An evaluation of shape indices as palaeoenvironmental indicators using quartzite and metavolcanic clasts in Upper Cretaceous to Palaeogene beach, river and submarine fan conglomerates

The feasibility of using quantitative shape measurements to discriminate between clast populations from different depositional settings was evaluated using samples from 11 fluvial, six submarine fan and four beach conglomerates from south-west California; these origins had been established previously by facies analysis. Quartzite and metavolcanic clasts were characterized by the following indices: modified Wentworth roundness (R_w), maximum projection sphericity (δ_p), oblate-prolate index (OPI) and long (L), intermediate (I) and short (S) axial ratios. These indices were compared with those documented previously for modern gravels. The results show that certain indices are useful palaeoenvironmental indicators, despite inherited differences in shape due to texture, provided that multiple sites are sampled and a statistical approach is used. Statistically, the most effective shape indices are δ_p and S/L which give good results with the Zingg classification (I/L vs. S/I); better results are also obtained using quartzite clasts. The OPI is useful for discriminating between beach and river conglomerates, which consist largely of oblate and prolate clasts, respectively. The relative abundance of blade-shaped clasts is a useful index of sediment maturity, being greatest for river clast samples and smallest for submarine fan clast samples. The latter are dominated by spherical particles. No correlation between palaeoenvironment and R_w is observed, hence the abundance of disc-shaped clasts in the beach conglomerates studied is attributed to selective transport in suspension and sediment by passing during fluvial transport prior to deposition in the surf zone. Selective transport of rollers (spheres and rods) by traction in a shallow marine setting, prior to redeposition by mass transport, may be responsible for the dominance of spherical particles in submarine fan conglomerates.     

SEDIMENTATION IN THE RIFT ZONE OF THE JUAN DE FUCA RIDGE

The sediments of the upper Swartkops River are almost exclusively gravels and boulder beds derived from the Cretaceous Uitenhage Group and the Paleozoic Cape Supergroup rocks. Many of the cobbles and boulders are second-cycle clasts, the great majority of which are quartzitic in composition. Pebble size and shape were examined and fabric analysis was performed on samples from 22 sites in the study area. Pebble imbrication planes dip consistently upstream at angles of 20? to 50? and pebble long axes generally are aligned normal to the flow direction. Clasts in the braid-plain deposits range from a few millimeters to tens of centimeters (large boulders over a meter in diameter are not uncommon). Pebble roundness ranges from 0.2 to 0.9 (averaging 0.43) and sphericity values range from 0.3 to 0.9 (averaging 0.59). The gravel clasts are angular to well-rounded, but are predominantly subrounded. Zingg diagram plots show a majority of discoidal pebbles, but there is a diversity of shapes reflecting the complex source area from which some resedimented clasts originated.

Channel and bar morphology is complex, with gravel bars often merging laterally and longitudinally with main and secondary channels. Both channels and bars are terraced stepwise downstream and across the braid plain. Bar tops are armored by both small and large clasts, whereas channels may be lined with cobbles or boulders, but often exhibit small pebble lags. Algal mats occur as fresh curtains in all standing pools of water and dried crusty deposits on pebbly substrates in inactive channels.

Imbrication studies demonstrate conclusively that pebble imbrication is the most meaningful indicator of flow direction in a gravel deposit and is far more reliable than rare cross-bedding encountered in bar-top sands, where bedforms often migrate laterally rather than downstream. The Swartkops braid-plain gravels resemble the ancient deposits of the Ventersdorp Contact Reef, both deposits being characterized by boulder-rich gravels, poor clast sorting, resedimented pebbles from a proximal fault-bounded source, and algal mats. Although heavy minerals are lacking in the Swartkops, trapping of fines by algal filaments appears to occur during low-flow conditions.     

