Meteor Zone Research Articles

Petrogenesis of Cenozoic Temperate Water Carbonates, South Australia: Model for Shallow-Burial or Meteoric Diagenesis of Phanerozoic Shallow Water Calcite Sediments: ABSTRACT

The Gambier Limestone is one of several extensive, shallow water, shelf carbonates of Eocene to Miocene age exposed along the southern margin of Australia. It is a muddy to grainy bryozoan calcarenite, with accessory benthonic foraminifers and echinoids. The sediments, originally composed almost entirely of calcite or Mg-calcite, have been in vadose and phreatic environments for over 10 m.y., yet are virtually unlithified. The only cements of any consequence are epitaxial on echinoids. Numerous karst features, dolines, caves, speleothems, and surface karren attest to prolonged residence in the meteoric zone. The Gambier is presently one of the best freshwater aquifers in Australia, with most flow intergranular and through sediments with over 30% porosity. Cementation is by minor intergranular pressure solution, which has developed under overburden of less than 100 m. The overlying Naracoorte Limestone (Miocene), a calcarenite of warmer water aspect, contains numerous aragonite molds and is cemented marble-hard. The authors propose that such temperate-water calcite limestones are a better model for the meteoric diagenesis of calcite sediments of all ages than aragonite-rich tropical sediments. It is probable that many similar early and middle Paleozoic calcite limestones may have been in the meteoric zone for prolonged periods, yet contain little more » or no petrographic or geochemical record of such exposure. « less

On the Correlation of the Midlatitude Lower Ionosphere with Solar and Meteorological Parameters during the MAP/WINE Winter of 1983/1984

The behaviour of the midlatitude lower ionosphere (Central Europe, USSR, Spain) is studied for the winter of 1983/84. The lower ionosphere is found to be governed by neutral atmosphere variability. The ionospheric data are divided into two groups according to altitude (not latitude or longitude). The principal component analysis method shows that the opposite response to the meteor zone (∼ 95 km) meridional and partly also zonal wind variability appears to be responsible for this division. There is a negative correlation of ionospheric data and zonal wind (∼ 95 km) with the 10 hPa temperature, stronger for the North Pole than for the quasi-local Berlin temperature. This seems to be caused mainly by longer-term trends, i.e., by the fact that the polar 10 hPa temperature reflects well the global behaviour of polar vortex.

Petrology of Upper Mississippian Denmar Formation, Greenbrier Group, East-Central West Virginia: ABSTRACT

The Denmar Formation of Randolph County is a package of interbedded marine carbonates and siliciclastics deposited on a broad subsiding carbonate platform. Seven major microfacies are recognized: terrigenous siltstone, dolomitized micrite, pelmicrite, biopelmicrite, quartzose pelmicrite, oosparite, and oomicrite. These microfacies characterize three major paleoenvironments: supratidal, intertidal, and shallow subtidal. Four major diagenetic realms are noted: the low supratidal flat containing evaporites and dolomite; the meteoric zone containing blocky calcite spar; the shallow marine phreatic zone with marine cement and micritized fossil fragments; and the burial zone containing mature stylolites. The Denmar Formation represents a sequence of alternating low-energy, shallowing-upward pelmicrite-mudstone and higher energy, shallowing-upward oosparite-grainstone shelf cycles. The pelmicrite-mudstone cycles are mainly composed of intertidal and supratidal sediment. The oosparite-grainstone cycles are mainly composed of low and higher energy subtidal sediments. Diagenesis occurred in both the marine and meteoric zones. Paleoenvironmental reconstruction of east-central West Virginia during deposition of the Denmar is attempted. End_of_Article - Last_Page 1449------------

Wave motions in the lower thermosphere during the Energy Budget Campaign 1980 from three locations in the northern Hemisphere

