Climatic control on stacking and connectivity of fluvial successions: Upper Cretaceous Bajo Barreal Formation of the Golfo San Jorge basin, Patagonia

PAREDES, JOSÉ MATILDO; GIORDANO, SERGIO ROBERTO; OLAZÁBAL, SABRINA XIMENA; VALLE, MAURO NICOLÁS; ALLARD, JOSÉ OSCAR; FOIX, NICOLÁS; TUNIK, MAISA ANDREA

MARINE AND PETROLEUM GEOLOGY

ELSEVIER SCI LTD

Año: 2020 vol. 113

0264-8172

Climate impact on alluvial organization owing to its control on water availability and sediment delivery withinthe catchment, but temporal changes in stacking patterns are often interpreted to reflect changes in subsidenceand base level. To test for evidence of climatic control on the stacking pattern, we study an outcrop successionwith two styles of stacking within the Upper Cretaceous Bajo Barreal Formation in the Cerro Ballena anticline,Golfo San Jorge Basin, Argentina. The 385 m thick and 2.5 km wide exposure has layer-cake geometry, lackingeither large-scale erosional surfaces, fluvial terraces, or evident paleosols, dismissing either local tectonic activityor base-levels shifts. Rooted in a paleohydrological study recognizing upward increasing in both channelwidth and flow depth of formative rivers, we use spectral gamma-ray logs, x-ray diffraction in mudstones, andsandstone petrography to understand the controls on the stacking pattern. At the base, Section A consists ofsmall-scale, isolated channels fills in a siliciclastic floodplain with sand:mud ratio of ~1:6, whereas the overlyingSection B has a sand-mud ratio of ~1:3, with larger-scale channels and greater inter-connectivity within avolcaniclastic floodplain. Upward reduction in K percentage through Section A parallel with increasing kaolinitecontent, and mudstone samples from Section B contains a higher proportion of kaolinite than Section A samples,evidencing an upward increase in paleo-weathering in humid conditions. Detrital components of Section Aindicate several volcanic sources (e.g., basic-intermediate components derived from the Middle Jurassic BahíaLaura Group and acidic components sourced from the Andes Cordillera), whereas Section B exclusively containsacidic clasts derived from the Andes Cordillera. Simultaneous changes in detrital constituents, suspended loadtype, and increasing scale of the rivers in Section B occur coeval with increases in channel inter-connectivity,here related to the increase of river discharge and sediment supply in a humid climate, favoring more frequentavulsions or higher channel migration rates in a relatively flat geomorphic scenario. The study demonstrates,combining independent lines of evidence, that climate change can impact the stacking and connectivity ofpotential sandstone reservoirs.