Lithofacies, Succession, and Their Genetic Interpretation of Lacustrine Gravel Beach-Bars

Based on the elaborate dissection of pro ﬁ le sections parallel and vertical with the shoreline and the forming beach-bars, the unique sedimentary succession of gravel beach-bars, “ ABC ” sequence, has been found, and their lithofacies and origin have been explained. The A interval is composed of poorly sorted sand and gravel, formed in the wave asymmetric zone. The B interval is composed of well-sorted gravel, formed in the breaker zone. The C interval is composed of normally graded sand, formed in the sur ﬁ ng zone. In the actual gravel beach-bars, three intervals are often presented in a variety of combinations, such as “ ABCABC, ” “ BCBCBC, ” “ ABABAB, ” and others. These ﬁ ndings provide an important basis for the identi ﬁ cation and distribution prediction of beach-bar reservoirs in the ancient continental lake basins.


Introduction
Lacustrine beach-bar deposits are characterized by well physical property, and it is easy to form large-scale oil and gas fields that is one of the important reservoirs in petroliferous basins in China [1][2][3][4][5][6]. So far, a considerable number of oil-bearing beach-bar deposits have been discovered in Bohai Bay Basin, Erdos Basin, Junggar Basin, and Pearl River Mouth Basin [7][8][9][10][11][12].
Most of the research are based on ancient beach-bar deposits, and the data used are multiple solutions, such as seismic and logging data. Different people can produce different conclusions. So many conclusions may be misinterpreted. In addition, the difficulty of the research of beachbars lies in how to differentiate between ancient beach-bar deposits and ancient delta-front deposits. The beach-bar deposits and delta-front deposits are similar in a sedimentary environment and hydrodynamic conditions and have similar lithology, sedimentary structure, and texture [1,15,[18][19][20]. In ancient layers, it is difficult to distinguish between beach-bar deposits and delta-front deposits. This seriously restricts the paleogeographical reconstruction and hydrocarbon prediction.
In view of the above problems, through the study of modern gravel beach-bars in the Qinghai Lake, the authors summarize sedimentary characteristics, successions, and model of gravel beach-bars and interpret the origin of sedimentary successions of gravel beach-bars. These findings provide an important basis for the identification and distribution prediction of beach-bars in the ancient continental lake basins.

Geological Setting
Qinghai Lake (36°32 ′ to 37°15 ′ N, 99°36 ′ to 100°46 ′ E), the largest extant closed-basin lake in China, is situated at an altitude of 3194 m above the sea level on the northeastern Qinghai-Tibetan Plateau. The lake area is surrounded by Datong Mountain, Sun Moon Mountain, and Qinghai South Mountain, which form the closed inland faulted basin. The long axis direction of Qinghai Lake (nearly east-westward) is about 106 km; the transverse direction (near north-southward) is about 63 km; the lake altitude is 3193~3198 m; the surface area is 4264~4473 km 2 ; and the perimeter of the lake is about 360 km. The lake shape is oval, and the east-west direction is longer than the north-south direction. The long axis of the north-west to about 315°. The average depth of the lake is approximately 21 m, its maximum water depth is approximately 32 m, and the water storage reaches 100 × 10 9 m 3 [21] (Figure 1(a)).
In this study, the beach-bar deposits is located at the Erhai on the southeast coast of Qinghai Lake (100°43 ′ 54.5 ″ E, 36°33 ′ 2.3 ″ N) ( Figure 1). The beach-bars formed by the action of longshore currents separate the Erhai from the Qinghai Lake. Therefore, the Erhai became a completely enclosed lagoon (Figure 1(b)). Because the lake level of the Qinghai Lake is currently declining [21], which formed a widely distributed, several rows are parallel to the shoreline and progradational type of beach-bar deposits in the Erhai (Figure 1(b)).

Methodology
The modern is the key to the past. Based on the field investigation of the Qinghai Lake, the authors have carried out the elaborate dissection of profile sections parallel and vertical with the shoreline on the southeast Qinghai Lake. The profile section parallel with the shoreline is located at 100°43 ′ 23.4 ″ E, 36°32 ′ 42.6 ″ N, and the azimuth angle of the section is 49°(the azimuth angle of the shoreline is 51°). The profile section vertical with the shoreline is located at 100°43 ′ 23.4 ″ E, 36°32 ′ 42.6 ″ N, and the azimuth angle of the section is 315°. Except   Figure 1: Geographic setting of Qinghai Lake and Distribution of beach-bars in the southeastern Qinghai Lake. 2 Lithosphere for the profile sections, forming beach-bars in the Qinghai Lake had been used to investigate the formation mechanisms of gravel beach-bar sedimentary succession.

