Breccia Unit Research Articles

Palaeointensity studies of a late Permian lava succession from Guizhou Province, South China: implications for post-Kiaman dipole field behaviour

SUMMARY Palaeomagnetic results are presented from a volcanic sequence in Zhijin County, Guizhou Province, Southern China. The lavas and associated volcanic breccias comprising the sequence represent a southern extension of the Emeishan volcanic province. Biostratigraphic dating of interbedded limestone units and stratigraphic constraints indicate that the section formed during the late Permian (~263‐255 Ma), and is thus somewhat older than the Emei stratotype section in Sichuan Province, and close in age to reported estimates of the termination of the Permo-Carboniferous (Kiaman) reverse superchron. Rock magnetic analyses and reflected light microscopy indicate that the magnetic mineralogy of the lava units is dominated by fresh, primary magnetites containing a significant fraction of single-domain grains. Thermal demagnetization behaviour of the breccia units is poor, but most lava samples have one or two components of remanence above 250°C. The normal polarity characteristic remanence held by the lavas implies a post-Kiaman age for this succession and suggests that the termination of the Kiaman occurred prior to 263 Ma, supporting recently published estimates. According to standard criteria, Thellier palaeointensity results from the lavas are of good quality and reveal that the dipole field strength was comparatively low shortly after the termination of the superchron. 80 per cent of samples record relative VDM values in the range 42‐52 per cent of the present-day value, supporting recent studies of mid-Kiaman field intensity. This suggests that a low-energy dipole existed at least between 300 and 255 Ma and does not appear to have been confined to the stable reverse polarity interval.

The Cretaceous–Tertiary transition in Guatemala: limestone breccia deposits from the South Petén basin

Limestone breccia deposits in southern Mexico, Guatemala and Belize have recently been interpreted as proximal to distal ballistic fallout deposits, generated by a bolide impact that struck Yucatan at K/T boundary time. We review the age, lithology and the depositional environment of five K/T boundary sections in the South Peten area of Guatemala (Caribe, Aserradero, Chisec, Actela, Chemal) in order to evaluate the nature and origin of K/T limestone breccia deposition. The sections are located 500 km south of the proposed impact site at Chicxulub and trend in an east–west direction from the Guatemala/Mexico border to southern Belize. In four of the five sections examined, a breccia unit up to 50 m thick overlies reef-bearing shallow-water limestones of late Cretaceous (Campanian-Maastrichtian) age. Rhythmically bedded limestones, marls and siltstones of early Danian age overlie the breccia and were deposited under middle-to outer-neritic conditions. The breccia consists of differently coloured layers of shallow-water limestones. Clast size generally decreases upsection to thin layers of predominantly rounded clasts, and these fine-grained rudstones grade into grainstones at the top. In at least one section (EI Caribe) diagenetically altered glass spherules are present in the uppermost layers of the grainstone. These glass spherules are of stratigraphic position and chemical composition similar to black and yellow glass from Beloc, Haiti and Mimbral, Mexico, which some workers have chemically linked to melt glass within the breccia of the Chicxulub cores. We suggest that breccia deposition in Guatemala may have been multi-event, over an extended time period, and related to the collision of the Yucatan and Chortis plates as well as related to a major impact or volcanic event at the end of the Cretaceous.

A submarine welded ignimbrite-crystal-rich sandstone facies association in the Cambrian Tyndall Group, western Tasmania, Australia

Three occurrences of rhyolitic welded ignimbrite are intercalated within a submarine, below-storm-wave-base sedimentary succession in the Cambrian Tyndall Group, Mount Read Volcanics, western Tasmania. These occurrences are closely associated with very thick crystal-rich sandstone facies that is present at this stratigraphic level throughout the Tyndall Group. This facies is interpreted to comprise deposits from syn-eruptive, crystal-rich, submarine sediment gravity flows that were generated by interaction of subaerial pyroclastic flows with seawater. Removal of fine ash and pumice from the submarine flows by hydraulic sorting and flotation resulted in marked crystal enrichment in the deposits. Rapid, essentially syn-eruptive aggradation of crystal-rich sand led to temporary shoaling so that in some cases, subsequent pyroclastic flows deposited welded ignimbrite in shallow marine or possibly subaerial settings (e.g., Zig Zag Hill welded ignimbrite). Breccia units composed of welded ignimbrite clasts and crystal-rich matrix (e.g., Comstock and Anthony Road ignimbrite breccias) imply that some welded ignimbrite was submerged, providing clasts to syn-eruptive, submarine, crystal-rich sediment gravity flows. One example of welded ignimbrite (Cradle Mountain Link Road) may have been deposited in an entirely below-storm-wave-base environment. The distinctive facies association of welded ignimbrite, crystal-rich sandstone and ignimbrite-clast breccia in the Tyndall Group exemplifies the submarine record of a major rhyolitic explosive eruption in the source volcanic terrane.

