Undissected Arc Research Articles

Petrography study of the recent sediments in Hareer’s region at Basrah Governorate - Southern Iraq

Two sites of Hareer’s region is located in the southern part of Iraq, specifically in the northern Basrah Governorate. The sites are at a depth of 5 meters. The results of heavy mineral analysis showed that there are fifteen types include; Opaque grain (Iron Oxide), Chlorite, Muscovite mica, Biotite, Orthopyroxene, Clinopyroxene, Amphibole (Hornblende and Actinolite), Epidote, Garnet, Kyanite, Staurolite, Zircon, and Tourmaline. On the other hand the light minerals and the results of the mineral analyses revealed that they were primarily composed of rock fragments, with a variety of quartz minerals, feldspar minerals, evaporate minerals, clay-coated grains, and trace amounts of unidentified minerals. The abundance and diversity of heavy minerals indicate the diversity of the sources of the parent rocks, such as igneous, metamorphic and sedimentary rocks. The large amount of opaque and small amount of other minerals in the study area samples is due to the oxidation process where many of the ferromagnesian minerals such as pyroxenes, and hornblende decompose to form iron oxides (e.g. hematite, limonite). Light minerals were visible as irregular, sharp, or occasionally semi-sharp crystalline forms, which suggests that the sources were close by and that the light minerals originated from the sedimentary basin or within it through the processes of deposit erosion and re-deposition. The study area, which is a part of the Undissected Arc, is primarily distinguished by a high content of quartz (mono- and polycrystalline quartz, including chert), and feldspar, which can occur in fluvial environments.

Provenance of Beach Sands in the Northern Gulf of Guinea, SW Cameroon

Abstract Petrographical, mineralogical and geochemical studies were carried out on the beach sands of Limbe (South-West Cameroon) to highlight their provenance, source rocks alteration and the tectonic framework. The Limbe beach sediments were classified as sandy litharenite type. The sediments were derived from mafic and andesite source rocks. The sediments are composed of quartz, olivine, augite, amphibole, pyroxene, plagioclase, potassium feldspar, muscovite and biotite, indicating the primary phases of magmatic source rocks; kaolinite and illite in the secondary phase; and rutile in their residual composition. The presence of ferromagnesian minerals indicates that they were derived from the mafic rocks, which are the basalts of the Cameroon Volcanic Line (CVL). However, the presence of quartz associated with plagioclase and feldspars suggests that alkaline granites played a minor role in the sedimentation of the Limbe beaches. Total Quartz - Total Feldspar - Total Lithic Fragments (Qt-Ft-Lt) mineralogical relationships demonstrate that these beach sediments would be formed in an undissected arc tectonic context, typical of volcanic sources with high relief producing black sands or sands enriched in iron oxides. Their microscopic characteristics of angular to subangular shapes suggest that the magmatic sources are proximal to the shoreline deposits. The characteristics of microporosity, iron oxide inclusions, conchoidal fracture, heavy mineral grains, and opaque minerals support this proximal source. The conchoidal fractures defining the angularity of these beach sediments would suggest its derivation from crystalline rocks. The predominance of heavy minerals suggests that the rocks proximal to the beach have been altered. The abundance of primary minerals associated with these secondary heavy minerals justifies the existence of unweathered lithic grains in the source rocks and an alteration that occurred under a tropical monsoon climate that ranged from semi-humid to humid. The use of the multi-element tectonic discrimination diagram implies an arc framework that is compatible with the geology of the study area.

Provenance and Paleoclimate Analysis of Sandstone from Meluhu Formation in Kendari, Southeast Sulawesi.

