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首页> 外文期刊>Journal of Volcanology and Geothermal Research >Origin and stratigraphy of phreatomagmatic deposits at the Pleistocene Sinker Butte Volcano, Western Snake River Plain, Idaho
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Origin and stratigraphy of phreatomagmatic deposits at the Pleistocene Sinker Butte Volcano, Western Snake River Plain, Idaho

机译：爱达荷州西部蛇河平原更新世的辛克比特火山火山岩浆沉积的成因和地层

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摘要

Sinker Butte is the erosional remnant of a very large basaltic tuff cone of middle Pleistocene age located at the southern edge of the western Snake River Plain. Phreatomagmatic tephras are exposed in complete sections up to 100 m thick in the walls of the Snake River Canyon, creating an unusual opportunity to study the deposits produced by this volcano through its entire sequence of explosive eruptions. The main objectives of the study were to determine the overall evolution of the Sinker Butte volcano while focusing particularly on the tephras produced by its phreatomagmatic eruptions. Toward this end, twenty-three detailed stratigraphic sections ranging from 20 to 100 m thick were examined and measured in canyon walls exposing tephras deposited around 180° of the circumference of the volcano. Three main rock units are recognized in canyon walls at Sinker Butte: a lower sequence composed of numerous thin basaltic lava flows, an intermediate sequence of phreatomagmatic tephras, and a capping sequence of welded basaltic spatter and more lava flows. We subdivide the phreatomagmatic deposits into two main parts, a series of reworked, mostly subaqueously deposited tephras and a more voluminous sequence of overlying subaerial surge and fall deposits. Most of the reworked deposits are gray in color and exhibit features such as channel scour and fill, planar-stratification, high and low angle cross-stratification, trough cross-stratification, and Bouma-turbidite sequences consistent with their being deposited in shallow standing water or in braided streams. The overlying subaerial deposits are commonly brown or orange in color due to palagonitization. They display a wide variety of bedding types and sedimentary structures consistent with deposition by base surges, wet to dry pyroclastic fall events, and water saturated debris flows. Proximal sections through the subaerial tephras exhibit large regressive cross-strata, planar bedding, and bomb sags suggesting deposition by wet base surges and tephra fallout. Medial and distal deposits consist of a thick sequence of well-bedded tephras; however, the cross-stratified base-surge deposits are thinner and interbedded within the fallout deposits. The average wavelength and amplitude of the cross strata continue to decrease with distance from the vent. These bedded surge and fall deposits grade upward into dominantly fall deposits containing 75-95% juvenile vesiculated clasts and localized layers of welded spatter, indicating a greatly reduced water-melt ratio. Overlying these "dryer" deposits are massive tuff breccias that were probably deposited as water saturated debris flows (lahars). The first appearance of rounded river gravels in these massive tuff breccias indicates downward coring of the diatreme and entrainment of country rock from lower in the stratigraphic section. The "wetter" nature of these deposits suggests a renewed source of external water. The massive deposits grade upward into wet fallout tephras and the phreatomagmatic sequence ends with a dry scoria fall deposit overlain by welded spatter and lava flows. Field observations and two new ~(40)Ar-~(39)Ar incremental heating dates suggest the succession of lavas and tephra deposits exposed in this part of the Snake River canyon may all have been erupted from a closely related complex of vents at Sinker Butte.
机译：信克比尤特（Sinker Butte）是位于中蛇新纪的大型大型玄武质凝灰岩的侵蚀残余物，位于西斯内克河平原西部边缘。吞噬岩特弗拉斯在斯内克河峡谷的壁中暴露出厚达100 m的完整区域，为研究该火山通过其整个爆发性喷发产生的沉积物提供了难得的机会。这项研究的主要目的是确定Sinker Butte火山的总体演变，同时特别关注其岩浆喷发产生的特非拉斯。为此，在峡谷壁上检查并测量了23个详细的地层剖面，厚度范围为20至100 m，露出暴露在火山圆周180°附近的特非拉斯的峡谷壁。在Sinker Butte的峡谷壁中可以识别出三个主要岩石单元：一个较低的层序，由众多稀薄的玄武岩熔岩流组成；一个中间层的吞噬岩特弗拉斯层序；以及一个由焊接的玄武岩飞溅形成的顶盖序列，以及更多的熔岩流。我们将磁岩沉积物细分为两个主要部分，即一系列经过改造的，大部分在水底沉积的提弗拉斯岩以及一系列数量较大的上层地下气旋和降落沉积物。大部分返修后的矿床为灰色，并具有诸如河道冲刷和填充，平面分层，高低角度交叉分层，低谷交叉分层以及布马德-浊积体序列等特征，这些特征与它们沉积在浅水区中一致或辫状流中。由于泛光作用，上覆的地下沉积物通常为棕色或橙色。它们显示出多种多样的层理类型和沉积结构，这些变化与基础浪涌，湿热至干热碎屑作用以及水饱和泥石流的沉积相一致。穿过地下气生球的近端断面显示出较大的回归交叉地层，平坦的层理和炸弹下陷，表明湿基潮和生球后尘的沉积。内侧和远端的沉积物由厚厚的床具良好的底弗拉组成。然而，交叉分层的基波涌沉积较薄，并且夹层沉积在沉积物中。交叉层的平均波长和幅度随着距通风口的距离而不断减小。这些层状的潮落矿床向上划分为主要为秋天的矿床，其中包含75％至95％的小泡状碎屑和局部的焊接飞溅层，表明水熔比大大降低。这些“较干燥”的沉积物上方是凝灰岩角砾岩，可能是水饱和的泥石流（拉哈斯）沉积而成。在这些块状凝灰岩角砾岩中首次出现圆形河砾石，表明地核的向下取心和夹带的岩层从地层下部开始。这些沉积物的“更好”性质表明有更新的外部水源。大量的沉积物向上渐变成湿润的沉积性特非拉斯，并在岩浆岩层中以干燥的火山灰沉积物结束，该沉积物被焊接飞溅物和熔岩流覆盖。实地观察和两个新的〜（40）Ar-〜（39）Ar增量加热日期表明，在蛇河峡谷这一部分暴露的熔岩和特非拉沉积物的继承可能全部是由Sinker的一个紧密相关的喷口喷发而来的。比尤特。

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