Sequence-stratigraphic dynamics: Variations of genetic stratigraphic units driven by basin subsidence
Introduction
The correlation between stratal stacking patterns and cyclic changes at the base level is a fundamental theme of sequence stratigraphy (e.g., Vail et al., 1977a; Haq et al., 1987; Mitchum and Van Wagoner, 1991; Catuneanu et al., 2009). The results of previous studies showed that the changes in the stratal stacking patterns and their associated sedimentary facies are mainly controlled by periodic sea level changes, tectonic uplift and subsidence, and changes in the sediment supply rates (e.g., Sloss, 1962; Vail et al., 1977b; Brown and Fisher, 1978; Haq et al., 1987; Mitchum and Van Wagoner, 1991; Posamentier and Allen, 1999; Eriksson et al., 2013; Murray-Wallace and Woodroffe, 2014; Miall, 2016; Catuneanu, 2019). The effects of these three factors on the sequence-stratigraphic architecture are primarily manifested in the following three ways. First, when the basin subsidence and the sediment rates are normal, a full base-level variation cycle can be classified into four phases: rapidly rising, highstand/slowly rising, falling, and lowstand/slowly rising. These intervals correspond to four genetic units (Catuneanu et al., 2009): transgressive (T), highstand normal regressive (HNR), forced regressive (FR), and lowstand normal regressive (LNR). Each genetic stratigraphic unit represents specific stratal stacking patterns and its associated sedimentary facies formed within a specific period of time. Second, when the basin subsidence rate increases rapidly or the sediment supply rate decreases rapidly, transgression is exaggerated while the regression decreases (Mitchum and Van Wagoner, 1991). The types of genetic units in each sequence are then reduced from four to three (Haq et al., 1987) or even two (Mei, 1996; Table 1). Third, when the basin subsidence rate is particularly high but the sediment rate is normal, the relative sea level (RSL) change of the entire third-order cycle increases rapidly, and the strata are stacked in a pattern of retrogradation, resulting in an indistinguishable sedimentary sequence (Wang and Shi, 1998). In contrast, when the crust is uplifted on a large scale, regression is exaggerated, and transgression decreases with the increasing uplift rate. In this case, the four genetic units of the sequence become FR units (Csato and Catuneanu, 2012).
In the above-mentioned three sequence-stratigraphic architectural patterns, the changes in the stratal stacking pattern can be classified into two main categories. The first category contains four genetic units resulting from the eustatic sea-level control of the third-order cycle itself. The genetic units in the second category vary due to rapid basin subsidence or uplift or a significant reduction or increase in the sediment supply rate. Eustasy can be considered to be an internal driving force of change in the sequence-stratigraphic architecture, where basin subsidence and sediment supply are external forces. Variations in the type of the genetic unit are driven by external forces and may differ as follows: (1) at least two driving modes: either dominated by basin subsidence or sediment supply; (2) two variations in direction: all four genetic units change into a transgressive unit (normal variation) or a forced regressive unit (reverse variation); and (3) multiple orders of variation. For example, as the subsidence rate of the basin continues to increase, the FR unit first changes to HNR and then to T. Therefore, the variations in genetic units are significantly complex problems that are difficult to understand.
In previous studies, the conceptual model of “relative sea level change = eustatic sea level change + subsidence” (Barrell, 1917; Van Wagoner et al., 1990) was used to define the relationship between the sea level change pattern and systems tract (Haq et al., 1987; Mei and Yang, 2000; Posamentier and Allen, 1999; Catuneanu et al., 2009). Stratal stacking patterns were identified by using the ratio of the sediment supply rate to the rate of accommodation (S/A ratio; Van Wagoner et al., 1988, Van Wagoner et al., 1990; Matenco and Haq, 2020) and the concept of “positive or negative accommodation space” (Catuneanu, 2002; Csato and Catuneanu, 2012) as the criteria. Although these concepts have been widely used for the past 40 years, the relevant models have been mostly qualitative and discussions focused on normal variations but rarely on reverse variations. The Type 4 sequence (Table 1) was proposed more than 20 years ago; however, no real case has been reported. In recent years, Csato and Catuneanu, 2012, Csato and Catuneanu, 2014 carried out mathematical simulations to study on variations in the stratigraphic architecture and provided valuable insights. For example, under the influence of abrupt events, such as plate tectonic movement or global climate change, the genetic units change in both positive and negative directions, and eight potential combinations of genetic units are possible (Csato and Catuneanu, 2012). However, these simulations were based on literature data and primarily focused on fourth-order cycles (~0.1 million years (Myr), aiming at understanding the last glacial period and other abrupt climate change events based on the changes in the stratigraphic architecture (Goodwin and Anderson, 1985; Csato and Catuneanu, 2014; Gong et al., 2018). Basic problems of the dynamics of the sequence-stratigraphic architecture at the third-order change scale have not been studied and numerous questions remain to be answered: (1) Are there other types of sequences corresponding to genetic unit combinations in nature, in addition to Types 1, 2, and 3? If so, what are their internal stratigraphic configurations? (2) How do the three forces drive the variation in the stratigraphic architecture? (3) What are the thresholds of the external driving forces of three levels of variations in genetic units? Thus, the study of such dynamics within the stratal architecture is still in its early stages.
