Blocks of fault-bounded imbricate stacks of Devonian limestones, a diagnostic field-criterion for a predominantly tectonic origin of chaotic rock fabrics in the Harz Mountains (Eastern Rhenohercynian Belt, Germany).

The distinction between sedimentary and tectonic processes in the formation of chaotic rock units is especially difficult in orogenic belts, where sedimentary structures are usually strongly overprinted by tectonic deformation (e.g. Fiesta et al. 2019). This also applies to the chaotic rock units, which are widespread in the allochthonous domain of the Harz Mountains. For these units, it has been assumed that their chaotic rock fabric was initially sedimentary in origin and was merely tectonically overprinted by subsequent Variscan deformation (e.g. Schwab & Ehling 2008). In contrast, it could be shown, that tectonic processes were crucial for the formation of the chaotic rock fabric (Friedel et al. 2019). This is particularly evident in the structural characteristics of Devonian limestone blocks.

In chaotic units, blocks of (hemi)pelagic condensed limestone of different Devonian age are widely incorporated in a slaty-clayey matrix. So far, the blocks were mostly regarded as olistoliths and thus considered as clear evidence for a sedimentary origin of the chaotic rock units (olistostromes). However, our investigations show that the limestone blocks are fault-bounded, folded and internally imbricated stacks of limestone strata, whose final fragmentation and isolation occurred subsequently to tectonic folding.

Such blocks of fault-bounded imbricate stacks of rock strata are a diagnostic field-criterion to identify a strong tectonic overprint or even a tectonic origin of chaotic rock fabrics. Since such blocks are regionally distributed, they support, together with other structural features, a predominantly tectonic origin of these units and argue against widespread submarine mass-flow deposits.

References:
Fiesta et al. 2019, Gondwana Research, 74, 7-30
Friedel et al. 2019, Intern. Journal of Earth Science, 108, 2295-2323
Schwab & Ehling 2008, Karbon, 110-140; in Bachmann et al. (Hrsg.) Geologie von Sachsen-Anhalt, Schweizerbart