Sea Level Change and Cyclicity

Eustasy, or global sea level, is the motion of the sea surface relative to a fixed

datum, such as the center of the earth (Posamentier et al., 1988). Evidence for sea level change within a sedimentary basin does not necessarily indicate eustatic change, so the term ‘relative sea level’ is used instead. Relative sea level refers to movement of the sea

surface in relation to a local datum, such as a buried stratigraphic surface, or the sea floor (Fig. 2.15; Posamentier et al., 1988). The relative change of sea level within a

sedimentary basin depends on the creation or removal of accommodation within the basin, which is in turn controlled by tectonic and eustatic mechanisms (Galloway, 1989; Plint et al., 1992).

Figure 2.15 Relationship between eustatic sea level and relative sea level. From Allen and Allen, 2005.

48

Sedimentary processes tend to be cyclical in nature, with depositional patterns recurring on many temporal and spatial scales (Helland-Hansen and Gjelberg, 1994). Some of these depositional patterns are observed to occur on a global scale, and suggest

eustasy as the main driving mechanism (e.g. Haq et al., 1988). Eustatic change is

primarily caused by changes in the volume of water in ocean basins (Miller et al., 2005). Factors controlling water volume variations are attributed to geological processes which occur on many time scales (Fig. 2.16).

Cycles of sea level change were originally categorized in terms of their

magnitude. Vail et al (1977), on the basis of seismic data, initially defined three orders of eustatic cycles, with fourth and fifth order cycles later recognized from well logs, cores and outcrops (e.g. Mitchum and Van Wagoner, 1990). More recently however, it has been shown that these ‘orders’ are often arbitrary and highly variable. Instead,

sedimentary processes are now described in terms of their episodicity (Miall, 2010). This episodicity is summarized below:

1) Sedimentary cycles that occur on time scales of 400-500 million years are referred to as global supercontinent cycles. A supercontinent cycle is

generated as a result of the assembly of continents and their subsequent rifting and dispersal. The assembly of a supercontinent inhibits radiogenic heat loss from the core and mantle, creating a thermal blanket underneath the

supercontinent and causing epeirogenic uplift over time. This may result in relative sea level fall on continental margins. There is still however,

uncertainty in the long term mechanism behind the cycle (Miall, 2010). 2) Sedimentary cycles that span 10-100 million years are caused by oceanic

spreading centers and plate movement (Plint et al., 1992; Miall, 2010). The changing volume of oceanic ridges is affected by the change in spreading rates that are controlled by mantle processes. Plate kinematics cause subtle yet noticeable movement in basement structures due to thermal changes in the crust and mantle, sediment loading, and/or crustal thickening and thinning (Miall, 2010).

3) Sedimentary cycles that operate on time scales of 0.1 to 10 million years are controlled by regional to local basement movement. The movements are

Figure 2.16 Timing and amplitude relationships for different geological mechanisms and their effect on eustatic change. SF- Sea Floor; Cont- Continental. From Miller et al., 2005.

50

controlled by regional plate kinematics, including changes in the intraplate stress regime. Such stress changes could lead to movement of crustal structures, such as the Vulcan Low and Medicine Hat Block described in section 2.4 (Miall, 2010).

4) Eustatic cycles that operate on timescales of 0.01 to 0.5 million years are recorded by sedimentary successions deposited in both nearshore and pelagic environments, and suggest several generating mechanisms. Many of these cycles are climatic in origin and are caused by changes in the amount of solar radiation and its global distribution across Earth. These cycles occur regularly, are known as Milankovitch cycles, and are affected by variations in three main components of Earth’s orbital rotation (Fig. 2.17) :

i) Eccentricity: the shape of Earth’s orbit around the sun, which operates on time scales of 400,000 years (yrs) and 100,000 yrs. ii) Obliquity: changes in the tilt of the earth, which operates on a time

scale of 41,000 yrs.

iii) Precession: changes in the ‘wobble’ of Earth’s orbit, which operates on a time scale of 21,000 yrs.

Orbitally-driven climate change on Milankovitch time scales drives the growth and decay of continental ice sheets, which is the principal mechanism responsible for high-frequency eustatic cycles (Plint et al., 1992; Miall, 2010). It is important to note however, that nonglacial sedimentary cyclicity is also possible. As an example, on an alternating time scale of 10,000 years, the interaction between the eccentricity and precession components of the Milankovitch band cause contrasting weather on Earth’s hemispheres (Miall, 2010). One hemisphere experiences short, hot summers and long, cold winters, while the other undergoes a period of long, cool summers, and short, mild winters. Such changes have a strong influence on oceanic circulation, wind patterns, and surface air-water temperature distributions (Miall, 2010). This leaves potential for effects on sedimentation (e.g. climate change) that are unrelated to eustatic change.

High frequency sequences have been recognized in the Cretaceous (e.g. Fischer, 1986; Plint, 1991; Sageman et al., 1997; Plint and Kreitner, 2007), but direct evidence for

Figure 2.16 Timing and amplitude relationships for different geological mechanisms and their effect on eustatic change. SF- Sea Floor. From Miller et al., 2005 Allen, 2005. Figure 2.17 The three components of earth’s orbital cycle that cause Milankovitch cycles: eccentricity, obliquity, and precession. See section 2.5 for details From Miall, 2010.

52

their origin due to glacioeustacy is still a matter of debate. Nevertheless, recognition of high frequency sedimentary sequences occurring over long distances and across different tectonic and climatic regimes strongly suggests a Milankovitch control (Miall et al., 2008; Varban and Plint, 2008b).