COLLABORATIVE RESEARCH: Sedimentology and Environment of Deposition of the Hadar Formation, Ethiopia

东非最重要的原始人类遗址哈达尔在15年后重新开放供研究。上新世哈达尔组提供了重要的地质机会，因为它位于埃塞俄比亚高原和阿法尔坳陷之间的构造边缘，突出的荒地暴露远远超出了哈达尔的研究地点。我们将根据非常好的地层/地质年代学格架，为该地层建立沉积相模型。以往的沉积学工作强调地层对比，只提出非常一般的沉积环境。在目前2.5公里高的高原陡坡以东仅50公里处，这些上新世沉积物显著地由70%的泥单元和30%的砂单元组成。大部分泥质为块状，含有早期的干旱古土壤。小型层状泥层序沉积在我们假设从东部（阿法尔中部）海侵的湖泊中。虽然巨大的泥砂层序似乎代表了一条曲流河的沉积，但这些沙在横向上过于广泛，而且过于渐变为鹅卵石砾岩（向西走向悬崖），无法用现有的曲流模型来解释。通过描绘和绘制详细的亚相，特别是在砂层中，通过编目横向和纵向层序以及第一个系统的古水流数据，我们可以评估砂滩与各种河道化流的选择。对于泥浆，我们希望描绘出不同成熟度的古土壤，以及湖泊是盐水还是淡水。沿着几个等时凝灰岩表面进行仔细的相图绘制，将提供贯穿哈达尔的环境马赛克时间片。本研究应提供：(1)阿法尔构造边界史的重要片段；(2)原始人生活和保存环境的知识，以及在哈达尔东部和西部广阔的暴露区寻找原始人的策略；(3)建立了厚层快速冲积沉积的一般相模式，其中片状砂与厚泥相结合。

The premier East African hominid site of Hadar recently reopened for research after a 15 year lapse. Major geologic opportunities are offered by the Pliocene Hadar Formation because of its setting along the tectonic margin between the Ethiopian Plateau and the Afar Depression with an outstanding badlands exposure that extends well beyond the studied site of Hadar. We will build a facies model of deposition for the formation pegged to a very good stratigraphic/geochronologic framework. The previous sedimentological work stressed stratigraphic correlation and suggested only very generalized environments of deposition. Being just 50 km east of the presently 2.5 km high Plateau escarpment, these Pliocene deposits remarkably consist of 70% mud units and only 30% sands. Most of the muds are massive with incipient arid paleosols. Minor laminated mud sequences were deposited in a lake we hypothesize transgressed from the east (central Afar). While the massive mud sand sequence seems to represent deposits of a meandering river the sands are too laterally extensive and too gradational into cobble conglomerates (westward toward the escarpment) to be explained by presently available meandering stream models. By delineating and mapping detailed subfacies, especially in the sands and by cataloging lateral and vertical sequences and the first systematic paleocurrent data, we can assess sandflat versus various channelized flow options. For the muds we hope to delineate paleosols of varying maturity, and whether the lake was saline or fresh. Careful facies mapping along the several isochronal tuff surfaces will provide environmental mosaic time-slices across Hadar. This research should provide: (1) a significant segment of the history of the Afar tectonic boundary; (2) knowledge of the environments where the hominids lived and were preserved, and a strategy for exploring for them in the vast exposures east and west of Hadar, and; (3) a contribution toward a general facies model for thick rapid alluvial deposition where sheets sands exist in association with thick muds.