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Eocene to Oligocene high paleolatitude neritic record of Oi-1 glaciation in the Otway Basin southeast Australia
Global and Planetary Change ( IF 3.9 ) Pub Date : 2020-08-01 , DOI: 10.1016/j.gloplacha.2020.103218
Stephen J. Gallagher , Bridget Wade , Li Qianyu , Guy R. Holdgate , Paul Bown , Vera A. Korasidis , Howie Scher , Alexander J.P. Houben , Brian McGowran , Tony Allan

Abstract Multiple stable isotope investigations from upper Eocene to lower Oligocene deep-water marine sequences record the transition from global greenhouse to the icehouse conditions (Oi-1 glacial). While Southern Ocean high latitude deep sea records of this transition are well known, their shallow marine equivalents are rare and have the potential to record the eustatic and oceanic consequences of Paleogene glacial variability. The well-known high paleolatitude (~55°S) neritic carbonate sequence at Browns Creek and Castle Cove in the Otway Basin in southeast Australia spans the Eocene-Oligocene boundary. During this time the area lay on the northeastern margin of the Australo-Antarctic Gulf facing the evolving Southern Ocean. The importance of this record has been hampered by a lack of a consistent stratigraphy and contradictory microfossil interpretations. To reconcile these issues we combine new bio-, chemo- and lithostratigraphic analyses of the outcrops and a new core (Colac-2) with pre-existing data to revise the stratigraphy. This confirms the middle/upper Eocene boundary is near the base of the section. The overlying upper Eocene siliciclastic strata are truncated by an unconformity (of ~0.8 Ma in duration) and overlain by glauconitic sand (the Notrostrea greensand) deposited after ~35.9 Ma. Subsequently deepening to middle to outer neritic depths deposited cyclic carbonates. Shallowing after ~35 Ma deposited laterally variable calcareous siliciclastic facies. These strata were tilted and eroded prior to 34 Ma leading to shallow water facies that may have been subaerially exposed during uplift. Brachiopod strontium isotope dates and an 0.5‰ carbon isotope excursion above this unconformity suggests the top of the Browns Creek and the base of the Castle Cove section correlate to Eocene-Oligocene transition (EOT-1) at ~34 Ma. The subsequent persistence of positive C/O isotope values above this level records the transition to the Oi-1 glaciation at ~33.7 Ma. Strong cyclicity in the inner shelf Castle Cove limestone is interpreted to record the commencement of obliquity dominated glacio-eustacy during the Oi-1 glacial phase. The shallowing from outer to inner shelf palaeodepths from the late Eocene to the early Oligocene is likely related to the onset of cryosphere expansion, however, palaeodepth estimates are complicated by the onset of regional compressional tectonism at the Eocene/Oligocene boundary that caused localized tilting and an unconformity with possible antisiphoning effects in this near-field site.

中文翻译:

澳大利亚东南部奥特韦盆地 Oi-1 冰川始新世至渐新世高古纬度浅海记录

摘要 从上始新世到下渐新世深水海洋序列的多重稳定同位素研究记录了从全球温室到冰库条件(Oi-1 冰川)的转变。虽然这种转变的南大洋高纬度深海记录是众所周知的,但它们的浅海等价物很少见,并且有可能记录古近纪冰川变化的浮海和海洋后果。澳大利亚东南部奥特威盆地的布朗斯溪和城堡湾著名的高古纬度 (~55°S) 浅海碳酸盐岩层序跨越始新世-渐新世边界。在此期间,该地区位于南大洋的南大洋东北缘。由于缺乏一致的地层和相互矛盾的微化石解释,这一记录的重要性受到了阻碍。为了解决这些问题,我们将露头和新岩心 (Colac-2) 的新生物、化学和岩石地层分析与预先存在的数据相结合,以修正地层。这证实了中/上始新世边界靠近该剖面的底部。上覆的始新世硅质碎屑岩地层被不整合面(持续时间约 0.8 Ma)截断,并被约 35.9 Ma 之后沉积的青海砂(Notrostrea 绿砂)覆盖。随后加深至中至外浅海深度沉积环状碳酸盐。在~35 Ma 沉积横向变化的钙质硅质碎屑相后变浅。这些地层在 34 Ma 之前发生倾斜和侵蚀,导致浅水相可能在抬升过程中暴露于地面。腕足类锶同位素日期和高于此不整合面的 0.5‰ 碳同位素偏移表明 Browns Creek 的顶部和 Castle Cove 剖面的底部与约 34 Ma 的始新世-渐新世过渡 (EOT-1) 相关。随后持续高于该水平的正 C/O 同位素值记录了在~33.7 Ma 时向 Oi-1 冰川作用的过渡。内陆架 Castle Cove 石灰岩的强周期性被解释为记录了在 Oi-1 冰期期间倾斜主导的冰川-潮汐的开始。从始新世晚期到渐新世早期,从外到内陆架古深度变浅可能与冰冻圈扩张的开始有关,然而,
更新日期:2020-08-01
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