Alfred C. Lenz

Wenlock (Silurian) graptolites, Cape Phillips Formation, Canadian Arctic Islands

Twenty six of the more important species of monograptids (s.l.), retiolitids and Cyrtograptus (from a total fauna of 52 species) are described from Wenlock strata of the Cape Phillips Formation, Canadian Arctic Archipelago. Of this fauna, eight new species or subspecies, Monograptus firmus festinolatus, M. instrenuus, M. opimus, M. testis incomptus, Cyrtograptus falcatus, C. hamatus brevis, C. kolobus and C. pseudomancki , are described and illustrated. Wenlock biostratigraphic zones comprise the Cyrtograptus centrifugus-C. insectus Zone (earliest Wenlock), M. instrenuus-C. kolobus Zone, tentatively divisible into lower and upper subzones, C. perneri-M. opimus Zone possibly divisible into lower and upper subzones, C. lundgreni-M. testis Zone divisible into a lower M. testis incomptus Subzone and an upper M. testis testis Subzone, and the Pristiograptus ludensis Zone (latest Wenlock).

Geochemical anomalies near the Ordovician‐Silurian boundary, Northern Yukon Territory, Canada 1

Basinal strata spanning the Late Ordovician extinction have been determined for major and trace elements, and isotopes of C, O and S. A marked increase in organic C, and siderophile (i.e., Ni, V) and chalcophile (i.e., Zn, Cd, Hg, As, Sb, Se and Mo) elements is observed at a level that is no older than the base of the G. persculptus Zone. In sections for which isotopic data are available, these anomalies coincide with decreases in δ 13 C values, suggesting a major biomass reduction, and δ18O values, indicating a warming trend. At the same time, δ34S values in pyrite become highly positive indicating reduced bottom‐water conditions. These results suggest that a rapid inversion of stratified ocean water causing a brief but widespread episode of anoxic surface waters may have been the immediate cause of extinction. The ultimate cause of extinction is unknown, although the magnitude and apparent global synchroneity of the extinction suggest that it was of catastrophic effect. The chalcophile and siderophile e...

UPPER WENLOCK AND LOWER LUDLOW (SILURIAN), POST-EXTINCTION GRAPTOLITES, VSERADICE SECTION, BARRANDIAN AREA, CZECH REPUBLIC

Abstract The Všeradice section was trenched and continuously sampled for graptolites from the base of the post-extinction upper Homerian strata to low in the Ludlow. The upper Homerian is divisible into three biozones, a lower parvus–nassa Biozone, a middle praedeubeli–deubeli Biozone, and an upper ludensis–gerhardi Biozone, the last named being succeeded by graptolites of the lowest Ludlow nilssoni Biozone. The graptolite diversity of nine species in the upper part of the parvus–nassa Biozone is the highest in the world. Five monograptid species, Pristiograptus parvus Ulst, 1974, P. dubius (Suess, 1851), Colonograptus deubeli (Jaeger, 1959), Colonograptus gerhardi (Kühne, 1955), and Colonograptus praedeubeli (Jaeger, 1990) are described from the upper Homerian. Twelve species of retiolitids, Gothograptus nassa (Holm, 1890), Neogothograptus cf. balticus (Eisenack, 1951), Spinograptus spinosus (Wood, 1900), Spinograptus clathrospinosus (Eisenack, 1951), Spinograptus munchi (Eisenack, 1951), Spinograptus reticulolawsoni (Kozłowska-Dawidziuk, 1997) Spinograptus? cf. nevadensis (Berry and Murphy, 1975), Spinograptus? sp. A and Spinograptus? sp. B, Plectograptus macilentus (Törnquist, 1887), and Plectograptus? karlsteinensis new species and Plectograptus? ovatus new species, are described and illustrated from upper Homerian and lowest Ludlow strata. The two new species occur in the upper parvus–nassa and lowest praedeubeli–deubeli, and ludensis–gerhardi biozones, respectively. Two morphs of Gothograptus nassa, a narrow (more typical) form, and a wide form, are recognized. While overlapping in their overall ranges, their occurrences in any particular small interval are sometimes mutually exclusive, suggesting ecological control.

