Early Transgression Research Articles

Unraveling marine phosphogenesis along the Miocene coast of Peru: Origin and sedimentological significance of the Pisco Formation phosphorites

Phosphorite deposits form through phosphogenesis, a mechanism of diffusion and concentration of interstitial phosphate in sediments leading to Ca-phosphate mineral precipitation in a favorable sedimentary regime at the seafloor. Stretching along the Peruvian coast, the East Pisco Basin is known to host phosphorite beds of Miocene age. In this study, we investigate the petrographic, mineralogical and chemical composition of the Miocene phosphorite layers of the East Pisco Basin, and discuss the phosphogenetic mechanism(s) and sedimentary dynamics that led to the genesis and accumulation of P-rich deposits. Phosphorite samples were collected from layers overlaying stratigraphic unconformities within the Pisco Formation and subsequently analyzed through multiple methods: optical microscopy, XRD, XRF, SEM-EDS, EPMA, ICP-MS and LA-ICP-MS. Our results indicate that these layers consist of phosphatic clasts and nodules along with dolomite clasts, basement boulders, shark teeth, mollusk molds and vertebrate bone fragments. The P-rich clasts are mainly composed of phosphatic intraclasts and small phosphatic nodules. They occur associated with minor amounts of dolomite clasts coated by Fe-oxyhydroxides and laminites. Clasts and nodules are cemented by both early and late diagenetic cements, such as dolomite, calcite, silica, gypsum/anhydrite and baryte. Our observations suggest that the Pisco phosphorite deposits are transgressive lags laying on ravinement surfaces. They formed just below the sediment-water boundary during the early transgression phases, when the porewaters were enriched in phosphorus due to the abundance of organic-rich sediments and the microbial activity, as typical of upwelling settings. These deposits formed in shallow marine conditions, characterized by high biological productivity and low net sedimentation rates, with suboxic conditions at and beneath the sediment-water interface. The concentration of phosphate-coated clasts was then favored by a sedimentary condensation mechanism of dynamic bypassing. Overall, this study provides new insights into the genesis of phosphorite layers in the East Pisco Basin and highlights the importance of understanding the complex interplay between authigenic and sedimentological processes in the formation of these economically and geologically relevant sedimentary rocks.

The impact of elevated nutrients on the Holocene evolution of the Great Barrier Reef

The initiation of the Holocene Great Barrier Reef coincided with rapid environmental change as sea level rose and inundated the shelf. Core data from One Tree Reef (southern Great Barrier Reef) shows coral growth started by ∼8.2 ka, but accretion between 8 and 7 ka was slower, occurred in deeper water, and comprised more sediment-tolerant coral communities compared to growth following sea-level stabilization. It has been postulated that environmental stressors (e.g. increased turbidity and nutrients) suppressed and delayed reef growth, however direct data supporting this hypothesis are scarce. Here we combine the isotopic composition of skeletal bound organic nitrogen (δ15N) and Ba/Ca ratios of coral skeletons with published geochemical proxies of terrestrial sediment discharge to constrain Holocene water conditions at One Tree Reef. Between 8 and 7 ka the skeletal δ15N values from multiple corals and genera were elevated (average of 8.45 ± 0.89‰) relative to the early transgression and following sea-level stabilization (average of 7.04 ± 0.82‰). We propose that elevated δ15N in corals reflects the discharge of deep terrestrial soil nitrogen resulting from high runoff. This is supported by Ba/Ca measurements and published rare earth element and yttrium (REE + Y) geochemical proxies in coral and reefal microbialites from the same cores. These data suggest that increased terrigenous discharge of sediment and nutrients did not inhibit reef growth, rather led to the establishment of slower-growing, deeper and more sediment-tolerant coral communities. Understanding the capacity for reef growth under adverse environmental conditions provides insight into thresholds and resilience of the GBR over centennial-millennial timescales.

Geochemical and Isotopic (Nd, Sr) Tracing of the Origin of REE Enrichment in the Cambrian Georgina Basin Phosphorites

AbstractPhosphorites of the Georgina Basin (northern Australia) are an established economic source of phosphate and have recently been recognized to be a potential source of rare earth elements (REE). Previous bulk‐rock geochemistry work focused on the eastern margin of the basin revealed that phosphorites from the southern region have significantly higher (up to 0.5%) REE contents than equivalent prospects further to the north. In this study, we examine the origin of REE enrichment in the Georgina Basin phosphorites using an integrated geochemical, petrographic, mineralogical, and isotopic (Sm‐Nd and Sr) approach. The trace element geochemistry of primary phosphate minerals is consistent with a seawater origin for the REEs in phosphorites, with minimal input from the underlying basement rocks. Variations in total REE concentrations are controlled largely by the seawater composition, depositional environment, and the texture of the phosphorite (grainstone vs. mudstone phosphorite). Grainstone phosphorite yields on average higher concentrations of REEs compared to the mudstone phosphorite, likely due to the higher surface‐to‐mass ratio of the grainstone phosphorite resulting in the uptake of higher concentrations of REEs during coastal reworking. The formation of the low‐REE northern and central deposits took place during early transgression in oxidized, shallow supratidal to subtidal platformal environments. During the subsequent main transgressive event, fault‐controlled topography in the southern domain allowed phosphogenesis along basement highs to be facilitated along a redox boundary, where more saline and REE‐rich deep anoxic bottom waters, along with Fe‐cycling and bacterial mediation played a major role in the enrichment of REEs in the southern phosphorites.

