Editorial
Processes in physical volcanology and volcaniclastic
sedimentation: modern and ancient
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The physical volcanology of modern and an-cient depositional systems, including the dynamics and features controlling eruption mechanisms, and the processes governing subaerial and sub-aqueous transport of pyroclastic and volcaniclas-tic debris are controversial subjects that have drawn much attention and debate in recent litera-ture (Fisher and Schmincke, 1984; Cas and Wright, 1987; McPhie et al., 1993). Physical vol-canological features in the ancient rock record, and especially the Archean, are significant because of their association with volcanic massive sulfide (VMS) deposits (Dimroth et al., 1985; Gibson et al., 1997, 1999). Defining proximal and distal morphological features of mafic and felsic lava flows is an important first order exploration tool, which, combined with hydrothermal alteration patterns (Franklin, 1990; Galley, 1993), and trace element geochemistry (Wyman, 1996), facilitates VMS exploration at depth and elaborates pre-ex-isting VMS models.
The physical volcanology of active volcanic edifices is commonly geared to explosive and effu-sive processes operating on emergent oceanic and continental arc volcanoes, and oceanic hot spots, such as Hawaii and Iceland. Emphasizing process-oriented features is a logical outgrowth because volcanic eruptions, such as those occurring at Surtsey (Thorarinsson, 1967), Mount St Helen’s (Criswell, 1987) or Myojinsho (Fiske et al., 1998) were witnessed, and eruption or eruption
mecha-nisms need not be contested, as is commonly the case in ancient volcanic sequences. The focus can be placed on volcanological processes, which cause eruptions and affect transport mechanisms. Volcanic rocks in ancient sequences are com-monly studied to establish a depositional setting or elaborate on a proposed geodynamic model that is linked to crustal-scale subduction- or hot spot-related mechanisms, rather than lava or magma genesis, as is often the case for modern volcanic deposits. Ancient volcanic sequences still represent a significant window for elucidating sub-aqueous eruptive mechanisms because the ex-posed section in ancient greenstone belts is commonly more complete than that those seen in modern subaqueous volcanic sequences. Subaerial volcanic rocks in Proterozoic and Archean vol-cano-sedimentary successions are poorly pre-served due to erosive processes, whereas submarine volcanic sequences are preferentially preserved in the ancient record. The exception to the rule appears to be the stable Kaapvaal craton, South Africa, spanning almost 2 Ga of Earth’s history. Abundant continental-type effusive and paroxysmal volcanism developed on this non-veg-etated (?) Archean to Proterozoic continent (Tankard et al., 1982), and represents a subject well worth further study in more volcanological and geochemical detail. Study areas on land are far more accessible to scientists than modern ocean floor lava fields via submersibles. Subaerial
eruptive counterparts are therefore generally em-phasized in modern deposits and are easier to document than subaqueous eruptive products.
This special volume in Precambrian Research represents a direct outgrowth of a special session ‘‘Processes in physical volcanology and volcani-clastic sedimentation: modern and ancient’’ at the meeting of the Geological (GAC) and Mineralogi-cal Association of Canada held in Que´bec City, May 1998, and sponsored by the Volcanology and Igneous Petrology Division of the GAC. The session attempted to combine many aspects of volcanology and volcaniclastic sedimentation in modern and ancient sequences. Numerous contro-versial aspects in volcanology were considered, irrespective of age, in which the overriding theme remained process-oriented, either associated with a specific depositional setting, an eruption nism and the resultant deposit, transport mecha-nisms in subaqueous eruption-fed deposits, the evolution of volcanic edifices and their geochemi-cal affinities, and hydrothermal fluids and related mineral deposits in conjunction with volcaniclastic sediments. Combining a session that deals with modern and ancient deposits was a gain for all the scientists involved because it created an awareness of specific problems that were not constrained to one epoch.
