General Geology of the Popigai Crater

The 100-km-diameter Popigai impact structure (Masaitis et al. 1980, 1998, 1999, and references therein) formed 35.7 Ma ago (Bottomley et al. 1997) at the northeast edge of the Anabar Shield, Northern Siberia. The crater was excavated within Archean crystalline rocks with overlying Proterozoic to Permian sedimentary cover with a thickness of up to 1.5 km. The crater is filled by various kinds of lithic breccias and impact melt rocks - both fragmental (suevites) and massive (tagamites) (Fig. 1). The total volume of impact rocks has been suggested to exceed 5000 km3, and impact melt rocks contribute up to 2800 km3.

The impact lithologies are subdivided into three main units with exceptionally variable thickness and strongly gradual transitions, from bottom to top in the vertical section: (1) a lower unit, which contains polymict megabreccia composed of fragments of sedimentary and crystalline target rocks, and crystalline megabreccia composed of fragments of shocked crystalline target rocks; (2) a middle unit, which contains a complex blanket of impact melt rocks, including thick (up to 600 m) tagamite sheets and overlying vitric and

Fig. 3. Panoramic view of a continuous bluff of impact rocks (with a height of 120 m), which forms the right side of the Chordu-Daldyn River valley, 11 km from the mouth. The bluff traces a concentric fault system in the southern part of the crater.

Fig. 4. Schematic geologic map of the Balagan River Basin, southwestern sector of the Popigai crater, compiled from deep drilling data. In the southeastern part of this area a 200-m-thick coptoclastite cover overlying tagamite has been mapped from 34 drillcores, and indicates subsidence of the southeastern block along a radial fault system.

Fine-grainEd lithic breccia

Shocked Arc ties n basement (peat ring]

Fine-grainEd lithic breccia

Fig. 4. Schematic geologic map of the Balagan River Basin, southwestern sector of the Popigai crater, compiled from deep drilling data. In the southeastern part of this area a 200-m-thick coptoclastite cover overlying tagamite has been mapped from 34 drillcores, and indicates subsidence of the southeastern block along a radial fault system.

Shocked Arc ties n basement (peat ring]

■ Massive impact melt Prpterozaic larget

■ Suevite Faults Allogenic megabreccia crystal-vitric suevites, which are composed mainly of black glass fragments (vitroclasts) and fragments of shocked crystalline rocks and their minerals; (3) an upper unit containing vitric-lithic suevites, which are composed mainly of greenish-gray vitroclasts and fragments of sedimentary rocks and of finegrained lithic breccia; thin shallow tagamite bodies also occur.

The main elements of the inner structure of the Popigai crater are: (1) a central depression, 40 km in diameter, filled mainly by vitric-lithic suevites

Fig. 5. Image of fault deformations within an allogenic megabreccia field on an aerial photograph (scale 1: 25 000), in the southeastern part of the crater, the Ongtu-Yuregue River valley. The fractures and faults are revealed by linear drainage patterns, zones of abundant vegetation (dark colors), and boundaries between lithologies that differ in their colors. Some photo-interpretation of faults is shown. The location of the area is given in Fig. 1.

and fine-grained lithic breccia with a thickness of >1.5 km; (2) a peak ring of disturbed and shocked crystalline target rocks of 45 km diameter; it is exposed below the impactites and impact breccia in the NW sector of the crater (Majachika Upland), as well as in some other areas; (3) an annular trough 15 to 20 km wide, filled with an impact rock sequence with a thickness of up to 1700 m, including coarse allogenic breccia, impact melt rocks (suevites and tagamites), and fine-grained lithic breccia; (4) an outer zone of deformed crystalline and sedimentary target rocks, 10-15 km wide.

In the present topography, the impact structure is expressed as a 100- to 200-m-deep circular depression. The depression is characterized by radial and

Fig. 6. Image of fault deformations within a suevite blanket on an aerial photograph (scale 1: 25 000) in the western part of the crater, the Namsique-Daldyn River valley. Suevites are indicated by light-colored areas. Some photo-interpretation of faults is shown. The location of the area is given in Fig. 1.

Popigai Crater

Fig. 5. Image of fault deformations within an allogenic megabreccia field on an aerial photograph (scale 1: 25 000), in the southeastern part of the crater, the Ongtu-Yuregue River valley. The fractures and faults are revealed by linear drainage patterns, zones of abundant vegetation (dark colors), and boundaries between lithologies that differ in their colors. Some photo-interpretation of faults is shown. The location of the area is given in Fig. 1.

concentric features of its inner structure that are emphasized by drainage patterns and near-parallel semi-circular uplands consisting of tagamite in the western part. Impact rocks within the depression are partly covered by Pliocene and Quaternary continental deposits, which are up to 160 m thick, as indicated by drilling data. Remnants of Pliocene-Lower Quaternary sands and gravels up to 20-30 m thick occur mainly along the southern rim of the crater, whereas younger Quaternary deposits are widespread throughout the depression area.

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