内陆湖盆“深水成盐”形成条件和识别标志:以东濮凹陷与现代盐湖为例

doi:10.13745/j.esf.sf.2020.5.4

摘要/Abstract

摘要：

位于我国东部的许多新生代盆地,在地质历史时期发育了巨厚的岩盐沉积。尽管这些盆地的成盐模式研究已经历了40多年,但由于缺乏“深水成盐”的现代沉积模式,到底是“深盆-深水”还是“深盆-浅水”成盐,一直难以定论。为了理解“深水成盐”的控制因素,很多现代盐湖开展了水文学和水化学调查,然而这些湖盆的矿物组合各不相同,“深水成盐”的形成条件和控制因素尚不清楚。本文对死海、佛瑞湖等“深水成盐”的现代盐湖以及我国东濮凹陷沙三段的成盐特征进行了解剖,从盐度、水深、湖平面波动以及卤水分层等角度探讨了“深水成盐”的形成条件,对比分析了“深水成盐”的相对位置以及沉积物特征,总结了“深水成盐”的识别标志。研究表明,盐岩层系在岩心和测井资料上显示出多尺度的旋回性。“深水成盐”为水深较大的洼陷中心成盐,与盆缘的“浅水成盐”同盆共存。沉积相对岩盐层系的结构和组成有明显的控制作用,其中洼陷带的“深水成盐”,主要在洼陷带的卤水-湖底沉积物界面附近,析出和增生粗晶的盐类矿物,常与富含有机质和黄铁矿的暗色泥岩共存;而湖滨附近的“浅水成盐”,单层厚度薄,结晶粒度细小,通常含有较多的陆源碎屑。死海为代表的现代盐湖以及东濮凹陷等古代成盐盆地的沉积特征表明,“深水成盐”发生于湖平面的下降期,而且卤水剖面的厚度对蒸发岩的形成分布有明显的控制作用,同时具有“深水”和“深盆”特征的内陆盐湖更容易形成单层厚度大、横向稳定的岩盐沉积。本研究有助于改变人们以往对内陆湖盆成盐机理的认知,尤其是现代盐湖的卤水分层析盐特征,对解读地质历史中的其他成盐事件具有重要启示。基于现代沉积实例的“深水成盐”识别标志,可以为古代岩盐沉积模式的建立提供限定条件。

关键词: 深盆, 深水成盐, 洼陷中心成盐, 卤水分层, 形成条件, 东濮凹陷, 死海

Abstract:

Eastern China holds many Cenozoic basins where saline giants developed episodically throughout geologic time. Yet, in part, because modern analogous for ancient halitic depositional environments are lacking, in terms of both size and mineralogy, the stratigraphy of evaporite deposition in eastern China, especially whether the depositional environments were “deep basin-deep water” or “deep basin-shallow water”, remains controversial after more than 40 years of research. In order to better understand the factors controlling the formations of ancient evaporite deposits, previous researchers have investigated the hydrology and hydrochemistry of many modern salt lakes with different mineral assemblages, but the forming conditions and indicators for deep-water evaporite deposits are still unclear. In this paper, we compared the salt-forming characteristics of modern salt lakes that possess deep-water evaporite deposits, such as the Dead Sea and Freefight Lake, with that of the Dongpu Sag in eastern China, and discussed the salt-forming conditions, including salinity, water depth, lake level fluctuation and brine stratification. In addition, we analyzed the fabrics in and distributions of the deep-water evaporite sediments and summarized the indicators for deep-water salt deposition. Studies have shown that multistage cyclicity in halite sequence core and log data can be used for salt rock identification. The thickness of the brine profile obviously controls the formation and distribution of evaporites, as thick single layered homogenous salt deposit with lateral stability is more likely to form in the inland “deep basin-deep water” type of salt lakes, such as the Dead Sea and Dongpu Sag. The deep-water evaporite deposits around the depocenter coexist with the sallow-water salt deposits along the lakeshore, and they are all formed by brine concentration as lake level recedes. The rock salt texture is closely related to the sedimentary facies. The coarse grained evaporite minerals mainly precipitate and aggregate at the brine-lakefloor sediment interface in the deep-water region, and are often interbedded with black shales rich in organic matter and pyrite. The shallow-water salt deposits along the lakeshore usually contain more finer grained terrigenous clastics. The evaporite deposits in both modern salt lakes, such as the Dead Sea, and ancient evaporite basins, like the Dongpu Sag, show that declining lake level and thicker brine profile promoted the “deep basin-deep water” type halite deposition. This study revised the current understanding of the mechanisms of salt deposition in inland lake, especially the brine stratification phenomenon in modern salt lakes. It has many implications for other evaporitic events in the geologic record. The indicators for “deep-water evaporite deposition” on the basis of modern salt lakes can provide constraints in building the depositional model of ancient rock salt.

