A fossilized Opal A to Opal C/T transformation on the northeast Atlantic margin: support for a significantly elevated Palaeogeothermal gradient during the Neogene?
A fossilized Opal A to Opal C/T transformation on the northeast Atlantic margin: support for a significantly elevated Palaeogeothermal gradient during the Neogene?
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DOI：
10.1046/j.1365-2117.2002.00184.x
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发表时间：
2002-12-01
期刊：
影响因子：
3.2
通讯作者：
Cartwright, J
中科院分区：
文献类型：
--
作者：
Davies, RJ;Cartwright, J
从最近在英国西北部法罗-设得兰海峡钻探的深水勘探井采集的岩石样本证实，横切上古近纪和新近纪序列并覆盖面积 10 000 km(2) 的独特高振幅地震反射体是蛋白石 A 到蛋白石 C/T（方英石/鳞石英）过渡的一个例子。对这些与广泛的二维和三维地震数据库相关的岩石碎片的分析限制了边界化石化的时间，此外还揭示了残留底部模拟反射器的不寻常的几何特征。生物硅 (Opal A) 到 Opal C/T 的成岩转化主要受温度控制，需要含有生物硅的沉积物。反射体（称为地平线 E）最初可能代表生物硅质软泥或粉砂岩，其中含有生物硅成分，在埋藏过程中随着成岩前缘迁移上切而发生转变。它与较浅的上新世早期反射层表现出的平行性及其在盆地挤压事件期间明显的上断迁移表明，它在中新世中晚期活跃，并在上新世早期当覆盖层在 200 至 400 m 之间时停止活动。目前边界的埋藏深度约为。 700 m，井位非活动成岩前缘温度为24℃。考虑到这些温度和深度限制，我们假设即使这是相对低温的蛋白石 A 到蛋白石 C/T 转变的例子，地温梯度也会暂时升高约 10%。在中新世中晚期，需要 60°C km(-1) 来启动和阻止边界的上断迁移。气候恶化和冷深水环流的出现等因素可能只是在阻止边界向上迁移方面发挥了重要作用。
Rock samples-collected from a recent deep-water exploration well drilled in the Faeroe-Shetland Channel, northwest of the UK-confirm that a distinctive high-amplitude seismic reflector that crosscuts the Upper Palaeogene and Neogene succession and covers an area of 10 000 km(2) is an example of a fossilized Opal A to Opal C/T (Cristobalite/Tridymite) transition. Analysis of these rock fragments tied to an extensive two-dimensional and three-dimensional seismic database constrains the time at which the boundary was fossilized and in addition reveals the unusual geometrical characteristics of a relict bottom-simulating reflector.The diagenetic transformation of biogenic silica (Opal A) to Opal C/T is predominantly temperature-controlled and requires sediments that contain biogenic silica. The reflector (termed as Horizon E) probably initially represented a biosiliceous ooze or a siltstone that contained a component of biogenic silica that underwent transformation as the diagenetic front migrated upsection during burial. The parallelism it shows with a shallower early Pliocene reflector and its apparent upsection migration during a compressional episode in the basin indicate that it was active during the middle and late Miocene and ceased activity during the early Pliocene when there was between 200 and 400 m of overburden. The present-day burial depth of the boundary is ca. 700 m and the temperature at the inactive diagenetic front at the well location is 24degreesC. Given these temperature and depth constraints, we hypothesize that even if this is an example of a relatively low-temperature Opal A to Opal C/T transformation, a temporarily elevated geothermal gradient of ca. 60degreesC km(-1) would have been required to initiate and arrest upsection migration of the boundary during the middle and late Miocene. Factors such as climatic deterioration and the onset of cold deep-water circulation are likely to only have had a contributory role in arresting the upward migration of the boundary.