The Meluhu Formation consists of quartz sandstone, red shale, siltstone, and mudstone at the bottom part, and alternating black shale, sandstone, and thin limestone in the upper part. Petrographic components composition can reveal the litho-tectonic setting of a region. The composition of sandstone can be done by determining and counting the proportion of a quartz minerals, feldspar, and rock fragment from thin sections using petrographic analysis. This result can guide to depositional setting and paleoclimates of a region. The Provenance and paleoclimates studies of Meluhu Sandstone are still relatively new. The purpose of this study was to find out the provenance and paleoclimate of sandstone of Meluhu Formation which is exposed in 14 sites along Kendari and adjacent area. The method used to make petrographic observations followed by data processing using the point counting method. The sandstone in this study area was classified into sublitharenite, lithic arenite, arkose, subarkose, and greywacke. The origin of rock was interpreted as located in the recycled orogenic province, interior craton, and undissected arc. The paleoclimate that occurred in the formation of these siliciclastic rocks was arid to humid. Keywords: Meluhu Formation, provenance, paleoclimate, sandstone.

Studi Batuan Asal (Provenance) Batupasir Formasi Simpangaur Daerah Way Krui, Lampung

Bengkulu basin is the one of fore arc basin which formed in Sumatra island. The basin formation is inseparable from tectonic activity of the Sumatra Island from the Pre-Tertiary era to present day. The relationship between tectonic activities and the basin filling can be identified by conducting a provenance study on the sedimentary rocks. The Simpangaur Formation is a Tertiary sedimentary rock that has filled the Bengkulu Basin in the southern Sumatra. The aim of this research is determining the provenance of Simpangaur Formation existed in Way Krui area, Lampung, and its tectonic environment. The observations of thin section were carried out on eleven sedimentary rock samples obtained from field. The provenance of these rocks is analyzed by quantifying the mineral composition microscopically. The result analysis show that the grains originated from a magmatic arc environment, specifically the transitional arc and undissected arc. Based on the presence of polycrystalline quartz in eleven rock samples, it is interpreted as originating from the Orogen Arc. In addition, it is also interpreted that the provenance for sandstone samples of the Simpangaur Formation are plutonic and metamorphic rocks in a semi-arid climate, based on ancient climate.

Tectonic settings of formation of volcanic and sedimentary rocks of the Itmurundy zone, central Kazakhstan

The paper presents new petrographic and geochemical data from volcanic and sedimentary rocks and first U-Pb ages of detrital zircons from sandstones of the Itmurundy zone of central Kazakhstan. The volcanic rocks are aphyric and porphyric basalts, andesibasalt and andesite. The major element composition of tuff and sandstone are close to that of andesite. The poorly sorted greenish grey sandstones carry numerous fragments of volcanic and sedimentary rocks suggesting its greywacke nature which is probably due to. The greywacke probably formed by the destruction of undissected arc. The distribution of U-Pb ages of detrital zircons spanning 505 to 432 Ma has unimodal character peaked at 445 Ma suggesting formation of the sandstones by the destruction and subsequent transportation of clastic material from a late Ordovician intra-oceanic arc. In geochemical diagrams, the tuffs and sandstone plot close to the volcanic rocks. All chondrite-normalized REE spectra show enrichment in LREE (LaN=38–367, La/YbN=4.0–16.9, La/SmN=2.1–3.3) and moderate to weakly differentiated HREE (Gd/YbN=1.4–4.0). However, the level of REE concentrations in the volcanic rocks, in particular, in basalts, is significantly higher than that in the sandstone and andesite. The primitive mantle normalized trace-element diagrams show peaks at Nb (Nb/Lapm=0.9–1.6, Nb/Thpm=0.8–1.6) in most basaltoids, but troughs at Nb for andesite, tuff and sandstone (Nb/Lapm=0.25–0.31, Nb/Thpm=0.17). The previous and new geochronological, petrographic and geochemical data show that the volcanic and sedimentary rocks of the Itmurundy zone formed in Ordovician time in an intra-plate oceanic setting and in a supra-subduction setting at a Pacific-type convergent margin.

