Sedimentary rocks tell stories about the Rogue Valley region’s geologic history

In the previous two posts, we explored the distribution of rock units in the Rogue Valley region, and how the different resistances of these rock types to weathering and erosion has created the topographic variations we see in the landscape. The different rock types also produce a large variety of soil types that are important for gardeners and vintners in the burgeoning wine industry. You may wish to review the previous two posts before continuing with this post, which will explore what was happening in our region throughout geologic time, based on evidence in the sedimentary rocks. As a sedimentologist, my specialty has been learning to “read” sedimentary layers, which are like pages of a history book.

Remember that the oldest rocks (Klamath terrane) are located SW of the valley—they formed about 300–150 million years ago. Toward the end of that period, granite, including the Ashland pluton, intruded into the pre-existing metamorphic rocks. These older rocks make up what we refer to as basement rocks—that is, they are the underlying “foundation” upon which the overlying sedimentary units were deposited.

The oldest sedimentary unit, which was deposited on the Klamath terrane basement rocks about 100–70 million years ago, is the Hornbrook Formation. We know that by the time the Hornbrook began to form, the Klamath terrane, including the Ashland pluton, had been uplifted and exposed at the surface because the oldest Hornbrook sediments contain pieces of the underlying granitic rock.

This cross section across the valley shows sediments of the Cretaceous-aged Hornbrook Formation (green colors), which has been divided into five parts. The oldest (e.g., lowest) part is the Klamath River Conglomerate that contains clasts of the underlying Basement complex (mostly Ashland pluton granite). The conglomerate was deposited in a fluvial (i.e., river) environment, whereas the rest of the formation was deposited in an ocean. The characteristics of the sedimentary layers tell us that the ocean got progressively deeper with time, from sandstone on the continental shelf to mudstone in the deep ocean. The overlying “Tertiary strata” refers to the Payne Cliffs Formation. (Figure from Elliot’s 2007 field guide)
This false-color image, of today’s Monterey Bay area, shows the different marine environments similar to those former conditions where the Hornbrook Formation was deposited. After the conglomerate was deposited, the land sank and the valley was then under the ocean for millions of years. The Osburger Gulch sandstone and Ditch Creek Siltstone were deposited on the continental shelf, where water depths range from 0 to about 150 meters (0–500 feet). The water level continued to deepen—the Rocky Gulch Sandstone was deposited on the continental slope, and the Blue Gulch Mudstone was next deposited in the deep ocean. There were submarine channels (as in Monterey Bay) that enabled some coarser-grained sediment (i.e., sand and gravel) to be carried to the deep ocean—the evidence is sandstone and conglomerate layers that are found within the mudstone, particularly to the north near Medford. SAF=San Andreas fault. (Figure from 2005 Scientific American article called “Panoramas of the Seafloor”)
In the Ashland area, the lower parts of the Hornbrook Formation are best exposed along the west side of I-5 (photo above), and can be viewed when heading south from Ashland toward the Siskiyou Summit. The Osburger Gulch sandstone is particularly well exposed; it consists of alternating layers of sandstone (the thick layers that jut out of the exposure) and mudstone (the thin layers that are recessive). On the continental shelf, beyond the surf zone, mud falls out of suspension during calm times with small waves. However, during storms, waves are large and can carry sand from the beach to the offshore zone. Higher sand/mud ratios indicate positions closer to the coast; in the surf zone, where waves are breaking, there is no mud, only sand or gravel. This is because the energy is always high and any mud brought to the coast gets carried farther offshore where it will eventually settle out of suspension.
Sand layers carried to the shelf by waves have distinctive broad structures called “hummocky cross stratification” (HCS)—see the sandstone layer above. This photo is from marine sediments in California that are about the same age as the Hornbrook Formation. Note blue pencil on the sandstone layer for scale; the lower sand/mud ratio indicates a location farther offshore than that shown in the Hornbrook photo above.
The Hornbrook Formation is filled with the fossils of marine organisms, such as the clams and scallops shown in the photo above. This photo is courtesy of Elizabeth Zinser, whose friend found the specimen exposed during highway construction. Construction sites are good places to see rocks of the Hornbrook that, because they are not very resistant, are not usually exposed for us to see.
This photo, taken near Hilt in northernmost California, is a rare exposure of the upper “Blue Gulch” mudstone part of the Hornbrook Formation that was deposited on the continental slope. The nodules that look like clasts are parts of the mudstone that were cemented together with calcite—because of the cement, these areas are more resistant than the rest of the mudstone. The calcite comes from shells of marine fossils that dissolve; changing chemical conditions can later cause the calcite to reform as cement between mud grains. These cemented parts of the mudstone are called “concretions”.
This photo, taken along the south side of (the unfortunately named) Dead Indian Memorial Highway near the Ashland airport, is the uppermost part of the Hornbrook Formation that was deposited in the deep ocean. This exposure shows alternating sandstone and mudstone layers that were deposited far below the action of waves. The sandstone is carried to the deep ocean via turbidity flows—the distinctive structure of each sand layer is an upward fining of sediment size that occurs because larger grains fall out first, and then smaller sizes fall out as the turbidity flow decelerates. See the blue arrow in the false color image of Monterey Bay above to see how flow down a submarine channel can carry coarser sediment (sand and even gravel) from the coastal zone to the deep ocean.

