August 27, 2022  

The Rest of the Story: The Harz Mountains

This post is a continuation of previous posts on northeast and central Germany, but we won’t be seeing direct evidence for glaciers in the Harz Mountains (Fig. 1).

Figure 1. Aerial view of the Harz Mountains (from Wikipedia).

Today’s post discusses some of the rocks exposed in the valleys, gorges, and road cuts that dissect the Harz Mountains (Fig. 2).

Figure 2. The large circle indicates the Harz Mountains (inset map). Locations, A through D, are approximate sites of photos and rocks discussed in this post.

We approached the Harz uplands from the east (site A in Fig. 2), where we encountered mines based on removing sedimentary rocks for use as building material (Fig. 3).

Figure 3. (A) Road and building gravel mine from the eastern end of Harz Mountains (site A in Fig. 2). (B) Close-up of tailings pile, showing uniformity of the conglomerate being removed from the open pit.

The mines in Fig. 3 were removing desired beds from the Tanner Graywacke ( age ~360-320 Ma), a poorly mixed sedimentary rock (e.g. graywacke) originally deposited in ocean trenches associated with volcanic island arcs. The Tanner graywacke ranges from mudstone to conglomerate. It forms medium beds with variable texture, and has been tilted to varying degrees (Fig. 4).

Figure 4. Photos of Tanner Graywacke near site A (see Fig. 2 inset for location). The beds in (A) are tilted about 30 degrees (unknown direction) and the outlined layer is ~6 inches thick. Panel (B) shows two nearly horizontal beds exposed within a kilometer of (A). These beds contain detailed sedimentary structures, like cross-beds, as indicated by white outlines in the lower bed.

Our route east of the Harz Mountains (red line in the inset of Fig. 2) took us to site B, where we encountered more facies of the Tanner Graywacke (Fig. 5).

Figure 5. Tanner Graywacke exposures at Site B (see inset of Fig. 2 for location). The massive layers in (A) are ~4 feet high. A thick layer of conglomerate (panel B) doesn’t form cliffs. Note the irregular cobbles exposed by weathering. (Image B is about 6 feet high.)

Our path (red line in Fig. 2 inset) led us into a valley between the small highlands where Figs. 4 and 5 were taken. This valley is filled with a lake, and probably follows a fault zone between Site B and the main Harz Mountains to the north (Fig. 6).

Figure 6. View looking north towards the Harz Mountains. This area is underlain by marine evaporites, including salt that occurs in domes (age ~250 Ma). Extensive karst development in carbonates has led to serious subsidence problems in the area as sinkholes continue to develop.

Our journey followed the southern margin of the Harz Mountains (red line in Fig. 2 inset), taking us by Site C, where we found nearly horizontal beds of Tanner Graywacke exposed along road cuts. We couldn’t stop until we found a rest area, where a large block was available for close examination (Fig. 7).

Figure 7. Images of a block of Tanner Graywacke (A) exposed at site C (see Fig. 2 for location). The boulder was not in place nor was it a glacial erratic. It was put their during highway construction. (B) Close-up of the weathered surface, showing a bright-white square in the circle; this is a grain of Na-feldspar encased in a matrix of clay minerals (weathered to black in the photo). Note the large, pinkish form in the extreme upper-right of the image. This is a block of what was probably a granitic source rock. (C) Lower magnification image of the same rock; the circled area includes dark spherules against a white matrix, which I cannot identify. If you zoom in closer by opening the image, you will see that they are actually rectangular and have smooth edges. The white weathering product is probably from feldspars whereas the dark minerals (entire crystals) may be amphibole or pyroxenes (more resistant to weathering). It is important to recall that a graywacke collects near the source, and thus includes minerals in every size and shape, and every stage of weathering. (D) The circle highlights a cavity that was occupied by a large piece of rock (instead of a mineral grain), similar in shape to the phenocryst displayed in plate (B).

We continued around the western end of the Harz Mountains and found exposures of the marine deposits (including evaporites and carbonates) that underlay the town of Kelbra (Fig. 6), including a thick sequence of either salt or anhydrite (Fig. 8).

Figure 8. Images of the marine sequence (age ~250 Ma) that is much younger than the Tanner Graywacke (age ~360-320 Ma), taken at approximately Site D (see Fig. 2 inset for location). (A) Contact between an evaporite (white rock) and overlying sediments (tan), showing several anomalous features. For example, there is some suggestion that the darker beds have been folded (zoom in on the image); several irregular blebs of evaporite (e.g. to the right of image) appear to be isolated. This may be a salt diapir (or some other ductile rock) that forced its way into younger sediments, folding them as it intruded. A fault zone is not out of the question, considering that site D is located at the margin of the Harz upland (compare to Fig. 4A). Plates B and C show medium beds (<1 foot in thickness) of resistant siltstone surrounded by mudstone/calcarenite. These beds may be tilted but not at such extreme angles as suggested by plate A.

This post reveals rocks that are widely separated in time while being found near each other, supporting the Harz uplift as they do (Fig. 2). As the title of this post suggests, geology is not a series of isolated events. Let’s get the rest of the story.

The Tanner Graywacke was deposited in an island arc, a tectonic region in which oceanic crust is being subducted beneath either a continental (or less often an oceanic) tectonic plate, about 350 million years ago. What was happening on the opposite shore of this proto-Atlantic ocean (aka Iapetus)?

I have encountered rocks of similar age in northern Virginia and discussed them in previous posts. On the west side of Iapetus, during a mountain-building event called (in America) the Acadian Orogeny, a series of island arcs were being subducted/accreted to form a series of suspect terranes. This orogeny was only a phase of the collision of Laurentia (porto-north America) and Avalon (proto-Europe), which endured for most of the Paleozoic era.

The next problem is what happened during the ensuing 100 million years, between deposition of the Tanner Graywacke and the evaporites and carbonates we encountered west of the Harz uplands (Fig. 8)? The collision was completed and Pangaea had been born of two continents…

The mountains rose and they were eroded almost as quickly by wind, rain, and ice, creating massive layers of sediment to the east (modern Europe) and the west (e.g. the Catskill Delta in New York). By 230 million-years ago, the earth’s upper mantle changed its mind and tore the newly formed supercontinent apart, creating rift valleys like that of East Africa in what is now Virginia. Splitting a continent can take as long as building one, but this was a relatively rapid event in geological time; by 200 million-years ago, diabase dikes were injected into the sedimentary and metamorphic rocks created by the closing of the porto-Atlantic Ocean (Iapetus) and the split was well under way. Alluvial and fluvial sediments were collecting in isolated basins in what is now Virginia, and evaporites were settling to the bottom of lakes and brackish coastal waters in Europe, as the ocean invaded…

Jump ahead 200 million years…

Figure 9. View looking upstream in the Elbe estuary, less than a hundred miles from the German port of Hamburg.

I love it when I can understand what the rocks are telling me…

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