Volcaniclastic Sedimentary Rocks Research Articles

Magmatic changes during the stabilisation of a cordilleran fold belt: the Late Carboniferous–Triassic igneous history of eastern New South Wales, Australia

In a 60 Ma interval between the Late Carboniferous and the Late Permian, the magmatic arc associated with the cordilleran-type New England Fold Belt in northeast New South Wales shifted eastward and changed in trend from north–northwest to north. The eastern margin of the earlier (Devonian–Late Carboniferous) arc is marked by a sequence of calcalkaline lava flows, tuffs and coarse volcaniclastic sedimentary rocks preserved in the west of the Fold Belt. The younger arc (Late Permian–Triassic) is marked by I-type calcalkaline granitoids and comagmatic volcanic rocks emplaced mostly in the earlier forearc, but extending into the southern Sydney Basin, in the former backarc region. The growth of the younger arc was accompanied by widespread compressional deformation that stabilised the New England Fold Belt. During the transitional interval, two suites of S-type granitoids were emplaced, the Hillgrove Suite at about 305 Ma during an episode of compressive deformation and regional metamorphism, and the Bundarra Suite at about 280 Ma, during the later stages of an extensional episode. Isotopic and REE data indicate that both suites resulted from the partial melting of young silicic sedimentary rocks, probably part of the Carboniferous accretionary subduction complex, with heat supplied by the rise of asthenospheric material. Both mafic and silicic volcanic activity were widespread within and behind the Fold Belt from the onset of rifting (ca. 295 Ma) until the reestablishment of the arc. These volcanic rocks range in composition from MORB-like to calcalkaline and alkaline. The termination of the earlier arc, and the subsequent widespread and diverse igneous activity are considered to have resulted from the shallow breakoff of the downgoing plate, which allowed the rise of asthenosphere through a widening lithospheric gap. In this setting, division of the igneous rocks into pre-, syn-, and post-collisional groups is of limited value.

The Bail Hill Volcanic Group: alkaline within-plate volcanism during Ordovician sedimentation in the Southern Uplands, Scotland

AbstractThe Bail Hill Volcanic Group (Caradoc) represents the largest, single volcanic complex exposed within the Ordovician turbidite succession of the Northern Belt in the Southern Uplands of Scotland. The group comprises a heterogeneous sequence of submarine lavas, volcaniclastic and intrusive rocks (up to 2 km thick), and crops out in a small area (c. 4 km2) around Bail Hill, north of Sanquhar. The Cat Cleuch Formation (older) is dominated by a sequence of autobrecciated basaltic lavas which contain large, zoned diopsidic clinopyroxene. The overlying Peat Rig Formation comprises a more mixed sequence of plagioclase-amphibole-phyric lavas, volcaniclastic rocks and contemporaneous volcaniclastic sedimentary rocks. The Cat Cleuch and lower part of the Peat Rig formations are cut by a vent breccia, the Bught Craig vent breccia, which formed part of the feeder to the upper part of the Peat Rig Formation. The Bail Hill volcanic rocks are alkaline in character, ranging from alkali basalt to trachyandesite in composition, possessing trace element characteristics and enrichment patterns typical of oceanic within-plate basalts. The Bail Hill Volcanic Group, although geochemically distinct, forms part of a mixed assemblage of tholeiitic and alkaline withinplate lavas within the Southern Uplands which are of broadly similar age, some of which are intercalated within the greywacke sandstone sequence. This assemblage clearly indicates that a period of extension and within-plate volcanism occurred during the early stages of the development of the Southern Uplands sedimentary basin.

Palaeomagnetism of the Borrowdale and Eycott volcanic groups, English Lake District: primary and secondary magnetization during a single late Ordovician polarity chron

