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Tephra Dispersal and Basaltic Tuff Cone Sedimentation in High-Energy Shallow Marine Environments

Gerald M. Ross

Our understanding of the processes of basaltic tuff cone construction and dispersal of volcaniclastic sediment is based on recent examples in which the preservation potential of the deposits is poor. A well-exposed sequence of ancient tuff cones and derivative reworked tephra deposits in the Northwest Territories of Canada can be used as an actualistic model for tuff cone sedimentation and preservation. Structures in interbedded nonvolcanogenic sedimentary rocks indicate that volcanism took place in an energetic shallow marine environment affected by wave, storm-wave, and tidal currents.

Pyroclastic rocks, deposited by hydromagmatic and less common strombolian eruptions, occur as remnants of tuff cones preserved in synvolcanic depressions. Lobate aprons of resedimented tephra flank the tuff cones and consist of near-vent subaqueous channel-fill facies deposited by high-concentration sediment gravity flows that grade into finer grained distal facies deposited by low-concentration sediment gravity flows. These rocks record erosion of the tuff cones by wave attack and subsequent off-cone transport of tephra by return flow from waves and tidal currents. Volcaniclastic sediments deposited marginal to the main fans are composed of reworked pyroclasts and admixed background marine detritus (e.g., ooids) and are characterized by the presence of traction current structures.

The depositional style and geometry of tephra dispersal are affected by the interaction of incoming marine currents with the topography of the cones which act to channelize and strengthen return flow. The abundance of unconsolidated tephra on the flanks of tuff cones favors development of a sediment gravity flow style of deposition, even in a shallow-water setting. A model which considers these factors, in addition to cementation and subsidence history of the cone, is applicable to tephra dispersal systems formed in shoaling oceanic island and ridge settings and coastal environments.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.