Origin of continental red beds: warming from above or heating from below?

The formation of continental red beds is generally considered to be related to an arid climate. Heating experiments (performed by L.J. and G.C.) using dried black mud sediment also demonstrate that the reddening may be caused by the transformation of goethite to haematite that begins at approximately 150℃ under anhydrous conditions, and increasing the temperature to 450℃ is positively correlated with the red colour and peak value of haematite. If this process applies to continental red beds, it implies a thermal origin of red beds as a result of high diagenetic temperatures rather than as the cause of their deposition under an arid climate. Namely, subsiding red-bed basins are heated from below rather than warmed from above. Here, we further strengthen this idea by new evidence from borehole cores drilled from red beds in SE China, showing a clear geological section from the surface soil to red beds to bottom granite. The data reveal that the continental red beds formed at least at a temperature within 150-400℃, and the underlying granite usually formed at temperatures greater than 600℃. Our results imply a possible relationship between continental red bed events and Earth’s thermal cycles.

大陆红层的形成通常被认为与干旱气候密切相关。由L.J.与G.C.开展的干燥黑泥沉积物加热实验同样证实，红化现象可能源于针铁矿（goethite）向赤铁矿（haematite）的相变——该相变在无水环境下约150℃时启动，且升温至450℃的过程与红色色度及赤铁矿峰值强度呈正相关。若该相变过程适用于大陆红层，则意味着红层的成因源于成岩温度升高所带来的热效应，而非干旱气候下沉积作用的结果。换言之，沉降中的红层盆地是由下方受热，而非上方升温。本研究基于中国东南部红层钻孔岩芯的新证据，进一步佐证了这一观点，该岩芯完整呈现了从表层土壤、红层到底部花岗岩的清晰地质剖面。数据显示，大陆红层的形成温度至少处于150~400℃区间，而其下伏花岗岩的形成温度通常高于600℃。本研究结果表明，大陆红层事件与地球热循环之间可能存在关联。