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Summary Red palaeosols of the late Pleistocene-early Holocene, both buried and non-buried, were studied recently in Sonora (NW Mexico) to reconstruct their pedogenesis as well as the palaeoenvironmental conditions. The alluvial palaeosol-sedimentary sequence of the La Playa archaeological site is a key locality for the buried San Rafael palaeosol, which exhibits a 2Ah-2Bw-2BCk-3Bgk profile and was defined as a Chromic Cambisol. Radiocarbon dates from pedogenic carbonates and charcoal set the soil formation interval between >18000 and 4300calibrated years before present (cal. year BP). Micro-morphological observations together with profile distribution of clay, carbonates, organic carbon, pedogenic iron oxides and rock magnetic properties indicated a strong eluvial-illuvial redistribution of carbonates, moderate silicate weathering and gleying in the lower horizon. Although this soil was much more developed than the overlying syn-sedimentary late Holocene Fluvisols, clay mineral composition and stable carbon isotope signatures of humus and carbonates were similar in both soils. We suggest that pedogenesis of the San Rafael palaeosol took place under a slightly more humid climate and relative geomorphic stability. This agrees with the regional palaeoclimate reconstruction, which indicates a moister climate during the Late Wisconsin glaciation (MIS 2). An abrupt termination of the San Rafael pedogenesis marked by disturbance and aridization features in the Ap horizon of the palaeosol could be linked to a global drought around 4200years cal. year BP. Surface Chromic Cambisols in northern Sonora show similar pedogenetic characteristics to the buried red palaeosols of La Playa. They appear to be a relict component of the present day soil mantle.

[1] Observations in trenches excavated across the Ekkara rupture zone (ERZ), the longest of the sporadic ground ruptures that accompanied the M 6.7–7.0 earthquake of 30 April 1954 on the Domokos fault zone (DFZ), identify a left-lateral oblique-normal fault with a slip vector aligned with the regional extension direction (verifying the tectonic origin of the ERZ). The paleoseismological interpretation of stratigraphic, soil-stratigraphic, and tectonic features is discussed with emphasis on the issues related to strongly oblique fault kinematics. Two pre-1954 events of ground rupture are recognized (E1 and E2) and an event of ground cracking only (E1a, shortly after E1). Event E2 was accompanied by larger displacement than E1 and 1954, but this does not necessarily imply a stronger earthquake. Archaeological dating of transported ceramic sherds constrains E1 between 6750 and 4450 B.P. (more likely, 6450–5750 B.P.). Luminescence dating of colluvial deposits and 14C dating of pedogenic carbonates (stone coatings) place E2 at ∼17,500 +− ∼2500 B.P. (preferred age). In the (hazard-wise) most conservative interpretation, recent recurrence intervals of ERZ activations exceed 3195 years. The sporadic nature of the 1954 ruptures precludes certainty on whether the ERZ has recorded every past activation of the DFZ unless data are also collected from other DFZ rupture zones. Minimum-limiting estimates of 0.3–0.5 mm/yr slip rate are derived for the ERZ, and the true slip rate is not expected to exceed 1 mm/yr. Slip rate estimates for the ERZ may be minima for the DFZ slip rate, however, because coseismic deformation in the past may have been distributed to more than one rupture zones at the surface.