RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 10, ES5001, doi:10.2205/2008ES000302, 2008

Siliceous Tuffaceous Type of Section

2008ES000302-fig05
Figure 5
[60]  The Kuragan Formation characterized by the siliceous tuffaceous type of section is widespread on the flanks of the Utyagulovo and Blyava synforms (Figure 5). The formation was distinguished in the 1930s [Lermontova and Razumovskii, 1933]. Its section was described in the range of all Ordovician series [Ruzhentsev, 1976] and dated by means of conodonts [Artyushkova et al., 1991; Maslov et al., 1993; Ryazantsev et al., 2000, 2005], (K. S. Ivanov and V. N. Puchkov, preprint, 1984).

2008ES000302-fig06
Figure 6
[61]  The Kuragan Formation is represented in the lower part by red or grey-green tuffaceous siltstones decoupled by basalt flows and sills. The grey and, rarer, red siliceous tephroids (Novokursk sequence) dominate in the upper part of the section. The close spatial association of the Kuragan Formation with the Tremadocian terrigenous Kidryasovo series is recorded (Figures 5 and 6). In places the formation is conformably overlain by carbonaceous shales of the Silurian Sakmara Formation (Figures 5 and 6).

[62]  Nearby the Blyava station (Figure 5a), on the slopes of the Kolnabuk gully, almost vertical occurrence of rocks is observed. The following units are recorded in westward direction upward from the base (Figure 6):

[63]  1. Aphyric basalts decoupled by 0.5-1-m-thick lenses of red tuffaceous siltstones, at Point 19K2 bearing Tripodus sp. A Tolmach. and Drepanodus arcuatus Pander; and 30 m westwards at Point PK1A with Acodus delicatus Branson et Mehl. and D. arcuatus Pander, referred to the Bergstroemognathus extensus and Drepanoistodus forceps Beds corresponding to the upper Lower-"Middle'' Arenigian (Figures 2 and 3). Thickness 100 m.

[64]  2. Red tuffaceous siltstones and siliceous tuffites decoupled by 2-10-m-thick beds and lenses of aphyric basalts and at Point 19K1 yielding Bergstroemognathus extensus (Graves et Ellison), Acodus delicatus Branson et Mehl, Drepanodus arcuatus Pander, and Drepanoistodus forceps (Lind.); at Point K66 bearing A. delicatus Branson et Mehl, Drepanodus conulatus Lindström, Scandodus furnishi Lindström, and numerous molds of inarticulate brachiopods and ostracodes. They are assigned to the same Bergstroemognathus extensus and Drepanoistodus forceps Beds, i.e. to the upper Lower-"Middle'' Arenigian analogs (Figure 3). Thickness 150 m.

[65]  3. Red tuffaceous siltstones with rare lenses of grey-green tuffaceous siltstones. Thickness 300 m.

[66]  4. Back of the fault, red siltstones similar to that of Bed 3, bearing gabbroid sills. Thickness 150 m.

[67]  5. Massive alternated pink, violet, and pistachio-colored tuffites. Thickness 50 m.

[68]  In the stratotype area, 1.4 km northeast of the Kidryasovo Village (Figure 5), on the right bank of the Pis'menka River, the cherry-colored tuffaceous argillites of the Kuragan Formation yield Arenigian-Llanvirnian conodonts identified by S. V. Dubinina [Korinevskii, 1988].

[69]  The younger units of the section were studied in the Shaitantau Mountains northwest of the Novokurskii Village (Figure 5b). The Shaitantau Mountains are mainly composed of the Silurian Sakmara Formation making up the core of the synform, on which flanks the Ordovician Kuragan rocks are exposed. The Ordovician and Silurian sediments are tectonically overlain by the Ordovician Guberlya Formation and by the Devonian siliceous basaltic complex.

[70]  The basal part of the section on the western synform flank is represented by poorly exposed red-colored tuffaceous siltstones bearing grey lenses. The rock is enriched with inarticulate brachiopod and ostracode detritus. The red siltstones at Point 7 (Figure 6) contain Drepanodus arcuatus Pander, Scalpellodus cf. S. viruensis (Löfgren), and Ansella jemtlandica (Löfgren) and are referred to the Ansella jemtlandica Beds that correspond to the Lower Llanvirnian (Figure 3).

