Hasil untuk "Geology"

Piero Poli, Josipa Majstorović, T. Dylan Mikesell

Broadband seismometers, though designed to record ground motion generated by earthquakes, are also sensitive to a wide range of other processes occurring at the interface between the solid Earth, oceans, and atmosphere, often considered noise. In the sub-seismic band (1–24 hours), they can detect tidal signals but are limited by self-noise for weaker Earth and atmospheric processes. By applying a coherence-based network stacking technique to large seismic arrays, we identify weak, periodic gravity signals at these frequencies. Using three years of collocated vertical seismic and pressure data from USArray, we demonstrate the atmospheric origin of these oscillations. Coherence and transfer function analysis reveal strong links between pressure and seismic acceleration at atmospheric tide periods. The transfer function shows frequency dependence consistent with superconducting gravimeter observations, and its consistently negative phase indicates that pressure increases correspond to decreases in gravitational acceleration. This confirms Newtonian attraction from atmospheric mass changes as the dominant mechanism. Our results show that network stacks of broadband seismometers can detect atmospheric gravity variations as small as 10–100 nanogals, demonstrating their value for gravimetry and for observing atmospheric dynamics. This approach also provides a framework to estimate atmospheric noise in the sub-seismic range, improving the detection of solid Earth signals once such contamination is removed.

Simon Ghyselincks, Valeriia Okhmak, Stefano Zampini et al.

Abstract Reconstructing the structural geology and mineral composition of the first few kilometers of the Earth's subsurface from sparse or indirect surface observations remains a long‐standing challenge with critical applications in mineral exploration, geohazard assessment, and geotechnical engineering. This inherently ill‐posed problem is often addressed by classical geophysical inversion methods, which typically yield a single maximum‐likelihood model that fails to capture the full range of plausible geology. The adoption of modern deep learning methods has been limited by the lack of large 3D training data sets. We address this gap with StructuralGeo, a geological simulation engine that mimics eons of tectonic, magmatic, and sedimentary processes to generate a virtually limitless supply of realistic synthetic 3D lithological models. Using this data set, we train both unconditional and conditional generative flow‐matching models with a 3D attention U‐Net architecture. The resulting foundation model can reconstruct multiple plausible 3D scenarios from surface topography and sparse borehole data, depicting structures such as layers, faults, folds, and dikes. By sampling many reconstructions from the same observations, we introduce a probabilistic framework for estimating the size and extent of subsurface features. While the realism of the output is bounded by the fidelity of the training data to true geology, this combination of simulation and generative AI functions offers a flexible prior for probabilistic modeling, regional fine‐tuning, and use as an AI‐based regularizer in traditional geophysical inversion workflows.

Andrea Guerrero, Thierry Smith, Adán Pérez-García

This study analyses the diverse types of bioerosion marks on the shells of two sea turtle specimens from the Lutetian (middle Eocene) of Belgium. The objective is to understand the physical stressors affecting these organisms in ancient marine environments and to identify the potential factors responsible for these changes. The first specimen, the holotype of Eochelone brabantica, presents multiple erosive anomalies on its costal plates. The second individual, corresponding to Puppigerus camperi, exhibits different types of shell deviations, also of an erosive character. The combination of macroscopic and imaging techniques (i.e., CT scanning and 3D surface scanning) allow us the precise observation and identification of the different alterations. Comparative analyses suggest several external factors as potential causes of the anomalies of the two specimens (i.e., invertebrate feeding traces and trauma). This detailed examination provides specific insights into the types and causes of shell alterations in these Eocene turtles, offering a clearer understanding of their interactions in the marine ecosystem in which they lived.

Brittany N. Hupp, Mohammed S. Hashim, Raquel Bryant et al.

