龙门山北段观雾山组地层时代分布、沉积充填及其古地理意义

Stratigraphic age distribution and sedimentary filling of the Guanwushan Formation in the northern segment of Longmenshan Mountains,and its palaeogeographic significance

自四川盆地西北部(川西北)泥盆系观雾山组油气勘探取得重要进展以来,观雾山组的沉积储层研究受到了广泛重视。然而,由于观雾山组地层划分方案的变动和地层的穿时性,导致目前对观雾山组的时代分布和充填过程认识不清,从而影响了沉积古地理格局的恢复。本文以四川广元西北乡、潜溪河剖面泥盆系观雾山组牙形石生物地层研究为基础,结合区域古生物地层资料,认为江油马角坝以北的观雾山组绝大多数沉积于晚泥盆世早—中弗拉期,且地层沉积时间自南西向北东逐渐变晚。龙门山北段观雾山组具有自西向东、自南向北地层厚度减薄和地层时代变新的特征,表明来自扬子地台西部的古特提斯洋海水向东侵入龙门山地区,因地形差异和渐进式海侵形成了自南西向北东的超覆沉积。由于岩相分异,超覆沉积形成的观雾山组与金宝石组在区域上既表现为上下叠置关系,也表现为同期异相的相变关系。该研究不仅对约束川西北地区观雾山组地层时代提供了可靠的古生物证据,而且对认识龙门山地区泥盆系的充填过程、沉积古地理格局均具有重要意义。

The study on sedimentation and reservoir of the Guanwushan Formation has been widely investigated in recent years since a great breakthrough was obtained in hydrocarbon exploration of the Devonian Guanwushan Formation in northwestern Sichuan basin.However,the stratigraphic age distribution and sedimentary filling pattern of the Guanwushan Formation remains controversial due to disputed stratigraphic division and stratum diachronism,which has hindered the reconstruction of the Devonian palaeogeographic configuration in the Longmenshan area.Based on the results of conodont biostratigraphy of the Guanwushan Formation at the Xibeixiang and Qianxihe sections in combination with regional geological material,this study reveals that the Guanwushan Formation distributed in the north of Majiaoba area(Jiangyou County) were generally deposited from the early to middle Frasnian(Late Devonian),and the depositional age trends are younger from the southwest toward the northeast.The Devonian strata in the Longmenshan area tend to be younger in stratigraphic age but decrease in thickness from the west to the east,suggesting that the Devonian seawater in Longmenshan area came from the Palaeotethys ocean that was located in the western margin of the Yangtze Platform.During the early to middle Frasnian,the episodic transgressions and topographic height differences caused sedimentary overlap in the Guanwushan Formation from the southwest to northeast at the northern segment of Longmenshan area.On the influence of overlap deposition,both superimposed and syndepositional relationships between the Guanwushan and Jinbaoshi formations in different areas could be identified due to facies differentiation.This study provides a robust palaeontological evidence for constraining the stratigraphic age of the Guanwushan Formation,which is of great significance in understanding of the depositional filling process of the Devonian strata as well as important for reconstructing the Devonian palaeogeographic configuration in the Longmenshan area.