The ups and downs of "Tectonic Quiescence " - recognizing differential subsidence in the epicontinental sea of the Oxfordian in the Swiss Jura Mountains

Recent studies in northern Switzerland have shown that epicontinental areas thought to have been tectonically stable during the Mesozoic were not necessarily as rigid as presumed. By comparing Oxfordian facies boundaries and depocenters in their palinspastic position with known faults in the basement, a direct relationship between the two can be demonstrated. Previously, the lack of obvious synsedimentary tectonic features has lulled scientists into believing that the realm of the Swiss Jura was tectonically stable during the Mesozoic. However, it can be shown that facies and sedimentary structures are largely influenced by tectonics. Subsurface data provide evidence for the presence of Paleozoic troughs in the basement which, apparently, were prone to reactivation during the Pan-European stress-field reorganization taking place in the Late Jurassic. This led to differential subsidence along pre-existing lineaments within the study area, which can be recognized in the distribution of Oxfordian epicontinental basins and their coeval shallow-water counterparts. Eustatic sea-level fluctuations played an important role in the development of shallow-water facies patterns, but a subordinate role in the control of accommodation space in basins. While tectonic activity is often recorded in the sedimentary record in the form of platform break-ups and associated sedimentary debris, more subtle indicators may be overlooked or even misinterpreted. Sedimentary structures and isopach maps, as well as subsurface data in the study area suggest that subtle synsedimentary tectonic movements led to the formation of two shallow, diachronous epicontinental basins during the Late Jurassic. It becomes possible to recognize and differentiate the combined effects of local and regional tectonism, eustasy and sedimentation. (C) 2002 Elsevier Science B.V. All rights reserved.