A “Local First” Approach to Glacigenic Sediment Provenance Demonstrated Using U-Pb Detrital Zircon Geochronology of the Permo-Carboniferous Wynyard Formation, Tasmanian Basin

Libby R. W. Ives, J. Isbell, K. Licht
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引用次数: 1

Abstract

We propose that a “local first” approach should be applied to the interpretation of provenance indicators in glacigenic sediments of all depositional ages, especially where the glacier flow path is poorly constrained and the records of potential source lithologies are incomplete. Provenance proxies, specifically U-Pb detrital zircon geochronology, of glacigenic sediments are commonly used to infer the size and distribution of past ice centers, which are in turn used to inform ancient climate reconstructions. Interpretations of these proxies often assume that similar provenance signals between glacigenic units of the same depositional age are evidence that they were deposited by the same glacier, even when those units are, not infrequently, separated by thousands of kilometers. Though glaciers are capable of transporting sediment great distances, this assumption is problematic as it does not acknowledge observations from the geologic records of Pleistocene ice sheets that show provenance proxies in glacial sediments are most likely to reflect proximal (within 100 km) sediment sources located along a specific flow path. In a “local first” approach, provenance indicators are first compared to local source lithologies. If the indicator cannot be attributed to proximal sources, only then should progressively more distal sources be investigated. Applying a local first approach to sediment provenance in ancient glacial systems may result in significant revisions to paleo ice sheet reconstructions. The effectiveness of the local first approach is demonstrated here by comparing new U-Pb detrital zircon dates from the Permo-Carboniferous glacigenic Wynyard Fm with progressively distal source lithologies along the glacier’s inferred flow path. The Wynyard Fm and source lithologies were compared using an inverse Monte-Carlo unmixing model (DZMix). All measured Wynyard Fm detrital zircon dates can be attributed to zircon sources within 33 km of the sample location along the glacier’s flow path. This interpretation of a proximal detrital zircon provenance does not conflict with the popular interpretation made from sedimentological observations that the Wynyard Fm was deposited by a large, temperate outlet glacier or ice stream that flowed south-to-north across western Tasmania. Overall, a local first approach to glacial sediment provenance, though more challenging than direct comparisons between glacigenic sedimentary deposits, has the potential to elucidate the complex histories and flow paths of glacial sedimentary systems of all depositional ages.
塔斯马尼亚盆地二石炭系Wynyard组U-Pb碎屑锆石年代学研究冰川期沉积物物源的“局部优先”方法
我们建议在所有沉积时代的冰川沉积物物源指标的解释中,应采用“局部优先”的方法,特别是在冰川流动路径限制较差和潜在物源岩性记录不完整的情况下。冰川期沉积物的物源代用物,特别是U-Pb碎屑锆石年代学,通常用于推断过去冰中心的大小和分布,进而用于为古代气候重建提供信息。对这些代用物的解释通常假设,相同沉积时代的冰川单位之间的相似物源信号证明它们是由同一冰川沉积的,即使这些单位经常相隔数千公里。虽然冰川能够远距离输送沉积物,但这一假设是有问题的,因为它没有承认更新世冰盖的地质记录的观测结果,这些观测结果表明,冰川沉积物中的物源代用物最有可能反映位于特定流动路径的近端(100公里以内)沉积物来源。在“本地优先”的方法中,首先将物源指标与本地物源岩性进行比较。如果该指标不能归因于近端来源,只有这样才应该逐步调查更多的远端来源。采用局部优先的方法研究古冰川系统的沉积物来源可能会对古冰盖重建结果进行重大修正。通过比较来自二叠纪-石炭纪冰期Wynyard Fm的新的U-Pb碎屑锆石日期与沿着冰川推断的流动路径的逐渐远端源岩性,证明了局部优先方法的有效性。使用反向蒙特卡罗解混模型(DZMix)对Wynyard Fm和源岩性进行了比较。所有测量的Wynyard Fm碎屑锆石日期均可归因于沿冰川流道样品位置33公里范围内的锆石来源。这种对近端碎屑锆石物源的解释与沉积学观察得出的普遍解释并不冲突,后者认为温亚德Fm是由一条从南到北流经塔斯马尼亚西部的大型温带出口冰川或冰流沉积的。总的来说,对冰川沉积物来源的局部第一方法,尽管比直接比较冰川沉积沉积物更具挑战性,但有可能阐明所有沉积时代冰川沉积体系的复杂历史和流动路径。
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