ABSTRACT Detailed logging of ichnological variations within parasequences of several Cretaceous (... more ABSTRACT Detailed logging of ichnological variations within parasequences of several Cretaceous (Upper Turonian) delta complexes from Wyoming and Utah are correlated with inferred short and long-term changes in depositional processes. These changes reflect various proportions of river, flood, wave, storm, and tide influences. Event beds, such as storm and river-flood deposits, tend to show low BI (Bioturbation Index) values of 0-2, owing to high accumulation rates, although this also depends on event frequencies. Upper surfaces of individual storm/river-flood beds may show BI values of 4-5, reflecting the transition to longer-lived fair-weather conditions. Fair-weather waves facilitate persistent agitation near the bed, buffering environmental stresses. Therefore, wave-dominated deposits that are not affected by storms yield climax communities with robust and diverse ichnofacies signatures reflecting "uniform and high" BI trend with values that average 4. River-dominated intervals show the least uniform trends of BI, because of the highly variable conditions related to river jet and plume behavior. BI values vary from 0 to 4, with generally low ichnogenera diversities. These alternations likely record seasonal to centennial fluctuations in sedimentation rate (river discharge) and water turbidity, which influences substrate conditions near distributary mouths. Tide-dominated intervals tend to show the most 'stressed' conditions, reflecting "non-uniform and low" trend of BI, with values of 0-2. These reflect salinity fluctuations, heightened water turbidity, rapidly shifting substrates, and narrow colonization windows associated with daily and monthly changes in tidal periodicity. Individual parasequences are characterized by either upward-increasing and upward-decreasing trends of BI, indicating protection from storm erosion and proximity to river input, respectively. Ichnological signatures change significantly across initial flooding surface, principally showing a marked increase in BI as delta lobes are quickly abandoned and transgressed. In contrast, across the maximum flooding surface, changes in ichnological signatures are subtle and rather uniform. We suggest that different parts of a single delta may experience marked differences in river, wave, and tide influence over time, reflecting the enormous complexity of operative processes at various temporal and spatial scales. Detailed intra-parasequence, bed-scale analyses of trace fossils help to reveal this complex evolutionary history of a single delta.
The regressive and subsequent transgressive transit of a shoreline across a clastic shelf generat... more The regressive and subsequent transgressive transit of a shoreline across a clastic shelf generates the standard reservoir flow unit in most marginal to shallow marine hydrocarbon reservoirs. The stratigraphic unit produced by the shoreline transits is commonly referred to as the high-frequency (10 4 to 10 5 years), regressivetransgressive sequence (RT sequence). The unit is usually bounded top and base by marine shales marking flooding surfaces, which can act as barriers to fluid flow. Stratigraphic architecture in the RT-sequence is also known to control the internal flow behavior of many reservoirs. Hence, an ability to consistently characterize reservoirs at a sub RTsequence scale is critical to enabling prediction of reservoir performance and optimization of resource extraction strategies. This study describes the internal architecture of one shallow-water (, 10 m), low-accommodation regressive shoreline succession from the Campanian of the Alberta Basin, Canada, based on an extensive outcrop and subsurface dataset that has been convolved into a 3D geocellular computer model. The architecture and evolution of the ancient mixed-process (waves, tides, and fluvial processes) regressive deltaic shoreline system is compared and contrasted with a partial Holocene analog from northeastern Australia. The same stratigraphic surfaces and units are identified in both the modern and the ancient regressive systems. The key architectural unit is the element complex set (ECS), which is a multi-kilometer-scale, discontinuity-bounded unit that is the product of the reorganization of the coastline, often caused by autogenic backwater-driven avulsions. Multiple avulsions during a regressive shelf transit episode lead to lateral offsets of ECS units in low-accommodation systems. These systems are termed ''avulsion-driven systems.'' An increasing component of vertical stacking of ECS units is observed in higher-accommodation regressive systems. The mechanisms for generating accommodation on a regressive-shoreline shelf-transit time frame may be allogenic (tectonic subsidence or eustatic sea-level change), autogenic, or a combination of the two mechanisms. A key, localized, autogenic mechanism is related to the distance of progradation during the transit of a deposystem across the shelf. In proximal shelf positions, ECS units tend to offset laterally due to limited available accommodation, whilst in more distal positions, early differential, load-induced, compactional subsidence of underlying prodelta and shelf muds can promote vertical stacking of ECS units. The critical down depositional-dip distance from the transgressive turnaround point at which ECS units become preferentially vertically stacked is a function of shelf gradient, shoreline trajectory, sandstone fraction, and prodelta and shelf mud rheology and is termed the ''critical autogenic ECS stacking distance.'' Vertical stacking of ECS units may also occur when ECS units overstep underlying shelf topography, such as the distal termination of an older RT sequence. Recognition criteria and nomenclature for intra-regressive-shoreline surfaces and stratigraphic units, as well as predictive models for the ancient record are detailed across a spectrum of types of deltaic systems.
