Non-marine Settings Research Articles

Desertic siliciclastic stromatolites in the Upper Jurassic Guará Formation from southwestern Gondwana: Trapping and binding in a non-marine setting

Siliciclastic stromatolites are rare in the geologic record, and their occurrence recorded in the literature is restricted to marine and coastal environments. The Upper Jurassic Guará Formation, from the Paraná Basin in southern Brazil, hosts unique non-marine siliciclastic stromatolites, providing a rare opportunity to study trapping and binding mechanisms by microbial mats in a continental setting. These microbialites occur interbedded with eolian and fluvial facies. The structural layering of the stromatolite domes is formed by the intercalation of sandy layers, resulting from trapping and binding of siliciclastic grains by microbial mats and in situ precipitation of amorphous to cryptocrystalline silica, which directly replaced the microbial colonies. The silica layers contain partially preserved spherical to ovoid bodies interpreted as colonies of coccoid microorganisms. These siliciclastic stromatolites were formed due to a specific balance of environmental factors, namely water chemistry and sediment supply, which enhanced the processes of trapping and binding and in situ precipitation. They record the presence of microbial life in a non-marine, silica-rich, fluvial-eolian environment in which there were no previous published occurrences of agglutinated stromatolites. These specimens record macroscopically identifiable evidence of microbial life in a continental environment that must be accounted for in the search for ancient life on Earth and Mars.

THE MICROBIAL “FINGERPRINTS” FROM THE CONTINENTAL LOWER PERMIAN OF POLAND

ABSTRACT Microbes colonize sediment and alter its properties creating a bio-mineral medium. The microbially induced sedimentary structures (MISS) are the fossil record of an interaction between the physical environment and such a medium. The present report documents bedding surface structures from the Cisuralian (Asselian) Sandstone Building Member (BSM) of the Słupiec Formation, a unit that outcrops in the south-western Poland, in the Sudetes Mountains. The BSM represents likely continental (fluvial) sedimentary settings. The sedimentary structures on bedding surfaces in the BSM are interpreted as the MISS. The observations of the bedding structures are supplemented with thin section data that support the microbial interpretation of the bedding surface structures. The Słupiec Formation MISS record supplements the global patchy fossil record of the post-Cambrian (Paleozoic) MISS from the non-marine settings.

Radiocarbon Chronology/Growth Rates of Ooids from Great Salt Lake, Utah

Ooids (calcium carbonate coated grains) are common in carbonate environments throughout geologic time, but the mechanism by which they form remains unclear. In particular, the rate of ooid growth remains elusive in all but a few modern marine environments. In order to investigate the rate of ooid growth in a non-marine setting, we used 14C to date ooids from Great Salt Lake, Utah, a well-known site of aragonitic ooids. Bulk ooids obtained from the northern shore of Antelope Island and the northeast shore of Great Salt Lake near Spiral Jetty were sieved into different size fractions and produced mean ages ranging between 2728±15 and 4373±20 14C yr BP. Larger ooids were older than smaller ooids, implying that larger ooids grew in the environment for a longer duration, with the caveat that bulk age dating integrates the growth history of an ooid. To better resolve growth history, ooids from the coarse fraction were sequentially dissolved, and 14C ages were obtained for each dissolution step to create a time series of ooid growth. The results of the sequential dating indicate that the coarse Great Salt Lake ooid growth began between 5800-6600 ± 60 14C yr BP while their outer cortices are nearly modern. Sequentially dated ooids from the South Arm of Great Salt Lake at Antelope Island record a nearly linear growth history (~ 10-15 µm/kyr), whereas ooids from Spiral Jetty record somewhat faster growth between ~6000 and 4000 years ago (0.03 – 0.06 µm/yr) followed by a 10x slower growth history for the remainder of their lifespan (0.003 – 0.008 µm/yr). The lifespan of Great Salt Lake aragonitic ooids is two to six times longer than those from modern marine environments, and thus provides a unique end member for understanding the mechanisms behind ooid formation. The ooid age range indicates that geochemical parameters measured from bulk ooid dissolution integrates over ~6000 years and thus does not represent a geochemical snapshot in time, as some previous studies have suggested.

Transition from stromatolite to thrombolite fabric: potential role for reticulopodial protists in lake microbialites of a Proterozoic ecosystem analog.

