ORANGE, D. L., J. YUN, Earth Sciences, Univ. California Santa Cruz, CA; G. GREENE, Moss Landing Marine Lab, CA; T. LORENSON and K. KVENVOLDEN, U.S.G.S, Menlo Park, CA; G. WHEAT, NOAA NURP; J. MARTIN, Univ. Florida; K. CAMPBELL, U. Auckland, New Zealand; J. BARRY and N. MAHER, MBARI, Moss Landing, CA

Abstract: California Cold Seeps Past and Present: Monterey Bay, the Eel River Basin, and Inland Paleo-Seeps

The present-day California margin is characterized by two tectonic settings: translation south of the Mendocino Triple Junction, and compression north of the MTJ. Both contain cold seeps, with similarities and differences between them. The subaerially exposed rocks of California also contain paleo-seeps.

Monterey Bay seeps occur on high angle faults, suspected mud volcanoes, outcrops of organic-rich units, and within headless canyons. Both active seeps (chemosynthetic clams and tube worms, bacterial mats) and dormant seeps (authignenic carbonate, no fauna) are found, and measured flow rates are low (meters per year). Pockmarks, evidence of episodic expulsion, are rare, and no active gas seeps have been found. The San Gregorio fault zone is covered by authigenic carbonate and brachiopods, similar to paleoseeps identified in the fossil record.

Eel River basin seeps occur on anticlines and on mud volcanoes over breached anticlines. On structures, active seeps are localized, whereas authigenic carbonate is widespread. Shelf seeps (40-50 m) show extensive bacterial mats on sand and bubbling gas, but no seep-specific megafauna. Seeps at 280-520 m, however, have large colonies of vesicomyid clams, bacterial mats, and bubbling gas. Significant portions of the seafloor between active structures are covered with pockmarks, but ROV investigations of these features failed to observe any active venting.

Eel River basin cold seeps differ from Monterey Bay in that they vent gas directly into the ocean and the sediments have greater concentrations of hydrocarbon gases and hydrogen sulfide.

AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California