Thin-section and X-ray diffraction analyses of 31 outcrop samples show that the early diagenesis of heterogeneous volcaniclastic and arkosic sandstone and conglomerate of the Middle Park Formation (Upper Cretaceous- Paleocene) was strongly influenced by detrital mineralogy. Rocks with abundant andesitic rock fragments experienced the most compaction and are characterized by an early generation of authigenic chloritic mixedlayer clays and relatively minor illite-smectite and chlorite, whereas rocks with moderate to minor percentages of volcanic rock fragments experienced less compaction than volcanic-rich rocks and are characterized by earlygeneration authigenic illite and lesser illite-smectite, chlorite, and chloritic mixed-layer clay. Compaction and cementation occluded most macroporosity in those samples richest in volcanic detritus, thereby sealing the rocks and preventing the development of later-stage cements or dissolution porosity. Unlike that of clay minerals, the distribution of authigenic heulandite was not influenced by variations in detrital mineralogy. This suggests significant calcium-ion transport during early diagenesis and shows that intervals rich in unstable constituents can influence the diagenesis of adjacent intervals without large percentages of plagioclase or volcanic rock fragments. Late diagenesis was strongly influenced by structural deformation that fractured, folded, and subaerially exposed the rocks. Fracturing may have caused local drops in pore-fluid pressure, causing laumontite to replace heulandite as the stable calcic zeolite. Penetration of the fractured rocks by oxidizing, slightly acidic, dilute meteoric water altered the pore-fluid chemistry and caused precipitation of minor percentages of illite-smectite, kaolinite, and hematite. Macroporosity is less than 5% in all but two samples. The Middle Park Formation illustrates several factors that can adversely affect porosity during diagenesis of volcaniclastic sandstone: (1) lack of early leaching by fresh meteoric water, (2) a prolonged episode of compaction before the first authigenic minerals precipitated, (3) cementation by first-generation clays and heulandite occluding most pores that had survived compaction, (4) minor burial dissolution, (5) minor late-stage cements that further reduced porosity and occluded multigrain fractures, and (6) absence of dissolution of framework grains or cement during uplift and subaerial exposure.