Prior observations suggest that foraminiferan protists use their reticulopodia (anastomosing pseudopodia) to alter sediment fabric by disrupting laminations of subtidal marine stromatolites, erasing the layered structures in an experimental setting. Because microbialites and foraminifera are found in non-marine settings, we hypothesized that foraminifera living in lakes could also disrupt layered microbialite fabric. With this aim and using a variety of multidisciplinary approaches, we conducted field surveys and an experiment on microbialites from Green Lake (GL; Fayetteville, New York State, United States), which has been studied as a Proterozoic ecosystem analog. The lake is meromictic and alkaline, receiving calcium sulfate-rich water in the monimolimnion; it supports a well-developed carbonate platform that provides access to living and relict microbialites. The living microbialites grow from early spring to autumn, forming a laminated mat at their surface (top ~5 mm), but a clotted or massive structure exists at depth (> ~ 1 cm). We observed a morphotype of "naked" foraminiferan-like protist in samples from GL microbialites and sediments; thus, considered the possibility of freshwater foraminiferan impact on microbialite fabric. Results of an experiment that seeded the cultured freshwater foraminifer Haplomyxa saranae onto the GL microbialite surface indicates via micro-CT scanning and anisotropy analysis that the introduced foraminifer impacted uppermost microbialite layering (n = 3 cores); those cores with an added inhibitor lacked changes in anisotropy for two of those three cores. Thus, it remains plausible that the much smaller, relatively common, native free-form reticulate protist, which we identified as Chlamydomyxa labyrinthuloides, can disrupt microbialite fabrics on sub-millimeter scales. Our observations do not exclude contributions of other possible causal factors.

Ediacaran microbial mats and microbially induced sedimentary structures in lacustrine deposits, Arabo-Nubian Shield, Egypt

Photoautotrophic microbial mats flourished on the substrates of the Precambrian marine environments but are barely reported from non-marine settings. Extraordinarily preserved microbial mats and microbially induced sedimentary structures (MISS) have been reported from the Ediacaran lacustrine deposits of the Hammamat Group in the Arabian Nubian Shield, Egypt. Biogenicity of microbial mats is affirmed by sporadically preserved filamentous cyanobacteria, copious remains of extracellular polymeric substances (EPS) and considerable organic‑carbon content. Locally abundant and diversified assemblages of 11 types of MISS signify optimal paleoecological conditions for proliferous photoautotrophic microbenthic growth, biostabilization and binding activities in oligotrophic fresh-water lakes. Microbial mats and MISS are so far the only terrestrial example in fresh-water lakes during the Ediacaran, specifically between Marinoan and Gaskiers glaciations, and possibly the most diversified assemblages in non-marine realms. Extreme paleoecological stresses throughout the Precambrian and repeatedly during the Phanerozoic reveal that microbial mats were the safe haven—and a primary source of food and oxygen—for early aquatic multicellular organisms. Accordingly, biosignatures of microbial mats and MISS in the Ediacaran lakes prove that the Precambrian non-marine aquatic environments were as important as marine settings in the evolution of ancient life on Earth and possibly on other planets such as Mars.

Authigenic clays as precursors to carbonate precipitation in saline lakes of Salar de Llamara, Northern Chile

The Salar de Llamara, Atacama Desert is host to saline lakes known as Puquios, which are possible analogs for lakes on early Earth or Mars. Here we examine associations between microbial communities, Mg-clay minerals, and carbonates in microbial mat samples from the Puquios using scanning electron microscopy, energy dispersive X-ray spectroscopy, focused ion beam nanotomography, and transmission electron microscopy. We observe micrometer-scale aggregates of nanocrystalline Mg-clay around cyanobacterial cell sheaths and throughout alveolar networks of microbial extracellular polymeric substances. High-Mn calcite occupies pore spaces and surfaces within the clay matrix. We propose a sequence whereby extracellular polymeric substance networks serve as surfaces for precipitation of Mg-clay, which permineralizes the original microorganisms. Formation of the clay reduces kinetic barriers and provides a suitable substrate for the formation of carbonate minerals. We therefore suggest that authigenic clays are important in the preservation of microbial communities and the precipitation of carbonates in non-marine settings such as the saline lakes of the Atacama Desert.

