in press.
AUTHOR’S CONTRIBUTION IN PAPERS
Paper I: The author is primarily responsible for collecting and analysing the data and for writing the manuscript.
Paper II: The author is primarily responsible for collecting and analysing the data and for writing the manuscript.
Paper III: The author is primarily responsible for collecting and analysing the data and for writing the manuscript.
1. INTRODUCTION
The Devonian Baltic Basin (BB) was a restricted shallow epeiric sea that developed as a back-bulge and later a foreland basin in front of the Scandinavian Caledonides (Plink-Björklund and Björklund 1999). The Middle- Devonian succession of the basin is composed mainly of tide-influenced siliciclastic deposits, except in the middle Eifelian Narva time, when mixed carbonate-siliciclastic deposits formed. These mixed carbonate-siliciclastic sediments and following siliciclastic deposits provide valuable information about (1) the depositional mechanism of the coeval carbonate and siliciclastic deposition (Tänavsuu-Milkeviciene et al. 2009 – Paper II), (2) processes occurred in the mixed carbonate-siliciclastic basin (Tänavsuu-Milkeviciene et al. 2008, 2009 – Papers I, II), and (3) depositional mechanism for tide- dominated deltaic deposits (Tänavsuu-Milkeviciene and Plink-Björklund 2009 – Paper III). In this thesis detailed facies and facies association studies were used to investigate the architecture, formation, palaeogeography, and evolution of the shallow marine mixed carbonate-siliciclastic deposits and tide-dominated delta deposits.
The deposition in the Devonian BB was influenced by the development of the Scandinavian Caledonides. After the extensional collapse and uplift of the Caledonides in the late Early Devonian and the early Middle Devonian (Roberts 2003) a shallow epicontinental sea covered the BB. The transgression gained its maximum during the Middle Devonian Narva time, when the wide-spread mixed muddy carbonate-siliciclastic deposits occurred in the BB. Similar shallow marine tidally influenced deposits are also found in the neighbouring areas in the Belarus, in the eastern part of the Moscow Syneclise and in the Timan-Pechora area (Valiukevičius et al. 1986; Valiukevičius and Kruchek 2000). The mixed carbonate-siliciclastic deposits were succeeded by the fine- grained tide-dominated deltaic deposits at the end of the Narva time and trough the following Aruküla time.
The geology and stratigraphy of the Middle Devonian Narva and Aruküla successions in the BB has been studied for more than 70 years. Earlier studies have been focused mostly on the lithological-mineralogical and palaeonto- logical aspects of the sediments (e.g. Orviku 1948; Tamme 1962; Kleesment 1998, Valiukevičius and Kruchek 2000), and much less on the palaeogeography or the basin evolution (e.g. Kuršs 1992; Kleesment 1997; Paškevičius 1997;
Narbutas 2005). This thesis presents the first study that focuses on detailed and systematic documentation of sedimentary facies, the distribution of facies associations, and interpretation of the basin evolution during the Middle Devonian Narva and Aruküla times according to the sequence stratigraphical approach.
Sequence stratigraphical analysis interprets the depositional origin of sedimentary rocks. This is in contrast to lithostratigraphy analysis, which maps lithofacies, which are commonly diachronous and have no time-significance.
Sequence stratigraphy has many definitions (e.g. Posamentier et al. 1988; Van Wagoner 1995; Posamentier and Allen 1999). Nevertheless, the simplest, and preferred by many authors, is ‘the subdivision of sedimentary basin fills into genetic packages bounded by unconformities and their correlative conformities’
(Emery and Myers 1996; Cattuneanu 2002). Sequence stratigraphy is used to provide a cronostratigraphic framework for the correlation and mapping of sedimentary facies and to understand how sedimentary basins accumulate and preserve sediments. Sequence stratigraphical model provides simplified, theoretical two- or three-dimensional representations of how the architecture of facies and stratigraphic surfaces is expected to be in the field and therefore makes it highly successful exploration technique in the search of natural resources.
There are different sequence stratigraphical models: three varieties of depositional sequences, genetic stratigraphic sequences, and transgressive- regressive sequences. Each model works best in particular tectonic settings, and one model is not applicable to the entire range of case studies (Catuneanu 2002) i.e. models for marine systems (Posamentier et al. 1988; Hunt and Tucker 1992;
Emery and Myers 1996; Yoshida et al. 2007), lake systems (Bohacs et al. 2000), carbonates (Schlager 2005), and siliciclastics (Weimer and Posamentier 1993).
In this study the depositional sequences were used to provide overall background information. No small-scale sequences were used.
Aims of the thesis:
1. to document a spatial and temporal variation of facies associations in the Narva and Aruküla successions, based on cm-scale descriptions of cores and outcrops;
2. to document the depositional processes in the described intervals;
3. to reveal a palaeogeographical context of the Baltic Basin during the Middle Devonian Narva and Aruküla times;
4. to study the causes of transgressive to regressive turnaround of the Middle Devonian Baltic Basin.
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2. GEOLOGICAL SETTING
The Baltic Basin is situated in the western part of the Baltica Plate and was at an equatorial position at the Devonian time (Cocks and Torsvik 2006). The Baltic Basin developed during the Devonian time as a back-bulge and later a foreland basin in front of the Scandinavian Caledonides (Plink-Björklund and Björklund 1999). The Devonian BB formed as a restricted shallow epeiric sea that was at times closed to the south, east and west, with the main denutation area in the north and the basin area in the south (Kuršs 1992; Alekseev et al. 1996;
Paškevičius 1997; Narbutas 2005; Marshall et al. 2007; Tänavsuu-Milkeviciene et al. 2009 – Paper II; Tänavsuu-Milkeviciene and Plink-Björklund 2009 – Paper III).
During the Early Devonian times the northern part of the BB was uplifted, and most of the basin experienced subaerial erosion (Kuršs 1992; Paškevičius 1997). The uplift has been attributed to propagation of the stress from the Baltica to Laurentia-Greenland collision (Plink-Björklund et al. 2004). At the end of the Early Devonian and during the Emsian, the basin started to subside anew (Plink-Björklund and Björklund 1999; Plink-Björklund et al. 2004).
A phase of coarse siliciclastic deposition occurred in the Devonian BB from the latest Early Devonian (Pragian) until the end of the Middle Devonian (end of Givetian). During the second part of the Eifelian, in the Narva time (Fig. 1), transgressive tidally influenced, mixed carbonate-siliciclastic subtidal to supratidal shallow marine deposits partially filled the BB (Plink-Björklund and Björklund 1999; Tänavsuu-Milkeviciene et al. 2009 – Paper II). This transgressive episode was followed by a southward progradation of tide-domi- nated deltaic deposits in the end of Narva time and by the following Aruküla time (Plink-Björklund and Björklund 1999; Tänavsuu-Milkeviciene et al. 2009 – Paper II; Tänavsuu-Milkeviciene and Plink-Björklund 2009 – Paper III). Tidal influence and deltaic to estuarine development continued through the Middle Devonian, up to the end of the Givetian (Kuršs 1992; Plink-Björklund and Björklund 1999; Plink-Björklund et al. 2004; Pontén and Plink-Björklund 2007;
Pontén and Plink-Björklund 2009).
