Pushing the boundary: A calibrated Ediacaran-Cambrian stratigraphic record from the Nama Group in northwestern Republic of South Africa
Pushing the boundary: A calibrated Ediacaran-Cambrian stratigraphic record from the Nama Group in northwestern Republic of South Africa
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DOI：
10.1016/j.epsl.2022.117396
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发表时间：
2022-02
影响因子：
5.3
通讯作者：
L. Nelson;J. Ramezani;J. Almond;S. Darroch;W. Taylor;D. Brenner;Ryan Furey;Madison Turner;E. F. Smith
中科院分区：
文献类型：
--
作者：
L. Nelson;J. Ramezani;J. Almond;S. Darroch;W. Taylor;D. Brenner;Ryan Furey;Madison Turner;E. F. Smith
在南非共和国西北部奥兰治河沿岸的Neint Nabbeep高原上发现的Nama群现在被认为是埃迪卡拉纪-寒武纪过渡的扩展记录，为以前所未有的分辨率研究跨越这一基本地质界线的地球化学和生物演化提供了机会。利用CA-ID-TIMS方法对本区那玛群(从匈奴段到诺姆萨斯组)的6个火山灰夹层进行了锆石U-Pb年代学研究，建立了高分辨率的贝叶斯年龄地层模型，直接对539.63 Ma±0.15 Ma至537.95 Ma±0.28 Ma的生物地层和碳同位素记录进行了时间刻度(2σ内部误差)。在越过边界的纳米比亚南部维特普斯次盆地，裸道组底部的火山灰层和纳赛普段内的火山灰层分别获得了545.27±0.11 Ma和542.65±0.15 Ma的新U-Pb年龄。我们的联合年代学揭示了Nama群在区域尺度上的详细沉积历史，表明Kuibis亚群和Schwarzrand下亚群的沉积速率相对较低，随后Schwarzrand亚群上部的沉积速度加快。这与前陆盆地典型的下沉指数增长模式是一致的。在整个Nama群中观察到的一些化学地层学趋势可能与这种沉积速率的增加推动的早期海洋成岩作用从海水缓冲状态向沉积物缓冲状态的转变有关。Neint Nabbeep高原内软体二叠体形态、钙化体化石和遗迹化石的出现与已知的全球生物地层范围大体一致。然而，我们记录了最年轻的埃迪卡拉型化石的产出，这些化石在地层上与大型复杂的双边遗迹化石重叠，介于539.18±0.17/−0.26 Ma和538.30±0.14/−0.14 Ma之间。然而，该剖面中的索引化石Treptichnus peduus仍然没有记录在案，我们认为它的首次区域出现可能比这些地层更年轻。尽管埃迪卡拉纪-寒武纪界线上的碳酸盐沉积相对连续且速率较高(如目前所承认的)，但Neint Nabbeep高原的上Nama群并不保留在其他寒武纪基底序列中观察到的特征的负碳同位素漂移。一种可能的解释是，这种化学地层学标志并不是在所有碳酸盐沉积环境中都普遍存在。另外，寒武纪的碳同位素漂移，以及首次出现的埃迪卡拉纪-寒武纪界线，可能是>；1 m.y。比目前认为的要年轻，断代为538 Ma，因此，表明早寒武世的辐射更加浓缩。在NAMA组确定索引化石Treptichnus peduin的首次出现基准方面的困难突出了寒武纪底部的全球生物地层学定义的挑战，并强调有必要采用综合地层学和放射性同位素年代学方法来了解埃迪卡拉纪-寒武纪边界的环境和生物演化的节奏和模式。
The Nama Group exposed on the Neint Nababeep Plateau along the Orange River in northwestern Republic of South Africa is now recognized as an expanded record of the Ediacaran-Cambrian transition that provides opportunity for an integrated stratigraphic approach in examining the geochemical and biologic evolution across this fundamental geologic boundary at unprecedented resolution. U-Pb zircon geochronology by the CA-ID-TIMS method on six intercalated volcanic ash beds in the Nama Group (from the Huns Member to the Nomtsas Formation) at this locality is used to construct a high-resolution, Bayesian, age-stratigraphic model, which allows a direct temporal calibration of the biostratigraphy and carbon isotope record from 539.63 ± 0.15 Ma to 537.95 ± 0.28 Ma (2σinternal errors). Across the border in the Witputs subbasin of southern Namibia, ash beds at the base of Nudaus Formation and within the Nasep Member yielded new U-Pb ages of 545.27 ± 0.11 Ma and 542.65 ± 0.15 Ma, respectively. Our combined geochronology reveals the detailed depositional history of the Nama Group at a regional scale, suggesting that a relatively low sediment accumulation rate in the Kuibis Subgroup and the lower Schwarzrand Subgroup was followed by accelerated sedimentation in the upper Schwarzrand Subgroup. This is consistent with a pattern of exponential increase in subsidence typical of foreland basins. Some of the observed chemostratigraphic trends throughout the Nama Group could relate to a shift from a seawater-buffered to a sediment-buffered regime of early marine diagenesis driven by this increase in sedimentation rate.Occurrences of soft-bodied erniettomorphs, calcified body fossils, and trace fossils within the Neint Nababeep Plateau are broadly consistent with known global biostratigraphic ranges. However, we document the youngest radioisotopically calibrated occurrences of Ediacaran-type fossils, which stratigraphically overlap with large and complex bilaterian ichnofossils, between 539.18+0.17/−0.26 Ma and 538.30+0.14/−0.14 Ma. Yet, the index fossilTreptichnus pedumremains undocumented from this section, and we suggest that its first regional occurrence may be younger than these strata. Despite relatively continuous and high rates of carbonate sedimentation across the Ediacaran-Cambrian boundary (as currently recognized), the upper Nama Group of the Neint Nababeep Plateau does not preserve the characteristic negative carbon isotope excursion observed within other basal Cambrian successions. One possible explanation for its absence is that this chemostratigraphic marker is not ubiquitous in all carbonate depositional environments. Alternatively, the basal Cambrian carbon isotope excursion, and perhaps the Ediacaran-Cambrian boundary as defined by the first appearance ofTreptichnus pedum, might be >1 m.y. younger than currently recognized, postdating 538 Ma and, thus, suggesting a more condensed early Cambrian radiation. Difficulties in determining with confidence the first appearance datum of the index fossilTreptichnus pedumin the Nama Group highlight the challenge of a global biostratigraphic definition for the base of Cambrian and underscore the necessity of an integrated stratigraphic and radioisotope geochronologic approach to understand the tempo and patterns of environmental and biologic evolution across the Ediacaran-Cambrian boundary.