Estructura y organización de las coladas volcánicas submarinas: relación entre coladas tabulares y lavas almohadilladas cretácicas en la Cordillera Vasco-Cantábrica (País Vasco)

Abstract

Sheet flows and pillow lavas are the most characteristic products of the submarine efusive volcanic activity which was developed in the Basque-Cantrabrian Basin during the Cretaceous. The morphology of the flows was strongly influenced by the flow rate. The high rates produced sheet flows, while the pillow lavas were generated when the magmatic contribution was lower. Nevertheless, both the sheet morphology and the pillow morphology appear locally related within the same lava flow. At the Meñacoz bay (Biscay), a single lava flow is made of interconnected columnar lava, welded megapillows and pillow lavas. This sequence of morphological facies corresponds to a system of distribution of lava flows similar to that of subaerial pahoe-hoe which would have developed at the end the tabular lava course when the melt was channeled in increasingly smaller units giving rise to pillow lavas. The drainage of the melt from the inner parts of the flow produced the formation of volcanic tunnels. The gravitatory collapse of the roof in the tunnels generated welded columnar breccias. These relations are clear evidence of the existence of two types of pillow lavas, one directly deeply rooted in the volcanic focus, commonly related with low lava flow, and the other, located in the distal parts of the sheet flows, and related to more important melt volume. The first type is located near of the issuing focus, while those of the second group could be generated in distant positions from the focus.