UNAM Scientific Drilling Program of Chicxulub Impact Structure‐Evidence for a 300 kilometer crater diameter

As part of the UNAM drilling program at the Chicxulub structure, two 700 m deep continuously cored boreholes were completed between April and July, 1995. The Peto UNAM‐6 and Tekax UNAM‐7 drilling sites are ∼150 km and 125 km, respectively, SSE of Chicxulub Puerto, near the crater's center. Core samples from both sites show a sequence of post‐crater carbonates on top of a thick impact breccia pile covering the disturbed Mesozoic platform rocks. At UNAM‐7, two impact breccia units were encountered: (1) an upper breccia, mean magnetic susceptibility is high (∼55 × 10−6 SI units), indicating a large component of silicate basement has been incorporated into this breccia, and (2) an evaporite‐rich, low susceptibility impact breccia similar in character to the evaporite‐rich breccias observed at the PEMEX drill sites further out. The upper breccia was encountered at ∼226 m below the surface and is ∼125 m thick; the lower breccia is immediately subjacent and is >240 m thick. This two‐breccia sequence is typical of the suevite‐Bunte breccia sequence found within other well preserved impact craters. The suevitic upper unit is not present at UNAM‐6. Instead, a >240 m thick evaporite‐rich breccia unit, similar to the lower breccia at UNAM‐7, was encountered at a depth of ∼280 m. The absence of an upper breccia equivalent at UNAM‐6 suggests some portion of the breccia sequence has been removed by erosion. This is consistent with interpretations that place the high‐standing crater rim at 130–150 km from the center. Consequently, the stratigraphic observations and magnetic susceptibiity records on the upper and lower breccias (depth and thickness) support a ∼300 km diameter crater model.

Grading in mud volcanic breccia from the Mediterranean Ridge

An analysis was made of mud breccia sampled from mud diapir fields on the crest of the Mediterranean Ridge to investigate whether they are emplaced by extrusion resulting in debris flows or by intrusion into the hemipelagic sediments. Descriptive sedimentological analyses was combined with the measurements of clast sizes and grading, and the determination of the ratio of clasts to the mud matrix in the upper and lower parts of the mud breccia units in 15 cores taken from the tops and flanks of mud volcanoes. Observed grading in clast size in the upper and/or lower boundary layers of most mud breccias is inferred to be the result of extrusion and flow on the seafloor. Such mud flows may be divided into three sections consisting of graded beds at the top and bottom separated by a massive intermediate part. The best developed grading may result from further reworking of extruded mud breccia through debris flows down the flanks of the mud structures. The composition of the clasts show five lithologies for which the dominant one is pebbles of silty mudstone.

Sedimentary model and high‐frequency cyclicity in a Mediterranean, shallow‐shelf, temperate‐carbonate environment (uppermost Miocene, Agua Amarga Basin, Southern Spain)

