Archive | Rocks and (no) Roads RSS for this section

The Subsiding Tidal Basin

Figure 1. View of Jefferson Memorial along the southeast part of the Tidal Basin.

In previous posts from northern Virginia I have discussed Precambrian orogenies, the rifting of Gondwana to form the Atlantic Ocean, erosion of the deep-seated roots of mountains, and the deposition of sediments along the modern Potomac River. With so much of earth’s history displayed here in my new home, I always keep my eyes open for the next geological adventure. I wasn’t disappointed when I walked along the banks of the famous Tidal Basin in Washington D.C. (Fig. 1), surrounded by monuments to Jefferson, Franklin Roosevelt, Martin Luther King Jr., Lincoln, George Washington…

The Tidal Basin itself (Fig. 2) is a monument, cherished by D.C.’s residents, because the cherry trees that line its banks, which were in bloom during my visit, were originally given to the United States by Japan in 1912. However, it has another symbolic meaning, as a reminder that the earth doesn’t stand still, not even long enough for our civilizations to flourish and decline naturally.

Figure 2. Google Map view of the Tidal Basin in the center of the image. The southern half (near the Jefferson Memorial) is the area discussed in the text.

The western side of the Potomac River (Fig. 3) comprises rolling hills and many creeks draining the deeply eroded topography. Capitol Hill (see Fig. 2 for location) is also high ground, but between these two topographic highs, the river’s channel has filled with fine-grained sediment (i.e. mud).

Figure 3. View looking westward to Virginia. Note the elevation on the other bank.

The northern end of the Tidal Basin (see Fig. 2 for location) is high and dry (Fig. 4).

Figure 4. Image along the northern end of the Tidal Basin, showing up to two feet of freeboard above the quiet water. Note the memorial cherry trees planted along the sidewalk.
Figure 5. View of the south shore of the Tidal Basin, west of Jefferson Memorial, at the bridge across the entrance to this enclosed body of water. Note the submergence of the walkway near the bridge and in the background of the image. This was near high tide. The land is undulating at this point, where the sediment consists of primarily mud.
Figure 6. Submerged sidewalk on the west side of the channel entering the Tidal Basin (see Fig. 2 for location). This photo was taken near high tide.

The southern end of the Tidal Basin is subsiding. This isn’t surprising, but the rate is alarming. For example, a back-of-the-envelope calculation, comparing Figs. 4 and 6, suggests subsidence of about three feet in less than a hundred years (since construction of the seawall). That works out to a burial rate on the order of 3/8 inches per year. That is substantial and explains why the National Park Service hasn’t kept up with the problem.

There is nothing surprising about the settling of muddy sediments at the apparent rate I observed in the Tidal Basin. The only confusing aspect of this predictable problem is why anyone would have thought it was a good idea to build massive stone edifices (e.g. Jefferson and FDR memorials) on muddy soil only a few feet above the high-tide line.

How long until Washington Monument becomes the leaning tower of America?

The Potomac River Floodplain

This post is going to talk about fluvial processes during the last few millennia, with the Potomac River as an example. A previous post discussed the geology of the Potomac’s fall line, where it drops out of the foothills of the ancestral Appalachian Mountains to the coastal plain before entering Chesapeake Bay. I’m going to keep this simple because, to be honest, fluvial geomorphology is not a straightforward topic. Rivers are constantly changing at time scales from years to millions of years. We won’t be walking back billions of years today, only a few hundred thousand, maybe a couple of million.

Figure 1. Schematic map, showing the location of the study area within northern Virginia (inset) and North America (double inset). Several features are labeled that will be referred to in the text and following figures. All photographs were taken within the black circled area. Note the labeled Holocene levee and Pleistocene terrace because they will be referred to later.

The lower Potomac River is braided, with multiple channels defining wooded islands (e.g. Van Deventer Island in Fig. 1). I won’t be talking about them but instead focus on what I saw, what the rocks (river sediment is unlithified rock to a geologist) tell me. The river flood plain extends to the Pleistocene terrace (yellow line in Fig. 1), which is about 80 feet higher in elevation than the river surface. No permanent structures have been constructed on the flood plain.

Some of the features we will examine are shown schematically in Figure 2. Note however, that the image shows a meandering stream whereas the Potomac is braided, which means that its channel doesn’t take those big loops shown in Fig. 2. That’s because the lower Potomac drops rapidly from Great Falls just upstream of the study area, to Washington D.C. in this area.

Figure 2. Schematic image of common river features.

We started out on the area labeled “Bluffs” in Fig. 2 and traversed the flood plain, following a tributary called Horsepen Run (see Fig. 1 for location). Note that Horsepen Run is a meandering stream, so we’ll see several features that scale downward from Fig. 2 as we cross the Potomac flood plain.

Figure 3. View looking downstream along Horsepen Run, showing fork in channel where debris collects during high flow. Note how shallow the flow is, the presence of large cobbles (some more than a foot in diameter), and erosion along the banks.

Horsepen Run (aka creek) drops quickly from the Pleistocene terrace (Fig. 1) but then crosses the Potomac flood plain and begins to meander. The photo in Fig. 3 is from a location just before this change in stream topography occurred.

Figure 4. View downstream of Fig. 3, showing coarse sediment inside of a meander forming a point bar.

The changes in stream morphology seen between Figs. 3 and 4 occur in larger streams (like the Potomac) but on much longer spatial scales.

Figure 5. Eroded bank of Horsepen Run, showing roots of a tree that cannot be more than a hundred years old. This indicates rapid downcutting and lateral movement of the channel. This kind of incision indicates a lowering of the stream’s base level, either due to uplift of the source or lowering of the receiving basin. For Horsepen Run, base level is where it enters the Potomac River (Fig. 6).
Figure 6. Confluence of Horsepen Run and Potomac River. The stream and river both have cut banks about 6 feet in height. There is no substantial delta at the mouth of Horsepen Run because of the coarseness of its sediment. Note the boulders seen in Figs. 3 and 4. This location is only a few miles from exposed Precambrian rocks.
Figure 7. View looking north, across the south fork of the Potomac River, towards Van Deventer Island which separates Virginia and Maryland. This is a rather large island in a terrain underlain by Precambrian rocks that are resistant to erosion. It is not a gravel bar as depicted in many representations of braided streams. Note the cut bank on the opposite shore, which is more than six feet in height (estimate only).