Scientific ocean drilling (SciOD) has been invaluable in advancing our understanding of Earth history. However, the most recent international SciOD programme ended in 2024, alongside the non-renewal of the riserless drilling vessel, the JOIDES Resolution. The US has not committed to joining a new SciOD programme despite prior efforts focused on important scientific priorities (e.g. climate change, assessing natural hazards). During this critical juncture, we argue that incorporating accessibility, justice, equity, diversity and inclusion (AJEDI) efforts will further develop a cohesive community that is well prepared to tackle questions critical to the US and global community. Herein we provide recommendations to develop a knowledgeable and diverse community of scientists in the changing landscape of US SciOD, as informed by historical participation data and recent efforts by early career scientists. Recommendations focus on accessible training opportunities, enhanced stewardship of archived materials, additional funding for research at all academic levels, inclusion of cultural advisors and social scientists, and a commitment to continuing SciOD education. By pursuing these recommendations, the US SciOD community could become a leader for modelling AJEDI principles and ensuring equitable knowledge transfer that is needed to reimagine and rebuild a new, inclusive SciOD programme.

Abdolvahab Afroogh, Jaber Shoghi, Mohammad Seraj et al.

Understanding the governing factors that influence structural style and fault-related folding mechanisms is crucial in the Dezful Embayment, an area of ∼ 60,000 km2 which accounts for most of oil production from Iran. Such studies enable the subsurface kinematic modeling of structures and structural geological analysis of hydrocarbon traps. In this study, variations in geometry and folding mechanism along the strike of the Mansourabad anticline are studied through field data, 2D and 3D seismic lines interpretation and well data. The displacement-distance profile of the forelimb thrust fault indicates that the anticline is a fault propagation fold in its central and NW parts. In the SE part of the anticline, there is a north-verging detachment fold, which is opposite to the southward vergence at the NW part. Due to structural variations, the amount of slip along the NW-SE trending Behbahan Fault varies. This variation in structural style results from changes in slip along the Behbahan Fault's forelimb. This blind thrust, which trends NW-SE, extends along the entire length of the Mansourabad anticline. The variable thickness of the syn-folding sediments controlled the structural style of the anticline, which interacted with the migration of the Gachsaran Formation and the deformation of the competent rocks.

T. A. Kireeva

The principles of using chlorine-bromine (Cl/ Br), boron-bromine (B/Br) and sodium-potassium (Na/K) ratios to determine the genesis of aqueous solutions are substantiated. Examples are given of clarifying the origin of waters, both natural and man-made, based on the relationship between the values of these coefficients and the mineralization of solutions (Mtot). It is shown that different values of the Cl/ Br coefficient make it possible not only to identify the difference between natural brines of concentration and brines of dissolution of halite strata, but also to identify groundwater contaminated with technogenic brines of hydrochloric acid treatments. Using the ratio B/Br > 1 makes it possible to diagnose the influx of high-temperature ( >100 °C) waters into deep aquifers, and is also a sign of the release of technogenic condensate waters. The value of the Na/K ratio, if reduced to 1 or less, makes it possible to reliably diagnose both technogenic brines and technogenically polluted fresh waters.

Pavel Burgert, Petr Šída, František Trampota et al.

In the 20 years since the important discovery of metabasite quarrying in the Jizerské hory Mountains (northern Bohemia, Czech Republic) a wealth of knowledge has been gathered on this raw material used to make Neolithic polished stone tools. A synthesis of these results now gives a more comprehensive view of how this raw material was employed in different periods and extracted directly at the source sites. The overview is at the same time a springing board for further research. Exploitation of metabasite and its widest spatial distribution was in the Linear Pottery culture (LBK; 5400–5000/4900 BC). In the post-LBK period of regionalisation, the sources of raw materials for the production of polished tools were diversified and the degree of utilisation of Jizera Mountains-type metabasite is demonstrably varied by region.

