Gravity effect of sediment compaction: examples from the North Sea and the Rhine Graben

Abstract

A Fourier domain expression for calculating the gravity effect of a continuously varying density structure is used to investigate the way in which sediment compaction modifies the shape of the gravity anomaly across a sedimentary basin. In general, sediment density increases with depth in a basin as the overburden thickness increases. The effect of the increase in sediment density is to reduce the gravity contribution from the density contrasts in the deeper parts of the basin relative to near surface contributions. For a theoretical uncompensated basin, the gravity effect of the sediments is calculated for a density-depth variation described by: (1) a simple exponential increase in sediment density with depth, and (2) an exponential modified to include a local density inversion representative of sediment overpressuring. It is shown that for both cases, the calculated gravity does not necessarily reflect the morphology of the sediment-basement interface. The gravity effect is most sensitive to the distribution of the youngest stratigraphic units within the basin. Results of modeling observed gravity anomalies across the Viking and Rhine Graben show that the small peak-to-trough amplitude of the gravity anomalies across these basins can be attributed to the increase in sediment density with depth rather than the compensation of the basin. For the Rhine Graben, it is further shown that the wavelength of the gravity anomaly is strongly controlled by the flexural strength of the lithosphere. Together these results suggest that while the amplitude of gravity anomalies across extensional basins may be primarily reflecting compaction of the sediment infill, the anomaly wavelength is more sensitive to the compensation mechanism.