The Fossil-Lagerstätte Enspel — a crater lake in a volcanic-influenced terrestrial environment of the Westerwald Basin (late Oligocene, Western Germany)

Abstract

The Westerwald region belongs to the Central European Volcanic Province (CEVP). As seen here, theWesterwald Basin is an aggregation of small basins (the smallest ones cover an area as little as 10,000 m), mainly situated within the Variscan Mosel, Lahn and Dill Synclines. Sedimentation started in the middle Eocene (Hottenrott 2002). A pre-existing landscape was filled first by coarse sediments, followed by clays and brown coal (Schafer et al. 2011). Tectonic activity seems to have increased during the middle late Oligocene, resulting in an increasing relief, displayed by sandy sediments (Butz and Vortisch 1987). Two watersheds separate the Westerwald Basin from the Lower Rhine Embayment (Schindler and Wuttke 2013) (Fig. 1). In the late Oligocene, volcanic activity commenced with trachytic intrusions, phreatomagmatic maar eruptions (Enspel) and pyroclastic eruptions. They were followed by basic shallow intrusions, lava flows, and continued maar formation (Elbtal, ?Elz) and the deposition of thick pyroclastics (Schreiber 1996; Haase et al. 2004). Locally, the eruptions and minor intercalated sediments covered the landscape up to 50 m high (Schafer et al. 2011). Reischmann (2011) relates the Oligocene volcanism to the Alpine orogeny and North Atlantic rifting; the main reason is extension and thinning of the lithosphere, resulting in a passive uplift of the asthenosphere; partial melting was caused by pressure release, which resulted in the eruptions in the CEVP. The volcanic activity led to changes in the local drainage systems, e.g. dammed valleys with subsequent lake formation. Caused by contemporaneous temporarily high Chattian sea level, palustrine facies became widespread, so that thick lignite seams were formed along with laminated lake sediments (e.g. Lake Norken, Uhl et al. 2011; fossil site Breitscheid, Schindler and Kolb 2010). Hygrophilous plant communities covered the lowlands. The macroflora of this riparian vegetation is dominated by leaves of Acer cf. tricuspidatum BRONN, twigs of Glyptostrobus europaeus (BRONGNIART) UNGER and charcoalified wood of Cupressaceae (Taxodiaceae s.l.), as seen in the lacustrine to swampy sediments of Lake Norken (Uhl et al. 2011). Upland or dryland floras and faunas were also influenced by volcanic activity. Forests died under heavy ash load, and their remnants were washed into the lowlands and partly preserved as silicified trunks. Isolated reptile and mammal bones and teeth (e.g. Teike and Tobien 1950) can be found within coarse volcanic ash (personal field observation, and unprepared material of the collections of the Museums of Breitscheid, Hachenburg, Wiesbaden and the Hessisches Landesamt fur Umwelt und Geologie, Wiesbaden, all Germany), demonstrating the influence of pyroclastic and/or lahar events on the biota. Floral recovery starts with pioneering plants like Cyperaceae, Poaceae and Betulaceae. This pioneering flora was succeeded by zonal assemblages of a mesophytic forest with strong East Asian influence (Herrmann et al. 2003, 2010; Kohler and Uhl 2014). This article is a contribution to the special issue “The Fossil-Lagerstatte Enspel reconstructing the palaeoenvironment with new data on fossils and geology” M. Wuttke (*) : T. Schindler Direktion Landesarchaologie, Referat Erdgeschichte, Generaldirektion Kulturelles Erbe Rheinland-Pfalz, Grose Langgasse 29, 55116 Mainz, Germany e-mail: michael.wuttke@gdke.rlp.de