Lower Silurian `hot shales' in North Africa and Arabia: regional distribution and depositional model

— The occurrence of shock metamorphosed quartz is the most common petrographic criterion for the identification of terrestrial impact structures and lithologies. Its utility is due to its almost ubiquitous occurrence in terrestrial rocks, its overall stability and the fact that a variety of shock metamorphic effects, occurring over a range of shock pressures, have been well documented. These shock effects have been generally duplicated in shock recovery experiments and, thus, serve as shock pressure barometers. After reviewing the general character of shock effects in quartz, the differences between experimental and natural shock events and their potential effects on the shock metamorphism of quartz are explored. The short pulse lengths in experiments may account for the difficulty in synthesizing the high-pressure polymorphs, coesite and stishovite, compared to natural occurrences. In addition, post-shock thermal effects are possible in natural events, which can affect shock altered physical properties, such as refractive index, and cause annealing of shock damage and recrystallization. The orientations of planar microstructures, however, are unaffected by post-impact thermal events, except if quartz is recrystallized, and provide the best natural shock barometer in terms of utility and occurrence. The nature of planar microstructures, particularly planar deformation features (PDFs), is discussed in some detail and a scheme of variations in orientations with shock pressure is provided. The effect of post-impact events on PDFs is generally limited to annealing of the original glass lamellae to produce decorated PDFs, resulting from the exsolution of dissolved water during recrystallization. Basal (0001) PDFs differ from other PDF orientations in that they are multiple, mechanical Brazil twins, which are difficult to detect if not partially annealed and decorated. The occurrence and significance of shock metamorphosed quartz and its other phases (namely, coesite, stishovite, diaplectic glass and lechatelierite) are discussed for terrestrial impact structures in both crystalline (non-porous) and sedimentary (porous) targets. The bulk of past studies have dealt with crystalline targets, where variations in recorded shock pressure in quartz have been used to constrain aspects of the cratering process and to estimate crater dimensions at eroded structures. In sedimentary targets, the effect of pore space results in an inhomogeneous distribution in recorded shock pressure and temperature, which requires a different classification scheme for the variation of recorded shock compared to that in crystalline targets. This is discussed, along with examples of variations in the relative abundances of planar microstructures and their orientations, which are attributed to textural variations in sedimentary target rocks. Examples of the shock metamorphism of quartz in distal ejecta, such as at the K/T boundary, and from nuclear explosions are illustrated and are equivalent to that of known impact structures, except with respect to characteristics that are due to long-term, post-shock thermal effects. Finally, the differences between the deformation and phase transformation of quartz by shock and by endogenic, tectonic and volcanic processes are discussed. We confirm previous conclusions that they are completely dissimilar in character, due to the vastly different physical conditions and time scales typical for shock events, compared to tectonic and volcanic events. Well-characterized and documented shock effects in quartz are unequivocal indicators of impact in the natural environment.

Shock metamorphism of quartz in nature and experiment: II. Significance in geoscience*

Ordovician and Silurian sea–water chemistry, sea level, and climate: A synopsis

Current information on the conodonts Clydagnathus windsorensis (Globensky) and Promissum pulchrum Kova! cs‐ Endro$ dy, together with the latest interpretations of conodont hard tissues, are reviewed and it is concluded that sucient evidence exists to justify interpretation of the conodonts on a chordate model. A new phylogenetic analysis is undertaken, consisting of 17 chordate taxa and 103 morphological, physiological and biochemical characters; conodonts are included as a primary taxon. Various experiments with character coding, taxon deletion and the use of constraint trees are carried out. We conclude that conodonts are cladistically more derived than either hagfishes or lampreys because they possess a mineralised dermal skeleton and that they are the most plesiomorphic member of the total group Gnathostomata. We discuss the evolution of the nervous and sensory systems and the skeleton in the context of our optimal phylogenetic tree. There appears to be no simple evolution of free to canal-enclosed neuromasts; organised neuromasts within canals appear to have arisen at least three times from free neuromasts or neuromasts arranged within grooves. The mineralised vertebrate skeleton first appeared as odontodes of dentine or dentine plus enamel in the paraconodont}euconodont feeding apparatus. Bone appeared later, co-ordinate with the development of a dermal skeleton, and it appears to have been primitively acellular. Atubular dentine is more primitive than tubular dentine. However, the subsequent distribution of the dierent types of dentine (e.g. mesodentine, orthodentine), suggests that these tissue types are homoplastic. The topology of relationships and known stratigraphic ranges of taxa in our phylogeny predict the existence of myxinoids and petromyzontids in the Cambrian.