The lundgreni Extinction Event: Integration of paleontological and geochemical data from Arctic Canada

Abstract The lundgreni Extinction Event (LEE) in Arctic Canada, as in other regions globally, exhibited an apparently abrupt and catastrophic reduction of graptolite and radiolarian taxa, followed by a short period of extremely low diversity, the dubius-nassa Biozone, followed in the subsequent praedeubeli-deubeli Biochron by the rapid evolutionary diversification of new taxa. The extinction event was very close to, or coincided with, a regression, as well as with a moderately strong positive δ13Corganic excursion. The excursion is bifurcated with the lower fork occurring at or near the LEE and the upper fork occurring in about the lower part of the praedeubeli-deubeli Biozone, possibly coinciding with a modest transgression. Radiolarians are diverse and abundant throughout the lundgreni Biozone, sparse to absent directly following the LEE (i.e., in the dubius-nassa Biozone), and found only sporadically within the praedeubeli-deubeli Biozone. Acritarch frequency throughout the entire investigated interval is extremely low, and only in one level in the lundgreni Biozone so-called “giant acritarchs” were observed. Sphaeromorph acritarchs and amorphous organic matter are moderately common in the lundgreni Biozone, extremely abundant in the brief time interval of the dubius-nassa Biozone at Rookery Creek, and only moderately common in higher strata. The presence of dark yellow and light brown-colored organic matter indicates that the absence of acritarchs is not due to reworking or thermal degradation but is, instead, suggestive of paleoecological control. Proximity to a shallower water shelf may have strongly influenced palynomorph/organic content.

Thermal Maturity of Lower Paleozoic Sedimentary Successions in Arctic Canada

Mean maximum graptolite reflectance values from numerous sections in Arctic Canada range from 0.6% in Cornwallis Island and northwestern Devon Island to 4.7% in Ellesmere Island. We attribute this great lateral reflectance variation to differing burial depths of the graptolite-bearing strata beneath thick synorogenic siliciclastic covers. We attribute the low maturity of rocks in northern Cornwallis and eastern Bathurst islands to a maximum burial of about 2 km. Elsewhere, we used the paleogeographic location and proximity to the Ellesmerian overthrust wedge to interpret the measured reflectance values. In northern Ellesmere Island, where the highest graptolite reflectance values (4.7%) occur, as much as 9.6 km of synorogenic siliciclastics accumulated on a tectonically loaded carbonate shelf. Initial synorogenic siliciclastics encountered substantial submarine-to-basin relief, and thus about 2 km of sediment were deposited prior to the initiation of deposition on the adjacent drowned shelf. Also, the deep-water sequence probably was underlain by attenuated continental crust adjacent to the southeastward-advancing Ellesmerian overthrust wedge. Together, these factors caused deposition of as much as 7.6 km of sediment in western Melville Island and, as much as 9.6 km in northeastern Ellesmere Island. On the drowned shelf, synorogenic stratal thicknesses were less, a feature we attribu e to a thicker, more rigid crustal sequence and greater distance from the Ellesmerian tectonic loading. We expect liquid hydrocarbons to be generated from the organic matter in the shales in areas with a graptolite R0 maximum (GR0 max) of less than 1.7%. The presence of two types of solid bitumens having different reflectances, morphologies, and optical textures suggests that hydrocarbons were generated and migrated through the graptolite-bearing strata. We expect only gaseous hydrocarbons in areas where GR0 max exceeds 2.0% R0.

UPPER HOMERIAN (UPPER WENLOCK, SILURIAN) GRAPTOLITES FROM ARCTIC CANADA

Abstract An extraordinarily well-preserved upper Homerian (uppermost Wenlock) post-extinction (post-lundgreni Event) graptolite fauna is described from Arctic Canada. The fauna of dendroids, retiolitids and monograptids, predominantly of uncompressed and isolated material, demonstrates almost totally new morphological features in comparison with the pre-extinction fauna. The two species of the lowest post-extinction fauna are followed by a rapid proliferation in the middle part of the interval, resulting in the maximum diversity, 21 species, for the entire upper Homerian. The upper Homerian is divided into three biostratigraphic units: the lower nassa-dubius Biozone, middle praedeubeli-deubeli Biozone, and the upper ludensis Biozone. The total fauna consists of ten species of monograptids and 11 species of retiolitids; of the latter group, three (Baculograptus, Papiliograptus and Doliograptus) are new genera, and six (B. batesi, P. papilio, P.? petilus, D. latus, Doliograptus sp. A, and Spinograptus praerobustus) are new species. Among the recognized species of monograptids, only Colonograptus schedidoneus and Lobograptus? cornuatus are confined to the Arctic, while five species of retiolitids (B. batesi, P.? petilus, D. latus, Doliograptus sp. A, and Spinograptus praerobustus) appear to be endemic.

