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2016 (vol. 16)

Evolution of Miocene siliciclastic deposits in central Tunisia: Case study of the Khechem El Artsouma section
Hanene BELGHITHI, Frédéric BOULVAIN, Chokri YAICH & Anne-Christine DA SILVA

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 Abstract:  After a NE-SW folding phase (Atlasic direction) during the Miocene, siliciclastic sediments from a deltaic environment were deposited in the south-western part of Tunisia while an internal offshore platform formed in the north-eastern part of the country. In this paper we focus on the Khechem El Artsouma Miocene section which is representative of central Tunisia deposits. This area is particularly interesting to study, because it corresponds to a transition zone between deltaic and shallow marine deposits. Facies analysis, magnetic susceptibility measurements and carbonate content measurements are combined to propose a reconstruction of the paleoenvironments and their evolution through the Miocene. Eight facies are identified, from fluviatile, swamp and evaporitic lagoon environments to shore environments. The deposits of the Ain Grab Formation (Langhian) are interpreted as transgressive, followed by the Beglia Formation (Serravalian), which are interpreted as regressive. The whole Saouaf formation (Upper Serravalian - Lower Tortonian) corresponds to a transgressive and regressive sequence. The Djebel Khechem El Artsouma section permits the proposal of a complete sedimentary model for the Miocene deposits from this central Tunisia transition zone.

Carnets Geol., vol. 16, no. 23, p. 557-568

Online since December 12, 2016


On the type species of Lingularia, and description of Eolingularia n. gen.
Maria Aleksandra BITNER & Christian C. EMIG

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 Abstract:  The material under study has previously been identified as Lingula krausei, collected from glacial erratics at Cape Rozewie, Poland. All specimens come from glauconitic sandstone of Cenomanian age. Dames (1874) identified his Lingula sp. as L. krausei which differs in shell outline from Lingula truncata. This species, dedicated to A. Krause, together with L. subovalis, is considered characteristic of the Cenomanian. Several new species and genera have recently been described or redescribed and compared to our specimens, here identified as Lingularia similis. By priority (ICZN, 1999), the type species L. similis is currently synonymized with Lingula krausei. A new genus Eolingularia, within the Family Lingulidae, is here described, with Lingularia siberica Biernat et Emig, 1993, as type species. This new genus ranges from the Carboniferous to the Triassic in Russia, China, and Spain. Possible synonymies are discussed.

Carnets Geol., vol. 16, no. 22, p. 543-555

Online since December 12, 2016


Albian rudist biostratigraphy (Bivalvia), Comanche shelf to shelf margin, Texas
Robert W. SCOTT, Whitney CAMPBELL, Rachel HOJNACKI, Yulin WANG & Xin LAI

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 Abstract:  Rudists were widespread and locally abundant carbonate producers on the Early Cretaceous Comanche Shelf from Florida to Texas, and on Mexican atolls. As members of the Caribbean Biogeographic Province, their early ancestors emigrated from the Mediterranean Province and subsequently evolved independently. Comanchean rudists formed biostromes and bioherms on the shelf interior and at the shelf margin. Carbonate stratigraphic units of the Comanche Shelf record rudist evolution during the Barremian through the Albian ages and an established zonal scheme is expanded.
This study documents new Albian rudist occurrences from the Middle-Upper Albian Fredericksburg and Washita groups in Central and West Texas. Rudists in cores at and directly behind the shelf margin southeast of Austin and San Antonio, Texas, complement the rudist zonation that is integrated with ammonites and foraminifers. These new rudist data test long-held correlations of the Edwards Group with both the Fredericksburg and Washita groups based solely on lithologies. Rudist and foraminifer biostratigraphy indicate that the Edwards Group is coeval with the Fredericksburg not the Washita Group.
In West Texas sections Caprinuloidea romeri occurs at the top of the Fredericksburg Group in the Fort Terrett Formation approximately 3 meters below the iron-stained hardground subaerial exposure surface. It is overlain by the Washita Group with the ammonite Eopachydiscus marcianus Zone. Caprinuloidea romeri is also documented from the type cored section of the Person Formation in the upper part of the Edwards Group. These species allow a precise correlation of the subsurface Person Formation in Central Texas with the Ft. Terrett Formation cropping out in West Texas. This evidence together with foraminifer and ammonite zones demonstrates that the Person Formation is coeval with Fredericksburg units underlying the basal Washita sequence boundary Al Sb Wa1. The upper Albian Washita Group Kimbleia and Mexicaprina caprinid zones overly the Caprinuloidea Zone and the successive Washita mortonicerid ammonite zones.