Key words: deep-basin, deep-water evaporite deposition, salt deposited in the depression center, brine stratification, forming conditions, Dongpu Sag, Dead Sea

中图分类号:

禚喜准, 郑旭, 陈骁帅, 徐田武, 崔建军. 内陆湖盆“深水成盐”形成条件和识别标志:以东濮凹陷与现代盐湖为例[J]. 地学前缘, 2021, 28(1): 43-59.

ZHUO Xizhun, ZHENG Xu, CHEN Xiaoshuai, XU Tianwu, CUI Jianjun. Forming conditions and indicators for deep-water evaporite deposits in inland lake basins: A case study of the Dongpu Sag and modern salt lakes[J]. Earth Science Frontiers, 2021, 28(1): 43-59.

图/表 11

图1 海相与陆相深盆的几何特征及岩盐的分布模式 a—海相“深水成盐说”,SL—海平面,H—海平面到海盆底面的深度,具有深水的局限深盆特征(H>200 m);b—陆相“高山深盆说”,LL—湖平面,H—母岩侵蚀区与湖盆洼陷带底面的海拔高差(H>1 000 m),为湖水很浅(小于10 m)的深盆,水体较浅的洼陷中心成盐。(图件a据参考文献[1],图件b为本文作者据柴达木盆地现代盐湖抽象而成)

Fig.1 (a) Geometric characteristics of marine and continental deep basins (adapted from [1]). (b) Schematic distribution pattern of rock salt (created based on the modern inland salt lakes in the Qaidam Basin)

图2 东濮凹陷沙河街组沙三中亚段盐岩平面分布特征 a—东濮凹陷沙河街组沉积期的前第三系古地质图特征,周围的母岩为古生代寒武系—奥陶系碳酸盐岩地层和石炭系—二叠系的碎屑岩地层,局部为含盐的潟湖沉积,可能为东濮凹陷的岩盐沉积提供了母质来源;b—沙三中地层厚度与盐岩沉积厚度的关系,即岩盐的最大厚度处位于富有机质暗色泥岩发育的湖盆洼陷中心,并非盆地的沉积中心。

Fig.2 Planar distribution characteristics of evaporates in central member 3, Shahejie Formation, Dongpu Sag

图3 东濮凹陷沙河街组沙三中3-沙三中5层段盐岩沉积特征(剖面位置见图2b) a—洼陷带卫79井、濮83井和卫69井在沙三中3和沙三中5的岩性组合横向变化对比,反映蒸发岩系与细粒沉积共存。b—蒸发岩系内部的泥岩夹层的薄片特征:b1,×100(-),显示了泥岩的纹层发育,并且发育大量黄铁矿,碎屑颗粒含量较少;b2,×100(+);b3显示荧光特征,纹层具有明显的黄色荧光,反映该页岩富含有机质。c2—c1素描图,显示盐岩内部发育薄层状的油页岩,页岩与盐岩呈竹节状。d—图3a中卫69井盐岩岩心特征,说明尽管蒸发岩系累积厚度较大,但盐岩的单层厚度小于1 m,盐岩沉积并不连续,存在泥页岩夹层。

Fig.3 Vertical distribution and textural characteristics of sediments in central member 3, Shahejie Formation, Dongpu Sag(Profile location see Fig.2b)

