- Email: [email protected]
New data on the structure of the Turukhan zone of deformation from the results of seismic survey and geological traverses
Available online at www.sciencedirect.com
ScienceDirect Russian Geology and Geophysics 58 (2017) 451–460 www.elsevier.com/locate/rgg
New data on the structure of the Turukhan zone of deformation from the results of seismic survey and geological traverses A.S. Efimov *, M.Yu. Smirnov, G.D. Ukhlova, E.V. Mosyagin, E.G. Keller, T.R. Kudrina Siberian Research Institute of Geology, Geophysics and Mineral Resources, Krasnyi pr. 67, Novosibirsk, 630091, Russia Received 20 July 2016; accepted 1 September 2016
Abstract The structure of the junction between the Turukhan zone of deformation and the Kureika Basin was studied using regional reflection profiling data acquired along the Lower Tunguska River, with reference to well logs and land-based geological surveys. The western flank of the line comprises four large blocks separated by faults, which produce different patterns in seismic sections. The eastern part of the line covers the margin of the Kureika Basin filled with plane-bedded sediments and its border with the Turukhan Uplift and the Turukhan tectonic zone with thrust sheets. © 2017, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: seismic section; reflector; fault; wave field; Riphean sediments; Lower Tunguska River
Introduction
Seismic surveys
The area of the Turukhan Uplift (Kureika–Baklanikha Megaswell) has been quite well documented in outcrops along rivers. Outcrops at the Lower Tunguska mouth expose a complete section of the East Siberian Neoproterozoic (Riphean and Vendian) and Cambrian reservoirs. Data from the area have been reported in many publications (Bazhenova and Kazais, 2011; Kazais, 2006; Kozlov et al., 1988; Mikutsky and Petrakov, 1961; Staroseltsev et al., 2012, and many others). The Turukhan Uplift is a zone of folds and thrusts composed of Late Proterozoic–Middle Cambrian sediments overriding the western margin of the Kureika Basin. It presumably consists of the Durnoi Mys, Turukhan, and Golyi Yar N–S elongated blocks which make an imbricate stack of sheets thrust one over another from west to east. The oldest rocks (Riphean Strelnye Gory Formation) crop out in the eastern uplifted parts of the blocks, while progressively younger sediments (up to Cambrian) appear westward. Folded Paleozoic rocks crop out east of the thrusts. The collected data are of exceptional value, given the absence of seismic profiling and drilling evidence for the area.
A unique 1471 km long regional seismic reflection profile was collected in 2014 along the Lower Tunguska River, jointly by teams of three companies (OOO Geofizicheskaya Sluzhba, OOO BGE, and OOO Dongeofizika), and processed at the geophysical center of the Siberian research Institute of Geology, Geophysics, and Mineral Resources (SNIIGGiMS) in Novosibirsk. The project is remarkable by an enormous amount of work far exceeding the previous experience in CMP profiling along rivers (Ob’, Yenisei, Lena, Vakh, Angara, Chunya, Biryusa, etc.), as well as by the highest-latitude location of the W–E profile as part of the East Siberian regional network. Most of the profile traverses thick Triassic basalts which poses problems to acquisition and impairs data quality (Fig. 1). In spite of the difficulties in field surveys along rivers, we have come to acquire high-quality seismic profiles more informative than most of other regional and local seismic profiles collected in the area (Fig. 2). Our results prove that the advanced technologies of field acquisition and processing can provide data of better quality than in the previous years.
Seismic stratigraphy and updated section structure * Corresponding author. E-mail address: [email protected] (A.S. Efimov)
This paper focuses mainly on data from the western flank of the Lower Tunguska profile interpreted along 140 km
1068-7971/$ - see front matter D 201 7 , V . S. So bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.rgg.2016.09.021 +
452
A.S. Efimov et al. / Russian Geology and Geophysics 58 (2017) 451–460
between the Erachimo River mouth and Turukhansk Village (Fig. 3). The seismic data were checked against well logs from Goloyarskaya 1 and Tungusskaya 1 (reference) holes in the eastern flank of the profile which strip, respectively, Riphean sediments at a depth of 1037 m below the ground surface and the Lower Cambrian Deltula Formation at a depth of 2751 m (the hole bottom is situated at 2945 m below the surface). Therefore, it is impossible to obtain unambiguous stratigraphic constraints of the reference reflectors B (Vendian top) and R0 (Vendian base). The seismic section of the eastern profile flank includes the reflector R0 traced according to amplitude peaks and inclined clinoform sets of older Riphean sequences that reach R0 from below and produce a pattern resembling a cutout (Payton, 1977) of the eroded Riphean surface (Fig. 4). For the lack of wells in the western flank of the profile, the correlation was with land geological data: a 1:200,000 geological map and cross sections of outcrops obtained in the course of the field surveys of 2008 (by S.V. Frolova, G.G. Akhmanova, M.K. Ivanova, E.V. Kozlova, and O.V. Krylova from Moscow University and T.R. Kudrina and M.V. Tabatchikov from SNIIGGiMS). See Fig. 5 for an example of stratigraphic correlation of reflectors with reference to geological surveys. According to the stratigraphic model of 1983 (Resolution…, 1983), Riphean rocks in the area belong to the Upper Riphean Tungusik Group (Durnoi Mys, Rechka, Turukhan, Miroedikha, Shorikha, Lower Tunguska (Burovoi), and Derevnya Formations) and the Middle Riphean Sukhoi Pit Group (Sukhaya Tunguska, Linok, and Strelnye Gory Formations). The overlying Upper Riphean Oslyany and Taseeva Groups are missing from the section, and the Middle Riphean Strelnye Gory Formation appears to lie on the Archean–Proterozoic basement. The known Riphean stratigraphy of the area is controversial: Mel’nikov et al. (2009) attribute the Sukhaya Tunguska, Linok, and Strelnye Gory Formations to the Upper Riphean, while the 1:2,500,000 Geological Map of 1980 shows Lower Riphean strata exposed within the Turukhan Uplift. The Upper Vendian–Lower Cambrian Platonovka Formation represents the Vendian section. It reaches a thickness of 400 m and consists mainly of dolomites, which are locally argillaceous and anhydrite-bearing, with scarce marl and sandstone interbeds. The Cambrian Kostino Formation, 1700 m of total thickness, is composed of dolomites, often organic-bearing, with limestone interbeds. The Cambrian Letnyaya Formation of limestone and purple mudstone and marl crops out in the zone of faults. The best differentiated Upper Riphean sediments form several seismic sequences. The youngest one (R3drn-mr) includes the Durnoi Mys, Turukhan, Rechka, and Miroedikha Formations composed of dolomites with sporadic stromatolites and variegated mudstones intercalated with limestone and marl. The observed total thickness is 280 m, but may reach 605 m according to the 1983 MSK stratigraphy.