In this study, we first introduce the concept of sequence-stratigraphic dynamics, including the dynamic equation of sequence-stratigraphic structural change, prototype sequence-stratigraphic model, and possible pathways of genetic unit change and underlying mechanisms. Subsequently, normal variations of the genetic units driven by basin subsidence are investigated using mathematical simulations and a case study based on the succession of genetic unit combinations and the dynamic parameters of eleven Eocene sequences in the northern part of the South China Sea. The third-order eustatic sea level change wavelet and the threshold of the external driving force due to mutations in the genetic unit combination are extracted and the validity of the dynamic equations and thresholds is verified using example parameters. Finally, the limitation of the dynamic equation, differences between old conceptual models and new dynamic equations, and geological significance of sequence-stratigraphic dynamics are discussed. The main purpose of our effort was to extend the study of sequence-stratigraphic architectural changes from a qualitative to a quantitative scope, thereby hopefully advancing the discipline.
Section snippets
Data
Several rapid basin subsidence and regional tectonic uplift events occurred in the Baiyun Sag in the northern South China Sea (see geological setting in Supplementary Data) during the Eocene (Li, 1993; Pang et al., 2007; Zhao et al., 2015). The Antarctic ice sheet formed at the end of the Eocene (Zachos et al., 2001), allowing for sedimentary strata in the study area to record various successions of genetic unit variations, including several indistinguishable sequences with only one type of
Methods
Dynamic equations for the variations in genetic units were established by referring to the Barrel diagram calculation of third-order cyclic RSL change (Barrell, 1917; Mitchum and Van Wagoner, 1991; Miall, 2016; Catuneanu, 2019), that is, based on the principle that sequence-stratigraphic structural changes are driven by a combination of three forces. The prototype and variation sequences were then defined using the dynamic equations and the sequence-stratigraphic model of the prototype sequence
Sequence stratigraphy dynamics equations
The change in the sequence- stratigraphic architecture is driven by the combined effect of cyclic sea-level change, generalized basin subsidence, and sediment supply. This basic principle can be expressed with the following set of equations:where △A(t) is the function of the residual accommodation space that changes with time; W(t) is the sea level change of third-order wavelet, which
Discussion
The above-mentioned case study and mathematical simulation verify the sequence stratigraphic dynamics hypothesis. Thus, the basic framework of the theory of sequence stratigraphic dynamics can be constructed. Subsequently, we will discuss the applicability of sequence stratigraphic dynamics and its scientific significance based on the analysis of the validity of the genetic unit change threshold, parameter error, and difference between mathematical equations and existing mathematical models.
Conclusions
- (1)
In third-order sequences, variations in the stratal stacking pattern are driven by three forces: third-order cyclic sea level change, overall basin subsidence, and sediment supply. These three forces cannot be ignored, nor can they be considered to be zero. The two old conceptual models, that is, “sea level change + basement subsidence” and “S/A ratio,” are not detailed enough because they do not consider the simultaneous sediment supply component in the calculation of the accommodation space.
Declaration of Competing Interest
This manuscript has not been published or presented elsewhere in part or in entirety and is not under consideration by another journal. We have read and understood your journal's policies, and we believe that neither the manuscript nor the study violates any of these. There are no conflicts of interest to declare.
Acknowledgment
This study was funded by the China National Science and Technology Major Project (No. 2016ZX05025-003). We thank the Shenzhen Branch of China National Offshore Oil Corporation for allowing us to use the relevant drilling and seismic data and the results of seismic stratigraphic correlation of the Eocene in the Baiyun Sag. After the first draft of this paper was completed, Professor O. Catuneanu reviewed it in early 2018, which not only emphasized the importance of evidence on the structure of
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