Shock compression studies in a dolomite rock (&rgr;0=2.84 g/cm3) have been conducted in the stress range 150--450 kbar. The entire loading and unloading history as well as the Hugoniot properties were investigated, using continuous recording piezoresistant stress gages. The following results were obtained: (1) The Hugoniot shock velocity-particle velocity curve shows deviation from linearity in a region corresponding to stresses between 100 and 250 kbar. (2) Comparison of the experimental Hugoniot with a Murnagham pressure-volume relation extrapolated from low-pressure ultrasonic data indicates anomalous compressibility. (3) Unloading experiments from peak stresses between 180 and 300 kbar show excessive hysteresis in the stress-volume plane. (4) Shock compression to a peak stress in excess of 400 kbar is followed by immediate stress relaxation. (5) Overtaking relief wave velocities measured at the Hugoniot state are found to be considerably higher than extrapolated bulk sound velocities. Our interpretation of the results is a rate-dependent, low- to high-density phsae transformation occurring in the stress range 100--500 kbar.