Clast-supported Breccia Research Articles

Hydrovolcanic eruptions of the Paraná Igneous Province: Insights from mafic volcaniclastic deposits in Sertanópolis, Paraná, Brazil

In many continental flood basalt provinces (CFBPs), primary volcaniclastic deposits formed by explosive hydrovolcanism have been shown to be significant counterparts of better-known effusive facies, a notion that challenged traditional views of CFB evolution. For a long time, the only exception to this was the Paraná Igneous Province in South America. In order to better characterize and understand this setting, detailed geological mapping and petrographic studies were carried out on interflow beds at Sertanópolis, a 250 km2 region in northwest Paraná. Four interflow units were identified at the transition between the high and low TiO2 volcanic sequences. Fine-grained interflow facies include vesiculated clastic rocks, with structures ranging from diffuse to well defined stratification, decimeter-scale dune-bedding with preserved backset and foreset laminations. They are formed mainly by fine accessory (non-juvenile) siliciclastic ash (20–70%) and extremely fine ash or mud (10–50%), followed by fine hydrothermal mica (<5–40%), commonly hydrated (clay-mineral-rich) ash aggregates (<5–20%) and lesser amounts of juvenile and accessory glass and basalt fragments (ash to block-sized, <5–30%). In the eastern portion of the research area, a 4 × 2.5 km region with outcrops of scoriaceous basaltic breccias was mapped. These are mainly matrix to clast-supported breccias with finely laminated matrix similar to the fine-grained units. These rocks are interpreted as accessory-rich mafic pyroclastic beds, and their features suggest that multiple pyroclastic density currents (PDCs) formed each interflow unit, with minor discrete fallout contribution in medial successions. Pyroclastic breccias represent proximal PDC and ballistic deposits that help outline a possible conduit in which magmatic-like fragments are dominant in the framework. Stratified tuff-breccia strata in medial deposits represent progressive dilution of PDCs and continuous reworking of scoriaceous fragments by multiple currents. The local stratigraphy is consistent with repeated explosive hydrovolcanic eruptions in a maar-diatreme-like system similar to hydromagmatic volcanoes of other CFBPs.

Uncommon preservation of dinosaur footprints in a tidal breccia: Eubrontes giganteus from the Early Jurassic Mongisty tracksite of Aveyron, southern France

AbstractA recent excavation yielded 118 large tridactyl footprints in the Lower Jurassic Dolomitic Formation of the Causses Basin, at Mongisty in southern France. Most of the tracks are ascribed to Eubrontes giganteus Hitchcock, 1845. They are preserved on a surface of 53 m2 and form parallel rows with a preferential orientation towards the north. Such an abundance and density of E. giganteus is observed for the first time in the Early Jurassic from the Causses Basin. Sedimentological and ichnotaphonomical analyses show that the footprints were made at different time intervals, thus excluding the passage of a large group. In contrast to all other tracksites from the Dolomitic Formation, where tracks are preserved in fine-grained sediments corresponding to low-energy depositional palaeoenvironments, the tracks from Mongisty are preserved in coarse-grained sediment which is a matrix- to clast-supported breccia. Clasts consist of angular to sub-rounded, millimetric to centimetric-scale (up to 2 cm), poorly sorted, randomly oriented, homogeneous dolostone intraclasts floating in a dolomudstone matrix. Sedimentological analysis shows that the depositional environments of Mongisty varied from subtidal to intertidal/supratidal settings in a large and protected flat marsh. The lithology of the track-bearing surfaces indicates that the mudflat of the Causses Basin was sporadically affected by large mud flows that reworked and redeposited mudstone intraclasts coming from the erosion of upstream, dry and partially lithified mud beds. Throughout the world, this type of preservation of dinosaur tracks in tidal matrix- to clast-supported breccias remains rare.

A lost carbonate platform deciphered from clasts embedded in flysch: Štramberk-type limestones, Polish Outer Carpathians