Simultaneous meteor radar wind observations were made at Budrio (Italy, 45° N), Sheffield (U.K., 53° N) and Stornoway (U.K., 58° N) during the Energy Budget Campaign (EBC), November–December 1980, in the altitude region between 75 and 115 km. Wind results are presented in terms of prevailing components and oscillations of different time scales: tidal (8, 12 and 24 h), gravity ((3÷6) h) and planetary ((2÷10) days) waves. The seasonal variation of the prevailing zonal components observed at the three stations in previous years shows the same maxima of Summer and Winter, as well as the equinoctial minima exhibited by the CIRA 1972 model for 50° N. The data show marked tidal activity with the semi-diurnal tide which exceeds in amplitude any other mode in the wind spectra. Tidal amplitudes recorded at Budrio appear significantly lower than those recorded at Sheffield and Stornoway. Variations of the amplitude of maxima of the semi-diurnal oscillation relative to the Budrio and Sheffield data show time scales generally greater than 5 days throughout the recording period. The diurnal and terdiurnal tides represent a small part of the total wind energy in the meteor zone and exhibit phases which disagree strongly at the quoted stations. The scatter in the hourly data is representative of small-scale wind disturbances due to internal gravity waves: at Budrio, by using the MEM method for individual day's data (November 23–24, 28–29, 1980), gravity waves with vertical wave-lengths of (20÷45) km, periods of 3.5, 4.5 and 6 h, and amplitudes of the order of 10 ms−1 have been observed. Long-period oscillations with periods in the (2÷10) day time interval have been identified in the wind patterns at Budrio and Sheffield, but the presence of planetary waves of quasi-2-day period was less significant than during a previous Summer 1980 run carried out at the same stations.

Chemical Diagenesis of Pennsylvanian Brush Creek (Pennsylvania) Carbonate Components: Trace Elements: ABSTRACT

The various (low-Mg calcite, intermediate-Mg calcite, high-Mg calcite, and aragonite) carbonate components from the Pennsylvanian Brush Creek Formation of Pennsylvania are preserved in different stages of diagenetic alteration. In general, these components follow the predicted diagenetic changes in structure, mineralogy, and chemistry deduced from theoretical considerations for progressively altered carbonates. The low-Mg calcite brachiopods show no signs of either structural or chemical alteration. The shell material is preserved as low-Mg calcite fibers with no apparent dissolution and/or infilling by diagenetic calcite. Also, the average Sr2+ content of the brachiopods is 820 ppm, which is in agreement with the chemical content of their Holocene counterparts. In contrast, the intermediate-Mg calcite rugose corals show signs of structural aggrading neomorphism. The trabecular fibers are in part recrystallized to small mosaic calcite grains. This structural alteration is concomitant with chemical changes in the Brush Creek rugose corals. The least-altered components contain about 1,770 ppm Sr2+, whereas the most altered components contain only about 1,030 ppm Sr2+ /SUP>. For the high-Mg calcite crinoids, diagenetic alteration is mostly a cementation process with minor mineralogical alteration. The open meshwork structure typical of the Echinodermata is infilled in the Brush Creek crinoids by diagenetic cement. This infilling cement has decreased the average Sr2+ content of 2,140 ppm of unaltered crinoids to that of 1,090 ppm Sr2+ for the most-altered Brush Creek crinoids. Scanning electron microscope analysis of the originally aragonitic mollusks (gastropods, pelecypods, and cephalopods) shows a complete structural diagenetic transition series. The original and least-altered mollusk material is preserved as nacre, which is always aragonite. The second phase of the transition series in the mollusks is represented by the aggrad ng neomorphism of the nacreous tablets into small, coarse mosaic calcite crystals. The structural transition is completed by the replacement of the mosaic calcite by coarse calcite spar. Mineralogically, the mollusk material changes from aragonite to aragonite-calcite to calcite, relative to the least- and to the most-altered specimens, respectively. The structural and mineralogical changes of the originally aragonitic mollusks are also confirmed by changes in their overall chemical composition. Average Sr2+ values measured for the least-altered Brush Creek mollusk material is 4,470 ppm. Mollusk material of the second phase of the diagenetic transition series contains on average about 2,170 ppm Sr2+. The most-altered mollusks, which are calcite, contain on average a out 1,110 ppm Sr2+. Similarily, the Na+ values follow the diagenetic trend of strontium. The least-altered material contains 750 ppm Na+, the intermediate-altered material contains 420 ppm Na+, and the most-altered material contains 250 ppm Na+. Thus the diagenetic alteration process and rate proceed in accordance with mineralogical stability. This sequence is aragonite, high-Mg calcite, intermediate-Mg calcite, and low-Mg calcite relative to fastest to slowest reaction, respectively. The diagenetic alteration and preservation process of the Brush Creek carbonate components is probably a two-stage event. The first stage occurs in the marine phreatic zone, and the second stage occurs in the meteoric phreatic zone. End_of_Article - Last_Page 520------------

