Ammonite Record Research Articles

Lower Valanginian ammonite biostratigraphy of the Cañón de la Vaca section, Taraises Formation, Coahuila state, northern Mexico

The present work carries out a biostratigraphic analysis of the Valanginian ammonite record from the Cañón de la Vaca section in the Sierra de Parras, Coahuila, Mexico. The collection of ammonites exhibits an abundance of specimens, representing a biodiverse range of taxa, and numerous occurrences throughout the biostratigraphic section. This record in the Taraises Formation allowed a solid identification and diagnosis of the ‘Thurmanniceras’ pertransiens, Neocomites neocomiensiformis and Karakaschiceras inostranzewi zones, along with the Neocomites premolicus, Neocomites neocomiensiformis, Busnardoites campylotoxus, Karakaschiceras inostranzewi and Neocomites platycostatus subzones. These biostratigraphic units corresponds to the lower Valanginian of the Standard Mediterranean Ammonite Zonation. Among the collected ammonites, numerous specimens of significant biostratigraphic importance remain scarcely known or lack a proper taxonomic identification. This situation has been already observed in other sections, such as Peña del Águila, Zacatecas, Mexico, and in the Betic Cordillera, southeast Spain. Fortunately, these unknown taxa can be compared with other sections in Mexico and with lower Valanginian ammonites from Spain, facilitating a more comprehensive biostratigraphic analysis. The present biostratigraphic analysis is expected to contribute to an increased understanding of lower Valanginian strata, thereby enabling future taxonomic and biostratigraphic research on the lower Valanginian in Mexico and globally.

The Orthaspidoceras uhlandi (Oppel) record and the maximum flooding in the Eastern Algarve during the Lower Kimmeridgian

Detailed field work on Upper Jurassic outcrops in all of the eastern Algarve Basin, together with puntual observa­tions, leads to the improvement of our knowledge of the Lower Kimmeridgian in the area. The sediments belong to the Peral Formation, the upper part of which shows buildup remains dominated by sponges and algae, and to the lowermost Jordana Fm. The previously favourable environment for ammonites therefore decreased. The consistent ammonite record in the Divisum Zone ( Uhlandi Subzone), within or directly above reefal com­plexes or related sediments, is taken as proof of an episodic improvement of the environment for cephalopods. This event coincides well with the transgressive interval in the eustatic curves proposed by the Exxon Group, and especially with the maximum flooding of the epicontinental eastern Algarve Basin during the uppermost Lower Kim­meridgian.

Ammonite biostratigraphy of two stratigraphic sections of the La Peña Formation (Aptian, Lower Cretaceous) in Nuevo León State, northeast Mexico

The ammonite record of two stratigraphic sections, La Boca Canyon and La Huasteca Canyon, of the La Peña Formation in Nuevo León State, northeast Mexico, was studied in this work. In Mexico, ammonite provincialism during the Aptian prevents the use of European zonations. We followed previous works where we proposed the critical need to define an independent ammonite zonation for the Central Atlantic Province. The current study constitutes an advance in the development of an Aptian ammonite zonation for Mexico. The methodology employed is the usual in this kind of contribution, but two additional methodologies must be highlighted, a semi-quantitative biostratigraphy and the use of 3D models in showing the more relevant specimens. These methodologies are not new, but are uncommon in the contributions of the majority of the Lower Cretaceous ammonite workers, and can be an interesting example to follow. Three ammonite zones and one subzone were recognized: Dufrenoyia justinae interval Zone, Caseyella aguilerae interval Zone, Kazanskyella minima interval Zone and Dufrenoyia scotti/Burckhardtites nazasensis concurrent range Subzone. In addition, taxonomic contributions in some species of the genera Cheloniceras, Epicheloniceras and Kazanskyella have been made. The lower/upper Aptian transition in Mexico and Europe is discussed. Several options of study have been proposed in order to better characterize the lower/upper Aptian transition in Europe and its correlation with Mexico. This work aims at stabilizing the Mexican zonation and its applicability/reproducibility for the whole Central Atlantic Province.