The deposits of the Narva and Aruküla successions are exposed in an east- west-oriented outcrop belt at the northern margin of the basin in Estonia and northwestern Latvia (Fig. 2). There are few outcrops available for a Narva succession, whereas the Aruküla succession is well exposed (Fig. 2). Thickness of the Narva and Aruküla successions is variable and increases generally southwards from the north-east in Estonia to the south-west in Latvia (Paškevičius 1997).
Figure 1. The Middle Devonian stratigraphy and main lithological units of the Baltic Basin (after Paškevičius 1997, and Kleesment and Shogenova 2005). Absolute ages after Kaufmann (2006).
Figure 2. Geographic location of the study area and present distribution of the Narva and Aruküla successions in the Baltic Basin. Black circles mark described cores; black stars mark described outcrops; grey circles mark cores described in literature; dark line marks cross-section; light shaded area marks outcrop area of the Vadja, Leivu, and Kernave Formations; dark shaded area marks outcrop area of the Aruküla Formation.
Marked cross-section is shown on Figure 3.
EifelianGivetian
Middle Devonian
Pärnu
391.9 Ma 388.1 Ma 383.7 Ma
Narva Aruküla Burtnieki Gauja Amata
Butkunai Kukliai Sventoji Estonia,
Latvia Lithuania Regional
Stage Global
Scale
Vadja
Kernave Aruküla Burtnieki Gauja Amata
Pärnu Ledai
Fine- to coarse-grained sandstones Mixed carbonate and siliciclastic mudstones Mudstones to very fine-
grained sandstones Fine-grained sandstones Fine- to medium-grained
sandstones Fine- to coarse-grained
sandstones Fine-grained sandstones
Leivu
Formations
Lithology
Narva Breccia
56°
Estonia
Latvia
Lithuania
A’
A
0 50 100 km 1
1 3
2
3
2
25 24 16
17
12
13 27 22
21 20
28
15 36
5 7
6 6
7 34 35 9 10
31
19 18 26
14
8
11 29
33
4 5 4
30
32 23
24° 26° 28°
58°
56°
LEGEND Outcrop area (Kernave Fm) Outcrop area (Aruküla Fm)
Distribution of the Kernave and Aruküla successions
Border of country Cross-section Core: own data/literature Outcrop
1415 17
8 1211 9 16 13 10
Tartu 1
1-15DESCRIBED CORES Mehikoorma 2
Valga 3
Talsi 12
Dobele 13 15Nida Svedasai
5 Butkunai 6
Ledai 7
Vidukle 10
Kunkojai 9
Palanga 14 Kriukai
8
Taurage 11 Ludza
4
Kallaste 7
Kalmistu 8
Vapramäe 9
Viljandi 11Paistu 12Õisu
13 1615LopaTarvastu Hendrikhansu
14 Maimu
Häädemeeste 1817
Tamme 10
7-18DESCRIBED OUTCROPS (ARUKÜLA SUCCESSION) 1-6DESCRIBED OUTCROPS (NARVA SUCCESSION)
3 1Gorodenka
Narva quarry
2 Poruni 5
6 Ruhnu Varnja Oore 4
16-36CORES FROM LITERATURE
Skoudas 27
Plunge 28
Dvorikai 36
Ukmerge 34
Pauksciai 35 Jurbakas 31 Aluksne 26
Gargzdai
29 33Paroveja
Stoniskiai 30
Geluva 32 Atasiene
25 Madona 24 Kihnu
16 Ruhnu 17
Adze 22
Piltene 21
Luzni 20 Võru 19
Värska 18
Katlakalns 23 18
3. STRATIGRAPHY
Division of deposits into formations and members is based on lithological- mineralogical and palaeontologaical data (Kleesment et al. 1987; Valiukevičius et al. 1986; Paškevičius 1997; Valiukevičius and Kruchek 2000; Kleesment and Shogenova 2005). The Narva succession in Estonia and Latvia includes three formations: Vadja, Leivu, and Kernave (Kleesment and Shogenova 2005). In Lithuania two formations are defined: Ledai and Kernave (Fig. 1, Paškevičius 1997). The Aruküla Formation is divided into three members in Estonia and Latvia; Viljandi, Kureküla, Tarvastu. In Lithuania the Aruküla Formation is correlative with the Kukliai Formation (see Fig. 1; Paškevičius 1997).
The Vadja, Leivu, and Ledai Formations are characterized by dominantly light, greyish-reddish carbonate mudstones with gypsum and siliciclastic mudstone interlayers, and siliciclastic mudstone beds. The Kernave Formation is dominantly siliciclastic, reddish-colored mudstone, siltstone, and very fine- grained sandstone. The boundary between Narva succession and below lying Pärnu Formation is in many places marked with distinctive breccia bed. The upper boundary of the Narva succession is gradational, but in places defined as the lowermost occurrence of a first (significant) uncemented reddish-brown sandstone bed above the dolomitic siltstone or dolomitic marls (Kleesment and Mark-Kurik 1997).
The Aruküla and Kukliai Formations consist of reddish-brown, grey, very fine- to fine-grained sandstones, interbedded with siltstones and in places, with dolomitic marl mudstones. In the Aruküla Formation each member starts with sandstones and grades upwards into mudstones and siltstones (Kleesment and Mark-Kurik 1997; Kleesment and Shogenova 2005). The upper boundary of the Aruküla Formation deltaic succession is not distinct, and is covered by the deltaic succession of the Burtnieki Formation (Plink-Björklund and Björklund 1999).
The existing formation and members boundaries are lithostratigraphical (Kleesment and Mark-Kurik 1997; Paškevičius 1997; Kleesment and Shogenova 2005) and do not agree in details with the sequence stratigraphical subdivision used here. In this thesis the deposits are subdivided into a lower, retrogradational, carbonate-rich part (composed mainly of the Vadja, Leivu, and Ledai Formations), and an upper, progradational, siliciclastic-rich part (composed mainly of the Kernave, Aruküla, and Kukliai Formations; Fig 1;
Tänavsuu-Milkeviciene et al. 2009 – Paper II; Tänavsuu-Milkeviciene and Plink-Björklund 2009 – Paper III). The lower boundary of the progradational succession is a downlap surface onto the maximum flooding surface that occurs at the top of the transgressive portion of the Narva succession (Tänavsuu- Milkeviciene et al. 2009 – Paper II; Tänavsuu-Milkeviciene and Plink- Björklund 2009 – Paper III). The upper boundary of the progradational succession is not distinctive and marks the end of tide-dominated delta (Tänavsuu-Milkeviciene and Plink-Björklund 2009 – Paper III).
Due to uncertainties in formation boundaries, the correlation between the core sections is based on the detailed facies and facies associations analysis. It is argued that in several occasions the earlier established formation boundaries follow changes in depositional environments and sediment input, which are diachronous, instead of representing actual timelines.
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4. METHODS
This study is based on cm-scale descriptions of outcrops and cores. Altogether 15 cores and 18 outcrops, with a total of 60 measured sections were documented across the BB, and combined with 30 previously published core descriptions from Estonia, Latvia and Lithuania (Fig. 2). Core recovery was ca. 75%, and they penetrated the whole thickness of the Vadja, Leivu, Ledai, Kernave, Aruküla, and Kukliai Formations. The outcrops are tens of meters to several kilometers wide and up to a several meters high. The sedimentary facies were defined by sedimentary structures, textures, and composition (carbonate- siliciclastic ratio, evaporite content). Based on lateral and vertical variation of sedimentary facies altogether 17 facies associations have been identified from studied sections. Defined facies associations are organized into three genetically related groups; (1) carbonate-rich facies associations, where carbonate deposits dominate, (2) mixed facies associations, where siliciclastic deposits dominate, and (3) siliciclastic-rich facies associations, which consist mainly of siliciclastic material. Carbonate-rich facies associations and mixed facies associations are mainly found in cores, whereas siliciclastic facies associations are in places well presented in outcrops.
