Last weekend I attended the fantastic Post-Graduate field tip of the British Sedimentological Research Group (BSRG) which was led by Dr. Sarah Davies and gave a short introduction of the Jurassic rocks exposed at the coastline around Scarborough! We saw some really interesting sedimentological and tectonical features and I thought it might be a good idea to share some of them!
In general the sediments you are about to see were deposited in the Peak Trough which is part of the Cleveland Basin. During the lower and middle Jurassic the area experienced the transition from marine to continental environments (and back). Marine sediments are often very fine grained (mudstones) while continental deposits consist of more sand-rich sediments.
Here you can observe several features: The most striking is probably the colour gradient from the bottom to the top of the picture (camera case is about 15 cm long): From dark-grey/black to deep red-brown. Additionally the grain size changes from clay to sand. The more or less horizontal bedding in the lower part is interrupted in the middle of the image where the sandstone seems to have formed some kind of depression. In the lower black-grey mudstones marine fossils can be found (I guess!) while the sandstone shows sedimentary features such as wave-ripples and sheet-flood stratification.
The change in colour reflects a change from a reducing to an oxidising environment. In a reducing environment organic matter is preserved (black) while in an oxidising environment sandstones often have a reddish colour due to the formation of iron-oxide minerals. The grain-size trend shows us energy level of the depositional environment: To transport a sand grain water has to flow faster as if it transports clay-particles. These two observations, together with the way the sediments are stratified tell us that the depositional environment changed from a deep marine (several 100’s m) to a shallow marine environment (near-shore). The deposition of the (heavy) sandstone on top of the mudstones led to the deformation you can observe in the middle part of the image.
One of the most interesting features we observed were relicts of a meander belt (in Scalby Bay). A meandering river has many sinuous beds that form the characteristic “snake” form of the river. At each bend two processes occur: Erosion on the concave bank and deposition at the convex bank of the river. The sediments deposited at the convex margin are called point bars.
At the wave-cut platform along the coastline in the Scalby Bay such point bar deposits can be observed. In the picture they dip away towards the sea and from a big half-circle. Each bar once was the convex bank of a big meandering river!
Dozen of such point bars can be seen, if looking from the top of the nearby cliff line they can be observed the best! Unluckily the tide was coming in too fast so I don’t have any good pictures from above. This is also the point where I should mention that I REALLY need a new camera (to give you better pictures 😉 ). Detailed work on the meander belt channels has been done as part of a PhD project about 20 years ago (paper can be found here). In the fluvial sediments of the meander belt several dinosaur footprints have been found (beautiful examples can be observed in this museum). And I found this footprint:
Yes, I agree, some imagination is needed to see the footprint. However, we are pretty sure it is one! Most likely the “true” footprint was several layers higher in the sedimentary sequence (1-2 cm) so what we observe here is the imprint of a footprint! For the ones with a bad imagination, this is what I am talking about:
While the tide was rising to fast to see the meander from above, we had a similar problem the next day when the tide did not fall fast enough when we visited the Robin Hood’s Bay/Ravenscar. In this area there are two interesting tectonical features: The Peak Fault which forms the western limit of the Peak Trough and doming within the Robin Hood’s Bay.
The Peak Fault is best observed in the wave-cut platform but as we were a bit too early in the morning (yep, some geologist rise early!) we could not see the platform perfectly. However, at the cliff face you can see some indications for syn-sedimentary activity of the fault: The sandstone bed in the middle of the cliff face thickens towards the right (where the actual fault is located, thickening is indicated in black). This means that the movement of the fault created additional space for sand to be deposited. Another fault-related feature is the down-wards rolling of all three “main-beds” (indicated in red) towards the fault. Movement along a fault often results in a drag of nearby beds, this is what can be observed here!
From these two obervations we can infer two directions of moving: During the deposition of the sandstone in the middle the section we are looking at was moved down in respect to the other side of the fault. The direction of drag indicates that the movement has been revered at one point and that the section was moved up.
On the first look it seems like just a normal coast line, but have closer look at the dipping and structure of the wave-cut platform emerging from the sea (tide is just falling). You can see that it forms a circular structure (of which only part is visible on the picture above). If you look at a satellite image (thanks Google Earth!) it becomes clearer (in red is the Peak Fault and in white a possible centre of the doming-structure):
The circular structure is a dome, an area of uplift. The reasons for the uplift are unknown, maybe the nearby Peak Fault plays a role or there is some salt below the brown-black rocks (?).
The rocks are black to grey mudstones and contain marine fossils. Within the layering a cyclicity can be observed. The dipping surfaces that can be observed in the wave-cut platform (see above) are strongly bioturbated and less prone to erosion than the less bioturbated mudstones in between. If I remember correctly there were some sandstones as well, indicating that the depositional environment changed from deep to shallow marine. Here a picture of a small (1 cm across) ammonite:
By the way, I did NOT take extensive notes during the trip, so many interpretations are based on what I remember and from the pictures I took. This means there are probably some errors ;-). Hope you enjoyed this entry. Thanks to everyone who mad this trip so much fun!