Cenomanian palaeosol from Neuquén Basin Patagonia, Argentina

Abstract

The Cenomanian Candeleros Formation, located in the Neuquén foreland Basin (32?40° SL), is a unit of Argentinean Patagonia that represent a continental fluvio-eolic system. In the study area, the outcrops are ~100 m-thick and records different stacked palaeosols. The aim of this work is to present the description and interpretation of these palaeosols, characterize the main palaeopedogenic processes, and infer preliminarily palaeoclimatic conditions. In the study area detail palaeopedological profiles were carried out. Palaeosols were identified in outcrops based on macroscopic pedofeatures such as structure, mottles, nodules, colour, slickensides, and rhizoliths. Palaeosol horizons, thickness, contact types, mean grain size, ped structure, type of nodules, and evidences of bioturbation were described. Approximately 65 palaeosols were identified in the measured sections. Based on prevalent pedogenic features, three pedotypes, listed in order of appearance, were defined. Classification of the palaeosols was based on the macrofeatures through a comparison with the USDA soil taxonomy (Soil Survey Staff, 1999) and with the palaeosol classification of Mack et al. (1993). The pedotype 1 shows well-developed horizons with reddish brown to red matrix (finegrained siltstone to fine-grained sandstone), and profiles with Bss-C horizons (0.60?1 mthick) with a very low relative abundance. Slickensides and very coarse wedge-shape peds with internal angular to subangular peds defined this pedotype. Small carbonatic concretions, and scare light greenish grey mottles are observed. The C horizon preserves relict sedimentary structures and mottles. This pedotype is classified as Vertisol-like palaeosols, and as Vertisols. The pedotype 2 is the most abundant throughout the succession. This has scarce horizon development with profiles Bw-C, Bk-C, or Bg-C (0.75?1 m thick). The Bw horizons are brown and weak to dusky red very fine-grained siltstone with subangular blocky peds, and scarce clay cutans. When light greenish grey mottles occur, the horizon is defined as a Bg horizon; meanwhile, a Bk horizon is defined when carbonatic features (rhizoliths) are present. The Chorizon is massive or preserve relict primary structures, with scarce gley mottles and desiccation cracks. This pedotype is classified as gley or calcic Inceptisol-like palaeosols, and as gleyic or calcic Protosols. The pedotype 3 presents a moderate relative abundance and shows profiles with Bss-Bssk-C or Bk-C horizons (0.60?1 m-thick). B horizons presents typical vertisol-like matrix with weak red to reddish brown very fine-grained siltstones, angular to subangular blocky peds, abundant slickensides, and in few cases light greenish grey mottles (Bss horizon). However, the most conspicuous feature of these palaeosols are the presence of carbonatic macrofeatures such as a network of rhizoliths, calcic concretions, typic nodules, aggregated nodules, and horizontal crust-like morphology (Bssk or Bk horizons). The C horizon is massive with scarce gley mottles. This pedotype is classified as vertic Aridisol-like palaeosols, and as vertic Calcisols. These data indicated that the main pedogenic processes in the Candeleros Formation were, in decreasing order of magnitude, gleization, calcification, vertization, and bioturbation. These processes took places under temperate and seasonal subhumid climate with a trendtowards drier conditions.