Figure 1 indicates the presence of natural levees (lower center of Fig. 1) near the main river channel. There is no “Yazoo Tributary” (see Fig. 2) at this location, so Horsepen Run cut across the Potomac’s natural levee. This can be seen beautifully in Fig. 8.

Figure 8. Meander in Horsepen Run cutting across the Potomac River’s natural levee, which is shown by the slope to the right side of the photo. The view is looking parallel to the Potomac’s channel at this location.
Figure 9. It is difficult to see the levee in this photo, which was taken from the top of the approximately 3-foot natural levee. Note the swale (low area) to the left side of the image. The levee isn’t as high as some (e.g., the Mississippi River) but it is quite noticeable, especially from ground level.

It was a beautiful February day to hike to across the Potomac River flood plain. I hadn’t expected to find so much dynamic geology so close to my new home, but there it was. The historic Potomac River transitions from its rocky confluence with the Shenandoah River at Harper’s Ferry, to the tidal river that defines Washington D.C., right here and, like America, it is not in equilibrium. The cut banks of the Potomac and its tributary, Horsepen Run, portend of rapid changes in the relative elevation of the land and the sea.

We are in for a wild ride…

A Typical Basin and Range

Figure 1. View of Piestewa Peak, the highest point of the Phoenix Mountains, from the parking lot. The top is about 1200 feet higher in elevation than where the photo was taken.

My last post examined some structures and petrology of a Metamorphic Core Complex (MCC), whereas the previous one discussed the geology of the Central Highlands of Arizona. I mentioned several times that these geologic provinces were but two manifestations of the profound tectonic change associated with uplift of the the Colorado Plateau.

Today’s post is from the Phoenix Mountains, a municipal park inside the city limits (Fig. 2). My geological interpretations and dates, etc, come from a report by the Arizona State Geological Survey.

Figure 2. Location of Phoenix Mountain Park is indicated by the pin marker.
Figure 3. Topographic representation of the Phoenix Mountains. Note the location of Piestewa Peak (labeled Squaw Peak in the image) in the center of the outcrops.

To the west (left of Squaw Peak in Fig. 3), a deep fault has been identified, which thrust the metasedimentary rocks comprising the eastern part of the range into juxtaposition with the metavolcanic rocks of Stoney Mtn and other outcrops to the west (Fig. 4).

Figure 4. Geologic map of the Phoenix Mountains. The dashed line indicates the approximate location of a fault that separates older Proterozoic rocks, with a higher metamorphic grade, from younger rocks.

Today’s post is focused on the circled area in Fig. 4 which, if we look back to Fig. 3, is the highest peak within the Phoenix Mountains. I was intrigued by the view from the parking lot (Fig. 1), and compelled to explore this fault-block in person. Note that the area discussed in this post in contained within the circle in Fig. 4.

The geology of Piestewa Peak is relatively simple. Schist. In this case, the metamorphic grade isn’t too high and the rocks preserve much of their original thin-bedded layering. However, they are standing on end (Fig. 5).

Figure 5. Exposure of metamorphic rocks in the lower part of Piestewa Peak. Sedimentary rocks were metamorphosed to a medium grade schist about 1.6 billion-years ago. The vertical lineation seen in the photo is caused by the characteristic alignment of platy minerals like muscovite mica.
Figure 6. View looking obliquely along the primary foliation of the schist, showing the preservation of primary sedimentary textures in the thicker layers near the center of the image. Note that the metamorphic fabric is not as steep here as in Fig. 5. Foliation varied by tens of degrees within the study area. Note the curvature of the layers, evidence of folding during a deformation event that may have coincided with metamorphism. A lot can happen in a billion years.
Figure 7. The foliation is nearly vertical at this location, much further up the trail than Fig. 5. However, the layers are fused more tightly and schistosity is reduced. Note the cross-cutting layer running from the center of the photo to the right, as well as the lighter-colored rock that is pinched in the center. This locality reveals multiple deformation events, including the intrusion of magmatic fluids rich in silica (i.e. quartz and feldspar).
Figure 8. This beautiful photo shows the preservation of primary sediment texture in the darker layers to the upper right. Lamination can be seen within each layer. Primary sedimentary texture and the metamorphic foliation are aligned. A second feature of this exposure is the complex relationship between the country rock (i.e. the schist) and the pink material invading it along bedding planes. Note the massive but jumbled appearance of this intrusive rock in the lower part of the image whereas it has been confined to thin veins in the upper part. This image and Fig. 7 are evidence for the occurrence of multiple metamorphic and intrusive events. Injection of granitic veins would have been later than the relatively low-grade metamorphism that produced the schist.
Figure 9. This photo shows the sharp contact between the intrusive granite (the orange hue suggests a potassium-rich source) and the schist, which would have been much older by this time. This is a thick vein that apparently is directed vertically, as suggested by the lack of a trace in the background.
Figure 10. Photo at the top of Piestewa Peak, showing the lower, eastern peaks of the Phoenix Mountains. Note the lack of erosion of the rocks exposed at the summit in this arid climate.
Figure 11. Image of nearly vertical schist intruded by granitic veins that cut across metamorphic and/or sedimentary lineations. Although I made no geometric measurements, I think this outcrop is approximately aligned with the veins shown in Figs. 8 and 9; if so, this could be near the termination of a splintered offshoot from the magma chamber that cut into these older metamorphic rocks. I didn’t find any ages for this granite but it is definitely younger than the 1.6 Ga schist, simply because it cuts across bedding planes and metamorphic lineations.

Summary

There isn’t much to say about this post after my road trip to Prescott, and then a hike in the White Tank mountains. The first thing I can say with confidence, however, is that Squaw Peak (aka Piestewa Peak) is squarely located within the Basin and Range, defined by faults that have brought disparate rocks into juxtaposition, but only in a small area. The Phoenix Mountains are nothing like the vast, overlapping fault-bounded mountains of the Central Highlands, but instead they are isolated in a sea of sand and gravel, sediment eroded from the long-gone rocks that encased them for eons. There was no superimposed shear evident in these rocks as in the White Tank mountains. They just rose from the earth’s bowels along nearly vertical faults.

These rocks aren’t as old as those we encountered in the Central Highlands — by about a billion years. Nevertheless, they suggest that plate tectonics determined the history of central Arizona, even so long ago. Because of the lack of suitable rocks, no plate reconstruction can be attempted for the Precambrian (neither a geologic period, era, or eon); thus, we can only assume that things were the same but different — upper mantle processes dragging crustal plates around, but without plants, oxygen, or animals to intervene in surface erosion.

