Diagenetic density inversions and the deformation of shallow marine chert beds in Israel

ABSTRACT Chert folds and 'dyke'-like boudinage in the Senonian Mishash Formation of Israel have orientations consistent with contemporaneous tectonic patterns whereas their amplitudes are anomalously high relative to strains calculated for adjacent layers. We suggest several means by which density inversions associated with chert diagenesis might amplify gentle structures whose wavelengths and orientations were determined by regional stresses.     

FORMATION OF LARGE-SCALE CROSS-BEDDING IN A CARBONATE UNIT

The type Lindsey Bridge Member of the Moorefield Formation of northeastern Oklahoma consists of 24 ft. of massively cross-bedded limestone. Cross-bed shape, lithologic variation, grain size and sorting, distribution of insolubles, and distribution of fossils and fossil burrows can be explained with reference to a hydrodynamic model developed in recent flume studies. Three facies can be distinguished in this unit: (1 Thei) foresets, thick-bedded, well-sorted, fine to medium crinoidal grainstones, dipping at angles up to 1° (2) toesets, which are thin-bedded, poorly sorted, skeletal packstones notably more fossiliferous than the foresets, with which they are laterally gradational; toesets dip at approximately 5°-8° (3) bottomsets, composed of argillaceous, fine-grained (mainly silt-size), skeletal limestones. Foresets overlie previously deposited bottomsets; this geometry is typical of regressive sedimentation. The exposure is adjacent to a pre-Moorefield topographic high. As currents crossing this high entered a basin on the downcurrent side, flow separation occurred. Bed material load was deposited mainly on the foreset slope, suspension material mainly in toeset and bottomset areas. The poor sorting of the toesets is in part due to reverse circulation, formed by the flow separation, which transported bottomset sediment back toward the foreset. Jopling (1965b) has shown that this depositional geometry produces tangential cross-beds similar to those seen in this outcrop. Differential settling velocity, substrate stability, and abundance of organic detritus influenced other sedimentologic properties of the deposit.     

Cosmic‐ray exposure history of the Norton County enstatite achondrite

Abstract– We report measurements of cosmogenic nuclides in up to 11 bulk samples from various depths in Norton County. The activities of ³⁶Cl, ⁴¹Ca, ²⁶Al, and ¹⁰Be were measured by accelerator mass spectrometry; the concentrations of the stable isotopes of He, Ne, Ar, and Sm were measured by electron and thermal ionization mass spectrometry, respectively. Production rates for the nuclides were modeled using the LAHET and the Monte Carlo N‐Particle codes. Assuming a one‐stage irradiation of a meteoroid with a pre‐atmospheric radius of approximately 50 cm, the model satisfactorily reproduces the depth profiles of ¹⁰Be, ²⁶Al, and ⁵³Mn (<6%) but overestimates the ⁴¹Ca concentrations by about 20%. ³He, ²¹Ne, and ²⁶Al data give a one‐stage cosmic‐ray exposure (CRE) age of 115 Ma. Argon‐36 released at intermediate temperatures, ³⁶Ar_n, is attributed to production by thermal neutrons. From the values of ³⁶Ar_n, an assumed average Cl concentration of 4 ppm, and a CRE age of 115 Ma, we estimate thermal neutron fluences of 1–4 × 10¹⁶ neutrons cm^?2. We infer comparable values from ε¹⁴⁹Sm and ε¹⁵⁰Sm. Values calculated from ⁴¹Ca and a CRE age of 115 Ma, 0.2–1.4 × 10¹⁶ neutrons cm^?2, are lower by a factor of approximately 2.5, indicating that nearly half of the ¹⁴⁹Sm captures occurred earlier. One possible irradiation history places the center of proto‐Norton County at a depth of 88 cm in a large body for 140 Ma prior to its liberation as a meteoroid with a radius of 50 cm and further CRE for 100 Ma.     

Minimum Bedrock Exposure Ages and Their Implications: Larsemann Hills and Neighboring Bolingen Islands, East Antarctica

<正>Considerable controversy exists over whether or not extensive glaciation occurred during the global Last Glacial Maximum(LGM) in the Larsemann Hills.In this study we use the in situ produced cosmogenic nuclide ~(10)Be(half life 1.51 Ma) to provide minimum exposure ages for six bedrock samples and one erratic boulder in order to determine the last period of deglaciation in the Larsemann Hills and on the neighboring Bolingen Islands.Three bedrock samples taken from Friendship Mountain(the highest peak on the Mirror Peninsula,Larsemann Hills;～2 km from the ice sheet) have minimum exposure ages ranging from 40.0 to 44.7 ka.The erratic boulder from Peak 106(just at the edge of the ice sheet) has a younger minimum exposure age of only 8.8 ka.The minimum exposure ages for two bedrock samples from Blundell Peak(the highest peak on Stornes Peninsula,Larsemann Hills;～2 km from the ice sheet) are about 17 and 18 ka.On the Bolingen Islands(southwest to the Larsemann Hills;～10 km from the ice sheet),the minimum exposure age for one bedrock sample is similar to that at Friendship Mountain(i.e.,44 ka).Our results indicate that the bedrock exposure in the Larsemann Hills and on the neighboring Bolingen Islands commenced obviously before the global LGM(i.e.,20-22 ka),and the bedrock erosion rates at the Antarctic coast areas may be obviously higher than in the interior land.     

