Bridging the Gap: Reconstructing the Evolution of the Grand Canyon from the Dating of Speleothems

弥合差距：根据洞穴化石年代重建大峡谷的演化

• 批准号: 0518602
• 负责人: Victor Polyak
• 金额: --
• 依托单位: University of New Mexico
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2005
• 资助国家: 美国
• 起止时间: 2005-08-15 至 2011-01-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0518602&HistoricalAwards=false
• 关键词: Bridging; Gap; Reconstructing; Evolution; Grand

The Grand Canyon is one of the most impressive geologic features on earth and one of the most, if not the most, recognized of our National natural landmarks. While a general consensus exists that the canyon formed since sometime after the middleMiocene (20 Ma), remarkably little absolute chronology on the evolution of the Canyon, and in particular the incision history, exists currently. Study of travertines and basalt flows at or near the bottom of the canyon has successfully produced incision rates, but only for the last half a million years. Surface materials deposited coeval with incision are rare or non-existent between 100 and 1000 meters above the Colorado River throughout the Grand Canyon. Caves in the Redwall Limestone exposed by the canyon, however, contain well-preserved deposits (speleothems) that are coeval with the incision of theGrand Canyon. Of these cave deposits, four particular speleothem types, mammillary coatings, folia, cave rafts, and gypsum rinds, are direct indicators of pre-existing water table levels. Many of these speleothems have unusually high uranium content and are thus ideally suited for combined uranium-series and uranium-lead chronology. The position of the water table can therefore be determined by dating these speleothems, yielding incision rates in different parts of the Grand Canyon. The overwhelming benefit of this method is that average incision rates can be obtained for periods of several million years, and for several hundred meters of vertical relief.Results from our pilot study show that these water table-indicator speleothems (especially mammillary coatings) are common and well preserved in the caves of theGrand Canyon. Preliminary U-series and U-Pb results demonstrate that mammillary coatings collected from Grand Canyon Caverns (at the southwestern end of the GrandCanyon at a higher elevation) formed ~19 Ma. This marks the timing of a drop in the water table through Grand Canyon Caverns suggesting early incision of the southwestern margin of the Colorado Plateau, consistent with the incision record from dating of basalt flows by Young (2004). Near Grand Canyon Village (eastern end of canyon), a mammillary coatings has been dated at ~3 Ma. The mammillary coatings are forming just prior to incision of the Redwall Limestone by the ground water associated with the Colorado River system. Recently exposed caves in Marble Canyon (e.g., FallsCave) should therefore have mammillary coatings with very young ages. In addition to the chronology, 87Sr/86Sr isotopic values of two of the dated mammillary samples show significant shift and suggest evolution of groundwater over the last 20 Ma, perhaps related to removal of overlying clay-rich deposits or caused by groundwater mixing. The fundamental process to be investigated by this proposed study is the rate of incision of the Grand Canyon, at several localities and from east to west, using U-series and U-Pb dating of calcite mammillary speleothem coatings. The proposed study includes two years of intensive field trips and concurrent U-Pb and uranium-series isotopic work. The investigators and their collaborators have deep expertise both in the field and laboratory based work. The Radiogenic Isotope Laboratory at the University of New Mexico has done extensive U-series and U-Pb isotopic work on carbonates.The value of this proposed study goes beyond geologic considerations. Almost half of the river miles in the canyon are within the boundaries of Grand Canyon NationalPark, perhaps the most popular and most visited national park in North America. The potential interpretative value of this proposed research is far reaching both scientifically and publicly. Another very important societal benefit is the link with the local Native Nations (please see letters of support). Although they lack local expertise for the kind of work proposed here, they recognize its value for their stewardship effort, thus are eager to work with us. We hope that part of the involvement will be in the form of student training around the project.

大峡谷是地球上最令人印象深刻的地质特征之一，也是最受认可的国家自然地标之一。虽然人们普遍认为峡谷是在中新世中期（20 Ma）之后的某个时间形成的，但目前关于峡谷演化的绝对年代学，特别是切割历史，却非常少。对峡谷底部或附近的石灰华和玄武岩流的研究已经成功地得出了切割率，但仅限于过去的 50 万年。在整个大峡谷，科罗拉多河上方 100 至 1000 米范围内，与切口同时沉积的表面材料很少或根本不存在。然而，峡谷暴露的红墙石灰岩洞穴中含有保存完好的沉积物（洞穴沉积物），这些沉积物与大峡谷的切口同时代。在这些洞穴沉积物中，四种特殊的洞穴沉积物类型：乳头涂层、浮叶、洞穴筏和石膏外皮，是预先存在的地下水位的直接指标。这些洞穴中的许多都具有异常高的铀含量，因此非常适合组合铀系和铀铅年代学。因此，可以通过对这些洞穴进行测年来确定地下水位的位置，从而得出大峡谷不同部分的切割率。这种方法的压倒性优势是可以获得数百万年时间和数百米垂直地形的平均切割率。我们的试点研究结果表明，这些地下水位指示洞穴（尤其是乳头涂层）很常见并在大峡谷的洞穴中保存完好。初步的 U 系列和 U-Pb 结果表明，从大峡谷洞穴（位于大峡谷西南端，海拔较高）收集的乳头涂层形成了约 19 Ma。这标志着大峡谷洞穴地下水位下降的时间，表明科罗拉多高原西南边缘的早期切割，与 Young (2004) 玄武岩流测年的切割记录一致。在大峡谷村（峡谷东端）附近，乳头涂层的年代约为 3 Ma。乳头涂层是在与科罗拉多河系统相关的地下水切割红墙石灰岩之前形成的。因此，大理石峡谷中最近暴露的洞穴（例如 FallsCave）应该具有非常年轻的乳头涂层。除了年代学之外，两个定年乳头样本的 87Sr/86Sr 同位素值显示出显着的变化，表明过去 20 Ma 中地下水的演变，可能与上覆富含粘土沉积物的去除有关或由地下水混合引起。这项拟议研究要调查的基本过程是使用方解石乳头洞穴涂层的 U 系列和 U-Pb 定年法，确定大峡谷从东到西几个地点的切割率。拟议的研究包括两年的密集实地考察以及同时进行的 U-Pb 和铀系列同位素工作。研究人员及其合作者在现场和实验室工作方面都拥有深厚的专业知识。新墨西哥大学的放射性同位素实验室对碳酸盐进行了广泛的 U 系列和 U-Pb 同位素工作。这项研究的价值超出了地质考虑因素。峡谷中几乎一半的河流都在大峡谷国家公园的范围内，这可能是北美最受欢迎和访问量最大的国家公园。这项拟议研究的潜在解释价值在科学和公众方面都具有深远的影响。另一个非常重要的社会效益是与当地原住民的联系（请参阅支持信）。尽管他们缺乏此处提出的此类工作的当地专业知识，但他们认识到其管理工作的价值，因此渴望与我们合作。我们希望部分参与将以围绕该项目的学生培训的形式进行。