Carnets Geol., vol. 16, no. 21, p. 513-541

Online since November 11, 2016


Jurassic-Cretaceous transition on the Getic carbonate platform (Southern Carpathians, Romania): Benthic foraminifera and algae
Cristian Victor MIRCESCU, George PLEŞ, Ioan I. BUCUR & Bruno GRANIER

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 Abstract:  The carbonate succession of the Piatra Craiului Massif, i.e., the eastern part of the Getic carbonate platform (Southern Carpathians, Romania), comprises reefal and peritidal limestones that contain a diverse microfossil assemblage composed mainly of calcareous green algae and benthic foraminifera. The biostratigraphically most significant benthic foraminifera found in the several sections studied are described; some (Anchispirocyclina lusitanica, Neokilianina rahonensis, Bramkampella arabica, Everticyclammina praekelleri) are reported for the first time in this area. Hence, assemblages of both foraminifera and calcareous algae characterize three biostratigraphic intervals in the studied succession, the Kimmeridgian - lower Tithonian, the upper Tithonian - lower Berriasian, and the upper Berriasian - ? lower Valanginian intervals. The main microfacies types (bioclastic rudstone, coral-microbial boundstone, bioclastic grainstone, interbedded mudstones and wackestones with cyanobacteria nodules) as well as both the foraminifera and the calcareous algae are paleoecological indicators that may contribute to the decipherment of the depositional environments and to building a depositional model for the eastern part of the Getic carbonate platform at the Late Jurassic-Early Cretaceous transition.

Carnets Geol., vol. 16, no. 20, p. 491-512

Online since October 24, 2016


The Weberg and Warm Spring Members of the Snowshoe Formation in Suplee area, Oregon: Lithofacies and Aalenian-early Bajocian ammonoid zonation
David G. TAYLOR

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 Abstract:  This paper documents the stratigraphic information upon which an ammonoid zonation from the lower part of the Snowshoe Formation in the Suplee area was originally established (Taylor, 1988). Ammonoid ranges from 22 measured stratigraphic sections reveal about 60 ammonoid species distributed among eight zones encompassing the late Aalenian and part of the early Bajocian stages. The sediments are dominantly volcaniclastic and, as a result, there are numerous tuff beds in the succession. As noted in Taylor (1982) the principal tuff beds (herein designated as units A through F) can be used jointly with the ammonite sequence to give precise correlations from section to section. Utilizing the ammonoids, tuff beds, and facies in combination gives a graphic representation of the westward transgression of the Snowshoe Formation as it enveloped an irregular topography, and lapped onto the topographically high Grindstone High to the west. The ammonoid zonation is significant in that it gives a standard of reference for the biochronology of the late Aalenian and early Bajocian ammonoids in the North American Western Cordillera.

Carnets Geol., vol. 16, no. 19, p. 449-490

Online since September 12, 2016


Plio-Pleistocene Ostracods from sedimentary sections along the eastern edge of Cape Bon (Wadi Lebna section, Eastern Tunisia)
Rim TEMANI, Driss NACHITE, Francesco SCIUTO, Saloua RAZGALLAH, Ratiba BEKKALI, Khayati HAYET & Nadia GAALOUL

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 Abstract:  This paper presents for the first time a detailed study of ostracods in the Upper Pliocene-Lower Pleistocene deposits of Wadi Lebna (Cape Bon eastern edge, northeastern Tunisia). The quantitative and qualitative analyses document the evolution of this area from a marine environment dominated by relatively deep or circalittoral species at the base of the succession, passing progressively up into shallower environments, and at the top infralittoral coastal environments. Particularly important from the ecological point of view is the presence in some samples of Bythocythere turgida. This species indicates deposition of these sediments during the cold phases of the Early Pleistocene.

