Abstract: Petrographic studies of Cretaceous-Tertiary red-beds in the Xiyaer Gang region of Northern Tibet indicate diagenetic alteration of the minerology and texture of the clastic sedimentary rocks, and suggest a diagenetic origin for their red-purple colour. The rocks were deposited in alluvial fan and upper fan delta environments. The mineralogy has been altered by addition of mechanically infiltrated clays (detrital in or-rgin), by dissolution of unstable minerals and rock fragments, and precipitation of authigenic minerals. The textures show alteration by the processes of: (i) infiltration of detrital clay and formation of authigenic clay minerals which form an interstitial clay matrix not present in the original sediment, (ii) dissolution of framework grains, (iii) replacement and recrystallisation. Two principal diagenetic environments are recognized: (1) the early shallow burial diagenetic environment which is controlled by depositional environment, sedimentation and climate. It is comparable to near-surface carbonate diagenetic environments; (2) deep burial environment of the later stages of diagenesis which is controlled by the gcochemical conditions of pore water, and pressure-temperature regimes.