The provenance of coarse-grained turbidite sandstones in the Kırkgeçit Formation(western part of the Elazığ Basin-east Turkey)

Modal analysis of sandstones for conglomerate rich marine basins is useful for investigating the provenance types surrounded by plate tectonics. Different kinds of siliciclastic rocks are important indicators for defining the paleogeography and geotectonic conditions of the sedimentary basin by provenance analysis. This paper, which focuses on the petrography and provenance of conglomerate rich turbidite systems, is one of the rarest studies on the sedimentary characteristics in the scientific literature. In this context, the study was carried out in the western part of the Elazığ Basin. The Middle Eocene?Oligocene basin infill called the Kırkgeçit Formation is characterised by the conglomerate rich submarine channel deposits and coarse-grained turbidite sandstones. Petrographically, the coarse-grained sandstones of the Kırkgeçit Formation are composed of litharenite and rarely feldspathic litharenite, lithic arkose and rock fragments. Mostly the components are derived from volcanic and plutonic rocks of the Elazığ Magmatites. Fossils and fossil fragments are transported from the shallow part of the western Elazığ Basin. The components transported from metamorphic rock fragments, which are rarely observed in the sandstone samples, are composed of recrystallized limestones and derived from Keban Metamorphites in the northern part of the basin. The provenance studies and petrographic analyses indicated that the conglomerate rich channels and coarse-grained sediments of turbidite sandstones in the Kırkgeçit Formation were plotted mostly to the recycled orogenic and undissected arc type source rocks. Therefore, this type of source rocks has been reflected as a back-arc basin deposit during Middle- Upper Eocene. The geotectonic evolution of the western Elazığ Basin has also supported this remark.

U–Pb ages and sandstone provenance of the Permian volcano-sedimentary sequence of the Hida Gaien belt, Southwest Japan: Implications for Permian sedimentation and tectonics in Northeast Asia

The Permian volcano-sedimentary strata of the Hida Gaien belt of SW Japan are characterized by Lower Permian felsic tuffs and tuffaceous clastic rocks, Middle Permian andesites and sandstones rich in volcanic detritus, and upper Middle to Upper Permian feldspathic sandstones and mudstones. A combination of petrographic and geochemical analyses together with zircon U–Pb dating for the sandstones reveals a provenance change from an immature undissected arc with active felsic and intermediate volcanism during the Early–Middle Permian to a mature arc with basement uplift during the Late Permian. The Upper Permian sandstone yields syn-depositional zircons with 500–400 Ma zircons. This age distribution suggests that sediment was derived from the Permian arc and early Paleozoic basement. Similar characteristics to Permian strata in the Hida Gaien belt are recognized in the eastern Central Asian Orogenic Belt in terms of detrital zircon ages and the change in dominant lithology from volcanic to clastic rocks. In particular, Permian strata of the Jilin area (NE China) are lithostratigraphically similar to those of the Hida Gaien belt and have similar detrital zircon U–Pb ages. As such, Permian strata in both areas would have been deposited proximal to each other in the same tectonic setting along a single subduction zone in the western Paleo-Pacific Ocean.

Erosional and Tectonic Evolution of a Retroarc Orogenic Wedge as Revealed by Sedimentary Provenance: Case of the Oligocene – Miocene Patagonian Andes

Sedimentary provenance techniques have been widely applied in foreland basin settings to understand tectonic and magmatic processes by tracking the exposure and erosion of distinct sediment source areas through time. We present a case example from the Magallanes-Austral retroarc foreland basin of Chile and Argentina (51°30’S), where modal sandstone and conglomerate compositional data, detrital zircon U-Pb geochronology, and sedimentology data from the Oligocene-Miocene Rio Guillermo Formation document a change in source areas during an important stage of orogenic development. In particular, our results from the ~24.3-21.7 Ma Rio Guillermo Formation record an abrupt shift from transitional to undissected arc provenance that indicate rejuvenated magmatism within the contemporary arc. Minor components of lithic grains suggest a subordinate source of recycled sediments that we interpret may have been derived from the intervening external fold-and-thrust belt, rather than directly from sources in the hinterland thrust domain. Detrital zircon U-Pb geochronology data show mostly Neogene (~20-40 Ma) and Cretaceous (~70-110 Ma) age groups, with minor amounts of Jurassic (~145-155 Ma) and Paleozoic (~260-540 Ma) age groups, which are consistent with a syndepositional arc and recycled external fold-and-thrust belt sources. Stratigraphic data suggest a vegetated, channelized braidplain environment developed above an erosional unconformity with the underlying shallow-marine Rio Turbio Formation. Upsection, the Rio Guillermo Formation locally transitions to a low-energy, organic-rich floodplain setting located within the upper reaches of a fluvial-tidal transition zone of the coastal plain, and the uppermost part of the formation is characterized by a coarse-grained sandy channelized braidplain environment along the foreland basin margin. Moderate sediment accumulation rates and coastal plain progradation during this period is consistent with sustained sediment flux from the Patagonian Andes and tectonic subsidence along the basin margin. Taken collectively, we propose that the abrupt provenance shift dominantly records erosion of the rejuvenated mafic volcanic arc, despite coeval changes in orogenic wedge dynamics brought about by increased plate convergence rates that drove uplift of the intervening external-fold-and-thrust belt along reactivated deep-seated high-angle basin structures.

