Igneous Detritus Research Articles

The Alps as the main source of sand for the Late Miocene Lake Pannon (Pannonian Basin, Croatia)

The provenance of the Upper Miocene sandstones from the Sava and Drava depressions of the North Croatian Basin was investigated using petrographic, geochemical, and heavy mineral analyses, including Raman spectroscopy. The study of these sandstones, which represent important oil and gas reservoirs in Croatia, allowed reconstruction of the Late Miocene source-to-sink model of the Lake Pannon drainage system and the evolution of the southwestern Pannonian Basin. The studied feldspatho-litho-quartzose sandstones consist of a mixture of sedimentary, metamorphic, and igneous detritus. Heavy-mineral assemblages are dominated by almandine-rich garnet with apatite, epidote, tourmaline, rutile, zircon, staurolite, and zoisite, indicative of low to medium-grade metamorphic source rocks. Higher concentrations of Ca and Mg than in the Upper Continental Crust standard (UCC) additionally reflect the abundance of limestone and dolostone rock fragments as well as carbonate cement. Geochemical compositional variations between sandstone samples from the Sava and Drava depressions primarily stem from diagenetic processes. CIX and alpha values indicate only minor weathering. Compositional features indicate an orogenic source located in the Eastern Alps and primarily represented by Austroalpine and Penninic nappes. This research offers a novel perspective to distinguish the Upper Miocene reservoirs from other sedimentary units within the basin, contributing to a more comprehensive understanding of the regional geological dynamics and supporting future exploration projects also related to energy transition.

Sedimentary provenance in continental rifts: U–Pb detrital zircon, Nd and Sr isotopes and lithogeochemistry of the Eocene alluvial sandstones of the Resende Basin, SE–Brazil

Multiproxy analysis of detrital minerals is a powerful tool for unravelling the source-to-sink evolution of syn-rift sedimentary basins that represent important depositional settings during major rift events. The Cenozoic Resende half-graben in SE Brazil is a classic example of a syn-rift basin developed under oblique extension, which preserves evidence of tectonic episodes and palaeoclimatic conditions during the opening of the Atlantic. Sediment in the basin was sourced from the surrounding basement rocks, which to the faulted margin consists of older Proterozoic orthogranulites and orthogneisses, whilst to the flexural margin is characterised by Neoproterozoic passive margin paragneisses. Cretaceous intra-plate alkaline stocks crop out at the western edge and in the central-south basement of the basin and represent a distinct provenance fingerprint. This study uses new detrital zircons U–Pb ages combined with previously published elemental and Nd and Sr isotopic data to characterise the sedimentary provenance of Eocene alluvial deposits and to understand how sediment sourcing has changed throughout the evolution of the basin. U–Pb detrital zircon analyses from samples collected across the Resende Basin are presented herein. Samples yield a spread of detrital age spectra from the Archaean to the Cretaceous. Precambrian zircons range from ca. 3228 Ma to ca. 542 Ma. Cretaceous zircons yield ages between ca. 138 Ma and ca. 67 Ma. The sediment source location is interpreted by comparing the yielded detrital zircon spectra with the age spectra of zircons from the basement rocks. The dominance of detrital zircons yielding ages of ca. 1500 Ma and ca. 1000 Ma is interpreted to be sourced from the Neoproterozoic basement rocks to the flexural border of the half-graben. Cretaceous zircons from samples with whole rock εNd40Ma > −10 are found in both the western and eastern portions of the half-graben but not in the central portions. It is interpreted that an along-axis fluvial system was responsible for a small increase in the compositional maturity of the Resende sandstones from west to east, resulting in the progressive dilution of Cretaceous alkaline igneous detritus along the basin axis. Preserved metamorphic rims in zircon grains indicate low transport abrasion, suggesting that Cretaceous zircons in the eastern depocentre could be associated with other small proximal sources. U–Pb zircon analyses and Nd and Sr isotopes provide reliable insights regarding sediment provenance and transport processes, whilst element geochemistry supplies evidence for hydraulic sorting effects. Consequently, the data represents a more robust estimation of the provenance patterns of the Resende Basin, showing chemical and geochronological differences that support the division of the basin into three depocentres (western, central, and eastern) connected by a longitudinal fluvial system sourced mostly by sediments coming from the south-western flexural border. This study highlights the usefulness of using multiple provenance proxies to unravel the complex development of rift basins, assessing multiple source terrains and the variable degrees of mixing in sedimentary basins.