The R3drn-mr sequence overlies more than 1400 m (mapped thickness) of dolomites with chert and clastic intercalations (R3sr-nt) that belong to the Shorikha and Lower Tunguska (Burovoi) Formations. The underlying R3drv sequence, of about 500 m, corresponds to the Derevnya Formation dolomites. The R2–3stn-ln sequence below, of 800 m total thickness, represents the Sukhaya Tunguska and Linok Formations composed of dolomites with chert and limestone intercalations. The oldest Strelnye Gory Formation has a mainly clastic composition: mudstone, marl, clayey limestone, and light gray quartz and feldspar-quartz sandstones. It is almost impossible to discriminate these sequences according to wave patterns. They all produce prominent strong events, which sometimes grade laterally into discontinuous often hummocky reflections, while the wave energy reduces abruptly. Such a pattern may result either from the presence of faults or from facies change. The wave amplitudes decrease near the ground surface, within the Riphean section, and the record even becomes chaotic, possibly, due to faults or to disintegration of rocks by weathering. Note especially that the reflectors remain continuous and strong within the near-surface Vendian–Cambrian interval (Kostino and Platonovka Formations). The interval corresponding to the Kostino and Platonovka Formations in the western flank of the profile (to well Goloyarskaya 1) includes long, quite strong and relatively high-frequency reflections, but unambiguous correlation of R0 is problematic for the lack of stratigraphic ties at depths, compositional similarity of Riphean and Cambrian rocks, and complex tectonics of the area. A high-amplitude wave, most likely, a reflection, appears about 20 ms. Reflections along this seismic interface form a pattern of erosion-like clinoform features, and reflectors appear to emerge almost vertically in places of large faults. This interface may correspond to a seafloor surface. The interpretation results indicate that prominent reflections record Riphean carbonates. This pattern is consistent with bedding observed in outcrops and with clastic-carbonate alternation.
Imaging faults in the seismic section The western flank of the profile images four well-pronounced large thrusts, as well as small faults which are too numerous to be shown all in the section. The westernmost thrust (sometimes called Durnoi Mys fault) separates the Turukhan and Durnoi Mys blocks and has its left wall thrown down and overlain by Jurassic sediments. There is a blind zone along the fault, which goes from the top to the bottom of the section between 0 and 20 km in the end of the profile running northward along the Yenisei River. The zone may trace the western edge of the Siberian craton, while the block as a whole is a synclinal fold filled with Riphean sediments spanning a stratigraphic range from the Durnoi Mys to Strelnye Gory Formations.
Fig. 1. Location of seismic reflection profile along the Lower Tunguska River in 1:2500,000 Geological Map (VSEGEI, 1980). 1, Quaternary; 2, Upper Cretaceous; 3, Lower Cretaceous; 4, Lower Jurassic; 5, Middle Ttriassic; 6, Lower Triassic; 7, Permian, undifferentiated; 8, Upper Permian; 9, Lower Permian; 10, Upper Carboniferous; 11, Middle Carboniferous; 12, Lower Carboniferous; 13, Lower and Middle Devonian; 14, Upper Silurian; 15, Lower Silurian; 16, Upper Ordovician; 17, Middle and Upper Ordovician; 18, Lower Ordovician; 19, Upper Cambrian; 20, Middle Cambrian; 21, Lower Cambrian; 22, Vendian; 23, Middle Riphean; 24, Lower Riphean; 25, granite intrusions; 26, gabbro-dolerite intrusions; 27, seismic profile along Lower Tunguska (2014); 28, study area.
A.S. Efimov et al. / Russian Geology and Geophysics 58 (2017) 451–460 453
454
A.S. Efimov et al. / Russian Geology and Geophysics 58 (2017) 451–460
Fig. 2. Time sections, compared. a, Fragment of seismic section along Lower Tunguska (processed at SNIIGGiMS); b, fragment of seismic section along profile 034691 (processed at a third-party institution).
The Strelnye Gory fault, with its western wall uplifted and thrust over the eastern one, separates the Turukhan and Golyi Yar blocks. The oldest Riphean Strelnye Gory Formation and the Vendian–Cambrian Platonovka Formation rocks crop out, respectively, on the left and on right of the fault. According to field data, rocks dip steeply at 70º–90º in the thrust zone
and at a lower angle (10º–40º) outside of it. The western half of the Golyi Yar block is a syncline composed of the Vendian–Cambrian Kostino and Platonovka Formations near the surface. The Strelnye Gory fault and the junction of Riphean and Vendian rocks are well evident in outcrops along the Lower Tunguska River. In the seismic image, the fault
A.S. Efimov et al. / Russian Geology and Geophysics 58 (2017) 451–460
455
Fig. 3. Fragment of the western part of the profile along the Lower Tunguska River in 1:1,100,000 Geological Map (VSEGEI, 1990). 1–32, age of rocks: 1, Lower Cretaceous (Aptian–Albian, Yakovlevo Formation); 2, Upper Jurassic; 3, Middle Jurassic; 4, upper Upper Permian; 5, lower Upper Permian; 6, Lower Triassic (Putorana sequence); 7, Lower Triassic (Dvurogino sequence); 8, Lower Triassic (Nadezhda Formation); 9, Lower Triassic (Khakanchan Formation); 10, Lower Triassic (Syverma Formation); 11, Upper Silurian; 12, Upper Silurian (Pridoli, Pankagir Formation); 13, Upper Silurian (Ludlow, Kongda Formation); 14, Lower Silurian (Wenlock, Mukten Formation); 15, Lower Silurian (Llandovery, Uguyuk Formation); 16, Middle Ordovician; 17, Middle Ordovician (Mangaseya, Zagornaya Formation); 18, Middle Ordovician (Krivolutsk stage, Angir, and Anarakan Formations); 19, Lower Ordovician; 20, Lower Ordovician (Chunya, Guragir Formation); 21, Lower Ordovician (Ust’-Kut–Chunya stages, Iltyk Formation); 22, Lower Ordovician (Ust’-Kut stage, Uigur Formation); 23, Upper Cambrian; 24, Upper Cambrian (Sak–Aksai stages, Kulyumba Formation); 25, Upper Cambrian (Ayusokkan stage, Orakta Formation); 26, Lower–Middle Cambrian (Bottomian, Toyonian, Amga stages, Shumnaya Group); 27, Lower Cambrian (Tommotian–Atdabanian, Krasnyi Porog Formation); 28, Upper Vendian (Platonovka Formation); 29, Upper Vendian (Graviy Formation); 30, Upper Vendian (Taseeva sequence, Iluchina Formation); 31, Upper Vendian (Turukhan Formation); 32, Upper Vendian (Shorikha Formation); 33–40, intrusions: Middle Triassic carbonates (33), V phase dolerite intrusions (34), Kureika-type IV phase intrusions (35), Katanga-type III phase intrusions (36), Tymer- and Letnyaya-type II phase intrusion (37), Late Permian Ergalakh-type I phase intrusions (38), alkaline syenites and porphyritic syenites (39), subalkaline gabbro-dolerite (40); 41, mainly extrusive (a) and tuff (b) mafic volcanic rocks; 42, contact hornblende; 43, observed (a) and inferred (b) boundaries of geological and lithological units; 44, observed faults of uncertain slip geometry; 45, river network; 46, seismic profile along Lower Tunguska (2014); 47, piston core holes; 48, deep holes.