Morphological Changes due to Anthropogenic Interferences in Gendol River Valley, Merapi Volcano

This research aims to identify the morphological changes in Gendol river, Merapi volcano. After the eruption 2010, Gendol river filled up by pyroclastic materials, then utilized as mining area. As the volcanic materials are abundance in Gendol river, sand mining activities are dominant, causing antrophogenic interferences that changes the morphology of Gendol valley. In this study, the morphological changes is measured by terrestrial survey, aerial mapping, and materials analysis through petrographic and granulometric analysis. Aerial mapping was conducted in February and August 2019 in order to identify the dynamic changes during 6 months observation. Vertical changes is found in the active mining zone, in western-side of research area, while horizontal changes was found in eastern-side. The comparison of widening and narrowing valley is 13:1 due to horizontal changes. Result of petrographic analysis showes that three different sampling locations originated from similar provenance, which is undissected arc provenance but may differ in period of sedimentation. Grain material found in research are categorized as suitable material for mining with the 0,733 mm grain size. However, the mining activities should be aware of material availability to minimize the riverbank failure.

Age and Provenance of the Nindam Formation, Ladakh, NW Himalaya: Evolution of the Intraoceanic Dras Arc Before Collision With India

AbstractThe Dras Arc in NW India Himalaya is a belt of basaltic‐andesites intercalated with arkose‐dominated volcaniclastic rocks of the Nindam Formation situated along the Indus Suture between India and Eurasia. Debates exist as to whether these rocks developed in a forearc basin to the Eurasian margin or as part of an intraoceanic island arc system that collided with either India or Eurasia before final continental collision. Detrital zircons from the Nindam Formation yield U‐Pb age spectra with dominant youngest age populations of ~84–125 Ma, corresponding with arc magmatism. Sandstone provenance analysis from the Nindam Formation indicates that the Dras Arc evolved from an undissected arc to dissected arc over a period of ~41 Myr. Slightly older, smaller populations occur at ~135–185 Ma, corresponding with reported ages of Neotethyan ophiolites (e.g., Spongtang). The basal section of the Nindam Formation reveals the presence of arc‐derived basaltic‐andesite and tonalite clasts, plus ophiolitic components sourced from an adjacent accretionary complex. There is a distinct absence of quartz or felsic granitic clasts, suggesting that the Nindam Formation did not develop as a forearc basin to the Ladakh Batholith of southern Eurasia but rather as separate intraoceanic island arc. A distinct “Gondwanan” signature occurs in all samples, with zircon age peaks at ~514–988, ~1000–1588, ~1627–2444, and ~2500 Ma. We suggest that the Dras and Spong arcs are the same intraoceanic island arc system that developed as a result of subduction initiation along NNE‐SSW transform faults perpendicular to the Indian and Eurasia continents.

Modal analysis of coastal sand in the Honshu arc-Izu arc collision zone, central Japan

The Izu Peninsula and its surrounding area, central Japan represent an arc-arc collision zone. This area is the region of ongoing arc-arc collision between the Izu arc, (=intra-oceanic island) and Honshu arc since the middle Miocene. The arc-arc collision zone provides a lot of significant information to understand the process of an arc-arc collision. We described a framework composition of modern beach sand in the arc-arc collision zone based on the characteristics of petrofacies. The provenance compositional field astriding the boundary line between undissected arc and lithic recycled represent a particular tectonic setting of the arc-arc collision between the Izu Peninsula and the Shimanto accretional prism.

