4.1 Lithofacies characteristics
Four types of facies—gravel, sandstone, siltstone, and mudstone—have been identified in the lithology and sedimentary structure of the Toutunhe–Qingshuihe Formation at the southern margin of the Junggar Basin. These facies include seven types of gravel facies, six types of sandstone facies, four types of siltstone facies and one type of mudstone facies(Fig. 2).See attached table 1 for details.
The gravel facies are dominated by gravel and pebbly sandstone developed in a high-energy sedimentary environment. Their colors are mainly reddish brown, grayish green, and grayish yellow. Based on their sedimentary structures, they can be divided into massive bedding gravel facies (Gm), plate cross-bedding gravel facies (Gp), trough cross-bedding gravel facies (Gt), graded bedding gravel facies (Gc), parallel bedding gravel facies (Gh), massive bedding gravelly sandstone facies (Gsp), and plate cross-bedding gravelly sandstone facies (Gsp).
The sandstone facies are mainly coarse to fine sandstone and their colors are mainly khaki, reddish brown, and gray. Based on their sedimentary structures, they can be divided into massive bedding sandstone facies (Sm), trough cross-bedding sandstone facies (St), convolute bedding sandstone facies (Sb), ripple bedding sandstone facies (Sr), parallel bedding sandstone facies (Sh), and plate cross-bedding sandstone facies (Sp).
The siltstone facies are mainly siltstone and argillaceous siltstone and their colors are mainly grayish yellow and reddish brown. Based on their sedimentary structures, they can be divided into massive bedding siltstone facies (Fm), horizontal bedding siltstone facies (Fh), ripple bedding siltstone facies (Fr), and cross-stratification siltstone facies (Fl).
The mudstone facies is dominated by mudstone and silty mudstone and the color is mainly reddish brown. The mudstone is fine-grained suspended material deposited under hydrostatic conditions, mainly massive bedding mudstone facies (Mh).
4.2 Sedimentary microfacies
Combined with the analysis of lithofacies sedimentary characteristics, it was determined that gravel, sandstone, siltstone, and mudstone are developed in the Toutunhe–Qingshuihe Formation in the study area. Various sedimentary structures such as massive bedding, parallel bedding, plate cross-bedding, and trough cross-bedding are developed. Four sedimentary facies—lacustrine facies, meandering river facies, shallow water delta facies, and alluvial fan facies—were identified vertically in the Toutunhe–Qingshuihe Formation in the central section of the southern margin of Junggar Basin [12/13/14](Fig. 3).
4.2.1 Lacustrine facies
Lacustrine facies can be divided into three basic subfacies: shore, shallow lacustrine, and deep lacustrine, according to characteristics of the landform and the lake water depth. Among these, the shore subfacies are located in the open area above the usual water line and flood elevation. Four sedimentary microfacies are developed, including lakeshore dunes, lakeshore bar, nearshore sandbar, and farshore sandbar [15/16].
1) Lakeshore dunes are developed in the open area above the flood surface, mainly formed by aeolian sand accumulation. The sedimentary rock is coarse-medium sandstone, and the grain size of the upper sediments gradually becomes finer. Trough cross bedding, parallel bedding and ripple bedding are developed. The entire outcrop is a positive grain sequence. Their lithofacies combination is combination A (Sp, St, Sh, Sr).
2) Lakeshore bars are developed between the flood surface and the low water elevation, where the wave action is strong. The erosion of gravel or sandy bedrock by the lake waves results in formation of gravel or lakeshore sandbars. Gh and Sh interbeds are developed at the bottom of the sedimentary microfacies, in which Gh is dominated by fine gravel. The gravel is well sorted and rounded, with obvious directionality. Their lithofacies combination is combination B (Gh, Sr, Sh).
3) Nearshore sandbars are developed between the low water elevation and the normal wave base surface. The water is shallow but always present. The sandbars are formed by the combined action of the waves and lake currents. At the bottom of the sedimentary microfacies in the study area, there are thick layers of ripple bedding fine sandstone. The upper part is mainly siltstone with good sorting and rounding. Small cross bedding and parallel bedding are developed. Their lithofacies combination is combination C (Sr, Fl, Fr, Fh).