Anatomy of a fluvial paleo-fan: sedimentological and architectural trends of the Paleocene–Eocene Wasatch–Colton System (western Uinta Basin, Utah, U.S.A.)

ABSTRACT Recent developments in fluvial geomorphology and sedimentology suggest that fluvial fans (also known as distributive fluvial systems) could be responsible for the accumulation of great volumes of clastic successions in continental basins. A general depositional model based on sedimentological and architectural trends has been formulated for these fluvial systems, however, their recognition in the stratigraphic record often relies on partially preserved, discontinuous successions. This study provides a sedimentological and architectural characterization of Paleogene alluvial strata of the Wasatch and Colton formations in the southwestern Uinta Basin, central Utah (U.S.A.), following an ∼ 120-km-long outcrop belt which records deposition from an alluvial wedge that prograded to the north and northwest, from the basin margin to distal lake-dominated environments, preceding the onset of ancient Lake Uinta. Lateral and vertical distribution of facies associations are presented from a dataset of field observations (thirteen logged stratigraphic sections, for a total of ∼ 2400 m) and virtual outcrop models along the proximal-to-distal extent of the Wasatch–Colton alluvial system. Four sectors are defined (proximal, medial, distal, and terminal) to mark the longitudinal heterogeneity of alluvial stratigraphy. Noteworthy trends comprise a downstream decrease in the overall thickness of the alluvial stratigraphic column, a reduction in the relative volume, architectural complexity, and amalgamation of fluvial-channel bodies away from the apex, a weak downstream-fining trend in channel sandstones, and a down-system increase in preserved overbank and floodplain deposits accompanied by increasing volume and facies complexity of preserved lacustrine and palustrine facies associations. A proximal-to-distal change in fluvial-channel architecture is noted, with proximal sections characterized by vertically and laterally amalgamated sheet-like channel fills, transitioning to a lesser degree of amalgamation towards the medial sector, whereas distal and terminal sections are dominated by floodplain fines enveloping a subordinate volume of isolated, ribbon-shaped channel-sandstone bodies. The temporal development of the stratigraphic succession is observed in its entirety throughout the field area and, albeit localized, channel-scale erosion and potential depositional hiatuses punctuate the stratigraphy. Two major system-scale trends have been described in the Wasatch–Colton System (WCS). A first-order, long-term progradational trend, especially evident in proximal and medial sections, encompasses the large-scale vertical patterns in facies and architecture vertically through most of the stratigraphy of the WCS. In contrast, the uppermost part of the stratigraphic succession is characterized by a reverse, retrogradational trend, possibly associated with the early transgression of Lake Uinta's southern margin, marking the base of the overlying Green River Formation. Albeit expressed by different vertical succession of facies, proximal-to-distal processes, and stratal patterns documented along a longitudinal transect of the WCS mirror substantially identical vertical trends through the stratigraphy, which are interpreted as Waltherian superposition of distinct depositional domains during fluvial-fan progradation. Analyses of sandstone petrography and zircon geochronology suggest a unified source for the fluvial system, also supporting an interpretation as a distributive, rather than a tributive, fluvial system. This study enhances our understanding of the current depositional model for extensive fluvial-fan successions via a regional-scale stratigraphic analysis of a fluvial paleo-fan based on integrated characterization of both vertical and down-system patterns in facies distribution and fluvial architecture, providing key insights on useful criteria for recognizing fluvial-fan successions from the rock record.

Stratigraphic architecture and fluvial interpretations of the Upper Cretaceous (Turonian?) Middendorf Formation, Chesterfield County, South Carolina, U.S.A.

