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Geochronology and Mammalian Biostratigraphy of Middle and Upper Paleocene Continental Strata, Bighorn Basin, Wyoming

机译:怀俄明州比格霍恩盆地中,上古新世大陆地层的年代学和哺乳动物生物地层学

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摘要

The Bighorn Basin of northwestern Wyoming preserves one of the most complete records of middle Paleocene to lower Eocene continental biota. The geochronology of this important interval depends partly on numerical calibration of the geomagnetic polarity time scale (GPTS), but the middle and late Paleocene parts of the GPTS have been poorly constrained radioisotopically. A new volcanic ash from the northern Bighorn Basin provides the first radioisotopic age for upper middle Paleocene strata (upper Selandian) and is an important calibration point for the base of polarity Chron C26n and the Selandian-Thanetian stage boundary in the GPTS. Sanidine from the ash has a weighted mean age of 59.00 ± 0.30 (2 σ) Ma, based on twenty-three 40Ar/39Ar bulk laser-fusion analyses. The ash corroborates the revised age estimate for the base of Chron C26n in the most recent geomagnetic polarity time scale (GPTS-04) and confirms that C26n is older than depicted in the 1995 GPTS.We place Paleocene mammalian biozones in the Bighorn Basin into a refined geochronologic framework. These biozones are the primary basis for biostratigraphic correlation of middle and upper Paleocene continental deposits in North America. New paleomagnetic data provide synchronous tie points for correlation among three stratigraphic sections in the northern Bighorn Basin and to the GPTS. Paleomagnetic correlation, study of new fossil material, review of known faunas, and refined stratigraphy enable us to reevaluate several biozones. We define three new zones based on first occurrences of taxa: the Phenacolemur (Ti-4b), Probathyopsis (Ti-5a, revised), and Copecion (Cf-3, revised) zones. The Phenacolemur zone includes some faunas previously placed in the Plesiadapis churchilli and Plesiadapis fodinatus zones (Ti-4 and Ti-5a, respectively). The Probathyopsis zone replaces the P. fodinatus zone and the Copecion zone replaces the Phenacodus-Ectocion acme zone (Cf-3).Seven of the biozones considered here are subdivisions of the Tiffanian land mammal age. The Tiffanian was originally typified by the Mason Pocket fauna in southwestern Colorado, but it is now much better known from faunas in the northern Bighorn Basin. Biostratigraphic evidence and new paleomagnetic data from the Bighorn Basin indicate that Mason Pocket is considerably older than was previously recognized and occurs in Chron C26r, rather than C25r. The geochronologic framework presented here helps to constrain the temporal ranges of species that occur in the middle and late Tiffanian, and it allows the Bighorn Basin record to be better compared to other faunal, floral, and paleoclimate records, both regionally and globally.
机译:怀俄明州西北部的比格霍恩盆地保留了中新世至下新世大陆生物群最完整的记录之一。这个重要间隔的地质年代学部分取决于地磁极性时标(GPTS)的数值校准,但是GPTS的中新世和晚新世部分受到的放射性同位素约束较弱。来自Bighorn盆地北部的新火山灰为上古新世地层(Selandian上部)提供了第一个放射性同位素年龄,并且是GPTS中Chron C26n极性基底和Selandian-Thanetian阶段边界的重要校准点。根据23份40Ar / 39Ar整体激光熔融分析,来自灰烬的Sanidine的加权平均年龄为59.00±0.30(2σ)Ma。灰烬证实了在最新的地磁极性时标(GPTS-04)中对Chron C26n基准年龄的修订估计,并证实C26n比1995年GPTS中描绘的年龄更旧。我们将比格霍恩盆地的古新世哺乳动物生物区放入一个完善的年代学框架。这些生物区是北美中上新世陆相沉积物生物地层学关联的主要基础。新的古磁数据为北比格霍恩盆地和GPTS的三个地层之间的关联提供了同步联系点。古地磁相关性,新化石材料的研究,已知动物区系的回顾以及精细的地层学使我们能够重新评估几个生物区带。我们根据首次出现的分类单元来定义三个新区域:苯酚(Ti-4b),Probathyopsis(Ti-5a,修订版)和Copecion(Cf-3,修订版)区。酚类动物区包括一些先前放置在Plesiadapis churchilli和Plesiadapis fodinatus区(分别为Ti-4和Ti-5a)的动物区系。 Probathyopsis区取代了P. fodinatus区,而Copecion区取代了Phenacodus-Ectocion acme区(Cf-3)。这里考虑的七个生物区是Tiffanian陆地哺乳动物年龄的细分。蒂法尼人最初以科罗拉多州西南部的梅森·帕克(Mason Pocket)动物群为代表,但如今在比格霍恩盆地北部的动物群中更为人所知。来自比格霍恩盆地的生物地层学证据和新的古地磁数据表明,梅森矿床的年龄比以前公认的大得多,发生在Chron C26r而不是C25r中。这里介绍的地质年代学框架有助于限制发生在蒂法尼时代中期和晚期的物种的时间范围,并且与其他动物,花卉和古气候记录相比,在区域和全球范围内,比格霍恩盆地的记录都更好。

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