shows up as quite a large 4–6 km wide zone of irregular reflections, i.e., a zone of disintegrated rocks (Fig. 6). Like the Strelnye Gory fault, the Voronovo fault east of it has an uplifted western wall thrust over the eastern wall; the oldest Riphean Strelnye Gory Formation, and the Cambrian Letnyaya Formation rocks crop out on the left and right of its axis. The eastern fault wall is an anticlinal fold with the Kostino Formation exposed in its hinge. The Voronovo fault plane shows up as a prominent reflector almost reaching the surface, unlike other dipping interfaces. It looks like an unconformity, with other reflectors on the right joining it at an angle about 0º. The fault remains hidden under sod in outcrops along the Nizhnyaya Tunguska but is exposed along the Sukhaya Tunguska which flows into the Yenisei 22 km further south (Fig. 6). The different seismic images of the two faults may result from the fact that the Voronovo fault plane dips at a lower angle (about 40º) than the Strelnye Gory one. Numerous dolerite intrusions crop out east of the Voronovo fault but they are irresolvable and impossible to trace in the seismic section where they are obscured by a dense network
of faults, while no subsurface ties of the intrusions are available. The Imangda–Letnyaya fault east of well Goloyarskaya 1, with its uplifted western wall thrust over the eastern wall, marks the boundary between the Kureika–Baklanikha Uplift and the Kureika Basin. The rocks exposed west and east of the fault are, respectively, the older Letnyaya and Kostino Formations and younger Silurian sediments. Another large fault east of the Imangda–Letnyaya fault likewise has its uplifted western wall thrust over the eastern one. There are no thrusts in the eastern flank of the profile, where sediments are plane-bedded. As noted above, the reflector R0 follows the peak amplitudes, and other dipping reflectors join it from below making an erosion-like clinoform pattern. At earlier times, there is a train of strong and long reflections, with its top corresponding to the Ordovician surface. Reflectors in the western flank of the profile, which is composed of the Vendian–Cambrian Kostino and Platonovka Formations, are weaker than those in the east where the Bakhta and Suringdakon zones represent, respectively, Vendian and
Fig. 4. Seismic geological model of the Turukhan zone of deformation. a, Seismic-geological cross section along the Lower Tunguska River; b, time section along the Lower Tunguska River. 1, reflectors and their symbols; 2, tentative boundaries of seismic sequences (in blind zone); 3, faults and their names; 4, zone of irregular reflections; 5, reflector (presumably seafloor); 6, facies change; 7, geological − upl-lt), Kostino (C − kst), Kostino–Platonovka (V–C − kst-pl), Platonovka (V–C − pl), Durnoi Mys–Miroedikha (R3drn-mr), Shorikha–Lower survey route shown in Fig. 5. Formations: Ust’-Pelyatka–Letnyaya (C Tunguska (R3sr-nt), Derevnya (R3drv), Sukhaya Tunguska–Linok (R2–3stn-ln), Strelnye Gory (R2–3str).
456 A.S. Efimov et al. / Russian Geology and Geophysics 58 (2017) 451–460
Fig. 5. Example of stratigraphic correlation of seismic reflectors with outcrops. Lithology: 1, limestone; 2, dolomite; 3, marl; 4, sandstone; 5, claystone; 6, siltstone; 7, limestone and clayey dolomite; 8, limestone and brecciated dolomite; 9, limestone and algal dolomite; 10, limestone and clay-bearing dolomite; 11, limestone and oolitic dolomite; 12, limestone and cavern dolomite; 13, dolomitic limestone and limy dolomite. Other symbols: 14, pyritization; 15, bitumen occurrences; 16, silicification; 17, blue schist; 18, calcite inclusions; 19, faults; 20, dip and strike; 21, observation and sampling sites; 22, geological survey line; 23, seismic reflectors. Formations: Strelnye Gory (PR3str), Linok (PR3ln), Sukhaya Tunguska (PR3sh), Derevnya (PR3dr), Lower Tunguska (Burovoi) (PR3nt), Shorikha (PR3sr), Miroedikha (PR3mr), − 1pl), Kostino (C − 1–2ks), Letnyaya (C − 2lt). Durnoi Mys (PR3dm), Platonovka (PR3-C
A.S. Efimov et al. / Russian Geology and Geophysics 58 (2017) 451–460 457
458
A.S. Efimov et al. / Russian Geology and Geophysics 58 (2017) 451–460
Fig. 6. Strelnye Gory and Voronovo faults: seismic time section (A), a photograph of outcrops in the right side of the Lower Tunguska (Strelnye Gory fault, B), and a photograph of outcrops at the Sukhaya Tunguska (Voronovo fault, C), after Staroseltsev et al. (2012).
