The Lower Permian Wasp Head Formation,Sydney Basin: high‐latitude,shallow marine sedimentation following the late Asselian to early Sakmarian glacial event in eastern Australia

The Lower Permian Wasp Head Formation (early to middle Sakmarian) is a ～95 m thick unit that was deposited during the transition to a non‐glacial period following the late Asselian to early Sakmarian glacial event in eastern Australia. This shallow marine, sandstone‐dominated unit can be subdivided into six facies associations. (i) The marine sediment gravity flow facies association consists of breccias and conglomerates deposited in upper shoreface water depths. (ii) Upper shoreface deposits consist of cross‐stratified, conglomeratic sandstones with an impoverished expression of the Skolithos Ichnofacies. (iii) Middle shoreface deposits consist of hummocky cross‐stratified sandstones with a trace fossil assemblage that represents the Skolithos Ichnofacies. (iv) Lower shoreface deposits are similar to middle shoreface deposits, but contain more pervasive bioturbation and a distal expression of the Skolithos Ichnofacies to a proximal expression of the Cruziana Ichnofacies. (v) Delta‐influenced, lower shoreface‐offshore transition deposits are distinguished by sparsely bioturbated carbonaceous mudstone drapes within a variety of shoreface and offshore deposits. Trace fossil assemblages represent distal expressions of the Skolithos Ichnofacies to stressed, proximal expressions of the Cruziana Ichnofacies. Impoverished trace fossil assemblages record variable and episodic environmental stresses possibly caused by fluctuations in sedimentation rates, substrate consistencies, salinity, oxygen levels, turbidity and other physio‐chemical stresses characteristic of deltaic conditions. (vi) The offshore transition‐offshore facies association consists of mudstone and admixed sandstone and mudstone with pervasive bioturbation and an archetypal to distal expression of the Cruziana Ichnofacies. The lowermost ～50 m of the formation consists of a single deepening upward cycle formed as the basin transitioned from glacioisostatic rebound following the Asselian to early Sakmarian glacial to a regime dominated by regional extensional subsidence without significant glacial influence. The upper ～45 m of the formation can be subdivided into three shallowing upward cycles (parasequences) that formed in the aftermath of rapid, possibly glacioeustatic, rises in relative sea‐level or due to autocyclic progradation patterns. The shift to a parasequence‐dominated architecture and progressive decrease in ice‐rafted debris upwards through the succession records the release from glacioisostatic rebound and amelioration of climate that accompanied the transition to broadly non‐glacial conditions.     

Faceted garnets from sandstones of the Munising Formation (Cambrian), northern Michigan: petrographic evidence for their origin by intrastratal dissolution

Imbricate wedge marks (facets) on garnets in sandstones of the Cambrian Munising Formation of northern Michigan are associated with mouldic secondary porosity developed at the expense of garnet. Mouldic pores surrounding faceted garnets indicate that garnets in these sandstones have been affected by intrastratal dissolution (retreat of the mineral surface from its original boundaries) rather than by grain enlargement, which would be expected if garnet overgrowths had formed. The association of garnet facets with textural evidence of garnet dissolution proves that garnet facets form by intrastratal dissolution. These results confirm similar findings in other recent studies, and extend the geographic and stratigraphic range of proven occurrences of facet formation by intrastratal dissolution.     

Open System Magmatic Evolution of the Taos Plateau Volcanic Field, Northern New Mexico: 3. Petrology and Geochemistry of Andesite and Dacite

Calc-alkaline olivine andesite and two-pyroxene dacite of theTaos Plateau volcanic field evolved in an open magmatic system.mg-numbers of spatially and temporally associated ServilletaBasalt (54–61) and ohvine andesite (49–59) are comparableand preclude fractional crystallization of ferromagnesian mineralsas the major differentiation process. If Servilleta olivinetholeiite is assumed to be the parental magma type, enrichmentsof highly incompatible trace elements (up to 17 ?) oVer concentrationsin the basalts require that andesitic and dacitic magmas containa substantial proportion of assimilated crust. Isotopic compositionsof andesite and dacite, which have slightly higher ⁸⁷Sr/⁸⁶Srratios than the basalts but lower ¹⁴³Nd/¹⁴⁴Nd, ²⁰⁶Pb/²⁰⁴Pb,²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb ratios, are consistent with contaminationof parental basalt by old, low Rb/Sr, low U/Pb, and low Th/Pbcontinental crust. Concentrations of highly incompatible traceelements in andesite and dacite lavas are decoupled from majorelement compositions; the highest concentrat ions of these elementsoccur in andesitic, rather than dacitic compositions, and andesitelavas are more variable in trace element contents. Assimilationof heterogeneous crust concurrent with fractional crystallizationof varying mineral assemblages could cause this decoupled behavior.High mg-numbers in andesite and dacite, skeletal olivine phenocrysts,and reversely zoned pyroxene phenocrysts are manifestationsof mafic replenishment and magma mixing in the Taos Plateaumagmatic system. Taos Plateau volcanoes are monolithologic and are distributedin a semi-concentric zoned pattern that is a reflection of thecomplex subvolcanic magmatic system. A central focus of basaltshields developed above the main basaltic conduit system; thesemagmas contain 10–35% admixed andesitic and dacitic magma.Basalt shields are surrounded by a partial ring of olivine andesiteshield volcanoes, where replenishment of basaltic magma providedthe heat necessary for prolonged assimilation of crust, resultingin intermediate-composition lavas. Dacite shields are locatedaround the periphery of the more mafic volcanoes and reflecta decrease in mafic input on the fringes of the magmatic system.     

