Miocene to Pleistocene sedimentation at ODP Site 133-823

More than 2000 turbidite, debris-flow, and slump deposits recovered at Site 823 record the history of the Queensland Trough since the middle Miocene and provide new insights about turbidites, debris flow, and slump deposits (herein termed gravity deposits). Changes in the composition and nature of gravity deposits through time can be related to tectonic movements, fluctuations in eustatic sea level, and sedimentological factors. The Queensland Trough is a long, relatively narrow, structural depression that formed as a result of Cretaceous to Tertiary rifting of the northeastern Australia continental margin. Thus, tectonics established the geometry of this marginal basin, and its steep slopes set the stage for repeated slope failures. Seismic data indicate that renewed faulting, subsidence, and associated tectonic tilting occurred during the early late Miocene (continuing into the early Pliocene), resulting in unstable slopes that were prone to slope failures and to generation of gravity deposits. Tectonic subsidence, together with a second-order eustatic highstand, resulted in platform drowning during the late Miocene.
The composition of turbidites reflects their origin and provides insights about the nature of sedimentation on adjacent shelf areas. During relative highstands and times of platform drowning, planktonic foraminifers were reworked from slopes and/or drowned shelves and were redeposited in turbidites. During relative lowstands, quartz and other terrigenous sediment was shed into the basin.
Quartzose turbidites and clay-rich hemipelagic muds also can record increased supply of terrigenous sediment from mainland Australia. Limestone fragments were eroded from carbonate platforms until the drowned platforms were buried under hemipelagic sediments following the late Miocene drowning event. Bioclastic grains and neritic foraminifers were reworked from neritic shelves during relative lowstands. During the late Pliocene (2.6 Ma), the increased abundance of bioclasts and quartz in turbidites signaled the shallowing and rejuvenation of the northeastern Australia continental shelf. However, a one-for-one relationship cannot be recognized between eustatic sea-level fluctuations and any single sedimentologic parameter. Perhaps, tectonism and sedimentological factors along the Queensland Trough played an equally important role in generating gravity deposits.
Turbidites and other gravity deposits (such as those at Site 823) do not necessarily represent submarine fan deposits, particularly if they are composed of hemipelagic sediments reworked from drowned platforms and slopes. When shelves are drowned and terrigenous sediment is not directly supplied by nearby rivers/point sources, muddy terrigenous sediments blanket the entire slope and basin, rather than forming localized fans. Slope failures affect the entire slope, rather than localized submarine canyons. Slopes may become destabilized as a result of tectonic activity, inherent sediment weaknesses, and/or during relative sea-level lowstands. For this reason, sediment deposits in this setting reflect tectonic and eustatic events that caused slope instabilities, rather than migration of different submarine fan facies.

在823站位回收的2000余套浊积岩（turbidite）、碎屑流沉积（debris-flow deposit）及滑塌沉积（slump deposit）记录了中新世中期以来昆士兰海槽（Queensland Trough）的演化历史，为浊积岩、碎屑流沉积与滑塌沉积（本文统称为重力流沉积）的研究提供了全新视角。重力流沉积的组分与特征随时间的变化，可与构造运动、全球海面升降（eustatic sea level）波动及沉积学因素相关联。昆士兰海槽是一条狭长的结构性坳陷，形成于澳大利亚东北大陆边缘白垩纪至古近纪的裂谷作用过程中。因此，构造作用奠定了该边缘盆地的几何形态，其陡峭的斜坡为多次斜坡失稳埋下了隐患。地震资料显示，晚中新世早期（持续至上新世早期）发生了新一轮断裂活动、沉降及伴随的构造倾斜，导致斜坡失稳，易于发生斜坡破坏并形成重力流沉积。构造沉降结合二级全球海面高位体系域，使得晚中新世时期台地发生淹没。
浊积岩的组分特征反映了其成因，并为相邻陆架区的沉积作用性质提供了认识。在相对海面高位期及台地淹没期，浮游有孔虫（planktonic foraminifers）从斜坡和/或被淹没的陆架区再搬运，并沉积于浊积岩中。在相对海面低位期，石英及其他陆源沉积物（terrigenous sediment）被搬运至盆地中。
石英质浊积岩与富黏土半远洋泥（hemipelagic muds）也可记录来自澳大利亚大陆的陆源沉积物供给增加。碳酸盐台地（carbonate platforms）的石灰岩碎屑被侵蚀搬运，直至晚中新世台地淹没事件后，碳酸盐台地被半远洋沉积物掩埋。相对海面低位期，浅海有孔虫（neritic foraminifers）与生物碎屑颗粒从浅海陆架区再搬运沉积。上新世晚期（2.6 Ma），浊积岩中生物碎屑与石英丰度的增加，标志着澳大利亚东北大陆架发生了浅化与活化。然而，全球海平面升降波动与任何单一沉积学参数之间并不存在一一对应的关系。昆士兰海槽一带的构造作用与沉积学因素或许在重力流沉积的形成中发挥了同等重要的作用。
823站位产出的浊积岩及其他重力流沉积未必代表海底扇沉积（submarine fan deposits），尤其当它们由来自被淹没台地与斜坡的再搬运半远洋沉积物（hemipelagic sediments）组成时。当陆架被淹没且附近河流/点源未直接供给陆源沉积物时，泥质陆源沉积物会覆盖整个斜坡与盆地，而非形成局域性海底扇。斜坡破坏会波及整个斜坡，而非局限于海底峡谷（submarine canyons）。斜坡失稳可能由构造活动、沉积物固有软弱特性，或在相对海面低位期引发。正因如此，该环境下的沉积记录反映的是引发斜坡失稳的构造与海面升降事件，而非不同海底扇相的迁移。