Abstract

Dolomite breccias from the Miocene Monterey Formation, Tepusquet area, California are composed of dolomitic siliceous mudstones that are extensively fractured and filled with white, coarse-grained saddle dolomites. Fracturing and brecciation are much more extensive and intense in the Tepusquet area than in most other outcrops of the Monterey Formation. Despite the intensity of brecciation and its potential importance as an analog to Monterey fractured hydrocarbon reservoirs, no detailed petrographic, crystallographic, or geochemical analyses have been performed on the breccias from the Tepusquet area. In the present study, petrographic, crystallographic, and geochemical analyses show that the vein-filling dolomites were precipitated from hydrothermal fluids that were associated with hydrocarbon migration, and that the early diagenetic matrix dolomites have been recrystallized, resetting their geochemical and crystallographic properties. Recrystallization of matrix dolomites is indicated by the uniformly negative delta 18 O compositions (x = -7.1 per thousand) low Sr contents (x = 230 ppm), low Na contents (x = 364 ppm), contracted unit cells (x: a = 4.812 A, c = 16.058 A), high degree of cation order, high Mg content (x = 46.1 mol% MgCO 3 ) as compared to most Monterey dolomites, and increasing 87 Sr/ 86 Sr ratios with decreasing Sr content. Vein dolomites have large variation in stoichiometry (41.7-49.0 mol% MgCO 3 ), Sr content (124-414 ppm), unit-cell dimensions ( a : 4.806-4.829 A, c : 16.013-16.178 A), and cation order (Ca site occupancy: 0.73-0.93, Mg site: 0.68-0.94). Recrystallization of some vein dolomites is indicated by the covariance of mol% MgCO 3 with Sr, delta 18 O, cation order, and unit-cell parameters. The covariance of delta 18 O with mol% MgCO 3 is the inverse of the trend expected for the recrystallization of dolomite during burial diagenesis, possibly because of recrystallization during uplift. Vein dolomites have higher 87 Sr/ 86 Sr values than matrix dolomites. The least radiogenic matrix dolomites (the least recrystallized as inferred from the delta 18 O composition) have 87 Sr/ 86 Sr apparent ages (13.0 and 15.5 Ma + or - 0.3) that agree well with the previously mapped age of the brecciated unit. The more recrystallized matrix dolomites have higher 87 Sr/ 86 Sr ratios (apparent ages too young for the lower Monterey Formation), thus recording the evolution of Sr isotopic composition of the pore fluid. The close chemical similarity of the vein dolomites and the most recrystallized matrix dolomites, the episodic association between hydrocarbons and the vein dolomites, and the recrystallization trends in the matrix dolomites, all indicate that evolved formation waters were the source of the hydrothermal fluids that precipitated the vein dolomites.

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