图4 现代青海尕斯库勒盐湖的浅水成盐特征 a,b—湖平面下降过程中滨岸带的盐类析出特征(滨湖缓坡带:A1、A2、A3、A4为卤水过饱和之后逐渐降低的各个时期湖平面,A5为现今的湖平面;滨湖陡坡带:B1、B2、B3、B4为卤水过饱和之后逐渐降低的各个时期湖平面,B5为现今的湖平面);c—盐湖滨湖环境的沉积物,岩盐单层厚度小于10 cm,与陆源碎屑沉积物共生;d—c图I区的局部放大图像,显示滨湖岩盐为细粒结构;e—湖盆边缘的薄层片状岩盐,与砾石等粗碎屑共存;f—湖盆边缘的薄层片状岩盐与滨岸砾石共存,向湖盆边缘,岩盐厚度逐渐减小。

Fig.4 Distribution and textural characteristics of sediments in the modern Gasikule Salt Lake, Qinghai

表1 “深水成盐”湖盆的地貌学、水化学和沉积学特征[10-11,24-27]

Table 1 Geomorphological, hydrochemical and sedimentalogic characteristics of deep- water salt deposition in lake basins. Adapted from [10-11,24-27].

湖泊名称	流域面积/ 湖区面积	平均水深/ 最大水深	表层盐度/ 恒温层盐度	析盐期的湖平面变化	卤水分层特征	卤水离子组成 (阴阳离子分别按含量顺序)	矿物组合	晶体结构特征
死海/ Dead Sea	40 000/645	300/320	①236.1/236.4 ②238.8/236.5	持续下降 (每年约1 m)	季节性分层	Na⁺,Mg²⁺,Ca²⁺, K⁺;Cl^-、Br^-	石盐	自形的粗粒晶体
佛瑞湖/ Freefight Lake	55.2/2.59	14.1/24.9 (1985年)	111.7/179.1	湖平面四季波动; 湖平面显著下降 (1956—1989年)	全年分层 (1984— 1987)	Na⁺,Mg²⁺,K⁺, Ca²⁺;S $O 4 2 -$ , Cl^-,OH^-	白钠镁矾、芒硝、泻利盐、石膏	自形的粗粒晶体
戴德湖/ Deadmoose Lake	70.19/10.5	9.9/48.1	34/85	湖平面四季波动; 湖平面显著下降 (1944—1974年)	全年分层 (1984— 1987)	Na⁺,Mg²⁺,K⁺, Ca²⁺;S $O 4 2 -$ , Cl^-,OH^-	芒硝、少量石膏	自形的粗粒晶体
利特湖/Little Manitou Lake	169/13.9	4.0/5.9	67/192	湖平面四季波动; 湖平面显著下降 (1944—1989年)	间隔性分层	Mg²⁺,Na⁺,K⁺, Ca²⁺;S $O 4 2 -$ , Cl^-,OH^-	泻利盐、芒硝、白钠镁矾、石膏和石盐	自形的粗粒晶体

表1 “深水成盐”湖盆的地貌学、水化学和沉积学特征[10-11,24-27]

Table 1 Geomorphological, hydrochemical and sedimentalogic characteristics of deep- water salt deposition in lake basins. Adapted from [10-11,24-27].

湖泊名称	流域面积/ 湖区面积	平均水深/ 最大水深	表层盐度/ 恒温层盐度	析盐期的湖平面变化	卤水分层特征	卤水离子组成 (阴阳离子分别按含量顺序)	矿物组合	晶体结构特征
死海/ Dead Sea	40 000/645	300/320	①236.1/236.4 ②238.8/236.5	持续下降 (每年约1 m)	季节性分层	Na⁺,Mg²⁺,Ca²⁺, K⁺;Cl^-、Br^-	石盐	自形的粗粒晶体
佛瑞湖/ Freefight Lake	55.2/2.59	14.1/24.9 (1985年)	111.7/179.1	湖平面四季波动; 湖平面显著下降 (1956—1989年)	全年分层 (1984— 1987)	Na⁺,Mg²⁺,K⁺, Ca²⁺;S $O 4 2 -$ , Cl^-,OH^-	白钠镁矾、芒硝、泻利盐、石膏	自形的粗粒晶体
戴德湖/ Deadmoose Lake	70.19/10.5	9.9/48.1	34/85	湖平面四季波动; 湖平面显著下降 (1944—1974年)	全年分层 (1984— 1987)	Na⁺,Mg²⁺,K⁺, Ca²⁺;S $O 4 2 -$ , Cl^-,OH^-	芒硝、少量石膏	自形的粗粒晶体
利特湖/Little Manitou Lake	169/13.9	4.0/5.9	67/192	湖平面四季波动; 湖平面显著下降 (1944—1989年)	间隔性分层	Mg²⁺,Na⁺,K⁺, Ca²⁺;S $O 4 2 -$ , Cl^-,OH^-	泻利盐、芒硝、白钠镁矾、石膏和石盐	自形的粗粒晶体