Cambrian strata separated by a facies boundary prominent in the wavefield. The Vendian section in the eastern profile part comprises the Katanga, Soba, and Tetere Formations and the Lower Precambrian rocks belong to the Yaseng, Moktakon, Mara, Abakun, Burus, Suringdakon, Bulai, Deltula, and Tanachi Formations. The position of the reflector H attributed to the base of the Letnyaya Formation can be checked against logs of reference well Tungusskaya 1. The Paleozoic section in the eastern profile flank includes multiple intrusions, of which twelve were stripped in Tungusskaya 1. One intrusion was found in Cambrian rocks of the Deltula Formation 60 m above the hole bottom but it is poorly detectable in the seismic section because the well is bounded by faults in the west and east and falls into a 4 km wide deformed zone. The sediments above show a complex wave pattern free from long prominent events.
Wavefield features The seismic image of the Letnyaya block contains a feature which may record a reef buildup within the interval of youngest Riphean sediments in the area (Durnoi Mys, Turukhan, Rechka, and Miroedikha Formations). It appears as a
series of asymmetric domes upon a flat surface, with one slope steeper than the other. The reflections inside the domes are discontinuous and hummocky, of low energy, with several oblique reflectors at the lower-angle slope. Such seismic features are typical of carbonate organic buildups. However, no bioherms have ever been reported for the plane-bedded Durnoi Mys, Turukhan, Rechka, and Miroedikha Formations. The reason may be that either large bioherms remain yet undetected in these younger formations or that the Shorikha rocks lie under the Platonovka Formation in the Golyi Yar structure. Other possibilities may be correlation or processing errors, or some nongeological causes. The idea of wedging Riphean sediments is consistent with a clinoform seismic pattern looking like an erosion cutout near well Goloyarskaya 1, with the wedges corresponding to Riphean sequences that accrete to the reflector R0 (Vendian base) from below. The Riphean rocks become thinner eastward along the profile and produce a characteristic wavefield pattern recorded, specifically, at point 121 km. This zone may be the boundary of the thinning Riphean sediments, while Vendian rocks east of the boundary may lie directly upon the basement. However, strong reflections and long events are prominent further east beneath R0 and gradually wedge out as well. These features do not look like multiple waves, and the precise
A.S. Efimov et al. / Russian Geology and Geophysics 58 (2017) 451–460
459
Fig. 7. Seismic image of possible reef buildups in a paleosection (brought to the reflector tentatively attributed to the Miroedikha Formation base).
location of the boundary of thinning Riphean sequences remains unknown. The train of strong reflections attributed to Riphean sediments deepens in the western direction and bifurcates, with its upper part pinching out (reflections disappear) and the lower part reaching a depth of 14,500 m. The wave pattern in the western flank of the profile is hard to interpret. The total thickness of Riphean sediments in the Turukhan region was previously estimated to approach 5 km. Our results show a seismically transparent zone (void of significant reflections), presumably, the basement, beneath the reflections identified with the Strelnye Gory Formation. Note that long and strong events reappear at the depths of 9 to 14 km and may be some discontinuity in the Archean–Proterozoic strata. However, this train of reflections grades into the Riphean reflections and accretes to R0 in the east. Therefore, the deep structure of the region is worth further investigation. The origin of the reflectivity zone in the 9–14 km interval, which is underlain by another transparent zone, remains unclear at the present stage of knowledge.
Conclusions 1. The advanced acquisition and processing technologies provide seismic data of a much better quality than in the previous years.
2. The encouraging results we have obtained allow planning future surveys in the central Kureika Basin along river valleys within the Putorana plateau. 3. The stratigraphy of seismic reflectors has been updated with reference to land-based geological data. 4. The Strelnye Gory fault shows up in the wavefield as a broad (4–6 km) zone of chaotic reflections. 5. The Voronovo fault produces a prominent reflector which almost reaches the surface, unlike other dipping reflections. Other reflectors accrete to the unconformity of the fault-related reflector at angles of about 0º making a clinoform pattern. 6. The seismic section contains anomalies looking like reef buildups within the Riphean sequences (Durnoi Mys, Turukhan, Rechka, and Miroedikha Formations). 7. Our surveys reveal a seismically transparent zone beneath the reflections attributed to the Strelnye Gory Formation, while long and strong reflection events reappear at the 9–14 km depths and rise gradually in the eastward direction to reach the reflector R0 (Vendian base). References Bazhenova, T.K., Kazais, V.I., 2011. The history of petroleum generation and accumulation in the northwestern Siberian craton (evolution of chemistry and structure of reservoirs). Neftegazovaya Geologiya. Teoriya i Praktika 6 (2), http://www.ngtp.ru/rub/1/17_2011.pdf. Kazais, V.I., 2006. The northwestern margin of the Siberian craton: A new tectonic model from geophysical (seismic–gravity–magnetic) modeling. Geologiya Nefti i Gaza, No. 5, 52–61.
460
A.S. Efimov et al. / Russian Geology and Geophysics 58 (2017) 451–460
Kozlov, G.V., Votakh, O.A., Alexandrov, V.S., 1988. Typical Precambrian formations in the Turukhan and Igarka uplifts, in: Tectonics of Cratonic Terrains [in Russian]. Nauka, Novosibirsk, pp. 9–51. Mel’nikov, N.V., 2009. A Vendian–Cambrian Salt-Bearing Basin of the Siberian Craton: Stratigraphy and History [in Russian]. Izd. SO RAN, Novosibirsk. Mikutsky, S.P., Petrakov, V.U., 1961. Tectonics of the Yenisei side of the Siberian craton, in: Geology, Geophysics, and Mineral Resources of West Siberia [in Russian]. Leningrad, pp. 46–58.
Payton, Ch.E. (Ed.), 1977. Seismic Stratigraphy: Applications to Hydrocarbon Exploration. AAPG Memoir, Volume 26. Resolution of the Stratigraphic Workshop on Precambrian, Paleozoic, and Quaternary, 1983. Part I: Upper Precambrian and Lower Paleozoic [in Russian]. SNIIGGiMS, Novosibirsk. Staroseltsev, V.S., Divina, T.A., Muratov, M.I., Staroseltsev, K.V., 2012. Discordant structures of the Turukhan horst as a feature of global-scale tectonics. Geologiya i Mineralno-Syr’evye Resursy Sibiri 4 (12), 3–7.
