关键词: 高密度电阻率法, 九斗洋干玛珥湖, 火山机构, 第四纪沉积, 雷州半岛

Abstract: 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.

Key words: electrical resistivity, Jiudouyang maar paleolake, volcanic crater, Quaternary sediment, Leizhou Peninsula