Shallow Marine Strata Research Articles

Middle–Late Pennsylvanian tectonosedimentary, climatic and biotic records in basins of Europe, NW Turkey and North Africa: an overview

Abstract In Europe, North Africa and Asia Minor, the remains of Pennsylvanian sedimentary basins bearing continental deposits either intimately mixed with shallow-marine strata or deposited in exclusively continental settings are preserved. Long-lasting research on these basins allowed the definition of regional stages and substages based on marine fauna and terrestrial flora, later extended by terrestrial and freshwater faunal biostratigraphies. Glacioeustatically driven marine bands provide laterally widespread correlation markers; however, where such bands are missing only biostratigraphic control exists. Resolution of biostratigraphic zonations combined with gaps in sedimentary successions and variable quality of the fossil record throughout the basin fills do not allow in all cases a precise correlation between the Pennsylvanian basins in Europe and, in turn, the timing of tectonic, climatic and biotic events, and thus an absolute complete understanding of the response of terrestrial and freshwater biota to climate changes across eastern tropical Pangaea. A helpful tool is new radioisotopic ages of intercalated volcaniclastics that reveal the partial diachroneity of some widely used biostratigraphies. We attempt to present the current state of the art to stimulate further research to mitigate gaps in our knowledge.

Low‐accommodation and backwater effects on sequence stratigraphic surfaces and depositional architecture of fluvio‐deltaic settings (Cretaceous Mesa Rica Sandstone, Dakota Group, USA)

AbstractThe adequate documentation and interpretation of regional‐scale stratigraphic surfaces is paramount to establish correlations between continental and shallow marine strata. However, this is often challenged by the amalgamated nature of low‐accommodation settings and control of backwater hydraulics on fluvio‐deltaic stratigraphy. Exhumed examples of full‐transect depositional profiles across river‐to‐delta systems are key to improve our understanding about interacting controlling factors and resultant stratigraphy. This study utilizes the ~400 km transect of the Cenomanian Mesa Rica Sandstone (Dakota Group, USA), which allows mapping of down‐dip changes in facies, thickness distribution, fluvial architecture and spatial extent of stratigraphic surfaces. The two sandstone units of the Mesa Rica Sandstone represent contemporaneous fluvio‐deltaic deposition in the Tucumcari sub‐basin (Western Interior Basin) during two regressive phases. Multivalley deposits pass down‐dip into single‐story channel sandstones and eventually into contemporaneous distributary channels and delta‐front strata. Down‐dip changes reflect accommodation decrease towards the paleoshoreline at the Tucumcari basin rim, and subsequent expansion into the basin. Additionally, multi‐storey channel deposits bound by erosional composite scours incise into underlying deltaic deposits. These represent incised‐valley fill deposits, based on their regional occurrence, estimated channel tops below the surrounding topographic surface and coeval downstepping delta‐front geometries. This opposes criteria offered to differentiate incised valleys from flood‐induced backwater scours. As the incised valleys evidence relative sea‐level fall and flood‐induced backwater scours do not, the interpretation of incised valleys impacts sequence stratigraphic interpretations. The erosional composite surface below fluvial strata in the continental realm represents a sequence boundary/regional composite scour (RCS). The RCS’ diachronous nature demonstrates that its down‐dip equivalent disperses into several surfaces in the marine part of the depositional system, which challenges the idea of a single, correlatable surface. Formation of a regional composite scour in the fluvial realm throughout a relative sea‐level cycle highlights that erosion and deposition occur virtually contemporaneously at any point along the depositional profile. This contradicts stratigraphic models that interpret low‐accommodation settings to dominantly promote bypass, especially during forced regressions. Source‐to‐sink analyses should account for this in order to adequately resolve timing and volume of sediment storage in the system throughout a complete relative sea‐level cycle.