Carnets Geol., vol. 16, no. 18, p. 431-447

Online since September 12, 2016


Comments on "Estimating the impact of early diagenesis on isotope records in shallow-marine carbonates: A case study from the Urgonian platform in western Swiss Jura" by A. Godet et al. [Palaeogeography Palaeoclimatology Palaeoecology 454 (2016) 125-138]
Bruno GRANIER, Bernard CLAVEL & Jean CHAROLLAIS

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 Abstract:  A recent paper by Godet et al. on Urgonian carbonates from the Swiss Jura concluded that diagenesis may hinder chemostratigraphic correlations of deep- to shallow-water facies. Although we agree with this conclusion we question their arguments and interpretations. These authors correctly identified diagenesis as the key factor, but we question the timing of events in their paragenetic sequence. In particular, they reported the leaching of originally calcitic oolitic cortices but failed to discuss this puzzling feature properly. In addition, the backbone of their argument is the stratigraphic correlation of a specific sequence from three dominantly shallow-water sections together with a basinal reference section. However, a reliable alternative biostratigraphic framework shows that this attempt of long-distance correlation might be erroneous.

Carnets Geol., vol. 16, no. 17, p. 417-429

Online since August 5, 2016


Taxonomy and distribution of the Cretaceous coral genus Eosiderastrea
Hannes LÖSER

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 Abstract:  The new genus Eosiderastrea Löser, 2016, was established for Cretaceous corals formerly assigned to the extant coral genus Diploastrea. It represents the oldest member of the Family Siderastraeidae. The genus is characterised by an astreoid arrangement, occasionally although some have a plocoid or a cerioid calicular arrangement, and a sub-regular hexameral, heptameral, octameral or decameral septal symmetry. The septal blades are made of medium-sized trabeculae. Synapticulae are present. Almost a hundred specimens were examined, among them numerous type specimens. In total, 26 Cretaceous species were separated. Fifteen species were assigned to formally established species, three species were compared to one of these species, two species were described as new and the remaining six species were reported in open nomenclature. The genus occurs in the Cretaceous worldwide and ranges from the late Valanginian to the Santonian. It shows its greatest diversity in the earliest Aptian and early to middle Cenomanian.

Carnets Geol., vol. 16, no. 16, p. 383-416

Online since July 7, 2016


Ivaldiceras, a new genus of heteromorphic ammonites from the Lower Aptian of southeast France
Gérard DELANOY, Cyril BAUDOUIN, Marc BOSELLI & Emmanuel PENAGÉ

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 Abstract:  New investigations in the Vocontian Lower Aptian deposits resulted in the discovery of very rare specimens of heteromorphic ammonites whose specific morphological and ornamental features have led to the erection of Ivaldiceras gen. nov. Represented by a very limited number of specimens this new genus consists of two species: I. baratteroi gen. & sp. nov. and I. divajeuensis gen. & sp. nov. The suprageneric position of the new genus is difficult to determine because of the poor preservation of its sutural elements and the incomplete state of the specimens. The genus is tentatively assigned to the Family Macroscaphitidae Hyatt, 1900, pending the study of better preserved material.

Carnets Geol., vol. 16, no. 15, p. 369-381

Online since July 7, 2016


Aptian ammonites from Mazapil, Zacatecas State (north-central Mexico) studied by Burckhardt in 1906: A revision
Ricardo BARRAGÁN, Josep Anton MORENO-BEDMAR & Celestina GONZÁLEZ-ARREOLA

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 Abstract:  Twelve specimens of Aptian ammonites reported and drawn by Burckhardt in 1906, from the Mazapil area (Zacatecas State, north-central Mexico), are reviewed for taxonomical purposes. The work by Burckhardt was the first report of Aptian ammonoids in Mexico. In addition, it represents the only record of Aptian ammonites known from Zacatecas State. The taxonomic review of the specimens allows for the recognition of Dufrenoyia sp., Colombiceras spathi Humphrey, Colombiceras mexicanus (Humphrey) n. comb., Colombiceras sp., Colombiceras spp., and Acanthohoplites aff. bigoureti (Seunes). This ammonite assemblage is indicative of a stratigraphic range that spans the uppermost lower Aptian to the upper Aptian interval.