ABSTRACTUppermost Tortonian to lower Messinian temperate carbonates crop out in the Agua Amarga Basin (SE Spain). They consist of four units. The lower three units can be tentatively assigned to the lowstand systems tract of a fourth‐order sequence, constituting in turn the lowstand (‘megatrough unit’), transgressive (‘breccia unit’) and highstand (‘bedded unit’) stages of a higher‐order cycle. All these materials were deposited in a small pull‐apart basin related to the sinistral Carboneras strike‐slip fault system. The best represented is the bedded unit (up to 25 m thick), which consists of bioclastic, bryozoan/bivalve‐dominated calcarenites/calcirudites with abundant fragments of echinoids, barnacles, benthic foraminifers, coralline algae, brachiopods and solitary corals. Facies trends within this unit are roughly arranged in an E‐W direction, with the coastline to the north of the basin. The depositional model is that of a gentle ramp with prograding beaches and shoals in its higher parts. Seaward of the shoals was the ‘factory area’, where most organisms lived and maximum carbonate production took place. From this area some of the skeletons were washed landwards by waves and/or currents during storms and incorporated into the shoals and beaches, and others moved downslope along the ramp as mass‐flows, accumulating to form the ‘fan‐bedded zone’.The factory‐area and fan‐bedded sediments intercalate five well‐defined, thick beds of calcarenites/fine‐grained calcirudites. They show bar morphologies (single or amalgamated), or make up sand‐waves with very consistent tabular cross‐bedding pointing landwards. These beds formed in a very shallow, wave/current‐influenced, coastal environment. The bars and sand waves in the fan‐bedded zone developed during lowstands, while those located higher up in the ramp interbedded with the factory facies are related to transgressive stages. Prograding beaches, shoals, factory facies and fan‐bedded layers developed during the highstands. Net skeletal production occurred mainly during the highstands.Sediment‐accretion values of these sediments are similar to those of present and ancient shallow‐marine, temperate carbonates considering that the whole bedded unit was deposited in a 100 000‐year interval (equivalent to the short eccentricity cycle). The five cycles inside the bedded unit would therefore correspond to the c. 20 000‐year precession cycles of the Milankovitch band.

Interaction between caldera collapse and eruptive dynamics during the Campanian Ignimbrite eruption, Phlegraean Fields, Italy

The Campanian Ignimbrite (36000 years B.P.) was produced by the explosive eruption of at least 80 km3 DRE of trachytic ash and pumice which covered most of the southern Italian peninsula and the eastern Mediterranean region. The eruption has been related to the 12-x15-km-diameter caldera located in the Phlegraean Fields, west of Naples. Proximal deposits on the periphery of the Phlegraean Fields comprise the following pyroclastic sequence from base to top: densely welded ignimbrite and lithic-rich breccias (unit A); sintered ignimbrite, low-grade ignimbrite and lithic-rich breccia (unit B); lithic-rich breccia and spatter agglutinate (unit C); and low-grade ignimbrite (unit D). Stratigraphic and componentry data, as well as distribution of accidental lithic types and the composition of pumice clasts of different units, indicate that coarse, lithic-rich breccias were emplaced at different stages during the eruption. Lower breccias are associated with fines-rich ignimbrites and are interpreted as co-ignimbrite lag breccia deposits. The main breccia unit (C) does not grade into a fines-rich ignimbrite, and therefore is interpreted as formed from a distinct lithic-rich flow. Units A and B exhibit a similar pattern of accidental lithic types, indicating that they were erupted from the same area, probably in the E of the caldera. Units C and D display a distinct pattern of lithics indicating expulsion from vent(s) that cut different areas. We suggest that unit C was ejected from several vents during the main stage of caldera collapse. Field relationships between spatter agglutinate and the breccia support the possibility that these deposits were erupted contemporaneously from vents with different eruptive style. The breccia may have resulted from a combination of magmatic and hydrothermal explosive activity that accompanied extensive fracturing and subsidence of the magma-chamber roof. The spatter rags probably derived from sustained and vigorous pyroclastic fountains. We propose that the association lithic-rich breccia and spatter agglutinate records the occurrence of catastrophic piecemeal collapse.

The eruption of the Breccia Museo (Campi Flegrei, Italy): Fractional crystallization processes in a shallow, zoned magma chamber and implications for the eruptive dynamics

The Breccia Museo Member (BMM) was formed by an explosive eruption that occurred in the SW sector of Campi Flegrei about 20 ka ago. The eruptive sequence consists of the Lower Pumice Flow Unit and the overlying Upper Pumice Flow Unit with its associated lithic Breccia Unit. Interlayered with the Breccia Unit is a welded deposit that mainly consists of spatter clasts (Spatter Unit).The products of this eruption range in composition from trachytic to trachyphonolitic with K2O decreasing from 9.5 to 7 wt.%; Na2O correspondingly increases from 2.6 to 7.2 wt.% with increasing differentiation (Nb from 23 to 122 ppm). The phenocrysts are mostly sanidine (Or88-63) with subordinate plagioclase (An33-27), clinopyroxene (Ca47Mg44Fe9 to Ca46Mg35Fe19), biotite, titanomagnetite, and apatite. The observed major- and trace-element variations are fully consistent with about 80% fractional crystallization of a sanidine-dominated assemblage starting from the least differentiated trachytes.The compositions of the erupted products are compatible with the progressive tapping of a shallow magma chamber that was thermally and chemically zoned. The incompatible trace elements indicate a slightly different magma composition with respect to trachytes of the Campi Flegrei mainland.The geochemical stratigraphy suggests that after an early eruptive phase during which the upper, most differentiated level of the magma chamber was tapped, the sudden collapse of the roof of the reservoir triggered drainage of the remaining magma, which ranged in composition from trachyte to trachyphonolite, and formed the Breccia Unit and the Upper Pumice Flow Unit.The strongly differentiated trachyphonolite composition of the spatter clasts also suggests that they likely originated from the uppermost part of the reservoir soon after the eruption of Lower Pumice Flow Unit and the collapse of the chamber roof. This is in agreement with the eruptive model proposed by Perrotta and Scarpati (1994).

