Carbonate ramp at Three Cliffs Bay

OUGS Severnside trip Sunday 11 June 2017

Carboniferous Carbonate ramp at Three Cliffs Bay, Gower

Gower is a good way from Hereford so we went down on Saturday and, after lunch with friends, headed onto the campsite. Saturday weather wasn't good. Fingers crossed for Sunday! The one consolation was that the shower block at the campsite was amazing - individual shower rooms with loo, basin and underfloor heating, and loads of hooks to hang your stuff on! A ridge of land on the seaward side gives a bit of shelter from the strong winds and a wander over this found it is made of resistant Devonian quartz conglomerate.
We headed up the lane into Penmaen in the morning and met up with the rest of the OUGS group in the car park there where our leader, Paul, told us something of what we were going to see before we headed down another lane towards the beach.

Professor Paul Wright making some preliminary explanations, Jan AJ taking it all in, and Andy Mitchell writing it all down for the newsletter report!

We stopped off on the way for a bit of archaeology, the dolmen on Penmaen Burrows: rather ruinous but enough of it can be seen to show that it too is made of the quartz conglomerate which forms the ridge running WNW - ESE from Reynoldstown to Penmaen.

The dolmen on Penmaen Burrows with Libby for scale

Mainly quartz conglomerate, but with occasional rogue clasts like this red one

The last half kilometer is down a sandy path over the dunes (easy down, especially as the gorse roots have thoughtfully been cut so's not to trip you up!) but the ascending the loose, deep sand is going to be a treat to look forward to later on.

The path down through the dunes, lots of little blue pimpernels which I'd seen in Spain but didn't remember seeing in the UK

Looking up Pennard Pill, from the foot of the Burrows, towards the ruins of Pennard Castle

Three Cliffs Bay from Penmaen Burrows

So, having got down to the beach, on to the geology!

The aim of the visit to this locality was to study the early Carboniferous ramp succession deposited during a "Greenhouse" interval when 3rd order sea level changes were taking place. We began at the outer ramp, working our way through oolite shoals to lowstand features including palaeokarst and palaeosols.

The first place we stopped was just above the Lower Limestone Shales (which aren't usually exposed here, but the beach was particularly low). The LLS overlie the Old Red Sandstone and they, and all the other strata we saw, dip steeply to the south making it remarkably easy to look at the entire section as we walked down the beach.

Stop 1, Lower Limestone Shales on the right, and Shipway Limestone behind us on the left

The Shipway Limestone is outer ramp facies, deposited at the start of a transgressive sequence tract.

Shipway Limestone on the right (north), looking south along the cliff exposure we investigated.

Paul gave us some excellent impromptu mini-tutorials - this was about the Early Carboniferous palaeogeography, using a map by Ron Blakey

In the Early Carboniferous we were on the southern side of St George's Land, looking across the Rheic Ocean to the rising Variscan mountains which would eventually reach around 4000 m height. When the Lower Limestone Shale was deposited though it was deep water muds and cherts 9they extend under the coalfield all the way to the north crop.
Then, as the sea shallowed, the younger sequence was deposited on top as a carbonate ramp, sloping very gently south, probably with some distal steepening

Cross stratification in the Shipway Limestone, part of Hummocky cross-stratification showing we were above storm wave base but below fair weather wave base in this distal section of the ramp

Shipway Limestone cross stratification and escape burrows

Another mini-tutorial while people investigated the  partially dolomitised Tears Point Limestone and the interfingered, shallower water, beds of oolite.

Not sure why but the oolite (above) seems to have more calcite veins in it than the Tears Point Limestone

Massive limestones were deposited in deeper water intervals, and oolite in shallower shoals when sea level receded., Libby is intrigued by the Milankovitch eccentricity cycle explanation for 120 ka sea level change cycle.

Quaternary raised beach above a faulted section which repeats the Tears Point Limestone

The Gully

At the Gully, there's evidence of a major drop in sea level, rather than the oscillations that had produced interfingered deep water limestone with shallower oolite. On the right is a karst surface (i.e. subaerial): this is "mammillated" - evidence of vegetation where water dripping off trees resulted in differential cement chemistry and weathering (a feature described in the Bahamas as "banana holes" )
In the Gully itself is the Gully Oolite, part of a transgressive sequence as shoals and then a barrier island built landwards over the karst. Paul explained the planar nature of the beds as resulting from cannibalisation of the barrier island as the water deepened and the sediments were reworked and the island moved inland as the sequence retrograded.
The High Tor Limestone on the left includes a basal transgression conglomerate.
A fascinating section, though the very strong winds, accompanied by rain showers, made it less appealing to study at length so we retreated up the beach and found a more sheltered spot for lunch before trudging back up the steep sand dunes to Penmaen!

Lunch back by the Lower Limestone Shale

Thanks to Jan Ashton-Jones for persuading Paul to lead this trip, and also for arranging a follow up lecture later in the year at our November "Day of Lectures".