We don’t know what happened that long ago, not to mention the billion years between the creation of these sedimentary/metamorphic rocks and the emergence of multicellular life. We can only view the rocks we’ve seen in Arizona through a glass darkly…

The White Tank Mountains: Anatomy of a Metamorphic Core Complex

My last post set the stage for this report, but this time I did a lot of walking to get the facts. A short drive took me to White Tank Mountain Regional Park, about 30 miles west of Phoenix. I studied geology at Arizona State University in Tempe…it must have been 40 years ago, and Metamorphic Core Complexes (MCC) were a big thing then. Let’s start with a map (actually several maps that focus ever closer on the field area).

Figure 1. Overview of the area. The White Tank MCC is located west of Phoenix (lower left inset). The mountain range is dissected in a SW-NE lineation and is steeper on the east side than the west. The lower-right inset shows the two locations where I examined the rocks. Site 1 is more of an overview, and Site 2 is where I looked closely at the mineralogical and textural variations within an intrusive rock body of Cretaceous age.

Now for a view from the ground.

Figure 2. View looking north from Site 2. The mountains in the distance are the Central Highlands, which I discussed in my last post. Many of the same Precambrian rocks are present in the White Tank Mountains, but with a different story to tell.

Site 1.

The White Tank MCC is located within the Basin and Range Province. In the middle of a low-lying flat desert, MCCs appeared within the last 60 my, in close proximity to a region defined by faulting and the uplift of Precambrian rocks on a huge scale. Time to look at some rocks.

Figure 3. A relatively small outcrop at Site 1 (see Fig. 1 for location). The entire road cut was about 200 yards in length.

Figure 3 reveals a complex pattern of deformation and magmatism. The most striking feature of this outcrop is the brilliant white veins that cut across the dark rocks, and folded in a crazy pattern on the right side. What is going on here?

The background is that the dark rocks are Precambrian metamorphic rocks and granites. As I discussed before, these rocks were deformed several times during the billion years spanning the resetting of their radiometric clocks and the tectonics associated with the uplift of the Colorado Plateau. The lighter-colored rocks are of Cretaceous age, injected as veins into preexisting weak fracture zones.

Figure 5. This photo shows the sharp contact between the white intrusion and the dark-colored Precambrian rocks, creating a weak zone that led to the erosional feature seen in the photo.
Figure 6. The dark rock is Precambrian gneiss that has been reheated and deformed ductile. Note the sharp contact with the intrusive granite. The older rock did not melt during intrusion.
Figure 7. Hand sample of the Cretaceous granite, showing alignment of platy minerals. This is a good example of syntectonic intrusion.The magma was intruded while the deeply buried rocks were being deformed.
Figure 8. I’m not certain how to interpret this sample, which is less than a foot in diameter. The best I can do (without getting carried away) is to suggest that none of these rocks were molten except for the white one on the left (the intrusive granite), which may have cooled sufficiently to behave the same as the Precambrian granites it was invading. It really is a remarkable juxtaposition of rock types and mineralogies. Note the block of intrusive granite embedded in a dark matrix of (supposedly) Precambrian gneiss.
Figure 9. This photo says it all. Note the arched intrusive rock in the lower center of the photo. This is an anticline, which results from compressional stress. It has been rotated because there is no source of compressional stress from that angle (about 20 degrees from vertical). Just above it, a similar vein of white rock has been kinked into a “V” pointing to the right. At the top of the photo, there appears to be an “X” which is a common jointing pattern. The only way to reconcile these disparate textures in such a small area (the image is about 4 feet across) identifying the folded veins as part of the Precambrian folding of metamorphic and igneous rocks. The unfolded veins are associated with intrusion of the White Tank granite in the late Cretaceous to Tertiary periods.

SIte 2.

Figure 10. Location map for textures seen along Mesquite Trail. See text for explanation.

Figure 10 reveals some of the field textures seen along the trail, starting at the uphill end, where medium and small blocks litter the landscape as the granite weathers in place (Fig. 11). Panel A (Fig. 10) shows a sharp contact between the main granite and a whiter material with microcrystalline structure similar to what we saw at Site 1 (Fig. 3). As the magma was intruded, the melt was fractionating into a component with a lower melting temperature and so it filled fractures, which indicates there was tectonic movement at that time. Panel B is evidence of syntectonic intrusion because it shows a lineation that was present in many of the rocks. Panel C is a close up (5x magnification) showing larger crystals in a finer matrix. All of it is feldspar (dominated by Na also known as albite). Panel D is a good exposure of the relationship between the finer grained material that forms veins in the main rock. Panel E shows a salt-and-pepper texture that dominates the rocks along the trail.

Figure 11. Hill side covered with large blocks

A close up of a fresh surface near Sample D (see Fig. 10 for location) shows simple mineralogy of the main rock body (Fig. 12). I would estimate 70-80% albite, >15% quartz, and minor biotite (a platy mineral, a variety of mica).

Figure 12. Close up (5x magnification) of granite, showing the dominance of feldspar.

The mineralogical composition can be used to classify this granite as tonalite. Tonalite is a granite that contains no potassium feldspar (no pink color), very little quartz, and mostly feldspar containing sodium and calcium. These rocks originate deep in the earth where ocean crust (basalt) melts and rises, losing much of the iron and magnesium it originally contained. The tectonic setting is an island arc, like Japan.

Figure 13. Close up (5x magnification) showing weathering of the granite in place. The larger crystals of feldspar and the quartz are more resistant, producing a gravel product with lots of fine grained material (mud).

Summary

Just as in the Central Highlands, sediments were deposited here in ocean basins as long ago as 2.8 by and subsequently buried and deformed, changing through heat and pressure into gniess, and injected with veins of quartz and feldspar during metamorphism. Several orogenic events followed, deforming the assemblage further. It was exhumed slowly over the ensuing billion years, and eventually injected with a tonalite granite created deep beneath an island arc. This was the time when the White Tank granite was emplaced. This magma was intruded when the region was undergoing extensional stress (pulling apart) as part of the adjustment to complex tectonic process associated with uplift of the Colorado Plateau.

