Recognition of strong seasonality and climatic cyclicity in an ancient, fluvially dominated, tidally influenced point bar: Middle McMurray Formation, Lower Steepbank River, north‐eastern Alberta, Canada
Recognition of strong seasonality and climatic cyclicity in an ancient, fluvially dominated, tidally influenced point bar: Middle McMurray Formation, Lower Steepbank River, north‐eastern Alberta, Canada
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在一个古老的、以河流为主、受潮汐影响的点坝中认识到强烈的季节性和气候周期性：加拿大艾伯塔省东北部陡岸河下麦克默里地层
DOI：
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发表时间：
2016
期刊：
影响因子：
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通讯作者：
R. Dalrymple
中科院分区：
文献类型：
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作者：
Bryce V. J. Jablonski;R. Dalrymple
在加拿大阿尔伯塔省东北部的陡岸河沿岸，下白垩统McMurray组的斜异石器层状沉积在河流-海洋过渡时期一条30 - 40米深的大陆尺度河流的点坝上。这种倾斜的异石器层序由砂层和细粒层两种交替的岩性组成。沙床在单向水流的作用下迅速沉积，很少或没有生物扰动。细粒地层含有罕见的潮汐结构，并受到低多样性的鱼化石组合的强烈生物扰动。砂层和细粒层之间的交替可能是由河流流量的强烈变化引起的；它们的持续时间被认为是季节性的（可能是每年的）。沙层在洪水期间积聚，当水是淡水时，在河流主导的条件下，而细粒床在洪水后期积聚，并在低河水流量时期（即洪水间期）在受潮汐影响的微咸水条件下继续沉积。这些变化反映了河流流量变化导致的河海过渡中潮汐和盐度极限位置的年迁移。在研究的露头岩层中，砂和细粒层呈循环组织，形成米级旋回。一个一米尺度的循环被定义为一个尖锐的底部，砂床厚度向上减少，细粒床的保存和生物扰动强度向上增加。米尺度周期被解释为河流流量长期（年代际）周期的产物，可能是由海洋或太阳动力学的波动引起的。在演替过程中，河流洪水沉积物的体积优势表明，在河流-海洋过渡过程中，堆积发生在相对靠陆的位置。这项研究表明，仔细的观察可以揭示河流-海洋过渡过程中的相互作用，这反过来又为确定河流-海洋过渡中沉积物的古环境位置提供了有力的工具。
Inclined heterolithic stratification in the Lower Cretaceous McMurray Formation, exposed along the Steepbank River in north‐eastern Alberta, Canada, accumulated on point bars of a 30 to 40 m deep continental‐scale river in the fluvial–marine transition. This inclined heterolithic stratification consists of two alternating lithologies, sand and fine‐grained beds. Sand beds were deposited rapidly by unidirectional currents and contain little or no bioturbation. Fine‐grained beds contain rare tidal structures, and are intensely bioturbated by low‐diversity ichnofossil assemblages. The alternations between the sand and fine‐grained beds are probably caused by strong variations in fluvial discharge; that are believed to be seasonal (probably annual) in duration. The sand beds accumulated during river floods, under fluvially dominated conditions when the water was fresh, whereas the fine‐grained beds accumulated during the late stages of the river flood and deposition continued under tidally influenced brackish‐water conditions during times of low‐river flow (i.e. the interflood periods). These changes reflect the annual migration in the positions of the tidal and salinity limits within the fluvial–marine transition that result from changes in river discharge. Sand and fine‐grained beds are cyclically organized in the studied outcrops forming metre‐scale cycles. A single metre‐scale cycle is defined by a sharp base, an upward decrease in sand‐bed thickness and upward increases in the preservation of fine‐grained beds and the intensity of bioturbation. Metre‐scale cycles are interpreted to be the product of a longer term (decadal) cyclicity in fluvial discharge, probably caused by fluctuations in ocean or solar dynamics. The volumetric dominance of river‐flood deposits within the succession suggests that accumulation occurred in a relatively landward position within the fluvial–marine transition. This study shows that careful observation can reveal much about the interplay of processes within the fluvial–marine transition, which in turn provides a powerful tool for determining the palaeo‐environmental location of a deposit within the fluvial–marine transition.
影响因子：
2
作者：
Baas, Jaco H.;Best, James L.;Wang, Mi
通讯作者：
Wang, Mi