Synthesis of Drilling Results from the Mid-Pacific Mountains: Regional Context and Implications
Synthesis of Drilling Results from the Mid-Pacific Mountains: Regional Context and Implications
复制标题
中太平洋山脉钻探结果的综合：区域背景和影响
DOI：
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发表时间：
1995
期刊：
影响因子：
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通讯作者：
W. Sager
中科院分区：
文献类型：
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作者：
E. Winterer;W. Sager
在中太平洋山脉（MPM）的Resolution和Allison guyots上钻探的结果记录了火山作用、沉降和Hauterivian-Albian浅水碳酸盐沉积物堆积的悠久历史。中白垩世后期出现了下沉和上层沉积物的堆积。Guyot分辨率下的基底为碱性玄武岩的陆基流，辐射测年平均值为127.6±2.1 Ma，位置约为14°S。上覆的浅水碳酸盐厚1620米。866站点的沉积物位于离台地边缘2 km处，沉积于浅层潮下至潮间带深度，间歇暴露于地面。在距离平台边缘0.5公里的867站点，海滩和风暴沉积物是常见的，而在距离平台边缘0.1公里的868站点，生活位置的海绵和芦苇表明平台边缘环境。在Allison Guyot，在865A孔底部取心碱性玄武岩岩基。地震剖面显示，基底下有多达600米的沉积物，其Ar/Ar辐射测年平均值为110.7±1.2 Ma。钻出的地层厚度为730米，从底部附近一直延伸到Albian的顶部。这一段始于大约200米的粘土石灰岩，沉积在安静的沼泽水域。向上，粘土逐渐消失，反映了随着海底山的沉降，火山山被掩埋。该系列的其余部分是沉积在潮下至潮间带环境中的碎屑岩。两个岩心和测井资料均显示浅上旋回厚度为3 ~ 10 m。傅里叶分析产生的估计约为100 ka作为最常见的频率。十米尺度上的相序列表明海平面的长期波动。成岩作用以文石溶解为主，胶结物为海相低镁方解石。孔隙水数据显示整个演替对现代海水是开放的。古约Resolution Guyot碳酸盐岩地层下部400 m以白云岩为主，sr同位素数据显示白云岩大部分形成于沉积后15 ~ 20 m。在埋藏的基底地形上，石灰岩的压实作用随着沉积而迅速进行，并在土屋被淹没后继续进行。在最近的阿尔比安时期，相对海平面下降了近200米，两个岩洞上的石灰岩地层暴露在地面和海浪的侵蚀下，但原因是构造还是浮力尚不清楚。到了土鲁纪中期，古岩群重新被淹没，但只积累了上层沉积物。为什么没有更多的浅水沉积物积聚在土丘上是一个谜。它们在出现时大约在8°S（分辨率）到11°S（艾利森）。上白垩纪的远洋沉积物只保存在阿拉伯石灰岩的洞穴中。Allison Guyot的始新世和古新世沉积物沉积在近赤道纬度，厚度约为120 m。下白垩纪台地被磷矿和氧化锰铁不同地包裹着，即使埋在远洋沉积物之下。声波速度的主要控制因素是成岩作用下密度的变化。盖特边缘周围混乱的反射可能是由厚厚的锰铁路面引起的，而不是由巨大的珊瑚礁引起的。钻探表明，周围的边缘不是框架礁，而是由沙洲和泻湖沉积物雕刻而成的侵蚀特征。在去除地形影响后，解析度盖尤特上空的自由空气重力异常有显著的残余。钻探结果表明，大部分多余的质量可以用白云化石灰岩和大厦周围低密度沉积物之间的密度对比来解释。MPM海山地磁异常反演显示了古地磁极位置的广泛地理分布。地磁极附近的两极可以归因于感应磁化。有些海山有复杂的异常，可能是由包含磁反转的复杂磁化结构来解释的。
The results of drilling on Resolution and Allison guyots, in the Mid-Pacific Mountains (MPM), document a long history of volcanism, subsidence, and accumulation of Hauterivian-Albian shallow-water carbonate sediments. Mid-Cretaceous emersion was followed by subsidence and accumulation of pelagic sediments. Basement beneath Resolution Guyot is subaerial flows of alkalic basalt, with radiometric dates averaging 127.6 ±2.1 Ma, emplaced at about 14°S. Overlying shallow-water carbonates are 1620 m thick. Sediments at Site 866, 2 km inward from the platform edge, were deposited in shallow subtidal to intertidal depths, with intermittent subaerial exposure. At Site 867, 0.5 km from the platform edge, beach and storm deposits are common, and at Site 868,0.1 km from the edge, sponges and rudists in life positions indicate a platform-margin environment. At Allison Guyot, alkalic basalt sills were cored at the bottom of Hole 865A. Seismic profiles suggest as much as 600 m of sediments underlie the sills, which have Ar/Ar radiometric dates averaging 110.7 ±1.2 Ma. The drilled strata, 730 m thick, extend from near the base to nearly the top of the Albian. The section begins with about 200 m of clayey limestone deposited in quiet, swampy waters. Upward, clays gradually disappear, reflecting the burial of volcanic hills as the seamount subsided. The rest of the series is wackestone deposited in subtidal to intertidal environments. Core and logging data at both guyots show shallowing-upward cycles of 3 to 10 m thick. Fourier analysis yields estimates of about 100 ka as the most common frequency. Longer-term fluctuations in sea level are suggested by facies successions at a decameter scale. Diagenesis was dominated by dissolution of aragonite, and cements are now marine, low-magnesium calcite. Pore-water data show the entire succession to be open to modern seawater. Dolomite dominates in the lower 400 m of carbonate strata at Resolution Guyot, and Sr-isotope data suggest much of it formed 15 to 20 m.y. after deposition. Compaction of limestone over buried basement topography proceeded apace with deposition and continued after drowning of the guyots. During the latest Albian, a fall in relative sea level of nearly 200 m exposed limestone strata on both guyots to subaerial and wave erosion, but whether the cause was tectonic or eustatic is not yet known. By mid-Turonian time, the guyots had re-submerged, but only pelagic sediments accumulated. Why no further shallow-water sediments accumulated on the guyots is a mystery. They were at about 8°S (Resolution) to 11°S (Allison) at the time of emergence. Upper Cretaceous pelagic sediments are preserved only in cavities within Albian limestone. Eocene and Paleocene sediments on Allison Guyot, about 120 m thick, were deposited at near-equatorial latitudes. The Lower Cretaceous platform is variably encrusted with phosphorite and ferromanganese oxides, even where buried beneath pelagic sediments. Primary control on acoustic-wave velocity is from diagenetic changes in density. Chaotic reflections around the guyot edges may be caused by the thick ferromanganese pavements, rather than by massive reefs. Drilling showed that the perimeter rims are not framework reefs, but erosional features carved from sand shoals and lagoonal sediments. A significant residual remains in the free-air gravity anomalies over Resolution Guyot after subtracting topographic effects. Drilling results imply that most of this excess mass can be explained by the density contrast between dolomitized limestone and low-density sediments surrounding the edifice. Inversion of magnetic anomalies over MPM seamounts gives a wide geographic scatter in paleomagnetic pole location. Poles near the geomagnetic pole can be attributed to induced magnetization. Some seamounts have complex anomalies perhaps explained by a complex magnetization structure containing magnetic reversals.