Late Carboniferous palaeodepositional changes recorded by inorganic proxies and REE data from the coal-bearing strata: An example on the Czech part of the Upper Silesian Coal basin (USCB)

In the Czech Republic, coal-bearing siliciclastic sediments have been deposited during the Serpukhovian and Bashkirian (Carboniferous). Until now, no attention has been paid to inorganic geochemical assessment of the coals and associated non-coal rocks from the mixed shallow-marine to continental sediments (Ostrava Formation), and continental non-marine settings (Karviná Formation). Samples were collected from a 750 m deep coal exploration borehole at the ČSM Mine. The bulk parameters, total organic carbon TOC, total inorganic carbon TIC, total sulphur TS, major elements, trace elements, and REEs were measured on these samples, and their mineral associations have been investigated using microscopy combined with the principal component analysis (PCA). Common redox proxies V/Cr, U/Th, Ni/Co, Mo/U, and the ratio S/TOC have been tested on the samples to investigate their usefulness for studying anoxia. Research concludes that redox proxies such as U/Th, Ni/Co and V/Cr have been strongly influenced by the clastic input and carbonates, which it hinders for them to be reliable indicators of anoxia. On the basis of Eu anomaly and REEs distribution, the primary source of detrital elements comes from the parent rock, being governed more by physical than redox processes.

Plesiosaurs from the fluvial Kem Kem Group (mid-Cretaceous) of eastern Morocco and a review of non-marine plesiosaurs

Plesiosaurs were a long-lived and widespread group of marine reptiles, with a worldwide distribution and a temporal range from the Late Triassic to the Late Cretaceous. Most occur in marine deposits, but some occur in low-salinity, brackish to freshwater environments. We report plesiosaurs from the freshwater fluvial deposits of the mid-Cretaceous (?Albian–Cenomanian) Kem Kem Group of Morocco. Remains include numerous shed teeth, vertebrae, and a humerus. The humerus represents a young juvenile; vertebrae likely belong to sub-adults. Teeth show heavy wear, similar to teeth of co-occurring spinosaurids. While coeval plesiosaurs from the Bahariya Formation of Egypt are members of Polycotylidae, the Kem Kem fossils show features of Leptocleididae, small-bodied plesiosaurs that were widely distributed in nearshore and non-marine settings in the Early Cretaceous. These fossils are the first freshwater plesiosaurs from Morocco, and are among the youngest representatives of Leptocleididae. The Kem Kem leptocleidids could have been infrequent visitors from the sea, freshwater-tolerant, or even freshwater-adapted, as in modern river dolphins. The abundance of shed teeth in the Kem Kem Group supports the hypothesis that they had some degree of freshwater tolerance. Furthermore, leptocleidids occur almost exclusively in shallow nearshore, brackish, or freshwater environments, suggesting adaptation to shallow, low-salinity environments. Other plesiosaur groups and other Mesozoic marine reptiles, including teleosaurids and mosasaurids, also occur in freshwater settings, suggesting plesiosaurs and other marine reptiles frequently exploited non-marine environments.

The structure of the nonmarine fossil record: predictions from a coupled stratigraphic–paleoecological model of a coastal basin

AbstractPresented here is a coupled model of the nonmarine fossil record, based on a geometric model of deposition, a random-branching model of evolution, and an ecological model based on an elevation gradient. This model provides testable predictions about the stratigraphy and fossil occurrences in coastal nonmarine settings under three scenarios of sea-level change. A slow relative rise in sea level causes a declining ratio of channel to floodplain deposits, plus changes in community composition that reflect an upward increase in elevation relative to sea level. A rapid relative rise in sea level drives increasing aggradation rates, decreases the ratio of channel to floodplain deposits, and triggers a shift from higher-elevation (more inland) to lower-elevation (more coastal) communities. A fall in sea level produces an unconformity, manifested by valleys separated by interfluves. The resumption of deposition following the sea-level fall causes an abrupt shift in community composition across the unconformity, reflecting the duration of the hiatus and the increased elevation relative to sea level. This produces a cluster of first and last occurrences at the unconformity, and it is the only sequence-stratigraphic source of such clusters in a nonmarine system, in contrast to the multiple mechanisms for generating these clusters in marine systems. A central prediction of these models is that the nonmarine fossil record preserves systematic changes in community composition that reflect elevation (or equivalently, distance from shore). Diagnosing these gradients in ancient systems is a promising avenue of future research.

