ON THE INFLUENCE OF SPECIFIC GRAVITY AND CHEMICAL WEATHERING ON THE DISTRIBUTION OF HEAVY-FRACTION MINERALS IN THE MESOZOIC DEPOSITS OF THE PUDIN REFERENCE BOREHOLE (TOMSK REGION)

A. F. Senakulis

(Presented by Prof. A. V. Aksarin)

The study of the distribution of heavy-fraction minerals in the Mesozoic deposits penetrated by the Pudin reference borehole made it possible to establish that the curves of percentage content have a random character. This made it possible to treat them as time series, for the study of which mathematical processing was used, with determination of probabilistic, correlation, and spectral characteristics. The listed parameters made it possible to reveal features of the process of sediment accumulation from its random realizations.

The initial material consisted of data on the mineralogical composition of Mesozoic rocks penetrated by the Pudin reference borehole in 1956. The mineralogical investigations were carried out by staff members of the petrographic laboratory of the Novosibirsk Geological Administration under the direction of A. G. Pod. The number of samples used is more than 450; the average sampling frequency is 1 sample per 6–7 m. The position of the samples in the section is characterized by the fact that samples from sandy, aleuritic, and clayey rocks are distributed uniformly through the section. Moreover, analysis of changes in the percentage content of such minerals as garnet, apatite, tourmaline, rutile, anatase, and zircon shows that they behave in the same way through the section in different types of rocks. Thus, in the depth interval from 700–1800 m, all minerals in the lower part of the interval show a maximum in sandy, aleuritic, and clayey samples, while in the upper part it changes to a minimum in all types of rocks. This permits the conclusion that the specific realizations of the random process were not substantially influenced by sample selection or by rock types.

The specific realizations, represented by curves of change in the percentage content of heavy-fraction minerals through the section of Mesozoic deposits, have a periodically rhythmic character. This is clearly revealed in the spectral and correlation characteristics of the random functions. On the graph of the normalized correlation function it can be seen that, for example, in the zircon content through the section, periodicity is often visible, recurring every 60–70 m and every 290–320 m. A similar pattern is also found in the correlation functions of other minerals.

Approximation of the data on the percentage content of heavy-fraction minerals through the section of the Pudin reference borehole makes it possible to establish features of the manifestation of the process of sediment accumulation in the Mesozoic…

zoic time. On all curves of percentage content, two first-order rhythms are distinguished for the most stable minerals (quartz, zircon, anatase, apatite, tourmaline, garnet, rutile, fragments of siliceous rocks). The lower one begins with maximum amounts of the indicated minerals at the base of the sedimentary cover and ends with minimum values at the boundary between the Kiyalinskaya and Pokurskaya formations (late Hauterivian–Barremian time). The beginning of the upper rhythm is confined to the upper part of the Kiyalinskaya Formation; the rhythm ends at the top of the Ipatovskaya Formation, preceding the Campanian deposits. It should be emphasized that the maxima of each mineral are shifted in time relative to one another. Thus, the upper boundary is marked by anatase at a depth of 1680 m, and by rutile at a depth of 1610 m. The onset of the maxima for other minerals of the heavy fraction falls within this 70-meter interval.

Each rhythm is subdivided into 3–4 second-order rhythms. The maximum values characterize the second-order rhythms that begin a first-order rhythm, and the minimum values characterize those that end it. The second-order rhythms are subdivided into smaller ones; therefore the curves represent periodic oscillations.

Analysis of the curves makes it possible to reveal their genetic essence. The maximum values of the amounts of stable minerals formed under conditions of the most intensive reworking, by chemical weathering, of rocks in the source area; the minimum values formed under conditions of denudation of the least weathered rocks. Analysis of the maxima also makes it possible to distinguish changes in the intensity of chemical weathering during the formation of the Mesozoic deposits. The greatest intensity is confined to the time of accumulation of the Tyumenskaya, Naunakskaya, and Kulomzinskaya formations of the lower rhythm, and to the upper part of the Kiyalinskaya and lower part of the Pokurskaya formations of the upper rhythm. In general terms, the data obtained confirm the cyclicity of Mesozoic sedimentation established by V. P. Kazarinov (1958), T. I. Gurova (1960), and others [1], [2].

Of interest is the similarity between the mineral-distribution curves and the values of apparent resistivity from the electric-log diagram, on which two maxima of AR values are recorded. One coincides with the base of the lower rhythm, the other with the base of the upper one. The appearance of AR maxima is a regular consequence of the greatest differentiation of material under conditions of maximum chemical reworking of rocks in the source area.