Evidence for the development of a superpermeability flow zone by bioturbation in shallow marine strata, upper Jubaila Formation, central Saudi Arabia

Superpermeability (super-k) flow zones in carbonate reservoirs are thought to be controlled by bioturbation—related burrow networks, e.g., burrow pores of Thalassinoides. To improve the current understanding, this study investigated Thalassinoides and associated passive fill in the Upper Jubaila Member of the Upper Jurassic Jubaila Formation, central Saudi Arabia. Analysis of the elemental and mineralogical compositions of powder samples from the passive fill and burrow matrix, and thin sections of selected burrow matrix samples were conducted to determine the origin of the passive fill. The goal, herein, is to infer the flow properties of the bioturbated strata of the Upper Jubaila Member based on the occurrence of this passive fill in the burrow pores of the Thalassinoides.Compared to the burrow matrix, the analyzed passive fill exhibit marked differences in elemental composition, suggesting a much younger and different origin than that of the marine carbonates of the Upper Jubaila Member. A CaO plus MgO concentration below 28% and an elevated SiO2 plus Al2O3 concentration above 45% suggest a clay-dominated passive fill. The reddish and brownish colorations of the passive fill are attributed to the presence of iron oxyhydroxides, as indicated by the elevated Fe2O3 concentration (a mean of 5.8% ± 1.7%), and a mineralogical composition with elevated percentages of goethite and hematite. The normalized rare earth element (REE) pattern of the passive fill is consistent with that of lithogenous (siliciclastic) sources, which are likely deposited in a well-oxygenated environment, as suggested by the redox-sensitive elements and normalized REEs. These sources probably originate in hot humid environment, as indicated by the chemical index of alteration (CIA) (>80).The above geochemical data suggest that the passive fill was derived from soils similar to Mediterranean Terra Rossa soils and postdate the formation of burrow pores in the Thalassinoides-bearing strata. This could have occurred after exposure of the Upper Jubaila Member. Terra Rossa soil most likely reached the Thalassinoides-bearing strata from the surface through vertical fractures and karst features, filled burrow pores, fractures along bedding plains, and even intercrystalline pores. The occurrence of Terra Rossa soil in the burrows and intercrystalline pores of the Thalassinoides-bearing strata implies that the pore system of the Upper Jubaila Member possessed connected pathways that enabled soil infiltration followed by simultaneous precipitation in the burrows and intercrystalline pores. The presence of these connected burrow pores containing shafts with diameters ranges from 1 to 5 cm suggests a superpermeable storage capacity and presence of a superflow unit in the Upper Jubaila Member. This study provides an excellent example to better understand the controls of super-k zones in carbonate reservoirs.

Late Mesozoic tectonic evolution of the central Bangong–Nujiang Suture Zone, central Tibetan Plateau

ABSTRACT The Bangong–Nujiang Suture Zone (BNSZ), located in the central Tibetan Plateau, experienced a Late Mesozoic transition from oceanic closure to continent–continent collision. Based on sandstone-composition analyses, U-Pb detrital-zircon dating and structural observations, we determined that this transition occurred coeval with the accretion of the Lhasa Terrane to Eurasia. According to our newly observed stratigraphic and geochronological data, we conclude the following: (1) the Upper Triassic-Lower Jurassic Muggar Gangri Group was mainly derived from the Qiangtang Terrane; (2) the Shamuluo Formation composed of Upper Jurassic-Lower Cretaceous shallow marine strata was sourced from both the Qiangtang Terrane and recycled Muggar Gangri Group; (3) the Lower Cretaceous (~125–110 Ma) Duoni Formation shows both Lhasa and Qiangtang detrital zircon fingerprints, indicating closure of the Bangong–Nujiang Ocean at ~120 Ma; and (4) the Upper Cretaceous Abushan Formation is a suite of intramontane molasse deposited after the Qiangtang–Lhasa collision. Therefore, these sedimentary strata record the transition from oceanic closure to continent–continent collision from 125 to 90 Ma within the central BNSZ. Moreover, based on relationships among strata distributions and thrust locations, we propose that thrust movements propagated southward during this transition and controlled sedimentary basin deposition. The Late Mesozoic tectonic transition within the BNSZ occurred well before the India-Asia collision and hence profoundly influenced the Cenozoic formation of the Tibetan Plateau.

