Extreme erosion and bulking in a giant submarine gravity flow

Abstract

Sediment gravity flows are ubiquitous agents of transport, erosion, and deposition across Earth's surface, including terrestrial debris flows, snow avalanches, and submarine turbidity currents. Sediment gravity flows typically erode material along their path (bulking), which can dramatically increase their size, speed, and run-out distance. Hence, flow bulking is a first-order control on flow evolution and underpins predictive modeling approaches and geohazard assessments. Quantifying bulking in submarine systems is problematic because of their large-scale and inaccessible nature, complex stratigraphy, and poorly understood source areas. Here, we map the deposits and erosive destruction of a giant submarine gravity flow from source to sink. The small initial failure (~1.5 cubic kilometers) entrained over 100 times its starting volume, catastrophically evolving into a giant flow with a total volume of ~162 cubic kilometers and a run-out distance of ~2000 kilometers. Entrainment of mud was the critical fuel, which promoted run-away flow growth and extreme levels of erosion.

Fig. 1.. Overview map of the Northwest African margin showing the pathway of the Bed 5 event and its erosional marks on the seafloor.

(A) Bathymetric map of the Agadir Canyon from the shelf edge down to the Canyon Mouth. Insert shows the wider Moroccan Turbidite System interconnected basins. DSDP, Deep Sea Drilling Project; GCs, Gravity Cores; GEBCO, The General Bathymetric Chart of the Oceans. Areal extent of erosion (gray overlay) across the Upper Canyon (B) and Lower Canyon (D). Slope maps (high/low slope = black/white color) detailing canyon floor knickpoint zones in the Upper Canyon (C) and Lower Canyon (E) with composite scours forming irregular (pocketed) erosional scarps (see fig. S1). Cores are marked with colored circles representing different research cruises. NT, Northern Tributary; ST, Southern Tributary; CT, Central Tributary.

Fig. 2.. Profiles (3.5 kHz) across the erosional trimlines at various points along the canyon margins.

(see Fig. 1) (A) Lower Canyon zone: a steep (and poorly resolved) step that cuts out 6 to 30 m of stratigraphy elevated between 90 and 150 m off the canyon floor. Note that core CD166/36 records Bed 5 without an erosional hiatus 210 m from the canyon floor. (B) Lower Canyon bend showing 6 to 24 m of erosion between 70 and 100 m above the canyon floor. TWTT, two‐way travel time; VE, vertical exaggeration.

Fig. 3.. Core correlation along the length of Agadir Canyon following the Bed 5 event to its source.

(A) Thalweg profile including bathymetric depth (black line) and slope gradient (blue line) from the Agadir Canyon head to its mouth (see Fig. 1 for profile position). Core locations marked with arrows with water depth (vertical number) and cruise code (45° labels). Several cores have elevated positions above the canyon floor (shown by dashed lines; a horizontal number gives height above the canyon thalweg). (B) Core correlation of Bed 5 and its erosion surface along the Agadir Canyon (A-coded beds within Agadir Basin and C-coded beds within the Agadir Canyon). Cores that are elevated above the canyon floor are highlighted with an asterisk. Note that core 61 ages on the basis of linear extrapolation from 72-ka coccolith biozone (see fig. S3).

Fig. 4.. Examples of Bed 5 deposits correlated from the Northeast Agadir Basin (Mouth of the Agadir Canyon) up into the head of the Agadir Canyon.

(see Figs. 1 and 5 for locations) (A) The Canyon Mouth records a thin gravel lag with basal erosion surfaces and several internal grain size breaks. (B) Along the canyon floor, it is characterized by a sharp basal erosion surface draped by very fine sand lag and a mud cap, which is often only a few centimeters in thickness. (C) Thirty meters above the canyon floor on the margins shows a steep erosion surface overlain by thicker ripple cross-laminated fine sand deposits. (D) Two hundred thirty meters above the canyon floor at the Tributary Confluence zone; Bed 5 is found as a thin gravel layer with large, outsized grains and mud clasts. (E) One hundred thirty meters above the Southern Tributary thalweg is a slightly thicker gravel layer with an erosive base highlighted by sheared mud clasts. (F) Bed 5 contains distinctive dark-red sandstone grains (1), armored mud clasts (2), some lithified mudstones (3), and a variety of shell fragments from the Moroccan Margin. Grain sizes: VFS represents very fine sand; FS, fine sand; MS, medium sand; CS, coarse sand; VCS, very coarse sand; G, gravel.

Fig. 5.. Bathymetric maps of the upper Agadir Canyon head zone highlighting the origin of the Bed 5 event.

(A) Bathymetry of the Agadir Canyon head zone showing the Northern, Central, and Southern Tributaries. (B) Zoom-in gradient map of large scours seen on the floor of the Southern Tributary [location shown with box in (A)]. (C) Canyon head drainage patterns highlighting the Northern (purple), Central (orange), and Southern (green) Tributaries. Thalwegs are shown with white lines. Cores MSM113-58, MSM113-59, and MSM113-60, and GeoB6006, GeoB6007, and GeoB6008 rule out the Northern and Central Tributaries as Bed 5 pathways (see the main text for details).

Fig. 6.. Bathymetric map of the Southern Tributary catchment with potential failure scenarios and associated volumes.

(A) Bathymetry across the Southern Tributary catchment with thalweg networks mapped with white lines. The source area for Bed 5 must originate upstream of core MSM113-61, which records Bed 5 as a coarse-grained gravel deposit. Bed 5 slope failure scenarios are presented assuming total catchment failure (B), restricted catchment failure (C), and canyon-floor failure only (D). Initial failure volumes are calculated from the failure thickness and the areal extent of the failure (see the main text for details). The example in (D) uses 30-m average failure thickness.