Igneous Clasts Research Articles

Thermal and impact metamorphism on the HED parent asteroid

The bulk texture and composition of four monomict eucrites, five polymict eucrites, and one howardite, as well as those of 16 separated clasts and lithological units from these samples were analyzed by optical and scanning electron microscopy and by electron microprobe. Bulk chemical compositions were obtained by INAA. The monomict eucrites Stannern, Millbillillie, Camel Donga, and Juvinas are recrystallized monomict breccias that probably originate from brecciated crater floors or ejecta blocks. The texture of igneous clasts from Juvinas can be explained by interactions of impact and igneous activity that led to disturbance of magma crystallization. Due to the presence of lithic clasts with highly variable chemical compositions and the occurrence of both equilibrated and unequilibrated pyroxenes, the monomict eucrite Pasamonte is redescribed as a polymict eucrite. Three clasts of impact-related lithologies in the polymict eucrite Pasamonte and the howardite EET 87503 contain considerable amounts of chondritic projectile contaminations. The textures of the investigated meteorites reflect a complex post-igneous history dominated by multistage thermal and impact metamorphism. The chronological sequence of thermal and impact events comprises up to six evolutionary phases. Phase I represents crystallization of primary magmas that led to the formation of unequilibrated basalts and other igneous rocks. Phase II represents slow subsolidus cooling or a period of reheating during which pyroxene equilibrated. Phases III and V represent periods of impact brecciation during which the rocks were brecciated in situ or, in the case of polymict HED breccias, mixed with various other rock types. During Phases IV and VI the breccias suffered annealing and recrystallization due to thermal metamorphism. The thermal events that caused recrystallization and equilibration of HED lithologies were active prior, during, and after the formation of impact breccias, indicating that the thermal input by impact might be responsible for thermal overprinting.

Provenance of igneous clasts in conglomerates of the Archaean Timiskaming Group, Kirkland Lake area, Abitibi greenstone belt, Canada

Four types of igneous clasts from Timiskaming conglomerates of the Kirkland Lake area are identified: calc-alkaline porphyry, trachyte (K2O + N2O > 7.25 wt.%), trondhjemite, and tholeiitic basalt. Clasts derived from quartz–carbonate veins and carbonatized porphyries and komatiites indicate CO2-rich hydrothermal activity along the Larder Lake – Cadillac Fault before sedimentation. Calc-alkaline porphyry, the predominant clast, is similar in appearance to feldspar porphyry intrusions widely exposed in the area, but the two are not related. The porphyry clasts are cogenetic with diorite–granodiorite intrusions and volcanic rocks of the greenstone belt. Andesite clasts are only observed near the base of the assemblage, and the ratio of basalt to andesite increases up stratigraphic sections. The change suggests progressive erosion of arc-related volcanic rocks during sedimentation. Composition, texture, and mineralogy of the trondhjemite clasts are consistent with their derivation from the marginal phases of the Round Lake batholith. Intrusive rocks of the Timiskaming Group (coarse-grained holocrystalline syenitic–monzonitic rocks, biotite-bearing feldspar porphyries, and mica-rich lamprophyres) were not observed in the clasts, which suggests sedimentation prior to unroofing of these rocks. Sedimentation of the conglomerates postdated unroofing of underlying plagioclase porphyry (~2685 Ma), but predated unroofing of Timiskaming intrusions. The distribution of alkaline clasts only in proximity to the fault implies that alkaline magmatism was confined along the fault. Derivation of clasts from both sides of the fault and proximity of inferred sources support a pull-apart basin for sedimentation with minor strike-slip movement during and after the sedimentation.

