The Eocene Huitrera Formation of northwestern Patagonia, Argentina, is renowned for its diverse, informative, and outstandingly preserved fossil biotas. In northwest Chubut Province, at the Laguna del Hunco locality, this unit includes one of the most diverse fossil floras known from the Eocene, as well as significant fossil insects and vertebrates. It also includes rich fossil vertebrate faunas at the Laguna Fría and La Barda localities. Previous studies of these important occurrences have provided relatively little sedimentological detail, and radioisotopic age constraints are relatively sparse and in some cases obsolete. Here, we describe five fossiliferous lithofacies deposited in four terrestrial depositional environments: lacustrine basin floor, subaerial pyroclastic plain, vegetated, waterlogged pyroclastic lake margin, and extracaldera incised valley. We also report several new 40Ar/39Ar age determinations. Among these, the uppermost unit of the caldera-forming Ignimbrita Barda Colorada yielded a 40Ar/39Ar age of 52.54 ± 0.17 Ma, ∼6 m.y. younger than previous estimates, which demonstrates that deposition of overlying fossiliferous lacustrine strata (previously constrained to older than 52.22 ± 0.22 Ma) must have begun almost immediately on the subsiding ignimbrite surface. A minimum age for Laguna del Hunco fossils is established by an overlying ignimbrite with an age of 49.19 ± 0.24 Ma, confirming that deposition took place during the early Eocene climatic optimum. The Laguna Fría mammalian fauna is younger, constrained between a valley-filling ignimbrite and a capping basalt with 40Ar/39Ar ages of 49.26 ± 0.30 Ma and 43.50 ± 1.14 Ma, respectively. The latter age is ∼4 m.y. younger than previously reported. These new ages more precisely define the age range of the Laguna Fría and La Barda faunas, allowing greatly improved understanding of their positions with respect to South American mammal evolution, climate change, and geographic isolation.

中文翻译：

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阿根廷巴塔哥尼亚丘布特省西北部不同始新世植物区系的相解释和年代学

阿根廷西北巴塔哥尼亚的始新世Huitrera组以其多样化，信息丰富且保存完好的化石生物群而闻名。在丘布特省西北部的拉古纳·德尔·洪科地区，该单元包括始新世以来最多样化的化石植物群之一，以及重要的化石昆虫和脊椎动物。它还包括LagunaFría和La Barda地区丰富的化石脊椎动物动植物群。对这些重要事件的先前研究提供的沉积学细节相对较少，放射性同位素年龄限制相对稀疏，在某些情况下已过时。在这里，我们描述了沉积在四个陆地沉积环境中的五个化石岩相：湖盆盆地底板，地下火山碎屑平原，植被，涝渍的火山碎屑湖边缘，和火山口切开的山谷。我们还报告了一些新的40Ar / 39Ar年龄确定。其中，形成火山口的Ignimbrita Barda Colorada的最上部单元产生的40Ar / 39Ar年龄为52.54±0.17 Ma，比以前的估计年龄年轻约6岁，这表明上覆化石湖相地层的沉积（以前受约束的年龄大于52.22。 ±0.22 Ma）必须几乎在沉降的火成岩表面上立即开始。Laguna del Hunco化石的最小年龄是由上覆的火成岩确定的，年龄为49.19±0.24 Ma，这证实了沉积是在始新世气候最佳时期发生的。拉古纳·弗里亚（LagunaFría）哺乳动物区系较年轻，分别受填满山谷的火成岩和40Ar / 39Ar顶盖玄武岩的限制，年龄分别为49.26±0.30 Ma和43.50±1.14 Ma。后者的年龄比以前报道的年龄小约4我。这些新时代更精确地定义了拉古纳·弗里亚（LagunaFría）和拉巴尔达（La Barda）动物群的年龄范围，从而大大提高了他们对南美哺乳动物进化，气候变化和地理隔离的立场的了解。