Influences of Storm Erosion and Deposition on Rhythmites of the Upper Wenchang Formation (upper Ordovician) around Tonglu, Zhejiang Province, China

Tonglu rhythmites (Upper Ordovician) in Zhejiang Province, east-central China, display three orders of cyclicity in sandstone and mudstone layer thickness. Millimeter-thick alternations of sandstone and mudstone laminae are ascribed to single tidal cycles. Centimeter-thick alternations of sand-dominated layers (SDLs) and muddominated layers (MDLs) are interpreted to be related to alternation of storm and calm weather conditions with a periodicity longer than that forced by neap–spring tidal cyclicity. The SDLs are interpreted as storm deposits on the basis of presence of scour structures, abundant intraformational mud pebbles, oscillation ripples, and thinning-upward trends in the sandstone laminae. A third, meter-thick cycle of variations in sandstone-lamina thickness is interpreted as a reflection of cross-shore changes in coastal dynamics and water depth in the subtidal–intertidal environment. Storm waves, usually considered to be random destructive factors to normal cyclic deposits, are here highlighted as effective agents of sediment transport and deposition of the sand-dominated layers. This study aims at improving our facies-level understanding of the genesis and preservation of storm-related tidal-flat rhythmites on open coasts, and highlights the fact that storm related facies can mimic the cyclicity that is commonly ascribed to neap–spring tidal variation.