ABSTRACT The Upper Cretaceous (Turonian?) Middendorf Formation is a sand-rich stratigraphic unit of fluvial origin that forms a large aquifer in the U.S. Atlantic Coastal Plain. In Chesterfield County (South Carolina), which is the site of the type locality, the formation ranges in thickness from 66.5 to > 119.7 meters. The base of the formation is an unconformity above Paleozoic metasiltstone, and the upper contact is an unconformity above which lies sand of the Quaternary Pinehurst Formation. Outcrops display the following five facies assemblages: 1) sandstone to conglomeratic sandstone (fluvial bar and channel deposits), 2) beds of alternating laminae of sandstone and mudstone (fluvial overbank or floodplain deposits), 3) ≥ 1 m-thick beds of clay (swamp deposits, floodplain deposits, and/or sediment that accumulated in abandoned fluvial channels), 4) 0.2–0.5 m-thick planar to slightly undulatory beds of framework-supported sandstone with a mud matrix (traction-dominated current deposits at the top of fluvial bars, upper-flow-regime bedform deposits in subsidiary fluvial channels, or coarse-grained overbank deposits), and 5) sandstone to conglomeratic sandstone cemented by iron (interpreted as fluvial bar and channel deposits, with the iron cement being a diagenetic “groundwater ferricrete” that formed via the circulation of shallow groundwater and the oxidation of iron-bearing minerals). Kaolinite in various forms is pervasive throughout the formation and is interpreted as an early diagenetic phenomenon that formed by prolonged postdepositional weathering and flushing by meteoric water under a warm and humid paleoclimate. The fluvial system that formed the Middendorf Formation prograded into the area from the west or northwest from uplifted margins of Mesozoic rift basins and/or the Appalachian Mountains. This progradation was a response to a base-level fall and the sediment accumulated during base-level lowstand and subsequent early transgression. In Chesterfield County, the Middendorf Formation can be subdivided into three fining-upward sequences. Each sequence consists predominantly of medium to coarse sand with a greater abundance of gravel in the lower part of the sequence and a greater abundance of clay and silt beds in the upper part. Each sequence is interpreted as either a response to autogenic processes or a response to allogenic sea-level changes, specifically a higher-order (higher-frequency) progression from relative lowstand conditions to early transgression whereby coarse sand and gravel (e.g., fluvial bar and channel deposits) were preserved during initial lowstand conditions and a greater proportion of mud and finer-grained sand (floodplain deposits) were preserved during subsequent early transgression. The Middendorf Formation is correlative with several other kaolinite-rich fluvial sandstones in North America including the Raritan Formation in New Jersey, the Tuscaloosa Formation of the eastern Gulf of Mexico (Alabama, Mississippi, Louisiana), the Woodbine Formation of the central Gulf of Mexico (Texas), and the Frontier Formation of Wyoming. The accumulation and preservation of these formations occurred in response to a Turonian eustatic sea-level fall and subsequent transgression, and the early diagenetic kaolinite in these formations is attributed to similar warm and humid paleoclimate conditions.

The evolution of catchment‐depositional system relationships on the dip slopes of intra‐rift basement highs: An example from the Frøya High, Mid‐Norwegian rifted margin

AbstractBasement highs form one of many potential sediment source areas during the evolution of continental rifts and rifted margins and add to the topographic complexity typical of active rifts. Footwall basement highs acting as a source area to sedimentary systems in the hangingwall of major faults have been documented in many systems worldwide. However, the back‐tilted footwall dip slopes of such highs have received comparatively little attention. Here, we investigate a subsurface case study from the Norwegian continental shelf, where catchments and shallow marine syn‐rift sedimentary systems on a dip slope are preserved due to early transgression of an intra‐rift high. At the onset of Late Jurassic rifting, the Frøya High emerged as a prominent, N‐S trending, 25 km‐wide basement high tilted towards the east in response to several kilometres of displacement along the Klakk Fault Complex, a major normal fault zone at the Frøya High's western edge. Using well‐calibrated 3D seismic reflection data, we observe a series of conspicuous Upper Jurassic wedges along the eastern edge of the Frøya High along the margin of the Froan Basin. Internally, these wedges show sigmoidal geometries marking top and foresets of clinoform packages with a maximum thickness of ca. 200 m with foresets between 30 and 200 m high, dipping ca. 10° towards the east, southeast and northeast. We interpret these wedges to represent a series of eastward prograding deltas positioned along a constructional shoreline, connected to E‐W trending valleys and river catchments up‐dip. The deltas show strong progradation, interpreted to reflect the impact of the continued uplift of their catchments prior to the abrupt termination of sediment supply from drainage capture by footwall scarp drainages. The presence of a connected, largely constructional shoreline has implications for Late Jurassic sediment distribution around the Frøya High, providing primary sedimentary input for longshore‐driven sedimentary systems in the Draugen Ridge to the north. Comparisons with other syn‐rift dip slope systems highlight a broadly similar evolution but shows a distinct lack of the protracted backstepping observed in other dip slope systems. We postulate that different structural configurations of dip slope systems, being footwall uplift or hangingwall subsidence driven, may drive the strongly progradational character of the deltaic systems on the Frøya High. The Frøya High example highlights the need to constrain primary sediment input points to aid the interpretation of volumetrically significant, but short‐lived and subtle depositional systems, especially within complex, tectonically active settings.

Biotic Response to Rapid Environmental Changes During the Permian–Triassic Mass Extinction

The divergent patterns of Permian–Triassic mass extinction (PTME) have been extensively documented in varying water depth settings. We here investigated fossil assemblages and sedimentary microfacies on high-resolution samples from two adjacent sections of the South China Block: Chongyang from shallow-water platform and Chibi from deeper-water slop. At Chongyang, abundant benthos (over 80%), including rugose corals, fusulinids, calcareous algae, and large foraminifers, disappeared precipitously at the topmost of Changxing Formation grainstone, which suggested complete damage of the benthic ecosystem, confirming a sudden single-pulse extinction pattern. The end-Permian regression, marked by a karst surface, provided a plausible explanation for this extinction pattern. Whereas for the fauna in Chibi, the benthos was relatively abundant (20%–55%) with more trace fossils and lacking calcareous algae. Benthic abundance in Chibi reduced by two steps at the two claystone beds (Beds 10 and 18): bioclastic content dropped from an average of 50% in Beds 1–9 to 10% in Beds 11–17 and then to less than 5% in Beds 19–23, suggesting a two-pulse extinction. At the first pulse, large foraminifers were prominent victims in both shallow- and deeper-water settings. A plausible survival strategy for small-sized foraminifers was to migrate to deeper water to avoid extreme heat in shallow water. The early Triassic transgression prompted some small foraminifers to migrate back to original platforms and flourish briefly as disaster forms. At the Early Triassic mudstone with bottom-water settings in Chibi, the appearance of abundant small pyrite framboids (diameters of 4.74–5.96 μm), an indicator of intensified oxygen deficiency, was simultaneous with the two-step reduction of benthic diversity and abundance. Thus, anoxic conditions might be the main cause of the PTME at deeper-water settings. Our study is an example of the wider debate about biotic response to rapid environmental change for both the Permian–Triassic transition and modern days.