ScienceDirect Russian Geology and Geophysics 58 (2017) 451–460 www.elsevier.com/locate/rgg
New data on the structure of the Turukhan zone of deformation from the results of seismic survey and geological traverses A.S. Efimov *, M.Yu. Smirnov, G.D. Ukhlova, E.V. Mosyagin, E.G. Keller, T.R. Kudrina Siberian Research Institute of Geology, Geophysics and Mineral Resources, Krasnyi pr. 67, Novosibirsk, 630091, Russia Received 20 July 2016; accepted 1 September 2016
Abstract The structure of the junction between the Turukhan zone of deformation and the Kureika Basin was studied using regional reflection profiling data acquired along the Lower Tunguska River, with reference to well logs and land-based geological surveys. The western flank of the line comprises four large blocks separated by faults, which produce different patterns in seismic sections. The eastern part of the line covers the margin of the Kureika Basin filled with plane-bedded sediments and its border with the Turukhan Uplift and the Turukhan tectonic zone with thrust sheets. © 2017, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: seismic section; reflector; fault; wave field; Riphean sediments; Lower Tunguska River
Introduction
Seismic surveys
The area of the Turukhan Uplift (Kureika–Baklanikha Megaswell) has been quite well documented in outcrops along rivers. Outcrops at the Lower Tunguska mouth expose a complete section of the East Siberian Neoproterozoic (Riphean and Vendian) and Cambrian reservoirs. Data from the area have been reported in many publications (Bazhenova and Kazais, 2011; Kazais, 2006; Kozlov et al., 1988; Mikutsky and Petrakov, 1961; Staroseltsev et al., 2012, and many others). The Turukhan Uplift is a zone of folds and thrusts composed of Late Proterozoic–Middle Cambrian sediments overriding the western margin of the Kureika Basin. It presumably consists of the Durnoi Mys, Turukhan, and Golyi Yar N–S elongated blocks which make an imbricate stack of sheets thrust one over another from west to east. The oldest rocks (Riphean Strelnye Gory Formation) crop out in the eastern uplifted parts of the blocks, while progressively younger sediments (up to Cambrian) appear westward. Folded Paleozoic rocks crop out east of the thrusts. The collected data are of exceptional value, given the absence of seismic profiling and drilling evidence for the area.
A unique 1471 km long regional seismic reflection profile was collected in 2014 along the Lower Tunguska River, jointly by teams of three companies (OOO Geofizicheskaya Sluzhba, OOO BGE, and OOO Dongeofizika), and processed at the geophysical center of the Siberian research Institute of Geology, Geophysics, and Mineral Resources (SNIIGGiMS) in Novosibirsk. The project is remarkable by an enormous amount of work far exceeding the previous experience in CMP profiling along rivers (Ob’, Yenisei, Lena, Vakh, Angara, Chunya, Biryusa, etc.), as well as by the highest-latitude location of the W–E profile as part of the East Siberian regional network. Most of the profile traverses thick Triassic basalts which poses problems to acquisition and impairs data quality (Fig. 1). In spite of the difficulties in field surveys along rivers, we have come to acquire high-quality seismic profiles more informative than most of other regional and local seismic profiles collected in the area (Fig. 2). Our results prove that the advanced technologies of field acquisition and processing can provide data of better quality than in the previous years.
Seismic stratigraphy and updated section structure * Corresponding author. E-mail address: [email protected] (A.S. Efimov)
This paper focuses mainly on data from the western flank of the Lower Tunguska profile interpreted along 140 km
1068-7971/$ - see front matter D 201 7 , V . S. So bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.rgg.2016.09.021 +
452
A.S. Efimov et al. / Russian Geology and Geophysics 58 (2017) 451–460
between the Erachimo River mouth and Turukhansk Village (Fig. 3). The seismic data were checked against well logs from Goloyarskaya 1 and Tungusskaya 1 (reference) holes in the eastern flank of the profile which strip, respectively, Riphean sediments at a depth of 1037 m below the ground surface and the Lower Cambrian Deltula Formation at a depth of 2751 m (the hole bottom is situated at 2945 m below the surface). Therefore, it is impossible to obtain unambiguous stratigraphic constraints of the reference reflectors B (Vendian top) and R0 (Vendian base). The seismic section of the eastern profile flank includes the reflector R0 traced according to amplitude peaks and inclined clinoform sets of older Riphean sequences that reach R0 from below and produce a pattern resembling a cutout (Payton, 1977) of the eroded Riphean surface (Fig. 4). For the lack of wells in the western flank of the profile, the correlation was with land geological data: a 1:200,000 geological map and cross sections of outcrops obtained in the course of the field surveys of 2008 (by S.V. Frolova, G.G. Akhmanova, M.K. Ivanova, E.V. Kozlova, and O.V. Krylova from Moscow University and T.R. Kudrina and M.V. Tabatchikov from SNIIGGiMS). See Fig. 5 for an example of stratigraphic correlation of reflectors with reference to geological surveys. According to the stratigraphic model of 1983 (Resolution…, 1983), Riphean rocks in the area belong to the Upper Riphean Tungusik Group (Durnoi Mys, Rechka, Turukhan, Miroedikha, Shorikha, Lower Tunguska (Burovoi), and Derevnya Formations) and the Middle Riphean Sukhoi Pit Group (Sukhaya Tunguska, Linok, and Strelnye Gory Formations). The overlying Upper Riphean Oslyany and Taseeva Groups are missing from the section, and the Middle Riphean Strelnye Gory Formation appears to lie on the Archean–Proterozoic basement. The known Riphean stratigraphy of the area is controversial: Mel’nikov et al. (2009) attribute the Sukhaya Tunguska, Linok, and Strelnye Gory Formations to the Upper Riphean, while the 1:2,500,000 Geological Map of 1980 shows Lower Riphean strata exposed within the Turukhan Uplift. The Upper Vendian–Lower Cambrian Platonovka Formation represents the Vendian section. It reaches a thickness of 400 m and consists mainly of dolomites, which are locally argillaceous and anhydrite-bearing, with scarce marl and sandstone interbeds. The Cambrian Kostino Formation, 1700 m of total thickness, is composed of dolomites, often organic-bearing, with limestone interbeds. The Cambrian Letnyaya Formation of limestone and purple mudstone and marl crops out in the zone of faults. The best differentiated Upper Riphean sediments form several seismic sequences. The youngest one (R3drn-mr) includes the Durnoi Mys, Turukhan, Rechka, and Miroedikha Formations composed of dolomites with sporadic stromatolites and variegated mudstones intercalated with limestone and marl. The observed total thickness is 280 m, but may reach 605 m according to the 1983 MSK stratigraphy.
