Distinguishing depositional environments in shallow-water mixed, bio-siliciclastic deposits on the basis of the degree of heterolithic segregation (Gelasian, southern Italy)

A new sedimentological technique has been tested in Gelasian cross-stratified, bio-siliciclastic deposits cropping out in southern Italy (Lucanian Apennine) to disentangle paleoenvironmental and model reconstructions for shallow-water mixed sediments. The proposed method suggests the use of the bioclastic/siliciclastic ratio (b/s), and the Segregation Index (S.I.) to evaluate the percentage of the dominant clastic component in a mixed deposit, and in order to estimate the degree of the heterolithic segregation between bioclastic and siliciclastic particles. The principle of this method is based on the experimental evidence that bioclasts and quartz grains reveal a different physical behavior if entrained by a hydraulic flow of a given energy. Thus, a different internal texture of bioclastic and terrigenous particles in the same sedimentary deposit is regarded as the result of the variation in the energy of a number of hydrodynamic processes that characterize shallow marine settings, including waves, currents, and tides. The studied mixed sediments, subdivided into facies associations, exhibit stratigraphic shoaling-upward successions recording repeated transitions from offshore to shoreface environments. These trends are documented through the vertical variations in the degree of heterolithic segregation between mixed clastic particles, quantified by using the S.I. introduced in the present study. The assessment of the degree of heterolithic segregation is thus proposed as a proxy to distinguish depositional environments related to different water depths in shallow-water mixed systems. Copyright © 2012, SEPM (Society for Sedimentary Geology).