Jurassic–Cretaceous arc magmatism along the Shyok–Bangong Suture of NW Himalaya: formation of the peri-Gondwana basement to the Ladakh Arc

The Jurassic–Cretaceous Tsoltak Formation of the eastern borderlands of Ladakh Himalaya consists of conglomerates, sandstones and shales, and is intruded by norite sills. It is the oldest sequence of continent-derived sedimentary rocks within the Shyok Suture. It also represents a rare outcrop of the basement rocks to the voluminous Late Cretaceous–Eocene Ladakh Batholith. The Shyok Formation is a younger sequence of volcaniclastic rocks that overlie the Tsoltak Formation and record the Late Cretaceous closure of the Mesotethys Ocean. The petrogenesis of these formations, ophiolite-related harzburgites and norite sill is investigated through petrography, whole-rock geochemistry and U–Pb zircon geochronology. The youngest detrital zircon grains from the Tsoltak Formation indicate an Early Cretaceous maximum depositional age and a distinctly Gondwanan, Lhasa microcontinent-related provenance with no Eurasian input. The Shyok Formation has a Late Cretaceous maximum depositional age and displays a distinct change in provenance to igneous detritus characteristic of the Jurassic–Cretaceous magmatic arc along the southern margin of Eurasia. This is interpreted as a sign of collision of the Lhasa microcontinent and the Shyok ophiolite with Eurasia along the once continuous Shyok–Bangong Suture. The accreted terranes became the new southernmost margin of Eurasia and the basement to the Trans-Himalayan Batholith associated with the India–Eurasia convergence. Supplementary material: Supplementary figures and Tables S1–S3 are available at https://doi.org/10.6084/m9.figshare.c.5633162

Detrital zircon provenance of Permian to Triassic Gondwana sequences, Zealandia and eastern Australia

ABSTRACT Permian Parapara Group and Triassic Topfer Formation are small clastic sedimentary outliers that rest on Cambrian to Devonian basement terranes in the Western Province of South Island, New Zealand. New detrital zircon U-Pb age datasets from three Parapara sandstones, one Topfer sandstone and four age-equivalent eastern Australian sandstones reveal age and provenance information about former Gondwana intracontinental basins. Youngest significant zircon age populations of 264 ± 6 and 261 ± 2 Ma in two Parapara Group sandstones and 215 ± 2 Ma in the Topfer sandstone match fossil ages and indicate broadly syn-depositional igneous activity. The provenance of the igneous detritus was probably the nearby Median Batholith and/or its volcanic equivalents. Zircons in a third Parapara sandstone are predominantly Cambrian and Precambrian in age and represent a separate petrofacies with a local Western Province basement terrane provenance. Intra-basin provenance switching between Gondwana margin arc and older basement sources is thus documented in Gondwana basin sequences in Zealandia, as well as those in Australia and Antarctica.

Provenance, tectonic setting, and weathering of sediments in Tumaco-1 ST-P well, Tumaco forearc basin, Colombia: Insights from petrography, heavy minerals, X-ray diffraction, and whole-rock chemostratigraphy