Rare Earth Element Geochemistry of the Zigzag - Klondyke Sedimentary Rock Formations: Clues to the Evolution of the Baguio Mineral District (Luzon), Philippines

. Petrographic analysis of the Oligocene-Miocene Zigzag Formation and Miocene Klondyke Formation of the Baguio Mineral District reveals a transitional arc source for the former and an undissected arc source for the latter. Whole rock geochemistry of these sedimentary rock formations show affiliation of the Zigzag Formation to active continental margin whereas the Klondyke Formation appears to have been derived from an oceanic island arc source. A different aspect of the geology of the district is further gleaned from the whole rock rare earth element geochemistry of these rocks. Samples from the Zigzag Formation are characterized by higher REE concentrations compared to the Klondyke Formation samples. The REE data indicate that the sedimentary rocks of the Klondyke Formation had geochemically-less fractionated igneous lithologies as their progenitor whereas the older Zigzag Formation was derived from a more differentiated source. This new set of information contributes to the understanding of the evolution of this district from a marginal basin to an island arc setting.

Unraveling Magmatic and Orogenic Provenance in Modern Sand: The Back-Arc Side of the Apennine Thrust Belt, Italy

Modern river and beach sands carried to and deposited along the coasts of the Tyrrhenian Sea are derived from multiple mag- matic-arc and orogenic sources, including ophiolitic sequences and tur- bidites issued from the Ligurian Ocean, carbonate-platform to pelagic sedimentary and metasedimentary successions originally deposited onto the Adria continental margin, and overlying foredeep clastic wedges. ''Undissected arc'' feldspatholithic signatures characterize limited areas (Capraia Island, Tarquinia and Napoli gulfs). Contrasting fea- tures, with respect to Circum-Pacific suites (low to very low P/F ratio), include sanidine as the most abundant detrital feldspar and common leucite and leucite-bearing lithic fragments, reflecting the potassic to ultrapotassic character of Neogene-Quaternary magmatism. Green au- gite, associated with oxyhornblende and either abundant hypersthene (Tuscan magmatic province) or minor olivine and spinel (Roman mag- matic province) is the dominant dense mineral. ''Dissected arc'' ar- kosic signatures reflect unroofing of Miocene monzogranitic stocks in the Tuscan archipelago. Sands from ophiolitic sequences and remnant-ocean sediments (''subduction-complex provenance'') are characterized by common cel- lular serpentinite with few chert grains, basaltic to diabase and gab- broic grains, and by abundant shale to slate lithic grains, respectively. Dense mineral suites are dominated by either diallage or chrome spinel. Sedimentaclastic sands ranging in composition from lithic to quartz- ofelspathic are widespread (''thrust-belt provenance''). Richest in lith- ic grains is detritus from Mesozoic platform (dominant carbonates with abundant dolostone) to pelagic (dominant limestone with abundant chert) sedimentary successions. Metamorphiclastic sands with poly- crystalline quartz, phyllite, and quartz-mica or marble lithic grains from denuded core complexes are found locally (Apuane Alps). Rela- tively quartzose sands with abundant feldspars and garnet-rich dense- mineral assemblages are recycled from foredeep turbidites derived, di- rectly or indirectly, chiefly from the Alps. This signature is foreign to the Apennine accretionary wedge, where continental basement is not structurally involved. We calculate that one-third of the sand grains deposited along Tyrrhenian shores today are ultimately derived, through long-distance multistage transport and repeated recycling, from the Alps.

北海道南東部，えりも地域に分布する，いわゆる“日高累層群”の砂岩組成

Characteristics of sandstones from so-called "Hidaka Supergroup" in the Erimo area were discussed based on modal compositions and chemical compositions of heavy minerals by means of an electron-probe microanalyser. The modal compositions suggest transitional to undissected arc provenance. Chemical features of detrital grains indicate that intermediate to basic rocks, partly metamorphosed to intermediate grade, and peridotites of arc or fore-arc origin were exposed on the land. These formations in the Erimo area can be correlated to the Nakanogawa Group in the Hidaka Belt.