Yucatán subsurface stratigraphy: Implications and constraints for the Chicxulub impact

Much of the discussion about the effects of an end-of-Cretaceous impact by a large extraterrestrial body in northwestern Yucatan has been done in the context of limited and partly erroneous published data on the Mesozoic stratigraphy of that area. Reexamination of cores and geophysical logs taken in several Pemex wells has produced improved lithologic and biostratigraphic correlation of the Jurassic to Maastrichtian section across the northern Yucatan peninsula. These data suggest that major disturbance of strata by an impact would have been confined to within about 100 km of the proposed impact center near Chicxulub. The only unusual lithologic unit is polymict breccia, which apparently was penetrated at or near the top of the Cretaceous section in all the deep wells of northern Yucatan. This breccia in Pemex wells Yucatan 1, 2, 4, 5A, and 6 is composed predominantly of detrital dolomite, limestone, and anhydrite clasts set in dolomitized carbonate mud matrix, which contains upper Maastrichtian foraminifers. These constituents, mixed with fragments of altered glass or melt rock, shocked quartz and feldspar, and basement rock, suggest an impact as the most likely origin for the breccia. The timing of brecciation is poorly constrained by biostratigraphic data. There is some evidence, however, that the breccia unit is overlain by about 18 m of uppermost Maastrichtian marls, suggesting an impact before the Cretaceous-Tertiary boundary. In addition, there may have been more than one episode of breccia deposition.

40 Ar/ 39 Ar dating of fluid migration in a mississippi valley-type deposit; the Gays River Zn-Pb deposit, Nova Scotia, Canada

The Gays River Zn-Pb deposit of southern Nova Scotia, Canada, represents an example of Mississippi Valley-type mineralization (reserves ca. 2.4 million metric tons 8.6% Zn and 6.3% Pb). The deposit is hosted by Visean-age, dolomitized carbonate rocks (bank and interbank facies) that form part of a series of carbonate banks (i.e., the Gays River Formation). The banks developed on paleotopographic highs underlain by lower Paleozoic metaturbidites of the Meguma Group, but locally there is an intervening basal breccia unit which contains fragments (centimeter to meter scale) of Meguma Group lithologies within a mineralized (Pb > or = Zn) dolostone matrix. Petrographic examination of fragments in the breccia unit indicate traces of hydrothermal alteration which is considered to have been coincident with mineralization at Gays River. Maximum temperatures associated with the alteration ranged from ca. 300 degrees (fluid inclusions) to ca. 350 degrees C (chlorite geothermometry). Five 40 Ar/ 39 Ar step-heating experiments of metasedimentary clasts in the basal breccia give nearly identical age spectra profiles, with low-temperature gas fractions indicating apparent ages of ca. 300 Ma followed by plateau ages of ca. 380 to 400 Ma for high-temperature gas fractions. Whereas the latter ages are consistent with previously determined whole-rock 40 Ar/ 39 Ar ages for regional deformation and metamorphism of the Meguma Group, the ca. 300 Ma age is considered to reflect thermal overprinting and is consistent with recent palcomagnetic determinations suggestive of a mid-Mississippian to late Pennsylvanian age for mineralization. Model calculations of the age spectra, assuming degassing of micaceous phases and volume diffusion, suggest that the most plausible interpretation is a reheating event of ca. 250 degrees to 300 degrees C of ca. 1- to 6-m.y. duration. The anomalously elevated temperatures determined from this study, in conjunction with the presence in Maritime Canada of Late Devonian-Carboniferous mafic-felsic magmatism and the widespread resetting of radiometric systems in the Meguma terrane at ca. 300 Ma, suggest that the Gays River mineralization may be related to the same broad-scale tectonic processes that formed the Martimes basin and involved regional-scale structures and probably magma generation.

