Integrated, multi-method provenance studies of siliciclastic sedimentary deposits are increasingly used to reconstruct the history of source-to-sink transport, paleogeography, and tectonics. Invariably, analysis of large-scale depositional systems must confront issues regarding how to best sample the system and adequately cope with the details of sediment mixing. Potential biases including variations in grain size, sediment flux, and mineral concentration may cause provenance tracking tools to misrepresent the contributions of source-areas that contribute to large drainage networks. We have acquired U-Pb detrital zircon data from modern sand and whole-rock geochemistry from mud sampled from the Sacramento-San Joaquin drainage of central California to elucidate conditions that can skew provenance trends along the course of a major river system. This drainage network is fed by headwaters that tap the Mesozoic pluton-dominated southern Sierra Nevada, the Paleozoic-Mesozoic wallrock and volcanic-dominated northern Sierra Nevada, the ultramafic-dominated eastern Klamath Mountains, and the intermediate to mafic Cascades volcanic arc. Analysis of the results indicates that detrital zircon provenance trends effectively record source variations for the southern, granite-dominated portion of the drainage network where contrasts in lithology and inferred zircon fertility are relatively minor. In these circumstances, mixture modeling of U-Pb detrital zircon data calibrated with a measure of zircon fertility approximates relative sediment flux contributed by individual drainages. Alternatively, in the northern parts of the system, source regions underlain by ultramafic and/or volcanic rocks are poorly represented, or entirely missing, in downstream detrital zircon records. In some cases, mud geochemistry data more faithfully represent sediment provenance trends. Sampling performed at the confluence of the Sacramento, American, Mokelumne, and San Joaquin Rivers within the Sacramento-San Joaquin Delta region yields a detrital zircon age distribution that is indistinguishable from that of an independently established database of Sierra Nevada batholith crystallization ages. However, when the combined river flows along a recently established passage to the Pacific through the San Francisco Bay region, dredged sediment is found to be significantly contaminated by locally eroded material from the Franciscan Complex and other rocks that crop out within the Coast Ranges. Large variation of Zr concentrations measured throughout the Bay area document that significant hydrodynamic fractionation impacts sediment delivery through this segment of the system. The more Sierra Nevada-like detrital zircon age distribution yielded by a piston-core sample from the continental slope may be explained by either early-stage unroofing of the Coast Ranges or more efficient sand delivery from the delta to the Pacific by a free-flowing river driven by a low stand in sea level.

中文翻译：

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沙泥物源趋势的稀释和传播：加利福尼亚中部（美国）现代沉积物的地球化学和碎屑锆石年代学

硅质碎屑沉积物的综合、多方法物源研究越来越多地用于重建源到汇输运、古地理和构造的历史。总是，大规模沉积系统的分析必须面临关于如何最好地对系统进行采样并充分处理沉积物混合细节的问题。包括颗粒大小、沉积物通量和矿物浓度变化在内的潜在偏差可能会导致来源追踪工具误报对大型排水网络有贡献的源区的贡献。我们已经从现代沙子中获取 U-Pb 碎屑锆石数据，以及从加利福尼亚中部萨克拉门托 - 圣华金排水系统中采样的泥浆中获取的全岩地球化学数据，以阐明可能使主要河流系统沿线的物源趋势发生偏差的条件。该排水网络由源头供给，这些源头开采以中生代岩体为主的内华达山脉南部、古生代-中生界围岩和以火山为主的内华达山脉北部、以超基性为主的克拉马斯山脉东部，以及中间到基性岩质的喀斯喀特火山弧。结果分析表明，碎屑锆石物源趋势有效地记录了排水网络南部以花岗岩为主的部分的源变化，在那里岩性和推断的锆石肥力的差异相对较小。在这些情况下，用锆石肥力测量校准的 U-Pb 碎屑锆石数据的混合建模近似于各个排水系统贡献的相对沉积物通量。或者，在系统的北部，在下游碎屑锆石记录中，位于超基性岩和/或火山岩之下的源区很少或完全缺失。在某些情况下，泥浆地球化学数据更能真实地反映沉积物来源趋势。在萨克拉门托-圣华金三角洲地区的萨克拉门托河、美国河、莫克勒姆河和圣华金河的汇合处进行的采样产生了碎屑锆石年龄分布，与独立建立的内华达山脉基岩结晶年龄数据库的分布无法区分。然而，当合并的河流沿着最近建立的通道流经旧金山湾地区到达太平洋时，发现疏浚沉积物被来自方济各会群和海岸山脉内其他岩石的局部侵蚀物质严重污染。整个湾区测量的 Zr 浓度的巨大变化表明，显着的流体动力分馏影响了通过系统这一部分的沉积物输送。来自大陆坡的活塞岩心样本产生的更像内华达山脉的碎屑锆石年龄分布可以通过海岸山脉的早期开顶或通过自由流动的三角洲到太平洋的更有效的沙子输送来解释由海平面低位驱动的河流。