Abstract These notes are designed to introduce the concepts and techniques of quantitative modeling of basin subsidence histories. The notes also describe some of the methods and results of modeling the development of sedimentary sequences generated by the interaction of subsidence, sediment supply, and sea-level changes. Analyzing and modeling basin subsidence can be a powerful tool for understanding how, when, and why basins form and, thus, compliments other basin analysis techniques. Subsidence analysis is useful for understanding regional tectonic history, history of sea-level changes, and the thermal history of basins that, in turn, are essential for diagenetic and hydrocarbon studies, and it also provides a basis for interpreting lithospheric structure and mechanics. In addition, quantitative models that take into account the competing effects of varying sediment supply rates, subsidence rates, and rates of sea-level change provide insights into the relative importance of tectonics versus eustatics in the generation of the basin-filling sequences. Comparison of the observed stratigraphy to that predicted by synthetic stratigraphic models of basin sequences can be used to help determine whether or not, and to what degree, basin deposition is controlled by tectonics or eustatics. This course concentrates on the theory and application of subsidence and stratigraphic modeling by working through specific examples from real or artificial basin sequences. By the end of the course, you should be able to apply some of the basin-modeling techniques to your own studies and have a strong enough introduction to other techniques that you can understand most of the available literature.

Abstract The application of Archimedes' principle to the earth suggests that continents are buoyed up by a force equal to the weight of the displaced mantle (Turcotte and Schubert, 1982). Adjacent blocks of different thickness and/or density structure will have different relative relief (Fig. 2.1). Typical lithospheric structure beneath the continents and the oceans are shown in Figure 2.2, these values will be used in most of our discussions. Below some depth, there is no density contrast between the two adjacent columns, and asthenosphere of equal density underlies both columns (Fig. 2.2). The weight of the columns above this depth of compensation must be equal. In this model of isostasy, we can calculate the relative relief between two adjacent continental columns of differing density structure (Fig. 2.3):

Abstract The goal of geohistory analysis is to produce a graphical representation of the vertical movement of a stratigraphic horizon in a sedimentary basin as an indicator of subsidence and uplift history in the basin since the horizon was deposited (Van Hinte, 1978; Fig. 3.1). Several types of stratigraphic data are needed to do a geohistory or subsidence analysis. These data include a stratigraphic column showing the present-day thickness of the stratigraphic units, types of lithologies, ages of horizons, and estimated paleowater depths. Other types of data that are useful, although not necessary, are porosity information for the units and thermal information, if your goal is to determine thermal history of the basin. In addition, there are several assumptions and uncertainties that are built into this analysis. Most of these problems can be overcome if thick stratigraphic sections of relatively shallow-water deposits are used and only long-term, large-scale changes are studied.

Abstract In an earlier section (Chapter. 2, part C), we discussed an isostatic balance for a section of rifted lithosphere. It was noted that the long-term subsidence of the rift was associated with cooling and thickening of the lithosphere. Cooling causes the lithospheric rock to become more dense. To a first approximation, the density of the mantle lithosphere (ρL) varies with temperature ( T ) according to: If the asthenosphere is nothing more than hot (T 1 = 1333 °C) mantle lithosphere, then we can calculate a density of 3184. kg/m 3 for the asthenosphere using Equation. 4.1. This suggests a density difference of only 4.4% between the coldest mantle lithosphere and the asthenosphere, but integrated over the entire thickness of the lithosphere temperature differences make a substantial contribution to the overall isostatic balance. In the following sections we will discuss a thermal model developed by McKenzie (1978) to describe the subsidence history of rift basins.

Abstract First, we need to go back and look at simple isostatic balances (discussed in Chapter. 2, part A) in a slightly different way. Consider the two columns shown in Figure 5.1. The column on the left is a reference column, and the column on the right shows the same crustal section thickened now by a factor of MP, where 0 < p < 1. The base of the thickened crust is deflected into the mantle by an amount w due to the weight of the added crust. A local isostatic balance requires that: This result says that the weight of the mountain belt (including the portion that lies below datum) is balanced by a buoyancy force from the mantle. This is a local isostatic balance in that the deflection of the crust at any location depends only on the local amount of crustal thickening at that location. One important shortcoming of the local isostatic balance is that it neglects the lateral strength of the lithosphere. A more realistic (and highly successful) model assumes that the lithosphere responds to loads like an elastic plate overlying an inviscid fluid. The elastic plate corresponds to some poorly defined, colder portion of the thermal lithosphere, whereas the inviscid fluid corresponds to a hotter portion of the lithosphere and the asthenosphere. You should not forget that the elastic plate model is just an extension of the local isostatic compensation model.

