James A. MacEachern

Colonization of Brackish-Water Systems through Time: Evidence from the Trace-Fossil Record

Abstract Trace fossils in estuarine deposits of different ages have been compared to evaluate colonization history of brackish-water ecosystems and to calibrate trace-fossil, brackish-water models with respect to geologic time. This comparative analysis reveals that, although the colonization of marginal-marine, brackish-water environments was a long-term process that spanned most of the Phanerozoic, this process of invasion of fully marine organisms into restricted, marginal-marine habitats did not occur at a constant rate. Five major colonization phases can be distinguished. The first phase (Ediacaran–Ordovician) represents a prelude to the major invasion that occurred during the rest of the Paleozoic. While Ediacaran–Cambrian ichnofaunas seem to be restricted to the outermost zones of marginal-marine depositional systems, Ordovician assemblages show some degree of landward expansion within brackish-water ecosystems. Intensity of bioturbation and ichnodiversity levels were relatively low during this phase. The second phase (Silurian–Carboniferous) is marked by the appearance of more varied morphologic patterns and behavioral strategies, resulting in a slight increase in ichnodiversity. While previous assemblages were arthropod dominated, brackish-water Silurian–Carboniferous ichnofaunas include structures produced by bivalves, ophiuroids, and polychaetes. Ichnofaunas from the third phase (Permian–Triassic) seem to be characterized by the presence of crustacean burrows, reflecting the late Paleozoic crustacean radiation and adaptation of some groups to brackish-water conditions. The fourth phase (Jurassic–Paleogene) is typified by a remarkable increase in ichnodiversity and intensity of bioturbation of estuarine facies. Colonization occurred not only in softgrounds and firmgrounds, but also in hardgrounds and xylic substrates. The fifth phase (Neogene–Recent) records the onset of modern brackish-water benthos. Although still impoverished with respect to their fully marine counterparts, brackish-water ichnofaunas may reach moderately high diversities, particularly in middle- and outer-estuarine regions, and degree of bioturbation may be high in certain estuarine subenvironments. Comparative analysis of brackish-water ichnofaunas through geologic time provides valuable evidence to understand colonization of marginal-marine environments through the Phanerozoic, and allows for calibration of ichnologic models that may aid in the recognition of estuarine valley-fill deposits in the stratigraphic record.

Vertebrate Burrow Complexes from the Early Triassic Cynognathus Zone (Driekoppen Formation, Beaufort Group) of the Karoo Basin, South Africa

Abstract A scratch-marked burrow complex with multiple branching tunnels and terminal chambers was excavated from the Lower Triassic Driekoppen Formation, northeastern Free State, South Africa. The burrow complex is attributed to the therapsid Trirachodon, based on disarticulated but fairly complete skulls and skeletons of at least 20 individuals recovered from a nearby, less well-preserved system. The entrance shaft slopes gently downward and is characterized by a bilobate floor and vaulted roof. The bilobate floor has a central flat-topped scratch-marked ridge flanked by two smooth grooves, each approximately the width of the occupant. At deeper levels, tunnels display tighter lateral curvatures, progressively decreasing burrow diameters, variable orientations, and some right angle branches. Burrow floors at these levels are vaguely bilobate. Distally, burrows flatten dorso-ventrally, becoming wedge-shaped before they terminate. These complexes are interpreted as colonial dwelling structures. Numerous branching tunnels and terminal chambers, as well as an enlarged entrance, constitute an unrealistically high expenditure of energy for a single occupant. Furthermore, the bilobate floor is atypical of a single occupant system. Absence of scratch marks in the depressions reflects the regular locomotory activity of Trirachodon. In single-occupant burrows, the center of the structure is worn. The bilobate floor with preferential wear along the flanks suggests routine travel on one side of the tunnel or the other, as multiple occupants moved past one another during daily activity. The presence of numerous fossilized Trirachodon individuals in terminal chambers at the second locality supports multiple cohabitation. There, the uniform sedimentary fill and vertebrate taphonomy suggest that the occupants were drowned in a flash flood. Vertebrate remains are absent at the first locality, where sediment influx was incremental, permitting the escape of burrow occupants. This constitutes the earliest record of multiple cohabitation of a burrow complex by tetrapods. The Trirachodon (Cynodontia) of the Early Triassic Cynognathus Zone probably displayed complex social behaviors previously regarded to be restricted to the mammals of the Cenozoic. This fossorial behavior may have been for thermoregulation, protection from predators, sites of reproduction, and the rearing of young.

