G&G Studies - Northern Siberia
The Northern Siberia Geology and Hydrocabon Systems: Project and the First Results
A.Khudoley1, V.Verzhbitsky2, A. Prokopiev3, E.Frantzen4, M.Tuchkova5, A. Egorov6, G. Serkina7, D. Vasiliev3, M.Rogov5, D.Zastrozhnov1, A.Li1
Geological map of the northern Siberia
The northern Siberia occupies a wide area with several sedimentary basins with high hydrocarbon potential. The most important tectonic domains are (Figure 1):
- Siberian craton with crystalline basement exposures (Anabar shield and Olenek uplift) and Mesoproterozoic to Mesozoic sedimentary cover;
- Enisey-Khatanga and Anabar-Lena depressions filled in with Mesozoic clastic rocks;
- Mesozoic Priverkhoyansk foredeep sedimentary basin filled in with clastic rocks;
- Taimyr, Olenek and Verkhoyansk fold belts;
- Grabens filled with Paleogene clastic rocks.
Sedimentary successions of the Siberian craton, Enisey-Khatanga and Anabar-Lena depressions as well as Priverkhoyansk foredeep basin contain both source rocks and collectors. Taimyr, Olenek and Verkhoyansk fold belts contain sedimentary and magmatic rocks varying in age from Mesoproterozoic to Mesozoic and show a multistage deformation history with the main compressional event in late Mesozoic.
The Northern Siberia Project is carried out by geologists from several geological institutions from St. Petersburg, Moscow, Yakutsk and TGSNOPEC Geophysical Company. The project is focused on the geological and hydrocarbon system study of sedimentary basins that span the Siberian craton from the north (Enisey-Khatanga and Anabar-Lena depressions) and east (Priverkhoyansk foredeep basin), as well as the Olenek fold belt.
Just to the north from the study area there is the opening of the Laptev Sea rift basin which is believed to be one of the most promising offshore areas for hydrocarbon discoveries, but is very poorly studied due to the absence of offshore wells and sparse grid of seismic lines. However, the geological map in Figure 1 clearly shows that tectonic structures of the Taimyr, Olenek and Verkhoyansk fold belts are cut by the Laptev Sea shoreline, and margins of the Laptev Sea rift sedimentary basin overlap onshore structures of fold belts. Rifting and formation of grabens in the near-shore areas are coeval with rifting and formation of the Laptev Sea rift system. Therefore, structural style and Mesozoic to Cenozoic sedimentary succession of the onshore tectonic domains reflects a complicated history of interaction between onshore tectonic domains and the Laptev Sea rift sedimentary basin, and onshore structural and sedimentary studies will provide more insight into the understanding of the Laptev Sea offshore rift basin evolution.
Key issues to be studied during the project implementation are:
- Late Proterozoic-Early Paleozoic geology and hydrocarbon systems of the northern margin of the Siberia craton;
- Late Paleozoic and Mesozoic geology, hydrocarbon systems and play elements;
- Tectonic events in the Olenek and Verkhoyansk fold belts;
- Main uplift/cooling/erosion pulses in the fold belts and adjacent parts of the Siberian craton;
- Main pathways of sediment supply into the Priverkhoyansk foredeep basin and the Enisey-Khatanga and Lena-Anabar depressions;
- History of the Paleo-Lena River drainage.
During decades of regional geological mapping and other geological studies carried out in 1950-1980 the study area was extensively studied in terms of stratigraphy and lithology. Therefore, our fieldwork is focussed on sampling the sedimentary succession to study collected samples using state-of-the-art equipment not available previously, and on studying sedimentary structures and structural elements to restore paleo-depositional environments and structural evolution of the region in accordance with modern geological concepts.
We believe that deciphering the tectonic history of the study will result in more accurate interpretation of offshore seismic data, as it may help to delineate seismic complexes and their inferred age, and we present here the main results of our structural studies.
One of the most important results of our fieldwork along the Olenek River was establishing an angular unconformity within Middle Triassic rocks, most likely between Ladinian and Anisian stages (Figure 2). This unconformity has neither been mapped previously, nor discussed in available publications. However, it points to a Middle Triassic tectonic event not documented earlier.
