Significant Hiatus Research Articles

Origin of atoll dolomites distinguished by geochemistry and crystal chemistry: Kita-daito-jima, northern Philippine Sea

Kita-daito-jima is a carbonate island located at the northwestern region of the Philippine Sea (25°55.6′–57.6′N, 131°16.9′–19.8′E). Dolomites extend from the island surface to a depth of 100 m below the ground surface (mbgs). Strontium isotope stratigraphy indicates that subsurface Units C1 (0–49.7 mbgs) and C2 (49.7–103.4 mbgs) were dolomitized at 5.5 Ma and 2.0 Ma, respectively, and that island-surface dolomites are products of dolomitization at 1.6–2.0 Ma. X-ray diffraction analysis indicates that the island-surface and borehole dolomites comprise variable mixtures of four and three dolomite crystal phases, respectively. Each of these phases is distinguished by a different Ca and Mg content. Three textural types can be recognized in the Kita-daito-jima dolomites, fabric-preserving crystalline nonmimetic (CNM), crystalline mimetic (CM), and fabric-preserving microsucrosic (MS). CNM dolomites contain more calcian phases, whereas MS dolomites commonly are richer in more stoichiometric phases. Backscattered electron images indicate that calcian dolomite phases were precipitated earlier than the more stoichiometric dolomite phases and that there is no significant hiatus between the phases, although they are diachronous. Both the island surface and borehole cores dolomites show linear relationships between whole-rock δ 18O composition and Mg contents and between whole-rock trace element concentrations (Sr and Na) and Mg contents. These covariances result from phase mixing, not stoichiometric effects. Deconvolution of whole-rock isotopic and elemental compositions based on the relative abundance of phases reveals that each phase has a distinct chemical and isotopic composition. Oxygen isotopic compositions of the island surface and borehole dolomites suggest that all dolomite phases formed in seawater and that dolomitization primarily occurred during glacio-eustatic sea-level lowstands and cooler ocean temperatures.

A stratigraphic network across the Subtropical Front in the central South Atlantic: Multi-parameter correlation of magnetic susceptibility, density, X-ray fluorescence and δ18O records

Abstract Computer aided multi-parameter signal correlation is used to develop a common high-precision age model for eight gravity cores from the subtropical and subantarctic South Atlantic. Since correlations between all pairs of multi-parameter sequences are used, and correlation errors between core pairs (A, B) and (B, C) are controlled by comparison with (A, C), the resulting age model is called a stratigraphic network. Precise inter-core correlation is achieved using high-resolution records of magnetic susceptibility κ, wet bulk density ρ and X-ray fluorescence scans of elemental composition. Additional δ18O records are available for two cores. The data indicate nearly undisturbed sediment series and the absence of significant hiatuses or turbidites. After establishing a high-precision common depth scale by synchronously correlating four densely measured parameters (Fe, Ca, κ, ρ), the final age model is obtained by simultaneously fitting the aligned δ18O and κ records of the stratigraphic network to orbitally tuned oxygen isotope [J. Imbrie, J. D. Hays, D. G. Martinson, A. McIntyre, A. C. Mix, J. J. Morley, N. G. Pisias, W. L. Prell, N. J. Shackleton, The orbital theory of Pleistocene climate: support from a revised chronology of the marine δ18O record, in: A. Berger, J. Imbrie, J. Hays, G. Kukla, B. Saltzman (Eds.), Milankovitch and Climate: Understanding the Response to Orbital Forcing, Reidel Publishing, Dordrecht, 1984, pp. 269-305; D. Martinson, N. Pisias, J. Hays, J. Imbrie, T. C. Moore Jr., N. Shackleton, Age dating and the orbital theory of the Ice Ages: development of a high-resolution 0 to 300.000-Year chronostratigraphy, Quat. Res. 27 (1987) 1-29.] or susceptibility stacks [T. von Dobeneck, F.Schmieder, Using rock magnetic proxy records for orbital tuning and extended time series analyses into the super-and sub-Milankovitch Bands, in: G. Fischer, G. Wefer (Eds.), Use of proxies in paleoceanography: Examples from the South Atlantic, Springer-Verlag, Berlin (1999), pp. 601-633.]. Besides the detection and elimination of errors in single records, the stratigraphic network approach allows to check the intrinsic consistency of the final result by comparing it to the outcome of more restricted alignment procedures. The final South Atlantic stratigraphic network covers the last 400 kyr south and the last 1200 kyr north of the Subtropical Front (STF) and provides a highly precise age model across the STF representing extremely different sedimentary regimes. This allows to detect temporal shifts of the STF by mapping δMn / Fe. It turns out that the apparent STF movements by about 200 km are not directly related to marine oxygen isotope stages.