5. RESULTS AND DISCUSSION 5.1. Carbonate-rich facies associations (1)
Facies Association 1.1: Carbonate sabkha
Facies Association (FA) 1.1 is found over the whole basin area (Fig. 3). This FA consists of crinkled, thinly laminated, brecciated, and massive carbonate and siliciclastic mudstones with evaporite fabrics. The breccia consists of angular to sub-rounded carbonate and siliciclastic mudstone clasts (up to 8 cm in diameter). In places, clasts are oriented randomly. In other places, breccia occurs as broken-up beds of carbonate or as contorted, soft-sediment deformed siliciclastic clasts. In the bigger clasts, the primary sedimentary structures like thin lamination, fenestrae, and desiccation cracks are preserved. The matrix of the breccia consists of carbonate and siliciclastic mudstone. Between brecciated beds, thinly laminated, wavy-bedded, crinkle-bedded or massive carbonate and siliciclastic mudstones with concave-up structures and desiccation cracks occur.
In places, vugs, veins, and cavities (1.5 mm deep) occur in the carbonates.
In the central and southern parts of the basin the breccia contains evaporites in the form of anhydrite and gypsum rosettes, nodules, interlayers, and halite crystals and pseudomorphs. In several places, sub-vertical or vertical cracks and fenestral porosity are filled with evaporites. Thin, up to 5 mm thick, organic- rich laminae are found within these deposits. In the northern part of the basin the breccias do not contain evaporites and they are interbedded with homogeneous, laminated, siliciclastic mudstones and massive carbonate mudstones.
Interpretation: The occurrence of brecciated deposits together with thinly laminated, wavy-bedded, and crinkle-bedded deposits as well as evaporite fabrics suggest deposition in an arid, supratidal environment (Warren and Kendall 1985; Kendall 1992; Pratt 2002). The occurrence of desiccation cracks and concave-up structures, i.e. tepee structures, indicate desiccation and deposition in supratidal conditions (Demicco and Hardie 1994). The broken-up carbonate breccia, crinkle lamination, and evaporite fabrics refer to the salt growth sedimentation deformations and to the solution-collapse features (Goodall et al. 2000). The breccia is thus interpreted as solution-collapse breccia formed through evaporite dissolution processes (Goodall et al. 2000;
Schreiber and El Tabakh 2000). Such breccias commonly occur in karst-related and in sabhka environments (Warren and Kendall 1985; Pomoni-Papaioannou and Karakitsios 2002). However, the fact that deposits formed in the basin margin during the transgression (Tänavsuu-Milkeviciene et al. 2008, 2009 – Papers I, II) suggests that deposition occured in sabkha environments. The breccias in the northern part of the basin lack evaporitic features. This could be resulted from the complete dissolution of evaporites in the deposits
Mehikoorma (2) Ludza (4) Svedasai (5) Butkunai (6) Ledai (7)
Tartu (1) Valga (3)
A - A’
50 100 175
km 180 55 83 10m
SU 1SU 2SU 3
Carbonate-rich facies associations (1) FA 1.1 Carbonate sabkha
FA 1.2 Intertidal- to supratidal carbonate tidal-flat
FA 1.3 Intertidal to supratidal shoals FA 1.4 Subtidal carbonates
FA 1.5 Subtidal shoals and channels FA 2.6 Subtidal mudstones FA 2.2 Supratidal flat FA 2.3 Intertidal flat deposits Mixed facies associations (2)
FA 3.2.2 Sand flat FA 3.2.1 Mixed and mud flat FA 3.2 Tidal flat
FA 3.1 Paleosol
FA 3.3 Tidal gullies and distributary channels
FA 3.5.2 Proximal tidal bar FA 3.5.1 Distal tidal bar FA 3.5 Tidal bar
FA 3.6 Prodelta
Boundary of Stratigraphic Unit Siliciclastic-rich facies associations (3)
Figure 3. Stratigraphic north-south oriented cross-section (A-A’) of the eastern part of the Baltic Basin of the Middle Devonian Narva and Aruküla successions. The cross-section show lower, retrogradational shallow marine tide-influenced mixed carbonate-siliciclastic deposits that are followed by progradational siliciclastic tide-dominated deltaic deposits. The lower transgressive part consists of two packages sand-rich deposits occur in the southern part of the basin, whereas in the upper package sand and mud rich deposits occur also in the central and southern part of the basin. In the upper progradational part, three progradational to aggradational vertically stacked packages that successfully thin upward, occur. North-south oriented cross-sections show southward progradation of the tide- dominated deltaic complex. The vertical bar is 10 m.
or alternatively, the breccia could have been formed by wave-cyclic loading causing deformation of previously deposited sediments (see Tänavsuu- Milkeviciene et al. 2008 – Paper I).
Facies Association 1.2: Intertidal- to supratidal carbonate tidal flats
Facies Association 1.2 occurs in the northern and southern parts of the basin (Fig. 3) and form highly variable depositional units 0.2 to 2 m thick. The lower part of the unit consists of planar-laminated, current ripple-laminated or homogeneous siliciclastic mudstones. The upper part of the unit consists of thinly laminated, wavy-bedded, nodular-bedded or massive carbonate mudstones with mud drapes and lenses, desiccation cracks, cavities, and vugs (1.5 mm deep). In many deposits sub-angular to sub-rounded carbonate mudstone clasts (up to 6.5 cm in diameter) occur. Brecciated deposits also are found in this FA. Anhydrite and gypsum rosettes and interlayers occur in a few deposits, as do occasional shells and fish scales.
Interpretation: The thin lamination and nodular or wavy-bedding, together with desiccation cracks and mud drapes, indicate deposition in intertidal to supratidal tidal flat environments (Elrick 1995; Osleger and Montañez 1996; Lehrmann et al. 2001). The carbonate clasts and brecciated deposits are interpreted to have formed under the influence of occasional storms or tidal currents (Pratt et al.
1992; Seguret et al. 2001; Horbury and Qing 2004).
Facies Association 1.3: Intertidal to supratidal shoals
Facies Association 1.3 is found in the central and southern parts of the basin (Fig. 3). These deposits form highly variable depositional units 0.2 to 2 m thick, and are characterized by the occurrence of (1) vugs, cavities (1.5 mm deep), desiccation cracks, and fenestrae; (2) abundant interlayers of current ripple- laminated up to fine-grained sandstones, and (3) carbonate mudstone intraclasts, tepee structures, and anhydrite or gypsum rosettes and interbeds.
The lower part of the unit consists of thinly laminated, lenticular, wavy- bedded, and ripple-laminated siliciclastic mudstones and siltstones. The upper, carbonate-rich part consists of flaser-bedded, wavy-bedded, and nodular-bedded deposits. In places, the depositional units grade upwards into sandy dolomites or sandstones with dolomitic cement. Mudstone interlayers, lenses and coarse to gravel-sized quartz grains occur in deposits. Shell fragments are found in a few places.