Petrology, Stratigraphy, and Depositional History of the Upper Triassic-Lower Jurassic Eagle Mills Formation, Choctaw County, Alabama: ABSTRACT

ABSTRACT In southwestern Choctaw County, Alabama, the Eagle Mills Formation consists of alluvial-fan red beds deposited in a Mesozoic graben. The Exxon No. 1 Smith Lumber Co. well penetrated the upper 2,210 ft of the Eagle Mills, and it is divided into eight lithostratigraphic units designated A through H from bottom to top. Unit A consists of numerous upward-fining and -coarsening sequences of sandstones, conglomeratic sandstones, and conglomerates of variable thickness separated by red mudstones. They were deposited on the distal fan by ephemeral streams and sheetfloods and are composed of low-grade metamorphic rock fragments probably derived from the Talladega slate belt. Unit B consists of red mudstone with interbedded upward-fining and -coarsening sandstones and conglomerates with abundant calcite concretions and paleocaliche. Thin-section analyses of three sandstones of Unit B show that two are phyllarenites and one is a subphyllarenite. They vary from very fine to coarse-grained and are moderately sorted and submature, with angular to subangular framework grains. Quartz (subequal monocrystalline and polycrystalline) is the dominant framework grain with lesser amounts of schist and phyllite. In the two shallower sandstones, quartz content decreases as chert and dolomite marble rock fragments increase. Scanning electron microscope examination of one sandstone shows the presence of large well-developed pores lined with illite and authigenic quartz. Examination of drill cuttings from other Unit B sandstones, however, shows them to be poorly sorted, highly indurated, and apparently of low porosity. Unit B was deposited on the distal fan by sheetfloods and ephemeral streams. Faulting and igneous activity related to rifting occurred during the latter stage of deposition of Unit B. Units C through G are dominantly conglomerates; Units C, D, and F contain abundant igneous rock fragments. Unit C contains plutonic (syenite) and volcanic (rhyolite and dacite) rock fragments, and the igneous clasts of Units D and F are volcanic. Rare poorly defined graded beds occur in Unit D cores. Units C and D exhibit a general upward coarsening of sediments, indicating progradation of the fan. Unit E coarsens upward to a sedimentary (chert and limestone) and metamorphic (dolomite marble and lesser amounts of phyllite and schist) rock-fragment-bearing cobbly pebble conglomerate. Unit F is a mixed volcanic and sedimentary-rock-fragment-bearing conglomerate. Unit G is a massive sedimentary and metamorphic rock-fragment-bearing cobbly pebble conglomerate. Cores taken from the upper half of this unit show this portion to be a poorly sorted clast- to matrix-supported conglomerate. Although Unit G is in general massive, rare poorly defined graded beds and rare thin-bedded sandstones are also present. Units D through G were deposited in the mid-fan subenvironment. In Unit H, high log porosities and well-rounded sand grains in drill cuttings indicate the presence of eolian deposits. Log responses and the presence of anhydrite in cuttings show that the Eagle Mills is overlain by the Werner Formation. The provenance for Eagle Mills sediments was the southern Appalachian system, with sedimentary rock fragments being derived from folded Paleozoic rocks, whereas metamorphic rock fragments came primarily from metasedimentary rocks of the Talladega slate belt. Igneous rock fragments were derived from rocks emplaced in the upper-fan area as a result of igneous activity associated with rifting. End_Page 117------------------------ FIGURE 1. [Grey Scale] Unit G cobbly pebble conglomerate. Clasts are chert and dolomite marble. FIGURE 2. [Grey Scale] Upward-fining conglomerate channel deposit. Light-colored clasts are quartzite; dark elongate clasts are hematite-stained phyllite and schist. FIGURE 3. [Grey Scale] Unit D porphyritic rhyolite clast. White phenocryst is alkali feldspar altered to illite. Matrix is hematite-stained mud. FIGURE 4. [Grey Scale] Thin-section photomicrograph of oolitic sandstone clast. Most ooid nuclei are quartz coated with hematite. Clast is from Silurian Clinton Group. End_of_Record - Last_Page 118-------

Sedimentation and extensional basin evolution in a Miocene metamorphic core complex setting, Alvord Mountain, central Mojave Desert, California, USA

