CRETACEOUS SLOPE DEPOSITS OF THE TRES PASOS FORMATION, SIERRA DOROTEA, CHILE: INSIGHT INTO THE SEDIMENTARY FILL OF DEEPWATER FORELAND BASINS

The preservation of sediment source areas and their associated depositional systems into the stratigraphic record is commonly limited, and at best discontinuous for the time scale at which sedimentary basins typically fill (millions of years). In such cases, preserved sections of deep-marine strata represent the most complete record of source-to-sink transfer of terrigenous sediment. The Cretaceous Tres Pasos Formation of southern Chile records the evolution of a continental margin-scale delta slope system. Prograding delta-fed slope systems represent an important phase of basin-filling in that they record the transition from deep- to shallow-marine sedimentation. An investigation of depositional processes and stratigraphic architecture of slope strata permits a qualitative evaluation of source-to-sink dispersal patterns and system evolution.

The Tres Pasos Formation outcrop belt extends for more than 100 km in the elongate, north to south Magallanes foreland basin. Detailed field analyses and mapping of units that outcrop along the paleo-foreland basin axis on Sierra Dorotea, north of the town of Puerto Natales, reveal evidence of a slope to base-of-slope depositional setting. Sedimentation in the region was characterized by mass wasting and deposition of thin-bedded tabular sandstone, siltstone, and mudstone. The most diagnostic depositional element in the stratigraphic succession, however, are various channelized systems, significant in that they represent the most efficient medium for sediment delivery into deepwater basins. Pinch-out of sandstone packages to the north (landward) indicates sediment back-filling within these slope conduits implying an early bypass phase followed by a waning cycle of deposition. At a larger scale, these sand-rich packages stack and step basinward reflecting the progradation of the delta-fed slope system and filling of the deep-marine basin.