Limestones designated the Štramberk-type are the most common carbonate exotic clasts (exotics) embedded in the uppermost Jurassic–Miocene flysch deposits of the Polish Outer Carpathians. About 80% of stratigraphically determinable carbonate exotics from the Silesian, Sub-Silesian and Skole units (nappes) are of Tithonian (mostly)–Berriasian (sporadically Valanginian) age. A study of these exotics revealed eight main facies types: coral-microbial boundstones (FT 1), microencruster-microbial-cement boundstones (FT 2), microbial and microbial-sponge boundstones (FT 3), detrital limestones (FT 4), foraminiferal-algal limestones (FT 5), peloidalbioclastic limestones (FT 6), ooid grainstones (FT 7), and mudstones-wackestones with calpionellids (FT 8). Štramberk-type limestones in Poland and the better known Štramberk Limestone in the Czech Republic are remnants of lost carbonate platforms, collectively designated the Štramberk Carbonate Platform. Narrow platforms were developed on intra-basinal, structural highs (some of them are generalized as the Silesian Ridge), with their morphology determined by Late Jurassic synsedimentary tectonics. An attempt was made to reconstruct the facies distribution on the Tithonian–earliest Cretaceous carbonate platform. In the inner platform, coral-microbial patch-reefs (FT 1) grew, while the upper slope of the platform was the depositional setting for the microencruster-microbial-cement boundstones (FT 2). Microbial and microbial-sponge boundstones (FT 3), analogous to the Oxfordian–Kimmeridgian boundstones of the northern Tethyan shelf (also present among exotics), were developed in a deeper setting. In the inner, open part of the platform, foraminiferal-algal limestones (FT 5) and peloidal-bioclastic limestones (FT 6) were deposited. Poorly sorted, detrital limestones (FT 4), including clastsupported breccias, were formed mainly in a peri-reefal environment and on the margin of the platform, in a high-energy setting. Ooid grainstones (FT 7), rarely represented in the exotics, were formed on the platform margin. Mudstones-wackestones with calpionellids (FT 8) were deposited in a deeper part of the platform slope and/or in a basinal setting. In tectonic grabens, between ridges with attached carbonate platforms, sedimentation of the pelagic (analogous to FT 8) and allodapic (“pre-flysch”) Cieszyn Limestone Formation took place. The most common facies are FT 4 and FT 1. Sedimentation on the Štramberk Carbonate Platform terminated in the earliest Cretaceous, when the platform was destroyed and drowned. It is recorded in a few exotics as thin, neptunian dykes (and large dykes in the Štramberk Limestone), filled with dark, deep-water limestones. Reefal facies of the Štramberk Carbonate Platform share similarities in several respects (e.g., the presence of the microencrustermicrobial-cement boundstones) with reefs of other intra-Tethyan carbonate platforms, but clearly differ from palaeogeographically close reefs and coral-bearing facies of the epicontinental Tethyan shelf (e.g., coeval limestones from the subsurface of the Carpathian Foredeep and the Lublin Upland in Poland; the Ernstbrunn Limestone in Austria and Czech Republic). Corals in the Štramberk Limestone and Štramberk-type limestones are the world’s most diverse coral assemblages of the Jurassic–Cretaceous transition. The intra-basinal ridge (ridges), traditionally called the Silesian Cordillera, which evolved through time from an emerged part of the Upper Silesian Massif to an accretionary prism, formed the most important provenance area for carbonate exotic clasts in the flysch of the Silesian Series. They are especially common in the Lower Cretaceous Hradiště Formation and the Upper Cretaceous–Paleocene Istebna Formation. The Baška-Inwałd Ridge and the Sub-Silesian Ridge were the source areas for clasts from the Silesian and Sub-Silesian units (e.g., in the Hradiště Formation), while the Northern (Marginal) Ridge was the source for clasts from the Skole Unit (e.g., in the Maastrichtian–Paleocene Ropianka Formation).

The Eureka Valley Landslide: Evidence of a Dual Failure Mechanism for a Long-Runout Landslide

Abstract Long-runout landslides are well-known and notorious geologic hazards in many mountainous parts of the world. Commonly encompassing enormous volumes of debris, these rapid mass movements place populations at risk through both direct impacts and indirect hazards, such as downstream flooding. Despite their evident risks, the mechanics of these large-scale landslides remain both enigmatic and controversial. In this work, we illuminate the inner workings of one exceptionally well-exposed and well-preserved long-runout landslide of late Pleistocene age located in Eureka Valley, east-central California, Death Valley National Park. The landslide originated in the detachment of more than 5 million m3 of Cambrian bedrock from a rugged northwest-facing outcrop in the northern Last Chance Range. Its relatively compact scale, well-preserved morphology, varied lithologic composition, and strategic dissection by erosional processes render it an exceptional laboratory for the study of the long-runout phenomenon in a dry environment. The landslide in Eureka Valley resembles, in miniature, morphologically similar “Blackhawk-like” landslides on Earth, Mars, and minor planet Ceres, including the well-known but much larger Blackhawk landslide of southern California. Like these other landslides, the landslide in Eureka Valley consists of a lobate, distally raised main lobe bounded by raised lateral levees. Like other terrestrial examples, it is principally composed of pervasively fractured, clast-supported breccia. Based on the geologic characteristics of the landslide and its inferred kinematics, a two-part emplacement mechanism is advanced: (1) a clast-breakage mechanism (cataclasis) active in the bedrock canyon areas and (2) sliding on a substrate of saturated sediments encountered and liquefied by the main lobe of the landslide as it exited the main source canyon. Mechanisms previously hypothesized to explain the high-speed runout and morphology of the landslide and its Blackhawk-like analogs are demonstrably inconsistent with the geology, geomorphology, and mineralogy of the subject deposit and its depositional environment.

Characterization and discrimination of paleokarst breccias and pseudobreccias in carbonate rocks: Insight from Ordovician strata in the northern Tarim Basin, China

Ordovician carbonates in the central and southern Halahatang area of the Tarim Basin host highly productive hydrocarbon reservoirs. Previous studies suggest that the Yingshan and Yijianfang Formations (the primary producing intervals) have been extensively karstified. However, the current author has not observed significant karst features in the formations based on the detailed study of 380 m long cores from 16 wells. In the current study, true breccias are only identified in two wells in the Lianglitage Formation, including crackle and mosaic breccias as well as chaotic, clast-supported breccias. The breccias are interpreted as karst-related breccias that formed in near-surface environments during pre-Silurian subaerial exposure, and timing of karstification probably coincided with eustatic sea-level fall caused by the middle Katian glaciation. Widespread “karst features” of the Yijianfang and Yingshan Formations identified as breccias or sediment-filled cavities in many previous studies are actually only a mottled fabric (pseudobreccia) caused by selective diagenetic alteration and infiltration of bitumen in the deeper subsurface. The color difference between pseudoclasts and pseudomatrix does not reflect a mixture of clasts and cave sediments, but instead is characterized by distinct, nevertheless commonly subtle, variations in the limestone texture and whether bitumen is present. One the basis of core and thin section observations, SEM (scanning electron microscope) and XRD (X-ray diffraction) analyses, the author presents criteria to distinguish karst breccias and pseudobreccias that can be applied to other similar carbonate deposits. Discrimination between karst breccias and nodular limestones is also presented. Differentiation of karst and pseudokarst features serves not only an academic interest, but also has important applications for the exploration and development of carbonate reservoirs.