The Dream Life of Ms. Dog: Anne Sexton's Revolutionary Use of Pop Culture

An exploration of Anne Sexton's examination of popular culture and her employment of commonly known contemporary references reveals her attempt to redefine the concept of culture. References to the popular film of Walter Mitty, suggestions of the comic strip Gasoline Alley, and verses from a quintessential pop song by Al Jolson (with significant hiatuses) create an alternative environment to the modernist classic of Eliot's "The Wasteland" with which Sexton creates a dialogue in her poem "Hurry Up Please, It's Time." Despite Sexton's awareness that she was perceived by others as lacking "taste," and despite her internalization of this deep criticism, she was determined to find her own way. Her use of the popular culture surrounding her—as opposed to the elitist culture of modernism—was part of her attempted redefinition of culture, her act of insurrection.

State Sovereign Immunity and Stare Decisis: Solving the Prisoners' Dilemma within the Court

This Comment argues that the liberal and conservative blocs on the U.S. Supreme Court are embroiled in a Prisoners' Dilemma with respect to whether they should follow precedent on the question of congressional abrogation of state sovereign immunity. The analytical consequence of this strategic situation within the Court is that, over the long run, all of the Justices would more fully realize their views of the merits of Eleventh Amendment cases by demonstrating more--not less--respect for the independent value of stare decisis. This Comment uses game theory to substantiate this claim, after which it offers a potential, contingent solution to the collective action problem that the Justices face. Most importanly, however, by focusing on the strategic dimension of stare decisis and identifying the long-term effects of the Court's tendency to overrule decisions when, and only because, a change in its composition empowers a new majority that thinks the precedent was wrongly decided, this inquiry brings into focus a significant hiatus in the current legal debate over the constitutional status of state sovereign immunity.

Jurassic dinoflagellate cyst stratigraphy of Hold with Hope, North-East Greenland

Dinoflagellate cysts of the Middle–Upper Jurassic succession on northern Hold with Hope have been studied in order to establish a biostratigraphic framework and to date the succession. The Pelion Formation is characterised by abundant Chytroeisphaeridia hyalina and Sentusidinium spp., with some Ctenidodinium thulium and Paragonyaulacysta retiphragmata in the lower part. Mendicodinium groenlandicum appears higher in the formation followed by Trichodinium scarburghense in the upper part. The succeeding Payer Dal Formation contains Scriniodinium crystallinum, Rigaudella aemula and Leptodinium subtile in the lower part and Dingodinium jurassicum and Prolixosphaeridium granulosum in the uppermost part. The Bernbjerg Formation contains abundant Sirmiodinium grossii and Gonyaulacysta jurassica. Adnatospahaeridium sp., Cribroperidinium granuligerum, Glossodinium cf. dimorphum and Scriniodinium irregulare appear in the lower part of the formation, followed by Avellodinium spp. in the highest part. The dinoflagellate cyst assemblages in the Pelion Formation indicate an Early–Late Callovian age (C. apertum – P. athleta Chronozones). This is supported by ammonites in the lower part of the formation, which refer to the C. apertum and P. koenigi Chronozones. A significant hiatus, from Late Callovian to Middle Oxfordian, is present between the Pelion Formation and the overlying Payer Dal Formation. The age of the Payer Dal Formation is Middle Oxfordian to earliest Late Oxfordian (C. tenuiserratum – A. glosense Chronozones). The Payer Dal Formation is conformably overlain by the Bernbjerg Formation of Late Oxfordian to possibly earliest Kimmeridgian age (A. glosense – P. baylei Chronozones). The A. glosense Chronozone is also documented by abundant ammonites in the lowermost part of the formation.