High-resolution stratigraphy of buchiid bivalves and ammonites from the Jurassic–Cretaceous boundary beds in the Paskenta area (California)

A bed-by-bed biostratigraphical study of two key Californian outcrop sections of the Jurassic–Cretaceous transition interval is presented. The studied succession is characterized by the occurrence of abundant buchiid bivalves and a discontinuous ammonite record. Several Lower Cretaceous ammonite zones were introduced in the literature, but the precise stratigraphic position of key ammonite records remains unclear. Consequently, the buchiid zonation established in the 1970s is still considered the prime biostratigraphic tool, and ammonites have supplementary value. The buchiid zonation is revised herein, and the nomenclature and boundaries are updated. The Buchia sublaevis zone (upper Valanginian (?) to lower Hauterivian) is for the first time recognised in California. Two further biostratigraphic units, i.e. ‘Buchia inflata beds’ (between B. uncitoides and B. pacifica zones) at McCarty Creek and ‘Buchia aff. volgensis-B. unschensis beds’ (between B. piochii and B. okensis zones) at Grindstone Creek, are proposed. The positions of zonal boundaries are determined more precisely than previously, and are based on the first or mass occurrence of index species. As a result, the Panboreal correlation of Tithonian to Hauterivian buchiid zones is improved. Ammonite biohorizons are used as an additional correlative tool, and the following succession is proposed for the Upper Tithonian to earliest Hauterivian interval of the study area: Fierrites dilleri, Paradontoceras storssi, Proniceras maupinense, Neocosmoceras euchrense, Paskentites paskentaensis, Kilianella crassiplicata, Neotollia mutabilis, Sarasinella hyatti, Thurmanniceras stippi, T. jenkinsi and Jeannoticeras jeannoti. A new genus, Howarthiceras n. gen. is established for “Groebericeras” baylei Imlay and Jones.

Recurrent hardgrounds and their significance for intra-basinal correlations: a case study of upper Bathonian rocks from the western margin of the Indian craton

A set of two to three prominent hardgrounds can be traced for more than 40 km from east to west within the Jurassic succession of the Jaisalmer Basin at the western margin of the Indian Craton. The hardgrounds started to form under subtidal conditions in a mixed carbonate–siliciclastic setting during the last phase of a transgressive systems tract, i.e. the maximum flooding zone. The age difference between the hardgrounds is very small, but they differ lithologically. Typically, the stratigraphically oldest hardground occurs at the top of a 1-m-thick calcareous sandstone. It is characterized by a spectacular megaripple surface encrusted with oysters and subsequently occasionally bored by bivalves. The hardground is overlain by 10–25 cm of biowackestone to biopackstone, at the top of which another hardground is developed. This second hardground is characterized by abundant bivalve (Gastrochaenolites isp.) and “worm” borings (Trypanites and Meandropolydora isp.) and occasional oyster encrustations. The third hardground can be found within the overlying 60-cm-thick, bioturbated, fossiliferous silty marly packstone. It shows common to abundant oyster encrustations and occasional borings together with reworked concretions. The individual hardground can be well recognized throughout the basin based on lithology and biotic components. The second hardground (biowackestone to biopackstone) with abundant bivalve and worm borings is most prominent and widespread. Lithostratigraphically, these three hardground surfaces belong to the uppermost part of the Bada Bag Member of the Jaisalmer Formation. Based on ammonites, such as Perisphinctes congener (Waagen), brachiopods, and corals, this interval of the Bada Bag Member has been assigned a late Bathonian age. The entire succession above the first hardground is bioturbated up to the overlying marly silt of the Kuldhar Member of the Jaisalmer Formation, which is already Callovian in age. The characteristic hardground lithologies, together with the ammonite record, allow long-distance correlations within the basin emphasizing their importance as valuable marker horizons. The biotic components associated with the hardgrounds and alternating sediments represent high diversity community relicts developed in shallow-water, open-marine environments.