Volcaniclastic resedimentation in distal fluvial basins induced by large-volume explosive volcanism: the Ebisutoge–Fukuda tephra, Plio-Pleistocene boundary, central Japan

The Ebisutoge–Fukuda tephra (Plio‐Pleistocene boundary, central Japan) has a well‐recorded eruptive style, history, magnitude and resedimentation styles, despite the absence of a correlative volcanic edifice. This tephra was ejected by an extremely large‐magnitude and complex volcanic eruption producing more than 400 km³ total volume of volcanic materials (volcanic explosivity index=7), which extended more than 300 km away from the probable eruption centre. Remobilization of these ejecta occurred progressively after the completion of a series of eruptions, resulting in thick resedimented volcaniclastic deposits in spatially separated fluvial basins, more than 100 km from the source. Facies analysis of resedimented volcaniclastic deposits was carried out in distal fluvial basins. The distal tephra (≈100–300 km from the source) comprises two different lithofacies, primary pyroclastic‐fall deposits and reworked volcaniclastic deposits. The resedimented volcaniclastic succession shows five distinct sedimentary facies, interpreted as debris‐flow deposits (facies A), hyperconcentrated flow deposits (facies B), channel‐fill deposits (facies C), floodplain deposits with abundant flood‐flow deposits (facies D) and floodplain deposits with rare flood deposits (facies E). Resedimented volcaniclastic materials at distal locations originated from unconsolidated deposits of a climactic, large ignimbrite‐forming eruption. Factors controlling inter‐ and intrabasinal facies changes are (1) temporal change of introduced volcaniclastic materials into the basin; (2) proximal–distal relationship; and (3) distribution pattern of pyroclastic‐flow deposits relative to drainage basins. Thus, studies of the Ebisutoge–Fukuda tephra have led to a depositional model of volcaniclastic resedimentation in distal areas after extremely large‐magnitude eruptions, an aspect of volcaniclastic deposits that has often been ignored or poorly understood.     

鄂尔多斯盆地西缘砾质冲积扇沉积学特征

晚三叠世鄂尔多斯西缘冲积扇沉积主要由碎屑流、颗粒流、片流及河道砾岩等组成。文中讨论了各类砾岩的沉积学特征,同时指出,不同成因的砾岩,其粒度分布曲线形态及某些粒度参数亦不同.据此本文提出了根据-系列新的粒度分布曲线形态和粒度参数Mdf,Rgs和Mdf/Rgs等甄别冲积扇中不同类型砾岩的设想。     

Petrography of isotopically-dated clasts in the Kapoeta howardite and petrologic constraints on the evolution of its parent body

Detailed mineralogic and petrographic data are presented for four isotopically-dated basaltic rock fragments separated from the howardite Kapoeta. Clasts C and ρ have been dated at ~4.55 AE and ~ 4.60 AE respectively, and Clast ρ contains ²⁴⁴Pu and ¹²⁹I decay products. These are both igneous rocks that preserve all the features of their original crystallization from a melt. They thus provide good evidence that the Kapoeta parent body produced basaltic magmas shortly after its formation (< 100 m.y.). Clast A has yielded a Rb-Sr age of ~ 3.89 AE and a similar ${}^{40}\text{Ar}{}^{39}\text{Ar}$ age. This sample is extensively recrystallized, and we interpret the ages as a time of recrystallization, and not the time of original crystallization from a melt. Clast B has yielded a Rb-Sr age of ~ 3.63 AE, and an ${}^{40}\text{Ar}{}^{39}\text{Ar}$ age of ? 4.50 AE. This sample is moderately recrystallized, and the Rb-Sr age probably indicates a time of recrystallization, whereas the ${}^{40}\text{Ar}{}^{39}\text{Ar}$ age more closely approaches the time of crystallization from a melt. Thus, there is no clearcut evidence for ‘young’ magmatism on the Kapoeta parent body.Kapoeta is a ‘regolith’ meteorite, and mineral-chemical and petrographic data were obtained for numerous other rock and mineral fragments in order to characterize the surface and near-surface materials on its parent body. Rock clasts can be grouped into two broad lithologic types on the basis of modal mineralogy—basaltic (pyroxene- and plagioclase-bearing) and pyroxenitic (pyroxenebearing). Variations in the compositions of pyroxenes in rock and mineral clasts are similar to those in terrestrial mafic plutons such as the Skaergaard, and indicate the existence of a continuous range in rock compositions from Mg-rich orthopyroxenites to very iron-rich basalts. The FeO and MnO contents of all pyroxenes in Kapoeta fall near a line with FeO/MnO ~ 35, suggesting that the source rocks are fundamentally related. We interpret these observations to indicate that the Kapoeta meteorite represents the comminuted remains of differentiated igneous complexes together with ‘primary’ undifferentiated basaltic rocks. The presently available isotopic data are compatible with the interpretation that this magmatism is related to primary differentiation of the Kapoeta parent body. In addition, our observations preclude the interpretation that the Kapoeta meteorite is a simple mixture of eucrites and diogenites.The FeO/MnO value in lunar pyroxenes (~60) is distinct from that of the pyroxenes in Kapoeta. Anorthositic rocks were not observed in Kapoeta, suggesting that plagioclase was not important in the evolution of the Kapoeta parent body, in contrast to the Moon. Both objects appear to have originated in chemically-distinct portions of the solar system, and to have undergone differentiation on different time scales involving differing materials.     

