Revisit origin of Cambrian flat-pebble conglomerate mounds in North China Platform
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Abstract: Intraformational carbonate breccias and conglomerates composed of flat intraclasts (often called “flat-pebble conglomerates”) are the universal phenomena in mixed carbonate and siliciclastic deposits of the Cambrian-Ordovician successions, suggesting unique paleoecologic, paleoceanographic, and paleoclimatic conditions during the Cambrian-Ordovician greenhouse periods. Flat-pebble conglomerates are often interpreted as storm deposits, yet depositional processes of them, especially of those with mound-shaped geometry and edgewise clastic fabric, still remain controversial. This paper revisits mound-shaped, edgewise flat-pebble conglomerates in the Furongian Chaomidian Formation (Shandong Province, China) using traditional sedimentological measures in order to illustrate the potential complexity of their depositional processes. Detailed field observations on the bed that contains flat-pebble conglomerate mounds were carried out in six outcrops of the Jiulongshan section. The conglomerate mounds are present in between two oolite layers, sitting on a typical, smooth and sharp hardground surface that truncates the underlying oolites. The conglomerate mounds are characterized by remarkable convex-up geometry with a relatively flat base, although irregular geometry also occurs in outcrops. The conglomerate consists dominantly of oligomictic pebble- to cobble-size clasts of finely laminated peloidal grainstone with subangular to subrounded corners, a few rounded homogeneous lime mudstone clasts with red rims, and a few bioclastic grainstone clasts and oolite clasts with glauconite grains. The matrix is composed mostly of white, crudely laminated and clotted microbialites (mainly micro-sparite) and orange dolomites. The matrix also contains many ooids (including broken and multigeneration ooids), fossil fragments (trilobites, echinoderms, brachiopods, and algae), peloids, and abundant glauconite grains. Fragments and debris of micritic Girvanella often occur in matrix. Calcified Girvanella is also identified present along the clast edges. The crudely laminated microbialites locally occur on the top of clasts and show small-scale (a few to 10s of cm wide and a few cm thick) domal macrostructures. The conglomerates are clast-supported and clasts are generally disorganized without preferred orientation. In some cases, however, some intact thin-bedded peloidal grainstones with thin (a few mm thick) shales or horizontal clasts occur in the basal part, whereas more inclined and vertical clasts in the upper part. Both clasts and matrix grains are truncated along the upper boundaries of the breccia lenses. Particularly when the flat-pebble conglomerate mounds are as thick as the oolite bed, they show a flat and smooth surface, overlain by calcareous shale facies. Flat-pebble clasts were derived from rip-up of thickly laminated to thinly bedded peloidal grainstones by strong currents and/or waves. The flat pebbles were transported from nearby and deposited as discrete dunes of various scales under the reworking of storm-induced combined flows. The abundant fossil fragments, broken ooids, and debris of calcified microbes in matrix of the conglomerates are collectively indicative of high-energy, strong-agitating conditions. During deposition of flat pebbles, microbes grew both on top of and in between pebbles, which acted as binding medium that prevented re-orientation of the edgewise clasts and further trapping flat pebbles, eventually forming flat-pebble mounds of various sizes. The flat-pebble mounds underwent substantial reworking by waves after deposition while ooids were being deposited. The study illustrates complex and dynamic depositional conditions for the mound-shaped flat-pebble conglomerates, suggesting multiple episodes of storm events. Detailed sedimentological studies on flat-pebble conglomerate will provide insights into frequency, intensity, and sedimentary processes of deep-time storm events.