Marginal-to-foreland basin stratigraphy in the Sierra de Beauvoir domain (Cretaceous- Paleocene, Fuegian Andes, Argentina)

D. R. MARTINIONI

Mendoza

Congreso; 18th International Sedimentological Congress; 2010

International Association of Sedimentologists

MARGINAL-TO-FORELAND BASIN STRATIGRAPHY IN THE SIERRA DE BEAUVOIR DOMAIN (CRETACEOUS-PALEOCENE, FUEGIAN ANDES, ARGENTINA). The Sierra de Beauvoir domain in the Fuegian Andes is a rugged mountainous area with limited access, dense forests, and a profuse Quaternary sedimentary cover of chiefly glacial and post-glacial deposits. This domain includes Sierra de Beauvoir, Sierra de Apen, and Sierra de las Pinturas. The pre-glacial geology of the area is rather hidden and fuzzy; the good exposures are sparse and remain quite restricted to the summits reaching above the timberline, or show up at some deep sitting valleys. After thorough geological field research in the Sierra de Beauvoir area, this part of the Andean fold and thrust belt happens to involve more than just a single mudstone dominated unit as originally described (Beauvoir Formation), instead comprises a Lower Cretaceous (s.l.), largely mudstone dominated, meta-sedimentary package (variably deformed and folded; host of igneous rocks), and an Upper Cretaceous-Paleocene coarsening upwards sedimentary succession. The Lower Cretaceous (s.l.) of Sierra de Beauvoir comprises: 1) Lago Fagnano Slates, clay-rich meta-sedimentary rocks (?uppermost Jurassic-lower Lower Cretaceous); 2) Beauvoir Formation, mudstone dominated meta-sedimentary unit, with Inoceramus spp. and Aucellina spp. (upper Lower Cretaceous-?lowermost upper Cretaceous, Aptian-Albian-?lowermost Cenomanian); and 3) mesosilicic and basic igneous rocks that intrude and crosscut the Lago Fagnano Slates and the Beauvoir Formation, respectively (104 ± 4 My, isotopic age of a basic dike that cuts the Beauvoir Formation). The Upper Cretaceous-Paleocene of the area includes: 1) Arroyo Castorera Beds, mudstone dominated sedimentary rocks with Inoceramus lamarcki among other inoceramids (Upper Cretaceous, ?upper Cenomanian-Turonian); 2) Río Rodríguez Beds, coarsening-, thickening-upward successions consisting of mudstone, siltstone, and very fine silty sandstone, with inoceramid remains attributable to Cremnoceramus sp. (Upper Cretaceous, Coniacian-?Campanian); 3) Cerro Fumando Beds, sedimentary unit dominated by sandy mudstone, increasingly sandier up-section, divided into a “Muddy” Member and a “Sandy” Member, with diagnostic genera of ammonites and dinocyst species, but no inoceramids (Upper Cretaceous, Maastrichtian-?Danian); and 4) Cerro Apen Beds, sandy to conglomeratic sedimentary rocks with intercalated mudstone and heterolithic packages that rest on an erosive unconformity over the Cerro Fumando Beds, have diagnostic dinocysts, and comprise a “Lower” Member (sandstone dominated; Paleocene, Danian) and an “Upper” Member Both the Lower Cretaceous and the Upper Cretaceous-Paleocene packages reflect, respectively, the main phases in the evolution of the marine sedimentary basins in the southernmost tip of South America from the latest Jurassic to the early Paleogene. The Lower Cretaceous (s.l.) units were accumulated after the main extensional period and subsequent fill of the Rocas Verdes Marginal Basin until its final closure during the earliest Late Cretaceous. The Upper Cretaceous to Paleocene sedimentary rocks accumulated during the development of the Austral Foreland Basin, having an initial transitional period with deep marine sedimentation, right after the closure and tectonic inversion of the marginal basin, followed by the progressive orogenic uplift of the Fuegian Andes under a contractional regime. The ongoing forward orogenic growth generated a coarsening upward succession that begun with the deposition of mudstone dominated units and continued with increasingly coarser, sandy to conglomeratic, sedimentary units. Additionally, coarse-grained facies recorded elsewhere in the southern Andes for the Foreland Basin inception seem not to have an analogous counterpart in the Sierra de Beauvoir domain, where mudstone facies dominate the Marginal-Foreland Basin transition without coarse sand or conglomeratic sedimentation until the latest Cretaceous-earliest Paleocene.