Interpretation of this tectonic event is not clear; it may represent both local compression and rifting-related block rotation. The unconformity was only recognized in outcrops located close to the mouth of the Olenek River, outside this area we did not see any evidence for angular unconformities within Triassic rocks.
Pre-Lower Jurassic unconformity was mapped throughout the project study area, and is typically represented by erosional contact at the base with an overlying conglomeratic unit. In the Tsvetkova Cape area regional-scale maps shows significant truncation of the Triassic section, and locally the Lower Jurassic conglomerates overly Lower Triassic rocks. Estimation of the pre-Jurassic erosion magnitude based on biostratigraphic correlations shows that for a distance of 6 km at least 300 m of Triassic rocks were eroded. This corresponds to an approximately 3º angular unconformity that should be recognized as a low-angle unconformity in the outcrop. However, detailed study of the relationship between the Lower Jurassic conglomerates and underlying Triassic rocks in several outcrops shows that beds in both units are parallel to each other with no evidence of truncation of the Triassic beds. Therefore, we interpret the pre-Jurassic tectonic event as a predominantly extensional one with a set of horst- and graben-like structures with highly variable amounts of erosion in adjacent blocks and an absence or a very low angular unconformity related to some block rotation (Figure 3).
The main compressional event occurred in the late Mesozoic era. This overprinted previously formed structures and created presently observed structural style represented by open to tight folds and thrust typical of the Olenek fold belt and the Tsvetkova Cape area. The latter is located on the boundary between the southern margin of the Taimyr fold belt and Enisey- Khatanga depression.
Timing of the main compressional event is not clear. In both the Olenek fold belt and the Tsvetkova Cape area no clear angular unconformities have been documented in the Jurassic – Lower Cretaceous section, although the intensity of folding is getting lower in the southern direction and from old rock to young. Local distribution of a thick Upper Jurassic conglomerate unit in the Tsvetkova Cape area points to significant erosion in the Taimyr fold belt. We interpret it as an evidence for the earliest stages of the late Mesozoic orogeny. In the Enisey-Khatanga and Anabar-Lena depressions transition from marine shale and sandstone facies to molasse-like units is not synchronous, but occurred in the late Early Cretaceous, likely marking the beginning of the late Mesozoic orogeny in the Taimyr and Olenek fold belts.
Post-orogenic extension has been identified in all study areas and extensional structures are superimposed on the general compression fabric. Most typical examples of post-orogenic extension are small-scale normal faults identified by slickenside striation study or by beds offset (Figure 4).
In the Tsvetkova Cape area normal faulting typically occurred along orogenicrelated thrusts and reverse faults, pointing to gravitational sliding along already formed fault surfaces. In the Olenek fold belt normal faults cut compressional structures and are likely to represent another extensional event.
The latest extensional event has been recognized in the Paksa Cape area. Here we identified several stages of deformation which are preliminary correlated with those in the Tsvetkova Cape area. However, in the Paksa Cape area all structures are cut by a regular system of north-south-trending joints with a well-preserved plumouse pattern on the surface which points to the predominance of extensional environments during the fracture system formation (Figure 5A,B). Joints are sub-parallel to the present-day shore. Therefore, we believe that the eastern coastline of the Paksa Cape is controlled by an offshore fault zone of extensional/transtensional kinematics. This fault system is parallel to normal faults forming the Laptev Sea rift system to the east from the study area. We interpret the north-south-trending joints on the east shore of the Paksa Cape as related to the westernmost normal faults of the Laptev Sea rift system (Figure 5C).
Figure 5C. Location of study area
Corresponding author: A.Khudoley and V.Verzhbitsky E-mail
References
Drachev, S.S., 2000. Tectonics of the Laptev Sea Rift System, Geotectonics, 6, 43–58 (in Russian).
Drachev, S.S., Savostin, L.A., Groshev, V.G. & Bruni, I.E., 1998. Structure and geology of the continental shelf of the Laptev Sea, Eastern Russian Arctic, Tectonophysics, 298, 357–393.
Petrov, O.V., editor-in-chief. 2004. Geological Map of Russia and Adjoining Water Areas, scale 1:2500000, VSEGEI and VNIIOkeangeologiya, St. Petersburg, 12 sheets (in Russian)