Large-Scale Sedimentary Characteristics of Gravel Beach-Bars in the Southeastern Qinghai Lake
Several beach-bar deposits are developed parallel to the shoreline on the southeastern Qinghai Lake, and the long axis azimuth angle of the beach-bars is about 43°. The maximum length of the beach-bars is able up to 8 km; the average is about 5 km. The maximum width of the beach-bars is able up to 0.7 km; the average is about 0.3 km. The maximum gravel thickness is more than 6 m. The geometry shape of the beach-bars is long and strip-like with a concave top and flat bottom ( Figure 2).
The southeastern beach-bars are mainly composed of medium gravel, fine gravel, and coarse sand. The grains are subround; the sorting is from poor to better, with parallel bedding, flushing cross-bedding, massive bedding, and normal grading bedding.

Sedimentary Characteristics of the Profile Parallel with the Shoreline
The profile is about 2 m in height and about 15 m in width (actually the beach-bar is 4800 m in length, 6 m in thickness, and 400 m in width) ( Figure 3). From bottom to top, the profile parallel with the shoreline can be divided into 17 layers on the basis of their lithology, texture, sedimentary structure, thickness of layers, lateral continuity, and contact relationship between top and bottom ( Figure 4). Because the profile is parallel with the   3 Lithosphere shoreline, the dip angle of layers is almost 0°, and the transverse continuity is well, and it is easy to track laterally (Figures 3 and 4). The abrupt transition can be seen in most of the layers. The transitions are gradual just between the 4th and 5th layers, between the 9th and 10th layers, between the 11th and 12th layers, between the 13th and 14th layers, and between the 15th and 16th layers ( Figure 4). The thickness of 17 layers is mostly about 10 cm, the thickest 1th and 6th layers can be more than 20 cm, and the thinnest 4th, 5th, and 16th layers are lower than 5 cm ( Figure 4). The layers are mainly composed of medium gravel, fine gravel, and coarse sand, and the massive bedding and the normal grading bedding are very developed. The massive bedding is only in the 5th, 12th, 14th, and 16th layers. In the 1st, 3rd, 6th, 8th, and 17th layers, gravels are oriented and floating in the coarse sands ( Figure 4).
Through the analysis of samples from each layer, it is found that 17 layers have two kinds of sorting coefficients. On the basis of sorting, 17 layers can be divided into two parts. One part is more than 2 (poorly sorted), including 1 st , 3 rd , 6 th , 8 th , and 17 th layers. The other is less than 2 (medium-sorted to well-sorted) ( Figure 4). The mean grain size of each layer is different. The coarse grain size layers are the 7 th , 9 th , 11 th , 13 th , and 15 th , and the mean grain size is lower than -2.5 phi. The fine-grain size layers are the 1 st , 5 th , 8 th , and 12 th , and the mean grain size is more than -1 phi ( Figure 4).
According to the lithology, sedimentary structure, grain-size parameter, thickness, and contact relation of the profile, 17 layers can be divided into three intervals: A, B, and C.
The A interval is dominated by gravels and sands, which are subround and poorly sorted and have massive bedding, with gravels oriented floating in the coarse sands. The thickness is more than 15 cm. The mean grain size is between -2 and -1 phi, and the sorting is more than 2 (Figures 4(d) and 4(e)). The 1 st , 3 rd , 6 th , 8 th , and 17 th layers all belong to the A interval ( Figure 4).
The B interval is dominated by gravels, which are subround and well-sorted and have massive bedding with a thickness of 7 to 15 cm. The mean grain size is between -4 and -1.5 phi, and the sorting is less than 2 (Figures 4(d) and 4(f)). The 2 th , 4 th , 7 th , 9 th , 11 th , 13 th , and 15 th are all belong to the B interval ( Figure 4).
The C interval is dominated by sands, which are subround and well-sorted and have normal grading bedding; the thickness is less than 9 cm. The mean grain size is between -1 and 0, and the sorting is less than 2 (Figures 4(d) and 4(g)). The 5 th , 10 th , 12 th , 14 th , and 16 th all belong the C interval ( Figure 4).