GEOCHEMISTRY, CLAY MINERALOGY, AND TAPHONOMY OF THE FOSSIL-RICH BUTTER SHALES

The Cincinnati Arch contains a suite of Upper Ordovician, mixed siliciclastic-carbonate, shallow marine strata, with alternating packages of packstones and shales that contain exceptionally well-preserved articulated fauna. The Waynesville Formation (C5 sequence) in Eastern Indiana includes distinctive clay-rich shales, which are locally referred to as butter shale. These bluish-green claystones are distinct in comparison to other shales because of their geochemistry, clay mineralogy, bulk composition, and well-preserved and biostratigraphically important fauna. This study focuses on an outcrop with exposed butter shale from the Waynesville Formation at Hanna Creek in Brookville, Indiana. Bulk-rock samples were collected and the geochemical and clay mineralogical composition of the claystones were analyzed using x-ray fluorescence (XRF) and x-ray diffraction (XRD) techniques. On average, the butter shale clay mineralogy is 74% illite, 25% chlorite/kaolinite, and less than 1% smectite. The butter shale lacks expandable smectite, likely from post-depositional alteration of original smectite through illitization and chloritization. The presence of iron in the system as a result of diagenetic processes allowed for the immediate mineralization, and in some cases pyritization, of fossils. Bulk chemical analysis coupled with clay mineral analysis provides an explanation for the Fe substitution into chlorite for the butter shales and supports the interpretation of rapid sediment sealing that allowed for the pristine preservation of articulated fauna and mineralization observed in the butter shale.

Ichnologic Note A New Ichnospecies of Cardioichnus from the Cretaceous (Albian) of New Mexico

ABSTRACTA new ichnospecies of Cardioichnus, Cardioichnus biloba isp. nov., is described and documented from shallow marine strata of Cretaceous (Late Albian) age in southern New Mexico, USA. It is a heart-shaped resting trace which, unlike all other ichnospecies of Cardioichnus, is epichnial and increases in width to a bilobate trace, often with a raised anterior terminus. Only the posterior end of this trace is associated with Bichordites burrows, and the anterior end is bilobate and undisturbed. This represents a resting position which differs from other known species of this cubichnium, which are hypichnial and subquadrate in symmetry. The tracemaker is a spatangoid echinoid, most likely Heteraster d'Orbigny, 1853.

Late Quaternary stratigraphy of the La Janda Basin (SW Spain) - first results and palaeoenvironmental significance

The La Janda basin in southern Spain is a near-shore geo-bio-archive comprising a variable Quaternary depositional history, with shallow marine, lacustrine, palustrine, and terrestrial strata. In the 1930s the lake was drained and is serving now as a huge agricultural area. The 33 m-core recovered in fall 2016 along with several shallower drill cores up to c. 15 m, reveals insights into a unique mixed terrestrial palaeo-environmental archive in Andalucia influenced by the Atlantic Ocean and hence the North Atlantic Oscillation (NAO) within the Gulf of Cadiz. The basin's evolution was influenced both by the postglacial marine transgression and by an active tectonic fault controlling most of the accommodation space by causing subsidence. Our long core was accompanied by further corings along an E-W striking transect in order to reveal also the relation of the influence of tectonic activity with sedimentary sequences. Multi-Sensor Core Logging has been completed. Results of sedimentological, geochemical and micropalaeontological analyses will be presented in the frame of the climate variations during the Late Pleistocene and the Holocene, along with a preliminary age-depth model based on radiocarbon (AMS-14C) and optical stimulated luminescence (OSL) dating techniques. Our investigations ultimately aim at providing valuable information on major Late Pleistocene to Holocene climatic and palaeo-environmental fluctuations in the southernmost part of the Iberian Peninsula.