Mega-debris flow deposits from the Oligo-Miocene Pindos foreland basin, western mainland Greece: implications for transport mechanisms in ancient deep marine basins

The turbidite dominated, Oligo-Miocene Pindos foreland basin of western mainland Greece contains two thick (60–72 m), matrix supported conglomerates. The conglomerates are ungraded and contain three clast types: (1) polymict, rounded, extrabasinal clasts (long axes 3–50 cm); (2) tightly folded, intrabasinal clasts (long axes 1–10 m); and (3) tabular, largely undeformed, intrabasinal blocks (long axes 18–300 m). Clasts are isolated within a slit dominated matrix. These chaotic, matrix supported conglomerates are interpreted as mega-debris flow deposits. During transport, extrabasinal clasts were supported by a combination of matrix cohesion and clast dispersive pressure, folded intrabasinal clasts were supported by a combination of buoyancy (Archimedes principle) and clast dispersive pressure. The large tabular clasts were transported by gravity sliding/gliding within the flow on films at high pore fluid pressure. These different clast support mechanisms were active simultaneously within the Pindos mega-debris flow deposits. As a result, the deposits have no systemic vertical stratigraphy, in contrast to many described large scale mass flow deposits. The mega-debris flow deposits are significantly thicker than most described ancient siliciclastic debris flow deposits and provide an ancient analogue for the thick Recent siliciclastic debris flow deposits on continental margins.     

Facies and provenance analysis of Paleogene‐age alluvial conglomerates from the northern part of the Mesta Basin,SW Bulgaria

The Mesta Basin in southwest Bulgaria is a graben that contains a Paleogene‐age siliciclastic and volcaniclastic succession deposited in alluvial and fluvial settings. A sedimentological analysis has shed light on conglomerate provenance, and the links between deposition and tectonic setting. Petrographical and chemical analysis of conglomerate clasts and matrix from the Dobrinishka, Gradinishka, Osikovo (or Osenovo) and Zlataritsa formations reveal both local, and more distal source provenance ages. The basal conglomerates are subdivided into three types, a lower and upper polymictic and a middle granitic conglomerate type. Petrographical and chemical analysis reveals granite, gneiss and amphibolite clasts that were sourced from the Sidironero–Mesta Unit of the Middle Allochthon of the Rhodope Metamorphic Complex, and the Rila–Rhodope Batholith to the east of the basin. Cathodoluminescence analysis of quartz sand grains reveals an increased input of red‐ and violet‐luminescent volcanic grains. Volcanic quartz in the oldest conglomerates indicates a hitherto unknown early (pre‐Oligocene) phase of volcanic activity in the vicinity of the Mesta Basin. The conglomerates were deposited in association with movement on the Ribnovo low‐angle normal fault during the Late Eocene, creating subsidence and the development of considerable accommodation space. The establishment of a fluvial environment followed subsequent Oligocene‐age volcanic activity. Copyright © 2012 John Wiley & Sons, Ltd.     

Resedimentation of cold pumiceous ignimbrite into water: facies transformations simulated in flume experiments

