Late Jurassic palaeoclimatic change from clay mineralogy and gamma-ray spectrometry of the Kimmeridge Clay, Dorset, UK

The Late Jurassic was a time of increasing aridity in NW Europe. Here, a new clay mineral dataset is presented from a 600 m thick composite core through the Kimmeridge Clay Formation, southern England. Clay mineral assemblages comprise mainly illite and kaolinite, with minor randomly interstratified illite–smectite mixed-layer clays. SEM observations indicate that clay minerals are mainly detrital, except in silty strata of late Tithonian age, which contain abundant pore-filling kaolinite aggregates. Th/K ratios determined from gamma-ray spectrometry mirror palaeoclimatically significant variations in kaolinite/illite ratios, with notable exception where diagenetic kaolinite occurs. Comparison with age-equivalent strata identifies regional cyclic variations in clay minerals at the 100–300 m scale. Small-scale (10–20 m) variations in clay minerals and Th/K also occur. It is proposed that the region experienced progressively intense humidity through the Kimmeridgian, followed by a return to more arid conditions during the early Tithonian. Following a mid-Tithonian peak in aridity (the ‘Hudlestoni Event'), more humid climatic conditions returned prior to the development of latest Tithonian intense aridity. Late Jurassic climate was apparently subject to synchronously cyclic changes across a broad area of the Laurasian continent. Such changes could not have resulted from mountain building or continental rotations.

晚侏罗世时期，欧洲西北部的干旱程度持续加剧。本研究基于英格兰南部金默里奇黏土组（Kimmeridge Clay Formation）一套厚度达600米的复合岩心，构建了全新的黏土矿物数据集。该数据集的黏土矿物组合以伊利石和高岭石为主，辅以少量随机无序层间伊利石-蒙脱石混层黏土。扫描电镜（SEM）观测结果显示，绝大多数黏土矿物为碎屑成因，仅在提通阶晚期的粉砂质地层中，可见大量充填于孔隙的高岭石集合体。通过伽马能谱法测定的钍钾（Th/K）比值，可反映高岭石/伊利石比值所记录的古气候显著变化，但成岩成因高岭石产出的层位为显著例外。将该数据集与同期地层对比后，可识别出100~300米尺度下黏土矿物的区域周期性变化，同时还存在10~20米尺度的黏土矿物与钍钾比值小规模波动。研究认为，金默里奇阶（Kimmeridgian）时期该区域湿度逐渐升高，随后在提通阶早期回归至更为干旱的环境；在提通阶中期干旱峰值（即“赫德尔斯通事件（Hudlestoni Event）”）之后，湿润气候再度回归，直至提通阶晚期再度出现极端干旱。晚侏罗世时期，劳亚大陆（Laurasian continent）广阔区域的气候均呈现同步周期性变化，此类变化无法由造山运动或大陆旋转所解释。