Integrated calcareous nannofossil and ammonite data from the upper Barremian–lower Albian of the northeastern Transdanubian Range (central Hungary): Stratigraphical implications and consequences for dating tectonic events

The transition of post-obduction Neotethyan contraction to Eo-Alpine (“Austroalpine”) nappe stacking that took place during the Early Cretaceous is an important event in the Cretaceous history of the Alpine-Carpathian-Dinaridic orogenic system. The Transdanubian Range (TR) in Hungary has been shown to have been impacted by both events. Dating of this transition has proved to be difficult; several interpretations were published during the last decades. Uniquely exposed in and around the Gerecse Mountains, the Lábatlan Sandstone Formation (LSF) and the Vértessomló Aleurolite Formation (VAF) bracket this time interval. The last phase of Neotethyan contraction occurred after the deposition of the LSF, and the Eo-Alpine nappe stacking started during the deposition of the VAF. Earlier stratigraphical studies provided valuable data for our understanding of geodynamics, but precise bio- and chronostratigraphy – to constrain the interval of deformations – became possible only with systematic correlation of macrofossil-poor outcrops to ammonite-rich series with the help of calcareous nannofossil data. Here new nannofossil data, and whenever still available, a re-examination of the original smear slides yields new constraints on the age of the younger part of the sequence. The nannofossil and ammonite record is now combined to create a local chronostratigraphical correlation, with records tied in to the integrated Tethyan nannofossil and ammonite biozonation, which, in turn, is numerically calibrated by international zonal standards and radiometric ages. Our study demonstrates that the youngest part of the sandstone-dominated LSF is in nannozone NC7B of late Aptian age. The overlying Köszörűkőbánya Conglomerate Member has a closely similar age in NC7A/B, noticeably older than previously suggested. On the other hand, the VAF clearly is of early Albian age (NC8). This latter unit is not represented in the inner Gerecse Mountains, in contrast to what has been suggested in previous studies. The observed earlier two sub-phases of Neotethyan compression with N–S to ENE–WSW shortening possibly are latest Aptian in age and are within nannofossil subzone NC7C, while the following Eo-Alpine deformation has an early Albian age (NC8) and is marked by W–E to NW–SE compression. The Aptian/Albian boundary (around 113 Ma) may indicate the switch of the TR from lower to upper position with respect to Neotethyan subduction to “Austroalpine” nappe stacking. Our data on deformational ages may support the idea that the onset of “Austroalpine” transpressional deformation at c. 114–112 myr ago could be related to the start of Penninic subduction, or, alternatively, to the onset of intra-oceanic subduction within the Austroalpine realm.

Response of benthic foraminiferal assemblages to contrasting environments during the Aalenian–Bajocian in the Iberia: a case study from the Talveila section (Iberian Range) and Murtinheira section (Lusitanian Basin)

This paper analyses the composition of the foraminiferal assemblages recorded in two different depositional contexts located in Iberia for the lower Aalenian–lower Bajocian stratigraphical interval: the Talveila section, corresponding to proximal facies within the Iberian Basin (NE Spain), and the Murtinheira section representing distal facies within the Lusitanian Basin (Western Portugal). The obtained specimens (10,736 in total) correspond to 6 suborders, 10 superfamilies, 16 families and 30 genera in Talveila, and 6 suborders, 9 superfamilies, 14 families and 26 genera in Murtinheira. Several biostratigraphic units based on the foraminiferal record and accurately calibrated with the ammonite record were recognized in both sections. Moreover, bioevents based on the first or last appearances of a taxon, significant changes in the abundance of one or several taxa or noticeable changes in diversity of the assemblages were also identified. The palaeoecological analysis shows that the foraminiferal assemblages from both sections were developed in a well-oxygenated and normal salinity shelf environment. The application of diversity indexes indicates that the paleoenvironmental conditions did not remain constant throughout the studied stratigraphic interval; changes recognized in both sections are similar, coeval and correspond to three intervals representing paleoenvironmental conditions more or less favourable for the development of the foraminiferal assemblages. As so, despite the different paleogeographical locations, the development of the assemblages in both sections during the Early Aalenian–Early Bajocian seem to have been conditioned by environmental changes of regional scale, which affected at the same time both the Iberian and the Lusitanian basins.

