SHALE MICRO-RIP-UP CLASTS – A NEW INDICATOR OF BED-LOAD TRANSPORT IN FINE-GRAINED SEDIMENTS

Mudstones, and especially black shales, were traditionally thought to be deposited in oxygen-poor to oxygen-free, largely tranquil environments. However, recent research has shown that mudstone floccules produce ripple structures in black shales. These ripples, in turn, indicate that bed load transport happens at times even in these distal environments. Here we go a step further: black shales can contain abundant micro-rip-up clasts throughout the succession, and therefore, that bedload transport is responsible for a significant portion of black shale deposition.

Not all black shales consist of or contain abundant rip-up clasts. Thin-section analysis of Alum Shale samples (Cambrian, Scandinavia) reveals few if any, definitive rip-up clasts. It is therefore speculated that the Alum Shale environment in many outcrops throughout Scandinavia represents a rather tranquil, likely distal shale environment without much bed load transport.

In contrast, the Bakken Formation (Devonian-Carboniferous, US) preserves potential rip-up clasts. These clasts range from elongate, imbricated mudstone fragments to rounded and unaligned grains scattered within the matrix. Such grains often fractionate with silt or even sand grains in black shales examined. The degree of rounding in these clasts reflects the extent of transport and clast reworking. The likely source of these is uncertain. However, these clasts must originate from the reworking of black shales, most likely from within the same basin, and eroded and re-deposited from the Bakken black shales elsewhere.

The results suggest that thin-section observations are necessary to distinguish between superficially similar shales and their associated environments of deposition. For black shales specifically, the results suggest that these shales can form in a variety of conditions, that range from the tranquil to the comparatively energetic. Additionally, this work suggests that similar processes may exist in other fine-grained strata, which can open a new avenue of research into mudstone deposition. This can aid in facies analysis, environmental reconstructions, and—albeit broadly—in inferring sediment supply sources.