Alejandro Lomoschitz

The maximum warmings of the Pleistocene world climate recorded in the Canary Islands

Abstract Evidence of the warmest Pleistocene climatic changes is preserved in the eastern Canary Islands. Although the existence of raised fossiliferous marine deposits in northern Gran Canaria has been known since the 19th century, their chronology and significance with regard to past sea levels and temperatures have remained uncertain. Here, we show three marine transgressions recorded by raised marine deposits. The highest (85 m above sea level (masl)) and oldest marine deposit is interbedded in basaltic lava flows and has been dated using both palaeomagnetic and radiometric methods. The age of these deposits, located on the northwest coast of Gran Canaria, near the town of Agaete, places the marine transgression in a first Pleistocene interglacial into the Plio–Pleistocene limit. The marine deposit at an intermediate altitude (35 masl) has been dated using the unspiked K–Ar method on a pillowed basaltic lava flow. The age of these deposits in Arucas places the transgression in the penultimate (European Mindel/Riss sense) interglacial. The deposits at 12 masl in Las Palmas are attributed to the last Pleistocene interglacial on the basis of their fossil content, which is similar to that of the Matagorda deposits in Lanzarote and Matas Blancas in Fuerteventura. Radiometric dating of a lava flow overlying similar deposits at El Cotillo in Fuerteventura has been carried out. The molluscan fauna associated with the deposits permits an estimation of the sea temperatures by reference to their present biogeographical distributions. The Plio–Pleistocene boundary coincides at Agaete (end of Olduvai event) with a high sea level between 1.8 and 1.75 Ma (oxygen isotopic stage (OIS) 63), the dated marine deposits at Arucas (0.42 Ma) place an interglacial in OIS 11.3 and finally we confirm that, in the Canaries, with only one high sea level with a Senegalese fauna (OIS 5.5), they are not an outpost for the Mediterranean Tyrrhenian (deposits with Strombus) oscillations.

The Barranco de Tirajana basin, Gran Canaria (Spain). A major erosive landform caused by large landslides

The Barranco de Tirajana (BdT), located on the island of Gran Canaria (Spain), has some specific features that differentiate it from the ravines of other volcanic islands in the Atlantic Ocean. The origin of this unusually wide upper basin (35 km 2 ) has been under discussion over the last century although its erosional origin is nowadays widely accepted. The purpose of this paper is to describe the landslide deposits that appear at the bottom of the basin and to assess their role in the geomorphological evolution of the basin. We suggest that the BdT basin is a major erosional landform initiated by important ravine incision and widened by a large number of landslides. There are 28 large landslides within the BdT basin. The main movements were of rock-slide and debris-slide types, affecting 70% and 25% of the total area, respectively. In addition, modes of displacement were predominantly translational (rock-, debris-, and earth-slides) consisting of 89% of the total, compared to rotations and flows that constitute only 11%. Three main periods of landslide activity have been established in the development of the BdT basin, using geomorphological criteria. Period I includes ancient movements that could have started at about 0.6 Ma or even 2.7 Ma ago and are considered as abandoned landslides. Period II corresponds to old landslides considered as dormant, which occurred within the Middle‐Upper Pleistocene. Finally, period III includes recent Upper Pleistocene landslides and Holocene landslides that are still active. We suggest that interglacials in the Canary Islands and NW Africa included humid and wet episodes that could account for the occurrence of periods of landslide activity in the BdT basin. D 2002 Elsevier Science B.V. All rights reserved.

Güímar and La Orotava Mega-Landslides (Tenerife) and Tsunamis Deposits in Canary Islands

More than 20 mega-landslides have been described in the Canary Islands affecting the flanks of the volcanic edifices. Guimar and La Orotava landslides, in Tenerife, are two exceptional cases due to their huge dimensions and outstanding geomorphological features. The estimated volume of these landslides exceed tens of cubic km. Tsunami deposits have been also identified in some of the islands of the archipelago probably associated to the large landslides of the islands flanks.

Evolution of Pajonales Landslide (Tirajana Depression, Gran Canaria): A Case of Advancing Landslide

We studied the evolution of different stages of Pajonales landslide (Tirajana Depression, Gran Canaria), based on the geotechnical investigations of both in situ and mobilized volcanic materials. The deposit extends over 560 ha and it has undergone successive reactivations, some during the 20th century. The landslide comprises four large bodies that have successively broken away from a single initial rock mass. The main scarp affects the lava flows with intercalations of pyroclastic materials of the Roque Nublo Group and later volcanic activity (5.5 Ma to present-day). The basal surface of the landslide developed in old rocks of the Mogan Group (14.0–13.3 Ma), which are rhyolitic and trachytic ignimbrites with hydrothermal alteration related to the infilling of the Tejeda caldera. This alteration caused silty-clay layers presenting a low friction angle, high plasticity and expansive behavior. Representative samples of pyroclasts and soils from landslides were collected and laboratory tests were performed to identify them, determine unit weight, grain-size, plasticity, and shear strength. The morphology of slope prior to sliding was reconstructed considering the location of the failure surfaces and scarps of first and second generation. Failure surfaces for each stage of sliding were identified using software of limit equilibrium analysis. These surfaces have been developed through clayey-silt levels that result from the alteration of pyroclastic materials, showing the most unfavorable geotechnical parameters (minimal or residual values). Moreover, the presence of water is a triggering factor, since total or partial saturation of the materials is required. Finally, the landslide is an advancing type: as successive reactivations occur, the sliding masses are broken down into smaller ones moving towards the Tirajana ravine.