Hutchison Medallist 1. Wave-Dominated to Tide-Dominated Coastal Systems: A Unifying Model for Tidal Shorefaces and Refinement of the Coastal- Environments Classification Scheme

Coastal depositional systems are normally classified based on the relative input of wave, tide, and river processes. While wave- through to river-dominated environments are well characterized, environments along the wave-to-tide continuum are relatively poorly understood and this limits the reliability and utility of coastal classification schemes. Two tidal shoreface models, open-coast tidal flats (OCTF) and tidally modulated shorefaces (TMS), have been introduced for mixed wave-tide coastal settings. Following nearly two decades of research on tidal shorefaces, a number of significant insights have been derived, and these data are used here to develop a unified model for such systems. First, OCTFs are components of larger depositional environments, and in multiple published examples, OCTFs overlie offshore to lower shoreface successions that are similar to TMS. Consequently, we combine OCTFs and TMSs into a single tidal shoreface model where TMS (as originally described) and TMS-OCTF successions are considered as variants along the wave-tide continuum. Second, tidal shoreface successions are preferentially preserved in low- to moderate-wave energy environments and in progradational to aggradational systems. It is probably difficult to distinguish tidal shorefaces from their storm-dominated counterparts. Third, tidal shorefaces, including both TMSs and OCTFs, should exhibit tidally modulated storm deposits, reflecting variation in storm-wave energy at the sea floor resulting from the rising and falling tide. They may also exhibit interbedding of tidally generated structures (e.g. double mud drapes or bidirectional current ripples), deposited under fairweather conditions, and storm deposits (e.g. hummocky cross-stratification) through the lower shoreface and possibly into the upper shoreface.The development of the tidal shoreface model sheds light on the limitations of the presently accepted wave-tide-river classification scheme of coastal environments and a revised scheme is presented. In particular, tidal flats are components of larger depositional systems and can be identified in the rock record only in settings where intertidal and supratidal deposits are preserved; consequently, they should not represent the tide-dominated end-member of coastal systems. Instead, we suggest that tide-dominated embayments should occupy this apex. Tide-dominated embayments exhibit limited wave and river influence and include a wide range of geomorphological features typically associated with tidal processes, including tidal channels, bars and flats.Les systèmes de dépôts côtiers sont normalement classés en fonction de l’apport relatif des processus liés à la houle, aux marées et aux rivières. Si les environnements dominés par la houle et les rivières sont bien caractérisés, les environnements le long du continuum houle-marée sont relativement mal compris, ce qui limite la fiabilité et l’utilité des systèmes de classification des côtes. Deux modèles d’avant-plages tidales, les estrans ouverts (open-coast tidal flats; OCTF) et les avant-plages modulées par la marée (tidally modulated shoreface; TMS), ont été introduits pour les milieux côtiers mixtes, houle-marée. Suite à près de deux décennies de recherche sur les avant-plages tidales, un certain nombre d’informations importantes ont été obtenues et ces données sont utilisées ici pour développer un modèle unifié pour ces systèmes. Tout d’abord, les OCTF sont les composants de systèmes de dépôt plus vastes et, dans de nombreux exemples publiés, les OCTF recouvrent des successions sédimentaires allant du large à l’avant-plage inférieure, similaires à celle des TMS. Par conséquent, nous combinons les OCTF et les TMS en un seul modèle d’avant-plage tidale où les TMS (tel que décrit à l’origine) et les successions TMS-OCTF sont considérés comme des variantes le long du continuum houle-marée. Deuxièmement, les successions d’avant-plages tidales sont préférentiellement préservées dans des environnements ayant une houle faible à modérée et dans des systèmes progradant et aggradant. Il est probablement difficile de distinguer les avant-plages tidales de leurs homologues dominés par les tempêtes. Troisièmement, les avant-plages tidales, incluant à la fois les TMS et les OCTF devraient présenter des dépôts de tempête modulés par la marée, reflétant ainsi la variation de l’énergie des vagues de tempête sur le fond marin liée à la marée montante et descendante. Les avant-plages tidales peuvent également présenter une interstratification de structures générées par la marée (par exemple, des doubles drapages argileux ou des rides de courants bidirectionnelles) déposées pendant des conditions de beau temps, et des dépôts de tempête (par exemple, des stratifications en mamelons) au niveau de l’avant-plage inférieure et éventuellement de l’avant-plage supérieure.Le développement du modèle d’avant-plage tidale met en lumière les limites de la classification tripartite (houle-marée-rivière) des environnements côtiers actuellement acceptée et une classification révisée est présentée. En particulier, les OCTF et les estrans sont des composantes de systèmes dedépôt plus importants et ne peuvent être identifiés que dans le registre sédimentaire dans les milieux où les dépôts intertidaux et supratidaux sont préservés; par conséquent, ils ne devraient pas représenter le membre extrême des systèmes côtiers dominé par la marée. Nous suggérons plutôt que les baies dominées par la marée occupent cette place. Les baies dominées par les marées présentent une influence limitée des vagues et des rivières et comprennent un large éventail de caractéristiques géomorphologiques généralement associées aux processus de marée, notamment des chenaux, des barres et des platiers tidaux