Differentiation of estuarine and offshore marine deposits using integrated ichnology and sedimentology: Permian Pebbley Beach Formation, Sydney Basin, Australia

Abstract This study integrates ichnology and sedimentology to refine the palaeoenvironmental and sequence stratigraphic interpretations of the Early Permian Pebbley Beach Formation, in the southern Sydney Basin, Australia. This succession has been interpreted previously to reflect entirely inner to outer shelf and slope environments of deposition. Detailed analysis of the formation reveals ichnological and sedimentological characteristics that contradict a fully marine interpretation. Instead, the interval reflects the vertical superposition and lateral juxtaposition of brackish-water and fully marine units. Marine facies comprise: (1) thoroughly bioturbated muddy siltstone (lower offshore); (2) thoroughly bioturbated sandy siltstone (upper offshore); (3) interbedded bioturbated sandy siltstone and laminated sandstone (delta-influenced offshore transition); (4) thoroughly bioturbated muddy sandstone (distal lower shoreface); (5) interbedded laminated sandstone, bioturbated muddy sandstone and dark claystone (delta-influenced lower shoreface); and (6) bioturbated, laterally variable sandstones (transgressive sand sheets). Estuarine facies comprise: (1) channelized heterolithic sandstone-mudstone (active estuarine channels); (2) sheet-like heterolithic sandstone-mudstone (active estuarine basins); and (3) laminated mudstone (abandoned estuarine channels and basins). The interpreted fully marine deposits contain ichnological suites that exhibit moderate to intense bioturbation, high diversities (31 ichnospecies belonging to 20 ichnogenera), uniform distributions of ichnogenera, and significant numbers of structures reflecting specialized feeding/grazing behaviours. In marked contrast, interpreted estuarine (brackish-water) deposits contain impoverished ichnological suites (9 ichnogenera), show variable but significantly reduced degrees of bioturbation intensity, pronounced variability in ichnogenera distributions and the predominance of a few, simple forms representing simple feeding strategies of resilient trophic generalists. The new analysis allows the recognition of a series of highly top-truncated and condensed sequences (cycles of relative sea-level fall and physical rise), which can be physically correlated over several kilometres. Sequence boundaries typically cut down through shoreface sandstones to directly overlie offshore facies, leading to an interface with little apparent lithological contrast. In the absence of laterally continuous exposure, these surfaces may be recognized by careful ichnofacies evaluation. Thus the re-evaluation presented herein has facilitated a more realistic sequence stratigraphic analysis of the Pebbley Beach Formation.

Firmground Zoophycos in the Lower Cretaceous Viking Formation, Alberta: A Distal Expression of the Glossifungites Ichnofacies

