RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 10, ES5001, doi:10.2205/2008ES000302, 2008
[86] Sections of this type are characteristic of the island arc Guberlya and Baulus formations; they are succeeding and are connected by gradual facies changes.
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Figure 7 |
[89] In the stratotype area (Figure 7) the Ordovician Kidryasovo and Guberlya formations were distinguished [Sharfman and Tsetlin, 1968]. The Tremadocian age of the Kidryasovo Formation was defined from the trilobite and brachiopod evidence, whereas the Guberlya Formation was estimated as Middle or Early-Middle Ordovician based on its correlation with the fauna-bearing Kuragan Formation.
[90] Subsequently, in the distribution area of the Guberlya Formation the Devonian organic remains were found in limestones [Sharfman and Tsetlin, 1968] and cherts [Puchkov and Ivanov, 1985]. As a result of finding in cherts of Early-Middle Devonian conodonts made by K. S. Ivanov and V. N. Puchkov, the Guberlya Formation was considered invalid and was eliminated from stratigraphic scales [Antsygin et al., 1993]. The volcanogenic sedimentary, mixtite-bearing rocks distributed in the area were suggested to consider the analogs of the Devonian Kosistek Formation.
[91] Our research revealed that the discussed area is mainly occupied by the Ordovician volcanogenic sedimentary rocks (Guberlya Formation) and by the olistostrome yielding cherty olistoliths with conodonts ranging in age from the Lochkovian to Lower Famennian inclusive [Borisenok and Ryazantsev, 2005; Ryazantsev et al., 2005]. The Tremadocian terrigenous sediments of the Kidryasovo Formation are locally distributed there.
[92] The Guberlya Formation is variable in composition. It is largely made up of tuffites and pillow basalts with lesser participation of rhyolites. The basalts contain limestone lenslike beds, small lenses, and interpillow bodies and are decoupled by horizons, members, and lenses of siliceous tuffites that commonly look like smears on pillow surface.
[93] The relatively undisturbed section occurs on the left bank of the Guberlya River, west of the Guberlya station (Figures 6 and 7). It begins with the pillow basalt member, less than 100 m thick, decoupled by 0.5-2-m-thick horizons of red tuffites, and is further built up by the 180-m-thick red tuffite member. The overlying sequence of over 500 m thick is represented by alternated lenslike light pistachio, rarer red, tuffites bearing beds of quartz rhyolite and basalt tuffs. The tuffites of the lower member at Point 424 (Figures 6 and 7) yield Periodon zgierzensis Dzik, Walliserodus ethingtoni (Fahraeus), Ansella jemtlandica (Löfgren), Oistodus? tablepointensis Stouge, Dapsilodus cf. mutatus (Branson et Mehl), Drepanodus robustus Hadding, and Protopanderodus cf. liripipus Kennedy et al. sensu Stouge [1984]. The association is characteristic of the Periodon zgierzensis and Ansella jemtlandica Beds corresponding to the mid-Llanvirnian (Figure 3).
[94] On the right bank of the Guberlya River, at Point B-87 (Figures 6 and 7), the 10-cm-thick red tuffite bed among basalts contains Ansella nevadensis (Ethington et Schumacher), Periodon aculeatus Hadding, Dapsilodus similaris (Rhodes), and Walliserodus sp. This assemblage is specific for the Pygodus anserinus, Periodon aculeatus, and Ansella nevadensis Beds (Figure 3) correlative with the uppermost Llanvirnian-Lower Caradocian analogs.
[95] Further northward the section is mainly composed of monotonous, red and pistachio-colored tuffites bearing rare members of basalts and basalt tuffs with carbonate cement. The following conodonts were found on the left bank of the Malaya Kayala River, in the 10-20-m-thick red tuffite bed, at Point B-102 (Figures 6 and 7): Pygodus serra (Hadding), Periodon aculeatus Hadding, Eoplacognathus cf. robustus Bergstrom, Dapsilodus viruensis (Fahraeus), Protopanderodus varicostatus (Sweet et Bergstrom), Drepanoistodus suberectus (Branson et Mehl), Plectodina sp., and Ansella sp. (Figure 2). The association is characteristic of the Eoplacognathus robustus and Pygodus serra Beds (Figure 3) and provides their correlation with the robustus Subzone of the serra Zone in Balto-Scandia, i.e. with the uppermost Llanvirnian.
