Origins of Bimodal Stratigraphy In Fluvial Deposits: An Example From the Morrison Formation (Upper Jurassic), Western U.S.A.
Origins of Bimodal Stratigraphy In Fluvial Deposits: An Example From the Morrison Formation (Upper Jurassic), Western U.S.A.
复制标题
河流沉积物中双峰地层的起源：美国西部莫里森组（上侏罗统）的一个例子
DOI：
10.2110/jsr.2015.93
复制
发表时间：
2015
影响因子：
2
通讯作者：
S. Huzurbazar
中科院分区：
文献类型：
--
作者：
P. Heller;Deidre Ratigan;S. Trampush;A. Noda;B. McElroy;J. Drever;S. Huzurbazar
河流河道带砂体堆积密度的变化是冲积建筑学最明显的特征之一。砂体彼此的接近程度已被用作表明外部因素（如盆地沉降速率、基准面的相对变化和气候变化）发生重大变化的现场标准。在某些地方，堆积密度在很高到很低的净砂岩之间的垂直序列中发生了快速变化。在不存在不整合面的情况下，这种变化不是两个不相关但并列的河流体系的结果，而是形成条件或保存方式的过渡性变化。为了研究堆积密度的快速变化是否受外部因素而不是自生行为控制，我们试图记录与堆积模式变化相一致的沉积中的其他可能变化。美国犹他州东部和科罗拉多西部的上侏罗统莫里森组，包含从平均63%净砂的盐水段到上覆的以泥质河漫滩沉积为主的灌木盆地段（10%净砂）的快速过渡。我们测量了9个沉积特征相关的河流系统的属性在两个成员沿着一个向下的古流向样带。两个成员之间发现的唯一显着变化是河漫滩粘土矿物的组成和丰度。堆积密度的转变与蒙脱石粘土的快速增加相一致，解释为火山灰流入量增加导致伊利石粘土的损失。无论是作为大气降落还是作为河流的冲刷负荷，火山灰都优先聚集在漫滩。几何学计算表明，为了仅通过添加火山灰来产生所观察到的堆积模式的变化，洪泛平原沉积物的一半以上必须由火山灰组成。增加灰的供应将增加漫滩沉积速率，减少撕脱的频率。这些效应加在一起可以将堆叠密度降低所需的50%。堆积密度对河漫滩沉积速率的依赖性突出了假设冲积层序中观察到的堆积密度变化主要受其他外部因素（如海平面波动、沉降速率或总体加积速率）控制的危险。
Abstract Changes in stacking density of fluvial channel-belt sand bodies are one of the most obvious aspects of alluvial architecture. The proximity of sand bodies to each other has been used as a field criterion to suggest major changes in external factors, such as basin subsidence rates, relative changes in base level, and changes in climate. In places rapid changes in stacking density are found in vertical succession between very high to very low net sandstone. In the absence of an intervening unconformity such changes are not the result of two unrelated, but juxtaposed, fluvial regimes but represent a more transitional change of formative conditions or preservation style. To study whether a rapid change in stacking density was controlled by external factors rather than autogenic behavior, we sought to document other possible changes in deposition that took place in concert with changes in stacking pattern. The Morrison Formation, Upper Jurassic of eastern Utah and western Colorado, U.S.A., contains a rapid transition from the Salt Wash Member, with an average 63% net sand, to the overlying Brushy Basin Member, dominated by muddy floodplain deposits (10% net sand). We measured nine sedimentary characteristics related to fluvial system properties in both members along a down-paleoflow-oriented transect. The only significant change found between the two members is the composition and abundance of floodplain clay minerals. The transition in stacking density is coincident with a rapid increase in smectitic clay, interpreted to result from increased volcanic ash influx, at the expense of illitic clay. The ash was preferentially accumulated in the floodplain, either as airfall or carried in the wash load of rivers. Geometric calculations suggest that in order to generate the observed change in stacking pattern by the addition of volcanic ash alone, more than half of the flood-plain deposition would have to be composed of ash. An increase in ash supply would increase floodplain sedimentation rate and reduce the frequency of avulsion. Together these effects could reduce the stacking density by the requisite 50%. Dependence of stacking density on floodplain sedimentation rate highlights the danger in assuming changes in stacking density observed in alluvial sequences are mainly controlled by other external factors such as sea-level fluctuation, subsidence rate, or overall aggradation rates.