This study considers the construction and functionality of biogenic structures made by marine, ve... more This study considers the construction and functionality of biogenic structures made by marine, vermiform nemerteans, polychaetes and hemichordates; marine crustaceans; motile bivalves; motile echinoderms; and sponges and sea anemones. We report on a range of modern biogenic structures similar to several known ichnogenera. Vermiform animals dominantly occupy vertical burrows that range from simple through helical shafts to Y-and U-shapes. Horizontal traces made by worms range in form, but are dominated by branching and variably sinuous to meandering burrows. Crustaceans primarily excavate open burrow systems that possess a range of architectures that are similar to either Thalassinoides or Psilonichnus. Smaller crustaceans, such as amphipods, mix the sediment. Bivalve traces vary in form, but generally preserve evidence of vertically oriented filter or interface-deposit feeding from a stationary location, rapid vertical escape, or horizontal grazing. Echinoderms dominantly preserve body impressions and motility traces, such as Asteriacites. An important class of biogenic structure, Scolicia and Bichordites, are made by urchins. Finally, sea anemones can generate large, penetrative, conical biogenic structures. Large, open horizontal networks serve as domiciles and deposit-feeding structures for crustaceans, but with worms similar burrow types are used more for passive carnivory and establishing an interface-feeding network. We report that the trace fossil Gyrolithes potentially represents mechanical ramps for shrimp, but is used as a sediment holdfast when made by worms. Finally, Y-shaped burrows are used for filter feeding in shrimp, and interface-deposit feeding in worms. These examples emphasize that inferences of behavior in the rock record are interpretive.
The Glossifungites ichnofacies is a substrate-controlled assemblage of trace fossils, characteriz... more The Glossifungites ichnofacies is a substrate-controlled assemblage of trace fossils, characterized by sharp-walled, unlined, passively filled, vertical to subvertical domichnia, excavated into semiconsolidated (firmground) substrates. In Cretaceous siliciclastic rocks of the Western Canada sedimentary basin, the assemblage is dominated by the ichnogenera Diplocraterion, Skolithos, Arenicolites, Thalassinoides, and Rhizocorallium. The Glossifungites assemblage demarcates discontinuity surfaces that reflect pauses in sedimentation accompanied
ABSTRACT X-radiographs of sediment box cores acquired from the western Gulf of Mexico reveal limi... more ABSTRACT X-radiographs of sediment box cores acquired from the western Gulf of Mexico reveal limited bioturbation in sediment deposited in bathymetries greater than 35 m. Between 15 and 35 m, sediments are thoroughly bioturbated with averaged bioturbation indices (for all beds in a core) between 2.1 and 5.6, and trace diversities between 2 and 9 distinct burrow forms. Below 35 m water depth, box cores exhibit trace diversities of 1–3, and core-averaged bioturbation indices range between 0.3 and 3.6. There is an overall decrease in trace diversity and bioturbation indices in the offshore direction. Cross-shore ichnological trends are compared to dissolved oxygen contents of bottom waters. Above 35 m, dissolved oxygen (DO) contents show pronounced variability, ranging from 100% DO saturation through to hypoxia (< 2.0 mg l-1), and reflect the periodic introduction of oxygen-depleted waters into otherwise fully oxygen-saturated seawater. Below 35 m, DO content of bottom waters is consistently at 60–75% saturation. DO decreases by an average of 0.117 mg l-1 per one-meter increase in water depth, such that bottom waters in 100 m water depth contain an average of 4.55 mg l-1 DO. The data reveal a direct correlation between: a) the density of infauna and the diversity and density of burrows, and b) DO of bottom water. The degree of bioturbation is significantly reduced in waters that are oxic, but below 80% DO saturation. Based on these observations, we suggest that it is inappropriate to link low bioturbation intensities and diversities to hypoxia (< 2.0 mg l-1), and by extension, anoxia (0 mg l-1). Instead, reduced oxygen contents (4.3 – 5.3 mg l-1) that lie well above hypoxic levels have a dramatic impact on the health of infaunal communities, and this is reflected by severe reductions in the ichnological character of sediments.
