Sedimentary signatures of the entrance of coarse-grained volcaniclastic flows into the sea: the example of the breccia units of the Las Palmas Detritic Formation (Mio–Pliocene, Gran Canaria, Eastern Atlantic, Spain)

The transformations of volcaniclastic gravity flows entering the sea remain poorly understood. They are recorded by sedimentary signatures in the volcaniclastic sedimentation of the Mio–Pliocene Las Palmas Detritic Formation (LPDF) of the volcanic island of Gran Canaria. These gravity flows are contemporaneous with an erosive period on the island succeeding the emplacement of the Fataga Volcanic Group, and the beginning of the evolution of the Roque Nublo stratovolcano. The sedimentary section that crops out at the mouth of a paleobarranco at Las Cuevas del Guincho comprises subtidal calcareous and clastic sediments affected by wave and storm action (Units 1 and 3), beach gravels (Unit 4), volcaniclastic deposits represented by reworked phonolitic pyroclastic deposits (Unit 1), breccia deposits that contain juvenile and phonolitic gravels and blocks mainly incorporated at the shoreline (Unit 2), and a debris avalanche deposit at the top (Unit 5). The breccia deposits display a reversely graded base which disappears distally as a result of seawater incorporation and liquidization of the volcaniclastic mass and mixing with marine sediment. This breccia unit is interpreted as having been emplaced by a granular mass flow after transformation of a block and ash flow deposit that entered the sea. In the debris avalanche deposits, thermal quenching of juvenile basaltic lava clasts and large inclined clastic intrusions of underlying marine sediments suggest that the debris avalanche entered the sea. Rapid subaqueous emplacement of the granular mass flow and of the debris avalanche deformed the sedimentary substratum. Water incorporation reduced the influence of the volcaniclastic flows on the substratum. AMS data confirm the effect of water ingestion and correlative loss of cohesion and allow also reconstruction of paleoflow directions that were normal to the paleoshoreline of the northern coast of Gran Canaria. All these volcaniclastic gravity flow deposits and their textural characteristics are particularly useful in the reconstruction of processes that occur at the subaerial–submarine transition and for the location of ancient shorelines.