Geoquímica inorgânica e orgânica dos folhelhos da Formação Pimenteiras: implicações para um sistema petrolífero não convencional

A Formação Pimenteiras registra a ingressão marinha Meso/Neodevoniano da Bacia do Parnaíba. Seus folhelhos representam o principal intervalo gerador para seu sistema petrolífero convencional. O objetivo deste trabalho é, baseando-se em amostragem de afloramento, integrar dados geológicos e geoquímicos para compreender e avaliar o potencial produtor de gás de folhelho da Formação Pimenteiras. Assim, nove fácies foram identificadas: folhelho com ostracodes (Fo), folhelho siltoso micáceo (Fm), folhelho tabular (F), arenito fino e muito fino maciço (Af), arenito com laminação hummocky (Ah), pares silte-argila com laminações heterolíticas linsen (Hl) e wavy (Hw), siltito escuro com níveis sulfetados (S) e brecha clasto sustentada (Bc), observada no topo dos afloramentos. Os argilominerais dominam a mineralogia, seguidos de quartzo. Os processos intempéricos pós-deposicionais são evidenciados pelos altos teores de ferro laterítico e presença de caulinita. A microscopia eletrônica de varredura permitiu classificar os poros como macroporos (&gt; 50 μm). Foram imageadas, também, feições como agregados de matéria orgânica e argilominerais com diferentes hábitos, destacando-se o maciço e o vermicular. Os valores de carbono orgânico total (COT) variam entre 0,02 e 1,31%. Cristais de pirita ocorrem associados à rocha com maior valor de COT e refletem o caráter anóxico da fácies S. Os argilominerais são predominantemente esmectitas e argilominerais interestratificados. Os resultados indicam uma provável dificuldade para o faturamento hidráulico devido à grande presença de argilominerais. A porosidade primária possui dimensão favorável, mas geometria desfavorável. A baixa maturidade térmica e os valores de COT pouco expressivos apontam para um baixo potencial produtor. Assim, as rochas deste estudo possuem um potencial de produção de gás não convencional pouco favorável, diante das atuais condições exigidas pelo mercado.

Microencruster-microbial framework and synsedimentary cements in the Štramberk Limestone (Carpathians, Czech Republic): Insights into reef zonation

The Stramberk Limestone (Tithonian–lower Berriasian) was developed on a northerly located, isolated intra-Tethyan carbonate platform. It is composed of various facies that can be observed in olistoliths and blocks embedded in the Cretaceous flysch of the Outer Carpathians in Moravia (Czech Republic). Corals, microbialites, microencrusters and synsedimentary cements contributed on various scales to the reef framework. The importance of corals and some microencrusters to the formation of the Stramberk reef complex is well recognized, while other components received less attention in previous studies. Two end members of boundstone types are described from the Kotouc Quarry, near Stramberk. Boundstone type A is dominated by phaceloid (branching-type) corals, encrusted by microbialites and microencrusters, in particular photophile species (“ Lithocodium-Bacinella”, Koskinobullina socialis Cherchi et Schroeder, Iberopora bodeuri Granier et Berthou). Boundstone type B is composed of microencrusters, microbialites and synsedimentary isopachous fibrous cements, while corals are absent or subordinate. Microencrusters [ Crescentiella morronensis (Crescenti) , Labes atramentosa Eliasova , Perturbatacrusta leini Schlagintweit et Gawlick , Radiomura cautica Senowbari-Daryan et Schafer , thin encrusting calcified sponges] are main biotic components of the microencruster-cement boundstone. Some identified microencrusters are known only or mostly from intra-Tethyan carbonate platforms. Except for C. morronensis, other common microencrusters in the coral-microbial boundstone (type A) are rare in the microencruster-cement boundstone (type B). The depositional setting of boundstone type A corresponds to a low-energy environment of an inner platform. Boundstone type B, until now not recognized in the Stramberk Limestone, was developed in a high-energy, upper fore-reef slope environment. Other important facies in the Kotouc Quarry are reef-derived breccias: matrix-supported breccia and clast-supported breccia with radiaxial-fibrous cement (showing some similarities to Triassic “evinosponges” cement), interpreted as being dominantly synsedimentary (pre-burial). The preliminary studies by the present authors, supported by observations under cathodoluminescence, highlight the significance of synsedimentary cementation for the formation of a boundstone framework (type B) and the stabilization of fore-reef, slope deposits.

Mass-transport deposits in a Variscan wedge-top foreland basin (Pisuerga area, Cantabrian Zone, NW Spain)

Large-scale mass-transport deposits (MTDs) from two wedge-top depocentres in a Variscan foreland-basin system (Cantabrian Zone, Iberian Massif, northern Spain) have been studied to gain a better understanding of the processes involved in comparable deposits found in modern submarine environments. The well-exposed outcrops allow documenting the composition, internal organization and lateral changes of the MTD units, and, additionally, to know their frequency and significance in a tectonically active depositional setting. The synorogenic 5000 m-thick succession that contains these resedimented units comprises seven major tectonically controlled sequences, whose bounding unconformities were recognised by seismic-scale stratal patterns, such as fault-propagation and growth-fold geometries (wedging, truncation and onlap). These major sequences range between 4 and 0.4 Myr in duration and between 1300 and 200 m in thickness and display an overall shallowing-upward trend, from a basal MTD and deep-water sediments (prodeltaic mudstone wedges, turbidites) to shallow-marine carbonates and deltaic deposits. Each major sequence consists of one or more transgressive–regressive high-frequency sequences, up to several hundred metres in thickness, showing rapid lateral changes of facies associations across the wedge-top depocentres. The MTD units are composed of megaclasts (up to 2 km long and 360 m thick) embedded in mudstones, matrix- and clast-supported breccias and conglomerates with variable internal organization, and subordinate turbidite sandstones, deposited from slides, and gravity flows from cohesive debris flows to high-density turbidites. These en masse deposits originated from the failure of shelf to slope reaches and some of them constitute multiepisodic units resulting from several large-scale collapse events. These collapses could have been triggered by several factors, mainly related to the growth of fault-propagation anticlines during thrust emplacement. The ensuing tectonic overload would lead to strong flexural subsidence and to the modification of the basin profile. Both the major sequences and the MTD units are a useful tool for correlating the highly deformed and faulted successions across the three fault-bounded synclines and establishing a coherent stratigraphic framework.

