Application of till geochemical and indicator mineral data to the interpretation of the thick till sequence at Muhos, northern Finland

Recent drilling at Muhos, northern Finland has revealed an exceptionally thick Quaternary sediment cover, overlying the unmetamorphosed Neoproterozoic Muhos Siltstone Formation in the Oulujoki River Valley. Here we report preliminary petrographical, geochemical and heavy mineral results from four till units from the Muhos drill-core. The drilling site is located at 27 m asl, c. 1 km south of Oulujoki River and 30 km SE from the city of Oulu and the present shore of the Bothnian Bay of the Baltic Sea. The drilling at this site was performed with a GM 200 GTT drilling platform using 1 -m long sample tubes (HDPV, diameter 5 cm) inside the stainless steel cover tubes. The coring penetrated down to the siltstone bedrock 54 m below the ground surface and the sediment core recovery was over 75%. The sedimentary sequence from the ground surface down to the local siltstone bedrock consists of four till units interbedded with sand/silt/clay beds. The upper two till beds, Mu-1 and Mu-2, are interpreted as being deposited during the Weichselian Glaciation between 115 000–10 000 years ago, the third till bed Mu-3 during the Saalian and the lowest till bed, Mu-4, during the Saalian or pre-Saalian glaciations, i.e. till beds Mu-3 and Mu-4 prior to c. 130 000 years ago. Petrography of the coarse fraction, geochemistry of the fine fraction and heavy minerals of the sand fraction were studied from all four till units and the Muhos siltstone samples. Coarse and fine fractions were separated by dry-sieving. The fine fraction was analysed by XRF and AAS. The mineralogical composition of the heavy fraction was studied by FESEM+EDS and SEM+EDS. The lowest till bed Mu-4 can be divided into three subunits. On the basis of clast content, geochemistry of till fines the ice movement direction was from the NW or NNW during the deposition of the lowermost brown and red till units. The composition of the upper grey unit of till bed Mu-4 shows that the ice flow direction was from the WNW. Petrographical composition of till beds Mu-3 and Mu-2 indicates that the ice flow again was from the NW whereas the petrographical composition of the uppermost till bed Mu-1 suggests that the ice flow during this deposition phase was from the WNW. Younger till beds studied have in part also some reworked and re-deposited material from the older till beds.

芬兰北部穆霍斯地区的最新钻探揭示了一套厚度异常可观的第四纪沉积盖层（Quaternary sediment cover），该盖层覆于奥卢约基河谷（Oulujoki River Valley）内未变质的新元古代（Neoproterozoic）穆霍斯粉砂岩组（Muhos Siltstone Formation）之上。本文报道了取自穆霍斯钻孔4个冰碛单元（till units）的初步岩石学、地球化学及重矿物分析结果。
该钻探点位海拔27米，距奥卢约基河以南约1公里，距奥卢市及波罗的海波的尼亚湾（Bothnian Bay）现今岸线东南方向30公里。本次钻探采用GM 200 GTT钻探平台（GM 200 GTT drilling platform），在不锈钢保护套管内使用长1米、直径5厘米的取样管（HDPV）完成钻进。
本次取心钻进至地表以下54米处的粉砂岩基岩，岩心采取率超过75%。从地表至本地粉砂岩基岩的沉积序列由4个冰碛单元与砂/粉砂/黏土层互层构成。
上部2个冰碛层Mu-1与Mu-2被认定形成于11.5万~1万年前的魏克塞尔冰期（Weichselian Glaciation）；第3个冰碛层Mu-3形成于萨勒冰期（Saalian）；最下部的冰碛层Mu-4则形成于萨勒冰期或萨勒冰期前，即Mu-3与Mu-4形成于约13万年前之前。
针对全部4个冰碛单元及穆霍斯粉砂岩样品，研究了其粗粒组分的岩石学特征、细粒组分的地球化学特征，以及砂级组分的重矿物组成。粗、细粒组分通过干筛法（dry-sieving）完成分离；细粒组分采用XRF (X-ray Fluorescence)与AAS (Atomic Absorption Spectroscopy)进行分析；重粒组分的矿物组成通过FESEM+EDS (Field Emission Scanning Electron Microscopy + Energy Dispersive X-ray Spectroscopy)及SEM+EDS (Scanning Electron Microscopy + Energy Dispersive X-ray Spectroscopy)完成表征。
最下部的冰碛层Mu-4可进一步划分为3个亚单元。基于砾石含量及冰碛细粒组分的地球化学特征，最下部的棕色与红色冰碛单元沉积时期的冰运移方向为西北（NW）或北北西（NNW）；而Mu-4上部的灰色冰碛单元的组成特征显示，其沉积时期的冰运移方向为西西北（WNW）。
冰碛层Mu-3与Mu-2的岩石学组成表明，其沉积时期的冰运移方向同样为西北（NW）；而最上部的冰碛层Mu-1的岩石学组成则显示，该沉积阶段的冰运移方向为西西北（WNW）。本次研究的较年轻冰碛层中，部分还含有来自较老冰碛层的再改造再沉积物质。