This volume represents an attempt at bridging the gap between volcanologists operating in an-cient and modern terranes. The oral and poster presentations indicated clearly that (1) volcanolo-gists conducting studies in modern settings should consider ancient rocks (including Archean) as a plausible source of information because textures, deposits and sequences are often remarkably well preserved and sections are more complete, and (2) geologists working in Precambrian terranes should not restrict themselves to documentation of volcanic rocks in order to distill a geodynamic model, but rather use a modern volcanological facies approach to better identify different pro-cesses operating on volcanic edifices and subse-quently constrain the depositional environment and thus geodynamic settings of Precambrian mountain belts.
Although the journal, Precambrian Research, emphasizes the Archean and Proterozoic, to
which most papers in this volume are geared, two papers concerning volcaniclastic rocks in prevege-tation times of early Paleozoic age, and a signifi-cant process-oriented synthesis paper are included in this special issue. These papers were integrated to demonstrate that volcaniclastic products in a subaqueous setting are controlled by the ambient water medium, and to show that subaqueous eruption-fed deposits should be common in the ancient rock record if a proper facies analysis is conducted. Submarine volcaniclastic material de-posited either directly from eruptions or remobi-lized and transported down-slope, are controlled by the same types of eruptions and governed by the same mass flow processes throughout time. In contrast, a striking difference is anticipated for reworked subaerial volcaniclastic deposits during the Archean and Early Proterozoic because of a weathering-intensive, CO2-rich atmosphere, and
lack of vegetation. As pointed out by Eriksson et al. (1998) and Mueller and Corcoran (1998) un-confined sheetflow and hyperconcentrated floodflow deposits may be the rule, not the excep-tion. Further studies concerning the role of the atmosphere on weathering, erosion, and transport of rocks should be considered.
The contributions in this volume show the close link between sedimentology and physical vol-canology because transport processes and resul-tant structures are strikingly similar. Determining the locus of deposition or tectonic setting cannot be resolved by one discipline alone and requires an interdisciplinary approach. Volcanology asso-ciated with sedimentology helps constrain the set-ting and facilitates reconstruction of the paleogeography. All papers display this combined approach.
1. Scientific contributions
disintegration of lava. Three principal categories were established for subaqueous deposits based on internal sedimentary structures and transport properties and mechanisms. Group 1 deposits in-clude pyroclastic deposits sensu stricto because they are gas-supported and contain typical heat retention structures. Group II volcaniclastic rocks, referred to as eruption-fed aqueous de-posits, are transported in a submarine milieu via high- to low-concentration turbidity currents, grain-flows and debris flows that originate directly from an eruption, but the transport medium is water-supported. Group III products are de-posited from lava flow-fed density currents, gener-ally derived from dynamo-thermal quenching and spalling of flows.
Two papers deal with subaqueous Paleozoic volcaniclastic deposits. Cousineau and Be´dard with ‘‘Sedimentation in a subaqueous arc/ back-arc setting: the Bobby Cove Formation, Snooks Arms Group, Newfoundland’’ propose a sub-aqueous eruption model for tuff turbidites, lapilli tuff-breccias and subordinate basaltic tuff-lapilli tuff deposits in a deep-water setting. It is inferred that some of the deposits could be eruption-fed products, but that many could be remobilized debris emanating from the vent. The deposits indicate that high- to low-concentration particu-late sediment gravity flows were prevalent with water as the transport medium. ‘‘Epiclastic vol-canic debrites-evidence of flow transformations between avalanche and debris flow processes, Middle Ordovician, Baie Verte Peninsula, New-foundland’’, by Kessler and Be´dard, describes mafic volcanic debrites detailing and interpreting their textural aspects in proximal and distal set-tings. The paper also includes estimates of the physical parameters such as the velocity of the flows.