Sedimentary facies and progradational style of a Pleistocene talus-slope succession, Northern Calcareous Alps, Austria

In mountain ranges, talus slopes are ubiquitous and typically represent the highest deposystem. The style of talus buildup from a low-dipping, immature slope to a high and steep, geomorphically mature slope, however, to date was not documented. Near Innsbruck city (Austria) a lithified talus-slope succession records progradation and downlap via talus-associated alluvial fans along its toe-of-slope; the fans linked progradation of the steep-dipping talus-slope segment over a lower-dipping substrate. The considered succession (‘Hötting Breccia’ Auct.) probably accumulated during the terminal Riss–Würm interglacial to early Würmian, and became lithified before the Last Glacial Maximum. The Hötting Breccia consists of alluvial-fan deposits which, in turn, are locally downlapped by a succession of aggrading to prograding talus slopes. Up-hill, the fossil talus slopes pinch out in onlap onto former rock cliffs. In the eastern part of outcrop, talus buildup is well-exposed along the flanks of a canyon; there, facies and depositional geometries record: (a) a basal, low-dipping alluvial-fan interval that accumulated near the toe-of-cliff, overlain and downlapped by (b) a steeper-dipping talus-slope succession. In the steep-dipping (25–35°), proximal talus-slope segment hundreds of meters in length, the talus successions consist mainly of: (i) clast-supported breccias of cohesive debris flows, intercalated with (ii) openwork breccias from grain flows and particle creep. Progradation of the steep-dipping segment of talus slopes took place via shingling of alluvial-fan depounits along the toe-of-slope. The fans linked the progradation of the steep-dipping, proximal talus-slope segment with the lower-dipping substrate ahead of the talus slope. The change from alluvial-fan deposition along the toe of initially high cliffs towards climbing onlap and progradation of talus slopes occurred when a slope segment dipping with the mean angle of residual shear of talus material had formed at the apex of the fan. Because the free cliff face supplying talus deposition diminishes with time, the progradation potential of talus slopes is inherently limited.

Internal structure, fault rocks, and inferences regarding deformation, fluid flow, and mineralization in the seismogenic Stillwater normal fault, Dixie Valley, Nevada

Outcrop mapping and fault-rock characterization of the Stillwater normal fault zone in Dixie Valley, Nevada are used to document and interpret ancient hydrothermal fluid flow and its possible relationship to seismic deformation. The fault zone is composed of distinct structural and hydrogeological components. Previous work on the fault rocks is extended to the map scale where a distinctive fault core shows a spectrum of different fault-related breccias. These include predominantly clast-supported breccias with angular clasts that are cut by zones containing breccias with rounded clasts that are also clast supported. These are further cut by breccias that are predominantly matrix supported with angular and rounded clasts. The fault-core breccias are surrounded by a heterogeneously fractured damage zone. Breccias are bounded between major, silicified slip surfaces, forming large pod-like structures, systematically oriented with long axes parallel to slip. Matrix-supported breccias have multiply brecciated, angular and rounded clasts revealing episodic deformation and fluid flow. These breccias have a quartz-rich matrix with microcrystalline anhedral, equant, and pervasively conformable mosaic texture. The breccia pods are interpreted to have formed by decompression boiling and rapid precipitation of hydrothermal fluids whose flow was induced by coseismic, hybrid dilatant-shear deformation and hydraulic connection to a geothermal reservoir. The addition of hydrothermal silica cement localized in the core at the map scale causes fault-zone widening, local sealing, and mechanical heterogeneities that impact the evolution of the fault zone throughout the seismic cycle.

The Gash Breccias of the Pembroke Peninsula, SW Wales

The thickest development of Carboniferous Limestone in Great Britain (about 1200 m) is in the Pembroke Peninsula of SW Wales. In various places, the regularity of the normally well-stratified limestone is broken by zones of disturbance, which are spectacularly displayed in magnificent near-vertical cliff sections. The zones generally occupy the whole of the 50 m-high cliffs and are up to 300 m wide. The chief component of these zones is a chaotic, clast-supported breccia, composed of angular limestone fragments welded together with varying degrees of firmness by sparry calcite veining or a normally sparse, red-pink sandy or silty matrix. The breccias are very easy to distinguish and form a striking contrast to the grey cliff scenery hereabouts (Figs 1, 2), yet they have not been discussed much—until now. Figure 1. Location map, showing localities mentioned in the text, the outcrop of the Carboniferous Limestone, the chief synclinal axes and the distribution envelope of the Gash Breccias (after Thomas 1971). 1. Flimston Bay; 2. Bullslaughter Bay (west); 3. Bullslaughter bay (east); 4. Trevallen; 5. Box Bay; 6. Draught; 7. Whitesheet Rock; 8. Lydstep Point; 9. St Margaret's Point; 10. Den Point; 11. Valleyfield Top; 12. Pembroke Castle. Download figure to PowerPoint Figure 2. The St Margaret's Point Gash (SS 119 974), viewing ESE along the Variscan strike direction. The contact is planar and faithfully follows a bedding plane in the Carboniferous Limestone succession. An exposure of the ‘undersurface’ of the breccia mass, examined by boat, displays a faint oblique slickensiding, which is inclined to the east at c. 30°. The height of the cliff at the junction is about 30 m. The breccia can be traced intermittently along the northern coast of St Margaret's Island for a distance of 220 m but it is not known where lie the northern, eastern and western boundaries beneath Caldey Sound. The air photograph reveals that either the southern boundary of this Gash is irregular, with sharp, near-right-angled indentations, or the Gash is affected by later cross faulting. Dixon records that the breccia mass is at least 45 m ‘thick’ so the implied minimum volume of this mass is c. 300 000 m3. Download figure to PowerPoint

