Formation and early history of Lakes Pepin and St. Croix of the upper Mississippi River
Formation and early history of Lakes Pepin and St. Croix of the upper Mississippi River
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密西西比河上游佩平湖和圣克罗伊湖的形成和早期历史
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发表时间：
2009
期刊：
影响因子：
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通讯作者：
V. Stefanova
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作者：
Dylan J. Blumentritt;H. E. Wright;V. Stefanova
对佩平湖和圣克罗伊湖的研究始于一个多世纪前，但新的信息使人们能够更近距离地了解这两个位于密西西比河上游的河流湖泊的地质历史。上一次冰川末期大型前冰川湖泊阿加西和德卢斯的排水帮助形成了现在的山谷。随着高流量出口水的退去，支流在切割的河谷中沉积了扇形泥沙。这些支流扇筑坝堵住了主干河，形成了江河湖泊。佩平湖以前被认为是一个单独的长而连续的湖，从奇佩瓦河扇的大坝一直延伸到圣保罗，湖的一只手臂沿着圣克罗伊山谷延伸。最近在桥梁和大坝位置进行的钻探显示了不止一段湖泊沉积物，这表明历史更加复杂。明尼苏达州和密西西比河并不总是沿着目前的路线前进。切割在基岩上但现在被冰川沉积物掩埋的山谷表明了以前的河流，最近的这些河流来自上一次间冰期，表面上有一连串的湖泊。密西西比河谷底的形态，以及塞满山谷的佩平湖的形态，在一定程度上反映在这些古老山谷的存在上，但也反映在冰川冲积阶地和支流冲积扇的存在上。在湖城附近的佩平湖拍摄的一个沉积物岩心，在湖泊沉积物下方的砾石中有一块木头，可以追溯到10.3公元前。BP，表明该湖是在阿加西湖和德卢斯湖的洪水退去后不久奇佩瓦河扇增长而形成的。来自新钻探的数据显示，在现代的佩平湖和圣保罗湖之间的密西西比河河谷，几个支流扇子后面筑起了小湖泊。一个支流湖泊，在这里被称为早期的弗米尔湖，可能已经通过水力筑坝堵住了圣克罗伊河，形成了一个新的圣克罗伊湖。来自朱砂河、坎农河和奇佩瓦河的支流扇子都将主要河谷分割成一系列河流湖泊。后来，奇佩瓦球迷的增长超过了朱砂和大炮球迷的增长，形成了一个单独的大湖，在这里被称为莱特佩平湖，它向上游延伸到圣保罗。从佩平湖采集的沉积物岩芯中没有重要的有机物质，无法通过放射性碳测年得出年代学。相反，磁性特征与圣克罗伊湖岩芯的磁性特征相匹配，后者确实有已知的放射性碳年代学。然后，通过将埋藏的湖泊沉积物顶部的高程投影到过时的佩平湖岩芯，使用估计的10厘米/公里的坡度，即佩平湖沉积物表面的当前坡度，估计了佩平三角洲的迁移率。根据这些近似值，佩平湖三角洲超过了黑斯廷斯6.0亿卡BP和红翼1.4亿卡BP。
Study of Lake Pepin and Lake St. Croix began more than a century ago, but new information has permitted a closer look at the geologic history of these two riverine lakes located on the upper Mississippi River system. Drainages from large proglacial lakes Agassiz and Duluth at the end of the last glaciation helped shape the current valleys. As high-discharge outlet waters receded, tributary streams deposited fans of sediment in the incised river valleys. These tributary fans dammed the main river, forming riverine lakes. Lake Pepin was previously thought to be a single long continuous lake, extending for 80 km from its dam at the Chippewa River fan all the way up to St. Paul, with an arm extending up the St. Croix valley. Recent borings taken at bridge and dam locations show more than a single section of lake sediments, indicating a more complex history. The Minnesota and Mississippi Rivers did not always follow their current paths. Valleys cut into bedrock but now buried by glacial sediment indicate former river courses, with the most recent of these from the last interglacial period marked at the surface by chains of lakes. The morphology of the Mississippi valley bottom, and thus the morphology of Lake Pepin as it filled the valley, is reflect in part by the existence of these old valleys but also by the presence of glacial outwash terraces and the alluvial fans of tributary streams. A sediment core taken in Lake Pepin near Lake City had a piece of wood in gravels just below lake sediments that dated to 10.3 ka cal. BP, indicating that the lake formed as the Chippewa River fan grew shortly after the floodwaters of Lakes Agassiz and Duluth receded. Data from new borings indicate small lakes were dammed behind several tributary fans in the Mississippi River valley between the modern Lake Pepin and St. Paul. One tributary lake, here called Early Lake Vermillion, may have hydraulically dammed the St. Croix River, creating an incipient Lake St. Croix. The tributary fans from the Vermillion River, the Cannon River, and the Chippewa River all served to segment the main river valley into a series of riverine lakes. Later the growth of the Chippewa fan surpassed that of the Vermillion and Cannon fans to create a single large lake, here called late Lake Pepin, which extended upstream to St. Paul. Sediment cores taken from Lake Pepin did not have significant organic matter to develop a chronology from radiocarbon dating. Rather, magnetic features were matched with those from a Lake St. Croix core, which did have a known radiocarbon chronology. The Pepin delta migration rate was then estimated by projecting the elevations of the top of the buried lake sediments to the dated Lake Pepin core, using an estimated slope of 10 cm/km, the current slope of Lake Pepin sediment surface. By these approximations, the Lake Pepin delta prograded past Hastings 6.0 ka cal BP and Red Wing 1.4 ka cal BP.