Interpretation: The depositional units that grade from ripple-laminated, lenticular, wavy-bedded siliciclastics up to flaser-bedded, wavy-bedded carbonates with desiccation cracks and fenestrial fabric indicate deposition in intertidal to supratidal conditions (Osleger and Montañez 1996; Lehrmann et al.
2001). The high occurrence of sand-rich deposits: ripple-laminated sandstone interlayers up to gravel-sized quartz grains, sandy dolomites, and sandstones
17
with dolomitic cement suggest that deposition occurred in high energy environments in the tidally influenced bars or storm- and tide-influenced shoals (Brooks et al. 2003; Rankey et al. 2006).
Facies Association 1.4: Subtidal carbonates
This Facies Association is found all over the basin (Fig. 3), as 0.2 to 0.9 m thick depositional units. The basal siliciclastic mudstones occur as homogeneous or current ripple-laminated beds. In places, rolling grain ripples are found. The upper part of this depositional unit consists of occasionally vuggy (1 to 2 mm deep), massive carbonates. Nodular-bedded and wavy-bedded carbonates also occur, but are less common. Anhydrite and gypsum rosettes and interlayers as well as angular to sub-rounded carbonate mudstone clasts (up to 6 cm in diameter) are found. Bioturbation of variable intensity, shells and fish scales occur in deposits. The presence of bioturbation and the lack of desiccation cracks, fenestrae and high structural variability make FA 1.4 different from the FA 1.2, described above. This FA is occurs in both, cores and outcrops. In outcrops FA 1.4 occurs as laterally continues depositional units that can be traced out over many kilometres wide area.
Interpretation: The dominantly homogeneous siliciclastic mudstones that grade upwards into massive carbonate mudstones are interpreted as subtidal deposits (Elrick 1995; Osleger and Montañez 1996; Jiang et al. 2003). The occurrence of carbonate clasts suggests occasional storm wave or tidal current action.
Facies Association 1.5: Subtidal shoals and channel deposits
Facies Association 1.5 consists of 0.2 to 2.2 m thick depositional units of interbedded siliciclastic and carbonate mudstones, sandstones with dolomitic cement and sandy dolomites. The siliciclastic-rich part occurs as homogeneous, planar or current ripple-laminated intervals. Carbonate-rich part occurs as massive, in places nodular-bedded or wavy-bedded intervals. Deposits are rich in sub-rounded and rounded quartz grains of various sizes; from very fine sand up to gravel sized (up to 6 mm). These quartz grains are commonly randomly distributed, but in a few places the quartz grains occur as thin layers, lenses or aggregations. In places, mud layers and lenses occur in sandy dolomites and in cemented sandstones. Anhydrite or gypsum rosettes and interlayers are also found. This FA occurs in the central and southern parts of the basin (Fig. 3).
Two different depositional units are found in FA 1.5, upward-fining and upward-coarsening.
Interpretation: The coarser grained deposits compared with the above described facies associations and the vertical continuation into FA 1.3 suggests deposition in a high-energy subtidal environment. The relatively thicker sand-rich deposits interbedded with thinner carbonate-rich deposits and the high occurrence of gravel sized quartz grains indicates deposition in a shoal environment (Gonzalez
and Eberli 1997; Rankey et al. 2006). The fining-up and coarsening-up depositional units are interpreted as tidal channel and subtidal bar deposits (Brooks et al. 2003).
5.2. Mixed facies associations (2)
Facies Association 2.1: Evaporitic mudflat/siliciclastic sabkha
Facies Association 2.1 consists of 0.1 to 4.8 m thick planar-laminated, crinkle- bedded, lenticular-, and wavy-bedded or homogeneous siliciclastic mudstone and siltstone depositional units. In places, deposits are brecciated and contain silicified or carbonate crusts. Very fine-grained to fine-grained homogeneous or ripple-laminated sandstones are found within the mudstones. In other places, massive, flaser-bedded, and wavy-bedded carbonate mudstones occur. Gypsum, anhydrite rosettes, interlayers, nodules, and halite crystals and pseoudomorphs occur in the deposits. This FA is found only in the western part of the basin.
Interpretation: The lenticular-bedding, wavy-bedding, flaser-bedding and dominance of mudstones and siltstones suggest deposition in the upper intertidal or lower supratidal area (James and Kendall 1992; Dalrymple 1992).
The crinkle-bedding and brecciated deposits, together with other depositional features and evaporite fabrics described above, indicate deposition in the evaporitic mudflat/siliciclastic sabkha (Kendall 1992; Schreiber and El Tabakh 2000). The brecciated intervals are interpreted to have formed as a result of solution of evaporite minerals by groundwater or by rain water (Goodall et al.
2000).
Facies Association 2.2: Supratidal flat
Facies Association 2.2 has been identified in the central part of the basin (Fig.
3). This FA consists of 0.07 to 3.7 m thick depositional units, where homogeneous, planar, and current ripple-laminated, siliciclastic mudstones grade upward into the planar-laminated, flaser-, wavy-bedded carbonate mudstones. In places, carbonates are gradationally laminated. The homogeneous or current ripple-laminated sandstone layers and lenses occur in deposits.
Anhydrite and gypsum rosettes, interlayers and vug fills as well as desiccation cracks are abundant in deposits. Shell fragments are also found.
Interpretation: The combination of planar-laminated and ripple-laminated siliciclastics with flaser-, wavy- or nodular-bedded carbonates, gradational lamination, and evaporite fabrics indicate deposition in arid, upper intertidal to supratidal flats (Reineck and Wunderlich 1968; Warren and Kendall 1985;
Alsharhan and Kendall 2003). The occurrence of desiccation cracks refers to the subareal conditions. The coarse grained interlayers and gradational bedding suggests tidal current or storm wave action.
Facies Association 2.3: Intertidal flat
Facies Association 2.3 is identified in the northeastern part of the basin (Fig. 3).
This FA consists of alternating homogeneous, current ripple-laminated or bioturbated siliciclastic mudstone units that pass upward into thin, massive carbonate mudstones with desiccation cracks. In a few places, wavy- and flaser- bedding occurs in the carbonates. In other places, current ripple-laminated and bidirectional cross-laminated sandstone interlayers are found. Sediments are partially or completely bioturbated and contain shell fragments.
Interpretation: The combination of siliciclastic mudstones, locally extensive bioturbation, wavy-bedding, flaser-bedding, and bidirectional cross-lamination suggests deposition in a setting subjected to rapidly changing energy conditions, such as the intertidal flat (Dalrymple 1992; Weimer et al. 1998).
Facies Association 2.4: Flood-tidal delta
Facies Association 2.4 occurs in the southwestern part of the basin. This FA consists dominantly of thinly bedded, planar, current ripple-laminated, and structureless siltstones and very fine-grained sandstones with siliciclastic mudstone interlayers and clasts (3 to 8 mm in size). Locally, fine-grained, cross- stratified sandstones are found. In many places, millimetre thick, very fine- grained sandstones to mudstones layers form thick depositional units.
Homogeneous, planar-laminated siliciclastic mudstone beds are less common.
In places, massive carbonate mudstones occur.
Interpretation: The relatively high sand content and the mixture of high-energy sandy facies with siliciclastic mudstones suggest deposition in a tidal environment, where the sand deposition rates were high, but episodic. The above mentioned structures and the mixture of laminated sandstones with mudstone layers and clasts suggest deposition in flood-tidal delta environment (see also Davis et al. 2003). The millimetre-scale sandstone to mudstone layers may indicate deposition during waning storm events (Walker and Plint 1992).
