Geomorphology of Cement Creek and its Relation to Ferricrete Deposits
Geomorphology of Cement Creek and its Relation to Ferricrete Deposits
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水泥河地貌及其与铁质混凝土矿床的关系
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发表时间：
2007
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通讯作者：
L. Wirt
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作者：
K. Vincent;S. Church;L. Wirt
作为阿尼马斯河流域研究的一部分，对水泥河下游 11 公里处由砾石和泥炭组成的最新第四纪地貌进行了测绘和测年，以便了解该地区的地貌历史，从而为了解铁质混凝土的形成提供地貌背景。在更新世末期，圣胡安山脉几乎被高山冰川覆盖，而且之前可能也被高山冰川覆盖过很多次。在最近的一次冰川作用（通常称为派恩代尔冰川作用）期间，西尔弗顿现址的冰厚约 520 米，格拉德斯通现址的冰厚约 430 米。那次冰川作用于大约 12,000 14 C 年前结束，除了冰碛之外，水泥溪沿岸的沉积物很可能是在那之后沉积的。其他沉积物显然是在冰川消融期间形成的，包括充满峡谷的溪流砾石沉积物、比谷底高约 20 米的局部溪流台地以及大型山体滑坡。位于大多数支流河口的显着冲积扇也可能在冰从水泥溪谷底消失后就开始堆积。这些扇可能会迅速积累，直到周围的山坡被植被稳定下来，这可能需要几个世纪的时间。最大的扇体或成对的扇体使谷底淤积了 20 多米，并导致水泥溪在其上游淤积。这导致水泥溪谷的分段纵向剖面仍然存在，因为大部分扇沉积物尚未被水泥溪清除。然而，6000 多年前，水泥溪部分冲破了扇的脚趾，切开了长达 5 米的扇沉积物。这反过来又导致粉丝们被他们的支流所割裂。公元前 3700 年之间公元 400 年（距今 5,650 至 1,550 年）水泥溪横向迁移，但其河床仍保持在现在河床的水平。大约从公元 400 年开始，即中世纪温暖期之前不久，水泥河开始水位上升，水位比之前的水位高出 3 米。水泥溪在公元 1330 年至公元 1700 年间切回到原来的高度，留下了一个突出的溪流台地。这个切入可能发生在公元 1500 年之前，也可能是在公元 1440 年左右。水泥溪台地的切入大致与小冰河时代的开始同步。水泥溪阶地的加积和切割不是由当地基准面过程引起的，可能是气候变化的结果，尽管所涉及的确切机制尚不清楚。相比之下，最近的人类活动对水泥河的形状和物理过程几乎没有影响。人类活动对地球化学过程的影响超出了本研究的范围，铁混凝土形成除外。铁化合物局部胶结所有来源的碎屑沉积物，形成砾岩型铁混凝土。大多数铁混凝土暴露物都是干燥的，并且可能是不活跃的，并且大多数是史前的。即使精确知道碎屑沉积物的沉积年龄，定义砾岩型铁混凝土胶结的时间也是有问题的，因为胶结可能在碎屑沉积后的任何时间发生。远端扇沉积物的胶结一定发生在 6000 多年前，因为那时砾石中的地下水被河流切割永久排出。弹簧沉积的铁混凝土包裹木炭，容量为 4,500 卡。年代，因此推测砾岩铁混凝土也形成于全新世中期。过去500年沉积的溪流砾石也被铁化合物胶结。我们得出的结论是，在整个全新世的不同时间和地点，水泥溪碎屑沉积物中形成了铁质混凝土，因此大多数铁质混凝土与采矿无关。铁混凝土的暴露在空间上是不连续的，并且与枯水季节地下水的出现率不对应。这表明流域内铁混凝土形成所需的地球化学条件并不均匀。潮湿的、可能活跃的铁混凝土的暴露几乎总是位于邻近阶地上有莎草湿地（下面是泥炭）的地方。我们得出的结论是，湿地和湿铁混凝土都是源自支流子流域的地下水的常年出现，而不是来自水泥溪本身。使用泥炭测定溪流切口时必须小心，因为泥炭在溪流切口后会继续在湿地中增生。这种持续增长的发生是因为湿地的供水来自于支流次流域的新兴地下水，而不是来自干流。因此，水泥溪等亚高山环境中莎草泥炭的存在是地下水从山谷两侧出现的古环境指标。
Latest Quaternary landforms, composed of gravel and peat, in the lower 11 kilometers of Cement Creek were mapped and dated in order to understand the geomorphic history of the area and thus provide a geomorphological context for understanding the formation of ferricrete, as part of the Animas River watershed study. The San Juan Mountains were nearly covered by alpine glaciers during the latest Pleistocene and presumably numerous times before. During the most recent episode, generally referred to as the Pinedale glaciation, ice was about 520 meters thick at the present site of Silverton and 430 meters thick at the present site of Gladstone. That glaciation ended about 12,000 14 C years ago, and except for till, the sediments along Cement Creek were likely deposited after that. Other deposits apparently formed during deglaciation, including a canyon-filling stream-gravel deposit, a localized stream terrace about 20 meters higher than the valley floor, and a large landslide. Prominent alluvial fans, located at the mouths of most tributaries, also probably began to accumulate as soon as ice disappeared from the floor of the Cement Creek valley. The fans probably accumulated rapidly until the surrounding hillslopes became stabilized by vegetation, which may have taken many centuries. The largest fans, or pairs of fans, aggraded the valley bottom perhaps more than 20 meters and caused Cement Creek to aggrade upstream of them. This resulted in a segmented longitudinal profile of the Cement Creek valley that is still present, because the bulk of the fan