Submarine deposits from pumiceous pyroclastic density currents traveling over water: An outstanding example from offshore Montserrat (IODP 340)
Submarine deposits from pumiceous pyroclastic density currents traveling over water: An outstanding example from offshore Montserrat (IODP 340)
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浮岩火山碎屑密度流在水面上形成的海底沉积物：蒙特塞拉特近海的一个突出例子 (IODP 340)
DOI：
10.1130/b31448.1
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发表时间：
2017
影响因子：
4.9
通讯作者：
Jutzeler M
中科院分区：
文献类型：
--
作者：
Jutzeler M
已经观察到火山碎屑密度流既进入海洋，又在水面上传播数十公里。在这里，我们确定了位于蒙特塞拉特U1396近海(综合大洋钻探计划[IODP]考察340)地点的1.2米厚的层状浮石火山灰岩芯，这是第一个提供证据表明它是由水面上方富含浮石的火山碎屑密度流形成的海底沉积形成的。海底沉积物的年龄约为4 Ma，其岩浆来源与更年轻的Soufrière Hills沉积物的岩浆来源相似，表明该岛经历了比陆地沉积物更早的大规模地下火山口形成的爆炸性喷发。沉积物的组合沉积学特征与海底喷发、火山碎屑落水或海底拥抱海底的浑浊流不相容，后者是由水下火山碎屑密度流在海岸线进入水中而产生的。层状浮石火山灰单元可细分为至少三个沉积单元，最低的一个被碎屑支撑。该装置包含五种不同大小的颗粒，并具有>；12 phi范围。颗粒主要是近圆形的浮石碎屑、岩屑、水晶碎片和玻璃碎片。浮石碎屑与其他颗粒类型的分离很差，整个矿床都有岩屑碎屑；细颗粒的密度分级很弱。我们对该单元的解释是记录了多个密集的热火山碎屑流，它们流经海洋，将火山碎屑释放到水面上，并将各种火山碎屑沉降到水柱中。我们的沉降和冷热浮选实验表明，浮石碎屑在水面上的内涝会立即发生。沉积物的水力分选总体较差，是由于水柱中多个垂直沉降脉冲的颗粒混合造成的，这证明了复杂的沉积作用。缓慢沉降的颗粒与较快下降的颗粒一起沉积在海底，这些颗粒通过随后的火山碎屑流输送到水面。最终的沉积脉冲在到达海底时最终发生偏转，并以横向连续的相沉积。这项研究强调火山喷发的产物与海洋之间的相互作用，并讨论了与向水中输送颗粒有关的沉积学复杂性和水动力学。
Pyroclastic density currents have been observed to both enter the sea, and to travel over water for tens of kilometers. Here, we identified a 1.2-m-thick, stratified pumice lapilli-ash cored at Site U1396 offshore Montserrat (Integrated Ocean Drilling Program [IODP] Expedition 340) as being the first deposit to provide evidence that it was formed by submarine deposition from pumice-rich pyroclastic density currents that traveled above the water surface. The age of the submarine deposit is ca. 4 Ma, and its magma source is similar to those for much younger Soufrière Hills deposits, indicating that the island experienced large-magnitude, subaerial caldera-forming explosive eruptions much earlier than recorded in land deposits. The deposit’s combined sedimentological characteristics are incompatible with deposition from a submarine eruption, pyroclastic fall over water, or a submarine seafloor-hugging turbidity current derived from a subaerial pyroclastic density current that entered water at the shoreline. The stratified pumice lapilli-ash unit can be subdivided into at least three depositional units, with the lowermost one being clast supported. The unit contains grains in five separate size modes and has a >12 phi range. Particles are chiefly subrounded pumice clasts, lithic clasts, crystal fragments, and glass shards. Pumice clasts are very poorly segregated from other particle types, and lithic clasts occur throughout the deposit; fine particles are weakly density graded. We interpret the unit to record multiple closely spaced (<2 d) hot pyroclastic density currents that flowed over the ocean, releasing pyroclasts onto the water surface, and settling of the various pyroclasts into the water column. Our settling and hot and cold flotation experiments show that waterlogging of pumice clasts at the water surface would have been immediate. The overall poor hydraulic sorting of the deposit resulted from mixing of particles from multiple pulses of vertical settling in the water column, attesting to complex sedimentation. Slow-settling particles were deposited on the seafloor together with faster-descending particles that were delivered at the water surface by subsequent pyroclastic flows. The final sediment pulses were eventually deflected upon their arrival on the seafloor and were deposited in laterally continuous facies. This study emphasizes the interaction between products of explosive volcanism and the ocean and discusses sedimentological complexities and hydrodynamics associated with particle delivery to water.
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