Shelf-derived mass-transport deposits: origin and significance in the stratigraphic development of trench-slope basins

Abstract

Continental shelves generally supply large-scale mass-wasting events. Yet, the origin and significance of shelf-derived mass-transport deposits (MTDs) for the tectonostratigraphic evolution of subduction complexes and their trench-slope basins have not been extensively studied. Here, we present highresolution, outcrop-scale insights on both the nature of the reworked sediments, and their mechanisms of development and emplacement along tectonically active margins, by examining the Middle Miocene shelf-derived MTDs outcropping in the exhumed southern portion of the Hikurangi subduction margin.Results show that periods of repeated tectonic activity (thrust propagation, uplift) in such compressional settings not only affect and control the development of shelfal environments but also drive the recurrent generation and destruction of oversteepened slopes, which in turn, favour the destabilisation and collapses of the shelves and their substratum. Here, these events produced both large-scale, shelfderived sediment mass-failures and local debris flows, which eventually broke down into a series of coalescing, erosive, genetically-linked surging flows downslope. The associated MTDs have a regional footprint, being deposited across several trench-slope basins. Recognition of tectonic activity as another causal mechanism for large-scale shelf failure (in addition to sea-level changes, high-sedimentation fluxes) has implications for both stratigraphic predictions and understanding the tectonostratigraphic evolution of deep-marine fold-and-thrust belts.

Lateral, longitudinal, and temporal variation in trench-slope basin fill: examples from the Neogene Akitio sub-basin, Hikurangi Margin, New Zealand

“…This association is found on both margins of the Akitio Sub-basin (Figure 5), such as on the western margin near Pongaroa (Figure 7D) and the northern end of the basin (Figure 5), on the eastern margin near Ware Ware (Figure 4 base of Coast Road section) and in the Pakowhai River (Figure 4). Similar successions are found at the base of other outcropping basins on the margin (Claussmann et al 2021). Rare imbrications in conglomerates and traction structures in sandstones indicate transport away from the adjacent basin margin.…”

“…Initially, basins form a narrow trough where sediments were ponded and mass-wasting occurs on newly formed basin margins (Figure 19A). Although one may expect activity first on the up-dip basin bounding structure later followed by activity on the down-dip structure, here, as with other basins on this margin (Claussmann et al 2021), activity on both basin bounding structures is documented at basin inception. This complex structural history may result from the reactivation of older structures that define the basins (Pettinga 1982).…”

“…Marine sedimentation predominates in the trench-slope basins (Van Der Lingen and Pettinga 1980;Neef 1992a;Field et al 1997;Bailleul et al 2007), the record of which exhibits a marked tectonic control, with sedimentations disrupted by a series of dis-and unconformities (Neef 1992a;Bailleul et al 2007Bailleul et al , 2013Burgreen and Graham 2014;Burgreen-Chan et al 2016;McArthur and McCaffrey 2019;. The fill of the basins includes widespread deep-marine marls, mudstones, turbidites, contourites, mass-transport deposits, and shallow marine siliciclastics and carbonates (Figure 2; Lee and Begg 2002;McArthur et al 2020;Claussmann et al 2021).…”

“…Some conglomerates may be monomict (LF4) (Figure 7A). These form packages at least tens of metres thick, but given only erosional remnants are preserved they may have formed considerably thicker intervals, as seen in the coastal exposures (Claussmann et al 2021). This association is found on both margins of the Akitio Sub-basin (Figure 5), such as on the western margin near Pongaroa (Figure 7D) and the northern end of the basin (Figure 5), on the eastern margin near Ware Ware (Figure 4 base of Coast Road section) and in the Pakowhai River (Figure 4).…”

“…The basin-wide unconformities and MTDs seen in the Middle and Upper Miocene strata represent major changes in the depositional system of the Akitio Sub-basin (Figure 15). The MTDs may be the cause of such changes, however, the MTDs themselves were likely triggered by an external event (Claussmann et al 2021). The Middle Miocene MTD can be correlated with a major tectonic pulse in the basin (Figure 16).…”