The late Aeronian graptolitesedgwickiiEvent, associated positive carbon isotope excursion and facies changes in the Prague Synform (Barrandian area, Bohemia)

Abstract

Study of the lower Silurian black shale succession of the Prague Synform has enabled detailed insight into graptolite faunal dynamics and diversity trends from the mid-Aeronian diversity maximum through to the late Aeronian crisis. Graptolite diversity decreased from 33 taxa in the Lituigraptus convolutus Biozone to 17 taxa in the upper part of the Stimulograptus sedgwickii Biozone and newly erected Lituigraptus rastrum Biozone. The graptolite assemblages of the latter biozones exhibit low species richness along with high dominance. Many graptolite species that became extinct in the early part of the sedgwickii Zone were promptly replaced by new forms. In the later part of the sedgwickii Zone, however, replacement of extinct species by new forms considerably decelerated. The increased rate of graptolite extinction recorded in the convolutus-sedgwickii biozone boundary beds coincided with subtle changes in black shale lithologies and a positive shift in δ 13 C org (of 2.2 ‰) compared to baseline values. Sea-level drawdown can be inferred from siltstones and/or temporary nondeposition in the middle sedgwickii Zone. This level also sees total organic carbon (TOC) fluctuations and a strong positive δ 13 C org excursion with a peak shift of at least 7 ‰. The sedgwickii Event exhibits substantial reorganization of the graptolite fauna, its taxonomic impoverishment and concomitant increase in species dominance rather than a sudden collapse of the pre-extinction assemblage. Associated changes in lithology, TOC and the pronounced δ 13 C org excursion suggest a relatively extended and probably multi-phase period of stressed conditions that affected the pelagic realm inhabited by graptolites in the course of the late Aeronian interval.

The Aeronian/Telychian (Llandovery, Silurian) boundary, with particular reference to sections around the El Pintado reservoir, Seville Province, Spain

“…The presence of Neolagarograptus tenuis at the base of this unit suggests, from the ranges presented by Štorch & Frýda (2012) through a continuously graptolitic convolutus/sedgwickii Biozone transition, that the base of these "Monograptus-sedgwicki shales" (Jones & Pugh 1916, Davies et al 1997 lies above that of the sedgwickii Biozone. Above the "Monograptus-sedgwicki shales" the succeeding upper Aeronian strata represent deposition under predominantly oxic conditions with thin graptolitic shales comprising a very small percentage of the total thickness of strata.…”

“…-Štorch & Frýda (2012) provided lithological, graptolite biostratigraphical and carbon isotope data through the upper Aeronian to lowermost Telychian of the Prague synform, Bohemia. The base of the Telychian here lies within a pale non-graptolitic mudstone unit.…”

“…Brief mention of preliminary results of work on the Aeronian of the El Pintado sections was included in Štorch & Frýda (2012). Other works on graptolites from El Pintado are those of Hernández Sampelayo (1932), Carvajal Acuña (1944, Simon (1951), Gutiérrez-Marco et al (1996), Lenz et al (1996) and Piçarra et al (1998).…”

“…In some places of course there are no graptolitic strata of latest Aeronian age, but in Arctic Canada Melchin & Holmden (2006) placed uppermost Aeronian strata yielding Stimulograptus halli in their upper sedgwickii Biozone. Štorch & Frýda (2012) erected a new biozone for the distinctive, abundant, short-ranging Lituigraptus rastrum. The latter is undoubtedly an extremely useful species, as can be seen from its stratigraphical range in El Pintado section 1 and for this reason the middle part of the halli Biozone at El Pintado is referred to the rastrum Subzone.…”

“…Although Me. undulatus is common at El Pintado, it does not attain the very high abundances that characterize some levels in the upper Aeronian in the Czech Republic (Štorch & Frýda 2012) and in mid Wales (Loydell 1991a(Loydell , 1992. Instead and unusually, Me.…”