Lithological influence on sill geometry in sedimentary basins: Controls and recognition in reflection seismic data

Sills play a leading role in the transport of magma in sedimentary basins. The contact between sills and host rocks reflects the acting emplacement processes during sill propagation and evolution. Recent studies have shown that the propagation of sills and dykes is strongly influenced by the lithology of the host rocks, but none have detailed documentation of marginal features in large-scale intrusive complexes. Three-dimensional seismic data is the primary method of mapping and investigating such complexes, but it is difficult to accurately image sills due to their low thickness compared to seismic resolution. By understanding the relationship between local lithology and marginal sill features, we can better understand the imaging of sills in seismic datasets and their resulting geometry. In this study, we present a seismic-scale sill analogue through multiple high-resolution three-dimensional models, with corresponding logs and field observations from Cedar Mountains, San Rafael Swell, US. This model was further used to develop a synthetic seismic dataset, providing us with a strong control on which marginal sill features fall beneath seismic resolution. We found that lithology plays a critical control in sill geometry and morphology. In Cedar Mountains, sills emplaced within massive sandstones frequently exhibit strata-discordant base contact with the host rock. Conversely, sills found within heterolithic intervals and mudstones typically display strata-concordant base contact with the host rocks. Sills within heterolithic intervals also tend to exhibit a more complex segmentation with multiple broken bridges. Furthermore, our findings show that sills are more than 3.7 times more likely to intrude in mudstone compared to sandstone and heterolithic intervals. These results suggest how sill geometries can be adapted to interpret lithology in seismic datasets from sedimentary basins with little to no well control. We anticipate that our findings may provide better knowledge for interpreting sills in sedimentary basins and contribute to developing more sophisticated geomechanical emplacement models for igneous intrusions.