Sediment Record of Falling Sea Level

Wolfgang Schlager¹ and Georg Warrlich^2
1 Vrije Universiteit/Earth & Life Sciences, Amsterdam, Netherlands
² PDO, Muscat, Oman

In the standard model of sequence stratigraphy, sea-level fall is recorded as an extensive erosional hiatus that constitutes the sequence boundary. Significant sedimentation is resumed near the sea-level minimum. However, subsequent observations revealed that commonly a sediment body is formed during relative fall – the FST (falling-stage systems tract or forced-regressive wedge systems tract). We employed the modelling programs STRATA and CARBONATE 3D to explore for carbonates and siliciclastics under what conditions an FST is produced and when the record resembles the standard model. It turns out that the parameter space for the formation of an FST is large compared with the parameter space for the standard model. The characteristic continuous unconformity of the standard model requires either a highly asymmetric sea-level curve with a very rapid fall or rapid erosion of the subaerially exposed highstand tract. We were unable to produce the anatomy of the standard model, i.e. a continuous unconformity and only minor erosion of the preceding highstand tract, with a sinusoidal sea-level curve. Observations in the Quaternary, where sea-level fluctuations are well constrained, are compatible with the modelling results: geometries resembling the standard model are common during extremely rapid sea-level falls (e.g. 22-18 ka B.P.), while falling-stage systems tracts were formed during slower sea-level falls (e.g. 60-22 ka B.P.). Discrimination between FST and HST is very difficult if the systems tracts are severely eroded. Interpretation of FST's as HST's may lead to wrong geobody geometries in reservoir models and, in carbonates, to underestimation of exposure-related porosity.