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Sedimentary cyclicity and dolomitization of the Raikkula Formation in the Nurme drill core (Silurian, Estonia)/Raikkula kihistu settetsuklid ja dolomiidistumine Nurme puursudamikus (Silur, Eesti).

INTRODUCTION

Complex studies of massive pervasive secondary dolomitization of Silurian rocks (Teedumae et al. 1999, 2001, 2003) have shown that dolomitization associates with the regressive phases of the evolution of the Baltic Palaeobasin and is related to the restricted area of inner shelf facies. Dolomitization has changed the primary composition of the sediments but relict lithological textures suggest its diagenetic origin. Studies of spatial dolomitization of primarily normal-saline calcareous sediments in the same area (Vishnyakov 1956; Jurgenson 1970; Kiipli 1983; Bityukova et al. 1996, 1998; Shogenova 1999) in general support its diagenetic origin with differences in details. Various sources of magnesium have been supposed for the Silurian secondary dolomites: Devonian sediments (Jurgenson 1970), hypersaline lagoonal waters of the Devonian basin (Vishnyakov 1956), mixed fresh ground water and marine water of the Devonian basin (Kiipli 1983), and contemporaneous Silurian seawater (Teedumae et al. 1999, 2001, 2003).

An extensive body of dolomites cross-cuts depositional sequences of the Raikkula and Nurmekund formations of the Raikkula Regional Stage (middle Llandovery). The pervasively dolomitized rocks of the Raikkula Stage are commonly underlain by the limestones of the Juuru Regional Stage. To the south the rocks are overlapped by the limestones or secondary dolostones of the Rumba Formation of the Adavere Regional Stage, or by the Devonian clastic sediments (Figs. 1, 2). In several drill sections (e.g. Nurme, Valgu) the totally dolomitized carbonate rocks of the Raikkula Stage are sandwiched between the undolomitized limestones of the Juuru and Adavere stages. This suggests the Raikkula Age for the diagenetic dolomitization, as was also supposed in previous studies on separate outcrop sections (Teedumae et al. 2001, 2003).

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

In the Nurme drill core the whole section of the Raikkula Formation is dolomitized and lies between the unaltered limestones of the Juuru and Adavere stages. The main objective of the research was complex study of the variability of the composition of dolostones, stoichiometry of dolomite and the nexus of these properties with the facies and sedimentary cyclicity in order to understand the evolution and timing of the process of dolomitization.

A. Teedumae studied the aspects of dolomitization, H. Nestor prepared the description of the studied section, made stratigraphical research and cyclicity analyses, T. Kallaste performed the X-ray diffractometry and X-ray fluorescence measurements. Titration and gravimetric analyses were made at the Central Laboratory of the Geological Survey of Estonia.

MATERIAL AND METHODS

Forty samples (Table 1) were collected from the Nurme drill core for complex study of the chemical composition and X-ray diffractometry. Of these, 33 samples are from the middle Llandovery Raikkula Formation, 3 samples from the underlying Tamsalu Formation, and 4 samples from the overlying Rumba Formation (Fig. 3). Samples were collected by lithological varieties, considering the changes within a variety.

CaO and MgO were analysed by titration, insoluble residue (henceforward i.r.) was determined by gravimetric analyses. [Fe.sub.2][O.sub.3] (total), Mn, and Sr were analysed by the X-ray fluorescence method with the VRA-30 analyser using an X-ray tube with Mo anode at 50 kV and 20 mA. Calibration of Mn and Fe was based on internationally intercalibrated dolomite reference materials Es-4 and Es-11 without matrix corrections. In calibration of Sr some additional silicate and limestone reference materials were used and, accordingly, matrix corrections (Compton scattering method) were applied. The precision of analyses was determined from 10 replicate measurements: [Fe.sub.2][O.sub.3] [+ or -] 0.005%, MnO [+ or -] 0.005%, Sr [+ or -] 2 ppm.

XRD measurements were carried out on a diffractometer HZG4, using Fe-filtered Co radiation. The rock powder was mixed in a mortar with Si in the ratio of 8 : 2, some drops of ethanol were added, and the mixture was evenly spread on a glass slide. The measured angular range 32-38[degrees]2[theta] reveals the 104 reflection of dolomite ([d.sub.104]) and calcite and 111 reflection of Si. The positions of reflections were calculated as weighted average. The instrumental shift was corrected according to the Si reflection (3.1355). The precision of the measurement of [d.sub.104] is [+ or -] 0.0005 [Angstrom].