A chemostratigraphic evaluation based on major and trace elements (including the REE group), heavy minerals, and whole rock petrographic analysis is applied on 300 samples from the Tumaco-1-ST-P well located in the Tumaco forearc basin, Colombia. The aim of the study is to gain a better understanding of the provenance, tectonic, and paleoweathering conditions of the succession, which contains three formations (Tumaco, Tangareal del Mira, and Cascajal) ranging in age from the late Oligocene to late Pliocene.SiO2 has the highest concentration of all major elements (mean at 52%), followed by Al2O3 (mean at 14.6%), Fe2O3, MgO, and CaO are in the range of 3.2–5.3%, Na2O and K2O oscillate between 1 and 2%, and TiO2, P2O5, MnO, and Cr2O3 are <1%. Out of trace elements, Ba, Sr, V, Cr, Cu, Zn, Ni and Zr are the most abundant; their enrichment is probably due to the marine/deltaic depositional environment conditions. Th, Cs, and Nb have the highest concentrations of all trace elements. In general, concentrations of light rare earth elements are greater than high rare earth elements (LREE > HREE). Both positive and negative Eu anomalies are present in Tumaco and Tangareal del Mira formations, suggesting that rocks have two sources, mafic and felsic; the presence of both is consistent with petrographic and heavy mineral analyses.Paleoweathering indices, such as the chemical index of alteration (CIA), plagioclase index of alteration (PIA), and index of compositional variability (ICV) in the Tumaco-1-ST-P well suggest that the source of these rocks experienced initial stages of weathering compared with Post-Archean Average Australian Shale (PAAS) and Upper Continental Crust (UCC) values, indicating that the Tumaco formation samples have undergone major alteration. The clay mineral assemblage is dominated by the occurrence of illite and illite/smectite mixed layers; when coupled with the CIA index and the Al2O3-CaO-Na2O-K2O (ACNK) plot this indicates post-depositional K-enrichment. The distribution of Al2O3, TiO2, and Zr suggests a limited recycling effect.The presence of first cycle mafic-to-intermediate igneous detritus, together with felsic to intermediate igneous and metamorphic minerals, suggest that the Tumaco forearc basin detrital material was supplied by two locations. One location corresponds to the orogenic island arc system of the Western Cordillera, which is dominated by mafic and intermediate extrusive rocks (basalt and andesite). The other source, which is volumetrically less important, is the Central Cordillera, which comprises intrusive felsic rocks and metamorphic rocks (skarn, marble, and quartz-micaceous schist). This subsidiary source represents a continental arc setting and is observed mainly in the Tumaco Formation.

U-Pb ages of detrital zircon from Cenozoic sediments in the southwestern Tarim Basin, NW China: Implications for Eocene–Pliocene source-to-sink relations and new insights into Cretaceous–Paleogene magmatic sources

A detailed investigation of potential provenance is still lacking in the southwestern Tarim Basin, which restricts our complete understanding of Cenozoic source-to-sink relations between the basin interior and the Pamir salient - western Kunlun Mountain Range. Debate also exists concerning the potential sources of the Paleogene and Cretaceous igneous detritus present in the Cenozoic sedimentary sequences. Here, we present U-Pb (LA-ICP-MS) ages of detrital zircons from the continuous Eocene-Pliocene sediment series in the well-exposed Aertashi section to investigate changes in sediment provenance through time. The U-Pb detrital zircon ages range widely from 45 to 3204 Ma and can be divided into seven main groups: 45–65 Ma (sub-peak at 49 Ma), 67–103 Ma (sub-peak at 95 Ma), 196–251 Ma (sub-peak at 208 Ma), 252–416 Ma (sub-peak at 296 Ma), 417–540 Ma (sub-peak at 446 Ma), 550–1429 Ma (sub-peaks at 614 Ma, 828 Ma and 942 Ma) and 1345–3204 Ma (sub-peaks at 1773 Ma and 2480 Ma). These zircons were mainly derived from the western Kunlun Mountain Range and northern Pamir salient to the west and south. The evolution of the provenance and source-to-sink relationship patterns in the southwestern Tarim Basin can be divided into three stages: (1) The Middle Eocene to Lower Oligocene sediments display a wide variety of detrital zircon ages, suggesting that the source area was extensive. (2) A major change in provenance occurred during the Late Oligocene to Early Miocene and was characterized by an abrupt increase in the proportion of Triassic and Lower Paleozoic igneous components, implying a significant adjustment in topography induced by the initial uplift and exhumation of the western Kunlun Mountain Range and northern Pamir salient. (3) In the Late Miocene, the source-to-sink system transformed again, and contributions of Triassic to Lower Paleozoic material weakened substantially due to the sufficient indentation of the Pamir salient. Our integrated analyses of zircon geochronology indicate that the main source terranes of the Paleogene and Cretaceous igneous detritus are the central and southern Pamir salient, respectively, which are speculated to have been continuously connected to the study area during Eocene-Pliocene times, although such detritus is scarce in certain formations and has not yet been detected.