Overlying the Hornbrook Formation (Kh) is the Payne Cliffs Formation (Tpc) that was deposited in rivers from about 50–35 million years ago. You will notice that there is about a 20-million-year gap in time between the top of Kh and the bottom of Tpc. During this time, the land was uplifted, and sea level may also have fallen, elevating the valley from beneath the ocean. Imagine a gently west-facing slope with meandering river channels—you might think of the Williamette Valley, with more of a westward slope.

An interesting aspect of the Payne Cliffs is that the fossils are mostly plants, including lots of petrified wood, that indicate the region had a tropical climate during that time period. Other sediments of similar age indicate a tropical climate, including fluvial (i.e., river) sediments on the western slope of the Sierra Nevada. [As a side note, the Sierran fluvial sediments were hydraulically mined for gold during the 19th-century California gold rush.]

This exposure of the Payne Cliffs Formation on the entry road to the park at Emigrant Lake shows a sequence of layers that are finer-grained upward, from conglomerate to sandstone to mudstone. This sequence is typical of sediments deposited in rivers—the gravel (now the rock called conglomerate) was deposited in a river channel where fast-moving flows can move large-sized sediments. Over time, the river channel migrates laterally, and the energy level decreases, until the location is in the floodplain where only mud-sized grains overflow the channels during floods. Eventually, a channel can migrate back to the same location, and another cycle starts—note that overlying the mudstone is more coarse-grained sediment that is the start of another fining-upward sequence.
This photo, taken in western Colorado, is part of the Morrison Formation, another fluvial deposit. This formation is older than the Payne Cliffs Formation and was deposited while dinosaurs still roamed the earth—it is one of the nation’s premier dinosaur fossil sites. This photo shows sandstone in a lens shape that was the shape of the channel; the underlying mudstone was deposited in a floodplain before the channel migrated to this location. Fossils are recovered in the floodplain mudstone, where dinosaurs would have been looking for food and water.

Volcanoes of the so-called Western Cascades became active around 40 million years ago—volcanic clasts are found in the uppermost sediments of the Payne Cliffs Formation, which is overlain by a thick sequence of volcanic rocks that indicate continued volcanism until ~10 million years ago. The Western Cascades volcanic rocks have been separated into a variety of different units such as the Roxy Formation and the Grizzly Peak Volcanics.

This photo is lava of the Grizzly Peak Volcanics at the summit of the peak. Standing on the lava is Southern Oregon University geology professor Jad D’Allura, who has contributed extensive knowledge about the Western Cascades volcanic rocks from his research. Lava is solidified magma that has reached Earth’s surface and flowed out of a volcano. The lava has compositions that are typically andesite to basalt, rock types with an intermediate to low amount of silica.
Beside lava, the other type of volcanic product is pyroclastic (hot clast) material, which is emitted from volcanoes during explosive eruptions that cause fast-moving firey flows and/or air falls. Unlike lava, with interlocking crystalline structure, pyroclastic layers are consolidated pieces of volcanic clasts that glue together because of their high temperature.
This diagram (from Wiley et al., 2011) is a cross section to illustrate the components of the Western Cascade volcanoes prior to their NE-ward tilting. Grizzly Peak was probably a shield-type volcano, which means that the silica-poor lavas created a volcano that had a broader, less peaked shape than more silica-rich volcanoes such as Mt. Shasta. Most Cascade volcanoes have an intermediate composition with higher levels of silica and more cone-like shapes than shield volcanoes. Notice that sedimentary layers are interleaved with volcanic layers.
This view is from Wagner Butte (part of the Ashland pluton) looking NE across the valley (Hornbrook and Payne Cliffs Formations) to the ridge with Grizzly Peak (Western Cascade volcanics). All of these units were progressively stacked on top of other until about 10 million years ago, when volcanism slowed and all of the rock units were tilted toward the NE. Then, starting about 7 million years ago, volcanism resumed to create the currently-active High Cascade volcanoes, including Mt. McLoughlin (snow-covered mountain in the distance). This chain of volcanoes is located farther east than the ancient volcanoes of the Western Cascades.
This sketch (adapted from a figure in the “Siskiyou Sundays” field guide) is a cross section perpendicular to the photo above. It shows the geologic units (Kh=Hornbrook Formation; Tpc=Payne Cliffs Formation; Tv=Western Cascade volcanic rocks) that overlie the Klamath terrane basement complex (gr=granite; m=metamorphic rock), all of which were tilted to the NE prior to development of the High Cascade volcanoes such as Mt. McLoughlin (Mt. McL). Mt. A=Mt. Ashland; GP=Grizzly Peak.

Important references (also Wiley et al., 2011; see April 19 post)

Elliot, Bill, 2007, Field trip guide to the Upper Cretaceous Hornbrook Formation and Cenozoic rocks of southern Oregon and northern CA.

Begnoche, Dan, 1999, Siskiyou Sundays—a tour of southwestern Oregon.

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