ABSTRACTThe Lower Miocene Clews Formation at Alvord Mountain, Mojave Desert, California, comprises an upward coarsening sequence of synextensional continental deposits of variable thickness; this sequence pinches out to the west against the Alvord Mountain pre‐Tertiary basement complex and thickens eastward to c.300 m over a distance of 7 km. Two stages of sedimentation are recognized in the formation. During the initial stage of sedimentation, the depocentre was delineated by a lacustrine system that was bounded to the west by small, sheetflood‐dominated alluvial fans and to the north by a southerly flowing fluvial braidplain. Lacustrine mudstone, siltstone and carbonate suggest an initially closed basin. Conglomerate to the west represents small, sheetflood‐dominated fans. Igneous clasts and a south‐east palaeoflow direction suggest a proximal, low‐relief source in the western Alvord Mountain area. Pebbly sandstone of the southward‐prograding braidplain has a metasedimentary provenance in the Paradise Range to the north. The second stage of deposition was dominated by coarse conglomerate and breccia. West to southwest palaeotransport indicators and a distinctive metaigneous petrofacies indicate a provenance in the Cronese Hills, 8 km to the east. These strata reflect rapid westward progradation of sheetflood‐ and debris flow‐dominated alluvial fans that advanced across the axial braidplain and lacustrine system.Basin development is interpreted to have been controlled by regional NE‐directed extension on a detachment fault associated with the central Mojave metamorphic core complex. 30 km to the west. The Alvord Mountain ‐ Cronese Hills region was initially transported as a single hangingwall block on the E‐dipping detachment. Initial lacustrine sedimentation reflects the development of a flexural or sag basin in the hangingwall. Subsequent westward progradation of alluvial fans out of the Cronese Hills is believed to record the propagation of a NW‐striking, SW‐dipping normal fault antithetic to the low‐angle E‐dipping detachment fault.

A ferroan region of the lunar highlands as recorded in meteorites MAC88104 and MAC88105

MacAlpine Hills 88104 and 88105 (MAC88104/5) are paired meteorites of noritic anorthosite composition from the lunar highlands. MAC88105 is a breccia composed mainly of melt-breccia clasts in a fine-grained, fragmental, and partly glassy matrix. The most abundant melt lithologies are feldspathic and are similar in composition to the bulk meteorite. Other melt lithologies include feldspathic melt rocks, mafic melt breccias, and a rare melt breccia relatively enriched in incompatible trace elements. Subordinate lithic clasts are granulitic breccias and ferroan (relatively low Mg [Mg + Fe] ) igneous lithologies, including troctolitic anorthosite, anorthositic norite, gabbronorite, and anorthosite. Igneous clasts having mafic mineral compositions more magnesian than Fo 55 and En 60 were not observed. Rare fragments of glass spheres and shards as well as glass clasts indicate that the meteorite was derived from an immature regolith. The bulk composition of MAC88105 is characterized by a molar Mg (Mg + Fe) ratio of 0.62, at the extreme low end of the range for meteorites from the lunar highlands. Its low concentrations of incompatible trace elements and feldspathic bulk composition (29% Al 2O 3), suggest that it, like the other lunar meteorites, formed at a site far removed from the areas sampled by the Apollo missions. Similarities in mineral compositions among the different lithologies of the breccia (melt-breccia clasts, granulitic breccias, igneous fragments) and the distribution of mineral fragments suggest that most components of the meteorite were derived from a crustal section dominated by material with a noritic anorthosite composition and an affinity to the ferroan suite of plutonic rocks. The mafic melt breccias have roughly peritectic melt compositions, similar to many “low-K Fra Mauro” melt breccias in the Apollo collection but much more ferroan, and represent a source material distinct from the noritic anorthosite. Moderate Ce anomalies, both positive and negative, occur in some separated clasts and to a lesser extent in some small chips dominated by matrix, but not in powder prepared from 20 g of bulk meteorite. These anomalies are almost certainly terrestrial weathering effects. One granulitic breccia clast is rich in siderophile elements as a result of a large amount of Fe-Ni metal. The Au Ir ratio of the clast (0.64, CI-normalized) is distinctly different from that of metal-rich impact melts from Apollo 16 (~3, CI-normalized). The compositional range observed among the nonmare lunar meteorites ALHA81005, Y791197, Y82192/3, Y86032, and MAC88104/5 is equivalent to that observed among representative samples of < 1 mm fines from Apollo 16. Thus, compositional data provide little constraint on the number of impacts required to eject these meteorites from the Moon.

