Sedimentary rocks tell stories about the Rogue Valley region’s geologic history
May 2, 2020
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.
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.]
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.
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.