Deposits and transport processes resulting from the resedimentation of cold, unconsolidated ignimbrite into water were simulated by flume experiments. The ignimbrite sample used was poorly sorted (σ = 2·4–3), fine ash‐rich (< 63 μm, 17–30 wt%) and included both dense lithic clasts (> 2000 kg m^?3) and pumice (500 to ca 1300 kg m^?3). As a result of the binding forces of the ash matrix, the experiments involved resedimentation from a steep front onto the floor (with or without an initial ramp) of the water‐filled tank under both still and wave‐generated conditions. Larger discrete collapse events were induced by oversteepening the sample front and by undercutting from wave action. The mass of the collapse and proportion of pore–space water strongly influenced the style of resedimentation and the deposits. Initial collapse events were from the top of the steep front and fell onto the floor. The largest, densest clasts were deposited as a lithic lag in a proximal sediment wedge or rolled down to a break‐in‐slope. Fine ash was transported in dilute turbidity currents, and coarse unsaturated pumice clasts floated off. Moderate collapse events generated high‐density turbidity currents, trapping pumice in the flow, causing them to saturate. These low‐density pumice clasts were easily remobilized by wave activity and passing currents and accumulated on the gentle slope at the bottom of the resedimented deposit. Large collapse events slumped, producing poorly sorted mounds similar in texture to the original starting material. As the matrix of the ignimbrite sample became saturated with water, moderate and large collapse events generated debrisflows and slurries that deposited massive, poorly sorted deposits. Furthermore, once more gentle slopes were established between the sample and deposit, small cascading grainflows deposited lithic clasts on the upper slopes and levees of pumice at the terminus of low‐relief, ash channels. The experiments show that, excluding large collapse events and debrisflows, resedimenting ignimbrite in water is effective at segregating low‐density pumice clasts from dense lithic clasts and fine ash. Experiments using fine‐ash poor ignimbrite and well‐sorted quartz sand for comparison formed an inherently unstable initial steep front that immediately collapsed by continuous grain avalanches. The grainflow deposits had textures similar to the fines‐poor starting material.     

The Palaeocene-early Oligocene Zacatecas conglomerate,Mexico: sedimentology,detrital zircon U–Pb ages,and sandstone provenance

ABSTRACT

This article presents detailed mapping results and the first U–Pb zircon dating and sedimentological characterization of the Zacatecas Conglomerate, which belongs to the Palaeogene red beds of central Mexico, deposited in fault-bounded basins during the Late Cretaceous to Eocene Laramide orogeny. The conglomerate was divided into five depositional facies associations according to their clast-type abundances and interlayered volcanic rocks. The lowermost member has a maximum depositional age based on young zircon grain ages varying from ca. 63 to 81 Ma. It is unconformably overlain by a continuous sequence characterized by a conglomerate rich in granite clasts at the bottom, with an interlayered tuff dated at 37.64 ± 0.36 Ma. Near the top, another tuff was dated at 30.84 ± 0.47 Ma, and a sandstone has a maximum depositional age of ca. 31.5 Ma. Normal grading, massive textures, channels, channel-form sandstone bodies, and upward-finning successions suggest that the Zacatecas Conglomerate is of fluvial origin. Late Jurassic to Early Cretaceous ages from zircons in plutonic rocks and sandstones bracket possible source regions for the Zacatecas Conglomerate. One possible source is Late Jurassic-Early Cretaceous granite derived from the Alisitos-Guerrero arc of western Mexico. Another possible source is the Tuna Manza Diorite, now exposed 250 km southeast of the study area. The lack of pre-Jurassic grains implies that possible sources such as the Nazas arc or the Potosí fan were not cropping out at that time, or at least that these areas were not affected by the fluvial system feeding the Zacatecas Conglomerate. It is possible that during the Palaeocene-early Oligocene the fluvial systems drained from west to east and from southeast to north, according to the above-mentioned constraints.     

Indian Continental Crust Recovered from Elan Bank, Kerguelen Plateau (ODP Leg 183, Site 1137)

At Site 1137 on Elan Bank of the Kerguelen Plateau, a largeigneous province in the southern Indian Ocean, a fluvial, volcaniclastic,polymict conglomerate and a fluvial sandstone are intercalatedwith subaerially erupted tholeiitic basalt flows. Clasts recoveredfrom the conglomerate have highly variable lithologies, includingalkali basalt, rhyolite, trachyte, granitoid and gneiss. Majorand trace element abundances and whole-rock isotopic data forthe sandstones, the conglomerate matrix and representative clastsfrom the conglomerate are used to infer the origin of thesediverse rock types. The gneiss clasts show an affinity to crustalrocks from India, particularly those of the Eastern Ghats Beltand its possible East Antarctic corollary, the Rayner Complex.The felsic volcanic clasts are not genetically related to theintercalated basalt flows, despite being erupted contemporaneouslywith these basaltic magmas. These felsic volcanic clasts probablyformed from partial melting of evolved upper continental crust.The granitoid also probably formed by partial melting of continentalcrust and could be an intrusive equivalent of the felsic volcanicrocks. In contrast, the alkali basalt clasts have isotopic compositionsthat are more similar to those of the tholeiitic basalt flowsrecovered at Site 1137; however, these clasts are highly alkalic(tephrite to phonotephrite) and have a distinct petrogenesisfrom the tholeiitic basalt flow units. KEY WORDS: geochemistry; Indian Ocean; Kerguelen Plateau; large igneous provinces; Ocean Drilling Program     