Conodont affinity and chordate phylogeny

On the causes of mass extinctions

Recent revisions to the biostratigraphic and chronostratigraphic assignment of strata from the type area of the Niagaran Provincial Series (a regional chronostratigraphic unit) have demonstrated the need to revise the chronostratigraphic correlation of the Silurian System of North America. Recently, the working group to restudy the base of the Wenlock Series has developed an extremely high-resolution global chronostratigraphy for the Telychian and Sheinwoodian stages by integrating graptolite and conodont biostratigraphy with carbonate carbon isotope (delta 13C(carb)) chemostratigraphy. This improved global chronostratigraphy has required such significant chronostratigraphic revisions to the North American succession that much of the Silurian System in North America is currently in a state of flux and needs further refinement. This report serves as an update of the progress on recalibrating the global chronostratigraphic correlation of North American Provincial Series and Stage boundaries in their type area. The revised North American classification is correlated with global series and stages as well as regional classifications used in the United Kingdom, the East Baltic, Australia, China, the Barrandian, and Altaj. Twenty-four potential stage slices, based primarily on graptolite and conodont zones and correlated to the global series and stages, are illustrated alongside a new composite delta 13C(carb) curve for the Silurian. Conodont, graptolite, isotope, New York, Ontario, series, Silurian, stage. (Less)

Revised correlation of Silurian Provincial Series of North America with global and regional chronostratigraphic units and delta 13C(carb) chemostratigraphy

When extracting or cleaning phosphatic fossils using acetic acid, the solutions must be buffered with calcium acetate to a pH greater than 3.6 for conodont and many fish taxa, and to a slightly greater pH for some conulariid, shark, and some fish taxa. Diagrams of the changes in pH and density during dissolution, rate of dissolution, and productivity per liter of acid provide a means for selecting the economically optimal process, monitoring the progress, and adjusting the composition of the solution when necessary.

The optimal acetate buffered acetic acid technique for extracting phosphatic fossils

The new standard conodont zonation includes, in succession from below: Lower Ps bicornis Zone, Upper Ps bicornis Zone, Lower P procerus Zone, Upper P procerus Zone, Lower K ranuliformis Zone, Upper K ranuliformis Zone, 0 s rhenana Zone, Lower K walliseri Zone, Middle K walliseri Zone, K patula Zone, uppermost K walliseri range, post K walliseri interregnum, K o ortus Zone and 0 s sagitta Zone Lower boundaries for each zone are defined or redefined A less detailed biostratigraphic nomenclature at the superzone level is suggested for use where collections are inadequate: The Pterospathodus Zonal group, the Upper Pterospathodus Zonal group, the P amorphognathoides Zonal group, the Ps bicornis Superzone, the P procerus Superzone, the K ranuliformis Superzone, the O s rhenana Superzone, the K walliseri Zonal group, the Upper K walliseri Superzone and the K o ortus Superzone These partly overlap and can be used according to the taxa present The proposed zonation is applied to most known conodont sequences The taxonomic concept of the taxa used is discussed; new taxa include Nudibelodina sensitiva, Ozarkodina paraconfluens and O martinssoni The species name Ozarkodina ortus is a senior synonym of Kockelella absidata

A new latest Telychian, Sheinwoodian and Early Homerian (Early Silurian) Standard Conodont Zonation

A succession of 26 conodont zones and 63 successive subzones and conodont faunas have been identified in the exposed latest Landovery to latest Ludlow strata of Gotland, Sweden. All zones, and several of the subzones and faunas, have been identified elsewhere on Baltica, as well as on other palaeocontinents, indicating that this zonation serves well as the standard zonation for calcareous successions. The average duration of the 24 Wenlock and Ludlow zones was 396±208 ka, using the latest radiometric timescale. However, most zones and subzones formed during the widely recognised Silurian oceanic events correspond to less time per unit—in the order of 30 to 100 ka—and most of those formed during intervening episodes correspond to a few 100 ka. The average duration of the 61 Wenlock and Ludlow zones, subzones, and faunas was only 156±82 ka, i.e., among the highest biostratigraphical resolution available for Phanerozoic epochs. On Gotland the combined known maximum thickness of the strata has increa...

A latest Llandovery to latest Ludlow high-resolution biostratigraphy based on the Silurian of Gotland - a summary

Formic acid must be buffered with both calcium carbonate and calcium phosphate to be suitable for extraction of conodont elements and other phosphatic fossils from dolomites. All unbuffered solutions and those buffered with only calcium carbonate or calcium phosphate destroy the phosphatic fossils. Experimental effects using different concentrations and different combinations of phosphate and calcium carbonate are diagrammatically illustrated. A 10 percent formic acid solution with 20 g calcium carbonate and 1.2 g tricalcium phosphate per liter leaves conodont elements undamaged even for a period of 129 days. One liter of this solution dissolves 95 g dolomite.

Lennart Jeppsson

Papers

Revised correlation of Silurian Provincial Series of North America with global and regional chronostratigraphic units and delta 13C(carb) chemostratigraphy

The optimal acetate buffered acetic acid technique for extracting phosphatic fossils

A new latest Telychian, Sheinwoodian and Early Homerian (Early Silurian) Standard Conodont Zonation

A latest Llandovery to latest Ludlow high-resolution biostratigraphy based on the Silurian of Gotland - a summary

A buffered formic acid technique for conodont extraction