Applied argument analysis, Nappe tectonics and Palynostratigraphy in the middle Lahn-syncline (Stratigraphy and facies relations in the Devonian and Lower Carboniferous of the middle Lahn-syncline between Weilburg and ruin Aardeck)

摘要

INTRODUCTION: The study area is situated in Germany in the federal state Hesse at the eastern margin of the Rheinisches Schiefergebirge within the Lahn syncline. It is part of the former Variscan geosyncline. In 1910 and 1921 AHLBURG developed a new concept for the interpretation of the geological situation in the Lahn region between Marburg and Limburg: the syncline-theory. Within this "Lahn-syncline" an idealised symmetrical succession of structural and facies features - most obvious within Middle Devonian strata - was proposed. He recognised a general trend to a basinal facies from Emsian to Upper Devonian times within the "syncline", which was divided from the Middle Devonian onwards by the build-up of volcanoes and reefs in its middle part. The syncline was thus 3-fold divided into: a) the "Southern Marginal Facies", b) a zone of volcanic ridges and reefs in the middle and c) the "Northern Marginal Facies" which nowadays is called Hörre-facies. Small remnants of proposed Upper Devonian basinal facies within the middle volcano-and-reef facies were attributed by him to the Northern Marginal Facies; but since no direct contact to the latter was observable, he gave them a separate name: "Gaudernbacher Schichten (layers)". Within this study the struggle which persisted over 80 years about the nature, origin, spatial distribution and palaeogeographic setting of the "Gaudernbacher Schichten" sensu AHLBURG (1918) will be settled: The Gaudernbach layers as a separate stratigraphic unit does not exist! It will be shown in this study that this conclusion can be, in the first instance, verified solely by logical inferences and historical contradictions. But what else - if not the occurrence of a "special facies" - could have forced AHLBURG to recognise something unique in this region? In order to solve this problem the following methods have been applied: a) reconstruction of palynostratigraphy and palynofacies, b) applied argument analysis, c) compilation of new correlation charts and historical analysis, d) analysis of Milankovitch cycles, e) fractal analysis, f) petrographic analyses, g) drill core descriptions. RESULTS: 1) The Gaudernbach layers do not exist. 2) Remnants of the Giessen nappe have been encountered in the area between Weilburg and Holzheim. 3) The Giessen nappe in the middle Lahn-syncline comprises a) late Viséan Kulm-slates (Kulmtonschiefer), b) the Viséan Bruchberg sandstone north of Limburg, c) Viséan light flinty-slates, d) Tournaisian dark flinty-slates, e) Tournaisian dark slates, f) Deckdiabas and Erdbach limestone, g) newly encountered early Famennian debris flow sediments and small occurrences of greywacke. 4) The clasts of the Viséan Bruchberg sandstone were derived from the north (Laurussia), whereas the components of the Famennian greywackes came from a southerly situated source region. 5) Milankovitch cycles have been detected within the background sediments of the Bruchberg sandstone formation and the Helle Lydite (Light flinty slate) formation. Sedimentation rates in the order of less than 10 mm/ka have been derived for both analysed sediments with the obtained data. 6) Autochthonous and allochthonous lithologies show no differences in their fractal properties. But apart from that, fractal analysis proved to be a valuable tool for the quantification of tectonic trends. 7) A new Lower Carboniferous correlation chart (LCC2003) is presented. 8) Applied argument analysis has been introduced to geology for the first time. In exemplary analyses the reliability of the Giessen-Harz Nappe concept is analysed and a scheme to make appropriate judgements on the speculative nature of two palaeogeographic texts is presented. 9) A scheme for the structured registration of biostratigraphic results in electronically processable data sets is presented. 10) 18 drill cores from mostly Upper Devonian and Lower Carboniferous lithologies, drilled north of Limburg, have been analysed and their strata graphically rotated to zero dip for palaeofacies analysis. 11) The palynostratigraphy has been - for the first time - successfully applied in the area between Weilburg and Holzheim. Palynofacies determinations have been undertaken. For the first time reliable statistical data about to what extent reworked fossils could be comprised in greywacke-bearing Upper Devonian debris flow sediments become available. 12) Special attention has been paid on the description of occurrence and formation of framboidal pyrite in pelites and in phycomata of acritarchs therein. 13) A geological sketch map 1:25000, featuring the area between Weilburg and Holzheim, is presented in enclosure 4. 14) Hypothetical reconnaissance maps showing the northern Gondwana southern Laurussia topography during the Middle Devonian and Lower Carboniferous are presented.