The R3drn-mr sequence overlies more than 1400 m (mapped thickness) of dolomites with chert and clastic intercalations (R3sr-nt) that belong to the Shorikha and Lower Tunguska (Burovoi) Formations. The underlying R3drv sequence, of about 500 m, corresponds to the Derevnya Formation dolomites. The R2–3stn-ln sequence below, of 800 m total thickness, represents the Sukhaya Tunguska and Linok Formations composed of dolomites with chert and limestone intercalations. The oldest Strelnye Gory Formation has a mainly clastic composition: mudstone, marl, clayey limestone, and light gray quartz and feldspar-quartz sandstones. It is almost impossible to discriminate these sequences according to wave patterns. They all produce prominent strong events, which sometimes grade laterally into discontinuous often hummocky reflections, while the wave energy reduces abruptly. Such a pattern may result either from the presence of faults or from facies change. The wave amplitudes decrease near the ground surface, within the Riphean section, and the record even becomes chaotic, possibly, due to faults or to disintegration of rocks by weathering. Note especially that the reflectors remain continuous and strong within the near-surface Vendian–Cambrian interval (Kostino and Platonovka Formations). The interval corresponding to the Kostino and Platonovka Formations in the western flank of the profile (to well Goloyarskaya 1) includes long, quite strong and relatively high-frequency reflections, but unambiguous correlation of R0 is problematic for the lack of stratigraphic ties at depths, compositional similarity of Riphean and Cambrian rocks, and complex tectonics of the area. A high-amplitude wave, most likely, a reflection, appears about 20 ms. Reflections along this seismic interface form a pattern of erosion-like clinoform features, and reflectors appear to emerge almost vertically in places of large faults. This interface may correspond to a seafloor surface. The interpretation results indicate that prominent reflections record Riphean carbonates. This pattern is consistent with bedding observed in outcrops and with clastic-carbonate alternation.
Imaging faults in the seismic section The western flank of the profile images four well-pronounced large thrusts, as well as small faults which are too numerous to be shown all in the section. The westernmost thrust (sometimes called Durnoi Mys fault) separates the Turukhan and Durnoi Mys blocks and has its left wall thrown down and overlain by Jurassic sediments. There is a blind zone along the fault, which goes from the top to the bottom of the section between 0 and 20 km in the end of the profile running northward along the Yenisei River. The zone may trace the western edge of the Siberian craton, while the block as a whole is a synclinal fold filled with Riphean sediments spanning a stratigraphic range from the Durnoi Mys to Strelnye Gory Formations.
Fig. 1. Location of seismic reflection profile along the Lower Tunguska River in 1:2500,000 Geological Map (VSEGEI, 1980). 1, Quaternary; 2, Upper Cretaceous; 3, Lower Cretaceous; 4, Lower Jurassic; 5, Middle Ttriassic; 6, Lower Triassic; 7, Permian, undifferentiated; 8, Upper Permian; 9, Lower Permian; 10, Upper Carboniferous; 11, Middle Carboniferous; 12, Lower Carboniferous; 13, Lower and Middle Devonian; 14, Upper Silurian; 15, Lower Silurian; 16, Upper Ordovician; 17, Middle and Upper Ordovician; 18, Lower Ordovician; 19, Upper Cambrian; 20, Middle Cambrian; 21, Lower Cambrian; 22, Vendian; 23, Middle Riphean; 24, Lower Riphean; 25, granite intrusions; 26, gabbro-dolerite intrusions; 27, seismic profile along Lower Tunguska (2014); 28, study area.
A.S. Efimov et al. / Russian Geology and Geophysics 58 (2017) 451–460 453
454
A.S. Efimov et al. / Russian Geology and Geophysics 58 (2017) 451–460
Fig. 2. Time sections, compared. a, Fragment of seismic section along Lower Tunguska (processed at SNIIGGiMS); b, fragment of seismic section along profile 034691 (processed at a third-party institution).
The Strelnye Gory fault, with its western wall uplifted and thrust over the eastern one, separates the Turukhan and Golyi Yar blocks. The oldest Riphean Strelnye Gory Formation and the Vendian–Cambrian Platonovka Formation rocks crop out, respectively, on the left and on right of the fault. According to field data, rocks dip steeply at 70º–90º in the thrust zone
and at a lower angle (10º–40º) outside of it. The western half of the Golyi Yar block is a syncline composed of the Vendian–Cambrian Kostino and Platonovka Formations near the surface. The Strelnye Gory fault and the junction of Riphean and Vendian rocks are well evident in outcrops along the Lower Tunguska River. In the seismic image, the fault
A.S. Efimov et al. / Russian Geology and Geophysics 58 (2017) 451–460
455
Fig. 3. Fragment of the western part of the profile along the Lower Tunguska River in 1:1,100,000 Geological Map (VSEGEI, 1990). 1–32, age of rocks: 1, Lower Cretaceous (Aptian–Albian, Yakovlevo Formation); 2, Upper Jurassic; 3, Middle Jurassic; 4, upper Upper Permian; 5, lower Upper Permian; 6, Lower Triassic (Putorana sequence); 7, Lower Triassic (Dvurogino sequence); 8, Lower Triassic (Nadezhda Formation); 9, Lower Triassic (Khakanchan Formation); 10, Lower Triassic (Syverma Formation); 11, Upper Silurian; 12, Upper Silurian (Pridoli, Pankagir Formation); 13, Upper Silurian (Ludlow, Kongda Formation); 14, Lower Silurian (Wenlock, Mukten Formation); 15, Lower Silurian (Llandovery, Uguyuk Formation); 16, Middle Ordovician; 17, Middle Ordovician (Mangaseya, Zagornaya Formation); 18, Middle Ordovician (Krivolutsk stage, Angir, and Anarakan Formations); 19, Lower Ordovician; 20, Lower Ordovician (Chunya, Guragir Formation); 21, Lower Ordovician (Ust’-Kut–Chunya stages, Iltyk Formation); 22, Lower Ordovician (Ust’-Kut stage, Uigur Formation); 23, Upper Cambrian; 24, Upper Cambrian (Sak–Aksai stages, Kulyumba Formation); 25, Upper Cambrian (Ayusokkan stage, Orakta Formation); 26, Lower–Middle Cambrian (Bottomian, Toyonian, Amga stages, Shumnaya Group); 27, Lower Cambrian (Tommotian–Atdabanian, Krasnyi Porog Formation); 28, Upper Vendian (Platonovka Formation); 29, Upper Vendian (Graviy Formation); 30, Upper Vendian (Taseeva sequence, Iluchina Formation); 31, Upper Vendian (Turukhan Formation); 32, Upper Vendian (Shorikha Formation); 33–40, intrusions: Middle Triassic carbonates (33), V phase dolerite intrusions (34), Kureika-type IV phase intrusions (35), Katanga-type III phase intrusions (36), Tymer- and Letnyaya-type II phase intrusion (37), Late Permian Ergalakh-type I phase intrusions (38), alkaline syenites and porphyritic syenites (39), subalkaline gabbro-dolerite (40); 41, mainly extrusive (a) and tuff (b) mafic volcanic rocks; 42, contact hornblende; 43, observed (a) and inferred (b) boundaries of geological and lithological units; 44, observed faults of uncertain slip geometry; 45, river network; 46, seismic profile along Lower Tunguska (2014); 47, piston core holes; 48, deep holes.