4) Shallow lake subfacies are developed within the weak reducing environment below the normal wave base and above the storm wave base. This subfacies is distributed in the interior of the lake on the plane. The sedimentary environment is relatively calm. Dark and reduced color rocks are developed, showing massive bedding and parallel bedding. The rock is abundant in organic matter. Their lithofacies combination is combination D (Fm, Fh, Mh).
4.2.2 Seasonal meandering river facies
A seasonal meandering river is one that dries up seasonally or for a longer period within a year. The width and depth of the seasonal meandering river vary with seasonally, and cracks or cross-sections may appear during the dry period. Depending on the sedimentary environment and sediment characteristics, meandering river deposits can be further divided into channel fill and flood plain sedimentary subfacies [16/17/18/19]. In this study area, vegetation cover is substantial at the site of development of the meandering river facies and the exposure is poor.
1) Channel fill subfacies are found at the deposition site of the bedload carried by the river. Massive sandstone is developed along the bottom and erosion filling structures can be seen. Above this bottom level, the sediments gradually become thinner and sorted by grain size. Sand ripple bedding, trough cross bedding, and massive bedding are developed. Their lithofacies combination is combination E (Sm, Sr, Sp, Fm).
2) Flood plain subfacies are accumulated when the riverbed overflows during floods. This subfacies is mainly composed of reddish-purple clay rock with development of massive bedding, parallel bedding, and small ripple bedding. Their lithofacies combination is combination F (Fr, Fh, Mh).
4.2.3 Shallow water delta facies
Shallow water deltas can be quite large, with substantial thickness and covering a wide area. Based on the sedimentary environment and sedimentary characteristics, shallow water delta facies can be divided into delta plain subfacies, delta front subfacies, and prodelta front subfacies [20/21/22]. Shallow-water delta plains exhibit distributary channels, with natural dikes and flood fans developed on both sides of the distributary channels, and swamps and lakes developed between the distributary channels. The delta front is the most active sedimentation area of a shallow water delta. Microfacies can be divided into underwater distributary channels, underwater natural dikes, front sheet sands, mouth bars, etc.
1) Distributary channels have sequences characterized by sandy deposition and gradual upwards thinning. The lithology is dominated by parallel bedded medium-fine sandstone, with upward development of massive bedding, plate cross bedding, and trough cross bedding. The scale gradually becomes smaller upward, with thin-layer plate cross-bedding gravel-bearing sandstone facies. Their lithofacies combination is combination G (Gsp, Sm, Sp, Sh, St).
2) Flood fans develop when a local breach of a natural dike causes the water to flow outward from the river channel. Suspended and bedload sediments overflow to the flood basin and form a flood fan beach. The sediments are dominated by siltstone, developing massive bedding, parallel bedding, and small cross bedding. Their lithofacies combination is combination H (Fm, Fh, Fl).
3) Natural dikes are formed by deposition of sediment carried by floods as they overflow. They exhibit development of small ripple bedding siltstone facies and very fine interbedded sandstone. Their lithofacies combination is combination I (Fr, Sr).
4) Distributary interchannels are the most widely distributed sedimentary microfacies in the delta plain. The lithology is dominated by dark organic mudstone and argillaceous siltstone. Massive bedding is developed, with thin parallel bedded siltstone. Their lithofacies combination is combination J (Mh, Fm, Fh).
5) Subaqueous distributary channels are underwater extensions of surface distributary channels. Their lithology is mainly sandstone and siltstone, with development of plate cross bedding, small cross bedding, parallel bedding, and ripple bedding. Their lithofacies combination is combination K (Sr, Sh, Sp, Fh, Fl). Horizontally parallel bedded sandstone facies are unstable. Vertically, they may be excessive for combination G.
