KUANG Hongwei,born in 1969,is a professor and doctoral supervisor at the Institute of Geology,CAGS. She is mainly engaged in sedimentology,Meso-Neoproterozoic stratigraphy and event sedimentology,as well as energy geology. E-mail: kuanghw@126.com.
The Earth’s evolutionary history has witnessed six major glaciations or glacial events,among which the earliest is believed to be the Pongola glaciation in South Africa,occurring approximately 2.9 billion years ago. A number of papers have been published on the last five major glaciations,yet understanding of the Pongola glacier remains limited. This raises key questions: did glaciation exist in the Archean,and how to identify it?By analyzing the characteristics of diamictites in the Mozaan Group(2.9 Ga)of the Pongola Supergroup in the Kaapvaal Craton,South Africa,this study summarizes the diagnostic markers of glacial deposits,and discusses whether the diamictites of the Archean Pongola Supergroup are glacial origin. Meanwhile,it aims to draw more attention from peers to the establishment and development of glacial sedimentology:a subdiscipline of event sedimentology in China. The Mesoarchean Pongola Supergroup is primarily distributed in the southeastern part of the Kaapvaal Craton and can be further divided into two groups: the Nsuze Group and the Mozaan Group. Petrological,sedimentological,and major/trace element geochemical analyses of the Mozaan Group diamictites support the interpretation of glacial origin. The Mozaan Group diamictites generally exhibit the following characteristics: (1)relatively thin thickness(<50 m);(2)massive mixed accumulation;(3)poor sorting and roundness of gravels;(4)gravels of varying particle sizes and compositions “floating” in a fine-grained matrix;(5)gravels with multiple sets of striations and polished surfaces;(6)quartz grains show elongation or deformation with silt-sized grains distributed around sandy grains in thin sections(referred to as a “nebular structure”);(7)dropstones developed in varves. Geochemical studies of elements reveal the following: the data points of Mozaan Group diamictites show a restricted distribution in the SiO2/Al2O3 diagram,which is significantly different from the typical linear negative correlation observed in modern and ancient sediments that have undergone significant weathering and sorting;The distribution range of TiO2/Al2O3 is also narrow,and the major element compositions are similar;The matrix of the diamictites is rich in total iron(Fe2O3(T))and shows a marked depletion of calcium(Ca)and sodium(Na). The mathematical inversion of measured δ18O and δ17O values in the diamictite and the underlying shale yields δ18O values of weathering water consistent with the isotopic characteristics of modern polar meteoric water,indicating cold climatic conditions. In addition,indicators such as the Chemical Index of Alteration(CIA)suggest that the Mozaan Group diamictites have barely experienced diagenetic alteration or chemical weathering,thus well representing the characteristics of their provenance. Therefore,the following glacial depositional model is proposed based on the above findings: during periods of low sea level,glaciers advanced along valleys toward the continental shelf. The till(glacial sediment)consisted of not only basal till,lateral till,and rock flour produced by glacial abrasion of the bedrock,but also rounded quartz grains mixed from shelf sands and fine-grained weathered materials derived from ferruginous mud. Some diamictites originated from the direct deposition of glacial sediments in periglacial zones and subsequent modification,characterized by a sharp contact between the diamictites and the underlying bedrock,with deformed layers present beneath parts of the diamictites. Another part of the thick diamictite layers,indicated by dropstones in the upper part,was transported by ice rafts to ice-distal glaciomarine environments.
| 科 Family | 属数 Number of genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) | 属 Genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) |
|---|---|---|---|---|---|---|
| 鹅膏菌科Amanitaceae | 2 | 11 | 5.26 | 鹅膏菌属 Amanita | 10 | 4.78 |
| 小菇科 Mycenaceae | 2 | 12 | 5.74 | 丝盖伞属 Inocybe | 5 | 2.39 |
| 多孔菌科 Polyporaceae | 8 | 14 | 6.70 | 蜡蘑属 Laccaria | 5 | 2.39 |
| 红菇科 Russulaceae | 3 | 23 | 11.00 | 小皮伞属 Marasmius | 6 | 2.87 |
| 小菇属 Mycena | 11 | 5.26 | ||||
| 光柄菇属 Pluteus | 5 | 2.39 | ||||
| 红菇属 Russula | 17 | 8.13 | ||||
| 栓菌属 Trametes | 5 | 2.39 |