Some Old Red Sandstone questions answered, and a diversion into Cretaceous chalk

In 2014 an "Old Red Sandstone symposium" was held in Brecon, under the auspices of the Fforest Fawr Geopark. Much to my regret I couldn't get to it as I was chairing an OUGS committee meeting so I jumped at the chance to hear Prof Brian Williams (who co-led our Gaspe trip in 2012) repeat his lecture at an OUGS Oxford branch meeting in Reading on 6th September.

The diversion

To get to Reading from Hereford takes a couple of hours: an opportunity to see places that are a bit far to justify visiting otherwise, so Jan and I thought we'd make a day of it, with a picnic lunch and settled on the Uffington White Horse as a target. I'd seen signposts to this, and to nearby Wayland's Smithy, many times on the way between Bristol and Milton Keynes but never had time to divert. 

Heading from the car park up towards Uffington Castle and White Horse Hill - can you spot the horse?

Numerous seedheads like this - around 5 cm across - I wonder if it is Goats Beard?

The horse is a Bronze Age hillside figure formed around 3000 years ago by turves being cut out to expose the chalk. It has had to be refurbished regularly to maintain its whiteness and this had just been done a week or so ago. Walking up to it from the car park was probably more a matter of having a goal for a post-lunch stroll since it can actually be seen better from below!

The Iron Age hill fort of Uffington Castle is just to the west of the white horse, and of course we had to visit the 257 m high summit, topped off by a trig point!

The horse is roped off, with signs asking people not to walk on it. Only portions can been seen from above

After visiting the Iron Age fort we walked across to the Horse which is roped off, and had recently been whitened with fresh chalk. The hill's summit is in the Upper Cretaceous (Cenomanian/Maastrichtian) White Chalk Subgroup. There's a strange hill just to the north - Dragon Hill - which looks rather artificial although said to be natural (with a bit of improvement). Slightly lower, this is the older Cenomanian Grey Chalk Subgroup which is "dirtier" in colour and lacks the flints of the White Chalk.

The Manger, a combe to the north of the White Horse

The north-east facing slope of the combe has an oddly gullied topography - I'd be interested to know more about this!
There's some general information on the area in an OUGS Wessex Footnotes newsletter 

Walking back down it was tempting to keep looking back in the hopes of a better view of the horse!

The rather dilapidated information board near the bottom of the hill

We rounded off the diversion with a "99" ice cream and then went on to Reading, meeting up first with Jan's brother-in-law and his wife, and then the Oxford Branch committee, at the Black Boy in Shinfield where they were giving Brian dinner before going on to the University for his lecture.

Back to the ORS

After the scampi and chips we went to the University's Chemistry building for the lecture - a really nice venue with space for the Branch library in a common room area just inside the door, and a nice lecture theatre around the corner from that. 

Brian opened the lecture by showing us a "Palaeo-satellite image" of the ORS continent. The ORS was deposited from top Silurian into the Lower Devonian but sedimentation was then interrupted by the Acadian orogeny moving northwards from south of this area.

"Palaeo-satellite image" of the Old Red Sandstone continent (Paul Wright)

Researchers have identified similarities and differences between rocks of the ORS Magnafacies on the southern edges of this continent, looking at exposures in Canada, Ireland and Wales. Notably, drainage was southwards in all areas, and they all display a similar upward coarsening pattern of sedimentation in response to the approaching Acadian deformation in the Mid/Late Emsian. In Canada there is 3.5 km of sediment from the Battery Point Fm up to the Malbaie Fm. In Ireland 2.5 km from Bulls Head Fm to Trabeg Fm and in Wales 3.5 km between the Milford Haven Fm and the Ridgeway conglomerate.

In Wales the sediments in the west, particularly Pembrokeshire, are more deformed as there is a Variscan overprint. This complicates drainage study as flow directions/lineations have to be corrected for plunge and dip of folds.