The molar concentration of CaC[O.sub.3] ([m.sub.Ca]) in dolomite (Table 1) was calculated by measuring the displacement of the [d.sub.104] peak relative to a standard (e.g. Lippmann 1973). The formula (1)

[m.sub.Ca] = ([d.sub.104] - 2.8840)/0.003 + 50 (1)

expresses the linear dependence of [d.sub.104] reflection with respect to the fix-point of ideal stoichiometric dolomite, the value of which (2.8840 [Angstrom]) was calculated (Teedumae et al. 1999) on the ground of the composition of two standards, Es-4 (Estonia) and SI-1 (former USSR). No siderite or rhodochrosite was revealed. Some samples showed slight traces of calcite (Table 1).

GEOLOGICAL SETTING

During the Silurian, Estonia was situated at the northern flank of the pericratonic Palaeobaltic basin characterized by the carbonate to fine-clastic type of sedimentation (Nestor & Einasto 1977, 1997). According to the latest stratigraphical charts (Nestor 1995, 1997) the Raikkula Regional Stage (middle Llandovery) is represented in Estonia by the Hilliste (partially), Raikkula, Nurmekund, and Saarde formations, laterally replacing one another in the southern direction (Figs. 1, 2). The most characteristic features of this stratigraphical interval are: (1) preservation of very shallow-water, nearshore deposits at the northern margin of the basin, which suggests a regressive phase of the basin development (Nestor & Einasto 1997); (2) low content of the clay material; (3) cyclic alternation of comparatively pure, almost barren micritic (micro- to cryptocrystalline) limestones or secondary dolostones with bioclastic limestones or marlstones, or their dolomitized analogues; (4) extensive pervasive dolomitization of rocks in central and eastern Estonia.

[FIGURE 3 OMITTED]

The barren micritic limestones or secondary dolostones cyclically alternate with marlstones in the Saarde Formation and with bioclastic limestones (packstones-wackestones) or their dolomitized analogues in the Nurmekund Formation (Nestor et al. 2003). Besides sea-level fluctuation, the sedimentary cyclicity of the meso-cycle level was probably induced also by alternation of arid and humid climate states (Nestor et al. 2003). Cyclicity in the Raikkula Formation is expressed by the alternation of micritic and bioclastic limestones (or their dolomitized analogues), and sedimentary dolostones, more thoroughly characterized below. The Hilliste Formation, which spreads only in northwestern Estonia, is dominated by pelmatozoan grainstones with bioherms. It corresponds to the uppermost Juuru and lower half of the Raikkula stages (Nestor 1995, 1997).

The stratigraphical subdivision of the formations in the Raikkula Stage is based on the cyclic alternation of rock types. The Nurmekund Formation is subdivided into more heterogeneous informal stratigraphical units: the Jarva-Jaani, Vandra, Jogeva, Imavere, and Mohkula beds (Fig. 2), each corresponding to a submesocycle. The Saarde Formation consists of more homogeneous lithostratigraphical units: the Heinaste, Slitere, Kolka, Ikla, Lemme, and Staicele members representing parts of the mesocycles (Nestor et al. 2003). The Raikkula Formation is divided only into the Lower and Upper Raikkula subformations (Nestor 1995, 1997), which correspond to the shallowing upward sedimentary mesocycles. The Lower Raikkula Subformation belongs to the Rhuddanian, the Upper Raikkula Subformation to the Aeronian (Nestor 1995).

The end of the Raikkula Age marked the termination of the differentiation stage in the development of the Baltic Palaeobasin, and also the end of the Lower-Middle Llandovery macrocycle of sedimentation accompanied by extensive regression (Nestor & Einasto 1997). It culminated with an extensive sedimentation break and denudation along the basin margins, in particular in western Estonia, where the gap increases in the northwestern direction so that the entire Upper Raikkula Subformation, corresponding to the Aeronian, is missing in drill cores of northern Saaremaa. The gap embraces also the distribution area of the Hilliste Formation on Hiiumaa Island and adjacent mainland. A pronounced unconformity is developed also on the opposite, southeastern flank of the Palaeobaltic Basin in eastern Lithuania, where the early and middle Llandovery deposits were subjected to denudation all over the carbonate shelf area (Nestor & Einasto 1997). The break had probably a glacio-eustatic origin as it temporally coincided with the Panuara unconformity in New South Wales (Jell & Talent 1989) and elsewhere, as well as with a glaciation in South America (Caputo 1998; Nestor & Nestor 2002a, 2002b). However, the variable span of the deposition break in different areas suggests that partly the break and denudation might have been influenced also by tectonic upheaval, induced by the beginning of the collision of the Laurentia and Baltica continents (Nestor & Einasto 1997).

