3 September 2019
The last outcrop I visited in Newfoundland this summer was an angular unconformity, exposed along the shore at a place called Bacon Cove, near the head of Conception Bay:
Here, I’ve traced out the unconformity surface in yellow:
Unconformities are gaps in the geologic record – structures which juxtapose two different geologic units along a surface formed through an extended period with no rock-forming activities at that site or (more commonly) erosion, which destroys rocks at the site. At Bacon Cove, erosion was the culprit. First a green shale formed, then it was tilted to a steep angle, then it was eroded, probably along a rocky coastline much like Bacon Cove appears today, and then it was buried in dark limy sediment, sand and pebbles and mud.
Here is another view of the unconformity where the different orientations of the strata are plain:
There are additional aspects of this site that are intriguing: note the differential weathering of the one layer in the upper unit, weathering out to form a series of little hollows. Also note the prominent joint set that transects both units, indicating it was imposed after both units existed and were lithified.
One nice thing about Bacon Cove is how high-relief the unconformity surface is: there are lots of swales and bumps on it.
It’s also kind of patchy: there were spots where ancient “potholes” had been filled in with the upper limestone, and were preserved today as elliptical patches of the younger unit. Here is a spot where erosion making the modern rock surface has penetrated through to the unconformity and into the lower unit, breaking the upper unit into several patches:
Here is a place where the upper unit is particularly sandy:
There were also boulders lying around that crossed the unconformity surface – what an amazing sample this would be to have in one’s rock garden! Earlier in the trip, I was impressed by several of these on display at the Johnson Geo Centre in St. John’s.
Here are a few more shots of in situ unconformity exposures, where I’ve cleaned up the unsightly coring holes using Photoshop:
I like these because they show the presence of substantial pebbles/cobbles as inclusions in the overlying unit. In this next shot, those all appear to be locally derived examples of the older greenish shale:
The lower (older) unit here is Ediacaran in age, and is correlated with the fossil-bearing rocks at Mistaken Point. The upper (younger) unit here is Cambrian. So in the grand sweep of geologic time, there’s not a huge amount of time missing at Bacon Cove. But it was enough to take horizontally-deposited mud, turn it to rock (lithify it) , and then rotate it and lift it up to where it could be partially eroded, before it was re-submerged and buried anew.