Stratigraphy and age of Karoo basalts of Lesotho and implications for correlations within the Karoo igneous province. Bushman's Pass

The Lesotho remnant contains the type succession for Karoo low-Ti basalts of central southern Africa. The 40Ar/39Ar dating indicates that the sequence was emplaced within a very short period at about 180 Ma and consists of a monotonous pile of compound basalt lava flows which lacks significant palaeosols and persistent sedimentary intercalations. We have used geochemistry to establish a stratigraphic subdivision of the lava pile. Thin units of basalt flows, the Moshesh's Ford, Golden Gate, Sani, Roma, Letele, and Wonderkop units, with diverse geochemical character and restricted geographical distribution, are present at the base of the succession. These are overlain by extensive units of compositionally more uniform basalt, the Mafika Lisiu, Maloti, Senqu and Mothae units, which build the bulk of the sequence. A single palaeomagnetic polarity reversal occurs within the lower third of the basalt succession and is consistently located within the Mafika Lisiu unit. This and the persistent and relatively uniform thickness of the stratigraphic units suggest that the pile was constructed in a uniform manner by eruption of basalt onto a generally planar surface from a widespread plexus of dykes. The stratigraphic sequence in Lesotho closely resembles that in the thinner sequence of low-Ti basalts of the Springbok Flats remnant, some 400 km to the north. A thin unit of high-Ti basalt within the upper part of the Springbok Flats sequence can be correlated with the thick high-Ti basalt suite along the rift-related Lebombo structure on the eastern margin of the Karoo province. This is the first established correlation between these two important outcrops of Karoo volcanic rocks and demonstrates that the low-Ti basalts of Lesotho and the cratonic interior are the approximate time equivalents of the lower part of the Lebombo sequence. This conclusion has important implications for models for the origin of the Karoo flood basalt province.Location of this section is described in Marsh et al. (1997) AGU Geophysical Monograph, 100, 247-272. Title of data set: Bushman’s pass.莱索托残余露头包含了非洲中南部卡鲁（Karoo）低钛玄武岩的典型层序。经⁴⁰Ar/³⁹Ar测年结果显示，该层序形成于约1.8亿年前的极短时间范围内，由一套单调的复合玄武岩熔岩流堆组成，几乎未发育显著的古土壤层与稳定的沉积夹层。本研究通过地球化学方法对该熔岩堆进行了地层划分。层序底部发育多套地球化学特征各异、分布范围受限的薄层玄武岩流地层单元，包括莫舍什福特（Moshesh's Ford）、金门（Golden Gate）、萨尼（Sani）、罗马（Roma）、莱特莱（Letele）与万德科普（Wonderkop）单元。上述薄层单元之上覆盖着分布广泛、地球化学组成更为均一的玄武岩地层单元，包括马菲卡利西乌（Mafika Lisiu）、马洛蒂（Maloti）、森库（Senqu）与莫泰（Mothae）单元，该套地层构成了整个层序的主体。在该玄武岩层序的下三分之一处存在一次单一的古地磁极性倒转，且该倒转稳定赋存于马菲卡利西乌单元内部。这一特征与地层单元稳定且相对均一的厚度共同表明，该熔岩堆是通过玄武岩浆从广泛分布的岩墙群喷发至近乎平坦的地表，以均一的方式堆积形成的。莱索托的地层序列与向北约400公里处的斯普林博克平原（Springbok Flats）残余露头的低钛玄武岩层序（厚度较薄）高度相似。斯普林博克平原层序上部的一套薄层高钛玄武岩单元，可与卡鲁省东缘与裂谷相关的莱邦博（Lebombo）构造带沿线的厚层高钛玄武岩组合进行对比。这是首次在这两处重要的卡鲁火山岩露头之间建立可对比关系，同时证明莱索托与克拉通内部的低钛玄武岩，其形成时间大致等同于莱邦博层序的下部地层。该结论对卡鲁溢流玄武岩省的成因模型具有重要意义。该剖面的位置详见Marsh等人（1997年）发表于《AGU地球物理专刊》（AGU Geophysical Monograph）第100卷，第247-272页的研究成果。数据集标题：布什曼关口（Bushman’s Pass）。