Albian angiosperm pollen from shallow marine strata in the Lusitanian Basin, Portugal

The evolution of angiosperms significantly changed the composition of the terrestrial vegetation during the mid-Cretaceous. In contrast to the wealth of information available on the biology and systematic relationships of early angiosperms, the temporal patterns of their evolution and radiation are poorly constrained. Here we present a continuous angiosperm pollen record from well-dated shallow marine deposits in the Lusitanian Basin, Portugal. The São Julião section provides a solid stratigraphic framework to track angiosperm pollen distribution patterns from the early Albian to early Cenomanian at mid-latitudes. In comparison to previous angiosperm pollen records from the Lusitanian basin, the section shows an extended late Albian succession and provides new insights into the diversification of early angiosperms during this important interval. Productive palynological samples were analysed and 79 different angiosperm pollen types have been recorded. Throughout the Albian angiosperm pollen represent only a minor component of the total palynoflora. The early Albian pollen record is characterized by highly diverse assemblages of monoaperturate pollen of monocot or “magnoliid” affinity and by the first appearance of polyporate and tricolpate pollen of eudicot affinity. A distinct diversification phase among tri- and poly-aperturate pollen (e.g., Cretacaeiporites, Retitrescolpites, Rousea, Striatopollis and Tricolpites) and the presence of conspicuous pollen grains assigned to Dichastopollenites characterize the middle and late Albian palynological assemblages. Thus, the section records a striking sequence of appearances of important angiosperm pollen morphologies. Monocolpates, polyporates and tricolpates appear in the early Albian whereas tricolporates appear from the early part of the late Albian onwards. Furthermore, well-constrained biostratigraphic ranges of selected angiosperm pollen from mid-latitudes are presented. In view of these new data, the temporal framework of the palynological Subzones II-B and II-C in the Potomac Group succession from the Atlantic Coastal Plain, eastern USA is revised to a middle to late Albian age.

Stratal Architecture and Morphodynamics of Downstream-Migrating Fluvial Point Bars (Jurassic Scalby Formation, U.K.)

Abstract Assessments of depositional architecture and paleodrainage in planform are increasingly recognized as critical for the reconstruction of ancient fluvial morphodynamics; however, planview exposures of ancient, undeformed meandering-fluvial deposits are scattered, and exceptional occurrences such as the exhumed meander plain of the Scalby Formation (Yorkshire, UK) deserve particular attention. Here we specifically focus on the planview style and morphodynamics of downstream-migrating point bars, providing parallels with modern meandering rivers sharing scale and processes. The Middle Jurassic Scalby Formation lies in the Cleveland Basin of Yorkshire (UK), and accumulated in incised valleys eroded onto shallow-marine strata. Five well preserved downstream-migrating point bars are exposed along a modern tidal platform. Through 1:2000-scale mapping, point bars are analyzed for sedimentology, stratal architecture, vectors of paleoflow and bar migration, and nature of adjacent and underlying deposits. Results indicate that downstream migration was facilitated by—but not strictly related to—the occurrence of resistant outer banks. Meander-bend migration promoted cannibalization of upstream-bar portions, subordinate preservation of central-bar portions, and widespread development of downstream-bar scrolls. The most reliable indicators of regional transport are upstream-bar paleoflow indicators and the dip direction of downstream-bar inclined beds. The generated sectional architecture overlaps in part with that of other fluvial depositional models, such as those for braid bars, indicating that lack of planform exposures may introduce bias when reconstructing ancient fluvial morphodynamics.

Along-Strike Variations In Stratigraphic Architecture of Shallow-Marine Reservoir Analogues: Upper Cretaceous Panther Tongue Delta and Coeval Shoreface, Star Point Sandstone, Wasatch Plateau, Central Utah, U.S.A.

Abstract Along-strike variations in sediment supply and process regime may result in different facies distributions, preservational biases, and stratigraphic architectures in shallow-marine strata. Data from a large (c. 100 km), nearly continuous outcrop belt aligned oblique to depositional strike was integrated with nearby subsurface well data and petrological analysis of distal fine-grained deposits, to analyze along-strike variations in stratigraphic architecture in a single parasequence in the upper Cretaceous Star Point Sandstone, Wasatch Plateau, central Utah, USA. The parasequence comprises wave-dominated shoreface deposits in the southern part of the study area and represents a single episode of shoreline regression and transgression. Four progradationally stacked bedsets (BS1–BS4) in the distal deposits of the parasequence are the expression of minor variations in relative sea level, sediment supply, and/or wave climate. The four bedsets can be traced to river-dominated deltaic deposits (Panther Tongue) in the northern part of the study area. We interpret the parasequence on the scale of the Wasatch Plateau outcrop belt as the record of deposition in a regressive river- and wave-influenced delta deflected asymmetrically towards the SSW, subparallel to the regional paleo-shoreline trend, and a coeval wave-dominated strandplain. The lateral transition from river- to wave-dominated facies is attributed to the localization of a major river acting as a sediment point source in the north, with wave-induced longshore transport to the south. Our results imply that characterizing lateral facies distributions and the expression of bedsets is vital for interpreting along-strike variability in controlling mechanisms of stratigraphic architecture, and that external forcing may be recorded differently along depositional strike.

