Crater formation and erosional truncation of Arkhangelskaya kimberlite pipe (Russia)

The Late Devonian kimberlite pipes Arkhangelskaya, Pionerskaya, named after Grib, as well as a number of other pipes, have full sections of crater tufogenic and terrigenous deposits in the Zimneberezhny diamondiferous region, the thickness of which is 120—127 m. The remains of the erosion bell, which was denuded at the stage of regional erosion in the Middle Carboniferous, are characteristic of the Arkhangelskaya pipe. Based on the systematic documentation of the walls of the pilot industrial pit of the Arkhangelskaya pipe, the authors have calculated the initial height of the bell (92 m). The calculation is based on the equality of the volume of host rocks eroded within the annular bell and the volume of terrigenous material contained in the crater deposits. The sequence and kinematics of the formation of crater deposits were reconstructed for the Arkhangelskaya pipe. Four phases of the development of the maar (explosion) crater were identified, which occurred after the injection of kimberlite magma and pyroclastics into the volcano’s vent before reaching the paleosurface: 1 — accumulation of sandstones, 27 m thick, due to erosion of the Vendian socle in a 50-meter-long annular bell with subsidence of the base of the crater with an amplitude of 77 m; 2 — the accumulation of tufogenic and terrigenous deposits with a thickness of 67 m, it was formed under conditions of repeated volcanic activity and a tephra annular shaft with a total subsidence amplitude of 169 m; 3 — terrigenous sedimentation with a total subsidence amplitude of 219 m; 4 — regional erosion in the period from the Late Devonian to the Early Carboniferous and the blocking of the pipe by deposits of Middle Carboniferous and Quaternary system. The depth of regional erosion practically coincides with the estimated height of the eroded bell, equal to 92 m. For the Pionerskaya and Grib pipes, the estimated heights of the erosion bell are 92 and 103 m, respectively. The subsidence of their bases with amplitude of 212—223 m was required to accumulate crater deposits. During the phases of sharp subsidence the collapse of the walls of the crater leads to the formation of coarse deposits and «reefs». After the completion of gravitational erosion, the maar crater switches to the planar erosion mode with the accumulation of fine clastic and chemogenic lake sediments. The causes of subsidence could be the outflow of degassed magma down the crater, ejection of pyroclastic, and compaction of tuff-sedimentary material in the crater.

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