Shahin E. Dashtgard

BRACKISH-WATER ICHNOLOGICAL TRENDS IN A MICROTIDAL BARRIER ISLAND–EMBAYMENT SYSTEM, KOUCHIBOUGUAC NATIONAL PARK, NEW BRUNSWICK, CANADA

Abstract A complex variety of marginal-marine microtidal environments from Kouchibouguac Bay, New Brunswick, Canada, present an opportunity to ichnologically and sedimentologically characterize microtidal settings in a high-latitude, temperate subarctic climate. Variations in bioturbate fabrics and distribution of infauna, analysis of the distributions of sediments and physical sedimentary structures, and the distribution of total organic carbon (TOC) can be associated with characteristic depositional processes. From these data typical sedimentary facies associations are produced. In outer estuary tidal inlets and areas of the flood-tidal deltas, strong currents and wave action eradicate the ichnological signature, resulting in variably laminated and bedded sand. In the central estuary, infauna activity coupled with generally low hydraulic energy levels lead to an absence of primary sedimentary structures. The inner estuary near bay-head deltas experiences riverine currents and freshwater influence. As a consequence, primary sedimentary structures are preserved. Mapping of infauna, sediment texture, TOC, and salinity reveals strong links between animal distribution and these three physicochemical parameters. Consequently, the distribution and type of bioturbation observed is at least passively related to grain size, TOC, and salinity. In outer estuaries and lower-central estuaries, salinity is near marine levels and fluctuates minimally. The distribution of infauna in these areas corresponds directly to sediment texture and TOC. Further up the estuaries, lower and fluctuating salinities—in addition to sediment texture and TOC content—control the distribution and diversity of infauna. Mapping of diversity and infaunal size up-estuary reveals two significant trends attributable to salinity stresses: (1) vermiform diminution, and (2) a significant decrease in infaunal diversity.

Chapter 4 – The Ichnofacies Paradigm

The ichnofacies paradigm has evolved over a six-decade period since its original inception by Dolf Seilacher. It is a multidimensional framework underpinned by recurring, facies-controlled groupings of biogenic structures that reflect animal responses to paleoenvironmental conditions. These constitute spatially and temporally extensive associations commonly regarded as “Seilacherian Ichnofacies.” The marine realm hosts five recurring softground ichnofacies (Psilonichnus, Skolithos, Cruziana, Zoophycos, and Nereites), generally distributed in a proximal–distal trend reflecting a passive response to increasing water depth (i.e., controlled by depositional factors that progressively change with bathymetry). There are three substrate-controlled ichnofacies (Trypanites, Teredolites, and Glossifungites), recording organism occupation of palimpsest substrates. Finally, there are six continental ichnofacies (Scoyenia, Mermia, Coprinisphaera, Termitichnus, Celliforma, and Octopodichnus–Entradichnus), mainly recording organism responses to temperature and the availability of moisture (i.e., climate-driven associations) in terrestrial settings or oxygenation, depositional energy, and substrate consistency in subaqueous settings. The Seilacherian ichnofacies operate as facies models, built through the distillation of ichnological characteristics derived from numerous modern and ancient case studies. Like lithofacies models, they serve as a norm for comparison, a framework for observations, a predictor in new situations, an integrated basis for interpretation, and a basis for teaching and communication.

Chapter 10 - Marine Invertebrate Neoichnology

Trace fossils are an in-situ record of the infaunal ecosystem and can be used to constrain paleoenvironmental conditions at the time of animal colonization. However, in paleoichnological studies, the behavior of the perceived tracemaker is derived solely from trace-fossil morphology, which introduces significant uncertainty to interpretations. Neoichnology removes some of that uncertainty by linking modern infauna to the traces they produce, and by comparing infaunal and trace distributions to the physical and chemical conditions of the environment. Polychaetes, bivalves and crustaceans are the dominant infaunal groups in marginal-marine and marine settings. Echinoderms, holothurians and cnidarians are increasingly significant tracemakers in more fully marine (shelf) settings. The burrows made by select genera in these six groups, and variations in the diversity and density of burrowing as a function of physical and chemical stresses, are presented as analogs for paleoichnological studies.

