5.1. Paleoclimate of MYJ Fm. in Sedimentary Period
In the late Cretaceous, the Simao Basin was located between 21.2 and 28.8° [
7]. The paleoclimate in this latitudinal zone was dominated by hot and dry [
33], with development of evaporites and calcareous breccia (
Figure 6). Under such climate, the weathering degree of parent rock is relatively moderate, and calcium oxide and sodium oxide are leached to a certain extent, which will lead to higher values of chemical index of alteration (CIA), plagioclase index of alteration (PIA), and chemical index of weathering (CIW) [
28,
30,
34].
Generally, the CIA values for basaltic parent rock not affected by weathering range from 30 to 45, those of granitic parent rock range from 45 to 55, and those of strongly weathered samples are close to 100 [
28]; the PIA value of parent rock not affected by weathering is generally 50, and that of strongly weathered sample is close to 100 [
29]; the CIW values of parent rocks not affected by weathering are usually range from 32 to 76, and those of strongly weathered samples are close to 100 [
30].
The CIA index of clay conglomerates in MYJ Fm. of MZK-3 well ranges from 60 to 87 (average = 75, standard deviation = 7); the PIA index ranges from 68 to 97 (average = 89, standard deviation = 8); and the CIW index ranges from 78 to 98 (average = 92, standard deviation = 5) (
Table 1). The calculated results are in accordance with the weathering strength of MYJ Fm. sandstone obtained by Wang [
35] and have reliable reference value. These parameters reflect that Na, K, and Ca in silicate minerals leach more from the parent rock, indicating slight to moderate chemical weathering at the source area and, consequently, may reflect the hot and dry paleoclimatic conditions.
The three indexes of upper member of salt and lower member of salt clay conglomerates are different. Among them, the average values of CIA, PIA, and CIW of upper member of salt clay conglomerates are 71 (standard deviation = 5), 88 (standard deviation = 10), and 92 (standard deviation = 7), respectively, while those of lower member of salt clay conglomerates are 79 (standard deviation = 6), 90 (standard deviation = 6), and 92 (standard deviation = 4), respectively, reflecting the feature that the weathering degree of upper member is weaker than that of lower member. It indirectly reflects that the drought degree of the upper paleoclimate is stronger than that of the lower member, and the leaching of Ca and Na is relatively weak.
Illite and illite/montmorillonite in clay minerals represent arid and semi-arid climatic conditions, while kaolinite represents humid climate with higher weathering degree [
36,
37].
The content of illite in MYJ Fm. ranges from 32.0% to 84.0% (average = 63.7%, standard deviation = 16.2), while the content of illite/montmorillonite mixed-layer ranges from 0.0% to 27.0% (average = 10.4%, standard deviation = 6.7) (
Table 3). The content of illite and illite/montmorillonite is dominant, which reflects a kind of arid paleoclimatic condition. Among them, the content of illite and illite/montmorillonite in upper salt clay conglomerates is relatively high, ranging from 82.0% to 93.0% (average = 88.4%, standard deviation = 4.5); the content of illite and illite/montmorillonite in lower salt clay conglomerates vary greatly, but the content increases from deep to shallow, reflecting the gradual increase of drought degree of paleoclimatic conditions (
Figure 7). The content of kaolinite in the samples decreased gradually with the increase in drought degree (
Figure 7).
The relative contents of some trace elements can also be used to analyze paleoclimate. Generally, Ga/Rb ratio values are relatively lower in arid climate conditions [
38,
39]; Sr/Cu > 5.0 in dry and hot climate, and 1.3 to 5.0 in warm and humid climate [
40,
41,
42].
The ratio values of Ga/Rb range from 0.12 to 0.27 (average = 0.17, standard deviation = 0.05), and ratio values of Sr/Cu range from 1.29 to 48.89 (average = 10.46, standard deviation = 12.15) (
Table 5), reflecting a dry and hot paleoclimatic condition. Among them, the average ratio value of Ga/Rb of upper salt clay conglomerates is 0.14 (standard deviation = 0.01), lower than that of lower salt clay conglomerates (average = 0.19, standard deviation = 0.05); the Sr/Cu ratio value of upper part of salt clay conglomerates is relatively stable; and the Sr/Cu ratio value of lower member of salt clay conglomerates changes greatly, reflecting the fluctuation of paleoclimate, which is consistent with the change of lithology (the occurrence of a small value is related to the increase in sandy or silty content).
Through the study of weathering degree of samples, we can indirectly understand the paleoclimatic conditions of MYJ Fm. during the sedimentary period. The weathering degree of clay conglomerates in MYJ Fm. of MZK-3 well is weak, which reflects the arid paleoclimate conditions. The paleolatitude of the study area has a hot paleoclimate background. Therefore, the paleoclimatic conditions of MYJ Fm. in the deposition period are hot and dry. This understanding has been confirmed by other studies such as sporopollen assemblage [
19], paleogeography [
43], fluid inclusion thermometry [
44].
The weathering degree of the lower part of salt clay conglomerates is stronger than that of the upper part of salt clay conglomerates, which may be caused by the warm and humid climatic conditions in individual periods. Those paleoclimatic conditions not only increased the content of detritus, but also increased the differentiation of major and trace elements.
