Colorado’s Turbulent Past: Tertiary Volcanics

We camped at Marshall Pass (location 3 in the photo below) among what we thought were Aspens, which apparently like the rocky soil that probably originated from weathering of the shales we saw in the previous post. The rocks were angular and uniformly about 8 inches in length; their size and shape are consistent with the origin of Poncha Creek following a fault through Marshall Pass. I will discuss this later.

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After descending from Marshall Pass, we followed some the TAT until we entered the San Juan volcanic field. Our route took us into the early phase of this volcanic period in CO’s history about 35 mya, when volcanoes in this area were producing large amounts of ash and a lava similar in composition to the granites we have seen in previous posts. This photo shows the sudden appearance of such a volcano as we entered the San Juan Mts.

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Note the rounded ridges and small peaks around the base of the main cone. The ridges are weathered lava flows, which often originate from the bottom of volcanoes and the small peaks are evidence of minor eruptions around the base of the main volcano. The flows are more visible in this photo that shows several such flows. We had the opportunity to follow a trail along one such flow, which still formed a scarp that is ~5-15 feet high after 30 my of erosion.

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This photo shows one of the many well-preserved cones from the early phase of volcanism.

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These volcanoes were producing mostly ash and debris that formed immobile layers that did not flow; however, they also produced ahs flows like Mt. St. Helens that would have moved rapidly down the slopes and destroyed anything in their path. These photos were taken at the approximate location indicated as “4” in the base map below.

san juan mts google image

I have some photos of these rocks that show some of the textures of these volcanic layers. This photo shows a tuff that does not appear to have melted together after the material was deposited from the air. If the fragments are hot enough and are buried under later volcanic material, they can become fused to form a “welded tuff” but that is not the case for this sample.

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It has the coloration of the orthoclase (K-Feldspar) we saw in the granitic rocks from previous posts but it also has many holes (vesicles), which originally contained gas that later escaped; some of these holes are left from weathered minerals and rock fragments that were entrained during eruption. I suggest using your browser to zoom in on this image to see the remarkable number of different particles it contains, many of which are probably from the sedimentary rock through which it passed during extrusion. Here is another sample that has a different weathering color but is essentially the same.

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The bulk of these tuffs is silica glass that didn’t have time to cool and form quartz crystals; it is about the same as the glass we produce for windows. I think I should again suggest that anyone looking at this post should click on these photos and zoom in with the browser to appreciate their composition.

The next photo shows a sample with easily identified quartz (the gray areas) and areas that show one of the weathering products, as shown by the white coloration. My first guess about the white material is a salt or carbonate formed from rapid decomposition of the glass to release common rock elements like Ca or Na, which formed minerals like NaCl (salt) or carbonates like calcite (CaCO3); possibly with metals included, e.g., Fe or Mg. Only laboratory analyses can identify these weathering products as well as the particles included in the original tuff.

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This photo shows what an exposure of these tuffs looks like. Note the cliff at the top and the substantial debris at its base because of erosion of these relatively weak ash flows.

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As we travelled to the SW, however, we entered a region where volcanism occurred several million years later, the middle phase of volcanism in the San Juan Mts. These volcanic rocks include layers of more resistant lava and welded tuffs. Here is a photo of a canyon formed from mixed layers of tuff, welded tuff, and lava flows.

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The nearest outcrop also shows what could be termed a volcanic breccia because it contains large (~12 inch) cobbles in a fine-grained matrix. This photo also shows the variability of the different flows in the cliff. Note the irregularity of the layers as if they were being deposited on an eroded terrain.

As a volcano releases pressure and material from a magma chamber, a number of different types of effusives are produced; for example, we have just seen examples of tuffs that are formed from exploding liquid magma and any material it picks up as it exits the magma chamber thru channels in the rock. Here is an example of a tuff that appears to have been so hot that this liquid rock melted together to form a welded tuff.

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Again, I urge the reader to zoom in on this photo, which shows fewer and smaller vesicles, as well as a layering and white minerals that I cannot identify from the photo.

The previous hand sample shows obvious layering but no evidence of flow; i.e., this material was deposited from the air and formed separate layers, possibly over a short period of time (e.g. days to weeks). I think this because there is some evidence of mixing between layers, which would not occur if it flowed or solidified between layers. This relationship is better seen in the following photo, which shows a kind of settling of an upper layer into a subjacent one.

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There is no geologic reason to expect to find useful minerals (valuable) in these volcanic layers because they do not show any evidence of veins or remineralization like we saw on the previous post. Nevertheless, as we passed thru the flows of the middle phase, we crossed “Mineral Creek”, which was an opaque green as it flowed from the adjacent mountains. I stopped and took a panoramic photo of the valley and zoomed in on the opposing hills to discover an interesting symmetry across the valley.

mineral creek composite

The panorama photo has white outcrops circled. The lower photos show close-ups of the opposing sides of the valley; similar exposures occur on both sides of the valley. These volcanic layers are horizontal and there is no obvious faulting (e.g. dipping layers). There were no trails to either exposure so any mining reconnaissance did not suggest economic remineralization (i.e. gold or silver) worth road building. Until I can find the time to collect samples from both exposures and compare them, this remains a geological mystery to me.

I have a footnote to this post. We drove the dirt roads in this area under both wet and dry conditions. The roads were all wide and smooth with very little washboard texture but they created a lot of dust when dry. Weathering of the tuffs produces silt-sized glass particles that drain moisture fast but stay in the air when disturbed by truck tires, especially offroad tires. It was quite different than the gravel roads we encountered in the Wet Mountains and at Marshall Pass, both of which were based on weathered granitic rocks that produce sand grains as they erode.


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