Palegeography and Strata

According to geologic explorations and relevant information, , the exposure strata in the project areas include, from old to new, pre-sinian system, sinian system, Cambrian system (upper and middle), Ordovician system, Silurian System (lower), Permian system, Triassic system (upper), Jurassic System, Cretaceous system, Tertiary system, Quaternary system, and the intrusive rocks from the late Proterozoic, Caledonian and Variscan stages. The strata are undeveloped, with many lancunas.

Due to the geotectonic conditions, the spatial distribution of the strata is not balanced:

(1) Pre-sinian system: rare and distributed exclusively near Riyue Mountain;

(2) Sinian system: widespread on the edge of the catchment and the mountainous areas in the west and northeast; the major layer of the base of the basin;

(3) Lower Paleozoic sediments: mainly distributed at Daban Mountain and Laoye Mountain;

(4) Upper Paleozoic sediments: mainly distributed to the south of Laji Mountain and Riyue Mountain;

(5) Trassic system of Mesozoic-Paleozoic sediments: mainly distributed to the south of Laji Mountain and Riyue Mountain; Jurassic system and Cretaceous system: dispersed across the basin.

(6) Tertiary system: can be divided into Paleogene system and Neogene system, according to its sedimentary and lithological features. Xining-group of Paleogene system (E) is widespread in the lower part of the Xining-Minhe basin. The rocks are: red or brownish red mud rock, silitite, thin gypsum rock interbedded with mirabilite, mud rocks containing gypsum, and alternating layers of sandrock, gravel and conglomerate. The contacts between Paleogene system and Cretaceous system are either conformable or overlapping.

Guide-group of Neogene system (N) is in the upper part of the northeast edge of Xining-Minhe basin. This group can be divided into two sub-groups according to their lithological characters. The top consists of mud rocks, and the alternating layers of silitite and mud rock containing gypsum. The bottom consists of grayish brown and brownish red mud rock, silitite and the alternating layers of river rock, stone rock and rock;

it is the result of river sedimentation and the contact between it and the underlying Xining-Group is continuous and conformable.

(7) Quaternary system: widespread in project areas. Due to the sedimentary environment and geological conditions, the genetic types of this system are very complex and the lithological characters and thickness of rocks vary significantly.

 Pliocene system (Q1): mainly consists of aeolian soil that covers hills and mountains above the high valley terrace like a cloak. Its’

thickness ranges between 10cm and 30cm. It is collapsible to some degree and a little permeable.

 Holocene alluvial system (Q4al): mainly found in river bed, flood plain, low-grade terrace and large gullies. It is often a layer consisting of boulders and stone rocks, void-structured, loose/a little compact, permeable (the permeability coefficient is often between 20-80m/d), and 0.6-2.5cm thick.

 Holocene proluvium sediments (Q4pl): mainly found in gullies and the mouths of gullies of tributaries, often in the form of proluvium fan, consisting proluvium stone rock and gravel soil. Its mineral composition includes: the sediments contain granite, sandstone,

incompact mud and sand. The permeability is good. The thickness often ranges between 0.5-2.5m.

 Holocene diluvium sediments (Q4dl): mainly spread at the frontier of Grade II and Grade III terraces and talus slope. It is a layer of diluvium gravel soil. The grain size and thickness vary greatly. The composition is mainly determined by the composition of rocks on the upstream. It consists of gravel and soil. It is void-structured and permeable. The thickness differs greatly.

 Gravity deposits: primarily the deposits as a result of landslide and collapse along the two sides of gullies and slopes, containing yellow earth or a mixture of yellow earth and clay. The thickness varies greatly with the size of the deposits. The deposits are incompact and loose, and they are the major matter source of a debris flow.

(8) Intrusive rocks

The distribution of intrusive rocks is determined by geotectonic conditions. In [project] areas, the exposed intrusive rocks include the rocks from the late Proterozoic, Caledonian, Variscan stages and pre-Xingkai period (前兴凯期).

 Rocks from the late Proterozoic: gneissic granite, migmatitic granite, and diorite-like dikes and suite.

 Rocks from Caledonian stage: diorite, quartz diorite, granodiorite, granite, euphotide, picrite-porphyrite, peridotite and ultrabasite.

 Rocks from Variscan stage: mainly granite.

 Rocks from the pre-Xingkai period: mainly granite.

Intrusive rocks mainly develop in the areas near fold axes and fractures. The extension direction of the rocks is similar to the direction of the tectonic line of the region. In the catchment, rocks from Caledonian stage are the most developed and widespread, compared with the others. Intrusive rocks from Variscan stage are mainly found near Laji Mountain (west section) and Riyue Mountain.

Tectonically, the Huangshui River catchment is located in the south of the uplift belt (dome) of the middle of Qilian Mountain Fold System. Geotectonically, [the catchment] lies in the uplift belts of Daban Mountain, Riyue Mountain and Lajishan which are at the fore-arc and west wing of the ‘multi-characters shaped’ Qilvhelan fold system, and the depression belts (or troughs) of Haiyan, Huangyuan, Riyue, Datong, Xining and Minhe basins. The tectonic line is mainly in the NWW direction.

Affected by the geologic conditions, the depression belts from the mid-Cenozoic era are also in the direction of NWW (as shown in the attached Figure 3: the geological structure of Huangshui River Catchment). During the Yan(燕) movement period, fractures and depression occurred in this region, thus many intermountain basins took shape and a thick layer of Neogene red clay accumulated there. After gentle folds and fractures occurred to Tertiary system due to the Himalayan movement, with the long-term erosion and denudation, a thick layer of yellow earth piled up during the Quaternary period.

The overall fold of Xining-Minhe Basin is a large a synclinal basin, composed of Tertiary system, with a 10-200 dip angle and undulation.