Geology and stratigraphy of forearc basin sediments in Hokkaido, Japan: Cretaceous environmental events on the north-west Pacific margin

Litho-, bio-, and chemostratigraphy of the Cretaceous forearc basin sediments exposed in Hokkaido, northern Japan allow a synthesis of the faunal, sedimentological, and environmental history of the north-west Pacific margin. Although the succession, named the Yezo Group, has yielded an abundant record of mid- to late Cretaceous invertebrates, monotonous lithologies of sandstone and mudstone, showing occasional lateral facies changes, have caused confusion regarding the lithostratigraphic nomenclature. Based on our wide areal mapping of the sequence, and analysis of litho- and biofacies, a new lithostratigraphic scheme for the Yezo Group is proposed. In ascending order, the scheme is as follows: the Soashibetsugawa Formation (Lower Aptian mudstone unit); the Shuparogawa Formation (Lower Aptian–lower Upper Albian sandstone-dominant turbidite unit); the Maruyama Formation (lower Upper Albian tuffaceous sandstone unit); the Hikagenosawa Formation (Upper Albian–Middle Cenomanian mudstone-dominant unit); the Saku Formation (Middle Cenomanian–Upper Turonian sandstone-common turbidite unit); the Kashima Formation (Upper Turonian–Lower Campanian mudstone-dominant unit); and the Hakobuchi Formation (Lower Campanian–Paleocene shallow-marine sandstone-conglomerate unit). In addition, we designate two further lithostratigraphic units, the Mikasa Formation (Upper Albian–Turonian shallow-marine sandstone-dominated unit) and the Haborogawa Formation (Middle Turonian–Campanian shelf mudstone/sandstone unit), which correspond in age to the shallower facies of the Saku and Kashima formations, respectively. Despite a lack of so-called “black shales”, because of siliciclastic dilution, our stratigraphic integration has revealed the horizons of oceanic anoxic events (OAEs) in the Yezo Group. The OAE1a horizon in the Soashibetsugawa Formation is characterized by a lack of foraminifers, macrofossils and bioturbation, and a prominent positive excursion of δ 13C org. A significant hiatus during the late Aptian and early Albian removed the OAE1b horizon. The OAE1c horizon in the Maruyama Formation shows a distinct negative excursion of δ 13C org with a concomitant high productivity of radiolarians. The OAE1d horizon in the middle part of the Hikagenosawa Formation consists of weakly laminated, pyrite-rich mudstone. Planktonic and calcareous benthic foraminifers are absent, whereas radiolarians are abundant above the OAE1d horizon. The mid-Cenomanian event (MCE) horizon is identified at the top of the Hikagenosawa Formation. Stepwise extinction of calcareous benthic foraminifers and a decrease in radiolarian diversity become apparent above the MCE horizon. In the study area, the OAE2 horizon has been well documented, and is placed in the middle part of the Saku Formation.

U-Th dating of Lake Lisan (late Pleistocene dead sea) aragonite and implications for glacial east Mediterranean climate change

U-series dating can be an effective means to obtain accurate and precise ages on Quaternary carbonates. However, most samples require a correction for U and Th in admixed detritus. This complication is often addressed through generation of U-Th isochrons, requiring analyses of several coeval samples. In addition, presence of water-derived (hydrogenous) Th in the carbonate can cause inaccuracies in isochron ages. This study reports a high-resolution U-series chronology of sediments deposited by Lake Lisan, the last glacial precursor of the Dead Sea. The strategy employed combines multiple measurements from a few stratigraphic heights and fewer analyses from many heights in a single described and measured section. The resulting chronology is based on ages at 22 heights in a ∼40-m-thick section covering the interval of ∼70–14 calendar ka BP. The effects of admixed detritus are evaluated using trace elements. Nearly pure aragonite samples, indicated by very low abundances of insoluble elements such as Nb and Zr, were found to contain hydrogenous Th, which causes the uncorrected U- 230Th age of a modern sample to be ∼2.5 ka. Nevertheless, accurate ages have been obtained by correcting for the detrital and aqueous interferences. The resulting ages are in stratigraphic order, and their accuracy is evidenced by consistency of Lisan Formation U-series and 14C ages with the coral-based calendar-radiocarbon age calibration. The U-Th ages provide a context to unravel the limnological history of Lake Lisan. Boundaries between the Lower, Middle, and Upper stratigraphic units correspond to the MIS 4/3 and 3/2 transitions, respectively. During MIS 2 and 4 the lake generally showed a stable two-layer configuration and a positive fresh-water balance, reflected by deposition of laminated aragonite-detritus. Dry intervals during MIS 2 and 4 are indicated by thick gypsum layers and an inferred depositional hiatus, which are temporally associated with Heinrich events H1 at ∼17 ka and H6 at ∼65 ka, respectively. During MIS 3 the lake level was unstable with intermittent dry periods indicated by abundant clastic layers and a significant hiatus between ∼43–49 ka. Clastic layers are associated with Dansgaard-Oeschger events during MIS 3, and indicate lake level declines during abrupt Northern Hemisphere warmings. Overall, the climate of the Eastern Mediterranean region shows a strong linkage to the Northern Hemisphere climate, with increasing lake size and stability during cold periods, and fluctuations and dessication during warmings and Heinrich events.

