Geochemical Evolution of Eocene Lakes in the Nevada Hinterland of the North American Cordillera
Geochemical Evolution of Eocene Lakes in the Nevada Hinterland of the North American Cordillera
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DOI：
10.1029/2021gc009863
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发表时间：
2021-10
期刊：
影响因子：
3.7
通讯作者：
A. Canada;E. Cassel;M. Elliot Smith
中科院分区：
文献类型：
--
作者：
A. Canada;E. Cassel;M. Elliot Smith
始新世的Elko组地层记录了北美科迪勒拉山脉腹地内华达州的湖泊沉积。我们提出了一个详细的地球化学地层学，通过高采样分辨率的地质年代学，从湖泊三角灰岩和埃尔克组的互层火山岩。两个湖相沉积的间隔，一个是早期始新世有限的空中延伸的“土坯湖”，一个是横向广泛的始新世中期的“埃尔科湖”，相隔1.55亿年。明显的不一致。沉积在明显短命（49.5-48.5 Ma）的早始新世湖Adobe的沉积物表现出高振幅的δ 18 O，δ 13 C和87 Sr/86 Sr的协变，这表明一个动态变化的集水和降水制度。埃尔科湖形成于始新世中期，其地层记录了三个地球化学上不同的阶段，表明它是一个单一的相互连接的水体，随着时间的推移，蒸发量越来越大。下Elko组（44.0-42.5 Ma）沉积于淡水湖。中Elko组（42.5-41.2 Ma）的岩相和地球化学表明，越来越咸和碱性的Elko湖经历了盐度分层引起的低层缺氧和富含12 C的有机质的埋藏。上埃尔科组（41.2-40.5马）记录了一个浅的埃尔科湖的最后阶段，经历了短暂的停留时间和故障分层。上埃尔科组的87 Sr/86 Sr急剧下降反映了附近破火山口的低87 Sr/86 Sr火山沉积物的空中范围不断增加。始新世中期地层记录pondering的古排水，增加水文隔离和火山活动，符合渐进的北向南移除的Farallon平板和/或分层的岩石圈地幔下部的北美板块。
Eocene strata of the Elko Formation record lacustrine deposition within the Nevada hinterland of the North American Cordillera. We present a detailed geochemical stratigraphy enabled by high‐sampling‐resolution geochronology from lacus trine limestone and interbedded volcanic rocks of the Elko Formation. Two intervals of lacustrine deposition, an early Eocene “Lake Adobe” of limited aerial extent and a laterally extensive middle Eocene “Lake Elko,” are separated by ∼5 m.y. of apparent unconformity. Sediments deposited in the apparently short‐lived (49.5–48.5 Ma) early Eocene Lake Adobe exhibit high‐amplitude covariation of δ18O, δ13C and 87Sr/86Sr, which suggests a dynamically changing catchment and precipitation regime. Lake Elko formed during the middle Eocene, and its strata record three geochemically distinct phases, indicating it was a single interconnected water body that became increasingly evaporative over time. The lower Elko Formation (44.0–42.5 Ma) was deposited in a freshwater lake. Middle Elko Formation (42.5–41.2 Ma) lithofacies and geochemistry suggest that an increasingly saline and alkaline Lake Elko experienced salinity stratification‐induced hypolimnion disoxia and burial of 12C‐rich organic matter. The upper Elko Formation (41.2–40.5 Ma) records a shallow final phase of Lake Elko that experienced short residence times and a breakdown in stratification. A sharp decline of 87Sr/86Sr in the upper Elko Formation reflects an increasing aerial extent of low‐87Sr/86Sr volcanic deposits from nearby calderas. Middle Eocene strata record ponding of paleodrainage, increasing hydrologic isolation and volcanism, consistent with progressive north to south removal of the Farallon flat slab and/or delamination of the lower lithospheric mantle of the North American plate.