Low-accommodation foreland basin response to long-term transgression: A record of change from continental-fluvial and marginal-marine to open-marine sequences over 60,000 km2 in the western Canada foreland basin

Low-accommodation foreland basins have been of recent interest because their stratigraphic architectures provide insight into the interaction of tectonic subsidence and eustasy. Aptian-aged strata in Alberta, Canada (McMurray Formation and Wabiskaw Member) were deposited under the influence of a continental-scale river that longitudinally transported and debouched sediment into the southward-transgressing Boreal Sea in the distal, low-accommodation Western Canada Foreland Basin (WCFB). In this study, we present a high-resolution stratigraphic framework for the McMurray-Wabiskaw interval, which was deposited during a long-term transgression. Using wireline logs from >20,000 wells and more than 500 cores, regional mapping focused on resolving stratigraphic relationships and the distribution of depositional environments across approximately 60,000 km2 in the Athabasca Oil Sands Region of northeastern Alberta. The studied stratigraphic interval is part of a third-order sequence: the McMurray Formation developed during a period of relative sea-level fall, lowstand, and early transgression, while the overlying Wabiskaw Member was deposited during late transgression. Within the McMurray Formation, superimposed fourth-order sea-level fluctuations in this low accommodation setting created a complex amalgam of deltaic strata vertically and laterally juxtaposed with valley-fill deposits. Relative sea-level rises resulted in rapid transgressions in which the shorelines migrated >400 km landward. Thin (5–15 m thick), widespread deltaic parasequence sets were deposited during subsequent sea-level highstands. During relative sea-level falls, fluvial meandering channel belts developed in downcutting valleys, with associated shorelines dramatically prograding basinward. In the Wabiskaw Member, continued sea-level rise coupled with potentially decreasing sediment supply resulted in the development of tide-influenced/dominated channels and bars in estuaries. The final phase of the third-order transgression was marked by a regional transgressive ravinement event and several southwestward backstepping wave-dominated shorefaces/strandplains. This low-accommodation foreland basin succession demonstrates a transition from deposition in continental fluvial and marginal-marine settings subject to high-frequency eustatic changes, to open-marine conditions over a long-term transgression.

Stratigraphic architecture of a saline lake system: From lake depocentre (Alès Basin) to margins (Saint‐Chaptes and Issirac basins), Eocene–Oligocene transition, south‐east France

ABSTRACTChanges in sedimentary profiles and stratigraphic architecture over time in lake basins are shown to be relevant markers of climate changes in terrestrial palaeoenvironments. The stratigraphic architecture of the Priabonian (late Eocene)–early Rupelian (early Oligocene) Alès–Saint‐Chaptes–Issirac saline lake system has been reconstructed from the sedimentological, biostratigraphic and magnetostratigraphic analysis of outcrops and well cores. As a function of the inflow–evaporation balance, three distinct depositional models have been proposed for the Priabonian Alès–Saint‐Chaptes–Issirac lake system which is characterized by both fault‐controlled and low‐angle ramp‐like margins: (i) freshwater to oligohaline lake; (ii) oligohaline to mesohaline closed‐lake; and (iii) hypersaline, evaporitic closed‐lake. Two major stages of lake spreading (possibly middle Priabonian and late Priabonian–early Rupelian) separated by two stages of significant lake closure represented by evaporite succession in the Alès Basin depocentre (middle to late Priabonian) have been evidenced. Regional tectonics and climate induced changes in inflow–evaporation balance of the lake and controlled the development of transgressive–regressive cycles with calcium‐sulphate sedimentation during regression in the lake depocentre (Alès Basin) and during early transgression in lake margins (Saint‐Chaptes and Issirac basins). The correlation between the palaeohydrological cycles inferred for the Alès–Saint‐Chaptes–Issirac lake system and the regional climate changes of Priabonian stage, prior to the Eocene–Oligocene transition, strongly suggests that climate significantly impacted the nature of lacustrine depositional models and the depositional architecture of Palaeogene, syn‐rift, saline lake basins from Western Europe.