The principal thrust of the special volume fo-cuses on the Earth’s Precambrian evolutionary period with two papers concerning the character-istics of shallow water and subaerial Pale-oproterozoic volcaniclastic rocks. ‘‘Shallow-water, eruption-fed, mafic pyroclastic deposits along a Paleoproterozoic coastline: Kangerluluk volcano-sedimentary sequence, southeast Greenland’’ by Mueller, Garde and Stendal presents a
well-pre-served volcano-sedimentary sequence. The bound-ing conglomerate-sandstone and volcanic lithofacies are indicative of a shallow-water coastal setting and the interstratified pyroclastic lithofacies, composed of a planar- to crossbedded tuff-lapilli tuff and a bedded lapilli-tuff breccia, are contemporaneous with the mafic pillowed suc-cession of the volcanic lithofacies. Both deposits are considered to result directly from shallow-wa-ter surtseyan-type eruptions, producing a combi-nation of gas-driven and water-laden, eruption-fed pyroclastic debris. Oberholzer and Eriksson in ‘‘Subaerial volcanism in the Pale-oproterozoic Hekpoort Formation (Transvaal Su-pergroup), Kaapvaal Craton’’, make a case for subaerially emplaced pyroclastic flows and their reworked counterparts. The intracratonic massive pyroclastic breccia and lapilli-tuff breccia deposits of mafic composition are considered primary eruptive debris, that on the distal and marginal portions of the pyroclastic flow, have been re-worked to produce the stratified lapilli-tuff brec-cias, an inferred sheetflood deposit, and massive reworked lapilli-tuff breccias interpreted as debris flow deposits. Ayres and Peloquin in a paper entitled ‘‘Subaqueous, Paleoproterozoic, metarhy-olite dome-flow cone complex, Flin Flon green-stone belt, Manitoba’’ give an excellent description and evaluation of a Proterozoic sub-aqueous rhyolite dome complex composed of two domes mantled by a cone-like mound composed of thin lava tongues extruded over and intruded into coeval hyaloclastite and resedimented pyro-clastic deposits. The authors show that a distinc-tion between endogenic and exogenic events is possible in the ancient rock record.
deep-water, a medial to distal, deep-water, and a medial, shallow-water seamount setting. Mafic subaqueous volcanic edifice construction con-ducted in Archean terranes because recognition of mafic-dominated centres is an important criterion for massive sulfide exploration. Chown, N’Dah and Mueller in the paper, ‘‘The relation between iron-formation and low temperature hydrother-mal alteration in an Archean volcanic environ-ment’’ suggest that some iron-formations in the Archean may represent a subsurface replacement deposit produced by extensive hydrothermal vent-ing. Some Archean banded iron-formations may therefore represent a precursor phase for high-temperature volcanic-hosted massive sulfide deposits.
Lafrance, Mueller and Daigneault in ‘‘Evolu-tion of a submerged composite arc volcano: vol-canology and geochemistry of the Norme´tal volcanic complex, Abitibi greenstone belt, Que´-bec, Canada’’ demonstrate the various construc-tive phases of a complex Archean subaqueous mafic to felsic volcanic edifice. The well-studied complex is composed of coalescing centres that display distinct physical volcanic and geochemical characteristics. The paper, ‘‘An Archean quartz-ite-andesite association in the Baltic Shield: Impli-cations for assemblage types and shield history’’ by Thurston and Kozhevnikov, describes an ande-site-quartzite assemblage representing the accu-mulation of shallow-water quartz-rich sedimentary rocks in a setting typical of the late stages of arc volcanism, as well as a part of the basin which subsided into a shallow marine basin with deepening in rifts. The paper has implica-tions for future correlaimplica-tions between Archean seg-ments of the Canadian and Baltic shields as well as on the role of quartz-rich sedimentary rocks as a correlation tool in the Archean. The final study ‘‘High temperature ash flow-wet sediment interac-tion in the Makawassie Formainterac-tion, Ventersdorp Supergroup, South Africa’’ by van der West-huizen and de Bruiyn describes the characteristics of an ash flow deposit in contact with an uncon-solidated sedimentary unit. Spherical structures at the base of an inferred high temperature ash flow are interpreted to be the results of a hot pyroclas-tic flow interacting with the underlying water-sat-urated sediments.
Acknowledgements
References
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Guest editors
W. Mueller and E.H. Chown Sciences de la Terre, Uni6ersite´ du Que´bec a` Chicoutimi,
Chicoutimi, Que´bec, Canada G7H 2B1 E-mail: wmueller@uqac.uquebec.ca
P.C. Thurston Precambrian Geoscience Section, Ontario Geological Sur6ey,
933 Ramsey Lake Road, Sudbury, Ont., Canada P3E 6B5