CYCLICITY OF THE SEQUENCE

Cyclicity in the Nurmekund and Saarde formations of the Raikkula Stage was recently discussed by Nestor et al. (2003). The studied section of the Nurme drill core shows a medium-rank cyclicity, which is rather characteristic of the Raikkula Formation. In this core the rocks of the Raikkula Formation are entirely dolomitized. The formation is under- and overlain by undolomitized limestones of the Tamsalu and Rumba formations, respectively (Fig. 3). Secondary dolomitization has changed the composition of rocks, but the primary lithological characteristics (skeletal remains, etc.) are more or less recognizable and enable identification of the primary origin of the carbonate rocks. This allows application of the textural classification of carbonate rocks (Dunham 1962), by adding the prefix "dolo-". However, due to dolomitization, the microfossils are very scarce (see V. Nestor 1994, fig. 12/1) and exact determination of the stage boundaries is difficult.

Three main rock types (lithofacies) alternate cyclically in the studied sequence. The lower part of a complete cycle is represented by rough- or wavy-bedded, fine-crystalline, micritic dolostone with a low, vertically variable content of relict skeletal detritus (dolomudstone to dolopackstone), and thin marly intercalations or partings at bedding planes. A more argillaceous layer (up to 1 m thick) of clayish dolostone commonly occurs at the base of such cycles. The type of bedding and content of skeletal and clay material are variable, causing obscure lower-rank cyclicity, but in general the role of the relict skeletal detritus increases upward in the sequence. The rocks of this part of the cycle were formed in the low-energy open shelf environment (facies zone III according to Nestor & Einasto 1977).

The middle part of the cycle is represented by horizontal-bedded, fine- to coarse-crystalline, porous dolograinstone with larger vugs and solution cavities after corals and stromatoporoids in coarse-crystalline intervals and by micro-lamination in fine-crystalline intervals. The uneven bedding planes are covered with discontinuous, wavy or stylolitic, greenish marly partings. Deposits of the middle part of the cycle were formed in the high-energy shoal environment (facies zone II by Nestor & Einasto 1977) and originally consisted mainly of pelmatozoan skeletal particles.

The upper part of a complete cycle is represented by thin-bedded to laminated, fine-crystalline dolostone with wavy to stylolitic greenish marly partings. The laminated dolostone, in places containing mud cracks, may be replaced by thicker-bedded bioturbated dolomudstone ("pattern dolomite"). This lithofacies was probably formed shorewards of the shoal facies zone, where calcareous silt (or pellets) and mud were alternately deposited in low-energy intertidal conditions (facies zone I by Nestor & Einasto 1977).

In the Nurme section, the Raikkula Formation consists of two complete shallowing upward cycles (I and III in Fig. 3), which contain all three cycle parts described above. They alternate with incomplete cycles (II and IV in Fig. 3) which are lacking some cycle elements.

The first cycle comprises the interval from 63.3 to 83.8 m. Its lower part (70.1-83.8 m) is represented by clayish and micritic dolostones (mainly dolowackestone). The middle part (67.0-70.1 m) consists of pelmatozoan dolograinstone, fine-grained in the lower and coarse-grained in the upper half. The upper part (63.3-67.0 m) of the cycle is formed by laminated and bioturbated dolostones; probable mud cracks occur at a depth of 66.3 m. The first cycle obviously correlates with the Jarva-Jaani Beds of the Nurmekund Formation (Fig. 4). The difference is that the cycle includes dolograinstone in the middle part and laminated dolostone in the upper part, but the Jarva-Jaani Beds consist of micritic limestone or dolomudstone with only rare interlayers of grainstone tempestites in the uppermost part.

The second cycle, occurring in the interval of 54.2-63.3 m, is incomplete in the studied section. It is represented only by relict pelmatozoan dolograinstone and corresponds to the middle part of a complete cycle. Distinction of the interval as a separate cycle is rather arbitrary and is based on the change of the shallowing upward trend of deposition at both its boundaries. The second cycle is correlatable with the Vandra Beds of the Nurmekund Formation, which consist of nodular wacke- and packstones (or their dolomitized analogues), formed in the low-energy environment of the open shelf facies zone (Nestor & Einasto 1977), whereas the rocks of the present cycle were formed in the high-energy shoal environment. On the other hand, the unit may be treated as a tongue of the Hilliste Formation, consisting of pelmatozoan and reef limestones and spreading in northwestern Estonia (Aaloe & Nestor 1977; Nestor 1997).