Rather than breaking into irregular blocks as in the Central Highlands, the White Tank mountains (and other MCCs in the western Cordillera), were formed by the uplift of Precambrian basement along deep sub-horizontal surfaces called detachment faults. In other words, the crust stretched in the Basin and Range rather than breaking into fragments.

I haven’t attempted to describe the complex tectonics of central Arizona, a task that is well beyond my experience. This is a topic that is hotly debated in the geological community to this day. This has only been a brief effort to relate the rocks I saw with my own eyes to what is known about the history of the earth.

Always listen to the rocks…

Road Trip: The Central Highlands of Arizona. Precambrian Rocks Torn Asunder.

To keep this post from going off the rails and becoming a treatise on metamorphic textures and their relationship to regional tectonic trends, I’m going to address the bullet list I ended the last post with. I hoped to find the following rocks, from youngest to oldest:

  1. Quaternary gravel
  2. Quaternary and Tertiary lava flows
  3. Tertiary stream deposits of sand, silt and gravel with rounded pebbles
  4. Tertiary lake sediments of horizontal, whitish, fine-grained rock with layers of volcanic ash
  5. Precambrian granite
  6. Precambrian gneiss and schist

The descriptions in The Roadside Geology of Arizona include unique features, from outcrop to hand sample scales. By the way, the “Roadside Geology” series books (available for many U.S. states) include introductory sections for the layman, and a level of detail that will increase anyone’s appreciation of the natural world, whether driving cross-country or on a daily excursion.

I’m going to dispense with my usual lengthy introduction. For the first figure, which becomes the de facto icon for the post, I’m going to skip maps and cut to the chase. I was driving north from Phoenix on Interstate 17, scoping for interesting exposures, checking the mileposts, and trying to remember several outstanding stops from the previous night’s reading. Frustrated at having passed the exit for Crown King, the ramp and interchange (beneath the I-17 roadbed) magnificently adorned by a long road cut that displayed interlayered dark and lighter colored rocks (Fig. 1), berating myself for not taking the path less traveled, I searched for the on-ramp and, when it came into view, I hit the brakes hard and cut onto the last fragment of the slip road. Traffic on the ramp was nonexistent and, with no unmarked Arizona Highway Patrol vehicles within sight, I backed several hundred yards while avoiding the drainage ditch and concrete barricade.

Figure 1. Road cut on I-17 at the exit to Crown King.

All I’m going to say about Fig. 1 right now is that this was the exposure that prompted me to drive — shall I say “recklessly?” But it is only the tip of the iceberg in the Central Highlands.

This post isn’t a typical report. Instead of a trip log, I’m going to show examples of the six rock-types from my list. I took a lot of photos but I will try (really hard) to limit the interpretation to a concluding paragraph.

The block faulted mountain ranges of the Basin and Range province, accompanied by Metamorphic Core Complexes like South Mountain or the White Tank Mountains, bounding Phoenix, Arizona, on the south and west, respectively, suggest extensional forces at play. The Colorado Plateau, on the other hand, reveals no significant evidence of either crustal extension or shortening, instead comprising relatively undeformed Paleozoic sedimentary rocks that appear to have been exhumed vertically. The earth’s crust had to accommodate not only 6000 feet of differential uplift, but a change in tectonic regime, within the distance from Phoenix to Prescott in the last 80 million years, creating the Central Highlands.

Quaternary Gravel

The Quaternary Period spans the last 2.58 my. It is identified with the most recent occurrence of extensive continental ice sheets.

Dating sediments deposited during the Quaternary is extremely difficult because of a scarcity of suitable material for radiometric dating of the time of deposition. Thus, stratigraphic relationships are used, along with sedimentary textures, to determine the relative age of sedimentary rocks. For example, Lithology (1), Quaternary gravel, contains very little organic debris for carbon-14 dating; Lithology (3), on the other hand, may occasionally contain some plant fragments in the finer-grained components; but carbon-14 dating can’t be used for rocks older than 500 ky (thousand years).

Gravel was present everywhere, filling topographic lows and even forming cliffs. Often, these sediments were found in thick intercalated beds of silt and gravel, but sometimes cross-cutting relationships reveal erosion of older sediments before infilling (Fig. 2).

Figure 2. Erosional surface (indicated by the sharp change from tan to reddish sediment, which is typical of Tertiary sedimentation. These sediments cannot be dated and age estimates are based on stratigraphic relationships. In other words, this is a channel eroded into an earlier flood plain sometime in the last million years.

The darker gravel in Fig 2 fills a channel in the finer, buff-colored silt. The difference in lithology, and presence of an erosion surface, suggest that the upper unit is representative of the Quaternary gravels of Lithology (1). The underlying rock is a siltstone representing the Tertiary (66-2.58 my) stream deposits of Lithology (3).

Quaternary and Tertiary Lava Flows

Several volcanic complexes were active during the last 66 my in the Central Highlands. The lava sometimes flowed from fissures and often from central cones. Their remnants can be seen in the arid landscape.

Figure 3. Mesas formed from lava flows. The once-continuous flows have been eroded by streams to form isolated mesas and ridges.

The lava flows are not very thick and individual flows can be identified. I’ll say more about the lava in the following discussions because of its relationship with the other lithologies.

Tertiary Stream Deposits

Figure 2 shows a silty bed that I have attributed to older Tertiary sediments, which could have been deposited over a 60 my time span. However, specific thickness of stream deposits can be dated by their relationship to the rocks of Lithology (2), because volcanic rocks can be easily dated. When such data are unavailable (as in this situation), we can’t be more specific.

Figure 4. Tertiary stream deposit consisting of gravel overlying silt.

An interesting thing about Fig. 4 is that ancient stream sediments are eroding to form new stream sediments in an endless cycle of deposition, erosion, etcetera.

Figure 5 Tertiary stream deposits consisting of intercalated gravel and silt layers.

Tertiary Lake Sediments

Figure 6. Lava flow over Tertiary fine-grained lake deposit.

Lithology (4) was broadly distributed near Black Canyon City but it wasn’t safe to stop and examine them. Figure 6 reveals some evidence of soft-sediment deformation in the underlying chalky sediment. The implication that the lava flowed into a lake is further supported by pillow structures (Fig. 7).

Figure 7. The contact between the overlying lava and lake sediments suggests some mixing during deposition. Note also the rounded form of the lava, which suggests that it flowed into water.

