GREINERT, J., C. JUNG, G. BOHRMANN and E. SUESS
; GEOMAR Research Center, KielAbstract: 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