The Aramachay Formation (Lower Jurassic) of the Peruvian Central Cordillera is made up of 300 m of organic-rich mudstones (Ichpachi Member) which grade up into 100 m of phosphatic siltstones, chart, and arenites, (Alata Member). The organic-rich mudstones are slightly enriched in P 2 O 5 (0.5% P 2 O 5 ) but do not contain visible authigenic phosphate; the organic maturity of the rocks as evidenced from kerogen elemental analysis may serve to explain this. Radiolaria in this facies are generally calcitized. In contrast, radiolaria in the phosphatic siltstones are phosphatized, occurring within apparently authigenic blabs and stringers of collophane (2% P 2 O 5 whole rock). In addition, this facies (although with a lower organic carbon content) contains the remains of sponges, benthic blue-green algae, and a few epibenthic bivalves (monospecific); it also contains more silt and some glauconite. It is inferred that the site of phosphate authigenesis lay wholly within this phosphatic siltstone facies, which records low or fluctuating oxygenation rather than fully anoxic conditions. This hypothesis is supported by studies of other similar sequences and by published carbon and sulfur isotope data. It is proposed that the facies sequence represents the fossilized upper margin of an oceanic O 2 -minimum zone, comparable with that observed on the modern Peruvian shelf. Such an environment has been suggested for phosphate authigenesis before but has not previously been identified in the stratigraphic record. The fine-grained sequence is sharply overlain by phosphatic lithoclastic arenites (8% P 2 O 5 whole rock) in which the rounded phosphate grains contain radiolaria and are therefore inferred to have been eroded from the underlying siltstone. These are, in turn, overlain by shallow-marine carbonates completing a sequence interpreted to represent a substantial marine regression.