Origin of Cambrian Flat-Pebble Conglomerate Mounds in the North China Platform
[Objective]Intraformational carbonate breccias and conglomerates composed of flat intraclasts(often called"flat-pebble conglomerates")are universal phenomena in mixed carbonate and siliciclastic deposits of 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 their depositional processes,particularly those with mound-shaped geometry and edgewise clastic fabric,remain con-troversial.[Methods]This study revisited mound-shaped,edgewise flat-pebble conglomerates in the Furongian Cha-omidian Formation(Shandong province,China)using traditional sedimentological measures to illustrate the poten-tial complexity of their depositional processes.[Results and Discussions]Detailed field observations on the bed containing flat-pebble conglomerate mounds were conducted in six outcrops of the Jiulongshan section.The conglom-erate 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 the outcrops.The conglomerate consists primari-ly 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 of white,crudely laminated,and clotted microbialites(mainly micro-sparite)and orange dolomites.The matrix also contains many ooids(including broken and multigener-ation 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 along the clast edges.The crudely laminated microbialites occur locally 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 mostly disorganized without preferred orientation.In certain cases,however,intact thin-bedded peloidal grainstones with thin(a few mm thick)shales or horizontal clasts occur in the basal part,with more inclined and vertical clasts in the upper part.Both clasts and matrix grains are truncated along the upper boundaries of the breccia lenses.When the flat-pebble conglomerate mounds are as thick as the oolite bed,they show a flat and smooth surface,overlain by cal-careous shale facies.Flat-pebble clasts were derived from the 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 and strong-agitating conditions.During deposition of flat pebbles,microbes grew on top of and in between pebbles,acting 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.[Conclusions]This study illustrates the complex and dynamic deposi-tional conditions for the mound-shaped flat-pebble conglomerates,suggesting multiple episodes of storm events.De-tailed sedimentological studies on flat-pebble conglomerate can provide insights into frequency,intensity,and sedi-mentary processes of deep-time storm events.
国家科技期刊平台
NSTL
万方数据
维普