[96] Sections of the Guberlya Formation were studied beyond the stratotype area as well.
[97] On the northeastern flank of the Utyagulovo synform (Figure 5) tuffs, tuffites, and basalts are more common in the section. The following units are recorded on the left bank of the Sakmara River, on the watershed of the Almash and Kyzyl Yar gullies, nearby their junction (Figure 6):
[98] 1. Frequently alternating fine-clastic tuffs of quartz and basic rhyolites and grey-green, turquoise-colored and, rarer, red ashstones and tuffites. Aphyric basalt lenses occur in the upper part. Total thickness is 300 m.
[99] 2. Red siltstones and tuffaceous siltstones, bearing grey-green tuffite lenses, numerous molds of ostracodes and inarticulate brachiopods, and at Point K-32 including Baltoniodus medius (Dzik), Strachanognathus parvus Rhodes, Periodon aculeatus Hadding, Drepanodus arcuatus Pander, and Protopanderodus sp. referred to the Baltoniodus medius and Strachanognathus parvus Beds (Figure 3), i.e. to the Lower Llanvirnian analogs. Thickness 20 m.
[100] 3. Grey-green, tinged with violet, coarse-bladed aphyric basalts. Thickness 200 m.
[101] Nearby the base of the section this structure is complicated with fault wedges composed of ultrabasites and siliciclastic rocks bearing Early Devonian (Emsian) conodonts. North of the structure the section is represented by tuffaceous rocks of the Kuragan Formation.
[102] The section of the Guberlya Formation was also studied at the eastern margin of the Churaevo Village (Figure 5). Here, at the northern foot of the mountain with a fire-prevention post at the top, above the poorly exposed red tuffaceous siltstones of the lowermost Kuragan Formation, the following units are recorded in the westward direction (Figure 6):
[103] 1. Grey-yellow tuffaceous sandstones and gravelstones with a peculiar pseudopillow shelly parting. Thickness 15 m.
[104] 2. Interbedded red and pink tuffaceous sandstones and siltstones, yellowish and greenish tuffites, and red radiolarites. The red-brown tuffaceous siltstones with 0.5-cm-thick tephra layers of mixed composition at Point E-26 (Figures 5 and 6) contain numerous large Spinodus spinatus (Hadding), Walliserodus ethingtoni (Fahraeus), Protopanderodus liripipus Kennedy et al., and Drepanodus robustus Hadding (Figure 2) and are referred to the upper part of the Pygodus anserinus, Periodon aculeatus, and Ansella nevadensis Beds (Figure 3) corresponding to the uppermost Llanvirnian-Lower Caradocian. Thickness 30 m.
[105] 3. Tuffaceous siltstones and sandstones decoupled by tuffaceous conglomerate lenses. Thickness 60 m.
[106] In the Shaitantau Mountains sections of the Guberlya and Kuragan formations are tectonically juxtaposed. The Guberlya Formation fringes the Shaitantau Mountains with a narrow stripe marking the common antiform structure (Figure 5b).
[107] South and east of the mountains the section is composed of rhythmically alternated, variegated, sand- and silt-sized tuffites. The rhythms commonly begin with a gravel-sized sediments. Southeast of the Shaitantau Mountains, 500 m southwest of the western margin of the Novokurskii Village, the tuffites terminate with a 5-mm-thick layer of grey cherts yielding at Point 3-15 (Figure 5b) Spinodus spinatus (Hadding), Periodon aculeatus Hadding, Protopanderodus cooperi (Sweet et Bergstrom), Drepanodus cf. robustus Hadding, Panderodus gracilis (Branson et Mehl), Drepanoistodus suberectus (Branson et Mehl), and Ansella sp. (Figure 2). These sediments correspond to the Protopanderodus cooperi Beds, i.e. to the Upper Llanvirnian analogs (Figure 3).