Late Ordovician volcanic rocks of the English Lake District typically have a magnetic remanence dominated by a single characteristic component. Previous investigations have interpreted this remanence as both of primary (pre-folding) and secondary origin. Palaeomagnetic field tests have been conducted on (a) andesite blocks from an autobrecciated lava top, (b) andesite blocks in mass-flow breccias, and (c) fault-blocks tilted during Ordovician caldera collapse to establish the time of remanence acquisition. All three tests show that the lavas retain a magnetization acquired during initial cooling: magnetizations of the breccias are coherent within clasts and random between clasts, whilst magnetizations of the tilted fault blocks converge with better that 95% confidence when corrected for the effects of caldera collapse. In contrast, the volcaniclastic sedimentary and pyroclastic rocks possess an Ordovician secondary remanence acquired after strata had been tilted by volcano-tectonic subsidence. A distributed sample of 65 andesite and basalt sheets through the Borrowdale Volcanic Group has a mean remanence direction D/I=341.9/−48.9° (α95=4.0°) yielding a positive fold test and a palaeomagnetic pole at 12.7°E, 4.3°S (dp/dm=3.5/5.3°). A progressive steepening of the palaeofield direction is recorded during emplacement of the Borrowdale Volcanic Group (∼I=−39° to I=−51°) which continued into the interval of volcanotectonic overprinting (I=−62°); the equivalent motion of Eastern Avalonia is ∼20° into higher southerly latitudes.Both the Eycott and Borrowdale volcanic groups exhibit uniform normal polarity throughout. Correlation with the geomagnetic time scale for the Ordovician restores the broad correlation between the two groups by constraining their emplacement and partial overprinting to a single long normal polarity chron occupying the Nemagraptus gracilis and earlier part of the Diplograptus multidens biozones (late Llandeilo and early Caradoc). All the volcanism, therefore, occurred within a period of no more than ∼5 Ma. The palaeomagnetic evidence confirms that the Borrowdale Volcanic Group was affected by both syn-volcanic deformation (caldera collapse) and regional compressive deformation prior to deposition of the (late Ordovician–Silurian) Windermere Supergroup. The succession of primary and secondary Ordovician palaeomagnetic poles from the Lake District inlier defines an anticlockwise apparent polar wander (APW) loop with the apex correlating with ‘soft’ closure of the Iapetus Ocean and late Ordovician deformation. The APW paths from Avalonia and Baltica converge at this point as subduction ceased and the arc subsided beneath the sea after mid-Caradoc times.

Paleomagnetism of the Permian Wallowa terrane: Implications for terrane migration and orogeny

A sequence of volcaniclastic sedimentary and pyroclastic rocks of Permian age (late Leonardian or early Guadalupian; ≈260 Ma) in the Wallowa terrane of Oregon and Idaho has been sampled for paleomagnetic study. Forty‐six sites (222 cores) were collected from folded, laminated siltstone, mudstone, and tuffaceous sandstone beds exposed along the Snake River in Hells Canyon. Several stratigraphic sections, within the 1500‐ to 2500‐m‐thick Hunsaker Creek Formation, were sampled over a stratigraphic interval of about 700 m. Thermal demagnetization revealed three components of magnetization. The A component has the lowest unblocking temperatures (generally less than about 200°C), has normal polarity, and fails the fold test. The B component is removed between heating steps of about 200°C and 550°C, is of normal polarity, and also fails the fold test. The C component, observed between heating steps of approximately 550°C and 660°C, is always of negative inclination. The grouping of this component improves slightly upon unfolding, with the optimum grouping occurring at 95% unfolding, using sites with n ≥ 3 and k ≥ 10, (k95/k0 = 1.18), or 100% using sites n ≥ 3 (k95/k0 = 1.51). This component is interpreted as the characteristic remanent magnetization acquired during deposition of the Hunsaker Creek Formation based on the following observations: The better grouping of the high‐temperature magnetic component after unfolding and the consistent reversed polarity seen in samples from the different sections. The resulting paleolatitude, calculated using inclination‐only statistics, of 24°±12° N (n ≥ 3, k ≥ 10), or 26°±9°N (n ≥ 3 sites), implies a northern hemisphere position of the Wallowa terrane during the Early Permian and possible southward transport of this terrane with respect to North America. If the Wallowa terrane was attached to the Wrangellia superterrane, our result indicates a northern hemisphere location for that superterrane during the Permian.

Seismic refraction measurements within the Peninsular terrane, south central Alaska

