Thickness and spatial distributions of clastic dykes, northwest Sacramento Valley, California

Abstract

The geometry and distribution of the clastic dykes of the Ono district, North Sacramento Valley are examined within stream sections. Five traverses along dry stream beds provide good exposure allowing the spacing, thickness and geometry of the dykes to be recorded. The spatial and thickness distribution of the dykes are considered using cumulative frequency plots, allowing a visual estimation of a best fit distribution. Dyke thickness conforms best to a log-normal distribution. There is also a characteristic minimum dyke thickness in a traverse and this is attributed to the minimum aperture that a fluid with sand clasts is able to exploit. Dyke spacing, however, shows a good correlation with a power-law distribution for four traverses, suggesting that there is a mechanistic control on the spatial distribution. Plotting dyke thickness against minimum dyke spacing reveals that thin dykes do not generally intrude in isolation. Unlike veins and igneous dykes, clastic dykes continue to provide preferential pathways for fluid flow, subsequent to their intrusion, thus inhibiting intrusion in the area surrounding a pre-existing dyke. A combination of this process and dyke branching provides the best model for the observed spatial and thickness distribution of clastic dykes seen in the Ono district, California.