Clay mineralogy of the Cenozoic Pagodroma Group, East Antarctica

The Cenozoic Pagodroma Group in the northern Prince Charles Mountains, East Antarctica, is a glaciomarine succession of fjordal character, comprising four uplifted formations of different ages. The composition of the <2 µm fraction of sediments of the Pagodroma Group was analysed in order to help identify source areas, past weathering conditions and glacial regimes. Both clay and non-clay minerals have been quantified. The assemblage of the upper Oligocene to lower Miocene Mount Johnston Formation is characterised by the dominance of illite and intermediate concentrations of chlorite. Similar to that assemblage is the clay mineral suite of the middle Miocene Fisher Bench Formation, where illite and chlorite together account for 95% of the clay minerals. The middle to upper Miocene Battye Glacier Formation is the only formation with significant and persistent smectite concentrations, although illite is still dominant. The kaolinite concentration is also high and is even higher than that of chlorite. The clay fraction of the upper Pliocene to lower Pleistocene Bardin Bluffs Formation is characterised by maximum kaolinite concentrations and relatively low illite and chlorite concentrations. The bulk of the clay fraction in each formation can be explained by the physical weathering and erosion of a nearby source under glacial conditions. In the case of Mount Johnston Formation and Fisher Bench Formation this source may be situated in the metavolcanic and gneissic rocks of Fisher Massif. The sediments of the Bardin Bluffs Formation indicate a local source within the Amery Oasis, where Proterozoic granitoid rocks and gneisses, and Permo-Triassic fluvial rocks of the Amery Group are exposed. These results suggest a strong local imprint on the glacial sediments as northwards flowing ice eroded the bedrock in these areas. The origin of the clay fraction of the Battye Glacier Formation is a matter of debate. The smectite and kaolinite content most easily can be explained by erosion of sources largely hidden beneath the ice upstream. Less likely, these clay minerals reflect climatic conditions that were much warmer and wetter than today, facilitating chemical weathering.东南极查尔斯王子山脉北部的新生代帕戈德罗玛组（Pagodroma Group）是一套具有峡湾型特征的冰海沉积序列，由四套不同时代的抬升地层构成。为明确物源区、古风化环境与冰川作用模式，研究人员对帕戈德罗玛组沉积物的<2 µm粒级组分开展了成分分析，并对其中的黏土矿物与非黏土矿物均进行了定量表征。 渐新统上部至中新统下部的约翰斯顿山组（Mount Johnston Formation）矿物组合以伊利石为主，且伴有中等含量的绿泥石。中中新世费希尔台地组（Fisher Bench Formation）的黏土矿物组合与其类似，该地层中伊利石与绿泥石合计占黏土矿物总量的95%。中中新统至上中新统的巴蒂冰川组（Battye Glacier Formation）是唯一一套含有显著且稳定蒙脱石含量的地层，尽管伊利石仍为优势矿物；该地层中高岭石含量同样较高，甚至超过绿泥石。上新统上部至更新统下部的巴丁断崖组（Bardin Bluffs Formation）的黏土组分以高岭石含量达峰值为特征，且伊利石与绿泥石含量相对较低。 各套地层的黏土组分主体均可通过冰川作用下近源区岩石的物理风化与侵蚀作用得到解释。就约翰斯顿山组与费希尔台地组而言，其物源可能来自费希尔山（Fisher Massif）的变火山岩与片麻岩地层。巴丁断崖组的沉积物则指示其物源来自阿姆里绿洲（Amery Oasis）的局部区域，该区域出露有元古代花岗岩类岩石与片麻岩，以及阿姆里群（Amery Group）的二叠-三叠纪河流相岩石。上述结果表明，当北向流动的冰川侵蚀上述区域的基岩时，冰川沉积物受到了强烈的局地物源印记影响。 巴蒂冰川组的黏土组分成因仍存在争议。其蒙脱石与高岭石含量最易通过上游冰盖下埋藏的源区岩石侵蚀作用得到解释。另一种可能性较低的解释是，这些黏土矿物反映了比现今更为暖湿的古气候条件，从而促进了化学风化作用。