Figure 1 shows a simplified geologic map of the field area with indication of the sample locations. Some of these samples were previously discussed by Linn et al. (1991, 1992) and DeGraaff-Surpless et al. (2002). They are labeled with the letters ``GV''. An additional six samples were collected along a transect on Joaquin Ridge and are labeled with the letters ``JR''. Table 1 summarizes the geochronological data. Apatites and zircons were separated from their host rock using the mineral separation techniques reported by DeGraaff-Surpless et al. (2002). All samples yielded abundant euhedral apatites and zircons. Both zircon and apatite fission track ages were measured with the external detector method (e.g., Dumitru, 2000). Figure 2 shows the apatite fission track data from Joaquin Ridge, arranged in order of decreasing stratigraphic age. Five of six samples on Joaquin Ridge came from the Great Valley Group, while the youngest sample (JR6) came from the Middle Miocene Temblor Formation, beneath the Middle Miocene Big Blue Formation. Figure 3 shows five zircon fission track samples labeled and arranged in decreasing order of stratigraphic age like the apatite fission track ages of Figure 2. The dark gray bands on the radial plots of samples GV21 and GV33 mark the range of crystallization ages measured by U/Pb SHRIMP dating (DeGraaff-Surpless et al., 2002). The light gray bands mark the depositional ages (Dibblee, 1971). Between n=20 and n=41 grains were dated per fission track sample. The probability p that either the oldest or the youngest population fraction of size f was missed by all n grains is given by:
For example, if n=30 and f=0.12, then p=5%. In other words, there is 5% chance that either the youngest or the oldest 12% of the detrital population was missed. In addition to the aforementioned outcrop samples, we also had access to material from two boreholes on Joaquin Ridge, the ARCO ``Christie #1'' and ARCO ``Joaquin Ridge #1'' wells. Vitrinite reflectance measurements were performed on 23 well cutting samples from ``Joaquin Ridge #1'' and three core samples from ``Christie #1''. Up to 100 reflected light points were measured on the vitrinite populations represented in each sample. The lowest reflectance values likely reflect contamination from organic matter in the drilling fluid, while some of the higher reflectance values may represent resedimented vitrinite. However, rather than arbitrarily rejecting some data, we have opted to just contour and plot all the data (Figure 4). The raw data for both the fission track and the vitrinite reflectance analysis are available in the Data Repository 1.
6'/120
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