Field descriptions, morphostratigraphic analysis, and geochronology

The complementary work of geomorphological mapping and field survey made it possible to recognize five tephra layers included in several morphostratigraphic profiles. These profiles were recorded from geomorphological units of different ages and genesis. The tephra levels were not continuous across the entire study area (note that we prospected two neighboring valleys), so stratigraphic positions were recorded in order to reconstruct a wide and confident morphostratigraphic context. The five tephras appear interbedded in several morphosedimentary contexts, from which we inferred their ages. Although about 150 profiles were described during the fieldwork, only seven were selected for this study (Figs. 1, 3, and 4), considering the presence of overlying tephra layers, absolute chronological data, and morphostratigraphic representativeness. We selected the RI01 profile (Fig. 3a) for the oldest tephra (V0) and RI03 (Fig. 3d) for V1a and V1b tephras. Besides, V1a, V1b, and V2 tephras were described and dated in CA2 and CA3 profiles (Fig. 3b and c), while V2 was described and dated in Inf2, El Paso 2, and CB7 profiles (Fig. 4, Table 1). After the stratigraphic records it was clear that the three tephras preliminary established were in fact five.

Figure 3. (color online) Representative stratigraphic profiles showing different morphosedimentary units: El Rincón (RI01) profile, with the chronostratigraphic location of the V0 tephras (a); Las Carreras (CA2 and CA3) and El Rincón (RI03) profiles, with chronological data of V0, V1a, V1b, and V2 tephras (b, c, d).

Figure 4. (color online) Stratigraphic profiles of the (a) El Infiernillo (INF2), (b) EL PASO2, and (c) Campo Blanco (CB7) showing tephra positions and chronologies in the different morphosedimentary units.

Table 1. ¹⁴C samples: u.s., underlying sediments; o.s., overlying sediments.

The cross sections recorded in several valleys contribute to improving the knowledge of the relative positions of the tephras (Fig. 2b). Several factors conditioned their positions and thickness. First, topography and wind circulation favored tephra accumulation in some specific locations (i.e., at present some deposits in the ravines reach 8 m in thickness), whereas there is no tephra accumulation in several outcrops because of environmental erosion. Another important factor related to tephra visibility is the ravine depth, with incisions in some cases reaching the oldest deposits (Fig. 2b). In all cases, tephra visibility is conditioned not only by its relative present position but also by the processes the tephra underwent during the geomorphological evolution of the area.

Tephra V0 never appears on the topographic surface. It is only visible in the incision scarps, rendering it an excellent marker for the transition between H1A and H1B accumulations (Figs. 2a, 3a). They are frequently visible in the bottom area of Las Carreras, El Rincón (Fig. 5a), the quarries exploited close to Tafí del Valle village (Fig. 5b), and the Carapunco and La Bolsa areas (Tafí valley). These tephras have a coarse ash texture and a high amount of biotite. Radiocarbon ages were obtained at El Rincón (RI01, Fig. 3a), on a 12-m-thick outcrop, where V0 tephra is located 5 m from its base (Fig. 3a). There are several peat layers under the ash level, the youngest dated to 11,802–11,192 cal yr BP (sample RI01-T5; Fig. 5a, Table 1). Accordingly, this tephra layer is younger than these ages (Fig. 6a). The tephra V0 was named El Rincón ash given the relevance of the information provided by the RI01 profile.

Figure 5. (color online) (a) RI01 profile showing V0 and V1 tephra positions and dated peat levels, El Rincón–Tafí valley (see Fig. 3a). (b) Profile from the quarries of La Costa (Tafí valley) showing V0 and V1 tephra levels. (c) V1 tephra from the top of an H1B unit in Tafí valley. (d) Lower section of CA2 profile showing V1a and V1b tephras with an interbedded peat in the Muñoz River, Tafí valley (see Fig. 3b). (e) INF2 profile showing V2 teprha lying over a peat level, El Infiernillo (Tafí–Santa María valleys water divide; see Fig. 4a). (f) Upper section of CA3 profile showing V2 position in the Muñoz River, Tafí valley (see Fig. 3b).