Lower Permian magmatism of the Iberian Chain, Central Spain, and its relationship to extensional tectonics

Abstract Late Variscan magmatism in the Iberian Chain (Central Spain) is recorded by the presence of both pyroclastic units and high-level intrusions (sills and dykes). This magmatism encompasses a variety of subalkaline igneous rocks, from basalt to rhyolite; andesitic rocks are, however, dominant. The pyroclastic units contain plant fossils and pollen that suggest an Autunian age, which is consistent with the available K-Ar radiometric age data (283–292 Ma) obtained for the hypabyssal intrusions. Earlier amphibole-rich andesite, rich in crustal xenoliths (metapelite, granitoid and quartzite fragments) and xenocrysts (e.g. garnet) suggest the occurrence of significant crustal assimilation in the petrogenesis of the intermediate magmas, whereas later intrusions (pyroxene-rich andesites, basalts and some rhyolites) lack any crustal xenoliths or xenocrysts. Crustal assimilation is also suggested by the radiogenic 87 Sr/ 86 Sr 290 and negative ɛNd 290 values for all the studied rocks. The cross-cutting relationships between the different rock types provide evidence for a multi-stage magmatic evolution, coeval with the post-orogenic transtensive evolution of the southern Variscan belt in Europe. A significant hiatus, covering the Middle Permian and most of the Upper Permian, separates this Lower Permian magmatism from the subsequent Triassic and Jurassic alkaline magmatism that represents different rifting events that affected the Iberian Chain, progressively thinning the Variscan crust as the Alpine cycle began.

Middle Permian fusulinoideans from the Moribu Formation in the Hida-gaien Tectonic Zone, Nyukawa Village, Gifu Prefecture, central Japan

This paper reports the following Middle Permian fusulinoideans from the Moribu Formation in the Hida-gaien Tectonic Zone, central Japan: Monodiexodina matsubaishi (Fujimoto), Monodiexodina sp. and Codonofusiella sp. These fusulinoideans clearly show that the lower part of the Moribu Formation is of Middle Permian age. It further suggests that a significant hiatus exists between the Moribu Formation and the underlying Arakigawa Formation.

