Pebble Conglomerate Research Articles

Diamond in the Atlin–Nakina region, British Columbia: insights from heavy minerals in stream sediments

The sources of rare diamonds reported in northwestern British Columbia, southwestern Yukon, and parts of Alaska are enigmatic. We carried out a heavy-mineral survey of 17 streams draining bedrock in the Atlin–Nakina region in northwestern British Columbia to determine if high-pressure igneous and metamorphic rocks exhumed in the area could be potential sources for the diamond. Heavy-mineral fractions returned flakes of gold but no diamond. The ferromagnetic fractions were examined optically and by electron microprobe analysis of key indicator minerals, namely olivine, orthopyroxene, clinopyroxene, garnet, spinel, and titanite. Detritus from a horizon of coarse pebble conglomerate in the Jurassic Laberge Group south of Atlin is the most likely source of the anomalous diamond. Garnets and pyroxenes in the latter sedimentary unit were derived by rapid erosion of peridotite and eclogite massif bodies exhumed from depths approaching the diamond stability field during collision and Pliensbachian uplift in the northern Cordillera. Evidence is shown for glacial transport of detritus from both the Laberge Group sediments and Neogene volcanics, the latter of which evidently covered a much wider area before the last glaciation.

Nd isotopic anatomy of a pebble conglomerate from the Murihiku terrane of New Zealand: Record of a varied provenance along the Mesozoic Gondwanaland margin

The Murihiku terrane is a volcano-sedimentary terrane of Late Permian to Early Cretaceous age that forms part of a collage of accreted terranes in the Eastern Province of New Zealand. These terranes record a history of deposition along the Gondwanaland margin, and include terranes of oceanic character (e.g. Brook Street terrane) and terranes dominated by continental detritus (e.g. Torlesse terrane). The Murihiku terrane is of particular interest because it is relatively large, one of the least structurally deformed terranes, and preserves a long (∼120 m.y) record of sedimentation. Previous studies have suggested that the Murihiku terrane preserves a record of change in provenance from predominantly mafic, juvenile sources to dominantly felsic volcanic sources in Middle to Late Triassic time interpreted as containing contributions from Precambrian continental crust, and modest amounts of detritus supplied directly from this old crust [Frost, C.D., Coombs, D.S., 1989. Nd isotope character of New Zealand sediments: Implications for terrane concepts and crustal evolution. American Journal of Science 289, 744–770.; Roser, B.P., Coombs, D.S., Korsch, R.J., Campbell, J.D., 2002. Whole-rock geochemical variations and evolution of the arc-derived Murihiku Terrane, New Zealand. Geological Magazine 139, 665-685.]. We present Nd isotopic data from a Late Triassic (Rhetian; Otapirian local stage; 200–206 Ma) roundstone cobble conglomerate from near Gore, South Island, New Zealand. The sandstone matrix of this conglomerate has an initial ɛ Nd of + 3.0 and Sm/Nd of 0.219, values that are intermediate between the more radiogenic Nd isotopic and higher Sm / Nd ratios of older Murihiku siltstones and less radiogenic, lower Sm / Nd, younger siltstones. Clasts within this matrix are mainly volcanic: we analyzed basaltic-andesite, andesite and dacite pebbles along with a volcanogenic sandstone pebble and hypabyssal quartz diorite clast. These pebbles have initial ɛ Nd between + 3.7 and + 2.6, and are probably derived from a Late Triassic volcanic arc. A felsic plutonic pebble with an initial ɛ Nd of − 0.5 and crustal residence age of 1.0 Ga may record erosion of Gondwanaland Proterozoic basement. A mudstone rip-up clast may be composed of fine-grained cratonic detritus; it has a Nd crustal residence age of 1.1 Ga. This conglomerate, containing both arc-derived and older cratonic-derived clasts, captures in a single sample the contribution of mafic arc, felsic arc and craton-derived sedimentary rocks to bulk sediment Nd budgets recorded in the Murihiku terrane.

Diagenetic silcrete and formation of silcrete ventifacts and aeolian gold placers in Central Otago, New Zealand