Abstract Having discussed the primary mechanisms of subsidence we can briefly focus on the plate tectonic settings of major sedimentary basins and examine their typical subsidence histories and mechanisms (Fig. 6.1). Much has been written about the driving mechanisms of basin formation in most tectonic settings. An early overview was provided by Dickinson (1976). The purpose of this chapter is not to attempt to summarize the state of knowledge of basin evolution. Instead, we simply define each basin type, following the basin classification scheme of Dickinson (1976), and focus on a few key points regarding the mechanisms of basin evolution. Due to space limitations we do not cite all of the relevant literature but provide just a few key references. The subsidence curves (Fig. 6.1) come primarily from the published literature, augmented by analyses done by ourselves or by students in our sedimentary basins course at the University of Wyoming. All the curves have been backstripped following the local-isostatic method described above (Steckler and Watts, 1978). However, inconsistencies arise from the use of different time scales, compaction corrections and paleowater depth estimates made by the various authors. Nonetheless, the overall consistency of the subsidence curves in each of the various tectonic settings suggests that use of different scales by different workers do not generate errors large enough to mask the overall trends.

Abstract Karl Popper once wrote, “Out of theories we create a world: not the real world, but our own nets in which we try to catch the real world.” One of the more exciting applications of quantitative analysis of basin development is its use as a tool in determining the relative importance of tectonic, eustatic and climatic effects on the development of basinal stratigraphy. The goal in modeling the development of basinal stratigraphy is not to try to explain every detail observed in the real world, but instead serves as a framework from which we can view the real world. As such, simplified basin-filling models can guide us to look for those critical field relations that can be used to distinguish between the fundamental factors governing the formation of the basinal stratigraphy. Geologists tend to think of certain factors as the primary controls on the development of basin fills, including: source area uplift rate, source area lithology, rain fall, temperature regime, sea level changes, and basin subsidence rates. In contrast, basin models can address only the most basic controls, such as: basin subsidence, eustatics, volume of sediment supply (flux), sorting of the sediment supply, and rates of sediment transport. The problem is that the relationship between the geologists' factors and the modeling parameters are far from straight forward. For example, changes in rainfall might affect sorting, flux and transportability of the sediment supplied to a basin. Changes in source area lithology can affect all of the same modeling factors. Because of

Abstract One of the more long-lived debates in the earth sciences concerns the origin of sedimentary sequences observed on continental margins (Barrell, 1917; Pitman, 1978; Seuss, 1906; Sloss, 1962; Vail et al., 1977; Vella, 1965; Watts, 1982). As defined by Vail et al. (1977), these sequences are packages of conformable sediments, representing a time span of 1 to 10 My, that are bounded by unconformities or horizons that can be correlated with unconformities. Strata within individual sequences show continuous onlap onto the continental margin, whereas sequence boundaries mark abrupt seaward shifts in coastal onlap. Originally, Vail et al. (1977) argued that onlapping sediments record a marine transgression, whereas sequence boundaries signal an abrupt regression. Subsequently, more detailed studies of a small number of sequences (Vail et al., 1984) showed that onlapping sediments in the upper parts are nonmarine, suggesting that regressions are gradual rather than abrupt. Based on the apparent global synchroneity of many sequence boundaries, Vail et al. (1977) and Haq et al. (1987) propose that transgressions and regressions are caused by oscillatory eustatic variations. Some workers (Hallam, 1984; Miall, 1986) question whether or not the resolution of biostratigraphic correlations is sufficient to show that sequence boundaries are truly synchronous between basins. A fundamental difficulty in understanding the origin of sedimentary sequences is that transgressions and regressions can be caused by changes