HR: 17:15h
AN: B54C-06    [Abstracts]
TI: A big old cold seep revisited: the Paleocene Panoche-Tumey Hills system
AU: * Schwartz, H
EM: hschwartz@pmc.ucsc.edu
AF: Hilde Schwartz, Earth and Planetary Sciences Dept., UC Santa Cruz, Santa Cruz, CA 95064, United States
AU: Sample, J
EM: James.Sample@NAU.EDU
AF: James Sample, Department of Geology, Box 4099, Northern Arizona University, Flagstaff, AZ 86011, United States
AU: Minisini, D
EM: daniel.minisini@bo.ismar.cnr.it
AF: Daniel Minisini, ENI, Exploration & Production, Via Emilia 1, Milan, 20097, Italy
AB: One of the largest ancient cold seeps known is exposed in the Panoche Hills and adjacent Tumey Hills, in central California. In outcrop the most striking components of the Panoche-Tumey Hills paleoseep (PTHP) are authigenic carbonate bodies representing methane-derived cementation at or just below the Paleocene seafloor, and sandstone injectites thought to delineate the subseafloor plumbing of the system. The carbonates and underlying injectites are largely contained and best displayed within the Moreno Formation, a dominantly shaley unit that was deposited at outer shelf depths on the western margin of the San Joaquin forearc basin in the Maastrichtian-Thanetian. Since our first (2002) publications about the PTHP we have learned a great deal about its scale and development. Field investigation has extended the length of the seep zone from an initial estimate of 5km to a minimum of 20km, beyond which the main seep horizon (the Cima Sandstone Lentil) is truncated by an unconformity. However, injectites lower in the Moreno Fm. persist for at least 100km along strike to the south, suggesting that the PTHP originally spanned much of the western margin of the Paleocene forearc. The stratigraphic range of the seep carbonates has also been revised upwards from 45m to at least 250m (representing approximately 3 my). The carbonates themselves have δ¹³C values between +3 and - 54‰ and δ¹⁸O values between -7 and +7‰(VPDB) and a range of habits, with irregular mounds and stratiform bodies volumetrically dominant. Mounds contain multiple fluid conduits, breccias, distinctive cement phases, and a zoned, low diversity chemotroph-rich paleofauna (including microbial mats, lucinid bivalves and vestimentiferan tubeworms). They represent focused, prolonged fluid flow, methane expulsion and ecosystem development, and are the best archives of individual seepage events. Stratiform bodies are uniformly micritic, laterally extensive and more biologically diverse. They represent pervasive seepage and locally preserve seafloor lag deposits. Vertical distribution of carbonates shows that the PTHP fluid system began, ended and was locally dominated by focused seafloor flow. Hydrate Ridge off of Oregon is an interesting modern analog for the PTHP. It is comparable in terms of size, tectonic setting, fauna and carbonate structures, though it formed at greater depth and overlies a different type of plumbing system.
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 0456 Life in extreme environments
DE: 3002 Continental shelf and slope processes (4219)
DE: 3021 Marine hydrogeology
DE: 4930 Greenhouse gases
SC: Biogeosciences [B]
MN: 2007 Fall Meeting