Debris flow and hyperconcentrated flow in a submarine channel • Y. K. Sohn et al.

No abstract

The submarine channel-fill system of the Cambrian Spurs Formation exhibits unique metre-scale cycles of breccia and diamictite. The studied sections, Eureka Spurs, are located at the Mariner Glacier in the central-eastern part of northern Victoria Land, Antarctica. A facies analysis of the channel-fill deposit has led to the recognition of four main lithofacies: breccia, diamictite, thin-bedded sandstone and mudstone. The channel-fill deposit consists of two architectural elements: hollow-fill (HF) and sheet-like (SL) elements. The SL has wide convex-up geometry and consists solely of a very thick bed of diamictite, and is interpreted as a submarine channel lobe. The HF has a concave-up erosional base and flat upper surface. The HF consists of nine cyclic alternations of underlying breccia (cohesionless debris flow) and overlying diamictite (cohesive debris flow). The deposition of breccia is interpreted to have been controlled by repeated allogenic processes such as earthquakes. In contrast, the abrupt vertical transition from breccia to diamictite in each cycle is interpreted to have resulted from an autogenic, slope instability-related process. The interaction of the allogenic and autogenic factors recorded in the metre-scale unique cyclic deposits provides new criteria to interpret cycles of submarine debris flow.

During early to middle Miocene times a sudden opening of the Ulleung (Tsushima) back‐arc basin in the East Sea (Sea of Japan) led to the development of intraslope basins along the rifted southwestern margin (southeast Korea). Abrupt subsidence resulted in the deposition of the 200 m thick Hunghae Formation (middle Miocene), a sand/mudstone sequence that can be divided into five facies. Facies I (sand and mudstone couplet) and II (coarse sand) are turbiditic in origin, as evidenced by massive, graded, crudely‐layered and parallel‐laminated sand beds. Facies III (homogeneous mudstone) is characterized by various lignite and plant fragments, clastic and biogenic grains that are randomly oriented, suggestive of hemipelagic deposition. Facies IV (chaotic deposit) is characterized by the disruption of beds, the presence of isolated siltstone blocks (or balls) and large clasts in the muddy matrix, indicative of retrogressive rockfall and slide/slump. Facies V (conglomerate) is of debris flow origin, as evidenced by clast‐ and matrix‐supported features, floating large clasts and absence of traction structures. Individual facies are organized into two types of facies association: (1) homogeneous mudstone (facies III) associated randomly with the rest (facies I, II, IV and V), indicative of hemipelagic and episodic sediment‐gravity flow processes, respectively; (2) conglomerate (facies V), coarse sand (facies II) and sand/mudstone couplet (facies I), representing the flow transformation from debris flow to high‐ and low‐concentration turbidity currents. These facies associations are similar in many respects to modern and ancient debris (or slope) aprons found elsewhere. Numerous isolated slide/slump blocks, wedged conglomerates with armoured mudstone balls, discontinuous lignite‐containing sand/mudstone beds, chaotic structure and growth faults suggest that the deposition occurred on a steep slope (intraslope basin) off coalescing fan‐deltas, mainly by unchannellized sediment‐gravity flows. Ancient deposits with irregular facies sequences can be viewed as debris‐apron systems, which provide alternatives to submarine‐fan models in many clastic basins with a line rather than point source.

No abstract