Late Barremian–early Aptian ammonite bioevents from the Urgonian-type series of Provence, southeast France: Regional stratigraphic correlations and implications for dating the peri-Vocontian carbonate platforms

This work provides a new ammonite age-calibration of the rudistid limestones of the Urgonian-type Provence carbonate platform (Southeast France) based on sampling along three ∼200 km-long platform-to-basin transects and re-examination of historical collections. Ammonite key findings indicate that the first rudistid platform stage (including the Agriopleura and requieniid–monopleurid beds) develops and spreads northward through the Toxancyloceras vandenheckii–Gerhardtia sartousiana zones interval (lower upper Barremian). This stage is interrupted by the tectonically-induced deepening of the southern Provence domain during the Imerites giraudi Zone while the northern regions records the massive deposition of Palorbitolina–Heteraster beds. Recovery of the rudistid carbonate system is illustrated by the development of caprinid-bearing rudistid limestones in the North Provence domain through the Martelites sarasini Subzone (lower Martelites sarasini Zone, uppermost Barremian), which shows a bidirectional progradation toward the South Provence and Vocontian basins. The caprinid-bearing limestones terminate at a short-term exposure and are overlain by cherty–oobioclastic deposits spanning the Pseudocrioceras waagenoides Subzone (upper M. sarasini Zone) to the lower Deshayesites forbesi Zone. A regional-wide flooding of the study area is illustrated by the abrupt change to a marl-dominated regime occurring in the upper D. forbesi Zone. Compared to the previous datings, the Barremian/Aptian boundary should be relocated in the lower part of the post-caprinid, cherty–oobioclastic deposits although its precise level cannot be fixed due to the lack of a continuous ammonite record. Ammonite age-calibration of the surrounding Urgonian rudistid platform series is discussed and gives evidence of a comparable twofold demise of the peri-Vocontian rudistid biota during the uppermost Barremian. Accordingly, the link between the final demise of the peri-Vocontian rudistid biota and the onset and/or culmination of the mid-early Aptian Oceanic Anoxic Event (OAE) 1a should be reconsidered.

Calcareous nannofossil and ammonite integrated biostratigraphy across the Jurassic – Cretaceous boundary strata of the Kopanitsa composite section (West Srednogorie Unit, southwest Bulgaria)

AbstractCalcareous nannofossil, calpionellid and ammonite occurrences have been directly constrained across the Jurassic–Cretaceous boundary interval in the section of Kopanitsa, SW Bulgaria. This section reveals a continuous and expanded sedimentary record through the Upper Tithonian and Lower Berriasian, besides an excellent calcareous nannofossil and ammonite record. The topmost part of the NJT 16b and the base of NJT 17a nannofossil Subzones correspond to the ammonite Microcanthum / Transitorius Subzone. The major part of the NJT 17a Subzone equates to the Durangites spp. ammonite Zone, whereas the NJT 17b Subzone correlates to the lower part of the B. jacobi ammonite Zone. The NKT nannofossil Zone approximately corresponds to the upper part of the B. jacobi Zone and the NK-1 nannofossil Zone correlates at least to the lowest part of the T. occitanica Zone. The FOs ofNannoconus globulus minor, N. wintereri, N. kamptneri minor, N. steinmannii minor, N. kamptneri kamptneriandN. steinmannii steinmanniiare confirmed as reliable bio-horizons for correlations in the Mediterranean Tethys area. The first occurrence ofNannoconus wintereriis regarded as an almost concomitant event with the first occurrence ofBerriasella jacobi. We suggest it could be the most useful nannofossil proxy for approximating the base of the B. jacobi Zone. Rare, but relatively well preserved calpionellids and calcareous dinoflagellates together with microfacies analysis were used additionally for stratigraphical and palaeoenvironmental interpretations. The investigated sediments are typical for the steep slope of a steepened ramp, with accumulation of hemipelagic and gravitational deposits.

