ABSTRACT It is widely accepted that most occurrences of inclined heterolithic stratification (IHS) in the rock record form by laterally accreting point bars in freshwater fluvial, tidally influenced fluvial, or tidally dominated estuary channels. Despite the widespread distribution of IHS in the subsurface and outcropping strata of the lower Cretaceous McMurray Formation, the large-scale depositional architecture and lateral facies variability of these deposits remains controversial. The relatively limited lateral extent of many of the outcrops is a challenge, particularly when point-bar deposits on the scale of hundreds of meters to kilometers are interpreted in outcrops spanning anywhere from 100 to 300 meters laterally. This has in turn led researchers to leverage other datasets such as 3-D seismic to analyze the large-scale depositional architecture of the IHS, leading to two main interpretations for the IHS in the McMurray Formation: 1) a fluvially dominated environment owing to geomorphological features comparable to those in large modern fluvial systems, or 2) an estuarine environment owing to the presence of trace fossils characteristic of marine-derived faunal colonization in brackish-water settings and strong evidence of tidal modulation. The purpose of this study is to investigate the sedimentology and depositional architecture of IHS in a unique, kilometer-wide outcrop exposure of McMurray Formation strata and compare it to IHS observed at other McMurray Formation outcrops previously interpreted as estuarine channels. This is achieved by combining traditional field-based methods with Unmanned Aerial Vehicle-based outcrop modeling to create a 3-D outcrop model to visualize and analyze large point-bar geobodies deposited in a channel upwards of 25 meters deep and 750 meters wide exposed in outcrop at Crooked Rapids of the Athabasca River, west of the City of Fort McMurray. Importantly, this methodology uses bed orientation trends, paleocurrent data, and sedimentological observations to identify and map architectural elements, which constitute an eastward-accreting point bar crosscut by a southwestward-accreting counter point bar in the outcrop. The results strongly suggest that the IHS at Crooked Rapids was deposited in a freshwater fluvial environment. When compared to IHS deposited in estuarine depositional environments, fluvial IHS is driven by seasonal river discharge as opposed to the interplay between river discharge and the extent of the tidal prism. Therefore, fluvial IHS is: 1) dominantly sandstone with very minor waning-flow siltstone interbeds resulting from erosion by the succeeding freshet phase, and 2) completely devoid of bioturbation until flat-lying bar top or overbank strata is encountered. Using 3-D outcrop modeling to supplement sedimentological and ichnological observations strengthens the interpretation of complex fluvial geobodies and increases the overall understanding of the large-scale depositional architecture of point bars across the tidal–fluvial transition zone.

One of the most important contributions that S. George Pemberton made to the field of ichnology was the identification that burrowed firmgrounds associated with the Glossifungites Ichnofacies, commonly demarcate important sequence stratigraphic allogenic surfaces, and more recently described autogenic surfaces. This study considers an outcrop example from the Turonian Ferron Sandstone of central Utah, wherein high and low abundance monospecific suites of Glossifungites isp. are preserved landward of marginal-marine settings, recording colonization in channels under low salinity conditions. High abundances of Glossifungites isp. are associated with sloped areas of the erosional surfaces due to environmental preference, and with clay-rich underlying lithologies owing to either substrate selection or toponomy. These suites of the Glossifungites Ichnofacies demarcate surfaces at the bases of small, stacked channels encased in coastal plain strata. Stacking suggests repeated colonization related to an autogenic process. The enclosing strata and estimated position of the shoreline indicate a more landward affinity than previously reported for the ichnogenus Glossifungites, which is normally related to erosional nearshore processes or, less commonly, to offshore submarine channel development. Glossifungites-like burrows are constructed in modern freshwater settings by subaqueous insects, such as mayfly nymphs, but the trace fossil widths reported from the Ferron Sandstone are up to one and a half times larger than these modern examples. This suggests that the described trace fossils were made by marine-recruited, brackish-water crustaceans that created similar-sized burrows, or if constructed by subaqueous insects, the tracemakers were divergent in size or body plan from known modern tracemakers.

Overpressure can be widely observed in the Jurassic, Cretaceous and Paleogene reservoirs in the southern margin of the Junggar Basin, northwest China. The study region was subjected to intense tectonic stress from the neighbouring North Tianshan Mountains since the Miocene. However, the causes of tectonic stress-induced overpressure are still poorly understood. In this study, we considered the plots of sonic velocity against density and vertical effective stress and applied numerical simulations and geological analysis to determine the origin of overpressure with respect to mudstone and sandstone in the area under study by investigating the compaction of mudstone and analysing the overpressure distribution. Our results indicate that the most overpressured mudstone in the second and third structural belts and the east of the Sikeshu Sag exhibits abnormally high porosity, which is obvious from the regional seal mudstone of the Paleogene Anjihaihe and Lower Cretaceous formations. The overpressure in these mudstones can be mainly attributed to the increase in vertical stress and horizontal tectonic stress (HTS). The current overpressures in the reservoir sandstone of the second and third structural belts as well as towards the east of the Sikeshu Sag are mainly induced by HTS, overpressure transfer and increase in vertical stress. The contribution of the overpressure associated with the generation of HTS to the total overpressure differs in different belts. This contribution ranges from 28.5% to 69.2%, 21.2%–46.2% and 16.3%–34.8% in the second, the third structural belts and the east of the Sikeshu Sag, respectively. Thus, HTS can be the primary contributor to the reservoir overpressure in many zones in the area under study.