Analyses of deltaic facies successions highlight recurring ichnological patterns that reflect a v... more Analyses of deltaic facies successions highlight recurring ichnological patterns that reflect a variety of physicochemical stresses imposed upon infaunal organisms by the interaction of various delta-front processes. Analysis of numerous ancient deltaic deposits in Canada, the United States, Australia, and offshore Norway persistently show reductions in bioturbation intensity and impoverishment in ichnological diversity, compared to those of nondeltaic shorelines. Some facies locally demonstrate sporadic colonization and recolonization of substrates left denuded by episodic depositional conditions. Deltaic ichnological suites also locally display size reductions of ichnogenera and a paucity of suspension-feeding ethologies. Resulting trace-fossil suites are overwhelmingly dominated by depositfeeding behaviors, even in sandy facies. Such ichnological characteristics largely reflect increased sedimentation rates and heightened fluvial discharge, which serve to impede infaunal colonization. River-derived stresses are profound: salinity changes, hypopycnal-flow-induced water turbidity, distributary flood discharges with accompanying phytodetrital (comminuted plant debris) pulses, hyperpycnal-flow-induced sediment gravity flows, and fluid-mud deposition all conspire to produce the overall depauperate nature of the ichnological assemblage. Freshet-discharge events during river floods, accompanied by hyperpycnal conditions, may lead to the episodic introduction of reduced-salinity waters immediately above the sediment-water interface in delta-front and prodelta depositional settings. Such conditions may facilitate development of synaeresis cracks and promote reductions in infaunal populations. Wave energy generally buffers fluvial effects, by dispersing suspended sediment offshore and encouraging the thorough mixing of waters of contrasting salinity. High mud concentrations near the delta front damp wave energy, however, limiting its effectiveness in remediating the benthic ecosystem, particularly immediately following distributary flood discharges and storm events. In wavedominated settings, strong alongshore drift also operates to extend river-derived stresses considerable distances downdrift from distributary mouths. Where asymmetric deltas are formed, markedly different ichnological expressions are expressed on either side of distributary-channel mouths. Updrift settings typically retain classic shoreface assemblages, whereas downdrift environments commonly acquire markedly stressed suites. Storm energy may be effective in dispersing mud and mixing waters, but it also results in erosion and episodic sediment deposition. Concomitant precipitation induces river floods, returning river-derived stresses to the delta front. Tidal energy and its effects on the infaunal communities of deltas are poorly documented. Tidal flux may trap mud plumes against the delta front, elevating water turbidity. Pronounced mud flocculation coupled with increased settling velocity associated with tidal mixing also leads to rapid deposition of thick fluid muds, particularly in low-lying areas, hampering or precluding colonization. Tidal energy also leads to marked changes in energy and salinity near the sediment-water interface at several time scales. Deltaic ichnological suites are characterized by structures of opportunistic trophic generalists, though mainly those of facies-crossing deposit feeders. High water turbidity, particularly near the sea floor, precludes most suspension-feeding behavior, and suppresses the development of the Skolithos ichnofacies, even in many proximal delta-front deposits. Ichnological characteristics record the dynamic interplay and relative importance of these different processes, both temporally and spatially. on delta systems.
Summary Cycle G of the Campanian Belly River Formation comprises two recurring and mappable facie... more Summary Cycle G of the Campanian Belly River Formation comprises two recurring and mappable facies associations (FA1 and FA2), both interpreted to record distal prodelta through proximal delta-front accumulation. FA1 is characterized by uniformly coarsening-upward successions, with abundant wave- and storm-generated sedimentary structures, subordinate syneresis cracks, convolute bedding, and claystone drapes. Trace fossils are sporadically distributed in low- to moderate-
ABSTRACT Tide-dominated and tide-influenced clastic depositional environments are typically inter... more ABSTRACT Tide-dominated and tide-influenced clastic depositional environments are typically interpreted to be associated with landward-stepping or transgressive systems. Here, we present an example of an overall regressive succession dominated by high frequency sequences that exhibit abundant sedimentary and ichnological evidence of tidal processes. These strata comprise the Campanian Bearpaw-Horseshoe Canyon Formation transition, exposed near Drumheller, Alberta, Canada. These clastic marine to marginal-marine sediments were deposited in mixed-process (wave-, tide- and fluvial-influenced) depositional environments along the western margin of a foreland basin. The deposits are subdivided into six, relatively thin (10 m on average), high frequency (~140,000 years), Transgressive-Regressive (T-R) sequences (A to F). The evolution of the successive T-R sequences was mapped on a sub-regional scale. The orientations of the paleoshorelines rotate in a predictable manner during transgression and regression. Transgressive shorelines are oriented approximately S–N and rotate clockwise during regression to approximately SW–NE. The rotations are attributed to the paleogeography of the region, with the study area interpreted to sit on the western flank of a large, structurally controlled embayment. The progradational high-frequency sequence set is dominated by tide-influenced strata and clearly demonstrates that tidal deposits can be preserved in both regressive as well as transgressive successions. Tidal influence in this regressive setting is attributed to shallow water depths and a wide shelf, which resulted in amplification of the tidal wave as it moved across the shelf. The embayed character of the coastline may also have contributed to this effect. Tidal range is estimated to have been meso- to macrotidal. A modern, partial analogue for the Horseshoe Canyon systems is identified from the tropical Mitchell River Delta in the Gulf of Carpentaria, Queensland, Australia. This delta system is analogous in terms of its tide-influenced facies, mesotidal range, low-gradient shelf with accompanying shallow water depths, the mixed-process character of its shoreline systems, and its horizontal to falling shoreline trajectory.
