Significance of Geometrical Configurations

The morphology of the boundaries of stratigraphic cycles can be used to interpret depositional environments. Facies (lithology) predictions require a wider geological and geophysical understanding, as we will see later. However, as all geoscientists know, the lithology encountered in onlapping and progradational seismic intervals varies according to :

(i) Water-depth ;

(ii) Sedimentary supply ;

(iii) Rate of relative change of sea level, which, in turn, is controlled by the subsidence and the global eustatic variations.

A few typical possibilities will be reviewed.

Plate 233 - A seismic interval can be characterized by a fraction formula, in which the upper members (numerator) correspond to the geometrical relationships associated with the upper and lower boundaries of the interval, in this particular example concordant (C) and onlap (On), while the lower member (denominator) describes the internal configuration of the interval, in this case, parallel (P).

Plate 234- As illustrated above, a concord onlap sand-shale interval with a parallel internal configuration, strongly, suggests either a shallow or a deep water depositional environment. According to the Walther's law (the distribution of environments represented by rocks in a vertical stratigraphic column is related to the distribution of those environments laterally), if a slope environment (oblique or sigmoid internal configuration) is found up-dip, the interval is a deep-water one. On the contrary, when a continental slope is found down-dip, the interval with a parallel configuration is shallow water, i.e., located landward of the shelf break.

Plate 235- As in plate 233, the upper member of the reflection configuration formula, that is to say, C and On correspond to the geometrical relationships (reflection terminations) associated, respectively, with the upper and lower boundaries. The lower member denotes the geometry of the reflectors (internal configuration) of the interval. The convention or expression is the same for sand-shale or carbonate intervals.

Plate 236 - Depending on the location of the continental slope, an onlapping carbonate interval {(C-On)/P} can suggest shallow-water or deep-water limestones. On seismic interpretation, interpreters progress from the general to the particular. Before concentrating on expensive and small size 3D data, explorationists should determine the regional geological context using regional 2D lines, on which the location of the continental slope, within the stratigraphic cycles, is, generally, possible and relatively easy to recognize, which is not often the case in 3D data. Since you know the location of the continental slope, in a seismic interval (generally a sequence-cycle), you can use the Walter's law to predict the environment and the more likely facies.

Plate 237- In this sand-shale reflection configuration, the upper member of the formula either toplap either downlap. The lower member, that is to say, the internal reflection configuration, is either sigmoid or oblique. It is sigmoid when aggradation is significant and oblique when aggradation is relatively small. In this kind of configuration the vertical and horizontal scales are indispensable to predict environments and facies.

Plate 238-  Prograding sand-shale intervals can be found in all depositional environments. In continental slope environments, they are, mainly, composed by deep marine (slope) shales. Landward of the shelf break (platform or shelf environment), they form mainly the regressive episodes (delta slope or retrogressive sandstones). The sand facies is generally associated with the toplaps. Landward of the bayline (seaward limit of continental deposits), this configuration is often associated with alluvial fans (a fan-shaped alluvial deposit formed by a stream where its velocity is abruptly decreased, as at the mouth of a ravine or at the foot of a mountain, http://dictionary.reference.com/browse/ alluvial+fans), in which the distribution of the sand facies is quite chaotic.

Plate 239 - On seismic lines, particularly in 2D lines, this reflection configuration is better recognized within limestone than in sand-shale intervals. The angle of the calcareous progradations is higher. If you identify clear and evident progradations landward of the shelf break, it is more like to be a limestone progradation and not deltaic.

Plate 240- As for shale-sand intervals, progradational limestone intervals are, mainly, associated with shallow water regressive episodes. Bioclastic limestones are, generally, associated with high-energy environments (shallow water) and so located in the upper part in connection with the toplaps.

Plate 241- The interval characterized by this type of configuration is difficult to recognize in short seismic lines, since its boundaries are not evident, particularly, in deep water seaward of the toe of the continental slope. The identification of incised valleys, located landward of the shelf break and canyons located seaward of the shelf break, allow interpreter to better define such {(C-C)/P} intervals. If such a configuration is located landward of the shelf break, the environment is, probably, shallow water. On the contrary, probably, it will be deep water. Don't forget the Walther's law says : the distribution of environments represented by rocks in a vertical stratigraphic column is related to the distribution of those environments laterally.

Plate 242 - Landward of the shoreline (roughly near the depositional coastal break), in a sand-shale context, a {(C-C)/P} sand-shale interval corresponds, often, to point-bars and meander belts, when located down-dip of the bay line and to alluvial fans when located up-dip. In the shelf or near the depositional coastal break, such a geometry can be associated, for instance, with offshore bars or beach deposits. In deep-water, seaward of and downward of the slope fans, it is, generally, associated with deep marine shales and turbidite depositional systems.

Plate 243 - When the facies is limestones, intervals with this type of reflection configuration are easier to recognize than the equivalent sand-shale intervals. This is particularly true when the upper and lower intervals are lower velocity intervals. In such conditions, the unconformities bounding the interval are enhanced by seismic reflectors. A positive reflection above and a negative reflection below. As in the case of sand-shale intervals, the recognition of incised valleys or canyon strongly increase of environmental prediction and the the accuracy of the location of the bounding unconformities.

Plate 244 - In a deepwater environment, the facies associated with a {(C-C)/P} limestone interval is, generally, a deep micrite limestone. In shallow environments, oolithic limestone and laminated algal limestone  are possible. Generally, in a transgressive episode, from bottom to top we get, often, the following stacking: laminated algal limestone, oolithic limestone and micrite limestone because the water-depth increase. On the contrary, in a regressive episode, as the water-depth decreases, from bottom to top, the normal succession is micrite limestone, oolithic limestone and laminated algal limestone.

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