Sedimentology and reservoir properties of tabular and erosive offshore transition deposits in wave-dominated, shallow-marine strata: Book Cliffs, USA

Facies models for wave-dominated shorelines include an ‘offshore transition zone’ between shelfal mudstones and nearshore shoreface sandstones. Offshore transition-zone deposits are commonly tabular sandstone beds interbedded with continuous mudstone beds. However, observations from the Blackhawk Formation show that the offshore transition zone locally consists of erosive-based sandstone beds with ‘pinch-and-swell’ geometries containing steep-walled gutter casts, in areas larger than 6 × 2 km along strike and dip. This increases the amount of sand-on-sand contacts, and leads to improved vertical permeability. Predicting the distribution of erosive offshore transition within the subsurface is therefore desirable. In this study, offshore transition-zone deposits have been studied using virtual outcrops. Tabular offshore transition-zone deposits have continuous sandstone and mudstone beds much longer than 500 m, and erosive offshore transition-zone deposits have discontinuous shales, on average, 60 m long. Reservoir modelling shows a 10- to two-fold increase in vertical permeability in erosive compared to tabular offshore transition deposits, the magnitude decreasing with increasing fraction of shale. Erosive offshore transition deposits occur near distributary channels, subaqueous channels and abrupt bathymetric breaks. A regional study shows that erosive offshore transition-zone deposits are mainly developed where parasequences prograde into deeper water offshore the platform break of the preceding parasequence, are commonly associated with basinal turbidites and may be related to erosion by bypassing turbidity currents.

Erosional surface generated by the 2011 Tohoku-oki tsunami in the shallow marine strata, and its preservation potential: A case of the research at Sendai Bay

Erosional surface generated by the 2011 Tohoku-oki tsunami in the shallow marine strata, and its preservation potential: A case of the research at Sendai Bay

Quartzite development in early Palaeozoic nearshore marine environments

AbstractQuartzites are especially characteristic of Proterozoic and Cambro‐Ordovician shallow marine strata, whereas equivalent age fluvial deposits are commonly arkosic. The absence of land vegetation in the pre‐Silurian influenced weathering processes and styles of fluvial deposition. It may also have had an impact on shallow marine sedimentation. Two field studies from the English Channel region are presented to investigate the processes leading to quartzite formation. On Alderney, nearshore marine and fluvial facies occur interbedded on a metre scale and are interpreted to represent deposition on the lower reaches of an alluvial plain, and in beach and upper shoreface environments. The marine and fluvial sandstones display marked differences in textural and mineralogical maturity, pointing to a process of sediment maturation by the destruction of feldspar and labile grains at the shoreline. At Erquy, fully mature, marine quartzites occur bounded above and below by alluvial deposits via sharp or erosional surfaces, and are interpreted to represent high energy, storm and tidally influenced lower shoreface and inner shelf deposits. A model for quartzite development is proposed where, under a cool climate, frequent storms in un‐vegetated, tectonically rejuvenated uplands provided an abundance of arkosic sand to fluvial basins and clastic shorelines. The model proposes that the marine basins were subject to high wave energies, frequent storm events and tidal currents. These were conditions conducive to transforming arkosic sand to quartz‐rich sand by the attrition of feldspar at the shoreline and in the shallow marine environment. On sediment burial, further feldspar destruction occurred during diagenesis. The proposed model highlights the potential for a step change in sediment maturity to occur at the shoreline in early Palaeozoic depositional systems tracts.