Geochemical evidence for a catastrophic biotic event at the Frasnian/Famennian boundary in south China

A strong {delta}{sup 13}C anomaly coincides with a weak Ir anomaly at the Frasnian/Famennian (F/F) boundary exposed at Xiangtian, Guangxi, south China. The maximum whole-rock Ir abundance is 0.23 ppb (0.35 ppb on a carbonate-free basis) compared with averages of 0.016 and 0.044 ppb above and below the boundary interval. The {delta}{sup 13}C in carbonate abruptly shifts from a late Frasnian level of about +1{per thousand} to {minus}2.49 {per thousand}in the boundary interval and then abruptly returns to pre-boundary levels, suggesting a temporary reduction of surface-water biomass. A widely distributed (over at least several hundred kilometers) limestone breccia unit of possible wave-deposit origin is present directly below the boundary in south China. Enrichments of Al, V, Cr, As, and U, and a dip in the Mn abundance in the boundary interval indicate that reducing conditions might have been associated with the element enrichment. However, an alternative hypothesis is that oceanic impact(s) at the F/F boundary near south China provided the excess Ir and caused the biotic crisis and the formation of breccia deposits observed in the Late Devonian South China Sea.

The Relief Canyon gold deposit, Nevada; a mineralized solution breccia

The Relief Canyon gold deposit in the Humboldt Range of western Nevada is a low-grade, high-tonnage orebody of Tertiary or younger age. The host rocks include limestones of the Triassic Cane Spring Formation, which are overlain by shales of the Triassic Grass Valley Formation. The rocks were folded and metamorphosed to greenschist grade during Jurassic and Cretaceous regional tectonic activity. Mesozoic thrusting may have occurred along the shale-limestone contact, but evidence has been obscured by later hydrothermal activity. The sedimentary rocks were nominally offset along several Late Tertiary normal faults related to uplift of the range.The upper part of the Cane Spring Formation is composed of a poorly sorted breccia composed of limestone clasts with a clay matrix. Irregular pockets within this zone are filled with clay- to pebble-sized fragments derived from the Grass Valley shale. The enclosing limestone beds were tilted moderately to the southwest during Mesozoic deformation, whereas bedding within these pockets is generally horizontal, indicating post-tilting deposition of the sediments. The sediments show graded bedding and other sedimentary features that indicate deposition from flowing water. Thermally mature carbon derived from the limestone is also concentrated in small pockets in the matrix. The breccia unit is likely the product of low-temperature solution brecciation. Ground water dissolved much of the limestone directly beneath the shales, progressively creating irregular cavities and the breccia. Sediments derived from the overlying Grass Valley shale were fiuvially deposited as a matrix to the developing solution breccia.Episodic pulses of hydrothermal fluids were introduced along faults and possibly mixed with the ground water in the breccia zone. Initially, jasperoids formed along the faults, but later hydrothermal pulses introduced gold, silica, and fluorine into both the early jasperoids and the unconsolidated cave-fill sediments to form the orebody. Continued solution-related brecciation chaotically disrupted the gold deposit.Gold, fluorite, pyrite, silver, calcite, and fine-grained silica are the principal hydrothermal minerals in the deposit. Gold was deposited as micron-sized flakes of native gold and rarely as electrum during a relatively late stage of silicification of the jasperoids, the carbon-rich zones, and the clay-rich matrix of the breccia. Fluorite was deposited with and later than the gold in the jasperoids, and it in part replaced the clay-rich breccia matrix. Antimony, arsenic, mercury, and thallium are directly associated with gold in the orebody.The deposit formed at a relatively shallow depth. On the basis of fluid inclusion data, late-stage hydrothermal fluids related to gold and fluorite deposition were extremely dilute and had temperatures near 200 degrees C. The fluid inclusions in fluorite show no evidence for boiling, but porous crackle breccias in the jasperoids suggest that hydrobrecciation took place.

