**3. Sequence stratigraphy**

*Ut* <sup>=</sup> <sup>2</sup> *<sup>Z</sup>* \_\_\_\_\_1

6 Seismic and Sequence Stratigraphy and Integrated Stratigraphy - New Insights and Contributions

*Ur* <sup>=</sup> *<sup>Z</sup>*<sup>2</sup> <sup>−</sup> *<sup>Z</sup>* \_\_\_\_\_1

the chronostratigraphic correlation [2, 9–12].

toplap and continuity (upper boundaries) [1, 2, 7–12].

a previous definition of the clusters [57, 60, 61].

while the reflected energy U<sup>r</sup>

*Z*<sup>1</sup> + *Z*<sup>2</sup>

*Z*<sup>1</sup> + *Z*<sup>2</sup>

The contrasts of acoustic impedance controlling the individuation of the seismic reflectors are located along surfaces corresponding to strata surfaces or to other discontinuities having a chronostratigraphic meaning. The strata surfaces represent the old surfaces of deposition, and then, they are coeval in the depositional area. The discontinuities are old erosional or nondepositional surfaces corresponding to significant stratigraphic gaps. Also if they represent events varying during the geological time, the discontinuities are considered as chronostratigraphic surfaces, since all the strata overlying the discontinuity are younger than the underlying strata [2, 9–12]. When identified on a seismic section, the discontinuities let to identify the most important lateral variations in the deposition of a stratigraphic succession. Moreover, they offer a geological basis in order to subdivide the stratigraphic successions in depositional

sequences, which are the basic stratigraphic units of seismic stratigraphy [2, 9–12].

The main steps of the seismo-stratigraphic analysis are represented by the identification of the discontinuities and consequently of the depositional sequences, by the reconstruction of the original geometry of the sedimentary bodies and related sedimentary environments and by

The seismic sequence analysis allows for the identification of the depositional sequences. The geometric relationships between the lateral terminations of the strata and the discontinuities or the correlative conformities define the boundaries of the depositional sequences [2]. The lateral terminations of the strata with respect to the sequence boundaries individuate the configurations of onlap, downlap, continuity (lower boundaries) and of erosional truncation,

The seismic facies analysis deals with both the individuation and the geologic interpretation of the geometry, continuity, amplitude, frequency and velocity of the seismic reflectors, more than the outer shape of the sedimentary bodies and the seismic facies associations in a depositional sequence [2, 56–61]. In the modern development of this methodology, one aim is represented by the recognition of clusters or groups, representative of significant variations in the properties of the rocks, in the lithology and in the content of fluids. The cluster analysis offers a significant instrument in order to perform the classification of the shapes of the seismic traces grouping them into clusters, often using an unsupervised process without

The analysis of relative sea-level fluctuations is based on the construction of chronostratigraphic diagrams and of curves of relative sea-level cycles [1, 6–10, 62]. In a chronostratigraphic section, reporting the chronological units in the ordinates of the graph, each layer has an equal time duration. Both erosional and non-depositional hiatuses may occur among the time surfaces corresponding to the layers of the depositional sequences. Three-dimensional

can be calculated through the following equation:

. *U* (1)

. *U* (2)

The concepts of depositional sequence, isochronous boundaries, and characteristic correlation geometry, which have been typically developed in the seismic stratigraphy, may be applied in the stratigraphic analysis of outcrops, representing, in that case, the sequence stratigraphy. Some beautiful examples of progradation, toplap, and other stratigraphic relationships have been described by Bosellini [66], an Italian geologist who has applied the concepts of the sequence stratigraphy to significant outcrops of the Triassic carbonate platforms of the Dolomites (Northern Italy). Bosellini [66] has described several types of progradational geometries occurring in spectacular outcrops located in the Dolomites at an outcrop scale comparable with one of the seismic sections. In the Dolomites, an episodic progradation of the carbonate platform has been suggested based on outcrop analysis. During the periods of high debris input, the progradation of the carbonate platform occurred, which was evidenced by the widening of shallow water carbonate depositional environments. On the contrary, during the periods of low debris input, the basinal sedimentation prevailed on the shallow water carbonate deposition. The onlap of the basinal facies at the toe of the carbonate slope may be observed in outcrop [66]. The progradational geometries have been interpreted accordingly with two different models, which have been named as two periods of Triassic times. In the Ladinian model, the progradation and the aggradation of the carbonate platform took place contemporaneously, indicating a phase of a relative sea-level rise. In the Carnian model, toplap geometries have been observed in the carbonate platform, indicating a phase of relative sea-level stand [66].

Numerous are the sequence stratigraphic studies carried out on carbonate platform outcrops. Stafleu and Schlager [67] have carried out a sequence stratigraphic study in which pseudotoplap geometries have been identified in the Schlern and Raibl Formations. Prograding clinoforms have been identified in the Schlern Formation coupled with topset geometries [67]. Two lithological models have been constructed to explain the geologic evolution of the carbonate platform, that is, (1) rapid progradation of the carbonate platform coupled with slow aggradation and (2) toplap of the prograding clinoforms against the topsets deposited in the inner platform. The seismic models have generated a pseudo-toplap, which is not coincident with a toplap in the outcropping sections [67].

The siliciclastic sequence stratigraphy, its concepts and application have been resumed by Posamentier and Allen [68]. The key concepts of siliciclastic sequence stratigraphy have been considered, including the key stratigraphic surfaces, such as the transgressive surface, the maximum flooding surface, the ravinement surface and many others, and their geologic meaning. The control factors on the deposition of sequences and system tracts have been considered, including the sea-level fluctuations, the sediment supply and the accommodation space [68].

Some applications of siliciclastic sequence stratigraphy have been given in the recognition of depositional sequences and system tracts from well logs coupled with seismic profiles and biostratigraphic data [69]. The integration of these stratigraphic methods has been applied to the Gulf of Mexico and has allowed for the prediction of reservoirs, seals and source rocks, useful in the petroleum exploration. The stratigraphic architecture has evidenced the occurrence of a complete depositional sequence, consisting of lowstand system tract (LST), transgressive system tract (TST), and highstand system tract (HST), whose stratigraphic signature has been identified based on well log interpretation. High-resolution paleobathymetric and biostratigraphic interpretation of well logs has detailed the general stratigraphic setting.

Some key concepts of sequence stratigraphy, particularly referring to the stratigraphic unconformities, are given in the Chapter 5 of this book. The stratigraphic unconformities are considered as main stratigraphic surfaces and their identification in outcrops can be constrained using the relative weathering maturity of the subaerial profile, the calibration through cyclostratigraphy, the absolute dating and the biostratigraphy. At the scale of the seismic profiles, the disconformities show concordant strata overlying and underlying the stratigraphic surface. In the sense of this chapter, they are considered to include the ravinement surfaces, which are important stratigraphic surfaces, related to the erosion during the transgressive movement of the landward margin of the transgressive system tract (TST) [70–72]. Moreover, the concept of drowning unconformity has been reviewed, considering this stratigraphic surface as one of the most important stratigraphic surfaces in carbonate platform settings [73–76]. These surfaces develop when the rate of vertical aggradation of the carbonate platform is lower than the rate of the accommodation space. Perhaps the deep water sedimentation tends to prevail on the shallow carbonate sedimentation, as evidenced by the individuation of the drowning unconformity. These kind of unconformities have been individuated offshore of the Apulian region in the Southern Adriatic Sea [62] and onshore in the Gargano Promontory, showing a well-developed carbonate platform margin-slope-basin succession [77].
