The Deposition Patterns And Geological Implication Of Early Paleozoic’s Mixed Sediments Of Terrigenous Clastics And Carbonate Deposits In Helan Mountain

Helan Mountain is located in the transition zone of Alxa block, North Chinablock and Qinling-Qilian orogenic, therefore its structure is complex and thesedimentary construction is unique, and its early Paleozoic develops a mixedsedimentary mode composed of terrigenous clastics and carbonate of marine facies.The mixed sediments of terrigenous clastics and carbonate are widespread in variablemarine sedimentary environments, its theoretical and practical significance are bothimportant, however, due to rather late start and the limit of dual mode, the research isrelatively weak. Domestic and foreign scholars have not do much work about themixed sediments of Helan mountain, however, the research is benefit to clarify theEarly Paleozoic’s sedimentary environment, provenance and regional evolution.Combined with regional tectonic setting, paleontology, petrological andsedimentary characteristics, the paper carries on a research on the early Paleozoic’ssedimentary characteristics, performance and patterns, and also expounds thegeological significance. From a micro point of view, the mixed sediments arecharacterized by mixed sedimentary rocks, and there are four types: argillaceouslimestone, limestone with mud, gray conglomerate and gray siltstone. From a macropoint of view, the mixed sediments are characterized by mixed layer system, and thereare three types: terrigenous clastics and carbonate rocks, mixed sedimentary rocks andterrigenous clastics, mixed sedimentary rocks and carbonate rocks, both graded mixeddeposition and abrupt mixed deposition reflect a complex genetic mechanism. On thebasic of partitions stratigraphic of study area, the paper emphasis in liithologic andlithofacies characteristics, sedimentary facies, strata contact relations, relationsbetween the residual strata distribution and the provenance analysis to analysis thepattern of early Paleozoic’s mixed sediments and proposes the model of early Paleozoic mixed sediments of study area which are collage mixed deposition ofCambrian and gradual mixed deposition of Ordovician.Tectonic movements, changes of sea level, the supply of terrigenous materialjointly controlled the early Paleozoic’s mixed sediments of study area. Consolidatedthe mixed sedimentary patterns and mode, the paper discusses the early Paleozoic’ssedimentary pattern of study area on the view of perspective of mixed sediments. TheEarly Paleozoic of study area experienced such an evolutionary process: thecoexistence of Oceanic and cratonic basin in Cambrian-confined cratonic basin inthe Early Ordovician-foreland basin prototype in the late Ordovician. The study areaof Cambrian presents abrupt contact with deepwater flysch formation and shallowwater platform construction, indicates that they belongs to different strata area.Tectonic movement controlls the mixed deposition, and there is an ancient oceanbasin between the Alashan block and the Qilian block. To the early Ordovician, thestudy area shows a shallow tableland carbonate deposition which covering the wholestudy area. It indicates that the study area is in the same tectonic environment. Alongwith the Qilian block approaching to the Alashan block, the North Qilian Ocean hasalready turned off in the early Ordovician, and the Qilian block and the Alashan blockhas been pieced together, the study area is limited cratonic basin. To the middleOrdovician, with the global transgression, the study area reappears mixed sediments,and presents a gradual mixed sedimentary model. To the late Ordovician, the studyarea presents the deep-water slope clastic deposition, and it is foreland basinprototype.