Circular, Cryogenic Structures from the Hirnantian Deglaciation Sequence (Anti-Atlas, Morocco)

Abstract More than a hundred circular structures are observed at the topmost part of the Late Ordovician glaciogenic record in the central Anti-Atlas, southern Morocco. The structures were formed in the outer glaciated platform and characterize the Hirnantian deglaciation sequence. They are distributed within a 20 meter thick stratal interval and, at present, relicts of them still occupy several square kilometers in map view. The circular to ovoidal structures, which comprise fluvial sandstone beds and are sealed by shallow marine strata, occur in closely spaced clusters. They are 13–320 m in diameter, with half of them in the 40–90 m range. Stratal wedging and bending, as well as extensional and reverse centimeter- to meter-scale soft-sediment fractures, demonstrate that formation processes initially involved synsedimentary uplifts. Subsequent subsidence and collapse resulted in the formation of circular structures, which essentially correspond to localized depocenters in the form of small-scale sag basins. Regarding the overall deglaciation context, the circular structures are interpreted as periglacial in origin, with vertical movements ascribed to cryogenic processes involving the growth and then decay of bodies of segregation ice. The size and diameter distributions, which are comparable to the recent (Younger Dryas) and current lithalsa structures, suggest that Hirnantian circular features relate to similar genetic processes and can be referred to as lithalsa-like structures. A conceptual emplacement model is proposed, which considers that a long-term sedimentary aggradation over the fluvial plain prevented any significant landforms (either a bump or a depression) from being expressed at the depositional surface at the time of the circular structure formation. The Hirnantian lithalsa-like structures constitute firm evidence for a Late Ordovician discontinuous permafrost during the deglaciation stage. Assuming that the late Quaternary meridional zonation is valid, it suggests the Anti-Atlas was at latitudes in the 55–65° interval. If combined with the rare pingo structures described in the literature, which attest that coeval continuous permafrost conditions occurred at higher paleo-latitudes, the Moroccan circular structures provide evidence for an alternate South Pole position during the Hirnantian (444 Ma).

Carbon-isotope stratigraphy from terrestrial organic matter through the Monterey event, Miocene, New Jersey margin (IODP Expedition 313)

The stratigraphic utility of carbon-isotope values from terrestrial organic matter is explored for Miocene siliciclastic sediments of the shallow shelf, New Jersey margin, USA (Integrated Ocean Drilling Program [IODP] Expedition 313). These shallow marine strata, rich in terrestrial organic matter, provide a record of deposition equivalent to the Monterey event, a prolonged interval of time characterized by relatively positive carbon-isotope values recorded from foraminiferal carbonate in numerous oceanic settings. Coherent stratigraphic trends and short-term isotopic excursions are observed consistently in palynological preparation residues, concentrated woody phytoclasts, and individually picked woody phytoclasts obtained from the New Jersey sediments. A bulk organic matter curve shows somewhat different stratigraphic trends but, when corrected for mixing of marine-terrestrial components on the basis of measured C/N ratios, a high degree of conformity with the woody phytoclast record is observed. However, assuming that the correlations based on strontium-isotope values and biostratigraphy are correct, the carbon-isotope record from the New Jersey margin contrasts with that previously documented from oceanic settings (i.e., lack of positive excursion of carbon-isotope values in terrestrial organic matter through the Langhian Stage). Factors that may potentially bias local terrestrial carbon-isotope records include reworking from older deposits, degradation and diagenesis, as well as environmental factors affecting vegetation in the sediment source areas. These possible factors are assessed on the basis of pyrolysis data, scanning electron microscope observations, and comparison to palynological indices of environmental change. Some evidence is found for localized degradation and/or reworking of older woody phytoclasts, but where such processes have occurred they do not readily explain the observed carbon-isotope values. It is concluded that the overall carbon-isotope signature for the exchangeable carbon reservoir is distorted, to the extent that the Monterey event excursion is not easily identifiable. The most likely explanation is that phytoclast reworking has indeed occurred in clinoform toe-of-slope facies, but the reason for the resulting relatively heavy carbon-isotope values in the Burdigalian remains obscure.

