再论华北寒武系竹叶状灰岩丘成因

Origin of Cambrian Flat-Pebble Conglomerate Mounds in the North China Platform

[目的]竹叶状灰岩通常被解释为风暴沉积,但其沉积过程,尤其是含竖直砾屑的竹叶状灰岩丘的成因,仍存在很大争议。[方法]对鲁西寒武系芙蓉统炒米店组的此类竹叶状灰岩开展了传统沉积学研究,以示其蕴含的复杂的沉积过程。[结果]该竹叶状灰岩发育在两中厚层鲕粒灰岩之间,以底平顶凸的丘形为特征,也见不规则形态。竹叶状砾屑是由先前沉积的薄板状球粒颗粒灰岩,被风暴引起的强水流或波浪破碎而成;这些砾屑在鲕粒灰岩硬底构造之上,由风暴混合流改造搬运,最终呈不连续的丘状或不规则状堆积起来。杂基中的化石碎屑、破碎的鲕粒、钙化微生物团块等也指示了高能动荡的水体环境。在竹叶状砾屑沉积时,微生物在其之上或之间生长、钙化,并将砾屑黏结,阻止了砾屑被再次改造而保存为一些竖直或倾斜的排列状态;同时钙化微生物黏结或障积更多的竹叶状砾屑,而最终形成大小不一的竹叶状丘或滩。竹叶状砾屑丘在沉积之后,其边缘和顶部不断地被强水流和波浪侵蚀,并被随之沉积的鲕粒覆盖。[结论]研究揭示了丘形竹叶状灰岩复杂动荡的沉积过程,指示其经历了多次风暴事件的沉积、侵蚀和改造。基于详细系统的沉积学特征,对竹叶状灰岩的成因解释,将对深时风暴的频率、强度及沉积过程等具有一定的指导意义。

[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.