The third cycle, in the interval of 41.0-54.2 m, is quite analogous to the first cycle, comprising wavy-bedded micritic dolostones in the lower (47.0-54.2 m), dolograinstones in the middle (43.4-47.0 m), and laminated dolostones with mud cracks in the upper part (41.0-43.4 m). This cycle is correlatable either with the Jogeva Beds or with the Jogeva and Imavere beds of the Nurmekund Formation, which both consist of wavy-bedded to nodular micritic dolostones (Nestor et al. 2003). In the latter case, the interval with numerous discontinuity surfaces at 49.0-51.0 m refers to a shallowing event, which may correspond to the regressive, upper part of the Jogeva Beds.

[FIGURE 4 OMITTED]

The fourth cycle, in the interval of 37.2-41.0 m, is represented only by dolomudstones characteristic of the lower part of a cycle. The interval contains pentamerids, which suggest correlation of this cycle with the Mohkula Beds of the Nurmekund Formation.

The described four medium-scale cycles, based on the changes in the shallowing upward trend of development, are treated as units of the submesocycle rank (Nestor et al. 2001, 2003). They may be grouped into pairs and in such case they form two cycles of the mesocycle rank (Nestor & Einasto 1997), widely used in the East Baltic stratigraphical practice and commonly termed as "Beds" (e.g. Aaloe et al. 1976). In the present case the two mesocycles are treated as subformations of the Raikkula Formation (Nestor 1997): the first and second submesocycles forming the Lower Raikkula Subformation and the third and fourth submesocycles forming the Upper Raikkula Subformation. The mesocycles reveal almost identical recurrence of the main cycle elements. They begin with a more argillaceous basal layer, followed by micritic, pelmatozoan, and laminated dolostones in submesocycles I and III. Only the uppermost parts of both mesocycles, corresponding to submesocycles II and IV, are different, in the first case being represented by pelmatozoan, in the second case--by micritic dolostone.

DISTRIBUTION OF DOLOSTONES

In central and eastern Estonia the rocks of the Nurmekund and Raikkula formations, in particular of their upper parts, are commonly dolomitized (Fig. 1), often also silicified. Dolomitization cross-cuts depositional sequences and is related to the inner shelf facies. The rocks of the Saarde Formation, formed in the deeper-water environment, are mostly undolomitized, except for the topmost metres below the contact with the overlying Devonian sediments. Judging by the spatial distribution and the degree of the alteration of the primary textures and by chemical analyses, the extension and intensity of dolomitization increase upward in the sequence. This coincides with the general regressive trend of the basin development during Raikkula time. The micritic limestones and wackestones in the lower parts of the Nurmekund and Raikkula formations are usually unaltered or only slightly dolomitized, grading upwards into totally dolomitized rocks. The lower limit of the totally dolomitized rocks is rather changeable in space and ranges stratigraphically from the base of the Tammiku Member of the Tamsalu Formation (e.g. Konnu core) to the uppermost part of the Ikla Member of the Saarde Formation (e.g. Abja core). Commonly the totally dolomitized sequence begins approximately near the base of the Jogeva Beds of the Nurmekund Formation, or the Upper Raikkula Subformation, and reaches about 30-50 m in thickness. It means that extensive pervasive dolomitization embraced mainly the upper half of the Raikkula Stage, which is not completely preserved in the denudated sections of western Estonia and at the margins of the present-day distribution area of the stage. Therefore it is difficult to estimate the primary lateral extent of the dolomitized rocks. It is likely that during Raikkula time the zone of dolomitization gradually migrated southwestwards in accordance with progressive shallowing and progradation of the carbonate shelf, especially in the second half of the time. The thickness of the dolomitized rocks may increase in the zones of tectonic disturbances and dolostones may occur there on lower stratigraphical levels. The very complicated picture of the temporal and spatial distribution of dolostones shows that probably there existed different types and sources of dolomitization.

RESULTS OF ANALYSES AND ASPECTS OF DOLOMITIZATION

The [d.sub.104] value of secondary dolomite throughout the section of the Raikkula Formation varies from 2.8857 to 2.8973 [Angstrom] (Table 1; Fig. 5a). The variability of the [d.sub.104] spacing has distinct regularities. The highest [d.sub.104] values (> 2.89 [Angstrom]) of dolomite occur near (within ca 2 m) the contacts with the over- and underlying marl and limestone. Through the rest part of the studied sequence [d.sub.104] is quite constant, varying between 2.8859 and 2.8888 [Angstrom]. The increase in the [d.sub.104] spacing of dolomite contacting with limestone is typical of secondary (Vingisaar & Utsal 1978; Kallaste & Kiipli 1995) as well as primary dolomite (Teedumae et al. 2003). At both boundaries of the Raikkula Formation the content of insoluble residue increases abruptly (Fig. 5b; Table 1). The following transgression of the Adavere Age ended the process of dolomitization and also inhibited the crystallographic ordering of contacting dolomite within about 2 m (Table 1). The contact of the Raikkula dolostone with the underlying limestone of the Tamsalu Formation (Figs. 2, 3) is less distinct and has a transitional character, showing the inter-bedding of highly clayey calcitic dolostone, dolomitic limestone, and marl. The [d.sub.104] value of dolomite coexisting with calcite in these interlayers is high and equals to that of dolostone contacting with limestone (Table 1; Fig. 5a). This regularity has been widely observed since Lippmann (1973).

