New exposure of the Ludwigskanal section near Dorlbach - a key section for the Lower Jurassic in the Franconian Alb, Southern Germany

Abstract

Slope instabilities and reinstatement of the Ludwigskanal ("Ludwig Canal") cutting near Dorlbach permitted a re-investigation of the poorly documented historic Lower Jurassic "Schwarzjura" section formerly exposed in 1840-1841 during the construction of the canal. Hence, for the first time, a detailed description of the exposed strata can be provided for this locality, covering the top 9 m of the Amaltheenton Formation, the 2.1 m thick Posidonienschiefer Formation, the 3.25 m thick Jurensismergel Formation, and the basal 1.3 m of the Opalinuston Formation. About 40 historic fossil findings described and illustrated from this locality by the famous palaeontologist Friedrich August Quenstedt in his monographs can now be assigned to specific beds In addition, new findings of semi-articulated ichthyosaur skeletons in the Bifrons Zone substantiate the conjecture that a 1.6 m sized Temnodontosaurus skull, excavated in 1841 at the Ludwigskanal, comes from bituminous marls of the Bifrons Zone. Apart from the historic fossil findings, the Ludwigskanal section is of special interest with respect to hypothesis on the seawater circulation system of the Lower Jurassic NW-European epicontinental sea. After sedimentation of the Upper Pliensbachian Amaltheenton Formation (with boreal faunal elements), coast-Parallel strong currents from north could have formed a submarine erosional relief including the "Altdorf high", where the condensed, limestone-dominated Posidonienschiefer Formation exposed in the Ludwigskanal section accumulated during Early Toarcian. Later, early during the Late Toarcian, bottom currents from southwest caused slow deposition or even temporary erosion, documented by current-aligned rostra of the "belemnite battlefield".