Petrology on Mars

Petrologic investigations of martian rocks have been accomplished by mineralogical, geochemical, and textural analyses from Mars rovers (with geologic context provided by orbiters), and by laboratory analyses of martian meteorites. Igneous rocks are primarily lavas and volcaniclastic rocks of basaltic composition, and ultramafic cumulates; alkaline rocks are common in ancient terranes and tholeiitic rocks occur in younger terranes, suggesting global magmatic evolution. Relatively uncommon feldspathic rocks represent the ultimate fractionation products, and granitic rocks are unknown. Sedimentary rocks are of both clastic (mudstone, sandstone, conglomerate, all containing significant igneous detritus) and chemical (evaporitic sulfate and less common carbonate) origin. High-silica sediments formed by hydrothermal activity. Sediments on Mars formed from different protoliths and were weathered under different environmental conditions from terrestrial sediments. Metamorphic rocks have only been inferred from orbital remote-sensing measurements. Metabasalt and serpentinite have mineral assemblages consistent with those predicted from low-pressure phase equilibria and likely formed in geothermal systems. Shock effects are common in martian meteorites, and impact breccias are probably widespread in the planet’s crustal rocks. The martian rock cycle during early periods was similar in many respects to that of Earth. However, without plate tectonics Mars did not experience the thermal metamorphism and flux melting associated with subduction, nor deposition in subsided basins and rapid erosion resulting from tectonic uplift. The rock cycle during more recent time has been truncated by desiccation of the planet’s surface and a lower geothermal gradient in its interior. The petrology of Mars is intriguingly different from Earth, but the tried-and-true methods of petrography and geochemistry are clearly translatable to another world.

Integrated provenance record of a forearc basin modified by slab‐window magmatism: detrital‐ zircon geochronology and sandstone compositions of the Paleogene Arkose Ridge Formation, south‐central Alaska

AbstractThis study presents an integrated provenance record for ancient forearc strata in southern Alaska. Paleocene–Eocene sedimentary and volcanic strata &gt;2000 m thick in the southern Talkeetna Mountains record nonmarine sediment accumulation in a remnant forearc basin. In these strata, igneous detritus dominates conglomerate and sandstone detrital modes, including plutonic and volcanic clasts, plagioclase feldspar, and monocrystalline quartz. Volcanic detritus is more abundant and increases upsection in eastern sandstone and conglomerate. U‐Pb ages of &gt;1600 detrital zircons from 19 sandstone samples document three main populations: 60–48 Ma (late Paleocene–Eocene; 14% of all grains), 85–60 Ma (late Cretaceous–early Paleocene; 64%) and 200–100 Ma (Jurassic–Early Cretaceous; 11%). Eastern sections exhibit the broadest distribution of detrital ages, including a principal population of late Paleocene–Eocene ages. In contrast, central and western sections yield mainly late Cretaceous–early Paleocene detrital ages. Collectively, our results permit reconstruction of individual fluvial drainages oriented transverse to a dissected arc. Specifically, new data suggest: (1) Detritus was eroded from volcanic‐plutonic sources exposed along the arcward margin of the sampled forearc basin fill, primarily Jurassic–Paleocene magmatic‐arc plutons and spatially limited late Paleocene–Eocene volcanic centers; (2) Eastern deposystems received higher proportions of juvenile volcanic detritus through time from late Paleocene–Eocene volcanic centers, consistent with emplacement of a slab window beneath the northeastern part of the basin during spreading‐ridge subduction; (3) Western deposystems transported volcanic‐plutonic detritus from Jurassic–Paleocene remnant arc plutons and local eruptive centers that flanked the northwestern part of the basin; (4) Diagnostic evidence of sediment derivation from accretionary‐prism strata exposed trenchward of the basin fill is lacking. Our results provide geologic evidence for latest Cretaceous–early Paleocene exhumation of arc plutons and marine forearc strata followed by nonmarine sediment accumulation and slab‐window magmatism. This inferred history supports models that invoke spreading‐ridge subduction beneath southern Alaska during Paleogene time, providing a framework for understanding a mature continental‐arc/forearc‐basin system modified by ridge subduction. Conventional provenance models predict reduced input of volcanic detritus to forearc basins during progressive exhumation of the volcanic edifice and increasing exposure of subvolcanic plutons. In contrast, our results show that forearc basins influenced by ridge subduction may record localized increases in juvenile volcanic detritus during late‐stage evolution in response to accumulation of volcanic sequences formed from slab‐window eruptive centers.