We present an interpretation of crustal seismic refraction data from the Peninsular terrane, one of the many exotic terranes that have been accreted to the continental margin of southern Alaska in the past 200 m.y. A seismic refraction line was collected along the Glenn Highway in the Copper River Basin of south central Alaska in 1984 and 1985, as part of the U.S. Geological Survey Trans‐Alaska Crustal Transect (TACT) program. P wave velocities of 2.7–3.5 km/s and thicknesses of 1–2 km characterize post‐Lower Jurassic sedimentary rocks that underlie most of the seismic refraction line. An average crustal velocity structure includes the following five velocity divisions. Beneath the sedimentary rocks lie 1–2 km of 4.0–4.6 km/s materials, correlating with andesitic volcaniclastic sedimentary rocks and lava flows of the Lower Jurassic Talkeetna Formation. Below these rocks, seismic velocity increases rapidly, from 5.0 to 6.1 km/s, in 2–3 km. At 7–8 km depth, velocity jumps to 6.3 km/s and increments to 6.6 km/s by 10–12 km depth. Velocities increase from 6.8 to 7.0 km/s between 12 to 20 km depth. At about 22 km depth, a jump in velocity from 7.0 to 7.4 km/s is inferred but is poorly resolved. Depth to the Moho discontinuity could not be determined from our data. The absence of clear PmP reflections may indicate that Moho is deeper than 40 km. Data from two offset shotpoints northeast of the line and within the Wrangellia terrane constrain the deep structure transition between Peninsular and Wrangellia terranes. The 6.3–6.6 km/s material thickens to the northeast, toward the suture between Peninsular and Wrangellia terranes, but southwest of its mapped trace at the West Fork fault. Peninsular terrane crustal structure appears dissimilar to that of continental interiors. It is similar to velocity structures determined for accreted island arc fragments in California, such as the basement of the Great Valley and the Klamath Mountains.

Ophiolitic and metamorphic rocks in the Percy Isles and Shoalwater Bay region, New England Fold Belt, central Queensland

Ophiolitic and metamorphic rocks of the eastern part of the New England Fold Belt in the Shoalwater Bay region and the Percy Isles are grouped in the Marlborough and Shoalwater terranes, respectively. Marlborough terrane units occur on South Island (Percy Isles) and comprise the Northumberland Serpentinite, antigorite serpentinite with rodingite and more silicic dykes and mafic inclusions, the Chase Point Metabasalt, some 800+ metres of pillow lava, and the intervening South Island Shear Zone containing fault‐bounded slices of mafic and ultramafic igneous rocks, schist, and volcaniclastic sedimentary rocks, and zones of melange. The Shoalwater terrane, an ancient subduction complex, consists of the Shoalwater Formation greenschist facies metamorphosed quartz sandstone and mudstone on North East Island and on the mainland at Arthur Point, the Townshend Formation, amphibolite‐grade quartzite, schist and metabasalt on Townshend Island, and the Broome Head Metamorphics on the western side of Shoalwater Bay, u...

COPPER-LEAD-ZINC: Geophysical signature of the Mons Cupri VMS deposit, Western Australia

Mons Cupri is an Archaean proximal, volcanogenic massive-sulphide deposit in the west Pilbara of Western Australia. Mineralisation occurs in the Mons Cupri Volcanics of the Whim Creek Belt. Stockwork copper-sulphides in altered Mount Brown Rhyolite are overlain by shallowly dipping, massive, copper-lead-zinc sulphides in volcaniclastic sedimentary rocks. Two airborne magnetic surveys detected a 70 nT anomaly in broad correlation with the outline of the mineralisation. The likely source of this anomaly is magnetite within a chlorite alteration pipe. One trial and two surveys with airborne electromagnetics have been conducted. None of the systems, including GEOTEM II, yielded anomalies over Mons Cupri itself. Seven trials and one survey with ground electromagnetics were also completed. Again, none produced anomalous results attributable to the mineralisation. Two induced polarisation surveys detected slightly lower resistivities over the stockwork mineralisation; a magnetic induced polarisation survey obtained the opposite result. However, higher percentage frequency effects, chargeabilities, and relative phase shifts clearly coincided with the stockwork mineralisation. None of the systems unequivocally detected the massive sulphides at depth. Mons Cupri is a difficult geophysical target. Low total-sulphide content, deep massive sulphides and a lack of electrical continuity combine to produce an enigmatic response and to defeat detection by electromagnetic systems.

Archean, deep-marine, volcanic eruptive products associated with the Coniagas massive sulfide deposit, Quebec, Canada

