PALEOMAGNETIC AND PETROPHYSICAL STUDIES OF THE PALEOZOIC BASEMENT DEPOSITS OF THE TOMSK UPLIFT

Z. A. Myshko, Yu. N. Lapin, L. A. Lapina

(Presented by Professor D. S. Mikov)

The study area is located in the zone where the northern spurs of the Kuznetsk Alatau, the Kuznetsk Basin, and the Kolyvan–Tomsk folded zone meet. Rocks of various ages, from the Proterozoic through the Cenozoic inclusive, take part in the geological structure of the area. The oldest rocks of the Upper Proterozoic occur among gabbroids as isolated bodies of marbles with interbeds of sandstones.

Undivided Cambrian deposits are represented by gabbro-diabase porphyrites, tuffs, and crystalline schists. On the eroded surface of the Cambrian rocks lie Devonian effusive-sedimentary rocks \((D_1)\), assigned to the Krasnogorsk Formation, composed of sandy-shaly rocks with lenses of porphyrites and tuffs.

The undivided Lower–Middle Devonian \((D_{1-2})\), 1000 m thick, is represented by the Omutninsk effusive-tuffaceous formations of the basaltic plagioclase-porphyrite type, tuff sandstones, and siltstones.

The undivided Middle–Upper Devonian, distinguished as the Pachinskaya Formation \(D_{2-3}\hat{p}\hat{s}\), is composed of a sequence of clayey shales and sandstones, with phyllites predominating in the lower part. The Pachinskaya Formation has a thickness of 1500 m.

In some parts of the area, Upper Devonian deposits overlie the Omutninsk sequence; they are represented by the Frasnian stage, the Yurginskaya and Podoninskaya formations.

The Frasnian stage, 800 m thick, is composed of limestones, marls, siltstones, and argillites. The Yurginskaya Formation includes sandstones, clayey and marly shales with rare interbeds of limestones. Deposits of the Podoninskaya Formation include conglomerates, siltstones, argillites, sandstones, gravelites, and, rarely, limestone horizons.

The Upper Devonian sediments are conformably overlain by deposits of the Tournaisian stage \((c, t)\), represented by clayey shales and limestones. The Visean stage \((c, v)\) is subdivided into the Lagernosadskaya \((c, lg)\) and Basandaika \((c, bs)\) formations. The Lagernosadskaya Formation is composed mainly of a sequence of clayey shales, while the Basandaika Formation is represented by interbedded sandstones, siltstones, and clayey shales.

All the above-mentioned deposits are folded into asymmetric folds of various orders, with a northeastern strike of the axes and limb dips of 25–45°. A large number of faults are encountered.

disturbances with fault-plane dip angles of 40–80°. The intrusive complex is represented by dikes of diabases, dolerites, and Triassic monzonites. Early Devonian dikes of microdiorites, gabbro, and Cambrian hyperbasites have been established.

Determining the age, subdivision, and correlation of the deposits of the Tomsk uplift under conditions of a complex geologic-tectonic setting requires the use of reliable criteria. In order to use and study paleomagnetic-petrophysical properties as one of these criteria, we selected 227 oriented samples from bank outcrops of the Tom River, exposing Devonian and Carboniferous deposits, as well as 483 core samples from boreholes exposing almost all stratigraphic subdivisions of the area.

In the laboratory of physical properties of TPI, the density—$\sigma$, magnetic susceptibility—$\chi$, and residual magnetization of rock samples $J_n$ were measured; for oriented samples the azimuth—$A_{\mathrm{cp}}$ and inclination—$J^\circ_{\mathrm{cp}}$ of the residual magnetization were determined. The magnetic properties of the samples were measured on an MA-21 astatic magnetometer by the standard method. The accuracy of the determinations of $\chi$ was 8.7%, and of $J_n$—15.7%. Rock density was measured on second-class VNI-1 balances by hydrostatic weighing with an accuracy of 0.02 g/cm$^3$.

The obtained parameters $\sigma$, $\chi$, $J_n$, $Q=\left(\dfrac{J_n}{J_c}\right)$ were plotted on geologic sections for boreholes and bank outcrops. Mean statistical values are given in the table.