E. Salganik, E. Salganik, B. A. Lange et al.

<p>Sea-ice ridges constitute a large fraction of the total Arctic sea-ice area (up to 40 %–50 %); nevertheless, they are the least studied part of the ice pack. Here we investigate sea-ice melt rates using rare, repeated underwater multibeam sonar surveys that cover a period of 1 month during the advanced stage of sea-ice melt. Bottom melt increases with ice draft for first- and second-year level ice and a first-year ice ridge, with an average of 0.46, 0.55, and 0.95 m of total snow and ice melt in the observation period, respectively. On average, the studied ridge had a 4.6 m keel bottom draft, was 42 m wide, and had 4 % macroporosity. While bottom melt rates of ridge keel were 3.8 times higher than first-year level ice, surface melt rates were almost identical but responsible for 40 % of ridge draft decrease. Average cross-sectional keel melt ranged from 0.2 to 2.6 m, with a maximum point ice loss of 6 m, showcasing its large spatial variability. We attribute 57 % of the ridge total (surface and bottom) melt variability to keel draft (36 %), slope (32 %), and width (27 %), with higher melt for ridges with a larger draft, a steeper slope, and a smaller width. The melt rate of the ridge keel flanks was proportional to the draft, with increased keel melt within 10 m of its bottom corners and the melt rates between these corners comparable to the melt rates of level ice.</p>

J. Mern, J. Caers

<p>Geoscientific models are based on geoscientific data; hence, building better models, in the sense of attaining better predictions, often means acquiring additional data. In decision theory, questions of what additional data are expected to best improve predictions and decisions is within the realm of value of information and Bayesian optimal survey design. However, these approaches often evaluate the optimality of one additional data acquisition campaign at a time. In many real settings, certainly in those related to the exploration of Earth resources, a large sequence of data acquisition campaigns possibly needs to be planned. Geoscientific data acquisition can be expensive and time-consuming, requiring effective measurement campaign planning to optimally allocate resources. Each measurement in a data acquisition sequence has the potential to inform where best to take the following measurements; however, directly optimizing a closed-loop measurement sequence requires solving an intractable combinatoric search problem. In this work, we formulate the sequential geoscientific data acquisition problem as a partially observable Markov decision process (POMDP). We then present methodologies to solve the sequential problem using Monte Carlo planning methods. We demonstrate the effectiveness of the proposed approach on a simple 2D synthetic exploration problem. Tests show that the proposed sequential approach is significantly more effective at reducing uncertainty than conventional methods. Although our approach is discussed in the context of mineral resource exploration, it likely has bearing on other types of geoscientific model questions.</p>

Sara Haro, Jonathan Jimenez-Reina, Ricardo Bermejo et al.

BioIntertidal Mapper is a user-friendly tool, with a graphical user interface, that automates the selection and processing of Sentinel-2 imagery, to generate intertidal habitat maps. The software uses Google Earth Engine API and the WorldTides API to select imagery acquired at low tide within a specified timeframe. These images are subsequently processed to calculate a Normalized Difference Vegetation Index, which is masked, based on a shapefile defining the area of interest. Maps are exported to a Google Drive folder. The program offers a simple solution for scientific and environmental manager to map intertidal habitats using free and publicly available satellite imagery.

