Clay mineralogy of Cenozoic sediments from the Peruvian continental margin

Analysis of the clay mineralogy of 537 samples from Cenozoic sediments of the Peruvian forearc reveals distinctive geographic and stratigraphic patterns in the distribution of clay minerals. Post-Oligocene clay mineral assemblages show relatively constant illite-chlorite-kaolinite ratios with a variable expandable mineral component (smectite, vermiculite, and mixed-layer chlorite-vermiculite and/or chlorite-smectite). The distribution of Quaternary clay minerals is primarily controlled by provenance. In Quaternary assemblages from the Salaverry, Lima, and Trujillo basins, illite is more abundant than expandable minerals, and the expandable phase is primarily vermiculite, whereas in the Quaternary of the West Pisco Basin (Site 686), expandable minerals are more abundant than illite, and the expandable phase shows a substantial smectite component. This reflects low abundances of smectite in the source area between 4° and 14°S, which is a consequence of the Quaternary \"volcanic gap\" in this region. A change from smectite- to vermiculite-dominated assemblages during the Quaternary at Site 687 suggests relative uplift of the outer-shelf high at this time and isolation of the West Pisco Basin from the Lima Basin. The greater abundance of expandable phases seaward within the Quaternary is attributed to mixing of fluvial smectite-poor material with either smectite-rich eolian material or authigenic smectite, rather than differential settling. Slope sites exhibit a gradual increase in expandable minerals within the Quaternary. This may be explained in terms of a greater reworked shelf-derived component in the lower Quaternary. High-resolution studies of clay minerals at Sites 680 and 686 may provide some evidence of cyclicity. Most sites exhibit a sharp decrease in expandable minerals at or within the Quaternary or Pliocene. This is attributed to the reduction of volcanic activity associated with the progressive southward migration of the Nazca Ridge. At all sites, older sediments are characterized by smectite-rich assemblages, with smectite generally more abundant at depth. This is caused by the addition of allogenic and authigenic smectite from alteration of volcanic material, produced extensively during the Miocene and late Eocene.对秘鲁前弧 (Peruvian forearc) 新生代沉积物 (Cenozoic sediments) 的537件样品开展黏土矿物学 (clay mineralogy) 分析，结果揭示了黏土矿物分布的显著地理与地层格局。渐新世后的黏土矿物组合呈现出相对稳定的伊利石 (illite)-绿泥石 (chlorite)-高岭石 (kaolinite) 比值，同时伴随可变的可膨胀矿物组分，包括蒙脱石 (smectite)、蛭石 (vermiculite) 以及混层绿泥石-蛭石和/或绿泥石-蒙脱石矿物。第四纪黏土矿物的分布主要受物源区 (provenance) 控制。在萨瓦里盆地、利马盆地与特鲁希略盆地的第四纪矿物组合中，伊利石含量高于可膨胀矿物，且可膨胀相以蛭石为主；而西皮斯科盆地686站位 (Site 686) 的第四纪沉积物中，可膨胀矿物含量高于伊利石，可膨胀相以蒙脱石为主要组分。这一现象反映出南纬4°至14°区间物源区的蒙脱石含量较低，而这正是该区域第四纪“火山空缺带 (volcanic gap)”所致。687站位 (Site 687) 在第四纪期间出现从蒙脱石主导到蛭石主导的组合转变，表明此时外陆架隆起 (outer-shelf high) 发生相对隆升，西皮斯科盆地与利马盆地之间出现阻隔。第四纪海域内可膨胀相含量更高的现象，被归因于贫蒙脱石的河流沉积物与富蒙脱石的风成沉积物或自生 (authigenic) 蒙脱石的混合作用，而非差异沉降。斜坡站位的第四纪可膨胀矿物含量呈逐渐升高趋势，这或许可通过下更新统中更多的改造型陆架源组分加以解释。对680站位 (Site 680) 与686站位的黏土矿物开展的高分辨率研究，或可为沉积旋回性提供若干证据。多数站位在第四纪或上新世期间或其界线上出现可膨胀矿物的急剧减少，这被归因于伴随纳斯卡海岭 (Nazca Ridge) 逐步向南迁移的火山活动减弱。所有站位的较老沉积物均以富蒙脱石组合为特征，且蒙脱石含量通常随深度增加而升高；这源于中新世 (Miocene) 与始新世晚期 (late Eocene) 广泛发育的火山物质蚀变所形成的外源 (allogenic) 与自生蒙脱石的叠加作用。