Uppermost Wenlock and lower Ludlow plectograptine graptolites, Arctic Islands, Canada; new isolated material

Beautifully preserved, isolated and uncompressed plectograptine graptolites from the uppermost Wenlock “Pristiograptus” ludensis Biozone and the lower Ludlow Lobograptus progenitor Biozone are described from the Arctic Islands. Described species comprise Agastograptus robustus Obut and Zaslavskaya from the ludensis and progenitor zones; Agastograptus sp., Gothograptus chainos Lenz, and Plectograptus (Sokolovograptus?) sp., from the ludensis Zone; and Holoretiolites (Balticograptus) erraticus (Eisenack) from the progenitor Zone. The dozens of specimens of A. robustus from one locality are covered with almost continuous periderm and interthecal tissue, a feature rarely seen in plectograptines.

Llandoverian and Wenlockian Cyrtograptus,and some other Wenlockian graptolites from Northern and Arctic Canada

Abstract Llandoverian and Wenlockian graptolite zonesin northern and Arctic Canada comprise the Sakmaricus-Laqueus Zone of latest Llandoverian age, Centrifugus Zone of earliest Wenlockian age, Rigidus Zone of about mid-Wenlockian age, possibly the Perneri Zone of late Wenlockian age, and the Testis-Lundgreni Zone of very late Wenlockian age. Species of Cyrtograptus described and illustratedin this study are C. centrifugus, C. cf. insectus, C. aff. lapworthi, C. laqueus, C. lundgreni, C. sp. (aff. C. lundgreni), C. mancki, C. radians, C. rigidus, C. sakmaricus, C. solaris n. subsp.? and C. sp. (similar to C. murchisoni bohemicus ). Other taxa comprise Monograptus cf. capillaceus, M. ? aff. deubeli, Gothograptus nassa, Plectograptus praemacilentus, Spinograptus cf. spinosus and Retiolites nevadensis .

Ludlow and Pridoli (Upper Silurian) Graptolites from the Arctic Islands, Canada

Graptolites flourished from earliest Ordovician to Early Devonian, a time range of about 90 million years, and were widely distributed as marine benthic and planktonic colonial organisms around the world. They were diverse and rapidly evolving and, as such, make excellent “index fossils” for relative age-dating of their enclosing basinal rocks. Rarely, however, graptolites are beautifully preserved in their original three-dimensional form, and can be extracted from the enclosing rock for detailed study under scanning electron microscope. Such a unique preservation is pervasive in the latest Ordovician to latest Silurian rocks of the central Arctic Islands of Canada, presenting an opportunity to study their complex morphology and evolution, making the Arctic graptolite sequence unique in the world. The present study examines the isolated, uncompressed as well as flattened forms from the Ludlow and Pridoli (Upper Silurian) graptolites of the Arctic, recognizing 46 species of monograptids and retiolitids of which seven are described as new species or subspecies.

Isotope Geochemistry and Plankton Response to the Ireviken (Earliest Wenlock) and Cyrtograptus lundgreni Extinction Events, Cape Phillips Formation, Arctic Canada

Several Canadian Arctic Silurian stratigraphic sections from the basinal facies of Cape Phillips Formation have been sampled across the Llandovery–Wenlock and early–late Homerian (late Wenlock) boundary intervals for integration of biotal (graptolite, radiolarian, palynomorph) and geochemical (13C) data for two well-known extinction events, the Ireviken and lundgreni Extinction (LEE) events. Graptolites, abundant and well preserved, provide a refined biostratigraphic base for other paleontologic and geochemical data. They were globally affected by both extinction events: about 64% reduction for the Ireviken and 90–95% for the LEE. Recovery from the LEE event was slow and diversity low through the late Homerian. Radiolarians—diverse (28 species), abundant, and beautifully preserved through the early Homerian―are sharply reduced slightly below the LEE boundary. Data for the late Homerian are more scattered, but it appears that diversity was low; few early Homerian taxa crossed the extinction boundary and new taxa appeared. Palynological studies around the LEE interval are at a preliminary level, but it appears that chitinozoans and microflora (acanthomorph acritarchs, prasinophytes, sphaeromorphs) were impacted by the extinction event. Chitinozoans, though seldom abundant, appear to disappear briefly across the LEE boundary, as do palynomorphs. Amorphous organic matter is abundant in the upper part of the lundgreni Zone; it is much less common in the early and middle–late Homerian and common in the latest part. Stable isotope geochemistry shows well-marked, positive excursions in the δ13Corganic fraction associated with the Ireviken event and LEE. The Ireviken excursion (C1) curve has a sharp base, reaches a peak in the early Wenlock, and then tapers more slowly. The LEE excursion (C4) peaks at, or slightly below, the early–late Homerian boundary. Both are positive excursions. Considering the limits of biostratigraphic placement of the boundaries, they were close to or coincident with regressions, particularly across the LEE interval. The δ13Corganic excursions are greater for inshore sections compared with the offshore section. The most parsimonious explanation for increased carbon content is accelerated weathering of carbonates exposed during a lowstand.