O dolomitach w wapieniach skalistych okolic Krakowa

Andrzej Łaptaś

Abstract


The dolomites in the Upper Jurassic limestones in the area of Cracow (southern Poland)

The dolomites discussed ocour in the Upper Jurassic limestones in the area of Cracow (Southern Poland — fig. 1). The dolomites have already been described by Gaweł (1947) and interpreted as resulted from infiltration of the Cretaceous saline brines into the Jurassic limestones. The Jurassic limestones enclosing the dolomitic bodies are poorly stratified and consist of micrite and detrital fragments. They also contain numerous calcified siliceous sponges and abundant siliceous concretions of early diagenetic origin. The dolomitization takes a shape of elongated, flat and lenticular bodies within the unaltered limestones. These bodies are oriented parallel to bedding surfaces. The dolomites tend to occur between the discernible bedding surfaces and seldom if ever develop along the bedding surfaces themselves. The dolomitic bodies can be traced through distances of several tens of meters. The lateral wedging out of dolomitic bodies is commonly observed (fig. 2). The thickness of these bodies changes through short distances, ranging from 0,5 to 5 m. No relation between the localization of the dolomitic bodies and the point fractures in the host rock have ever been observed. The contact between the dolomite and limestones is usually not sharply defined, but gradational through a short distance. The incipient dolomitization is characterized by selective replacement of limestones, whereby the calcified sponges and their algal calcareous crusts remain unaltered. Also the brachiopods shells may escape the dolomitization. It is the micrite that is dolomitized preferentially. The final stages of dolomitization are characterized by the appearance of a mosaic of hypidiomorphic dolomite crystals (Pl. II fig. 4). The size of these crystals ranges from 0,09 to 0,35 mm. The primary structures tend to be abscured or oblitered. The only traces of organic structures preserved are small openings i.g. molds left after the dissolution of shells during the dolomitization. The pure dolomite, devoid of limestones relics forms irregular bodies surrounded by incompletely dolomitized limestones showing a mottled appearance (Pl. IV fig. 2). The lover and upper surfaces of dolomitic bodies are irregular (fig. 5, 7). However,, no traces of any submarine erosion have ever been observed along such surfaces. It is of importance to note that early diagenetic siliceous concretions are totally absent in the dolomitized parts of the rock, although they are very abundant in the unaltered limestone (fig. 2). There are also no cavities which would point to dissolution of the preexistent concretions in the limy sediment subjected to dolomitization. This indicates that the formation of the silica concretions and the dolomitization were essentially penecontemporaneous. It is suggested that the high pH of the dolomitization processes prevented the formation of the silica concretions. The calcification of primarily siliceous sponges must have taken place shortly before the formation of the dolomite. The dolomite discussed have been subjected to later silification. This silification tend to occur in the dolomitic bodies showing an advanced stage of dolomitization. Under the microscope it is seen that the fine grained silica replaces the undolomitized relics of organic structures (chiefly sponges) and the calcite matrix between the dolomite rhombohedra. However, not all of the undolomitized structures are subject to replacement. During the process of secondary silification the dolomite rhombohedra remain unaltered. The silification discussed is much younger than the dolomitization and is of post-Jurassic age. It is known that the crystalline carbonates frequently show increased porosity and are subject to solutional disaggregation. Such disaggregation results from dissolution of crystal edges and the limy matrix between the crystals. The final product of such disaggregation is variously called: „sanded dolomite” , dolomitic sand” , „powdered dolomite”, „dolomitic flour” etc. The disaggregation is commonly observed in the dolomite here described, and resulted from the action of meteoric waters. These waters dissolved the dolomites in preference to limestones and the dolomitic bodies are riddled by countless karst cavities. Concluding the foregoing discussion the following statements can be made: 1. The dolomitization of the Upper Jurassic limestones occured prior to the Cretaceous transgression (the Cretaceous abrasion surfaces truncate the already formed dolomite). 2. The dolomitization took place in unconsolidated sediments below the water-sediment interface so that the forming dolomite was not affected by submarine erosional processes. 3. The dolomitization was essentially penecontemporaneous with the formation of the siliceous concretions of early diagenetic origin. 4. The dolomitization was presumably effected by connate brines entrapped in the primary pore space of calcareous sediment. 5. The preferred localization of dolomitic bodies in the middle parts of limestone layers is not clearly understood. It may be tentatively assumed that the cementation first affected the areas adjacent to bedding surfaces leaving the central parts of the limy layers more accesible for migrating brines.

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