shows up as quite a large 4–6 km wide zone of irregular reflections, i.e., a zone of disintegrated rocks (Fig. 6). Like the Strelnye Gory fault, the Voronovo fault east of it has an uplifted western wall thrust over the eastern wall; the oldest Riphean Strelnye Gory Formation, and the Cambrian Letnyaya Formation rocks crop out on the left and right of its axis. The eastern fault wall is an anticlinal fold with the Kostino Formation exposed in its hinge. The Voronovo fault plane shows up as a prominent reflector almost reaching the surface, unlike other dipping interfaces. It looks like an unconformity, with other reflectors on the right joining it at an angle about 0º. The fault remains hidden under sod in outcrops along the Nizhnyaya Tunguska but is exposed along the Sukhaya Tunguska which flows into the Yenisei 22 km further south (Fig. 6). The different seismic images of the two faults may result from the fact that the Voronovo fault plane dips at a lower angle (about 40º) than the Strelnye Gory one. Numerous dolerite intrusions crop out east of the Voronovo fault but they are irresolvable and impossible to trace in the seismic section where they are obscured by a dense network
of faults, while no subsurface ties of the intrusions are available. The Imangda–Letnyaya fault east of well Goloyarskaya 1, with its uplifted western wall thrust over the eastern wall, marks the boundary between the Kureika–Baklanikha Uplift and the Kureika Basin. The rocks exposed west and east of the fault are, respectively, the older Letnyaya and Kostino Formations and younger Silurian sediments. Another large fault east of the Imangda–Letnyaya fault likewise has its uplifted western wall thrust over the eastern one. There are no thrusts in the eastern flank of the profile, where sediments are plane-bedded. As noted above, the reflector R0 follows the peak amplitudes, and other dipping reflectors join it from below making an erosion-like clinoform pattern. At earlier times, there is a train of strong and long reflections, with its top corresponding to the Ordovician surface. Reflectors in the western flank of the profile, which is composed of the Vendian–Cambrian Kostino and Platonovka Formations, are weaker than those in the east where the Bakhta and Suringdakon zones represent, respectively, Vendian and
Fig. 4. Seismic geological model of the Turukhan zone of deformation. a, Seismic-geological cross section along the Lower Tunguska River; b, time section along the Lower Tunguska River. 1, reflectors and their symbols; 2, tentative boundaries of seismic sequences (in blind zone); 3, faults and their names; 4, zone of irregular reflections; 5, reflector (presumably seafloor); 6, facies change; 7, geological − upl-lt), Kostino (C − kst), Kostino–Platonovka (V–C − kst-pl), Platonovka (V–C − pl), Durnoi Mys–Miroedikha (R3drn-mr), Shorikha–Lower survey route shown in Fig. 5. Formations: Ust’-Pelyatka–Letnyaya (C Tunguska (R3sr-nt), Derevnya (R3drv), Sukhaya Tunguska–Linok (R2–3stn-ln), Strelnye Gory (R2–3str).
456 A.S. Efimov et al. / Russian Geology and Geophysics 58 (2017) 451–460
Fig. 5. Example of stratigraphic correlation of seismic reflectors with outcrops. Lithology: 1, limestone; 2, dolomite; 3, marl; 4, sandstone; 5, claystone; 6, siltstone; 7, limestone and clayey dolomite; 8, limestone and brecciated dolomite; 9, limestone and algal dolomite; 10, limestone and clay-bearing dolomite; 11, limestone and oolitic dolomite; 12, limestone and cavern dolomite; 13, dolomitic limestone and limy dolomite. Other symbols: 14, pyritization; 15, bitumen occurrences; 16, silicification; 17, blue schist; 18, calcite inclusions; 19, faults; 20, dip and strike; 21, observation and sampling sites; 22, geological survey line; 23, seismic reflectors. Formations: Strelnye Gory (PR3str), Linok (PR3ln), Sukhaya Tunguska (PR3sh), Derevnya (PR3dr), Lower Tunguska (Burovoi) (PR3nt), Shorikha (PR3sr), Miroedikha (PR3mr), − 1pl), Kostino (C − 1–2ks), Letnyaya (C − 2lt). Durnoi Mys (PR3dm), Platonovka (PR3-C
A.S. Efimov et al. / Russian Geology and Geophysics 58 (2017) 451–460 457
458
A.S. Efimov et al. / Russian Geology and Geophysics 58 (2017) 451–460
Fig. 6. Strelnye Gory and Voronovo faults: seismic time section (A), a photograph of outcrops in the right side of the Lower Tunguska (Strelnye Gory fault, B), and a photograph of outcrops at the Sukhaya Tunguska (Voronovo fault, C), after Staroseltsev et al. (2012).