Precambrian Granite

The oldest rocks I’ve discussed so far were less than 70 my, but now we’re going to jump back to between 2.5 and 1.5 billion-years ago. There are no rocks from the Paleozoic or Mesozoic Eras in this part of the Central Highlands.

Granite outcrops weren’t ubiquitous along the highway until Prescott, although they are everywhere within the area — just not next to Interstate 17. I had an opportunity to see them up close on the return, which took me through Peeples Valley to Congress (Fig. 8).

Figure 8. Route map for the return to Phoenix.

I drove through overlapping plutons, with variations in the granite composition indicated by differences in their color, which varied from orange (orthoclase feldspar) to light gray (plagioclase feldspar). Most were weathered to a reddish color by the oxidation of iron-containing minerals, and they were all rounded as seen in Fig. 9. The one-billion year duration of magmatism suggests that this was a time of intense continent building.

Figure 9. Typical knob of weathered granite within the Central Highlands.
Figure 10. Image of quartz vein and slight offset along a joint. This pluton cooled and was uplifted enough to fracture, before being injected with fluids from subsequent intrusion.
Figure 11. The darker, rounded rock is a piece (~6 inches in diameter) of wall rock that was entrained during emplacement of the magma.

Precambrian Metamorphic Rocks

Arizona (as well as most of the earth’s surface) is underlain by a crumbled, jumbled, layer of metamorphic rocks — reflections of long-forgotten oceans that teemed with life, collecting fine particles eroded from the slowly emerging continents, as well as the bodies of earth’s first inhabitants. In other words, for metamorphic rocks to have formed about two-billion years ago, sedimentary particles must have collected in ocean basins a couple hundred million years earlier. The earth is only 3.8 billion-years old, so there could have been only so many cycles of metamorphism in any given location. I’m guessing…maybe two or three?

Figure 12. Exposure of Precambrian metamorphic rocks on I-17 north of Phoenix.

The title of this post refers to Precambrian rocks being torn apart during the separation of the Colorado Plateau from the lowlands represented by the Basin and Range province. Figure 12 exemplifies this dynamic process. To the north (left side of Fig. 12), it is difficult to ignore the nearly vertical lineation of rocks that are more like schist, with a sheen caused by the alignment of muscovite minerals. The tan color suggests that the original sediment contained a lot of quartz. The rocks exposed on the south (right in Fig. 12) end of the road cut are dark in color and include lenses of quartz (white flecks beneath the yellow arrow), suggesting a very different post-depositional history. They were probably mudstones before alteration.

The dark rocks (below the white arrow…) that conjoin these discordant blocks exhibited fine-scale jointing (scales of inches) and conchoidal fractures, an indicator of microcrystalline structure. I haven’t shown the photographic evidence of my description, to shorten the report. (I really wanted to.) These metamorphic textures indicate stress regimes with different orientations. Figuring it out would require substantial, detailed field work to measure stress indicators, and geochemical analyses. Not being in a position to do any of that, and unaware of any reports on this area, I’m going to treat this as a fault that cuts the plane of the road-cut at an angle similar to the steep dip of the lighter beds on the left of Fig. 12.

It is time to address the geological questions raised in Fig. 1 which, it turns out was a cause of confusion in deciphering the geological history of the Central Highlands.

Figure 13. Tertiary stream deposits (light-red layers) overlying Precambrian metamorphic rocks. The upper foreground reveals dark-red boulders of basalt in contact with the Precambrian rocks while lying on the sediments in the background.
Figure 14. Hand sample view of the metamorphic rock seen in Fig. 13. Some foliation is seen at the bottom-center of the photo, but it is generally featureless at the macroscopic scale. (The image is ~2 feet across.)
Figure 15. Good exposure of the metamorphic rocks, showing overlapping joint patterns superimposed on quartz veins. This outcrop is indicative of the multiple metamorphic events that impacted the region during the billion years between original deposition of the sedimentary rocks and their exposure during the Tertiary.

The black, metamorphic rock in Fig. 14 gave the name Black Canyon to the region because it outcrops throughout the area, underlying both volcanic and sedimentary rocks.

Figure 16. Contact between Tertiary volcanics and sedimentary rocks. The dark gray zone contains blocks of volcanic rock, which is unusual for a baked zone often seen below lava flows. This may have been underwater at the time of eruption. (Note the rounded — pillow lava — form of the overlying volcanics.)

Summary

Almost three-billion years ago, the western half of what is today North America was an ocean margin, possibly like the East Coast. Mud and sand was accumulating in environments much as we find today except there were no plants so erosion was probably more intense. By about 2.5 Ga (billion years ago) these sediments were buried to depths as great as 35 km (~20 miles) and were compressed as tectonic plates collided. This process continued, no doubt in pulses, for another billion years, producing the complex textures we see in Figs. 12-15. This area was on the trailing edge of proto-North America during the formation of the supercontinent Gondwana, between ~500 and 200 Ma. When the modern Atlantic Ocean began to open, subduction of oceanic crust began and new continental crust was created, forming modern California.

Streams crisscrossed the area much as we see today, but in a different climate. The volcanism associated with subduction of the Pacific Ocean crust didn’t reach Arizona until ~30 Ma, when volcanoes spewed out numerous lava flows such as those that cap the mesas seen in Fig. 3, and the Colorado Plateau began to rise by thousands of feet. This is when the Central Highlands (aka Transition Zone) formed, a buffer between the block-faulted Basin and Range of central Arizona and much of the western deserts of Nevada, and the uniform and mostly undisturbed Paleozoic sedimentary rocks of the Colorado Plateau.

Figure 17. View from Sunset Point, on top of a thick sequence of lava flows produced during the Tertiary.

Road Trip: The Central Highlands of Arizona. Introduction

I haven’t posted much about the geology of Arizona, where I grew up and wandered the back ways while studying Geology at Arizona State University because, unfortunately, most of these excursions occurred before the internet and are nothing more than dim memories, supported by fuzzy photographs enclosed in aging plastic as part of my photo album collection. Many of those albums were damaged by water during storage. Recent trips to the Grand Canyon State were in a motor home, which isn’t conducive to unplanned photo stops on busy highways. The good news is that I am back for a few days and ready to explore one of the state’s most interesting provinces, the Central Highlands.

