Cretaceous-Paleogene biomagnetostratigraphy of ODP Leg 121 sites@en

Broken Ridge, in the eastern Indian Ocean, is a shallow-water volcanic platform which formed during the Early to middle Cretaceous at which time it comprised the northern portion of the Kerguelen-Heard Plateau. Rifting during the middle Eocene and subsequent seafloor spreading has moved Broken Ridge about 20 infinity N to its present location. The sedimentary section of Broken Ridge includes Turonian-lower Eocene limestone and chalk with volcanic ash, an interval of detrital sands and gravels associated with middle Eocene rifting and uplift, and a middle-late Oligocene unconformity overlain by a thin section of Neogene-Holocene pelagic calcareous ooze. This paper summarizes the available post-cruise biostratigraphic and magnetostratigraphic data for the Cretaceous-Paleogene section on Broken Ridge. The synthesis of this information permits a more precise interpretation of the timing of events in the history of Broken Ridge, in particular the timing and duration of the middle Eocene rifting event. Paleontologic data support rapid flexural uplift of Broken Ridge in response to mechanical rather than thermal forces. Other highlights of the section include a complete Cretaceous/Tertiary boundary and an opportunity for first-order correlation of Paleogene diatom stratigraphy with that of the calcareous groups.破碎海脊（Broken Ridge）位于东印度洋，是一处形成于早白垩世至中白垩世（Early to middle Cretaceous）的浅水火山台地，彼时它曾是凯尔盖朗-赫德高原（Kerguelen-Heard Plateau）的北部组成部分。中始新世（middle Eocene）的裂谷作用（Rifting）及随后的海底扩张（seafloor spreading），使破碎海脊移动至现今的北纬20度附近区域。该海脊的沉积剖面包含赋存火山灰（volcanic ash）的土仑阶（Turonian）至下始新统石灰岩与白垩（chalk）、与中始新世裂谷及抬升（uplift）作用相关的碎屑砂与砾石层段，以及被薄层新近纪-全新世（Neogene-Holocene）远洋钙质软泥（pelagic calcareous ooze）覆盖的中晚渐新世（middle-late Oligocene）不整合面（unconformity）。本文梳理了破碎海脊白垩系-古近系（Cretaceous-Paleogene）剖面已有的航次后生物地层学与磁地层学数据。对这些信息的综合分析，可更为精准地解析破碎海脊演化历史中的事件时序，尤其是中始新世裂谷事件的发生时间与持续时长。古生物学数据表明，破碎海脊的快速挠曲抬升是受机械作用力而非热动力作用驱动。该剖面的其他重要发现还包括完整的白垩系/第三系（Cretaceous/Tertiary）界线，以及首次将古近系硅藻地层与钙质类群地层进行一级对比的契机。