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

大峡谷是地球上最令人印象深刻的地质特征之一，也是我们国家自然地标中公认的最多的地标之一。虽然普遍的共识是峡谷是在中新世(20 Ma)之后形成的，但目前关于峡谷演化的绝对年代学，特别是切割历史的绝对年代学研究非常少。对峡谷底部或附近的导线和玄武岩流动的研究已经成功地得出了切割率，但只在过去的50万年里。在科罗拉多河上方100米到1000米之间，整个大峡谷都很少或根本不存在与切割同时代沉积的表面材料。然而，峡谷暴露出的红墙石灰岩中的洞穴，包含了保存完好的沉积物(洞穴)，这些沉积物与大峡谷的切割是同时代的。在这些洞穴沉积物中，有四种特殊的洞穴沉积类型，它们是哺乳动物涂层、叶状、洞筏和石膏皮，是先前存在的地下水位的直接指标。其中许多洞穴具有异常高的铀含量，因此非常适合于铀系列和铀-铅联合年代学。因此，地下水位的位置可以通过测定这些洞穴的年代来确定，从而得出大峡谷不同地区的切削率。这种方法的压倒一切的好处是，平均切割率可以达到数百万年的周期，以及数百米的垂直起伏。我们的初步研究结果表明，这些地下水位指示洞穴(特别是哺乳动物涂层)在大峡谷的洞穴中很常见，保存得很好。初步的U系列和U-Pb结果表明，采集自大峡谷洞穴(位于大峡谷西南端，海拔较高)的哺乳动物覆盖物形成于~19 Ma。这标志着通过大峡谷洞穴的地下水位下降的时间，表明科罗拉多高原西南边缘早期切割，与Young(2004)玄武岩流动测年的切割记录一致。在大峡谷村附近(峡谷东端)，一种哺乳动物覆盖物的年龄为~3 Ma。在红墙石灰岩被与科罗拉多河水系有关的地下水切割之前，哺乳动物的涂层正在形成。因此，最近暴露在大理石峡谷中的洞穴(例如瀑布洞穴)应该有非常年轻的哺乳动物涂层。除年代学外，其中两个哺乳动物样品的~(87)Sr/~(86)Sr同位素值显示出显著的位移，表明20 Ma以来地下水的演化，可能与上覆富粘土沉积物的迁移有关，也可能是地下水混合造成的。这项拟议研究的基本过程是利用方解石哺乳动物沉积涂层的U系列和U-Pb测年，从东到西，在几个地点对大峡谷的切割速度进行研究。拟议的研究包括为期两年的密集实地考察以及同时进行的铀-铅和铀系列同位素工作。研究人员和他们的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成因同位素实验室已经对碳酸盐做了大量的U系列和U-Pb同位素工作。这项拟议的研究的价值超出了地质学的考虑。峡谷中近一半的河流英里位于大峡谷国家公园的范围内，大峡谷国家公园可能是北美最受欢迎和游客最多的国家公园。这项拟议的研究的潜在解释价值无论在科学上还是在公开方面都是深远的。另一个非常重要的社会利益是与当地土著民族的联系(请参阅支持信)。尽管他们缺乏当地的专业知识来从事这里提出的工作，但他们认识到这项工作对他们的管理工作的价值，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。

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.