Ammonite biostratigraphy of the Sierra del Patrón section (Durango State, Mexico) and its bearing on the lower/upper Aptian boundary of the Central Atlantic Province

The present contribution documents the ammonite record of the La Peña Formation from the la Sierra del Patrón (Durango State, Mexico). Twenty one taxa have been recognized among the 354 specimens collected bed-by-bed. Our biostratigraphic analysis of the ammonite distribution allow us to improve the zonation for the transition of the lower/upper Aptian boundary of northeast Mexico, composed of the upper lower Aptian Dufrenoyia justinae interval Zone (including the Dufrenoyia scotti/Burckhardtites nazasensis concurrent range Subzone at its top) and the lower upper Aptian Caseyella aguilerae interval Zone (including the Gargasiceras? adkinsi total range Subzone at its base). The lower/upper Aptian boundary is herein well characterized by means of endemic or quasi-endemic taxa and a strong ammonite faunal turnover marked by a renewal at the generic and family levels comparable with those of the Mediterranean-Caucasian regions (end of Deshayesitidae balanced by the renewal in the douvilleiceratid, desmoceratid and acanthohoplitid stocks). The proposed zonation of the Sierra del Patrón section is an important step forward in the development of the Aptian zonation for the Central Atlantic Province and its correlation with the Standard Mediterranean ammonite zonation.

Integrated biostratigraphy (Ammonoidea, Foraminiferida) of the Aalenian of the Lusitanian Basin (Portugal): A synthesis

This work reports a synthesis of biostratigraphic outputs resulting from over two decades of comprehensive analysis on the expressive stratigraphic record of the Aalenian of the Lusitanian Basin (Western Iberian Margin; Portugal). The rich and diverse ammonite record studied from ten marly and marlylimestones sections, outcropping in different sectors of the basin ranging from distal to proximal facies, has enabled the establishment of a detailed ammonite-based biostratigraphical framework correlated with other basins located around the Iberian Plate. The study of the benthic foraminiferal assemblages occurring in the basin at the same time interval has allowed the establishment of a biostratigraphic scale based on microfossils accurately calibrated with the ammonite record, an important tool in log interpretation in the context of hydrocarbon exploration activities in sedimentary basins of Jurassic age. The corresponding biostratigraphic units are characterized, as well as the bioevents recognized in the benthic foraminiferal record, which represent another additional proxy of local and/or regional rank that can be used to determine both the age and depositional environment assigned to core samples.

Late Maastrichtian cephalopods, dinoflagellate cysts and foraminifera from the Cretaceous–Paleogene succession at Lechówka, southeast Poland: Stratigraphic and environmental implications

The Lechówka section comprises the most complete Cretaceous–Paleogene (K-Pg) boundary succession in Poland and is among 29 sites worldwide with the youngest ammonite record. Here, cephalopods (ammonites and nautilids), organic-walled dinoflagellates (dinocysts) and foraminifera from the uppermost Maastrichtian interval are studied. In terms of ammonite biostratigraphy, the upper Maastrichtian Hoploscaphites constrictus crassus Zone is documented up to a level 120 cm below the K-Pg boundary. There is no direct, ammonite-based evidence of the highest Maastrichtian H. constrictus johnjagti Zone. However, the predominance of the dinocyst marker taxon Palynodinium grallator suggests the presence of the equivalent of the uppermost Maastrichtian Thalassiphora pelagica Subzone, which is correlatable with the H. c. johnjagti ammonite Zone. The planktonic foraminiferal assemblage is coeval with that from the H. c. johnjagti Zone as well. These data indicate that the top of the Maastrichtian at Lechówka is complete within the limits of biostratigraphic resolution, albeit slightly condensed. The dinocyst and foraminiferal assemblages are dominated by taxa that are characteristic of high-energy, marginal marine environments. A reduction in test size among the calcareous epifaunal benthic foraminifera is observed at a level 50 cm below the K-Pg boundary, which is possibly related to environmental stress associated with Deccan volcanism.