[108] On the flanks of the Blyava synform (Figure 5) the tuffaceous sediments typical for the Guberlya Formation are associated with tuffaceous sandstones and siltstones bearing basalt flows, which are characteristic of the lower part of the Kuragan Formation, and occupy a higher structural and, most likely, stratigraphic position.
[110] At 250 m southeast of the Absalyamovo Village (Figure 1) the 40-m-thick block of grey-green aphyric basalts bears the 15-m-thick, greenish, siliceous tuffite bed. The tuffites at Point 669 (Figures 1 and 6) contain Pygodus protoanserinus Zhang, P. serra (Hadding), Periodon aculeatus Hadding, Protopanderodus varicostatus (Sweet et Bergstrom), Drepanodus arcuatus Pander, Panderodus sp., and Ansella sp. (Figure 2) and are assigned to the Pygodus protoanserinus and P. serra Beds (Figure 3) correlative with the uppermost Llanvirnian.
[111] The serpentinite melange located 2.2 km northwest of the Yal'chigulovo Village (Figure 1) is overlain by the 20-m-thick mixtite horizon bearing ophiolite rock fragments. The mixtites are replaced upwards by the 200-m-thick grey-green aphyric basalts and basaltic andesites. In the upper part of the section, at Point 6033 (Figures 1 and 6), the 6-10-m-thick lenses of grey-green, brown and dark grey cherts yield Periodon aculeatus Hadding, Pygodus cf. anserinus Lamont et Lindström, Drepanodus robustus Hadding, and Ansella sp., referred to the Pygodus anserinus, Periodon aculeatus, and Ansella nevadensis Beds (Figure 3), i.e. to the analogs of the uppermost Llanvirnian-Lower Caradocian.
[112] Thus, the research of sections of the Guberlya Formation located within and beyond the stratotype area provided reason enough to re-establish the formation in the Ordovician stratigraphic scheme, with more refined composition and age. According to composition, the Guberlya Formation is intermediate between the Ordovician volcanogenic Baulus Formation (see below) and the siliceous tuffaceous Kuragan Formation. The Guberlya Formation is of Llanvirnian-Early Caradocian age.
[114] The stratigraphic successions of the Periodon, Ansella, and Pygodus species
revealed in the section of the formation does not contradict the present notion on
morphophylogeny of these genera. In the composite section one can record the
chronological successions Periodon zgierzensis
P. aculeatus (Figure 2); Ansella jemtlandica
A. nevadensis; and Pygodus serra
P. protoanserinus
P. anserinus.
The concurrent occurrence of P. protoanserinus Zhang and P. serra (Hadding) permitted
the distinction of the Pygodus protoanserinus and P. serra Beds of a narrow stratigraphic
range for the Upper Llanvirnian analogs (Figure 3), which is of fundamental importance for
development of high-resolution Ordovician conodont scale in the South Urals.
[115] The Baulus Formation distinguished by V.T. Tishchenko in 1988 is represented by acidic, basic, and, to a lesser degree, intermediate extrusive rocks. Its sediments make up cores of the large Blyava and Utyagulovo synforms (Figure 5), between which the stratotype section was described in a small synform at the Blyava and Kuragan Rivers confluence [Artyushkova et al., 1991; Maslov et al., 1993].
