Laguna Palacios Formation (San Jorge Basin, Argentina): an Upper Cretaceous loess-paleosol sequence from Central Patagonia

BELLOSI E. S. Y J. C. SCIUTTO

Cordoba

Congreso; IX Reunión Argentina de Sedimentología; 2002

Asociacion Argentina de Sedimentología

   The sedimentary scenario for LPF was similar to the loess plains with a few rivers and ponds. Pyroclastic and subordinated mixed sediments, supplied as distal ash fallout (from plinian explosive eruptions) and by subordinated fluvial streams, were intensively pedified. Attributes shared with Pleistocene-Holocene loess of Pampean plain are numerous: yellow-brown (reddish) hue, absent or crude bedding, vertical walls, abundant glass shards, fine sand-silt volcaniclastic particles (air-fall tephra), prevailing herbaceous vegetation (grassland/savanna), advanced pedogenesis, eolian reworking, fluvial and lacustrine/ palustral intercalations. Another resemblance with the pampean loess andosols and mollisols is the variation from entisols (stage 1-2) to freely drained andosols and alfisol/mollisols (stage 3-4). In contrast, wetter conditions are responsible for carbonate absence in Cretaceous paleosols. The development of the LP loess-paleosol sequence was regulated by pedogenesis/supply balance in a temperate subhumid-seasonal climate and grass-covered landscape. The abundance and stacking of paleosols generally indicate low sedimentation rate. However the accumulation of the Cretaceous tuffs was probably episodic but not slow (controlled by the intensity of explosive volcanism and wind direction), according to the rapid weathering and pedogenesis of holocene pyroclastic loess (0,5 k.y.). Other calculations estimate 2-20 k.y. for moderately developed pyroclastic paleosols. On the other hand, incised channels also suggest a decrease in the accommodation space. Unlike of the usual alluvial architecture model, they are not laterally related to mature paleosols since the prevailing process in LPF was eolian, not fluvial.