The Recognition of Ophiomorpha irregulaire on the Basis of Pellet Morphology: Restudy of Material from the Type Locality

Serial grinding of a specimen of Ophiomorpha irregulaire collected from Coal Creek Canyon in the Book Cliffs of Utah, USA—the type locality of the ichnospecies—allows us to assess the validity of pellet morphology as an ichnotaxobase for species-level identification. The importance of O. irregulaire stems from its abundance in petroliferous shallow marine strata of post-Paleozoic age. Our three-dimensional reconstruction of the burrow wall of O. irregulaire demonstrates for the first time that it is composed of sand-cored pellets. The typically spiky nature of the pellets in cross-section is inferred to result from differential compaction of the sand core and pelletal lining. Rupturing of the thin outer clay coating of the sand pellets is considered to produce the distinctively attenuated, spiky outer surface to the burrow wall of this ichnospecies. This study demonstrates the utility of pellet morphology in species-level classification of Ophiomorpha and lends support to O. irregulaire being a cosmopolitan ichnospecies present in post-Paleozoic strata. It also demonstrates the need to ensure that ichnotaxonomic identifications are based on three-dimensional knowledge of the trace fossil in question.

Mid-Carboniferous deglaciation of the Protoprecordillera, Argentina recorded in the Agua de Jagüel palaeovalley

The Agua de Jagüel Formation near Uspallata, Mendoza Province, Argentina, was deposited within a palaeovalley along the southeastern margin of the Calingasta–Uspallata Basin adjacent to the Protoprecordillera. The basal glacigenic sequence of the formation was deposited in the Serpukhovian–Early Bashkirian, near the beginning of the late Palaeozoic ice age. The sequence is composed of diamictite, conglomerate, sandstone, and mudrock and records four depositional stages within a palaeofjord: 1. morainal bank deposition by a wet-based tidewater glacier, 2. glacial retreat succession where ice retreated up the fjord out of the immediate area, allowing iceberg deposition of dropstones and dump deposits, 3. continued glacial retreat with ice receding onto land, allowing a shoreface to develop within the palaeovalley, and 4. transgression across the shoreface and resumption of iceberg deposition. The thickness and facies of this succession are similar to deposits within modern Alaskan fjords housing temperate tidewater glaciers. Comparison of the Agua de Jagüel Formation with Alaskan fjords indicates that sequence stratigraphy used for low latitude deposits must be applied with caution, as glacimarine ice retreat within a fjord produces strata surfaces that are easily misidentified as flooding surfaces resulting from changes in water depth. Likewise, the transgression during stage 4 is significant as it indicates a rise in sea level that occurred during glaciation of the Protoprecordillera, thus suggesting that the transgression between stages 3 and 4 was not driven by input of glacial meltwater. Additionally, the sediment geochemistry suggests that the bottom waters of the palaeovalley were anoxic, which may help explain the absence of bioturbation in the sequence as well as in other ancient glacimarine palaeovalleys. The deglaciation succession and transgression are also recorded in the nearby Hoyada Verde and Tramojo Formations, so it is proposed that all three formations record one glacial event in the Protoprecordillera. Overlying fluvial and shallow marine strata in the Agua de Jagüel, Hoyada Verde, and Tramojo Formations show no indication of continued glaciation in the Protoprecordillera following stage 4 and equivalent strata. The deglaciation succession of the Agua de Jagüel Formation affirms the emerging concept that the late Palaeozoic ice age was characterized by alpine glaciers, ice caps, and small ice sheets that were not massive enough to have driven eustatic fluctuations of 100 m+ as previously understood, and that ice never covered westernmost Gondwana during later LPIA events.

Bioturbation levels during the end-Ordovician extinction event: a case study of shallow marine strata from the Welsh Basin