A morphometric analysis of terrace gravels in Santa Ynez basin,Santa Barbara County,California

An analysis of 6,300 pebbles from eighteen river terraces within the five subareas of the Santa Ynez River basin has revealed significant differences in particle sphericity and roundness of the various terrace gravels.Using the Cailleux technique of pebble measurement, Krumbein's sphericity ratio formula (Y) and the author's own roundness ratio or the I/L¹ ratio (R) the computed values derived for $\text{Y}$ range from 0.92 to 1.11, and for $\text{R}$ from 0.66 to 0.79 on these benches.An analysis of variance, based on the F-test applied to samples has established that: (a) gravel sphericity and roundness increase significantly from upstream to the river mouth; (b) in the subareas themselves pebble sphericity and roundness vary significantly from bench to bench indicating varied energy conditions of deposition; (c) all terrace gravels measured show higher sphericity and roundness than the underlying Orcutt gravels. This suggests a reworking of the latter by the Santa Ynez River.     

Slump blocks, intraformational conglomerates and associated erosional structures in Pennsylvanian fluvial strata of eastern Canada

Pennsylvanian fluvial channel sandstones in New Brunswick and Nova Scotia contain numerous examples of eroded mudstone surfaces, including in situ mudstone beds, boulders and slumped blocks. The eroded surfaces bear a variety of structures including linear scours, flutes, longitudinal furrows and rill marks. A block of interchannel mudstone up to 40 m in extent, displays a basal slip-plane, slump-related deformation and evidence of intense corrasion on a channel floor. Mudstone clasts from small pebbles to boulders over 4 m long are common immediately above channel-base erosion surfaces and represent a lag. Clasts over 20 cm diameter are commonly fluted, occasionally on all sides, suggesting clast rotation. Rill marks occur on large mudclasts and in situ mudstone surfaces and indicate emergence and erosion by surging water or surface runoff. Preservation of the delicate erosional structures depended on a highly cohesive mud substrate and subsequent rapid burial. A previous interpretation of the mud blocks and their surficial features as the result of mud intrusion is inconsistent with the field evidence.     

Texture and structure of resedimented conglomerates: examples from Książ Formation (Famennian—Tournaisian), southwestern Poland

The Famennian-Tournaisian conglomerates and sandstones of the Ksiaz Formation are interpreted as marine resedimented deposits. Matrix- and clast-supported conglomerate beds are equally common, and two textural sequences (motifs) have been recognized: (I) matrix-rich conglomerate → pebbly sandstone → sandstone, and (II) clast-supported conglomerate → sandstone. Variation in clast type partly controls motif type, and therefore, to some extent, matrix percentage in the conglomerates generally. Grading is extremely common in both clast- and matrix-supported conglomerates: inverse (19%), inverse-to-normal (14%) and normal (26%). The studied succession, itself part of a 4 km thick, fan delta, basin-fill sequence, is organized into large (110–150 m) and small-scale (5–30 m) sequences, both of which show (1) upward coarsening and thickening, (2) upward trend of sandstones and pebbly sandstone → matrix-rich conglomerates → clast-supported conglomerates and (3) a less clear upward tendency of massive and normally graded beds → inversely graded beds. Variation in matrix percentage in beds is therefore also partly controlled by fan processes, during the progradation of fan bodies and lobes. It is predicted that individual resedimented conglomerate beds or motifs show general downfan trends in thickness, texture and structure opposite to those evident in the vertical sequences.     

The distribution of U and Pu in the St. Severin chondrite

A detailed study of the U distribution of the St. Severin chondrite has been made by fission track radiography in order to clarify the interpretation of fission Xe thermal release data in terms of the mineralogical location of the fission Xe within the meteorite. This is of importance because the ${}^{244}\text{Pu}{}^{238}\text{U}$ ratio for St. Severin has been widely adopted as the average solar system value. The U contents of the constituent minerals cannot account for the total rock U which, instead, appears to be primarily localized on grain boundaries. The greatest localizations of U are in olivine-poor, orthopyroxene-rich ‘clasts’. Our data coupled with those of Podosek (1970a) show that ²⁴⁴Pu in St. Severin was also located on grain boundaries and that the bulk of Pu and U are unfractionated within this meteorite. Due to recoil, the ²⁴⁴Pu fission Xe is found in 10 micron surface layers on major phases. Assuming that the grain boundaries (on which the Pu was located) was formed during metamorphism, the ${}^{244}\text{Pu}{}^{238}\text{U}$ ratio for St. Severin applies to a time subsequent to the textural recrystallization of the meteorite. Our data support the interpretation of Podosek and our best estimate of the solar system ${}^{244}\text{Pu}{}^{238}\text{U}$ is 0.015.     

