Stratigraphy, conodont taxonomy and biostratigraphy of Upper Cambrian to Lower Silurian platform to basin facies, northern British Columbia
Date
2018-02-26
Authors
Pyle, Leanne
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Abstract
This study establishes the stratigraphic framework and conodont biostratigraphy
of Lower Paleozoic strata of the Northern Canadian Cordilleran Miogeocline, which
document a non-passive tectonic evolution of the rifted margin of Laurentia. Only a few
reconnaissance stratigraphic studies have been conducted previously in the study area.
Nine key sections span an east-west transect from the Macdonald Platform to the Kechika
Trough (platform-miogeocline-basin) and 3 key sections comprise a transect across the
parautochthonous Cassiar Terrane. Over 12 000 m of strata from the Kechika and Skoki
formations and Road River Group in northeastern British Columbia were measured and
described, from which a total of 405 conodont samples (4-5 kg each) were taken. A total
of 39 526 conodonts have been used to refine the Upper Cambrian to Lower Silurian
conodont biostratigraphy across the transect.
The stratigraphy is revised to divide the Kechika Formation (late Cambrian to
early Arenig in age) into 5 formal members: Lloyd George, Quentin, Grey Peak.
Haworth and Mount Sheffield members. The Skoki Formation (early to late Arenig in
age) comprises 3 new formal members defined as: Sikanni Chief. Keily and Redfern
members. The Road River Group is divided into 3 new formations: Ospika (early
Arenig to Llanvim in age), Pesika (Lower Silurian in age) and Kwadacha (formerly the
Silurian Siltstone). The Ospika Formation is further subdivided into 5 formal members:
Cloudmaker, Finlay Limestone, Chesterfield, Finbow Shale and Ware.
Conodonts of Late Cambrian to Early Silurian age are described taxonomically from the Kechika, Skoki, Ospika and Pesika formations across the transect. A total of 39 526 identifiable conodonts recovered from 142 productive samples indicate high species diversity and abundance in shallow water facies and less diversity and abundance
with in deeper water facies. Elements are moderately to well preserved, typically with a colour alteration index (CAl) of 3-5.
A total of 197 species, representing 73 genera are identified and illustrated among
which 6 new genera and 39 new species are described. Fifteen of the 39 new species had
too little material and were described in open nomenclature. The new genera are
Graciloconus, Kallidontus, Planusodus and 3 new genera (A, B, C) treated in open nomenclature. The new species are Acodus kechikaensis n. sp., A. quentinensis n. sp., A. warenesis n. sp., Cordylodus delicatus n. sp., Colaptoconus
greypeakensis n. sp., ?Diaphorodus n. sp., Drepanoistodus minutus n. sp., Graciloconus
concinnus n. gen. n. sp., Kallidontus serratus n. gen. n. sp., K. nodosus n. gen. n. sp., K.
princeps n. gen. n. sp., Laurentoscandodus sinuosus n. sp., Macerodus cristatus n. sp., M
lunatus n. sp., Microzarkodina n. sp., Oepikodus n. sp., Oistodus n. sp., Paroistodus n.
sp., Planusodus gradus n. gen. n. sp., ?Prioniodus n. sp., Protoprioniodus n. sp.,
Rossodus kwadachaensis n. sp., R. muskwaensis n. sp., R. sheffieldensis n. sp.. R. subtilis
n. sp., Scolopodus amplus n. sp., Striatodontus strigatus n. sp., Triangulodus akiensis n.
sp., Tricostatus infundibulum n. sp., T. terilinguis n. sp., 3 unnamed new genera and 3
new species and 5 new species of Drepanoistodus (A, B. C, D, E).
The conodont zonation for Upper Cambrian to Lower Silurian strata is refined, using
Sections 4, 5, 13 and Grey Peak as reference sections. It allows close dating of
stratigraphic boundaries. The oldest zones in the Kechika are cosmopolitan and include
the Eoconodontus Zone (upper Cambrian), Cordylodus proavus and Cordylodus
lindstromi zones (uppermost Cambrian) and lapetognathus Zone (base of Tremadoc).
Ten higher zones are recognized and redefined for shallow water platform facies
containing faunas of the Midcontinent Realm. Four of these are new (Polycostatus
falsioneotensis, Rossodus tenuis, Scolopodus subrex and Acodus emanualensis zones)
and 10 new subzones are established. Those for the Kechika Formation include, in
ascending order, the Polycostatus falsioneotensis Zone (lower Tremadoc). Rossodus
tenuis Zone (lower Tremadoc); Rossodus manitouensis Zone with R. muskwaenesis and
R. sheffieldensis subzones (middle Tremadoc), Low diversity interval (upper Tremadoc), Scolopodus subrex Zone with Graciloconus concinnus and Colaptoconus bolites subzones (lower Arenig) and Acodus kechikaensis Zone with Kallidontus serratus.
Diaphorodus russoi and Kallidontus nodosus subzones (lower Arenig). Those for the
Skoki Formation include the Oepikodus communis Zone with Tropodus sweeti,
Bergstroemognathus extensus and Juanognathus variabilis subzones (middle Arenig).
The O. communis Zone spans the Kechika-Skoki boundary and the uppermost Kechika
lies within the lowermost part of the O. communis zone underlying the T. sweeti Subzone.
The Skoki Formation also contains the Jumudontus gananda Zone (middle Arenig) and Tripodus laevis Zone (upper Arenig). The Phragmodus undatus Zone (Upper
Ordovician) lies within the Road River Group in the Cassiar Terrane.
Thirteen deep water zones are recognized for basinal facies containing faunas of
predominantly the North Atlantic Realm. Five new zones are established
(Drepanoistodus nowlani, Acodus deltatus, Paracordylodus gracilis, Paroistodus
horridus and Dzikodus tableheadensis zones) and one new subzone within the P. gracilis
Zone is proposed. Those within the Kechika Formation include Cordylodus angulatus
Zone (lower Tremadoc), Paltodus deltifer Zone (middle Tremadoc), Drepanoistodus
nowlani Zone (middle Tremadoc), Acodus deltatus Zone, (middle Tremadoc),
Paroistodus proteus Zone (upper Tremadoc), Paracordylodus gracilis Zone with
Oelandodus elongatus Subzone (upper Tremadoc) and Prioniodus elegans Zone (base of
Arenig). Those within the Skoki and Ospika formations include Oepikodus evae Zone
(Skoki Formation, middle Arenig), Paroistodus originalis Zone (Skoki and Ospika
formations, upper Arenig), Paroistodus horridus and Dzikodus tableheadensis zones
(both within the Ospika Formation, lower Llanvim). The Amorphognathus tvaerensis
Zone lies within the Road River of the Cassiar Terrane (Upper Ordovician). The
Distomodus staurognathoides Zone lies within the Pesika Formation (middle
Llandovery).
The conodont faunas therefore provide detailed temporal constraints for the
stratigraphic framework. Some evolutionary remarks are made for selected species
involved in radiations, especially in the Tremadoc and Arenig, that are useful in further
refining the standard Midcontinent Realm zonation. The Midcontinent Realm conodont
faunas are used for regional correlations within North America and those of the Atlantic
Realm provide calibration on an interregional scale, for example, with Baltica.
Description
Keywords
Geology, Stratigraphic, Cambrian, Silurian, Conodonts