An Upper Ordovician sponge-bearing micritic limestone and implication for early Palaeozoic carbonate successions

The Montréal area belongs to the St Lawrence Lowlands, a Cambrian Early Ordovician passive margin of the Iapetus Ocean, later covered by Appalachian Middle to Upper Ordovician foreland deposits. A structural and palaeostress analysis has been carried out in order to reconstruct its tectonic evolution. The structural map has been revised with new data. Palaeostresses are reconstructed based on inversion of fault slip data, and these results are independently corroborated by the microstructural study of calcite mechanical twinning. Field relationships are used to establish the relative chronology of fractures and to deduce the motion on regional faults. The reconstructed structural and tectonic evolution brings to light some relationships between structural inheritance and tectonic events that have affected the area since Early Palaeozoic times. An early NW–SE extension is responsible for N040-trending faults along the northern border of the St Lawrence Lowlands, and for N090- and N120-trending faults cross-cutting the Montréal area. This extension is followed by WNW–ESE and NNW compressions, which have induced reverse motion on pre-existing faults and generated strike-slip conjugate faults. Subsequent NE–SW and NNW–SSE-directed extensions have reactivated previous faults with normal to strike-slip motions. A late NE–SW compression is recorded in the Monteregian plutons. Compressions in WNW–ESE and NNW directions are consistent with Appalachian collisional tectonism, but N040- and N090-trending faults cross-cut Appalachian folds and foreland deposits. Although the early NW–SE extension is consistent with the collapse of the Iapetan margin in Early Palaeozoic times, most of the present geometry of the St Lawrence Lowlands could be attributed to Mesozoic tectonism, recorded as nearly N–S-directed extensional events.

Download Full-text

A lower Silurian (Llandovery) diplobathrid crinoid (Camerata) from mid-Wales

Geological Magazine ◽

10.1017/s0016756819001511 ◽

2020 ◽

Vol 157 (7) ◽

pp. 1176-1180 ◽

Cited By ~ 1

Author(s):

Fiona E. Fearnhead ◽

Stephen K. Donovan ◽

Joseph P. Botting ◽

Lucy A. Muir

Keyword(s):

Turbidity Current ◽

British Isles ◽

Upper Ordovician ◽

Environmental Fluctuation ◽

Lower Silurian ◽

Sedimentary Succession ◽

Early Palaeozoic ◽

Cover Plates

AbstractEarly Palaeozoic crinoids are known only patchily from the British Isles except for accumulations at starfish beds. A single, exquisitely preserved crinoid is reported from the Telychian (Llandovery, Silurian) of the Pysgotwr Grits Formation of the Llangurig area, Powys, mid-Wales. This sedimentary succession is turbiditic in origin and poorly fossiliferous. The crinoid is a diplobathrid camerate, Euptychocrinus longipinnulus sp. nov., preserved as an external mould without counterpart. It has a high, shuttlecock-like crown; a conical, unsculptured cup; low infrabasals; a pair of long, slender, feather-like arms on each ray, each bearing numerous long pinnules; and a heteromorphic column. Most previous reports of this genus have been from the Upper Ordovician – lower Silurian series of Laurentia; uncertainly, it is described from the Upper Ordovician deposits of Morocco (Gondwana). Euptychocrinus longipinnulus is the first Avalonian occurrence. The beautiful preservation of the arms, including cover plates of pinnules, contrasts with the proxistele, which is a series of ‘broken sticks’. This crinoid responded to an adverse environmental fluctuation, probably a turbidity current, by autotomizing the stem, but it was carried downslope and buried alive.

