湖南茶陵盆地上白垩统戴家坪组红层的古地磁研究

Paleomagnetism of the Upper Cretaceous Daijiaping Formation red beds in the Chaling Basin,Hunan Province,China

由于红层能够携带稳定的天然剩磁,因此在古地磁研究中被广泛采用。以往对于红层的研究大多集中于河湖相,对风成相红层的研究较少,尤其是风成沉积过程及沉积环境对剩磁记录的影响仍缺乏清晰的认识。对湘东南茶陵盆地上白垩统戴家坪组红层开展了古地磁研究,通过对比分析研究了风成相和河湖相红层携带剩磁的稳定性及其剩磁记录的可靠性。岩石磁学结果显示载磁矿物为磁铁矿、磁赤铁矿、赤铁矿。退磁结果显示,仅有约1/6的样品能够分离出特征剩磁(ChRM)。虽然两者的ChRM平均方向总体一致(风成相样品磁偏角Ds=222.7°,磁倾角Is=-43.3°,Fish统计精度参数κ=5.9,95%置信区间的半顶角角度α_(95)=20.6°,数据量n=11;河湖相样品D_(s)=204.6°,I_(s)=-47.8°,κ=2.4,α_(95)=23.1°,n=28),但两者单个样品ChRM方向的分布均较为分散。岩石薄片分析表明,相较于河湖相红层而言,风成相样品以细颗粒为主,部分粒度较粗,几乎无填隙物,易于受到物理扰动和后期化学作用影响,使得其剩磁记录的稳定性变差。对取自同一前积纹层不同部位风成相红层样品的退磁结果分析表明,当前积纹层与层系界面夹角大于20°时,其剩磁记录受到沉积过程的影响显著。对华南白垩纪红层样品古地磁结果的统计分析表明,华南白垩纪风成相红层剩磁的稳定性和可靠性总体较弱,可能是由于其结构松散且赤铁矿含量少,既不利于原生剩磁的稳定获得,也易于受到后期化学作用的改造而影响其可靠性。研究结果为风成相红层剩磁记录的稳定性和可靠性研究及其在古地磁研究中的应用提供有利指导。

Red beds are widely used in palaeomagnetic studies as they can carry stable natural remanence.Most previous studies on red beds have mainly focused on lacustrine-fluvial facies,with limited targeting the aeolian red beds.Consequently,the influence of aeolian deposition processes and sedimentary environments on remanence records remains poorly understood.In this study,we conducted a palaeomagnetic investigation of the red beds from the Upper Cretaceous Daijiaping Formation in the Chaling Basin,Hunan Province,China,to compare the stability and reliability of remanence in aeolian and lacustrine-fluvial red beds.Rock magnetic results indicate that magnetite,maghemite and hematite are the dominant magnetic remanence carriers.Specimens were subjected to stepwise thermal demagnetization,and only 1/6 of the specimens yielded characteristic remanent magnetisations(ChRMs).Although the mean directions of ChRM in both aeolian(D_(s)=222.7°,I_(s)=-43.3°,κ=5.9,α_(95)=20.6°,n=11)and lacustrine-fluvial(D s=204.6°,I_(s)=-47.8°,κ=2.4,α_(95)=23.1°,n=28)samples are statistically indistinguishable,the distribution of individual ChRM directions is scattered,as evidenced by the largeα_(95)values.Thin section observation reveals that the aeolian samples are predominantly composed of fine grains,with some coarser grains and almost no interstitial fillings compared with the lacustrine-fluvial red beds.This makes them susceptible to physical disturbance and subsequent chemical alteration,which compromises the stability of their remanence records.Analysis of palaeomagnetic data of specimens from different positions of an aeolian foreset bed reveals that the reliability of remanence is significantly influenced by aeolian depositional processes in case the angle between the foreset bed and the bedding plane is larger than 20°.Statistical analysis of palaeomagnetic data previously published from the Cretaceous red beds in South China and these in this study suggests that the remanence records in the Cretaceous aeolian red beds in South China tend to be less stable and less reliable,compared with that of the coeval lacustrine-fluvial red beds.It is probably because of their loose structure and low hematite content,which hinders the acquisition of primary remanence and makes them susceptible to diagenetic alteration.These findings provide valuable insights for studying the stability and reliability of remanence in aeolian red beds and their application in palaeomagnetic study.