The mafic-dominated volcanic and related volcaniclastic sedimentary rocks, which host the Archean Coniagas Zn–Pb–Ag massive sulfide deposit, are inferred to be the result of submarine explosive and effusive eruptions at depths of approximately 1000 m, as suggested by the presence of volcaniclastic turbidites, the absence of wave-induced sedimentary structures, pillowed lava flows, the sulfide deposit itself, and the incipient arc setting. The rock assemblage includes massive, pillowed and brecciated, basaltic to andesitic flows, massive, andesitic to rhyodacitic lapilli tuffs, andesitic stratified lapilli tuffs, and bedded tuffs. Preserved fragments and delicate volcanic textures, such as angularity of clasts, chilled clast margins, and clast vesicularity, and sedimentary structures are consistent with a subaqueous hydroclastic origin for the volcaniclastic sedimentary rocks. Explosive degasification of magma and (or) lava, in conjunction with fragmentation due to the interaction of magma–water, or nonexplosive hydroclastic fragmentation can account for the observed characteristics in the volcaniclastic deposits.The 280 m thick Coniagas volcano-sedimentary succession, used to reconstruct the volcanic history of the deposit, records two explosive–effusive volcanic cycles. The initial stage of each cycle is envisaged to have commenced with a small fire fountain or boiling-over eruption. Transport and deposition of the fragmented debris along the flanks of the volcanic edifice is attributed to high-concentration particulate gravity flows. The massive lapilli tuffs are interpreted as laminar debris flows, whereas the stratified lapilli tuffs may reflect turbulent flow deposits. The bedded tuffs were produced during the waning eruptive stages or elutriated from high-concentration syneruption flows. Ingestion of water, causing hydroclastic fragmentation, occurred during the eruptive and (or) the transport process. Calm, effusive mafic volcanism, characterized by massive, pillowed and brecciated flows and reworked counterparts, terminates each volcanic cycle. The massive, felsic lapilli tuffs, which host the mineralization, are inferred to represent locally reworked hydroclastic products of explosive or nonexplosive origin. The Coniagas mine deposit may serve as a guide for future exploration of small Archean volcanic-hosted massive sulfide deposits with a restricted alteration halo.

Geophysical Signature of the Mons Cupri VMS Deposit, Western Australia

Mons Cupri is an Archaean proximal, volcanogenic massive-sulphide deposit in the west Pilbara of Western Australia. Mineralisation occurs in the Mons Cupri Volcanics of the Whim Creek Belt. Stockwork copper-sulphides in altered Mount Brown Rhyolite are overlain by shallowly dipping, massive, copper-lead-zinc sulphides in volcaniclastic sedimentary rocks. Two airborne magnetic surveys detected a 70 nT anomaly in broad correlation with the outline of the mineralisation. The likely source of this anomaly is magnetite within a chlorite alteration pipe. One trial and two surveys with airborne electromagnetics have been conducted. None of the systems, including GEOTEM II, yielded anomalies over Mons Cupri itself. Seven trials and one survey with ground electromagnetics were also completed. Again, none produced anomalous results attributable to the mineralisation. Two induced polarisation surveys detected slightly lower resistivities over the stockwork mineralisation; a magnetic induced polarisation survey obtained the opposite result. However, higher percentage frequency effects, chargeabilities, and relative phase shifts clearly coincided with the stockwork mineralisation. None of the systems unequivocally detected the massive sulphides at depth. Mons Cupri is a difficult geophysical target. Low total-sulphide content, deep massive sulphides and a lack of electrical continuity combine to produce an enigmatic response and to defeat detection by electromagnetic systems.

Late Palaeozoic arc flank and fore‐arc basin sequence of the New England fold belt in the stanage bay region, central Queensland

Devonian and Carboniferous (Yarrol terrane) rocks, Early Permian strata, and Permian‐(?)Triassic plutons outcrop in the Stanage Bay region of the northern New England Fold Belt. The Early‐(?)Middle Devonian Mt Holly Formation consists mainly of coarse volcaniclastic rocks of intermediate‐silicic provenance, and mafic, intermediate and silicic volcanics. Limestone is abundant in the Duke Island, along with a significant component of quartz sandstone on Hunter Island. Most Carboniferous rocks can be placed in two units, the late Tournaisian‐Namurian Campwyn Volcanics, composed of coarse volcaniclastic sedimentary rocks, silicic ash flow tuff and widespread oolitic limestone, and the conformably overlying Neerkol Formation dominated by volcaniclastic sandstone and siltstone with uncommon pebble conglomerate and scattered silicic ash fall tuff. Strata of uncertain stratigraphic affinity are mapped as ‘undifferentiated Carboniferous’. The Early Permian Youlambie Conglomerate unconformably overlies Carboniferou...