A diagenetic silcrete horizon from c. 1 to >6 m thick is commonly present within fluvial quartz sandstones and pebble conglomerates of Eocene Hogburn Formation and early‐middle Miocene Dunstan Formation in Central Otago. The silcrete was formed by precipitation of crystalline quartz‐overgrowth cement from groundwater focused within permeable, moderately well sorted or well‐sorted fluvial sands and silts that were deposited on floodplains that broadened ahead of marine and lacustrine transgressions. The silcrete is hosted near the base of these sandstone and siltstone beds in both formations, and extends downward into the conglomeratic, incised‐channel strata that locally underlie them. Downward advance of the cementation front was inhibited by reduced porosity arising from the higher clay content of the basal sandy or conglomeratic beds. Late Miocene‐Recent uplift has exposed the highly resistant silcrete horizons at the range margins. In situ silcrete undergoes cantilevering and tensile failure as the underlying weakly cemented sediments or kaolinitised schist that support it are eroded. Resultant large slabs and boulders of silcrete are locally preserved on range slopes and on the surface of fans in the adjacent basins. Aeolian erosion and formation of silcrete ventifacts, stone pavements, and aeolian gold placers has occurred locally and episodically in Central Otago in the rain‐shadow east of the Southern Alps since they began to rise in the late Miocene. Many of these aeolian features are relict, but they are actively forming near some range fronts exposed to the prevailing westerly wind. Sand eroded from quartz‐rich fluvial sediments exposed at these range fronts is reworked backwards and forwards across the range front in a cyclic process: basinward by mass movement and in ephemeral streams, and rangeward by the westerly winds. Ventifacts are less common on leeward slopes facing away from the westerly wind, because the supply of aeolian sediment is limited, and colluvial, fluvial, and aeolian processes all transport potential aeolian sediment in the same direction without cyclic reworking.

The fluvial megafan of Abarkoh Basin (Central Iran): an example of flash-flood sedimentation in arid lands

Abstract The Abarkoh Basin is situated in central part of the NW-SE-trending Gavkhoni-Sirjan depression of Central Iran. The studied fluvial megafan represents a Quaternary fan, which is located in the western part of this intermontane, extensional basin. This fan covers an area of 940 km 2 and with a gentle slope (about 0.5°) over a 45 km-length terminates in a playa lake in the centre of the Abarkoh Basin. Its catchment is the Abadeh Basin (> 2000 km 2 ), which is a NW-SE-trending intermontane basin and mainly floored with the Quaternary alluvial-fan sediments. The studied megafan was built by stream-dominated processes, as revealed by the presence of thick-bedded, clast-supported gravels-sandy gravels (gravelly-sandy conglomerates) in the proximal-fan areas that grade into thick marls with sinuous, lenticular channel-filled, grain-supported, imbricated, pebbly conglomerates in the proximal-medial fan regions. Thick calcareous marls and lenticular, single channel-fill sandstone-conglomerate deposits characterize the distal part of the fan and pass laterally into sediments of the playa lake fringe. The aim of this paper is to present an example of a fluvial megafan in an arid-semi-arid setting, to highlight the flash-flood, sheetflood-channelized sedimentation in megafans, and to stress their importance in water resources in arid lands. Fluvial fans are frequently identified in the rock record. The fluvial megafan of the Abarkoh Basin represents one of only a few described, large-scale, stream-dominated, Quaternary megafans in arid-semi-arid settings.

Exploration potential of the Falher G shoreface conglomerate trend: evidence from outcrop

ABSTRACT The lower Cretaceous Falher Member shoreface conglomerate trends are the most prolific natural gas reservoirs of the Alberta and British Columbia Deep Basin. Individual pools can be in excess of 100BCF with discovery wells yielding AOFs of more than 100 MMCFD. Since the 1980s, five conglomerate shoreface trends (Falhers A, B, C, D and F) have been successfully drilled and mapped in the subsurface through observations of cores, cuttings, and well logs. Subsequent to their initial discovery and drilling in the subsurface each trend has been correlated to an outcrop equivalent in the Rocky Mountain Front Ranges of northeastern British Columbia. Until recently, no other Falher conglomerate shorelines have been recognized. South (paleolandward) of the Falher A-F trends, an older shoreface conglomerate fairway, designated the Falher G, has been discovered through outcrop observations at Holtslander Ridge near Belcourt Creek, British Columbia, 110 km southeast of the southernmost described Falher shoreface conglomerate outcrop. This trend, which is oriented oblique to younger trends, can be extrapolated into the subsurface plains at the Narraway Field, Alberta, approximately 35 km south of the nearest (Falher F) shoreline conglomerate reservoir fairway. Detailed analyses of measured sections, gamma log profiles, and photo mosaics show that the Falher G is a prograding shoreface facies association of clast-supported and sand matrix chert pebble conglomerate 8 to 12 m thick. The outcrop is approximately 5.0 km in length, along a north-south-trending depositional dip exposure that grades basinward from clast B supported conglomerate to pebbly sandstone. The Falher G shoreface outcrop contains several internal ravinement surface that dip at 5-10° to the NNW (320°). Analysis of locally available coal exploration core limits the paleoshoreline width of the Falher G conglomeratic shoreface trend to less than 5.0 km, with a paleoshoreline orientation of 285°/105°. Extrapolation of this trend into the subsurface may yield a new and significant shoreface conglomerate reservoir. 1 Also at: Department of Geology and Geophysics. University of Calgary, 2500 University Drive NW, Calgary, AB T2N IN4 End_Page 23-------------------------

Early-stage volcanism and sedimentation of Jeju Island revealed by the Sagye borehole, SW Jeju Island, Korea

