Characterisation and diachronous initiation of coarse clastic deposition in the Magallanes-Austral foreland basin, Patagonian Andes

Magallanes-Austral Basin (MAB) fill is preserved along a >1000km north-south trending outcrop belt in the southern Patagonia region of Argentina and Chile. Although the stratigraphic evolution of the MAB has been well documented in the Chilean sector (referred to as the Magallanes Basin), its northern terminus in southern Argentina (Austral Basin) is poorly constrained. We present new stratigraphic and geochronologic analyses of the early basin fill (Aptian-Turonian) from the Argentine sector (49-51 degrees S) of the MAB to document spatial variability in stratigraphy and timing of deposition during the initial stages of basin evolution. The initiation of the retroarc foreland basin fill is marked by the transition from mudstone to coarse-clastic deposition, which is characterised by the consistent presence of sandstone beds>ca. 20cm thick interpreted to represent sediment gravity flows deposited in a submarine fan system. Depositional environments within the early fill of the basin range from lower to upper deep-water fan settings as well as previously undocumented slope deposits. These facies are present as far north as El Chalten, Argentina (ca. 49 degrees S), indicating that facies-equivalent rocks can be traced along-strike for at least 5 degrees of latitude, based on correlation with strata as far south as the Cordillera Darwin (ca. 54 degrees S). Eight new U-Pb zircon ages from ash beds reveal an overall southward younging trend in the initiation of coarse clastic deposition. Inferred depositional ages range from ca. 115 +/- 1.9Ma in the northernmost study area to not older than 92 +/- 1Ma and 89 +/- 1.5Ma in the central and southern sectors respectively. The apparent diachronous delivery of coarse detritus into the basin may reflect (1) gradual southward progradation of a deep-water fan system from a northerly point source and/or (2) orogen-parallel variations in the timing and magnitude of thrust-belt deformation and erosion that provided more local sources for sediment delivery.