Characteristics of the tsunami deposits

Tsunami deposits are preserved at different locations around Isla Baja and Teno Bajo (Fig. 1). The stratigraphy of the sedimentary sequences slightly differs from one site to another (Fig. 2), due to the nature of the substratum underlying the tsunami deposits, and to the lateral and longitudinal discontinuity of the different tsunami units. At Playa de la Arena, two units of tsunami deposits can be distinguished within a paleo-valley (Fig. 3a). Lower unit A is a massive gravel that is clast-supported, very coarse-grained and poorly sorted (from fine pebbles to 1 m large boulders). The dominant population of clasts comes from basaltic lavas that forms nearby coastal cliffs and platforms, but there are also numerous marine bioclasts (Fig. 3b: bivalve and gastropod shells, foraminiferas, calcareous algae, coral fragments), rare rounded pebbles from the beach, and rare pumices. The thickness of unit A ranges from 40 cm to 1.5 m, but its base is not visible. Upper unit B is also a coarse gravel, but it is slightly finer than unit A, matrix-supported, and considerably enriched in pumices relative to unit A. Unit B is thus a pumiceous gravel, and its composition is much more varied than unit A: local-derived basalts are mixed with phonolites, hydrothermally altered rocks, syenites, obsidian and subangular pumices. The dominant type of pumices is light-green coloured, highly vesiculated, and fibrous. The matrix is sand to silt-sized, and contains abundant bioclasts. The contact between units A and B displays large scour-and-fill features. Unit B is particularly thick (up to 2 m) and crudely structured into subunits with local variations of grain size and composition. The subunits are often separated by a thin layer of very fine material (ash size). There is no vertical size grading in the individual units and subunits, except in some inversely-graded clast-supported lenses. The two units of tsunami deposits are overlain by a consolidated lithic-rich pumiceous gravel that is less than 1 m thick. No marine bioclasts could be found in this gravel, which could correspond to the reworked facies of a lithic-rich ignimbrite (Fig. 3a). Another coastal section of tsunami deposits was found near El Puertito, on the northern coast of Isla Baja (Fig. 1), where tsunami unit A overlies a palaeosoil on a lava flow dated 194 ka (ref. 20).

Figure 2: Stratigraphic logs of the main sections. Tsunami unit A is a very coarse-grained gravel that is clast-supported, and poorly sorted (from fine pebbles to boulders). Composition of tsunami unit A is dominated by locally-derived basaltic lavas, but there are also numerous marine bioclasts (bivalve and gastropod shells, foraminiferas, calcareous algae, coral fragments) and rare pumices. Tsunami unit B is a coarse gravel, matrix-supported, and considerably enriched in pumices relative to unit A. Unit B and the underlying El Abrigo breccia have a similar composition: local-derived basalts are mixed with phonolites, hydrothermally altered rocks, syenites, obsidian, pumices, and few marine bioclasts. The contact between tsunami units A,B displays large scour-and-fill features. Unit B is crudely structured into subunits with local variations of grain size and composition. Note that tsunami unit A is absent at elevations higher than 21 m a.s.l. See Fig. 1 for location of the sections. Full size image

Figure 3: Tsunami deposits at Playa Arena and Teno Bajo. (a) Succession of tsunami units A,B at Playa de la Arena (Isla Baja); (b) Thin section of tsunami deposits at Playa Arena (tsunami unit A). Note the numerous marine bioclasts (fragments of bivalve shells, bryozoans, coralline algae, and foraminifers); (c) General view of the tsunami deposits at Teno Bajo (altitude 18 m a.s.l.) with a lava flow dated 178 ka outcropping on the lower right corner; (d) Tsunami unit A eroded by tsunami unit B with floating boulders from the underlying 178 ka lava flow at Teno Bajo. Note the difference between tsunami units A,B in terms of texture, mostly due to the abundance of pumices in unit B. Scale bars in centimeters. Full size image