Jeju Island comprises extensive lava flows and hundreds of monogenetic volcanic cones with rare and thin sedimentary deposits. However, a number of boreholes reveal that the lavas are only 50 to 120 m thick along the coastal regions and are underlain by a 100 m-thick volcaniclastic sedimentary formation, which is correlative with the Seoguipo Formation. Detailed examination of the subsurface Seoguipo Formation, as recovered from the Sagye borehole in the southwestern part of the island, reveals that the formation consists of primary hydrovolcanic facies (massive or inclined stratified lapilli tuff) emplaced by Surtseyan fallouts and debris flows in subaerial or subaqueous settings and reworked hydrovolcanic facies (stratified, massive, or normally graded pebble conglomerate and sandstone) and nonvolcanic facies (homogenous or laminated mudstone) that were deposited in subaerial to submarine (nearshore to offshore) settings. The predominance of volcaniclastic deposits in the formation demonstrates that the early stage of Jeju volcanism was characterized by repetitive hydrovolcanic activity probably because of an abundance of external water for hydroexplosion. The Seoguipo Formation is envisaged to consist of multiple, superposed hydromagmatic volcanoes with intervening, marine or nonmarine sedimentary sequences. This finding warrants redefinition of the Seoguipo Formation as a sedimentary succession, above the U Formation, composed of fossil-barren and and fossiliferous, subaerial and subaqueous, volcaniclastic and nonvolcanic sedimentary deposits beneath the plateauforming lavas.

Timing of Upper Carboniferous-Permian horst-basin formation and magmatism in the NW Thuringian Forest, central Germany: a review

Abstract During Late Carboniferous-Early Permian times dextral transtensional movements along the NW-trending Franconian Fault System and parallel faults caused complex block faulting in the Thuringian Forest region, Germany, accompanied by intense magmatism. This is well constrained by geochronological data ( 207 Pb/ 206 Pb single zircon, SHRIMP, 40 Ar/ 39 Ar mica, zircon fission-track ages), field relations, and the sedimentary record from the Ruhla Crystalline Complex (RCC) and surroundings. The combined dataset indicates that the Ruhla Crystalline Complex was faulted into three nearly N-S-trending segments, which underwent different exhumation histories during Late Carboniferous-Permian times. The central segment of the RCC was exhumed by several kilometres as a horst block, while the eastern and western segments subsided simultaneously, forming the basement to the Oberhof and Eisenach molasse basins, respectively. Late Carboniferous-Permian uplift of the central segment is constrained by 40 Ar/ 39 Ar cooling ages of 311 ± 3 (muscovite) and 294–288 ± 3 Ma (biotite), a weighted zircon fission-track age of 272 ± 7 Ma and overlying Zechstein sediments. In contrast, the eastern segment shows much older 40 Ar/ 39 Albiotite cooling ages between 336 ± 4 and 323 ± 3 Ma, was exposed at c. 300 Ma, and subsequently covered by molasse sediments and volcanic rocks between 300 and c. 275 Ma. A similar Late Carboniferous evolution is inferred for the western segment, as it is also overlain by Lower Permian volcanic rocks and has a 297 ± 29 Ma single zircon fission-track age. Simultaneous horst and basin formation is additionally constrained by granite pebbles in conglomerates of the Oberhof and Eisenach basins. These pebbles can partly be derived from granites in the central segment of the RCC. Age data and the orientation of granitoid bodies and dykes in the Ruhla Crystalline Complex and its surroundings provide evidence for the opening of NE-trending structures between 300 and 294 Ma, and formation or reactivation of W- to NW-trending structures between 290 and 275 Ma. Magmatic activity in the Thuringian Forest region may have been caused by widespread mantle upwelling in central Europe during the Late Carboniferous-Early Permian.

Giant Placers of the Victorian Gold Province

ABSTRACT The Victorian gold province has yielded 2500 tonnes (t) Au, nearly 2 percent of cumulative world gold production, mostly mined between 1851 and 1910. Fifty-five percent (1375 t) was placer gold from modern and paleostream systems, and from eluvial deposits, and the remainder came from primary quartz vein-related deposits. Most of the alluvial gold placers are in unconsolidated or weakly cemented quartz pebble conglomerate and gravel, dominated by hydrothermal quartz, although a few paleoplacers are within duricrusted conglomerate that required crushing. Large and abundant gold nuggets were common. Placer gold deposits formed in three intervals following uplift in the Late Cretaceous, Late Eocene, and Pliocene. An important factor in the preservation of the paleoplacers has been their burial by younger sediments and basalt flows, with consequent protection from erosion and dispersal. Factors in the formation of the giant gold placers of Victoria include the following: (1) the existence of a major primary gold province with several multimillion-ounce gold deposits; (2) uplift and reactivation of older faults; and (3) high rainfall and deep Paleogene weathering.

