深水斜坡背景下的重力流水道—朵叶体系沉积特征及演化OA

[Objective]A typical gravity-flow sedimentation set is present in the Ordovician Lashizhong Formation at the western margin of the Ordos Basin.This study investigated the formation process of gravity flows in the third mem-ber of the Lashizhong Formation with the aim of summarizing the sedimentary characteristics and evolutionary pat-tern,and to establish a sedimentary model.[Methods]Outcrop observation,thin-section examination,grain size analysis and paleocurrent measurement were used in the analysis.[Results]Eight kinds of lithofacies and six litho-facies associations were found in the study area:horizontal bedded shale(A1);lens-shaped massive bedded calciru-dite(B1);lens-shaped parallel-bedded sandstone(B2);lens-shaped cross-bedded sandstone-siltstone(B3),wedg-es of cross-bedded sandstone-siltstone(C1),graded beds of sandstone-siltstone(D1),parallel-bedded siltstone(D2),and bedded pebbly sandstone(D3).These indicate channel axis deposition(B1),vertical aggradation chan-nels(B2,C1,A1),distributary channels(B3,C1,A1),proximal lobes(D1,A1),distal lobes(D2,A1)and mass transport deposits(D3).Channel axis deposition,vertical aggradation channels,distributary channels,proxi-mal lobes and distal lobes have been successively developed in the lowest part of the Third member of the Lashizhong Formation,which is divided into five stages.The middle and upper parts of the Third member of the Lashizhong For-mation show the development of mass transport deposits,proximal lobes and distal lobes,and is divided into seven stages.[Conclusions]The evolution of these strata is closely related to the type and energy of gravity flow.At the ini-tial stage,gravity flow was dominated by debris flow,then,as the debris flow energy weakened,turbidity current dominated.The vertical aggradation channels were developed during multi-stage periods of erosion and sedimentation.Distributary channels and lobes then developed with further weakening of the turbidity flow.Tectonic movement in-creased the sand content of sediment supply,leading to the initial development of massive transport deposition fol-lowed by large-scale lobes with higher sand content.The vertical evolution of gravity flow sedimentation was governed by a combination of factors,including sea-level fluctuations,sediment supply and its composition,as well as tecton-ic movement.An initial rise in sea level followed by a sub-sequent fall are suggested by a decrease and then increase in the scale of gravity flow development.Variations in sediment supply types gave rise to diverse sedimentary units,including channel-axis deposition composed mainly of calcirudite,as well as vertical aggradation channels,distribu-tary channels,proximal lobes and distal lobes mainly comprising fine sandstone and siltstone.Moreover,the magni-tude of sediment supply significantly impacted the morphological features,dimensions and sand-to-mud ratio of these sedimentary units.During the depositional period of the Lashizhong Formation,relatively subdued tectonic activity fa-cilitated the formation of a channel-lobe system that is relatively modest in size compared to the scale typically en-countered in exploration contexts.The interplay of the above factors-sea-level changes,sediment-supply dynamics,and tectonic setting-together shaped the architectural complexity and distribution patterns of the gravity flow depos-its in the basin.Understanding these controls is crucial for accurate reservoir characterization and prediction in hydro-carbon exploration and production.This study helps in the understanding of gravity-flow evolution in the study area,and provides a theoretical reference for oil and gas exploration.