Rocks within rocks, and rocks within rocks within rocks
Final update: Jimmy Hague has drawn my attention to a full report on the region by the University of Malaga. But my photos of rocks are nicer.
Benalmadena, Costa del Sol, Spain, some 20 miles West of Malaga, and perhaps readers can enlighten me about what I’m seeing: Update: one reader (Professor Craig Jones of University of Colorado, Boulder) has done just this. His comments (below) have persuaded me to add two further images, of coarse sandstone/gravel, including schist fragments, with slumping, one mile to the east of the folded rocks. So we seem to have a transition between compression (Betic-Rif orogeny) and extension (either connected to formation of the Western Mediterranean basin, or, as sometimes happens, secondary to the compression).
Rocks within rocks within rocks; red sandstone matrix (no stratification or bedding apparent), containing fragments of varied origin and degree of processing; some examples include fragments of quartz-veined basalt. Note at far left, and also beneath scaling coin, pebbles of quarts-containing conglomerate.
“Coglomerate” says my friendMichaelRoberts, and this is undoubtedly correct, but opens the way for the next level of questioning. How did this conglomerate form and in what kind of environment? The diversity of the pebbles in composition and processing suggests rapid river transport, but what process would leave so much sand between them? I haven’t seen an outcrop of this kind of rock, but there are chunks of it all along the coastline, and some examples (e.g. those to left and right in this picture) are far poorer in pebbles.
Rocks within rocks: quartz vein within a very strange looking rock indeed; dark, micaceous, bands grossly distorted, presumably by sideways compression. The quartz seems to me to have been inserted after the folding (or it would be more fragmented), and retains signs of the many separate nucleation events. At other locations there is horizontal jointing, and veining with soft, rust-tinted, textured material (calcite, I presume), penetrating the quartz veins and therefore more recent:
So we have a sequence of events:
- Formation of original rock (suggestions? Is that even knowable at this stage?)
- Metamorphosis to schists
- Lateral compression causing bending of bedding planes
- Hydrothermal deposition of silica
- Jointing under stress (remember that North Africa is still moving towards this area, at the rate of about a centimeter a year)
- Deposition of calcite, as a subsequent episode and from a different, probably low-temperature, source.
The folding is best seen (left and right above) on the boulders carved from the rock to make room for hotels, and used to build a sea wall. The image left matches the rock at the base of the exposure, while that on the right is from higher up, matching Wikipedia’s description of the area as within what it calls the outer Betics and more specifically “the Alpujárride Thrust Sheet”, which
“spreads from western Málaga province to Cartagena in the east. It was buried from 35 to 50 km deep. At its base is mica schist, with some gneiss and migmatite formed from sediments older than the Permian. Above this is a bluish grey schist from the Permian, and the next layer is carbonate from the Middle to Late Triassic. Above this is a black mica schist, and the top layers are a brown coloured metapelite and a quartzite.”
Which had me racing for the definitions, and when I found them I felt a lot better about not having been sure whether the lower material was sedimentary or igneous.
And Professor Jones’ comments (below) have persuaded me to add further images, of an exposure just one mile to the east of the schists, of what I take to be coarse off-shore stratified sandstones and gravels, actually containing fragments (above) of the mica schist as well as seashell fragments, with slumping (below), possibly associated with run-off into the newly forming Alboran Sea (Western Mediterranean), although most of this basin is now under water. In any case, it looks as if we have both compression and extension regions close together, accounting for the extraordinary varety of rock types found. More about how this could have happened here.
Posted on June 19, 2016, in Geology and tagged Alboran Sea, Alpujarride thrust, Benalmadena, Betics, geology of Spain, mica, orogeny, quartz vein, schist, slumping. Bookmark the permalink. 10 Comments.