Facies Association 2.5: Tidal inlet
Facies Association 2.5 is dominated by planar, current ripple-laminated, and thinly bedded, fine-grained sandstones with siliciclastic mudstone lenses and interlayers. In places, cross-stratified sandstones occur. Mud clasts (up to 9 mm in diameter) are found in some deposits and in a few places thin, massive carbonate mudstones occur. This FA occurs in the southwestern part of the basin. Two different depositional units are found in this FA: upward-fining and upward-coarsening.
Interpretation: Thinly bedded, cross-laminated and stratified sandstones with mudstone lenses and clasts indicate deposition in a high-energy tidal environment (Reading and Collinson 1996). The upward-fining and upward-
coarsening depositional units are interpreted as channel and bar deposits. Facies Association 2.5 is similar to FA 2.4. However, the occurrence of bar and channel deposits and the higher amount of coarser-grained sandy material suggests deposition in a tidal inlet.
Facies Association 2.6: Subtidal mudstones
Facies Association 2.6 is dominated by homogeneous siliciclastic mudstones.
Locally, siltstone and sandstone interlayers occur. In places, thin, massive carbonate mudstone beds are found. No trace fossils have been observed. This FA is found in the northeastern and southeastern parts of the basin (Fig. 3).
Interpretation: Predominance of homogeneous siliciclastic mudstones indicates deposition in a low-energy environment. Such homogeneous sediments can indicate an extremely high degree of bioturbation (see Enos 1998; Malpas et al.
2005). However, no tracks or burrows were found. The silty and sandy interlayers indicate occasional wave or tidal current action. The FA 2.6 is interpreted as subtidal mudstones (Fig. 3; Pratt et al. 1992; Osleger and Montañez 1996; Jiang et al. 2003).
5.3. Siliciclastic-rich facies associations (3)
Facies Association 3.1: Paleosol
Facies Association 3.1 occurs mainly in the southern part of the basin. Thin units of FA 3.1 occur also in the northern and middle parts of the basin (Fig. 3).
This FA is divided into two types: (1) type one that contains of red, purple, and ochre coloured homogeneous, bioturbated, and brecciated mudstones to very fine- and fine-grained sandstones, and (2) type two that contains of cemented, structureless, normally graded medium- to coarse-gained sandstones. Ferric minerals occur as accumulations on bed tops and as thin layers within the deposits. Root traces and desiccation cracks filled with coarser grained sediments occur in the first type. Carbonate concretions, nodules, and mud clasts, in places interbedded by networks of calcite veins occur in second type.
This Facies Association is found only in cores, associated with FA 3.2.
Interpretation: The bioturbated and brecciated intervals together with root traces, ferric mineral accumulations, and carbonate concretions indicate paleosol units formed during periods of subaerial exposure, landscape stability, and nondeposition (Ruskin and Jordan 2007). The brecciated, bioturbated mudstone and siltstone units with root traces and ferric mineral accumulations suggest paleosol formation in arid climatic conditions as ferruginous soils (Collinson 1996). Cemented sandstones with carbonate concretions, nodules, and networks of calcite veins are interpreted to be formed in semi-arid or arid
21
conditions with low precipitation level (Mack 1992), as calcretes (Collinson 1996).
Facies Association 3.2: Tidal flats
Facies Association 3.2 is in association with FAs 3.3 and 3.4, gradationally overlies FA 3.5, and is capped by FA 3.1 (Fig. 3). This FA consists of heterolitic, rhythmical deposits of mudstones and sandstones, in depositional units up to 10 m thick. Small-scale soft-sediment deformation and vertical to subvertical simple trace fossils occur throughout the units. Root traces, shell fragments, and fish-scale fragments are also common. In many places, deposits are red and rich in ferric mineral accumulations that occur as thin layers or aggregates. Facies Association 3.2 can be divided into two types: (1) deposits where mudstones and siltstones dominate, and (2) deposits where sandstones dominate. The first type, FA 3.2.1 consists of homogeneous, thinly laminated, lenticular-, and wavy-bedded mudstones and siltstones, with planar-laminated and current ripple-laminated sandstone interlayers or interbeds. Locally, grey, lenticular and wavy-bedded siltstones occur together with structureless and bioturbated red siltstones. Carbonate-cemented siltstones and dolomitic marlstones occur.
The second type, FA 3.2.2, consists of flaser-bedded, planar-laminated, current ripple-laminated, and planar cross-stratified sandstones that are rich in mica and mud drapes. In places, structureless sandstones with carbonate clasts and carbonate cemented bed tops occur. Sandstones are interbedded with planar-laminated and current ripple-laminated or homogeneous mudstones and siltstones.
Facies Association 3.2 is best exposed in cores. In outcrops, FA 3.2 is not always a cliff former and tends to be partially covered. In outcrops, FA 3.2 occurs as a laterally discontinues beds with maximum lateral extent of 70 m and a height of up to 2.5 m.
Interpretation: Flaser-, wavy-, lenticular-bedded, and thinly laminated heterolithic and rhythmical deposits suggest deposition by low-energy tidal currents, most likely on tidal flats (e.g. Reineck and Wunderlich 1968; Yoshida et al. 2001). The mudstone- and siltstone-rich heterolitic deposits in the FA 3.2.1, indicate deposition on a lower energy muddy supratidal or mixed, upper- intertidal flats (Dalrymple et al. 1991). The overall sandy heterolitic deposits in the FA 3.2.2 indicate strong tidal currents and deposition on a higher-energy sandy, lower-intertidal flat near the channel or tidal creek thalweg (Dalrymple 1992; Davis and Flemming 1995).
Facies Association 3.3: Tidal gullies and distributary channels
Facies Association 3.3 occurs as laterally discontinuous, erosionally based up to 3 m thick units within FAs 3.2 and 3.4 (Fig. 3). This FA consists of upward- fining depositional units. The lower part of unit consists of plane-parallel,
planar- and trough cross-stratified, and structureless fine- and medium-grained sandstones that grade upwards into planar-laminated and current ripple- laminated siltstones and mudstones and very fine-grained sandstones. Locally, mud-clast conglomerate, shell and fish fragments occur at the base of depositional units or along the bases of the beds. Bidirectional dip directions in adjacent cross sets are common in laminated and cross-stratified deposits. Mica and mud drapes, and mud lenses with variable thickness, are also common. FA 3.3 is only documented in cores located in the northern and central parts of the basin (Fig. 3).
Interpretation: The small-scale upward-fining depositional units of dominantly cross-stratified sandstones with conglomerate beds, and shell and fish fragments at the bases of the units, indicate deposition from high-energy currents in channels (Dalrymple et al. 1991; Willis et al. 1999). Bidirectional dip direction of cross-strata and mica and mud drapes suggest deposition by tidal currents (Nio and Yang 1991). In addition to the latter, FA 3.3 is laterally equivalent with FAs 3.2 and 3.4, which suggests deposition in tidal gullies or tidally influenced distributary channels.
Facies Association 3.4: Supratidal muds
Facies Association 3.4 occurs in the western part of the basin, in association with FAs 3.2 and 3.3, and passes laterally basinward into FA 3.6. Facies Association 3.4 consists of homogeneous, brecciated, planar-laminated mudstones and siltstones that form up to 10 m thick depositional units. Planar- laminated and ripple-laminated very fine- to fine-grained sandstones occur in places. Locally, deposits contain mud clasts, isolated root traces, and shell fragments. Bioturbation is very rare. Structureless carbonate interbeds occur in few places. This FA is represented only in cores in the western part of the basin.