Pleistocene glaciolacustrine breccias of seismic origin in an active graben (central Poland)

Varved layers in the partly consolidated glaciolacustrine Early Saalian deposits exposed in the Bełchatów mine (central Poland), show specific types of deformations. Two types dominate: half-varve deformations constituting the uppermost, clayey part of a single varve, and multi-varve deformations affecting a few varves. The latter are found either as clast-supported breccia beds with varve fragments in a quasi-horizontal position within a clayey matrix, or as deformed beds resulting from subsequent plastic deformation and folding. The deformed beds are interpreted as seismites derived from earthquakes related to the deep graben structure. The magnitudes of the earthquakes have determined the type of deformations. The seismites are thought to have originated intraformationally.

Lithofacies and depositional processes on a high, steep-margined Carboniferous (Bashkirian–Moscovian) carbonate platform slope, Sierra del Cuera, NW Spain

The depositional slope of the Sierra del Cuera, a Carboniferous (Bashkirian–Moscovian), high, steep-margined carbonate platform, provides excellent cross-sections of lithofacies zonations and associated stratal geometries. The steep (30–40°) and nearly planar upper slope is dominated by massive sheet-like layers of microbial, cement-rich boundstone, which alternate with intercalations of red-stained bryozoan cementstone with crinoids and brachiopods. The slightly gentler (20–26°) and concave-upward lower slope is characterized by clast-supported resedimented deposits. The upper slope extended from platform break to ca. 300 m water depth, whereas lower slope sediments were deposited in water depths up to 600–700 m, at which level the slope beds flatten to a few degrees (toe-of-slope) and interfinger with spiculitic and argillaceous basinal sediments. Between 250 and 450 m water depth, boundstones and breccias alternated in a transitional zone. The lower slope sediments include clast-supported breccias with radiaxial fibrous cement in interparticle space, mud- to clast-supported breccias with red-stained carbonate mud matrix and packstone to grainstone and rudstone beds. Most of the clasts comprise boundstone reworked from an upper slope setting and smaller grains are platform and slope derived. A pervasive submarine cementation occurs along the upper two-thirds of the flank and this stabilized the slope. Slope deposition is interpreted as follows. During active boundstone accretion, microbial boundstone layers slid off and formed breccia tongues extending from the lower upper slope down to the toe-of-slope. Rock falls and avalanches were generated whenever the shear strength of the substrate of loose (or partly lithified) sediment was exceeded. Upper-slope boundstone accretion and shedding, independent of the depth of light penetration, controlled most of the depositional processes on the slope. Cement-dominated intervals are considered to be related to early highstand (and/or flooding) phases. Relative sea-level fluctuations and/or associated changes in the water conditions are believed to be responsible for intervals of low boundstone production or cement precipitation. Whether the in situ boundstone and breccia are preferentially related to lowstand or highstand periods is, as yet, unclear.

Microbial boundstone dominated carbonate slope (Upper Carboniferous, N Spain): Microfacies, lithofacies distribution and stratal geometry

The Carboniferous, particularly during the Serpukhovian and Bashkirian time, was a period of scarce shallow-water calcimicrobial-microbialite reef growth. Organic frameworks developed on high-rising platforms are, however, recorded in the Precaspian Basin subsurface, Kazakhstan, Russia, Japan and Spain and represent uncommon occurrences within the general trend of low accumulation rates and scarcity of shallow-water reefs. Sierra del Cuera (Cantabrian Mountains, N Spain) is a well-exposed high-rising carbonate platform of Late Carboniferous (Bashkirian-Moscovian) age with a microbial boundstone-dominated slope dipping from 20° up to 45°. Kilometer-scale continuous exposures allow the detailed documentation of slope geometry and lithofacies spatial distribution. This study aims to develop a depositional model of steep-margined Late Paleozoic platforms built by microbial carbonates and to contribute to the understanding of the controlling factors on lithofacies characteristics, stacking patterns, accumulation rates and evolution of the depositional architecture of systems, which differ from light-dependent coralgal platform margins. From the platform break to depths of nearly 300 m, the slope is dominated by massive cement-rich boundstone, which accumulated through the biologically induced precipitation of micrite. Boundstone facies (type A) with peloidal carbonate mud, fenestellid and fistuliporid bryozoans, sponge-like molds and primary cavities filled by radiaxial fibrous cement occurs all over the slope but dominates the deeper settings. Type B boundstone consists of globose centimeter-scale laminated accretionary structures, which commonly host botryoidal cement in growth cavities. The laminae nucleate around fenestellid bryozoans, sponges, Renalcis and Girvanella-like filaments. Type B boundstone typically occurs at depths between 20–150 m to locally more than 300 m and forms the bulk of the Bashkirian prograding slope. The uppermost slope boundstone (type C; between 0 and 20–100 m depth) includes peloidal micrite, radiaxial fibrous cement, bryozoans, sponge molds, Donezella, Renalcis, Girvanella, Ortonella, calcareous algae and calcitornellid foraminifers. From depths of 80–200 m to 450 m, 1–30 m thick lenses of crinoidal packstone, spiculitic wackestone, and bryozoan biocementstone with red-stained micrite matrix are episodically intercalated with boundstone and breccias. These layers increase in number from the uppermost Bashkirian to the Moscovian in parallel with the change from a rapidly prograding to an aggrading architecture. The red-stained strata share comparable features with Lower Carboniferous deeper-water mud-mound facies and were deposited during relative rises of sea level and pauses in boundstone production. Rapid relative sea-level rises might have been associated with changes in oceanographic conditions not favourable for thecalcimicrobial boundstone growth, such as upwelling of colder, nutrient-rich waters lifting the thermocline to depths of 80–200 m. Downslope of 150–300 m, boundstones interfinger with layers of matrix-free breccias, lenses of matrix-rich breccias, platform- and slope-derived grainstone and crinoidal packstone. Clast-supported breccias bound by radiaxial cement are produced by rock falls and avalanches coeval to boundstone growth. Matrix-rich breccias are debris flow deposits triggered by the accumulation of red-stained layers. Debris flows develop following the relative sea-level rises, which favour the deposition of micrite-rich lithofacies on the slope rather than being related to relative sea-level falls and subaerial exposures. The steep slope angles are the result of in situ growth and rapid stabilization by marine cement in the uppermost part, passing into a detrital talus, which rests at the angle of repose of noncohesive material. In the Moscovian, the aggradational architecture and steeper clinoforms are the result of increased accommodation space due to tectonic subsidence and due to a reduction of slope accumulation rates (from 240±45−605±35 m/My to 130±5 m/My). The increasing number of red-stained layers and the decrease of boundstone productivity are attributed to environmental changes in the adjacent basin, in particular during relative rises of sea level and to possible cooling due to icehouse conditions. The geometry of the depositional system appears to be controlled by boundstone growth rates. During the Bashkirian, the boundstone growth potential is at least 10 times greater than average values for ancient carbonate systems. The slope progradation rates (nearly 400–1000 m/My) are similar to the highest values deduced for the Holocene Bahamian prograding platform margin. The fundamental differences with modern systems are that progradation of the microbial-boundstone dominated steep slope is primarily controlled by boundstone growth rates rather than by highstand shedding from the platform top and that boundstone growth is largely independent from light and controlled by the physicochemical characteristics of seawater.

