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Abstract 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 and87Sr/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 of12C‐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 of87Sr/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.
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Microcrystalline dolomite in a middle Permian volcanic lake: Insights on primary dolomite formation in a non‐evaporitic environment
Abstract Lacustrine dolomite nucleation commonly occurs in modern and Neogene evaporitic alkaline lakes. As a result, ancient lacustrine microcrystalline dolomite has been conventionally interpreted to be formed in evaporitic environments. This study, however, suggests a non‐evaporitic origin of dolomite precipitated in a volcanic–hydrothermal lake, where hydrothermal and volcanic processes interacted. The dolomite occurs in lacustrine fine‐grained sedimentary rocks in the middle Permian Lucaogou Formation in the Santanghu intracontinental rift basin, north‐west China. Dolostones are composed mainly of nano‐sized to micron‐sized dolomite with a euhedral to subhedral shape and a low degree of cation ordering, and are interlaminated and intercalated with tuffaceous shale. Non‐dolomite minerals, including quartz, alkaline feldspars, smectite and magnesite mix with the dolomite in various proportions. The 87 Sr/ 86 Sr ratios (0.704528 to 0.705372, average = 0.705004) and δ 26 Mg values (−0.89 to −0.24‰, average = −0.55‰) of dolostones are similar to those of mantle rocks, indicating that the precipitates mainly originated from fluids that migrated upward from the mantle and were subject to water–rock reactions at a great depth. The δ 18 O values (−3.1 to −22.7‰, average = −14.0‰) of the dolostones indicate hydrothermal influence. The trace and rare earth element concentrations suggest a saline, anoxic and volcanic–hydrothermally‐influenced subaqueous environment. In this subaqueous environment of Lucaogou lake, locally high temperatures and a supply of abundant Mg 2+ from a deep source induced by volcanic–hydrothermal activity formed favourable chemical conditions for direct precipitation of primary dolomite. This study's findings deepen the understanding of the origin and processes of lacustrine primary dolomite formation and provide an alternative possibility for environmental interpretations of ancient dolostones.
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- Award ID(s):
- 1714749
- PAR ID:
- 10443752
- Date Published:
- Journal Name:
- Sedimentology
- Volume:
- 70
- Issue:
- 1
- ISSN:
- 0037-0746
- Page Range / eLocation ID:
- 48 to 77
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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The Yangtze Platform borders the Nanpanjiang Basin on its north and west. During the Early Triassic the platform evolved from low-relief ramp with oolitic margins to a steepening platform with a relatively flat-topped geometry with margin shoals and evaporitic interior facies. At the Zhenfeng margin the precursor depositional facies include: oolitic grainstone to packstone, skeletal peloidal packstone, clotted microbialites and fenestral laminites. The Anshun strata range from undolomitized to partially dolomitized oolite and microbial facies to partially and completely dolomitized facies such as fenestral laminites. Dolomitization changes upward through the section with fenestral laminate facies being more pervasively dolomitized than the oolitic and skeletal packstone facies. The diagenetic evolution (paragenetic sequence includes: neomorphic alteration of aragonite, marine cementation, replacement dolomite, euherdral dolomites, saddle dolomites, calcite veins, stylolites, and late-stage fractures with calcite and oxide fill. Previous data from the Yangtze Platform include dolomite showing δ¹⁸O values ranging from -7.7‰ to 0.75‰ (VPDB) and δ ¹³C values ranging from 0.77‰ to 4.0‰ (VPDB). Vein calcite values range from δ ¹⁸O -18.4‰ to -5.2‰ and δ ¹³C -6.1 to 3.4‰. ⁸⁷Sr/⁸⁶Sr values from dolomite ranges from 0.707677 to 0.708601 with the exception of elevated ⁸⁷Sr/⁸⁶Sr in three samples. Homogenization temperatures (Th) and freezing point depressions (Tmice) from primary fluid inclusion assemblages from dolomite crystals indicate entrapment of saline brines (9.5 to 16 wt. % NaCl) over temperatures of 80-185°C. The preferential dolomitization of mud-rich platform interior facies and preservation of oolitic limestone facies at the platform margin points to interior derived dolomitizing fluids consistent with evaporative reflux. The range in δ ¹⁸O is consistent with enrichment by evaporative concentration of seawater, but also includes negative values consistent with high temperature fluids. ⁸⁷Sr/⁸⁶Sr values in dolomite are consistent with modified seawater including a radiogenic contribution of hydrothermal fluids. The geothermometric data, oxygen isotope values and radiogenic Sr contribution indicates that early dolomites were recrystallized at high temperatures by burial fluids.more » « less
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Abstract The Chisana Formation consists of Lower Cretaceous volcanic rocks that occur in the Nutzotin Mountains of eastern Alaska. New stratigraphic analysis indicates that the volcanic succession is >2 km thick at the Bonanza Creek type section. We present stratigraphic, geochemical, Sr‐Nd‐Pb isotope, and U‐Pb age data from samples collected from various stratigraphic levels of the Chisana Formation. We demonstrate that the Chisana Formation can be divided into a lower subaqueous unit, a middle transitional unit, and an upper subaerial unit. Chisana Formation lavas range from transitional to subalkaline basalts through andesites. Trace element geochemistry shows high field strength element depletions relative to large ion lithophile elements and hydrous mineral assemblages with calc‐alkaline to tholeiitic chemistries, all consistent with a magmatic arc origin. Chisana lavas yield geochemical compositions and isotope characteristics that overlap with magmas from volcanic suites formed within juvenile continental crust and immature island arcs. Volcanism occurred between ~131 and 117 Ma judging from previously reported lava ages and new U‐Pb ages of detrital zircons recovered from sandstones that conformably underlie the lowermost Chisana Formation lavas. Our results support existing tectonic models in which an east dipping subduction zone existed beneath Wrangellia during Early Cretaceous time. The upsection shift from marine to terrestrial depositional conditions in the Chisana Formation and the overlying ~117–93 Ma Beaver Lake Formation was coincident with regional shortening. Together, the geologic evidence for shortening and terrestrial deposition are interpreted to reflect accretion/suturing of Wrangellia against inboard terranes.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1111/sed.13031