Petrography of Ordovician and Silurian sediments in the western Irish Caledonides: tracers of a short-lived Ordovician continent-arc collision orogeny and the evolution of the Laurentian Appalachian-Caledonian margin

Abstract Ordovician orogeny affected the Laurentian margin of the Appalachian-Caledonian Belt from the Southern Appalachians to the British Isles and is dated, stratigraphically, as post-uppermost Lower Cambrian and pre-upper Llandovery. Geochronological data favour a short Grampian orogeny from c . 470 to 460 Ma during the late Arenig-mid-Llanvirn, which is supported by the late Arenig-earliest Llanvirn termination of the Laurentian rifted margin carbonate shelf. The likeliest plate tectonic model for the Grampian Orogeny is of a continent-facing oceanic (Laurentia) arc, created as an infant mafic arc during the Middle Cambrian, evolving into an Early Ordovician intermediate-silicic arc with suprasubduction-zone ophiolites. This arc collided with the Laurentian margin and forearc ophiolites were obducted across the margin creating an orogen and accreting the arc to the margin. Continued plate convergence was accommodated by a flip in subduction polarity that terminated late orogenic retrocharriage, and led to the rapid final stages of uplift and unroofing of the orogen and to subduction accretion in the new mid-Ordovician trench. Presented in this paper are definitive new detrital heavy mineral evidence that supports and enhances this model of a short-lived orogenic event involving ophiolite obduction, and the rapid development and unroofing of a Grampian Barrovian metamorphic complex over c . 10 Ma. High-resolution heavy mineral analysis (HRHMA) has been undertaken on most of the Ordovician and Silurian stratigraphical units in the South Mayo Trough in western Ireland. It is shown that ophiolite unroofing began during the Arenig and that, by the early Llanvirn, a Barrovian complex was being eroded. The Killadangan ‘Formation’, of the Clew Bay Complex is confirmed as part of a pre-Grampian early Ordovician accretionary prism by both HRHMA and by quartz-felspar-lithic (QFL) analysis, which show it to have been derived from Precambrian rocks of the Laurentian margin. QFL analysis also shows that the Ordovician provenance of the South Mayo Trough sequence evolves from an undissected arc through a dissected arc to a recycled orogenic detrital pattern, except for the earliest sediments (Letterbrock), which were derived from the transitional continental sediments of the Killadangan accretionary prism, which, in turn, were derived from Laurentia. The problem of timing the Grampian deformation and metamorphism of the Dalradian is now regarded as being solved. It was an Arenig-Llanvirn event lasting c . 10 Ma, which occurred during and is recorded, faithfully, by the detrital heavy mineral assemblage in the conformable Ordovician sequence of the South Mayo Trough. The Miocene evolution of New Guinea is strikingly similar to the Arenig-Llanvirn evolution of the Laurentian margin and was, analogously, the short-lived result of the collision of an arc with the north Australian margin followed by subduction polarity flip.

Detrital record of the Gravina arc, southeastern Alaska: Petrology and provenance of Seymour Canal Formation sandstones