Geological Setting of the Meadowbank Gold Deposits, Woodburn Lake Group, Nunavut

The Meadowbank gold deposits are hosted by a series of polydeformed and metamorphosed auriferous iron formations located in the Neoarchean Woodburn Lake group, western Churchill province, Nunavut. The supracrustal rocks in the Meadowbank area consist of a thick sequence of intermediate-composition volcanic rocks with intercalated iron formation, ultramafic volcanic rocks, and quartzite to quartz arenite. U-Pb geochronology gives an interpreted age of 2711 ± 3 Ma for the host strata at Meadowbank. The intermediate volcanic rocks are predominately volcaniclastic and have a geochemical composition that is consistent with an active continental margin setting. Iron formation deposition was coeval with the intermediate volcanism, as indicated by incorporation of volcanic detritus in the chemical precipitate. The geochemistry of the iron formation indicates that it precipitated from a fluid that at one time was at high temperature (>250°C), reducing, and likely acidic. Intercalated ultramafic rocks rarely show spinifex textures and have trace element ratios that are consistent with mantle plume-related undepleted mantle asthenosphere, likely recording episodic mantle upwelling associated with rifting in an active continental margin. The contact between the ultramafic volcanic rocks and the overlying quartz arenite is a disconformity marked by a quartz pebble conglomerate. However, similar geology on the structural hanging wall and footwall of the contact suggests that the conglomerate does not represent a significant hiatus in deposition, and is more likely a prograding terriginous siliciclastic unit. Three metamorphic events are recognized. The first, M1, is a cryptic greenschist facies event that is pre-D2. The second event, M2, is a mid-greenschist to amphibolite facies, syn-D2 event. M3 is the last event recognized, and is a post-tectonic greenschist facies event that is regional but inhomogeneous in extent, possibly reflecting thermal aureoles around post-tectonic 1.7 to 1.8 Ga Nueltin granites. The structural geology of the area is complex with four phases of deformation recognized, two of which had a significant effect on the geometry of the deposit. All of these regional events are interpreted as Paleoproterozoic in age. Relationships between deformation fabrics and mineralization, as well as the overall geometry of the mineralized envelopes, suggest that the deposit formed during syn- to late-D2. Superimposed on the mineralization are D4 structural elements that postdate gold mineralization. The main control on gold mineralization is replacement of magnetite by pyrrhotite and pyrite in high-strain corridors. The composition of amphiboles and chlorites associated with gold mineralization is remarkably consistent and shows no spatial or temporal variation, suggesting that it was buffered by the iron-rich nature of the host rocks. The bulk metasomatic effect on the intermediate volcanic rocks is characterized by the addition of K2O and the loss of CaO, Na2O, and MgO, with little variation in the total iron content. The elemental losses are likely the result of destruction of feldspars, stilpnomelane, and ferroactinolite, which are relatively common in the unmineralized rocks, and the formation of sericite, chlorite, and grunerite. Textural and timing relationships suggest that the mineralization is syn- to late-D2, and by inference M2. Fluid inclusions, chlorite, and arsenopyrite geothermometry all suggest that mineralization took place at about 325° to 375°C and 1.3 kbar pressure. These are lower P-T conditions than is indicated by M2 mineral assemblages, suggesting that the deposit formed during the waning stages of metamorphism. Strain partitioning due to the mechanical contrast of the iron formation preferentially localized dilational settings during D2 deformation. This allowed fluid influx, sulfidation of the iron formation, and resulting precipitation of gold to be concentrated in high-strain corridors. Geologic relations and geochronology collectively suggest that a Paleoproterozoic (ca. 1.8–1.9 Ga) deformation event was responsible for the introduction of gold into the Neoarchean supracrustal sequences.

Fragments of the earliest land plants.

The earliest fossil evidence for land plants comes from microscopic dispersed spores. These microfossils are abundant and widely distributed in sediments, and the earliest generally accepted reports are from rocks of mid-Ordovician age (Llanvirn, 475 million years ago). Although distribution, morphology and ultrastructure of the spores indicate that they are derived from terrestrial plants, possibly early relatives of the bryophytes, this interpretation remains controversial as there is little in the way of direct evidence for the parent plants. An additional complicating factor is that there is a significant hiatus between the appearance of the first dispersed spores and fossils of relatively complete land plants (megafossils): spores predate the earliest megafossils (Late Silurian, 425 million year ago) by some 50 million years. Here we report the description of spore-containing plant fragments from Ordovician rocks of Oman. These fossils provide direct evidence for the nature of the spore-producing plants. They confirm that the earliest spores developed in large numbers within sporangia, providing strong evidence that they are the fossilized remains of bona fide land plants. Furthermore, analysis of spore wall ultrastructure supports liverwort affinities.

Santa Bárbara Formation (Caçapava do Sul, southern Brazil): depositional sequences and evolution of an Early Paleozoic postcollisional basin

The Santa Bárbara Formation, located in southernmost Brazil, comprises red-colored conglomerates, sandstones, and siltstones. It was deposited in the Early Paleozoic under continental conditions in a restricted, strike-slip basin (Santa Bárbara basin) in response to the postcollisional stresses of the Brasiliano/Pan-African cycle (900–500 Ma). Five facies associations are recognized: alluvial fans, fan-delta front, lacustrine, sandy braided, and gravel bed braided river deposits. The application of sequence stratigraphy enables the recognition of three depositional sequences. The two basal sequences (I and II) represent a coherent depositional pattern, with axial fluvial and fan-delta systems that deposit northeastward and lateral contribution from alluvial fans. Such coarse deposits are composed mainly of metamorphic clasts derived from the erosion of ‘Caçapava high’, the eastern steep margin of the Santa Bárbara basin. Sequence III lies unconformably over the basal subunits and reflects the inversion of the axial systems, in that the paleocurrents of the gravel bed deposits systematically point south/southwestward. The alluvial fan deposits of Sequence III also suggest a tectonic rearrangement of the basin, with partial erosion of the basal sequences and the presence of granitoid fragments, which reflects a deeper denudation stage for the Caçapava high and possibly a significant hiatus at the base of Sequence III.

