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The Huolinhe Formation in the Huolinhe Basin, eastern Inner Mongolia is one of the most important Lower Cretaceous coal-bearing strata in China, yielding abundant, diverse, and well-preserved plant fossils. Its precise age, however, is poorly known due to lack of associated marine deposits and volcanic beds. Here we present U-Pb zircon ages, and the associated palynological assemblages of an ash layer of the Huolinhe Formation recently discovered at the Zhahanaoer open-cast coal mine in Jarud Banner. Stratigraphic analyses based on boreholes suggest that the ash layer occurs near the bottom of the “lower coal-bearing member” of the Huolinhe Formation. U-Pb zircon geochronology using the SIMS method constrains the depositional age of the ash layer to be 125.6 1.0 Ma (late Barremian–earliest Aptian), and this is consistent with the result from LA-ICP-MS analyses of the same sample. A late Barremian–earliest Aptian age for the ash layer is also supported by the palynological assemblage associated with the layer, in which the pollen of gymnosperms and the spores of ferns and bryophytes are dominant, angiosperm pollen is very rare and represented by only Clavatipollenites. This study contributes important new data for understanding the age of the entire Huolinhe Formation and also provides a more precise maximum age for the key plant fossils preserved in the deposits above the ash layer.
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Paleoenvironments, taphonomy, and stable isotopic content of the terrestrial, fossil-vertebrate–bearing sequence of the El Disecado Member, El Gallo Formation, Upper Cretaceous, Baja California, México
Abstract The Late Campanian (Late Cretaceous), upper part of the El Disecado Member, El Gallo Formation, Baja California, México, preserves a rich fossil assemblage of microvertebrates and macrovertebrates, silicified logs, macroscopic plant remains, and pollen that was likely deposited as the distal part of a subaerial fan. The unit was episodic and high energy, with its salient features deriving from active river channels and sheet, debris-flow deposits. Landscape stability is indicated by the presence of compound paleosol horizons, containing Fe2O3 mottling in B horizons, cutans, and calcium carbonate concretions. All of these features indicate wet/dry cyclicity in subsurface horizons, likely attributable to such cyclicity in the climate. Drainage was largely to the north and to a lesser extent, the west; however, some current flow to the south and east is preserved which, in conjunction with the proximal location of marginal marine deposits, suggest the influence of tides in this setting. The fossil vertebrates preserved in this part of the El Disecado Member are almost exclusively allochthonous, preserved as disarticulated isolated clasts in hydraulic equivalence in the braided fluvial system. A relatively diverse microvertebrate assemblage is preserved, the largest components of which are first, dinosaurs, and second, turtles. Non-tetrapod fossils are relatively uncommon, perhaps reflecting an absence of permanent standing water in this depositional setting. Here we report a high-precision U-Pb date of 74.706 + 0.028 Ma (2σ internal uncertainty), obtained from zircons in an airfall tuff. The tuff is located low within the sequence studied; therefore, most of the sedimentology and fossils reported here are slightly younger. This date, which improves upon previously published 40Ar/39Ar geochronology, ultimately allows for comparison of these El Gallo faunas and environments with coeval ones globally. Primary stable isotopic nodules associated with roots in the paleosols of the terrestrial portion of the El Disecado Member are compared with ratios from similar sources from coeval northern and eastern localities in North America. Distinctive latitudinal gradients are observed in both δ13C and δ18O, reflecting the unique southern and western, coastal geographic position of this locality. These differences are best explained by differences in the floras that populated the northern and eastern localities, relative to the southern and western floras reported here.
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- Award ID(s):
- 1424892
- PAR ID:
- 10215411
- Date Published:
- Journal Name:
- Geosphere
- Volume:
- 16
- Issue:
- 4
- ISSN:
- 1553-040X
- Page Range / eLocation ID:
- 991 to 1011
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1130/GES02207.1
