Regressive Surface Of Marine Erosion

Figure 2.8 Current status of the development of a new, internationally accepted chronostratigraphy for the Ordovician System. New global series and stages are correlated with the comparable chronostratigraphic divisions used in North American and the United Kingdom and Ireland. GSSP, global standard section and point.

Figure 2.8 Current status of the development of a new, internationally accepted chronostratigraphy for the Ordovician System. New global series and stages are correlated with the comparable chronostratigraphic divisions used in North American and the United Kingdom and Ireland. GSSP, global standard section and point.

The dividing lines between transgressive and regressive system tracts are marked by various types and degrees of unconformities that may be recognized on seismic profiles. Whereas most major sequence boundaries are probably due to global eustatic changes in sea level associated with climatic change or fluctuations in seafloor spreading processes, sequences can also be generated by more local tectonic controls. Research teams in the Exxon Corporation expanded the concept of sequence stratigraphy to build global sea-level curves for the entire Phanerozoic during the 1980s and 1990s. The description of succes sions defined within unconformity-bounded sequences has proved valuable in hydrocarbon exploration, where sequence boundaries can be recognized at depth using seismic geophysics.

Sequence stratigraphers have developed their own specialist terminology (Fig. 2.10). A sequence is a unit of similar strata bounded by unconformities. Sequences are laid down in three-dimensional assemblages of lithofa-cies linked by common depositional processes that can be divided into individual systems tracts. The architecture of sequences is controlled by changes in sea level, whether eustat-

Figure 2.9 North American Phanerozoic sequences: the recognition of these large packages of rock or what are termed "megasequences" formed the basis for the modern discipline of sequence stratigraphy, established by the Exxon Corporation. (Based on various sources.)

ically or tectonically driven, or perhaps a mixture of both, and the room available for sediment, termed accommodation space. Normal regressions, driven by increased sediment supply, and forced regressions, driven by base level fall, will both generate falls in sea level, where base level is the level above which deposition is temporary and prone to erosion. Transgressions are prompted by base level rise, when this of course exceeds sedimentation rates. There are also six main types of surface: subaerial unconformity, basal surface of forced regression, regressive surface of marine erosion, maximum regressive surface, maximum flooding surface and ravinement surface; the first three are associated with base level fall and the last three with base level rise. Finally there is a variety of systems tracts (Fig. 2.10): lowstand, transgressive, highstand, falling stage and regressive systems tracts. Changes in sea level seem to have had major effects on the planet's marine biotas through time and sequence stratigraphy provides a framework to describe these effects (Box 2.4). For example, shell concentrations may be associated with stratigraphic condensation at maximum flooding surfaces, i.e. the deepest-water facies where deposition is very slow or they may lie near the top of highstand system tracts. Firmgrounds (see p. 522) and their biotas, that include usually burrowers and encrusters, favor major flooding surfaces. Moreover, diversity increases are often associ ated with marine transgressions as more shallow-water habitats are created when continents are flooded. On the other hand, marked regressive events have been associated with major extinctions through habitat loss. Nevertheless it has been suggested by some authors that such diversity changes are artificial. Transgressive units are generally more widespread across continental areas, so increasing the chance to collect fossils; the converse may be true for regressive events. But sampling biases alone cannot account for apparent changes in biodiversity through time; processes related to sea-level change and the formation and destruction of marine habitats have also provided controls on the origination and extinction of marine taxa (Peters 2005).