Synextensional Basin Evolution, Lower Miocene Clews Formation, Central Mojave Desert, California

The lower Miocene Clews Formation at Alvord Mountain, Mojave Desert, California, consists of up to 400 m of coarsening-upward continental deposits. During the initial stage of basin development, the depocenter was delineated by a lacustrine system that was bound to the west by alluvial fans and graded northward into a south-flowing fluvial braidplain. Lacustrine strata, composed of claystone, siltstone and carbonate, suggest an initially closed basin. Conglomerate and sandstone to the west represent small, sheetflood-dominated fans and contain igneous clasts and southeasterly paleoflow indicative of a proximal, relatively low relief source in the western Alvord Mountains. The second stage of deposition was dominated by coarse conglomerate and megabreccia with west-southwesterly paleotransport indicators and a distinctive metaigneous petrofacies that indicate a provenance in the Cronese Hills, 8 km to the east. These strata reflect rapid westward progradation of debris flow-dominated alluvial fans that advanced across the facia braidplain, eventually onlapping the smaller Alvord Mountain sourced fans across the basin. Influx of sediment from the paradise Range ceased. Based development resulted from regional northeast-southwest extensional deformation. The Alvord Mountain-Cronese Hills region was transported northeast as a single block on an east-dipping, low-angle normal fault associated with the Waterman Hills core complex 30more » km to the west. Development of westward prograding fans is believed to record subsequence propagation of a northwest-trending, west-dipping normal fault associated with, but antithetic to the low-angle master fault.« less

Provenance of Lower Old Red Sandstone conglomerates, SE Kincardineshire: evidence for the timing of Caledonian terrane accretion in central Scotland

Northerly-derived conglomerates dominate much of the thick Lower Old Red Sandstone (ORS) succession of the NE Midland Valley of Scotland. These contain a diverse suite of granitoid boulders, the provenance of which can be used to constrain the timing of the post-Caradoc displacements which juxtaposed the Highland Border Complex against the Grampian terrane. Rb-Sr muscovite-whole rock ages for two-mica granite boulders from the Crawton Group are 473–457 Ma, confirming the persistence of an Ordovician two-mica granite source first recognized in the Dunnottar Group. The Rb-Sr muscovite ages are supported by a U-Pb age of 475 ± 5 Ma for monazite separated from one of the clasts. A Rb-Sr biotite age of 412 ± 4 Ma for a clast of porphyritic biotite granite and geochemical evidence linking this clast to many of the other biotite granite clasts, suggests that late Silurian granitoids and related hypabyssal intrusions have also made an important, hitherto unrecognized, contribution to the conglomerates. Mica-whole rock ages for accompanying psammitic clasts suggest the source included metamorphic rocks (of up to at least staurolite grade) whose uplift overlapped with the intrusion of crustally-derived melts. The chemistry, chronology and Sr and Nd isotopic compositions of the various igneous clasts are shown to resemble closely the Ordovician through to Silurian magmatic history of a small area of the Grampian terrane in NE Scotland. This is taken as strong evidence for a pre-Lower ORS juxtaposition of the Highland Border Complex and the Grampian terrane, to within several tens of kilometres of their present position. Ashgill–Silurian terrane displacements are invoked to achieve this. Discrepancies in the proportions and types of clast occurring in the conglomerates are explained by: uplift and unroofing of the Grampian terrane prior to docking; the involvement of polycyclic sediment, perhaps as a local veneer of pre-existing sediment on the Grampian terrane; and the likelihood that the source involved parts of the local Grampian terrane which are no longer exposed.

