High-resolution sequence stratigraphy from piezocone tests: an example from the Late Quaternary deposits of the southeastern Po Plain

Abstract

Cone penetration tests are traditionally regarded as a major tool for geotechnical investigations. This study, based upon interpretation of 234 cone penetration tests with pore-pressure measurements (piezocone tests or CPTU), carried out in the Late Quaternary deposits of the southeastern Po Plain, shows how CPTU tests can also be used for sedimentological purposes, including detailed facies characterization, subsurface stratigraphic correlations, and identification of the key surfaces for sequence-stratigraphic interpretation. The Late Quaternary depositional history of the southeastern Po Plain, reconstructed on the basis of data from seventeen continuously cored boreholes, includes (1) alluvial plain development during the Late Quaternary lowstand and the early stages of transgression, (2) formation of a rapidly migrating barrier–lagoon system during the late transgressive phases (8800–6000 y BP), (3) construction and progradation of a wave-dominated delta (ancient Po delta) during the following sea-level highstand (6000–800 y BP), and (4) development of the present-day alluvial plain, following the delta lobe abandonment in the 13th century A.D. Local execution of CPTU tests in coincidence with drilling sites enables the calibration of borehole data with piezocone penetration profiles. Lithofacies characterization is based upon estimation of three major parameters: corrected cone resistance (qt), sleeve friction (fs), and pore water pressure (u). Plotting of qt versus the ratio of cone friction to cone bearing (FR) is adopted as the major tool for sediment texture classification. Eight major facies associations for the southeastern Po Plain are identified. These are, for decreasing qt values: (1) fluvial channel sands, (2) beach-ridge sands, (3) transgressive barrier sands, (4) crevasse sands and silts, (5) levee silts and sands, (6) floodplain silts and clays, (7) prodelta clays, and (8) marsh clays and peats. CPTU profile interpretation also provides the basis for the identification of the three major key surfaces within the Late Quaternary 4th-order depositional sequence. (1) The transgressive surface (TS), marking the boundary between the locally pedogenized, stiff Pleistocene alluvial clays and the overlying Holocene transgressive paralic deposits, has a distinctive pore-pressure response, with very low u values, and is characterized by a sharp downward increase in fs, which is paralleled by a moderate increase in qt. (2) The ravinement surface (RS), corresponding to the boundary between fine-grained back-barrier deposits and the overlying transgressive barrier sands, is invariably marked by a sharp upward increase in qt (and decrease in FR). (3) The maximum flooding surface (MFS), which is not clearly recognizable on the sole basis of core data, is identified within shallow-marine (prodelta) clays in combination with minor peaks of qt, which have been interpreted to reflect a laterally extensive fossil lag, with transition in proximal areas to sand (shell-rich?) layers. Simplicity, speed, and comparatively low costs of CPTU tests imply that an extensive use of this method, when used in conjunction with core programs, can be a very attractive alternative to economically less convenient methods for the geological mapping of alluvial/coastal plain areas consisting of non-gravel deposits.