西藏南部白垩纪深海沉积地层——冲堆组及其地质意义

藏南日喀则地区,有一套深海沉积地层断续出露于吉定—白朗一带。在西段,它基本上位于蛇绿岩北侧。在东段则由于构造原因而夹在蛇绿岩中间(图1)。

THE CONGDU FORMATION——CRETACEOUS DEEP-SEA DEPOSITS IN SOUTHERN XIZANG (TIBET) AND ITS SIGNIFICANCE

In Xigaze rigion,southern Xizang,the ophiolite pillow lava are overlain directly by a ophiolitic flysch-Congdu Formation.The Congdu Formation may be subdivided into three parts.The lower part is consisted of green and purplish red rhythmic alternations of siliceous mudstones,silty mudstones,siliceous siltstones,silty cherts and fine-grained sandstones,intercalated with medium to coarse-grained sandstones and few limestones.In its basal part there are some radiolarian cherts.It has a total thickness of 20-140 m.The clastic compositions in this part are mainly composed of plagioclases and detritus of basic volcanics,with some pyroxenes and detritus of chert and serpentine,generally lacking in quartz grain.These clasts,containing ra-diolaria shells,often show well-sorting and orientation.The abundant radiolaria fossils,including Patulibracchium grapevinensis Pessagno,Halesium sp.,Crucella sp.,Archaeospongoprunum tehamaensisPessagno,Vitorfus sp.,C Pessagno,Dicroa sp.,Cassideus sp.,Petasiforma sp.,Ultranapora cf.praespinifera Pessagno,Holocryptocapsa sp.,Thanarla praeveneta Pessagno,Dictyomitra sp.,Pseudodictyomitra lodoganensis Pessagno,Novixitus sp.,Pseudoeucyrtis micropoma (Squinabol) etc.,indicate the Late Albian to Early Cenomanian age.The upper part is consisted of 100 m of gray green or dark gray rhythmic alternations.Its clastic compositions are similar to those of the lower part,but argillaceous composition is increased,the clastic grain size is reduced and radiolaria fossils are more less.The top part comprises 20 m of calcareous shales and limestones,containing abundant Rotalipora? sp.or radiolaria shells replaced by calcite within micrite.Its age may still be Cenomanian.Upwards,the Congdu Formation is followed conformable by the Angren Formationa Turonian flysch.In its basal part about 20 m of olistostrome contains blocks and pebbles of Rotalipora limestone,chert,sandstone and basic volcanics derived from Congdu Formation and ophiolite,that suggests a syndepositional tectonic movement.The detritus of the Congdu Formation were obviously derived from a near active oceanic ridge,and were deposited in a basin closed to the carbonate compensation depth (CCD).However,the basin’s depth were gradually reduced,and finally was obviously lower than CCD.The detritus of the Angren Formation,except those derived from ophiolite,also contain many clasts of intermediat-acidic magmatic rocks,which were obviously derived from the Gandise on the north.Such change of the sedimentary environment may be caused by the northward shift of the South Tethys.The thin carbonate member and the entrance of terrigenous detritus in a short time all mean that the northern part of the Xigaze sea might not be wide during this time.It might be similar to a small ocean or even a marginal sea,and that is suported by the characters of the chemical compositions of the sheeted dykes and pillow lavas^[3,15].