Sedimentary Characteristics of the Profile Vertical with the Shoreline
The profile is about 2.5 m in height and about 15 m in width (actually the beach bar is 4800 m in length, 6 m in thickness, and 400 m in width) ( Figure 5). The profile vertical with the shoreline is mainly composed of medium gravel, fine gravel, and coarse sand; the whole is a low-angle swash bedding; and each lamina dip is about 4~6°. The lateral continuity of each lamina is unstable and difficult to track laterally. To better describe the profile sedimentary characteristics, the profile is divided into two lithologic columns: Column 2 and Column 3 ( Figures 5 and 6).
From bottom to top, each lithologic column can be divided into several layers on the basis of their lithology, texture, sedimentary structure, thickness of layer, lateral continuity, and contact relationship (Figures 5 and 6). Some of layers can be tracked laterally. The layers change greatly in thickness, and the overall trend is to reduce toward the lake. The thickness of layers is about 20 cm, the thickest is able up to 60 cm, and the thinnest is able up to 5 cm. Abrupt transitions can be seen between most of the top-bottom interface of each layer. Each layer extends laterally 4 m to the left, the longest can reach 7 m, and the shortest can reach 2 m. The dip angle of each layer to the lacustrine direction is about 4~6°( Figure 5).
Each layer is composed of medium gravels, fine gravels, and coarse sands, respectively, and the grains are subround. The layers have mainly massive bedding and normally graded bedding. On the basis of the lithology, sedimentary structure, size parameter, thickness, and contact relation between top and bottom of the profile, the layers can be divided into three intervals: A, B, and C ( Figure 6).
The A interval is dominated by gravels and sands, which are subround and poorly sorted and have massive bedding with gravels oriented floating in the coarse sands. The thicknesses are from 5 to 60 cm and from 3 to 6 m in lateral width ( Figure 6).
The B interval is dominated by gravels, which are subround and well-sorted and have massive bedding. The thicknesses are from 5 to 40 cm and from 2 to 7 m in horizontal width (Figure 6).
The C interval is dominated by sands, which are subround and well-sorted and have normal graded bedding. The thicknesses are less than 10 cm and from 1 to 3 m in horizontal width ( Figure 6).

Sedimentary Succession and Genetic
Interpretation of Gravel Beach-Bars Through a detailed study of the profile vertical and parallel with the shoreline, it is found that gravel beach-bars have a fixed sedimentary succession, named "ABC" sequence ( Figure 7): (1) The A interval, the bottom of the sedimentary succession, is dominated by coarse sands and gravels, which are subround and poorly sorted and have massive bedding, with gravels oriented floating in the coarse sands  In the actual gravel beach-bars, three intervals are often presented in a variety of combinations, such as "ABCABC," "BCBCBC," "ABABAB," and other combinations (Figures 4  and 6).
Through the study of forming beach-bars in the Qinghai Lake, the "ABC" sequence is also found in the surface (Figure 8). The A interval is located in the wave asymmetric zone (the wave shoaling zone called by other researchers; the wave can impact the lake bottom, and the wave height begins to rise but is not broken enough) (Figure 8). The oscillation amplitude and energy of waves are weak so that sands can be transported, but gravels cannot be transported. The authors think that the gravel of the A interval is transported by intermittent storm waves. After the storm, the wave cannot move the gravel of the A interval, so the A interval is poorly sorted and gravels are oriented floating in the coarse sands ( Figure 8). The B interval is located in the breaker zone ( Figure 8). In this zone, the wave is broken diving into the bottom of the lake. The energy is strongest. The sediments are coarsest, because the relatively fine sediments cannot deposit in this zone. Therefore, the B interval is well-sorted ( Figure 8). The C interval is located in the surfing zone ( Figure 8). After the wave is broken, most of the energy is consumed. Due to the effect of inertia, the wave continues to move landwards until the energy is exhausted. The sediments deposit orderly form large to small, leading the normal grading in the C interval ( Figure 8). Due to the difference of wind, wave, and lake level at each season, even every year, the location of the wave asymmetric zone, the breaker zone, and surfing zone can change back and forth. Therefore, the ABC intervals in the actual beach-bars are displayed in various combinations, such as "ABCABC," "BCBCBC," and "ABABAB."

Sedimentary Model of Gravel Beach-Bars in Qinghai Lake
Through the three-dimensional anatomy of the gravel beachbars on the southeastern part of the Qinghai Lake, including the profile vertical to the shoreline, the profile parallel to the shoreline, and the depositing surface, the sedimentary model of gravel beach-bars was established finally (Figure 9). The hydrodynamic conditions of the formation of gravel beach-bars can be divided into three zones: the wave asymmetric zone, breaker zone, and surfing zone (Figure 9). Above the wave base, the wave begins to impact the bottom of the lake; the frictional force between the lake bottom and the wave causes the asymmetric deformation of waves; this area is the wave asymmetric zone. The wave is seriously deformed until broken and plunges to the lake bottom; this area is the breaker zone. After the wave is broken, due to the effect of inertia, the water continues to move to the shore until the energy is depleted; the area is the surfing zone.
The sediments of the asymmetric wave zone are mainly sandstone, and the flat gravels are scattered sporadically.   Figure 6: Photography, sketch, and interpretation of profile section of gravel beach-bars in the southeastern Qinghai Lake vertical with the shoreline. 6 Lithosphere The sediments of the breaker zone are mainly conglomerates and well-sorted. The surfing zone sediments are mainly sandstones and normally grading.
With the rising and falling of the lake level, the three hydrodynamic zones migrate back and forth on the plane, resulting in the sediments of three zones vertical overlaying each other.

