A facies model for a submarine volcaniclastic apron; the Miocene Manukau Subgroup, New Zealand

Submarine volcaniclastic aprons of emergent volcanoes provide a long-term record of their history and are preserved long after the source has been eroded following the cessation of volcanism. A facies model for submarine volcaniclastic aprons is necessary to guide interpretation of ancient uplifted successions and to understand the sedimentary and volcanic processes associated with apron formation. The Manukau Subgroup, New Zealand, includes a well-exposed Miocene submarine apron consisting of the products eroded and erupted from a basaltic-andesite volcanic island complex. The depositional environment, setting, and stratigraphy of the volcaniclastic apron are well constrained, allowing us to conduct a detailed facies analysis and to assess the apron architecture. Lateral downslope and vertical progradational sequences reflect erosion of the subaerial edifice and the direct and subsequent products of contemporaneous volcanism. The volcaniclastic facies vary largely as a function of proximity to source and the types of rock resedimented. The dominant transport method was high-density turbidity currents, with lesser contributions from debris avalanches, debris flows, and dilute turbidity currents. In addition, eruptions from volcanic vents located within the apron are recorded as interbedded water-settled and resedimented pumice and scoria deposits (explosive eruptions), lavas, hyaloclastite, and crosscutting dikes (effusive eruptions). The proximal apron is largely composed of coarse breccia-conglomerate that is locally chaotically disrupted due to the instability promoted by steep depositional slopes and rapid sedimentation rates. The subangular cobbles and boulders originated from nearby (probably coastal) variably vesicular and variably oxidized lavas and were deposited as lithic lags. In this highly energetic environment, very little sandstone was deposited or preserved, and freshly erupted pumice deposits were quickly resedimented. In the medial apron, poorly sorted conglomerate and thickly bedded sandstone, which includes pumice lapilli, built up over the distal apron sequence of thinly bedded, fine-grained volcaniclastic mudstone/sandstone. The presence of pumice lapilli suggests that progradation was a response to voluminous explosive eruptions. Due to their low densities and easy entrainment, pumice lapilli largely bypassed deposition on the steep proximal apron and were sedimented onto the more gently sloping medial apron. Hence, coarse pumice clasts were transported much farther from their source than their dense lithic counterparts. The apron records a shallowing-upward sequence, and the final submarine volcaniclastic facies was deposited within channels that incised the medial and distal apron. The channels record an initial eruption hiatus and erosion of the edifice. Well-rounded clasts recycled from the uplifted breccia-conglomerate were initially deposited followed by pumiceous and scoriaceous pebbly sandstone from renewed explosive eruptions. Submarine volcaniclastic aprons involve rapid and prolific sedimentation from multiple source locations, clasts of different densities and origins, and can include the direct products of contemporaneous volcanism. The facies architecture, however, can be clearly divided into proximal, medial, and distal volcaniclastic apron environments. The proximal apron is dominated by the coarsest clasts. Steep slopes, rapid sedimentation rates, and seismic activity in the steep proximal apron promote slumping. The medial and distal aprons are more likely to record explosive activity, debris flow events, and eruption hiatuses due to the gentler slopes and depositional rather than erosional setting.