Ayers Rock

For years the world's most famous inselberg was plagued by the deleterious effects of human activity. Now a massive investment, stimulated by the international tourist industry, has ensured preservation of the rock's unique geomorphological features and its fragile ecology.     

Generation of Porphyritic and Equigranular Mafic Enclaves During Magma Recharge Events at Unzen Volcano, Japan

Mafic to intermediate enclaves are evenly distributed throughoutthe dacitic 1991–1995 lava sequence of Unzen volcano,Japan, representing hundreds of mafic recharge events over thelife of the volcano. This study documents the morphological,textural, chemical, and petrological characteristics of theenclaves and coexisting silicic host lavas. The eruptive productsdescribed in this study appear to be general products of magmamingling, as the same textural types are seen at many othervolcanoes. Two types of magmatic enclaves, referred to as Porphyriticand Equigranular, are easily distinguished texturally. Porphyriticenclaves display a wide range in composition from basalt toandesite, are glass-rich, spherical and porphyritic, and containlarge, resorbed, plagioclase phenocrysts in a matrix of acicularcrystals and glass. Equigranular enclaves are andesitic, non-porphyritic,and consist of tabular, medium-grained microphenocrysts in amatrix glass that is in equilibrium with the host dacite magma.Porphyritic enclaves are produced when intruding basaltic magmaengulfs melt and phenocrysts of resident silicic magma at theirmutual interface. Equigranular enclaves are a product of a moreprolonged mixing and gradual crystallization at a slower coolingrate within the interior of the mafic intrusion. KEY WORDS: mafic enclaves; quenched mafic inclusions; magma mingling; Unzen volcano; Unzen Scientific Drilling Project; resorbed plagioclase     

Residence, Resorption and Recycling of Zircons in Devils Kitchen Rhyolite, Coso Volcanic Field, California

Zircons from the Devils Kitchen rhyolite in the PleistoceneCoso Volcanic field, California have been analyzed by in situPb/U ion microprobe (SHRIMP-RG) and by detailed cathodoluminescenceimaging. The zircons yield common-Pb-corrected and disequilibrium-corrected²⁰⁶Pb/²³⁸U ages that predate a previously reported K–Arsanidine age by up to 200 kyr, and the range of ages exhibitedby the zircons is also approximately 200 kyr. Cathodoluminescenceimaging indicates that zircons formed in contrasting environments.Most zircons are euhedral, and a majority of the zircons areweakly zoned, but many also have anhedral, embayed cores, witheuhedral overgrowths and multiple internal surfaces that aretruncated by later crystal zones. Concentrations of U and Thvary by two orders of magnitude within the zircon population,and by 10–20 times between zones within some zircon crystals,indicating that zircons were transferred between contrastingchemical environments. A zircon saturation temperature of 750°Coverlaps within error a previously reported phenocryst equilibrationtemperature of 740 ± 25°C. Textures in zircons indicativeof repeated dissolution and subsequent regrowth are probablycaused by punctuated heating by mafic magma input into rhyolite.The overall span of ages and large variation in U and Th concentrations,combined with calculated zircon saturation temperatures andresorption times, are most compatible with crystallization inmagma bodies that were emplaced piecemeal in the crust at Cosoover 200 kyr prior to eruption, and that were periodically rejuvenatedor melted by subsequent basaltic injections. KEY WORDS: zircon geochronology; residence time; rhyolite; ion microprobe; California     

Inter- and supratidal sedimentology of a fjord-head estuary, south-western British Columbia

Detailed examination of inter- and supratidal delta and floodplain sediments exposed in eroding bank sections at 52 locations along the Squamish River estuary provides the basis for recognizing seven distinct facies within the 5500 m-long estuary. Estuary sedimentation is initially driven by the development of sand bar complexes along the seaward edge of the intertidal delta. Sedimentation continues within interdistributary bay environments as intertidal sandflats and then tidal marshes develop, Aggradation of the delta within interdistributary bay environments results in a gradual transition from delta to alluvial plain. Of the seven facies identified, only the intertidal sands and tidal marsh deposits provide evidence of their tidal origin. Examination of deposits throughout the riverine estuary reveals a number of gradual yet distinct changes of sediment size, structure, and sequence architecture. These trends record the changing nature of tidal and riverine control on sedimentation along the tidal gradient. Generally, with increasing distance up-estuary, sediment grain-size increases, the thickness of fine-grained overbank deposits decreases, and bedding changes from fine parallel bedding to higher energy bedforms. In addition, fining-upward successions become capped by coarser sands, facies contacts change from gradational to abrupt and occasionally erosional, and facies successions become increasingly complex and less predictable. Squamish River estuary has been divided into four zones based on sedimentological and stratigraphical evidence, each zone reflecting changes in the relative influence of tidal and riverine control on sedimentation. Each zone contains distinctly different facies sequences, although zone boundaries generally are gradational.