Budowa geologiczna "Wypiętrzenia Rzeszotarskiego" w świetle najnowszych danych wiertniczych i geofizycznych

Konrad Konior

Abstract


Geological structure of the Rzeszotary elevation in the Wight of recent geophysical and drilling data

A drilling carried out in 1909 SW of Wieliczka at Rzeszotary encountered metamorphic rocks underlying directly Jurassic deposits. It was however not earlier than in 1927 that J. Nowak drew any general conclusions from this discovery, recognizing the metamorphic rocks of Rzeszotary as an elevation of the crystalline basement which he termed the Rzeszotary anticline. According to this author, the anticlinal axis runs from NW to SE. J. Stemulak and E. Jawor (1963) considered the Rzeszotary elevation to be a horst structure. K. Konior (1966) thought it to be a continuation to SE of the Silurian Kraków—Myszków zone, distinguished by St. Siedlecki (1962). Konior (1966, 1969) named this regional tectonic structure the Myszków—Kraków—Rzeszotary— Tymbark anticline. The name „Rzeszotary elevation” refers to the southern part (south of Kraków) of this structure, which has been ascertained within the range of over 100 km from Myszków to Wiśniowa. The flanks of the anticline are made up of Paleozoic rocks that thin away as they approach the core (Konior, 1966; 1967).

STRATIGRAPHY
In the southern part of the Rzeszotary elevation the highest tectonic element is formed by two overthrust Carpathian flysch units, i.e. the Silesian and SübsileSian nappes. Total thickness of the two units increases towards the south and varies from 176.6 m in the borehole Rzeszotary 2 (Bur tan, 1962) to 2263.5 m in the borehole Wiśniowa 1 (Burtan, 1964). The localization of boreholes is given in Fig. 9 and Plates I, II. Flysch formations are thrust over autochtonous and paraautochtonous Miocene rocks, the thickness of which is insignificant on the foreland and increases to 1198—1120 m in the region of the Carpathian overthrust. Towards the south, the thickness of the Miocene diminishes to 694 and 378.5. Under the uniform Miocene cover there are Cretaceous deposits, their thickness ranging from 10.3 m (borehole Zabierzów) to 237 m (borehole Słomniki IG-1). The Cenomanian is complete whereas stratigraphical gaps have been recorded in the Turonian; the whole Coniacian is missing (Panow, 1934; Bukowy, 1956). The individual Cretaceous profiles in the neighbourhood of Kraków are incomplete in one way or another. Cretaceous deposits in the area in question occur within ’’embayments” protruding to the south or south-west and separated by ’’islands” or ’’peninsulas” built of Malm deposits (Figs. 1, 2). Jurassic rocks occupy the whole area in question, forming the oldest, uniform element of the Mesozoic cover. Broadly speaking, they are represented by two types of sediments, carbonate sediments representing Malm and sands and mudstones belonging to Dogger and uppermost Lias. Thickness of the Jurassic deposits varies from 90.8 m (borehole Bębło) to 401 m (borehole Liplas 2), which is the maximum thickness in this area. A comparison of the thickness distribution of the two contrasting series, i.e. the clastic one of Dogger and Lias and the carbonate series of Malm (Figs. 3, 4), permits to put forward a hypothesis that before the transgression of the Dogger sea, the area of the Rzeszotary elevation and its surroundings were morphologically differentiated. The existing depressions favoured a more intensive accumulation of sediments, which result ed in the levelling of the area. The carbonate Malm sediments already accumulated in the levelled area. If the Cainozoic-Mesozoic cover is removed, its varied Palaeozoic basement is readily observed (Fig. 5). Permian deposits appear in the form of red mudstones with anhydrite nests containing sandstone intercalations in the top and conglomerates in the bottom part. They have been assigned to Zechstein (Mor yc and Senkowi c z owa , 1968). Their thickness amounts to 1368.9 m (borehole Liplas 2). The occurence of Zechstein deposits in the neighbourhood of Kraków seems to indicate that, at the time of their formation, there was SE of Kraków a sea bay open to the south-east (Моrус, 1971). A rapid increase in the thickness of Permian deposits SE of Kraków evidences that the sedimentary basin of Zechstein deposits was bounded both from SW and NE by tectonic dislocations running from NW to SE. Considerable differences may be observed in the distribution of the Permian and Carboniferous deposits in the area under study. While the Zechstein deposits reach as far as the central zone of the Rzeszotary elevation, the Carboniferous rocks occur only on its both flanks. In the west they are bounded by the Upper Silesian Coal Basin (Konior , 1971) and in the east they form two emibayments (Fig. 6). From the data available it appears that the central zone of the Rzeszotary elevation was a sedimentary area neither during the Lower Carboniferous nor the Devonian transgressions, which was already stated by J. Nowak (1927). Being a relic of Caledonian folding, this zone could have been a barrier for both the Lower Carboniferous and Devonian transgressions. Except for the narrow, most elevated central zone, which was very likely devoid of Devonian deposits from the very beginning, the Rzeszotary elevation is occupied by uniform, homogeneous Devonian cover (Fig. 7). It consists of carbonate rocks representing the Upper and Middle Devonian at the top, and of sandstone-mudstone rocks belonging to the Lower Devonian at the bottom. Thickness of the carbonate Devonian ranges from 40 m (borehole Wyciąże 5) to 1084 m (borehole Wyciąże 1) on the NE flank of the elevation, and from 663 m (borehole Mogilany 1) to 947 m (borehole Wysoka 1) on its SW flank. The Lower Devonian is from 28 m (borehole Wyciąże 1) to 115 m (borehole Niepołomice 11) thick on the NE flank of the Rzeszotary elevation, whereas on the SW flank its thickness exceeds 1385 m (borehole Mogilany 1). So enormous a thickness of the Lower Devonian must have been due to particularly favourable conditions. The basic condition was the existence of a suitable basin whose bottom had a tendency to sink, most likely along faiult zones, as it was filled with sediments. In the area discussed, Silurian deposits constitute the lowermost, unmetamorphised Palaeozoic unit. They were found by drillings NW of Kraków (Siedlecki, 1962; Roszek and Siedlecki , 1963; Bukowy, Ślósarz 1968) and in the borehole Piotrowice 1 far to the south (Konior, 1970). The Silurian deposits of the Myszków — Kraków — Rzeszotary — Tymbark elevation, known so far from boreholes, can be divided into two parts. In the older one schists prevail while the younger part is represented by mudstones, sandstones and the so-called conglomerates from Łapczyca (Łydka, Siedlecki and Tomczyk, 1963; Znosko, 1963; 1965 a, b). Thickness of the older schistaceous part, confirmed so far by drillings, exceeds 334.7 m (borehole Bębło), whereas the younger mudstone-sandstone-conglomerate series is from 10 m (borehole Wyciąże 1) to over 217.1 m thick NE of Kraków. Basing on the recent exploration of the Silurian deposits it may be assumed that they accumulated in a sea bay that was then in the region of Kraków (Fig. 8). A change in the character of the sediments being formed, caused by orogenetic movements, resulted in the formation of conglomerates, particularly in the coastal parts of the disappearing bay. The conglomerates are regarded to be of Middle Ludlow age. They would therefore represent molasse formations frofri the Cracovian phase of the Caledonian orogenetic period (Łydka, Siedlecki and Tomczyk, 1963), thought it is conceivable that they might be assigned as well to the youngest, Ardenian, phase. The Silurian deposits overlie directly unmetamorphised Precambrian rocks in the northern part of the area and metamorphic rocks in the southern part. The metamorphic rocks are Precambrian sediments metamorphised in the Sandomirian phase of the Caledonian orogeny (Heflik and Konior, 1971). The boundary between the metamorphosed and unmetamorphosed Precambrian has probably the same direction as the Caledonian folds, i.e. WNW-ESE.

