Factores micro ambientales

The North American Commission on Stratigraphic Nomenclature (NACSN) defines an allostratigraphic unit as a “mappable body of rock that is defined and identified on the basis of its bounding discontinuities” (NACSN, 1983, Article 58). In one sense, the discussion of allostratigraphy by NACSN (1983) means that its use is better defined and regulated than sequence stratigraphy—sequence stratigraphy was intentionally omitted from the North American Stratigraphic Code due to a lack of consensus. However, the definition of an allostratigraphic unit is also very vague, because it is up to the author to define the nature of the ‘bounding discontinuity’. With no set definition of surfaces, it may be difficult to compare the results of various allostratigraphic studies.

The allostratigraphic approach has been widely used, and may be more useful than traditional sequence stratigraphy in tectonically active areas (Martinsen et al., 1993).

Relevant to this thesis is the allostratigraphic framework for the Cardium Formation, established by Plint et al. (1986). The Cardium allostratigraphic framework uses erosion

surfaces related to sea-level fall and transgressive surfaces as the ‘bounding discontinuities’ described by the NACSN (Fig. 2.16).

Figure 2.15—Shoreline trajectories as they relate to transgression and regression. Accretionary trajectories are controlled by sediment supply, whereas non-accretionary trajectories are controlled by pre-existing topography. A normal regression occurs during slow sea-level rise, with aggradation and progradation producing a seaward-upward trajectory of the shoreline. A normal regression can only be accretionary, because, by definition, normal regression is driven by progradation due to sediment supply

(Posamentier et al., 1992). Forced regressions, on the other hand, may be accretionary or

non-accretionary, depending on the relative rates of sediment supply and relative sea- level fall. Forced regressions in all cases are characterized by a seaward-downward trajectory of the shoreline. Finally, transgressions are marked by landward-upward trajectories, and may be accretionary or non-accretionary. Helland-Hansen and Martinsen (1996).

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Figure 2.16— Summary of key surfaces and systems tracts using the depositional sequence (bounded by sequence boundaries and their correlative conformities) and allomembers (bounded by transgressive surfaces). Summary based on the Cretaceous Dunvegan Formation of Western Canada. Plint et al. (2001).

the Cardium Formation. Relative sea-level fluctuations (probably of eustatic origin; e.g. Wadsworth and Walker, 1991) are evident throughout Turonian and Coniacian time, and they created regionally extensive marine flooding surfaces. These flooding surfaces may be erosional due to transgressive ravinement and may therefore overprint pre- existing subaerial or submarine erosion surfaces. In addition to the (probable) eustatic fluctuations during Cardium deposition, tectonic uplift and subsidence in the active foreland basin may modify the expression of relative sea-level changes (e.g. Leggitt et al.,

1990; Hart and Plint, 1993a). For example, a forced regressive unit, recognized by an abrupt basinward shift in facies, may pass along basin strike or dip into a gradational facies succession, typical of a normal regression. Using sequence stratigraphic methods, this distinction is critical to differentiate a sequence (associated with a sea-level fall) from a parasequence (in which accommodation is lost only by sediment infill, not by relative sea-level fall). Using allostratigraphy, an allomember could be defined by the flooding surface and the transition from forced to normal regression could be described

qualitatively, perhaps invoking differential tectonic activity as the reason for the inconsistency. Because of the flexibility provided by ‘bounding discontinuities’, allostratigraphy is better suited for tectonically active areas than traditional sequence stratigraphy (Martinsen et al., 1993).

Marine flooding surfaces are attractive allostratigraphic markers because they have minimal diachroneity (Cross and Lessenger, 1988). Surfaces related to relative sea- level fall, such as the subaerial unconformity and correlative conformity, or the regressive surface of marine erosion, are highly diachronous (Plint and Nummedal, 2000) and so are less suited for use as proxy time-lines. Bentonites (volcanic ash beds) are the only true time lines in the rock record, because they form from a single volcanic eruption. However, bentonites are not tied to relative sea-level changes, so while they may be useful

correlation tools, they are limited in their use for interpretation of depositional controls. Where bentonites are traceable, they nearly parallel—but never cross— marine flooding surfaces, demonstrating the merit of flooding surfaces as proxy time-lines (Varban and Plint, 2005; Tyagi et al., 2007).

Marine flooding surfaces provide an effective means of correlation of marine strata, but the flooding event may also be recognized in the contiguous non-marine record. Plint et al. (2001) demonstrated that the marine transgressive surface/ravinement surface

merges landward with a subaerial unconformity that is unmodified by marine processes. The subaerial unconformity consists of the basal unconformity of incised valleys, and the related interfluve paleosols (McCarthy and Plint, 1998).

The relationship between marine and non-marine stratigraphy is poorly

understood in the Cardium Formation. The Musreau Member of the Cardium Formation is a non-marine wedge that is either restricted in time between the E4 and T4 surfaces (Plint et al., 1986), onlapped by younger regional erosion surfaces E5, E6 and E7 (Hart,

1990) or interfingers with time-equivalent marine sediments (Duke, 1985).Portions of the Musreau Member are probably contemporaneous with erosion surfaces E4, E5, E6, and E7; the relationship could be better understood if the Musreau Member were revisited using the non-marine to marine correlation methods described by Plint et al. (2001).

Other surfaces used in traditional sequence stratigraphy, such as the basal surface of forced regression and the maximum flooding surface, may also be recognized even when using an allostratigraphic approach. These surfaces are helpful for interpreting depositional history, but they are not used as correlation tools in an allostratigraphic method.