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，也是最被认可的国家自然地标之一。虽然普遍认为峡谷形成于中中新世（20 Ma）之后的某个时间，但目前关于峡谷演化的绝对年代学，特别是切口史，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口率，但这只是最近50万年的结果。在科罗拉多河上方100 ~ 1000米的大峡谷地区，与切口同时沉积的地表物质很少或根本不存在。然而，峡谷暴露出的红墙石灰岩中的洞穴，包含了与大峡谷切口同时代的保存完好的沉积物（洞穴）。在这些洞穴沉积物中，有四种特殊的洞穴沉积物类型：乳头状覆盖物、树叶、洞穴筏和石膏皮，是预先存在的地下水位的直接指标。这些洞穴中有许多具有异常高的铀含量，因此非常适合铀系列和铀铅组合年表。因此，地下水位的位置可以通过确定这些洞穴的年代来确定，从而得出大峡谷不同地区的切口率。这种方法最大的好处是可以得到几百万年的平均切割速率，以及几百米的垂直地形。我们的初步研究结果表明，这些指示水位的洞穴特征（尤其是乳头状涂层）在大峡谷的洞穴中很常见，保存得很好。初步的U-series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）收集的乳状膜形成了~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的早期切口，与Young（2004）从玄武岩流的年代测定中得出的切口记录一致。在大峡谷村附近（峡谷东端），一层乳状膜的年代为~3 Ma。乳突涂层是在红墙石灰岩被科罗拉多河系统相关的地下水切割之前形成的。因此，最近在大理石峡谷（例如，FallsCave）暴露的洞穴应该具有非常年轻的乳状涂层。除年代学外，其中两个乳状样品的87Sr/86Sr同位素值显示出明显的变化，表明地下水在过去20 Ma的演化，可能与上覆富粘土沉积物的移除或地下水混合有关。本研究的基本过程是大峡谷的切割速度，在几个地方，从东到西，使用u系列和U-Pb定年方解石乳头状洞穴涂层。拟议的研究包括两年密集的实地考察和同时进行的U-Pb和铀系列同位素工作。研究人员及其合作者在实地和实验室工作方面都具有深厚的专业知识。新墨西哥大学的放射性成因同位素实验室对碳酸盐进行了广泛的u系列和U-Pb同位素研究。这项拟议研究的价值超出了地质考虑。峡谷中几乎一半的河流里程都在大峡谷国家公园的边界内，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。这项拟议研究的潜在解释价值在科学上和公众上都是深远的。另一个非常重要的社会效益是与当地土著民族的联系（请参阅支持信）。虽然他们缺乏在这里提出的工作方面的当地专门知识，但他们认识到其管理工作的价值，因此渴望与我们合作。我们希望参与的一部分将以学生围绕项目进行培训的形式进行。

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.