Fine-scale biostratigraphy within the Stimulograptus sedgwickii Zone (Silurian: Llandovery) at Dob’s Linn, Southern Uplands

Abstract

Synopsis A 1.89-m section excavated from the Stimulograptus sedgwickii Biozone (Silurian: Llandovery, Aeronian) at Dob’s Linn, Southern Uplands, Scotland, representing some 0.2 Ma of deposition, shows pronounced, quantitatively established patterns of graptolite abundance and diversity. The most striking signature comprises several marked peaks in abundance of the distinctive taxon Neolagarograptus tenuis (Portlock) within this interval. Such ‘bloom events’, if recognized further afield, may provide very fine-scale stratigraphic resolution, seemingly akin to the ammonite biohorizons of the Jurassic. Taxonomic richness as a whole decreases upwards and appears associated with bentonite layers, suggesting that volcanic ash falls might have led to local extinctions of graptolite populations with little or no subsequent recovery. Repetition of the analysis on a duplicate section along-strike at Dob’s Linn showed comparable patterns, demonstrating the robustness of the methodology.

Early Silurian (Llandovery) graptolite assemblages of Saudi Arabia: biozonation, palaeoenvironmental significance and biogeography

“…Else where the in terval of the lower sedgwickii Biozone has been iden ti fied as a discrete tenuis Subzone that is char ac ter ized by the abun dance of the epon y mous spe cies, and which is rec og nized in Bo he mia and North Af rica (Štorch and Massa, 2006), and also in the lower part of the sedgwickii Biozone in Saudi Ara bia. Pannell et al (2006) sug gested that this in ter val rep re sented about 0.2 Ma, be ing of com pa ra ble du ra tion to the subzones of the Telychian guerichi Biozone. This pro vides a highly re solved stra tig ra phy that al lows cor re la tion from palaeocontinental Gond wana at high south ern palaeo-lat i tude to palaeocontinental Laurentia in the palaeo-trop ics.…”

“…In Saudi Ara bia the sedgwickii Biozone com prises about 19 taxa, mostly rang ing from the pre ced ing convolutus Biozone. The low er most in ter val of the sedgwickii Biozone is char ac ter ized by Neolagarograptus tenuis (Portlock) that is char ac ter is tic of the N. tenuis as sem blage Biozone in Libya (Štorch and Massa, 2006; see also Pannell et al, 2006): this in ter val also in cludes the first oc cur rence of Parapetalolithus altissimus (Elles and Wood). The ear li est N. tenuis slightly pre cedes S. sedgwickii in Saudi Ara bia, but the two graptolites over lap in the lower part of the biozone (through circa 4 m of strati graphic thick ness in one core).…”

“…SUBDIVISION AND CORRELATION OF THE SEDGWICKII BIOZONE Pannell et al (2006) iden ti fied a dis crete strati graphi cal inter val at the base of the sedgwickii Biozone in Scot land that was char ac ter ized by sev eral peaks of abun dance of Neolagarograptus tenuis. They sug gested that these peaks might rep re sent widely cor re lat able events.…”

“…Thus, the Saudi Ara bian graptolite as sem blage of the sedgwickii Biozone contains spe cies that are also pres ent in south ern Scot land (Pannell et al, 2006;Wil liams et al, 2003), Eng land (e.g., Hutt, 1975), Wales (Loydell, 1992(Loydell, , 1993 and Bo he mia (Štorch, 1994). This pat tern con tin ues into the halli and guerichi biozones, en abling sub-zonal cor re la tion be tween Wales and Saudi Ara bia.…”

Graptolites in British stratigraphy

“…tenuis seems to be restricted to the lower part of the biozone (Rickards, 1976;Davies et al. At Dob's Linn this species shows marked stratigraphical variations in abundance within its range (Pannell, Clarkson & Zalasiewicz, 2006). At Dob's Linn this species shows marked stratigraphical variations in abundance within its range (Pannell, Clarkson & Zalasiewicz, 2006).…”