TECTONICS A ND PALAEO GEO GR APH Y OF THE RZESZOTARY ELEVATION
The tectonic structure of the Rzeszotary elevation is shown in Plates I and II. The lengths of the cross-sections and their directions depend on those of the seismic profiles (Fig. 9). Over the distance from Kraków to the Raba river valley, the Rzeszotary elevation is an anticline whose flanks are thrown down along faults (Nowak, 1927; Stemulak and Jawor, 1963; Konior, 1966; 1969). In its central zone the elevation is made up of Precambrian rocks overlain by Silurian deposits. Before the ingression of the Silurian sea, there was in the region of Kraków a Caledonian range built up of Precambrian deposits to the north and the metamorphised Precambrian to the south. It was the Ardenian phase of the Caledonian orogenetic period that gave rise to the Rzeszotary elevation. From the Lower Devonian onwards, the Rzeszotary elevation was Controlling the extent of the Devonian, Lower Carboniferous and Zechstein transgressions. The Lower, Middle and Upper Devonian deposits wedge out in the limbs of the elevation and do not cover its core (K onior, 1966). During the Bretonian phase, the Devonian deposits were subjected to strong faulting. The Bretonian faults have two main directions, NW—SE and SW—NE. Denudation processes prior to the Visean transgression removed sediments on elevated horsts down to the top part of Eifelian dolomites (Konior 1967) and, farther to the east, down to the Lower Devonian, the latter included (Kwiatkowski , Moryc and Tоmczyk, 1966). In the period of denudation preceeding the transgression of the Lower Carboniferous sea, the peninsula formed within the Rzeszotary elevation was not inundated even by the Visean sea, the extent of which was particularly wide. While in the NE limb of the elevation the sea regressed by the end of the Visean (Fig. 6), the northern bay became a sedimentary basin for the lowermost Namurian sediments (Korejwo and Teller , 1968). In the western limb of the Rzeszotary elevation sedimentation lasted throughout the Upper Carboniferous at the time coalbearing deposits of the Upper Silesian Coal Basin were formed. After the regression of the Visean and Lower Namurian seas from the north-eastern neighbourhood of the elevation, there began a prolonged period of denudation, which in the youngest Carboniferous expanded over the whole area of the Upper Silesian Coal Basin, lasting to the end of the Early Permian (Rotliegendes). The Zechstein sea left its sediments only in the SE part of the area. Despite their local character, the sediments have an enormous thickness of 1368.9 m in the borehole Liplas 2 (Moryc and Senkowiczowa , 1968). Throughout the Triassic the Rzeszotary elevation was submitted to denudation processes that brought about its morphological differentiation reflected in the variable thickness of the clastic Lower Jurassic deposits. They cover the whole area, being deposited on all the constituents of the elevation, from the Precambrian to Zechstein. In the part situated between the borehole Bębło in the north and borehole Wiśniowa IG in the south (Fig. 9, Pl. I, II), the Rzeszotary elevation is a geological feature of complex structure and pre-Jurassic morphology. Within the discussed distance of 48 km, it dips from + 290.6 m to —2418.0 m, i.e. by 2708.6 m. The dip is relatively gradual on a length of 31 km between the Bębło (+290.6 m) and Rzeszotary 2 (—505.0 m) boreholes, and only 5 km S of the borehole Rzeszotary 2 the pre-Jurassic deposits dip down to —1718.0 m (borehole Dobczyce 1) to gently reach a depth of —2418.0 m in the borehole Wiśniowa IG 1, about 12 km to the south. This form of the Palaeozoic metamorphic substratum is due to huge fault zones that are responsible for the divisions of the elevation into three principal, stepJdipping blocks. The present edge of the overthrust flysch units overlies the southermost part of the upper block as well as the middle and lowermost blocks. The thickness of the Miocene separating flysch units from the Cretaceous (locally) and Jurassic ones is considerably varied since it depends on the position of the underlying tectonic blocks. The main faults run from NW to SE and SW to NE. These two directions are conspicuous, the former having affected to a high degree the run of sedimentary troughs, particularly in the Lower Devonian and ZechStein. The overthrusting of flysch units on the Miocene foreland rejuvenated the old, Hercynian dislocations and, moreover, was responsible for the formation of new, ’’Carpathian” ones, their direction being W—E (Nowak, 1927). The latter seem to be characteristic of the Cracow region, but, on the other hand, there are probably no such dislocations in the substratum of the Carpathians south of Wieliczka. The run of more important faults is shown in Fig. 9. The Upper Miocene faults, which are very numerous indeed, caused the greater blocks to be fissured already after older Miocene rocks had been deposited. One of the principal faults, the direction of which is SW-NE, i.e. transverse to the axis of the Rzeszotary elevation, throws the top of the Jurassic deposits by about 1150 m. The present author surmised its existence as early as in 1966. The second important fault running from WSW to ENE throws the top of the Jurassic deposits by about 750 m. It is not unlikely that, being parallel to the previous one, this fault extends farther to the south and is responsible for the specific tectonic relations observed in the region of the Żywiec elevation in the Flysch Carpathians. Other transverse faults of interest are shown in Fig. 9, I-I, III-III, IV-IV. Longitudinal faults having NW—SE, sometimes passing into NWN—SES, directions caused smaller or greater displacements of the individual blocks. Being most rejuvenated, they decided on the formation and survival of this elevated tectonic structure that was the Rzeszotary elevation during the Palaeozoic age. In the area under study there are five longitudinal faults (Fig. 9, V—IX). Except the IX-th one, these faults determine the general direction of the. Rzeszotary elevation, leading together with the interesecting transverse faults and the whole network of less important Miocene faults to additional fracturing of this structure into several blocks of different size and to the present mosaic pattern of the pre-Jurassic substratum.

