Decoding Deep-Time Rhythms: Probing the Limit of Stratigraphic Correlation in the Time-Specific Facies of the Late Devonian Usseln Limestone (Rhenish Massif, Germany)

The iso- or diachronous character of a geologic unit is scale-dependent, especially for time-specific facies. The Usseln Limestone is a Late Devonian time-specific facies from Germany, occurring immediately below the Lower Kellwasser black shale. Here, we investigate whether cm-scale rhythmical bands within the Usseln Limestone are correlatable across its depositional basin. Its facies were studied at three locations ca. 50 km apart, representing different depositional settings. Its cm-scale alternations in lithological facies and elemental content (μXRF) form an excellent target for correlations on millennial timescales. Correlation attempts failed to converge to a solution at the cm-scale of individual rhythmites. Dynamic Time Warping, however, provided convincing correlations at the dm-scale, supporting its use as a high-resolution correlation tool. The Usseln Limestone base may be diachronous, but the top is likely isochronous. This finding is in agreement with sudden basin-wide black shale deposition at the onset of the Kellwasser Crisis.