Figure 1

Figure 1 reveals how Arizona can be divided based on the most-recent geological event in its history, the problematic uplift of the Colorado Plateau during the last 80 million years by approximately 2.5 km (about 8000 feet). Today, Flagstaff is located at an elevation of 7000 feet while Phoenix, located in the Basin and Range province sits at a mere 1000 feet above sea level. The Transition Zone (Fig. 1) reflects how the earth’s crust adjusted to this 6000 foot difference in elevation. (Note that I am using the older terminology — Central Highlands — mostly as a nod to this area being identified as a distinct geographic province long before its tectonic underpinning was understood.) It’s also a nod to the motivation for this blog, in this case The Roadside Geology of Arizona, by Halka Chronic, published in 1983 by Mountain Press Publishing.

This post is an introduction to the Central Highlands. I will be driving (in a car and not a motor home) from Phoenix, in the Basin and Range province (see Fig. 2) on Interstate 17 to the middle of the volcanic complex shown in a burnt orange in Fig. 1, and then follow AZ-69 NW to Prescott, before returning to Phoenix on AZ-89 by a more westerly route.

Figure 2

The dramatic change in elevation from Phoenix to Flagstaff suggests that the earth’s crust was deformed substantially when the Colorado Plateau was uplifted (to the right in Fig. 3), much of the difference accommodated by faulting.

Figure 3

Blocks of crust the size of mountains slide up and down along the endless number of faults that characterize the Central Highlands, which is why its geological name is the Transition Zone (see Fig. 1), from low elevation to high. Most of the rocks we will see are Precambrian gneisses and granites. In other words, sedimentary rocks originally deposited far more than 500 Ma in the past, buried and subsequently altered by extreme heat and pressure more than a billion years ago, when they were penetrated and partially melted by molten material from deep within the earth (at least ten miles below the surface). Much later (within the last 100 million years) magma filled the faults crisscrossing the Central Highlands and flowed onto the ancient surface to form volcanoes and lava flows.

I will cross this Transition Zone at two locations, separated by about 40 miles, and attempt to identify variations in the rocks caused by a number of geological factors. That is the (hypothetical) purpose of this road trip.

There are six kinds of rocks and sediments we will be looking for on this trip, and we expect to find them in wildly confusing juxtapositions because of so many faults and the immense spans of time represented by the rocks, not to mention taking different routes on our journey.

  1. Quaternary gravel
  2. Quaternary and Tertiary lava flows
  3. Tertiary stream deposits of sand, silt and gravel with rounded pebbles
  4. Tertiary lake sediments of horizontal, whitish, fine-grained rock with layers of volcanic ash
  5. Precambrian granite
  6. Precambrian gneiss and schist

I will try to show examples of these with photographs but…well, roadside geology is a lot more dangerous than it used to be, with so many vehicles on the roads, and everyone in such a hurry…not to mention reading my blog on their cellphones (I wish).

Cloistered in Washington Heights

We were spending Thanksgiving week in NYC, visiting our daughter, and went to see a unique collection of European artifacts from the Middle Ages (~1100 to 1500 CE) in an annex operated by the Metropolitan Museum of Art. They call the castle built in 1935 (this was common during that era) The Cloister. To our surprise and delight, this modern fortress was constructed on a ridge of metamorphic rock call Washington Heights, standing out high above the Hudson River.

Figure 1. The Cloisters “castle” built on top of Washington Heights in Manhattan, NY.

Thick sequences of eugeosyncline sediments were deposited as the Iapetus Ocean was closing about 600 Ma (million years ago), weathered from rapidly eroding volcanoes (e.g. Japan today). Between 541 and 459 Ma, these sediments were buried deeply enough to be heated and semi-melted as the island arc collided with the mainland during the Taconic Orogeny. Figure 2 shows a schematic of the result of this process, which created the Manhattan Formation, the topic of today’s post.


Figure 2. Schematic of subsurface geology alongInterstate 95, from New Jersey to Long Island.

It was a cold November morning when we hiked over the Manhattan Formation (red layer outlined by the rectangle in Fig. 2. The castle is right under the arrow for Washington Heights. When metamorphosed at several miles depth (a guess) under extreme pressure and heat, these immature sediments formed schist containing large garnet phenocrysts.

Figure 3. Outcrop of Manhattan Formation on trail to the Cloisters (Fig 1).

The overall dry gray schist has irregular layers of quartz showing rotation of inclusions (Fig. 4).

Figure 4. Close-up from left side of Fig. 3, showing swirling structure around inclusion (light gray over at upper-center of image). The image is about three inches across.

Hexagonal phenocrysts occur in widely separated areas (Fig. 5).

Figure 5. Image of segment from Fig. 3 revealing many hexagonal phenocrysts of garnet.

The garnet crystals (Figs. 4 and 5) were very large and well formed. The brownish surface indicates they are rich in iron, which is literally rusting (oxidizing).

Figure 6. Detail of garnet (brown hexagon in upper-center of photo), probable albite feldspar (white mineral surrounding the crystal), quartz (gray blebs throughout), biotite mica (black flecks), and orthoclase feldspar (orange-pink flecks and streaks). The central crystal is the size of a quarter.
Figure 7. Photo taken from Washington Heights, showing Hudson River, George Washington Bridge, and a bluff of presumably the same rock on the other side, in New Jersey.

The Manhattan Formation is a hard rock even though it has a preferred plane of inherent weakness as to all schists. For example the plane of weakness in this outcrop (Fig. 3) is approximately vertical, being aligned with original bedding as suggested in Fig. 2 (red layer).

As we recall from previous posts, the closing of the Iapetus Ocean occurred in pulses that began with the Taconic Orogeny and the deformation of the Manhattan Formation. A series of island arcs were thrust onto photo-North America until photo-Europe collided in the Alleghanian Orogeny about 250 Ma.

It All Started Here

You don’t have to go far to find some rocks here in Fairfax County, Northern Virginia, and the rocks tell a fascinating story of earth-shaking scale; continents colliding and splitting asunder, right beneath our feet. Today’s post goes straight to the middle of it all, not far from our apartment.

Figure 1. Trail head for today’s adventure.

It was a few degrees above freezing, but that doesn’t matter to the rocks or to me and my stalwart field companion. Figure 2 shows the field area and our approximate path along Sugarland Run, a meandering creek that passed through a narrow, rocky channel before entering a wide, boggy area as it flowed north (towards the Potomac I presume).

