Genesis of fossil concentrations in foreland basins: A case study of the middle Eocene Man Aike Formation, Magallanes Basin, Chilean Patagonia

Eduardo Guareschi Müller , Rodrigo Scalise Horodyski , Karlos Guilherme Diemer Kochhann , Gerson Fauth , Leslie Marcela Elizabeth Manríquez , Rodrigo do Monte Guerra , Hugo Schimidt Neto , Cristine Trevisan , Marcelo Adrian Leppe Cartes
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Abstract

Foreland basins are tectonically active, and the accommodation for sediment deposition in shallow marine environments is continuously altered due to rapid variations in the proximal area, influencing fossil concentration. The middle Eocene Man Aike Formation in the northwest region of the retroarc foreland Magallanes/Austral Basin, Chilean Patagonia, is represented by coarse siliciclastic deposits interbedded with dense fossil concentrations. To better understand the depositional and paleoenvironmental context recorded by these strata and fossil concentrations, this study presents a multi-proxy characterization, based on facies analysis, taphonomy and geochemical analysis of bulk sediments. In the study area, the base of the sedimentary succession is interpreted as stacked transgressive facies deposits of the shoreface zone. Taphonomic features as articulation, fragmentation, orientation and bioincrustation suggests that shell concentrations were influenced by fair weather and storm waves (mass flows started by gravitational effects) in three types. The fossil concentrations that characterize episodic deposits, we interpreted as proximal and distal tempestites. The Lowest shell fossil concentrations are interpreted as fossiliferous lags, suggesting the onset of a transgression in the Magallanes Basin. The fossil concentrations composed of closed and articulated bivalves, observed in the middle portion of succession, were interpreted as event concentrations. Results of geochemical analyses (terrigenous elements/Ca, Al/Ca, Ti/Al and Sr/Ca ratios) support the occurrence of a deepening trend towards the top of the sedimentary succession. Taphonomic control related to the low accommodation of the foreland basin, combined with relatively high sea-level, was a determining factor in the genesis of the middle Eocene fossil concentration.
前陆盆地化石富集成因:以智利巴塔哥尼亚麦哲伦盆地中始新世Man Aike组为例
前陆盆地构造活动频繁,浅海环境中沉积物沉积的容纳空间由于近端区域的快速变化而不断改变,影响了化石浓度。智利巴塔哥尼亚Magallanes/Austral盆地弧后前陆西北地区中始新世Man Aike组以粗质硅屑沉积互层和致密化石为代表。为了更好地了解这些地层和化石浓度所记录的沉积和古环境背景,本研究在对大块沉积物进行相分析、地层学和地球化学分析的基础上,进行了多指标表征。在研究区,沉积演替的基础被解释为岸面带海侵相堆积沉积。衔接、碎裂、定向和生物结壳等声学特征表明,贝壳浓度受天气和风暴波(由重力效应引发的质量流)的影响有三种类型。化石浓度表征幕式沉积,我们解释为近端和远端风暴。最低的贝壳化石浓度被解释为化石滞后,表明麦哲伦盆地海侵的开始。在演替中段观察到的由闭合双壳类和铰接双壳类组成的化石浓度被解释为事件浓度。地球化学分析结果(陆源元素/Ca、Al/Ca、Ti/Al和Sr/Ca比值)支持沉积演替有向顶部加深的趋势。与前陆盆地的低可容性有关的地学控制,结合相对较高的海平面,是中始新世化石集中成因的决定性因素。
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