图5 死海的湖平面波动和卤水密度变化曲线(据文献[11,33-35]修改)

Fig.5 Lake level and brine density changes in the Dead Sea. Modified from [11,33-35].

图6 死海、佛瑞湖、戴德湖不同季节的温度和盐度分层特征 a,b—死海(3月、6月、9月份)的温度和盐度分层特征,为了制图方便,50~300 m深处的纵向比例缩小;c,d—佛瑞湖(1985年8月份)、戴德湖(1972年8月22日、1980年3月19日)的温度和盐度分层特征。(图a死海数据来自参考文献[33],图b佛瑞湖数据来自参考文献[24],戴德湖数据来自文献[26,37])

Fig.6 Characteristics of seasonal temperature and salinity of different layers in the Dead Sea (data adapted from [33]), Freefight Lake (data adapted from [24]) and Deadmoose Lake (data adapted from [26,37])

图7 “浅水成盐”与“深水成盐”在“深水深盆”类盆地中的时空分布关系 a—岩盐未饱和的咸水湖泊,仅湖盆边缘的局限水域存在浅水成盐,LL代表湖平面;b—岩盐过饱和的盐湖,湖盆边缘的“浅水成盐”与洼陷带的“深水成盐”同盆共存。

Fig.7 Relationship between the temporospatial distributions of shallow- and deep-water salt depositions in the “deep-water deep-basin” type basin

表2 青藏高原的咸水湖泊的地貌学、水化学特征[40,41,42]

Table 2 Geomorphological and hydrochemical characteristics of saltwater lakes in the Qinghai-Tibet Plateau (adapted from [40,41,42])

湖泊名称	流域面积/ 湖区面积	最大水深/m	盐度/ (kg·m^-3)	表层电导率/ (μS·cm^-1)	恒温层电导率/ (μS·cm^-1)
纳木错湖	10 610/1 980	99	1.780	1 872	1 852
当惹雍错湖	8 185/835.3	214.48	未测	12 900	12 600
扎日南木错湖	15 452/996.9	71.55	未测	18 500	18 000

图8 我国青藏高原几个淡水—咸水湖泊的水化学剖面特征[40,41,42] a-e—扎日南木错湖、当惹雍错湖、纳木错湖(2006年9月19日)、纳木错湖(2007年8月18日)的温度、pH值、电导率、环境光量和溶解氧量剖面特征。c—用双坐标表达电导率数据,顶部坐标为扎日南木错湖、当惹雍错湖电导率刻度,底部坐标为纳木错湖(2006年9月19日、2007年8月18日)的刻度值;e—用双坐标表达溶解氧量数据,顶部坐标为扎日南木错湖、当惹雍错湖溶解氧量刻度,底部坐标为纳木错湖(2006年9月19日、2007年8月18日)溶解氧量的刻度值。

Fig.8 Hydrochemical profile characteristics of representative freshwater-saltwater lakes in the Qinghai-Tibet Plateau. Adapted from [40,41,42].

图9 “浅水”与“深水”湖盆的岩盐分布和结晶特征 a,b—两盐湖为岩盐过饱和卤水,LL代表湖平面,其中a为缓坡浅水型,b为陡坡深水型。

Fig.9 Distribution and crystalline properties of rock salt in deep-water and shallow-water lake basins (LL—lake level)

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