Milankovitch-scale multi-proxy records from fluvial sediments of the last 2.6 Ma, Pannonian Basin, Hungary

The continuous cores of two nearly 500 m deep boreholes, cutting Pleistocene fluvial sequences in the eastern part of the Pannonian Basin, Hungary have been studied. Magnetostratigraphy indicates that the sediment sequences span the last 2.6 Ma without significant hiatuses. Comparison of systematic variations in magnetic susceptibility with changes in grain size distribution revealed Milankovitch-scale cyclicity in sedimentation of 40 and 100 kyr cyclicity, which also shows a correspondence with the δ 18O variations of the ODP 677 site. Comparison of pollen and gastropod paleoecological data with the magnetic susceptibility proxies suggests that climate was a major allogenic control on the cyclicity. Zones of high magnetic susceptibility correspond closely with sandy and coarse silty parts of the borehole successions which largely coincide with pollen and molluscs indicating a warm climate. On the contrary zones with low magnetic susceptibility correspond to silts and clays, and in most cases coincide with cold climate indicator pollen and molluscs. Heavy mineral and XRD studies revealed that magnetic susceptibility reflects the absolute amount of detrital Fe-oxide minerals, mostly magnetite. Variations in the amount of magnetite are related to changes in heavy mineral content, which arise primarily from changes in sediment supply to the basin by rivers. The depositional model for the Pleistocene fluvial sediments suggests that sediment flux increased towards the basin during interglacials, due to the increased discharge and transport capacity of the rivers, while a decreased sediment supply to the distal parts of the basin occurred during glacials. A delay between erosion and storage of weathering products near to the source area and subsequent transportation and re-deposition of sediments in the basin interior setting is an important cause of this effect.

Pedogenesis and geochemical alteration of the Picture Gorge subgroup, Columbia River basalt, Oregon

Reddened, clay-rich horizons between basalt flows have historically been identified as ‘‘baked zones’’ or as zones of lateral groundwater movement. The reddening has been attributed to thermal or chemical alteration of the permeable portion of the basalt flow. Many of these red horizons are instead paleosols that record significant hiatuses in basalt flow emplacement. Paleosols were identified in the field on the basis of features such as horizonation, ped morphology, and root traces. Terrestrial gastropods, fossil plants, and peats also occur locally. Plots of weathering indices, such as base loss, are consistent with a pedogenic origin for these horizons. Ca, Na, Mg, Fe 21 , and P have been extensively lost from the A and Bt horizons of the paleosols, consistent with modern weathering patterns. K and Rb have been added to the upper portions of the profiles and lost in the lower portions of the profiles relative to the parent material, with Rb added disproportionately. Historically, this pattern of enrichment has been cited as evidence of K metasomatism. The apparent metasomatism can be explained using a new model in which there was extensive felsic volcanism contemporaneous with pedogenesis, and K and Rb were likely added by windborne ash. The apparent discordance between K and Rb contents of the paleosols can be explained by the presence of land plants because K is an important electrolyte and Rb does not serve any major biological role. This model could account for apparent K metasomatism in other paleosols following the advent of land plants and may be applicable to older paleosols as well.

Glaciers and Climate in the Andes between the Equator and 30° S: What is Recorded under Extreme Environmental Conditions?