The Lerokis and Kali Kuning submarine exhalative gold-silver-barite deposits, Wetar Island, Maluku, Indonesia

Wetar, an island in the southern part of the Banda Arc, is made up submarine volcanic rocks, with the oldest rocks exposed being subvolcanic intrusions and flows dated at 12 Ma. Basaltic andesite pillow lavas and intercalated volcaniclastic sedimentary rocks grade upward into more felsic volcanic lavas, tuffs and breccias, and sedimentary rocks and epiclastic mudflows cap the sequence. Gold-silver mineralization occurs at Lerokis and Kali Kuning, 3.5 km apart on the north coast of the island, in stratiform barite sand, clay or silt. The sediments are underlain by Cu-rich massive pyrite in volcanic breccias and overlain by a limestone dated at about 4 Ma. Foot wall volcanic breccias and lavas show intense clay-pyrite alteration indicating temperatures higher than 230°C. In situ geological resources, prior to the inception of mining in 1990, totalled 2.9 Mt at 3.5 g/t Au, 114 g/t Ag and 40% barite at Lerokis and 2.2 Mt at 5.5 g/t Au, 146 g/t Ag and 60% barite at Kali Kuning. Most of the Au occurs as electrum, associated with limonite, jarosite and goethite, and most of the Ag is in tetrahedrite and sulphosalts. High Pb contents, between 0.5% and 1.4% Pb on average, derive from plumbojarosite, sulphosalts and cerrussite, and there is, on average, between 200 ppm to 900 ppm Cu, 0.2% Sb, 0.1% As, 100 ppm to 200 ppm Zn, and 18 ppm Hg. Underlying massive sulphide mineralization is mainly pyrite-marcasite with zones of Cu enrichment. A significant part of the Cu is contained in enargite. Formation likely took place at less than 600 m water depth in a sea floor caldera setting similar to the Kuroko district in Japan. The ferruginous sediments hosting the Au-barite deposits may have originated through erosion like the ochres of Cyprus. These stratiform Au-barite deposits highlight a new style of sea floor mineralization not clearly recognized before. Modern-day analogues have been described from a variety of sea floor settings.

Gamilaroi Terrane: A Devonian rifted intra-oceanic island-arc assemblage, NSW, Australia.

Abstract The Gamilaroi terrane comprises part of the New England orogen of eastern Australia and contains Devonian strata preserving evidence of their development in a variety of arc-related environments. Radiolarian studies provide new and more precise constraints on stratigraphy and together with geochemistry allow development of a new model for evolution of the terrane. A diverse suite of volcaniclastic sedimentary rocks intercalated with regionally extensive extrusive and high level intrusive meta-felsic igneous rocks of low-K and calc-alkaline volcanic affinity is present. These rocks constitute the oldest in-situ volcanic rocks in the Gamilaroi terrane and are similar to those found in modern intra-oceanic volcanic island-arc settings. Basaltic intrusions and lavas occur at higher stratigraphic levels. Major and trace element compositions, together with relatively flat chondrite-normalized REE distributions, resemble those from rocks formed in modern intra-oceanic island-arc rift zones. The close spatial association of these rock types together with an absence of continent-derived detritus is consistent with interpretation of development of the Gamilaroi terrane in an intra-oceanic island arc which experienced local rifting. This interpretation is a departure from previous models for the New England orogen, which interpret the Gamilaroi terrane as a forearc basin that developed above a long-lived (Cambrian to Permian), west-dipping subduction zone fringing the continental margin of Gondwana. By analogy with modern arc-continent collision zones in which crust between colliding continents and island arcs is preferentially subducted beneath the approaching arc, we suggest that oceanic crust intervening between Gamilaroi terrane and Gondwana was subducted eastwards under the western margin of the Gamilaroi terrane arc. During a latest Devonian collision event the Gamilaroi terrane was obducted onto the Gondwana margin. This resulted in subduction flip and subsequent development of an east-facing continental margin arc system on top of the Gamilaroi terrane.

Is there evidence for Cretaceous-Tertiary boundary-age deep-water deposits in the Caribbean and Gulf of Mexico?

Over most of the Gulf of Mexico and Caribbean a hiatus is present between the lower upper Maastrichtian and lowermost Tertiary deposits; sedimentation resumed ∼200 ka (upper zone Pla) after the K-T boundary. Current-bedded volcaniclastic sedimentary rocks at Deep Sea Drilling Project (DSDP) Sites 536 and 540, which were previously interpreted as impact-generated megawave deposits of K-T boundary age, are biostratigraphically of pre-K-T boundary age and probably represent turbidite or gravity-How deposits. The top 10 to 20 cm of this deposit at Site 536 contains very rare Micula prinsii, the uppermost Maastrichtian index taxon, as well as low values of Ir (0.6 pbb) and rare Ni-rich spinels. These indicate possible reworking of sediments of K-T boundary age at the hiatus. Absence of continuous sediment accumulation across the K-T boundary in the 16 Gulf of Mexico and Caribbean sections examined prevents their providing evidence of impact-generated megawave deposits in this region. Our study indicates that the most complete trans-K-T stratigraphic records may be found in onshore marine sections of Mexico, Cuba, and Haiti. The stratigraphic records of these areas should be investigated further for evidence of impact deposits.