Table

IV Middle and Upper Devonian \(D_{2-3}\)

V Upper Devonian \(D_3\)

VI Lower Carboniferous \(C_1t\)

Triassic T

According to the data in the table, the sandy-clayey deposits of the Devonian and Carboniferous, phyllite-like shales, marbles, and Cambrian gabbro possess weak magnetic properties, with variations of \(\chi\) from \(12 \cdot 10^{-6}\) CGS to \(60 \cdot 10^{-6}\) CGS and \(J_n\) from \(0.5 \cdot 10^{-6}\) CGS to \(20 \cdot 10^{-6}\) CGS. The exceptions are the conglomerates of the Upper Devonian and the lower Carboniferous, in which \(\chi\) reaches \(700 \cdot 10^{-6}\) CGS, and \(J_n\)—\(100 \cdot 10^{-6}\) CGS. Higher magnetic parameters \(\chi\) \((1500—2200) \cdot 10^{-6}\) CGS and \(J_n\) \((300—2200) \cdot 10^{-6}\) are characteristic of effusives of the Cambrian, Devonian, and Triassic—porphyrites and dolerites. The parameter \(Q\) for all the rocks studied does not exceed 1, except for the Triassic dolerites \((Q = 1.8)\).

Among the weakly magnetic gabbro, varieties with \(J_n = 124 \cdot 10^{-6}\) CGS and \(\chi = 862 \cdot 10^{-6}\) CGS stand out; they give rise to characteristic sharply differentiated curves of \(\chi\) and \(J_n\) along the section of the Cambrian rocks. The Podoninskaya Formation, in comparison with the underlying and overlying Yurginskaya and Lagernosadskaya formations, owing to the presence in it of frequent interbeds of magnetic conglomerates, is marked by distinctly differentiated curves of \(\chi\) and \(J_n\) against an overall elevated background. The same character is inherent in the curves of \(\chi\) and \(J_n\) for the volcanogenic-sedimentary deposits \(D_1\) and \(D_{1-2}\), which is associated with the presence in them of frequent lenses of porphyrites.

From the distributions of the directions of \(J_n\) in rock samples \(D_{2-3}pc\), \(D_3jr\), and \(C_1lg\) and \(C_1bs\) from riverbank outcrops, a general normal polarity has been established—

…variable character. Against this background, two zones of direct magnetization are clearly distinguished in the Pachin Formation. The \(J_n\) vectors of the first direct zone are grouped in the southeastern octant with \(A_{\mathrm{av}} = 113^\circ\) and \(J_{\mathrm{av}} = 43^\circ\). The second zone is characterized by grouping of \(J_n\) in the northwestern octant with \(A_{\mathrm{av}} = 300^\circ\) and \(J_{\mathrm{av}} = 35^\circ\). The coordinates of the pole calculated for these zones are, respectively, \(\omega_1 = 38^\circ\) N, \(\lambda_1 = 201^\circ\) E, and \(\omega_2 = 10^\circ\) N, \(\lambda_2 = 142^\circ\) E, which corresponds in time to \(D_{2-3}\).

In terms of density, the rocks are only weakly differentiated. The densest are the Cambrian deposits—\(2.82\)–\(2.92\ \mathrm{g/cm^3}\)—and the least dense are the Lower Carboniferous rocks—\(2.4\ \mathrm{g/cm^3}\). The remaining rock varieties are characterized by an average density of \(2.7\ \mathrm{g/cm^3}\).

Analysis of the results of paleomagnetic–petrophysical studies of rocks of the Tomsk uplift provides grounds for noting the following:

1. The presence of a noticeable difference in magnetic properties:

1) between rocks of the Podonin Formation—\(D_{?pc}\)—and the underlying deposits \(D_3\);

2) between rocks of the Pachin Formation—\(D_{2-3pc}\)—and the volcanogenic-sedimentary deposits \(D_{1-2}\) that underlie it.

1. Effusive rocks assigned by age to the Triassic differ from Devonian and Cambrian effusive rocks in having higher remanent magnetization and the parameter \(Q\).

2. In terms of the direction of the remanent-magnetization vector, two zones of direct magnetization occur in the rocks of the Pachin Formation, corresponding in time to \(D_{2-3}\).

The features noted above may be used for subdivision and correlation of the deposits studied, as well as for estimating the age of the effusive rocks and deposits of the Pachin Formation. Therefore, it is advisable to apply the method of paleomagnetic–petrophysical investigations and kappametry in core-drilling wells in conjunction with geological exploration work.