Carnets Geol., vol. 16, no. 14, p. 355-367

Online since June 23, 2016


Possible vertebrate coprolites from the Upper Cretaceous (Coniacian) of the Sudetes Mountains (southern Poland)
Tomasz BRACHANIEC & Anna WIECZOREK

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 Abstract:  Possible coprolites from the Upper Cretaceous (Coniacian) of Waliszów Stary in the Sudetes Mountains (southern Poland) are described for the first time. They are relatively small, irregular in outline, and preserved as goethite, limonite, siderite and hematite. Although it is difficult to identify the producer of these coprolites, they were most probably formed by some fish.

Carnets Geol., vol. 16, no. 13, p. 349-354

Online since June 23, 2016


Late Albian cactus in shallow carbonate shelf, West Texas
Robert W. SCOTT, Zexuan WANG, Xin LAI & Yulun WANG

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 Abstract:  Well-preserved vertical molds of a stalk-like fossil are in growth position in the upper part of the Upper Albian Fort Lancaster Formation exposed in West Texas. The Fort Lancaster Formation is the uppermost Albian carbonate unit deposited on the shallow water Barremian-early Cenomanian Comanche shelf. The Fort Lancaster represents one long-term transgressive-regressive cycle capped by a regional unconformity overlain by lower Cenomanian deeper water Del Rio Shale.
The erect stalk-like molds are up to 1 m tall and about 6 cm in diameter. The mold walls are composed of six pairs of angular ridges and grooves evenly spaced at an angle of 60° to each other. The ridge crests are dotted by paired ringed depressions with small central nodes; these negative impressions indicate the presence of oval rings with small depressions inside of each ring. The diameter of the stalk is the same at its base as at its top indicating that growth did not expand the organism but extended it upward. These stalk-like organisms appear to be rooted on disconformable bedding surfaces that were exposed to the atmosphere. Carbon isotope data shows the classic negative shift below subaerial contacts. Subsequently sea level rose and muddy carbonate sediment buried the stalk. The absence of original wall material suggests that the wall was composed of readily decayed or dissolved material such as organic matter or aragonite. Stalk-like organisms of this size and shape are rare and potential types are cacti, plant reeds, Porifera, or Scleractinian Cnidaria.

Carnets Geol., vol. 16, no. 12, p. 337-346

Online since May 19, 2016


The taxonomy of middle Miocene red algae from the Gârbova de Sus Formation (Transylvanian Basin, Romania)
Ramona CHELARU & Ioan I. BUCUR

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 Abstract:  This study describes seventeen non-geniculate coralline algal species (orders Corallinales, Hapalidiales and Sporolithales) from the middle Miocene (lower-middle Badenian) red-algal limestones of the Transylvanian Basin, Gârbova de Sus Formation. For the description and identification at species level, we follow the common diagnostic features used for fossil species and some characters that are used as diagnostic for modern species (roof morphology for asexual conceptacles, the presence/absence of a layer of elongated cells below sporangial compartments and number of cells in paraphyses for Sporolithon, and measurements of gametangial and carposporangial conceptacles). Female conceptacles of Spongites fruticulosus Kützing are for the first time described in fossil material. We propose the attribution of Lithophyllum platticarpum Maslov to Spongites fruticulosus Kützing as a gametangial thallus with male conceptacles.

Carnets Geol., vol. 16, no. 11, p. 307-336

Online since May 19, 2016


Environmental control on shell size of Middle Triassic bivalve Plagiostoma
Krzysztof R. BROM, Robert NIEDŹWIEDZKI, Tomasz BRACHANIEC, Bruno FERRÉ & Mariusz A. SALAMON

 | HTML  | PDF [705 KB]  | DOI : 10.4267/2042/60118

 Abstract:  Fossil shells of the marine bivalve Plagiostoma striatum Schlotheim sampled from the Middle Triassic (so-called Muschelkalk) of Poland demonstrate that, under unfavourable environmental conditions, this species commonly occurring in Triassic German basins exhibits a dwarfed shell. As a consequence of a marine regression episode resulting in a significant increase of salinity and a partial emersion of seafloor these bivalves vanished. The next transgressive pulse caused a re-emergence of these bivalves. They were initially characterized by half-size shells than in the population living prior to the regression episode and, subsequently, during progressive transgression, their shells returned to normal size. Coincidence between eustatic curve and changes in bivalve shell size and their disappearance may be attributed also to biotic interactions, such as a biotic collapse in primary bioproductivity or/and a competition for space or any other resources due to shelf habitat loss during regressive periods.

