Rock Creek Park, Maryland: Early Cambrian Marine Sediments

Figure 1. This post comes from the NW end of Rock Creek Park, which begins in Washington DC. I discussed volcanic rocks and sediments there in a previous post. The star indicates home, so this is another local trip, examining some rocks we’ve seen before although by a different name. The inset map shows Lake Frank and the dam that created it. This is a flood control dam built in the sixties. The dash line shows our walking path, which followed the lakeshore a little, but climbed several ridges as we crossed a small creek that feeds the NW end of the lake.

Figure 2. Photo of Lake Frank from the dam. The main inflow (North Branch Creek) is to the right at the other end of the lake in this image. The outflow exits through the structure near the dam and continues a short distance before joining Rock Creek and flowing to DC.

Figure 3. This exposure of Sykesville Formation rocks (538-511 Ma) is tilted about 30 degrees in a westerly direction, as indicated by the yellow line. It is obscured by shadows, so I took a photo of a loose sample lying nearby (inset). The foliation is considered to be tectonic rather than original (according to Rock D). This unit is about 9000 feet thick and is uniformly conglomerate with metasedimentary clasts. As with any sedimentary rock, however, there is going to be spatial variability. These rocks are contemporary with Harpers Formation I examined at Bull Run Nature Preserve. However, the Harpers Formation contains interbedded schist (mud), quartzite (sand), and conglomerate (gravel) in a coarsening upward trend.

Figure 4. (A) Photo of loose boulder along the trail near Fig. 3, showing what looks like quartz filling a vein in a host rock that seems to contain some clasts, although quartz has penetrated pretty thoroughly. The dash lines are meant to suggest where the quartz is less massive; there are also several locations where a thin layer composed of fine-grained, gray material is found. Not a great example. (B) This is a better example, but the trees weren’t cooperating. The arrows point to irregular clasts contained in a gray matrix. However, the gray could be a weathering color.

SUMMARY

Overall, I’d say that the Sykesville Formation is pretty similar to the Harpers Formation at Bull Run Nature Preserve (~40 miles SW), including their radiometric ages. These rocks represent the erosion of highlands for more than 20 million years and, if the coarsening upward sequence seen at Bull Run NP is any indication, the source moved closer to VA and MD. This suggests that mountains were still rising during this period and hadn’t peaked yet. I’m certain that I’ll eventually encounter more of these rocks. The source of the metasedimentary clasts contained in the Sykesville Formation would have been previously buried sediments that were metamorphosed during burial and (probably) deformation.

There are several cycles visible here: (1) deposition of sediments during the Precambrian, with subsequent burial, deformation and exhumation over tens of millions of years; (2) these rocks were exposed to the atmosphere and weathered during the Cambrian period. Some particles collected on the seafloor to form the Sykesville and Harpers Formations not far from the source. These were buried and slightly metamorphosed; (3) they are now being exposed as the overlying rock has been removed by erosion.

Around and around we go …