Distinguishing Diagenetic Environments of Equant Calcite Cementation: Example from Lower Cretaceous Pearsall Formation in South Texas: ABSTRACT

Equant calcite is the most common cement in limestones. It occludes more pore space, and hence has more control over reservoir quality than any other carbonate cement. It is important, therefore, to be able to delineate the environments of precipitation of equant calcite cements. The Lower Cretaceous Pearsall formation in south Texas has a history of equant calcite cementation that began early in the shallow-subsurface meteoric environment and is continuing today in the deep-subsurface basinal environment. Trace-element analysis of Mg+2 and Fe+2 along with morphological characteristics of the Pearsall equant calcite cements delineate environments of precipitation. Cements precipitated in the early, shallow-subsurface meteoric environment are generally very fine to medium-crystalline equant (less commonly bladed) calcites. The very fine to fine-crystalline calcite cement generally rims grains and is gradational into the medium-crystalline equant calcite cement that fills intergranular and moldic porosity. Early cements commonly have irregular crystal boundaries as seen in thin section. Late, deep-subsurface precipitated cements are generally coarse to very coarse-crystalline equant calcites. Commonly they have straight crystal boundaries and form one or several large crystals in a pore space. There is generally a sharp change in crystal size with the fine to medium-crystalline equant calcites precipitated in the shallow-subsurface meteoric zone. Trace-element analysis (electron microprobe) shows a statistically valid difference of Mg+2 and Fe+2 content between the early and late equant calcite cements. The early calcite cements have a higher Mg+2 content (1.8 ± 0.3 mole % MgCO3) and a lower Fe+2 content (785 ± 184 ppm) than the late calcite cements (1.3 ± 0.3 mole % MgCO3 and 2.618 ± 1,952 ppm Fe+2). Also, semiquantitative probe analysis, due to low count rate, indicates that the early calcite cement is richer in Sr+2 (738 ± 286 ppm) than the late calcite cement 472 ± 189 ppm). Magnesium in the fine-crystalline rim cement shows a pronounced trace-element distribution pattern. In the incipient crystal growth next to the grain, a high Mg+2 peak usually occurs, followed by a decrease in Mg+2 in the late rim cement and coarser equant calcite cement. This initial high Mg+2 peak is attributed to early meteoric diagenesis as described by Benson in 1974 in meteoric cements from Barbados. The Fe+2 trace-element distribution pattern shows an opposite trend from that of the magnesium. The early cements show low Fe+2, whereas the late cements are Fe+2 rich, indicating a reducing environment of precipitation for the late cements. Early, shallow-subsurface equant calcite cements can be distinguished from late, deep-subsurface equant calcite cements by relative position in pores, crystal size, straightness of crystal boundaries, gradation between crystal sizes, and by trace-element content and distribution patterns. These parameters may be valid environmental indicators of equant calcite precipitation in other limestone formations. End_of_Article - Last_Page 506------------

Prevailing Wind in the Meteor Zone (80–100 km) over Atlanta and its Association with Midwinter Stratospheric Warming

Abstract The wind data generated by an all sky, continuous wave radio meteor wind facility at Atlanta (34°N, 84°W) is analyzed over the period of August 1974 through July 1975. Zonal and meridional components of the prevailing wind over the height range of 80–100 km, at 2 km intervals represent 5–10 day averages where the tidal components have been removed. Large southerly wind during winter and weak northerly wind during summer at 80–100,km altitude is consistent with other observations and mesospheric circulation models. Various phases of the 1974–75 midwinter stratospheric warming, including a pre-warming pulse during the second half of November 1974, are indicated to affect the prevailing Wind in the meteor zone over Atlanta in a consistent manner, by making use of the latitudinal and vertical compensation of temperature and also the movement of pressure systems in the stratosphere and above.