Resolving the Architecture and Early Evolution of a Forearc Basin (Georgia Basin, Canada) Using Detrital Zircon

Convergent-margin basins (CMBs) are commonly associated with active arcs, and hence are rich in detrital zircon (DZ) whose ages closely reflect the timing of deposition. Consequently, maximum depositional ages (MDA) from DZ geochronology can be employed to resolve the stratigraphy and evolution of CMBs. Herein, we use DZ to revise the internal architecture of the lower Nanaimo Group, which partially comprises the fill of the (forearc) Georgia (or Nanaimo) Basin. Maximum depositional ages and multi-dimensional scaling of DZ age distributions are employed to determine chronologic equivalency of strata and assess sediment provenance variability within the pre-existing lithostratigraphic framework. The results are compared to a recently developed sequence stratigraphic framework for the lower Nanaimo Group. The basal lithostratigraphic unit of the Nanaimo Group, the Comox Formation (Fm), comprises strata that are neither time correlative nor genetically related. The three lithostratigraphic units directly overlying the Comox Fm (Haslam, Extension, and Protection formations) comprise strata with similar genetic affinities and MDAs that indicate deposition of these units was not always sequential and locally was contemporaneous. Through this work, we provide an example of how MDAs from DZ geochronology in CMBs can resolve basin-scale stratigraphic relations, and identify chronological changes in sediment provenance

Insolation-paced sea level and sediment flux during the early Pleistocene in Southeast Asia

Global marine archives from the early Pleistocene indicate that glacial-interglacial cycles, and their corresponding sea-level cycles, have predominantly a periodicity of ~ 41 kyrs driven by Earth’s obliquity. Here, we present a clastic shallow-marine record from the early Pleistocene in Southeast Asia (Cholan Formation, Taiwan). The studied strata comprise stacked cyclic successions deposited in offshore to nearshore environments in the paleo-Taiwan Strait. The stratigraphy was compared to both a δ18O isotope record of benthic foraminifera and orbital parameters driving insolation at the time of deposition. Analyses indicate a strong correlation between depositional cycles and Northern Hemisphere summer insolation, which is precession-dominated with an obliquity component. Our results represent geological evidence of precession-dominated sea-level fluctuations during the early Pleistocene, independent of a global ice-volume proxy. Preservation of this signal is possible due to the high-accommodation creation and high-sedimentation rate in the basin enhancing the completeness of the stratigraphic record

Of sand and mud: sedimentological criteria for identifying the turbidity maximum zone in a tidally influenced river