Structure and Stratigraphy Beneath a Young Phreatic Vent: South Inyo Crater, Long Valley Caldera, California

An 861‐m‐long hole has been cored on a slanted trajectory that passed directly beneath South Inyo Crater in the west moat of Long Valley Caldera, California. The purpose of the hole was to investigate the magmatic behavior that led to surface deformation and phreatic activity during the 600‐year‐old eruption of the Inyo vent chain. The hole was sited 216 m southwest of the crater, passed beneath its center at a depth of 566 m, and terminated 79 m northeast of the crater center at a depth of 810 m. Metamorphic basement was encountered at a depth of 779 m. The volcanic and sedimentary sequence consists solely of post‐Bishop Tuff caldera fill, including 319 m of moat basalt and 342 m of early rhyolite, and is nearly 900 m thinner than in a Unocal Corporation well 900 m to the southeast. Apparently, a major fault lies between the two holes and forms part of the western structural boundary of the caldera, 3–4 km inboard of its topographic boundary. Breccia zones that intrude the caldera fill were intersected at 12.0–9.3 m and 1.2–0.8 m SW and 8.5–25.1 m NE of the crater center. The largest breccia unit is symmetrically zoned from margins rich in vesicular rhyolite and locally derived rhyolite wall rock, with most clasts ≤ 0.1 m, to a center of up to 50 vol % basalt, with individual basalt clasts to 1 m in intersected length. The basalt appears to be an early feeder for the moat basalt sequence. The vesicular rhyolite is chemically distinct from both the rhyolite wall rock and previously recognized 600‐year‐old Inyo eruptives but matches fresh‐appearing pyroclasts in the crater ejecta. This component of the breccia is therefore interpreted as representing juvenile magma that vesiculated and fragmented as it rose along the margins of an older basalt intrusion. During this process, wall rock fragments were ejected from depths extending to at least 800 m. Progression to a magmatic eruption was probably prevented by rapid influx of groundwater into the feeder. The chemical data do not support the hypothesis that a single, simple dike exists under the entire segment of the Inyo chain that was active 600 years ago. If such a dike exists, it apparently tapped different magma chambers along its ∼ 10‐km length. The excess of surface extension over that needed to accommodate the intruded juvenile material may reflect the presence of a larger intrusion at depth, below the level of fragmentation, or the contribution of tectonic activity to the surface deformation.

Deep marine arc apron deposits and syndepositional magmatism in the Alisitos group at Punta Cono, Baja California, Mexico

ABSTRACTRocks exposed at Punta Cono include very fine‐grained to coarse‐grained tuffs, lapilli tuffs, and tuff breccias deposited in a deep marine environment. Syndepositional basaltic intrusive activity was common. In one locality a hyaloclastite‐peperite complex formed. Slumped sections with fluidal basalt ‘clasts’, derived from intrusions that entered the sediment pile from below, are present elsewhere. Abundant soft‐sediment folds in fine‐grained laminated subaqueous fall‐out tuff suggest steep gradients; these are cut by shallow channels filled with coarse‐grained tuff, lapilli tuff, and rare tuff breccia. The combination of marine fossils, extreme textural immaturity, abundant slump features, and syndepositional magmatism indicates deposition upon the submarine flanks of an active volcano.Recognition of magma‐wet sediment interaction is hampered in volcaniclastic rocks because of the similarity between host and intrusive fragments. Products of magma‐water‐sediment interactions at Punta Maria include: (1) jigsaw‐puzzle hyaloclastite, formed by non‐explosive hydroclastic fragmentation of magma upon contact with water and water‐bearing sediment; (2) peperites, produced by mixing of magma with sediment; and (3) an unusual tuff breccia unit, the result of non‐explosive mixing of ‘wisps’of lava with sediment during remobilization of an unconsolidated section. Low‐explosivity magma‐water‐sediment interactions are favoured by relatively high hydrostatic pressures in sub‐wave base settings.

An Oligocene submarine rockfall/avalanche breccia, Fiordland, New Zealand

Abstract Fan and canyon sediments of the Bal-leny Group (Cretaceous-Oligocene) of southern Fiordland, New Zealand, were deposited in small, fault-controlled, extensional basins and contain numerous examples of coarse mass-flow deposits. At Green Islets Peninsula, a 180 m thick breccia is well exposed in coastal outcrops. The massive, poorly sorted breccia is composed of pebble- to boulder-sized, angular blocks of the local basement and overlies, and locally interdigitates with, pelagic carbonate of the Chalky Island Formation. The breccia apparently formed as a rock avalanche^) which cascaded off a basement high into a basin margin setting. The total volume of the breccia is c. 108 m3, and the most distal portion exposed appears to have travelled about 1000 m. Field relations do not unequivocally indicate whether the main breccia unit represents a single event consisting of an amalgam of several surges which forcibly interdigitated with the marl, or multiple events representing basin margin, debris-cone l...