Walker Creek Revisited: Reinterpretation of Diagenesis of Smackover Formation, Walker Creek Field, Arkansas: ABSTRACT

End_Page 1675------------------------------Recent petrographic, trace element, isotopic, and fluid inclusion work has necessitated a major revision of the diagenetic model for the Smackover Formation at Walker Creek field, southern Arkansas. Prior studies concluded that all diagenetic changes occurred within a few tens of feet of the earth's surface. This investigation has shown that near surface diagenesis is confined to very minor marine cementation. The equant calcspar and baroque dolomite cementation occurred much later at temperatures between 60 and 120°C during the initial stages of hydrocarbon migration in the deep subsurface. Earlier workers also concluded that the equant cementation is unrelated to depositional facies. They attributed the layered permeable and impermeable zones to permeability preservation in a meteoric vadose zone and occlusion in a meteoric phreatic zone. Because the equant cement is of a late origin, it can no longer be attributed to a meteoric zone. The equant cement does occur in all depositional facies, but the effect of this cement upon the permeability of the rock is related to the original depositional facies. Permeability has been preserved in the coarser grained ooid and ooid-intraclast grainstones. This may be related to the size of the pore throats; cementation could block the pore throats completely in the finer grained material only. The coarser rock remained permeable and was made more permeable by late stage dissolution. This new-found relation of permeability and original depositional facies has several implications regarding future hydrocarbon exploration in the southern Arkansas-northern Lousiana area. End_of_Article - Last_Page 1676------------

Hydrostratigraphic Framework and Flow Dynamics of Uraniferous Aquifer-Oakville Sandstone of South Texas Coastal Plain: ABSTRACT

Synthesis of geologic, hydrochemical, and hydrologic data for the Oakville Sandstone provides a framework for effective hydrochemical exploration, leach mine planning, and evaluation of mining impact. Quantitative lithofacies maps were combined with structural and well-pump test data to produce a regional Oakville transmissivity map. Interpreted groundwater flux patterns within the resultant three-dimensional transmissivity framework were mapped, and quantitatively illustrate several hydrodynamic principles. (1) The complete groundwater system includes meteoric, elisian (compactional), and thermobaric connate zones. Boundaries separating zones are dynamic and evolve through time. (2) Recharge of meteoric systems occurs directly at outcrop by seepage through overlying confining units, and by seepage from underlying aquifers. (3) Flow occurs within a hierarchy of cells, ranging from unconfined local (n × 10 sq mi) to intermediate confined (n × 103 sq mi) to regional confined (n × 104 sq mi) flow cells. Shallow groundwater may move in any direction with respect to dip. (4) Discharge occurs by direct groundwater flow upward to the water table and by vertical seepage, commonly localized by fault zones, into adjacent aquifers. (5) Topography and the three-dimensional permeability framework localize recharge and discharge zones and control flow direction. Recharge occurs in topographically high areas; discharge in lowest areas. Groundwater flux in the Oakville is highly complex. Major uranium deposits occur within the realm of local (Ray Point district), intermediate (George West district), and regional (Duval County) flow cells. Extant flow direction commonly does not agree with geochemically-inferred mineralization-front polarity. End_of_Article - Last_Page 711------------

The mean zonal circulation in the meteor zone above Garchy (France) and Kiruna (Sweden)

The large set of data obtained from 6 years of observations by the Garchy meteor radar yields a picture of the mean zonal circulation in the 75–105 km range at 47°N. The annual wind pattern shows that the lower meteor heights (80–90 km) are characterized by strong vertical and seasonal changes and mark a region of transition between the mesospheric monsoonal circulation observed up to around 85 km and another type of circulation above 90 km characterized by a strong steady eastward circulation in the summer and a weak circulation during the rest of the year. These features were also observed over Kiruna (68°N) by another CNET meteor radar situated at this place for 1 yr. The annual wind component prevails in the meteor range and is nearly out of phase below and above the 85–90 km level. Comparisons made with the results from other stations show that these characteristics are probably planetary in character.

