Volcanic rocks as discriminants in evaluating tectonic versus climatic control on depositional sequences, Permo-Carboniferous continental Saar-Nahe Basin

Abstract

The third- and fourth-order sequential arrangement of continental sedimentary strata in the Permo-Carboniferous Saar–Nahe Basin preserves a detailed record of contemporaneous faulting and volcanism. Transgressive lacustrine facies of the Upper Carboniferous coincide with the stratigraphic position of extrabasinally derived pyroclastic fallout layers and features of synsedimentary tectonism such as unconformities, thickness and facies changes across faults, and seismogenic deformation structures. The tectonic imprint on fourth-order cycle initiation is further documented by abrupt vertical facies transitions during lake transgression, implying rapid and episodic rather than gradual basin subsidence. This interaction becomes much more pronounced within the fluvially dominated Lower Permian, when tectonism controlled both the ascent of large volumes of magma within the basin as well as episodic enlargement of accommodation space and depositional area. Major extrusive volcanic units are preferentially associated with sequences recording transgression that are only preserved in fault-bounded hanging wall traps starved from clastic sediment input. It is suggested that the stratigraphic position of pyroclastic fallout within the sequential framework of basins elsewhere, provides an important analytical tool for the identification of contemporaneous tectonic events and as such form a discriminant between purely climatically induced Milankovitch band cycles versus those involving a tectonic control.