A big old cold seep revisited: the Paleocene Panoche-Tumey Hills system
Abstract
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.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- Bibcode:
- 2007AGUFM.B54C..06S
- Keywords:
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- 0454 Isotopic composition and chemistry (1041;
- 4870);
- 0456 Life in extreme environments;
- 3002 Continental shelf and slope processes (4219);
- 3021 Marine hydrogeology;
- 4930 Greenhouse gases