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This trip will take you through one of the world's best exposures of oceanic lithosphere (ophiolite). We will begin in the mantle (peridotite) and work our way upward to pillow basalts and overlying sediments.

An ophiolite is a sequence of rock that is interpreted as representing oceanic lithosphere (the ocean portion of a tectonic plate). The ophiolite sequence, from bottom to top, includes peridotite, gabbro (often layered), mafic dikes, and pillow basalt. This sequence is frequently overlain by oceanic sedimentary deposits such as bedded chert, mudstone, limestone, and graywacke sandstone. Many ophiolite outcrops are missing a portion of this sequence and may not include overlying sedimentary strata.

By definition, an ophiolite is simply the rock sequence. However, most geologists interpret these sequences as representing oceanic crustal and upper mantle material that has been pushed up onto continents when slivers of the sea floor are caught between converging plates. The ophiolite we will observe in the Smith River region has been formed through complex igneous and tectonic processes. Although the general concept of an ophiolite is simple, we will discover that the details of its genesis are complex and not always clear. Those interested in the detailed geologic interpretations of this exposure are encouraged to read the research done by Harper (listed below). Also, a link to another web site on ophiolites is given below.

The following is a generalized interpretation of an ophiolite: The dense, iron- and magnesium-rich peridotite (stop #1) is interpreted as uppermost mantle material. The peridotite (harzburgite) is often altered to the rock serpentine (“serpentinized”) during the convergent process. Serpentine is a hydrated (water added) iron/magnesium-rich rock that is thought to form when peridotite is squeezed upward along faults. The peridotite and serpentinized peridotite is overlain by (or transitional into) gabbro.

The overlying gabbro is interpreted as originally forming at a divergent plate boundary. Partial melting of the underlying mantle produces the gabbro-rich magma chamber above the peridotite. Layering of the gabbro may be due to either crystal settling (by density?) within the magma chamber or a segregation process related to the chemical stability of minerals. Good examples of layered gabbro exist at stop #2. Above the gabbro is a transitional area of gabbro and intruding mafic (iron and magnesium-rich) dikes. Above these exposures are exposures composed almost entirely of sheeted dikes with only a small portion of gabbro.

The sheeted dikes are interpreted as “feeder tubes” that carry the gabbroic magma (a dark, iron-magnesium-rich, silicate mixture) upward from the magma chamber. These dikes cut through the overlying gabbroic rock. Excellent examples of dark (“mafic”) dikes cutting gabbro can be observed at stop #3. Ideally, a mass of vertical dikes that represent the axis of spreading should exist in this portion of the ophiolite. However, this axis region may be more diffuse in nature.

The sheeted dikes bring magma upward to the surface where it erupts on the ocean floor to form pillow basalts. The rapid cooling in marine waters produces the bulbous pillow shapes. This process produces deposits of pillows, pillow flows, and pillow breccia above the sheeted dikes. Excellent exposures of pillows and pillow breccia can be observed at stop #4. In addition, the transition from pillow basalt to overlying marine sediments also can be observed at stop #4.

Bedded chert and argillite (mudstone) are interpreted as deep-water marine sedimentary deposits that formed on top of the pillow basalt. Fossils from the radiolarian-rich chert or the argillite can be used to date the ophiolite. Example of chert, argillite, and even limestone nodules are present at stop #4. Interbedded sandstone layers also exist farther up section.

Reference: Harper, G.D., The Josephine Ophiolite, Geological Society of America Bulletin, volume 95, pages 1009-1026, 1984.

Let's go see the Ophiolite. Watch your step, the rocks are slippery. Did you remember your hand lens?