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2003 (vol. 3)
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Letter 3 [2003]: Note on magnesite
formation (Studies on irreversible geochemical reactions Nº 9), by John C.
DEELMAN.-
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250 KB] Reference: [CG2003_L03]
DOI:10.4267/2042/305
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Abstract: In recent laboratory experiments magnesite (MgC03) has been synthesized at a temperature of
313ºK (= 40ºC). The experiments have demonstrated that irreversible reactions are involved in the low-temperature formation of
magnesite. Fundamental to such irreversible reactions is a requirement for fluctuations, i.e., alternations between precipitation and dissolution. But unequivocal evidence for the necessity for fluctuations in order to produce such irreversible geochemical reactions can be demonstrated only by static control experiments. The present note describes several static control experiments on the low-temperature synthesis of
magnesite. The first experiment consisted of adding the total amount of ammonia (used in the original experiment in 14 different titration steps) in a single action: only magnesium hydroxide carbonate formed, not
magnesite. In the second experiment the possible reaction between magnesium chloride, ammonia and carbon dioxide in solution was studied at
318ºK (= 45ºC): magnesium hydroxide formed, not magnesite. The third static control experiment involved the reaction between magnesium chloride and ammonium carbamate; this time nesquehonite formed, not magnesite. The implications of these static control experiments in relation to the low-temperature formation of magnesite and dolomite in the sedimentary environment are discussed.
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Online since December 4, 2003
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Article
7 [2003]: Pristiograptus (Graptoloidea) from the
perneri-lundgreni biozones (Silurian) of Lithuania, by Sigitas Radzevičius.- Format [HTML]
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1,749 KB] Reference : [CG2003_A07]
DOI:10.4267/2042/300
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Abstract:
Two new forms of Pristiograptus dubius, here designated as varieties
"A" and "B", are described and figured. Both were found in
core samples representing the
perneri through lundgreni biozones of the Wenlock epoch. The
cores are from boreholes Šiupyliai-69, Parovėja-9, Likėnai-396,
Paežeriai-222 and Sutkai-87 in central and northern Lithuania. The associated
graptolites Cyrtograptus perneri Bouček, C. radians Törnquist, C. lundgreni Tullberg,
Monograptus flemingii flemingii (Salter), M. testis
testis (Barrande), Monoclimacis flumendosae
(Gortani) and Pristiograptus pseudodubius
(Bouček) (= P. parvus)
are all indicative of the perneri, radians and
lundgreni biozones of the Wenlock. Two species of Pristiograptus
occur in this time-stratigraphic interval: P. dubius and P. lodenicensis.
In P. dubius the degree and manner of extension of the thecal apertural
lip (thecal hood) onto the succeeding theca distinguish it from otherwise very
similar taxa. The thecal morphology of P. lodenicensis differs from that
of a typical Pristiograptus in that the apertural lip has a central
depression and is rounded into small lobes laterally.
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Online since November 22, 2003
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Reply
to Article 6 [2003]:
Reply to L.E. Popov and L.E. Holmer (CG2003_A06):
Obolid taxonomy, by Christian C. EMIG.
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516 KB] Reference : [CG2003_A06_Reply]
DOI:10.4267/2042/299
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Online since October 4, 2003
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Article 6 [2003]: Understanding linguloid brachiopods: Obolus and
Ungula as examples, by Leonid E. POPOV & Lars E. HOLMER.-
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2,806 KB] Reference: [CG2003_A06]
DOI:10.4267/2042/298
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Abstract: EMIG (2002) re-examined the taxonomy
of the genus Obolus EICHWALD from the Middle Cambrian - earliest Ordovician of the
East Baltic region as part of a proposal for a wholesale revision of the
principles of linguloid systematics. He contended that previous taxonomic
studies on Obolus and related forms were carried out erroneously using characters
that have no taxonomic value. EMIG's proposed revision is based mainly on the
limited morphological diversity between fossil and Recent taxa within a single
linguloid Family, the Lingulidae. However, the present study demonstrates the
taxonomic validity of the diagnostic characters used for classification within
the mostly extinct families of the Superfamily Linguloidea, for they exhibit far
more variation in morphology. This study also shows that EMIG has provided no
satisfactory basis for his radical changes and revisions to the existing widely
accepted taxonomy of the Cambrian to earliest Ordovician Obolidae of the East
Baltic.