Geological Setting of the Meadowbank Gold Deposits, Woodburn Lake Group, Nunavut

The Meadowbank gold deposits are hosted by a series of polydeformed and metamorphosed auriferous iron formations located in the Neoarchean Woodburn Lake group, western Churchill province, Nunavut. The supracrustal rocks in the Meadowbank area consist of a thick sequence of intermediate-composition volcanic rocks with intercalated iron formation, ultramafic volcanic rocks, and quartzite to quartz arenite. U-Pb geochronology gives an interpreted age of 2711 ± 3 Ma for the host strata at Meadowbank. The intermediate volcanic rocks are predominately volcaniclastic and have a geochemical composition that is consistent with an active continental margin setting. Iron formation deposition was coeval with the intermediate volcanism, as indicated by incorporation of volcanic detritus in the chemical precipitate. The geochemistry of the iron formation indicates that it precipitated from a fluid that at one time was at high temperature (>250°C), reducing, and likely acidic. Intercalated ultramafic rocks rarely show spinifex textures and have trace element ratios that are consistent with mantle plume-related undepleted mantle asthenosphere, likely recording episodic mantle upwelling associated with rifting in an active continental margin. The contact between the ultramafic volcanic rocks and the overlying quartz arenite is a disconformity marked by a quartz pebble conglomerate. However, similar geology on the structural hanging wall and footwall of the contact suggests that the conglomerate does not represent a significant hiatus in deposition, and is more likely a prograding terriginous siliciclastic unit. Three metamorphic events are recognized. The first, M1, is a cryptic greenschist facies event that is pre-D2. The second event, M2, is a mid-greenschist to amphibolite facies, syn-D2 event. M3 is the last event recognized, and is a post-tectonic greenschist facies event that is regional but inhomogeneous in extent, possibly reflecting thermal aureoles around post-tectonic 1.7 to 1.8 Ga Nueltin granites. The structural geology of the area is complex with four phases of deformation recognized, two of which had a significant effect on the geometry of the deposit. All of these regional events are interpreted as Paleoproterozoic in age. Relationships between deformation fabrics and mineralization, as well as the overall geometry of the mineralized envelopes, suggest that the deposit formed during syn- to late-D2. Superimposed on the mineralization are D4 structural elements that postdate gold mineralization. The main control on gold mineralization is replacement of magnetite by pyrrhotite and pyrite in high-strain corridors. The composition of amphiboles and chlorites associated with gold mineralization is remarkably consistent and shows no spatial or temporal variation, suggesting that it was buffered by the iron-rich nature of the host rocks. The bulk metasomatic effect on the intermediate volcanic rocks is characterized by the addition of K2O and the loss of CaO, Na2O, and MgO, with little variation in the total iron content. The elemental losses are likely the result of destruction of feldspars, stilpnomelane, and ferroactinolite, which are relatively common in the unmineralized rocks, and the formation of sericite, chlorite, and grunerite. Textural and timing relationships suggest that the mineralization is syn- to late-D2, and by inference M2. Fluid inclusions, chlorite, and arsenopyrite geothermometry all suggest that mineralization took place at about 325° to 375°C and 1.3 kbar pressure. These are lower P-T conditions than is indicated by M2 mineral assemblages, suggesting that the deposit formed during the waning stages of metamorphism. Strain partitioning due to the mechanical contrast of the iron formation preferentially localized dilational settings during D2 deformation. This allowed fluid influx, sulfidation of the iron formation, and resulting precipitation of gold to be concentrated in high-strain corridors. Geologic relations and geochronology collectively suggest that a Paleoproterozoic (ca. 1.8–1.9 Ga) deformation event was responsible for the introduction of gold into the Neoarchean supracrustal sequences.

Stratigraphic architecture and depositional setting of the coarse‐grained Upper Cambrian Owen Conglomerate, West Coast Range, western Tasmania

The Upper Cambrian Owen Conglomerate of the West Coast Range, western Tasmania, comprises two upward‐fining successions of coarse‐grained siliciclastic rocks that exhibit a characteristic wedge‐shaped fill controlled by the basin‐margin fault system. Stratigraphy is defined by the informally named basal lower conglomerate member, middle sandstone member, middle conglomerate member and upper sandstone member. The lower conglomerate member has a gradational basal contact with underlying volcaniclastics of the Tyndall Group,while the upper sandstone member is largely conformable with overlying Gordon Group marine clastics and carbonates. The lower conglomerate member predominantly comprises high flow regime, coarse‐grained, alluvial‐slope channel successions, with prolonged channel bedload transport exhibited by the association of channel‐scour structures with upward‐fining packages of pebble, cobble and boulder conglomerate and sandstone, with abundant large‐scale cross‐beds derived from accretion in low‐sinuosity, multiply active braided‐channel complexes. While the dipslope of the basin is predominantly drained by west‐directed palaeoflow, intrabasinal faulting in the southern region of the basin led to stream capture and the subsequent development of axial through drainage patterns in the lower conglomerate member. The middle sandstone member is characterised by continued sandy alluvial slope deposition in the southern half of the basin, with pronounced west‐directed and local axial through drainage palaeoflow networks operating at the time. The middle sandstone member basin deepens considerably towards the north, where coarse‐grained alluvial‐slope deposits are replaced by coarse‐grained turbidites of thick submarine‐fan complexes. The middle conglomerate member comprises thickly bedded, coarse‐grained pebble and cobble conglomerate, deposited by a high flow regime fluvial system that focused deposition into a northern basin depocentre. An influx of volcanic detritus entered the middle conglomerate member basin via spatially restricted footwall‐derived fans on the western basin margin. Fluvial systems continued to operate during deposition of the upper sandstone member in the north of the basin, facilitated by multiply active, high flow regime channels, comprising thick, vertically stacked and upward‐fining, coarse‐grained conglomerate and sandstone deposits. The upper sandstone member in the south of the basin is characterised by extensive braid‐delta and fine‐grained nearshore deposits, with abundant bioturbation and pronounced bimodal palaeocurrent trends associated with tidal and nearshore reworking. An increase in base‐level in the Middle Ordovician culminated in marine transgression and subsequent deposition of Gordon Group clastics and carbonates.

