Ambient-Noise HVSR signatures of deep and shallow impedance structure in a quaternary Plain (Pampean Region, Argentina)

Sedimentary plains host extensive Quaternary sequences whose internal impedance architecture is often poorly constrained, limiting the resolution of subsurface layering and its lateral continuity. We conducted a dense single-station ambient-noise HVSR survey (82 sites) across the La Plata–Berisso–Ensenada area, in the northeastern Pampean Region of Argentina, to characterize both deep and shallow impedance contrasts. The data consistently reveal two frequency modes. The fundamental resonance (f₀ ≈ 0.30–0.40 Hz) exhibits a compact spatial distribution and a smooth basinward increase, indicating a laterally continuous, deep impedance contrast that reflects the large-scale sedimentary thickening toward the Salado Basin. In contrast, the higher-frequency peak (f₁ ≈ 2–12 Hz) displays strong geomorphic and lithogenic controls. Inland, heterogeneous values are consistent with stiffness contrasts produced by pedogenic horizons and loess–paleosol variability within the Pampeano Formation. Across the coastal plain, lower and more organized f₁ values form shore-parallel bands that delineate the Holocene cover overlying Pampeano sediments. A short vertical transect at a quarry constrains the inland f₁-generating contrast to the upper ∼10–15 m, supporting its interpretation as a near-surface mechanical boundary. Together, these patterns show that systematic HVSR mapping can resolve both basin-scale impedance gradients and intra-sedimentary contrasts within thick Quaternary successions. The approach provides a scalable, non-invasive framework for integrating subsurface impedance structure, surface geomorphology, and stratigraphic interpretation in data-limited plain environments.