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Drainage Area, Bedrock Fracture Spacing, and Weathering Controls on Landscape‐Scale Patterns in Surface Sediment Grain Size
Journal of Geophysical Research: Earth Surface ( IF 3.9 ) Pub Date : 2020-09-08 , DOI: 10.1029/2020jf005560
Alexander B. Neely 1 , Roman A. DiBiase 1, 2
Affiliation  

Sediment grain size links sediment production, weathering, and fining from fractured bedrock on hillslopes to river incision and landscape relief. Yet models of sediment grain size delivery to rivers remain unconstrained due to a scarcity of field data. We analyzed how bedrock fracture spacing and hillslope weathering influence landscape‐scale patterns in surface sediment grain size across gradients of erosion rate and hillslope bedrock exposure in the San Gabriel Mountains (SGM) and northern San Jacinto Mountains (NSJM) of California, USA. Using ground‐based structure‐from‐motion photogrammetry models of 50 bedrock cliffs, we showed that fracture density is ~5 times higher in the SGM than the NSJM. 274 point‐count‐surveys of surface sediment grain size measured in the field and from imagery show a drainage area control on sediment grain size, with systematic downslope coarsening on hillslopes and in headwater‐colluvial channels transitioning to downstream fining in fluvial channels. In contrast to prior work and predictions from a hillslope weathering model, grain size does not increase smoothly with increasing erosion rate. For soil‐mantled landscapes, sediment grain size increases with increasing erosion rates; however, once bare bedrock emerges on hillslopes, sediment grain size in both the NSJM and SGM becomes insensitive to further increases in erosion rate and hillslope bedrock exposure, and instead reflects fracture spacing contrasts between the NSJM and SGM. We interpret this threshold behavior to emerge in steep landscapes due to efficient delivery of coarse sediment from bedrock hillslopes to channels and the relative immobility of coarse sediment in fluvial channels.

中文翻译:

表层沉积物粒度的流域,基岩断裂间距和景观尺度上的风化控制

沉积物的粒径将沉积物的产生,风化和细化(从山坡上的裂隙基岩与河道切口和景观缓解联系在一起)。然而,由于缺乏现场数据,向河流输送的沉积物粒径模型仍然不受限制。我们分析了美国加利福尼亚州圣加布里埃尔山(SGM)和北部圣哈辛托山(NSJM)的侵蚀速率和山坡基岩暴露梯度对基岩裂缝间距和山坡风化如何影响地表沉积物粒度的景观尺度格局。使用50个基岩悬崖的地面结构从运动摄影测量模型,我们发现SGM的裂缝密度比NSJM高约5倍。实地和图像测量的274个点计数的表面沉积物粒度显示,流域控制了沉积物粒度,在坡道和源水冲积河道中系统性地使下坡变粗,过渡到河流河道的下游细化。与先前的工作和山坡风化模型的预测相反,随着侵蚀速率的增加,晶粒尺寸不会平滑增加。对于土壤覆盖的景观,沉积物粒度随着侵蚀速率的增加而增加;然而,一旦裸露的基岩出现在山坡上,NSJM和SGM中的沉积物粒度对侵蚀速率和山坡基岩暴露的进一步增加变得不敏感,而是反映了NSJM和SGM之间的裂缝间距对比。我们将这种阈值行为解释为在陡峭的景观中出现,这是由于从基岩山坡到河道的粗沙的有效输送以及河道中的粗沙的相对不动。
更新日期:2020-10-06
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