Evolution of the Breccia-Hosted Porphyry Cu-Mo-Au Deposit at Agua Rica, Argentina: Progressive Unroofing of a Magmatic Hydrothermal System

Detailed geologic mapping has been used to show that Agua Rica is a porphyry-style Cu-Mo-Au deposit that was first overprinted by polystage brecciation associated with a high sulfidation epithermal event and then by a barren surface-venting phreatomagmatic diatreme, prior to a final stage of supergene enrichment. It was emplaced in the Miocene (~8–5 Ma) as an outlier of the Farallon Negro Volcanic Complex in northwestern Argentina. The Agua Rica deposit lies next to the contact between Precambrian or lower Paleozoic metasedimentary rocks and coarse-grained Ordovician granites. In a first pulse of Miocene magmatism, equigranular to porphyritic intrusions were emplaced, with minor potassic alteration and weak Cu-Mo mineralization. Subsequent intrusion of feldspar porphyries was associated with intense porphyry-style stockwork veining, potassic and propylitic alteration, and disseminated Cu-Mo-Au mineralization (molybdenite, chalcopyrite ± bornite ± pyrite). The present alteration and mineralization pattern is dominated by an almost pervasive overprint of high sulfidation epithermal assemblages (phyllic and advanced argillic alteration and Cu-Au-Ag-As-Pb-Zn mineralization) in breccia cements and as void fillings. Covellite is the dominant copper mineral in the ore and seems to have partly or completely replaced chalcopyrite and bornite of the earlier porphyry events. The high sulfidation epithermal assemblages are closely related to the emplacement of a largely clast-supported hydrothermal breccia. Three major bodies of this breccia have been mapped on the basis of clast lithology, clast shape and size, degree of alteration, and composition of breccia matrix. Igneous breccia with a fine-grained porphyritic matrix is intimately associated and interfingers with the base of the hydrothermal breccia columns. A final phase of magmatic hydrothermal activity formed a matrix-supported and commonly bedded crater infill breccia. It formed by a surface-venting phreatomagmatic eruption, as shown by a continuous downward transition from bedded breccias to clast-supported breccias with sandy or pumiceous matrix to a solid igneous breccia with a fine-grained porphyritic matrix in the lower core of the conical crater infill breccia body. Graded, matrix-rich epiclastic sediments subsequently filled the crater. Magmatic activity was terminated by a dike of unmineralized biotite porphyry, which intruded the crater infill breccia. Talus breccia was shed into the crater from the rim. Supergene leaching and enrichment, which replaced covellite, pyrite, chalcopyrite, and bornite by chalcocite and secondary covellite, formed an enrichment blanket that was dissected by the present-day, steeply incised topography. The distinctive feature of the Agua Rica hydrothermal system is the occurrence of early, weakly mineralized intrusions, later feldspar porphyries with stockwork-hosted chalcopyrite-bornite-molybdenite mineralization, hydrothermal breccias with an epithermal pyrite-covellite overprint, and barren surface-venting breccias—all exposed at one location within 1,000 m of vertical exposure. Reconstruction of the time sequence of these geologic elements indicates that Agua Rica is the result of a protracted history of magmatic hydrothermal activity with superposition of several intrusion events that probably extended over several million years during progressive regional uplift, erosion, and explosive unroofing.

