Severn estuary heavy minerals support Irish Sea Glacier thesis

With a certain amount of generous help (and not revealing my sources) I have now been able to see some scans of pages from a Bristol University doctorate thesis from 1974.  The results of the doctorate research were never published, because the author obtained a job immediately after its completion and never, thereafter, managed to find the time to write things up.  At the base of the page I reproduce an OCR document, with many corrections because parts of the scans are indecipherable.  Please forgive the inevitable errors, but this is the best I can do.......

The most interesting point to come out of all this is that the heavy minerals do not appear to have come from secondary or sedimentary rocks in the immediate vicinity of the Severn Estuary, so they must have come from primary sources either in North Wales or in the North of Scotland.  So they must have been carried in sediments by the Irish Sea Glacier as it flowed southwards initially and then broadly eastwards up the Bristol Channel and into the Severn estuary. This is of course confirms what many geologists and geomorphologists have been saying for many years.  Dr Griffiths suggests that the "immature heavy mineral suite" and the relative abundance of the heavy minerals in question, accords with Kellaway's assumptions about ice movement -- and this also supports his assertion that there was a distinct ice stream within the glacier which carried erratic material from Scotland, Northern Ireland and North Wales.

This brought to mind the work of CB Crampton on the heavy minerals in the glacial and other deposits of the Vale of Glamorgan, in which he concluded that there were various heavy mineral suites which bore little resemblance to those of the underlying rocks, and which must have been carried either by southward-flowing Welsh ice or by eastward-flowing Irish Sea ice..


Reports and transactions (Cardiff Naturalists' Society), 1900-1981 -
Vol. XCI 1961-63
CB Crampton: Certain aspects of soils developed on calcareous parent materials in south Wales
http://welshjournals.llgc.org.uk/browse/viewpage/llgc-id:1373290/llgc-id:1379464/llgc-id:1379470/get650

 In my mind this confirms that Irish Sea ice must not just have affected the coasts of Somerset, Devon and Cornwall, but that it must have reached Mendip and the Bristol area as well.  The heavy mineral assemblage studied by Elwyn Griffiths cannot in its entirety have been transported upstream from the Bristol Channel into the Severn Estuary by tidal streams, since the prevailing flow and direction of sediment transport in this sort of situation has to be downstream, towards the open sea.

Now I would like to know how this research ties in with the work on Flat Holm a couple of years ago.  How many of those pebbles collected from the beaches came from Scotland and North Wales?  And what does the evidence tell us about erratic comminution over distance and over time?

The evidence accumulates........

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(Note:  thanks to Dr Rob Ixer for correcting various spelling mistakes in the following -- now updated.)

Extract:  Dr Elwyn Griffiths, Ph D Thesis, University of Bristol (1974): “Sedimentary Response To The Tidal regime Of The Upper Severn Estuary. "

Extract:  Dr Elwyn Griffiths, Ph D Thesis, University of Bristol (1974): “Sedimentary Response To The Tidal regime Of The Upper Severn Estuary. "

P 10
Study of the interactions ??  between wave and tidal energy indicates that wave activity, when present, only modifies the distribution of heavy mineral concentrations already produced by tidal currents.

This implies that the degree of selective sorting is mainly attributed to differences in current velocities. The highest parts of the sandbars, areas of deposition,  experience the lowest overall velocities, and channels and constrictions of the study area experience the highest overall velocities.

Finally, an analogy to the process of current selective sorting is seen on a smaller scale at Lydney sandbar. Allen (1968) illustrates heavy mineral concentrates along the crests of straight asymmetrical ripples from this site. Segregation must have taken place, during a decreasing current when the critical erosional velocity was surpassed for the light fraction but not for the heavy fraction. As the shear stress is lowest in the troughs of ripples (Raukivi,1963) quartz grains once deposited here will remain as long as the critical erosional velocity is not surpassed.

1.18.  PROVENANCE

The provenance of the upper Severn Estuary sands is best determined by the categorisation of certain heavy mineral associations.  These associations, indicative of major source rocks, have been defined by Krumbein and Pettijohn (1938); Feo-Codecido (1956); and many others.

Using Feo-Codecido’s provenance scheme it is evident that the heavy mineral suite is immature and that the major source rocks are contact and dynamothermal metamorphics and basic igneous rocks. The low content of tourmaline, RUTILE and zircon (RTZ. index) suggest that reworked source rocks are minor. The perfect crystalline forms of tourmaline, garnet and hornblende, and the angular fragments of kyanite, staurolite, diopside and enstatite, rule out prolonged weathering and recycling from a secondary source. Also, the high proportions of hornblende, garnet, kyanite, staurolite, and andalusite (see Appendix 4) and the presence of glaucophane suggest that the dynamothermal metamorphic rocks are a primary and dominant source. The only metamorphic rocks within the catchment area of the Severn river are the Malverns but the linear outcrop does not contain the above assemblage.

Because of the lack of primary source rocks in the estuary drainage basin and the catchment areas of rivers flowing into the upper reaches of the Bristol Channel, reworked sedimentary rock sources must be considered. To produce an almost saturated immature heavy mineral suite plus metamorphic rock fragments the presumed sedimentary rock source must be undergoing relatively rapid erosion and have a large areal extent within the drainage basin. Also, the former source of these rocks must have been primarily metamorphic and igneous rocks.

The Old Red Sandstone and the Triassic rocks are the most common, outcropping along the shores of the estuary, sometimes forming the estuary bedrock, and within the catchment areas of the two major rivers, the Severn and the Wye.  Allen (1965) showed that the transport path and source rocks of the Upper Old Red Sandstone of the Welsh Borderland, based on feldspar associations, was from North Wales and the Mona Complex in Anglesey. It was noted, however, that undoubtedly contributing quartz, orthoclase, some of the opaques and iron oxides, the Old Red Sandstone of the Welsh Borderland is low in garnet and other high ranking metamorphic minerals.  Similarly, the Triassic rocks which possibly contribute fluorite, apatite, and sedimentary rock fragments again do not contain the relevant igneous and metamorphic heavy mineral suites. The Carboniferous Limestone, which in the Bristol district is rich in haematite and magnetite, and the Jurassic rocks of the Cotswolds, are considered to be unlikely sources.

When trying to determine source rocks which are not within the present sedimentary basin, and in this case with no direct connections, the ensuing discussion becomes rather tenuous. The nearest possible source rocks of any real extent is the Mona Complex of Anglesey and more  probably the metamorphic rocks of N. Scotland, which are 200 km and 500 km distant, respectively. The only possible method of transport from such an isolated region would be ice action.

There are known extensive and thick glacial deposits in the lower reaches of the Severn Estuary (Hawkins, pers. comm.). Kellaway (1970) proposed that the glacial erratics and the Bluestones of Stonehenge, which are believed to be derived from Pembrokeshire, were brought into the area by eastward movement of the Irish Ice. Large garnetiferous boulders have been found at Porthleven in Cornwall and at Portishead, near Bristol, which are thought to have been transported by the Irish Sea Ice from Scotland. (Hamilton, pers. comm.)

The glacial deposits ??  found in the Bristol region may, possibly, have been dragged by the ice from the floor of the Celtic Sea. It is possible that the sands of the study area have a similar mode of origin; but to confirm this, further exacting provenance studies are necessary.