Isotopic and trace element evidence for submarine lithification of hardgrounds in the Jurassic of eastern England

Geochemical and petrographic data suggest early submarine cementation of hardgrounds from the Lincolnshire Limestone Formation, Middle Jurassic, England. The three hardgrounds, from Cowthick, Castle Bytham and Leadenham quarries, developed in tidal-inlet, on-barrier and lagoonal sub-environments of a carbonate barrier-island complex. At Cowthick early composite (acicular-bladed) radial-fibrous cements, which pre-date aragonite dissolution, completely fill intergranular pore-space at the hardground surface; away from it isopachous fringing cements decrease in thickness. Microprobe analyses demonstrate zoning within the fringes with magnesium concentrations (> 2 wt % MgCO₃) higher than those in allochems or later, ferroan cement (?0.5 wt % MgCO₃, 1.7 wt % FeCO₃). At Castle Bytham early granular isopachous cements, which post-date aragonite dissolution, occur within 5 cm of the surface. At Leadenham early lithification is superficial and represented by ferruginous crusts and micritic internal sediment. Late blocky cement fills residual pore-space in all three examples. Carbon and oxygen isotopic composition of whole-rock samples taken at intervals away from each hardground surface demonstrate the increasing proportion of late ¹⁸O depleted cements (δ¹⁸O – 8 to – 10). Early cements must have a marine isotopic composition; different δ¹⁸O values from each hardground reflect the intensity of early lithification and exclusion of late cements at the hardened surface. There is no isotopic evidence for subaerial cement precipitation during possible emergence at Castle Bytham. Oyster samples (with δ¹⁸O, – 2.9 and δ¹³C, 2.4) give estimated palaeotemperatures of 22–25°C. Early cements from Cowthick are enriched in ¹⁸O and ¹³C (δ¹⁸O = 0 δ¹³C ? 3‰) compared to the oyster values. In conjunction with trace element data this is interpreted as evidence for high-magnesium calcite precursor cements which underwent replacement in a system with a low water: rock ratio. The intensity of early lithification is related to depositional environment: maximum circulation of sea-water producing the most lithified hardground (Cowthick). This is directly analogous to the formation of Recent hardgrounds.     

NCGIA education activities: the core curriculum and beyond

Abstract

Education has been part of the NCGIA's mission from the earliest discussions of the concept of the Center at the National Science Foundation. To respond to the need for short-term solutions to the shortage of adequately trained personnel in GIS, the Center developed a set of teaching materials or core curriculum. The steps in its development are described and an analysis of initial distribution statistics is presented. Current efforts to develop a framework for laboratory materials are outlined. The paper ends with an assessment of the project and comparison with other disciplines.     

Intra- and extrabasinal controls on fluvial deposition in the Miocene Indo-Gangetic foreland basin, northern Pakistan

The Miocene Siwalik Group (upsection, the Chinji, Nagri, and Dhok Pathan Formations) in northern Pakistan records evolving fluvial systems within the Himalayan foreland basin. Sedimentological variations are evaluated with respect to local, regional, and global controls on fluvial deposition and basin filling. Thick (5 m to tens of metres) sandstones are composed of channel bar and fill deposits of low-sinuosity, meandering and braided rivers which formed large, low-gradient sediment fans (or ‘megafans'). River flow was dominantly toward the south-east. The proportion of thick sandstones varies in all Siwalik sections on three scales, and reflects similar variations in palaeochannel size and grain size: (1) small-scale variations are generally tens of metres thick, and reflect the alternation of thick sandstones (channel-belt deposits) and mudstone-dominated strata (overbank deposits) through the section; (2) medium-scale variations are roughly one-hundred to a few hundreds of metres thick, and primarily correspond to changes in channel-deposit thickness, but also to the degree of superposition of channel deposits and/or to changes in the number of channel-belt deposits per unit of section; and (3) large-scale variations (formation-scale) are greater than one km thick, and primarily correspond to changes in channel-deposit thickness. Time-scales of small-, medium-, and large-scale variations appear to be on the order of 10⁴, 10⁵ and 10⁶ years, respectively. The Chinji-Nagri transition is characterized by increases in channel-deposit proportion, sandstone thickness, palaeochannel size and discharge, mean grain size of sandstones, and sediment accumulation rates; and a decrease in avulsion period. The Nagri-Dhok Pathan transition is characterized by decreases in channel-deposit proportion, sandstone thickness, palaeochannel size and discharge, mean grain size of sandstones, and avulsion period; and a further increase in sediment accumulation rates. Formation boundaries across the Potwar Plateau decrease in age toward the west. The Chinji-Nagri transition ranges in age from ～ 10·9–12·7 Ma, and the Nagri-Dhok Pathan transition ranges in age from ～9·3–10·1 Ma. Small-scale variations are attributable to repeated river avulsions triggered by autocyclic processes and/or mountain-front tectonism (e.g. faulting, earthquakes). Medium-scale variations are attributable to local changes in the position of large sediment fans, also triggered by autocyclic processes and/or mountain-front tectonism. The Chinji-Nagri transition records the diversion or establishment (possibly due to river piracy) of a larger river system in the area. River diversion or piracy probably took place within the mountain belt and is attributable to increasing and spatially variable mountain-belt uplift rates, and possibly the development of associated mountain-front deformational structures. The Nagri-Dhok Pathan transition records the diversion of the larger river system out of the area and the establishment of a smaller river system. This diversion is attributable to progressively increasing rates of mountain-belt uplift and basin subsidence. The regional palaeoclimate throughout the time interval studied was apparently constant, and eustatic sea level changes apparently had no effect on deposition in the area.     

