Geologic Survey of the Northern Cascades: Deformed Trench Sediments

Figure 1. The ridges in the background of this photo represent the western foothills of the Cascades Range (Site 1 in Fig. 2). Today’s post will follow Highway 20 as it winds its way into the heart of the orogen, exploring sedimentary rocks that were deposited when Washington’s coast was located east of Puget Sound.

Figure 2. (A) We drove a loop around the Northern Cascades and stopped at the numbered locations. Today’s post will report on sites 1-3, which form the western flank of the central peaks. (B) The Northern Cascades are not primarily volcanic, although there are active volcanoes in the region, e.g. Mt. Baker. The box outlines the larger map area. Note that these mountains are not primarily composed of intrusive rocks; and they are northwest of the Columbia River volcanics.

Figure 3. This geologic map is derived from the Cooperative National Geologic Map (https://ngmdb.usgs.gov/nationalgeology). The sediment prism represents lithified sediments that collected within the subduction trench. These are called metasedimentary and metavolcanic rocks because they are only slightly modified by heat and pressure.

The wide range of ages results from the difficulty of applying radiometric dating to such deformed rocks, as well as the assortment of material that is mixed together within the trench. For example, the maroon area north of Site 2 is described as melange–a mixture of different rock types that have been folded and faulted as if put in a blender.

The dashed line is thought to be a strike-slip fault along which the eastern block has moved southwards; we don’t know which side moved, of course, without studying the regional distribution of rocks. That is beyond the sope of this post. The rocks to the east side of this fault are the core of the Northern Cascades: metamorphic rocks that have lost all evidence of their origins, and younger igneous rocks that we’ll see in the next post.

Figure 4. This is a typical exposure of these deformed sediments from Site 2 (see Fig. 3 for location). These are sedimentary rocks yet no bedding can be seen from this distance.

Figure 5. This photo from Site 2 was taken in a small quarry, which permitted examination of a fresh surface on a boulder that was removed from somewhere on the hillside. Note the absence of bedding. I am pointing to the next photo.

Figure 6. Close-up of the boulder in Fig. 5, showing foliation in lieu of bedding (top to bottom in photo). There is a thin quartz vein running from left-to-right near the top of the image, and several micro-veins throughout the sample. The original sediment was heated sufficiently, under pressure, for quartz to become mobile, but there wasn’t a lot of quartz in the original sediment. It may have been either mud or possibly volcanic layers.

Figure 7. This road cut near Site 3 is representative of the melange found in the western foothills of the Northern Cascades. There is a suggestion of bedding but nothing is persistent, and there are no straight lines. This is typical for deformed sedimentary prism sediments.

Figure 8. This close-up of the rocks in Fig. 7 is about 12 inches across. It reveals quartz filling a cavity within a larger mass of breccia. Quartz is very mobile under moderate heat and pressure.

Figure 9. This close-up is a few inches across. It reveals a mixture of mineral crystals (seen as rectangles and shiny grey areas throughout the image) that are feldspar (another mobile mineral) and quartz, but no lineation. Deformation was pervasive, from the outcrop scale (Fig. 7) to the microscopic scale. These rocks behaved like mush on geological time scales (e.g. millions of years).

Final Thoughts

This schematic block diagram of a subduction zone has to be advanced a few million years to explain the rocks we discussed in this post. The Trench (orange area with white arrows on the front of the diagram) in this photo represents the last few million years whereas the rocks we saw are located closer to the Cascade Volcanoes today.

The rocks haven’t moved that far from where they were deposited as sediment and lava in the submarine trench; instead the trench has moved westward as the thickening continental crust (right side of diagram) forced the ocean plate to sink further to the west. It’s a simple question of conservation of mass and energy; as powerful as plate spreading is, it has limits on geologic time and space scales. In other words, N. America was bigger and heavier.

The melange being created in the current trench will be exposed in tens of millions of years, and we probably won’t be around to observe it.