[71]  On the eastern flank of the synform (Figures 5b and 6) the red tuffaceous siltstone sequence is overlain by the 50- to 150-m thick member of red and grey-green siliceous tephroids bearing scarce lenses of variegated tuffaceous sandstones. The tephroids at Point 1 yield Hamarodus brevirameus (Walliser), Protopanderodus liripipus Kennedy et al., Panderodus gracilis (Branson et Mehl), Scabbardella altipes (Henningsmoen), and redeposited Periodon aculeatus Hadding and are referred to the Hamarodus brevirameus, Dapsilodus mutatus, and Scabbardella altipes Beds corresponding to the Upper Caradocian-Ashgillian (Figure 3).

[72]  Crowns of the eastern ridges in the Shaitantau Mountains are made up of the thrust sheet, in the lower part of which grey siliceous tephroids at Point 2 ast (Figures 5b and 6) contain redeposited conodont elements of Periodon aculeatus Hadding that serve as a marker of the Periodon aculeatus Beds of the Kuragan Formation, corresponding to the Upper Llanvirnian analogs (Figure 3). Further upwards the grey siliceous tephroids at Point 3 (Figures 5b and 6) yield Hamarodus brevirameus (Walliser), Scabbardella altipes (Henningsmoen), Dapsilodus mutatus (Branson et Mehl), Periodon grandis (Ethington), Protopanderodus liripipus Kennedy et al., Panderodus sp., Ansella sp., and Plectodina furcata (Hinde); at Point 4, Hamarodus brevirameus (Walliser), Protopanderodus liripipus Kennedy et al., Periodon grandis (Ethington), and Spinodus spinatus (Hadding) (see Appendix, Plate 5, fig. 18), and are assigned to the Hamarodus brevirameus, Dapsilodus mutatus, and Scabbardella altipes Beds corresponding to the Upper Caradocian-Ashgillian (Figure 3). The grey-colored member is 50- to 150-m-thick.

[73]  It is conformably overlain by carbonaceous, clayey siliceous and dolomitic shales of the Sakmara Formation, 250-300 m thick (Figures 5b and 6). At Point 5 they contain Llandoverian conodonts Ozarkodina aldridgei Uyeno et Barnes and Dapsilodus obliquicostatus (Branson et Mehl) and at Point 6, Llandoverian graptolites Lagarograptus acinaces (Tornquist).

[74]  According to composition, the red-colored tuffaceous siltstone sequence at the base of the Shaitantau section is correlated with the Kuragan sediments from the Blyava Station area (Figure 6). The overlying siliceous tephroids are recognized as the Novokursk [Khvorova et al., 1978] or Pis'menka [Puchkov, 2000; Puchkov et al., 1990] sequence in the upper part of the Kuragan Formation. In this paper we accept the first variant.

[75]  Both sequences of the Kuragan Formation, i.e. the lower (red-colored tuffaceous siltstone with basalt flows) and the upper (siliceous tephroid) Novokursk sequence, make up the northeastern flank of the Utyagulovo synform on the left bank of the Sakmara River, east of the Churaevo and Yumaguzino 1 Villages (Figure 5).

[76]  Nearby the Yumaguzino 1 Village, on the right bank of the Tereklya River (left tributary of the Sakmara River) (Figure 6), the red tuffaceous sandstones and siltstones are exposed at the ridge foot. The slope and top of the ridge are composed of mainly grey and grey-green siliceous tephroids. The siliceous rocks are decoupled by variegated tuffaceous sandstones and gravelstones. The beds steeply (60o-70o) dip westward (overturned occurrence).

[77]  The grey tephroids (enriched in sand-sized tephra) at Point K8a (Figure 6), 40 m above the top of the red sequence, yield Hamarodus brevirameus (Walliser), Scabbardella cf. altipes (Henningsmoen), Periodon grandis (Ethington), Protopanderodus cf. liripipus Kennedy et al., Amorphognathus cf. superbus (Rhodes), Prioniodus cf. gerdae Bergstrom, and Panderodus gracilis (Branson et Mehl), and are referred to the Hamarodus brevirameus, Dapsilodus mutatus, and Scabbardella altipes Beds (Figure 3) corresponding to the Upper Caradocian-Ashgillian. The similar rocks exposed 50 m upward from the top, at Point K8 (Figures 2 and 6), and bearing H. brevirameus (Walliser), P. liripipus Kennedy et al., P. grandis (Ethington), Drepanodus robustus (Hadding), Istorinus sp., Baltoniodus sp., Amorphognathus sp., and Panderodus sp., are assigned to the same faunal beds. The similar conodont assemblage was recorded 70 m above, in the member of manganese-bearing siliceous rocks. This association at Point K6 (Figures 2, 5, and 6) includes H. brevirameus (Walliser), Dapsilodus mutatus (Branson et Mehl), P. liripipus Kennedy et al., P. grandis (Ethington), Drepanoistodus suberectus (Branson et Mehl), Panderodus gracilis (Branson et Mehl), and Spinodus sp. and is peculiar to the same faunal beds corresponding to the Upper Caradocian-Ashgillian analogs (Figure 3). The siliceous tephroid (i.e. Novokursk) sequence is overlain there by the carbonaceous shales of the Sakmara Formation (Figure 6).