Figure 2. Map of Sugarland Run Park (approximately the darker area). The star is the approximate location of our apartment in Herndon. The creek forms the boundary between Reston and Herndon.

Towards the southern end of our trail (dashed red line in Fig. 2), we encountered very-large boulders adjacent to the creek (Fig. 3), which had a bed composed of angular gray casts showing little evidence of transport (Fig. 4).

Figure 3. Typical assemblage of large boulders found throughout the study area.
Figure 4. View looking downstream, showing angular nature of boulders lining Sugarland Run.

We didn’t find any large boulders (some were as large as automobiles) that were definitively in place (i.e., visibly attached to subjacent basement rock), but these huge stones weren’t moved far at least not all of them. In several places, they were obviously pushed aside by bulldozers to make way for apartment buildings. Nevertheless, there is no obvious reason for some of them to have been displaced (e.g., Fig. 3), so it is reasonable to assume that the boulder fields represent the outer layer of unexposed rocks supporting the area, fractured and eroded but too resistant to have been weathered since exhumation, probably during the last ice age.

The drainage has been disrupted by construction, resulting in previous stream beds that have been cut off.

Figure 5. Field of smaller, angular boulders indicating the previous presence of a tributary flowing into Sugarland Run. Soil was pushed into the relict stream from the left side of the image to clear land for homes. Compare to Fig. 4 for the original depositional environment.

In keeping with the Rocks and (no) Roads principle of studying the earth as we see it, I couldn’t find any freshly broken rocks, but we can say for certain that this is a relatively uniform exposure of homogeneous, fine-grained, gray rocks with no obvious phenocrysts.

Figure 6. Photo at 5x magnification of weathered surface of typical boulder from Fig. 4.

A couple of interesting characteristics of this rock are visible in Fig. 6: first, it weathers to a reddish color from an original gray tone (compare to Fig. 5, which shows boulders not exposed to constant water); secondly, the texture is fairly uniform. This last observation requires clarification. The hand sample in Fig. 6 was covered with whitish biological material of unknown composition, but if you stare at Fig. 6 long enough (or enlarge the image) you will notice that it does not consist of identifiable “grains” of sand. As I learned from my travels around Australia, when you can’t see sedimentary particles, and there is no apparent structure, you are looking at an igneous rock of some kind.

The lighter, bluish gray area in Fig. 2 indicates the extent of a rather special but widely distributed rock along the east coast of North America. According to several field reports, summarized in the Rock-D app, this is a high-titanium quartz-normative diabase. A traditional classification places it in the tholeiitic magma series. It has been reliably dated to have been emplaced between 200 and 174 Ma. Yet another unique rock type found within the Newark Supergroup.

In a previous post, we examined some of the sedimentary rocks deposited in subsiding basins during this time period. Meanwhile deep beneath the surface, magma squeezed into fractures in the weakened crust as Pangea began to split apart. Another piece of the puzzle falls into place. A single continent containing all of the earth’s land (look at a globe to imagine what that must have been like), was torn apart by slowly convecting rocks within the mantle (like a pan of water boiling), heated by the primordial molten iron of the earth’s core.

To help you visualize it, here’s a plate construction from just before all hell broke loose (geologically speaking) created by Fama Clamosa (I couldn’t find a solid reference but this is about what I’ve seen elsewhere).

Figure 7. Plate reconstruction at 200 Ma, just before Pangea split along the border between what would later become North America and Africa.

Jump ahead 200 million years.

Figure 8. Downstream of the rocky section seen in Figs. 4 and 5, Sugarland Run enters a wide meadow, forming several branches, and gravel replaces angular boulders.

That completes our journey back in time. One last point I’d like to make, and explain the title of this post, is that Northern Virginia was ground zero (see Fig. 7) and every time I go for a walk or short ride, I’m reliving more than a billion years of the earth’s history. It is a humbling experience.

One last thought: our planet transformed from the Paleozoic Era (Fig. 7) to the modern world in 200 million years; at a spreading rate of about 1 inch per year (total widening of the basin) and a little math (200 million inches equals ~3200 miles), I see that Herndon, Virginia is about 1700 miles from the mid-Atlantic ridge, the point where it all began in the early Jurassic Period.

I’m feeling dizzy…

The Closing of Iapetus: Island Arcs and Metamorphism in Rock Creek Park, Washington D.C.

I’ve spoken before about the Iapetus Ocean that separated North America from Europe. It is more than conjecture because there are pieces of the sediments and volcanic rocks underlying it to be found along the east coast of N. America. These ophiolites are proof of the existence of ocean crust and upper mantle pushed onto what later became Maryland (albeit at great depths beneath the surface) during the closing of an ocean basin between approximately 500 and 300 Ma.

Figure 1. Map of Washington DC area, showing the study area. The photos below were taken at the marker within the circled area in the inset map.

The green area in the inset map of Fig. 1 has been classified as the Sykesville Formation. These are sedimentary rocks, formed from a melange that collected behind an island arc. They contain a crazy mixture of sediments eroded from volcanic islands and exhumed rocks when these islands were smeared onto a continent at geological time scales (e.g. Japan and the Philippines today). This is a very slow process. However, this entire episode was nothing more than the prelude to closing of the Iapetus Ocean when two continental land masses collided, neither of which could be subducted beneath the other. Thus, these marine sediments were buried and subjected to incredible pressure and heat, producing the metasedimentary rocks of the Sykesville Formation.

Figure 2. Exposure of Sykesville Formation rocks along the Rock Creek Parkway (see inset of Fig. 1). The photo is approx. six feet across.

The original sedimentary texture (layers of sediments deposited horizontally) has been overprinted by foliation, a metamorphic texture caused by compression deep beneath the surface, usually at high temperature. The nearly vertical, yellow line in Fig. 2 indicates this metamorphic foliation, which is probably close to the orientation of original bedding. The circled areas show striations on a foliation surface (lower right) and perpendicular (upper left).

After being metamorphosed at great depth these rocks were folded and faulted, so that nearly horizontal bedding and foliation were tilted to nearly vertical. Figure 2 is looking towards the NW (field estimate) and is consistent with similar deformed rocks observed at Great Falls. In other words, these metasedimentary rocks have been folded and faulted (there are several faults within the area) when two continental land masses collided.

A closer look reveals a hint at what occurred more than 300 million years ago.

Figure 3. Close up from the outcrop in Fig. 2, showing quartz lens in a dark background The field of view is about two inches.

