Pragian–Emsian successions in Uzbekistan and Bohemia: magnetic susceptibility logs and their dynamic time warping alignment

Abstract

High-resolution low-field magnetic susceptibility (MS) logs of the Lower Devonian limestone beds of the Zeravshan–Gissar Mountain Region (Uzbekistan in Central Asia) and Prague Synform (Barrandian area, Bohemia, Czech Republic, Europe) were stratigraphically correlated using dynamic timewarping (DTW) alignment techniques that are sensitive to local variations in sediment accumulation rates. A method of adjustment of raw data before the actual DTW alignment was developed in order to eliminate multiple sources on the MS signal, and a successful MS-DTW correlation was performed between the Zinzilban Gorge section in Uzbekistan and the Pozar-3 section in Bohemia. The results basically corroborate some of the most recent biostratigraphic correlations, especially those suggested by P. Carls and J.I. Valenzuela-Rios, and provide a 10- to 100-fold better resolution. The projection of the basal Emsian Global Boundary Stratigraphic Section and Point (GSSP) in the Zinzilban Gorge (Uzbekistan) to the succession of Bohemian strata points to a level in the lower parts of the Lodenice Limestone, i.e., to the lowermost one-third of the Praha Formation. Hence, more than two-thirds of the original Pragian in Bohemia (~Praha Formation) remain above the basal Emsian GSSP level in Uzbekistan. This implies that the decision about this GSSP ratified by IUGSin August 1996 led to a significant reduction of the extent of the original Pragian to only its oldest strata. Many other stratigraphically correlatable levels were identified within the sequences of MS stratigraphic patterns in these two widely separated sections in spite of the fact that the Pragian–Emsian palaeoclimatic conditions of the northern hemisphere were different from those of the southern hemisphere. The base of the Norbonak Beds at Zinzilban corresponds to a stratigraphic level lying ~3.6m below the base of the Zlichov Limestone in the Pozar-3 section. The Bohemian anoxic Graptolite Event correlates to a level about 3m below the boundary of the Zinzilban and Norbonak beds at Zinzilban.