[78]  North of the Yumaguzino 1 Village and 1.3 km northwest of Point K8a, the Novokursk sequence becomes up to 230 m thick [Ruzhentsev, 2005]. The thickness of the overlying carbonaceous shales of the Sakmara Formation increases as well.

[79]  Twenty meters above the top of the red-colored sequence, at Point NK-2 (Figure 6), the grey siliceous tephroids of the Novokursk unit contain Pygodus anserinus (Lamont et Lindström), Periodon aculeatus Hadding, Panderodus gracilis (Branson et Mehl), and Protopanderodus sp. and are referred to the Pygodus anserinus and Periodon aculeatus Beds correlative with the uppermost Llanvirnian-Lower Caradocian (Figure 3).

[80]  The light grey tuffosilicites 100 m above the top of the red-colored sequence, at Points 264 and 264a (Figures 2 and 6) bear Hamarodus brevirameus (Walliser), Protopanderodus liripipus Kennedy et al., Scabbardella altipes (Henningsmoen), Periodon cf. grandis (Ethington), Belodina compressa (Branson et Mehl), Panderodus gracilis (Branson et Mehl), and Drepanoistodus suberectus (Branson et Mehl) and are assigned to the Hamarodus brevirameus, Dapsilodus mutatus, and Scabbardella altipes Beds (Figure 3) corresponding to the Upper Caradocian-Ashgillian.

[81]  The fragment of the section that includes the contact between the red-colored siltstone and siliceous tephroid Novokursk sequences is recorded further southward, nearby the crossing of the roads to Churaevo and Kuvandyk [Ryazantsev et al., 2005, Fig. 9]. The grey siliceous tephroids at Point B-38 yield Protopanderodus liripipus Kennedy et al., Periodon grandis (Ethington), Panderodus gracilis (Branson et Mehl), and Plectodina sp. The association is characteristic of the Hamarodus brevirameus, Dapsilodus mutatus, and Scabbardella altipes Beds (Figure 3) corresponding to the Upper Caradocian-Ashgillian analogs.

[82]  Thus the siliceous tuffaceous type of section (Kuragan Formation) was accumulated from the upper Lower Arenigian to Ashgillian inclusive.

The composite section of the Kuragan Formation.
[83]  It includes the succession of five faunal beds (Figure 3) that are correlated with the conodont zones of Balto-Scandia [Cooper and Sadler, 2004] and traditional Ordovician stages of Britain, and embrace the interval from the upper Lower Arenigian to Ashgillian. In two points of the Kuragan section the continuity of faunal beds is broken (Figure 3).

[84]  The stratigraphic succession of the species Periodon aculeatus to P. grandis does not contradict the present notion of morphophylogeny of the genus.

Paleoenvironments during the formation of the siliceous tuffaceous complex.
[85]  As shown above, in the Ordovician siliceous tuffaceous complex (Kuragan Formation) the lower and upper parts are clearly distinguished. The lower part represented by red-colored tuffaceous sandstones and siltstones with basalt flows, was likely formed in a distal part of the marginal basin (Figure 6) that was located west of the Ordovician Guberlya ensimatic island arc [Ryazantsev et al., 2005]. Formation of the upper part of the complex, which is composed of red- and grey-colored siliceous tephroids (Novokursk sequence of the Kuragan Formation), was associated with environments of an island arc foot (Figure 6).


RJES

Citation: Dubinina, S. V., and A. V. Ryazantsev (2008), Conodont stratigraphy and correlation of the Ordovician volcanogenic and volcanogenic sedimentary sequences in the South Urals, Russ. J. Earth Sci., 10, ES5001, doi:10.2205/2008ES000302.

Copyright 2008 by the Russian Journal of Earth Sciences

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