Middle Ordovician trilobites from the Houping Formation of South China: implications for palaeoecology and palaeobiogeography

ABSTRACT Seventeen trilobite species belonging to 16 genera and 13 families are systematically documented from the Houping Formation (upper Dapingian-lower Darriwilian, Middle Ordovician) at the Datangkou and Dacao sections, Chengkou, northern Chongqing. Of these, a new species, Mioptychopyge chengkouensis sp. nov., is established and described. New morphological information is provided for previously known species on the basis of new specimens. Two trilobite associations based on faunal composition and dominant forms are distinguished from the Houping Formation, in ascending order, the Agerina Association (from the lower and middle members) and the Nileus-Illaenus Association (from the upper member). The Agerina Association lived in an inner shelf margin environment with carbonate substrate above storm wave base (<70 m deep), whereas the Nileus-Illaenus Association developed in a shallow outer shelf environment below storm wave base (>70 m deep), indicating a rise of relative sea level corresponding to the early Darriwilian transgression. The cluster and nonmetric multidimensional scaling analyses of the trilobites in the formation show that the fauna had a strong endemicity to South China and a close biogeographic connection with east Peri-Gondwana, especially Tarim, Annamia and central Iran.LSIDurn:lsid:zoobank.org:pub:E4E6AD0B-417F-4522-83E3-4DC44B647760

Crowded Rosselia ichnofabric in estuarine settings recording early transgressions in lowermost Permian post-glacial Gondwana (Rio Bonito Formation, Paraná Basin, S Brazil)

Ichnology has widely been used in paleoecological, paleoenvironmental, and evolutionary analysis. Rosselia is a dwelling burrow of infaunal, detritus-feeding terebellid polychaetes and is common in Phanerozoic shallow-marine deposits. The crowded Rosselia ichnofabric (CRI, i.e., a dense occurrence of Rosselia) provides itself in this study as a valuable guide to recognizing high-energy and high-frequency shoreface settings but is poorly recorded in estuarine settings. This study aims (1) to interpret the occurrence of Rosselia and the accompanying ichnofauna in the early Permian postglacial estuarine deposits of the Rio Bonito Formation (Paraná Basin) in southern Brazil and (2) to evaluate the potential of the CRI as an ichnomarker of transgressive events in estuarine settings. The study area presents a glacially incised paleovalley (the Capané paleovalley) near Caçapava do Sul town in the south of Brazil and exhibits 8 facies associations that tell the depositional history of the paleovalley-fill sediments. The succession indicates a confined and retrogradational stacking pattern and constitutes two high-frequency depositional sequences. The CRI is recurrent in three distinct intervals of Sequence 1; it occurs in both the sandy to clayey heterolithic facies and the sandstone facies. The CRI indicates relatively high-energy sedimentation that is likely associated with the storm events influencing the paleovalley during a marine transgression. The Rosselia specimens are either vertical or inclined to the bedding plane. The causative tubes vary from 5 to 10 mm in diameter and the spindle diameter ranges from 20 to 90 mm. Most of the specimens are preserved as funnel-shaped burrows; however, spindle-shaped burrows also occur. The density of burrows is high (60–80 per m2), representing a crowded Rosselia ichnofabric (CRI). The CRI reflects the high energy and the high frequency of the storm surges that impacted the Capané paleovalley estuary during the Early Permian. This scenario inhibited the presence of other tracemakers that colonized the surrounding subenvironments of the paleovalley, resulting in the preservation of a monotypic R. socialis assemblage in the storm-related deposits. These results provide new insights about the distribution of CRI in shallow marine environments and reinforce its value as a stratigraphic ichnomarker of high-energy, high-frequency storm deposits also in estuaries impacted by large-scale episodic marine incursions.

Refinement of the stratigraphic framework for the Regional C depositional unit of the McMurray Formation and implications for the early transgression of the Alberta Foreland Basin, Canada

ABSTRACTThe Lower Cretaceous McMurray Formation in Alberta and Saskatchewan, Canada, comprises a series of depositional units (DUs) consisting of stacked parasequences bounded by flooding surfaces and incised by fluvio-estuarine channel belts. The fluvio-estuarine channel belts of the McMurray Fm have been the focus of numerous studies whereas the regional DUs have received substantially less attention. Of the regional DUs, Regional C (equivalent to the middle McMurray) is the most understudied, yet this interval records the history of the McMurray Formation between deposition of fluvial strata in the lower McMurray and marine facies in the upper McMurray and overlying Clearwater Formation. Determining the history of the Regional C DU is fundamental for accurately reconstructing the stratigraphic evolution of the McMurray Fm and, by extension, the early evolution of the Alberta Foreland Basin.The Regional C is divided into two DUs separated by a regionally mappable flooding surface. This surface occurs 11 to 15 m below the top of the Regional C and is traceable over a 2,550 km2 area. This flooding surface divides the thick interval of undifferentiated Regional C into a lower C2 DU and an upper C1 DU, each with a maximum thickness of &amp;lt; 15 m. The thickness of the C2 and C1 DUs indicates that deposition at this time also occurred in a setting of low to moderate accommodation creation, which is consistent with the rest of the McMurray Formation. The limited available accommodation space was easily surpassed by sediment supplied by the paleo-distributive channel system, leading to a basinward progradation of the shoreline.The C2 and C1 DUs are retrogradationally stacked, with the maximum regressive paleo-shoreline of C1 lying landward of that of C2. This stacking arrangement indicates that the shoreline backstepped during the early stages of transgression of the Boreal Sea. The backstepping of the paleo-shoreline from C2 to C1 time is consistent with previous studies that show continued and stepwise retrogradation and/or transgression of the paleo-shoreline from the onset of deposition in the lower McMurray Formation through to maximum transgression in the Clearwater Formation. Together, these studies demonstrate that the early drowning of the Alberta Foreland Basin was persistent and slow.

