Provenance Discussion of Loess-Like Silty Clay
The occurrence of suspended rivers in the lower Yellow River is mainly attributed to the sediment accumulation, a result of the significant load of silt and sand transported by the Yellow River. While there is ongoing academic discussion about the precise origins and distribution of the mentioned sediments, there is a widespread agreement that the Loess Plateau plays a substantial role in contributing to the sediment load of the Yellow River. Rivers play a pivotal role in the transportation process that contributes to the formation of silty clay resembling loess. Consequently, it is reasonable to infer that loess serves as one of the primary origins of loess-like silty clay.
Loess is a distinctive soil variety predominantly consisting of silt-sized particles with diameters ranging from 0.05 mm to 0.005 mm. Loess is characterized by a relatively high silt particle content, with a lower content of clay particles. A significant proportion of the sediment transported by the Yellow River in its basin comprises clay-sized particles derived from loess. These fine particles are subsequently carried downstream and deposited in low-lying areas or stagnant water bodies, where they accumulate to form strata of silty clay. The coarse mineral composition of loess comprises quartz, feldspar, calcite, and mica, with the predominant clay minerals being illite, kaolinite, and montmorillonite. The loess contains a relatively high calcium content, which is mainly derived from the calcite and other calcium-bearing minerals in the parent material. These minerals release calcium ions as a result of weathering.
In particular, there is a uniformity in the mineral composition of loess and silty clay; however, loess is primarily comprised of silt particles and contains fewer clay particles. However, the widespread occurrence of loess-like silty clay layers in many urban areas along the middle and lower sections of the Yellow River cannot be exclusively ascribed to the deposition of clay particles derived from loess and transported by the river. Consequently, the development of silty clay with loess-like characteristics is not solely attributed to the loess transported by the Yellow River, but also has a direct correlation with the geological sediments of the Loess Plateau.
During experimental investigations, it was observed that loess-like silty clay, which appears yellow-brown when in situ and dry, turns reddish upon oxidation in a moist environment. Analysis of the synchrotron radiation X-ray diffraction (XRD) spectrum indicated the presence of iron oxides with low crystallinity in loess-like silty clay, despite the presence of a weak diffraction peak at 0.242 nm. Owing to its slightly alkaline pH, the iron primarily exists in an insoluble inorganic state, leading to limited reactive iron and influencing the mobility of iron in the soil. Under arid conditions, the reactivity of iron oxides with water is limited, resulting in the soil retaining its yellow color in a relatively constant state. However, when the soil becomes moist and undergoes oxidation, the Fe2+ in the iron minerals undergoes oxidation to Fe3+, resulting in the formation of new crystalline structures that modify the soil's light absorption and reflection properties. This alteration causes the moist soil to exhibit a reddish-brown appearance.
The loess strata are known for containing the most comprehensive and complete geological information dating back 2.5 million years. Further investigation into the Quaternary of the Loess Plateau is required to examine the origins of iron oxides and clay particles. Taking the Luochuan loess profile as a case study, the Luochuan area commenced the accumulation of approximately 20 m of thick Hipparion Red Clay (Tertiary Red Beds) by the conclusion of the Pliocene. Studies have indicated that this clay contains high proportions of clay particles (37%-55%), as well as substantial quantities of montmorillonite and illite, which are expansive clay minerals exhibiting robust physicochemical activity. A 135-m-thick layer of loess is deposited on top of the red clay, consisting of alternating loess layers, paleosols, and weathered loess layers. The paleosols display ferruginization, frequently manifesting as reddish-brown or brownish-red, and containing microstructures of optically oriented clays that have been documented. This indicates that the paleosols found in the loess and the Hipparion Red Clay are probable substantial contributors of iron oxides and clay particles.
Sun et al.30 conducted a study on paleosols intercalated within loess layers. They gathered multiple paleosol samples from the Liujiapo loess profile near Xi'an for analysis of granulometry and mineralogical composition. The clay particle content in the previously mentioned paleosols was found to be over 60%, as indicated in Table 7. Liu et al.31 conducted an analysis of the mineralogical characteristics of different paleosol layers, and the findings are detailed in Table 8.