The early to mid-Campanian Basal Belly River Formation in the Ferrybank, Keystone, and eastern Pe... more The early to mid-Campanian Basal Belly River Formation in the Ferrybank, Keystone, and eastern Pembina fields of central Alberta, reflects a mixed wave-and river-influenced deltaic succession with strong storm overprinting. Prodelta deposits consist of mud-dominated heterolithic successions, characterized by a low abundance yet moderately diverse trace fossil assemblage attributable to a "stressed" expression of the Cruziana Ichnofacies. Proximal prodelta to distal delta front intervals comprise interbedded sandy siltstones and very fine-to fine-grained sandstones exhibiting convolute bedding and sporadic bioturbation. Trace fossil assemblages range from very low to moderate abundance, and low to moderate diversity; the suites are attributable to a "stressed" expression of the Cruziana Ichnofacies. Distal delta front deposits coarsen upwards into fine-to medium-grained sandstones of the proximal delta front. High-energy conditions, coupled with strong storm influences, resulted in erosional amalgamation of tempestites, and led to sporadic distribution of ichnogenera. Proximal delta front intervals are weakly bioturbated, and trace fossil assemblages are characterized by low abundances and low to moderate diversity. Most forms within the sandstone facies represent the structures of deposit-feeding organisms. Suites reflect a "stressed" infaunal community and contain a mixture of elements characteristic of both the Skolithos and Cruziana ichnofacies. Analysis of more than fifty cored wells has revealed prodelta and delta front deposits that vary markedly along depositional strike. The along-strike variations fit well with the recently proposed asymmetric delta model. The model is based on observations of modern wave-influenced deltas such as the Danube. This study provides an ancient analogue. Continued research will seek to further delineate delta lobe asymmetry and concomitant along-strike facies variations, both attributable to longshore drift and deflection of river-induced stresses downdrift of distributary channel mouths. Organisms are exceedingly sensitive to fluvial influence and this "stress" is reflected in the relatively low diversity and low abundance assemblages that characterize many deltaic successions. Ethological preferences and burrow sizes further reflect the level of "stress" imparted on infaunal organisms within the subaqueous delta environment. Trace fossil suites of river influenced deltaic successions signify a departure from the archetypal ichnofacies characteristic to shoreface successions, and their mapped distributions may serve as a predictive tool for determining distributary channel proximity.
Preliminary ichnological-sedimentological study from 51 cored wells of the Lower Cretaceous (Earl... more Preliminary ichnological-sedimentological study from 51 cored wells of the Lower Cretaceous (Early to mid-Albian) Mannville Group (Sparky, Waseca, and McLaren formations) in west-central Saskatchewan and west-central Alberta yield ten recurring facies that reflect conditions of reduced and fluctuating salinity, typical of brackish-water settings. The successions display physical sedimentological features dominated by abundant organic-rich mudstone drapes of probable fluid-mud origin, syneresis cracks, soft-sediment deformation, carbonaceous detritus, normally graded mudstones, and wave, current and combined-flow ripples. Facies are generally heterolithic, attesting to autogenic fluctuations in depositional processes. Ichnologically, bioturbation intensities range widely (BI 0-5), and facies are characterized by abundant diminutive ichnogenera comprising low-diversity (locally monogeneric) trace fossil suites. Common ichnogenera include Gyrolithes, Teichichnus, Planolites, Palaeophyc...
The Lower Cretaceous (early to mid-Albian) Mannville Group (Sparky, Waseca, and McLaren formation... more The Lower Cretaceous (early to mid-Albian) Mannville Group (Sparky, Waseca, and McLaren formations) in west-central Saskatchewan is up to 60m thick, and consists of weakly consolidated sandstones, shales, mixed sands and shales forming heterolithic bedding, and minor coals. These units were deposited in shallow marine to coastal-plain environments. The study area is located in west-central Saskatchewan (Lloydminster area) from Townships 48 -53, and Ranges 19W2 to 28W3, covering a total area of 5,400 km² (Fig. 1). Figure 1: The study area is located in West-Central Saskatchewan, Canada, shown in the above figure. The figure illustrates the core distributions employed in this study.