Abstract The substrate-controlled Glossifungites ichnofacies is a firmground suite of trace fossils that commonly demarcates erosional discontinuities in sedimentary successions. The Glossifungites ichnofacies typically is characterized by biogenic structures that are vertical to subvertical, sharp-walled, unlined and commonly passively infilled. The structures hitherto have been restricted to permanent or semi-permanent domiciles, and predominantly to vertical to subvertical burrows of suspension-feeding organisms. Several cores of the late Albian (Lower Cretaceous) Viking Formation from the Hamilton Lake Field of Alberta, Canada contain an atypical Glossifungites ichnofacies dominated by the foraging, probing and deposit-feeding/dwelling structures of firmground Thalassinoides, Rhizocorallium, and Zoophycos. The Thalassinoides are passively infilled with coarse grains of sand and chert granules. Rhizocorallium displays active stages of infill, although the final, horizontal U-shaped tube is passively filled. The Zoophycos, however, demonstrates active infill manifest by chert granules and coarse sand distributed in the spreite, and this is a departure from the normal expression of the Glossifungites ichnofacies. The firmground structures subtend from the regional stratigraphic discontinuity BD4, excavated into black, silt- and sand-poor offshore and shelf mudstones. BD4 is interpreted to reflect a transgressively modified sequence boundary. Most Zoophycos, Thalassinoides, and Rhizocorallium penetrate no deeper than 3–4 cm below the discontinuity into the underlying mudstones. Where BD4 overlies a sandier substrate, firmground Zoophycos are not present within the suite. The facies directly overlying BD4 consist of fining-upward, pebble-, granule-, and very coarse-grained sand-bearing, thoroughly burrowed muddy sandstones to sandy mudstones, typically 5–25 cm thick. This basal transgressive lag contains Teichichnus, Planolites, Palaeophycus, Diplocraterion, Chondrites and rare Helminthopsis, and is the source of the coarse-grained material incorporated into the fill of the firmground ichnogenera. The granule-bearing sandy mudstones overlying the transgressive lag contain abundant Teichichnus, Planolites, Helminthopsis, Anconichnus, Chondrites, and Terebellina, and reflect rapid deepening to proximal offshore conditions. Progressive deepening is recorded in the accumulation of distal offshore and shelf mudstones of the late Albian Westgate Formation. Although excavation of BD4 occurred above fairweather wave base, its colonization occurred under much lower energy conditions. Within distal softground settings, ichnogenera capable of deeply penetrating muddy substrates are largely restricted to Zoophycos, Thalassinoides, Rhizocorallium, and Chondrites. These ichnogenera constitute forms capable of being excavated, albeit shallowly, within a firm substrate. This is a low energy, distal expression of the Glossifungites ichnofacies.

Stratigraphic applications of substrate-specific ichnofacies: delineating discontinuities in the rock record

Abstract Trace fossils represent both sedimentological and palaeontological entities, providing a unique blending of potential environmental indicators in the rock record. Trace fossils and trace fossil suites can be employed effectively to aid in the recognition of various discontinuity types and to assist in their genetic interpretation. Ichnology may be employed to resolve surfaces of stratigraphic significance in two main ways: (1) through the identification of discontinuities using substrate-controlled ichnofacies (the firmground Glossifungites ichnofacies, the hardground Trypanites ichnofacies and the woodground Teredolites ichnofacies); and (2) through careful analysis of trace fossils in vertical (soft-ground) successions (analogous to facies successions). Integrating the data derived from substrate-controlled ichnofacies (so-called omission suites) with palaeoecological data from vertically and laterally juxtaposed softground ichnological successions greatly enhances the recognition and interpretation of a wide variety of stratigraphically significant surfaces. When this is coupled with conventional sedimentary facies analysis and sequence stratigraphy, a powerful approach to the interpretation of the rock record is generated.

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.

Modern Perspectives on the Teredolites Ichnofacies: Observations from Willapa Bay, Washington

Abstract This study details occurrences of Teredolites ichnocoenoses from modern bay-margin settings in Willapa Bay, Washington. In particular, the paper scrutinizes in situ log-grounds from two intertidal zones situated immediately seaward of low-lying, supratidal, forested marshlands that are fed by small streams. Vestiges of remnant marshes and streams are preserved on intertidal flats as in-situ roots, broken stumps, strewn logs, abundant organic detritus, and organic sandy mud. Xylic material and organic sediment were deposited in the supratidal marshes: tide and wave processes truncated the swamps, exposing in situ tree-root networks and the lowermost supratidal sediments. Stream and swamp deposits overlie and incise older Pleistocene strata. Both units are overlain by discontinuous, modern intertidal deposits. The intertidally exposed stumps and logs support a diverse community of animal and plant life. Boring organisms, encrusters, and refugium seekers are found on and within the xylic substrates. Some encrusting animals and all of the boring fauna produce traces that are comparable to ichnofossils reported by palichnologists. Wood-boring traces reported in this study are similar morphologically to the ichnogenera Caulostrepsis, Entobia, Meandropolydora, Psilonichnus, Rogerella, Teredolites, Thalassinoides, and Trypanites. Most of these ichnogenera have not been reported from rock-record examples of the Teredolites ichnofacies. The stratigraphic and environmental significance of the reported (modern) locales is consistent with previous studies that associate Teredolites ichnofacies with base-level rise in marginal-marine environments. At Willapa Bay, bored xylic media form a coeval surface with adjacent, burrowed firmgrounds as well as softgrounds.

