ABSTRACT Flat‐pebble conglomerates (FPCs), typical event deposits developed in the Lower Palaeozoic successions of North China, have been widely researched for their formation processes and characteristics during the Cambrian. However, similar studies focusing on the Ordovician are limited. In this study, based on integrating detailed field and thin‐section observations with petrography and sedimentary association analysis, five types of FPC were identified in the Lower Ordovician based on the morphology, colour and arrangement of pebble clasts, including chaotic flat‐pebble with oxidized edge conglomerate (FPC1), oriented arranged flat‐pebble with oxidized edge conglomerate (FPC2), chaotic flat‐pebble with reduced edge conglomerate (FPC3), oriented arranged flat‐pebble with reduced edge conglomerate (FPC4) and chaotic angular to subangular flat‐pebble conglomerate (FPC5). Interbedded with ribbon rocks, these FPCs form four vertical depositional associations: (1) layered or layered‐like FPCs, (2) mound‐shaped FPCs lenses, (3) channel‐shaped FPCs lenses and (4) irregularly shaped FPCs. The clasts of FPCs are mainly derived from ribbon rock fragmentation induced by combined seismic and storm events, with minor contributions from supratidal sediment desiccation cracking. Multi‐stage layered or layered‐like FPCs were deposited in mid‐ to high‐energy settings under the influence of repeated storm events. Mound‐shaped FPCs lenses formed through small‐scale slope failures of unconsolidated intertidal deposits, whereas channel‐shaped FPCs lenses resulted from storm‐induced clast fallout. Irregularly shaped FPCs originated from in situ diagenesis under burial conditions. The disappearance of FPCs after the Middle Ordovician is closely related to marine biodiversity and associated bioturbation. This study provides comprehensive genetic insights into the Early Ordovician flat‐pebble conglomerates.
Xiang et al. (Tue,) studied this question.