Brian went on to detail a number of Lower ORS features:

1) The lithography is actually around 70% mudrock in Pembrokeshire

2) There is a tuff horizon, pyroclastic flow and fall deposits, which stretches from Pembrokeshire to Ludlow.

3) Calcretes, which indicate basing shutdown, are similarly widespread. The basin was starved of sediment and soils developed.

4) Where there is sandstone it may indicate ephemeral events, such as flash floods, in some places but perennial "big river" systems in others.

5) Right at the top there are monster rivers flowing through rifted valleys, with fans on the slopes at the sides. Seen in Gaspe, also Ireland, and right through to Portishead.

Brian's original lecture was twice as long so we, necessarily, had an abbreviated version.

He discussed the mudrocks in some detail, illustrating points with examples from the Moor Cliffs Fm near Manorbier in Pembrokeshire. This is 70% mudrock with only minor sands and none of the classic fining upwards sequences.
There may be a variety of mudrocks: those originally soils, some pelleted, some heterolithic and some subaqueous but underlying this is the question of how you produce so much mud in the first place! Paul Wright's suggestion of pelleted mud being transported is interesting but studies have shown that the pellets disappear on compaction and lithification.
There appears to be some bioturbation, and also large plant roots but these are apparently giant fungi (Hillier, Edwards, Morrisey, 2008)

After we had been out for the day with John Davies a couple of weeks ago Jan and I were left with a question at the back of our minds: how did he correlate strata between Pembrokeshire and Herefordshire? Brian shed light on this question when he went on to describe two extensive marker horizons, the Townsend Tuff, and a series of calcretes.

The Townsend Tuff Bed (Marriott 2009)

The Townsend Tuff records three events of ash generation (Falls A, B and C), erupted from a centre some 100 miles to the SW. It provides a precise and dateable marker at 420.03 Ma (Ludlow - top Silurian). Sedimentary structures such as shallow water ripples from waves on a lake indicate the ash is likely to have been deposited in shallow water on an alluvial plain.

The calcretes occur 90 m above the ash beds and indicate a period when the sediment supply was cut off "basin shut down". In the absence of sedimentation, calcretes developed along with features such as the bowl-shaped "gilgai structures". (Love and Williams, 2000) Some, e.g. the Chapel Point calcrete, are 25 m thick. The question here is where the supply of calcium carbonate came from. It might be loess - wind-blown dust. Triassic loessite is being studied, but it would be hard to prove that wind blown dust is the source of these calcretes at the Silurian-Devonian boundary (c 417 MA). Again the calcretes are extensive, from Pembrokeshire in west Wales to Portishead in the east indicating widespread basin shut-down at this time (Williams and Hillier 2004).

After this came the sandstones and appearance of fish! Brian gave the Freshwater West Fm as an example and mentioned Tredonning Quarry as a place where there are "Eoarthropleurid" (beaconites to you and me) burrows. This Lower Devonian was a time of big rivers (25 km wide flood plains), flowing along fault bounded troughs with fans developing on the scarps at the sides. There is 900 m of sediments in Dingle, 2 km in Wales, the New Shipping Fm, the Woodhill Bay Conglomerate, Llanishen conglomerate etc.

A dryland river environment, based on the Freshwater West Fm (Marriott and Hillier, 2014)

Perennial river environment based on multi-story sandstones. The raised water table allows groundwater calcretes to develop. (Marriott and Hillier, 2014)

Alluvial fan and axial river system, based on Ridgway Conglomerate Fm (Marriott and Hillier, 2014)

Fans from the south built down syntectonically across the W-E Bristol Channel fault but what fascinated me was that, although Brian discussed these trough axial "big rivers" with reference to southern Ireland and Wales, he had shown us something very similar in Gaspe, also west-east flowing and with fans on the bounding scarps.

Lots of papers to read! And John's trip for OUGS to look at the ORS in the Brecon Beacons in October to look forward to! I certainly feel much happier now I know about the two big marker horizons!