Laurentia

1. Literatur/Quellen..................................................................................1
2. Was ist Erdgeschichte..........................................................................2
3. Altersmessungen..................................................................................3
4. Ur-Atmosphäre.....................................................................................5
5. Präkambrium: Ediacara Fauna.............................................................5
6. Archaische Grünstein Gürtel................................................................6
7. Entwicklung der Tektonik....................................................................7
8. Ende Prä-Kambrium.............................................................................8
9. Der Übergang vom Präkambrium zum Kambrium...............................8
10. Kambrium.............................................................................................9
11. Kambrische Fossil-Lagerstätten.......................................................10
12. Ende des Ordoviziums.......................................................................11
13. Die Bildung von Laurentia und dem Old Red Kontinent...................12
14. Devon..................................................................................................13

The Paleoproterozoic Wernecke Supergroup of Yukon was deposited when the northwestern margin of Laurentia was undergoing major adjustments related to the assembly of the supercontinent Columbia (Nuna) from 1.75 to 1.60 Ga. U-Pb detrital zircon geochronology coupled with Nd isotope geochemistry and major and trace element geochemistry are used to characterize the evolution of the Wernecke basin. The maximum depositional age of the Wernecke Supergroup is reevaluated and is estimated at 1649 ± 14 Ma. Detrital zircon age spectra show a bimodal age distribution that reflects derivation from cratonic Laurentia, with a prominent peak at 1900 Ma. Going upsection, the late Paleoproterozoic peak shifts from 1900 Ma to 1850-1800 Ma, and the proportion of Archean and early Paleoproterozoic zircon decreases. These modifications are a consequence of a change in the drainage system in western Laurentia caused by early phase of the Forward orogeny, several hundred km to the east. The exposed lower and middle parts of the Wernecke Supergroup are correlated with the Hornby Bay Group. Zircon younger than 1.75 Ga appear throughout the sedimentary succession and may have originated from small igneous suites in northern Laurentia, larger source regions such as magmatic arc terranes of the Yavapai and early Mazatzal orogenies in southern Laurentia, and possible arc complexes such as Bonnetia that may have flanked the eastern margin of East Australia. Basins with similar age and character include the Tarcoola Formation (Gawler Craton) and the Willyama Supergroup (Curnamona Province) of South Australia, the Isan Supergroup of North Australia, and the Dongchuan-Dahongshan-Hondo successions of southeast Yangtze Craton (South China). Nd isotope ratios of the Wernecke Supergroup are comparable with values from Proterozoic Laurentia, the Isan and Curnamona assemblages of east Australia, the Gawler Craton, and the Dahongshan-Dongchuan-Hondo successions of the Yangtze Craton of South China. These similarities are compelling evidence for a shared depositional system among these successions. Western Columbia in the Late Paleoproterozoic may have had a dynamic SWEAT-like configuration involving Australia, East Antarctica and South China moving along western Laurentia

Biostratigraphy underpins the Phanerozoic time scale, but its application to pre-Ediacaran
strata has remained limited because Proterozoic taxa commonly have long or unknown stratigraphic
ranges, poorly understood taphonomic constraints, and/or inadequate geochronological
context. Here we report the discovery of abundant vase-shaped microfossils from the
Callison Lake dolostone of the Coal Creek inlier (Yukon, Canada) that highlight the potential
for biostratigraphic correlation of Neoproterozoic successions using species-level assemblage
zones of limited duration. The fossiliferous horizon, dated here by Re-Os geochronology at
739.9 ± 6.1 Ma, shares multiple species-level taxa with a well-characterized assemblage from
the Chuar Group of the Grand Canyon (Arizona, USA), dated by U-Pb on zircon from an
interbedded tuff at 742 ± 6 Ma. The overlapping age and species assemblages from these two
deposits suggest biostratigraphic utility, at least within Neoproterozoic basins of Laurentia,
and perhaps globally. The new Re-Os age also confirms the timing of the Islay δ13Ccarbonate
anomaly in northwestern Canada, which predates the onset of the Sturtian glaciation by
>15 m.y. Together these data provide global calibration of sedimentary, paleontological, and
geochemical records on the eve of profound environmental and evolutionary change.