The XRD results show the excess of Ca. Provided that additional Ca replaces Mg, the growth of the [d.sub.104] value calculated by the formula (2)

[DELTA][d.sub.104] = [m.sub.Ca] x (3.035 - 2.742) = [m.sub.Ca] x 0.293 [Angstrom] (2)

is 0.0023-0.0047 [Angstrom].

As the calculated and measured [d.sub.104] values are, in general, in good accordance, it is most likely that additional Ca is bound in dolomite structure and expands the lattice parameters. The calculated [d.sub.104] spacing for 50 mol% CaC[O.sub.3] is 2.884 [Angstrom], for 52 mol% - 2.890 [Angstrom], for 54 mol% - 2.896 [Angstrom].

The presence of Fe and Mn in the dolomite lattice may also affect the value of [d.sub.104]. The possible concentration of Fe in the dolomite lattice (Fig. 6, trendline) is 0.42% [Fe.sub.2][O.sub.3] (0.45 mol% FeC[O.sub.3]), which corresponds to the variation of the [d.sub.104] value of 0.0002 [Angstrom], calculated by the formula (3),

[DELTA][d.sub.104] = [m.sub.Fe] x (2.79 - 2.742) = [m.sub.Fe] x 0.048 [Angstrom] (3)

and is below the precision of the X-ray diffractometry.

The impact of Mn on replacement of Mg and Ca is calculated by the formulas (4) and (5), respectively, where Mn m is the molar concentration of MnC[O.sub.3] in dolomite:

[DELTA][d.sub.104] = [m.sub.Mn] x (2.85 - 2.742) = [m.sub.Mn] x 0.108 [Angstrom], (4)

[DELTA][d.sub.104] = [m.sub.Mn] x (2.85 - 3.035) = -[m.sub.Mn] x 0.185 [Angstrom]. (5)

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

The concentration of Mn in dolostone is low. The maximum content of Mn (600 ppm; Table 1) in dolostone corresponds to 0.10 mol% MnC[O.sub.3]. The calculated variations of [d.sub.104] of mineral dolomite, if Mn replaces Mg as well as Ca, are below the precision of the measurement, being respectively 0.0001 [Angstrom] (formula (4)) and 0.0002 [Angstrom] (formula (5)).

As follows from above, the changes in lattice parameters of the studied dolomite are mainly induced by the Ca/Mg ratio in the dolomite lattice. Two general groups of dolomite, based on stoichiometry, can be distinguished (Fig. 7), showing bimodal distribution of [Ca.sup.2+]. One group clusters between 2.885 and 2.888 [Angstrom] (50-52 mol% CaC[O.sub.3]) and the other between 2.939 and 2.973 [Angstrom] (53.3-54.4 mol% CaC[O.sub.3]). The first group includes secondary dolomite in dolostone not contacting with limestone. The second group comprises the dolomite in dolostone contacting with limestone and dolomite coexisting with calcite in limestone and marl. The same regularities have been established in a previous study of dolomites of different genesis (Teedumae et al. 2003). The Ca/Mg ratio of dolomite reflects environmental factors of dolomitization and its bimodal set would generally be interpreted as the reflection of environmental changes during diagenesis.

The stoichiometry can be used for the distinction of different types of dolomite (Goldsmith & Graf 1958; Searl 1994; Kallaste & Kiipli 1995) and it correlates well with the genesis (Teedumae et al. 2003). The results of previous (Teedumae at al. 1999, 2001, 2003) and present studies have shown that the secondary dolomite, formed in the course of Silurian pervasive dolomitization, is the most completely ordered dolomite so far known in the Estonian sequence. This may point, besides the environmental characteristics and total recrystallization, to the role of the crystallization rate, as very slowly growing crystals are closer to the stoichiometric composition (Morrow 1982).

[FIGURE 7 OMITTED]

The stability of the lattice of Ca-rich dolomite depends on the character of the ordering of Ca ions within Mg layers. Bimodal ranges of Ca/Mg variation reflect the preferred levels of Ca uptake of the most stable ordering.

The content of insoluble residue of dolostone has a positive correlation with the growth of [d.sub.104] spacing (Fig. 5a,b) for the mineral dolomite with [d.sub.104] < 2.890 [Angstrom] (< 52 mol% CaC[O.sub.3]). For dolomite with the expanded lattice (53 mol% CaC[O.sub.3]) no correlation could be revealed (Fig. 7).

