Several of my posts in Northern Virginia have noted the presence of old canals which were part of the earliest transportation system to reach beyond the Atlantic Seaboard (e.g. The Potowmak Canal and Goose Creek canal). Today we crossed the Potomac River and followed the Chesapeake & Ohio Canal for several miles (between Locks 28 and 29), which took us along a scenic path through a metamorphic terrain in Maryland.
The cut for the tunnel (Fig. 1) reveals a dark rock with foliation oriented nearly vertical in the plane of the cut (Fig. 3) with blebs of pink and white material (Fig. 4).
These basalts originally flowed out of fissures and volcanoes, probably at the seafloor, during rifting of a continent (according to Rock-D‘s summary). The origin of magma can be correlated to tectonic regime (e.g., mid-ocean ridge, rifting continent, island arc) based on the chemical signature of the whole rock (not minerals, which are altered during metamorphosis). At any rate, the age range of the Catoctin Formation (1000-541 Ma) spans the final closing of an unnamed ocean to form a hypothesized supercontinent called Rodinia during the Grenville Orogeny between 1100 and 900 Ma — and the subsequent rifting of Rodinia, which occurred between 750 and 633 Ma. Unraveling that time discrepancy is beyond the scope of this blog.
Let’s just say that these basalts (and minor sedimentary rocks) were created during the tumultuous assembly, pleasant life, and violent breakup of a hypothesized supercontinent.
The Catoctin Formation (basalts with some sediments) were buried deeply and metamorphosed by heat and burial pressure but not compressed, at least not enough to produce folds visible at the scales of Figs. 3 through 6. This suggests that Rodinia wasn’t created by huge horizontal forces, crashing tectonic plates together like putty.
Jumping ahead in time to the present, we see these rocks still controlling the Potomac and the development of transportation during the early days of the United States.
The outcrops seen in Fig. 7 occur very near the contact between the Catoctin Formation and a Pink leuocratic metagranite (1600 – 1000 Ma) with gneissic foliation (L3 in Fig. 2).
It is important to note that the outcrop in Fig. 8 has been exposed to the vagaries of the Potomac since the Pleistocene, so its irregular surface is not unexpected. Nevertheless, the estimated orientation is radically different than that of the Catoctin Fm, which is nearly vertical (see Fig. 6). One possibility is the presence of a fault between the two rock formations, as suggested by the geologic map (Fig. 2). The fault (indicated by the nearly vertical, black line south of Site B) may extend under the alluvial sediments around the Potomac and not be visible in Maryland.
We have one last stop today, Site C (see Fig. 2 for location), where we will try and get some closure on the fate of Rodinia (pun intended).
Let’s take a look at some samples that were conveniently made available by the Maryland DOT, probably during construction of the park.
The rocks we saw today were of similar ages (determined by radiometric dating, not guesswork), and were deposited, deformed, and intruded during the Grenville Orogeny. In other words, based on my brief examination of the literature, these rocks represent the collision of continents to form Rodinia (1100-900 Ma), its long lifespan (900-750 Ma), and its breakup to form the precursor of the Modern Atlantic Ocean (Iapetus) between 750 and 633 Ma.
I’m going out on a limb here (again). It is significant that there aren’t a lot of sedimentary rocks (even metasediments) here, which means they were removed by erosion. Keep that in mind, when I add that even the oldest metamorphic rocks (the granite gneiss, L2) aren’t folded like the schists at Great Falls. These rocks were buried deeply, relative to the impact they survived, which apparently didn’t affect the deeper parts of the crust (at least not along the modern Potomac River). There probably was a lot of folding of the sedimentary rocks that covered these igneous and metamorphic rocks, but they were eroded away during the 150 million years that Rodinia was a (quasi) stable supercontinent. That’s a long time. The youngest of the Catoctin rocks (~540 Ma) were deposited just as the so-called Taconic Orogeny was starting up in modern-day New England…
It is extremely difficult to wrap our minds around such four-dimensional events playing out on such long time scales but, to make it even more fun, the mantle and core are dancing to their own tune and directly influencing everything we experience and can observe from our perilous seat atop the earth’s crust.
Just look at all those canals we dug to avoid the rocks…