中國雲南楚雄前陸盆地系統之地層架構與堆積區劃分

Abstract

楚雄盆地位於中國雲南省雲貴高原西部，揚子地塊西南緣，盆地西側為麗江台緣褶皺帶，西南側為哀牢山造山帶，受到多次造山運動作用影響，現今已演變為高原地形，而無凹陷的盆地形貌。在前人的研究之中，對於楚雄盆地的形成有著不同的看法，本文藉由建立楚雄盆地之區域地層架構，與DeCelles and Giles (1996)所提出的前陸盆地系統概念作對照，為楚雄盆地是一周緣型前陸盆地系統之假說提出佐證。 楚雄盆地為一中生代的沉積盆地，形狀呈北寬南窄的楔狀，沉積物厚度可達十公里，盆地主要發育期在三疊紀。盆地內之綠汁江斷裂帶發育在基底隆起區範圍內，可沿此斷裂帶將盆地大略分為兩個基底較為凹陷的區域，故可以綠汁江斷裂帶為界，將三疊系地層分為東、西兩個地層區。西區地層發育較完整、厚度較厚，且較少缺失，中、下三疊統之地層僅出現在西區；而東區之沉積物堆積時間較晚，在晚白堊紀的馬頭山組之前未有和西區共通之地層。 因西區之沉積地層較完整，可藉由西區之縱向地層分布看出盆地演化過程所經歷的不同階段。低充填階段的代表地層為晚三疊紀卡尼期的雲南繹組及晚三疊紀諾利期的羅家大山組，已充填階段之代表地層為晚三疊紀諾利期的花果山組，而過充填階段之代表地層為早侏羅紀的馮家河組。低充填階段所沉積的岩套為巨厚的深海相複理層，已充填及過充填階段所沉積的則是淺海至陸相的磨礫層岩套。 楚雄盆地西區基底較深，沉積物厚度大，而東區基底較淺，地層厚度較薄，沉積物厚度由楚雄盆地西南側的哀牢山造山帶向東逐漸增厚，最厚的區域臨近哀牢山造山帶前緣，而再向東逐漸變薄，形成一中間較厚、兩邊較薄的角柱狀，符合DeCelles與Giles(1996)所提出之前陸盆地系統內的沉積物側向分布。 藉由文獻資料之蒐集統整，本文將楚雄盆地劃分為四個區域，此四個區域之地層特徵符合前陸盆地系統之四個堆積區。紅河斷裂帶向東至三街─沙橋斷裂帶之基底為造山楔形體，有許多向東的逆衝斷層及褶皺發育，沉積物由西向東增厚，故可對應至楔形體上方堆積區。而盆地西部由三街─沙橋斷裂帶向東至洒芷村地區，堆積巨厚沉積物，少有不整合面發育，可對應至前淵堆積區。而洒芷村以東至綠汁江斷裂帶，有一南北向延伸的基底隆起帶，有穩定大陸之岩石出露，自三疊紀晚期始有沉積物堆積，缺失侏羅紀中期至白堊紀早期之地層，地層中常有不整合面，推測此區應屬於前凸起堆積區。而此區以東至普渡河斷裂帶，其沉積物厚度較薄，且在白堊紀馬頭山組以下之地層和西部地區並無相通，故判斷此區為後凸起堆積區。

The Chuxiong Basin is located northeast of Yangtze platform, west of Lijiang fold belt, southwest of of Ailaoshan orogenic belt in the western Yunnan Province in southwest China. The Chuxiong Basin is characterized by a narrow and elongated zone in N-S direction between Ailaoshan orogenic belt, Lijiang fold belt and Yangtze platformwith an area of 36500 km2 area. The Chuxiong basin has has experienced multiple orogenic events and has result in an uplifted plateau without characteristics of depressed basin. There are many hypothesis of the forming mechanism of Chuxiong Basin. This study established the stratigraphic framework of Chuxiong basin. The framework of Chuxiong Basin can be compared with the foreland basin system by Decelles and Giles (1996) to provide evidence supporting the Chuxiong foreland basin as a peripheral foreland basin system. Chuxiong Basin is a Mesozoic sedimentary basin developed mainly during Late Triassic, receiving sediments nearly 10 km thick. This basin can be separated by the Luzhijiang fault into two depressed parts. The stratigraphic sections of the western part are thicker and more complete. Early and middle Triassic strata only appear in the western part. The sediments were deposited later in the eastern part. There are no strata in the eastern part as same as in the western part before late Cretaceous. The vertical stratigraphic sequences of the western part show the evolutional stages of the Chuxiong Basin. The representive strata of underfilled stage are Yunnanyi Formation in Carnian and Luojiadashan Formation in Norian. The stratum of filled stage is Huaguoshan Formation in Norian and the stratum of overfilled stage is Fengjiahe Formation in early Jurassic. The sediments in underfilled stage is mainly deep marine flysch, the sediments in filled and overfilled stage are molasse characteristic of shallow marine to continental environments. The base of western part of the Chuxiong Basin is deeper and sediments in western part are relatively thick, the sediment thickens toward Ailaoshan belt and the thickest part is near the front of Ailaoshan belt, then the sediment thickness decreases eastward. The lateral distribution of sediments forms a sedimentary wedge that tapers both toward and away from the orogenic belt (Ailaoshan belt), and is comparable to the sediment distribution proposed by DeCelles and Giles(1996). Through literature research, this paper concludes that the Chuxiong foreland basin can be divided into four depozones. These depozones can be correlated to the four depozones in foreland basin system. The basement rock from Red River Fault to Sanjie-Saqiao fault is an orogenic wedge with northeast propagating thrust sheets. The sediment on top of the orogenic wedge thickens eastward. This zone can be considered as a wedge-top depozone. From Sanjie-Saqiao fault to Sazhi village, the sediments are very thick (6km~10km) and almost no unconformity occurs in the stratigraphic sections. This zone can be considered as a foredeep depozone. The basement from Sazhi village to Luzhijiang fault is uplifted from north to south. Sediments started to deposit on top of the basement since late Triassic in this zone and is lack of Middle Jurassic to Early Cretaceous sediment. Unconformities appear often in the strata, hence, this paper suggests that this depozone may be a forebulge depozone. The sediments are thinner from Luzhijiang fault to Puduhe fault. The strata below Matoushan formation in Cretaceous are not correlated to the west part of Chuxiong basin, so this depozone could be determined to back-bulge depozone.