Obolus EICHWALD and Ungula PANDER are shown to constitute
distinctive and discrete genera comprising the species
Obolus apollinis EICHWALD, O. ruchini KHAZANOVITCH et POPOV, O. transversus
(PANDER), Ungula ingrica (EICHWALD), U. inornata (MICKWITZ),
and U. convexa PANDER.
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Online since September 12, 2003
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Article 5 [2003]: New insight on the stratigraphy of the "Upper Thamama" in offshore Abu Dhabi
(U.A.E.), by Bruno GRANIER, Ahmed Saqer AL SUWAIDI, Robert BUSNARDO, Sabah Karim
AZIZ & Rolf SCHROEDER.-
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7,938 KB] Reference: [CG2003_A05]
DOI:10.4267/2042/297
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Abstract:
An integrated case study of field "A" in offshore Abu Dhabi found that the
stratigraphic framework for the uppermost part of the so-called "Thamama
Group" required revision. Detailed sedimentological work permitted a subdivision of the
succession into lithostratigraphic units (more accurately "allostratigraphic
units") and the fossil content permitted their allocation to standard age-related units ranging from
Late Barremian through Middle Aptian times. Additional work focused on the so-called
"Shu'aiba Formation" and resulted in a new and comprehensive interpretation on a
regional scale which differs from published interpretations based on
onshore studies in Abu Dhabi, Qatar, and Oman.
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Online since July 31, 2003
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Article 4 [2003]: A new approach in rock-typing, documented by a case study
of layer-cake reservoirs in field "A", offshore Abu Dhabi (U.A.E.), by Bruno GRANIER.-
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436 KB] Reference: [CG2003_A04]
DOI:10.4267/2042/296
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Abstract: In carbonate reservoirs,
the relationship between porosity Ø, a measure of the combined volumes of several kinds of pore space
(e.g. interparticle and separate-vug), and permeability K are neither
linear nor logarithmic, hence only weakly correlatable. Approaches to an estimation of permeability that employ both petrographical and petrophysical parameters, the so-called rock-typing techniques, have proven to be the most nearly precise. However in many studies
simple K/Ø cross-plots are used for each rock-type to provide trendlines from which K values are derived as a function of Ø values; this is common practice even though the coefficient
of correlation r2 departs significantly from 1.
This paper describes
and provides examples of an improved technique of rock-type classification in which each rock-type is characterized by a discrete and unique Gaussian distribution
of log K. It facilitates upscaling for it suggests the use of a single geometric mean value
for permeability and a corresponding standard deviation (variance, or coefficient of variation) for each rock-type.
This new technique can be used in uncored wells by extrapolating these
determinations into wireline logs as documented below in a case study of layer-cake reservoirs in a field
of the Abu Dhabi offshore (U.A.E.).
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Online since July 12, 2003
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Letter
2 [2003]: Lower sea levels in the Middle Cenomanian, by Jake M.
HANCOCK.-
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280 KB] Reference: [CG2003_L02]
DOI:10.4267/2042/304
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Abstract: It has been known since the 1970's that the relatively high sea levels during the Cenomanian in southern England and northern France were interrupted by a strong fall in sea level early in the Middle Cenomanian. This was a eustatic trough whose effects can be found not only in north-west Europe, but also from western Kazakhstan in central Asia to Texas, Colorado and South Dakota in the U.S.A.
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Online since July 3, 2003
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Letter 1
[2003]: Proof that Lingula (Brachiopoda) is not a living-fossil,
and emended diagnoses of the Family Lingulidae, by Christian C. EMIG.-
Format
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1,204 KB] Reference: [CG2003_L01]
DOI:10.4267/2042/303
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Abstract: Lingula
is often considered a "living-fossil" based on its supposed lengthy
morphological conservatism owing to its absence of evolution, and its
remarkable survival for more than 550 M.Y. This conclusion is based on the
typical apparently unchanged "linguliform" shape of the shell.
However the taxa of the family Lingulidae show morphological evolutionary
changes despite the fact that the group appears panchronic among the Recent
Brachiopoda. Consequently, traditional opinion that Lingula is a "living-fossil"
should be rejected. Diagnoses of the Family Lingulidae and of its three genera are herewith emended.