The role of fluid- and sediment-gravity flow processes during deposition of deltaic conglomerates (Cardium Formation, Upper Cretaceous), west-central Alberta

ABSTRACT Most previous studies on conglomerates of the Carrot Creek Member (Cardium Formation) in the Cyn-Pem/Carrot Creek area of west-central Alberta have focused on understanding the stratigraphic context of these informally termed conglomerate However, a detailed understanding of the depositional mechanisms that controlled the spatial distribution of conglomerate facies, which in turn control reservoir quality and related production characteristics within and between different pools, is presently lacking. As a result, a number of important questions still remain. However, many of these issues can be resolved by differentiating distributary-channel from delta-front conglomerates, and also appreciating the sedimentological importance of the steep depositional gradient (>15°) on the delta front. Distributary-channel and delta-front conglomerates commonly consist of interstratified, matrix-rich and matrix-poor conglomerate. In distributary-channel conglomerates, this bipartite texture is most likely related to the spatial variability of gravel and sand deposition on the leeside of gravel-bed, subaqueous dunes and/or bars. Reservoir quality in these conglomerates was significantly reduced by a later infiltrated sand matrix. Marine conglomerates, on the other hand, were deposited on a steep delta-front by sediment-gravity processes. Planar-based, ungraded, matrix-supported conglomerate overlain abruptly by better-sorted, very coarse sandstone or granule/fine pebble conglomerate represent, respectively, en masse deposition from high-concentration sediment-gravity-flow dispersions and grain-by-grain gravity-driven transport and deposition. Most dispersions collapsed on the delta slope, but others flowed to the base of the slope and deposited their sediment in a granular jump. Much of the sand-sized sediment deposited at the mouths of the distributary-channel network was winnowed and remobilized by waves and transported alongshore and away from the delta front and, as a consequence, preserved the high-reservoir quality that characterizes delta front conglomerates. End_Page 426------------------------

Sequence Stratigraphy and Carbonate-Siliciclastic Mixing in a Terminal Proterozoic Foreland Basin, Urusis Formation, Nama Group, Namibia

ABSTRACT Superb three-dimensional exposures of mixed carbonate and siliciclastic strata of the terminal Proterozoic Urusis Formation in Namibia make it possible to reconstruct cross-basin facies relations and high-resolution sequence stratigraphic architecture in a tectonically active foreland basin. Six siliciclastic facies associations are represented: coastal plain; upper shoreface; middle shoreface; lower shoreface; storm-influenced shelf; and pebble conglomerate. Siliciclastic shoreface facies pass seaward into and interfinger with facies of an open carbonate shelf. Four carbonate facies associations are present: mid-shelf; shelf crest; outer shelf; and slope. Facies are arranged hierarchically into three scales of unconformity-bounded sequences. Small-scale sequences are one to tens of meters thick and span a few thousand years. They consist of shelf carbonate with or without shoreface siliciclastic facies near the bottom. Medium-scale sequences are tens of meters thick and span a few hundred thousand years. They consist of shoreface siliciclastic facies in their lower parts, which grade upward and pass seaward into shelf carbonate. Large-scale sequences are tens to hundreds of meters thick and span 1 to 2 million years. They are identified by widespread surfaces of exposure, abrupt seaward shifts in shoreface sandstone, patterns of facies progradation and retrogradation, and shoreline onlap by medium-scale sequences. Patterns of carbonate-siliciclastic mixing distinguish tectonic from eustatic controls on the evolution of large-scale sequences. Characteristics of eustatically controlled large-scale sequences include: (1) basal unconformities and shoreface sandstone that extend across the shelf to the seaward margin; (2) retrograde carbonate and siliciclastic facies belts that onlap the shoreline together, symmetrically, during transgression; and (3) upper shoreface sandstone that progrades seaward during highstand. In contrast, tectonically controlled sequences feature: (1) basal erosion surfaces and upper shoreface sandstone that are restricted to near the landward margin and pass seaward into zones of maximum flooding; and (2) asymmetric stratigraphic development characterized by landward progradation of carbonate from the seaward margin coincident with backstepping and onlap of the shoreline by siliciclastic facies. A two-phase tectonic model is proposed to account for the stratigraphic asymmetry of tectonically controlled sequences. Increased flexural bending during periods of active thrust loading caused submergence of the seaward margin and uplift of the landward margin. Rebound between thrusting episodes flattened the basin gradient and submerged the landward margin, causing expansion of carbonate facies from the seaward margin and simultaneous transgression of the landward margin. Although the two-phase model should apply to single-lithology successions deposited in active foreland basins, the mixing of carbonate and siliciclastic facies provides a particularly sensitive record of tectonic forcing. The sensitivity may be sufficient for medium- and small-scale sequences to record higher-frequency variations in flexural warping.