Results of wind velocity measurements at middle and high latitudes by the meteor radar method.

The Institute of Experimental Meteorology carries out regular wind measurements by the meteor radar at three stations, Heiss Island (from 1964), Obninsk (from 1964) and Molodezhnaya, the Antarctica (from 1967). The seasonal course of wind regime parameters in the meteor zone in different years was found to be relatively stable in comparison with site-to-site variations of these parameters. It is shown that wind velocity variations with the periods from 2-5 to 10-20 days are typical of the meteor zone and are connected with corresponding variations in stratomesosphere. The analysis of dates for different years showed that in the meteor zone the spring reversal of circulation begins much earlier in stratosphere; more earlier (later) dates of the reversal in stratosphere correspond to more earlier (later) dates of the reversal beginning in the meteor zone.

Interpretation of the origin and diagenesis of Pleistocene chalk, Eniwetok Atoll, Marshal Islands

ABSTRACT Three Pleistocene carbonate samples from a core from Eniwetok Atoll, Marshall Islands, were analyzed in order to describe and determine the origin of clay found extensively in the subsurface of the atoll. The samples represent a lagoonal depositional environment as evidenced by relatively high abundances of coral and Halimeda, and the lack of coralline algae. Sample depths of 51, 63, and 79 m below MSL occur below three distinct solution unconformities at 22, 60 and 70 m Consequently, each sample represents a zone with a different diagenetic history. Eight size fractions of the samples were grouped into three size intervals based upon the following compositional characteristics observed on scanning electron microscope micrographs: (1) generally biogenic carbonate grains (>5 µ), (2) significant amounts of calcite cement (2-5 µ), and (3) abundant aragonite needles (.1-2 µ), some of which are believed to be aragonitic cement. The mineralogy of the >5 µ fractions is determined largely by the mineralogic composition of the biogenic grains and their degree of neomorphism, whereas the mineralogy of the 2-5 and .1-2 µ fractions is determined largely by the addition of calcitic and aragonitic cements, respectively, superimposed upon neomorphosed remnants. Each sample has some calcite cement (most in the 79 m sample) which probably precipitated in the vadose and/or meteoric phreatic zones during periods of atoll emergence. On the other hand, differences in the strontium and magnesium concentration values for 100% aragonite suggest that: (1) the aragonite cement in the 51 m sample precipitated in a closed system, e.g., from hypersaline solutions during times of atoll emergence, and (2) the aragonite cement in the 63 and 79 m samples precipitated in an open system, e.g., from the marine phreatic zone during times of atoll submergence. Evidence for cycles of atoll emergence and submergence consists of growth relations between equant calcitic and aragonitic cement grains.

A study of the variations of atmospheric tides in the meteor zone

The Garchy meteor radar has now yielded a very large amount of data, which are used to draw up a detailed picture of the general mean and seasonal averages of the semi-diurnal and diurnal tides in the 80–100 km altitude range. The semi-diurnal tide is generally dominated by the fundamental, S 2 ω 2,2 mode, but higher-order modes prevail in winter, and spring and autumn averages cannot both be described by a single ‘equinoctial’ model. The diurnal tide is alternately dominated by the S ω 1,1 mode and by evanescent modes (in summer). Comparisons with theoretical predictions show off strong disagreements. Short-term variations of the tides are investigated with the help of two-dimensional spectral analysis and regression methods: several cases of complex tidal structure can be interpreted as reflection or wave superposition phenomena.

