Outcrop characterization of a submarine channel-lobe complex: The Lower Mount Messenger Formation, Taranaki Basin, New Zealand

Abstract The Upper Miocene Lower Mount Messenger Formation (LMMF) exposed in cliffs along the west coast of the North Island, New Zealand, includes 650 m of thick-bedded, fine-to very fine-grained sandstone, interbedded sandstone and mudstone, mudstone, and mass transport deposits. Deposition was from low- and high-density turbidity currents and small to very large, intraformational slumps, slides, and debris flows. The LMMF comprises five major facies associations (FA) representing the principal depositional settings within which the sediments accumulated: (FA1) distal to medial frontal lobes and lobe complexes, (FA2) more proximal frontal lobes and crevasse and/or overbank splays; (FA3) submarine channels; (FA4) levees; and (FA5) mass transport deposits (MTDs). The lower, northern part of the LMMF is dominated by FA1 and FA2 deposited as part of a large, amalgamated frontal lobe system. This section is overlain and, near the top, interbedded with FA3 levee deposits. The top of the LMMF includes a channel complex that interfingers with the levees and related lateral splay deposits. The submarine channel belt was nearly 3 km wide and delivered the sediments to the lobe complex more than 15 km downslope. The channels are filled largely with the deposits of low-density turbidity currents characterized by plane lamination and climbing-ripple cross-lamination. In contrast, lobes contain thick-bedded massive and cross-stratified sandstones deposited by high-density flows. This contrast of apparently higher energy, high-density turbidite divisions in the lobes and lower energy, low-density deposits in the upslope channel fill is thought to reflect diachronous deposition of these otherwise contiguous parts of the deposystem. When sand deposition was occurring on the lobe complex, large, high-density flows moved through and largely bypassed or actively eroded the upslope channels. As flow volumes and strength declined, deposition on the lobe complex waned and sand deposition from low-density currents occurred within the channels, with little sediment other than mud reaching the lobe complex. The overall northward paleoflow direction in the outcrop window is parallel to the inferred eastern paleoslope. The abundance of massive turbidites in the most downslope deposits and evidence for correlative deposits with south- and southwest-directed paleocurrents a short distance to the north suggest that the basin may have been ponded.