Towards an Aptian (Lower Cretaceous) ammonite biostratigraphy of the Mina Texali section, Central Atlantic province (Puebla State, Central Mexico)

This work is a study of the ammonite record of a new stratigraphic section of Aptian age, at the Mina Texali (Puebla State, Central Mexico). A detailed biostratigraphic analysis was carried out on 309 specimens systematically sampled on a bed-by-bed basis. An Aptian ammonite zonation is proposed for the Mina Texali (= MT) section with two interval zones, Dufrenoyia justinae and Caseyella sp., and one informal biostratigraphic unit represented by the Huastecoceras trispinosoides beds. We also analyze the taxonomic composition, paleoecology and some systematic issues of the ammonite record of the MT section. The first record in Mexico of the genera Pseudosaynella, Xerticeras and the nautiloid Heminautilus is identified in this section. The ammonite assemblage is assigned to the proximal part of the outer neritic region of the continental shelf. The ammonite record of the studied section is diagnostic in establishing the lower-upper Aptian transition, and the local zonation of the MT provides important data for the development of an Aptian standard ammonite zonation for the Central Atlantic province.

Lower Aptian ammonites of the Sierra de Parras, Coahuila State, northern Mexico

We report an interesting Aptian ammonite record from the La Pena Formation in the Sierra de Parras, Coahuila State. This assemblage is analyzed from a paleoecological perspective. It contains the first reported occurrence of a macroconch of Dufrenoyia from Mexico, and the largest known specimen of 'Gargasiceras' adkinsi. Such an assemblage yielding large ammonites is unusual in deposits of this age in Mexico. To properly document this ammonite record, we review 'Gargasiceras' adkinsi, formerly misidentified as Rhytidoplites robertsi, and allied taxa. From our analysis, we conclude that 'Gargasiceras' adkinsi exhibits strong intraspecific variability. We also analyze in detail the differences that exist between 'Gargasiceras' adkinsi and Rhytidoplites robertsi, and emend the concept of the genus Rhytidoplites. These revisions are important from a biostratigraphic point of view since 'Gargasiceras' adkinsi is an index species for the lower Aptian ammonite zonation of Mexico.

High resolution ammonite–benthic foraminiferal biostratigraphy across the Aalenian–Bajocian boundary in the Lusitanian Basin (Portugal)

This work describes the ammonite and benthic foraminiferal assemblages recorded across the Aalenian–Bajocian boundary of the Serra da Boa Viagem II section, located about 6 km to the east of the Bajocian GSSP (Murtinheira, Portugal), and calibrated to the standard ammonite zonation previously established for the Lusitanian Basin. A total of 220 ammonite specimens referred to 30 fossiliferous levels were collected and identified throughout the section, enabling the recognition of the Concavum Zone (Concavum and Limitatum subzones) of the upper Aalenian, and the Discites Zone of the lower Bajocian. A total of 2356 foraminifers were obtained from the 16 samples collected along the section, corresponding to 4 suborders, 8 families, 16 genera and 44 species.The occurrence of Lenticulina quenstedti (Gümbel) has enabled the recognition of the Lenticulina quenstedti Zone, ranging from the Bradfordensis Zone (middle Aalenian) to the lower Discites Zone (lower Bajocian). The first record of Ramulina spandeli Paalzow, whose occurrence, up to now, was limited in the Lusitanian Basin to the Murtinheira section (the Bajocian GSSP), highlights the usefulness of the Ramulina spandeli Zone, with its lower boundary referred to the lower Discites Zone (lower Bajocian). Other bioevents displaying local, basinal or regional biostratigraphic interest have also been identified.The benthic foraminiferal record here presented, accurately calibrated with the ammonite record, aims at contributing to support the recognition of the Lenticulina quenstedti Zone and the Ramulina spandeli Zone as formal biostratigraphic units integrating the biostratigraphic scale based on benthic foraminifers for the Aalenian–Bajocian boundary in the Lusitanian Basin (Portugal). Copyright © 2014 John Wiley & Sons, Ltd.