Tsunami deposits are also exposed on the Teno Bajo peninsula at altitudes between 15 and 50 m a.s.l. (Fig. 3c). Some outcrops have been briefly described23,24 and a tsunami origin was proposed25,26. Post-depositional erosion reduced the deposits to small patches on top of a basaltic lava flow dated 178 ka (ref. 27). A 40 cm thick sand layer is intercalated between the lava flow and the tsunami deposits. The stratigraphy of the deposits slightly differs from one section to another, but a synthetic log can be traced as follows (Fig. 2). Lower unit A is a coarse gravel fining landward (from very coarse to medium pebbles, in a very coarse sand matrix), with rare pumice clasts, and particularly rich in fragments of bivalve shells. As in Playa de la Arena, Unit A has no internal structure. The majority of the clasts are angular to subangular fragments of basaltic lavas coming from the underlying lava flows. Unit A is clearly residual here and its thickness ranges between 5 and 50 cm (Fig. 3d). There is no dominant orientation of the elongated clasts. The basal contact between unit A and the sand layer is erosional. Compared to unit A, upper unit B is considerably enriched in pumice clasts, thus forming a 0.5–1.5 m thick pumiceous gravel where different subunits can be distinguished (depending on variations of grain size and proportion of pumices). The indurated matrix is essentially made of fine pumice fragments. Marine bioclasts are also present in various abundances from one subunit to another. Mean grain size ranges from fine to coarse pebbles, but larger clasts up to metric boulders can be found. The lower part of unit B is characterized by a crude stratification forming discontinuous lenses or trains of imbricated and inversely graded clasts. The orientation of the elongated or imbricated clasts is alternatively landward and seaward (Fig. 3d), except in the uppermost subunit where clast imbrication is clearly seaward. It is worth noting that this uppermost subunit is also particularly rich in rounded pumice lapilli and marine bioclasts. Units A and B can be traced respectively up to 320 and 700 m inland (21 and 50 m a.s.l.).

Traces of the tsunami deposits were identified at several locations around a volcanic cone named Taco and dated 706 ka (ref. 27; Fig. 1). On the western flank of the cone, tsunami unit B was identified at altitudes between 115 and 132 m a.s.l. The main section (quarry at 132 m a.s.l.) displays tsunami unit B as a massive pumiceous gravel eroding a succession of pyroclastic deposits, colluvial deposits and palaeosoils (Fig. 2). The tsunami also truncates a layer of cream-coloured pumice lapilli interpreted as a Plinian fall deposit (Fig. 4a). Thickness of the tsunami gravel is irregular, ranging from 50 cm to 3 m. Erosion of the substratum is illustrated by 20–40 cm large rip-up clasts of the underlying palaeosoil (Fig. 4b). The base of the tsunami deposit is characterized by a 2–5 cm thick fine-grained traction carpet. Composition of the gravel is similar to unit B downslope (Playa de la Arena section): fragments of lava flows of different lithology (from basalts to phonolites), pumices and rare obsidians, and marine bioclasts. Different facies of pumices are present: light green pumices (dominant facies), cream pumices, and banded crystal-rich (feldspars) pumices.

Figure 4: Tsunami deposits at Taco and Lomo de las Campanas. (a) Tsunami deposit (pumiceous marine gravel, corresponding to tsunami unit B) at 132 m a.s.l. on the flanks of Taco volcanic cone, eroding pumice fall deposits of the Diego Hernandez III Formation (pre-Abrigo eruption); (b) Rip-up clasts of soil as evidence of substrate erosion at the base of tsunami unit B (Taco outcrop); (c) Contact between the Abrigo breccia and tsunami deposits (tsunami unit B) at Lomos de las Campanas (50 m a.s.l.). Both units have a similar composition but tsunami unit B is matrix-supported and cemented by carbonates; (d) Detailed view of the erosional contact between the Abrigo breccia and tsunami unit B. The contact is characterized by downward injections of the tsunami in the breccia, suggesting basal amalgamation. Scale bars in centimeters. Full size image

At Lomo de las Campanas (50 m a.s.l.) tsunami unit A is also absent and tsunami unit B directly scours a clast-supported breccia (Fig. 4c,d), which corresponds to the uppermost subunit of the Abrigo ignimbrite (last eruption of the third Diego Hernandez cycle28,29,30,31,32). The dominant facies of the Abrigo eruption on Isla Baja is a massive lithic-rich ignimbrite, as observed along the coast, but the Abrigo breccia outcrops locally28. Tsunami unit B and the Abrigo breccia share the same heterogeneous composition (basaltic and phonolitic lavas, hydrothermally altered lavas, syenites, obsidian, crystal-rich juvenile clasts), but the tsunami gravel is matrix supported and cemented by carbonates. The contact between tsunami unit B and the Abrigo breccia is clearly erosional (Fig. 2), but its interpenetrative geometry (amalgamated contact) suggests that the two events are closely spaced in time, if not simultaneous (Fig. 4d). There is no weathering horizon between the Abrigo breccia and tsunami unit B.

The abundance of bioclasts differs from one site and subunit to another (upper tsunami unit A being richer than lower unit B), and tends to decrease landward. Bioclasts are never in growth position. The terrestrial fauna is represented by rare gastropod shells and two bones of a giant endemic lizard (Gallotia goliath). More than 1,000 marine bioclasts were analysed and 123 taxons were determined: 85 gastropods, 31 bivalves, 6 corals and 1 scaphopod (Supplementary Table 1). Bivalve shells such as G. glycymeris and Anadora gibbosa, and scaphopods Laevidentalium caudani are particularly abundant. Biodiversity of the marine fauna is particularly rich and represents a mixing of faunas from different environments (depth, substratum), species of the infra-circalittoral zones being dominant. All taxons can be found nowadays in the Canary Islands. Fragmentation of shells is moderate for the gastropods (25%) and high for the bivalves (46%). Bioturbation affects 28% of the marine bioclasts (incrustation, bioperforation).