Detrital zircon (DZ) provenance analysis is used to resolve the age of sandstone injectites together with source sandstones that form fault-bounded, tabular bodies within Mesoproterozoic crystalline rocks of the Colorado Front Range. Named Tava sandstone (informal), the unit is a product of liquefaction and remobilization of mature quartz sediment within source bodies having volumes ≥1x106 m3 into dikes up to 6m in width. To surmount the indeterminate age of emplacement, we obtained new U-Pb DZ age data for two source sandstones, three dikes and one sill, for comparison to four Paleozoic arenites.  Tava age distributions feature a dominant 1.33 to 0.97 Ga broad age group and narrow ca. 1.11, 1.46, and 1.68 Ga groups, with several smaller age groups >1.5 Ga. The Tava DZ results are dissimilar to Paleozoic sandstones but closely resemble published DZ reference data for Grenville-orogen-derived siliciclastic units of western USA. The similarity in age distributions is borne out by statistical comparisons between Tava sandstone, Paleozoic samples, and Neoproterozoic strata that reveal a high probability of correlation of Tava sandstone to ca. 800-680 Ma strata deposited during intracontinental extension. We conclude that Tava sandstone is Neoproterozoic in age and provides a new avenue to investigation of Rodinia’s terrestrial paleoenvironment.

0. Abstract                               1
1. Introduction                               2
2. Paleomagnetic constraints on the Neoproterozoic supercontinent  3
2.1 1100–900 Ma: Laurentia and Baltica               3
2.2 1100–900 Ma: Amazonian craton (AC) and Baltica       3
2.3 1100–900 Ma: Kalahari craton               5
2.4 1100–900 Ma: Congo–Sao Francisco (CSF) craton       5
2.5 1075–990 Ma Siberian craton                       5
2.6 India 1100–1000 Ma                               5
3. True polar wander?                       5
4. The breakup of Rodinia: 800–700 Ma               5
5. 580 Ma and younger reconstructions               7
6. Conclusions                             7

Stephen R. Westrop, Jennifer D. Eoff, Tin-Wai Ng, Alyce A. Dengler and Jonathan M. Adrain

All recent work has assigned the genera Cheilocephalus Berkey, 1898 and Oligometopus Resser, 1936 to the family Cheilocephalidae Shaw, 1956, a small group of largely Laurentian trilobites from the Late Cambrian Steptoean Stage. New data from Nevada and Newfoundland indicate that the Cheilocephalidae is polyphyletic and that Cheilocephalus and Oligometopus are allied with different superfamilies. The Cheilocephalidae, which includes Cheilocephalus and Pseudokingstonia Palmer, 1965, is assigned to the Dameselloidea Kobayashi, 1935, and a pygidium from the Cow Head Group of Newfoundland indicates that Cheilocephalus first appears in the late Marjuman. The Dameselloidea disappeared on other continents at an extinction interval at the end of the Marjuman, but Cheilocephalus survived and underwent modest diversification in Laurentia in the succeeding Steptoean Stage. Oligometopus is transferred to the leiostegioidean family Leiostegiidae Bradley, 1925. The genus shows closest affinities with such Gondwanan genera as Idamea Whitehouse, 1939, and its appearance in mid-Steptoean strata records immigration of the Leiostegoidea into shelf-margin environments of Laurentia.

Circum-Laurentian middle Cambrian (Cambrian Series 3) deposits in Greenland and British Columbia yield a new hipponicharionid bradoriid arthropod, Flumenoglacies n. gen., characterized by a comarginal, ramp-like structure which is crested by a continuous lobe. The narrow lobe is the result of the medial fusion of anterior and posterior lobes, seemingly a recurrent theme in hipponicharionid evolution. The type species, F. groenlandica n. sp., is described from the Ekspedition Brae Formation (Drumian Stage) of Peary Land but the description of two unnamed species from slightly older middle Cambrian strata of the Stephen Formation of British Columbia provides additional evidence for the wide distribution of Small Shelly Faunas during the Cambrian.

