雷州半岛九斗洋干玛珥湖火山机构与第四系空间展布——基于高密度电阻率法勘测和钻孔研究

为查明雷琼火山群中九斗洋干玛珥湖的第四纪地层空间展布，以及埋藏古火山形态，为后续研究提供地质背景资料，采用高效、便捷的高密度电阻率法对九斗洋干玛珥湖湖盆区进行勘测，并结合钻孔岩芯的研究进行验证。得出的主要结论为：1）盆地深部地层最高电阻率可达到300 Ω?m以上，盆地中央基岩以下存在高阻地质体，推测为岩浆通道位置。岩浆经过通道溢出后横向展布，覆于沉积地层上，随后在火山口内形成湖泊，堆积第四纪湖相沉积，其火山机构与玛珥湖型火山一致；2）九斗洋干玛珥湖第四纪松散沉积地层的电阻率通常＜60 Ω?m，泥炭层电阻率最低；3）第四纪沉积总厚度为25~50 m，盆地松散沉积的岩浆岩基底总体平整，但尚有波状起伏。研究结果表明，高密度电阻率法结合钻孔验证是研究火山机构形态及火山口湖第四纪沉积空间展布的有效方法。

Electrical Resistivity Tomography of Volcanic Crater Structure and Quaternary Lacustrine Sediments of Jiudouyang Maar Paleolake,Leizhou Peninsula

Maars, formed by phreatomagmatic eruptions, are major concern of volcanology and paleoenvironment research. There are many Quaternary maars located in Leiqiong Volcanic Field, south China, which provide excellent archives for paleoenvironmental reconstructions. To clarify the spatial distribution of Quaternary sediments and volcanic structure of Jiudouyang maar paleolake, Leizhou Peninsula, we conducted electrical resistivity tomography (ERT) imaging surveys in the crater, accompanying with core drilling that was used to verify the resistivity results. Two profiles along the major and minor axis of the crater respectively, comprised of five segments with lengths varying from 900 to 1 200 m, were collected by means of Wenner array with 10 m electrode spacing. They were inverted by using Res2dinv software, and the result allowed imaging the uppermost 150 m strata. The basalt bedrock with higher resistivity (more than 80 Ω?m) is distinct from the upper Quaternary sediments and the interface is approximately flat, revealing that the crater basin has a bowl-like shape rather than a funnel shape. Moreover, magma conduit is characterized by abnormal high resistivity (up to 300 Ω?m), which is surrounded by the underlying Quaternary sediments (e.g., the Zhanjiang Formation or the Beihai Formation). Volcanic structure as above is corresponding with universal model of soft-substrate maars but not hard-substrate maars. In contrast, the upper Quaternary lacustrine sediments are characterized by low resistivity (less than 60 Ω?m) areas where the thickness varies between 25 and 50 m. Based on variations in lithology, the sediments sequence can be divided into five units from top to bottom: Unit 1 (0-6.18 m), reddish yellow clay and fine sand with occasionally irregular gravel (1.96-3.07 m); Unit 2 (6.18-11.70 m), green-greyish clay; Unit 3 (11.70-13.37 m), organic-rich clay with wood fragments and organic detritus; Unit 4 (13.37-38.51 m), green-greyish diatomaceous clay to diatomite, with some varve-like laminae (29.23-31.86 m); Unit 5 (38.51-47.60 m), diatomite laminae with a layer of tephra and scoria (38.51-39.55 m); Unit 6 (47.60-49.49 m), tephra and pyroclastic breccia, overlying the unweathered basalt. Despite of some slight offsets in depth resulted from the limit of spatial resolution of ERT, it shows agreement between the coring stratigraphy and electrical resistivity profiles. For instance, organic-rich clay layer (Unit 3) is corresponding to the lowest values of resistivity result (<5 Ω?m), because of its lower density and higher water content. Overall, we have successfully imaged the volcanic structures and sediment spatial distributions, and testified that electrical resistivity tomography would be an ideal method to provide fundamental information for other researches.