Age of the tsunami

Tsunami deposits are younger than the 178 ka (Teno Bajo, Playa Arena) and 194 ka lavaflows (El Puertito) on which they rest (Fig. 1). The 40 cm thick sand layer intercalated between the tsunami deposits and the 178 ka lava flow at Teno Bajo suggests that the tsunami did not occur immediately after the emplacement of the lava flow. We could not find evidence of tsunami deposits on the 153 ka lava flow of El Palmar volcano27, which is very close to Playa Arena (Fig. 1). A chronological link between the tsunami, the last major explosive eruption in Tenerife (El Abrigo28,29,30,31,32) and a massive collapse of the north flank of the Las Cañadas central edifice (Icod collapse33,34,35) can be established. The age of the uppermost breccia of the Abrigo ignimbrite, which is stratigraphically concomitant with the tsunami, is still controversial. 40Ar/39Ar ages of feldspars range between 196±6 ka (ref. 29) and 169±1 ka (ref. 36), with discrepancies related to the presence of partly degassed xenocrysts29. Nepheline syenites of the breccia were dated at 179±11 ka (ref. 37) (K-Ar), 183±8 ka (ref. 37) (40Ar/39Ar) and 175±3 ka (K-Ar)38. The Icod submarine debris avalanche has been dated ∼170 ka from shallow seismic33. The age of the Icod turbidite in the Agadir Basin was estimated at 155–175 ka (ref. 39) (coccolithophore biostratigraphy) and 165±15 ka (ref. 40) (Oxygen Isotope Stages). This is concordant with the 161±5 ka (ref. 38) and 158±5 ka (ref. 41) ages of the oldest post-collapse lavas filling the Icod embayment onshore (collected in two different water galleries below the Teide volcanic complex). Thus, published ages point to a major suite of large-magnitude events (explosive eruption, flank collapse and tsunami) affecting Tenerife ca. 170–175 kyr ago. Note that the present-day altitudes of the different tsunami outcrops are out of the range of documented MIS 5.5 marine highstands in the Canary Islands (<12 m a.s.l.)42, except for the Playa de la Arena and El Puertito outcrops.

Origin of the pumices

The pumice clasts found in the tsunami deposits resemble the great variety of pumices of the third Diego Hernandez Formation (DHF III), in terms of colour (light green, grey, cream pumices), crystallinity (from near-aphyric pumice to crystal mush), and texture (fibrous, coarsely to finely banded). The phenocryst assemblage is similar to the DHF phonolites (alkali feldspars, clinopyroxene, biotite, magnetite, titanite). Major and trace elements analyses (and especially the Si/Al and Nb/Zr ratios) are consistent with a DHF III origin of the pumices incorporated in the tsunami deposits (Fig. 5). Despite small differences between the DHF phonolitic units in terms of mineralogy and geochemistry, Si/Al and Nb/Zr values allow us to distinguish two lineages43,44. DHF III products have significantly higher Si/Al and higher Nb/Zr ratios than DHF I and DHF II (Fig. 3). Complex history of titanite fractionation, melting of pre-existing syenite plutons, and mingling with mafic magma explains these variations in trace elements within the different DHF units43,44.

Figure 5: Pumice geochemistry. Nb versus Zr (a) and Nb versus Si/Al (b) of pumices collected in the Tenerife tsunami deposits, compared to pumices of the Diego Hernandez Formation43,44. Error bars (2σ for major elements and ±3% for trace elements) are within the size of the symbols. Full size image

Light green and cream pumice collected in the tsunami deposits both have a typical DHF III signature (Fig. 5). The green pumice is tentatively interpreted as the northern counterpart of plinian fall deposits identified at the base of the Abrigo ignimbrite in the south of the island28. Both deposits share the same composition, colour and texture (high-Nb/Zr, fibrous, near-aphyric, light green pumice). The occurrence of a plinian fall phase at the onset of the Abrigo eruption remains controversial because fallout deposits are absent (or not preserved) below the ignimbrite for most of the outcrops28,30,31. The cream pumice in the tsunami deposits comes from a Plinian fall deposit that is clearly eroded by the tsunami on the western flank of Taco (Fig. 3a: pumice lapilli). The upper part of the cream pumice fall deposit displays traces of pedogenesis suggesting that it is not synchronous with the tsunami and Abrigo eruption (it was thus possibly deposited by a DHF III pre-Abrigo eruption such as Benijos29). The pumice gravel overlying tsunami unit B at Playa de la Arena, Lomo Campanas and Taco (Fig. 2) represents a pumice-rich reworked facies of the Abrigo breccia.