Lower Jurassic marine sandstones in the Hasle Formation on Bornholm Island can easily be separated in two different petrographic types. In the lower part of the type locality, type 1 sandstones are friable with layers and lenses of concretionary siderite. They are separated by a major erosional surface from the overlying type 2 well-cemented sandstones.All parts of the sandstones were subjected to chloritization of original grain coatings, neomorphism of chlorite and local precipitation of authigenic pyrite. During early diagenesis, diagenetic pathways differed between the two sandstone types. Type 1 sandstones were subjected to compaction before kaolinitization and some quartz authigenesis occurred. Type 2 sandstones immediately above the major erosional surface were cemented by concretionary siderite at a very early stage, and thus further diagenetic modifications were prevented. Above this zone, sandstones are tightly cemented by various carbonate minerals. Diagenetic modifications mainly involved alternating events of carbonate precipitation and dissolution. The sequence of carbonate precipitation was: (1) zoned stable/metastable siderite; (2) siderite, gradually with significant Ca- and Mg-substitution; (3) ankerite; and (4) calcite, reflecting a sequence of precipitating carbonates containing progressively lower amounts of iron. A period of kaolinitization predated calcite cementation.Six diagenetic stages were recognized: (1) early burial reducing conditions resulting in precipitation of pyrite; (2) precipitation of chlorite and siderite caused from compactional flow from underlying brackish and fresh-water sediments; (3) dissolution of carbonate and partly oxidation of siderite caused by fresh-water flushing related to uplift; (4) precipitation of siderite with gradually increasing substitution by CaCO3 and MgCO3 and eventually precipitation of ankerite caused by renewed burial; (5) fresh-water diagenesis with precipitation of kaolinite and quartz; (6) precipitation of calcite in remnant porosity.