CYCLIC DEPOSITS OF DEBRIS FLOWS IN A SUBMARINE CHANNEL SYSTEM, THE SPURS FORMATION, MARINER GROUP (CAMBRIAN), ANTARCTICA

The submarine channel-fill system of the Cambrian Spurs Formation exhibits a unique meter-scale cyclic deposit of breccia and diamictite. The studied section at the Mariner Glacier, northern Victoria Land (NVL), Antarctica exposes the Spurs Formation which consists of fissile mudstone, thin-bedded sandstone, and channel-fill conglomerates. A channel-fill unit is ca. 190 m in width and ca. 20 m in thickness. A detailed facies analysis of the channel-fill deposit leads to four main lithofacies: 1) breccia (cohesionless debris flow), 2) diamictite (cohesive debris flow), 3) thin-bedded sandstone (high-density turbidity currents), and 4) mudstone (low-density turbidity currents). The channel-fill deposit consists of two architectural elements: hollow-fill (HF) and sheet-like (SL) elements. The HF has concave-up erosional base and flat upper surface, and consists mostly of cycles of breccia and diamictite beds with minor thin-bedded sandstones. This element is interpreted as a deposit infilling a scoured hollow. 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 channel-fill deposit is formed by a succession of 3 depositional stages: 1) channel-wide erosion and hollow-fill deposition, 2) deposition on the over-filled channel, and 3) small-scale channel erosion and fill.

Nine cyclic successions occur in the HF. The erosive lower boundary of each cycle incises into the underlying mudstone and the diamictite of the preceding cycle, whereas the diamictites are confined to the dimension of the underlying breccia. The initial phase of each cyclic succession may have been controlled by repeated allogenic processes such as tectonic activities, eustatic sea-level changes. In contrast, abrupt vertical transition from breccia to diamictite in each cycle is interpreted to have resulted from an autogenic process, channel wall collapse, causing muddy cohesive debris flow. It is interpreted that the interaction of the allogenic and the autogenic factors recorded in the meter-scale unique cyclic deposits provides a hint for understanding paleoenvironment of the NVL.