Pliocene pedosedimentary cycles in the southern Pampas, Argentina

Well-developed Bt horizons of five palaeosols (P1-P5) have been recorded pr eviously within a 20-m-thick succession of Pliocene siltstones and clayey s iltstones in the southern part of the Buenos Aires Province of Argentina. T his paper reports a detailed field and micromorphological (thin section) in vestigation of a 6-m portion of the sequence encompassing P2 and P3. Large- scale faunal burrow infillings occur throughout: other bioturbation feature s in the form of channel and spongy microstructures are mainly confined to the siltstones. The intervening clayey siltstones (Bt horizons) have been a ffected more by shrink-swell disruption, as evidenced by slickensides and a range of striated b-fabrics in thin sections. Clay coatings, indicative of illuvial accumulation of clay translocated in suspension from overlying A or E horizons, occur in both the siltstones and clayey siltstones. The type s, microstratigraphic associations and depth functions of features are inte rpreted in terms of changing interactions, balances and dominances between sedimentary, pedogenic and erosional processes over time, thus providing th e basis for the pedosedimentary reconstruction of landscape evolution in th e region during part of the Pliocene represented by the whole P1-P5 sequenc e (4-5 Ma BP). It is envisaged that this period was dominated by aeolian de position, although fluvial and mass movement processes probably led to rewo rking and redistribution of some of the materials. Overall rates of subaeri al deposition, however, were not substantial: pedogenic processes were acti ve throughout, the balance between sedimentation and pedogenesis varying ov er time in a cyclical fashion. Phases of reduced deposition and establishme nt of relatively stable land surfaces were marked by the development of arg illic soil profiles with clearly defined eluvial and illuvial horizons. Int ervening periods of more rapid accumulation of coarser material were charac terized by accretionary soil development and welding of new pedological fea tures on existing soils as the surface accreted, first transforming existin g eluvial horizons into BCt/AE horizons (siltstones) and then encouraging t he syndepositional upward extension of these complex horizons. The primary basis of the alternating units of siltstones (BCt/AE horizons) and clayey s iltstones (Bt horizons) lies in the cyclical change in size of particles de posited, although pedogenic translocation processes enhanced these textural differences. The underlying driving mechanism behind the pedosedimentary c ycle can only be speculated upon, although it is tempting to relate the sed imentation pattern to climatic fluctuations linked to glacial advances and retreats in the Patagonian Andes during the Pliocene.