Mid-Cretaceous crustal extension recorded in deep-marine half-graben fill, Cedros Island, Mexico

Marine, mid-Cretaceous fore-arc strata on Cedros Island (Vargas and Pinos Formations) record the onset of extensional brittle deformation of the upper crust concomitant with initial uplift of blueschist-grade rock from great depth. The pre-extensional Vargas Formation is chiefly fine grained, and it contains facies interpreted as basin-plain, sand-rich-fan, and slope deposits that onlapped a Jurassic unconformity toward the north. The syn-extensional Pinos Formation is coarser grained than is the Vargas and records the initiation and filling of a northeast-southwest-trending, deep-marine, half-graben structure that formed by reactivation of a Jurassic fault zone located along the northern margin of the basin. The axis of the half graben acted as a submarine canyon that funneled gravelly sediment-gravity flows toward the south-west, whereas the shoulder of the half graben was draped by sandy turbidites and mud. A second episode of faulting on a fault parallel to, and synthetic with, the older half-graben fault produced a stress-transfer zone that steepened the intervening block, producing a catastrophic slope failure in the basin. Several points of evidence suggest that the canyon-bounding fault zone mapped at the northern margin of the basin was syndepositionally active. Basin-floor scarps are indicated by the presence of large, angular blocks derived from the Jurassic strata; these blocks occur within Cretaceous conglomerate and olistostromes, and as clast-supported breccia. Asymmetric basin subsidence is recorded in monotonic lithofacies trends that occur laterally away from the inferred fault zone. These trends include a decrease in average grain size and bed thickness, and an increase in interbedded mudstone and in lateral continuity of individual beds. Abundant slope-failure features, including slide blocks as much as 100 m wide by 40 m thick, also point to frequently recurring intrabasinal seismicity. The facies distribution and basin structures exposed on Cedros Island are analogous to aspects of the southern Viking Graben and the Ebro turbidite complex. Modern tectonic and physical analogues for the half graben are found at the Kuril margin and in the Aleutian are, where oblique convergence has produced fault-bounded submarine canyons of the same lateral and vertical scale as inferred for the Cretaceous basin on Cedros Island. The temporal coincidence between brittle deformation in the upper plate with peak blueschist metamorphism in the lower plate of the convergent margin suggests that initial blueschist uplift began soon after deepest burial.

Basalt Breccias and Conglomerates in the Lower Jurassic McCoy Brook Formation, Fundy Basin, Nova Scotia: Differentiation of Talus and Debris-Flow Deposits

ABSTRACT Basalt talus breccias and debris-flow conglomerates are interbedded with terrestrial redbeds in the Lower Jurassic McCoy Brook Formation in the synrift Fundy basin. The redbeds formed in a semiarid to arid climate in alluvial-fan, eolian, fluvial, playa, and lacustrine environments. The basalt talus breccias formed adjacent to cliffs of North Mountain Basalt produced by synsedimentary faulting on the northern side of the basin. The breccias lack bedding planes, contain basalt boulders up to 8 m in length, and are interstratified with fluvial, playa, eolian, and lacustrine sandstones and mudstones. At Wasson Bluff, 24 m of talus breccia with a matrix of sandy mud comprise a sequence that is matrix-supported at the base and rapidly grades upward to the typical clast-supported fabric. The matrix of clast-supported breccia locally contains horizontal to inclined sandy gravel layers and, in a few places, abundant bones of small reptiles that lived within the talus, indicating that matrix infiltrated open spaces around the boulders. A slope of 34° to the southeast f the paleosurface of the talus is inferred from the orientations of the lower surfaces of discoidal, and plate-shaped boulders. The basalt debris-flow conglomerates total 21 m in thickness and comprise 51 beds. Some beds are separated by centimeter-scale sandstone layers deposited by streamflows. The conglomerates have a sandy-mud matrix and a fabric that varies from matrix-supported to clast-supported. The mean orientation of discoidal clasts is subhorizontal, with substantial scatter of individual clasts. The base of each flow is non-erosional, and many boulder project above flow surfaces. We attribute the debris flows to remobilization of debris during heavy rainstorms in the semiarid to arid climate.

Karst-Controlled Reservoir Heterogeneity in Ellenburger Group Carbonates of West Texas: Reply (1)

Petroleum production from restricted shelf carbonates of the Lower Ordovician Ellenburger Group is commonly considered to have been a result of a pervasive, relatively homogeneous tectonic fracture system within the reservoir rock. However, regional facies and diagenetic (paleokarst) studies of Ellenburger strata, based on cores and wireline logs, have demonstrated that significant reservoir compartmentalization was caused by karst modification in the upper part of the unit. Ellenburger Group carbonates, which attain a thickness of over 1,700 ft (520 m), record sedimentation on a shallow-water restricted shelf occupying most of west Texas. Logging of over 10,000 ft (3,050 m) of core from 63 wells has allowed recognition of six facies assemblages: (1) lithic arenite, (2) mixed siliciclastic-carbonate packstone-grainstone, (3) ooid-peloid grainstone, (4) mottled mudstone, (5) laminated mudstone, and (6) gastropod-intraclast packstone-grainstone. These facies assemblages record initial transgression and subsequent progradation and aggradation. Paleoslope was generally south and east from the Texas arch toward the Ouachita and Marathon orogenic belts. All facies assemblages, with the local exception of the ooid-peloid grainstone assemblage, are characterized y very low intergranular and intercrystalline porosity. Porosity development in Ellenburger Group carbonates is directly related to a prolonged period of subaerial exposure that coincided with a Middle Ordovician eustatic lowstand prior to transgression of Simpson Group siliciclastics. During this episode, a widespread system of caves, sinkholes, joint-controlled solution features, and collapse breccias developed. Of particular importance to reservoir development was the formation of a regionally extensive cave system between 100 and 300 ft (30 and 90 m) beneath the exposed Ellenburger surface. Infill of this cave system by Simpson Group sand and clay segmented the upper Ellenburger into three karst facies, which are, in descending order, (1) cave-roof dolomites (fracture and mosaic breccias), (2) laterally persistent cave-fill facies (siliciclastic-matrix-supported and carbonate-matrix-supported breccias), and (3) lower collapse facies (chaotic clast-supported breccias) of the cave floor. Pronounced vertical segregation of permeable zones defined by the three karst facies is evident in the Emma, Andector, Martin, Block 13, and several other major Ellenburger reservoirs. Lateral reservoir heterogeneities formed by localized laterally extensive collapse structures, such as in the Shafter Lake reservoir, also contribute to compartmentalization of producing zones within the upper Ellenburger Group. Secondary and tertiary recovery programs in these Ellenburger reservoirs can be optimized by integrating concepts of lateral and vertical heterogeneity predicted by the karst model.

