Estimating aggradation rates from palaeosol type: the Punta San Andrés Alloformation (Argentina) as a case study

Abstract

The degree of soil development depends on how long sediment is exposed to pedogenic processes (Marriott and Wright, 1993). That is to say that, whenever sedimentation rate is low, palaeosols will develop more mature profiles than those developed under higher sedimentation rates. Also, certain pedogenic features are very indicative of sedimentation rates. Hydromorphic and vertic features such as nodules, slickensides and cutans develop under relatively high aggradation rates while the development of mature calcrete profiles require lower aggradation rates (Marriott and Wright, 1993; Balin 2000). The continental deposits of the Punta San Andrés Alloformation (Plio-Pleistocene) crop-out in the marine cliffs of south-eastern Buenos Aires Province (Mar del Plata City, Argentina). Deposits of this unit have been assigned to different sub-environments such as floodplains (tabular and wedge-like silty sandstones), fluvial channels (mono and multi-episodic ribbon sandstones) and temporary water bodies (tabular, laminated silty sandstones). The aggradation rate of the floodplain deposits in which palaeosols developed was never calculated. The aim of this presentation is to provide information about this important feature in order to contribute to the better understanding of the sedimentary dynamics and the influence of allogenic factors in the accumulation of this unit. As the Punta San Andrés palaeosols are overlain by sheet-flood deposits, their superficial horizons have been eroded. Therefore, a proper classification of these palaeosols is difficult to establish. However, two main groups can be differentiated based on the sub-superficial horizons identified. The first group is characterized by poorly developed Bw horizons (2-2.5 m) with typical, concentric, nucleic and amiboidal strongly impregnated Fe-nodules (0.2-0.5 mm), evidence of root activity (micro and mesoscopic channels with scarce clayey coatings and hypo-coatings) and weakly developed subangular blocky peds (10-50 mm). When identified, b-fabric is porostriated around a plane (slickensides) and granostriated. The nature of this profile seems to be cumulic since identified Bw horizons are composed of three or four sheet-flood pulses (30-40 cm each) affected by pedogenesis. The second group is dominated by strongly developed calcic horizons (Bk), 1.5-2 m thick, with stage III (30-125 µm micritic peloids, micritic coated grains and 3-5 cm nodules), IV (6-8 cm laminated horizon with 0.5-1 mm micritc laminae) and V (brecciation) morphologies. As carbonatization is rather intense, primary structures can not be identified and the nature of these profiles can not be assigned. Even when the decapitated nature of the palaeosol profiles makes estimation of aggradation rates difficult, other characteristics such as palaeosols maturity and type can be used to estimate a relative aggradation rate. In the Punta San Andrés case, aggradation rates could have oscillated between periods of 2-5 mm/y and periods of less than 0.2-0.5 mm/y aggradation rates (Marriott and Wright, 1993; Balin 2000). The Punta San Andrés palaeosols show a cyclic vertical alternation between Bw and Bk horizons, indicating cyclic variations between relatively high aggradation rates (Bw horizons) and lower aggradation rates (Bk horizons). The combination of these observations with a facies and architectural analysis of the litosomes could help to develop a model for sedimentation in the area and could provide information on whether alternation between palaeosol types is due to climatic instability or other allogenic factors.