Diagenesis of volcanogenic sandstone and mudstone of the 8,500 m thick Great Valley sequence (Tithonian to Hauterivian) involved extensive mass transfer during burial in the outer-arc basin.

The clay mineral assemblage of mudstone is characterized by (1) disappearance of discrete smectite at a relative stratigraphic depth of 6,600 m, and (2) a gradational increase of illite/smectite with increasing stratigraphic depth over the lowermost 4,500 m of strata. ^dgr¹⁸O for illite/smectite changes with descending stratigraphic position from 21.9 to 15.5 ppm and is temperature dependent. Potassium and sodium enrichment and uniform aluminum composition in the clay-size fraction of the mudstone relative to the whole rock indicates that the dominant reaction was: smectite + potassium ^rarr illite/smectite + silica. Authigenic quartz with ^dgr¹⁸O ^cong 21.6 ppm throughout the sequence indicates that this reaction is temperature triggered. Calcite derived from pr mary biogenic carbonate acquired ^dgr¹⁸O values from +0.2 to -10.5 ppm PDB during successive mobilizations as temperature increased with burial.

In sandstones, the sequence of mineral authigenesis is: (1) chlorite cutans (^dgr¹⁸O = 13.2 to 13.9 ppm) around framework grains formed during shallow burial; (2a) precipitation of radiating pore-fill chlorite (^dgr¹⁸O = 11.1 to 13.3 ppm) as iron and magnesium were released from mudstone by illitization of smectite at temperatures as low as 60°C, or (2b) precipitation of calcite (^dgr¹⁸O = -4.2 to -10.8 ppm PDB) as calcium was released from nearby shales and albitized plagioclase; (3) late replacement of framework grains by chlorite (^dgr¹⁸O = 6.5 to 13.0%) or by calcite (^dgr¹⁸O = -8.3 to -12.7 ppm PDB) at more elevated temperatures.

Prehnite, laumontite, and quartz veins (^dgr¹⁸O = 17.2 to 20.5 ppm) are found only in the most basal strata, and were derived from the ophiolitic basement. These phases do not imply zeolite-grade burial metamorphism of the basal sediment.

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