Lacustrine high resolution sequence records from Upper Cretaceous Nenjiang Formation in Songliao Basin

摘要

The Nenjiang Formation was continuously cored in the SK-1 borehole(China Cretaceous Continental Scientific Drilling Project-SongKe1well).Over 95.7%recovery rate,the obtained continuous core is 827.6 m in length,covering the geological age ranges from late Santonian to middle Campanian.This is the most complete and continuous sedimentary deposits in the Songliao basin till now.Here we systematically presented the lithological results of the observation of the cores in 2--5 cm thickness scale at the drilling spot,and the analysis on lithofacies and sedimentary sequence,cyclic stratigraphy.13 types of sedimentary rocks deposited mainly in lacustrine,delta and fluvial environments were found in the Nenjiang Fromation,and they were subdivided into 19 microfacies including dolomite,marl,argillaceous limestone,turbidite,mudstone of still water,volcanic ash,channel-lag deposit,point bar,natural levee,crevasse splay,crevasse channel,floodplain,flood lake,distributary mouth bar,subaqueous distributary channels,subaqueous natural levee,subaqueous inter-distributary bay,distal bar and adjacent shore sand bar.The whole sedimentary column is composed of 630 sixth-order depositional cycles in the meter-scale,which can be further organized in 148 fifth-order depositional cycles,38 fourth-order depositional cycles and 5 third-order depositional cycles.Meticulous depiction(centimeter level)of the whole Formation provides an approach to study its sedimentary process with high precision.The centimeter-scale description of the sequent core presented some previously unidentified dolomite and volcanic ash layers.The cyclostratigraphy of the Nenjiang Formation was controlled by Milankovitch cycled,which is helpful to discover the climatic changes of the late Santonian to the middle Campanian in Songliao Basin.In addition,the detailed description of the sequent section is significant for the upcoming oil and gas exploration in the Songliao Basin if regarding the new type of oil/gas systems.