Cambrian strata separated by a facies boundary prominent in the wavefield. The Vendian section in the eastern profile part comprises the Katanga, Soba, and Tetere Formations and the Lower Precambrian rocks belong to the Yaseng, Moktakon, Mara, Abakun, Burus, Suringdakon, Bulai, Deltula, and Tanachi Formations. The position of the reflector H attributed to the base of the Letnyaya Formation can be checked against logs of reference well Tungusskaya 1. The Paleozoic section in the eastern profile flank includes multiple intrusions, of which twelve were stripped in Tungusskaya 1. One intrusion was found in Cambrian rocks of the Deltula Formation 60 m above the hole bottom but it is poorly detectable in the seismic section because the well is bounded by faults in the west and east and falls into a 4 km wide deformed zone. The sediments above show a complex wave pattern free from long prominent events.
Wavefield features The seismic image of the Letnyaya block contains a feature which may record a reef buildup within the interval of youngest Riphean sediments in the area (Durnoi Mys, Turukhan, Rechka, and Miroedikha Formations). It appears as a
series of asymmetric domes upon a flat surface, with one slope steeper than the other. The reflections inside the domes are discontinuous and hummocky, of low energy, with several oblique reflectors at the lower-angle slope. Such seismic features are typical of carbonate organic buildups. However, no bioherms have ever been reported for the plane-bedded Durnoi Mys, Turukhan, Rechka, and Miroedikha Formations. The reason may be that either large bioherms remain yet undetected in these younger formations or that the Shorikha rocks lie under the Platonovka Formation in the Golyi Yar structure. Other possibilities may be correlation or processing errors, or some nongeological causes. The idea of wedging Riphean sediments is consistent with a clinoform seismic pattern looking like an erosion cutout near well Goloyarskaya 1, with the wedges corresponding to Riphean sequences that accrete to the reflector R0 (Vendian base) from below. The Riphean rocks become thinner eastward along the profile and produce a characteristic wavefield pattern recorded, specifically, at point 121 km. This zone may be the boundary of the thinning Riphean sediments, while Vendian rocks east of the boundary may lie directly upon the basement. However, strong reflections and long events are prominent further east beneath R0 and gradually wedge out as well. These features do not look like multiple waves, and the precise
A.S. Efimov et al. / Russian Geology and Geophysics 58 (2017) 451–460
459
Fig. 7. Seismic image of possible reef buildups in a paleosection (brought to the reflector tentatively attributed to the Miroedikha Formation base).
location of the boundary of thinning Riphean sequences remains unknown. The train of strong reflections attributed to Riphean sediments deepens in the western direction and bifurcates, with its upper part pinching out (reflections disappear) and the lower part reaching a depth of 14,500 m. The wave pattern in the western flank of the profile is hard to interpret. The total thickness of Riphean sediments in the Turukhan region was previously estimated to approach 5 km. Our results show a seismically transparent zone (void of significant reflections), presumably, the basement, beneath the reflections identified with the Strelnye Gory Formation. Note that long and strong events reappear at the depths of 9 to 14 km and may be some discontinuity in the Archean–Proterozoic strata. However, this train of reflections grades into the Riphean reflections and accretes to R0 in the east. Therefore, the deep structure of the region is worth further investigation. The origin of the reflectivity zone in the 9–14 km interval, which is underlain by another transparent zone, remains unclear at the present stage of knowledge.
Conclusions 1. The advanced acquisition and processing technologies provide seismic data of a much better quality than in the previous years.
2. The encouraging results we have obtained allow planning future surveys in the central Kureika Basin along river valleys within the Putorana plateau. 3. The stratigraphy of seismic reflectors has been updated with reference to land-based geological data. 4. The Strelnye Gory fault shows up in the wavefield as a broad (4–6 km) zone of chaotic reflections. 5. The Voronovo fault produces a prominent reflector which almost reaches the surface, unlike other dipping reflections. Other reflectors accrete to the unconformity of the fault-related reflector at angles of about 0º making a clinoform pattern. 6. The seismic section contains anomalies looking like reef buildups within the Riphean sequences (Durnoi Mys, Turukhan, Rechka, and Miroedikha Formations). 7. Our surveys reveal a seismically transparent zone beneath the reflections attributed to the Strelnye Gory Formation, while long and strong reflection events reappear at the 9–14 km depths and rise gradually in the eastward direction to reach the reflector R0 (Vendian base). References Bazhenova, T.K., Kazais, V.I., 2011. The history of petroleum generation and accumulation in the northwestern Siberian craton (evolution of chemistry and structure of reservoirs). Neftegazovaya Geologiya. Teoriya i Praktika 6 (2), http://www.ngtp.ru/rub/1/17_2011.pdf. Kazais, V.I., 2006. The northwestern margin of the Siberian craton: A new tectonic model from geophysical (seismic–gravity–magnetic) modeling. Geologiya Nefti i Gaza, No. 5, 52–61.
460
A.S. Efimov et al. / Russian Geology and Geophysics 58 (2017) 451–460
Kozlov, G.V., Votakh, O.A., Alexandrov, V.S., 1988. Typical Precambrian formations in the Turukhan and Igarka uplifts, in: Tectonics of Cratonic Terrains [in Russian]. Nauka, Novosibirsk, pp. 9–51. Mel’nikov, N.V., 2009. A Vendian–Cambrian Salt-Bearing Basin of the Siberian Craton: Stratigraphy and History [in Russian]. Izd. SO RAN, Novosibirsk. Mikutsky, S.P., Petrakov, V.U., 1961. Tectonics of the Yenisei side of the Siberian craton, in: Geology, Geophysics, and Mineral Resources of West Siberia [in Russian]. Leningrad, pp. 46–58.
Payton, Ch.E. (Ed.), 1977. Seismic Stratigraphy: Applications to Hydrocarbon Exploration. AAPG Memoir, Volume 26. Resolution of the Stratigraphic Workshop on Precambrian, Paleozoic, and Quaternary, 1983. Part I: Upper Precambrian and Lower Paleozoic [in Russian]. SNIIGGiMS, Novosibirsk. Staroseltsev, V.S., Divina, T.A., Muratov, M.I., Staroseltsev, K.V., 2012. Discordant structures of the Turukhan horst as a feature of global-scale tectonics. Geologiya i Mineralno-Syr’evye Resursy Sibiri 4 (12), 3–7.