Shallow subsurface diagenesis of Pleistocene periplatform ooze: northern Bahamas
Shallow subsurface diagenesis of Pleistocene periplatform ooze: northern Bahamas
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更新世台台周围软泥的浅层地下成岩作用：巴哈马北部
DOI：
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发表时间：
1985
期刊：
影响因子：
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通讯作者：
D. Siegel
中科院分区：
文献类型：
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作者：
H. T. Mullins;S. Wise;A. Gardulski;E. Hinchey;P. M. Masters;D. Siegel
先前对台地周围碳酸盐的早期海底成岩作用的研究表明，这些最初的多矿物（文石、镁方解石、方解石）沉积物仅在海流冲刷的开放海道或因海底侵蚀而暴露在表面的地方才容易受到早期成岩作用的影响。也有人提出，在浅层地下，平台周围的软泥至少在 200,000-400,000 年的时间内保留其原始矿物学，并且在数千万年的时间内保持未成岩状态。
这里报道的证据表明，在小巴哈马浅滩以北约 1,000 m 水深收集的两个活塞岩心中，晚更新世时代的台地周围渗出物存在广泛的方解石化和选择性岩化。结果表明，浅层（<30 m）地下成岩作用可以在不到 440,000 年的时间内显着改变平台周围软泥的原始矿物学，以方解石为主，并且方解石的胶结作用可以在同一时间范围内产生白垩软泥序列。最初含有高比例源自河岸的镁质方解石的台地周边渗出物似乎比那些最初含有少量镁质方解石的渗出物具有更高的成岩潜力。浅层地下方解石化作用和固定化作用大大降低了台地周围碳酸盐岩的成岩潜力，白垩软泥序列显然可以持续数千万年，埋藏深度至少为 300 m。
浅层地下成岩作用，在水深 > 1,000 m 处，通过镁质方解石和文石的溶解以及方解石再沉淀作为化金填充物、外部增生物和胶结物进行。据推测，密度驱动的“科胡特对流”是浅层地下成岩作用的主要驱动机制，其中海水中镁质方解石和文石处于欠饱和状态，而方解石则处于饱和/过饱和状态。在深海海底和浅层地下成岩作用的早期阶段，镁的去除应该会增加间隙水中的镁含量，这可能会增加科胡特对流流体流的“白云石化潜力”。
Previous studies on early submarine diagenesis of periplatform carbonates have implied that these originally polymineralic (aragonite, magnesian calcite, calcite) sediments are susceptible to early diagenesis only in current-swept open seaways or where surficially exposed by erosion on the seafloor. It has also been proposed that while in the shallow subsurface, periplatform oozes retain their original mineralogy for at least 200,000–400,000 yr and remain unlithified for tens of millions of years.
Evidence is reported here for extensive calcitization and selective lithification of periplatform oozes of late Pleistocene age in two piston cores collected from water depths of ∼ 1,000 m north of Little Bahama Bank. It is shown that shallow (<30 m) subsurface diagenesis can significantly alter the original mineralogy of periplatform oozes to predominantly calcite in less than 440,000 yr, and that cementation by calcite can produce chalk-ooze sequences within the same time-frame. Periplatform oozes that originally contain a high percentage of bank-derived magnesian calcite appear to have a higher diagenetic potential than those originally low in magnesian calcite. Shallow subsurface calcitization and fithification greatly reduce the diagenetic potential of periplatform carbonates, and chalk-ooze sequences apparently can persist for tens of millions of years and to burial depths of at least 300 m.
Shallow subsurface diagenesis, at water depths > 1,000 m, proceeds via dissolution of magnesian calcite and aragonite and reprecipitation of calcite as allochem fillings, exterior overgrowths and cement. It is speculated that density-driven ‘Kohout convection‘, where seawaters under-saturated with respect to magnesian calcite and aragonite and saturated/supersaturated with respect to calcite flow through the margins of carbonate platforms, is the primary driving mechanism for shallow subsurface diagenesis. Removal of Mg during early stages of deep seafloor and shallow subsurface diagenesis should increase the Mg content of interstitial waters which is likely to increase the ‘dolomitizing potential’ of Kohout convection fluid flow.