5.2. Redox Conditions
It has been found by scholars that black shales cannot all be assigned to anoxic environments and red-brown shales cannot all be assigned to oxidation environment [
45], which indicates that the oxidation–reduction conditions of sedimentary environment cannot be accurately identified only based on the color of sedimentary rocks. Therefore, the redox conditions of the red-brown clay conglomerates of MYJ Fm. were studied in this paper. V, Ni, Mo, Cu, Cr, and Mn can be used as sensitive elements for redox conditions to indicate a paleomarine environment [
46,
47,
48,
49]. In an oxidation environment, V, Ni, Cr appear in the form of soluble ions; in the reduction environment, V and Cr appear in the form of insoluble oxides or hydroxides [
46,
49]. Ni is still soluble under moderate reduction conditions, but at the sulfate reduction stage, it will appear in pyrite in the form of NiS, and the reduction sensitivity of Ni is weaker than that of V and Cr [
49].
Based on the above geochemical properties of V, Ni, and Cr, V/(V + Ni) and V/Cr can be used to indicate the redox properties of sedimentary water bodies. Higher values indicate stronger reduction conditions [
50,
51,
52]. Among them, a V/(V + Ni) value greater than 0.84 indicates a reduction environment, less than 0.60 indicates an oxidation environment, and between 0.60 to 0.84 indicates a weak oxidation and weak reduction environment; a value for V/Cr ratio greater than 4.25 indicates a reduction environment, less than 2.0 indicates an oxidation environment, and between 2.0 to 4.25 indicates a weak oxidation and weak reduction environment [
50,
53,
54].
The values of the V/(V + Ni) ratio of clay conglomerates in the MYJ Fm. of the MZK-3 well range from 0.61 to 0.82 (average = 0.73, standard deviation = 0.05), which reflects that the sedimentary environment is in the condition of weak oxidation and weak reduction; the value of V/Cr range from 1.18 to 1.43 (average = 1.29, standard deviation = 0.08), which reflects the partial oxidation of sedimentary environment. Based on the V/(V + Ni) ratio value and V/Cr ratio value, it is considered that the clay conglomerates of MYJ Fm. were deposited in and oxidation environment but which had not reached the degree of strong oxidation. Among them, the V/(V + Ni) ratio values of upper salt clay conglomerates range from 0.72 to 0.76 (average = 0.74, standard deviation = 0.02); the V/Cr ratio values range from 1.21 to 1.39 (average = 1.31, standard deviation = 0.08); the V/(V + Ni) ratio values of lower salt clay conglomerates range from 0.61 to 0.82 (average = 0.73, standard deviation = 0.06); and the V/Cr ratio values range from 1.18 to 1.43 (average = 1.28, standard deviation = 0.07). The average ratio values of V/(V + Ni) and V/Cr of the lower clay conglomerates are slightly lower than those of the upper samples, indicating that the redox potential of the sedimentary environment may be relatively high.
Manganese (Mn) in sedimentary water is not easily adsorbed by organic matter or combined with other minerals, and its content increases with the increase in environmental oxidation [
55]. Under oxidation conditions, Mn forms insoluble Mn
3+ or Mn
4+ hydroxides or oxides, which are co-deposited with detrital particles in the environment; under reduction conditions, Mn is reduced to soluble Mn
2+, and Mn content in sediments is relatively lower [
46,
48,
49]. In the samples, the average enrichment factor of Mn is slightly higher than 1.0 (
Table 4), which is also a reflection of weak oxidation environment.
In combination with the relative contents of Fe and V in sediments, some scholars have proposed that [
45] (1) under strong oxidation conditions, Mn and Fe appear in the form of oxides, and the relative contents are relatively high (Mn
average = 1300 ppm, Fe
average = 56,000 ppm); (2) under the condition of neutral pH nitrate sulfate reduction, the content of Mn in sediments is lower (average = 310 ppm), and Fe is still in the form of oxides; and (3) under the conditions of strong reduction, the V content in the sediment is particularly high, while the content of Mn and Fe is relatively lower.
The content of Mn varies from 243.4 to 933.9 ppm (average = 619.8 ppm, standard deviation = 213.5 ppm); the content of Fe varies from 35,000 to 74,000 ppm (average = 52,000 ppm, standard deviation = 11,000 ppm). The content of V varies from 78.2 to 128.2 ppm (average = 96.8 ppm, standard deviation = 14.9 ppm). This reflects the overall partial oxidation of the sedimentary environment, but has not reached the degree of strong oxidation. This conclusion is consistent with that obtained from the relative contents of V, Ni, and Cr.
In order to identify the redox conditions of sedimentary environment by using the relative contents of Fe, Mn, and V, we firstly reduced the relative content of Fe by 100 times, and then constructed the ternary scatter plot (
Figure 8). In
Figure 8, area 1 represents strong oxidation environment, area 2 represents weak oxidation environment, and area 3 represents reduction environment. The data points of clay conglomerates in MYJ Fm. of MZK-3 well fall in areas 1 and 2, indicating that the sedimentary environment is partial oxidation, and some periods are still in strong oxidation environment. The understanding that MYJ Fm. is in a strong oxidation environment in some periods is also supported by the results of high and low frequency susceptibility tests [
56].