High-resolution sequence stratigraphy of early transgressive deposits, Viking formation, Joffre Field, Alberta, Canada

The Lower Cretaceous Viking Formation of the Joffre field is characterized by complex reservoir architecture. Deposits of three discrete sequences were delineated using high-resolution sequence stratigraphy. The coarse-grained deposits of sequence 3, lying between BD-2 and an overlying open marine flooding surface, comprise the main reservoir interval within the Viking Formation of the Joffre field. This succession has previously been interpreted as an incised conglomeratic shoreface, stranded in a basinal position during transgression; however, sequence 3 displays characteristics difficult to reconcile with a shoreface interpretation, including an abundance of brackish mudstone interbeds and rip-up clasts, dominance of trough cross-stratification in the coarse clastics, and large-scale interfingering of the coarse clastics with fine-grained marginal-marine deposits. Despite the incised basal contact and brackish-water characteristics of the deposits, the succession does not reflect an estuarine incised valley complex, as conventional sequence stratigraphic wisdom might suggest. The shore-parallel orientation of the deposit, the lack of a convincing valley margin to the northeast, and parasequence orientations lying parallel to the strike of the deposit are inconsistent with an incised valley interpretation. Instead, the succession is interpreted as a broad brackish-water embayment of the shoreline, into which coarse-clastic bayhead delta and distributary channel complexes were deposited during incremental transgression. Such lagoonal or brackish bay complexes are ubiquitous in modern transgressive shorelines, but previously have been recognized only rarely in the ancient record.

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.

The ichnological expression of marine and marginal marine conglomerates and conglomeratic intervals, Cretaceous Western Interior Seaway, Alberta and northeastern British Columbia

ABSTRACT Although uncommon, conglomeratic successions and their associated interbeds do contain trace fossils that attest to their marine or marginal marine origins. Intervals within the Cretaceous of the Western Interior Seaway of Alberta and northeastern British Columbia contain ichnological assemblages consistent with open bay, estuarine incised valley, shoreface, and transgressive ravinement settings. Conglomerates and conglomeratic sandstones of open bays are largely confined to bay margins, and are markedly heterolithic, commonly trough cross-stratified, and moderately bioturbated with Diplocraterion, Skolithos, Palaeophycus, Conichnus and Ophiomorpha. Interbedded sandstones and mudstones contain Teichichnus, Planolites, Terebellina, Arenicolites, Bergaueria, Rosselia, Asterosoma, Cylindrichnus, Thalassinoides, and fugichnia. Burrowed incised valley conglomerates and pebbly sandstones mainly record channel fills of tidal inlets or flood-tidal deltas at wave-dominated estuary mouths. Trace fossil suites comprise Teichichnus, Ophiomorpha, Skolithos, Diplocraterion, Arenicolites, Rosselia, Cylindrichnus, and fugichnia. Intercalated sandstone and mudstone beds are moderately burrowed with Planolites, Teichichnus, Terebellina (sensu lato), Ophiomorpha, Palaeophycus, Asterosoma, Chondrites, Conichnus and fugichnia. Upper shoreface and foreshore conglomerates are well sorted, clast supported, and display good clast segregation. Bioturbation is uncommon, comprising Palaeophycus, Ophiomorpha, Cylindrichnus, Rosselia and fugichnia. Interbedded sandstones and pebbly sandstones may contain Macaronichnus and Palaeophycus. The Cardium Formation locally contains conglomeratic intervals interpreted to reflect the entire shoreface succession. These conglomeratic intervals are unburrowed, though they overlie Glossifungites ichnofacies-demarcated discontinuities, and grade upward into burrowed shelf mudstones. Conglomeratic transgressive lags and pebbly sandstones mantle wave or tidal scour ravinement surfaces, and commonly infill palimpsest Diplocraterion, Skolithos, Arenicolites, Thalassinoides, and Rhizocorallium of the Glossifungites ichnofacies, which demarcate these discontinuities. Transgressive lags may be variably burrowed with virtually any softground suite, depending on the depositional setting. Tidal scour ravinement lags at incised valley mouths, for example, may be weakly burrowed with an impoverished Skolithos ichnofacies, whereas wave ravinement lags produced during regional transgression can contain open marine, diverse mixed Skolithos-Cruziana ichnofacies assemblages. End_Page 77-------------------------