Figure 6. (color online) Synthesis of available chronological data and relative position of tephras showing the most probable sedimentation time and the reconstruction of the different events: V0 tephra (a), V1a and V1b events (b), V2a and V2b events (c).

Previous works identified in the study area one single Middle Holocene tephra that was dated from underlying peats to after 4290±40 ¹⁴C yr BP (4955–4618 cal yr BP) by Fernández-Turiel et al. (Reference Fernández-Turiel, Saavedra, Pérez-Torrado, Rodríguez-González, Alias and Rodríguez-Fernández2012, Reference Fernández-Turiel, Saavedra, Perez-Torrado, Rodríguez-González, Carracedo, Osterrieth, Carrizo and Esteban2013) and later to after 3763 ± 36 ¹⁴C yr BP (4228–3927 cal yr BP) by Sampietro-Vattuone and Peña-Monné (Reference Sampietro-Vattuone and Peña-Monné2016). However, during geomorphological mapping and field survey, two different tephras related to the top of the H1 morphostratigraphic unit were observed in several profiles. The two identified tephras were named V1a (Carreras 1a ash) and V1b (Carreras 1b ash), and the representative profile is the CA2 section in Figure 3b. The V1a and V1b tephras were much easier to locate and distinguish than the other tephras. However, they are virtually impossible to differentiate in the field, unless they are in the same profile. While recording V1a and V1b tephras, we found catena sequences across alluvial fans where the ashes lie on top of the outcrops at the apex of the alluvial fan (Figs. 2b, 5c) and in the middle section of the outcrops recorded in the middle-low fan area (Fig. 5d). The longitudinal profiles recorded, as described in the methodology, allowed us to understand the evolution of the landforms, reinforce the value of the tephras as sedimentary, chronological, and evolutionary indicators and differentiate between H1 and H2 units when they overlap (CA2 and CA3 profiles in Fig. 3b and c, 5d). In general, V1a and V1b tephras indicate the limit between the Middle and Late Holocene, coinciding with a change in the geomorphological dynamics of the region, because the incision period separating H1 and H2 accumulation phases was just starting when the tephras fell (Sampietro-Vattuone and Peña-Monné, Reference Sampietro-Vattuone and Peña-Monné2016) on top of H1B accumulations, filling shallow incisions (Fig. 2b).

As in some cases the outcrops of V1a and V1b developed interbedded peats (Fig. 5d), it was possible to make radiocarbon dating in the intermediate organic matter layers (Fig. 3b and c). In V1a, ages are later than 4789–4289 cal yr BP and earlier than 3830–3470 cal BP, while V1b tephra was dated later than 3830–3470 cal yr BP (Table 1; Figs. 3b and c, 6b). The estimated age of V1b tephra is younger than that of H2 unit, with H2 accumulation starting earlier than 2760–2188 cal yr BP, which is the radiocarbon dating of a paleosoil (s1; Fig. 2a and b) interbedded in the lower section of H2 (Sampietro-Vattuone and Peña-Monné, Reference Sampietro-Vattuone and Peña-Monné2016; Peña Monné and Sampietro Vattuone, Reference Peña Monné and Sampietro Vattuone2018c). Figure 6 offers a graphical synthesis of all available chronological data related to V1a and V1b tephras, demonstrating the existence of two eruptions very close in time and very similar in composition (see “Physicochemical characterization” section).

Finally, thin tephra layers related to the H3 morphostratigraphic units were observed in 48 profiles (i.e., Fig. 5e and f), in some cases overlying archaeological materials of the Late Period (Fig. 4b). Despite its similar compositional and macroscopic features, two different tephras were recognized, as they were dated in several outcrops (Figs. 4, 6c; Table 1). Considering only radiocarbon evidence from the CA2 profile (Fig. 4), it is possible to establish that V2 tephra is older than 991–774 cal yr BP and younger than 655–624 cal yr BP (Fig. 6c). However, other radiocarbon information from Inf2-T5 suggests that V2a tephras occurred after 716–553 cal yr BP (Figs. 4, 6c; Table 1). Moreover, the V2b tephras are lying above the paleosoil (s2) dated to 497–468 cal yr BP (Sayago et al., Reference Sayago, Collantes and Niz2012) and 490–333 cal yr BP at El Paso 2 (Sampietro-Vattuone et al., Reference Sampietro Vattuone, Peña Monné, Maldonado, Sancho Marcén, Báez and Sola2018a). There is also another V2 sample interbedded in an H3 unit at El Paso 3. This unit was estimated to be younger than 600 yr BP (Fig. 6c) according to the evolutionary model of Sampietro-Vattuone and Peña-Monné (Reference Sampietro-Vattuone and Peña-Monné2016) and Peña Monné and Sampietro Vattuone (Reference Peña Monné and Sampietro Vattuone2018c). All this evidence points to the existence of two upper Holocene tephras named V2a (Carreras 2 ash) and V2b (El Paso 3 ash) (Fig. 6c).