Ammonite-benthic Foraminifera turnovers across the Lower-Middle Jurassic transition in the Lusitanian Basin (Portugal)

This paper describes and characterises the co-occurrence of ammonite and benthic foraminiferal assemblages across the São Gião outcrop (Central Portugal), a reference section for the Lower-Middle Jurassic boundary in the Lusitanian Basin. The upper Toarcian-lower Aalenian marls and marly-limestones in this section provide a precise and detailed ammonite-based biostratigraphic zonation, with a mixed assemblage of northwest European and Mediterranean faunal elements, associated with benthic foraminifera assemblages with northern hemisphere affinities, both correlatable with the Aalenian GSSP at the Fuentelsaz section (Iberian Cordillera, Spain). A total of 447 well-preserved ammonite specimens and 13.116 foraminifera have been studied; no evidence was detected of any taphonomic processes that could have changed the original assemblages. From a biostratigraphic point of view, the ammonite record has enabled four biostratigraphic units to be recognised (the Mactra and Aalensis subzones of the Aalensis Biozone in the upper Toarcian, and the Opalinum and Comptum subzones of the Opalinum Biozone in the lower Aalenian). With regard to the benthic foraminifera, the taxa identified have enabled the Astacolus dorbignyi Zone and 11 bioevents to be identified, most of which representing local biostratigraphic proxies. However, the increase in the relative abundance of Lenticulina exgaleata Dieni from the upper part of the Opalinum Subzone to the lower part of the Comptum Subzone has a regional value. The constant and continuous ammonite record of northwest European taxa, together with typical Mediterranean taxa – namely Grammoceratinae – throughout the section, the high relative abundance of Miliolina representatives – generally interpreted as foraminifers typical of shallow waters – and the absence of foraminiferal forms typical of cool waters, do not support the inference of cool seawater temperatures attributed to the Early Aalenian, or the global character of the “Comptum cooling event”, at least with reference to the Lusitanian Basin.

Ammonites and biostratigraphy of the Cuyo Group in the Sierra de Reyes (central Neuquén Basin, Argentina) and their sequential significance

This paper deals with the stratigraphy, the systematics of the ammonite faunas and the biostratigraphy of the Cuyo Group in the Sierra de Reyes (southern Mendoza Province, Argentina). The 230 m-thick marine siliciclastic succession of the Cuyo Group is composed, from base to top, of the Bardas Blancas Formation (mudstones, sandy mudstones and fine-grained sandstones), the Los Molles Formation (black shales) and the Lajas Formation (coarse- to fine-grained sandstones). Four ammonite levels have been recognised in the Cuyo Group. The first two levels (I and II) are located in the lower section of the Bardas Blancas Formation. The monospecific level I yielded Dumortieria cf. pusilla and is regarded as upper Toarcian-lower Aalenian, whereas level II is composed of an association of Podagrosiceras cf. athleticum, Planammatoceras sp., Phylloceras sp. and Eudmetoceras? sp. which can be assigned to the upper Aalenian-lower Bajocian Malarguense Standard Zone. The third level appears in the middle part of the Bardas Blancas Formation and contains Morphoceras sp., Phylloceras sp. and Lytoceras cf. eudesianum, and is assigned to the lower Bathonian. The younger level (IV) was identified in the lower section of the Lajas Formation and yielded Iniskinites crassus, Stehnocephalites gerthi and Lilloettia cf. steinmanni. This association corresponds to the upper Bathonian Steinmanni Standard Zone. The ammonite record of the Cuyo Group in the Sierra de Reyes allowed dating the transgressive succession represented by the Bardas Blancas and Los Molles formations as late Toarcian-early Bathonian. The ammonite fauna identified in the lower section of the Lajas Formation (level IV) indicates that the change from transgressive to regressive conditions recorded at the upper part of the Cuyo Group occurred around early-late Bathonian times. The uppermost strata of the Cuyo Group (upper section of the Lajas Formation), having a well defined regressive signature, can be dated as lower Callovian. This relatively well constrained biostratigraphic framework of the Cuyo Group in the Sierra de Reyes and its comparison with neighbouring areas allowed for a better understanding of the sequential evolution in central Neuquén Basin during the late Lower to Middle Jurassic.