The geochemistry of Archaean shales derived from a Mafic volcanic sequence, Belingwe greenstone belt, Zimbabwe: provenance, source area unroofing and submarine versus subaerial weathering

Shales of the ∼2.7 Ga Zeederbergs Formation, Belingwe greenstone belt, Zimbabwe, form thin (0.2–2 m) horizons intercalated with submarine lava plain basalts. Shales of the overlying Cheshire Formation, a foreland basin sedimentary sequence, form 1–100 m thick units intercalated with shallow–water carbonates and deep-water, resedimented basalt pebble conglomerates. Zeederbergs shale is characterized by high contents of MgO and transition metals and low concentrations of K 2O and LILE as compared to average Phanerozoic shale, indicative of an ultramafic to mafic source terrain. Cheshire shales have similar major and trace element contents, but MgO and transition metals are less enriched and the LILE are less depleted. Zeederbergs shales have smoothly fractionated REE patterns (La N /Yb N = 2.84–4.45) and no significant Eu anomaly (Eu/Eu* = 0.93–0.96). REE patterns are identical to those of the surrounding basaltic rocks, indicating local derivation from submarine reworking. Cheshire shales have rather flat REE patterns (La N /Yb N = 0.69–2.19) and a small, negative Eu anomaly (average Eu/Eu* = 0.85), indicative of a mafic provenance with minor contributions of felsic detritus. A systematic change in REE patterns and concentrations of transition metals and HFSE upwards in the sedimentary succession indicates erosion of progressively more LREE-depleted basalts and ultramafic volcanic rocks, followed by unroofing of granitoid crust. Weathering indices confirm the submarine nature of Zeederbergs shale, whereas Cheshire shale was derived from a source terrain subjected to intense chemical weathering.

The Lago Sofia Conglomerate : Debris Flow to Hyperconcentrated Flow Deposits in a Cretaceous Submarine Channel, Southern Chile

The Lago Sofia conglomerates encased in the Cretaceous Cerro Toro Formation, southern Chile, represent a gigantic submarine channel system developed along a foredeep trough. The channel system consists of several tributaries along the trough margin and a trunk channel along the trough axis. Voluminous debris flows were generated ubiquitously along the tract of the submarine channel mainly by the failure of nearby channel banks or slopes. The flows transformed immediately into multiphase flows and resulted in very thick-bedded mass-flow deposits with a peculiar structure sequence. The mass-flow deposits commonly overlie fluted or grooved surfaces and consist of a lower division of clast-supported and imbricated pebble-cobble conglomerate with common basal inverse grading, and an upper division of clast- to matrix-supported and disorganized pebble conglomerate or pebbly mudstone with abundant intraformational clasts. The structure sequence suggests a temporal succession of a turbidity current, a bipartite hyperconcentrapted flow with active clast collisions near the flow base, and a cohesive debris flow probably with a rigid plug. The multiphase flow is interpreted to have resulted from transformation of clast-rich but cohesive debris flows. Cohesive debris flows appear to transform more easily into dilute flow types in subaqueous environments because they are apt to hydroplane. This is in contrast to the flow transitions in subaerial environments where noncohesive debris flows are dominant and difficult to hydroplane.

Storms versus tsunamis: Dynamic interplay of sedimentary, diagenetic, and tectonic processes in the Cambrian of Montana

The Upper Cambrian Deadwood Formation of north-central Montana is composed mainly of fine-grained calcareous sandstone and intercalated silty shale, and contains scattered intraclastic flat- pebble conglomerate. These rocks, typical of many mixed carbonate- siliciclastic, subtidal shelf successions of Proterozoic through Early Ordovician age, are distal storm deposits in which the conglomerates are conventionally thought to record occasional, very high energy events that scoured incipiently cemented layers. In the Deadwood Formation, most sandstone beds have a linear to crudely reticulate pattern of vertical cracks filled with injected silt and clay. The cracks exhibit a consistent northwest-southeast trend, and are very shallow burial deformation structures caused by the passage of seismic waves possibly originating from strong earthquakes in Idaho. Most conglomerates are composed of angular to subangular sandstone intraclasts, the polygonal plan view of which is similar in outline to the areas between cracks in the sandstones. The deformation implied by the cracks, sporadic distribution of conglomerates, high degree of scouring indicated, and angularity of the intraclasts argue that, rather than storms, the conglomerates were generated by occasional tsunamis. The sweep of these tsunamis across the shallow intracratonic sea created extraordinarily strong oscillating bottom currents at the deeper reaches of storm wave base. “Tsunamites” are therefore identified with confidence virtually for the first time in shallow subtidal shelf deposits.

