Manifestation of Salt Tectonics in the Western Part of the Orenburg Region
Abstract Generated abstract
This paper examines small but pronounced structural depressions in Upper Permian deposits in the western platform part of Orenburg Oblast, where salt tectonics had previously been considered limited mainly to adjacent foredeep and depression zones. Using drilling and stratigraphic evidence from the Mustafino depression on the Sharlyk area, the authors argue that the depression is not caused by basement subsidence, since it is absent at the top of the underlying Iren horizon and is associated instead with reduced salt thickness in the depression center. They interpret these features as resulting from migration of salt from beneath the depressed area into adjacent zones after deposition of the Iren and overlying units. The study concludes that shallow negative or positive structures related to salt movement may not reflect deeper structures, so exploratory drilling should test horizons below the Iren horizon.
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Manifestation of Salt Tectonics in the Western Part of Orenburg Oblast
A. G. Bedin, L. A. Pukhlyakov
(Presented by Professor A. V. Aksarin)
In the western platform part of Orenburg Oblast, salt-bearing deposits are widely developed in Permian-age sediments. Despite this, the Upper Permian rocks occur relatively calmly, with a general dip in the southern and southwestern directions, complicated by low-amplitude zones of dislocations. It was formerly believed that salt tectonics was developed only in the extreme southeastern and southern parts of the oblast, in the zone of the Cis-Ural foredeep and the Caspian depression, where salt domes and similar formations are known. It turns out that salt tectonics, only in several other forms, is also manifested in the platform part of the oblast. Here there are a number of small in area, but sharply expressed in the Upper Cretaceous deposits, depressions with amplitudes of up to 200 m—these are the Mustafinskaya, Alekseevskaya, Staronikolskaya, and others.
For a long time it was believed that the formation of these depressions was connected with subsidence of the basement in one area or another. However, this hypothesis is contradicted by the fact that the dimensions of these depressions are comparatively small. Due to vertical movements of the basement, depressions of much larger dimensions should arise.
Having analyzed a large amount of factual material, the authors came to the conclusion that the depressions under consideration formed as a result of the displacement of salt from beneath the areas located under them. Particularly convincing data in this respect are provided by a section of the Mustafinskaya depression, situated on the Sharlykskaya area (Fig. 1). Here, in particular, it is seen that a depression with an amplitude on the order of 200 m in the area of well No. 329 is observed only along the tops of the following suites (from top to bottom): Amanakskaya, Bolshekinelskaya, Sokskaya, Transitional, Sosnovskaya, Hydrochemical, and Kalinovskaya; and also along the top of the Ufimian stage. Along the top of the horizon lying below, the Iren horizon—
Fig. 1. Geological profile of the Sharlyk area through the Mustafino depression by wells No. 330, 322, 329, and 317.
Tatarian stage: 1 — Amanak suite, 2 — Bolshoi Kinel suite, 3 — Sok suite; Kazanian stage: 4 — transitional unit, 5 — Sosnovka suite; 6 — hydrochemical suite; 7 — Kalinovka suite; 8 — Ufa stage; Kungurian stage: 9 — Iren horizon, 10 — Filippov horizon.
this depression is not traced. And if its formation had been caused by vertical movements of the basement, then here as well it would have been expressed no less distinctly than in the overlying horizons.
In accordance with the foregoing is the fact that, in the section of the Iren horizon in wells No. 330, 332, 317, etc., there are thick salt beds. In well No. 329, which is located in the center of the depression under consideration, the salt beds have an insignificant thickness. The interbeds of other rocks of the Iren horizon, among which the leading role belongs to anhydrites, have approximately the same thickness in all wells of this area.
Thus, it may be assumed that at the time of formation of the Iren horizon and the overlying Upper Permian deposits, the thickness of the salt beds in the area of well No. 329 was approximately the same as over the entire remaining area, and that the Mustafino depression, within which it is located, did not then exist. Subsequently, the salt from this area began to migrate to adjacent areas, and the surfaces of the units lying above it formed a depression.
Unfortunately, in the course of drilling the Sharlyk area, determination of the bedding elements of the strata was not applied, and the drilling itself was carried out on too sparse a grid. As a result, only the most general idea of the Mustafino depression was obtained.
From what has been said there follows one very important practical conclusion. When carrying out structural drilling, in the event that similar negative as well as positive structures are discovered, exploratory drilling should be conducted to horizons situated below the Iren horizon, since these structures may not correspond to structures in deeper deposits.