Miocen w Trzebuni kolo Myślenic (Polskie Karpaty Zachodnie)

Abstract

W profilu otworu wiertniczego Trzebunia 2 około 6 km na SW od Myślenic, pod fliszem karpackim i parautochtonicznymi warstwami skawińskimi występują osady mioceńskie karpatu i badenu. Osady karpatu rozpoczynają się mułowcami, ponad nimi występują zlepieńce wapienne i piaskowce rozwinięte u podnóża klifu kopalnego, powstałego na regionalnym uskoku Rzeszotary-Żywiec. Ponad osadami karpatu leżą utwory badeńskie: seria podewaporatowa (iłowce i mułowce), ewaporaty (ogipsowane margle) i warstwy chodenickie (iłowce i mułowce). Po raz pierwszy pod fliszem Karpat Zachodnich natrafiono na osady badenu środkowego -ewaporaty i warstwy chodenickie. W kilku otworach wiertniczych na południe od Wieliczki pod fliszem występują parautochtoniczne osady górnego badenu. Implikuje to pogórnobadeński wiek nasunięcia Karpat.THE MIOCENE FORMATION IN TRZEBUNIA NEAR MYŚLENICE (THE POLISH WFSTERN CARPATHIANS) The Western Flysch Carpathians have been thrusted over the Miocene formations occurring over the Mezozoic, Palaeozoic and Precambrian rocks. The Skawina formation or the underevaporite series (the Lower Badenian) has been hitherto known as the youngest sediments of the Miocene occurring under the Western Carpathians. The Middle Badenian sediments series of evaporites and Chodenice Beds have been drilled for the first time in the Trzebunia 2 borehole. The Middle Badenian beds occurs over the Lower Badenian beds (the underevaporites series). under which the Carpathian beds occurs. The bottom of Miocene consists of the Oxfordian limestones (Fig. 1). The Carpathian sediments arc bipartite. The lower part consists of conglomerate and sedimentary breccia consists of the Jurassic and Palaeozoic fragments of limestones and limy sandstones. The upper part is represented by siltstones and claystones monotonously shaped. The coarse clastic material originated from erosion of a cliff developed on the regional Rzeszotary-Żywiec fault (Fig. 2) which limited the Carpathian beds sedimentation zone from the north. A sedimentary cone occurred in the foot of the cliff, reaching the Trzebunia 2 borehole. The siltstones occur further to the south (Figs 2. 3). The Lower Badenian sediments (the underevaporite series) consists of claystones and siltstones with shallow laminae of silty sandstones. The shallw inserts of anhydrite and gypsum occur in the upper underevaporite series. The anhydrite and gypsum presence showed the beginning of chemical sedimentation at the end of Lower Badenian. In the large scale, sedimentation of evaporites begun in Middle Badenian. They occur in the Trzebunia 2 borehole as gypsum-covered claystones. The series of evaporites sedimented in the western marginal zone of the saline basin, in the field of the clastic-sulfate facies. in the margin of the bay reaching south in the Carpathians land (Fig. 2). The Chodenice Beds (the upper part of Middle Badenian) in the form of claystones and siltstones with sandstones laminae occur over the evaporite series. The Carpathian Flysch with a tectonic detached block of the underevaporite series is overthrusted the autochthonous Miocene sediments. The Carpathian Flysch belongs to three following tectonic units: Subsilesian, Silesian. Magura (Fig. 3). The lack of the Silesian Unit in the Trzebunia 2 profile is caused by local tectonic conditions. Parautochthonous deposits of Upper Badenian occur under the Flysch in some boreholes south of Wieliczka (Fig. 4). Drilling data enable to reconstruct the geological development of the area. This may be showed in several stages (Fig. 5): Stage I - Carpathian. Carpathian begun with siltstone sedimentation. Then the cone built of the background rock formed at the foot of the cliff developed on the Rzeszotary-Żywiec fault. Stage II - the end of Carpathian. Spreading of marine transgression caused flooding the cliff and stopped its erosion. Over the sedimentary cone deposits developed siltstones with no coarse-clast is material. Stage III -the lower part of Lower Badenian. The sea transgressed the cliff edge and spread over the area located to the north of the Rzeszotary-Żywiec fault (the Western Carpathian Foreland recently). Monotonously formed claystones and siltstones with sandstones laminae. The Dębowiec series coarse-clastic deposits sedimented locally in the south of Cracow in the lowest part of Lower Badenian. Stage IV - the upper part of Lower Badenian and the lower part of Middle Badenian. Chemical sedimentation (anhydrite and gypsum laminae in the Trzebunia 2 profile) begun at the end of Lower Badenian. Evaporites sedimented in the lower part of Middle Badenian. The marine basin regressed from almost whole western part of the Carpathian Foredeep. A saline basin. with accumulation of anhy-drite. gypsum and halite formed to the east and south-east of Cracow. Stage V - the upper part of Middle Badenian and Upper Badenian. Increasing the marine transgression caused decreasing mineral salt concentration in seawaters and ending chemical sedimentation. Claystones, siltstones and sandstones of the Chodenice Beds (Middle Badenian) and the Grabowiec Beds (Upper Badenian) formed in the Carpathian Foredeep to the east of Cracow. Stage VI - the end of Upper Badenian and Lower Sarmatian. Uplifting movements in the Western Carpathians caused withdrowing the marine basin and beginning gravitational movement of the Car-pathian nappes to the north. Rejuvenation of the Rzeszotary-Żywiec fault took place and a morfological edge on the fault formed in Lower Sarmatian. The Chodenice and Grabowiec Beds sedimented in the fault upper side zone were destructed by erosion. Stage VII - after Lower Sarmatian. On reaching the edge of the fault by the Flysch Carpathians. the Subsilesian Unit rocks accumulated and the Carpathians front overthrusted to the recent position. Occurrence of the Middle Badenian sediments in the Trzebunia 2 profile and parautochthonous Upper Badenian sediments shows post Upper Badenian overthrusting of the Carpathians. It may result from reconstruction of geological development of the area that thrusting the Carpathian edge to the recent position took place after Lower Sarmatian.