GREINERT, J., C. JUNG, G. BOHRMANN and E. SUESS

Abstract: Cold Vent-Induced Carbonate Types and Their Relation to Isolated CHEMOHERM Complexes and dewatering patterns at the Cascadia Margin

The second accretionary ridge at the Cascadia margin is an area known for cold fluid venting and carbonate precipitation. During a cruise in 1996 we sampled carbonates in three isolated carbonate complexes (300 to 500 m.y.) between 800 and 600 m water depth. Two of these complexes were observed east of ODP Site 892 while the third was the 'bioherm' area west of this Site. This complex as well as a second one is built up by vent induced inorganic carbonate precipitation because of which we prefer the name CHEMOHERM for this kind of carbonate complexes. The samples show three different petrographic main types which can be classified as mudstones, breccias and gas hydrate-related carbonates. Concretion-like mudstones cemented by dolomite and high Mg-calcite show slab- to block-like forms of gravel to boulder size. These carbonates are the main types at the easternmost third complex and were formed as a diagenetic precipitation in the sediment column. The most important carbonates at the two Chemoherm complexes are irregular and chaotically sorted ruditic breccias with matrix-like intraclasts, bioturbation trails, shells and mudclasts which are strongly cemented by aragonite and less strongly cemented by high Mg-calcitic. The associated gas hydrate-related carbonates are porous ruditic breccia with botryoidal aragonite pore cements and aragonite layers. The brecciation is induced by a process similar to hydro, fracturing of semiconsolidated sediment due to ascending gas bubbles and the crystallisation of irregular gas hydrate layers. All these carbonate types show a different scattering at the carbonate complexes which give useful hints in addition to geochemical and isotopical investigations for the cold vent and dewatering processes at the Cascadia margin.

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