Chapter 19 – Shorefaces

The shoreface is a seaward sloping, sandstone depositional wedge, and can be subdivided into a lower, middle, and upper shoreface. The sediment wedge grades basinward into offshore sandy and silty shales and landward into foreshore sandstones and/or conglomerates. The lower shoreface lies within fair-weather wave base. The trace-fossil assemblages typically reflect a diverse and abundant Cruziana Ichnofacies. The middle shoreface contains swaley and lesser hummocky cross-stratified sandstones with a trace-fossil assemblage dominated by the Skolithos Ichnofacies. The upper shoreface is dominated by multidirectional trough cross-bedding. Trace fossils are rarely abundant and consist of deeply penetrating structures of the Skolithos Ichnofacies. Under exceedingly high-energy conditions, Macaronichnus may be developed near the transition with the foreshore. Shoreface successions display a wide range of variability, largely restricted to the lower and middle shoreface intervals. This variability appears to be controlled by relative storm dominance and the episodic nature of associated storm-bed deposition.

Computer modeling bioturbation: The creation of porous and permeable fluid-flow pathways

Computer modeling of trace fossils (Skolithos, Thalassinoides, Planolites, Zoophycos, and Phycosiphon) and ichnofacies (Skolithos, Cruziana, and Zoophycos ichnofacies) is undertaken to assess the impact of bioturbation on porosity and permeability trends in sedimentary media. Model volumes are randomly populated with the digitally modeled trace fossils to test for connectivity between burrows. The probability of vertical and lateral interconnections is compared with bioturbation intensity. The results of the simulations indicate that biogenic flow networks develop at low bioturbation intensity, between 10 and 27.5% bioturbation (BI-2). However, the efficiency of connectivity is controlled by the architecture of the burrows. For all trace-fossil and ichnofacies models, regardless of trace-fossil orientation, continuous horizontal and vertical connectivity across the sediment volume is achieved within a 0 to 10% range in bioturbation. In subsurface aquifers and petroleum reservoirs, the presence of bioturbation can significantly influence fluid flow. In particular, for marine sedimentary rocks, where burrows are more permeable than the surrounding matrix, a greater degree of three-dimensional burrow connectivity can produce preferred fluid-flow pathways through the rock. Recognizing these flow conduits may enable optimization of resource exploitation or may contribute to increasing reserve estimates from previously interpreted nonreservoir rock.

Chapter 16 – Estuaries

Estuaries are characterized by the mixing of fluvial and marine waters and the presence of energy gradients associated with wave, tide, and river processes. This chapter explores the influence that the juxtaposition of unsteady through reduced salinities and variable energy distributions has on the distribution and composition of trace fossils in estuary settings. Due to the wide physiographic range of estuary occurrences, a simple summary model for the identification of estuaries is not presented. However, some common ichnological and sedimentological elements of estuaries are explored: (1) the presence of brackish-water bioturbation, (2) the observance of bioturbated intertidal flat deposits, (3) the presence of burrowed inclined heterolithic stratification, and (4) the identification of Glossifungites-demarcated omission surfaces, which may be associated with transgressive erosion in marginal-marine settings. More generally, the distribution of food resources and brackish water—a result of tidal mixing and energy distributions—dictates the longitudinal distributions of tracemaking organisms and their biogenic structures. Considering the above ichnological characteristics, the identification of estuaries from ichnological datasets very much depends on a large number of observations from a spatially significant dataset.

Relationships between Organic Carbon and Pascichnia Morphology in Intertidal Deposits: Bay of Fundy, New Brunswick, Canada

Abstract Grazing trails of the Valviferan isopod Chiridotea coeca are examined to test the relationships between trail morphology and the distribution of food (organic carbon). These isopods burrow up to 1 cm beneath the surface within ripple troughs and planar-bedded sand in the upper intertidal zone. The burrows are grouped into three forms based on the tortuosity of their course and degree of looping and trail crossover in plan view. Sediment samples taken directly from the trail furrows are used to establish the total organic-carbon content associated with each burrow morphology. There is an increase in organic-carbon content from burrows of low tortuosity (linear burrows) to burrows of higher tortuosity (convolute burrows with many crossovers), suggesting that benthic food content directly influences the behavior of C. coeca. Detailed study of trace emplacement further reveals a relationship between C. coeca and food content leading to the recognition of three grazing styles. These are directly related to the plan-view morphology of the trail and reflect the depth at which the isopod tunnels in the sediment. Graphical analysis of weight percent organic carbon against grazing style shows a positive correlation between deep burrowing and high benthic food content. The morphology of C. coeca trails differs from deep-sea turbidite and flysch deposits. In the deep-sea environment resources are replaced slowly, and food distribution is comparatively low and uniform, leading to regular meanders and complex traces that do not crossover existing trails. In the intertidal zone, C. coeca encounters sporadically distributed resources that are replenished semidiurnally. The resultant burrows exhibit increasing crossovers with increasing food content, representing a once-over feeding strategy designed to rapidly harvest a high-value, renewable resource.