A pre-caldera plateau-andesite field in the Borrowdale Volcanic Group of the English Lake District

The 6 km thick Ordovician Borrowdale Volcanic Group is readily divisible into a lower 2.2–2.7 km thick predominantly pre-caldera succession dominated by basalt, andesite and dacite sheets, and an upper succession of caldera-related ignimbrites and volcaniclastic sedimentary rocks of c. 3 km thickness. The lower Borrowdale Volcanic Group rocks, here included within a single lithostratigraphical unit, the Birker Fell Formation, affords a well-exposed section through a pre-caldera sequence. The Birker Fell Formation is dominated by andesites which comprise 60% of the stratigraphy. Thin sequences of reworked volcanic detritus are commonly interbedded with the andesites; locally there are thicker units of volcaniclastic sedimentary rocks. Interpretation of the stratigraphy and eruptive history of the formation has been aided by the recognition of some distinctive lithological units including: (1) a 200-600 m thick sequence of weakly parallel-bedded basaltic tuffs resting on the pre-volcanic basement, (2) a 200-600 m sequence of single flow unit aa basalts, (3) dacite flows and ignimbrites up to > 1100 m thick in the middle and upper parts of the formation and (4) aphyric to pyroxene-phyric, simple and compound flows of basalt, 50-400 m thick, locally present in the uppermost parts. Analysis of the facies comprising the Birker Fell Formation indicates that it was emplaced as a sub-aerial, plateau-andesite sequence, formed by the coalescence of products erupted from a number of centres or fissures in an extensional, subsiding volcano-tectonic rift zone. Volcano-tectonic faulting and eruption of thick ignimbrites locally influenced development of this field.

A shallow marine volcaniclastic facies model: an example from sedimentary rocks bounding the subaqueously welded Ordovician Garth Tuff, North Wales, U.K.

Volcaniclastic sedimentary rocks bounding the Ordovician Garth Tuff in North Wales were deposited in a shallow marine setting adjacent to a magmatic arc. The volcaniclastic sedimentary rocks are primarily of angular to euhedral quartz, angular euhedral feldspar, and volcanic rock fragments. These grains are of a pyroclastic origin, but have been reworked to various degrees. Sedimentary rock fragments, rounded quartz, muscovite, biotite, iron-rich chlorite and various clay minerals occur in lesser amounts. The sedimentary rocks can be divided into a proximal offshore to foreshore facies association (POFFA) and an offshore to lower shoreface facies association (OLSFA). The sand-dominated POFFA, exposed near Capel Curig, is characterized by large wave ripples, hummocky cross-stratification, co-sets of trough cross-beds, reactivation surfaces, plane beds, and graded turbidite layers in suspension-deposited mudstone. Water depths calculated from bedding plane exposure of large wave ripples and determined by the presence of hummocky cross-stratification and by facies association vary from a few meters to around 30 m (45–50 m theoretical maximum) in a 45 m-thick stratigraphic section beneath the tuff. Eight kilometers to the southeast, the Garth Tuff is bounded by a thick sequence of suspension-deposited laminated mudstone with thin graded beds of siltstone and fine-grained sandstone. This facies association (OLSFA) represents the introduction of material by distal turbidites and by pelagic sedimentation. The OLSFA was deposited in water depths well below storm wave base, possibly in excess of 200 m. No evidence of storm waves or surface-generated currents occur until over 40 m above the tuff. This study documents marine sedimentary rocks, deposited in water depths ranging from foreshore to offshore, as the bounding facies of the Garth Tuff. It is, thus, reasonable to conclude that the Garth Tuff was emplaced and welded in water depths greater than the thickness of the tuff in the area of this study.