6) Debouch bars are formed by accumulation of sand and mud carried by rivers to the estuary due to a decrease in flow velocity. The lithology is mainly medium-thick gray sandstone and siltstone. The sorting is good, and massive bedding, parallel bedding, and ripple bedding are developed. Their lithofacies combination is combination L (Mh, Fm, Fh, Fr).
4.2.4 Alluvial fan facies
There are two sedimentary types within alluvial fans. One is mudflow caused by gravity flow and related sedimentation; the other is sedimentation from ephemeral or seasonal water flow caused by traction. Based on the location and characteristics of the sediments, they can be divided into mudflow deposition, channel deposition, overflow deposition, and sieve deposition [23]. Among these, channel sedimentary microfacies are developed at the head, middle, and end of the fan, with differing sediment grain size and structural scale.
1) Mudflow deposition is formed by rapid accumulation of high-density and high-viscosity fluids mixed with clastic sediments and water. The sorting is very poor, with gravel, sand, and mud mixed accumulation; the bedding is not obvious and is generally bulky. The upper part develops massive bedding pebbly sandstone facies. The top may contact parallel bedding or trough cross-bedding gravel, and it may also repeat the lithofacies assemblage A sequence.
2) Channel deposition refers to deposition of sediments into river channels temporarily cut into the alluvial fan. A typical root-fan channel is deep and straight. Their lithology is mainly gray-green coarse-medium gravel, with poor sorting and rounding, with development of thick layered massive bedding gravel facies. Upward visible plate cross-bedding gravelly sandstone facies and trough cross-bedding gravel facies are vertically superpositioned. The gravel in the plate cross-bedded gravel-bearing sandstone facies is distributed along the layer with obvious directionality. Thin parallel bedding and low angle cross-bedded gravel facies are developed at the top. Their lithofacies combination is combination N (Gm, Gt, Gp, Gh, Gsp).
The river channel in the fan becomes a shallow, mostly braided river. The sediments are mainly medium-fine gravel, sorting and rounding. Thick layered rhythmic bedding is developed in the lower part. Its interior shows multiple sets of positive cycles superimposed upward. Parallel bedding, trough cross-bedding, and trough cross-bedding can be seen upward. The contact relationship between Gp and Gt is erosion-erosion. Its lithofacies combination is combination O (Gc, Gh, Gp, Gt) Corresponding to lithofacies assemblage O ( Gc, Gh, Gp, Gt ), and with upward transition to assemblage M.
There are many sheet flows and unrestricted rivers at the fan end. The sediments are mainly fine gravel and coarse-medium sand and fine-grained sediments significantly increase. The gravel layer and the sandy gravel layer are superimposed and developed. The gravel layer develops parallel bedding and low angle cross-bedding, and the sandy gravel layer develops parallel bedding and low-angle cross bedding. The lithology of the gravel layer is mainly fine gravel, which is well sorted and rounded. The gravel is developed along the bedding in the parallel bedding with obvious orientation. Their lithofacies combination is combination R (Gt, Gh, Gsh, Gsp).
3) Overflow deposits, also known as sheet deposits, are formed by flooding from the end or both sides of a channel on an alluvial fan. The sediments are dominated by overlapping development of gravel layers and sandy gravel layers. The parallel bedded gravel facies is dominated by medium gravel arranged along the layer, which is subangular and directional. Small plate bedding is developed in the gravel-bearing sandstone facies, with the gravel evenly distributed and mixed with coarse sand. Their lithofacies combination is combination P (Gc, Gh, Gsh, Gsp).
4) Sieve deposits are formed when the water carries fine sediments through the pores between the gravel of the alluvial fan gravel layer, while coarse-grained debris is retained in the gravel pores. The gravel is mainly angular-subangular and is filled with sand. The interlayer boundary is vague and the top layer is a massive structure. The sieve deposits are composed of graded bedded gravel facies, trough cross-bedded pebbly sandstone facies, massive bedded gravel facies, and massive bedded gravelly sandstone facies. This sedimentary microfacies type is developed in the middle area of the alluvial fan. Their lithofacies combination is combination Q (Gc, Gsp, Gm, Gsm). Combination Q is a local accumulation phenomenon, often accompanied by combination O.