ABSTRACT Detailed logging of ichnological variations within parasequences of several Cretaceous (... more ABSTRACT Detailed logging of ichnological variations within parasequences of several Cretaceous (Upper Turonian) delta complexes from Wyoming and Utah are correlated with inferred short and long-term changes in depositional processes. These changes reflect various proportions of river, flood, wave, storm, and tide influences. Event beds, such as storm and river-flood deposits, tend to show low BI (Bioturbation Index) values of 0-2, owing to high accumulation rates, although this also depends on event frequencies. Upper surfaces of individual storm/river-flood beds may show BI values of 4-5, reflecting the transition to longer-lived fair-weather conditions. Fair-weather waves facilitate persistent agitation near the bed, buffering environmental stresses. Therefore, wave-dominated deposits that are not affected by storms yield climax communities with robust and diverse ichnofacies signatures reflecting "uniform and high" BI trend with values that average 4. River-dominated intervals show the least uniform trends of BI, because of the highly variable conditions related to river jet and plume behavior. BI values vary from 0 to 4, with generally low ichnogenera diversities. These alternations likely record seasonal to centennial fluctuations in sedimentation rate (river discharge) and water turbidity, which influences substrate conditions near distributary mouths. Tide-dominated intervals tend to show the most 'stressed' conditions, reflecting "non-uniform and low" trend of BI, with values of 0-2. These reflect salinity fluctuations, heightened water turbidity, rapidly shifting substrates, and narrow colonization windows associated with daily and monthly changes in tidal periodicity. Individual parasequences are characterized by either upward-increasing and upward-decreasing trends of BI, indicating protection from storm erosion and proximity to river input, respectively. Ichnological signatures change significantly across initial flooding surface, principally showing a marked increase in BI as delta lobes are quickly abandoned and transgressed. In contrast, across the maximum flooding surface, changes in ichnological signatures are subtle and rather uniform. We suggest that different parts of a single delta may experience marked differences in river, wave, and tide influence over time, reflecting the enormous complexity of operative processes at various temporal and spatial scales. Detailed intra-parasequence, bed-scale analyses of trace fossils help to reveal this complex evolutionary history of a single delta.
The regressive and subsequent transgressive transit of a shoreline across a clastic shelf generat... more The regressive and subsequent transgressive transit of a shoreline across a clastic shelf generates the standard reservoir flow unit in most marginal to shallow marine hydrocarbon reservoirs. The stratigraphic unit produced by the shoreline transits is commonly referred to as the high-frequency (10 4 to 10 5 years), regressivetransgressive sequence (RT sequence). The unit is usually bounded top and base by marine shales marking flooding surfaces, which can act as barriers to fluid flow. Stratigraphic architecture in the RT-sequence is also known to control the internal flow behavior of many reservoirs. Hence, an ability to consistently characterize reservoirs at a sub RTsequence scale is critical to enabling prediction of reservoir performance and optimization of resource extraction strategies. This study describes the internal architecture of one shallow-water (, 10 m), low-accommodation regressive shoreline succession from the Campanian of the Alberta Basin, Canada, based on an extensive outcrop and subsurface dataset that has been convolved into a 3D geocellular computer model. The architecture and evolution of the ancient mixed-process (waves, tides, and fluvial processes) regressive deltaic shoreline system is compared and contrasted with a partial Holocene analog from northeastern Australia. The same stratigraphic surfaces and units are identified in both the modern and the ancient regressive systems. The key architectural unit is the element complex set (ECS), which is a multi-kilometer-scale, discontinuity-bounded unit that is the product of the reorganization of the coastline, often caused by autogenic backwater-driven avulsions. Multiple avulsions during a regressive shelf transit episode lead to lateral offsets of ECS units in low-accommodation systems. These systems are termed ''avulsion-driven systems.'' An increasing component of vertical stacking of ECS units is observed in higher-accommodation regressive systems. The mechanisms for generating accommodation on a regressive-shoreline shelf-transit time frame may be allogenic (tectonic subsidence or eustatic sea-level change), autogenic, or a combination of the two mechanisms. A key, localized, autogenic mechanism is related to the distance of progradation during the transit of a deposystem across the shelf. In proximal shelf positions, ECS units tend to offset laterally due to limited available accommodation, whilst in more distal positions, early differential, load-induced, compactional subsidence of underlying prodelta and shelf muds can promote vertical stacking of ECS units. The critical down depositional-dip distance from the transgressive turnaround point at which ECS units become preferentially vertically stacked is a function of shelf gradient, shoreline trajectory, sandstone fraction, and prodelta and shelf mud rheology and is termed the ''critical autogenic ECS stacking distance.'' Vertical stacking of ECS units may also occur when ECS units overstep underlying shelf topography, such as the distal termination of an older RT sequence. Recognition criteria and nomenclature for intra-regressive-shoreline surfaces and stratigraphic units, as well as predictive models for the ancient record are detailed across a spectrum of types of deltaic systems.