The sedimentary record of Quaternary glacial to interglacial sea‐level change on a subtropical carbonate ramp: Southwest Shelf of Australia

AbstractIn the last decades, the understanding of temperate carbonate systems has improved considerably, but their development over glacial–interglacial timescales is still understudied in comparison to their tropical counterparts. A key question is how do temperate carbonate platforms respond to high‐amplitude, glacial–interglacial sea‐level changes? Integrated Ocean Drilling Program Site U1460 was drilled at the uppermost slope of the Southwest Shelf of Australia at the transition between the subtropical Carnarvon Ramp and the warm–temperate Rottnest Shelf. The origin and composition of the sediments in the upper 25 m below seafloor at Site U1460 were investigated using X‐ray diffraction, scanning electron, and light microscopy. The Middle Pleistocene to Holocene sequence at Integrated Ocean Drilling Program Site U1460 contains a record of sea‐level controlled sedimentary cycles. Carbonate sediments deposited during interglacial sea‐level highstands (Marine Isotope Stages 1, 5, most of 7, 9 and 11) are mainly fine‐grained (&lt;63 µm) and dominated by low‐Mg calcite from pelagic bioclasts such as planktic foraminifera. The glacial lowstand intervals (Marine Isotope Stages 2 to 4, 6, 8, 7d, 10 and 12), instead are coarser‐grained and relatively rich in aragonite and high‐Mg calcite from neritic bioclasts, such as bryozoans. These changes in texture, mineralogy and composition are best explained by the deposition of neritic bioclasts closer to the shelf edge during glacial sea‐level lowstands. During early transgression, reworking of bioclast‐rich coastal dune deposits likely leads to transport and redeposition of neritic clasts on the upper slope. In contrast, dominantly pelagic sediments characterize deposition at the platform edge during interglacial highstands. These results highlight regional differences in the response of temperate carbonate systems to sea‐level change: A previously published model developed for early Pleistocene temperate carbonates from the Great Australian Bight indicates that shelfal material was exported to the upper slope during sea‐level highstands. It is argued that this difference is related to the change in duration and amplitude of glacial–interglacial sea‐level cycles before and after the Mid‐Pleistocene transition.

The Late Cretaceous phosphatized ichnofauna fossils from the Eocene basal horizon of the Middle Dnieper area

The object of our study was phosphated remains of ichnofaunas from the basal horizon of the Kanivska suite (Eocene), which transgressively onlaps here the eroded top of the Burimska suite. The goal of the research was to reconstruct some sedimentation features of the final stages of the Late Cretaceous basin’s existence and during of the Early Eocene transgression on the Middle Dnieper area (Ukraine). The specimens of the basal horizon of the Kanivska suite (Р2kn) were sampled from the outcrops in the Melanchin Potik gulley and in a branch of the Glyadovy gulley. It is a complex of rough-medium grained gravel sandstones with shark teeth, fish bone fragments, detritus of Bivalvia and Brachiopoda. The nature of preservation and arrangement suggests that the fossils were relocated. The sample from the Glyadovy gulley is an agglomeration of phosphatized debris of various types of ichnofauna, cemented with a phosphate and sand substrate. The first type, in our opinion, is a fragment of a burrow nucleus of a crayfishlike decapod Thalassinoides Ehrenberg ichnogenus, as indicated by the surface , absence of a constructed wall and its size. The phosphorite features of the second type are cores of the burrows of the Ophiomorpha Lundgren genus with a characteristic ophiomorphic sculpture in the form of knobbles on the outer surface. The structures of the second type have certain differences in the distribution and nature of phosphate material. The described cores are interpreted as domichnia: dwelling places of fossil fauna. They were not known earlier within the Middle Dnieper area. The animals constructed a system of void ducts in the well ventilated middle ichnofossil layer with depth up to 60 cm (as can be inferred from the diameter of Thalassinoides). Although crayfish burrows can be present in any formation, it is generally thought that they are more specific for the littoral and shallow water environments. Based on the fauna composition and the phosphated debris preservation, we can assume the age of ichnofaunas is Late Cretaceous. An indirect indication of this can be the bones and teeth of Cretaceous vertebrates, found in the same basal layer, in particular, ichthyosaurus genus (Platypterygius sp.) and a phosphate concretion with ammonite remnants. Besides phosphatized ichnofauna debris, the basal level contains black odd-shaped nodules and microconcretions, composed of amorphous phosphate minerals. They contain a considerable amount of microscopic fossils of presumably phytoorganic nature and fragments of undeterrmined groups. From the fact that the phosphate material is not completely decrystallized, and that some microfossils have their initial composition, we infer that they belong to the Early Eocene.