CONCLUDING REMARKS
The origin of the Rzeszotary elevation as a tectonic element may be traced back to the Lower Devonian, most likely to the Ardenian phase. It was not inundated by transgressions in the Late Palaeozoic. The Breto nian phase of the Hercynian orogenetic period imposed the basic features on the tectonics of the Rzeszotary elevation. Throughout the Late Carboniferous, except the Stefanian, continental coal-bearing deposits were being formed SW of the elevation. In a small area east of the elevation, only lowermost Namurian deposits were formed. The next, Asturian, phase ultimately completed the Hercynian structure of the Rzeszotary elevation. At that time the elevation attained its maximum width and, simultaneously, the greatest morphological differentiation, shown by the extent of Triassic rocks. It was not earlier than in the Jurassic that the Rzeszotary elevation, as a tectonic structure, did not affect any more the extent of seas of the individual transgressions; the whole area was then covered by Jurassic and later, locally, by Cretaceous deposits. In the Miocene, this Jurassic- Cretaceous substratum was overlain by Miocene, mainly Tortonian, sediments of variable thickness. Orogenetic movements connected with the overthrusting of the Carpathians on the Miocene foreland resulted in the formation of transverse faults running WSW—ENE in the discussed part of the Rzeszotary elevation. This was the last stage in the history of this tectonic structure, leading to its present state.

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