Sedimentary cycles in the Surat Basin and global changes of sea level

Six sedimentary cycles, each hundreds of metres thick, have been recognised in the Surat Basin. The Jurassic cycles (nos. 1-4) typically started with high-energy deposition of coarse sediments, and ended with labile sand, silt, and mud. The environments are thought to have been braided streams, followed by meandering streams, swamps, lakes, and deltas. After a period of non-deposition or erosion, followed by a phase of high-energy deposition, the first Cretaceous cycle (no. 5) ended with marine mud; the second (cycle 6) started with paralic silt and sand and ended with shallow marine silt. The cycles are thought to be the result of global sea-level changes, characterised by rapid falls of sea level followed by slow rises, which, respectively, lowered and raised the base of erosion in the Surat Basin. During the Jurassic, the open sea lay several hundred kilometres to the east, and the sea only occasionally entered the basin, via the Brisbane and Toowoomba Straits. In the Cretaceous, the sea level was relatively higher and eustatic falls and rises of sea level led to alternating marine regressions and transgressions. The six Surat Basin cycles correspond in time to nine global sea-level oscillations. We think that some of the latter may have been too slight to be identified in the basin. There is also evidence that local isostatic movements may have exaggerated the impact of some global cycles and obscured that of others.

苏拉特盆地已识别出六个沉积旋回，每个旋回厚度达数百米。侏罗纪旋回（编号1-4）通常以高能粗碎屑沉积起始，以不稳定的砂、粉砂和泥质沉积结束，其沉积环境被认为依次为辫状河、曲流河、沼泽、湖泊及三角洲。经历一段无沉积或侵蚀期后，首个白垩纪旋回（编号5）以高能沉积起始，最终形成海相泥岩；第二个白垩纪旋回（编号6）则以海陆交互相粉砂和砂岩起始，以浅海相粉砂沉积结束。这些旋回被认为是全球海平面变化的产物，其特征为海平面快速下降后缓慢上升，这两种过程分别降低和抬高了苏拉特盆地的侵蚀基准面。侏罗纪时期，开阔海域位于盆地以东数百公里处，海水仅偶尔通过布里斯班海峡和图文巴海峡进入盆地。白垩纪时期海平面相对较高，全球性海平面升降（eustatic sea-level rises and falls）导致海退与海侵交替发生。苏拉特盆地的六个旋回在时间上对应九次全球海平面振荡，其中部分振荡可能因幅度过小而未在盆地中被识别。另有证据表明，局部均衡运动（isostatic movements）可能放大了某些全球旋回的影响，同时掩盖了其他旋回的作用。