Interpretation: The homogenous, planar-laminated mudstone units suggest deposition in low-energy conditions. The occasional sandstone beds and mud clasts indicate rare current events. The occurrence of brecciated deposits and isolated root traces suggests subareal or supratidal conditions.
Facies Associations 3.5: Tidal bar
Facies Association 3.5 occurs across the whole basin (Fig. 3) and is well exposed in both cores and outcrops. Facies Association 3.5 passes laterally southward (basinward) to FA 3.6 and is gradationally overlain by FA 3.2. This FA consists of gradationally based, upward-coarsening successions, up to 30 m thick. Cross-laminated and cross-stratified siltstones and sandstones are common throughout the succession. Complex cross-sets with single and double mica and mud drapes, reactivation surfaces, and bidirectional cross-sets occur in deposits. Overturned ripples and soft sediment deformations occur in sandstones. In a few places, structureless carbonate beds and carbonate
cemented concretions up to 0.7 cm in diameter occur. Fish remains and shell fragments are abundant, but bioturbation is less common. This FA consists of upward-coarsening depositional units, couple of meter thick, separated by inclined master surfaces. In places inclined, low-angle master surfaces with dips of 7–15° can be traced across outcrops. The amount and frequency of mudstone and siltstone layers, interbeds, and beds decrease upwards in the depositional units. Facies Assiciation 3.5 is divided into two types: (1) FA 3.5.1, which consists of finer grained deposits and forms the lower part of the depositional unit, and (2) FA 3.5.2, which consists of coarser grained deposits and forms the upper part of the depositional unit.
Facies Association 3.5.1 consists generally of plane-parallel, current ripple- laminated, flaser-, wavy-, and lenticular-bedded mudstones, siltstones, and very fine-grained sandstones. In very few places, wave ripple lamination occurs.
Facies Association 3.5.1 is not always a cliff former and in outcrops tends to be partially covered. There is limited number of exposures of inclined master surfaces, and it is thus difficult to evaluate the dip angles and dip orientations.
Erosional surfaces with an erosion depth up to 0.7 m, filled with horizontal beds, occur locally. Palaeocurrent directions derived from cross-strata and ripples are bidirectional. The dominant group of palaeocurrents varies between 260-300°, with most currents between 280–300°. The subordinate group varies between 60–180°, with most current directions between 60–90° and 160–180°.
Facies Association 3.5.2 consists generally of fine-grained, plane-parallel- stratified, planar- and trough cross-stratified sandstones. Large-scale cross- stratified sandstones with bed thickness of 0.6–1.5 m occur in places. Mud clasts, with a diameter of 0.5–7 cm, occur along the base of the cross sets.
Sigmoidal bedding, with foreset thickness that thickens and thins between 0.2–
1.5 cm within the individual sets, occurs locally.
Type two is laterally extensive in outcrops. The inclined low-angle master surfaces with dips of 7–15° can be traced across outcrops up to 250 m wide.
Cross-set thickness between master surfaces varies from 25 cm to 150 cm.
Macroforms up to 4 m high and 40 m long occur. Locally, concave-up erosion surfaces up to ca. 30 m wide and 2.5 m deep occur. Mud pebbles up to 25 cm in diameter occur on erosional surfaces.
Palaeocurrent directions derived from cross-strata and ripples are bidirectional. The dominant group of palaeocurrents varies between 90–170°, with most currents between 130–170°. The subordinate group varies between 320–30°, with most currents between 10–30°. The palaeocurrent directions derived from the master surfaces vary between 140–230°, most currents between 140–200°.
Interpretation: The flaser-, wavy-, lenticular-bedded deposits, bidirectional cross-strata, reactivation surfaces, sigmoidal bedding and abundant mica and mud drapes indicate deposition from tidal currents with fluctuating current speed and direction, and suggest deposition in a high-energy tidal environment
(Reineck and Wunderlich 1968; Nio and Yang 1991; Willis 2005). Cyclic thickening and thinning of foresets within cross-sets indicates rhythmic changes of neap and spring tides (Nio and Yang 1991). Large-scale inclined master surfaces with superimposed cross-strata indicate deposition in large macroforms. The large difference of dip azimuth between the master surface dip directions and the migration directions of superimposed macroforms suggests lateral migration and deposition in delta front tidal bars (Dalrymple 1992:
Dalrymple and Choi 2007) that occur as more discontinues deposits present across the basin.
The heterolithic stratification, relatively high mud content, flood dominated palaeocurrents, and the more seaward position of FA 3.5.1 compared with FA 3.5.2 indicates lower energy conditions and suggests deposition in distal tidal bars.
The coarser grain size and ebb dominated palaeocurrents of FA 3.5.2 compared with FA 3.5.1 suggests more significant fluvial influence (Dalrymple and Choi 2007). Therefore it could also be a mouth bar. However, due to the dominance of tidal signatures, such as sigmoidal bedding, cyclic thickening and thinning of foresets within cross sets, and high occurrence of single and double mica and mud drapes, FA 3.5.2 is interpreted as proximal tidal bar.
Facies Assocition 3.6: Prodelta
Facies Association 3.6 is present only in cores, and occurs southward (basinward) of FA 3.5, primarily in the western and southern parts of the basin (Fig. 3). This FA consists of interbedded homogeneous, planar-laminated, and ripple-laminated mudstone and siltstone depositional units 0.4–4.6 m thick that stack vertically, forming successions up to 20 m thick. Very fine- to fine- grained structureless or ripple-laminated sandstones layers and interbeds occur within mudstones and siltstones. Structureless carbonate beds and thin mm thick mica interlayers, occur locally. In places, shell fragments occur on bedding planes.
Interpretation: The dominantly laminated mudstones and siltstones with coarser grained sandstone layers and beds, together with the seaward position from FA 3.5, suggest deposition in a prodelta environment. Fine-grained sandy layers are interpreted as occasional fluvial input (Reading and Collinson 1996). Current- rippled sandy beds and shell fragments suggest occasional fair-weather wave influence. The structureless character of mudstone and siltstone beds may indicate a high degree of bioturbation (Enos 1998).
25
5.4. Basin evolution during the Narva and Aruküla times
The Baltic Basin formed during the Middle Devonian time as a restricted shallow epeiric sea. The basin became shallower to the north and also to the south towards the Mazurian-Belarusian uplift (Kuršs 1992; Alekseev et al.
1996; Paškevičius 1997; Narbutas 2005; Marshall et al. 2007; Tänavsuu- Milkeviciene et al. 2009 – Paper II; Tänavsuu-Milkeviciene and Plink- Björklund 2009 – Paper III). Most of the Middle Devonian time siliciclastic- dominated sedimentation prevailed in the BB. However, the middle Eifelian time marks an abrupt change in the development of the BB, as well as in the most areas across the East European Platform, from siliciclastic-dominated sedimentation to the carbonate-dominated sedimentation (Nikishin et al. 1996).