Fast sediment underplating and essentially coeval juvenile magmatism in the Ordovician margin of Gondwana, Western Sierras Pampeanas, Argentina

Metasedimentary high-pressure upper amphibolite facies gneisses (1.2±0.1GPa and 780±45°C) at Las Chacras, Sierra de Valle Fértil, are tectonically juxtaposed to the westernmost parts (outboard) of the Famatinian (Early Ordovician) magmatic arc, which underwent syn-plutonic middle crust high-grade metamorphism at lower pressure. U–Pb SHRIMP zircon data suggest that the gneisses contain Famatinian igneous detritus, so that their sedimentary protoliths were probably deposited in a forearc basin and then rapidly underthrust and accreted to the lower crust of the arc, essentially coevally with arc magmatism at 468±4Ma. Chemically and isotopically juvenile garnetiferous amphibolites within the gneisses are recognised as representing the most primitive magmas so far observed in this belt, which has often been considered to be a continental arc derived from isotopically mature sources. This is consistent with the idea that at least part of the dominant Famatinian magmatism originated in depleted mantle but was heavily contaminated by crustal components.

The Green Beds of the SW Highlands: deposition and origin of a basic igneous-rich sedimentary sequence in the Dalradian Supergroup of Scotland

Synopsis The Green Beds are distinctive chlorite- and epidote-rich metasedimentary rocks, which collectively represent a series of important lithostratigraphical marker units in the Southern Highland Group of the dominantly late Precambrian Dalradian Supergroup of Scotland. In a recent re-evaluation of the nature and origin of the Green Beds, integrated mapping, sedimentological, petrological and geochemical studies have been carried out. The Green Beds form relatively thin but laterally extensive units within thick sequences of siliciclastic metasedimentary rocks, and were deposited from high-density turbidity currents and debris flows in a deep-sea slope apron or ramp setting. They comprise a physical mixture of basic and siliciclastic material, representing periodic influxes of basic igneous detritus into the ‘background’ siliciclastic sedimentary system. The mineralogy of the Green Beds is largely the result of metamorphism, although rare relict detrital minerals from both granitic and basic sources are preserved. Geochemical studies show that the Green Beds and other Southern Highland Group metasedimentary rocks have a similar trace element signature, indicating a common siliciclastic source area. Two suites of Dalradian lavas, the Tayvallich and Loch Avich pillowlavas, were also analysed. Although it is unlikely that these particular volcanic units were the source of the Green Bed basic material, it is believed that the mafic component of the Green Beds was derived from episodes of near-contemporaneous volcanic activity.

Petrology and provenance of the Neogene Usibelli Group and Nenana Gravel; implications for the denudation history of the central Alaska Range

ABSTRACT Neogene basins exposed along the north-central flank of the Alaska Range record the geologic development of the highest mountain range in North America. Strata from these basins consist of the Miocene Usibelli Group ( 600 m thick) and the Pliocene Nenana Gravel ( 1000 m thick). Petrographic analyses of 54 sandstone thin sections and 36 conglomerate clast counts from one of these basins, the Healy Creek basin, delineate two distinct petrofacies. A stratigraphically older, quartz-rich petrofacies defines the Healy Creek, Suntrana, and Lignite Creek Formations of the Usibelli Group. A stratigraphically younger, lithic-rich petrofacies defines the Nenana Gravel. Sandstones of the lower petrofacies are characterized by a dominance of quartz (Q60F27L12; Qm33F28Lt39) and a high proportion of polycrystalline quartz (Qp68Lvm21Lsm11). Schist and quartz clasts are common in conglomerates from this petrofacies. Imbricated conglomerate clasts and planar cross-stratification indicate southward to southwestward paleoflow for this quartz-rich petrofacies. Relative to the lower petrofacies, sandstones of the upper petrofacies are defined by a dominance of lithic fragments (Q3919L43; Qm16F21Lt63), a lower proportion of polycrystalline quartz, and a higher proportion of volcanic lithic fragments (Qp37Lvm55Lsm8). Conglomerate clast compositions in this petrofacies consist mainly of sedimentary and igneous clasts. This lithic rich petrofacies is dominated by northward paleocurrent indicators. The compositional and paleocurrent data of the Usibelli Group and Nenana Gravel suggest a regional paleodrainage reversal from southward-flowing fluvial systems transporting sediment from the quartz-rich, metamorphic Yukon-Tanana terrane, to northward-flowing fluvial systems transporting sedimentary and igneous detritus from the growing Alaska Range. Superimposed on the first-order tectonic signal are second-order compositional differences. These differences may be related to a combination of factors but appear to be consistent with well documented changes in paleoclimate. For example, sandstones deposited during the warmest and wettest paleoclimates are characterized by a relative enrichment in quartz, especially polycrystalline quartz (Healy Creek Formation, Q81F16L3, Qp93Lvm3Lsm4; Suntrana Formation, Q61F25L14, Qp66Lvm25Lsm9). Sandstones deposited during cooler and drier climates, in contrast, have lower proportions of quartz and higher proportions of feldspar and lithic fragments (Lignite Creek Formation, Q45F33L14, Qp56Lvm28Lsm17). Our analysis cannot account for all the possible influences on sandstone compositions during deposition of the Usibelli Group, but we contend that the apparent correlation between sandstone composition and changes in paleoclimate suggest that climatic modification of sandstone composition may have been significant.