A. J. Mason, A. Vaks, A. Vaks et al.

<p>We describe a new method for the measurement of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M2" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">U</mi><mo>/</mo><mi mathvariant="normal">Pb</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="30pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="e043b0a717ed7605fa31d32304cccc94"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gchron-4-33-2022-ie00001.svg" width="30pt" height="14pt" src="gchron-4-33-2022-ie00001.png"/></svg:svg></span></span> ratios by isotope dilution multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) for the dating of geologically young clean carbonates, particularly speleothems. The method is intended for materials containing little or no initial <span class="inline-formula">²³²</span>Th. We illustrate and validate the method with four examples ranging from 0.57 to 20 Ma. The new method is capable of applying the <span class="inline-formula">²³⁵</span>U–<span class="inline-formula">²⁰⁷</span>Pb and <span class="inline-formula">²³⁸</span>U–<span class="inline-formula">²³⁴</span>U–<span class="inline-formula">²⁰⁶</span>Pb chronometers, common Pb and quantifiable residual <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msup><mi/><mn mathvariant="normal">234</mn></msup><mi mathvariant="normal">U</mi><msup><mo>/</mo><mn mathvariant="normal">238</mn></msup><mi mathvariant="normal">U</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="52pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="a9cbbcfe4acccbf09a9fda48c3a2a269"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gchron-4-33-2022-ie00002.svg" width="52pt" height="15pt" src="gchron-4-33-2022-ie00002.png"/></svg:svg></span></span> disequilibrium permitting. These provide an alternative to the more widely used <span class="inline-formula">²³⁸</span>U–<span class="inline-formula">²⁰⁶</span>Pb chronometer, which can be highly inaccurate for samples that are <span class="inline-formula">&lt;</span> ca. 20 million years old, owing to uncertainties in the excess initial <span class="inline-formula">²³⁴</span>U (hence, excess radiogenic <span class="inline-formula">²⁰⁶</span>Pb) commonly observed in speleothems.</p>

Fatwa Ramdani, Putri Setiani

Abstract The global pandemic of COVID‐19 has been insisted by many countries in the world to implement social distancing, including Indonesia. This research measured the environmental impact before and during the COVID‐19 pandemic in Indonesia. Variables, such as nitrogen dioxide (NO2) of Sentinel‐5P Tropospheric Monitoring Instrument (Tropomi), nighttime light condition of Visible Infrared Imaging Radiometer Suite (VIIRS) Day/Night Band (DNB) and land surface temperature (LST) of Thermal InfraRed Sensor (TIRS) of Landsat‐8 Operational Land Imager (OLI), were monitored using the cloud‐based computing platform of Google Earth Engine (GEE). This study found that all monitored variables decreased during the period of the COVID‐19 pandemic compared to the period before the pandemic. However, the pattern of reduction of NO2 concentration and nighttime light was not observed to be coherent with large‐scale social distancing regulation enforced by the government. This study found that the reduction has begun 2 months earlier than the official regulation enforced by the government. The lower LST shown in both the time‐series map and the graph demonstrated changes before and during the pandemic. The hypothesis was tested using the t‐test method, and the statistical results show the significant difference between two groups of observed variables (e.g. before and during the pandemic) with the lower p‐values and higher Cohen's d. Results of this study indicate that it should be feasible to monitor the environmental impact of the COVID‐19 pandemic using the combination of NO2 concentration, nighttime light and LST information as a proxy.

Robert Anders, Matthew K. Landon, Peter B. McMahon et al.

Study region: Santa Barbara County, California, USA. Study focus: To analyze a wide array of newly collected chemical, isotopic, dissolved gas, and age dating tracers in conjunction with historical data from groundwater and oil wells to determine if water and/or thermogenic gas from oil-bearing formations have mixed with groundwater in the Orcutt Oil Field and surrounding area. New hydrological insights for the region: Three of 15 groundwater samples had compositions indicating potential mixing with water and/or thermogenic gas from oil-bearing formations. Relevant indicators included salinity tracers (TDS, Cl, Br), NH3, DOC, enriched δ13C-DIC, δ2H-CH4, δ13C-CH4, and δ13C-C2H6 values, and trace amounts of C3-C5 gas. The potential sources/pathways for oil-bearing formation water and/or thermogenic gas in groundwater overlying and adjacent to the Orcutt Oil Field include: (1) upward movement from formations developed for oil production due to: (a) natural migration; or (b) anthropogenic activity such as injection and/or movement along wellbores; and (2) oil and gas shows in overlying non-producing oil-bearing formations. Groundwater age tracers, elevated 4He concentrations, and isotopic compositions of noble gases indicated legacy produced water ponds were not a source. This phase of the study relied on samples and data from existing infrastructure. Additional data on potential end-member compositions from new and existing wells and assessments of potential vertical head gradients and pathways between oil and groundwater zones may yield additional insight.