Crystal Accumulation and Magma Mixing in the Petrogenesis of Tholeiitic Andesites from Fukujin Seamount, Northern Mariana Island Arc

Fukujin Seamount is a large, active, submarine volcano on thevolcanic front in the northernseamount province (NSP) of theMariana island arc (MIA). Five dredge hauls from the summitand upper flanks of Fukujin recovered mainly highly porphyriticbasaltic andesites. A few nearly aphyric samples are medium-Ksiliceous andesites (SiO₂ = 62%, K₂O = 1•5%). Fukujin andmost other large arc-front volcanoes of the northern MIA havetholeiitic (iron-enrichment) fractionation trends. This contrastswith the calc-alkaline trends of many smaller seamounts. A negativecorrelation of modal plagioclase content with bulk-rock SiO₂,as well as bulk-rock major and trace element variation trends,and glass analyses, suggests that lavas with >30 vol.% phenocrystsand <55 wt.% SiO₂ are partial cumulates. The presence ofbimodal phenocryst populations along with reversed to normalzoning of phenocrysts is explained by magma mixing of andesiticand basaltic liquids. Hybrid basaltic andesites probably formedby the accumulation of plagioclase in a tholeiitic magma chamberundergoing replenishment and mixing at a shallow crustal level.A petrogenetic model is presented for the origin of basalticandesite by combined magma mixing and fractional crystallization.Aphyric siliceous andesites can be modelled by simple fractionationof basaltic andesite. The early fractionating assemblage consistedmainly of plagioclase and clinopyroxene, with lesser olivineand minor magnetite, but plagioclase remained suspended in themelt. The later fractionating assemblage was dominated by plagioclasewith orthopyroxene instead of olivine. ^*Present address: 2260 rue Panet, Montreal, Quebec, H2L 3A6, Canada.     

Zircon microstriators in the sand of auriferous Andean tills: fine tools and noble metals

Chemically inert and physically hard minerals, of which zircon is universally present and usually abundant, are minor but important components of glacial gold and tin placer deposits. Zircons and other much less common resistant minerals inflict major damage on light minerals, of which quartz is the dominant, chemically resistant member. Because of its sharpness inherited from a strong crystal morphology, and overall prismatic form, zircon is especially important as an abrasive mineral in glacial systems. Its chemically inert nature, its dominancc in terms of hardness over light minerals, and its abundance amongst other hard minerals makes it unique and important as a microstriator. Transported in a highly viscous glacial medium, it is capable of damaging other softer grains with aggressive crushing, chipping, striating, abrading and polishing processes. These occur in both coarse-grained gravelly sand and in fine-grained clayey silt matrices at the base of the icc. Zircon grains tend to serve many functions, initially as inclusion tools in larger feldspar grains and as 'studs' in quartzite grains. Wearing first on points, and later, following liberation, they assume a shape by honing, faceting and fracturing as tools and as grit that allows them to act as microstriators, inflicting damage on other particles in the basal ice. With a hardness of 7.5, lacking significant cleavage, and exhibiting strong crystal form. the finer-grained zircons appear to abrade and striate quartz (hardness 7.0). feldspars (hardness 6.0). garnets (hardness 6.5–7.5), and gold (hardness 2.5-3.0). A detailed study of Bolivian tills shows the dominant form of the zircon striator to be an elongate, pencil shape (euhedral polygonal prism with sharp, pyramidal terminations) that shows various degrees of abrasion, and ranges from wide grains with dull edges to narrow grains with sharp edges (typical pencil form).