The thickness and lateral distribution of sand and mud beds and bedsets on channel bars from the tidally influenced Fraser River, British Columbia, Canada, are quantitatively assessed. Fifty-six vibracores totalling ca 114m of vertical section are used to tabulate bed thicknesses. Statistical calculations are undertaken for nine channel bars ranging from the freshwater and tidal zone, to the sustained brackish water and tidal zone. The data reveal that thickness trends can be organized into three groups that broadly correspond to time-averaged hydrodynamic and salinity conditions in the various distributary channels. Thick sand beds (up to 30cm) and thin mud beds (up to 5cm) characterize the freshwater tidal zone. The tidal and freshwater to brackish-water transition zone comprises thin sands (up to 10cm) and thicker muds (up to 19cm), and the sustained brackish water tidal zone consists of thin muds (up to 6cm) with relatively thicker sands (up to 25cm). The results suggest that the locus of mud deposition occurs in the tidal freshwater to brackish-water zone, probably reflecting mud flocculation and deposition at the turbidity maximum. Landward of the turbidity maximum, mud deposition is linked to tidal influence (tidal backwater effect and reverse eddy currents on channel margins) as mud beds thin in the landward direction. These results support the hypothesis that mud deposition is greatest at the turbidity maximum and decreases in both the seaward and landward direction. This study also showcases that mud-bed thicknesses are greatest towards the turbidity maximum and thin in both the landward and seaward direction. In the rock record, the apex of mud deposition probably marks the position of the palaeo-turbidity maximu

Sedimentological trends across the tidal–fluvial transition, Fraser River, Canada

The tidal–fluvial transition (TFT) is a complex depositional zone in rivers, where tidal- and river-flow interact. However, subdivision of the TFT into discrete zones is not well established. Studies of the lower Fraser River, Canada, reveal criteria for subdividing the TFT into zones that experience similar hydrodynamic and salinity variations from year-to-year, and three zones are identified: (1) mixed tidal–fluvial with persistent brackish-water zone (mixed tidal–fluvial); (2) fluvially dominated and tide influenced, freshwater to brackish water transition zone (turbidity maximum); and, (3) fluvially dominated and tidally modulated, freshwater zone (tidal backwater). Of the four main datasets (sedimentological, ichnological, palynological, and geochemical) evaluated across the TFT of the Fraser River, only sedimentological and ichnological datasets can be used reliably to distinguish between depositional zone within the TFT. Mud volume is highest in the turbidity maximum zone and decreases in both the landward and seaward direction. Rhythmic (tidal) bedding is common in the mixed tidal-fluvial zone, and sand–mud interbedding occurs in both the turbidity maximum and mixed tidal–fluvial zones. Significant sand–mud interbedding is not expected in the tidal backwater, nor landward of the tidal backwater. Higher salinity and longer residence times of saline water at the bed are manifested ichnologically in larger diameter burrows, higher bioturbation intensities, and a more diverse trace assemblage. Variations in river discharge result in a heterogeneous distribution of burrows, where burrows are mainly concentrated in mud beds. Increased fluctuations in depositional energy and increasing sedimentation rate reduce trace density. The depositional trends presented from the lower Fraser River are intended as a basis for comparison to trends defined from the TFTs of other major river systems. This work demonstrates that it is possible to predict relative depositional position across the TFT, and that a comprehensive model for TFT deposits can be constructed

Using a modern analogue to interpret depositional position in ancient fluvial-tidal channels: example from the McMurray Formation, Canada

