IMPLICATIONS FOR THE POST-DEPOSITIONAL HISTORY OF THE GREAT VALLEY GROUP

Partial annealing of apatite fission track sample JR2 and complete annealing of JR1 at $\sim$14 Ma by burial-induced heating alone is improbable for at least two reasons:

• Maximum temperatures of the Great Valley Group on Joaquin Ridge did not coincide with the timing of maximum burial. In the Panoche Hills area, north of Joaquin Ridge, subaerial exposure and erosion occured intermittently in the Paleocene and Eocene, and almost continuously since the Oligocene (Moxon, 1990; Bartow, 1991). Furthermore, deposits as old as the Eocene Kreyenhagen Formation of Oil Canyon, 10 kilometers north of Coalinga, contain biogenic silica in the form of opal-A (Milam, 1985), indicating that they were never heated above 28-56$^oC$ (Murata and Larson, 1975). The total thickness of Tertiary deposits presently outcropping at the nose of Joaquin Ridge is only $\sim$1.5 km (Dibblee, 1971).
• The age of the annealing (as dated by sample JR1) exactly coincides with the exhumation of the nearby New Idria serpentinite body (see the next paragraphs). It is unlikely that this coincidence is mere chance.

An alternative explanation for the vitrinite reflectance and fission track data is that the Great Valley Group was heated by the serpentinite diapir. Three lines of evidence suggest that the serpentinite body was hot when it breached the surface. Most importantly, mineral assemblages of Franciscan inclusions in the serpentinite body indicate that it rose from depths of as much as $\sim$20 km which, even under the lowest thermal gradients, would make them relatively hot ($>$200$^o$C; Coleman, 1996). Secondly, the serpentinite diapir rose very rapidly. Evidence for the massive size and sudden nature of this event is contained in the Middle Miocene Big Blue Formation, which crops out $\sim$15 km east of the serpentinite dome. The Big Blue Formation consists almost entirely of serpentinite clasts, some of which are house-sized (Anderson and Pack, 1915). Paleocurrents indicate flow towards the east, in the opposite direction of the ``normal'' Great Valley Group paleocurrents (Casey and Dickinson, 1976; Bate, 1985). The facies gradient from sheared protrusive serpentinite through braided stream deposits to marine tidal flat facies evinces an eastward facing paleoslope (Bate, 1985). The fluvial deposits preserving paleocurrents were shed from the gradually spreading flank of a New Idria serpentinite protrusion that breached the suface to form a dome-like mass that spread laterally as additional serpentinite was supplied to the surface by upward diapiric flowage from within the crust. If the serpentinite body rose to the surface extremely rapidly, it is likely to have remained hot all that time. Finally, serpentinization reactions are exothermic:

Antigorite: $\rm 34 Mg_2SiO_4 + 51 H_2O \rightarrow Mg_{48}Si_{34}O_{85}(OH)_{62} + 20Mg(OH)_2$

Chrysotile: $\rm 2 Mg_2SiO_4 + 3 H_2O \rightarrow Mg_3Si_2O_5(OH)_4 + Mg(OH)_2$

Both of these reactions require a lot of water. This could be the reason why the serpentinization did not happen before the Middle Miocene. At that time, the Mendocino triple junction passed the latitude of Coalinga. The faulting and folding caused by the San Andreas fault might have introduced a pathway for fluids in the ophiolitic crust that underlies the Great Valley Group. Both the reaction enthalpy $\Delta H$ of the serpentinization reactions and the heat capacity C$_p$ of the serpentinite minerals vary with temperature (Holland and Powell, 1998). At the conditions relevant to the New Idria serpentinite dome, the pressure effect is negligible. Making the simplifying assumption that serpentinization of the entire body occurred at the same time, we can calcuLate a first-order approximation of the maximum temperature increase that could be caused by the serpentinization:

$$\rm\Delta T = \frac{\Delta H}{C_p}$$

Figure: Serpentinization reactions are less exothermic as the ambient temperature increases. The New Idria serpentinite body must have been relatively hot for one of two reasons: (1) it formed under high ambient temperatures, or (2) it generated the heat itself.

The evolution of this reaction temperature as a function of ambient temperature is shown in Figure 5. The lower the ambient temperature, the more exothermic the serpentinization reactions are, but above a few hundred degrees, they can even become endothermic. The buffering thermodynamics of the serpentinization reactions are such that the serpentinite body must have been hot when it formed, either because the ambient temperature was high or because of its own reaction heat. A rough estimate of the thermal effect that a hot, spherical body the size of the New Idria diapir would have on the adjacent country rock can be calculated assuming simple conductive cooling:

$$T(r,t) = \frac{T_i}{2} \left(erf\left (\frac{R-r}{2 \sqrt{\k... ...t}\right) - exp\left(-\frac{(r-R)^2}{4 \kappa t}\right)\right)$$

with $T_i$ the initial temperature difference between the serpentinite and the country rock, R the radius of the sphere ($\sim$10km), r the distance from the center of the sphere, $\kappa$ the diffusivity ($\sim 10^{-6}$m$^2$/s), and t time (modified from Carslaw and Jaeger, 1959). Figure 6 shows the result of this calculation. It indicates that during $\sim$10$^6$ years after the intrusion of a hot body the size of the New Idria diapir, the rocks within a few km of the contact would experience a transient heating spike. Added to the pre-existing background geothermal gradient, this spike could explain both the vitrinite reflectance data and the apatite fission track annealing behavior. Thermal halos around protrusive serpentinite bodies of west-central California have been described by Murata et al. (1979), who traced the Marca Shale Member of the Upper Cretaceous and Paleocene petroliferous Moreno Shale over a distance of 120 km and found that biogenic silica in this unit was cristobalitic everywhere, except for the northern flank of Joaquin Ridge, where it comes within less than 1 km of the New Idria serpentinite body. This is the only place where quartz-phase silica exists, indicating maximum temperatures $> \sim 80^o$C.

Figure: Conductive cooling of a hot sphere surrounded by a cooler material creates a transient heating signal in the latter. Let the sphere (radius = 10km, left side of the figure) represent the New Idria serpentinite body, rapidly rising in a Great Valley Group country rock (right side of the figure) with thermal diffusivity $\kappa$ = 10$^{-6}$ m$^2$/s. Then the thin black lines show the evolution with time (from 0 to 1Ma) of the thermal contact. The thick black line connects the maximum temperatures reached at different distances. The location of three of the fission track samples is also marked on the figure.

We have not attempted to rigorously model petroleum generation and trapping in the Vallecitos syncline, but current studies by the United States Geological Survey may better constrain the petroleum history (Peters et al., 2005). Nevertheless, various relations provide general constraints on petroleum generation and accumulation, and thus provide a point of comparison for our interpretation. Biomarkers recently collected from the Vallecitos oil field by the United States Geological Survey show biomarker and isotope compositions indicative of Upper Eocene Kreyenhagen source rocks (written communication, Kenneth E. Peters). However, the small pools in the syncline occur mainly in fault traps located under the Kreyenhagen Formation (California Division of Oil and Gas, 1982). Therefore, the Maastrichtian-Danian Moreno Formation might be a more plausible source of these hydrocarbons. Although not understood in detail, the fault traps likely developed when the syncline folded in the Late Middle Miocene (Rentschler, 1985). Thus, maturation, migration and entrapment likely occured no earlier than Late Miocene, consistent with our data and interpretation.