230 Recent Advances in Stratigraphy Geology I

Wednesday, 3 November 2010
8:00 AM - 12:00 PM
Room 303

A DIRECT QUANTITATIVE TEST OF THE SEQUENCE STRATIGRAPHIC MODEL

Paper number: 230-4
Program: Technical Sessions
Time: 8:45 AM – 9:00 AM

Authors

Abstract

The sequence stratigraphic approach, a major geological paradigm, is used to interpret and predict spatio-temporal distribution of sedimentary rocks, fossils, and resources. Well established qualitatively, the model is largely untested quantitatively. When applied to shallow-marine coastal and tectonically stable siliciclastic settings experiencing (glacio-)eustatic sea-level changes the models make several explicit predictions regarding the stratal architecture (and the associated fossil record) that constitutes a depositional sequence. In particular rate of sedimentation, depositional resolution and frequency of preservable depositional events, is predicted to decrease upwards through the transgressive systems tract (TST), reaching its minimum at the condensed section, and then increases upwards through the highstand systems tract (HST).

Using 309 shells of two corbulid bivalves, individually dated by 14C-calibrated amino-acid racemization ratios, we evaluate these general predictions in the specific regional context of the Holocene sedimentary record of the Po Plain. The 4th-order regional sequence displays a distinct succession of facies consisting of brackish to marginal marine retrogradational deposits (early-TST), overlain by fully marine sand and clay deposits (late-TST), and capped with progradational deltaic clays and sands (HST).

All predictions were confirmed: the temporal resolution, the frequency of depositional events and the rate of deposition decreased upward during the TST, reaching minima at the turnaround between TST and HST. The reverse trend, with increasingly more frequent, thicker, and less time-averaged beds, was observed for the overlying highstand systems tract HST. The results quantify a strong TST-HST asymmetry in the resolution of the sedimentary and fossil records predicted by the sequence stratigraphic model. The results provide a compelling case for applicability of amino acid racemization method as a tool for evaluating changes in depositional dynamics, sedimentation rates, time-averaging and temporal resolution of the fossil record across sequence stratigraphic cycles.