Sedimentary Characteristics And Paleogeography Of The Hekou Group In Lanzhou-minhe Basin In The Early Cretaceous

Late Mesozoic, extensional structure is global events, when appeared a series ofMesozoic basin in Northwest of China, and the Lanzhou-Minhe basin which locatedat northern of Qilian mountain was a typical intracontinental rift sedimentary basin, sothe research on basin evolution, provenance, paleogeography and paleoecology in theEarly Cretaceous is significance.Based on the basins filler, we system analysed the characteristics of rocksequences, stratigraphic correlation, characteristics of the provenance, basin evolutionand paleogeography by using of the rock assemblages, stratigraphic sequence,paleocurrents, gravel components, sandstone detrital components of the Heko Group.We can consider that:The Heko Group of the Lanzhou-Minhe basin in the Early Cretaceous was giantthick terrestrial clastic sedimentary, which has an evolution of alluvial fan-fandelta-lacustrine/fluvial-delta for lengthwise, and characteristic of alluvial fan-fandelta-fluvial-lacustrine along transverse. The study indicates that the DatongheFormation was ascribed to the Late Jurassic-the Early Cretaceous is better,furthermore, the Lanzhou-Minhe basin has the earliest deposition in the EarlyCretaceous. Through the analysis of paleocurrent direction, gravel components andsandstone detrital components, we can consider that the provenance direction of theHeko Group was from the basin margin to the center, and the main source area wasthe uplifted Qilianshan Mountains around the basin during the Early Cretaceous. Onthe background of the regional tectonic movement, the evolution of the basin in theEarly Cretaceous can be classified into three phases: initial rift subsidence stage,expanding period and contraction period. Early stage, basin margin has coarse clasticsedimentary which standed for a set of alluvial fan; medium-term, the scale of basinexpanded with the sedimentary center migrated to the north, and developed anextensive and stable fluvial/lacustrine sedimentary, it was thriving period of plantsand animals; later stage, scope of lake shrinked, the whole basin uplifted and wasdenuded, then the basin evolution in Early Cretaceous was over. From the research ofsedimentary facies, sedimentary environment, basin evolution, paleontology and eolian sands of the Heko Group in the Early Cretaceous can insist that the wholeclimate was hot arid-semiarid, and the characteristics of paleogeography were underthe restraint of basin tectonic evolution and paleoclimate, so the paleogeographicenvironment pattern can be restored from the foregoing researches.