ABSTRACT

Carbonate minerals filling fractures in the Monterey Formation at four locations along the Santa Barbara Channel coast display a ¹⁸O(SMOW) range from 20.7 to 35.7 and ¹³C range from - 18.6 to + 16.6. Many fracture fills display a covariation of ¹⁸O and ¹³C, and successive generations of carbonate have lower ¹⁸O values. These isotope systematics can be interpr ted in terms of ¹⁸O values that were controlled by crystallization temperatures and the ¹⁸O of connate pore water, and ¹³C values that were controlled by depth-sequential organic matter degradation reactions. Each successive generation of carbonate precipitated at progressively higher burial temperatures from isotopically evolving connate pore water. These carbonate-filled fractures developed during burial and not during uplift.

Using crystallization temperature estimates from fluid inclusion data, the change in ¹⁸O of conate pore water with increasing temperature in the burial environment is modeled. From this model, the fracture fill ¹⁸O values indicate precipitation over the following temperature intervals: 25-70°C (Carpinteria Beach), 7-15°C (Goleta Slough Beach), 45-100°C (Refugio Beach), and 53-103°C (Jalama Beach).