Sublimation and melt disturb the environmental information obtained from ice core records in the Andes. In two case studies we demonstrate to what extent these post-depositional processes may remove major parts of the accumulated snow cover. Dark ash layers from the Tungurahua eruption changed the albedo of surface snow on Chimborazo glacier (6268 m, 1°30′ S,78°36′ W, Ecuador) between two ice core drilling campaigns and forced substantial melt. Re-distribution and washout of the chemical constituents shifted the concentration profiles obtained in December 1999 as compared to an equivalent core drilled in December 2000. The stable isotope records showed that approximately the water equivalent (weq) of an annual layer had melted, and that the percolating melt water penetrated within the firn layer to a depth of at least 16.5 m without refreezing. In the second example, from a site on the dry axis between the tropical and extra-tropical precipitation belts, significant loss of accumulated snow layers occurred by sublimation. A surface experiment at Cerro Tapado glacier (5536 m, 30°08′ S,69°55′ W, Chile) revealed that losses of ≈2 mm weq (≈5 mm snow) per day occurred during the dry period following the 1997/98 El Niño. This loss generally included the entire surface layer enriched in stable isotopes, and thus caused minimal disturbance of the isotopic signature (and hence climatic information) of the net accumulation, yet chemical constituents again experienced considerable changes in concentration. From annual layer counting and direct dating it is obvious that the major part of the accumulated ice on both glaciers is younger than 100 years; however, isotopic and chemical variations at least in the basal ice from Cerro Tapado clearly point to climate conditions different from the recent centuries. This evidence is supported by mass balance considerations derived from a glacier-climate model. The possibility of a third type of disturbance aside from sublimation and melting – in this case a significant hiatus in the environmental chronology – also deserves consideration for other icecore records from this region. Potential disruptions or discontinuities need to be carefully evaluated given the profound changes in climatic and glaciological conditions since the Last Glacial Maximum throughout Holocene times.

Southeastern margin of the middle Paleozoic shelf, southwesternmost Appalachians: Regional stability bracketed by Acadian and Alleghanian tectonism

Lower Devonian–Lower Mississippian(?) metasedimentary rocks in the southernmost Appalachian orogen near the southeast corner of Laurentia record a period of continental-margin stabilization bracketed by orogenic events best assigned to the early Acadian and earliest Alleghanian orogenies. These strata are in the Talladega belt, a far-traveled allochthon forming the southwestward extent of the western Blue Ridge. In the allochthon, lower Paleozoic trailing-margin carbonate-shelf rocks are unconformably overlain by a >2.5-km-thick early Acadian (Silurian(?)–Lower Devonian) clastic wedge, derived from basement-cored uplifts to the south and southeast. Deposition of this clastic wedge was followed by continental-margin stabilization and deposition of a starved-basin sequence containing widespread Lower Devonian–Lower Mississippian(?) chert near the outer shelf margin. The chert sequence grades northeastward through a shelf–slope–basin-plain transition into a thick black-shale basinal facies. These units were deposited during the classic Acadian orogeny in New England and the Catskill delta's progradation across the central Appalachian foreland—the major clastic response to Acadian uplift. The Talladega sequence subsequently was subjected to low-grade regional metamorphism and pervasive deformation, possibly as late as middle Mississippian, during what were probably the initial stages of the Alleghanian orogeny. Deposition of the Lower Devonian–Lower Mississippian(?) sequence of the Talladega belt thus represents a significant hiatus in deformation along southeast Laurentia, bracketed first by orogenic activity related to early Acadian (Silurian(?)–Early Devonian) foundering of the early Paleozoic shelf and deposition of a clastic wedge and later by earliest Alleghanian regional dynamothermal events.

The loess profile at Albertirsa, Hungary—improvements in loess stratigraphy by luminescence dating

A very detailed Upper Pleistocene record is exposed in a loess/palaeosol sequence at the section at Albertirsa in Hungary. Luminescence dating was carried out to establish a more reliable chronological framework for the Hungarian loess record. The lowermost palaeosol of the section, probably an equivalent of the MF2 horizon at the Mende key section, is superimposed on penultimate glacial loess. An IRSL age estimate of about 65 ka BP was determined for the lower part of this palaeosol, indicating soil formation late in oxygen isotope stage (OIS) 5 or early in OIS 3. A significant hiatus most likely occurs in the lower part of the lower fossil soil. The loess between the upper and and lower palaeosol yielded a mean luminescence age of about 50 ka BP. The upper palaeosol is probably an equivalent of the MF1 horizon at the Mende type section, formed between 37 and 25 ka BP, and so this palaeosol represents the Hengelo and/or Denekamp interstadial of northwestern European stratigraphy. The loess is intercalated by a humic horizon, which yielded a mean luminescence age estimate between 21 and 19 ka at the Albertirsa section.