4.3 Lower combination sedimentary facies
Based on previous literature and detailed descriptions of outcrop profiles, the Middle Jurassic Toutunhe Formation, Upper Jurassic Qigu Formation, Karaza Formation, and Lower Cretaceous Qingshuihe Formation in the southern margin of the Junggar Basin mainly developed four sedimentary facies: shallow water delta facies, meandering river facies, alluvial fan facies, and lacustrine facies [24]. A lacustrine gravel beach bar is developed in the Nananjihai outcrop of the Qingshuihe Formation, but is absent in the Queergou outcrop and the Honggou outcrop, possibly due to the type of bedrock on the lake bank.
4.3.1 Qingshuihe formation
The lithology at the bottom of the Qingshuihe Formation in the Nananjihai outcrops is brownish red gravel and yellow sandstone soil, with gravel sorting, general roundness, and miscellaneous base support. The gravel layer is straight and has developed parallel bedding. Gm, Gt, Gh, Gsp, Sr, St, and other lithofacies are common. Combination B is developed, which is the shore lake gravel beach bar microfacies. The lithology of the middle and lower parts of the Qingshuihe Formation is yellow fine sandstone and siltstone(Fig. 4), with sedimentary structures such as small ripple bedding, parallel bedding, and plate cross bedding. This lithology shows the development of ripple marks and mud cracks. In the longitudinal direction, changes in particle size are not obvious and the level is straight. Sh, Sr, Sp, Fm, Fr, Fh, and other lithofacies are common, and combination D is developed, which is the shallow lake subfacies type.
The lithology of the Qingshuihe Formation in the Queergou outcrops is mainly gray medium-fine sandstone with siltstone, with sedimentary structures such as massive bedding, deformed bedding, and parallel bedding. Sm, Sr, Sh, Sb, Fm, and other lithofacies are common, and combination D is developed, which is lake facies deposition(Fig. 5). The lithology at the bottom of the Qingshuihe Formation in Honggou is mainly medium-thick fine gravel. The upward grain size becomes finer, and the whole is dominated by sand and the development of massive bedding, parallel bedding, and ripple bedding. Sm, Sr, Sh, and other lithofacies are common, and combinations C are developed, which are shore lake and shallow lake sedimentary subfacies.
In summary, the Qingshuihe Formation develops shore-lake gravel beach-bar microfacies and shallow lake subfacies, demonstrating the sedimentary model of the lake.
4.3.2 Karaza Formation
Among the three outcrops in the central southern margin of the Junggar Basin, the Lower Cretaceous Qingshuihe Formation of the Nananjihai outcrops is covered by the Upper Jurassic Qigu Formation, while the Upper Jurassic Karaza Formation is missing. The Jurassic Karaza Formation in the Honggou and Queergou outcrops covers the Qigu Formation and underlies the Lower Cretaceous Qingshuihe Formation.
The lithology at the bottom of the Karaza Formation is mainly brownish red coarse-medium gravel. The gravel is poorly sorted, poorly rounded, angular, and supported by miscellaneous bases. The particle size gradually decreases moving upward, and the color becomes gray and gray-green. Block bedding, large plate cross bedding, trough cross bedding, parallel bedding and other sedimentary structures are developed. Gm, Gsm, Gp, Gt, Gc, and other lithofacies are common(Fig. 6). A combination of M and N is developed, which includes mudflow and riverbed filling microfacies.
The lithology in the middle of the Karaza Formation is mainly gray-green and gray-yellow medium-fine gravel, with thin sandstone. The water discharge structure can be seen in the gravel layer and parallel bedding, graded bedding, trough cross bedding, and so on are developed. Lithofacies such as Gc, Gp, Gt, Gsm, Gm, and Gsp are common(Fig. 7). A combination of O, P and Q is developed, which includes braided channel, overflow deposit, and sieve sedimentary microfacies.