[116] In the Blyavtamak Village area, on the southern slope of the Baulus Mountain
(Figures 5 and 6) upward from the foot, the basalts and basaltic tuffs grade into rhyolites
and rhyolite tuffs. The beds dip gently at
20o. Nearby the contact between acidic and basic
rocks, in the 20-m-thick member, extrusive rocks are decoupled by beds of fine-clastic tuffs,
tuffites, and red cherts. The lower chert stratum, at Point 10a (Figures 2 and 6) bearing Spinodus spinatus (Hadding), Periodon aculeatus Hadding, Protopanderodus liripipus Kennedy et
al., and Ansella sp. identified by Dubinina
[Artyushkova et al., 1991],
is referred to the Spinodus spinatus and Periodon aculeatus Beds (Figure 3) corresponding
to the mid-Caradocian. The upper chert bed with burrow molds yields at Point 10 (Figure 6)
Protopanderodus insculptus (Branson et Mehl) (see Appendix, Plate 5, fig. 19),
Scabbardella altipes (Henningsmoen), Dapsilodus mutatus (Branson et Mehl),
Walliserodus cf. nakholmensis (Hamar), Panderodus gracilis (Branson et Mehl),
Cornuodus longibasis (Lindström), Icriodella sp., and Istorinus erectus Knupfer
(Figure 2) and is assigned to the Hamarodus brevirameus, Dapsilodus mutatus, Scabbardella altipes, and Istorinus erectus Beds correlative with the Upper Caradocian-Ashgillian
(Figure 3).
[117] On the left bank of the Blyava River, southwest of the Blyavtamak Village, an aphyric basalt block with grey chert beds is recorded. It is likely a fragment of section of the Baulus Formation. At Point 11 (Figure 2) the cherts contain Hamarodus brevirameus (Walliser), Periodon grandis (Ethington), Dapsilodus mutatus (Branson et Mehl), Plectodina furcata (Hinde), Panderodus gracilis (Branson et Mehl), and Belodina sp. and are also referred to the Hamarodus brevirameus, Dapsilodus mutatus, Scabbardella altipes, and Istorinus erectus Beds that correspond to the Upper Caradocian-Ashgillian (Figure 3).
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Figure 8 |
[119] In the western wall of the Blyava quarry (Figure 8), above the ore stratum, the member of interbedded rhyolites, their tuffs, and basalts is crowned by the 0.5-2-m-thick bed of red siliceous siltstones and hematite-siliceous shales. At Point 38 (Figure 6) the red siliceous siltstones bearing Dapsilodus mutatus (Branson et Mehl), the form transitional from Periodon cf. aculeatus Hadding to P. cf. grandis (Ethington), Drepanodus robustus Hadding, Protopanderodus cf. liripipus Kennedy et al., and Panderodus sp. (Figure 2), are referred to the Hamarodus brevirameus, Dapsilodus mutatus, Scabbardella altipes, and Istorinus erectus Beds of the Baulus Formation section (Figure 3), which are correlative with the Upper Caradocian-Ashgillian. Previously the findings in this horizon of inarticulate brachiopods [Korinevskii, 1992] and, evidently in the same bed, of Ordovician conodonts [Ivanov et al., 1989; Maslov et al., 1993] were reported. The siliceous rocks terminating the Baulus Formation are overlain by the 0.5-m-thick bed that represents the base of the Blyava Formation. It is composed (upward from the base) of siliciclastic sandstones, basaltic tuffites, and carbonaceous siliceous shales. The latter are recorded above, among the pillow basalts as well. The Llandoverian, including Late Llandoverian, graptolites were encountered in the shales at different levels [Borodaev et al., 1963].
[120] At the Komsomol'skoe deposit (Figure 8), on the left bank of the Khersonka River, the member overlying the ore-bearing sequence is composed of interbedded basalts, quartz rhyolite tuffs and tuffites, red flaggy siltstones, cherts, jaspers, and hematite shales. This level is analogous to the supraore bed recovered in the Blyava quarry. The Middle and Lower-Middle Ordovician conodonts were found there previously [Ivanov et al., 1989]. At Point 3-37 (Figure 8) we collected in red jaspers the Late Caradocian-Ashgillian Hamarodus brevirameus (Walliser), Panderodus sp., and Protopanderodus sp., characteristic of the above-mentioned beds of the Baulus Formation (Figure 3). This confirms the validity of correlation of the supraore horizons in the Blyava and Komsomol'skoe deposits.