Sedimentary pyrite formation: An update

Sedimentary pyrite formation during early diagenesis is a major process for controlling the oxygen level of the atmosphere and the sulfate concentration in seawater over geologic time. The amount of pyrite that may form in a sediment is limited by the rates of supply of decomposable organic matter, dissolved sulfate, and reactive detrital iron minerals. Organic matter appears to be the major control on pyrite formation in normal (non-euxinic) terrigenous marine sediments where dissolved sulfate and iron minerals are abundant. By contrast, pyrite formation in non-marine, freshwater sediments is severely limited by low concentrations of sulfate and this characteristic can be used to distinguish ancient organic-rich fresh water shales from marine shales. Under marine euxinic conditions sufficient H₂S is produced that the dominant control on pyrite formation is the availability of reactive iron minerals.Calculations, based on a sulfur isotope model, indicate that over Phanerozoic time the worldwide average organic carbon-to-pyrite sulfur ratio of sedimentary rocks has varied considerably. High $\text{C}\text{S}$ ratios during Permo-Carboniferous time can be explained by a shift of major organic deposition from the oceans to the land which resulted in the formation of vast coal swamps at that time. Low $\text{C}\text{S}$ ratios, compared to today, during the early Paleozoic can be explained in terms of a greater abundance of euxinic basins combined with deposition of a more reactive type of organic matter in the remaining oxygenated portions of the ocean. The latter could have been due to lower oceanic oxygen levels and/or a lack of transportation of refractory terrestrial organic matter to the marine environment due to the absence of vascular land plants at that time.     

Coniacian carbonate-conglomerate event on carbonate ramps from the Northern Chotts ranges,South Tunisia: facies geometry and tectono-sedimentary evolution

Conglomerate bodies are wide spread in the Lower Coniacian carbonate of the southern of Tunisia. This paper presents an examination of the stratigraphic architecture of these Coniacian conglomerates. It aims to the understanding of the processes leading to their genesis and the relationship and context with the late Cretaceous sedimentation. These conglomerates are related to the incision of Aptian palaeovalleys in a carbonate platform in an extensional setting. The sedimentary analysis and the geometry of these bodies show that the Cretaceous succession in this area are formed by eight facies within four facies associations, representing a series of distinct depositional environments ranging from alluvial plain to the open marine environment. The sedimentary analysis allowed the recognization of elementary sequence which start with the conglomerate bodies corresponding to the incised valley fills.     

Evaluation of a siliciclastic diamictite from the Maya Mountains of Belize

A diamictite unit 9.2 m thick and 1.5 × 1.0 km in aerial extent lies unconformably on siliciclastics of the Santa Rosa Group in the Maya Mountains of Belize. The diamictite has an irregular scour base with 1.5 m of erosional relief. Matrix accounts for 24–27% of the deposit and consists of coarse sand to clay-sized particles of lithics and mineral grains of various origins. Clasts are granule to boulder in size, sub-rounded, poorly sorted, matrix-supported, oriented randomly and lack grading. Clasts are composed exclusively of sandstone, siltstone and shale lithologies derived from the Santa Rosa. Trace element concentrations are relatively higher in the Santa Rosa sediments, but Cu and Zn are elevated in the diamictite. Some clasts exhibit alteration rinds related to post-depositional hydrothermal processes. The age of the diamictite is unknown, it overlies the latest Pennsylvanian to Middle Permian Santa Rosa Group, and has a present-day erosional top of the unit which prevents further age refinements. The diamictite is interpreted to be of sediment-gravity flow origin related to tectonic activity on the Northern Boundary fault. However, this study points to the potential for future re-evaluation of the deposit with refinement of the depositional models themselves. The diamictite deposit also allows for further interpretation of the broader geologic history of Northern Central America.     