The Jacobina Paleoproterozoic gold-bearing conglomerates, Bahia, Brazil: a “hydrothermal shear-reservoir” model

The N–S Jacobina foreland basin is a Paleoproterozoic basin (2080 to 1900 Ma) of stacked detrital and schist formations developed along a sinistral wrench–overthrust fault of the Transamazonian orogeny. At least five successive stages are identified in the westward progradation of the basin, wherein bedding-parallel strike–slip thrusts and prograde metamorphism indicate progressive involvement in the Transamazonian collision. Shearing and associated metamorphic recrystallization indicate a high geothermal gradient and a gradual loading of the underthrust units, with those to the east being deeper seated than those to the west. Basic–ultrabasic dykes and granite stocks cross-cut the lithologies and contributed to the thermal evolution. Conglomerates and chaotic breccia layers found in several formations mark periods of tectonic instability accompanied by erosion of the earlier stages of basin development. Multistage hydrothermal activity is indicated by a widespread alteration with gold-bearing quartz lodes, conglomerates showing a sulphide–gold–Cr–mineral association, leaching of ultrabasic rocks, and emerald mineralization related to late-tectonic granite emplacement. The economic gold-bearing clastic sediments (conglomerates and minor quartzites deposited during the fourth unit/stage of basin development) have generally been interpreted according to the “modified paleoplacer” model. Such a model, which implies the existence of a gold preconcentration within the quartz–pebble conglomerates, is supported by the lithological controls, reworked pebbles (e.g. cataclastic quartz, quartz–pyrite mylonite and rare fuchsite-bearing rocks) that had been hydrothermally altered during an earlier hydrothermal stage, and lead isotopic compositions revealing an Archean inheritance that is partly preserved in galena inclusions. New basin-scale structural and metallogenic data obtained by the authors, suggest a basin model characterized by successive cycles of sedimentation–faulting/thrusting–erosion (“cannibalism” model) that favours both a “paleoplacer” model and a “mesothermal shear-zone-related epigenetic” model. Heterochronous shear zones, formed between 1988 and 1912 Ma during the development of the basin, acted as feeder zones for postsedimentary mesothermal quartz–tourmaline–muscovite/fuchsite lodes, which constitute a possible source of the reworked hydrothermally altered pebbles. These shear zones controlled (a) an acid leaching of the sediments and the subsequent development of Al-rich assemblages interpreted as advanced argillic alteration zones, and (b) a sulphidation process affecting detrital ilmenite and magnetite and responsible for the high concentration of gold within shear veins containing assemblages of Cr–Ni-bearing minerals (euhedral chromite, Cr–rutile, linnaeite, Cr–tourmaline and fuchsite) and euhedral pyrite that crystallized over andalusite. The conglomerates, with their pore and fissure permeability, acted as a “reservoir and trap” for the mineralizing fluids to give a “hydrothermal shear-reservoir” model. The homogeneous sulphur isotopic composition of the sulphides (between −0.5‰ and 1.3‰) and the δ 18O–δD values of the hydrothermal fluid (+8±1‰ and −36±3‰, respectively) suggest a magmatic source for the fluids. Moreover, the high thermal regime that characterizes the Jacobina basin could reflect a contribution from both the regional granitic magmatism and the basic–ultrabasic dyke network responsible for the Cr–(Ni–Co) mineralogical signature of the mineralization. Under the oxidation conditions of the Paleoproterozoic, the sulphidation of the gold-bearing conglomerates would depend largely on the extent of the hydrothermal–magmatic processes.

Paleoproterozoic Huronian basin: product of a Wilson cycle punctuated by glaciations and a meteorite impact