The Jurassic-Cretaceous Paleogeography Of The Sula Area, North Maluku

The study of paleogeography and hydrocarbon potentiality of the Sula area, North Maluku has been conducted by the Lemigas Exploration team. This paper specifically presents a paleogeography of the Jurassic-Cretaceous age of the Sula area as a part of the result of this study. In this paper, paleogeography means palaeoenvironment which is defined based on biostratigraphy. Data used in this paper are mostly secondary data obtained from National Data Center which is combined with primary data collected during field work campaign. The subsurface data analysis allows subdivision of 7 depositional sequences throughout Jurassic-Cretaceous succession. In fact, each sequence mostly consists of transgressive and highstand system tracts. Lowstand system tract only occurs in the earliest sequence. Sequences 1 (Bobong Formation), 2, 3 and 4 (Buya Formation) are assigned to the Jurassic age, whilst sequences 5, 6 and 7 (Buya Formation) are attributed to the Cretaceous age. Generally, the depositional environment of most sequences is getting deeper toward the North. The shallowest environment takes place in non-marine setting, whereas the deepest environment occurs in outer neritic (100m-200m). It is most likely that Jurassic-Cretaceous depocenter was situated in the northern part of the study area. However, it is required additional data to confirm this interpretation.

Palaeoenvironments Of The Permian-Cretaceous Sediments Of The Bintuni Bay, Papua

This paper presents a part of the result of Lemigas in-house research entitled Paleogeography and Hydrocarbon Potentiality of the pre-Tertiary Sediment of the Bintuni Basin, Papua, which is financially supported by the Government through a project so called DIPA 2009. In addition, this paper specifically discusses a palaeoenvironment of the Late Permian-Cretaceous sediment of the Bintuni Bay based on biostratigraphy. Data used in this paper is mostly secondary data obtained from National Data Center and an oil company which is therefore, classified as a confidential information. Paleoenvironment appeared in a non-marine setting during Permian-Triassic to form Ainim Formation in which, shale of this formation acts as a primary sourcerock for the studied area. Early Jurassic is characterised by a non-deposition, whilst Middle Jurassic is indicated by the occurrence of non-marine environment to deposit fluvial sandstone of the Lower Kembelangan Formation which is considered to be a main reservoir. Subsequently, environment shifted to the marine setting where deep marine shale occurred to form caprock. Mean while, the Early Cretaceous sediment disappears from the studied area suggesting massive erosion. Finally, depositional environment took place in the marine setting during the Late Cretaceous resulting in the formation of deep marine shale of Jass Formation which may contribute as a caprock in the element of petroleum system

First record of the Permian nonmarine helical trace fossil Augerinoichnus from outside of New Mexico

Augerinoichnus refers to successions of horseshoe-shaped structures interpreted as sedimentary remains of horizontal helical burrows of worm-like animals. The monospecific trace fossil was hitherto only known from early Permian intertidal to lowland coastal-plain deposits of New Mexico, U.S.A. Recent discoveries of conspecific material in early Permian nearshore lacustrine deposits of southwestern Germany represent the first record of Augerinoichnus outside of New Mexico and extend the paleogeographic and paleoenvironmental distribution of the ichnotaxon. Augerinoichnus evidentally was made in a wide array of nonmarine settings that ranged from tidal flats to inland floodplains where substrate cohesion and stability as well as stable and predictable food resources allowed the tracemaker to forage in open burrows.

Source rock characterization and oil grouping in the NW Java, Central Java and NE Java Basins, Indonesia

This study reveals the detailed organic geochemistry from crude oils (acquired from wells and seepages) and rock extracts from NW Java and NE Java Basin that have been gathered and compiled from previous publications. The interpretation was conducted from geochemical data value and plot, GC-MS fingerprints, and agglomerative-hierarchical cluster analysis using the Euclidean algorithm. Various source rocks from those basins were deposited under fluvio-lacustrine to the marine environment. Six groups of crude oils are also distinguished. Groups 1, 2, and 6 are oils from deltaic source rocks, Groups 3 and 4 are oils from marine source rocks, and Group 5 is from lacustrine and/or fluvio-lacustrine source rocks. Groups 1, 2, and 6 could be distinguished from the pristane/phytane (Pr/Ph) ratio and C29 sterane composition, while Groups 3 and 4 differ from the distribution of C27 sterane. The schematic depositional environment of source rocks is also generated from this study and suggests that Group 5 is deposited during early syn-rift non-marine settings, while the remaining groups are deposited in the deltaic (Group 1,2 and 6) and marine settings (Groups 3 and 4). The main differences between those groups are including the distributions of C27-C28-C29 steranes.

