Substrate lithification in early Cambrian carbonates triggered by intense bioturbation

Burrowing organisms in the early Cambrian are often known for disturbing the microbial mat dominated seafloor, promoting the cycling of nutrients and oxygen and creating a mixed, loose layer in the substrate. Here we provide new insights on the influence that bioturbation had in modifying substrates in the Cambrian. Dense, morphologically complex burrows with thick diagenetic halos are manifest at several horizons in the Houjiashan Formation (Cambrian Stage 4) of North China. Fragmented burrow parts with their diagenetic halo indicate early and preferential lithification starting from the burrow margins and affecting the surrounding matrix. With increasing amount of bioturbation and the development of diagenetic halos, as well as depleted d13C values within the micrite halos suggest that these originally open conduits (i.e., burrows) facilitated the gradual consolidation and lithification of the soft substrate and transformed it into a firmground and, locally, incipient hardground. The possibility that bioturbation drives and expedites substrate lithification during the Cambrian may be a plausible explanation for the widespread occurrence of consolidated carbonate substrates and their subsequent erosion and reworking during this geological period