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Abstract

UDC (552.333.4:552.14):551.73(571.51)

INTRUSIONS OF DOLERITES ON TANACHINSKAYA AREA OF THE BAKHTINSKY MEGANOSE, WEST OF THE SIBERIAN PLATFORM

N. V. Melnikov

In deep and core wells drilled in the central part of the Bakhtinsky meganose intrusions of dolerites are ubiquitously stripped. Thicknesses of the intrusions vary from decimeters to a kilometer. Most of them are in the upper part of the sedimentary cover (Permian-Ordovician). It is common for the intrusions to change their stratigraphic position and thickness. In the middle of the cover they are isolated, more persistent over the territory and occur more often in saliferous-carbonate Cambrian formations. There are no intrusions in the lower part of the cover (Cambrian, Upper Vendian lowermost strata). Features of dolerite intrusion distribution are studied in the Tanachinskaya area in the Deltul river basin, the right inflow of the Bakhta. Nine Tanachinskaya and one Ust-Deltulinskaya wells 1,844-3,833m deep and fourteen core wells 1,065-1,495m deep are drilled there in the area of 240 sq. km. Four sections present the distribution of dolerite intrusions in wells. A rather complex distribution of dolerites is observed in the upper cover in wells down to 1,100 m marks. Over this interval of depths in each well intrusion thicknesses and their location in the stratigraphic section of Ordovician-Upper Paleozoic change abruptly. There are no persistent intrusions. Isopachs of intrusive bodies from 1,000 to 1,360 m form a narrow strip of a northeastern direction. That strip may be over channels by which magmatic melt penetrates into the sedimentary cover. In the middle of the sedimentary cover (Upper, Middle, the top of Lower Cambrian, 1,100–2,700 m mark interval) there are two-four intrusive sheets up to 450 m in total thickness. Isopach orientations are various. There is no connection with those in the shallow subsurface. The mentioned features of the distribution of dolerite intrusions allow one to assume that at the first stage of penetration the melt from the center of basaltic magma rises to the day surface under a high pressure. When the melt pressure exceeds lithostatic one at depths less than 1,000–1,400 m, magma lifts rocks and fills appearing cavities. The rise of sedimentary rocks lasts until the magmatic melt pressure does not decrease to lithostatic one. At the following stage the pressure in the magmatic chamber decreases and becomes lower than the pressure of the rocks overlying the chamber. In the middle cover there appear cleavage cracks close to layer-by-layer ones into which magma remaining in the chamber flows. Magma acts as hydraulic wedge and makes the cracks wider. Thus, intrusions in the top of the sedimentary cover tear it to blocks and lift them at various levels. Intrusions in the middle of the sedimentary cover were emplaced as a result of magma transition from the chamber to the cracks (interstices) close to layer-by-layer ones in the sedimentary cover. The interstices become wider as the rocks sink to the magmatic chamber.

Key words: dolerite intrusions, Tanachinskaya area, blocks of sedimentary cover, intrusion blocks, mechanism of intrusion emplacement.