Carnets Geol., vol. 16, no. 10, p. 297-305

Online since May 21, 2016


Lithostratigraphic, paleoenvironmental and sequential evolution of the Cenomanian-Lower Turonian in the Guir area (western Algeria)
Madani BENYOUCEF, Christian MEISTER, Kaddour MEBARKI, Émilie LÄNG, Mohammed ADACI, Lionel CAVIN, Fatima-Zohra MALTI, Djamila ZAOUI, Amine CHERIF & Mustapha BENSALAH

 | HTML  | PDF [2,328 KB]  | DOI : 10.4267/2042/59926

 Abstract:  The lithostratigraphy, paleoenvironment and sequence stratigraphy of the Cretaceous succession outcropping at the southern foot of the Saharan Atlas (south-western Algeria) are discussed in this work. Our current study focuses on the analysis of fifteen sections distributed along to the northern and southern limits of the Guir Basin, between the Ben-Zireg to the East and the Meridja-Boukais axis to the West. In this area, three lithostratigraphic formations were identified and interpreted from an environmental and a sequential points of view: 1 - The lower "Grčs rouges" Formation attributed to the Lower Cenomanian, is dominated by fluvial or coastal detrital deposits; 2 - The middle "Marnes ŕ gypse inférieures" Formation assigned to the Lower-Middle Cenomanian is characterized by littoral setting with some storm influences, especially towards the base; 3 - The upper "Calcaires de Sidi Mohamed Ben Bouziane" Formation of upper Cenomanian-early Turonian in age, is represents a laterally extensive, shallow carbonate platform environment. The three formations constitute an overall transgressive megasequence built of a number of transgressive-regressive minor sequences. The opening of the deposits towards the North is confirmed by the organization of the lithostratigraphic units and by the Tethyan paleobiogeographic affinity of the echinoids and bivalves, and especially the occurrence of Neolobites vibrayeanus ammonite.

Carnets Geol., vol. 16, no. 9, p. 271-296

Online since April 14, 2016


Some steps toward a new story for the Jurassic - Cretaceous transition in Mount Lebanon
Bruno GRANIER, Christopher TOLAND, Raymond GÈZE, Dany AZAR & Sibelle MAKSOUD

 | HTML  | PDF [2,066 KB]  | DOI : 10.4267/2042/59924

 Abstract:   The stratigraphic framework of the Upper Jurassic and Lower Cretaceous strata of Lebanon that dates back to Dubertret's publications required either consolidation or full revision. The preliminary results of our investigations in the Mount Lebanon region are presented here. We provide new micropaleontological and sedimentological information on the Salima Oolitic Limestones, which is probably an unconformity-bounded unit (possibly Early Valanginian in age), and the "Grčs du Liban" (Barremian in age). Our revised bio- and holostratigraphic interpretations and the new age assignations lead us to emphasize the importance of the two hiatuses in the sedimentary record below and above the Salima, i.e., at the transition from the Jurassic to the Cretaceous.

Carnets Geol., vol. 16, no. 8, p. 247-269

Online since April 14, 2016


On the fossil alga Marinella lugeoni Pfender, 1939, nom. cons., and its seven unfortunate avatars. Revision of the Juliette Pfender Collection. Part 2. Revision of the Jesse Harlan Johnson Collection. Part 2
Bruno GRANIER & Dimas DIAS-BRITO

 | HTML  | PDF [1,378 KB]  | DOI : 10.4267/2042/59922

 Abstract:   A review of eight lookalike fossil species led to their being synonymized. Although Marinella lugeoni Pfender, 1939, is not the senior synonym, it is proposed to ascribe it the status of a "nomen conservandum". The age of its type-locality in Spain is Late Jurassic, not Early Jurassic. We also document small Marinella lumps found in Albian-Cenomanian strata of Brazil.

