Special thanks to field trip leaders Alain Tremblay and Francine Robert
Last month the Canadian Tectonics Group (CTG) held their annual meeting at Charlevoix, Quebec, the site of a Devonian [Lemiux et al. 2003] impact structure. The field trip portion of the meeting centered around learning about and seeing impact structures in outcrop.
This post feature some impact structures we observed while cutting a transect (Figure 2) from the center of the impact crater (mylolisthenite) to the crater rim (normal fault with backthrusting).
Setting and background
The impact structure, 54Km in diameter [“Charlevoix”], is approximately half-exposed. The other half is under the St. Laurent River (Figure 1,2,3). The crater straddles the cystalline Grenville province, the Cambrian-Ordovician sediments, and accreted Appalachian Orogen. Supra-crustal faults make up the impact cratering. Major fault systems trend Northwest and Northeast, consisting largely of normal faults. Polymictic breccias provide the best evidence form impact, though other impact related rock types are present (cataclastic gouge, pseudotachylite, shatter cones) [Lemiux et al. 2003].
Shattercones. 19T 408942.00 m E, 5263137.00 m N
At this stop we observed shattercones on many scales in limestone and mudstone. The lineations on a shatter cone point in the direction of impact. Basically a shock wave travels through the rock, creating a network of fine fractures, often arranged in a conical shape (Figure 4).
Along the railroad tracks, heading back to the parking lot, shattercones could be observed in coarser grained crystalline rock. This was a great opportunity to show how much better formed shattercones are in fine grained rock vs. coarse grained.
Mylolisthenite. 19T 407510.00 m E, 5260539.00 m N
What, you’ve never heard of mylolisthenite? The term mylolisthenite is used to distinguish a specific type of breccia. It is pale grey to green (Figure 5), fine grained, breccia contained various clasts within the non-fused matrix (differentiating it from pseudotachylite), even though melt fragments can be found within the rock [Rondot, 1989].
Here we are in the inner ring of the crater, beside the center peak. Our location during impact is within what is known as the transient bowl, just below the impact surface.
Soft-Sediment Deformation. 19T 409437.00 m E, 5267768.00 m N
Here we saw some crazy sediment deformation on various scales (Figures 6, 7). This outcrop was previously interpreted as an undersea debris flow. I don’t think the group came to a consensus on whether this was from gravity slope slumping or a result of impact. We did agree that the formation was VERY wet during deformation.
Breccia. 19T 410858.00 m E, 5271299.00 m N
This area is near a large open fold, presumably related to impact. This breccia could be of tectonic (St. Laurent fault) or impact origin (Figure 8).
Normal fault with backthrusting. 19T 414296.00 m E, 5275355.00 m N
Okay, I know that sounds crazy… but hear me out. First off, I tried to stitch together a panorama of this whole outcrop that I could annotate, but Photoshop was giving me trouble stitching it together. So instead he’s some pictures.
Two hypothesis were presented for this outcrop. The first being that these quartzite beds were thrust up onto the crystalline basement. The second being that these are drag folds (Figure 9) associated with normal motion of the crater collapse. I support the normal fault hypothesis, here’s why.
When a crater impacts, a transient bowl is created. This bowl then relaxes, collapsing downward, creating normal faults along the crater rim.
What about the backthrusting and folding? Typically when normal faults are observed, they are associated with an extensional stress regime, and are the result of accommodating this extension. Here though, we have a semi-sphere bounded area. Extension isn’t being accommodated, collapse is. So even though we have normal motion at our crater margins, there is local shortening as this material collapses into the crater (Figure 12), giving rise to the backthrust and open folds (Figures 9, 10, 11) observed at this outcrop.
Rondot, Jehan. “Pseudotachylite and mylolisthenite.” Meteoritics. 24. (1989): p320.