**4. Stratigraphy of paleovolcanic successions**

An important aspect in the characterization of paleovolcanic terrains is the correct description and interpretation of their stratigraphy. We have already highlighted some of the most relevant difficulties in merely identifying ancient volcanic deposits, not to mention the challenges to establish their lateral relationships and the relative ages. The development of a detailed stratigraphy can help alleviate these difficulties and correctly interpret the succession of events that make up a given succession of deposits. Likewise, the completion of such a stratigraphy is essential to be able to interpret such successions in terms of eruptive sequences.

The stratigraphic divisions to be established, as well as their geological mapping, will depend on the type of study to be carried out. However, there must be some general criteria that allow us to establish a stratigraphy that can be compared with other similar examples and, therefore, can always be interpreted in the same way. When investigating a given area, the first step must always be the objective description of the local stratigraphic succession in terms of lithostratigraphic units able to be mapped and correlated. Lithostratigraphy consists of the description, identification, and interpretation of rock units (see [55]). Individual units must be described and defined based on their general lithological characteristics and their interrelationships with adjacent units. Stratigraphy of volcanic terrains, both modern and ancient, should try to identify the stratigraphic and chronological order in which the products of an eruption, a series of eruptions, or of an interruptive period (epiclastic deposits or reworked volcanics) appear in the geological record (**Figure 11**) [2]. Therefore, the most logical way to describe the stratigraphy of volcanic terrains is by using the same principles as classical stratigraphy (e.g., [56]), that is, to identify and group the different existing units based on a hierarchy that allows for the identification of a

temporal succession of events or units of eruptive activity [2, 8]. In recent volcanism, the identification of the different lithostratigraphic units can be done without too much difficulty since the products of the different eruptions can nearly always be easily distinguished. Likewise, the variations in the compositional trend of the magmas, eruptive styles, or other characteristics that allow the deposits from different eruptions to be grouped in cycles of volcanic activity are equally identifiable. However, in older terranes, due to the complications discussed above, establishing the correct lithostratigraphy is not always possible. Despite this, attempts should be made to use the same lithostratigraphic subdivisions, since an accurate interpretation of a paleovolcanic zone must include (or at least attempt) the identification and interpretation of the different volcanic episodes recorded in the succession of deposits. Martí et al. [2] have provided a detailed review of the principles of volcanic stratigraphy and how they should be applied in field studies of volcanic terrains. I direct the reader to this contribution to be informed about the methods used in volcanic stratigraphy.

As we have seen previously, the different deposits can be identified based on their lithology (mineralogy, petrology, alteration, color, degree of welding, grain-size distribution), geometry, and relative stratigraphic position. Within a volcanosedimentary succession, the existence of different deposits corresponding to the same eruption (i.e., Member) and of different members constituting a formation can be established based on the presence of first-order discontinuities such as paleosoils, erosional surfaces, or interbedded epiclastic deposits. However, the presence of erosional surfaces or interbedded epiclastic deposits does not always indicate a significant interruption in eruptive activity. This can be especially important when trying to reconstruct an eruptive sequence from a poorly exposed succession of deposits. Recall that some pyroclastic deposits are emplaced in a highly turbulent regime (e.g., [57]), so

#### **Figure 11.**

*Example of a volcano-sedimentary succession in which different volcanic units (lava flows (L), primary pyroclastic deposits (P), and epiclastic deposits (E)) all having originated by reworking of volcanic material (Tenerife, Canary Islands). The presence of paleosoils (Pa) separating some of the deposits is also visible. (credit: Joan Martí).*

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

they can erode previously formed deposits without indicating a change of eruption. In the same way, the existence of rainfall, sometimes torrential, associated with volcanic eruptions is a common fact, and this may cause some primary pyroclastic deposits to be partially reworked during the eruption itself (e.g., [58]). In this case, the maturity of the epiclastic deposit (degree of reworking) will be a criterion to consider in its identification.

Establishing the age of the deposits forming a particular stratigraphic succession is crucial to determine the eruptive history of a particular volcanic system. This will permit distinguishing between several eruptions and also establishing the existence of possible cycles of activity. However, knowing the "absolute" (radiogenic) age is not always possible, since it will depend on the quality (degree of alteration) of the samples, their mineralogy, and the limitations of the method itself. The same restrictions or uncertainties apply with dating based on flora found in ancient pyroclastic or associated deposits. Therefore, what is essential is to establish at least the relative chronology of the set of deposits studied.