Several features associated with the tephras, especially V1a and V1b, were also recorded in order to have a better outcrop characterization. In many cases, the thickness of each tephra could vary from a few centimeters to more than 8 m (Fig. 7a). Besides their thickness and position (Figs. 5c and d, 7a), some tephras show laminar structures at their base, followed by massive, thick deposits (Fig. 7b). In other cases, when exposed on the surface as mantles, they later developed different and distinctive erosive features such as yardangs (Fig. 7c) and taffonis and crusts in vertical exposures (Fig. 7d). There are also bioturbations represented by roots, worms, and rodent galleries filled with ashes (Fig. 7e–g). Some deposits show interbedded sand facies (Fig. 7h) and oxides, the latter normally associated with peat formations.

Figure 7. (color online) Features associated with tephra deposits: thick V1 deposits (a), V1 basal laminations with deformation (b), V1 tephra yardangs (c), weathered scarp (taffonis) and crust formed on exposed V1 massive tephra deposits (d), worm bioturbation on V1 tephra deposit (e), roots on V1 tephra level (f), rodent galleries crossing V1 tephra deposit (g), and V1 tephras interbedded with sand facies including oxides (h).

Physicochemical characterization

The mineralogical, textural, and geochemical features (Supplementary Table 2) of the five tephras recognized in the Tafí and Santa María valleys are described in the following paragraphs. Because V1a-V1b and V2a-V2b pairs are impossible to differentiate by their mineralogy, glass shard-pumice morphology, or geochemistry, both pairs are described together.

The V0 tephra is light brown and composed mainly of pumice particles and glass shards (~70%) and, to a lesser extent, crystal and lithics. The main mineral assemblage is biotite, quartz, and feldspars (mainly plagioclase) (Fig. 8a). The abundance of large crystals of biotite is an important feature that makes it possible to differentiate V0 from the other four tephras. In addition, magnetite, titanite, pyroxene, and muscovite are present as accessories. The lithics are granitic rocks, and, together with muscovite crystals, they represent a low degree of contamination of the original deposits. In V0 tephra, there is evidence of an incipient edaphization process such as the presence of oxide patinas, silty material, and organic matter, in concordance with the bioturbation observed at outcrop scale. The V0 tephra presents different morphological types of juvenile clasts. The most abundant ones are highly vesicular, irregular to subrounded pumice particles with small spherical to subspherical vesicles (Fig. 9a) or more elongated and tubular shapes separated by thin glass walls. The high vesicular pumice particles are usually covered by fine ash particles that hinder the characterization of the vesicle morphology (Fig. 9a). Another morphological type is represented by subangular, low vesicular to dense glass shards (Fig. 9b). The fine ash fraction is composed of small glass shards with flat plate and curved morphologies (Fig. 9c) derived from the fragmentation of large, highly vesicular pumice particles. In addition, we identified a low percentage of rounded, massive, slightly prolate aggregates of fine ash (Fig. 9d).

Figure 8. (color online) Photographs taken with binocular magnifying glass: abundance of large biotites (Bt) in V0 tephra (a); abundance of glass shards with respect to the crystal proportion in V1 tephra (b), and V2 tephra characterized by the high content of hornblende (Hbl) (c).