The Record of Environmental and Microbial Signatures in Ancient Microbialites: The Terminal Carbonate Complex from the Neogene Basins of Southeastern Spain

The Messinian microbialites of the Terminal Carbonate Complex (TCC) from the Neogene basins of southeastern Spain show both diversified morphologies and an excellent preservation of primary microbial microstructures. Their stratigraphic architecture, fabric (micro-, meso-, and macro-fabric), and mineralogical composition were investigated in eight localities from three sedimentary basins of southeastern Spain: The Sorbas and Bajo Segura basins and the Agua Amarga depression. Two recurrent microbialite associations were distinguished. Laterally linked low relief stromatolites predominated in Microbialite Association 1 (MA1), which probably formed in low energy lagoons or lakes with fluctuating normal marine to hypersaline water. The microfabrics of MA1 reflected the predominance of microbially induced/influenced precipitation of carbonates and locally (Ca)-Mg-Al silicates. Microbialite Association 2 (MA2) developed in high energy wave and tidal influenced foreshore to shoreface, in normal marine to hypersaline water. High-relief buildups surrounded by mobile sediment (e.g., ooids or pellets) dominated in this environment. MA2 microbialites showed a significant proportion of thrombolitic mesofabric. Grain-rich microfabrics indicated that trapping and binding played a significant role in their accretion, together with microbially induced/influenced carbonate precipitation. The stratigraphic distribution of MA1 and MA2 was strongly influenced by water level changes, the morphology and nature of the substratum, and exposure to waves. MA1 favorably developed in protected areas during third to fourth order early transgression and regression phases. MA2 mostly formed during the late transgressions and early regressions in high energy coastal areas, often corresponding to fossil coral reefs. Platform scale syn-sedimentary gypsum deformation and dissolution enhanced microbial carbonate production, microbialites being thicker and more extended in zones of maximum deformation/dissolution. Microbial microstructures (e.g., microbial peloids) and microfossils were preserved in the microbialites. Dolomite microspheres and filaments showed many morphological similarities with some of the cyanobacteria observed in modern open marine and hypersaline microbialites. Dolomite potentially replaced a metastable carbonate phase during early diagenesis, possibly in close relationship with extracellular polymeric substances (EPS) degradation. Double-layered microspheres locally showed an inner coating made of (Ca)-Mg-Al silicates and carbonates. This mineral coating could have formed around coccoid cyanobacteria and indicated an elevated pH in the upper part of the microbial mats and a potential dissolution of diatoms as a source of silica. Massive primary dolomite production in TCC microbialites may have resulted from enhanced sulfate reduction possibly linked to the dissolving gypsum that would have provided large amounts of sulfate-rich brines to microbial mats. Our results open new perspectives for the interpretation of ancient microbialites associated with major evaporite deposits, from microbe to carbonate platform scales.

Sequence stratigraphy in the Middle Ordovician carbonate successions (Yeongheung Formation), mid-east Korea: Paleogeographic implications for a land-detached ramp-type platform

This study reveals the origin of the stratigraphic discrepancy between the Yeongweol and Taebaek groups and reconstructs paleogeographic model for the Taebaeksan Basin on the Sino-Korean Craton (SKC). The global sea-level fall in the earliest Middle Ordovician would have subaerially extensively exposed the Lower Ordovician peritidal successions, forming the solution-collapsed breccia zones on the Sauk-Tippecanoe bounding surface. The successive rise in sea level generated the fine-grained siliciclastic successions in the Taebaek Group during early transgression since the carbonate production of the group was hampered by the strong input of siliciclastic sediments from terrestrial sources. However, such successions are absent in the Yeongweol Group because the Yeongweol platform was distant from the siliciclastic sources and sustained carbonate production due to isolation from terrestrial influences. The Yeongweol Group is interpreted as a “land-detached (isolated) ramp-type platform” as it responded to sea-level fluctuations with restricted terrestrial influence, whereas the Taebaek Group can be described as a “land-attached ramp-type platform” affected by a large input of siliciclastic sediments during lowstand and early transgression. Based on the comparison of stratigraphic patterns and configurations of land-attached and -detached ramp-type successions, this study concludes that the Yeongweol sequence would have been formed on a land-detached (isolated) carbonate ramp-type platform located at the SKC along the northeastern Gondwana margin.

ПРОИСХОЖДЕНИЕ “ВРЕЗАННЫХˮ ИЛИ СЛЕПЫХ ДЕЛЬТ НА ВОЛГО-УРАЛЬСКОМ МЕЖДУРЕЧЬЕ В СВЕТЕ НОВЫХ ПРЕДСТАВЛЕНИЙ ОБ ИСТОРИИ КОЛЕБАНИЙ УРОВНЯ КАСПИЯ

The relief of the Volga-Ural interfluve is considered, where the rivers in the mouths form “embedded” or blind deltas. These deltas terminate close to the Volga-Ural Sands. During spring snowmelt floods, large lakes appear at the mouths of rivers. Therefore, it is difficult to agree with the opinion that the rivers disconnected from the sea during the regression phases due to the low water levels in rivers. Sea regression led to the downcutting of the delta channels into the alluvial and marine deposits. Analysis of the outcrops, taking into account the coastal geomorphology and the impact of the sea level fluctuations on the coast has led to the conclusion that there was no deep Atelian regression and Great Early Khvalynian transgression. It was a Great Khazarian transgression, after which lagoon-transgressive terraces formed sequentially at the background of the gradual lowering of the Caspian Sea. Positive oscillations of sea level led to the formation of lagoons, where the chocolate clays accumulated. All tributary rivers formed deltas that prograded into such lagoons. Therefore, Sea the rivers could not follow the coastline during the subsequent lowering of the Caspian Sea. The shallowing of the lagoons resulted in the drainage of the deltas. When the water level in the lagoons dropped, rivers cut in deltaic deposits and “embedded” or blind deltas were formed.

