Development history of Middle Jurassic neptunian dykes in the High-Tatric series, Tatra Mountains, Poland

Abstract

Neptunian dykes filled with Middle Jurassic sediments from the High-Tatric series in the Tatra Mountains are described. Six types of dykes, which constitute two groups, are distinguished, basing on the type of filling deposits, external shape of the systems and character of the walls. Group I embraces all of the dykes with sharp-edged walls and predominantly vertical structures. Group II consists of dykes with smooth walls and predominantly horizontal structures. The distribution of particular types of dykes in the High-Tatric tectonic units is discussed. The processes of the initiation of dykes, their development and filling with deposits are reconstructed. The distribution of the dykes of Group I, their orientation, shape, size and relationship to the host-rocks, indicate the mechanical nature of the initiation phenomena. The dykes of Group II underwent major reconstruction during the development stage, which made studies of their initiation impossible. Neptunian dykes belonging to Group I display features indicating the sole
role of mechanical processes during their development. In the dykes of Group II chemical erosion played an important role. Different dykes were filled in different ways. Processes of injection of loose sediments into fissures opening in the solid substrate, rapid burying of open dykes by migrating dunes of crinoidal sand, and slow sieving of fine material into vast systems of voids, are proposed to explain the filling of dykes by various types of sediments. The interpretation of the formation of internal breccias at the footwall of a fault scarp is presented. The influence of pressure dissolution phenomena on the post-depositional history of the neptunian dykes is evaluated. Field and microscopic observations were complemented by isotopic analyses of the sediments and calcite cements filling the dykes. Paleogeographic conclusions based on the characters of the neptunian dykes are given.