Stabilization of calcium-rich dolomite to a more ideal type will mostly take place by dissolution and reprecipitation, since solid-state diffusional processes operate only at the submicron scale and are slow (Tucker & Wright 1994). The lower content of insoluble residue, characteristic of the agitated water sediments, suggests the promoting role of the activity of seawater in the dolomitization process. This supposition is supported by the fact that the most stoichiometric dolomite (< 51 mol% CaC[O.sub.3]) in general belongs to the intervals of primarily bioclastic sediments (Table 1; Figs. 3, 5a), which are more permeable to dolomitizing fluids.

The content of Fe compounds ([Fe.sub.2][O.sub.3] total) shows a positive correlation with the content of insoluble residue for all types of the studied rocks (Fig. 6). This regularity, observed also in all previous studies (Teedumae et al. 1999, 2001, 2003), indicates that iron compounds are mainly of primary, sedimentary origin. Their distribution is controlled by the facies pattern (Jurgenson 1988).

The concentration of Mn in dolostone is low (366-601 ppm; Table 1). In general, there is a covariant trend of increasing concentration with increasing stoichiometry (Fig. 5a,c) of mineral dolomite. Mn is highly soluble in an anoxic environment and, if available, should be readily incorporated into the dolomite lattice, but in the present case its impact on the dolomite structure was below the detection limit. There is a negative correlation between the contents of Mn and insoluble residue of dolostone (Figs. 5c, 8). Lower Mn concentrations would suggest a lower primary supply of Mn and fluctuation in Eh. For other types of rocks no correlation is observed.

Low concentrations of Fe and Mn, characteristic of seawater, support the idea of the absence of external dolomitizing fluids and early diagenetic dolomitization. Early, near-surface dolostones, tend to have low Fe and Mn contents, since most near-surface fluids are oxidizing, in contrast with late, deep-burial dolomites, which might have high Fe and Mn concentrations through precipitation from negative Eh porefluids in which Mn and Fe are in solution (Tucker & Wright 1994).

The content of Sr in dolostone is low, ranging from 29 to 74 ppm (Table 1). The Sr concentration covaries positively with the [d.sub.104] spacing of mineral dolomite (Fig. 5a,d). The highest concentrations associate with the intervals near the contact of dolostone with limestone (Table 1), where the poorly ordered dolomite occurs. The concentrations (30-40 ppm) are the lowest in the intervals of the coarsercrystal dolostones, which means that Sr is lost in the recrystallization process of dolomite. The mineralogy of precursor carbonate may also have an important role. The Sr contents are high when aragonite is being dolomitized, whereas calcite with its much lower Sr content will be replaced by very Sr-depleted dolomite (Tucker & Wright 1994). It suggests that carbonate sediments might have undergone early diagenetic stabilization to low magnesian calcite before dolomitization.

The components discussed above and their interrelations refer to early diagenetic dolomitization (Einsele 2000) in a normal-saline environment, where the only source for Mg ions was seawater. All components discussed above and their interrelations refer to the early diagenetic near-surface dolomitization of the Raikkula Formation. The only possible source for Mg ions for the dolomitization of these primarily normal-saline sediments could have been seawater. Bacterial sulphate reduction, which has been supposed to have a potential role in the dolomitization process (Garrison et al. 1984; Baker & Burns 1985; Slaughter & Hill 1991; Tucker & Wright 1994; Wright 1999; Einsele 2000; experimentally demonstrated by Warthman et al. 2000), can be treated as the main dolomitizing factor also for the studied dolomites.

[FIGURE 8 OMITTED]

Massive pervasive dolomitization, cross-cutting the Silurian depositional sequences, was associated with the regressive phases of the evolution of the Baltic Palaeobasin and related to the restricted migrating zone of the normal-saline shallow-water inner shelf facies (Teedumae et al. 1999, 2001, 2003). The Raikkula Stage represents such a regressive part of the Early-Middle Llandovery macrocycle (Nestor & Einasto 1997) that ended with extensive local sedimentation brakes and denudation.

The Nurme drill section, represented by the sediments of the shallow, inner shelf facies, was located in the zone of diagenetic (pervasive) dolomitization during the whole Raikkula Age. In the course of general regression and shallowing in Raikkula time, this zone expanded and migrated southwards, where dolomitization commenced somewhat later, approximately in the middle of Raikkula time. The occurrence of totally dolomitized carbonates of the Raikkula Formation between the undolomitized carbonates of the underlying Tamsalu and overlying Rumba formations (Fig. 3) limits the time span of diagenetic dolomitization in the Nurme section within the Raikkula Age.

