Here is an old friend. The Waterfall Conglomerate, from the Jurassic age Waterfall Formation in the Culpeper Basin, Virginia
Aside from being charming and beautiful, this rock has a very interesting personality as well as an interesting source area. The discovery of this unit in outcrop last spring, was a major achievement for myself and my partner in regional geology, Aaron. This is a charismatic rock which we had seen as float, but never in out crop. As a goal in our study of regional geology, we challenged ourselves to find it in situ. We finally found this elusive conglomerate after some reading and exploring. When we took a closer look and cut a few pieces open, it raised many more questions than it answered. This rock launched me and the geology super-crew (me and Aaron) into an ongoing search through several miles of section, stacks of literature and millions of years of geologic time to find the source area.
But before blitzing into a discussion of source area, which is heavy with regional geology jargon, I’m going to first attempt to provide a little bit on the geologic history and regional geology.
A bit about the Culpeper Basin:
On the surface, the Culpeper Basin is recognizable as an elongate, NE-SW trending, mostly flat region, located in the Piedmont province of Virginia. The southern extent of the basin is somewhere around Culpeper, Virginia and continues northward into Maryland.
Here is a Google map image on which I drew the approximate location of the Culpeper Basin.
Geologically, the Culpeper Basin is an ancient rift basin, a down-slumping block bound by normal faults on both sides. This basin formed in the Mesozoic period as the first phases of rifting which took place during the break up of Pangaea.
During this period, extensional forces on the Earth’s crust tore giant rips across the continent. These rift basins spread to the point at which continental flood basalts spilled out onto the surface, but not far enough that they allowed the admittance of seawater.
Here is a general cross-section of Culpeper Basin. By C.M. Bailey of William and Mary.
Here is a view of the 5 geophysical provinces of Virginia. From William and Mary’s website The Geology of Virginia.
And here is a Google Earth image that I created using the USGS Google Earth overlay for the state of Virginia. This has pretty much been an indispensable tool in my campaign of local geologizing. The white star located on the edge of the Blue Ridge Province (Blue Ridge) and Piedmont Province (Piedmont) marks the approximate location of the Waterfall Conglomerate.
Before we move on, there are just two more diagrams that I feel obligated to put up.
One is this general cross-section of the Blue Ridge from James Madison University’s website The Geological Evolution of Virginia and the Mid-Atlantic Region. I included this to illustrate how the Blue Ridge is an up-folded anticlinal structure.
I think this is a cross-section from the central section of Blue Ridge and does not show the Culpeper Basin. However, it is a region with in the Piedmont Province, and in Northern Virginia rests against the division between the Blue Ridge and Piedmont.
The next graphic is the same Google earth image from before, except this time it is made to correspond with this above cross-section diagram.
The Waterfall Conglomerate is a western border conglomerate and the youngest unit in the Culpeper Basin, it has been interpreted as alluvial fan deposits also known as a fanglomerate. The Waterfall Conglomerate is not laterally continuous across the basin margin, rather there are several different fan conglomerates along the western border.
The source area for these Triassic-Jurassic border conglomerates is interpreted to be primarily from the older rocks to the West, mostly from the Blue Ridge province (Lindholm, 1979).
So we would be expecting to find (listed in order of proximity).
- Quartzite and other metasediments from the Chilhowee Group
- Metabasalt from the Catoctin Formation
- Igneous and metmorphic rocks from the Basement complex
- A few pieces of the Valley & Ridge thrown in.
So here is the rock once again, but cut open this time to reveal the internal mysteries.
What a beauty! A lovely conglomerate containing a lot of different clast types. Among the more interesting to me are the finely laminated red and grey/black calcareous/ fine-grained siliclastics, and the unidentified pale-pink and off-white mystery-clast in the upper right corner that fizzes in hcl. Actually everything here fizzes…everything.
What a minute… What ?
I don’t see anything in there that looks like pieces of the Blue Ridge. The next calcareous unit moving westward from here is the Valley and Ridge. But I don’t think that the dominant source area for this rock is from the west at all. Here’s why.
1. Lack of Chilhowee clasts.
The first thing that jumped out at me once I cut this rock open was the lack of any quartzite chunks. The next unit immediately to the west of this location is the Chilhowee group, a “tough” package of rocks, made of quartzite and other resistant sedimentary rocks which stand topographically higher than the surrounding lithologies. If this conglomerate was brought from a western source area, wouldn’t we expect to find pieces of its nearest westward neighbor?
A similar western border conglomerate, the Leesburg Conglomerate is to the north and has quartzite clasts derived from the Chillhowee group, but there are little to none at this location in the Waterfall Conglomerate.