ABSTRACT: The Ordovician-Silurian succession of the Llandovery area, mid-Wales, preserves shal-low marine sediments deposited during the Hirnantian (end-Ordovician) extinction event. Ichno-logical analysis of the rocks shows that the degree and depth of bioturbation in this part of the WelshBasin was often low across this interval, both during glacioeustatic sea level fall and the early part ofthe subsequent transgression. This pattern suggests that stratigraphic dilution was not the primarycontrol on trace fossil abundance: an increase in sediment supply during the regression could explainlow abundance in early Hirnantian units, but not in the post-glacial transgressive strata. A significantdecrease in oxygen levels during sea level rise might be invoked instead, but this cannot have beenthe sole cause, as the occurrence of burrows up to 20 mm in diameter in the Bronydd Formation (lateHirnantian–Rhuddanian) shows that seafloor oxygen levels were at least intermittently high. Anabsence of vertical bioturbation through much of the succession indicates that sessile, suspension-feeding organisms were generally scarce. The overall pattern probably reflects a decline in benthicinfauna during the extinction event, but the 2 pulses of extinction described previously in body fossils are not evident ichnologically in this part of the Welsh Basin.KEY WORDS: Ichnofabrics · Benthos · Infauna · Mass extinctions · Trace fossils

Palynology of the NBP03–01A transect in the Northern Basin, Western Ross Sea, Antarctica: A late Pliocene record

Fifty‐seven samples taken from ten piston cores collected along a transect off the continental margin of the Northern Basin, Ross Sea, Antarctica were analyzed for palynomorphs. Moderately diverse assemblages of marine microplankton and terrestrial palynomorphs were recovered. The palynomorph assemblages have been subdivided into two main groups: the in‐situ flora (including acritarchs, dinoflagellate cysts, leiospheres and prasinophyte algae mainly composed of cymatiosphaerids), and the reworked flora (including dinoflagellate cysts, pollen and spores). The leiospheres are the most abundant palynomorphs. This prominence in the relative abundance of leiospheres has been reported as typical of assemblages found today at the limit between seasonal and pack ice in the Arctic. In‐situ dinoflagellate cysts are sparse. They are mainly represented by Lejeunecysta, which, based on species similarities to those from Cape Roberts, are believed to be of Oligocene to Pliocene age. All other dinoflagellate cysts recovered are the result of reworking from Eocene to Oligocene sediments. Reworked spores and pollen comprise the second most abundant group. They are of moderate diversity and include an Eocene or older assemblage of Nothofagidites, Podocarpaceae and Proteaceae. Other taxa are representative of warmer rainforest vegetation, with Oligocene and Neogene taxa that include representatives of woodland to herbaceous/low shrubby tundra vegetation growing in colder subpolar climates. These assemblages indicate either different periods of deposition or reworking from diverse sources. Through seismic correlation and diatom analysis, the sediments are believed to be Late Pliocene in age. On this basis, it is postulated that the major glacial advance, RSU 2 of Brancolini et al. (1995) or Unconformity 10 (U10) of Bart et al. (2000), occurred before 2.3 Ma, which is the oldest age of in‐situ species recovered in units above U10. As both terrestrial and marine reworked taxa include assemblages of Eocene to Oligocene age, it is assumed that those reworked components were yielded from a single source; most probably Eocene to Oligocene shallow marine strata eroded and transported from the area of Ross Island to the shelf margin through ice streams located in the Drygalsky and Joides basins.

Faunal affinity of Toarcian-Aalenian (Early Jurassic) bivalves from Mae Sot and Umphang (Tak Province), Northwestern Thailand

In the Mae Sot and Umphang areas (NW Thailand), Jurassic strata seal the Permian and Triassic substratum of the Shan-Thai terrane with a brecciated conglomerate. Pliensbachian to Early Bajocian shallow marine strata are intercalated within the partly terrestrial Jurassic sequence. Thirty-five Toarcian-Aalenian bivalve species from Mae Sot and Umphang were identified. Among these,Eomiodon chumphonensis Hayami is known only from Thailand, whereasParvamussium donaiense (Mansuy),Myophoriella saurini Hayami,Thracia loducensis Hayami,Pholadomya (Bucardiomya) fontainei Hayami,Thracia loducensis Hayami andModiolus sestiniae Hayami were originally described in Vietnam.M. sestiniae also occurs in Iran. The greater part of the fauna, however, shows varying degrees of affinity with Myanmar (6 species), Cutch (10 species), Japan (13 species) and Europe (9 species), Levant (2 species). In view of these, it appears that these highly endemic Toarcian-Aalenian bivalves from Thailand characterize the Southeastern Asian Province of Tethys.