摘要
本文从研究磨拉石与造山带的关系入手,分析了扬子板块东缘志留系磨拉石的沉积学特征和物质成分特点,进而推测出在磨拉石堆积区东侧(相当于现代长江口外)有一条近南北向的古造山带,该造山带始形成于奥陶纪末,现在已被剥蚀并被深埋于地下。这对重新认识华南大地构造演化及古生代油气分布规律都是很有意义的。
The erogenic belt that rises straight up in relief is easy to recognize. However, some fossil orogenic belt, having been strongly eroded, now are absent from surfac of the earth and covered by later sediments. Of course it is very difficult to identify this kind of orogenic belt, but studying molasse-deposited closely related to the orogenic belt-can provide a good method for identifying the orogenic beltz.Different orogenic belt may produce different kind of molassed.The authors concluded the characteristics of molasse related to the colliding orogenic belt, such as :1 .The sedimentary body is a very thick wedge which is thicker close to the orogenic belt and pinches out in the direction of the craton.2.The clastic sediments are coarse, and thick strata of kconglomerates may occur near by the orogenic belt.3. The sediments of the molasse are of recyclic character, and the chemical composition is rich in SiO2 and so the ratio of AL2O3 SiO2 is low.4. The molasse is deposited mainly on land where the proximal portion of molasse mainly exists or in neritic environments where the distal portion of molasse often occurs.5. Abundant sedimentary structures show that paleocurrent direction is perpendicular to the orogenic belt.6. Sedimentary sequences are rhythmic ones in which the thickness of one rhythmic unit is about several cm to several m.7.The molasse is unconformable with underlying strata which is often flysch in the proximal region, and is disconformable or conformable whith underlying strata which is platform formation in the distal area.The authors analyzed the Silurian molasse formation in the Northwest of Zhejiang, Southeast of Anhui, and South of Jiangsu Provinces, and deduced that there was a Later Caledonianfossil orogenic belt in the east edge of Yangzi plate. Comparing the molasse with the underlying strata-the Upper Ordovician flysch formation in sedimentological characteristics and tectonic settings, the authors found many important facts, some of them are listed here:1.The underlying Ordovician flysch deposited in an aulacogen, and it' s clastic materials were supplied from both (southeast and northwest) sides of the basin and transported mainly along the basin axis in the northeast direction; but the sediments of the molasse came from east side and were transported mainly westward.2.The thickness of sedimentary body of the molasse may be up to 6000m in the east area, and decreases rapidly in the west of the molasse basin. 3. The mineral composition ot tne molasse apparently diners from that ot tne underlying tryscn, came from a recyclic erogenic belt.On the basis of above-mentioned study, the authors made a careful study of the sedimentological char-acteristics of Maoshan formation (Upper Silurian) in the Southern Jiangsu and Southern Anhui.The results show tht the Maoshan formation is the uppermost and sthe westpart of the molasse.The sediments come from easternside, and the bottom of the sedimentary basin sloped to the west at that time.So the source area in the east is the orogenic belt uplfted since the end of Ordovician. Because the deeper part of the orogenic belt were eroded, the slightly metamorphosed debris are more abundant and the igneous, sedimentary de-bris are fewer in the Maoshan Group than those in the lower part of the molasse.The Maoshan Group can be treated as fresh water deposited upper moiasse and the other strata (including Middle, Lower Silurian and the uppermost part of Ordovician) can be considered as the lower moiasse deposited in neritic zone. Comparing the western boundary of the lower moiasse with that of the upper moiasse, it is clear that the sedimentary axis of the moiasse basin migrated westward for at least 90km in the period from the end of Ordovician to the end of Silurian.The speed fmigration is about 3mm per year.The study on Maoshan Group further proved that there existed a colliding orogenic belt (almost from south to north in the east side of the basin ) in the area nearby the outfall of Yangzi River.The area adjacent to Jiangsu, Zhejiang and Anhui Province was the f
出处
《沉积学报》
CAS
CSCD
北大核心
1990年第2期1-8,共8页
Acta Sedimentologica Sinica
