**6. Volcanism, basin dynamics, and sedimentation**

The presence of volcanic deposits is frequent in many sedimentary basins. The existence of volcanic episodes in the sedimentary record represents an important source of information to understanding the geological evolution of that particular time frame. Specifically, the presence of volcanism responds to geodynamic conditions that favor the genesis and rise of magmas and that, on the other hand, can translate into suitable tectonic conditions for the development of a subsidence structure [78]. In paleovolcanic terrains, where erosion and tectonics may have obliterated their original characteristics, the location of vent zones or proximal areas will help to infer the position of the main fault zones that controlled volcanism and that occasionally may also be associated with basin subsidence—even when these faults may have been reactivated in subsequent tectonic movements (e.g., [54, 79]). Likewise, volcanic deposits, and especially those derived from explosive eruptions, constitute a valuable tool for establishing stratigraphic correlations within and outside the limits of the basin, as well as a precise geochronology of the host sedimentary successions (e.g., [80]). On the other hand, the interaction between volcanic processes (which are generally catastrophic) and sedimentary processes sometimes leads to the appearance of "anomalous" deposits within the sedimentary successions, which can be predisposed to erroneous interpretations if the nature of volcanic processes is not well known [81, 82]. Thanks to the current approach of volcanology that pursues the study and understanding of volcanic processes, bringing together many other aspects besides the pure identification and classification of volcanic rocks, it is possible to obtain a much broader vision of these phenomena, which helps in the interpretation of other geological problems, such as basin analysis.

The formation of a sedimentary basin is a geodynamic process that frequently implies the existence of a fracture network that allows the progressive subsidence of the blocks it delimits, thus accommodating sedimentation [83]. Similarly, volcanic episodes respond to geodynamic processes that lead to the formation of magmas at depth and hence facilitate their rise to the surface. However, the formation and ascent of magmas do not always imply the existence of eruptive processes. Only when tectonic conditions are adequate (especially in the upper crust) can magma reach the surface. These conditions imply a locally distended stress field that favors the opening of fractures and consequently the rise of magma through them (e.g., [84]). In many cases, these fractures are the ones that delimit the basin and control its subsidence, so the location of the volcanic centers is directly related to the structure of the basin. Therefore, in paleovolcanic terrains, the reconstruction of volcanic stratigraphy that indicates the position of vents may help to infer the structure of the basin where volcanic deposits have been emplaced.

Due to the fact that the physical conditions that control the release of magma to the surface do not vary with time, the study of volcanic processes is useful, above all, in the reconstruction of those basins that have undergone subsequent tectonic transformations. In current basins, the application of geophysical methods allows obtaining an adequate understanding of their structure and dynamics—although these methods may be of little value in the interpretation of ancient basins. However, the reconstruction of the volcanic episodes helps to know the initial structural conditions that controlled them and, therefore, allows one to deduce which were the tectonic features that directed the dynamics of the basin. Moreover, in deeply eroded paleovolcanic terrains, it is sometimes possible to observe the roots of large volcanic complexes (stratovolcanoes, collapse calderas, etc.) represented by different sets of

## *Volcano Geology Applications to Ancient Volcanism-Influenced Terrains: Paleovolcanism DOI: http://dx.doi.org/10.5772/intechopen.108770*

subvolcanic intrusions and faults [85–87], whose orientation may depart from the regional ones; hence, the structural reconstruction of such paleovolcanic settings needs to be conducted and understood at different spatial and temporal scales when these complexities appear.

The influence of volcanic activity on sedimentation can be significant in various aspects. The sedimentation rate in a basin with volcanism can be much higher than in a non-volcanic basin with similar characteristics. This may represent a significant increase in the rate of subsidence of the basin, while it can significantly reduce the time required to become clogged. This fact can be accentuated in the case of volcanotectonic basins or especially in the case of large collapse calderas, where the deposition of successions of volcano-sedimentary materials, several hundreds of meters thick, is carried out over very short periods of time (e.g., [43]). This can be misleading if the observer does not properly separate both processes on the timeline.