Carnets Geol., vol. 16, no. 7, p. 231-245

Online since April 14, 2016


On the fossil alga Elianella elegans Pfender & Basse, 1948, and its so-called lookalikes, with description of Elianella brasiliana n.sp. Revision of the Juliette Pfender Collection. Part 1
Bruno GRANIER & Dimas DIAS-BRITO

 | HTML  | PDF [2,885 KB]  | DOI : 10.4267/2042/59920

 Abstract:  According to some authors, the three fossil algal forms discussed in this paper, Cordilites cretosus (Reus, 1846), Elianella elegans Pfender & Basse, 1948, and Parachaetetes asvapatii Pia in Rao & Pia, 1936, are synonyms. However, we document that they represent three discrete species. The genus Elianella Pfender & Basse, 1948, gets unique features that make it easy to discriminate; we describe a new species from Albian limestones of Brazil. The other two genera, Cordilites Počta, 1887, and Parachaetetes Deninger, 1906, share some common features, such as having a thallus made of a lattice-network with columns (filaments) and rows. However, on the basis of their diagnostic criteria, the species Cordilites cretosus and Parachaetetes asvapatii should not be confused.

Carnets Geol., vol. 16, no. 6, p. 213-229

Online since April 14, 2016


Revision of the Early Cretaceous genera Heminautilus Spath, 1927, and Josanautilus Martínez & Grauges, 2006 (Nautilida, Cenoceratidae)
Cyril BAUDOUIN, Gérard DELANOY, Josep Anton MORENO-BEDMAR, Antoine PICTET, Jean VERMEULEN, Gabriel CONTE, Roland GONNET, Patrick BOSELLI & Marc BOSELLI

 | HTML  | PDF [34,156 KB]  | DOI : 10.4267/2042/58977

 Abstract:  In spite of recent interest, the various species belonging to the genera Heminautilus Spath, 1927, and Josanautilus Martínez & Grauges, 2006, have never been the object of a general revision. More than 160 specimens belonging to various species of these two genera were studied, with the aim of identifying the specific characteristics and the stratigraphical and paleogeographical distribution of the various species which compose them, as well as to propose a phylogeny for the group. This study confirms the non-synonymy of Heminautilus saxbii (Morris, 1848) and H. lallierianus (Orbigny, 1841), establishes that H. tejeriensis Martínez & Grauges, 2006, and H. verneuilli (Vilanova, 1870) represent junior synonyms of H. saxbii (Morris) and suggests the creation of the species H. ? japonicus sp. nov. The origin of the genus Heminautilus Spath from Pseudocenoceras Spath, 1927, proposed by Tintant, and the origin of Josanautilus Martínez & Grauges from Heminautilus Spath are both confirmed.

Carnets Geol., vol. 16, no. 5, p. 61-212

Online since April 14, 2016


Sedimentologic study of palustrine continental carbonate deposits of the Tajerouine area, NW Tunisia
Naoufel GHANNEM, Faouzia TLILI, Chadia RIAHI & Kamel REGAYA

 | HTML  | PDF [1,885 KB]  | DOI : 10.4267/2042/58976

 Abstract:  The palustrine limestones of Quaternary (Middle to the Late Pleistocene) age described in this study are located in the El Gara massif Tajerouine (Northwest of Tunisia). The study of these deposits at different scales, both macroscopic (lithological description) and microscopic (petrography and mineralogy), shows that they have a tabular structure and an angular unconformity contact on the marly – calcareous series from the Aptian-Albian. They show lateral and vertical changes in facies, including conglomerates, limestones and stromatolites. Layer thickness tends to increase gradually from East to West and also to the South of the study area. Because of fluctuations in the groundwater table, these carbonates have undergone periods of flooding and emersion, causing many pedogenic changes. The nature of these changes was controlled by the height of emersion (nodulisation, roots traces, pseudo-microkarst). These pedogenic changes in turn affected cementation during diagenesis (alternations of phreatic cement), justifying the rock classification as palustrine limestone.