Volcanic clasts in Silurian conglomerates of the Midland Valley (Hagshaw Hills inlier) Scotland, and their meaning for Caledonian plate tectonics

34 igneous clasts of predominantly rhyolitic composition of the Parishholm- and Hareshaw-Conglomerate (Silurian, Hagshaw Hills inlier, Midland Valley, Scotland) have been analysed for major and trace element contents. The geochemical composition of the clasts indicates different source areas. Firstly, some clasts are derived from rhyolites of a within-plate setting (peralkaline rhyolites of oceanic islands or continental rifts) and secondly, other clasts are derived from calc-alkaline to high-K rhyolites. Such rhyolites erupt mainly above convergent plate boundaries, but occur also in continental areas of crustal extension. Both conglomerates were shed from the direction of the Southern Uplands into the Midland Valley. Since no calc-alkaline rhyolites are known in the Southern Uplands, a volcanic terrane should have been situated during the Silurian in the area where the Southern Uplands lie now. The geotectonic character of this terrane cannot unequivocally be identified with the help of the trace element data of the clasts. Therefore several models are discussed.

Lower Devonian age for the ‘Great (basal) Conglomerate’, Scottish Borders

The ‘Great Conglomerate’ covers over 200 km2 of SE Scotland in several outcrops (Fig. 1), and reaches 600 m thick (Watt 1963). Though marking the base of the local Old Red Sandstone (ORS) succession, it was long mapped as Upper ORS (Clough et al. 1910; Watt 1963; Greig 1971). However, such thick and extensive conglomerates are atypical of the Upper ORS (Mykura 1983), so that recent BGS 1:625,000 and 1:50,000 maps (25W: Galashiels and 33E: Dunbar) have now reassigned it to the Lower ORS (Davies and McAdam 1986). This note highlights misleading published information, and illustrates for the first time how a Lower Devonian (c.400–410 Ma) age tends to be confirmed by intrusions cutting, and igneous clasts within, the Conglomerate. We deal first with localities which are less critical than they appear on published maps. The largest Conglomerate outcrop, south of Dunbar (Fig. 1), is cut by many intrusions (Watt 1963; Davies and McAdam 1986) which merely show it is pre-Carboniferous: (1) the Carboniferous Dirrington Great Law and adjacent felsites (325 ± 10 Ma; Snelling and Chan 1971); (2) Carboniferous quartz-dolerites (cf. Macdonald et al. 1981); (3) Carboniferous and/or Tertiary basalts (‘K’ symbols on BGS sheet 33E); and (4) Carboniferous-Permian olivine-basalts and ‘monchiquites’ (‘D’ symbols). In marked contrast, the Lauder Conglomerate outcrop (Fig. 1) is cut by no such intrusions, nor by regional NE–SW dyke-swarms (now dated to c. 420–395 Ma; Rock et al. 1986), which invade surrounding Ordovician-Silurian strata (Fig. 1). Dykes shown on BGS maps . . .

Provenance and setting of some Ordovician and Silurian conglomerates in Northern Ireland

Synopsis The marine Caradoc–Llandovery deposits of the Pomeroy and Lisbellaw inliers in counties Tyrone and Fermanagh, which accumulated in a basin that deepened southwards towards the Longford–Down accretionary prism, contain rare conglomerates of predominantly vein quartz, quartzite and undeformed igneous clasts. These rocks have a similar setting and dispersal to the Llanvirn–Wenlock deposits of the Girvan area of Scotland, and in Ireland it is likely that coarse grained debris was also derived from pre-Dalradian metamorphic basement and an island arc complex to the north. It is possible, though not certain, that this source area is represented by Ordovician igneous rocks and metasediments of the Tyrone Igneous Complex.

Submarine‐fan deposited sandstone and rudite in a mid‐Cenozoic interarc basin in Maewo, Vanuatu (New Hebrides)

ABSTRACTThe volcaniclastic, Lower Sarava Formation of north‐eastern Maewo was deposited in a Late Oligocene‐Early Miocene deep sea fan environment. Eight lithofacies can be recognized from a westward‐trending, deep‐sea fan which prograded at a depth of &gt;4.25 km within an extinct interarc basin. These lithofacies include massive spilite‐rich rudite, graded coarse rudite‐arenite, graded fine rudite‐arenite, intraformational rudite, brown and grey siltstone, minor calcarenite and a lithofacies composed of turbidites with Bouma sequences. The sediment was essentially derived from the Vitiaz palaeoarc to the east. Non‐calcareous red and green siltstone and minor airfall tuff was deposited distal to the arenite and rudite, in areas, or during periods, of slower sedimentation.Slightly more than half of the lower part of the Sarava Formation is represented by massive rudite, graded coarse rudite‐arenite, and graded fine rudite‐arenite. These deposits, and some of the calcarenites and massive arenites. were deposited from decelerating high density turbidity currents. Some of the thicker rudite‐arenites may have been induced by tsunamis. Classical turbidites deposited, perhaps, from low density turbidity currents are rare.The petrography of igneous clasts indicates that Late Oligocene‐Early Miocene volcanism on the Vitiaz palaeoarc was largely basaltic‐andesitic but there were some dacitic eruptions.

