Origin of Dolomites in the Monterey Formation

Abstract The Monterey Formation in the Pismo and Huasna Basins is characterized by four lithofacies (siliceous, calcareous, phosphatic, and siliciclastic). Each lithofacies contains dolomite as matrix, cements, concretions, replacement features, and as fracture fillings. Some dolomites apparently represent a single diagenetic event, whereas others clearly result from multiple diagenetic events. Due to its complex diagenetic history the dolomite exhibits a wide range of isotopic compositions (δ13C = -25.5 to +16.8 °/oo and δ180 = -7.2 to +1.2 °/oo, relative to PDB). In the various facies of the Pismo Basin the dolomite replacing microfossils typically exhibits δ13C values of about -15.0 °/oo and an estimated isotopic temperature of formation (T.F.) of 50°C (δ180 values from -3 to -4; using a water δ18q composition of 0.0). In the Huasna Basin the dolomite in the calcareous facies exhibits a range of compositions and T.F. {δ13C = -7.3 to +l.l°/oo and δ180 = -2.0 to -4.0, T.F. of 40 to 50°C). The siliceous facies from both basins contains dolomite, as matrix and framework grain replacements, with the following compositions: δ13C = +6.5 to +12.0 and T.F. of 35 to 50OC (δ*80 = -7.0 to +0.8). The above isotopic compositional differences are interpreted as resulting primarily from variations in burial rates for different lithofacies. Other conclusions drawn from the isotopic data pertain to concretion growth and the spatial distribution of high temperature dolomite. Concretions and replacement structures from both basins and from various lithofacies show very large variations in isotopic composition [δ13C = -7.6 to +16.8 0/00) and T.F. (from 26 to 52°C, 6I80 = +1.2 to -4). Some concretionary growth occurred at or near the sediment-water interface, whereas other concretionary structures grew at greater depth and higher temperatures. All dolomite samples characterized by high temperatures of formation (>70°C) are spatially associated with regionally significant fluid conduits, either faults or large sand bodies.