Discussion of ‘brackish water faunas from the St Maughans Formation: the old red sandstone section at Ammons Hill, Hereford and Worcester, UK’ by W.J. Barclay, P.A. Rathbone, D.E. White and J.B. Richardson
✍ Scribed by V. P. Wright; S. B. Marriott
- Book ID
- 101281660
- Publisher
- John Wiley and Sons
- Year
- 1996
- Tongue
- English
- Weight
- 248 KB
- Volume
- 31
- Category
- Article
- ISSN
- 0072-1050
No coin nor oath required. For personal study only.
✦ Synopsis
In their paper, Barclay et al. (1994) carefully documented the presence of brackish water faunas in the St Maughans Formation of part of the Welsh Borderland. The faunal evidence of marine incursions in the Old Red Sandstone (ORS) has been noted by many workers, but sedimentological evidence is sparse. Barclay et al. (1994: 377) note that 'discounting of marine and tidal influences in the deposition of beds of this age in South Wales by Marriott and Wright (1993) cannot be supported in the light of the faunal evidence.'
We believe that one of our conclusions on a study of palaeosols and palaeogeomorphology of the Lower ORS of Manorbier, Dyfed has been taken out of context and evaluated in a sequence far removed from our study area and to a unit of very different style to those we documented. In our study of the Manorbier Rat Island Mudstone (upper member of the Freshwater West Formation) and Moor Cliffs Formations we found no evidence of marine incursions either as brackish or marine faunas, or as tidal signatures in the sediments such as bipolar cross-sets, reactivation surfaces, mud drapes and bundling, or features related to wave and tidal interaction. Allen and Williams ( I 982) had, however, noted evidence of marine influences in the Sandy Haven Formation north of Milford Haven (this formation is correlated with the Moor Cliffs Formation). Furthermore, we emphasized in our study the absence of evidence of marine hydromorphism in the abundant palaeosols in the Lower ORS. Such evidence has been extensively documented in similar palaeosols in other Palaeozoic sequences by Driese and Foreman (1991; 1992), Driese el al. (1992) and Wright and Robinson (1988).
We did not imply that marine incursions did not occur universally in the Lower ORS in the region, but that we found no evidence in the Manorbier sections. The assertion by Barclay et al. (1994) that our conclusions ' . . . cannot be supported' is incorrect as it applies to another area, in a different facies sequence, and it presupposes our statement was meant to cover all sequences of this age. It clearly does not, as our references to the paper by Allen and Williams (1982) show.
We feel that there is a broader issue here other than whether or not minor marine incursions were restricted in their distribution, and we wish to take this apparent disagreement as evidence for the unusual nature of the marine to non-marine transition in the ORS. This transitional system may have been a key factor facilitating the changes in the biosphere which took place at this time.
The case of marine and brackish incursions based on faunal evidence is irrefutable, but what remains to be explained is the apparent lack of clear sedimentological evidence. This may reflect a shoreline transition zone lacking the typical foreshore, estuarine or deltaic environments seen in most such zones in the geological