Frequency-reliant correlative patterns of asymmetric lacustrine-paralic sequences: A genetic approach to the late miocene Bithynia Marlstones of the southeastern Volterra Basin, Italy

摘要

The occurrence of maximum flooding events with marine affinity in terrestrial settings is critical in sequence stratigraphy, in that it sets firm isochrons that enable correlations between coeval marine and nonmarine realms. This paper focuses on the variable and frequency-dependent correlative patterns of the Bithynia Marlstones of the SE Volterra Basin (Italy), a lensoid body of shallow lacustrine strata that contains evidence of episodic marine incursion. Sedimentologic analyses depicted three associations of respectively profundal, marginal lacustrine, and distributary system facies. The rhythmic occurrence of prominent limestone beds with marine affinity and the up-dip absence of subaerial unconformities together allow a pragmatic genetic sequence stratigraphic approach. A two-fold sequence stratigraphic hierarchy is developed. A set of limestones with marine affinity that can be correlated throughout the lensoid body bound four low-frequency sequences thought to have been generated by regional high-amplitude base-level fluctuations. These low-frequency sequences are composed of less arealy extensive high-frequency sequences, generated in response to localized low-amplitude base level perturbations. Both low-frequency and high-frequency sequences are structured in lower progradational and upper retrogradational systems tracts, and are in most cases characterized by asymmetric development. Asymmetry is expressed by thinner progradational and thicker retrogradational systems tracts, as well as by thicker retrogradational and thinner retrogradational systems tracts. This thickness asymmetry, likely induced by differential interplays of subsidence and sediment supply on an inherited topography, is interpreted as symptomatic of complex interactions between regional and local base-level oscillations. In the Bithynia Marlstones depositional system, the lacustrine facies distribution and respective architecture were mostly unrelated to the low-frequency base-level trend, but rather appear to be significantly correlated with the high-frequency trend. The latter largely affected the Bithynia Marlstones deposition, ultimately forcing the rhythmic alternation between hydrologically balanced and underfilled lacustrine phases in the high-frequency sequences.