The first day of the trip, Friday, September 7, 2007, was led by Dr. Jad D’Allura, geology professor at Southern Oregon University (SOU), and a specialist in igneous and metamorphic petrology, amongst other research pursuits.  Dr. D’Allura took the GSOC party westward along the Klamath River in northern California to observe the metamorphic rock exposures in several terranes of the Klamath Mountains.  These included the Permian-Jurassic North Fork, Permian-Carboniferous Salmon River, Permian-Jurassic Eastern Hayfork, Permian-Jurassic Western Hayfork, Permian-Jurassic Marble Mountain, and Jurassic Condrey Mountain terranes.  All of these terranes are included in the Western Paleozoic and Triassic Belt.

The oldest of the terranes is the Salmon River (primarily basalts) which formed the basement for the Eastern Hayfork and North Fork terranes.  The Marble Mountain and Western Hayfork Terranes represent an accretionary wedge (mélange) and a volcanic arc.  The assembly of these terranes began in Late Middle Jurassic times with westward thrusting with associated deformation and metamorphism.  Plutonic bodies intruded into the terranes and across their boundaries, and are thus referred to as “stitching plutons”.  These intrusions occurred in Early Late Jurassic time and mark the latest possible assembly of the Western Paleozoic and Triassic Belt.

In the late Jurassic, a major tectonic event underthrust the Condrey Mountain terrain beneath the other layers, which were now stacked over one another similarly to roof shingles (geologists use the term imbrication to describe this), and further deformed by metamorphism.  Later, in the Miocene, uplifting caused a dome structure in the Condrey Mountain terrane to rise past the other terrane layers to the surface.  Metamorphism is greatest in the Marble Mountain terrane, closest to the Condrey Mountain dome, and decreases toward the Salmon River terrane.  This doming during the Miocene also explains why the late Cretaceous Hornbrook formation dips to the northeast. This is the tie-in between the field trips taken on Friday and Saturday by the GSOC group.

The GSOC field trip observed the rocks in these terranes at Stops 1, 3, 4, 6, 7 and 7b of Dr. D’Allura’s “Geological Field Guide Along the Klamath River” (see reference below).  The terranes viewed were the North Fork at Stop 1, Eastern Hayfork at Stops 3 and 4, Marble Mountain at Stops 6 and 7b, and Condrey Mountain at Stops 7 and 7b.  The rocks observed at these locations included a large spectrum of metamorphic and intrusive examples.  Lower grade metamorphic rocks included dark argillite, greenstone, chert, and phyllite at Stops 1 and 3, and somewhat deformed and altered pillow basalts at Stop 4.  Stop 6 was dominated by higher grade amphibolite, a dark rock which sparkled with crystals of metamorphic minerals.  Stops 7 and 7b included graphitic schists and greenschists of the Condrey Mountain terrane as well as an example of retrograde metamorphism of ultramafic rock in the Marble Mountain terrane at the contact boundary.

Between stops Dr. D’Allura pointed out the changes in topography that accompanied moving from one terrane to another.  For example, between Stops 1 and 3 the party traveled through a rugged canyon whose narrow walls consisted of Salmon River terrane rocks.  As the field trip passed into the Eastern Hayfork terrane, the canyon widened and became less steep and rugged.

At Stop 6, in addition to the outcrop observation, the GSOC party went down to the Klamath River floodplain to observe the myriad variety of river cobbles and pebbles to be found there.  Examples of grano-diorite, quartzite, schist and marble cobbles, some with very large and interesting crystals, were to be found.

At the end of a hot afternoon of rock observation and sample collection, the GSOC party decided to make a Stop 8 at Quigley’s Market to relax in the shade on the patio, re-hydrate and prepare for Saturday’s trip.

See slide show of Day 1.

The following day, Saturday, September 8, 2007, the GSOC group was ably led by Dr. Bill Elliott, assistant professor of geology at SOU, and an expert in sedimentary petrology.  Dr. Elliott took the group on a route in the area near Hilt, California, to observe sedimentary rocks in several members of the Late Cretaceous Hornbrook, the Oligocene Colestin, and the Late Oligocene to Early Miocene Roxy Formations.  The Tertiary Colestin and Roxy Formations unconformably overlie the Hornbrook Formation in the Cottonwood Creek Valley of northern California and are part of the Western Cascades group.

The Hornbrook Formation, which consists of mainly marine clastic sediments (i.e., derived from the fragments of other rocks), are believed to be formed in a forearc basin in a subduction zone during the late Cretaceous.  (A forearc basin will be to the oceanic side of a volcanic arc in a subduction zone.)  The members of the Hornbrook Formation, all of which were viewed by the GSOC party, are the oldest unit, the Klamath River Conglomerate Member, which also has the only non-marine sediments, the Osburger Gulch Sandstone Member, the Ditch Creek Siltstone Member, the Rocky Gulch Sandstone Member, and the youngest Blue Gulch Mudstone Member.  Each of the members consist of alternating units of varying texture – the member names refer to their general characteristics.  The Hornbrook Formation was viewed in Stops 1-7 of the field trip, following the “Field Trip Guide to the Upper Cretaceous Hornbrook Formation and Cenozoic Rocks of southern Oregon and northern California” field trip guide which was written specifically for this trip by Dr. Elliott.

The highlights of the Hornbrook Formation seen by the group included fossil lenses of cephalopods, gastropods, and other marine fauna in the extremely hard matrix of the Osburger Gulch Sandstone, the beautiful arkosic sandstone matrix for the Klamath River Conglomerate, worm burrows in the Ditch Creek Siltstone, interesting turbulent depositional traits and composition of the conglomerates in the Rocky Gulch Sandstone, and the oddly solid Hilt Bed amongst the Blue Gulch Mudstone units.

The remaining Stops 8-11 contained material from the Colestin and Roxy Formations of the Western Cascades.  The Colestin Formation consists of volcanic arc material.  Layers of this formation were viewed at the top of Siskiyou Pass along I-5.  Volcanic tuffs, including vitric and crystal types, and volcanic sandstones were present.  Dikes with an assortment of clastic xenoliths cut through the layers.  Stop 9 included a landslide and Colestin conglomerates, and Stop 10 consisted of alternating lava flow and paleosol layers.  Stop 11 gave the group an opportunity to excavate some Metasequoia fossils from a whitish colored layer of mudrock in the Colestin.

After another hot day the group headed back to a little restaurant in Hilt.  Some of the members made plans to cap their interesting field trip days with further exploration of the area, including a trip to Oregon Caves.  This area of Oregon is a very interesting place to study geology.

References and Additional Reading

Orr, Elizabeth L., Orr, William N., and Baldwin, Ewart M., Geology of Oregon, Fourth Edition, Kendall/Hunt Publishing Company, Dubuque, Iowa, 1992.  Refer to the Klamath Mountains chapter pp. 51-78.

D’Allura, Jad Alan, “Geologic Field Guide Along the Klamath River; From Interstate 5 Near Hornbrook to Seiad Valley,” California Geology, March 1990, pp. 58-67.

Elliott, Bill, “Field Trip Guide to the Upper Cretaceous Hornbrook Formation and Cenozoic Rocks of southern Oregon and northern California,” Saturday, September 8, 2007.

SOU Geology Department:
Dr. D’Allura: http://www.sou.edu/geology/faculty/JDallura.htm
Dr. Elliott: http://www.sou.edu/geology/faculty/ElliottB.htm