This investigation combines traditional and newly available investigative techniques to characterize the hydrocarbon system of the Fruitland Formation coals, both at outcrop and in the subsurface. These analyses indicate that the Fruitland coal hydrocarbon system began with Late Cretaceous–early Tertiary deposition and maturation of the coal source rocks; Late Cretaceous–early Paleocene tilting of the basin; Eocene uplift, exposure, and erosion of the basin margins; Eocene groundwater recharge, which maintained hydrodynamic pressure in the reservoirs; and continued uplift, which caused occlusion of permeability to occur ca. 35 Ma. Present-day erosion is slowly breaching biosome-scale reservoirs and allowing methane to escape to the atmosphere at the outcrop. Oligocene opening of the Rio Grande rift changed the stress regime of the San Juan Basin, allowing fractures to open and fluid to migrate from pre-Cretaceous rocks to the surface. Outcrop seeps have been ongoing throughout Recent geologic time and probably have been active since the coals were first exposed at the outcrop. Methane production from the coal in deeper parts of the basin has not contributed to methane gas seeps at the outcrop. Our analysis calls into question hydrologic assumptions regarding the flow of water in coalbed aquifers and finds that a reexamination of coalbed aquifers in other basins is also warranted.

Abstract The mobilization of volatile elements during the subduction of marine crust and sediments is an important question for the understanding of the marine element budget. Geochemical behavior and halflife (15.7 Ma) of 129 I make this isotopic system a useful tracer for processes associated with the subduction and recycling of marine sediments. The North Island of New Zealand is particularly suited for testing this system because samples of volcanic and geothermal waters are accessible from a variety of settings across the volcanic arc. We report here on waters recently collected from locations sampled earlier by Giggenbach et al. (1993) , specifically from the East Coast, the Taupo Volcanic Zone, and the Northland, representing the fore arc, main arc, and the zone behind the volcanic arc, respectively. Although a significant number of the samples showed the presence of anthropogenic 129 I, preanthropogenic ratios could be determined for waters from the East Coast and the Taupo Volcanic Zone. Waters from the Taupo Volcanic Zone had the lowest iodine concentrations and 129 I/I ratios close to 300 × 10 −15 . In contrast, waters sampled in the East Coast had the highest iodine concentrations coupled with 129 I/I ratios as low as 55 × 10 −15 . Whereas samples from the Northland had intermediate values in iodine concentrations, the initial 129 I/I ratios for this set of samples could not be determined due to the presence of anthropogenic 129 I. The ratios for the Taupo Volcanic Zone are compatible with iodine derived from marine sediments, mobilized from the entire sediment column undergoing subduction in this area. The ratios in the East Coast suggest the presence of a substantially older component in these fluids, also of marine origin, but mobilized from formations in the accretionary wedge. The ages derived for these samples (~70 Ma) are in good agreement with ages estimated for hydrocarbons found in this area, suggesting a common source; the positive correlation observed between iodine and methane concentrations supports this interpretation. Results from this study demonstrate that processes leading to release of geothermal fluids are substantially different in fore- and main-arc settings. A comparison of the data from New Zealand to results from volcanic areas in Japan, Central America, and South America shows that the 129 I system gives site-specific results and can be used successfully to determine origin and history of fluids in subduction zones.