5.3. Paleosalinity
B in sediments mainly exists in clay minerals, which may replace Si or Al atoms in the lattice of clay minerals, and its content is mainly controlled by the B content in sedimentary water besides the parent B content. The concentration of boron in sediment water is linearly related to salinity, so it can be used to restore the salinity of sedimentary water.
The relationship between B content in sediment and salinity of sediment water can be established as follows (Freundlich isotherm adsorption equation):
In the equation,
Bk is the B content of kaolinite (ppm);
B is the measured content (ppm);
Xi,
Xm,
Xk are the relative contents of illite, montmorillonite, and kaolinite in clay minerals, respectively;
SP is the paleosalinity (‰); and C
1 and C
2 are constants. According to the relationship between B content of kaolinite in modern sedimentary water and salinity, C
1 and C
2 are 1.28 and 0.11, respectively [
57].
According to (1) and (2), the paleosalinity of the clay conglomerate sedimentary water body in MYJ Fm. of MZK-3 well vary from 10‰ to 92‰ (average = 30‰, standard deviation = 23‰), which is similar to the salinity of modern seawater (
Table 6). Among them, the paleosalinity of upper salt clay conglomerate sedimentary water body ranges from 12‰ to 38‰ (average = 21‰, standard deviation = 12‰); The paleosalinity of lower salt clay conglomerate sedimentary water body ranges from 10‰ to 92‰ (average = 35‰, standard deviation = 27‰). The numerical characteristics show that (1) the paleosalinity of the upper part of the salt is lower than that of the lower part of the salt, which reflects the gradual desalination process of the water body in the clastic rock sedimentary environment; (2) in the lower part of salt clay conglomerates deposition period, the paleosalinity of the water body in the sedimentary environment is relatively high, but it fluctuates violently, and the lowest values of Sr/Cu appear, which is consistent with the occurrence of gray-green siltstone intercalation and is obviously affected by the warm and humid climate in individual periods; and (3) the salinity of sedimentary water does not reach the stage of salting out in both the upper member of salt and the lower member of salt clay conglomerates.
The main reason for the difference of salinity between upper member of salt and lower member of salt clay conglomerates may be paleoclimatic conditions. Under the background of arid and hot paleoclimate, the salinity of catchment basin mainly comes from recharge water body, and the salinity depends on the type and quantity of recharge water. If the supply of hydrothermal brine and salt spring water is not considered, only atmospheric precipitation can supply, then the strong rainfall can increase the debris flow into the basin, the size of debris at the same location, and the total amount of salt dissolved in the water body. The salinity of this kind of water body is lower in the initial formation period, but since the large amount of salt dissolved in the water, the salinity of the sedimentary environment after evaporation and concentration is higher than that in the period of less precipitation supply. The paleoclimate background of the above salt and lower part of salt clay conglomerate sedimentary period is exactly like this: in the upper part of the salt clay conglomerates’ sedimentary period, the paleoclimate was dry and hot, the salt supply in the catchment basin was relatively less, and the salinity of the sedimentary environment was relatively low; in the lower part of the salt clay conglomerates’ deposition period, the paleoclimate was dry and hot as a whole, and only a few periods of atmospheric precipitation supplied more salt, which led to the increase of salt supply in the catchment basin, and the water body sank after evaporation and concentration. The salinity of the sedimentary environment is relatively high.
The characteristics of paleosalinity in the sedimentary period of clay conglomerates in MYJ Fm. indicate that the contact relationship between the salt rock and the clay conglomerates is transilient, and the sequence of evaporation between them has not formed. It can be interpreted as the clay conglomerates not being clastic sediment in the critical brine (A critical state that before evaporate into halite.) based on the paleosalinity recovery and mineral combination characteristics, which show that the paleosalinity of the sedimentary environment of the clay conglomerates does not reach the stage of evaporate into halite. The critical brine were not concentrated in the original evaporation basin after the clay conglomerate deposition, because there is no carbonate rock and gypsum on the clay conglomerate, and they should be the salt that precipitates before the seawater concentrates to the critical brine.
A reasonable explanation for the genesis of the potash deposits in the clastic rocks of MYJ Fm. is the metallogenic model of “deep source and shallow mineralization” [
18,
58]. The core idea of this theory is that the potash deposit in Simao Basin was formed by the diapir of the Middle-Jurassic Hepingxiang Fm. source salt compressed by tectonic activity to MYJ Fm. This genetic model can not only reasonably explain the objective phenomena—such as the source of salt being seawater, the salt-related clastic rocks in continental environment, the lack of carbonate and sulfate rocks in the corresponding scale, and the special structural morphology of salt bodies—but also reasonably explain the abrupt contact between salt rock and clastic rock without the transition of the evaporation sequence and the salinity difference in the salt-related clastic rock sedimentary system. Another reasonable explanation is that the concentrated brine which has reached the stage of precipitating halite evaporates and concentrates in the basin, but the source of this brine is still difficult to trace.