Low latitude meteor wind observations

A meteor wind station has been constructed in Jamaica (18°N, 77°W), West Indies, and neutral winds in the altitude range 80–110 km measured for 24 hours at approximately weekly intervals from March 1971 to February 1972. A total of 13,000 individual meteor echoes have been analysed, including nearly 3,000 which have enabled a ±5 km height estimate to be made from the echo decay rate. A least squares fourier analysis has been carried out on the data averages, and the prevailing wind and first six harmonic components extracted. Very high wind speeds, well in excess of 100 m/s, have been observed to occur almost continuously, except during the Spring. These high velocities often show a characteristic sudden onset and evidence has been obtained that they are associated with strong oscillations of about 4 h period. The diurnal tide usually dominates the oscillatory wind spectrum. This oscillation shows evidence of a 15 km vertical wavelength during the Spring and there is a clear indication that the oscillatory wind energy diminishes with increasing height in the meteor zone. The prevailing meridional winds show a seasonal variation in good agreement with the general circulation based on rocket measurements, and agreement in the case of the zonal component, though poor during late Winter and Spring, is satisfactory for the remainder of the year.

On the scale sizes of wind systems in the meteor zone

In view of certain limitations associated with single-station meteor wind observations a two-station experiment has been carried out in order to establish the medium and large scale features of the upper atmospheric wind. It emerges from this study that the major constituents of the wind in the meteor region have considerable scale sizes in the horizontal, usually exceeding several hundred kilometres, and it appears that the dominant structure results from the presence of tidal oscillations and planetary waves while turbulence and internal atmospheric gravity waves are likely to be responsible for the residual wind features. This is also evident from the vertical structure of the wind which has been investigated in this study with the aid of interferometric height finding equipment.

The Sheffield meteor wind experiment

AbstractAn account is given of the development and application of a system for the study of high altitude winds using the radio meteor method. Three main subjects have been investigated : atmospheric tides and short term wind oscillations possibly resulting from internal atmospheric gravity waves, wind shears in the meteor zone and finally the nature of drifts obtained with the spaced receiver ionosonde technique. The Sheffield system has been steadily expanding and current developments as well as other projects linked with the Sheffield experiment are described.

Atmospheric circulation in the meteor zone

Atmospheric circulation in the meteor zone

Wind shears in the meteor zone

Records of meteor drifts obtained during the I.Q.S.Y. with the radio-meteor technique have been used to resolve wind shears in the 80–100-km region of the upper atmosphere. Some theoretical aspects relevant to the technique of observation are being dealt with and comparison is made with wind shear data obtained by different methods. Two main shear generating mechanisms, turbulence and internal gravity waves, are taken into consideration and it is shown that the present results are more in keeping with a regular wave structure of the upper atmosphere.

Paleoecology and Diagenesis of Key Largo Limestone, Florida

The Key Largo Limestone is a Pleistocene reef limestone that forms the upper Florida Keys. It consists of an organic framework of coral colonies and an interstitial skeletal calcarenite, both of which are homogeneous throughout the upper part of the formation. The Pleistocene reef was formed chiefly by massive and rounded coral heads, the principal frame builder being Montastrea annularis. No organic zonation is visible perpendicular to the long axis of the reef. Both a well-sorted and a poorly sorted calcarenite facies can be distinguished. The poorly sorted facies, which predominates, surrounds coral heads and is characterized by its high calcilutite content. The well-sorted facies forms channel deposits between coral buildups. In organic composition, the ancient Key Largo reef is roughly equivalent to the deep-water Montastrea annularis zones of living West Indian bank reefs and to West Indian patch reefs. It grew under semi-restricted conditions, possibly as a series of coalescing patch reefs, but more probably below the surf zone in op n water, where it spread laterally over large areas of the sea floor. The constituent composition of the two calcarenite facies indicates that aragonite and high-magnesium calcite originally were the chief mineralogic components. Fossils that initially were high-magnesium calcite have been altered in place to low-magnesium calcite through the removal of excess magnesium by fresh water in the meteoric zone; such fossils have otherwise remained intact. Aragonit, on the other hand, currently is being dissolved in the meteoric zone, and the dissolved CaCO3 is being redeposited near the site of dissolution as low-magnesium calcite cement. Many fossils that originally were aragonite now remain only as molds of sparry calcite. The rate of aragonite dissolution and calcite cementation is accelerated by artificial exposure. The diagenetic fabrics exhi ited by the Key Largo Limestone should be useful in recognizing ancient limestone units that were subjected to subaerial exposure soon after deposition.