四川海相克拉通盆地显生宙演化阶段及其特征

四川叠合盆地是在四川海相克拉通盆地基础上形成的。本文利用最新的钻井资料、地震资料及其研究成果,详细阐述了四川海相克拉通盆地在显生宙的演化阶段及其特征。研究结果发现,四川海相克拉通盆地显生宙演化可分为早晚两期,早期为晚震旦世-石炭纪,晚期为二叠纪-中三叠世。两期克拉通演化都经历了早期弱拉张,后期弱挤压阶段。弱拉张初始阶段都有一次海相碳酸盐岩的大面积稳定沉积(震旦系灯影组和二叠系栖霞-茅口组)和随后的隆升剥蚀作用及风化壳岩溶作用。其后进入弱拉张期,发育拉张槽,拉张强度最大的部位均位于克拉通的西北部,都是从克拉通的西北部边缘向克拉通内部减弱。然而,两期拉张槽的充填特征不同,早寒武世绵阳-长宁拉张槽是补偿型充填,与拉张槽周缘相比,拉张槽内沉积厚度巨大;晚二叠世-早三叠世开江-梁平拉张槽为欠补偿型充填,与拉张槽周缘相比,拉张槽内沉积厚度非常薄。拉张期结束后进入弱挤压阶段,形成古隆起,挤压强度最大的部位均位于克拉通的西南部,都是从克拉通的西南边缘向克拉通内部减弱。弱拉张阶段的拉张槽与弱挤压阶段的古隆起均为大角度相交关系;然而,拉张槽和古隆起的规模差别较大,早寒武世绵阳-长宁拉张槽面积约5.4×10~4km~2,对应的加里东期乐山-龙女寺古隆起面积6×10~4km~2;晚二叠世-早三叠世开江-梁平拉张槽面积约2.0×10~4km~2,对应的印支期开江古隆起面积0.8×10~4km~2;晚二叠世-早三叠世蓬溪-武胜拉张槽面积约1.5×10~4km~2,对应的印支期泸州古隆起面积4.2×10~4km~2。绵阳-长宁拉张槽的规模比开江-梁平拉张槽、蓬溪-武胜拉张槽要大,乐山-龙女寺古隆起的规模也大于泸州-开江古隆起的规模。四川海相克拉通盆地显生宙演化特征在很大程度上控制了四川叠合盆地海相油气地质条件的发育和油气藏的形成分布。

Evolutionary episodes and their characteristics within the Sichuan marine craton basin during Phanerozoic Eon, China

The Sichuan superimposed basin, located at western China, is founded on the Sichuan marine craton. The Phanerozoic evolution within the Sichuan marine craton can be divided into two episodes. The first episode is from Late Sinian (Edicarian) Epoch to Carboniferous Period, while the second one from Permian Period to Middle Triassic Epoch. Both of the two episodes are characterized with a gentle extensional stage and a subsequent gentle shortening stage. However, there are intense extension and shorting at the correspondent episodes in the margins of the Sichuan marine craton. The beginning of the both gentle extensional stages within the Sichuan marine craton accommodated widespread stable deposits of marine platform facies (i.e., Upper Sinian Dengying Formation, Middle Permian Qixia and Maokou formations), which were superimposed by subsequent uplifting and erosion with typical karstification. Furthermore, an extensional stage is characterized by the formation of intracratonic sags (e.g., the Early Cambrian Mianyang-Changning sag, the Late Permian to Early Triassic Kaijiang-Liangping sag) with maximum extension at the northwestern part of the craton, decreasing into the center of the craton. It should be noted that the intracratonic sags may be different in sedimentation and geodynamics. For example, the Early Cambrian Mianyang-Changning intracratonic sag is with thick mudstone, indicating of compensation-type filling process, in contrast to the Late Permian to Early Triassic Kaijiang-Liangping sag dominated with thin mudstone as a result of starvation-type filling process. In a gentle shortening stage, it is usually characterized by the formation of paleo-uplifts (i.e., the Caledonian Leshan-Longnvshi paleo-uplift, Indosinian Kaijiang paleo-uplift), of which the maximum highhills located at the southwestern part of the craton, indicating of a geodynamic decrease from the southwest to the center of the craton. In particular, there is a substantial intersection angle between the intracratonic sage and the correspondent paleo-uplift, even to perpendicular. Besides, the areas of the intracratonic sags and the correspondent paleo-uplifts are different. The Early Cambrian Mianyang-Changning sag and Caledonian (Early Paleozoic) Leshan-Longnvshi paleo-uplift are with area of 5.4×10⁴km² and 6×10⁴km², respectively; while the Late Permian to Early Triassic Kaijing-Liangping sag and Indosinian (Triassic) Kaijiang paleo-uplift with area of 2.0×10⁴km² and 0.8×10⁴km², and the Late Permian to Early Triassic Pengxi-Wusheng sag and Indosinian Luzhou paleo-uplift with area of 2.0×10⁴km² and 4.2×10⁴km², respectively. The evolutionary characteristics mentioned above within the Sichuan marine craton have controlled the hydrocarbon accumulations and their distributions across the Sichuan superimposed basin.