LATE PALEOZOIC STRATIGRAPHY AND PETROGRAPHY OF THE THINI CHU GROUP (MANANG, CENTRAL NEPAL): SEDIMENTARY RECORD OF GONDWANA GLACIATION AND RIFTING OF NEOTETHYS

In the Manang area (north Annapurna Range; Nepal Himalaya), the Permo-Carboniferous succession is 1000 to 1500 m thick. Crinoidal biocalcarenites (Tilicho Lake Fm.) pass upward to alternating black shales and sharp-based white quartzose sandstones (Thini Chu Group). Detailed stratigraphic analysis of this unit allowed us to recognize and establish 5 new formations and 8 new members. The Marsyandi Fm., of Visean age, records an increase of subarkosic terrigenous detritus during the initial stage of Neotethyan rifting, and is capped by two sequences Syringotbyris beds) characterized by transgressive sandstones rich in Serpukhovian brachiopods. The overlying black shales with subordinate quartzarenites (Col Noir Shale) are followed in the more proximal Bangba section by diamictites yielding dolostone rock fragments, documenting the first advance of glacial ice on rift shoulders, actively uplifted since the Bashkirian-Moscovian (Bangba Fm.). Next, glaciomarine to transgressive shelfal deposits are enriched first in igneous detritus and then in arenaceous rock fragments (Braga Fm.). Mafic to felsic magmatism during the climax of rifting was thus followed by active erosion of sedimentary successions, probably around the Carboniferous/Permian boundary. A major Early Permian transgression, coinciding with ameliorating climates at the end of the Gondwana glaciation, was followed by mainly estuarine chert-bearing quartzose pebbly sandstones capped by richly bioclastic shelfal deposits (Puchenpra Fm.). This second major transgression, associated with quartzose sandstones documenting subdued rift reliefs, is dated as Bolorian at Bangba, as Murgabian-Midian at Col Noirand as Djulfian at Tilicho. The base of the condensed outer shelf/upper slope carbonates capping the Thini Chu Group (topmost biocalcarenites) is also strongly heterochronous, being dated as Bolorian to Kubergandian-Murgabian at Bangba and as Djulfian-Dorashamian at Col Noir and Tilicho. Thermal subsidence associated with the opening of Neotethys thus began as early as the Early Permian.

Provenance mixing in an intraoceanic subduction zone: Tonga Trench-Louisville Ridge collision zone, southwest Pacific

Dredging on the lower slope of the Tonga forearc, at the intersection between the Louisville Ridge hotspot chain and the trench, yielded both Late Tertiary volcaniclastic sediments and Late Cretaceous slightly tuffaceous pelagic sediments. Petrography and phase chemistry of volcanic debris of Tertiary age samples indicates derivation from a low-K tholeiitic arc: volcanic glass has a low K 2O content and shows an Fe enrichment trend; plagioclase grains are high in An and low in Or components; pyroxene grains (calcic clinopyroxene, orthopyroxene and pigeonite) have low TiO 2 contents and show an Fe enrichment trend; alkali feldspar and biotite are absent. The composition of the Tertiary samples is similar to other Tonga forearc sediments and is consistent with their derivation from the adjacent magmatic arc. The character and composition of volcanogenic debris in Cretaceous age samples indicates derivation from an intraplate alkali igneous source: plagioclase compositions show an increasing Or component with increasing Ab; alkali feldspar is a rare additional component of these samples; calcic clinopyroxene has high TiO 2 contents and is titanaugite; amphibole is also rich in TiO 2 and is kaersutite; minor volcanic glass is rich in alkalis. Samples were dredged from the seaward slope of a small terrace on the lower trench slope. A planar reflector, located 2.6 km below the terrace is marked by an abrupt jump in seismic velocity and is interpreted as the top of an essentially undamaged Late Cretaceous guyot of the Louisville chain that was subducted at the Tonga trench about 0.5 Ma. This guyot is the likely source of the Late Cretaceous, intraplate igneous detritus collected at the dredge site. Some dredge samples yielded a mixed volcanic arc/intraplate provenance and/or Late Tertiary/Late Cretaceous ages. This probably represents the mixing of the Cretaceous seamount and Tertiary magmatic arc sources during post-collision slumping of the wedge of lower slope material as it re-establishes its critical taper.

