Virtual Field Trip from virtual-geology.info

Locality 4.1 - Amroth East and adjacent areas

It is important that you access this field trip on a laptop or desktop PC. Click on any image to enlarge it.

At this location, we will study some interesting sections through Westphalian (Upper Carboniferous) strata. Explore the area in Google Maps.

A. General view of the Westphalian strata at Saundersfoot, west of Amroth. The Amroth and Telpyn Point sections are in the background.

There is excellent exposure of the Coal Measures along the coast for several km from east of Amroth towards Tenby. Some of it shows complex Variscan folding and thrusting, but there are good, relatively undeformed sections on the long limbs of major folds.

The aim of visiting this location is to become familiar with Upper Carboniferous sedimentary sequences, in preparation for the exercise at Location 4.2. As always, you should collect and synthesize information of a range of different types - lithology, thickness and geometry of sedimentary units, sedimentary structures and palaeocurrents, fossils and trace fossils (or absence thereof), lateral and vertical changes etc. From these observations, you should be able to attempt interpretations of the processes and environments involved in the formation of the section. You have seen a similar section in a practical earlier this semester.

Identifying and interpreting sedimentary structures and features is an important part of this exercise.

Handout: refer to the handout for today, which contains essential diagrams and information. Don't forget to use the information in the Field Guide too.

Tasks: See today's handout, and the photo captions.

I thank Huw Williams and Paul Davies of Reservoir Geology Consultants for their company during many visits to these and other sections along the Pembrokeshire coast. I have learned a lot!

Field evidence

To complete the task at this location, carefully study the photographs below of outcrops and samples. Describe them, and use them as evidence on which to base your interpretations. Pay particular attention to field relationships. Some of the close-up photos are from other nearby sections in the Westphalian of Pembrokeshire.

Take a virtual 3D tour of these outcrops on the eRocK virtual field trip by Adam Cawood and Clare Bond at the University of Aberdeen.

B. The section viewed from the storm beach at the east end of Amroth beach. The cliff is approximately 20m high.	C. A closer view of the 20m thick section. The geologist is sitting near the top of a sandstone unit. Try to estimate grain-size changes in the cliff section - this can be approximated from colour changes. Sandstones tend to be lighter, but some are iron-stained. Describe any bedding or sedimentary structures.	D. View to the right of Photo C. This may help to determine grain-size changes.	E. The mudstones within this section often contain organic material, similar to that seen in these core samples.
F. What is the black unit about halfway up the section?	G. We are now going to focus on these bedding planes in the sandstones low down on the right of Photo F. The palaeocurrent exercise below uses measurements from this surface.	H. A lab sample of a similar structure. This is rib-and-furrow (trough cross-lamination). What size and shape of bedforms does it represent? Look carefully: can you pick out individual troughs? There is enough 3D relief that foresets within those troughs can be seen to dip to the right. So which way was the current flowing?	I. The same structure seen on the bedding surface in Photo G. Trough boundaries and palaeocurrent directions have been marked. Flow regime?
J. The sharp bases of some thin sandstone beds show this trace fossil, Pelecypodichnus. We also have a worm's eye view of rib and furrow. What biological activity does the trace fossil represent? Why do these structures appear to be aligned?	K. Let's move higher up the cliff, and pay attention to the iron-stained, rather structureless yellowish unit below the black bed.	L. View down onto a bedding surface in a similar unit to Photo K. Colour mottling is present. The dark rusty objects are made of siderite. What do their shapes suggest? What can we deduce about conditions after deposition of this unit?	M. View down onto a bedding surface in a similar unit to Photo K. Colour mottling is again present. What is the dark structure running left to right across the photo? Can you see smaller dark material running at right angles to it? These structures are preserved in a thin film of carbon, representing organic material.
N. This cylindrical sand-filled structure has a thin coating of coaly material around it (not seen in the photo). It is perpendicular to bedding. It is closely related to the dark structure in Photo M. - they both belong to a similar organism. Thanks to Huw Williams for pointing these out.	O. Several objects like the one in Photo N have been extracted and used as mooring posts in Saundersfoot Harbour. Applied geology in action! Thanks again, Huw.	P. A better preserved sample of part of one of the structures shown in Photos N and O.
Q. Next question: what is the black unit likely to be? Might it be related to what we saw in Photos K to P? Under what conditions did it form? To what extent might its thickness have changed during burial, compared with other lithologies in the section?	R. Closer view of the black unit and the units immediately below and above it. Describe the base of the overlying sandstone unit.	S. What structures can you see in this thick sandstone? What sedimentary environment might the sandstone represent?

Palaeocurrent data set, measured from rib-and-furrow seen in plan view in Photos G and I (data collected 1993). Use the palaeocurrent data sheet supplied in your base maps file. Since the bedding surface dips gently we do not need to make any recalculations for tectonic dip.

Plot these data on rose diagram paper. Calculate the vector mean and vector strength, and record these in your virtual notebook. What can we conclude from these results?

Interpretation

Based on all the evidence we've looked at, it's now time for an overall interpretation of the section, in terms of changing processes, conditions and environments through time. What was the overall depositional setting?

Just for fun...

• There's a lot of wildlife to see on the coast. Take a break from the geology, and look for signs of life on the shore. Information leaflets from the National Park will help you to identify coastal wildlife.

Next locality

Make sure you've completed all the work for this locality. Now we can walk back along the storm beach, get back on the virtual coach and head off to our next stop, a coastal traverse from Marros Sands back to Amroth.

This page is maintained by Roger Suthren. Last updated 19 May, 2021 2:33 PM . All images © Roger Suthren unless otherwise stated. Images may be re-used for non-commercial purposes.