Late Tertiary Era - Miocene Epoch

Basic Concept:

While the first 4.45 By (billions of years) were relatively uniform (our area was basically underwater for most of those billions of years), the Miocene Epoch was a period of intense activity throughout western North America. Basin and range faulting began in the western United States around 17 Ma (millions of years ago), fracturing plates, and lifting or dropping rocks into long narrow mountain ranges separated by long narrow valleys.

A hotspot activated in eastern Oregon at the same time -- the same hot spot that produced undersea and subsurface volcanoes offshore during the early Eocene. During the mid Miocene, flood basalts from this hotspot under eastern Oregon began to cover large areas of eastern Washington and Oregon. These flows of hot liquid magma repeatedly reached the shallow coastal sea and ocean in the first million years, then occasionally in later flows over the next few millions of years. The basalt floods ran down river valleys and under water, between sediment layers, and flooded up in cracks in sediments and rocks. The Snake and Columbia Rivers found new paths through fresh basalt several times, and continued to flow west to Willamette Sound and the Pacific Ocean.

The lowlands of Puget Sound and Willamette Valley began to sink around 16 Ma. This is called the Puget - Willamette Trough, and it continues to sink today. It is bracketed by uplifting areas to east and west - the Olympics, Coast and Cascade Ranges.

Early in the Miocene Epoch the climate was warm and dry, but it became cooler and wetter. Around 16 Ma, climate warmed again, and was much drier. The High Cascades in Washington and Oregon began to erupt during this period, and the Coast Range emerged from the shallow sea. The Coast Range was finally on its way up into the air, driven up in part by an active subduction zone to the west.

The rocks that formed the Olympic Mountains began to crumple and fold at this time; major folding, lamination and uplift commenced around 15 Ma. Beds were crumpled up from horizontal to vertical, and pushed up at the same time, folding around in a large horseshoe shape with oldest layers on the outside, including the underlying basalt layer, and youngest sedimentary layers on the inside towards the center of the range. The top curve of the horseshoe faces northeast, with the open ends to the west and southwest. Further south in the Coast Range, rotation of our part of the plate continued.

In our area, with new mountains of the Coast Range emerging to the north and south, a shallow sea to large estuary with a delta persisted around the Columbia River entrance, which was east of its present position and west of the Cascade Range. A distinct thick sedimentary series named the Astoria Formation formed during the middle Miocene in the delta of the Columbia River. The Astoria Formation is composed of many different sediment layers, including fine-grained sandstones, mudstones and siltstones; these are typical of delta sediments and reflect low to high water flows and energy.

In the late Miocene, a second very distinctive rock series formed north of the Columbia River, the Montesano Formation, which is lacustrine (of lakes) to estuarine (of bays) sediments, containing sandy, silty and clayey layers. Today the Montesano Formation appears as soft, poorly consolidated layers that erode easily, and may contain shell layers and soft limestone. Towards the end of the Miocene, estuarine areas were common along the west edge of the continent on the west side of the emerging Coast Range, though none were in the exact position of estuaries today.

Local Elements:

Miocene flood basalts are found along the Columbia River from the east side of the Cascade Range to the coast. Miocene flood basalts pushed through sediments under the shallow sea. Modern mountains were formed, such as Saddle and Neah-Kah-Nie Mountains, Naselle and Bear River Ridges, by subsequent uplift. Miocene sediments are found throughout the Coast Range, and form many of the soft, slumping rocks beneath regional roads. Erosion of soft sedimentary rocks carved away thousands of feet of material, leaving behind the harder basalt rocks of the Eocene and Miocene Epochs. Some Miocene flows reached the ocean, to form pillow basalts that make up modern headlands, such as parts of Tillamook Head, and Cape Falcon.

Local Examples:

Cathlamet to Longview along Highway 4 is a good area to view Miocene flood basalts. Saddle Mountain State Park is a good site to see Miocene basalt dikes, tuffs and sediments along the main trail. Neah-Kah-Nie Mountain also has a nice dike running along the spine of the mountain. Most Miocene sediments are in high elevation sites in the Coast Range and not easily accessible.

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