Middle Ordovician conodonts from the Buchans Group, central Newfoundland, and their significance for regional stratigraphy of the Central Volcanic Belt: Reply

Conodonts have been recovered from carbonate clasts within breccia units in the Buchans Group of central Newfoundland. The assemblage is characterized by specimens of Cordylodus horridus Barnes and Poplawski and Histiodella holodentata Ethington and Clark. These and other species recovered suggest a Whiterockian (latest Arenig–early Llanvirn) age for the clasts. This is somewhat older than the whole-rock Rb–Sr age of 447 ± 18 Ma on volcanics of the Buchans Group. The carbonate clasts are considered to be derived from the area of deposition of the Buchans Group rather than from some exotic source. This interpretation is based on knowledge of local tectonics, inferred paleogeogaphy, and geographic distribution of the conodont assemblage. All other clasts in the breccia units are considered of local origin. If the conodonts are of local Buchans Group origin then they indicate a Whiterockian age for the unit. In view of this, current regional models for the Central Volcanic Belt may require substantial revi...

Middle Ordovician conodonts from the Buchans Group, central Newfoundland, and their significance for regional stratigraphy of the Central Volcanic Belt

Conodonts have been recovered from carbonate clasts within breccia units in the Buchans Group of central Newfoundland. The assemblage is characterized by specimens of "Cordylodus" horridus Barnes and Poplawski and Histiodella holodentata Ethington and Clark. These and other species recovered suggest a Whiterockian (latest Arenig–early Llanvirn) age for the clasts. This is somewhat older than the whole-rock Rb–Sr age of 447 ± 18 Ma on volcanics of the Buchans Group. The carbonate clasts are considered to be derived from the area of deposition of the Buchans Group rather than from some exotic source. This interpretation is based on knowledge of local tectonics, inferred paleogeogaphy, and geographic distribution of the conodont assemblage. All other clasts in the breccia units are considered of local origin. If the conodonts are of local Buchans Group origin then they indicate a Whiterockian age for the unit. In view of this, current regional models for the Central Volcanic Belt may require substantial revision. A Whiterockian age for the Buchans Group is consistent with the observation that all significant volcanogenic massive sulphides of the Canadian Appalachians are of Caradoc or older Ordovician age. It is suggested that the Buchans Group is thrust southeastwards over the Victoria Lake Group, which is a probable distal facies equivalent.

Mechanisms of Sandstone Deposition in a Late Proterozoic Submarine Canyon, Adelaide Geosyncline, South Australia

Late Proterozoic submarine canyon fills in the Adelaide Supergroup (Flinders Range, South Australia) are asymmetrical in terms of their facies. Coarse breccia units, commonly associated with coarse-grained channelized turbidite sandstone units, generally occur adjacent to north walls of all the east-west-trending canyon incisions. In contrast, fine-grained sandstones and mudstones within the canyon fill are generally associated with south walls. In one canyon (Patsy Springs canyon), an additional element of asymmetry is associated with the prevalence of northward-climbing sets of climbing ripples (southward-dipping stoss sides) within channelized turbidites, in what are interpreted to be major thalweg channels and their associated levees. Flute casts at the bases of indiv dual turbidite sandstones invariably indicate initial turbidity current flow to the west throughout the vertical sequences of the channel fills. Parallel laminations above the flute casts in each flow pass upward into climbing ripples with south-dipping stoss sides, implying southward lateral accretion across the channel of a levee or bar as each turbidity current decelerated. The asymmetries outlined could be explained by: (1) Coriolis force acting on the turbidity currents, or (2) the existence on a steep slope of a meandering canyon gorge, confining a thalweg channel developed within subsequent canyon fill. In such a situation, roller-coastering turbidity currents would seek outer bends of the meandering primary gorge. The single accessible cross section of Patsy Springs canyon does not provide a unique solution; however, studies still in progress on a related series of canyons to the north suggest that a strongly meandering gorgelike canyon with an internal thalweg channel complex, situated in an upper fan or basin slope, provides the best explanation for all observations. A significant Coriolis force influence appears to be ruled out by observations on sandstone-body geometries and associated paleocurrents. The preferred model is one in which high-energy turbidity currents were confined within and influenced by the steep walls of the primary canyon gorge during aggradation of canyon fill.