Ancient Gravel Beach-Bar Identification Mark
In ancient formations, there are many similarities between the beach-bar deposits and the delta-front deposits, which is confusing. Delta-front deposits and beach-bar deposits are mainly developed in the shallow water area of the Qinghai Lake; the beach-bars accounted for about 60%, and the delta-fronts accounted for 40%. In many researches of ancient lacustrine basin deposits, generally, the delta-front deposits occupy the majority. The authors think that many delta-front deposits may be misinterpreted as beach-bar deposits.
Through the detailed study of the modern lacustrine gravel beach-bars, the identification mark of the ancient beach-bars has been supplemented and perfected ( Table 1). The specific identification marks are as follows: (1) The difference in sedimentary succession: the gravel beach-bar deposits have a fixed sedimentary succession, the "ABC" sequence. With the wave size and lake level changes, gravel beach-bar deposits are actually presented in variously combined patterns, such as "ABCABC," "ABABAB," and "BCBCBC." (2) The difference in sedimentary structures: there is low-angle swash bedding in beach-bar deposits. There is much parallel bedding in the profile parallel to the shoreline, with flat gravels floating in the sands.
(3) The difference in textures: the textural maturity of the beach-bars is higher, the granules are subround, and the sorting can change from poorly to well.
(4) The difference in top-bottom interface: the bottom interface of the beach-bar deposits is generally a flat abrupt surface.
(5) The difference in geometries: the trend of the beachbars is parallel to the shoreline, and the geometry shape is long stripe-like with convex tops and flat bottoms.

Discussion
These viewpoints of gravel beach-bar deposits developed in the lacustrine basin differ from the previous research of beach-bars [18,[22][23][24][25]. Jiang Figure 9: Depositional model of gravel beach-bars in the Qinghai Lake.  [6]. However, both of them neglected the effect of different wave zone to sediments. Although the lake waves are not as big as the sea waves, the effect of different wave zone to sediments is still obvious.
Our study shows that the lake waves are obviously divided into three parts, wave asymmetric zone, wave breaker zone, and wave surfing zone (Figure 9). The different wave zones have different sedimentary characteristics (Figures 7 and 8).
At present, no one has proposed the particular depositional succession of lacustrine beach-bars. Most beachbar researches pay their attention to the summary of lithology, texture, sedimentary structure, distributional pattern [15,18,24], or the applied research of lacustrine beach-bars [5,25,26]. Through our study on the several beach-bar profiles in the Qinghai Lake, the lacustrine gravel beach-bars are composed several particular depositional successions, the "ABC" succession ( Figure 7). The gravel beach-bars can present various combinations: "ABABAB," "ABCABC," "BCBCBC," etc. (Figures 4 and 6).
The sedimentary environment of lacustrine beach-bars is similar to the sedimentary environment of lacustrine delta fronts [27,28]. It is difficult how to distinguish between lacustrine beach-bar sediments and delta front sediments in ancient basins. Through our study, the particular identification marks have been proposed. In terms of the bottom contact relationship, the beach-bars are distinct, and the delta fronts are gradual. In terms of the depositional succession, the beach-bars have the "ABC" succession and show a finecoarse-fine succession from bottom to top. The delta fronts have no special succession and show a fine-coarse succession from bottom to top. These new viewpoints are greatly important for the recognition of lacustrine beach-bar.

Conclusion
Gravel beach-bars have a fixed sedimentary succession, "ABC" sequence. The A interval, the bottom of the sedimentary succession, is composed of coarse sands and gravels, which are subround and poorly sorted and have massive bedding, with gravels oriented floating in the coarse sands; the B interval, the middle of the sedimentary succession, is composed of gravels, which are subround and well-sorted and have massive bedding; the C interval, the top of the sedimentary succession, is composed of coarse sands, which are well-sorted and have normally graded bedding. In the actual gravel beach-bars, three intervals are often presented in a variety of combinations, such as "ABCABC," "BCBCBC," and "ABABAB." The hydrodynamic conditions of gravel beach-bars can be divided into three zones, wave asymmetric zone, breaker zone, and surfing zone. The sediments of the asymmetric wave zone are mainly sandstones, and flat gravels are scat-tered sporadically, that is, the A interval. The sediments of the breaker zone are mainly conglomerates and well-sorted, that is, the B interval. The surfing zone sediments are mainly sandstones and normally grading, that is, the C interval.

Data Availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.

Disclosure
This work has been presented as a conference abstract in IOP Conference Series: Earth and Environmental Science Volume 300 Issue 2.

Conflicts of Interest
The authors declare that they have no conflicts of interest.