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Online since May 1, 2003
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Article 3 [2003]: The Cretaceous of the Elbe valley in Saxony (Germany)
- a review, by Karl-Armin TRÖGER.-
Format
[HTML]
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377 KB] Reference: [CG2003_A03]
DOI:10.4267/2042/295
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Abstract: In Central Europe one of the most
important interchanges between the North Temperate Realm and the Tethyan Realm
took place during the Cenomanian along the course of the Elbe Valley Geosuture
that separates the Erzgebirge block (part of the Mid European Island during the
Cretaceous) from the Lusatian block (West Sudetic Island). Strata of the Lower
Cretaceous and the basal portion of Lower Cenomanian are absent in the Elbe
Valley Geosuture but a marine transgression in a NW-SE direction occupied a
portion of this tract during the late Early Cenomanian and a second incursion
from SE to NW occurred during the Late Cenomanian. These transgressions are
known to have occurred because of faunal migrations southwards from the North
Temperate Realm and northwards from the Tethyan Realm across the Bohemian Basin.
Sediments of the first transgression (Lower Cenomanian, dixoni zone) are
restricted to the northwestern part of the Elbe Valley Geosuture in the Meissen
area for at that time the fluvial Niederschöna Formation occupied the southern
part of the Elbe Valley Geosuture and the adjacent Erzgebirge block. Most of the
rivers in this system ran eastward but drained toward northern Bohemia. This
fluvial environment predominated in the southern part of the Elbe Valley
Geosuture and on the adjacent Erzgebirge block.
But the uppermost levels of the fluviatile Niederschöna strata are influenced
by marine ingressions. The main transgression from the North Temperate Realm
toward the Tethyan Realm took place in the basal Upper Cenomanian (naviculare
zone) along the Elbe Valley Geosuture. A small regression followed (plenus
event, geslinianum zone): Indications of this regression are visible on
the flanks of small islands in the Elbe Valley Geosuture. Four marine sequences
are recognized in strata dated Late Cenomanian - earliest Turonian.
These sequences are important for the comparison of Upper Cretaceous formations
in Saxony with those of the Upper Cretaceous in northern Bohemia and in the
peripheral areas of the NW German-Polish Basin, in the Münsterland,
Subhercynian, Eichsfeld regions and in the Anglo-Paris Basin. These sequences
are described and their influence on the changes in the paleogeography in the
Elbe Valley Geosuture is discussed. Slight variations in sea level as a
consequence of local tectonic movements in the Elbe Valley Geosuture cannot be precluded.
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Online since April 26, 2003
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Article 2 [2003]: Micropaleontological investigations in the modern Mahakam delta, East Kalimantan (Indonesia), by Bernard LAMBERT.-
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4,845 KB] Reference: [CG2003_A02]
DOI:10.4267/2042/294
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Abstract:
The Mahakam delta is a mixed, fluvial and wave dominated delta located in the eastern part of the island of Borneo (the East Kalimantan province of Indonesia). The distribution of benthic fauna in this delta system is influenced by the combined or antagonistic action of three main parameters: fluvial input of fresh water and sediment, tide, and a strong regional north to south drift current. A model of the present-day faunal distribution has been established taking into account the perturbations induced by the tide and by the regional drift current. This model has been extrapolated into the recent past through the study of shallow cores that permit the reconstruction of late Holocene deltaic regression patterns.
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Online since March 8, 2003
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Article 1 [2003]: The Cenomanian: stage of hindlimbed snakes, by
Jean-Claude RAGE & François ESCUILLIÉ.-
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[HTML]
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1,578 KB] Reference: [CG2003_A01]
DOI:10.4267/2042/293
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Abstract:
Three "snakes with legs" are known: Pachyrhachis problematicus, Haasiophis terrasanctus and Eupodophis descouensi. They have short posterior limbs but lack an anterior girdle and forelimbs. Moreover, Pachyophis woodwardi, Mesophis nopcsai and Simoliophis ssp. appear to be closely related to the hindlimbed
taxa; consequently, although the presence of posterior limbs has not been demonstrated for these genera, it is presumed that they too were
hindlimbed. All these snakes have been recovered only from the Cenomanian. Moreover, these six genera come from a restricted area (western Europe and
northwesternmost Africa to the Middle East). This limited geographic range suggests that snakes might have originated in the "Mediterranean" part of the Tethys, but the restricted stratigraphical range remains
unexplained.
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Online since February 17, 2003
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