Petrographic examination shows authigenic chabazite and calcite in the ice-free or weathering horizon of glacially deposited diamictites in the Sirius Group on Table Mountain in the Dry Valleys area of Antarctica. Some samples contain as much as 18% chabazite, which fills pores in matrix clays. These pores are thought to have formed by repeated cycles of freezing and thawing during a warmer and wetter climate than the present. The chabazite is sodium-rich and has an average composition of (Ca (sub 0.9) Na (sub 1.9) K (sub 0.2) )Al (sub 3.9) Si 8 O 24 .6H 2 O. The presence of authigenic minerals in the Sirius Group suggests that chemical weathering is active and may take place in frozen ground along interfacial films of brine. Precipitation of the minerals is likely to occur at the boundary between the ice-free and ice-cemented horizons. As the ice cement sublimates, the film of brine becomes more concentrated until precipitation occurs in the process of efflorescence. Precipitation is kinetically driven by sublimation, allowing the crystals to grow in the open pore space of the ice-free horizon just above the ice cement. Because the depth to the ice-cement boundary fluctuates in response to major climatic changes, authigenic minerals that mark the position of this boundary may be a proxy indicator of past climates.