Scopelophila cataractae in Texas

Scopelophila cataractae (Mitt.) Broth. is reported for the first time from Texas on a collection from an igneous substrate in Brewster Co., in Big Bend National Park. Small gemmae are borne on branching, green, protonema-like filaments in leaf axils of the Texas plants. Locality records in the United States of Scopelophila cataractae (Mitt.) Broth., a rare copper moss, include Arizona, California, North Carolina, Pennsylvania, and Tennessee (Shaw & Beer 1989). Copper mosses are noted for growing on substrates enriched with metals. The present collection is sterile, as are all others from North America (Shaw & Beer 1989). TEXAS. BREWSTER CO.: Big Bend National Park, Burro Mesa, in Apache Canyon near its head, ca. 29*17'N, 103*25'W, on moist igneous detritus and in seepage cracks below intermittent waterfall, Reese 17344 (BUF, DUKE, LAF, MICH, MO, NY, US). Burro Mesa is largely composed of volcanic material, including lava and ash beds (Maxwell 1968). The general area of the collection site is composed of soft, tuff-like strata with brilliant hues varied from reddish through yellow-orange to violet, perhaps indicative of mineral enrichment. Apache Canyon is relatively remote from human influences today, although it was apparently heavily used by Indians (based on abundant artifactual evidence in the vicinity) and, more recently, by cattle ranchers (Maxwell 1968). Mercury (cinnabar-HgS) mines and smelters-no longer operating-are located in Brewster Co., including one in the Park itself, but there has never been mining activity at Apache Canyon. The nearest mercury mine site is at Study Butte, ca. 11.5 km NW of Apache Canyon. Thus the Apache Canyon population of S. cataractae is growing on a presumably metal-rich subtrate that has not been modified by human activity, as is the case for some locales for this moss. Although protonemal gemmae have apparently not been reported for American material of S. cataractae, they have been reported for this moss from other parts of the world (reviewed by Arts 1988, with illustrations). What may be termed protonemal gemmae (Fig. 1) are produced sparingly on ascending, green, branching, protonema-like structures in leaf axils of the Texas plants; they readily dehisce from the filaments that bear them. The minute gemmae, to ca. 14.5 x 60 #Am, are composed of 1-3 cells in linear arrangement, and have thick walls and dense contents. Some of the larger gemmae are FIGURE 1. Outlines of protonemal gemmae of Scopelophila cataractae (Mitt.) Broth., one still attached to protonemal filament; all from Reese 17344. Scale bar = 50

Late Mesozoic to Early Cenozoic Foreland Sedimentation in Southwest Montana: ABSTRACT

Stratigraphic variations in the petrology of Upper Jurassic through lower Tertiary sandstones from southwest Montana, and inferred dispersal patterns and depositional environments, reflect the tectonic evolution of a foreland basin in response to gradually increasing orogenesis. Upper Jurassic to Lower Cretaceous (Aptian) chert-litharenites were largely derived from an older folded and faulted miogeoclinal prism of terrigenous and carbonate rocks farther west in Idaho. Volcanic and plutonic sources were negligible. Deposition was on a broad coastal plain extending east toward the craton. Subordinate admixtures of sediment containing low-rank metamorphic rock fragments derived from an intrabasinal reactivated Precambrian structure (Belt arch) and regionally extensive inter alated fresh to brackish-water limestones are unique characteristics of this retro-arc sequence. Marked upward increase in the quantity of locally derived plutonic and volcanic rock fragments in Blackleaf (Albian) and younger sandstones indicates the progressively more important role of nearby comagmatic sources. A transition in time from paralic sedimentation to dominantly alluvial sedimentation, was associated with the increase in igneous detritus. Dispersal patterns were complex because of local topographic barriers. Early Tertiary sedimentation occurred in extensional fault-bounded basins in an intra-arc setting. Highly immature lithic sandstones and arkoses with a complex plutonic-volcanic-gneissoid provenance accumulated in a variety of high- and low-energy alluvial environments. End_of_Article - Last_Page 791------------