Horseshoe crab trace fossils from the Upper Cretaceous Two Medicine Formation of Montana, USA, and a brief review of the xiphosurid ichnological record

AbstractA locality in the Upper Cretaceous Two Medicine Formation of Montana preserves abundant and variable horseshoe crab tracks and trails of the ichnotaxon Kouphichnium isp. These specimens span six morphologies differing in track form and trail configuration. These differences likely reflect variations in track-maker locomotion and behavior, substrate consistency, epichnial versus hypichnial preservation, and undertrack versus true tracks. Several tracks preserve the first clear appendage impressions for an extinct horseshoe crab. This discovery adds new information to the fossil horseshoe crab diversity in the Cretaceous Period. Trackway dimensions, such as the external width across the pusher legs or of the prosomal drag mark, provide information on the track-maker size. Most trackways correspond with crabs 9–14 cm wide; the abundance but limited size range of the traces suggests the large assemblage corresponds to a mating aggregation. The trace fossil record of xiphosurids indicates that throughout their history, horseshoe crabs inhabited both marine and nonmarine settings. They were definitively present in freshwater habitats from the lower Carboniferous through at least the Paleogene. Horseshoe crab trace abundance is highest from the upper Carboniferous through the Jurassic and likely reflects two factors: true upper Carboniferous taxonomic diversity and a preponderance of suitable sites for trackway preservation in the Late Triassic and Jurassic. Cretaceous traces are uncommon, and this Two Medicine locality is the first occurrence of horseshoe crab traces in the Late Cretaceous worldwide. Overall, track abundance and diversity would seem to correspond well with the reported horseshoe crab body fossil record.

Early-Middle Triassic boundary interval: Integrated chemo-bio-magneto-stratigraphy of potential GSSPs for the base of the Anisian Stage in South China

The Wantou and Youping sections of Guangxi, South China provide a detailed high-resolution integrated calibration of the Early-Middle Triassic boundary succession for lithostratigraphy, volcanic episodes, conodont first occurrences (FOs), ammonoid biostratigraphy, geomagnetic polarity, inorganic carbon isotopes, sea-surface temperatures derived from conodont-apatite oxygen-isotopes, and ID-TIMS U–Pb radiometric dating. The upper Spathian (late Early Triassic) magnetostratigraphy is characterized by normal polarity (magnetozone LT9n) that encompasses the FOs of the typical Spathian conodonts Triassospathodus homeri and Gladigondolella carinata, the late Spathian Neopopanoceras haugi ammonoid zone and the beginning of a progressive positive shift in inorganic carbon isotopes. The overlying reversed polarity interval (LT9r) contains two brief normal-polarity subzones (MT1n and MT2n) that can be recognized in several other marine and terrestrial sections. The FO of conodont Chiosella timorensis sensu stricto, a proposed base-Anisian global marker, is near MT1n and near the end of the positive δ13Ccarb excursion. Sea-surface temperatures were reported to have cooled by 4°C during this rise in δ13Ccarb, suggesting a sequestration of carbon dioxide. The lowermost Anisian at Wantou and Youping is dominated by normal polarity (MT3n, with the presence of one major reversed-polarity subzone MT3n.1r), contains the FO of typical Anisian conodonts (Gladigondolella tethydis/Magnigondolella alexanderi), and has the onset of a plateau in inorganic carbon isotopes values (stabilizing around +4‰). The combination of the FO of conodont Chiosella timorensis s.s., the brief normal polarity zone (MT1n) and the last portion of the rising carbon-isotope trend are suitable for primary proxies for global correlation of the Early-Middle Triassic boundary (base of Anisian) to other marine and non-marine settings. Radiometric dates at the Wantou and at the Guandao sections, coupled with a composite cyclostratigraphy for Early Triassic through Anisian, indicate that the FO of the conodont Chiosella timorensis s.s. is at approximately 246.7 Ma.