The Huronian Supergroup (∼2.4–2.2 Ga) comprises up to 12 km of mainly sedimentary supracrustal rocks. The oldest Huronian unit, the Livingstone Creek Formation, is chemically and mineralogically immature. Significant chemical weathering and deposition of U-rich quartz pebble conglomerates followed deposition of the Livingstone Creek Formation and extrusion of the overlying, rift-related lavas of the Thessalon Formation. Much of the succeeding Huronian comprises climatically(?)-controlled tripartite cycles, each of which begins with glaciogenic diamictites, followed by mudstones and cross bedded arenaceous units. Huronian deposition is interpreted as the result of a partial Wilson cycle, involving rifting and development of a southward-facing passive margin. The ∼2.4 Ga Murray and Creighton granites have been considered by others to be co-eval with an early orogenic episode (Blezardian). Expected unconformities are, however, lacking and these granites, together with co-eval volcanic rocks and abundant soft sediment deformation structures are considered to be due to anorogenic processes during Huronian basin subsidence. The main deformation of the Huronian is probably related to the Penokean orogeny (∼1.89–1.8 Ga). Geochemical investigations have contributed to paleoclimatic and provenance studies and have helped to define basin-wide metasomatic events. A large impact at ∼1.85 Ga likely played an important role in formation of the Sudbury Igneous Complex (SIC) and deposition of fallback breccias of the Onaping Formation (basal Whitewater Group in the Sudbury basin). Overlying sedimentary rocks of the Whitewater Group are considered to be a fortuitously-preserved portion of a widespread flysch apron that spread across the southern margin of the Superior Province as a foreland basin fill, in response to the closure phase of the Wilson cycle during the Penokean orogeny. Geochronological data suggest that these events occurred during the Penokean orogeny.

Depositional environment of the Campanian (Upper Cretaceous) strata at Maltesholm, Scania, southern Sweden

Campanian sedimentary rocks of the Maltesholm quarry consist of mostly coarse-grained, poorly consolidated, ruditic grainstones, almost free of micrite and lime mud, with interbedded clay horizons. The main constituents of the grainstones are coralline algae and quartz. Abundance of coralline algae and lack of micrite indicate a shallow, nearshore, high-energy environment. Two different main facies may be recognised, viz. fair weather facies with rudists in abundance, and storm facies with pebble and shell fragment conglomerates, scour and fill structures, and hummocky cross-bedded deposits. The conglomerate pebbles are well rounded and consist mostly of Precambrian rocks, limonite nodules, and quartz. Palaeocurrent data of belemnite rostra indicate a storm driven current direction perpendicular to an inferred palaeoshoreline. Compared with other localities in the Kristianstad basin, the same type of environments are found in the Ullstorp and Ignaberga quarries located in the northern part of the basin.

Effects of object ellipticity on strain, and implications for clast–matrix rocks

We examine the influence of object shape on strain, taking the example of an elliptical object in a matrix of different viscosity. The Eshelby–Bilby equation for the cross-sectional strain of circular cylindrical objects in a matrix of different viscosity is differentiated, to express the relationship of developing sectional ellipticity on incremental strain. This leads to an expression for the strain of initial elliptical objects whose axes are parallel or perpendicular to pure shearing. Graphs show that competent elliptical objects with axial ratios of R i =3 or more, will strain significantly more than circular objects of the same viscosity; less if the objects are incompetent. While the effect is likely to be insignificant for competent objects with initial ellipticity of <2, which is indicated from statistics for undeformed pebbles in conglomerates, any clast with greater ellipticity, such as R i =5 to 10, could deform significantly more than an equant clast, and thus appear to be materially less competent. These principles have implications for geological strain studies and for competence contrasts in rocks.

Basin-internal derivation of hydrocarbons in the Witwatersrand Basin, South Africa: evidence from bulk and molecular δ13C data

Gold in the quartz–pebble conglomerates of the late Archean Witwatersrand Basin, South Africa, is often intimately associated with carbonaceous matter of organic/biogenic origin which occurs in the form of stratiform carbon seams and paragenetically late bitumen nodules. Both carbon forms are believed to be formed by solidification of migrating hydrocarbons. This paper presents bulk and molecular chemical and stable carbon isotope data for the carbonaceous matter, all of which are used to provide a clue to the source of the hydrocarbons. These data are compared with those from intra-basinal shales and overlying dolostone of the Transvaal Supergroup. The δ 13C values of the extracts from the Witwatersrand carbonaceous material show small differences (up to 2.4‰) compared to the associated insoluble organic matter. This suggests that the auriferous rocks were stained by mobile hydrocarbons produced by thermal and oxidative alteration of indigenous bitumens. A contribution from hydrocarbons derived from intra-basinal Witwatersrand shales cannot be excluded. Individual aliphatic hydrocarbons of the various carbonaceous materials were subjected to compound specific isotope analysis using on-line gas chromatography/combustion/stable isotope ratio mass spectrometry (GC/C/IRMS). The limited variability of the molecular parameters and uniform δ 13C values of individual n-alkanes (−31.1±1.7‰) and isoprenoids (−30.7±1.1‰) in the Witwatersrand samples exclude the mixing of oils from different sources. Carbonaceous matter in the dolostones shows distinctly different bulk and molecular isotope characteristics and thus cannot have been the source of the hydrocarbons in the Witwatersrand deposits. All the various forms of Witwatersrand carbon appear indigenous to the Witwatersrand Basin, and the differences between them are explained by variable, in general probably short (centimeter- to meter-scale) hydrocarbon migration during diagenesis and subsequent hydrothermal infiltration.