Upper Jurassic to Lower Cretaceous(?) synorogenic sedimentary rocks in the southern Spring Mountains, Nevada

A newly recognized sequence of Upper Jurassic to Lower Cretaceous(?) terrigenous rocks in the Good-springs district, Nevada, was deposited during the emplacement of the Contact thrust plate. Two facies are recognized: (1) interbedded conglomerate and sandstone derived from Mesozoic igneous and terrigenous platform rocks and (2) interbedded carbonate and sandstone-clast conglomerate, quartz sandstone, and red shale. No igneous detritus occurs in the facies with carbonate-clast conglomerate. Carbonate clasts could only have been derived from the Paleozoic carbonate sequence, which was exposed in the area by latest Jurassic to earliest Cretaceous thrusting. The age of rocks from a volcanic unit within the synorogenic sequence was determined radiometrically to be 150 ± 10 m.y. (K-Ar on biotite). The sequence was deposited disconformably on deeply eroded rocks of the early Mesozoic platform and ultimately overridden from the west by the Contact thrust plate. Information from the sequence corroborates previously reported regional data regarding the timing and nature of the Contact-Red Springs thrust event.

Radiocarbon date for the largest extinct bird

IN December 1973 two fragments of an eggshell of the extinct giant elephant bird, Aepyornis sp., were obtained from a site 50 km west south-west of Fort Dauphin (21°1′S, 47°E), southern Madagascar. The bird, similar in appearance to, but not directly related to, the ostrich (Strathio camelus) was the largest ever to exist; the largest species had a height of 3 m, and laid eggs up to 35 cm long1. The site from which the small fragments were obtained is located in coastal sand dunes some 100 m from the shoreline. The immediate region is underlain with granitic rocks capped by igneous detritus. The climate is semi-arid with a mean annual rainfall of 500–600 mm. The vegetation consists of bush interspersed with many endemic succulents including the very tall, stalk-like plants of the Didiraeceae family2.

Application of Geochemistry to Stratigraphic Problems in Lower Cretaceous of Western Canada

The detrital sediments that form the Lower Cretaceous series of the Interior Plains of western Canada were sampled at close vertical intervals in fourteen widely spaced sections in Alberta and western Saskatchewan. The soda and potash contents of these samples were determined and then corrected to a carbonate-free basis. logs are plotted for each section and show that throughout the area studied the series can be separated into three stratigraphic divisions, each of distinctive soda content. The lower division is poor in soda. The middle division is principally composed of sediments that are rich in sodic igneous detritus, and which interfinger with low-soda strata in the eastern part of the area. The upper soda division consists of sediments that have a low to intermediate soda content. Samples of different grain size, such as sand and shale, share a distinctively similar soda content if they come from the same division or sediment suite. Thus chemical contacts may be traced from section to section without being affected by the lateral facies changes from sand to shale that are so common in the Lower Cretaceous. The potash content of the series is controlled by grain size variations to a much greater extent than soda, and thus this oxide is much less useful for stratigraphic correlation. Correlations made on the basis of the chemical data are related to the different stratigraphic units into which the series has been divided in different parts of the area. Compositional variation between the sediment suites is ascribed principally to varying provenance, rather than to influences within the depositional basin. Plots showing regional chemical variation within the middle division indicate a western derivation for the highly sodic detritus and an eastern derivation for the soda-poor sediments, with the two suites mixing in eastern Alberta and western Saskatchewan. The upper division shales show a consistent decrease in potash and an increase in soda away from the presumed western shoreline of the time. Chemical methods of correlating detrital sediments appear to be useful, particularly when applied to shales or to mixed assemblages that include shales. The rapid development of more efficient methods of analyzing rocks makes the application of geochemical methods to stratigraphic studies increasingly attractive.