Fluidized mixtures of magma and rock in a late Caledonian breccia dyke and associated breccia pipes in Appin, Scotland

AbstractThis paper describes an unusual occurrence of igneous material as clasts in dyke and pipe breccias associated with late Caledonian minor intrusions. It is shown that the clasts were in a plastic condition when incorporated into the breccia rock. These igneous clasts were derived from magma disrupted at depth and then transported into the fluidized breccia columns where they were mixed with large numbers of clasts derived from the quartzite wall‐rocks. Textures and planar fabrics developed during collapse of the fluidized system are described and shown to be separable from the later compaction associated with extensive pressure solution of the fine matrix. Most Caledonian breccia pipes lack igneous clasts and it is considered that this group of breccias represent the rarely‐preserved boundary zone between active magma and breccia systems.

The magma ocean from the Fra Mauro shoreline: An overview of the Apollo 14 crust

Numerous coarse‐grained, polymineralic, texturally‐monomict, igneous clasts from the Apollo 14 breccias have been studied petrographically and mineralogically. The aims of these clast studies were to evaluate the range of crustal lithologies present at the Apollo 14 site, to bracket their constituent mineralogical compositions, and to assess their relative abundances. Troctolite and troctolitic anorthosite are the most abundant rock types. These are related to the Mg‐rich trend. Three distinct plutons may be represented. There is a distinct paucity of noritic lithologies. The second most abundant group consists of plagioclase cumulates (alkali‐rich anorthosites) with more evolved mineral compositions than the Mg‐rich trend cumulates; their provenance is indeterminate. Coarse‐grained ilmenite gabbros and mineralogically‐evolved monzonoritic and granitic clasts are widespread in occurrence but not abundant. An olivine‐gabbronorite clast (14305,92) is pristine. Petrographically and mineralogically, it shows a striking resemblance to Apollo 12 plivine mare basalts. The paucity of ferroan anorthosites at this and the Apollo 17 site is attributed to a higher density of Mg‐rich and related intrusives in the primitive anorthositic crust in comparison with other sites. Our data provide further support for widespread regional heterogeneities within the early lunar crust.

The Antarctic achondrite ALHA 76005: a polymict eucrite

ALHA 76005 is a basaltic achondrite containing few. if any, orthopyroxenes. Its bulk major and trace element composition is like that of a non-cumulate eucrite, and unlike that of a howardite. It contains a variety of igneous clasts which differ in their textures, pyroxene/plagioclase ratios and pyroxene and plagioclase compositions. One clast, No. 4, was found to have the REE pattern of a cumulate eucrite and an oxygen isotopic composition different from that of the bulk meteorite. Both the chemical and oxygen isotopic composition of clast No. 4 suggest that it was derived from a source different from its host. These observations lead to the conclusion that ALHA 76005 is a polymict eucrite.