Figure 9. Scanning electron microscopy images: highly vesicular irregular to subrounded pumice particles in V0 tephra (a), subangular low vesicular to dense glass shards in V0 tephra (b), glass shard with flat plate and curved morphologies from V0 tephra (c), aggregates of fine ash from the V0 tephra (d), pumice clasts from V1 tephra characterized by rounded vesicle morphology separated by thick glass walls (e), pumice clasts from V1 tephra with fluid forms and elongated to tubular vesicles (f), highly vesicular pumice with complex vesicle shapes from V2 tephra (g), and tubular pumice in V2 tephra (h).

The V1a and V1b tephras are white and composed mainly of pumice particles and glass shards (~70%) and, to a lesser extent, crystals and lithics. The main mineral assemblage is quartz, feldspars (mainly plagioclase), and biotite (Fig. 8b). Magnetite, pyroxene, and amphibole are present as accessories. Edaphization and rework processes are not evident. The main compositional difference from V0 tephra is the higher percentage of pumice and glass shards and the lower proportion of crystals and crystal fragments. As in V0 tephra, the most conspicuous particle type is the highly vesicular, irregular to subrounded pumice. However, the pumice clasts from V1a and V1b tephras are characterized by more rounded vesicle morphology separated by thick glass walls (Fig. 9e) and a better development of clasts with fluid forms related to elongated and tubular vesicles (Fig. 9f). Large, subangular, low vesicular to dense glass shards and irregular prolate aggregates of fine ash are also present. The fine ash fraction consists of small glass shards with flat plate and curved morphologies.

The V2a and V2b tephras are light gray and composed of pumice particles and glass shards, with a relatively high proportion of crystals (~30%) and lower percentages of lithics. The main mineral assemblage is amphibole, quartz, feldspars (mainly plagioclase), and biotite (Fig. 8c). The abundance of amphibole is the key feature to differentiate V2a and V2b tephras from the other three tephras. Clinopiroxene, magnetite, apatite, muscovite, and rutile are present as accessories. The V2a and V2b tephras present similar types of juvenile components in terms of morphology and size. However, the most relevant feature of these tephras is the occurrence of irregular, highly vesicular pumice with complex vesicle shapes (Fig. 9g) and tubular pumice (Fig. 9h).

The data generated by pXRF are not suitable for classical geochemical classifications like the Total Alkali Silica (TAS) diagram (e.g., Le Bas et al., Reference Le Bas, Le Maitre, Streckeisen and Zanettin1986) because some elements such as Na are not measured. However, Sr/Rb versus K/Sr ratios are essentially controlled by feldspar proportion and thus reflect approximately the igneous rock type (Sola et al., Reference Sola, Báez, Bustos, Hernandez, Sampietro Vattuone, Peña Monné and Becchio2016). In this sense, Figure 10a shows the trace element ratios measured in our samples (Sr/Rb; K/Sr) compared with a database of volcanic rocks of the Central Andes (Mamani et al., Reference Mamani, Wörner and Sempere2010). Our geochemical data, along with the mineralogy of each ash level, indicate that V0, V1a, and V1b are probably rhyolitic in composition, whereas V2a and V2b are probably more dacitic in composition. This inference is consistent with the mineral assembly previously described. In addition, scatter plots combining trace elements with contrasting behavior, such as Rb-Sr or Zr- Sr, were used to identify the geochemical pattern for each tephra (Fig. 10b). The V1a and V1b tephras are characterized by high Rb content (>200 ppm) and low Sr content (<100 ppm). In contrast, V0, V2a, and V2b have low Rb content (<200 ppm) but can be differentiated by their Sr content: V0 < 300 ppm and V2a and V2b >300 ppm. In addition, V1a and V1b have lower Zr content (<100 ppm) than V0, V2a, and V2b (between 100 and 250 ppm).

Figure 10. (color online) (a) Relative geochemical classification using Sr/Rb versus K/Sr diagram proposed by Sola et al. (Reference Sola, Báez, Bustos, Hernandez, Sampietro Vattuone, Peña Monné and Becchio2016), with dacites and rhyolites of the Central Andes compiled by Mamaní et al. (Reference Mamani, Wörner and Sempere2010) and the samples analyzed in this work. (b) AL/BM: geochemical data of the Alemanía and Buey Muerto ashes taken from Hermanns et al. (Reference Hermanns and Schellenberger2008).