This study considers the construction and functionality of biogenic structures made by marine, ve... more This study considers the construction and functionality of biogenic structures made by marine, vermiform nemerteans, polychaetes and hemichordates; marine crustaceans; motile bivalves; motile echinoderms; and sponges and sea anemones. We report on a range of modern biogenic structures similar to several known ichnogenera. Vermiform animals dominantly occupy vertical burrows that range from simple through helical shafts to Y-and U-shapes. Horizontal traces made by worms range in form, but are dominated by branching and variably sinuous to meandering burrows. Crustaceans primarily excavate open burrow systems that possess a range of architectures that are similar to either Thalassinoides or Psilonichnus. Smaller crustaceans, such as amphipods, mix the sediment. Bivalve traces vary in form, but generally preserve evidence of vertically oriented filter or interface-deposit feeding from a stationary location, rapid vertical escape, or horizontal grazing. Echinoderms dominantly preserve body impressions and motility traces, such as Asteriacites. An important class of biogenic structure, Scolicia and Bichordites, are made by urchins. Finally, sea anemones can generate large, penetrative, conical biogenic structures. Large, open horizontal networks serve as domiciles and deposit-feeding structures for crustaceans, but with worms similar burrow types are used more for passive carnivory and establishing an interface-feeding network. We report that the trace fossil Gyrolithes potentially represents mechanical ramps for shrimp, but is used as a sediment holdfast when made by worms. Finally, Y-shaped burrows are used for filter feeding in shrimp, and interface-deposit feeding in worms. These examples emphasize that inferences of behavior in the rock record are interpretive.
The Glossifungites ichnofacies is a substrate-controlled assemblage of trace fossils, characteriz... more The Glossifungites ichnofacies is a substrate-controlled assemblage of trace fossils, characterized by sharp-walled, unlined, passively filled, vertical to subvertical domichnia, excavated into semiconsolidated (firmground) substrates. In Cretaceous siliciclastic rocks of the Western Canada sedimentary basin, the assemblage is dominated by the ichnogenera Diplocraterion, Skolithos, Arenicolites, Thalassinoides, and Rhizocorallium. The Glossifungites assemblage demarcates discontinuity surfaces that reflect pauses in sedimentation accompanied
ABSTRACT X-radiographs of sediment box cores acquired from the western Gulf of Mexico reveal limi... more ABSTRACT X-radiographs of sediment box cores acquired from the western Gulf of Mexico reveal limited bioturbation in sediment deposited in bathymetries greater than 35 m. Between 15 and 35 m, sediments are thoroughly bioturbated with averaged bioturbation indices (for all beds in a core) between 2.1 and 5.6, and trace diversities between 2 and 9 distinct burrow forms. Below 35 m water depth, box cores exhibit trace diversities of 1–3, and core-averaged bioturbation indices range between 0.3 and 3.6. There is an overall decrease in trace diversity and bioturbation indices in the offshore direction. Cross-shore ichnological trends are compared to dissolved oxygen contents of bottom waters. Above 35 m, dissolved oxygen (DO) contents show pronounced variability, ranging from 100% DO saturation through to hypoxia (< 2.0 mg l-1), and reflect the periodic introduction of oxygen-depleted waters into otherwise fully oxygen-saturated seawater. Below 35 m, DO content of bottom waters is consistently at 60–75% saturation. DO decreases by an average of 0.117 mg l-1 per one-meter increase in water depth, such that bottom waters in 100 m water depth contain an average of 4.55 mg l-1 DO. The data reveal a direct correlation between: a) the density of infauna and the diversity and density of burrows, and b) DO of bottom water. The degree of bioturbation is significantly reduced in waters that are oxic, but below 80% DO saturation. Based on these observations, we suggest that it is inappropriate to link low bioturbation intensities and diversities to hypoxia (< 2.0 mg l-1), and by extension, anoxia (0 mg l-1). Instead, reduced oxygen contents (4.3 – 5.3 mg l-1) that lie well above hypoxic levels have a dramatic impact on the health of infaunal communities, and this is reflected by severe reductions in the ichnological character of sediments.