deposits have not been removed by Cement Creek. More than 6,000 years ago, however, Cement Creek partially breached the toes of the fans, incising through as much as 5 meters of fan sediment. This, in turn, caused the fans to become incised by their tributary streams. Between 3700 B.C. and A.D. 400 (5,650 to 1,550 cal. yr B.P.) Cement Creek migrated laterally, but its bed remained at the level of the present streambed. Starting about A.D. 400, which is slightly before the Medieval Warm Period, Cement Creek began to aggrade and rose as much as 3 meters above its previous level. Cement Creek incised back to its previous level, leaving behind a prominent stream terrace, between A.D. 1330 and A.D. 1700. This incision may have occurred before A.D. 1500 and perhaps about A.D. 1440. Incision of the Cement Creek terrace is roughly synchronous with the beginning of the Little Ice Age. Aggradation and incision of the Cement Creek terrace were not caused by local base level processes, and may have been the result of climate change, although the precise mechanisms involved are not known. Recent human activities, in contrast, have had little influence on the shape and physical processes of Cement Creek. The influence of human activities on geochemical process is beyond the scope of this study, with the exception of ferricrete formation. Iron compounds locally cement clastic sediment of all origins, creating conglomerate-type ferricrete. Most ferricrete exposures are dry and presumably are inactive, and most are prehistorical in age. Defining the timing of conglomerate-type ferricrete cementation is problematic, even where the depositional age of the clastic sediment is precisely known, because cementation could have occurred at any time after clastic deposition. Cementation of distal fan sediment must have occurred more than 6,000 years ago, because that is when the ground water in the gravel was permanently drained by stream incision. A spring-deposited ferricrete encased charcoal that is 4,500 cal. years old, so presumably conglomerate ferricrete also formed during the middle Holocene. Stream gravels deposited in the past 500 years are also cemented by iron compounds. We conclude that ferricrete formed in Cement Creek clastic sediment at various times and locations throughout the Holocene, and thus most ferricrete is unrelated to mining. Exposures of ferricrete are spatially discontinuous and do not correspond to the rate of emergence of ground water during the low-flow season. This suggests that the geochemical conditions necessary for ferricrete formation are not uniform in the watershed. Exposures of wet, possibly active ferricrete are almost invariably located where there are sedge wetlands (underlain by peat) on an adjacent terrace. We conclude that both wetlands and wet ferricrete result from the perennial emergence of ground water that originated in tributary subbasins, rather than from Cement Creek itself. One must take care when using peat to date stream incision because peat can continue to accrete in wetlands after stream incision. This continuing accretion occurs because the water supply to wetlands is from emerging ground water that originated in tributary subbasins, not from the main stream. Thus, the presence of sedge peat in subalpine settings like Cement Creek is a paleo-environmental indicator of the emergence of ground water from the valley sides.