Vitrinite reflectance data

Figure: Contoured vitrinite reflectance results for two wells on Joaquin Ridge (for their location, see Figure 1).

Additional evidence for a heating event comes from vitrinite reflectance data from two wells on Joaquin Ridge (see Figure 1 for the location of the wells, and Figure 4 for the vitrinite reflectance data). The ``Joaquin Ridge #1'' well is 4,390 m deep and located in the vicinity of sample JR3. The ``Christie #1'' well is 1,380 m deep, and located next to sample JR2. Micro-paleontological ages have been obtained for both wells (Martin B. Lagoe, written communication). ``Joaquin Ridge #1'' sediments are of Coniacian (at 910 m) to Cenomanian age (at TD), whereas the ``Christie #1'' samples at TD are Cenomanian (Figure 4). Twenty-three samples from the ``Joaquin Ridge #1'' well were analyzed, from depths of 300 to 4,390 m. They show $R_o$ values of 0.6-1.5%, explaining why apatite fission track samples JR3, JR4, and JR5 have not been thermally reset. These samples are all located upsection from the shallowest ``Joaquin Ridge #1'' vitrinite reflectance samples and should, therefore, correspond to $R_o$ values $<$ 0.6%, or maximum paleo-temperatures less than $\sim$85$^oC$ (Sweeney and Burnham, 1990). The ``Joaquin Ridge #1'' vitrinite reflectance data imply a thermal gradient of $\sim$14 $^o$C/km, which was normal in the Great Valley forearc basin (Dumitru, 1988). There exists substantial evidence that the geothermal gradient in the Great Valley Group was very low during the Cretaceous and the beginning of the Tertiary. This is postulated to have been caused by the refrigerating effect of the subducting Farallon pLate (Dumitru, 1988). The ``Christie #1'' well is located near sample JR2. Three cores taken in ``Christie #1'' at 1,250-1,380 m are characterized by vitrinite reflectance values of $\sim$1.9-2.0%, or maximum paleotemperatures of $\sim$180$^oC$, very hot for the Great Valley Group. However, apatite fission track sample JR2, located about 1,500-2,000 m upsection from the ``Christie #1'' vitrinite reflectance samples, has not been reset. Assuming a thermal gradient similar to that inferred from the ``Joaquin Ridge #1'' well, this would lower the predicted vitrinite reflectance value for sample JR2 to about $R_o$ = 1.3% (maximum paleo-temperature $\sim$150$^oC$). This rough estimate, if correct, conflicts with the observation that apatite fission track sample JR2 has not been completely annealed. This would imply that the high $R_o$ values of ``Christie #1'' are due to a thermal anomaly, that the thermal gradient in this well was not equal to that of ``Joaquin Ridge #1'', and/or that this gradient was not linear. Although not reset, sample JR2 has been at paleo-temperatures above those of samples JR3, JR4, and JR5. The high paleo-temperatures of the ``Christie #1'' samples are unlikely to be the result of simple burial, but are instead interpreted to be the result of a Middle Miocene heating spike, associated with the upward protrusion and tectonic denudation of the New Idria serpentinite body.