CONCLUSIONS

Four shallowing up submesocycles were distinguished and first described in the Raikkula Formation of the Nurme drill core. They in turn group into two mesocycles, treated as the Lower and Upper Raikkula subformations. The intensity of dolomitization increases upwards in the cycles in accordance with the shallowing up trend of development. Such a trend is more conspicuous in the lower submesocycles (I and III) of both Raikkula subformations, where stoichiometry of dolomite increases upwards in the sequence. The trend is less distinct in the incomplete submesocycles (II and IV) that were formed in more uniform facies and bathymetrical conditions. In the latter case (submesocycle IV) the dolomitization process at the top of the Raikkula Stage was obviously inhibited by the succeeding, transgressive calcitic sedimentation environment of the Adavere Age.

Intense pervasive dolomitization associated with the regressive phases of the development of the Baltic Palaeobasin, including the middle Llandovery Raikkula Age. Initially dolomitization could have been related to the restricted area of the inner shelf facies, which expanded in accordance with the progressive shallowing of the basin towards the end of the Raikkula Age.

The studied Nurme drill section was located in the zone of pervasive dolomitization during the whole Raikkula Age. The occurrence of totally dolomitized carbonates of the Raikkula Formation between the undolomitized carbonates of the underlying Tamsalu and overlying Rumba formations limits the time span for early diagenetic dolomitization within the Raikkula Age.

The whole set of the studied components is consistent with the normal-marine water environment of that time. There are no signs of possible inflow of external dolomitizing fluids.

Stoichiometry of dolomite correlates with the cyclicity of the sedimentation environment, increasing in accordance with the regressive, shallowing up trend of development. The most completely ordered dolomite is associated with the primarily bioclastic sediments in the upper part of regressive sedimentary cycles, formed in a high-energy environment. It shows the promoting role of the activity of marine water, and primary porosity of sediments in the process of dolomitization.

ACKNOWLEDGEMENTS

The present study was financed by the governmental target funding project No. 0332088s02 and supported by the Estonian Science Foundation (grant No. 5088). Special thanks go to Tarmo Kiipli for assistance in sampling and Kaie Ronk for drawing figures. We are sincerely grateful to the referees Dimitri Kaljo and Kalle Kirsimae for valuable comments and recommendations.

Received 22 May 2003, in revised form 4 November 2003

REFERENCES

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Aada Teedumae, Heldur Nestor, and Toivo Kallaste

Institute of Geology at Tallinn Technical University, Estonia pst. 7, 10143 Tallinn, Estonia; teedumae@gi.ee
Table 1. Major, minor, and trace element composition of rocks
and [d.sub.104] values of dolomite

Sample Depth, Titration Gravimetric
number m analyses analyses

 Ca[O.sub.2] MgO, Insoluble
 % % residue, %

 1 33.60 39.75 3.01 20.86
 2 34.80 32.01 3.44 33.10
 3 35.10 42.25 3.09 15.60
 4 36.90 36.49 3.27 25.46
 5 37.30 30.37 16.93 8.62
 6 39.00 27.54 15.99 13.50
 7 40.70 29.19 19.60 4.26
 8 41.10 28.95 18.22 7.98
 9 42.00 29.31 19.00 5.68
 10 43.00 29.54 20.37 4.28
 11 45.70 28.72 20.28 3.64
 12 48.28 36.25 14.70 3.84
 13 48.40 29.01 20.34 4.32
 14 49.40 29.78 19.85 3.56
 15 50.40 29.66 20.20 3.42
 16 52.30 25.66 17.02 15.78
 17 54.80 29.43 20.71 2.42
 18 57.50 29.54 19.85 4.52
 19 58.50 29.31 20.97 2.06
 20 59.70 29.90 20.71 1.84
 21 60.60 28.37 19.94 6.80
 22 61.50 29.66 20.63 2.08
 23 63.20 29.90 21.14 1.16
 24 64.80 28.95 20.37 4.58
 25 66.40 29.07 19.17 5.76
 25A 68.80 30.48 20.63 1.78
 26 70.90 28.37 19.51 6.20
 27 73.00 28.95 20.43 4.26
 28 74.90 27.66 15.99 14.52
 29 76.30 29.07 17.96 9.50
 31 77.90 28.38 20.46 3.58
 30 78.50 27.50 19.53 7.74
 32 78.90 27.06 19.61 8.38
 33 82.20 27.94 19.53 4.92
 34 83.70 29.15 17.05 8.28
 35 84.60 33.77 10.70 14.46
 36 85.40 23.98 8.14 34.82
 37 85.80 26.62 9.22 28.02
 38 86.35 36.52 6.05 16.62
 39 88.50 39.16 5.66 14.24
 40 90.50 40.26 5.50 12.32