Analyses of deltaic facies successions highlight recurring ichnological patterns that reflect a v... more Analyses of deltaic facies successions highlight recurring ichnological patterns that reflect a variety of physicochemical stresses imposed upon infaunal organisms by the interaction of various delta-front processes. Analysis of numerous ancient deltaic deposits in Canada, the United States, Australia, and offshore Norway persistently show reductions in bioturbation intensity and impoverishment in ichnological diversity, compared to those of nondeltaic shorelines. Some facies locally demonstrate sporadic colonization and recolonization of substrates left denuded by episodic depositional conditions. Deltaic ichnological suites also locally display size reductions of ichnogenera and a paucity of suspension-feeding ethologies. Resulting trace-fossil suites are overwhelmingly dominated by depositfeeding behaviors, even in sandy facies. Such ichnological characteristics largely reflect increased sedimentation rates and heightened fluvial discharge, which serve to impede infaunal colonization. River-derived stresses are profound: salinity changes, hypopycnal-flow-induced water turbidity, distributary flood discharges with accompanying phytodetrital (comminuted plant debris) pulses, hyperpycnal-flow-induced sediment gravity flows, and fluid-mud deposition all conspire to produce the overall depauperate nature of the ichnological assemblage. Freshet-discharge events during river floods, accompanied by hyperpycnal conditions, may lead to the episodic introduction of reduced-salinity waters immediately above the sediment-water interface in delta-front and prodelta depositional settings. Such conditions may facilitate development of synaeresis cracks and promote reductions in infaunal populations. Wave energy generally buffers fluvial effects, by dispersing suspended sediment offshore and encouraging the thorough mixing of waters of contrasting salinity. High mud concentrations near the delta front damp wave energy, however, limiting its effectiveness in remediating the benthic ecosystem, particularly immediately following distributary flood discharges and storm events. In wavedominated settings, strong alongshore drift also operates to extend river-derived stresses considerable distances downdrift from distributary mouths. Where asymmetric deltas are formed, markedly different ichnological expressions are expressed on either side of distributary-channel mouths. Updrift settings typically retain classic shoreface assemblages, whereas downdrift environments commonly acquire markedly stressed suites. Storm energy may be effective in dispersing mud and mixing waters, but it also results in erosion and episodic sediment deposition. Concomitant precipitation induces river floods, returning river-derived stresses to the delta front. Tidal energy and its effects on the infaunal communities of deltas are poorly documented. Tidal flux may trap mud plumes against the delta front, elevating water turbidity. Pronounced mud flocculation coupled with increased settling velocity associated with tidal mixing also leads to rapid deposition of thick fluid muds, particularly in low-lying areas, hampering or precluding colonization. Tidal energy also leads to marked changes in energy and salinity near the sediment-water interface at several time scales. Deltaic ichnological suites are characterized by structures of opportunistic trophic generalists, though mainly those of facies-crossing deposit feeders. High water turbidity, particularly near the sea floor, precludes most suspension-feeding behavior, and suppresses the development of the Skolithos ichnofacies, even in many proximal delta-front deposits. Ichnological characteristics record the dynamic interplay and relative importance of these different processes, both temporally and spatially. on delta systems.
Summary Cycle G of the Campanian Belly River Formation comprises two recurring and mappable facie... more Summary Cycle G of the Campanian Belly River Formation comprises two recurring and mappable facies associations (FA1 and FA2), both interpreted to record distal prodelta through proximal delta-front accumulation. FA1 is characterized by uniformly coarsening-upward successions, with abundant wave- and storm-generated sedimentary structures, subordinate syneresis cracks, convolute bedding, and claystone drapes. Trace fossils are sporadically distributed in low- to moderate-
ABSTRACT Tide-dominated and tide-influenced clastic depositional environments are typically inter... more ABSTRACT Tide-dominated and tide-influenced clastic depositional environments are typically interpreted to be associated with landward-stepping or transgressive systems. Here, we present an example of an overall regressive succession dominated by high frequency sequences that exhibit abundant sedimentary and ichnological evidence of tidal processes. These strata comprise the Campanian Bearpaw-Horseshoe Canyon Formation transition, exposed near Drumheller, Alberta, Canada. These clastic marine to marginal-marine sediments were deposited in mixed-process (wave-, tide- and fluvial-influenced) depositional environments along the western margin of a foreland basin. The deposits are subdivided into six, relatively thin (10 m on average), high frequency (~140,000 years), Transgressive-Regressive (T-R) sequences (A to F). The evolution of the successive T-R sequences was mapped on a sub-regional scale. The orientations of the paleoshorelines rotate in a predictable manner during transgression and regression. Transgressive shorelines are oriented approximately S–N and rotate clockwise during regression to approximately SW–NE. The rotations are attributed to the paleogeography of the region, with the study area interpreted to sit on the western flank of a large, structurally controlled embayment. The progradational high-frequency sequence set is dominated by tide-influenced strata and clearly demonstrates that tidal deposits can be preserved in both regressive as well as transgressive successions. Tidal influence in this regressive setting is attributed to shallow water depths and a wide shelf, which resulted in amplification of the tidal wave as it moved across the shelf. The embayed character of the coastline may also have contributed to this effect. Tidal range is estimated to have been meso- to macrotidal. A modern, partial analogue for the Horseshoe Canyon systems is identified from the tropical Mitchell River Delta in the Gulf of Carpentaria, Queensland, Australia. This delta system is analogous in terms of its tide-influenced facies, mesotidal range, low-gradient shelf with accompanying shallow water depths, the mixed-process character of its shoreline systems, and its horizontal to falling shoreline trajectory.