[121] At the northern closure of the Blyava synform (Figure 8) the section of the Baulus Formation is overturned. With the normal succession of rocks of the formation the quartz and basic rhyolites are overlain by basalts (100-m-wide outcrop). In the mid-section the basalt member is decoupled by a 2-10-m-thick, red, brown, and grey siliceous tuffaceous siltstone bed. At Point 599 (Figures 2 and 8) the siltstones contain Hamarodus cf. brevirameus (Walliser), Scabbardella cf. altipes (Henningsmoen), and Periodon sp. of the Late Caradocian-Ashgillian age. The basalts are further replaced by siliceous and siliciclastic rocks of the Lower-Middle Devonian Akchura Formation. The same siliceous tuffaceous siltstone bed, 200 m to the northeast, at Point 5-184 (Figures 2 and 8) yields H. cf. brevirameus (Walliser), Protopanderodus cf. liripipus Kennedy et al., Drepanodus robustus (Hadding), and Panderodus sp. In both points the conodont associations are characteristic of the Hamarodus brevirameus, Dapsilodus mutatus, Scabbardella altipes, and Istorinus erectus Beds of the Baulus Formation (Figure 3), which correspond to the Upper Caradocian-Ashgillian. The overturned occurrence previously resulted in the mistaken inference that the volcanites belong to the Devonian section, and they were distinguished as the Utyagulovo Formation [Maslov et al., 1993].
[122] The question of the age of ore-bearing sequences and the massive sulfide ore deposits, was debated over a long period [Artyushkova et al., 1991; Ivanov, 1996; Ivanov et al., 1989; Korinevskii, 1992]. From the available records we infer the Ordovician and Silurian age of the ore-bearing volcanogenic complex and the Late Ordovician age of the massive sulfide ore deposits (Figure 6). The schematic section of the ore-bearing volcanogenic complex is described in the other paper [Ryazantsev et al., 2005].
[123] The data on composition and age of the ore-bearing volcanogenic complex of the Sakmara zone permit its comparison with that of the Tagil zone [Karetin, 2000] (see inset map Figure 1). Their similarity is in association of the ore deposits with the Ordovician-Silurian boundary and in the occurrence of siliceous rocks at this level.
[127] In the 2-4-m-thick and about 200-m-long bed of jaspers and red siliceous siltstones, at 600 m from the Baiguskarovo Village at Point 037 (Figure 2) we collected Periodon grandis (Ethington), Panderodus sp. A Dzik, and Dapsilodus sp. The phthanite lens among basalts, at 1.5 km from the Baiguskarovo Village margin, at Point 1-184 (Figure 2), yields Scabbardalla altipes subsp. B Orchard, Panderodus gracilis (Branson et Mehl), and Plectodina sp. Five meters upward from the base the bluish-green cherts alternating with phthanites, at Point 029 (Figure 2) bear S. altipes (Henningsmoen) and D. mutatus (Branson et Mehl).
[128] According to composition and age, these sediments are correlative with the Baulus Formation of the Sakmara zone, namely, with the Hamarodus brevirameus, Dapsilodus mutatus, Scabbardella altipes, and Istorinus erectus Beds (Figure 3) corresponding to the Upper Caradocian-Ashgillian.
[130] The stratigraphic succession Periodon aculeatus
P. grandis established in
the section of the Baulus Formation does not contradict the present notion on evolution of
this genus.
[132] Volcanites of both formations are strongly heterogeneous in chemical composition. Basalts of the Guberlya Formation are correlative with intraplate oceanic and island-arc tholeiites [Borisenok and Ryazantsev, 2005]; basaltoids of the Baulus Formation, with island-arc tholeiites, MORB, and intraplate oceanic basalts. All the available data indicate that the volcanogenic complex reflects the evolution of the ensimatic island arc which experienced splitting, riftogenesis, and died off by the beginning of the Silurian.
Citation: 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.
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