Keywords: 
• tidal rhythmites /
• storm deposits /
• conglomerates /
• sedimentary succession /
• Late Ordovician /
• Zhe-Wan Marginal Sea Basin

Abstract: The Late Ordovician Wenchang Fm. is subdivided into two members. Member I (M I, the lower section), characterized by presence of thick massive sandstone layers intercalated with thin mudstone layers and brachiopodous, bryozoan and crinoidea assemblage, is interpreted as nearshore to shallow-marine deposits. Member Ⅱ(M Ⅱ, the upper section), is further subdivided two parts: M Ⅱ-1 (the lower one), consisting of sandstone intercalated with conglomerate lenses; M Ⅱ-2 (the upper section), composed of muddy sandstone or sandy mudstone with well-developed tidal bedding. There are two opposite opinions concerning the sedimentary environment of M Ⅱ. Some consider that M Ⅱ-1 is debris-flow deposits on the basin slope owing to biological assemblage containing graptolite, and M Ⅱ-2 is deep-water internal-tide deposits. Others interpret M Ⅱ-1 as near-shore subaqueous channel deposits based on its biological assemblage dominated by shallow-water species and conformity contact with the underlying M Ⅰ in the region, and M Ⅱ-2 as tidal-flat deposits. M Ⅱ-2 is characterized by alternations of sand and mud laminae, and of sand- and mud-dominated layers. In this study, raindrop imprints were observed on some bedding-planes, exhibiting the periodic subaerial exposure of sedimentation facies. The presence of symmetrical oscillation ripples shows wave activities were common at the study area. Mud pebbles are inferred to form on the intertidal flats during storms in comparison with studies on the modern tidal flats margining the Changjiang Delta. M Ⅱ-2 is, therefore, not deep-water internal-tide deposits but normal tidal-flat deposits with strong storm influence. The sand-dominated layers (SDL), several to several tens of cm thick, are interpreted as storm deposits based on the presence of: 1) scour features, 2) numerous intraformation mud-pebbles, 3) oscillation ripples, 4) relatively thick sand laminae at the lower part with an upward trend of thinning toward the adjacent mud-dominated layer. The mud-dominated layers (MDL), consisting of thin sand/mud alternating laminae, deposit after storms when sedimentary environments return to tidal regime. A small sequence, consisting of a SDL and a MDL, is thus a storm succession. Facies transition is gradual among the three units of the formation (M I, M Ⅱ-1,M Ⅱ-2). Dip directions and angles are close to each other. Detrital composition analyses show that they have similar provenance from the Cathaysian Block. It is inferred that sedimentary environments should have no much change during deposition of three different units. Sedimentological study suggested a shallowing-upward progradational succession from high-energy shallow-marine deposits to low-energy tidal-flat deposits. Upward increases of clay content indicate that sedimentary environments get closer to the shoreline. M Ⅱ-1, interlayered between neritic facies and tidal-flat deposits, is thus interpreted nearshore facies with conglomerate layers of event subaqueous channel deposits.