Palynology of a lower Wenlock (Silurian) shelf-basin transect, Wales and the Welsh Borderland
Well-exposed lower Wenlock sections and borehole sequences, representing various facies along a shelf-basin transect in Wales and the Welsh Borderland, have been sampled for palynomorphs. Primary attention is paid to the type area in Shropshire, including stratotypes of both lower and upper boundaries of the Sheinwoodian Stage, with sampling as close as 10cm through the boundary horizons. The study has been extended into other sequences on the shelf to nearshore facies in the Bristol area and to basinal sections in North Wales. Total organic residues were recovered using quantitative processing techniques and absolute palynomorph abundances were determined. Both transmitted light and scanning electron microscopes were used to work on strew-mounted residues allowing detailed morphological study of the palynomorphs. Techniques were developed for allowing remounting of gold coated SEM cover slips, for transmitted light study and for permanent records. Taxonomic focus is on the acritarchs and the chitinozoans; forty-one acritarch genera and one hundred and seven species and eleven chitinozoan genera and twenty-eight species are systematically described. Ten acritarch species, one chitinozoan genus and three chitinozoan species are new. One genus and species of trilete spore is also systematically described. In addition scolecodonts, graptolite fragments, melanosclerites, chitinous hydroids and amorphous kerogen were recovered and their distribution noted. The exceptionally well-preserved assemblages recovered from the deeper water shelf sections (including the Eastnor Park and Lower Hill Farm boreholes and Whitwell Coppice section) contain 80 2000 acritarchs/g, and 10-60 chitinozoans/g, while the species diversity index (Fisher et al. 1943) for these sections varies between 0.35 and 30.2. The nearshore/shallow water sections (including Tortworth and Dolyhir) yield a well preserved palynomorph assemblage of low abundance (0.024 to 1.14 palynomorphs/g) and low species diversity (0.35 to 3.8). The poorly preserved assemblages of the basin (including the Pistyll Quarry section and the Llanrwst and Conway composite sections) contain 0 to 4.6 palynomorphs/g and species diversity varies between 0 and 4.8. Palynomorph absolute abundances and species diversity are compared and contrasted, both are considerably higher in the inner shelf and shelf sections than in the nearshore/shallow water and outer shelf and basinal sections. Distribution of the organic residues through the different sections is illustrated and discussed and the acritarchs and chitinozoans are used for biostratigraphical refinement. Taxa ranges and relative frequencies are illustrated by computer drafted figures for each section; graphical techniques are also used for correlation purposes as are summary logs of range data. In addition to vertical palynomorph distributional patterns through a studied section, palynomorph assemblage distributional patterns are also discussed and illustrated by graphical representations for the different palaeoenvironments represented by the shelf-basin transect. It is noted that the chitinozoans generally prefer deeper water; on the outer shelf the genera Ancyrochitina Eisenack 1955a and Cingulochitina Paris 1981 and in the basin the genera Sphaerochitina Eisenack 1955a and Conochitina Eisenack 1931 are dominant. With the acritarchs thin-walled leiospheres and shortspined MlchrystrId1um Deflandre 1937 emend. Staplin 1961 appear to have a preference for nearshore/shallow water environments. The acritarchs are most abundant and diverse on the shelf with the acanthomorphs being the dominant group. Basinal sections are dominated by small thick-walled leiospheres and relatively abundant short-spined fat-bodied Veryhachium Deunff ex Downie 1959. Marine and inshore indices adapted from Richardson & Rasul (1990) are also used to highlight assemblage contrasts over the shelf and basin. From the biostratigraphical results a new biozonational scheme for the early Wenlock is proposed, based on the recorded stratigraphical ranges of diagnostic taxa. Three existing acritarch biozones (the Deunffia brevispinosa, Deunffia furcata and Eisenackidium wenlockensis biozones) have their boundaries changed on new stratigraphical range information and one new zone, the Helosphaerldium malvernensis Biozone is proposed. Two new chitinozoan biozones, the Calplchitlna (Densichitina) densa and Clngulochitlna cingulata biozones are also proposed. The palynomorph biozones are related to the established graptolite biozones (see Bassett at al. 1975) in the Whitwell Coppice section and the Lower Hill Farm borehole in the Wenlock type area. The thermal maturity of the different sections is calculated by the use of the Acritarch Alteration Index (AAI) of Legall et al. 1981, which is a method of calibrating palaeotemperatures. For consistency in results only the acritarch genus Leiosphaeridia Eisenack 1958 emend Downie & Sarjeant 1963 was used. For the shelf sections the AAI is low and varies between 2 and 4 (indicating palaeotemperatures of 60-70⁰C ), in contrast the thermal maturity of the basinal sections is much higher, with an AAI of 5 showing palaeotemperatures in the range 90-460⁰C and probably towards the higher end of that range.