The highlighter in Fig. 3 indicates a bleb of quartz (the gray mineral) with a distinctive pinching (note the lower and upper parts of the circled area). Although this does indicate very high temperatures and pressure, the rock has not melted and there is no indication of veining or other evidence of contact metamorphism.

Figure 4. Close up of Fig. 2 looking along the beds (i.e. downward into the ground). Note the rectangular, white segment, and the irregular mineralogy and (sedimentary?) relationship of the darker rock to the lower right of the image.

Textures like those seen in Figs. 3 and 4 are associated with depositional/tectonic environments where clumps of rock are either being eroded by surface processes (e.g. rain and erosion) or plucked from a thick layer of overlying rock as it slides over a given sequence (in this case the Sykesville Formation) when it is buried deeply enough for the rock to be ductile. This poorly understood process produces allochthons like ophiolites. The textures of these rocks have been interpreted (using radiometric and textural data) as resulting from the latter process.

Take a long look at Fig. 4 and imagine sediments deposited in the Sea of Japan being jammed into Korea, buried deeply and sliding over each other for millions of years.

The earth is relentless…

The Newark Supergroup: From Continental Rifting to Glaciation

We didn’t go very far from home today, just down the road to Frying Pan Park; the field area was the hill and a creek near a Baptist Church constructed in 1792 and still in original condition.

Figure 1. Fying Pan Meeting House. The exposures we discuss are behind the photographer.

A small cemetery, with one gravestone as recent as 1938, marks the entrance to a series of trails wandering over the hilltop where Confederate soldiers bivouacked less than 20 miles from Washington DC. There were a few skirmishes but no battles.

Figure 2. The study area (red circle) lies along a creek as seen in the inset photo.

The geologic map of the area identifies the rocks exposed on hilltops and along the creek (indicated in the center of Fig. 2) as being part of the Newark Supergroup. These rocks were originally deposited between 237 and 174 Ma in shallow basins defined by block-faulted mountains similar to the Basin and Range province of western North America. This was during the early stages of the breakup of the supercontinent Pangea. The sediments were thus immature, i.e., conglomerates, coarse sandstone, siltstone, etc, all mixed together in restricted basins and their deposition changing rapidly over time as the surrounding mountains eroded.

Not long after deposition in rapidly subsiding basins, magma from the upper mantle welled up and filled fractures within the crust. These diabase intrusions heated the sediments of the Newark Group and metamorphosed them by temperature. They were not deeply buried. Thus the rocks we found didn’t look that different from sandstones although they are technically metasedimentary rocks.

Figure 3. Bedding surface of quartzite rocks exposed along the creek shown in the previous figure.

The beds are relatively flat here because the rocks were never subjected to compression, so they weren’t folded. Instead, as the crust split apart, they tilted slightly along normal faults to form grabens. In Frying Pan Park, they were horizontal. Zooming in on a bed we can see how sharp the edges are.

Figure 4. Photo of creek bed, showing details of bedding and coloration.

There are a couple of details to notice: First, the beds are about six inches thick; second, the rocks show a darkening that isn’t due to surface staining, seen in the block just right of center in the photo; third, the blocks have sharp edges. The darkening is caused by “cooking” of the original sediments when igneous rocks were injected into the pile of sediments. This process is called contact metamorphism. Another effect of coming in such close proximity to magma is that the mineral grains in the sediments become more tightly cemented, producing very hard rocks. Thus the sharp, knifelike edges.

In accordance with the “Rocks and (no) Roads” ethos, I don’t break open hand samples to examine the minerals. I accept what is available because this is about amateur geology, not data collection. There were no freshly broken samples so this is the best I could do.

Figure 5. Image of the rock from the creek at 5x magnification.

The salt and pepper color is caused by organic stains having nothing to do with the mineralogy. What can be gleaned from this poor field sample is that the individual mineral grains are not rounded and none of them appear to be large, so this is an immature sandstone (greywacke) and not a conglomerate. If it were from an environment like a beach, the grains would be visibly smoother, even to the naked eye. (For example, check out orthoquartzite.)

The sediments that comprise the Newark Group collected in intermontane basins. The Sierra Nevada is an example of what the topography may have looked like when these sediments were deposited about 200 million-years ago.

Figure 6. Photo of the Sierra Nevada mountains, an example of intermontane basins filling with sediment eroded from the surrounding peaks. Note that the Newark Group sediments would have been collecting between the snow-covered peaks.

After being buried several miles (nobody knows exactly how deeply) beneath the surface for 200 million years, these rocks were exhumed when ice sheets advanced into Pennsylvania during the last ice age, which began at least 2.5 million years ago. Northern Virginia was never covered by ice, but it was within a hundred miles of an ice sheet that reached two-miles in thickness. Two miles! The result was felt far to the south, where massive seasonal floods at the leading edge of the ice would have transported very large blocks of stone down rapidly eroding valleys.

Figure 7. View looking downstream in Frying Pan Branch (See Fig. 2 for location).

This is a great picture. Four things leap out of this pastoral image of a NoVA forest: (1) The graffitied block reveals bedding much thicker than that seen in Fig. 4, suggesting a dynamic environment, possibly an alluvial fan, when these grains were washed down the sides of rising mountains (see Fig. 6) and came to their final resting place; (2) the larger blocks show a joint pattern that determined how the rocks would break down and weather when the third event occurred; (3) the rounded blocks juxtaposed on top of the jointed bedrock outcrops indicate fast-flowing water that physically eroded them and transported them some distance (less than a mile); and (4) the modern creek flows weakly over its boulder-strewn bed. This short stream, its watershed consisting of a few hilltops, didn’t transport these behemoths anywhere.

But this creek isn’t relict, it’s simply operating on a different time scale.

Figure 8. View looking downstream on a currently dry channel of Frying Pan Branch.

This boulder field, littering a dry fork of Frying Pan Branch, suggests that the smooth flow over the upstream reach shown in Fig. 3 is capable of transporting large rocks; however, it’s all relative from a geological perspective. If I were to guess, I’d say that the boulders in Fig. 8 haven’t moved in several thousand years. My reasoning is that the clear path seen in Fig. 3 suggests that there are no more large stones to roll downhill and dislodge others, like dominoes. But we never know what comes next.

A summary of the impacts of the most-recent ice age in Virginia is available at this web site. Take a look.

See you next time.