The lithology at the top of the Karaza Formation is mainly fine gravel and sand. The grain size of the sediment is clearly smaller. Small plate cross bedding and parallel bedding are developed. Gm, Gsp, Gp, Gh, and other lithofacies are common. The combination R is developed, which is the underwater distributary channel microfacies.
In summary, the Karaza Formation develops mudflow deposits, channel deposits, overflow deposits, sieve deposits, and other microfacies, exhibiting the sedimentary model of alluvial fans.
4.3.3 Qigu Formation
The Qigu Formation in the Honggou outcrops is in integrated contact with the overlying Karaza Formation. The lithology at the bottom of this ancient formation is mainly brown-red mudstone, siltstone, and sandstone. Sedimentary structures such as trough cross bedding, plate cross bedding, parallel bedding, ripple bedding, and massive bedding are developed. The layer thickness is 10–50 cm. Mud crack phenomena can be seen in the mudstone. Lithofacies such as Sp, St, Fh, Fm, and Mh are common. The combinations E and F are developed, including distributary channel and flood plain microfacies.
The Qigu Formation in the Nananjihai outcrops is in unconformity contact with the overlying Qingshuihe Formation and the Karaza Formation is missing in the middle. The main lithology of the Qigu Formation is red mudstone and argillaceous siltstone, with calcareous cementation. There are few fossils and plant rhizomes, reflecting the dry and rainy climate at that time. Sp, St, Fh, Fm, Mh, and other lithofacies are common(Fig. 8). The combinations E and F are developed, including distributary channel and flood plain microfacies.
In summary, the Qigu Formation develops distributary channel and flood plain microfacies, exhibiting the sedimentary model of a seasonal meandering river. The overall color of the Qigu Formation is red and purplish red. It is judged that climate was dry and hot during its deposition.
4.3.4 Tuotoutunhe Formation
The lithology of the bottom of the Toutunhe Formation is mainly gray-yellow and gray-green fine-medium sandstone, pebbly sandstone, and siltstone. Sedimentary structures such as wave ripple bedding and plate cross bedding are developed. The scour surface at the bottom of the river and a thick layer of “mud-coated sand” structure are visible. Lithofacies such as Sr, Sp, Fr, Fh, and Mh are common. The combinations G, H, I, and J are developed, including distributary channel and interdistributary channel microfacies in a shallow water delta plain(Fig. 9).
The lithology in the central part of the study area is mainly gray-yellow, gray fine-medium sandstone, and gravelly sandstone. Sandy bedding, parallel bedding, and trough cross bedding are developed. Lithofacies such as Sp, Sh, Sr, and Gsp are common(Fig. 10). The combinations K and L are developed, including underwater distributary channel and debouch bar sedimentary microfacies in a shallow water delta front.
The top sediments are mainly khaki, purplish red, and gray-white siltstone and sandstone, with a thin layer of gray-green mudstone. Sedimentary structures such as plate cross bedding, trough cross bedding, parallel bedding, and ripple bedding are developed, with interbedded siltstone and sandstone. Lateral variations in the thin sandstone are unstable, with lengths of about 20–30 m. Horizontal variations in the two red sandy bands are stable, reflecting frequent rise and fall of the lake surface and indicating that the environment of the Toutunhe Formation experienced a short period of hot drought. Lithofacies such as Sp, Sr, Sh, Sm, Fm, Fh, and Mh are common(Fig. 11). The combination A, C, and D is developed, including lakeshore dunes, farshore sandbar, and shallow lake subfacies.
In summary, the color of the sediments in the Toutunhe Formation changed from gray-yellow and gray-green to khaki and purplish red, reflecting that the climate gradually changed from warm and humid to dry and hot. Microfacies such as distributary channel, natural dike, interdistributary channel, underwater distributary channel, lakeshore dunes, farshore sandbar, and shallow lake subfacies are developed, demonstrating the sedimentary model of a shallow water delta-lake.