Abstract

The Hoh rock assemblage of the western Olympic Peninsula, Washington, was Deposited in Eocene to Miocene time, and accreted, deformed, uplifted, and eroded by the late Miocene. Northeast-dipping fault zones within the Hoh are characterized by intensely deformed mud matrix mélange, and they range from ∼200 m to >1,000 m thick; kinematic indicators within the matrix indicate thrusting to the west-southwest. Both hanging-wall and footwall sections are strongly folded and faulted. Wall-rock sections are lithostratigraphically distinct, with hanging walls composed of massive coarse sandstone and conglomerate, and footwall sections composed of thin- to medium- bedded sandstone and siltstone. Folds within the wall-rock sections are oblique to, and are truncated by, the later fault-zone mélanges, indicating that the fault zones postdate folding.

Evidence of fluid involvement in the Hoh rocks includes petroliferous outcrops, carbonate and clay cement, and veins of carbonate and laumontite. The progressive evolution of veins and cements (paragenetic sequence) can be interpreted in terms of crystallization under conditions of increasing consolidation, decreasing salinity, and changing fluid composition. Suppressed carbonate luminescence may reflect crystallization under reducing conditions, perhaps related to the sulfate reduction zone, the presence of hydrocarbons, or clay dehydration. Although deformational features may be associated with the earliest phase of veining, most of the localized grain breakage and matrix shear zones formed after the early micritic veins and before the last phase of sparitic carbonate (+laumontite) veins. Laumontite occurred syn-deformationally and in equilibrium with carbonate, and also post-deformationally.

The paragenetic sequences observed in all samples from the Hoh rock assemblage can be correlated, suggesting that the sources and processes controlling the fluid emplacement were similar across a wide region. In addition, the paragenetic sequence in the Hoh rocks is similar to that documented in autochthonous coeval basins of southwest Washington. This similarity suggests that the precipitates, formed during the evolution of the Hoh rocks, involved fluids that originated within the matrix and did not involve exotic or far-traveled fluids.

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