Subhanil Guha, Himanshu Govil

Coronavirus disease (COVID-19) has changed the human lifestyle just like a disaster in 2020. Many people died throughout the world due to its severe attack. Lockdown is the most common term used in today's life to prevent the adverse effect of COVID-19. However, during the lockdown period, a significant improvement in the urban environment was noticed in almost every part of the world. During the lockdown period, the decrease in the number of running vehicles and moving people on the road lowers the pollution level and it has a direct positive impact on the urban environment. The study examines the changes found in land surface temperature (LST) and normalized difference vegetation index (NDVI) during the lockdown period in Raipur city, India with the earlier periods (2013–19) to compare the environmental status. The results indicate that the LST is reduced and NDVI is increased significantly during the lockdown period, and the negativity of the LST-NDVI correlation is increased remarkably. The study also shows a better ecological status of the city during the lockdown period. The study is useful for environmental strategists and urban planners.

Y. Cohen, Y. Cohen, Y. Cohen et al.

<p>A wide variety of observation data sets are used to assess long-term simulations provided by chemistry–climate models (CCMs) and chemistry-transport models (CTMs). However, the upper troposphere–lower stratosphere (UTLS) has hardly been assessed in these modelling exercises yet. Observations performed in the framework of IAGOS (In-service Aircraft for a Global Observing System) combine the advantages of in situ airborne measurements in the UTLS with an almost-global-scale sampling, a <span class="inline-formula">∼20</span>-year monitoring period and a high frequency. Even though a few model assessments have been made using the IAGOS database, none of them took advantage of the dense and high-resolution cruise data in their whole ensemble yet. The present study proposes a method to compare this large IAGOS data set to long-term simulations used for chemistry–climate studies. As a first application, the REF-C1SD reference simulation generated by the MOCAGE (MOdèle de Chimie Atmosphérique à Grande Echelle) CTM in the framework of Chemistry-Climate Model Initiative (CCMI) phase I has been evaluated during the 1994–2013 period for ozone (<span class="inline-formula">O₃</span>) and the 2002–2013 period for carbon monoxide (CO). The concept of the new comparison software proposed here (so-called Interpol-IAGOS) is to project all IAGOS data onto the 3-D grid of the model with a monthly resolution, since generally the 3-D outputs provided by chemistry–climate models for multi-model comparisons on multi-decadal timescales are archived as monthly means. This provides a new IAGOS data set (IAGOS-DM) mapped onto the model's grid and time resolution. To get a model data set consistent with IAGOS-DM for the comparison, a subset of the model's outputs is created (MOCAGE-M) by applying a mask that retains only the model data at the available IAGOS-DM grid points.</p> <p>Climatologies are derived from the IAGOS-DM product, and good correlations are reported between with the MOCAGE-M spatial distributions. As an attempt to analyse MOCAGE-M behaviour in the upper troposphere (UT) and the lower stratosphere (LS) separately, UT and LS data in IAGOS-DM were sorted according to potential vorticity. From this, we derived <span class="inline-formula">O₃</span> and CO seasonal cycles in eight regions well sampled by IAGOS flights in the northern midlatitudes. They are remarkably well reproduced by the model for lower-stratospheric <span class="inline-formula">O₃</span> and also good for upper-tropospheric CO.</p> <p>Along this model evaluation, we also assess the differences caused by the use of a weighting function in the method when projecting the IAGOS data onto the model grid compared to the scores derived in a simplified way. We conclude that the data projection onto the model's grid allows us to filter out biases arising from either spatial or temporal resolution, and the use of a weighting function yields different results, here by enhancing the assessment scores. Beyond the MOCAGE REF-C1SD evaluation presented in this paper, the method could be used by CCMI models for individual<span id="page2660"/> assessments in the UTLS and for model intercomparisons with respect to the IAGOS data set.</p>

Essa Georges Lwisa

Renbo Song, X. Qin, Yeqing Tao et al.

Hadis Khalilizadeh, Abbas Ghaderi, Ali Reza Ashouri et al.

