Collaborative Research: Timing and Controls on Plio-Pleistocene Erosion and Sedimentation in the Eastern Peninsular Ranges, Southern California

来自加州大学戴维斯分校、俄勒冈大学和西华盛顿大学的一个研究小组，正在对加利福尼亚南部菲什克里克-瓦莱西托盆地保存的一份非常完整的档案进行多学科研究，该档案记录了上新世晚期到现代的古侵蚀速率。源区的侵蚀速率由碎屑石英中的宇宙成因核素浓度确定，并对盆地沉积物快速埋藏和挖掘过程中沉积后核素生长的最小值进行了修正。通过对比现代基岩暴露于盆地沉积物碎屑矿物的(U-Th)/He冷却年龄，评价源区凹陷生产。利用高精度地磁反演和古强度地层学标定了沉积年龄和沉积速率。这些数据产生了冲积沉积物中宇宙形成核素生长的高分辨率时间序列，记录了流域平均侵蚀率随时间的重要变化。将源区侵蚀速率的时间变化与盆地相结构变化和沉积物积聚速率的地层记录进行比较，从而评估将侵蚀强迫与地层结果联系起来的过程响应函数。使用这些方法，该团队正在测试构造与气候驱动的基岩源侵蚀变化和基岩源沉积物沉积到邻近盆地的三个假设：(1)大约250万到300万年前北半球冰川开始时，气候条件变冷，时间变化更大，导致侵蚀和本地沉积物的产生增加；(2)构造隆升的增加导致地形起伏，侵蚀加剧；(3)无论源区侵蚀速率如何变化，构造沉降速率的减小都推动了局部沉积的进积。在为期一年的试点研究中收集的数据表明，自2.8 Ma以来，侵蚀率下降了40%。这一初步结果与气候或沉降速率驱动的退化最为一致，尽管目前还不能排除抬升驱动的模式。这项新研究将进一步分析这一记录，并从第二个沉积物源区复制这一信号。有充分的证据表明，基岩侵蚀和沉积物产生的速度与气候强迫和地形起伏的构造构造密切相关，尽管这些控制的确切性质和反馈动力学仍然不完全清楚。现代水系沉积物中的碎屑宇宙成因核素浓度被广泛用于量化短期流域侵蚀速率，以及评估构造强迫、岩性和侵蚀过程对沉积物生成速率的作用。虽然这种方法在现代沉积集水系的研究中很常见，但它在古代沉积物中的应用相对较少，而在连续沉积序列中的应用更少。这项研究是通过将这些方法应用于更古老的沉积物来绘制新的地面。如果成功，地质学家将有新的工具来区分气候和构造在景观演化中的作用。

A research team from the University of California at Davis, University of Oregon, and Western Washington University is conducting a multi-disciplinary examination of a remarkably complete and well-exposed archive of late Pliocene to modern paleo-erosion rates preserved in the Fish Creek - Vallecito basin in southern California. Erosion rates in the source are determined from cosmogenic nuclide concentrations in detrital quartz, with corrections for minimal post-depositional nuclide in-growth during rapid burial and exhumation of basinal sediments. Relief production in the source areas are assessed by comparison of (U-Th)/He cooling ages in modern bedrock exposures to detrital minerals in basinal sediments. Depositional ages and sedimentation rates are calibrated with high-precision magnetic reversal and paleointensity stratigraphy. These data yield a high-resolution time series of cosmogenic nuclide in-growth of alluvial sediments, which record important changes in catchment-averaged erosion rates through time. Temporal changes in source-area erosion rate are compared to the stratigraphic record of changes in facies architecture and sediment accumulation rates in the basin, allowing assessment of the process-response functions that link erosional forcing to stratigraphic outcomes. Using these approaches, the team is testing three hypotheses for tectonic - versus climate driven changes in erosion of the bedrock source and progradation of sediment derived from this source into the adjacent basin: (1) a shift into cooler and more temporally variable climate conditions at the onset of northern hemisphere glaciation approximately 2.5 to 3.0 million years ago caused increased erosion and production of locally derived sediment; (2) an increase in tectonically driven uplift led to relief production and enhanced erosion; and (3) a reduction in tectonic subsidence rate drove progradation of locally derived sediments irrespective of changes in source-area erosion rate. Data collected during a 1-year pilot study show that erosion rates decreased by 40% since 2.8 Ma. This preliminary result is most consistent with either a climatic or subsidence-rate driver of progradation, though an uplift-driven model cannot be ruled out at this time. This new study will further analyze this record and replicate this signal from a second sediment source-area.It is well documented that rates of bedrock erosion and sediment production are closely linked to climate forcing and tectonic construction of topographic relief, though the exact nature and feedback dynamics of those controls remain incompletely understood. Detrital cosmogenic nuclide concentrations in modern stream sediments are widely used for quantifying short-term catchment erosion rates, and for assessing the role of tectonic forcing, lithology, and erosion processes on sediment production rates. While this method is common in studies of modern sediment-catchment systems, its application to ancient sediments is relatively rare, and its use in continuous sedimentary sequences is rarer still. This study is charting new ground through the application of these methods to older sediments. If successful, geologists will have new tools to distinguish the role of climate versus tectonics in landscape evolution.