New single crystal 40Ar/39Ar ages improve time scale for deposition of the Omo Group, Omo–Turkana Basin, East Africa

Six tuffaceous beds within the Omo Group of the Omo–Turkana Basin have been dated using the 40Ar/39Ar single crystal total fusion method on anorthoclase, yielding eruption ages. The Omo Group constitutes up to 800 m of subaerially
exposed sediments surrounding Lake Turkana within the East African Rift system in northern Kenya and southern Ethiopia. Rhyolitic
explosive eruptions produced tuffs and pumice clasts that are considered to have been deposited shortly after eruption. The
new age data on feldspars from the pumice clasts range from 4.02 ± 0.04 Ma for the Naibar Tuff of the Koobi Fora Formation
to 1.53 ± 0.02 Ma for Tuff K of the Shungura Formation. The Orange Tuff in the KBS Member of the Koobi Fora Formation was
dated at 1.76 ± 0.03 Ma, providing good control in this part of the sequence where formerly there was a >200 ka gap. Data
are consistent with earlier measurements and significantly improve age resolution within the Omo Group, which has yielded
many vertebrate fossils, including hominin fossils comprising a number of species. We suggest new age estimates for a limited
number of hominin specimens.

通过对位于肯尼亚北部和埃塞俄比亚南部的东非裂谷系图尔卡纳湖周围的奥莫组中的六层凝灰岩层采用40Ar/39Ar单晶总融合法进行年代测定，获得了喷发年龄。奥莫组由至800米的露出地表的沉积物组成，这些沉积物环绕着图尔卡纳湖。流纹岩爆发产生的凝灰岩和火山弹被认为是喷发后不久沉积的。来自火山弹中的长石的新年龄数据范围从库比福拉组纳伊巴凝灰岩的4.02 ± 0.04百万年至舒贡拉组凝灰岩K的1.53 ± 0.02百万年。位于库比福拉组KBS成员中的橙凝灰岩被测定为1.76 ± 0.03百万年，为该序列的这一部分提供了良好的控制，因为在之前存在一个超过20万年的空白期。这些数据与先前的测量结果一致，并在很大程度上提高了奥莫组内部的年代分辨率，该组已产出许多脊椎动物化石，包括多种人属化石。我们为有限数量的人属样本提出了新的年龄估计。