The exceptionally well exposed Lago Sofia conglomerate and sandstone lenses in the Upper Cretaceous Cerro Tor0 Formation of southern Chile are interpreted as the channel and channel margin facies of a deep-sea fan. The north-to-south oriented channels formed on an elongate fan in a narrow retroarc basin between a rising cordillera to the west and the South American craton to the east. The great length of some of the channels ( > 120 km) seems to reflect the long duration ( > 30 m.y.) and stable nature of the basin. Enclosing the lenses is the fine-grained Cerro Tor0 Formation which represents overbank turbidite flows and hemipelagic sedimentation on levee and levee flank areas. Foraminifera1 assemblages suggest deposition in 1000-2000 m of water. Most of the conglomerate has features developed by tractive currents (parallel-and cross-stratified conglomerate). Most is moderately well sorted, imbricated, and has parallel to inclined stratification; large-scale dunes up to 4 m high are exposed. Typical sediment gravity flow structures and bedding styles (e.g. pebbly mudstones, graded conglomerate, giant flutes) are not as common in the channel deposits as are tractive features. Tractive features in the gravels apparently were developed by rolling, sliding, and saltation as the bed-loadcomponent of highly turbulent, moderateto low-density turbidity currents flowinginaconfined channel. Graded-to-massive conglomerates appear to have been deposited rapidly from fully turbulent flows; diamictites were deposited from debris flows in which fluid viscosity, yield strength, and buoyancy of the fluid were dominant. The three major conglomerate classes recognized do not occur in a systematic manner; vertical and lateral heterogeneity is the rule. lNTRODUCTIONExtensive, thick sequences of modern turbidites are found on submarine fans built in deep water beyond either submarine canyons or networks of smaller feeder channels, from the overflow of marine basins filled with turbidites, o r in front of growing deltas (see Nelson & Kulm, 1973;Normark, 1974). The depth a t which modern fans are being deposited, however, makes much direct observation unpractical and detailed lithological and morphological studies very expensive. Although huge thicknesses Colorado 80160 U.S.A.

Rocks on South Georgia Island at the eastern end of the North Scotia Ridge are no older than late Mesozoic. The Cumberland Bay and Sandebugten graywacke and mudstone sequences there are comparable in general lithology and structural style to the Lower Cretaceous Yahgan Formation of the Beagle Channel area in southernmost South America. The Cumberland Bay rocks, which form most of South Georgia Island, were thrust northeastward over the Sandebugten sequence. The Cumberland Bay and Yahgan sequences contain Cretaceous fossils, whereas the Sandebugten rocks are unfossiliferous. The dominant dispersal of Cumberland Bay detritus was toward the northwest. The Sandebugten dispersal pattern was more complex but was dominated by a southdirected component. In Early Cretaceous time, however, the South Georgia microcontinent apparently was attached to South America along the present southern margin of the Burdwood Bank. The Cumberland Bay, Sandebugten, and farther westward along strike, the Yahgan, apparently were deposited in a marginal small ocean basin between a calc-alkalic volcanic arc built on a sliver of old South American continental crust and the main part of the South American continent from which the sliver moved away. According to this interpretation, deformation of the sediments occurred when the arc moved back toward the continent in middle Cretaceous time, and the basin was closed and uplifted with the arc.

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