<strong>Abstract</strong> This study led to identify 30 species and 16 genera of eight different plant macrofossils orders including Equisetales, Cycadales, Bennettitales, Coniferales, Peltaspermales, Ginkgoales, Corystospermales, and Filicales at the Deh-Shiykh section, north of Kerman. According to stratigraphic ranges of the index taxa such as<em> Otozamites furoni, Nilssoniopteris mosafolia, Nilssonia tenuicaulis, Anomozamites major, Coniopteris hymenophylloides, Nilssonia bozorga</em> and<em> Elatides thomasii</em> the age of early Middle Jurassic (Bajocian–Bathonian) is assigned to the Deh-Shiykh section. Middle Jurassic ichno fossils of the Hojedk Formation in the Deh-Sheikh section have been studied for the first time. The study also led to the determination of terrestrial ichno fossils including eight ichno species, belonging to four ichnogenera such as<em> Planolithes</em>, <em>Rhizoliths</em>, <em>Taenidium</em> and <em>Thalassinoides</em>. They have been found alongside the stems of Equisetals. Combination of the results of ichno fossil studies with plant fossils help us to identify the palaeoenvironment. These plant fossils are generally indicator of humid conditions. Moreover, the identified ichnofossils in accordance with the sedimentological studies, represent deltaic environments influenced by the river systems and channels. This is the first study of the combination of three studies of paleontology (Plant macrofossils), sedimentology and ichnology in east central Iran. The results have contributed to the accurate understanding of the paleoenvironment of fossiliferous horizons of the Hojedk Formation. <strong>Keywords</strong>: Plant macrofossils, Tracefossils, Middle Jurassic, Delta, Kerman. <strong> </strong> <strong><br clear="all" /> </strong> <strong>Introduction</strong> The northern region of Kerman is one of the richest areas of Iran for the study of plant fossils and so far more than 80 taxa have been reported (Khalilizadeh 2010). The first studies on coal deposits in the Kerman region have been carried out by Houtum-Schindler (1881), Stur (1886) and Stahl (1897). In recent years, Mirzaei-Ataabadi (2002) has studied the plant macrofossils of the Hojedk Formation in the Pabdana, Bab-Nizu and Hashuni areas. Vaez-Javadi and Mirzaei-Ataabadi (2006) studied the plant macrofossils of the Hojedk Formation. Ameri et al. (2014a, b) have also studied the biostratigraphy of the Hojedk Formation deposits in the Bab-Hutk section. Studies on these plant macrofossils have been more focused on coal areas and mines, non-coal areas in this respect has been received less attention. In addition, there are no reports of ichnofossil studies of the Jurassic layers of the Kerman region, especially in association with plant fossils. Trace fossils are very useful for understanding the paleoclimates and generally interpreting paleosedimentary environments (Buatois and Mangano 2004). In the present study, which focuses on the study of plantmacrofossils of the Hojdak Formation in Deh-Sheikh section north of Kerman, trace fossils have also been studied along with this macrofossils and the general sedimentology of these layers is also considered. <strong> </strong> <strong>Material & Methods</strong> Important factors in the identification of plant macrofossils include the characteristics of veins and pinnules, rachis, the grooves on the stem as well as the nodes and inter nodes in sphenophyte. In this study, about 100 plant macrofossil specimens were studied and they were photographed using a Nikon 90D digital camera. The images were processed using Adobe Photoshop CS5. All these specimens are kept in the Museum of Ferdowsi University of Mashhad. Trace fossil studies are usually carried out in the field on a centimeter scale and samples are taken to the lab for further and more detailed examination. In this study, about 20 trace fossil samples were collected. The lithofacies described in the field are divided mainly on the sedimentary texture (especially grain size) and structures; according to the Miall (1985) classification. The identified petrofacies are based on Folk (1980) classification scheme with polarizing microscope. Also, identification of sedimentary environment and subenvironments were interpreted based on the facies characterizes. <strong> </strong> <strong>Discussion of Results & Conclusions</strong> Based on the plant macrofossils, especially <em>Coniopteris hymenophylloides</em>, <em>Elatides thomasii </em>and <em>Nilssonia friziensis</em>, Middle Jurassic age (Bajocian–Batonian) is confirmed for the studied section. The studied plant macrofossils, especially filicales and horsetails, grow in humid environments. The in situ Equisetal stems have paleoecological significance. Horsetails, are hygrophytes, and adapted to humid conditions (Brea et al. 2009). Osmundaceae have been to warm and humid conditions along the rivers or marshes (Gordenko 2008). Due to the abundance of cycadophyta and ferns in the study area, tropical to subtropical paleoclimate have been reported (Salyukova et al. 2013). The ichno fossil diversity of the deltaic regions is very low (Hobday and Tavener-Smith 1975). Salinity changes, water turbulence, mud influx and etc. are the most important stress in the delta area. The study of 8 ichnospecies in the Deh-Sheikh section shows these traces, are simple. They are similar to those of deltaic systems (Buatois and Mangano 2011). These trace fossils resemble specimens of deltaic systems and are related to the heteropod invertebrates of Scoyenia ichnofacie. According to sedimentological studies, the sedimentary environment of the Hojedk Formation in the Deh-Sheikh section is often interpreted as a deltaic environment (coarsening upward cycles) under the influence of the river (fining upward cycles) which indicate of channel filling and energy depletion periods. From this study, the combination of paleontological, sedimentological and lithological studies with the study of ichnofossils will be very useful and effective in better understanding the palaeoenvironment and with proper interpretation, it will helpful to recognition details and create a clear picture of the environment. <strong><br clear="all" /> </strong> <strong> </strong>