Uppermost Lower Aptian transgressive records in Mexico and Spain: chronostratigraphic implications for the Tethyan sequences

Terra Nova, 24, 333–338, 2012AbstractA widespread marine transgression, which began at the very end of the Early Aptian, is well recorded in Mexico and Spain. In Mexico, this transgression was the most important Aptian transgressive event and its record begins in the Dufrenoyia justinae Zone, whereas in Spain the corresponding transgression is registered in the uppermost part of the Dufrenoyia furcata Zone. The basal age of this Tethyan transgression does not correspond exactly to any of the Lower Aptian third‐order sequences as reported in the literature. In Spain, the most important Aptian transgressive event was earlier, corresponding to the Tethyan sequence Ap3, which is well‐defined below this transgression reported herein. Consequently, it is possible to correlate this later transgressive event with the third‐order Ap4 sequence, which has commonly been attributed to the Upper Aptian. The available ammonoid data allow us to correct the basal age of the transgression, and to correlate the start of the ammonite record in Mexico with the Tethyan sequence Ap4.

Otapirian and Aratauran sequences (latest Triassic and earliest Jurassic) along the northern Marokopa coast (SW Auckland, New Zealand) and observations on the Triassic/Jurassic boundary in New Zealand

Data on the changes in lithology and fauna across the Otapirian/Aratauran (Triassic/Jurassic) boundary are provided by 4 representative sections along the northern Marokopa coast. The latest Otapirian is characterised by a thick bed (40+ m) of indurated sandstone, extensively bioturbated. Aratauran sediments consist of mudstone, siltstone and sandstone, with some intercalated tuffaceous beds. In the absence of ammonites, the transition from the bivalve Otapiria dissimilis (Otapirian) to O. marshalli (Aratauran) is used as faunal control points across the boundary. The first appearance of Otapiria marshalli also serves as a proxy for the Triassic/Jurassic boundary. At Marokopa, ammonite assemblages first appear some 15–20 m above the boundary and the occurrence of Metophioceras is indicative of the base of the Conybeari Subzone (earliest Sinemurian). The hiatus in the ammonite record in New Zealand is a reflection of the worldwide biotic crisis across the Triassic/Jurassic boundary.

Evolution of pelagic swells from hardground analysis (Bathonian–Oxfordian, Eastern External Subbetic, southern Spain)

The Middle Bathonian to Middle Oxfordian interval in the Eastern External Subbetic (Betic Cordillera, SE Spain) is characterized by Ammonitico Rosso facies including various stratigraphic breaks. Five hardground-bounded units are recognized in relation to hiatuses in the ammonite record at the following stratigraphic boundaries: Hg1 (Lower–Middle Bathonian), Hg2 (Middle–Upper Bathonian), Hg3 (Lower–Middle Callovian), Hg4 (Middle–Upper Callovian), and Hg5 (Callovian–Oxfordian). Interesting features of these hardgrounds include their microfacies, ferruginous crusts and macro-oncoids, taphonomy of macroinvertebrates, trace fossils, neptunian dykes, and the hiatuses associated with each of them. The main hardgrounds (Hg1, Hg2, and Hg5) contain trace fossils of the Cruziana and Trypanites ichnofacies as well as abundant fossil macroinvertebrates with taphonomic features evidencing corrasion, early diagenesis, and reworking, indicating substrate evolution from softground to hardground. Neptunian dykes affected the trace fossils and ammonoid moulds, and their walls and the hardground surfaces were colonized by ferruginous microbial crusts. These features are characteristic of the External Subbetic pelagic swells, where the absence of sedimentation, sediment bypassing and erosion, and early diagenesis during relative sea-level falls produced hardgrounds. The neptunian dykes are indicative of tectonic activity in the areas of pelagic swells. Ferruginous crusts and macro-oncoids developed only on hardground surfaces and neptunian dykes walls prior to deposition of condensed bioclastic beds, which are interpreted as the first deposits after hardground development and are related to the onset of transgression. The varying ranges of the gaps as well as lateral facies changes are related to different local paleobathymetry controlled by the activity of listric faults.