Table 7
Particle size content of paleosols
Stratigraphic Position | Soil Type | Particle Size Content (%) |
>0.05 mm | 0.05 − 0.01 mm | 0.01 − 0.005 mm | <0.005 mm | <0.002 mm |
Holocene | Paleosol | 0.5 | 25.0 | 11.1 | 63.4 | 53.2 |
Upper Lishi Loess | Paleosol | 0.7 | 24.4 | 9.8 | 65.2 | 55.8 |
Lower Lishi Loess | Paleosol | 0.6 | 23.6 | 11.2 | 64.6 | 53.8 |
Wucheng Loess | Paleosol | 0.3 | 21.1 | 11.1 | 67.7 | 55.2 |
Lantian Formation | Red Clay | 0.5 | 15.0 | 8.0 | 76.5 | 63.0 |
Table 8
Overview of mineralogical components and characteristics in loess paleosols
Loess Paleosol Layer | Mineral Composition |
Holocene | Illite, Montmorillonite, Kaolinite, Quartz, Calcite, Organic Matter |
Upper Pleistocene | Illite, Montmorillonite, Kaolinite, Glauconite, Calcite, Quartz |
Middle Pleistocene | Illite, Montmorillonite, Kaolinite, Montmorillonite, Quartz, Fe2O3 |
The analysis presented in Table 8 demonstrates that the main mineral constituents of the paleosols found in the loess are largely similar to the silty clay minerals. The unique coloration of paleosols found in loess deposits, which is defined by a high colloidal particle content, closely resembles that of silty clays. The substantial thickness and similar mineral composition, in conjunction with the aforementioned attributes, strongly indicate that paleosols are a significant source material for the formation of silty clay.
Hipparion Red Clay, is characterized by its striking purple-red, brick-red, and yellow-brown coloration. It is extensively distributed in the middle and upper reaches of the Yellow River region. The red color of this clay is attributed to its iron oxide or hydroxide content. The differences in red and yellow shades are indicative of the diverse climatic conditions during deposition and the varying proportions of Fe2+ to Fe3+ ions, which contribute to the soil's distinct colors. Following the diagenetic process of the Hipparion Red Clay, natural processes such as weathering, unloading, and shrinkage played a significant role in the formation of various jointing and fracture patterns, including structural and weathering joints. The previously mentioned changes have weakened the clay's ability to resist erosive and deflationary forces. Moreover, the soil's high salinity and dispersivity increase its susceptibility to generating substantial solid runoff when exposed to rainwater. The mentioned runoff materials are carried by the Yellow River, offering a substantial source of material for the downstream loess silty clay layers.
Li et al.32 conducted an analysis of the particle size and mineral composition of Hipparion Red Clay. Their findings revealed a predominance of clay particles, with the clay-size fraction (< 0.005 mm) typically ranging from 38–54%, as presented in Table 9.
Table 9
Granulometric composition (%) of Hipparion Red Clay.
Particle Size Range | > 0.075mm | 0.075 ~ 0.005mm | < 0.005mm | < 0.002mm |
Content % | 0.08 ~ 3.41 | 45.02 ~ 62.71 | 37.12 ~ 54.40 | 30.24 ~ 48.04 |
Average % | 1.54 | 55.2 | 43.3 | 37.1 |
The soil is primarily composed of minerals such as feldspar, quartz, and calcite in terms of its mineral content. The predominant clay minerals are primarily illite, with the presence of kaolinite and montmorillonite as well 32. This composition is fundamentally in line with the mineral composition of loess-like silty clay. Given the erodibility of Hipparion Red Clay, its distinctive red hue, high concentration of clay-sized particles, and its mineral and chemical composition resembling that of loess-like silty clay, Hipparion Red Clay serves as a significant source material for silty clays.
In summary, the formation of loess-like silty clay is influenced by a multitude of factors and encompasses a complex process that incorporates contributions from various sources, including the loess from the Loess Plateau, paleosol layers, and Hipparion Red Clay.