The sedimentary history of a debris-flow dominated, Devonian alluvial fan–a study of textural inversion

Karlskaret fan, with a radius of less than 11/2 km and dominated by debris-flow conglomerates, is one of numerous alluvial fans built out from the fault margins of Hornelen Basin (Devonian, Norway). The fan body is more than 170 m thick proximally, consists of four main coarsening-upwards segments and thins distally by a rising of its base and by a vigorous interfingering with very fine-grained sediments originating from an adjacent, impinging floodbasin system. Within the entire fan body, and within individual lobes, is a proximal-distal (and vertical) facies change from sheet-like, polymodal debris-flow conglomerates through matrix-rich conglomerates that are commonly distorted by loading, slumping and faulting, to remarkably sheet-like, matrix-rich granule sandstone of subaqueous debris-flow origin. Because the alluvial fan prograded into an actively aggrading floodbasin the primary fanglomerates, themselves having been subject to some sorting on the fan surface, incorporated large quantitites of very fine sediments. This inclusion of fines, effectively a textural inversion on the lower fan reaches, frequently led to remobilization and resedimentation of material beyond the fan toe. Anomalous maximum particle size/bed thickness relationships and a variety of graded textures within these resedimented beds suggest deposition in lacustrine areas of the adjacent floodbasin.     

Paleogene island arc collision-related conglomerates, Yarlung–Tsangpo suture zone, Tibet

Coarse clastic rocks in the Liuqu Conglomerate, formed in both terrestrial and subaqueous settings, record a Paleogene phase in the tectonic evolution of Tibet. Facies changes are commonly abrupt with rapid changes in clast types, grain size and stratal patterns. Sediments were derived from the leading (northern) edge of the Indian margin and a Late Jurassic–Cretaceous intraoceanic island arc that lay within Tethys. The coarse clastic sedimentary rocks of the Liuqu conglomerates are extremely proximal, but are locally offset relative to their original source terranes. They record aspects of the history of collision between these terranes and are interpreted to have been deposited in oblique–slip basins that developed along the zone of collision. The absence of clasts derived from terranes to the north of the Yarlung–Tsangpo suture suggests that basins associated with deposition of the Liuqu Conglomerate developed prior to the final collision between India and Asia.     

Geochronologic investigations of the Sudbury Nickel Irruptive and the Superior Province granites north of Sudbury

RbSr ($\text{λ}\text{Rb}\text{= 1.39 × 10}{}^{\mathrm{?11}}\text{yr}{}^{\mathrm{?1}}\text{)}$ and U-Pb ($\text{λ 238 = 1.54 × 10}{}^{\mathrm{?10}}\text{yr}{}^{\mathrm{?1}}\text{, λ235 = 9.72 × 10}{}^{\mathrm{?10}}\text{yr}{}^{\mathrm{?1}}$) measurements were undertaken in the Sudbury area, Sudbury, Ontario to determine the ages of the Sudbury Nickel Irruptive, Superior Province granites north of Sudbury, Sudbury Breccia and subsequent metamorphism. The Sudbury Nickel Irruptive norite whole rock Rb-Sr data yield an age of $\text{1883 ± 136}\text{Myr}$ ($\text{I.R}\text{. = 0.7071 ± 0.0005}$; all results quoted at 2π level) while the Nickel Irruptive micropegmatite Rb-Sr system has been disturbed and does not yield an isochron. A plagioclase-whole rock pair from the norite near the norite-micropegmatite transition yields an age of 1866 Myr, which when taken in conjunction with field (Stevenson and Colgrove, 1968) and geochemical (Naldrettet al., 1970, 1972) data does not support the conclusion of gibbins and McNurr (1972) that the micropegmatite is a later intrusion rather than a differentiate of the magma which produced the norite. Rb-Sr studies of the Superior Province granites north of Sudbury yield an age of $\text{2698 ± 162}\text{Myr}$ ($\text{I.R.}\text{= 0.7019 ± 0.0012}$). U-Pb zircon studies of these granites and granitic clasts within the Sudbury Breccia yield an age of $\text{2.71 ± 0.05}$ Byr and suggest the breccia granitic clasts were derived from the Superior Province granites. The granitic rocks ~150 km north of Sudbury have been undisturbed for ~ 2.6 Byr based on Rb-Sr mineral studies, whereas the granites and Sudbury Breccia within ~ 15 km of the Nickel Irruptive, as well as the Sudbury norite at the perimeter of the Irruptive have been disturbed by the Penokean Orogeny 1.7–1.75 Byr ago. The Penokean event appears to have overprinted isotopic evidence of the Sudbury impact event at least in the area studied.