A. McKirdy

ANDREA ALLASINAZ

One of the major mass extinctions of the Phanerozoic took place at the Rhaetian-Hettangian boundary. According to some researchers, it might have been preceded by a minor event at the end of the Carnian. The faunal association present in the Late Trixsic formations of Lombardy and their changes related with the lithofacies changes are analysed here. In the Carnian Gorno Formation and Val Sabbia Sandstone, five bivalve assemblages have been distinguished. From the trophic analysis of these molluscs, we observe the infaunal suspension feeders predominance in the 1st and 2nd assemblage. They are gradually replaced by epibyssate species in the following associations, closely related with the lithofacies changes. In the overlying Norian Dolomia Principale, most bivalves disappear. They are replaced by species of Neomegaladon, Isognomon and gastropods. It is pointed out that this decline is not due to mass extinction, but to the settlement of different conditions in the basin which fitted better the euryhaline bivalve assemblages. In the overlying Riva di Solto Argillite, Zu Limestone and Conchodon Dolomite, 4 bivalve assemblage zones have been distinguished. Both, the new genera appearance and the number of species increase are closely linked to the substitution of calcareous facies of peritidal platform with argillaceous and arenaceous sequences of inner basin. In the Hettangian Sedrina Limestone, the fauna treated by Gaetani (1970), rich in bivalves, is examined. The diversity and the phyletic relationship of this species with those of the Rhaetian zones are pointed out. The diversity, the origination and extinction rate are calculated for the bivalve species of Lombardy and for the Megalodontidae and Dicerocardiidae revised by Végh-Neubrandt (1982). Eventually, some morphologic changes in the phyletic lineage of megalodontids have been pointed out too. Concluding, faunal crisis like mass extinction is not recorded by the bivalve assemblages of the Late Triassic of Lombardy and Southern Alps in general. They reflect pseudoextinctions, sudden and severe turnover, certainly depending on the changing environmental conditions. The affinity among the late Rhaetic bivalves and those of the Hettangian demostates that the big faunal crisis at the Rhaetian-Hettangian boundary in Lombardy is traceable back to a rapid faunal turnover consequent to the rifting and sinking of the carbonate platform.