Carnets Geol., vol. 16, no. 4, p. 43-59

Online since April 1, 2016


Lithothamnion crispatum: long-lasting species of non-geniculate coralline algae (Rhodophyta, Hapalidiales)
Giovanni COLETTI, Juraj HRABOVSKÝ & Daniela BASSO

 | HTML  | PDF [2,096 KB]  | DOI : 10.4267/2042/58720

 Abstract:  The examination of fossil Lithothamnion specimens ranging in age from the Early Miocene to the Pleistocene revealed the occurrence of multiporate conceptacle chambers pitted with depressions. This is the diagnostic feature of Lithothamnion crispatum, a cosmopolitan hapalidiacean with a wide depth range in modern oceans. The comparison of the microscopic anatomy of both the fossil and modern L. crispatum confirmed that they are conspecific. Therefore, this species has a long stratigraphic distribution starting at least 20 My ago, without significant morphological changes in either reproductive or vegetative anatomy.

Carnets Geol., vol. 16, no. 3, p. 27-41

Online since February 20, 2016


Insights into the permeability of polygonal faults from their intersection geometries with Linear Chimneys: a case study from the Lower Congo Basin
Sutieng HO, Daniel CARRUTHERS & Patrice IMBERT

 | HTML  | PDF [483 KB]  | DOI : 10.4267/2042/58718

 Abstract:  Layer-bound arrays of polygonal compaction faults have long been considered as important migration routes for hydrocarbon fluids leaking to the surface across thick shale sequences. A classic example is the deep offshore of the Lower Congo Basin where numerous fluid-venting structures are present above a Pliocene polygonal fault system. In this paper we present a detailed seismic analysis of a newly recognised system of Quaternary-aged Linear Chimneys and their intersection geometries with pre-existing Pliocene-aged polygonal faults (PF). Most (73%) of the 209 chimneys analysed intersect the lower portions of polygonal faults and almost half of these are rooted in strata below the PF interval. This indicates that fluid (in this case gas) migrated vertically, cross-cutting polygonal faults as it ascended through the tier. This is a strong indicator that PFs did not provide viable migration pathways otherwise chimneys would terminate at the upper tip of the fault, which would be the most likely migration exit point. Only twice in the whole system of Linear Venting Systems did this occur. A sub-set of chimneys stems from or above PF planes but these are restricted to either the lower footwall or from the apex area of hanging wall. At best they are evidence of fluids migrating up the lower part of polygonal faults and exiting deep within the tier, then migrating through most of the tier in their own vertical leakage vents. These results provide strong indicators that at least within this part of the Lower Congo Basin polygonal faults were the least effective/favoured migration pathway and that it was more energy-efficient for migrating gas to hydrofracture its fine-grained overburden than to re-open polygonal faults.

Carnets Geol., vol. 16, no. 2, p. 17-26

Online since February 20, 2016


A simple technique to establish sequences of datums and to highlight transgressive–regressive cycles
Jean GUEX & Federico GALSTER

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 Abstract:  The relative diachronism of first or last local occurrences (FOs and LOs) of fossil species may highlight transgressive/regressive cycles. A simple technique allowing the extraction of this information by means of the UAgraph program is discussed in the present paper. The technique consists in modifying a usual database of UAgraph by augmenting it with the restricted data concerning only the FOs (or LOs) of the taxa under consideration. The resulting data set combines the information on total ranges and those concerning the FOs and LOs only. Calculating the UAs of such a duplicated database produce a range chart in which we can read the maximal ranges of all the taxa and, in addition, the biochronological dispersion of the FOs and LOs. For a given transgressive/regressive cycle, the UAs defined by the species related to sea level fluctuations migrate with time from distal to proximal sections and inversely. This trend can be detected visually by the means of the UAs reproducibility chart, output of the UAgraph program. In a more general frame, the same holds true for species whose regional dispersion is related to specific conditions and when these conditions migrate in space with time (e.g., water temperatures and diatoms). The above discussion is strictly related to FOs and LOs that for a given section are definitive, however well constrained ephemeral appearances and disappearances can be easily integrated in the database for the same purposes.

Carnets Geol., vol. 16, no. 1, p. 1-16

Online since January 14, 2016


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