The significance of karst unconformities on overlying resource shales: Lessons learned from the Devonian Woodford Formation applied to the Permian Wolfcamp Shale

We have summarized the threefold significance of karst unconformity boundary: (1) The development of a sequence stratigraphic model for the Devonian Woodford Shale Formation is transferable to the Upper Wolfcamp in the Permian Basin, (2) demonstration of the more general application of that model beyond the Woodford to other resource shales, and (3) illustration of a modification of common sequence stratigraphy models specifically to unconventional resource shales. During early transgression, marine encroachment into the paleolows created anoxic, hypersaline marine “pockets” conducive to the preservation of organic matter. The result is deposition of thick, laterally discontinuous, organic-rich strata stratigraphically at or near the unconformity surface. This pattern of deposition and distribution of the organic-rich shale has been well-documented in the Devonian Woodford Shale and is applicable to other resource shales, in this case to the Permian Upper Wolfcamp Formation in the Central Basin Platform of the Permian Basin. The stratigraphy of the distribution of the Upper Wolfcamp on top of the Upper/Middle Wolfcamp Unconformity is similar to that of the Woodford, suggesting a similar origin and distribution. The resulting stratigraphy in both cases resembles that of the classical Exxon sea slug model except that rather than a single organic-rich deposit defining the condensed section and maximum flooding surface, a second organic-rich deposit occurs stratigraphically lower, at or near the unconformity surface. This theoretical summary can support the discovery of potential drillable target zones in the Woodford Shale and the Wolfcamp Shale.

New data on the biostratigraphy of the Early Devonian “Spirifer” tonkinensis brachiopod fauna in South China and adjacent region

Conodont and brachiopod biostratigraphic studies have been conducted at the Mode and Poyuan sections, Guangxi Province, South China. Combining these with seven previously studied sections, it is suggested that the appearance of the “Spirifer” tonkinensis Fauna in different areas of South China and adjacent areas is diachronous, showing an evident chronological trend from south to north. This trend is consistent with the northeastward transgression during the Early Devonian in South China. The “Spirifer” tonkinensis fauna may originate from northern Vietnam and then spread northwards into the South China Block. The persistence of the Early Devonian transgression resulted in the disappearance of favourable biotopes of the “Spirifer” tonkinensis Fauna (i.e. the near-shore, shallow marine environment with muddy substratum and plentiful terrigenous input), due to either a deepening of seawater or the development of a carbonate platform. This might ultimately have triggered the extinction of the fauna.

Tracking the isostatic and eustatic signals in an end Ordovician–early Silurian deglacial sedimentary record (Algeria – Libya border)

AbstractTo elucidate the signature of isostatic and eustatic signals during a deglaciation period in pre‐Pleistocene times is made difficult because very little dating can be done, and also because glacial erosion surfaces, subaerial unconformities and subsequent regressive or transgressive marine ravinement surfaces tend to amalgamate or erode the deglacial deposits. How and in what way can the rebound be interpreted from the stratigraphic record? This study proposes to examine deglacial deposits from Late‐Ordovician to Silurian outcrops at the Algeria–Libya border, in order to define the glacio–isostatic rebound and relative sea‐level changes during a deglaciation period. The studied succession developed at the edge and over a positive palaeo‐relief inherited from a prograding proglacial delta that forms a depocentre of glaciogenic deposits. The succession is divided into five subzones, which depend on the topography of this depocentre. Six facies associations were determined: restricted marine (Facies Association 1); tidal channels (Facies Association 2); tidal sand dunes (Facies Association 3); foreshore to upper shoreface (Facies Association 4); lower shoreface (Facies Association 5); and offshore shales (Facies Association 6). Stratigraphic correlations over the subzones support the understanding of the depositional chronology and associated sea‐level changes. Deepest marine domains record a forced regression of 40 m of sea‐level fall resulting from an uplift caused by a glacio‐isostatic rebound that outpaces the early transgression. The rebound is interpreted to result in a multi‐type surface, which is interpreted as a regressive surface of marine erosion in initially marine domains and as a subaerial unconformity surface in an initially subaerial domain. The transgressive deposits have developed above this surface, during the progressive flooding of the palaeo‐relief. Sedimentology and high‐resolution sequence stratigraphy allowed the delineation of a deglacial sequence and associated sea‐level changes curve for the studied succession. Estimates suggest a relatively short (&lt;10 kyr) duration for the glacio‐isostatic uplift and a subsequent longer duration transgression (4 to 5 Myr).