In a sedimentary basin where there is a direct influence of volcanic activity, sedimentation will be significantly affected by the simultaneous presence of eruptions that generate large volumes of pyroclastic materials and by the growth and subsequent dismantling of volcanic edifices. When studying the response of the sedimentary system to the presence of volcanism, we must make a distinction between syn-eruptive periods and inter-eruptive periods [82]. The syn-eruptive periods are characterized by the instantaneous production, geologically speaking, of large volumes of volcaniclastic sediments and other volcanic products that may be remobilized and deposited through different sedimentation processes (e.g., [88]). These periods are short and are separated by relatively longer inter-eruptive periods during which volcanism has little or no influence on the sedimentary system and which will consequently be characterized by a significant decrease in sediment production. In the stratigraphic record, the existence of these syn- and inter-eruptive periods can be identified based on the lithological and sedimentological characteristics of the deposits. In this sense, we must take into account that the resulting deposits will depend on the relative importance of these two types of periods.

The presence of volcaniclastic sedimentation implies some notable differences with respect to typical siliciclastic sedimentation [9, 82, 89, 90]. First, the resulting deposits will mostly be made up of fragments derived directly from eruptive activity rather than by weathering of preexisting rocks (**Figure 14**). In contrast, pyroclastic deposits are sediments generated over very short intervals of time and can be emplaced in the form of thick layers that may cover the topography more or less homogeneously or fill valleys and topographically depressed areas, resulting in efficient erosion. In a volcanic terrain with a predominance of explosive activity, erosion rates are high not only due to the existence of a high volume of unconsolidated material, but also to the destruction of the vegetation, which acts as a regulating agent for sedimentation from volcanic processes [82, 88, 91]. The loss of vegetation and the relative impermeability of the pyroclastic material, due to its fine grain size or poor sorting compared with soils, causes an increase in the amount of material that can be remobilized, which significantly increases the volume and periodicity of the total discharge into the basin [82].

The style of volcanism is of great importance to the development of the basin infilling successions. The volume of volcaniclastic material determines the extent of the influence of eruptions on sedimentation. In paleovolcanic terrains where a large part of the vent areas may have been eroded, it is necessary to identify the primary or secondary character of all the deposits that form the stratigraphic record if we

#### **Figure 14.**

*Field example of an ignimbrite deposit (Ig) eroded by an epiclastic crystal-rich sandstone (E) which incorporates fragments of the ignimbrite (Igf). The lack of a paleosoil separating both deposits is indicative of a short time lapse between the two (credit: Joan Martí).*

want to know the evolution of the non-volcanic sedimentation in the basin and the influence of eruptive activity on it. The identification of the syn- and inter-eruptive periods serves to interpret the stratigraphic record in terms of cycles of eruptive activity, which, when combined with the identification of compositional criteria, allows establishing the relationship between sedimentation and magma compositions. In this way, we can observe how variations in the volume and extension of volcanic material supplied by the eruptions to the basin may depend on variations in the degree of explosiveness of magmas, this in turn being related to variations in their chemical composition (and volatile content).

Finally, we should note that the epiclastic processes that act in volcanic terrains do not differ from those that can be found in non-volcanic terrains. However, differences may exist in the resulting deposits due variations in the density of fragments, as a consequence of their variable degrees of vesiculation; this may affect their hydraulic classification and, consequently, the texture and sedimentary structures of the resulting deposits [1, 92]. A common feature of most paleovolcanic sequences is the presence of crystal-rich deposits of a different nature (e.g., [1, 58, 66, 93–95]). The main features that these rocks present can be relatively similar despite their potential diversity of origins (**Figure 9c, d, f**), making it possible that they could be materials of pyroclastic or epiclastic origin or a combination of both. The correct characterization of these crystal-rich deposits, particularly those of epiclastic origins, will permit us to know the influence of volcanism on sedimentation in the basin, its nature, and quite possibly the location of the source areas of the volcaniclastic materials. For example, in the description of many ancient terrains we can find rocks of an ambiguous nature, rich in crystals and with a clay matrix, which are generically called graywackes (e.g., [96, 97]), and which in many cases, among other origins, correspond to volcaniclastic (pyroclastic or epiclastic) deposits. The presence of these deposits in the stratigraphic record remark the importance of volcanism as a source

of sediments and offer good samples for radiometric dating because of the minerals (e.g., zircon) they usually contain.