Cydostratigraphy: finding the rhythm_

Quaternary geologists have accepted for some time that recent climate change follows repeated cycles of astronomical change. These short-term patterns are called Milankovitch cycles, named after the Serbian mathematician Milutin Milankovitch (1879-1958). Such cycles are controlled by the additive effects of the Earth's movements through space (Fig. 2.12a) and can directly affect global sedimentation patterns. Three main types of movement occur: eccentricity (variation in the shape of the Earth's orbit from nearly circular

Figure 2.10 Sequences, system tracts and stratigraphic surfaces defined in relation to base level and transgression-regression curves: (a) stratal architecture across a non-marine to marine transect is related to (b) sequence stratigraphies in the non-marine and marine parts of the transect. (A), positive accommodation (base level rise); BSFR, basal surface of forced regression; c.c., correlative conformity; c.u., coarsening upward; DS, depositional sequence; FR, forced regression; FSST, falling stage systems tract; f.u., fining upward; GS, genetic stratigraphic sequence; HST, highstand systems tract; IV, incised valley; LST, lowstand systems tract; MFS, maximum flooding surface; MRS, maximum regressive surface; NR, normal regression; R, ravinement surface; RST, regressive systems tract; SU, subaerial unconformity; TR, transgressive-regressive sequence; TST, transgressive systems tract. (Based on Catuneanu, O. 2002. J. African Earth Sci. 35.)

Figure 2.10 Sequences, system tracts and stratigraphic surfaces defined in relation to base level and transgression-regression curves: (a) stratal architecture across a non-marine to marine transect is related to (b) sequence stratigraphies in the non-marine and marine parts of the transect. (A), positive accommodation (base level rise); BSFR, basal surface of forced regression; c.c., correlative conformity; c.u., coarsening upward; DS, depositional sequence; FR, forced regression; FSST, falling stage systems tract; f.u., fining upward; GS, genetic stratigraphic sequence; HST, highstand systems tract; IV, incised valley; LST, lowstand systems tract; MFS, maximum flooding surface; MRS, maximum regressive surface; NR, normal regression; R, ravinement surface; RST, regressive systems tract; SU, subaerial unconformity; TR, transgressive-regressive sequence; TST, transgressive systems tract. (Based on Catuneanu, O. 2002. J. African Earth Sci. 35.)

to elliptical; 100 kyr cycle), obliquity (wobble of the Earth's axis; 41 kyr cycle) and precession (change in direction of the Earth's axis relative to the sun; 23 kyr cycle). Throughout the stratigraphic record there are many successions of rhythmically alternating litholo-gies, for example limestones and marls (calcareous shales), that may have been controlled by Milankovitch processes. Apart from their obvious value for correlation, such rhythms probably also effected changes in community compositions and structures together with the extinction and origination of taxa.

Some of the most extensive and remarkable decimeter-scale rhythms, probably controlled by precession cycles, have been detected in the Upper Cretaceous chalk facies, where individual couplets can be tracked from southern England to the Caucasus, a distance of some 3000 km. A cyclostratigraphic framework can be related to well-established ammonite, inoceramid bivalve and foraminiferan biozones together with carbon isotope excursions, providing a high-resolution and composite stratigraphy (Fig. 2.12b). The dark marly sediments may have been deposited during precession minima at eccentricity maxima during intervals of cool, wet climates (Gale et al. 1999).

Geological time scale: a common language_

If we are to understand global events and rates of global processes, geologists must talk the same language when we correlate and date rocks (Box 2.5). Rapid developments in stratigraphy during the last few years (Gradstein & Ogg 2004) have prompted publication of GTS2004, an updated geological time scale (Gradstein et al. 2004). Over 50 of the 90 Phanerozoic boundaries are now properly defined in stratotype sections (GSSPs) and the new scale uses a spectrum of new stratigraphic methods, such as orbital tuning, together with more advanced radiometric dating techniques and new statistical tools (Fig. 2.13). Although traditional stratigraphic methods form the basis of the geological column and our understanding of the order of key biological events, the prospect of precisely defined radiometric dates makes it possible to determine the rates of many types of biological process. Not all the recommendations have met with universal approval, and they are only recommendations. For example, GTS2004 removed the Tertiary and Quaternary epochs from the chronostratigraphic column without the approval of the IUGS; but these terms are widely used and deeply embedded in the literature and are thus unlikely to disappear

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