40Ar- 39Ar chronology of isolated phases from Bununu and Malvern howardites

High resolution 40Ar- 39Ar age spectra have been measured on plagioclase and glass from two howardites. Both the plagioclase and glass from the gas-rich Bununu howardite show well-defined age plateaux, yielding distinct ages of 4.42 ± 0.04 and 4.24 ± 0.05 AE, respectively. These age patterns are rather well behaved and are interpreted as representing the distinct times of formation of plagioclase from igneous processes and of glass fragments produced by impact on the meteorite body. The release pattern for the glass from the heavily shocked Malvern howardite is undulating at low and intermediate temperatures but does have a high-temperature plateau. Its age spectrum indicates little apparent diffusion loss, but rather an extensive redistribution of either 40Ar during the shock event or of 39Ar during the neutron irradiation or both. The total K-Ar age of Malvern glass is 3.64 ± 0.04 AE and the high-temperature plateau is 3.73 ± 0.05 AE. The age spectrum of the Malvern plagioclase has an intermediate temperature “plateau” at 3.80 AE that represents 20% of the total 40Ar content and increases towards a high-temperature plateau at 4.29 ± 0.04 AE containing 26% of the total gas release. It seems likely that the event which formed the Malvern glass also reset part of the plagioclase. The distinct histories observed for the different phases of these howardites are consistent with their formation from a regolith. The present results along with similar young ages for igneous clasts from Kapoeta clearly show that the regoliths were extant on the parent bodies of howardites and that they were subjected to violent impact events at least as recently as 3.7 AE ago.

Source and Depositional Environments of the Moose Channel Formation, Northwest Territories

ABSTRACT The Moose Channel Formation of the Fish River area, Northwest Territories, comprises over 8200 ft of clastic rocks. The strata, of Maestrichtian-Paleocene age, are genetically equivalent to much of the thick clastic section underlying the Mackenzie Delta. The sandstones in the Moose Channel Formation are mineralogically and texturally immature. An abundance of feldspars, chert grains, and sedimentary and volcanic rock fragments indicates granitic, sedimentary and volcanic source terranes. The Old Crow granites and the volcanic rocks of the British Mountains are possible sources of the igneous clasts. Many of the sedimentary rock fragments are assumed to have been derived from the Richardson Mountains south of the study area. Paleocurrent measurements show that northerly and easterly transport directions prevailed during deposition of the Moose Channel clastics. Grain-size analyses of 125 sandstone samples indicate that unidirectional current energy and gravitational energy were the two basic kinds of energy responsible for deposition of most of the sandstones. These two energy types are compatible with fluvial depositional processes. The Moose Channel Formation has been divided into five units after Young (1971, 1972). The depositional environment of each unit has been interpreted as follows: Unit 1, the oldest, is a regressive sequence containing prodelta and braided-stream deposits. Unit 2, a transgressive sequence, contains prodelta deposits grading upwards into nearshore marine silty shale and fine-grained sandstone. Unit 3, dominantly sandstone, is interpreted as fluvial deposits. Unit 4 is interpreted as a transitional marine-nonmarine delta-plain complex. The youngest strata, in Unit 5, are interpreted as nearsource channel and overbank deposits. An analogy is drawn between the depositional settings of these units and those of the Wilcox Group of Texas (Fisher and McGowen, 1969).

Upper Precambrian (?) Diamictite of Florida Mountains, Southwestern New Mexico

Diamictite in the Florida Mountains is about 12 m thick, is probably of younger Precambrian age, and consists of igneous and sedimentary clasts as much as 1.2 m in diameter in a fissile shale matrix. The lower 9 m probably formed from ice-rafted pebbles, cobbles, and boulders that were dropped into marine mud. The clasts were not locally derived, as they consist of rock types not present in the older Precambrian rocks of the Florida Mountains. The upper 3 m appears to be a conglomeratic arkose derived locally after faulting of the lower part of the diamictite.

A major unconformity in the Archaean, Jones Creek, Western Australia

Abstract Evidence is presented to show that a granite conglomerate of the Archaean Yilgarn succession rests uncomformably on granite in the Jones Creek area of Western Australia, 400 km north of Kalgoorlie. South of Jones Creek the conglomerate evidently overlies with angular discordance the lower section of Archaean greenstone (mainly metabasalt and layered metagabbro). The relationship between this lower greenstone and granite is obscured by recent cover, hence the granite is either ‘middle’ Yilgarn in age or represents the basement on which the Yilgarn succession was deposited. The conglomerate is fairly well stratified, characteristically with rounded granite clasts and an arkosic or mafic matrix. Other types of igneous clast are locally present but sediment clasts are absent. Its proposed formation name is Jones Creek Conglomerate.