The early to mid-Campanian Basal Belly River Formation in the Ferrybank, Keystone, and eastern Pe... more The early to mid-Campanian Basal Belly River Formation in the Ferrybank, Keystone, and eastern Pembina fields of central Alberta, reflects a mixed wave-and river-influenced deltaic succession with strong storm overprinting. Prodelta deposits consist of mud-dominated heterolithic successions, characterized by a low abundance yet moderately diverse trace fossil assemblage attributable to a "stressed" expression of the Cruziana Ichnofacies. Proximal prodelta to distal delta front intervals comprise interbedded sandy siltstones and very fine-to fine-grained sandstones exhibiting convolute bedding and sporadic bioturbation. Trace fossil assemblages range from very low to moderate abundance, and low to moderate diversity; the suites are attributable to a "stressed" expression of the Cruziana Ichnofacies. Distal delta front deposits coarsen upwards into fine-to medium-grained sandstones of the proximal delta front. High-energy conditions, coupled with strong storm influences, resulted in erosional amalgamation of tempestites, and led to sporadic distribution of ichnogenera. Proximal delta front intervals are weakly bioturbated, and trace fossil assemblages are characterized by low abundances and low to moderate diversity. Most forms within the sandstone facies represent the structures of deposit-feeding organisms. Suites reflect a "stressed" infaunal community and contain a mixture of elements characteristic of both the Skolithos and Cruziana ichnofacies. Analysis of more than fifty cored wells has revealed prodelta and delta front deposits that vary markedly along depositional strike. The along-strike variations fit well with the recently proposed asymmetric delta model. The model is based on observations of modern wave-influenced deltas such as the Danube. This study provides an ancient analogue. Continued research will seek to further delineate delta lobe asymmetry and concomitant along-strike facies variations, both attributable to longshore drift and deflection of river-induced stresses downdrift of distributary channel mouths. Organisms are exceedingly sensitive to fluvial influence and this "stress" is reflected in the relatively low diversity and low abundance assemblages that characterize many deltaic successions. Ethological preferences and burrow sizes further reflect the level of "stress" imparted on infaunal organisms within the subaqueous delta environment. Trace fossil suites of river influenced deltaic successions signify a departure from the archetypal ichnofacies characteristic to shoreface successions, and their mapped distributions may serve as a predictive tool for determining distributary channel proximity.
Preliminary ichnological-sedimentological study from 51 cored wells of the Lower Cretaceous (Earl... more Preliminary ichnological-sedimentological study from 51 cored wells of the Lower Cretaceous (Early to mid-Albian) Mannville Group (Sparky, Waseca, and McLaren formations) in west-central Saskatchewan and west-central Alberta yield ten recurring facies that reflect conditions of reduced and fluctuating salinity, typical of brackish-water settings. The successions display physical sedimentological features dominated by abundant organic-rich mudstone drapes of probable fluid-mud origin, syneresis cracks, soft-sediment deformation, carbonaceous detritus, normally graded mudstones, and wave, current and combined-flow ripples. Facies are generally heterolithic, attesting to autogenic fluctuations in depositional processes. Ichnologically, bioturbation intensities range widely (BI 0-5), and facies are characterized by abundant diminutive ichnogenera comprising low-diversity (locally monogeneric) trace fossil suites. Common ichnogenera include Gyrolithes, Teichichnus, Planolites, Palaeophyc...
The Lower Cretaceous (early to mid-Albian) Mannville Group (Sparky, Waseca, and McLaren formation... more The Lower Cretaceous (early to mid-Albian) Mannville Group (Sparky, Waseca, and McLaren formations) in west-central Saskatchewan is up to 60m thick, and consists of weakly consolidated sandstones, shales, mixed sands and shales forming heterolithic bedding, and minor coals. These units were deposited in shallow marine to coastal-plain environments. The study area is located in west-central Saskatchewan (Lloydminster area) from Townships 48 -53, and Ranges 19W2 to 28W3, covering a total area of 5,400 km² (Fig. 1). Figure 1: The study area is located in West-Central Saskatchewan, Canada, shown in the above figure. The figure illustrates the core distributions employed in this study.
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Papers by J. Maceachern