Evidence for episodic cementation and diagenetic recording of seismic pumping events,North Coles Levee,California, U.S.A.

Bands of calcite and dolomite cements alternating with zones of nearly carbonate-free sand occur in the Stevens sandston aat North Coles Levee, San Joaquin, Valley, California. Temperatures calculated from O isotopes suggest that the calcite cement bands were emplaced episodically as a result of repeated injections of hot water from deeper in the section. Burial analysis suggests that these cements precipitated from 7 Ma to the present over the temperature range of 45 to ～95°C.Carbon isotope data suggest that the C in the cements is a mixture derived from two sources, detrital shell material (δ¹³C(PDB)≈) and CO₂ liberated from maturing kerogen (δ¹³C ≈ ?24). Plots of δ¹³C vs time and depth of crystallization show that the cementation sequence was: (1) dolomite cements, possibly concretionary, precipitated at depths <1–2 km and at temperatures <45°C; (2) calcite cements with δ¹³C(PDB) values as low as ?13, crystallized from depths between 1220 and 1820 m (4000 and 6000 ft) and at temperatures between 45 and 80°C; (3) calcite cements with δ¹³C(PDB) values approaching zero and calculated temperatures of crystallization up to the present reservoir temperature of 95±3°C.A log of δ¹³C vs calculated depth of crystallization correlates with the stratigraphic column at North Coles Levee. If the correlation is valid the light δ¹³ in each cement sample can be tied to its source. A model based on this interpretation suggests that the early, light C was derived from maturing kerogen in the Kreyenhagen Formation (Eocene) as it passed through the oil window between 4 and 5 Ma. The subsequent passage of younger sediments with less organic material produced correspondingly smaller amounts of light CO₂ which was reflected in the relatively heavier C isotopes in the later cements.It is suggested that the epidsodic injections of hot water carried dissolved gases and minerals, principally calcite, upward from rocks as deep as 2–3 km below the Stevens sandstone and reprecipitated the calcite in more permeable zones in the rock. Degassing of CO₂ from rising pore waters likely triggered the precipitation and accounts for the relatively large volumes of cement. The Sibson model for seismic pumping of pore fluids is considered a likely explanation for the observed cementation.