The fluvial–tidal transition (FTT) is a complex depositional zone, where fluvial flow is modified by tides as rivers approach a receiving marine basin. Variations in the relative importance of tidal versus fluvial processes lead to a distinctive distribution of sediments that accumulate on channel bars. The FTT generally consists of three broad zones: (1) a freshwater-tidal zone; (2) a tidally influenced freshwater to brackish-water transition; and (3) a zone of relatively sustained brackish-water conditions with stronger tides. A very common type of deposit through the fluvial–tidal transition, especially on the margins of migrating channels, is inclined heterolithic stratification (IHS). At present, a detailed account of changes in the character of IHS across the FTT of a paleo-channel system has not been reported, although a number of modern examples have been documented. To fill this gap, we quantitatively assess the sedimentology and ichnology of IHS from seven cored intervals in three geographic areas situated within the youngest paleovalley (“A” Valley) in the Lower Cretaceous McMurray Formation of Alberta, Canada. We compare the data to trends defined along the FTT in the present-day Fraser River in British Columbia, Canada to interpret paleo-depositional position in the ancient fluvial–tidal channels. Analysis determined that the mean mudstone thickness is 8.2 cm in the southern study area (SA). Mean thickness increases to 11 cm in the central study area (CA), and decreases again to 4.4 cm in the northern study area (NA). The proportion of mudstone is 31% in SA, 44% in CA, and 27% in NA. Thickness-weighted mean bioturbation intensity in sands varied from 0.29 in SA and CA, to 0.28 in NA. On the other hand, thickness-weighted mean bioturbation intensity (BI) in mudstone increases from 1.46 in SA, to 1.77 in CA, and is 1.94 in NA. The ichnological diversity also increased from south to north. Sedimentological results show similar trends to those of the Fraser River, enabling the identification of a freshwater to brackish-water transition zone with tidal influence. The interpreted position of the transition is underpinned by the bioturbation intensity and trace-fossil diversity trends, indicating periodic brackish-water conditions throughout SA in the McMurray Formation during low river flow conditions. Together, these data suggest that a broad FTT existed in the “A” Valley, with fluvial-dominated channels to the south that experienced seasonal brackish-water inundation during base flow, and channels experiencing increasing brackish-water influence lying further north towards a turbidity maximum zone. The FTT zone appears to have extended for several hundred kilometers from south to north. Based on the sedimentological and ichnological data, as well as estimations of lateral accretion rates, we refute the commonly applied Mississippi River depositional analogue for McMurray Formation channels. Rather, we show that while not a perfect fit, the tidally influenced Fraser River shows much greater agreement with the depositional character recorded in McMurray Formation IHS. Future work on the McMurray system should focus on characterizing tide-dominated deltaic and estuarine systems, such as the Ganges-Brahmaputra, and on forward-modeling the evolution of tide-dominated and tide-influenced river systems

X-ray fluorescence data of the trace fossil Rosselia 'Da'an-3' from the Pliocene Yutengping Sandstone Member in Taiwan

In October 2019, the Rosselia samples were collected from outcrops beside the Da'an River in Taiwan, belonging to the early Pliocene Yutengping Sandstone Member of the Kueichulin Formation. Samples of the Rosselia were analyzed to explore the potential of the Rosselia as a natural archive for recording paleoenvironmental shifts in seabed sediment origin. X-ray fluorescence signals were collected from the samples to understand the compositional shifts in the concentric lamination of the Rosselia. The Itrax core scanner was used to conduct non-destructive XRF elemental analysis and produce semi-quantitative results in elemental content

Organic carbon data of the trace fossil Rosselia 'Da'an-4' from the Pliocene Yutengping Sandstone Member in Taiwan

In October 2019, the Rosselia samples were collected from outcrops beside the Da'an River in Taiwan, belonging to the early Pliocene Yutengping Sandstone Member of the Kueichulin Formation. Samples of the Rosselia were analyzed to explore the potential of the Rosselia as a natural archive for recording paleoenvironmental shifts in seabed sediment origin. Organic carbon measurements (total organic carbon or TOC; and the stable isotopic signature δ13C) were collected from the samples to understand the compositional shifts in the concentric lamination of the Rosselia. An elemental analyzer (Flash EA, Thermo) connected with an isotope ratio mass spectrometry (Delta V, Thermo Finnigan) were used to analyze TOC and δ13C of hand-drilled rock powder samples of the lamination

X-ray fluorescence data of the trace fossil Rosselia 'Da'an-4' from the Pliocene Yutengping Sandstone Member in Taiwan

In October 2019, the Rosselia samples were collected from outcrops beside the Da'an River in Taiwan, belonging to the early Pliocene Yutengping Sandstone Member of the Kueichulin Formation. Samples of the Rosselia were analyzed to explore the potential of the Rosselia as a natural archive for recording paleoenvironmental shifts in seabed sediment origin. X-ray fluorescence signals were collected from the samples to understand the compositional shifts in the concentric lamination of the Rosselia. The Itrax core scanner was used to conduct non-destructive XRF elemental analysis and produce semi-quantitative results in elemental content