The Lipalian or Vendian Period (600-541 million years ago) begins and ends with global environmental perturbations. It begins as the worst glaciation on record draws to a close. It ends with a sudden appearance of abundant skeletonized animals that mark the beginning of Cambrian ecology. Several key events in Earth history occur during the Lipalian, bracked between severe glaciation (white or snowball Earth) and the initiation of modern marine ecosystems. The most notable of these events is the appearance of an unusual and conspicuous marine biota ("Garden of Ediacara"). This biota appears to have characteristics inherited from its sojourn beneath the ice. This chapter examines the role the cryophilic biota, consisting largely of cyanobacteria and chryosphyte and chlorophyte algae, may have played in ending the great ice age. It is hypothesized here that these microbes induced albedo reductions and other changes that rapidly improved global climate.

0. ABSTRACT                                                                               1
1. INTRODUCTION                                                                       2
2. THE RODINIA HYPOTHESIS                                                       3
3. Break-up of Rodinia                                                               3
4. Assembly of southern Gondwana                                       4
5. Rodinia and true polar wander                                               4
6. LOW LATITUDE GLACIATION AND THE SNOWBALL EARTH 5
7. The snowball Earth hypothesis                                                 5
8. Criticisms of the snowball hypothesis                                 5
9. Snowball Earth and early animal evolution                         6
10. CLOSING REMARKS                                                                 6

Contents
Abstract .............………………………………………………...…………………………………..2

1. The Armorican Massif within the framework of the Variscan Belt

2. The precollision history …………………….....……………………………………… 4
2.1. A Proterozoic basement of Gondwanan affinity
2.2. The Early Palaeozoic rifting ………………………………………………………….. 5

3. The northern suture zone(s) ……………………………………………………….. 6
3.1. Key evidences in respect of the northern suture zone(s)
3.2. Geometry and kinematics of the nappe stack ………….........……… 7

4. Tectonic interpretations of the Léon Domai.……………….……………….. 8
4.1. Potential models
4.2. The Léon Domain and the Normannian Nappe: a single crustal unit
4.3. Tectonic evolution of the Léon Domain ……………………………………… 9
The southern suture zone(s)

5.1. Key evidence in respect of the southern suture zone(s)
5.1.1. Palaeobiogeographical data
5.1.2. Ophiolite complexes …………………………………………………………….… 10

5.2. Geometry and kinematics of the nappe stack (Fig. 3 and 5 to 7 .  11
5.2.1. North of the Nort-sur-Erdre Fault
5.2.2. South of the Nort-sur-Erdre Fault ………………………………………………… 12

5.3. Age of the high-pressure relics ……………………………………………………… 13
5.4. Erosion of a mountain belt

6. Tectonic interpretation of the South-Armorican domai…………..… 14

6.1. Correlations between the Iberian Massif and the Armorican Massif
6.1.1. The Audierne nappe stack ……………….……………………………………….. 15
6.1.2. The Champtoceaux-Mauges nappe stack
6.1.3. The Vendée nappe stack
6.1.4. The structural position of the Île-de-Groix blueschists

6.2. Correlations between the Armorican Massif and the Massif Central
6.2.1 From Champtoceaux to Aigurande
6.2.2 2.2. From the Vendée to the Limousin ………………….………….…… 17

7. Tectonic models of the Armorican Massif

Circum-Laurentian middle Cambrian (Cambrian Series 3) deposits in Greenland and British Columbia yield a new hipponicharionid bradoriid arthropod, Flumenoglacies n. gen., characterized by a comarginal, ramp-like structure which is crested by a continuous lobe. The narrow lobe is the result of the medial fusion of anterior and posterior lobes, seemingly a recurrent theme in hipponicharionid evolution. The type species, F. groenlandica n. sp., is described from the Ekspedition Brae Formation (Drumian Stage) of Peary Land but the description of two unnamed species from slightly older middle Cambrian strata of the Stephen Formation of British Columbia provides additional evidence for the wide distribution of Small Shelly Faunas during the Cambrian.

Section 3-Earth/Environmental Science Basic Research Grant 2002 Provenance of the Colonsay Group, SW Scotland Claire A. McAteer1 J. Stephen Daly (Principal Investigator) 1, MJ Flowerdew2, JN Connelly3, TB Housh3 1 School of Geological Sciences, University ...