In the BB changes in sedimentation are marked in numerous places with distinctive breccia beds that separate below lying siliciclastic-rich estuarine deposits of the Pärnu Formation (Tovmasyan 2004) from the carbonate-rich deposits of the Vadja and Ledai Formations (see Fig. 1). The origin of these breccia beds, known as the Narva Breccia, has been discussed for almost 50 years. Brecciated beds are suggested to be formed, as (1) submarine slumping and sliding deposits initiated by earthquakes (Karajajute-Talimaa and Narbutas 1964; Kuršs 1992; Kleesment 1997; Paškevičius 1997), (2) tsunami triggered beds, associated with the Kaluga impact event in the Russian Basin (Masaitis 2002), and (3) due sulphate inhydration with an increase in the volume of sediments related to sedimentary and diagenetic processes (Paškevičius 1997).
However, Paškevičius (1997) did not associate the breccia with a specific depositional environment. Based on the sedimentary characteristics as fenestrae, desiccation cracks, thin lamination, soft-sediment deformed and broken beds of siliciclastics and carbonates, organic-rich laminas, and evaporite fabrics, Tänavsuu-Milkeviciene et al. (2008 – Paper I) suggested formation of brecciated beds as solution-collapse breccias in sabkha conditions due to evaporite mineral formation and dissolution, and in places possibly also due to wave-action. Detailed analysis of facies and facies associations (Tänavsuu et al.
2007; Tänavsuu-Milkeviciene et al. 2009 – Paper II) suggests that brecciation is not associated with one single tectonic or meteorite impact event as suggested earlier (e.g. Masaitis 2002), because the occurrence of brecciated beds in at last 11 different stratigraphical horizons, association with other supratidal facies, and its systematic occurrence on the basin margins during the transgression (see also Fig. 3; Tänavsuu-Milkeviciene et al. 2008, 2009 – Papers I, II).
The brecciated beds were covered with thick succession of the mixed carbonate-siliciclastic deposits (composed mainly of the Vadja, Leivu, and Ledai Formations; Fig. 3). In the previous studies, carbonate-rich deposits were interpreted to have been accumulated either in a lagoonal environment (Paškevičius 1997) or in shallow marine, tidally-influenced environments (Kleesment 1997; Plink-Björklund and Björklund 1999; Narbutas 2005).
Tänavsuu-Milkeviciene et al. (2009 – Paper II) suggest that the BB expanded first from south-west to north-east and, later during the transgression, also to the north, south, and east. Carbonate-rich sabkha and supratidal to intertidal deposits formed on the basin margins, whereas subtidal carbonates were deposited in the basin centre. During the first part of transgression overall muddy deposition occurred in the BB, except in the southwestern and southern parts of the basin where silt- and sand-rich tidal delta (FA 2.4), tidal inlet (FA 2.5), intertidal to supratidal shoal (FA 1.3), and subtidal shoal and channel (FA 1.5) deposition occurred (see also Fig. 3). The occurrence of the sand-rich deposits in the southern and southwestern parts of the BB is also reported in the earlier studies (Paškevičius 1997; Narbutas 2005). Narbutas (2005) suggest the inflow of sand-rich material also from the south, from Mazurian-Belarusian uplift. Based on distribution of the facies associations, Tänavsuu-Milkeviciene et al. (2009 – Paper II) suggested that the sand-rich material was derived from southwest through a tidal inlet and flood-tidal delta complex by tidal currents and storm waves. During later stages of the transgression the amount of siliciclastic material increased also in the northern and central parts of the basin.
This is indicated by the appearance of intertidal to supratidal shoals (FA 1.3), siliciclastic-rich tidal flat (FAs 2.1 to 2.3), and subtidal mudstone (FA 2.6) deposits in the northern part of the basin (Fig. 3; Tänavsuu-Milkeviciene et al.
2009 – Paper II).
The upper, progradational part that comprises mainly of the Kernave, Aruküla, and Kriukai Formations, is dominated by silty to fine-grained siliciclastic deposits. These fine-grained sandstones are interpreted to be deposited in shallow marine and deltaic settings (Kleesment 1997; Paškevičius 1997; Plink-Björklund and Björklund 1999; Tänavsuu-Milkeviciene and Plink- Björklund 2009 – Paper III). In Estonia, the Aruküla Formation is divided into three depositional units based on lithological and mineralogical data (Kleesment and Mark-Kurik 1997). Each depositional unit starts with sandstones and grades upwards into mudstones and siltstones and is interpreted to indicate sea-level fluctuations (Kleesment 1997; Kleesment and Mark-Kurik 1997). Tänavsuu- Milkeviciene and Plink-Björklund (2009 – Paper III) subdivided sand-rich progradational part also into three Stratgraphical Units (see Fig. 3). These stratigraphical units represent three gradationally based progradational to aggradational vertically stacked packages that successively thin upward (Tänavsuu-Milkeviciene and Plink-Björklund 2009 – Paper III). However, in their subdivision Stratigraphical Units 1–3 (Fig. 3) are based on the vertical and lateral changes of facies associations instead of lithological changes. Also formation of stratigraphical units is caused by sediment supply and infilling of accommodation space rather than fluctuations of the sea-level.
During the progradation, the coarse siliciclastic material was river-derived from the northern part of the basin and redistributed later by the tidal currents (Kleesment 1997; Plink-Björklund and Björklund 1999; Tänavsuu-Milkeviciene et al. 2009 – Paper II; Tänavsuu-Milkeviciene and Plink-Björklund 2009 –
Paper III). The detailed analysis (Tänavsuu-Milkeviciene and Plink-Björklund 2009 – Paper III) reveal that main volume of deposits, tidal bars (FA 3.5) and sandy tidal flats (FA 3.2.2), accumulated on the delta front and on the subaqueous delta plain. The subareal deposits, paleosols (FA 3.1), muddy and mixed tidal flat (FA 3.2.1), and supratidal muds (FA 3.4) occur at the top of the prograding tidal bars (FA 3.5) and adjacent to the distributary channels.
Similarly to the other authors Tänavsuu-Milkevicene and Plink-Björklund (2009 – Paper III) suggested that these sandy deposits formed in the shallow sea. However, the overall subaqueous character of the deposits, occurrence of marine fauna (see Mark-Kurik 1995), and high amount of tidal signatures as single and double mica and mud drapes, reactivations surfaces, sigmoidal bedding, and bidirectional cross-stratification suggests that the progradational succession formed as a dominantly subaqueous, tide-dominated delta with restricted river-dominated delta plain. Main depocenter of delta occurred in the eastern part of the basin, were thick tidal bar (FA 3.5) deposits formed. In the end of the Aruküla time, thick siliciclastic tidal flat deposits (FA 3.2) formed in the northern margin of the basin, indicating development of locally subareal delta plain and changing of tide-dominated delta to tide-influenced delta in the end of Aruküla time (Tänavsuu-Milkeviciene and Plink-Björklund 2009 – Paper III).
5.5. Causes of transgressive to regressive turnaround
The retrogradiation of carbonate-rich deposits in the BB during the Eifelian is coeval with the overall global transgressive trend (Elrick et al. 2009). It also correlates with the Choteč Event in Middle Devonian episodes of marine dysoxia/anoxia and with associated extinctions (Fig. 4; House 2002). However, the presence of fine siliciclastic material in mixed facies associations (FAs 2.1 to 2.6) and also in the carbonate rich-facies associations (FAs 1.1 to 1.5) indicates variable, but relatively continuous influx of terrigeneous material throughout the retrogradation. Occurrence of sand-rich deposits in the southern and southwestern parts of the basin and later during the transgression also in the northern part of the basin, suggests that main siliciclastic inflow area changed from southwest to north at the end of transgression (see also Fig. 3).