ABSTRACT: Sequence Stratigraphy and Seismic Stratigraphy of the Lower Cretaceous Section in the Northeastern Gulf of Mexico

ABSTRACT The Lower Cretaceous (LK) section in the offshore Alabama and Mississippi area has been a petroleum target since the 1970's in the Main Pass and the Viosca Knoll areas. To date, no comprehensive stratigraphic analysis of this section in these locations has been published. This work focuses on the characterization of the sequence stratigraphy and seismic stratigraphy of the LK strata offshore Alabama and Mississippi. Approximately 3,500 kilometers of 2D multi-channel seismic reflection data are interpreted and integrated with gamma ray, lithologic well log descriptions, and well log lithostratigraphic picks from more than 50 wells, and with 527 meters of core from wells in the study area and surrounding areas. Seven check-shot surveys are used to integrate the seismic and well log data, and software for PC is used to integrate and interpret the various data sets. In the study area, the LK section is composed mostly of carbonates with minor siliciclastic beds that were deposited in inner-middle shelf, shelf margin, slope, and basin environments. Diagnostic well log signature patterns were associated with different paleoenvironments. Associations of seismic facies were utilized to recognize the various depositional settings. Eight major third-order depositional sequences were recognized. This section overlies a Lower Cretaceous (Valanginian) unconformity associated with a significant hiatus and is overlain by an Upper Cretaceous (mid-Cenomanian) unconformity associated with a deposition hiatus of more than 10 million years in the Main Pass 253#6 well. Highstand systems tract and lowstand systems tract deposits are generally the only strata seismically recognizable. Transgressive systems tract deposits consist of a single seismic cycle or are not resolvable. The shelf margin, characterized by discontinuous skeletal reefs in the carbonate sequences, shifted basinward during the Early Cretaceous. The shelf edge experienced greater subsidence with respect to the back-reef and middle shelf areas. Tectonic activity, probably associated with salt movement, was restricted to the shelf margin and slope settings.

Internal structure of a trough blowout, determined from migrated ground‐penetrating radar profiles

Ground‐penetrating radar (GPR) was used to investigate the relationship between the geomorphological development of a large aeolian trough blowout and the stratigraphy and internal sedimentary structure of its associated deposits. Although analogous, many of the data‐processing techniques routinely applied in seismic reflection are very rarely applied in GPR studies. In this study, a simple migration program was used that significantly enhanced the quality of GPR images from a large trough blowout at Raven Meols on the Sefton coast, northwest England. These improvements aided subsequent data interpretation, which was achieved through application of the principles of radar stratigraphy. GPR shows the pre‐blowout dunes to have a complex internal structure that suggests they were formed in the presence of at least a partial vegetation cover. Subsequent to stabilization of these dunes a thin soil developed. This dune soil forms an important radar sequence boundary and delineates a complex topography beneath the depositional lobe of the blowout. The internal structure of the depositional lobe of the blowout does not conform to a model of simple radial foreset deposition, as derived from contemporary process studies reported in the literature. Instead, the pattern of deposition has been extensively modified by the antecedent dune topography and by varying spatial and temporal exposure to important sand‐transporting winds that is partly controlled by interactions between the regional wind pattern and local dune morphology. Trough blowout deposits in coastal aeolian sedimentary sequences are likely to be recognized by the presence of laterally continuous packets of relatively high‐angle cross‐strata, which often display a spatially‐variable radial dip pattern that is only very poorly or partially developed. In addition, a soil, or other surface representing a significant hiatus in dune deposition, is likely to underlie the blowout deposits, the topography of which will show a clear relationship to the dip and orientation of the overlying cross‐strata.

State Sovereign Immunity and Stare Decisis: Solving the Prisoners' Dilemma within the Court

This Comment argues that the liberal and conservative blocs on the U.S. Supreme Court are embroiled in a Prisoners' Dilemma with respect to whether they should follow precedent on the question of congressional abrogation of state sovereign immunity. The analytical consequence of this strategic situation within the Court is that, over the long run, all of the Justices would more fully realize their views of the merits of Eleventh Amendment cases by demonstrating more--not less--respect for the independent value of stare decisis. This Comment uses game theory to substantiate this claim, after which it offers a potential, contingent solution to the collective action problem that the Justices face. Most importanly, however, by focusing on the strategic dimension of stare decisis and identifying the long-term effects of the Court's tendency to overrule decisions when, and only because, a change in its composition empowers a new majority that thinks the precedent was wrongly decided, this inquiry brings into focus a significant hiatus in the current legal debate over the constitutional status of state sovereign immunity.