1、遼西八道壕煤盆地層序地層和火山巖層位及找煤遠(yuǎn)景研究        普查勘探資料顯示阜新盆地外圍的彰武-黑山地區(qū)分布著18個中生代盆地,其中大部分盆地被第四紀(jì)或火山巖覆蓋嚴(yán)重。其含煤性和層序地層格架不清。本文選取區(qū)內(nèi)的八道壕煤盆地為例,綜合運用了地震地層學(xué)、層序地層學(xué)、測井、煤巖層對比、火山巖同位素測年等技術(shù)手段對其進行了層序劃分、沉積相對比以及聚煤規(guī)律的研究。同時采得區(qū)內(nèi)大面積分布的火山巖樣品20余件,對其中的11件進行了SHRIMP U-Pb同位素分析。研究結(jié)果表明:區(qū)內(nèi)八道壕煤盆地的含煤地層八道壕組屬早白堊世,

2、可與阜新盆地的沙海組對比。地震剖面、測井和巖芯鑒定成果證實八道壕組形成和保存于西側(cè)有同沉積斷裂活動的小型掀斜構(gòu)造盆地之中。該盆地西深東淺,不對稱性明顯。八道壕組可分為三個三級層序,下部第1層序( Sq1)為沖積扇注入湖盆形成的水下扇-扇三角洲沉積體系,湖侵域和高位域各含一個煤組,湖侵域退積的水下扇邊緣和高位域扇三角洲平原都有厚煤層發(fā)育。成煤早期由于盆地西側(cè)醫(yī)巫閭山的上隆,沖積扇自盆地西南注入湖盆,盆地西北部湖泊深部被濁積扇粗碎屑沉積物充填淤淺首先出現(xiàn)了成煤環(huán)境,發(fā)育了厚的煤層并自西側(cè)盆緣斷裂附近向東超覆。隨著沖積扇的繼續(xù)充填,湖盆覆水較深的部位自西向東遷移,成煤沼澤隨之向東移動,因此煤層之上被

3、一套進積的扇三角洲沉積體系所覆蓋;Sq1湖侵域的煤層比高位域的煤層分布面積要大。待湖盆逐漸變淺和淤積,碎屑沉積物的顏色由黑、黑灰、灰、灰綠逐漸變?yōu)樽匣液妥霞t,表明晚期陸上沖積扇沉積已經(jīng)代替了扇三角洲沉積,成煤作用隨之結(jié)束。所取的11件火山巖樣品測得的U-Pb同位素年齡都在122Ma-132Ma之間,且大部分為125Ma-127Ma,時代均屬早白堊世中期。其間包括3次火山噴發(fā)活動,第1次和第3次為中基性玄武安山巖的裂隙式噴發(fā),為構(gòu)造活動期的產(chǎn)物;第2次為中性粗安巖的中心式噴發(fā),為構(gòu)造穩(wěn)定期的產(chǎn)物。與阜新盆地南部義縣附近的義縣組火山巖噴發(fā)的時代大致相當(dāng)。因此除非存在構(gòu)造逆沖的情況,該火山巖下找中生

4、代煤層的希望渺茫。研究表明區(qū)內(nèi)這些盆地的呈“隆、凹”相間的格局,且展布方向有一定的規(guī)律性,而且呈NEE方向展布的6個是早侏羅世盆地,其中有的可能存在北票組的含煤地層和可采煤層;而呈NNE方向展布的12個均是早白堊世盆地,其中如八道壕、雷家和謝林臺等含煤性較好,可作為找煤的遠(yuǎn)景區(qū)Ther are 18 Mesozoic basins distributed in Zhangwu-Heishan region, west Liaoning province. Most of which are severely covered with the Quaternary strata or volca

5、nic rocks. This paper takes Badaohao Basin in this region for example and combines the seismic stratigraphy, sequence stratigraphy, well logging, correlation of coal seam and strata, and other techniques to research its sequence division, sedimentary facies correlation and the coal accumulation rule

6、. Volcanic rock is widely distributed in this region. Over 20 volcanic rock samples are collected, 10 of which the SHRIMP U-Pb isotope dating analysis is conducted to. The research result shows:Badaohao Formation - coal-bearing strata - in Badaohao Coal Basin is of early Cretaceous Period and can be

7、 correlated to Shahai Formation in Fuxin Basin. The seismic profiles, well logging and core identification show that Badaohao Formation formed and is preserved in small-scaled tilting structure with synsedimentary faulting events in the west. This basin is deep in the west and shallow in the east, a

8、nd shows obvious unsymmetry. Badaohao Formation can be divided into 3 third-order sequences. The first sequence in the lower part is one subaqueous-fanfan-delta depositional system which was caused by alluvial fan intruding lake basin; the lake transgressive system tract and high-stand system tract

9、each contain one coal-seam group; and thick coal seams develop at the edge of subaqueous fan retrograded by lake transgressive system tract and in the region appropriately covered with water in the fan delta of high-stand system tract. At the early stage, due to the uplift of Yiwulv Mountain to the

10、west of the basin, the alluvial fan intruded the lake basin from the southwest of the Basin; and the deep part of the lake in the northwest of the basin is filled by coarse clastic sediments in turbidite fan to form the coal-forming environment, develops thick coal seams and overlaps eastwards from

11、the place near the west basin margin. As the alluvial fan continued filling the deep part of the lake, the part which was covered with deep water in the lake basin migrated from the west to the east and the coal-forming swamp moved eastwards, so the coal seam is covered with a prograding fan-delta s

12、edimentary system and the coal seam in Sq1 lake transgressive system tract is more widely distributed than that in high-stand system tract. As the lake basin shoals and silts up, the color of clastic sediment gradually changes from black, black gray, gray and grayish green to purplish grey and red p

13、urple, meaning that the subaerial alluvial fan deposit of late stage has replaced the fan-delta deposit and that the coal-forming process ends.The U-Pb isotope ages of the 11 volcanic rock samples are 122Ma132Ma and mostly 125Ma127Ma, which belongs to the middle period of early Cretaceous epoch. 3 v

14、olcanic eruptions happened: the 1st and 3rd are fissure eruptions of intermediate-basic basaltic andesite and are resulted from the structures of active period, while the 2nd belongs to central eruption of neutral trachyandesite and is derived from the structure of stabilized period. They are almost

15、 cotemporary with the volcanic eruptions of Yixian Formation near Yi County in the south of Fuxin Basin. Therefore, it is unpromising to find Mesozoic coal seams under volcanic rock unless thrust structure exists.The research shows that these basins alternate between uplift and concave and that their spreading directions follow certain rule. The 6 NEE-trending basins are of early Jurassic epoch, some of which may contain coal-bearing strata and minable coal seams of Beipiao Formation. The 12 NN