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2016 (vol. 16)

Late Albian cactus in shallow carbonate shelf, West Texas
Robert W. SCOTT, Xexuan 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

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 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

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 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

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 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

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 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

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 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)

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 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

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 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

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 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

 | HTML  | PDF [394 KB]  | DOI : 10.4267/2042/58207

 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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