2. Is that Catoctin?
This conglomerate has beautiful chunks of basalt in it, but where are they from? The relatively unaltered nature of the basalt clasts strikes me as odd. Sure, I haven’t seen all of the Catoctin in existence, but I’ve seen a fair bit. I know from field experience that almost all of the Catoctin around here is really easy to spot as it has the characteristic green color. I think all of the Catoctin I’ve seen out on the “flanks” of the anticline was green and clearly altered. The only place I’ve seen really black Catoctin, is in the feeder dikes intruding into basement rocks exposed along skyline drive. So it seems mysterious to me, that if these are in fact Catoctin, how they escaped metamorphic alteration received by everything else in the local source area.
3. What about the red and grey/black finely laminated calcareous siltstone/mudstone clasts?
Here is a closer picture at the original image of the cut sample. Apologies for lack of scale. Ball park, the laminated clast on the bottom left is about as wide as my thumbnail and just a little bit longer.
This clast type is very common in this conglomerate accounting for I’d estimate more than 50 percent in this sample. The problem I find is that if we look westward for a source area, we must go all the way to the Valley and Ridge to find carbonate clasts. And we need to go deep into the Valley and Ridge, the first couple units adjacent to the Blue Ridge like the Tomstown Dolomite and the Beakmantown are both dolomitic and in outcrop look nothing like these clasts in the conglomerate. Although I have less personal experience in the Valley and Ridge than in the Blue Ridge, I don’t know if I’ve ever seen anything in outcrop in either province that looked like these finely laminated clasts.
Perhaps this is a matter of an inconsistency between my own current regional picture and the actual regional picture during the deposition of this conglomerate… A lot has changed since the Jurassic Period. There is also a lot I still don’t fully understand about the sequence of events in East Coast Geology. I am not yet an expert at mentally reconstructing ancient environments at a given point in geologic time and then pressing the “play” button to watch things unfold. I work with more or less a growing set of snapshots, that continue to build the larger narrative for me.
That being said, I still think there is something odd about these clasts.
Maybe the source area is not from the west, but instead from the east. Or I should say rather, intrabasinal instead of extrabasinal. The red and black/grey laminated clasts seem to resemble the fine-grained siliclastics described lower in the strat column. With in the Waterfall formation there is laminated lacustrine sediments (Gore, 1988). These lacustrine sequences record the history of a 40 million year lake during the early Jurassic Period, which was located on or adjacent to, the western boundary of the Culpeper Basin (Hentz 1985). I think that some of these clasts are intrabasinal lacustrine sediments which were reworked and deposited during periods of shoreline fluctuations. If these are who I think they are, then the tiny laminations record cyclical changes in sedimentation in the ancient lake.
An intrabasinal source area would also explain the presence of the unaltered black basalt. In the western Culpeper Basin in the Buckland Formation there are several basalt flows which seem to have been a part of the source area for this rock (Lindholm 1979).
This could also explain the identity of the strange mystery clasts. Remember these?(The pink and off white clasts in the upper right corner)
I think these could be related to the lacustrine calcareous fine siliclastics and basalt occurring right next to each other. Perhaps these could be thermally altered calcareous lime mud. They fizz just like everything else and have a strange crystalline type structure. This is sort of a leap for me, and pushes me right to the limit of my geologic understanding on the topic. I don’t really know what these are and cant find anything in my research that really nails it down. If anyone with more experience in this area has any ideas I’d be very interested to hear them. With out thin sections it is hard for me to say.
Here are a few more pictures of the mystery clasts.
As an undergraduate geology student and enthusiast of local geology, I can say that this rock unit did more for my understanding of regional geology (especially the Mesozoic) than almost any other single unit. For several reasons. 1 This rock is a conglomerate on the border of 2 geophysical regions in Virginia and potentially contains pieces of both of them. So I learned a lot about a broad geographic and geologic range. And 2) Because (most) sedimentary rocks are made from the broken down and reworked material of other rocks, I think there is a natural mystery that begs to be solved in each one. I’m still not all the way sure about everything in this rock, but I’ve learned a lot in the process that has led me here.
Gore, P.J.W.(1988) Lacustrine sequences in an early Mesozoic rift basin: Culpeper Basin, Virginia, USA. Geological Society of London Special Publications. 40,247-278
Gore, P.J.W. (1988) Toward a Model for Open and Closed Basin Deposition in Ancient Lacustrine Sequences: The Newark Supergroup (Triassic-Jurassic), Eastern North America. Paleogeography,Paleoclimatology,Paleoecology, 70, 29-51
Hentz, T.F. (1985). Early Jurassic sedimentation of a rift-valley lake : Culpeper Basin, Virginia. Bulletin of the Geological Society of America,96,92-107
Lindholm, R.C. (1979). Geologic history and stratigraphy of the Triassic-Jurassic CCulpeper Basin, Virginia. Buttetin of the Geological Society of America,90,995-997, 1702-1736
Lindholm, R.C. Hazlett, J.M. (1979). Petrology of Triassic-Jurassic Conglomerates in the Culpeper Basin, Virginia. Journal of Sedimentary Petrology, v49, n4, 1245-1262