Study On The Provenance And Sedimentary System Of The Upper Carboniferous Benxi Formation In The Central-eastern Ordos Basin

Based on the outcrop,core,logging,heavy minerals,cathode luminescence,main trace elements,rare earth elements and detrial zircon,this study analyzed the characteristics of provenance system and tectonic evolution,determined the palaeoenvironmental characteristics,identified the sedimentary facies types and the distribution of sedimentary facies and further studied the sandbodies types and distribution.Finally,we constructed sedimentary evolution model and the tide-controlled delta sedimentary model of the Benxi Formation.The results show that:(1)There are two main provenance systems which can be further divided into four provenance systems in the northern and southern basin.The source rock types in the northwest are metamorphic rocks,sedimentary metamorphic rocks and a small amount of meso-basic magmatic rock,the tectonic setting is generally stable and the continental island arc is locally developed in the passive continental margin.The source rocks in the northeast are meso-basic magmatic rocks and sedimentary metamorphic rocks,the tectonic setting is generally active,the continental island arc is well developed in the passive continental margin.The source rock types in the southwest are basal metamorphic rocks,sedimentary rocks and a certain amount of meso-acid magmatic rocks.As continental island arc and active continental margins,it is generally active.The source rock types in the southeast are metamorphic quartz sandstones,quartzites,partial sedimentary rocks and a small part of magmatic rocks,it is relatively stable passive continental margin with localized island arcs.After analyzing it,it is believed that the North Qinling microcontinental terrane which was collided with the North China Plate supplied sediments to the southern part of the basin during this period.(2)During the Benxi Formation deposited,the water was gradually transited from anaerobic fresh water at the edge of the basin to the reductive brackish water and salt water in the interior of the basin.The climate is generally warm and humid.From early to the late,the salinity gradually decreases,the reductivity is strongest in the medium,and weak in the early and late stages.At the end of Benxi phase,it gradually dried up.(3)The Benxi Formation mainly developed braided river delta,tidal flat,tidal controlled delta and coastal shallow sea facies.The Hutian member is mainly composed of alluvial plain,iron-aluminum layers and marsh mudstone deposits.During the Pangou member deposited,the southern basin margin developed tide-controlled delta and the northern margin developed braided delta.The Jinci member developed the same sedimentary facies with the Pangou member,but the northern braided delta extend further and the southern delta shrinked.During this period,river channals,estuary dams,tidal sand dams,tidal flat sandbodies were developed.(4)The first tectonic control on sedimentary facies are characterized by the geomorphological reversal.The southern palaeogeomorphy was higher than the north in the Hutian and Pangou phase.The river was northward in the Hutian member,and the tidal controlled delta extended to the center of the basin in the Pangou phase.In the Jinci phase,the northern palaeogeomorphy was higher than the south,so the northern braided delta extended further than before and southern delta shrinked.The second tectonic control is that the river developed in the gully of the tectonic geomorphology,and the low point of the palaeogeomorphy of southern margin is the sediment entry point.The sediment supply on the sedimentary facies is mainly manifested in that the stronger northern sediment supply corresponded to the braided delta and the weaker southern sediment supply corresponded to the tidal controlled delta,the undeveloped sediment supply margin developed tidal flat deposition.The climate control is manifested in that the climate was warm and humid,the vegetation was prosperous,there was a higher weathering and stronger source supply in the source area,forming the delta on the southern and northern margin.(5)The Benxi Formation sedimentary evolution model and the tidal-controlled delta sedimentary model were established.During the Benxi Formation sedimentary period,the sea level was gradually increased.From the Hutian member to the Jinci member,the sedimentary range was gradually expanded.The Hutian member mainly developed the weathered residual layer of the alluvial plain,the river flowed eastward and finally flowed to the north.In the Pangou phase,the tide-controlled delta was developed in the south and braided delta was developed in the north.During the Jinci member deposited,the sea level briefly declined,and the geomorphology reversed to become north-high and south-low,and the northern sediment supply increased,so that the northern delta extended further and the southern delta shrinked.The tidal-controlled delta sedimentary model shows that there are different sand stacking sequences in defferent sub-facies,and the sandbodys were mainly developed in the delta front.