Sample X-ray fluorescence analyses
number
 [Fe.sub.2] Mn, Br, Sr,
 [O.sub.3], % ppm ppm ppm

 1 1.39 172 4 184
 2 1.67 150 3 169
 3 1.04 142 4 243
 4 1.45 165 3 273
 5 1.09 505 5 67
 6 5.46 482 3 52
 7 0.69 466 15 37
 8 0.79 445 18 41
 9 0.66 389 23 37
 10 0.56 417 27 38
 11 0.67 453 15 31
 12 0.61 433 11 46
 13 0.65 462 16 32
 14 0.56 508 15 31
 15 0.65 568 12 36
 16 1.21 476 20 51
 17 0.66 601 16 29
 18 0.60 452 22 35
 19 0.43 517 34 37
 20 0.45 522 26 41
 21 0.61 580 25 31
 22 0.46 527 33 44
 23 0.69 507 19 31
 24 0.66 445 20 39
 25 0.59 460 18 36
 25A 0.41 516 23 40
 26 0.77 445 19 39
 27 0.49 509 15 32
 28 1.47 444 20 47
 29 1.06 420 20 42
 31 0.48 387 13 30
 30 0.89 383 27 39
 32 0.95 470 28 54
 33 0.85 366 32 44
 34 1.05 434 5 57
 35 1.08 270 6 77
 36 2.69 248 2 74
 37 2.26 237 3 73
 38 1.52 194 5 109
 39 1.30 157 6 158
 40 0.98 153 6 208

Sample X-ray diffraction analyses
number
 [d.sub.104], CaC[O.sub.3], Calcite
 [Angstrom] mol%

 1 ++
 2 ++
 3 ++
 4 ++
 5 2.8973 54.4 (+)
 6 2.8954 53.8 +
 7 2.8875 51.2 (-)
 8 2.8888 51.6 (-)
 9 2.8872 51.1 +
 10 2.8864 50.8 (-)
 11 2.8863 50.8 (-)
 12 2.8866 50.9 ++
 13 2.8859 50.6 (-)
 14 2.8860 50.7 (+)
 15 2.8860 50.7 (-)
 16 2.8873 51.1 (-)
 17 2.8861 50.7 (-)
 18 2.8863 50.8 (-)
 19 2.8862 50.7 (-)
 20 2.8871 51.0 (-)
 21 2.8863 50.8 +
 22 2.8866 50.9 (-)
 23 2.8857 50.6 (-)
 24 2.8863 50.8 +
 25 2.8866 50.9 +
 25A 2.8875 51.2 (-)
 26 2.8870 51.0 (-)
 27 2.8858 50.6 (+)
 28 2.8881 51.4 (+)
 29 2.8874 51.1 +
 31 2.8866 50.9 (-)
 30 2.8870 51.0 (-)
 32 2.8883 51.4 (-)
 33 2.8882 51.4 (-)
 34 2.8958 53.9 +
 35 2.8959 54.0 ++
 36 2.8962 54.1 ++
 37 2.8953 53.8 ++
 38 2.8961 54.0 ++
 39 2.8955 53.8 ++
 40 2.8939 53.3 ++

Sample Rock type
number (Vingisaar et al. 1965)

 1 Clayey limestone
 2 Marlstone
 3 Clayey limestone
 4 Marlstone
 5 Dolostone
 6 Clayey dolostone
 7 Dolostone
 8 Dolostone
 9 Dolostone
 10 Dolostone
 11 Dolostone
 12 Calcitic dolostone
 13 Dolostone
 14 Dolostone
 15 Dolostone
 16 Clayey dolostone
 17 Dolostone
 18 Dolostone
 19 Dolostone
 20 Dolostone
 21 Dolostone
 22 Dolostone
 23 Dolostone
 24 Dolostone
 25 Dolostone
 25A Dolostone
 26 Dolostone
 27 Dolostone
 28 Clayey dolostone
 29 Dolostone
 31 Dolostone
 30 Dolostone
 32 Dolostone
 33 Dolostone
 34 Dolostone
 35 Clayey calcitic dolostone
 36 Dolomitic marlstone
 37 Dolomitic marlstone
 38 Clayey dolomitic limestone
 39 Clayey dolomitic limestone
 40 Clayey dolomitic limestone

(-) below the detection limit; (+) < 0.5%; + order of
values 1%; ++ high in calcite.
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Author:Teedumae, Aada; Nestor, Heldur; Kallaste, Toivo
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Date:Mar 1, 2004
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