Cambrian trace fossils from North Africa and their contribution to Gondwana's paleobiogeography and depositional history

Cambrian successions in northern Africa are widely distributed in the subsurface, but of limited exposure. Over large areas they are dominated by nonmarine deposits free of any body fossil content, which hampers stratigraphic and paleogeographic conclusions for subdivision and interregional correlation. Hitherto, the occurrence of trace fossils was mentioned sporadically, but research focused on this special issue was practically absent until recent years. Here we report results of the first ichnological investigation combined with sedimentological observations from the northern Sahara Desert (Hasawnah Formation, Al Qarqaf Arch, Jebel Hasawnah mountain region, west-central Libya), and we summarize the recent state of knowledge on the Cambrian trace-fossil record in northern Africa in general and compare this with the adjacent Middle East succession in Jordan and Israel. Five trace-fossil assemblages of low diversity are present in the Al Qarqaf Arch, belonging to the Skolithos and Cruziana ichnofacies, namely Skolithos assemblage, Diplocraterion assemblage, Rusophycus assemblage, Psammichnites assemblage and Treptichnus assemblage. Fossil content and sedimentological criteria point to a predominantly nonmarine setting for the Hasawnah Formation only sporadically affected by limited marine influence. The depositional area represents an unusually large fluvial to estuarine system with a transition to marginal-marine and finally to fully marine conditions. Due to the new findings and by co-occurrence of trace fossils and body fossils in Jordan and Israel, an ecostratigraphic approach is suggested for the Cambrian of the North Africa – Middle East sector of western Gondwana. According to this, a stratigraphic position of the exposed North African Cambrian successions within the (early) Middle Cambrian interval is concluded. Based on distributed trilobite trace fossils we suggest an “aegypticus-burjensis level” as a useful stratigraphic tool for correlation in this North African – Middle East paleogeographic region. Due to paleontological and lithofacies data a North-to-South gradient from low or moderate (Jordan and Israel) to high southern latitude positions (Egypt and subsequently Libya) during the Cambrian is concluded. In general, the presented successful methodology exemplifies the combination of critical evaluation of the ichnological and ichnostratigraphic Cruziana concept with sedimentological facies investigation of early Paleozoic transitional land–sea areas hitherto assumed to be unfossiliferous or unworthy of detailed research. The so concluded data are of great value for geological applied aspects, as field work, mapping, correlation of strata, reconstruction of transregional depositional history, and paleogeographic models.

Evaluating Late Cretaceous OAEs and the influence of marine incursions on organic carbon burial in an expansive East Asian paleo-lake

Expansive Late Cretaceous lacustrine deposits of East Asia offer unique stratigraphic records to better understand regional responses to global climate events, such as oceanic anoxic events (OAEs), and terrestrial organic carbon burial dynamics. This study presents bulk organic carbon isotopes (δ13Corg), elemental concentrations (XRF), and initial osmium ratios (187Os/188Os, Osi) from the Turonian–Coniacian Qingshankou Formation, a ∼5 Ma lacustrine mudstone succession in the Songliao Basin of northeast China. A notable δ13Corg excursion (∼+2.5‰) in organic carbon-lean Qingshankou Members 2–3 correlates to OAE3 in the Western Interior Basin (WIB) of North America within temporal uncertainty of high-precision age models. Decreases in carbon isotopic fractionation (Δ13C) through OAE3 in the WIB and Songliao Basin, suggest that significantly elevated global rates of organic carbon burial drew down pCO2, likely cooling climate. Despite this, Osi chemostratigraphy demonstrates no major changes in global volcanism or weathering trends through OAE3. Identification of OAE3 in a lake system is consistent with lacustrine records of other OAEs (e.g., Toarcian OAE), and underscores that terrestrial environments were sensitive to climate perturbations associated with OAEs. Additionally, the relatively radiogenic Osi chemostratigraphy and XRF data confirm that the Qingshankou Formation was deposited in a non-marine setting. Organic carbon-rich intervals preserve no compelling Osi evidence for marine incursions, an existing hypothesis for generating Member 1's prolific petroleum source rocks. Based on our results, we present a model for water column stratification and source rock deposition independent of marine incursions, detailing dominant biogeochemical cycles and lacustrine organic carbon burial mechanisms.

