Rock successions can be named, analyzed and understood using two fundamentally different approaches. The first approach, lithostratigraphy, involves correlating similar lithotypes and "packaging" the rocks into lithostratigraphic units. Examples of formal lithostratigraphic units include groups, formations, and members (North American Commission on Stratigraphic Nomenclature (NACSN) 1983). The second approach, which incorporates time stratigraphy, involves the correlation of time markers through rocks of potentially varying lithologies and the packaging of rocks into units bounded by unconformities and other types of surfaces. Examples of formal lithostratigraphic units in a time-stratigraphic framework include allogroups, alloformations, and allomembers (NACSN, 1983). This paper outlines these concepts and discusses their application to strata in the foreland succession of the Western Canada Sedimentary Basin.
Allostratigraphy is defined as the packaging of rocks bounded by discontinuities within a time-stratigraphic framework (NACSN, 1983). These bounding discontinuities can include unconformities, ravinement surfaces, flooding surfaces, and omission surfaces (Bhattacharya and Walker, 1991a). The elevation of unconformities to a higher level of significance and then repackaging of the rocks into sequences using those surfaces embodies the sequence stratigraphic approach. "Sequence Stratigraphy is the study of rock relationships within a chronostratigraphic framework wherein the succession of rocks is cyclic and is composed of genetically related stratal units" (Posamentier et al., 1988).
Understanding genetic relations affords a better understanding of coeval depositional systems and hence more meaningful paleogeographic reconstructions. Historically, much of the stratigraphic succession in the Western Canada Sedimentary Basin has been subdivided on a lithostratigraphic basis and many of the formation and member boundaries in both the Energy Resources Conservation Board (ERCB) and Atlas data bases are diachronous across the basin. For example, in analyzing the Devonian Winterburn-Woodbend strata, Switzer et al. (this volume, Chapter 12) used a time-marker approach and repicked markers across the basin in order to better map the genetic units, rather than utilize the essentially lithostratigraphic ERCB and Atlas data bases.
Figure 25.1 contrasts the two approaches as applied to the Upper Cretaceous Dunvegan Formation in northwestern Alberta. In the lithostratigraphic interpretation (Fig. 25.1a), the Dunvegan Formation is shown as a sandy clastic wedge that interfingers with shales of the Shaftesbury Formation below and the Kaskapau Formation above. Farther east, the Dunvegan pinches out into shales of the La Biche Formation (Singh, 1983). Several marine tongues are shown within the Dunvegan Formation.
An allostratigraphic interpretation of the same rocks elucidates the nature of this interfingering (Fig.25.1b) and incorporates the time-marker approach. The cross section (Fig. 25.1b) shows that the Dunvegan Formation has been subdivided into seven allomembers (lettered A to G), representing regressive sedimentary events, each capped by a marine flooding surface associated with transgression (Bhattacharya, 1988; Bhattacharya and Walker, 1991a; Bhattacharya, this volume, Chapter 22). These seven flooding surfaces are interpreted as representing chronostratigraphically significant bounding discontinuities that can be correlated over hundreds of kilometres. More importantly, the surfaces can be correlated as time-markers into the Kaskapau and Shaftesbury shales. The allomembers thus represent lithologically heterogenous units that contain strata previously included in different formations. The allomember boundaries cut across the conventional lithostratigraphic boundaries but better illustrate the genetic relations between the different lithostratigraphic units.
Ideas regarding changes in base level as a control on sedimentation can be traced back to the work of Grabau (1906), Blackwelder (1909), and Barrell (1912, 1917). Their ideas were formalized into the concept of time-stratigraphy by Wheeler (1958), who recognized that the duration of hiatuses in sedimentary successions is as important as the rocks actually present. Wheeler (1958) depicted geological cross sections in terms of time represented. An example of a time-stratigraphic diagram for the Dunvegan example shown above is illustrated in Figure 25.2. Sloss (1963) applied Wheeler's ideas to the North American craton and developed the concept of continent-wide unconformity-bounded units, which he termed sequences.
The availability of high-quality seismic data, acquired for petroleum exploration during the latter half of the 20th century, provided a data base that gave the geologist a continuous subsurface image of stratigraphic relations in areas not previously accessible. Scientists at Exxon Production Research Co. recognized the stratigraphic significance of the seismic tool. In their landmark publication, Vail et al. (1977) recognized that major unconformities could be identified by reflection terminations on seismic data and developed the concepts of seismic stratigraphy. They termed the depositional units bounded by these unconformities seismic depositional sequences (Mitchum et al., 1977). Vail et al. (1977) also observed that there was a striking similarity of coastal onlap geometry between widely separated ocean basins. They suggested that the cause of this apparent synchroneity of events was global sea-level change (eustasy). Their observations led to the publication of global sea-level charts (Vail et al., 1977; Haq et al., 1987, 1988).
The application of these new stratigraphic principles to outcrop, core, and well log data has led to the broader concept of sequence stratigraphy (Wilgus et al., 1988). With the publication of Jervey (1988), Loutit et al. (1988), Posamentier et al. (1988), Posamentier and Vail (1988), Sarg (1988), and Van Wagoner et al. (1990) the use of sequence stratigraphic concepts as a tool for lithology prediction came into its own. It also became apparent that sequence stratigraphic concepts were applicable at all spatial and temporal scales and could be applied using a variety of data bases including conventional seismic, outcrop, and flume studies (e.g., Baum and Vail, 1988; Greenlee and Moore, 1988; Posamentier et al., 1992a; Wood et al., in press). A strength of the sequence stratigraphic concepts has been to allow integration of diverse data bases including biostratigraphic data, geochemical data, and tectono-structural data (e.g., Leckie et al., 1990, 1992).
Last modified: August 20, 2008
