Interpretation of neovolcanic versus palaeovolcanic sand grains: an example from Miocene deep-marine sandstone of the Topanga Group (Southern California)
Interpretation of neovolcanic versus palaeovolcanic sand grains: an example from Miocene deep-marine sandstone of the Topanga Group (Southern California)
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DOI：
10.1111/j.1365-3091.1995.tb00409.x
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发表时间：
1995-10
期刊：
影响因子：
3.5
通讯作者：
S. Critelli;R. Ingersoll
中科院分区：
文献类型：
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作者：
S. Critelli;R. Ingersoll
尽管有大量关于火山碎屑砂（石）的资料，但沉积记录中火山碎屑的组成、时空分布记录却很少。对含火山颗粒的砂（石）进行光学分析时，最复杂的任务之一是区分由古火山岩侵蚀产生的颗粒（即古火山颗粒，非同世颗粒）和由沉积过程中活火山作用（水下和/或陆上）产生的颗粒（新火山颗粒，同世颗粒）。南加州中托潘加群的深海火山碎屑砂岩与3000米厚的火山沉积物（包括玄武岩、安山岩和英安岩等水下和陆上熔岩和火山碎屑岩）互层。这些岩石位于下托潘加群的石英岩相（岩相1）之上，形成于中生代岩浆弧的深蚀作用。中托潘加群砂岩成分的变化为同世火山作用对深海沉积的影响提供了例证。火山碎屑地层沉积在浊积岩杂岩（火山碎屑围裙）的深海部分，这些杂岩（火山碎屑围裙）建立在一连串的基底内熔岩流上，以及位于地下的熔岩流和火山碎屑岩的陡峭侧翼上。独特的岩相（2-5）。岩相2直接上覆玄武岩和玄武岩-安山岩熔岩流，为纯火山石器砂岩，包括玻璃状、微晶状和条状火山颗粒，以及新火山晶体（斜长石、辉石和橄榄石）。大量的淬火玻璃（palagonite）碎片表明物源为水下新火山，而包括安山岩和少量玄武岩颗粒在内的砂岩则表明物源为陆上新火山。该岩相可能是在协同作用时期沉积的，证明物源可能来自于盆内和盆外火山事件。不纯火山岩相3沉积于喷发间期，既包括新火山岩（85%），也包括源自深成岩、变质岩和古火山岩的更老碎屑。火山喷发后，上覆石英岩相4和石英岩相5表明新火山碎屑逐渐减少（48-14%），而深部变质岩和古火山碎屑增加。上托潘加群（卡拉巴萨斯组）整合覆于中单元之上，主要发育深部碎屑砂岩（岩相6）。新火山碎屑大幅减少（4%），而古火山碎屑的百分比与下托潘加群（岩相1）相似。中托潘加群砂岩在纵向上分为4个
Despite abundant data on volcaniclastic sand(stone), the compositional, spatial and temporal distribution of volcanic detritus within the sedimentary record is poorly documented. One of the most intricate tasks in optical analysis of sand(stone) containing volcanic particles is to distinguish grains derived by erosion of ancient volcanic rocks (i.e. palaeovolcanic, noncoeval grains) from grains generated by active volcanism (subaqueous and/or subaerial) during sedimentation (neovolcanic, coeval grains). Deep-marine volcaniclastic sandstones of the Middle Topanga Group of southern California are interstratified with 3000-m-thick volcanic deposits (both subaqueous and subaerial lava and pyroclastic rocks, ranging from basalt, andesite to dacite). These rocks overlie quartzofeldspathic sandstones (petrofacies 1) of the Lower Topanga Group, derived from deep erosion of a Mesozoic magmatic arc. Changes in sandstone composition in the Middle Topanga Group provide an example of the influence of coeval volcanism on deep-marine sedimentation. Volcaniclastic strata were deposited in deep-marine portions of a turbidite complex (volcaniclastic apron) built onto a succession of intrabasinal lava flows and on the steep flanks of subaerially emplaced lava flows and pyroclastic rocks. distinctive petrofacies (2-5). Directly overlying basalt and basaltic-andesite lava flows, petrofacies 2 is a pure volcanolithic sandstone, including vitric, microlitic and lathwork volcanic grains, and neovolcanic crystals (plagioclase, pyroxene and olivine). The abundance of quenched glass (palagonite) fragments suggests a subaqueous neovolcanic provenance, whereas sandstones including andesite and minor basalt grains suggest subaerial neovolcanic provenance. This petrofacies probably was deposited during syneruptive periods, testifying to provenance from both intrabasinal and extrabasinal volcanic events. Deposited during intereruptive periods, impure volcanolithic petrofacies 3 includes both neovolcanic (85%) and older detritus derived from plutonic, metamorphic and palaeovolcanic rocks. During post-eruptive periods, the overlying quartzofeldspathic petrofacies 4 and 5 testify to progressive decrease of neovolcanic detritus (48-14%) and increase of plutonic-metamorphic and palaeovolcanic detritus. The Upper Topanga Group (Calabasas Formation), conformably overlying the Middle unit, has dominantly plutoniclastic sandstone (petrofacies 6). Neovolcanic detritus is drastically reduced (4%) whereas palaeovolcanic detritus is similar to percentages of the Lower Topanga Group (petrofacies 1). The Middle Topanga Group sandstones are vertically organized into four