陆丰南地区断拗转换界面厘定及其地质意义

Determination and geological significance of fault-depressiontransformation interface in the southern Lufeng area

珠江口盆地经历裂陷阶段向拗陷阶段的转变,同期沉积地层记录了该构造变革和沉积演化过程,为厘定断拗转换界面提供了可能。基于地震反射、沉积旋回特征,识别出陆丰南地区恩平组内部存在全区可追踪对比的构造沉积转换面——T72界面,也是区分上下恩平组界面。对陆丰13洼洼间隆钻井LFA开展上下恩平组碎屑锆石U-Pb定年、重矿物组分、元素分析,提出恩平组T72界面上下地层物源体系发生转变。结合区域沉积充填特征,表明陆丰南地区T72界面为断-拗转换界面。具体表现为:下恩平组沉积期湖盆继承文昌期裂陷盆地特征,湖盆水体较深,物源主要为盆地内部的中生代基底凸起,沉积层序明显受盆内低凸起控制,在盆内低凸起四周发育一系列近源的辫状河三角洲,井区主要接受来自惠陆低凸起物源;上恩平组沉积期间,湖盆进入断拗过渡期,湖盆扩大、水体变浅,盆内低凸起四周的断裂活动减弱,沉积层序逐渐向盆内低凸起超覆,盆外太古宙—古生代物源组分供给不断加强,近源的三角洲砂体逐渐向远源的浅水三角洲砂体转化,同时发育宽广的滨浅湖环境,原供给物源的惠陆低凸起为水体淹没,井区接受了北部隆起外的物源。

The Pearl River Mouth basin experienced a transition from rifting stage to depression stage. The synsedimentary strata recorded the tectonic transformation and sedimentary evolution process, thus providing the possibility of determining the fault-depression transformation interface. The characteristics of seismic reflection and sedimentary cycles were used to identify the T72 unconformity that is widely recognized as a traceable and comparable tectono-sedimentary transitional unconformity in the Lufeng south area, and is also the interface to distinguish the upper and lower Enping Formation. Through comprehensive analysis of detrital zircon U-Pb dating, heavy mineral composition and element analysis of sediment in the upper and lower Enping Formation of well LFA, it was proposed that the provenance system of the upper and lower Enping Formation had changed. Combined with the regional sedimentary filling characteristics, it shows that the T72 unconformity in the southern Lufeng area is a fault-depression transformation interface. The study shows that the lake basin inherited the characteristics of the Wenchang rifting lake basin during the deposition period of the lower Enping Formation, with relatively deepwater, and provenance was mainly local and Mesozoic igneous rock basement inside the basin. Sedimentary sequence was obviously controlled by the lower uplifts inside the basin and faults around them, and the proximal braided river delta sandbodies which were distributed around the low uplift in the basin. The well LFA area mainly received the provenance from the Huilu low uplift. During the deposition period of the upper Enping Formation, when lake basin was in the fault-depression transformation period, the lake basin was large with shallow water.Fault activities around the lower uplifts inside the basin were obviously weakened and depositional sequence overlapped toward low uplifts inside the basin. As the supply of Archean to Paleozoic provenance components from outside the basin continued to strengthen, the proximal delta sand bodies transformed gradually to distal braided river deltas and bodies, and at the same time, a broad shore shallow lake environment was developed. The Huilu low uplift was inundated by water, and the well LFA area received sediment from the source outside the northern uplift belt.