Testing whether early diagenesis of skeletal carbonate is different in non-marine settings: Contrasting styles of molluscan preservation in the Upper Jurassic of Portugal

This study tests the hypothesis that the early diagenesis of aragonite shells should differ fundamentally between marine and freshwater environments. This is predicted to be the case because aragonite is highly susceptible to dissolution in the TAZ (Taphonomically Active Zone) in low energy marine settings due to acidity caused largely by the oxidation of H2S generated by sulphate-reducing bacteria, but reduced sulphide activity in freshwater settings should result in less early dissolution of aragonite. To test this hypothesis a range of fresh-brackish-hypersaline and marine limestones were sampled from the Upper Jurassic (mid Oxfordian) Cabaços Formation of central western Portugal. In these freshwater and brackish deposits, molluscs are preserved mostly as sparite shell replacements indicating that the original aragonite was preserved through the TAZ and was later replaced during subsequent burial by calcite cement. In limestones deposited in more marine to hypersaline settings, molluscan remains mostly consist of the calcitic layers of bimineralic bivalves, as shell where the original was wholly calcitic, or as gastropod steinkerns. Exceptions occur and reflect other factors such as higher energy conditions during deposition whereby organic matter, as the drive for microbial decay processes, was removed. The implications for molluscan preservation including some hydrocarbon reservoirs are discussed.

Ichnological evidence of a horseshoe crab hot-spot in the Early Triassic Buntsandstein continental deposits from the Catalan Pyrenees (NE Iberian Peninsula)

Xiphosurids (horseshoe crabs) are aquatic chelicerate arthropods commonly related to marine environments. Although today only four marine species exist, in the geological record they were much more diverse (especially during Carboniferous and Triassic periods), and even inhabited non-marine settings. Here we analyze an exceptional xiphosurid trace-fossil site preserved in the lowermost Buntsandstein red-beds from the Early Triassic of the Catalan Pyrenees. Ichnological and sedimentological analyses were mainly conducted during fieldwork. Fossiliferous surfaces were cleaned and mapped in order to recognize distribution and orientation patterns. Two main ichnotaxa were identified: (1) Selenichnites isp. (>400 trace fossils), consisting of a lunate, anteriorly convex lobe (prosoma imprint) followed by a rounded to triangular impression (opisthosoma imprint) and a posterior straight thin groove (telson imprint); and (2) Kouphichnium isp. (about 70 trace fossils), consisting of long (up to 2 m), often curved, complex series of parallel rows of morphologically variable tracks and imprints. Selenichnites isp., interpreted as resting and/or feeding traces (cubichnia/fodinichnia), were mostly oriented against the current, preventing tracemakers being overturned. Kouphichnium isp. correspond to locomotion traces (repichnia). Trace fossils are found at the uppermost succession of fining- and energy decreasing- upwards fluvial red beds with no marine influence. These ichnofossils are the earliest Triassic evidence of life from the Catalan Pyrenees, and thus shed light on ecosystems composition at the dawn of the Mesozoic era, when biota recovered from the greatest mass extinction in Earth history.

Comment on the paper of Gierlowski-Kordesch and Cassle “The ‘Spirorbis’ problem revisited: Sedimentology and biology of microconchids in marine–nonmarine transitions” [Earth-Science Reviews, 148 (2015): 209–227

Gierlowski-Kordesch and Cassle (2015, Earth-Science Reviews 148, 209–227) interpreted microconchids (extinct tentaculitoid encrusters) as strictly marine, rejecting the previous conclusions that these organisms occupied a variety of habitats ranging from marine to non-marine. We argue that due to misunderstanding of previous statements concerning the affinity of microconchids, the authors used an actualistic approach and incorrectly compared microconchids with modern phoronids, which led to oversimplifications and misinterpretations. Also their idea that microconchids, as supposedly strictly marine organisms, were preserved in non-marine settings only because of transport by storm surges and tsunamis is unsupported on paleontological grounds. These errors led to mistaken conclusions about microconchid paleoecology that must be discussed in order to avoid an erroneous interpretation of microconchid paleoenvironments.