Study of the specific realizations shows that, along with similarity, they reveal noticeable differences. We have attempted to identify differences in the probability distributions of specific realizations and the factors operating in this process. The probability distribution of instantaneous values of random processes can be represented by the density and distribution function. The distribution density was determined from the relative residence time of a realization within a specified interval, and the distribution function from the relative residence time of a specific realization above a specified level [4]. The methods of analysis considered apply chiefly to stationary random processes possessing the ergodic property. In this connection, the entire described section was divided into four intervals: two correspond to the lower and upper maxima, and two to the upper and lower minima.

In each part of the section, within which a specific realization could be regarded as stationary and ergodic, the probability-distribution density was calculated, and from it the mode and amplitude were determined in normalized form. If the width of the interval of values of the percentage content of a mineral is taken as unity, then the mode shows—

shows in which part of this interval the random function remains longer. If the entire residence time of the function in a given interval is taken as unity, then from the amplitude we can determine the duration of the function’s residence at the modal value. In other words, the mode and the amplitude of the distribution-density graph show the interval of values on which the maximum residence time of the random function falls, and what the duration of its residence at this level is.

The curves of the contents of heavy minerals along the section are characterized by different values of the mode and amplitude of the probability density

Table

Principal parameters of the probability-density graphs for minerals in the Mesozoic deposits of the Pudin reference borehole

probabilities (table). We attempted to identify the factors determining the behavior of the minerals in the section. Analysis of the table shows that the maximum value of the mode characterizes the most stable minerals. In this connection, we attempted to trace the nature of changes in probability densities depending on the resistance of minerals to chemical weathering. It should be said that there is no single opinion in the literature on the resistance of minerals to chemical weathering [3], [5], which is explained by differences in the methods used to resolve this question. Nevertheless, a number of minerals—garnet—apatite—tourmaline—rutile—anatase—zircon—can be represented with a certain de-

Fig. 1. Graphs of the dependence of the mode of the probability-density curves on the resistance of minerals to chemical weathering. Legend: 1–4 are curves for different depth intervals; 1—from 400 to 1100 m—upper minimum; 2—from 1100 to 1700 m—upper maximum; 3—from 1600 to 2250 m—lower minimum; 4—from 2250 to 2940 m—lower maximum.

with certainty as a series from the least resistant minerals to the most resistant to chemical weathering.

In Figs. 1 and 2 graphs are given of the dependence of the mode and the amplitude in the indicated series for four parts of the section. The most distinct increase in mineral resistance is observed for the lower and upper minima of the percentage-content curves (Fig. 1). For the intervals of the section characterized by maximum mineral contents, this dependence is less distinct; here garnet, apatite, tourmaline, and rutile have almost the same modal value, and only for anatase, and especially for zircon, does its value increase sharply.

The change in amplitude in the indicated series has the opposite character (Fig. 2). It reaches its maximum value among the least resistant minerals, and its minimum among the most resistant. This dependence is most clearly manifested in the intervals with minimum mineral contents.

Fig. 2. Graphs of the dependence of the amplitude of the probability-density curves on the resistance of minerals to chemical weathering. Symbols as above.

It is of interest to compare the probability characteristics with the specific gravity of the minerals studied. Here a fairly clear relationship is observed only with the mode; moreover, the minerals are subdivided into two groups:

Fig. 3. Plot of the dependence of the mode of the probability-density curves on the specific gravity of minerals. The designations are the same.

zircon, anatase, tourmaline fall into one; garnet, apatite, rutile, and ilmenite with magnetite into the other (Fig. 3). In all the graphs one and the same relationship is established: the mode increases as the specific gravity of the minerals increases.

Thus, analysis of the densities of probability distributions shows that the character of the distribution of minerals along the section is influenced by the specific gravity of the minerals and by their resistance to processes of chemical weathering. The greatest influence of these factors is evident during the period of least intensity of chemical processes; during periods of maximum development of chemical weathering in the source areas, this relationship becomes obscured.

REFERENCES

1. Gurova T. I. On terrigenous-mineralogical complexes of the Mesozoic deposits of the West Siberian Lowland.—Proceedings of SNIGGIMS, issue 9, 1960.
2. Kazarinov V. P. Mesozoic and Cenozoic deposits of Western Siberia. Moscow, Gostoptekhizdat, 1958.
3. Logvinenko N. V. On the question of sedimentary differentiation of matter.—In: On the State of the Science of Sedimentary Rocks. Publishing House of the Academy of Sciences of the USSR, 1951.
4. Mirskii G. Ya. Instrumental Determination of the Characteristics of Random Processes. Moscow, “Energiya,” 1972.
5. Reference Manual on the Petrography of Sedimentary Rocks. Leningrad, Gostoptekhizdat, 1958.