Volcaniclastic Megaturbidites: the Footprint of Pyroclastic Flows Entering Bodies of Water

Abstract:

There are many scenarios where pyroclastic flows erupted on land flow into large bodies of water such as lakes or the sea, or where they form directly from eruptions underwater. The nature of the final deposits will depend on the degree of mixing with ambient water. Close to entry points into water (shorelines, subaqueous vents) the deposits may be very similar to subaerial pyroclastic flow deposits, preserving the poor sorting and thermal characteristics (welding, high TRM) because the flows have not mixed external water into their interiors.

With increasing distance from such entry points many flows will mix thoroughly with the ambient water mass leading to transformation of hot, gas supported pyroclastic flows into water transformed mass flows. Water-supported mass flows are able to better hydraulically sort their components and to lose heat. Many deposits show a vertical grain size and componentry grading, and enhanced levels of hydraulic sorting compared with subaerial pyroclastic flow deposits. They can usually be recognised as being syn-eruptive because they are monomictic, consisting overwhelmingly of juvenile pyroclasts (pumice, glass shards, dense juvenile clasts), and a variety of accessory lithics that are mostly volcanic in provenance. Their thickness is indicative of the magnitude of the eruption event. Deposits cored off Montserrat from recent small volume pyroclastic flows entering the sea, are thin, but the rock record preserves individual beds/deposits that are 10s of metres or more thick, indicating the entry of very large volume pyroclastic flows into water. Such deposits can be called syn-eruptive volcaniclastic megaturbidites, because the final depositional mechanism was turbidity current-like water-supported flow and not gas-supported (pyroclastic flows, sensu stricto).

They can be distinguished from normal terrigenous turbidites which are polymictic, usually < 1-2m thick. Even terrigenous megaturbidites or seismo-turbidites are < 10m thick, unless channelized (<25m). Very thick (mega)turbidites thicker than 10m and dominated by juvenile pyroclasts can only have been generated by extraordinarily large, instantaneous supply events, such as very large volume explosive eruptions, sometimes super-eruptions.

Modern and ancient examples will be discussed.

There are many scenarios where pyroclastic flows erupted on land flow into large bodies of water such as lakes or the sea, or where they form directly from eruptions underwater (Fig.). The nature of the final deposits will depend on the degree of mixing with ambient water. Close to entry points into water (shorelines, subaqueous vents) the deposits may be very similar to subaerial pyroclastic flow deposits, preserving the poor sorting and thermal characteristics (welding, high TRM) because the flows have not mixed external water into their interiors.

With increasing distance from such entry points many flows will mix thoroughly with the ambient water mass leading to transformation of hot, gas supported pyroclastic flows into water transformed mass flows. Water-supported mass flows are able to better hydraulically sort their components and to lose heat. Many deposits show a vertical grain size and componentry grading, and enhanced levels of hydraulic sorting compared with subaerial pyroclastic flow deposits. They can usually be recognised as being syn-eruptive because they are monomictic, consisting overwhelmingly of juvenile pyroclasts (pumice, glass shards, dense juvenile clasts), and a variety of accessory lithics that are mostly volcanic in provenance. Their thickness is indicative of the magnitude of the eruption event. Deposits cored off Montserrat from recent small volume pyroclastic flows entering the sea, are thin, but the rock record preserves individual beds/deposits that are 10s of metres or more thick, indicating the entry of very large volume pyroclastic flows into water. Such deposits can be called syn-eruptive volcaniclastic megaturbidites, because the final depositional mechanism was turbidity current-like water-supported flow and not gas-supported (pyroclastic flows, sensu stricto).

They can be distinguished from normal terrigenous turbidites which are polymictic, usually < 1-2m thick. Even terrigenous megaturbidites or seismo-turbidites are < 10m thick, unless channelized (<25m). Very thick (mega)turbidites thicker than 10m and dominated by juvenile pyroclasts can only have been generated by extraordinarily large, instantaneous supply events, such as very large volume explosive eruptions, sometimes super-eruptions.

Modern and ancient examples will be discussed.