The Temporal Significance of Bioturbation in Backshore Deposits: Waterside Beach, New Brunswick, Canada

Abstract This paper provides an estimate of the temporal nature and environmental implications of bioturbation in upper-foreshore and backshore deposits resulting from the burrowing activity of talitrid amphipods at Waterside Beach, Bay of Fundy, Canada. Short-term preservation potential is assessed by comparing shore-normal variations in burrow morphologies and densities to grain-size distribution, and hydraulic and eolian processes. The intensity and distribution of burrowing is linked to depositional environment and provides a temporal framework for bioturbation indices in ancient upper-foreshore and backshore deposits. Bioturbation in foreshore and backshore successions provides a means for predicting short-term, autocyclic processes in the rock record, in that: (1) the initial occurrence of burrowing marks the transition from the foreshore to the backshore, and (2) the degree of bioturbation may be used to predict depositional rates and processes. At Waterside, bioturbation indices (BI) of two or less indicate a single season of colonization; BIs of two (upper end of range) and higher suggest multiple seasons of colonization. A high BI may indicate that the backshore environment was protected from wave reworking or erosion (particularly during storms) and/or was subjected to minor eolian sedimentation.

Unburrowed mudstones may record only slightly lowered oxygen conditions in warm, shallow basins

Unbioturbated mudstones and highly bioturbated silty and sandy mudstones from the late Albian of Alberta, Canada, are characterized by their ichnological, foraminiferal, and geochemical signatures. A comparison of these data sets is undertaken to isolate the dissolved oxygen (DO) conditions that led to the preservation of unbioturbated mudstones versus highly bioturbated silty and sandy mudstones. Highly diverse and abundant benthic foraminiferal assemblages, coupled with conclusive geochemical signatures, indicate that unbioturbated mudstones were deposited under oxic bottom waters. The paucity of bioturbation in these rocks is attributed to the persistence of low-oxic conditions (5 > DO > 2 mg L–1) at the seafloor, comparable to the present-day Gulf of Mexico. We assert that unbioturbated mudstone should not automatically be attributed to oxygen deficiency (<2 mg L–1). Instead, it may reflect oxygenation sufficient to support benthic microfauna (foraminifera) but insufficient to sustain a diverse ecosystem of macrofauna (burrowing fauna). Moreover, we propose that the distribution of unburrowed mudstones deposited below low-oxic waters is predictable. A paucity of bioturbation is normal in shallow marine (below fair-weather wave base to ∼200 m water depth) deposits of subtropical to tropical ocean basins and/or semienclosed seaways.

Chapter 6 - Sequence Stratigraphy

Sequence stratigraphy is a methodology that employs stratal stacking patterns and key bounding surfaces to erect a framework allowing depositional facies to mapped and interpreted paleogeographically. Historically, sequence stratigraphy has been focused on allogenically induced changes such as eustasy, tectonics, and climate. Like sedimentology, ichnology assists in the identification and interpretation of bounding surfaces separating stratal units. These are recognized through the use of trace-fossil omission suites and the juxtaposition of ichnological suites recording shifts in depositional settings that contravene Walthers Law. Two siliciclastic case studies from the Lower Cretaceous Viking Formation of Canada and one carbonate case study from the Middle Permian–Lower Triassic Khuff Formation, offshore Iran, are employed to showcase the utility of ichnology in sequence-stratigraphic evaluations. Case study 1 deals with incised shorefaces and the identification of forced regressive, lowstand, and transgressively incised shallow-marine sand bodies. Case study 2 addresses wave-dominated estuarine valleys that are incised into highstand marine parasequences and incrementally infilled during early transgression. Case study 3 focuses on sequences from a carbonate platform built of transgressive systems tracts and highstand systems tracts separated from one another by maximum accommodation zones and capped by maximum regressive surfaces that are locally transgressively modified.