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Modern and Ancient Alluvial Systems
Sheet Debris Flow and Sheetflood Conglomerates in Cretaceous Cool-Maritime Alluvial Fans, South Orkney Islands, Antarctica
Small, steep, and laterally coalescing alluvial fans that formed under a cool and maritime climate along the sides of a 10 km wide trough are dominated by two types of planar, 1–2 m thick rudites. Sorted, gradually aggraded rudites are interpreted as traction-carpet deposits of sheetfloods. Their characteristics include clast support, moderate organization, sorted matrix, and planar stratification outlined by variable grain size and sorting; normal grading and crossbedding are rarely developed. In contrast, unorganized clast-supported conglomerates were sheet debris flows. Lack of internal partings suggest that they were instantaneously deposited. Clay is mostly less than 5% of matrix; perhaps incorporated snow was an important lubricant.
Sheetfloods and debris flows both appear to have been large, unchannelled, choked with sediment and virtually non-erosive. They seem to have carried all available material (maximum clast sizes and bed thickness are unrelated). Rapid loss of capacity maintained steep fan slopes, as shown by steep primary dips and low-angle down-fan crossbeds. The conglomerates and thin, discontinuous interbedded sandstones are primarily mid-fan deposits.
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