Karst-Controlled Reservoir Heterogeneity in Ellenburger Group Carbonates of West Texas

Petroleum production from restricted shelf carbonates of the Lower Ordovician Ellenburger Group is commonly considered to have been a result of a pervasive, relatively homogeneous tectonic fracture system within the reservoir rock. However, regional facies and diagenetic (paleokarst) studies of Ellenburger strata, based on cores and wireline logs, have demonstrated that significant reservoir compartmentalization was caused by karst modification in the upper part of the unit. Ellenburger Group carbonates, which attain a thickness of over 1,700 ft (520 m), record sedimentation on a shallow-water restricted shelf occupying most of west Texas. Logging of over 10,000 ft (3,050 m) of core from 63 wells has allowed recognition of six facies assemblages: (1) lithic arenite, (2) mixed siliciclastic-carbonate packstone-grainstone, (3) ooid-peloid grainstone, (4) mottled mudstone, (5) laminated mudstone, and (6) gastropod-intraclast packstone-grainstone. These facies assemblages record initial transgression and subsequent progradation and aggradation. Paleoslope was generally south and east from the Texas arch toward the Ouachita and Marathon orogenic belts. All facies assemblages, with the local exception of the ooid-peloid grainstone assemblage, are characterized y very low intergranular and intercrystalline porosity. Porosity development in Ellenburger Group carbonates is directly related to a prolonged period of subaerial exposure that coincided with a Middle Ordovician eustatic lowstand prior to transgression of Simpson Group siliciclastics. During this episode, a widespread system of caves, sinkholes, joint-controlled solution features, and collapse breccias developed. Of particular importance to reservoir development was the formation of a regionally extensive cave system between 100 and 300 ft (30 and 90 m) beneath the exposed Ellenburger surface. Infill of this cave system by Simpson Group sand and clay segmented the upper Ellenburger into three karst facies, which are, in descending order, (1) cave-roof dolomites (fracture and mosaic breccias), (2) laterally persistent cave-fill facies (siliciclastic-matrix-supported and carbonate-matrix-supported breccias), and (3) lower collapse facies (chaotic clast-supported breccias) of the cave floor. Pronounced vertical segregation of permeable zones defined by the three karst facies is evident in the Emma, Andector, Martin, Block 13, and several other major Ellenburger reservoirs. Lateral reservoir heterogeneities formed by localized laterally extensive collapse structures, such as in the Shafter Lake reservoir, also contribute to compartmentalization of producing zones within the upper Ellenburger Group. Secondary and tertiary recovery programs in these Ellenburger reservoirs can be optimized by integrating concepts of lateral and vertical heterogeneity predicted by the karst model.

Significance of xenocrystic Precambrian zircon contained within the southern continuation of the Josephine ophiolite: Devils Elbow ophiolite remnant, Klamath Mountains, northern California

The Josephine ophiolite of the western Jurassic belt, Klamath Mountain province of California and Oregon, is the expression of a well-documented Late Jurassic suprasubduction zone rift basin that formed between an active Late Jurassic arc to the west and a remnant Middle Jurassic arc to the east. The Devils Elbow ophiolite remnant (DEO) exposed along the South Fork of the Trinity River represents the southernmost continuation of the Josephine ophiolite and provides important new constraints on the early history of this rift basin. The DEO consists of pillow lavas and breccias and a well-developed sheeted dike complex that are depositionally overlain by a clast-supported breccia derived almost exclusively from ophiolitic detritus. Dikes and irregular pods of plagiogranite are conspicuous elements of the DEO and exhibit mutually crosscutting relations with mafic dikes; thus, they are interpreted as genetically related elements of the dike complex. Zircon separates from two widely separated plagiogranite dike localities yielded two distinct zircon populations: a clear, euhedral, magmatic population and a reddish, rounded xenocrystic population. The isotopic systematics of four zircon fractions from these two dike localities indicate a crystallization age for the DEO of 164 ±1 Ma and an age of ∼1.7 Ga for the xenocrystic zircon component. The occurrence of a xenocrystic Precambrian zircon component within the plagiogranites of the DEO provides unequivocal evidence that rifting occurred within preexisting zircon-bearing crustal rocks of the Klamath Mountains. As there is no Precambrian crust within the Klamath Mountains, the xenocrystic zircon population must have been derived from supracrustal sedimentary sequences. The most likely source for the older zircon component is a terrigenous metasedimentary sequence contained within the Rattlesnake Creek terrane, which formed part of the structural basement of the rifted Middle Jurassic arc. The incorporation of xenocrystic Precambrian zircon from the rifted basement of the Middle Jurassic arc could only have occurred during the earliest stages of rifting. Thus, the DEO must represent a fragment of an initial rift that ultimately formed the Late Jurassic basin now floored by the Josephine ophiolite. In support of the zircon data, dikes and lavas of the DEO have geochemical characteristics transitional between island arc and mid-ocean ridge magma series, as would be expected within an initial rift setting of a suprasubduction zone ophiolite.