However, the progradation of sandstones at the end of the Narva time does not correlate with any globally-recognized sea-level fluctuations. It corresponds to a time of well-defined world-wide eustatic sea-level rise named the Kačak Event, identified close to the Eifelian-Givetian boundary (Fig. 4; House 2002;
Buggisch and Joachimski 2006; Marshall et al. 2007; Haq and Schutter 2008).
Also the overall aggradational character of the prograding deltaic complex and
Figure 4. Relative sea-level chart of the Devonian time, positions of the Choteč and Kačak Events, and approximate position of the Kernave-Aruküla deltaic succession.
Note that the Kernave and Aruküla deltaic suggession correlates well with overall relative sea-level rise. Modified after Marshall et al. (2007), Haq and Schutter (2008).
gradational boundaries between Stratigraphical Units 1–3 suggest a balance between sedimentation and relative sea-level rise. The Kačak Event occurred during deposition of the Kernave Formation (Marshall et al. 2007), which forms the main part of Stratigraphic Unit 1. However, the upward-thinning, from Stratigraphic Unit 1 to Stratigraphic Unit 3, and progradation of the deltaic succession indicates a relative decrease in the relative sea-level rise rate through the delta evolution (Tänavsuu-Milkeviciene and Plink-Björklund 2009 – Paper III).
Therefore, we suggest that the turnaround in the BB from retrogradational carbonate-dominated to progradational siliciclastic-dominated infill occurred due to changes in tectonic regime in the hinterland that caused the increase of siliciclastic input rates that exceeded the rates of relative sea-level rise.
The turnaround from retrogradation to progradation during the Narva time additionally marks a significant change in the compositional and textural properties of the siliciclastic input into the BB (Plink-Björklund and Björklund 1999; Plink-Börklund et al. 2004). The Emsian and earliest Eifelian coarse siliciclastic succession, underlying the Narva succession, is characterized by poor sorting, a high proportion of angular or sub-angular grains, as well as a
Devonian Early
Lochkovian Pragian Emsian Eifelian
Kernave-Aruküla deltaic succsession
Kaèak EventChoteè Event
Givetian Frasian Famennian
Stage Onlap curve
Landward Basinward
MiddleLateEpoch
Period
29
comparison, the coarse siliciclastic succession from upper Narva (Eifelian) to Givetian deposits is texturally and compositionally mature (consists up to 99.9% of quartz). Moreover, the textural and compositional maturity increases upward from the upper Narva to Gauja successions. Plink-Björklund and Björklund (1999) and Plink-Börklund et al. (2004) suggested that the compositional and textural maturity changes in the BB were caused by reorganization of hinterland in the Scandinavian Caledonides. They suggested that the Emsian and early Eifelian deposition occurred in a back-bulge depocenter to the Scandinavian Caledonian foreland basin, and the coarse siliciclastic material was derived from the erosion of the forebulge or the adjacent Precambrian terranes (Plink-Björklund and Björklund 1999; Plink- Börklund et al. 2004). In contrast, the upper Eifelian and Givetian deposits are interpreted to be mainly cannibalized from the Scandinavian foreland basin (Plink-Björklund and Björklund 1999; Plink-Börklund et al. 2004). The end of the Eifelian coincides with the main phase of orogenic collapse and uplift in the Scandinavian Caledonides (see Roberts 2003) that reduced loading on the foreland, and the foreland basin was uplifted. As the result the forebulge ceased and the sediment supply into the BB was opened from the uplifted foreland basin to the northwest and west (Plink-Björklund and Björklund 1999; Plink- Björklund et al. 2004).
Tänavsuu-Milkevciene et al. (2009 – Paper II) confirms the above interpretation and suggests that the forebulge may have migrated to the north- western margin of the BB during the earliest Eifelian, as indicated by the lack of sand-rich deposits in the northern part of the BB during the early transgression (Fig. 3). They suggested that the forebulge migration ceased and the forebulge started subsiding during transgression. In agreement with the latter, the siliciclastic material input into the northern and western part of the basin increases in the end of transgression (Fig. 3). The progradation of the deltaic complex is interpreted to be associated with the orogenic collapse and uplift in the Scandinavian Caledonides that caused the erosion of the foreland basin fill and the coarse sediment transport into the Baltic Basin (Tänavsuu-Milkevciene et al. 2009 – Paper II).
relatively high content of weakly resistant minerals, like feldspars and micas (quartz content roughly 75 to 85%; Kuršs 1992; Kleesment 1997). In
6. CONCLUSIONS
The detailed and systematic study of cores and outcrops from the Middle Devonian Narva and Aruküla successions were carried out to reconstruct the evolution of the shallow, epeiric, tide-influenced Middle Devonian transgressive to regressive BB. This is the first study where the sequence stratigraphy was used to reconstruct processes and the palaeoegeography of the Middle Devonian BB.
Seventeen Facies Associations were separated and divided into three groups (1) carbonate-rich facies associations, (2) mixed facies associations, and (3) siliciclastic-rich facies associations. The vertical and lateral facies transitions show carbonate dominated transgressive basin fill that was followed by the regressive siliciclastic dominated deposition. The transgressive part of the basin infill consists of carbonate and siliciclastic sabkhas and intertidal to supratidal flats in the basin margin areas and subtidal muds in the deeper parts of the basin. In the southern and southwestern parts of the basin shoal, tidal inlet and flood-tidal delta deposits occurred. The mixing of carbonate and siliciclastic deposits was controlled by grain size, volume and location of siliciclastic input rather than relative sea-level changes as suggested in widely used reciprocal mixing models.
The progradational part of the basin fill consists of three gradationally based vertically stacked stratigraphical units. Each stratigraphical unit consists of progradational and aggradational successions that form two characteristics portions. The lower upward-coarsening portion consists of prodelta to distal tidal bar and proximal tidal bar deposits. The upper upward-fining portion consists of tidal flat, supratidal muds, occasional tidal gully and distributary channel, and paleosol deposits. The frequent occurrence of tidal signatures, such as mica and mud drapes, tidal bundles, heterolitic stratification, and bidirectional palaeocurrents, as well as dominance of subaqueous depositional environments indicates deposition in a tide-dominated delta.
The turnaround from the transgressive carbonate dominated environments to the regressive siliciclastic dominated environments occurred during the relative sea-level rise and was caused by the tectonic movements in the hinterland, rather than relative sea-level changes.
ACKNOWLEDGEMENTS
This work was financed by grant 2090/2002 from the Swedish Institute, by grant 2003–3391 from the Swedish Research Council, and Estonian Science Foundation grant 5372. Additional financing was received from IAS postgraduate grant scheme 2004, NordForsk, and Doctoral School of Ecology and Environmental Sciences.
I thank Anne Kleesment, Girts Stinkulis, Kristine Tovmasyan and Estonian Geological Survey, Institute of Geology at University of Tallinn Technical University, University of Latvia, Latvian State Geological Survey, Lithuanian Geological Survey, Institute of Geology and Geography of Lithuania and Museum of Geology of Lithuania who provided help concerning the regional geology and core data.
Acknowledgements to my supervisors Piret Plink-Björklund, Kalle Kirsimäe and Leho Ainsaar whos thoughtful comments have helped me a lot.
I would also like to thank my family, especially my spouse Martynas Milkevičius, who supported me on my way.
