The initiation of the early Paleozoic Cordilleran miogeocline: evidence from the uppermost Proterozoic – Lower Cambrian Hamill Group of southeastern British Columbia

Abstract

The uppermost Proterozoic -Lower Cambrian Hamill Group of southeastern British Columbia contains geologic evidence for a phase of extensional tectonism that led directly to the onset of thermally controlled subsidence in the Cordilleran miogeocline. Moreover, the Harnill Group contains the sedimentological record of the passage of the ancient passive margin from unstable tectonic conditions associated with rifting and (or) the earliest phases of thermal subsidence to post-rift conditions characterized by stabilization of the margin and dissipation of the thermal anomaly generated during the rift phase (the rift to post-rift transition). Widespread uplift that occurred prior to and during the deposition of the lower Hamill Group is indicated by an unconformable relation with the underlying Windermere Supergroup and by stratigraphic relations between Middle and Upper Proterozoic strata and unconformably overlying upper Lower Cambrian quartz arenites (upper Hamill Group) in the southern borderlands of the Hamill basin. In addition, the coarse grain size, the feldspar content, the depositional setting, and the inferred provenance of the lower Hamill Group are all indicative of the activation of basement sources along the margins of the Hamill basin. Geologic relations within the Hamill Group that provide direct evidence for extensional tectonism include the occurrence of thick sequences of mafic metavolcanics and rapid vertical facies changes that are suggestive of syndepositional tectonism.Evidence of extensional tectonism in the Hamill Group directly supports inferences derived from tectonic subsidence analyses that indicate the rift phase that immediately preceded early Paleozoic post-rift cooling could not have occurred more than 10-20 Ma prior to 575 + 25 Ma. These data, together with recently reported isotopic data that suggest deposition of the Windermere Supergroup began -730-770 Ma, indicate that the rift-like deposits of the Windermere Supergroup are too old to represent the rifting that led directly to the deposition of the Cambro-Ordovician post-rift strata. Instead, Windermere sedimentation was apparently initiated by an earlier rift event, probably of regional extent, that was part of a protracted, episodic rift history that culminated with continental breakup in the latest Proterozoic -Early Cambrian.Le Groupe de Hamill, d'lge prottrozoique tardif -cambrien hltif, dans le sud-est de la Colombie-Britannique, exhibe des phtnomtnes gtologiques qui ttmoignent en faveur d'une phase de tectonisme d'extension qui serait directement responsable du dtmarrage d'une subsidence, crontr6lte par des conditions thermiques, dans le miogtoclinal de la Cordilltre. En outre, les sediments du Groupe de Hamill portent les marques de la transition d'une ancienne marge passive a des conditions tectoniques instables associks a la formation d'un rift et (ou) de phases pkcoces de subsidence thermique passant ?I des conditions postrift caractCrisCes par une stabilisation de la marge et une dissipation de l'anomalie thermique e n...

Chemostratigraphy of predominantly siliciclastic Neoproterozoic successions: a case study of the Pocatello Formation and Lower Brigham Group, Idaho, USA

“…The Bannock Volcanic Member has traditionally been correlated with mafic volcanic and diamictite-bearing strata from northeast Washington that have been dated using Sm-Nd and Rb-Sr whole-rock analyses at 762 ± 44 Ma (2<x; Devlin & Bond, 1988), a date consistent with the ages of Neoproterozoic volcanism along the length of the Cordillera. Most correlations suggest that the principal diamictite horizons found in northwestern Canada, southern British Columbia and adjacent Washington, Idaho, Utah and California are equivalent and represent the Sturtian glaciation.…”

“…Subsidence analysis of other Neoproterozoic to Cambrian successions along the Cordilleran miogeocline has been used to date the initiation of crustal subsidence at approximately 575 + 25 Ma (assuming a date of 545 Ma for the base of the Cambrian; Armin & Mayer, 1983;Bond, Kominz & Devlin, 1983;Bond et al 1985;Bond & Kominz, 1984;Devlin & Bond, 1988;Christie-Blick & Levy, 1989a;Levy & Christie-Blick, 19916). The discrepancy between this date and the age of the initiation of subsidence and rifting implied by the Pocatello Formation and correlative units (perhaps as much as 200 Ma older) was first noted by Link (1984).…”

Chapter 39 The Edwardsburg Formation and related rocks, Windermere Supergroup, central Idaho, USA

“…of southeastern Idaho (Harper & Link 1986) and the broadly correlative Huckleberry Fm. of northeastern Washington and southern British Columbia (Devlin et al 1988) are chemically similar to these metabasalts of the Edwardsburg Fm.…”

“…However, it is possible that the geochronological data for the Edwardsburg Fm. constrains Aitken (1991), Rainbird et al (1996) Ross et al (1995), Kendall et al (2004); 2 Devlin & Bond (1988); Hein & McMechan (1994); 3 Devlin & Bond (1988), Devlin (1989), Hein & McMechan (1994), Colpron et al (2002); 4 Evans (1987), Miller & Whipple (1989), Lindsey et al (1990), Miller (1994); 5 Lund et al (2003);6 McCandless (1982), Evans & Green (2003), Lund et al (2010); 7 Crittenden et al (1971Crittenden et al ( , 1983, Link et al (1993), Christie-Blick (1997); 8 Crittenden et al (1971Crittenden et al ( , 1983, Link et al (1993).…”

Sedimentology and regional significance of the ‘argillite unit’, a probable Cryogenian map unit in southeast Yukon, Canada

“…Some workers have viewed rifting as essentially synchronous with the base of the Windermere Supergroup (Ross, ; Narbonne and Aitken, ; Dalrymple and Narbonne, ; Thorkelson et al, ), whereas others have emphasized latest Neoproterozoic or earliest Cambrian extension (Devlin and Bond, ; Devlin, ; Lickorish and Simony, ; Macdonald et al, , ). Rifting likely was protracted and complex (e.g.…”