54 SEDIMENTARY ROCKS/Chert Miocene–Pliocene of the Pacific margin, and formed in small rifted and back-arc basins where there was strong nutrient supply, high phytoplankton activity, and a lack of detrital sediment input The Monterey Formation (Miocene, California) contains chert derived from diatomaceous sediments (Figure 2A and B), and is associated with hydrocarbon source rocks The maturation process from diatomaceous sediment to chert involves porosity loss to the extent that the cherts can form a diagenetic reservoir seal at depth In the Mediterranean, cherts occur in the Late Miocene at the time when the area was characterized by small, restricted basins in the build-up to the Messinian salinity crisis at the end of the Miocene In general, cherts of Tertiary age are widespread in oceanic areas, and occur in a wide variety of settings satisfying the conditions described above Figure Bedded cherts Laminated diatomite (A) and lamin ated diatomite with slump fold (B) Monterey Formation, Miocene, Gaviotta Beach, near Ventura, California, USA (C) Deformed bedded radiolarian cherts within the Khabarovsky Complex, ex posed on the banks of the Amur River in the city of Khabarovsk, Russian Far East These Upper Triassic Lower Jurassic cherts are associated with black mudstone and metabasalt units of the same age, and are tectonically interleaved with Cretaceous tur bidite units This complex association formed in a Late Mesozoic accretionary complex during collision of the North China Block with Siberia Photographs courtesy of David Macdonald sediment-starved basins and submerged carbonate platforms on passive continental margins are potential sites for deposition when upwelling currents bring nutrients and calcareous plankton is scarce The Gulf of California (