Theoretical wave modeling of large wave ripples in volcaniclastic sediments, Ordovician Llewelyn Volcanic Group, North Wales

Volcaniclastic sedimentary rocks of Ordovician age underlying the Garth Tuff member of the Capel Curig Volcanic Formation, North Wales were deposited on a shallow marine shelf. Deposition at the Capel Curig anticline 2 km southwest of Capel Curig was on a high-energy proximal offshore to foreshore. Analysis of large wave ripples and associated sedimentary structures suggests a shallow marine depositional environment, above storm wave base and in water depths of 10–30 m. This estimate of water depth from wave calculations is similar to that estimated by facies analysis.

The Patagonian Orocline: New paleomagnetic data from the Andean magmatic arc in Tierra del Fuego, Chile

The Hardy Formation is a 1300‐m‐thick succession of Upper Jurassic‐Lower Cretaceous volcaniclastic sedimentary rocks interbedded with lava flows on Hoste Island at the southernmost tip of South America (55.5°S, 291.8°E). The strata are gently folded and metamorphosed to the prehnite‐pumpellyite grade. A well‐defined characteristic direction of magnetization, carried by magnetite, was readily identified in 95 samples from seven sites. At a given site, the directions group slightly better without structural correction. However, the means of the seven sites cluster better without tilt correction at the 99% significance level, implying that the magnetization postdates the folding event. It is most likely that the magnetization was acquired during the mid‐ to Late Cretaceous Andean orogeny that involved the folding and emplacement of the Patagonian Batholith. The fact that all samples are normally magnetized supports this age assignment. The pole position of 42.9°N, 156.6°E, α95=3.3° implies that the sampling area has rotated counterclockwise relative to cratonic South America by 90.1±11.9° with no significant flattening of inclination (F=1.9 ± 3.7°). Geologic considerations indicate that the rotation involved the entire Andean magmatic arc in Tierra Del Fuego. The results support interpretation of the Hardy Formation as part of the Andean magmatic arc deposited on the Pacific side of the Late Jurassic‐Early Cretaceous Rocas Verdes marginal basin. Oroclinal bending of the arc in southernmost South America accompanied inversion of the marginal basin and the development of a Late Cretaceous‐Cenozoic left‐lateral transform system (South America‐Antarctica) that later developed into the North Scotia Ridge.

A Late Cretaceous-Paleogene cauldron cluster: the Nôhi Rhyolite, central Japan

The Nohi Rhyolite now occupies an area of about 5000 km2 in central Japan. It is the largest cluster of volcanic piles of Late Cretaceous to Paleogene age on the Inner Side of SW Japan. It unconformably overlies Paleozoic to Jurassic rocks and is intruded by Paleogene granitic intrusions in its southern and northern parts. The Nohi Rhyolite consists chiefly of rhyolitic to rhyodacitic welded-tuff sheets with subordinate amounts of volcaniclastic sedimentary rocks. Each welded-tuff sheet is usually 200 to 700 m thick, extends laterally 20 to 60 km, and is composed of nearly homogeneous densely welded tuff. Six volcanic sequences (I–VI) are recognized in the southern and central parts of the Nohi Rhyolite. Each sequence generally comprises successive accumulations of welded-tuff sheets and volcaniclastic sedimentary layers underlying the sheets. Sequence I, III and V volcanism was derived from zoned magma bodies and was accompanied by granodiorite porphyries. Sequence II, IV and probably VI volcanism was derived from homogeneous magma bodies and was not accompanied by granodiorite porphyries. The estimated total volume is of the order of 300–700 km3 for Sequences I, II and IV, 1600 km3 for Sequence III, and 50 km3 or less for Sequences V and VI. The volume of Sequence III is one of the largest volumes of ashflow tuffs so far recognized. One or more polygonal cauldrons, each 15–40 km across and 400–1000 m deep, formed during each of Sequence I through IV. These cauldrons were generally formed by two successive depressions. The first depression in each sequence is of the Motojuku-type, characterized by collapse prior to the principal eruption. The second depression was formed during the principal eruption. The cauldron of Sequence V has an unusual shape and was probably formed during or after the eruption. The development of a cauldron in Sequence VI is uncertain. The Nohi Rhyolite is elongate in a NW direction and contains a cluster of seven or eight cauldrons. These cauldrons formed in a tension field trending N and NE, and their volcanism migrated northward. The main regional tectonic framework controlling the cauldrons resulted from uplifting of the basement, which is ascribed to the ascent of a large volume of magma in Late Cretaceous to early Paleogene time.