The Impact of Climate on Mineral Characteristics
Loess-like silty clays exhibit differences in both visual characteristics and mineral composition compared to those present in the Yangtze River and Pearl River basins. The differences mentioned above stem from diverse material sources and are also influenced by regional climatic conditions, especially fluctuations in temperature and humidity. The factors mentioned above play a crucial role in determining the formation and transformation of minerals. The Yellow River Basin, Yangtze River Basin, and Pearl River Basin in China each exhibit unique climatic characteristics. The Yellow River Basin is characterized by a temperate continental monsoon climate, featuring cold winters, hot summers, and dry conditions with minimal rainfall. In contrast, the Yangtze River Basin experiences a subtropical monsoon climate, with mild and moist weather, ample precipitation, and different seasonal variations. The Pearl River Basin displays a subtropical to tropical monsoon climate, characterized by warm and humid conditions and abundant rainfall. Distinct climatic regions result in different weathering patterns and soil compositions, contributing to geographical differences in mineral compositions and formations, as illustrated in Table 10.
Table 10
Characteristics of silty clay in different climatic regions
Characteristic | Loess-like Silty Clay | Grey Silty Clay | Red Silty Clay |
1. Exhibits a higher sand content, with an appearance that ranges from yellow-brown to yellowish-brown. 2. Rich in quartz and feldspar, contains a significant amount of montmorillonite, with considerable calcite and minor amounts of amphibole; kaolinite is less common. | 1. Typically has a color spectrum ranging from grey to greyish-brown. 2.Abundant in illite and kaolinite, with montmorillonite being relatively scarce. | 1. Coloration generally spans from red to reddish-brown. 2. Enriched in kaolinite, with minimal montmorillonite content, and a certain presence of gibbsite and goethite. |
The sediment of the Yellow River originates mainly from the Loess Plateau and contains a high concentration of minerals, including quartz, feldspar, and mica. The arid and cold climate in this region restricts the scope of chemical weathering, while the alternating dry and wet climatic conditions promote the development and conservation of minerals such as calcite, montmorillonite, and glauconite, but are not favorable for the formation of kaolinite. The high calcium content of loess facilitates the development of calcite in alkaline or slightly alkaline conditions. Under soil conditions ranging from neutral to strongly alkaline, with minimal leaching, the potassium ions located within the layers of the illite lattice may undergo leaching and subsequently be substituted by magnesium ions. This process results in the conversion of illite into montmorillonite.
Conversely, the climatic conditions in the Yangtze River basin, characterized by elevated temperatures and increased precipitation, as well as the weakly acidic or acidic composition of the soils, enhance chemical weathering. This environment is conducive to the formation of kaolinite in warm and moist climates with sustained and vigorous hydrolysis. This process leads to a decrease in the montmorillonite content and an increase in the kaolinite content, as well as the leaching of alkaline earth metals such as Ca, resulting in reduced presence of calcite in the clay.
Illite forms under less intense leaching conditions in continental, neutral, or slightly alkaline aqueous environments. As the climate becomes wetter and hotter, there is an intensification of chemical weathering and ion exchange, resulting in an increase in Al3+ ions within the illite. Furthermore, the progression of chemical weathering leads to the strengthening of hydrolysis, which further transforms illite into kaolinite.
In the Pearl River basin, specifically in the Xijiang area, the warm and rainy climate has resulted in heightened chemical weathering processes. In this acidic soil environment, montmorillonite undergoes dissolution, leading to the abundant formation of kaolinite. This process is a consequence of extensive leaching in a humid climate under acidic conditions and can also involve the transformation of montmorillonite into kaolinite. Mica-type minerals may undergo sequential transitions from illite to montmorillonite and finally to kaolinite, as a result of varying degrees of weathering and leaching. This is associated with the appearance of iron and aluminum hydroxides, such as gibbsite and goethite. Alkali and alkaline earth metals, such as calcium, undergo significant leaching, resulting in the absence of calcite in the sediments of the Pearl River.