A proximal ignimbrite breccia facies on santorini , Greece

Prominent among the pyroclastic deposits of Santorini are several thick, widespread lithic breccia deposits, which are found in intimate association with ignimbrite. At least three of these breccias are interpreted, on the basis of field and grain-size criteria, as having originated by the segregation of lithic clasts from active pyroclastic flows. They therefore record the occurrence of three large, previously unrecognized ignimbrite-forming eruptions of the volcano. The breccias of the 18,500 yr. B.P. Cape Riva eruption include two types. The first type is a thin, basal ground breccia, which overlies a strong erosion surface. This breccia shows pinch and swell structures and is strongly enriched in lithic and crystal components. It is considered to have formed by strong fluidization due to incorporation of air into the head of an active pyroclastic flow. The second, and predominant, type consists of thick co-ignimbrite lag breccias (up to 25 m), which overlie the ground breccia. These deposits are generally clast-supported, poorly sorted breccias which in places grade both vertically and laterally into non-welded pumiceous ignimbrite. They consist of well-defined, normally graded units which show coarse tail grading of lithic and pumice clasts. Each breccia unit is underlain by a thin, inversely graded ignimbrite basal layer, and correlates laterally with a flow unit of the associated ignimbrite. The lag breccias are therefore thick equivalents of the 2b lithic concentration zones of Sparks et al. The lag breccias and ignimbrite contain abundant lithic segregation structures that are characteristic of strong gas fluidization. These structures, the presence of basal layers, and the gradation into normal ignimbrite, suggest that the lag breccias originated by the segregation of lithic clasts within the bodies of dense, but strongly fluidized pyroclastic flows. The Cape Riva breccias occur within a few kilometers of their source vent and are interpreted as proximal facies of their associated ignimbrite. The presence of the ground breccia indicates that within this distance, the pyroclastic flows had developed the head and body regions characteristic of gravity currents. The deflation of the pyroclastic flow bodies, within a few kilometers from source, to particle concentrations sufficient to permit the generation of basal layers and coarse tail grading, is incompatible with present theories of column collapse. It is postulated that high pressures at the base of the collapsing Cape Riva eruption column were sufficient to significantly compress the dilute particle-gas mixture of the column close to the source vent. Subsequent sedimentation, as the pyroclastic flows moved laterally, increased the density further to the point where the observed sedimentary features could form. Simultaneous decompression of the gas phase resulted in strong fluidization, and the segregation of the lag breccias.

Comparative Morphology of Ancient and Modern Pillow Lavas

Pillow lava is a common feature of volcanic stratigraphy throughout geologic time. A comparison is made between the morphology of oceanic pillows from the FAMOUS area of the Mid-Atlantic Ridge, recent pillows from Iceland, and Archean pillows from the Rouyn-Noranda area of northwestern Quebec. In two dimensions the shapes and sizes of pillows from the three areas are similar but not identical. Abyssal and subglacial lava flows, when viewed in three dimensions, appear to be tubular lava forms similar to subaerial pahoehoe flows. Archean pillows also appear to be interconnected. Evidence of this includes broken selvages, lenticular quartz-filled cavities representing fossil lava levels, and the "pinch and swell" nature of elongate pillows. The stratigraphy of Archean volcanics and modern-day oceanic crust as seen in DSDP holes is similar. Both consist of interlayered massive, pillowed, and breccia units in an apparent random order. However, there appears to be a preponderance of pillowed flows in the modern volcanics compared to the ancient examples. Archean flows appear to be longer and, possibly, thicker than those at present-day ocean ridges and individual Archean pillows are in some places flattened and welded. These observations suggest that the rate of eruption in the Archean may have been greater than in known areas of the Mid-Atlantic Ridge. Pillow morphology and vesicle size and distribution in the Archean examples are consistent with a relatively shallow depth of eruption for the Archean sequences. Overall, the styles of eruption of both the Archean and modern pillow sequences appear to have been similar.