Sandstone dikes in dolerite sills: Evidence for high-pressure gradients and sediment mobilization during solidification of magmatic sheet intrusions in sedimentary basins

Sediment dikes are common within dolerite sill intrusions in the Karoo Basin in South Africa. The dikes are subvertical and as much as 2 m wide, sometimes with abundant fragments of sedimentary rocks and dolerite. The matrix consists of contact-metamorphic sandstone. There is no petrographic evidence for melting within the sediment dikes. The maximum temperature during heating is restricted to the plagioclase and biotite stability field, or above ∼350 °C. Thermal modeling of a sandstone dike in a dolerite sill shows that a temperature of 350–450 °C is reached in the dike after a few hundred years of sill cooling. The calculated pressure history of a cooling sill and its contact aureole shows that substantial fluid pressure anomalies develop on a short time scale (1–15 yr) and are maintained for more than 100 yr. Calculated pressure anomalies in the sill (-7 to -22 MPa) and the aureole (4–22 MPa) are significant and may explain sill fracturing and sediment mobilization from the aureole into the sill. We conclude that sediment dikes represent common features of sedimentary basins with sill intrusions in which fluid pressure gradients have been high. Sediment dikes thus signify that pore fluids may escape from the aureoles on a short time scale, representing an intermediate situation between fluid loss during formation of microfractures and fluid loss during violent vent formation.