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 similar to 350 degrees C. Thermal
modeling of a sandstone dike in a dolerite sill shows that a temperature
of similar to 350-450 degrees 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.