The importance of establishing a correct stratigraphy for correlation purposes relies on the fact that pyroclastic materials can be deposited over wide extensions, sometimes exceeding the limits of the basin itself, which means that these volcaniclastic horizons occasionally constitute excellent correlation levels. As previously mentioned, we should also consider that a volcanic eruption represents a very short period of time (generally hours or a few days), which, when translated to the geological scale, means an instant; in this way they can be considered as a physical representation of an isochron.

In recent years, magnetostratigraphy, which uses variations within the stratigraphic sequence of the magnetic properties of rocks (magnetic susceptibility and direction of remanent magnetism), has emerged as an excellent method for geological correlations (see [59]) and particularly in old volcanic terranes (see e.g., [60]). In this sense, we must consider that magnetostratigraphy, together with radiometric or fossil dating, allows us to obtain not only relative ages but also an absolute timescale of the volcanic succession.

In any case, it must be kept in mind that when going backward (toward older terrains) in the examination of paleovolcanic terrains, the geological timescale is progressively less well defined, so we may find that simple stratigraphic unit levels are representing very important periods of time, even several million years long, as the degree of preservation of volcanic materials becomes worse proportional to the age of the terrain. However, in fact, a volcanic level—and especially those of pyroclastic origins—represents an instant not only on the geological timescale, but also on the human timescale. For this reason, we must be very careful in interpreting the chronostratigraphic value of volcanic units in ancient terrains since each deposit by itself represents a single event in geological time but corresponds to the culmination of long geodynamic and magmatic processes that may extend significantly longer than the observed stratigraphic succession.

Volcanic deposits may show significant lateral variations from the vicinity to the vent to the areas away from it (e.g., [61]). In this sense, it is worth mentioning that proximal to distal definition is far not as fixed as in normal sedimentary environment. In volcanic systems, these can be in a very broad range, even with similar eruption styles but different eruption intensity, eruption rate, etc. This is particularly important in paleovolcanic systems where we have limited spatial knowledge about them system. So what we see in the cross sections offered by most outcrops needs to be scale up to 3D to be able to provide an "intelligent guess" for the location's position relevant to the source.

#### **Figure 12.**

*Example of a) proximal (co-ignimbrite lag breccias), b) intermediate (units of ignimbrites showing a characteristic planar basal contact), and c) distal (distal, > 100 km away from the vent, strongly indurated (silicified) ash fallout deposits), deposits from the Permo-carboniferous volcanism of the Catalan Pyrenees (NE Spain) (credits: Joan Martí).*

Depending on the distance to the vent, volcanic deposits can be proximal, intermediate, or distal. For example, fall deposits will progressively decrease in thickness and grain size with the distance from the vent. For ballistically emplaced deposits, a more or less radial distribution can be observed around the vent, but the deposits associated with the horizontal dispersion of the eruptive column will present a distribution that will depend on the orientation of the prevailing winds, although the proximal to distal distribution will be as mentioned before. The deposits generated by PDCs may also present significant lateral variation with distance from the vent. In the case of deposits emplaced from dense PDCs, they may correspond to thick units (intra-formational breccias) in the proximal zones (**Figure 12a**), massive ignimbrites in the intermediate zones (**Figure 12b**), and co-ignimbritic ash layers in the most distal areas (**Figure 12c**). Deposits emplaced from dilute PDCs, especially those associated with tuff ring or cone-type edifices, present a very characteristic distribution from the vent to the distal zones, with significant changes in their internal sedimentary structures. However, in ancient volcanic terrains, especially for those in which later tectonic processes have been important, it is possible that only parts of the geological record corresponding to volcanic activity have been preserved, so that these variations from proximal to distal will probably only be assumed on the basis of variations in the deposit's thickness, grain size, or rock type [61–63].

Within this ideal model of proximal-distal (referred to the vent area) variations in volcanic terrains (**Figure 1a** and **12**), proximal areas are mainly represented by lava flows, domes, and coarse-grained primary pyroclastic deposits or epiclastic volcaniclastic materials generated by erosion and gravitational processes, which act on the steep slopes of the volcanic edifice. In deeply eroded terrains, proximal areas may also include different groups of subvolcanic intrusive rocks (stocks, sills, and dykes). The presence of a significant fumarolic alteration is also a good guide to identify proximal zones in paleovolcanic terrains. The intermediate areas are mostly represented by the terminal parts of lava flows and thick successions of PDC, as well as some fallout deposits and their reworked products. Increasing the distance from the vent also increases the amount of re-sedimented pyroclastic material and epiclastic deposits. Finally, the distal areas will be formed by fine-grained fallout deposits interbedded with abundant non-volcanic sedimentary material.