Download Full-text

Intra-basaltic soil formation, sedimentary reworking and eodiagenetic K-enrichment in the Middle to Upper Ordovician Dunn Point Formation of eastern Canada: a rare window into early Palaeozoic surface and near-surface conditions

Geological Magazine ◽

10.1017/s0016756811001026 ◽

2011 ◽

Vol 149 (5) ◽

pp. 798-818 ◽

Cited By ~ 4

Author(s):

P. JUTRAS ◽

J. J. HANLEY ◽

R. S. QUILLAN ◽

M. J. LEFORTE

Keyword(s):

Soil Formation ◽

Upper Ordovician ◽

Eastern Canada ◽

Near Surface ◽

Soil Material ◽

Orbital Cycles ◽

Enhanced Mobility ◽

Early Palaeozoic

AbstractMafic flows of the Middle to Upper Ordovician Dunn Point Formation of eastern Canada were deeply weathered under warm and relatively humid conditions before being buried by subsequent flows. In the absence of superior plants, and in the context of relatively low atmospheric carbon levels, the soils developed alkaline groundwater conditions through mineral–water interactions, which resulted in an enhanced mobility of Al relative to Si in most palaeosols of that formation. Although the vegetation cover was volumetrically insignificant compared with that of subsequent geological times, it was apparently producing very efficient chelates, which, for most palaeosols of the succession, generated a well-defined cheluviation pattern for not only Al and Fe, but also and mainly Ti, which is typically immobile in modern soils. The resulting soils developed an Al–Fe–Ti-depleted upper horizon that was enriched in Si, probably through periodic ground saturation. Long-term climatic variations related to orbital cycles are inferred to have accounted for a second type of soil in the succession, which contrasts with the former by showing a Si-depleted and less Al–Fe–Ti-depleted upper horizon. Some soil material was substantially reworked by surface runoff, but such occurrences can be easily differentiated from in situ soil material in terms of texture, structure and composition. A thick overlying rhyolite flow is thought to be responsible for providing abundant K in solution, which was incorporated in the underlying basalt palaeosols as exchangeable cations within a probably montmorillonitic clay precursor to the Fe–Mg-rich phengite that later developed during deep burial and orogenic compression.

Download Full-text

Detrital zircon provenance of the Lower Yangtze foreland basin deposits: constraints on the evolution of the early Palaeozoic Wuyi–Yunkai orogenic belt in South China

Geological Magazine ◽

10.1017/s0016756812000969 ◽

2013 ◽

Vol 150 (6) ◽

pp. 959-974 ◽

Cited By ~ 19

Author(s):

HAI-BIN LI ◽

DONG JIA ◽

LONG WU ◽

YONG ZHANG ◽

HONG-WEI YIN ◽

...

Keyword(s):

South China ◽

Volcanic Rocks ◽

Foreland Basin ◽

Primary Source ◽

Upper Ordovician ◽

Palaeomagnetic Data ◽

Lower Yangtze ◽

Depositional Age ◽

Early Palaeozoic ◽

Geological Constraints

AbstractThe Lower Yangtze foreland basin is situated to the northwest of the early Palaeozoic Wuyi–Yunkai orogen in South China. To demonstrate its provenance history and the denudation of the orogen, seven sandstone samples were collected from the upper Ordovician to Silurian strata for U–Pb dating. The zircons show a broad range of ages that can be linked with the ages of specific units in the Wuyi–Yunkai orogen. The zircon spectra in the late Ordovician samples are similar to those in the pre-orogenic strata, suggesting a recycled source. The dominant age population of 880–740 Ma in the early Llandovery samples indicates that the middle Neoproterozoic volcanic rocks were the primary source. A significant age population of 460–425 Ma in the late Llandovery to Wenlock samples reflects the fact that the synorogenic magmatic and metamorphic rocks were exposed to provide detritus. The youngest zircons from the uppermost Silurian strata yield an age of 425 Ma, which approximates the inferred depositional age. This age, together with available biostratigraphic data, indicates that the foreland basin was formed 448–425 Ma ago. We surmise a possible link between the Wuyi–Yunkai orogen and the Appalachian–Caledonian orogen based on the geological constraints and palaeomagnetic data.

Download Full-text