Sandstones of the Upper Jurassic to mid-Cretaceous Seymour Canal Formation (northern Gravina belt, southeastern Alaska) are volcaniclastic wackes. They exhibit a variably developed solution cleavage and zeolite- to low-greenschist-grade metamorphic assemblages. Point counts of 37 lithofeldspathic sandstones, spanning the unit9s stratigraphic range, yield framework modes typical of detritus shed from undissected arc sources (Q 4 F 40 L 56 , Qm 3 F 40 Lt 57 , Qm 8 P 91 K 1 , Qp 3 Lvm 91 Lsm 6 ) with only minor detritus characteristic of older terranes or continental-margin sources. Sandstones of the dominant thinly bedded turbidite lithology were derived almost exclusively from the contemporaneous Gravina arc. In contrast, sandstones of channelized conglomeratic facies are richer in quartz, sedimentary, metasedimentary, and plutonic detritus derived from basement terrane(s). The compositional heterogeneity and grain size of conglomeratic facies are consistent with derivation from local source areas, and we interpret them to reflect minor unroofing of the Alexander terrane and the superposed Gravina arc, mostly during Early Cretaceous time. A distinctive, although volumetrically minor, clinopyroxene-rich sandstone petrofacies signals a change in modal expression of source-area volcanism during the Cretaceous, and it probably records initial eruption of pyroxene-rich basalts of the mid-Cretaceous Douglas Island Volcanics. Supracrustal records preserved in the Nutzotin Mountains sequence, Dezadeash flysch, and Gravina sequence are also dominated by shallow-level arc detritus, although coeval supracrustal units on the Insular terrane of British Columbia reflect deeper incision of arc basement, possibly due to along-strike variation in arc geometries or tectonics of the Jura- Cretaceous Insular terrane.

Late Precambrian and early Paleozoic tectonism and associated sedimentation in northern Victoria Land, Antarctica

Northern Victoria Land, part of the Trans-antarctic Mountains, occupies a key position in the reconstructed Pacific margin of Gondwana, lying near the Oligocene rift between Antarctica, Tasmania, and Australia. Sedimentology, sedimentary petrology, and structural history of the Robertson Bay Group and the Bowers Supergroup are here used to develop a regional interpretation of these rocks in plate-tectonic terms. The Robertson Bay Group, a distal shale-turbidite sequence, is interpreted to have originated in a continent-continent collision environment and was com-pressionally deformed after the Cambrian-Ordovician boundary but before Devonian time. The Sledgers Group of the Bowers Supergroup, a proximal shale-turbidite and volcanic sequence, was derived from an undissected arc environment and was deformed during the same time interval as was the Robertson Bay Group and possibly also during the Silurian-Devonian. The Cambrian Mariner and overlying Leap Year Groups represent shallow-marine and terrestrial deposition. Reworking and selective removal of unstable clasts obscure provenance, but, although these units may be derived from dissected arc sources, they are quartz-rich. They appear to have been deformed by Silurian-Devonian tectonism. The Robertson Bay Group and the Bowers Supergroup occur in large, fault-bounded terranes. The plate-tectonic interpretations developed for these units in this paper place the arc-related Bowers Supergroup between the Antarctic craton to the west and the recycled orogen-related Robertson Bay Group to the east. This geometry is not readily related to in situ tectonic environments; therefore, it is concluded that the possibility exists that these terranes are “suspect,” that is, they may be entirely allochthonous to the Antarctic craton and have been added by later accretion.

The Morphology and Relationships of Oncotheca Balansae

The anatomy and morphology of vegetative and reproductive structures of the monotypic, dicotyledonous genus Oncotheca were studied to evaluate the systematic relationships of this New Caledonian endemic. Wood has remained at a low level of advancement, as evidenced by the solitary, angular pores and exclusively scalariform perforation plates. Imperforate tracheary elements are fiber tracheids, and axial parenchyma is diffuse to diffuse in aggregates in distribution. Sieve tube elements possess oblique to very oblique, compound sieve plates. Nodal anatomy is pentalacunar, and petioles contain a single, undissected arc of vascular tissue. A number of apparently xeromorphic leaf features, including general foliar coriaceousness, may result from the poor availability of soil minerals. Leaves are characterized by a thick cuticle, uniseriate epidermis with nearly anomocytic stomata having paracytic or tetracytic tendencies, and a bifacial mesophyll with two or three hypodermal layers. Foliar venation is brochidodromous. High-order venation is characterized by freely ending branched veinlets and imperfect areolation. Flowers are relatively advanced, as evidenced by extensive fusion of parts and the union of major floral bundles such as the ventral carpellary traces. A close relationship between Oncotheca and the Aquifoliaceae and Ebenaceae is refuted. Support is given to the elevation of the genus to familial status and its placement near the Theaceae.