Rewizja stratygrafii dolnego ordowiku w Górach Świętokrzyskich

Jerzy Znosko, Roman Chlebowski


Revision of the Lower Ordovician stratigraphy in the Holy Cross Mts. (Central Poland)

Stratigraphie subdivisions of the Ordovician rocks established in the Holy Cross Mts. have been for a long time regarded as standard for the whole Poland. Recently, stratigraphie sequences established beyond the limits of the Holy Cross Mts. make it necessary to revise the stratigraphy of the mountains themselves. In the Holy Cross Mts. two regions in which the Palaeozoic sediments are represented by different fades have been recognized for a long time. These are: the Kielce and Łysogóry regions. In the latter there is a continuous sedimentation from the Upper Cambrian to Tremadocian manifesting itself in day and mudstone sediments of the Upper Cambrian and Lower Tremadocian age (Łysogóry beds). There is, however, a dear discordance between the Lower Tremadocian or even Upper, Middle or Lower Cambrian and the overlying rocks of the Didymogratus extensus zone (Lower Arenigian). However, in the majority of profiles of this region rocks not older than of Llaind'eilian or Caradocian age overlie those of the Lower Tremadodan or different Cambrian horizons. There is then a hiatus originally comprising the Bryograptus zone of the Upper Tremadodan and the Didymograptus zone of the Lower Arenigian. More frequently, however, the hiatus is greater due to the subsequent tectonic and erosive events, so that it practically also comprises the Llanvimian, Uandeilian and sometimes the lower part of the Caradocdan. In the Kielce region, Middle or Lower Camibrian sediments are overlain with a great stratigraphie hiatus and angular discordance by conglomerates (s. c. Międzygórze Conglomerate) with an admixture of tuffogenic material and glauconite or else directly by the so-called chalcedonites or glauconitic day cherts containing well preserved Bryograptus detritus (Lower Ordovician). In the Międzygórze Conglomerate with distinct volcanogenic haibit, fragments of black day shales of different size and degree of roundinig (sharp-edged fragments included) have been noted beside various pebbles of pre-Ordovician rocks. The shales reveal the presence of fragments of the Dictyonema colonies as well as very numerous Chitinozoa fragments. Towaridls the top the Lower Ordovician sediments pass into glauconitic clay-mudsitone-sandy tuffogenic rocks with bentonite intercalations. The palaeontologic data prove that the conglomerate complex from Międzygórze must be younger in age than the Lower (Dictyonema) Tremadodan because it contains fragments of Dictyonema shales. The lack of Bryograptus microdetrituis in day cherts and glauconitic chalcedonites that have been also regarded to be of Tremadodan age indicates that they, too, are younger than the Tremadocian. Furthermore, the fact that the reworked Bryograptus microdetritus is missing altogether even in the younger rocks demonstrates that Sandomirian folding occurred in the Bryograptus zone. In consequence, the sedimentation process was interrupted and erosion of the previously deposited sediments started. This revolution must have 'lasted from the close of the Bryograptus zone throughout the greater part of the Dichograptus zone. This thesis is supported by the results of investigations carried out on the pre-Cambrian platform of north-eastern Poland, where there is a continuity of the Upper Cambrian and day^shaly Dictyonema and Bryograptus Tremadocian. The Bryograptus day shales are overlain by glauoonitite with dear indications of erosion; its palaeontologic stock explicitly assigns its age to the Didymograptus extensus zone of the Lower Arenigian. Glauoonitite, in turn, is overlain by glauconitic limestones and dolomites with the Lower Arenigian index fauna. Erosion of the Sandomirian revolution, which is very well dated on the platform, resulted in shearing and destruction of the ipre-Arenigian profile in a various degree. Sometimes glauconitite overlies Tremadocian sandstones with Obolus, sometimes Upper Cambrian sandstones or even Lower Cambrian daystones, this stratigraphie sequence being similar to that in The Kielce region of the Holy Cross Mts. Glaucoinitite .owes its origin to volcanic activity that was common on the turn of the Tremadocian and Arenigian. Glauconitic clay cherts, chalcedonites and bentonites have the same genesis. Glauconite of these beds originated due to the transformation of volcanic dust rich in biotite and potassium feldspars. Most recent stratigraphie studies have demonstrated that glauconite rocks recorded in the Lower Ordovician in the different part of Poland are coeval and belong entirely to the Lower Arenigian. The hiatus caused by the Sandomirian revolution is between the Bryograptus zone of the Upper Tremadocian and the Dtidymogiraptus extensus zone of the Lower Arenigian. Thus, the Sandomirian foldings and diastrophism is contemporaneous with the Early Caledonian revolution in the other parts of the Caledoaman geosyncline. Dictyonema shales of the Łysogóry beds appearing in situ in the Łysogóry region as well as their reworked fragments in the Międzygórze Conglomerate of the Kielce region testify to the closing period of the sedimentation cyclothem and demonstrate that the declining sedimentation manifested in the Łysogóry beds must have primarily extended over much vaster territories. From the palaeogeographic point of view it is necessary to accept the primary presence of Dictyonema shales and of the Łysogóry beds in the Kielce region. Their actual lack is a secondary feature and is due to a violent though short-lived erosional activity caused (by the Sandomirian révolution.

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