Volcanism and related slope to shallow-marine volcaniclastic sedimentation: an Archean example near Chibougamau, Quebec, Canada

Volcaniclastic sedimentary rocks and K-rich lava flows of the Archean Hauy Formation are discontinuously exposed in a 850 m thick sequence in the Waconichi synclinorium northwest of Chibougamau, Quebec. The interstratified sedimentary rocks comprise two major sedimentary facies associations: the Graded Bedded Facies Association (WBFA), indicative of deposition by sediment gravity flows, and the Wavy Bedded Facies Association (WBFA), defined by wave-induced structures. The GBFA is near the base of the stratigraphic section and is interstratified up-section with the WBFA. In the GBFA the abundance of synsedimentary folding, truncated slump folds, load casts, and low-angle erosional surfaces, as well as sediments deposited by turbidity currents and debris flows, support designation as a volcanic slope deposit. In the WBFA trough crossbeds, wavy bedded gravels, and plane-bedded volcaniclastic sandstones are indicative of a nearshore setting. The abundance of angular to subangular lithic volcanic fragments, and broken and euhedral pyroxene crystals in the sediments is suggestive of a pyroclastic origin. Some beds may represent primary pyroclastic material, but unequivocal petrographic recognition of pumice or glass shards is not possible because of metamorphic overprint. Composition suggests an initial magmatic or phreatomagmatic eruption, subsequently reworked along the shoreline and transported downslope. Lava flows comprising massive to lobate andesites and massive to blocky basalts constitute most of the stratigraphic sequence; pillowed flows are strikingly absent from the section. Both compositional types have an increase in vesicularity and change in flow form up-section from massive to lobate or blocky, suggesting progressively shallower water depths. The composite character represented by Hauy lava flows and associated volcaniclastic sediments in characteristic of stratovolcanoes. The studied section is an integral part of an evolved arc with an adjacent backarc basin. The volcano-sedimentary facies indicate the development of a volcanic island in one of these basins.

Paleozoic evolution of active margin basins in the southern Central Andes (northwestern Argentina and northern Chile)

The geodynamic evolution of the Paleozoic continental margin of Gondwana in the region of the southern Central Andes is characterized by the westward progression of orogenic basin formation through time. The Ordovician basin in the northwest Argentinian Cordillera Oriental and Puna originated as an Early Ordovician back-arc basin. The contemporaneous magmatic arc of an east-dipping subduction zone was presumably located in northern Chile. In the back-arc basin, a ca. 3500 meter, fining-up volcaniclastic apron connected to the arc formed during the Arenigian. Increased subsidence in the late Arenigian allowed for the accomodation of large volumes of volcaniclastic turbidites during the Middle Ordovician. Subsidence and sedimentation were caused by the onset of collision between the para-autochthonous Arequipa Massif Terrane (AMT) and the South American margin at the Arenigian-Llanvirnian transition. This led to eastward thrusting of the arc complex over its back-arc basin and, consequently, to its transformation into a marine foreland basin. As a result of thrusting in the west, a flexural bulge formed in the east, leading to uplift and emergence of the Cordillera Oriental shelf during the Guandacol Event at the Arenigian-Llanvirnian transition. The basin fill was folded during the terminal collision of the AMT during the Oclóyic Orogeny (Ashgillian). The folded strata were intruded post-tectonically by the presumably Silurian granitoids of the “Faja Eruptiva de la Puna Oriental.” The orogeny led to the formation of the positive area of the Arco Puneño. West of the Arco Puneño, a further marine basin developed during the Early Devonian, the eastern shelf of which occupied the area of the Cordillera Occidental, Depresión Preandina, and Precordillera. The corresponding deep marine turbidite basin was located in the region of the Cordillera de la Costa. Deposition continued until the basin fill was folded in the early Late Carboniferous Toco Orogeny. The basin originated as an extensional structure at the continental margin of Gondwana. Independent lines of evidence imply that basin evolution was not connected to subduction. Thus, the basin could not have been in a fore-arc position as previously postulated. Above the folded Devonian-Early Carboniferous strata, a continental volcanic arc developed from the Late Carboniferous to the Middle Triassic. It represents the link between the Choiyoi Province in central Chile and Argentina, and the Mitu Group rift in southern Peru. The volcanic arc succession is characterized by the prevalence of silicic lavas and tuffs and volcaniclastic sedimentary rocks. During the latest Carboniferous, a thick ostracod-bearing lacustrine unit formed in an extended lake in the area of the Depresión Preandina. This lake basin originated in an intra-arc tensional setting. During the Early Permian, marine limestones were deposited on a marine platform west and east of the volcanic arc, connected to the depositional area of the Copacabana Formation in southern Peru.