A proximal distributary channel deposit in the Turonian Ferron Sandstone of central Utah contains a trace-fossil assemblage comprised of turtle tracks (Chelonipus), vertebrate swim trace fossils (Characichnos), repichnial traces of mollusks (Archaeonassa), Rhizocorallium, and Teredolites. Mayflies are abundant producers of horizontal, spreiten, U-shaped burrows in modern freshwater channels, but rarely ascribed as producers of Rhizocorallium in the rock record. The in situ trace fossils studied are interpreted as a freshwater assemblage. Teredolites occurs within allochthonous debris reworked from stratigraphically lower brackish-water deposits. This Ferron Sandstone assemblage provides an example of the juxtaposition of freshwater ichnocoenose with exhumed brackish-water clasts. Caution must be exercised in environmental interpretation when taking into consideration both adjacent floodplain and the sedimentary structures surrounding the clasts. Complex trace-fossil relationships such as these may play a role in the scarcity of Rhizocorallium reports from freshwater deposits in the rock record, and may be more common than presently recognized. This study illustrates the importance of addressing the detailed ichnology above and below the base of channel deposits in progradational environments (e.g., delta plain), where reworked, brackish-water deposits (and bored clasts) may be entrained within overlying lower salinity facies.

Spectacular examples of well-preserved, stacked Rosselia occur in cores of the Lower Cretaceous Bluesky Formation from Alberta's Peace River oil-sands deposit. Stacked Rosselia segments reflect burrow readjustments of single tracemakers following erosion and sedimentation events. Where present, these traces help to refine paleoenvironmental interpretations and can be used as proxies for assessing the magnitude and frequency of depositional events. In this study, six cores were logged in detail. Two representative cores containing densely packed (crowded), stacked Rosselia were described sedimentologically and ichnologically and burrow lengths and numbers were measured. The Bluesky sections are interpreted to represent storm-influenced, wave-dominated delta-front deposits on the basis of modal sedimentation measurements and the presence of storm- and wave-generated sedimentary structures. The stacked Rosselia burrows record up to four post-depositional re-establishments per tracemaker, with each readjustment representing a response to abrupt sediment accumulation. Decimeter-length burrows occurring in the core datasets suggest multiple decimeter-scale depositional events in a relatively short time frame, i.e., months to perhaps a few years, depending on the lifespan and growth rate of the organism. The use of Rosselia in this study illustrates a method of evaluating sedimentation events within a brief temporal window, one lying beyond the resolution of more traditional dating methods. Furthermore, the assemblages of crowded, stacked Rosselia are presented as the first documented occurrence from the Mesozoic and the first recognized in a cored succession.

The late Aptian to early Albian Bluesky Formation of Alberta is characterized by complex vertical and lateral associations of siliciclastic sediments deposited during overall transgression of the Boreal sea. As the Bluesky Formation is host to substantial subsurface bitumen deposits of the Peace River oil sands, a refined understanding of vertical and lateral facies distributions is essential for exploration and exploitation activity. To aid in achieving this goal, high-resolution core logging was completed on a 40 core dataset within Ranges 16-17W5M, and Townships 82-84 (approximately 215 km(2)). We identify 16 distinct facies comprising 5 facies associations (FA1-FA5). These facies associations include: 1) FA1 - Wave-dominated, fluvially influenced embayment delta; 2) FA2 - Fluvially-dominated, tidally-influenced distributary channel; 3) FA3 - Fluvially and tidally influenced delta; 4) FA4 - Marine-embayment shoreface to offshore; and, 5) FA5 - Mixed energy estuary. The evolution of these facies associations suggest periodic progradation within an overall back-stepping, transgressive, marine-embayment system. Through combination of sedimentary and ichnological characteristics, this research has led to the identification of wave-influenced deltaic and marine-embayment sedimentary environments previously un-documented within the Peace River oil sands. Additionally, proximal-distal depositional trends obscured by the complex facies distributions were identifiable using ichnological criteria. It is intended that the sedimentological and ichnological characteristics identified herein will aid in the recognition of similar embayment-type settings in other ancient datasets.

Process ichnology emphasizes the use of trace fossils as proxies for sedimentary processes and conditions. The advantage of this method is that depositional stresses can be identified based on several process-ichnological parameters. The purpose of this paper is to demonstrate the use of process-ichnology data, with a focus of establishing how process ichnology metrics can be visualized with geomodeling to aid spatial interpretation.For this study, process ichnology metrics (including bioturbation index and size diversity index), which is the product of interval ichnogenera diversity and interval maximum burrow diameter, are presented from a core dataset of the Cretaceous McMurray Formation. These data are modeled using standard geostastistical techniques for effective visualization of spatial trends. The modeled ichnology data are compared to sedimentary facies in order to interpret the dominant stresses occurring at the time of infaunal colonization.Several interpretations are made from the process ichnology model. The size diversity index and bioturbation index values from inclined heterolithic stratification show strong spatial variability related to variable depositional conditions across and along inclined heterolithic stratification bar forms. Facies interpreted to represent tidal flat deposition are distinguishable on the basis of relatively high bioturbation index values coupled with intermediate to low size diversity index values. Overall, we interpret variability in salinity and sedimentation rates to be the dominant infaunal stresses in the studied stratigraphic interval.

Abstract The process ichnological methodology was applied to a core dataset from the late Aptian to early Albian Bluesky Formation to identify the ichnological characteristics of ancient, marginal marine environments. This methodology has proven effective in recognizing the response of trace making organisms to various physico-chemical stresses in modern environments, but its application to ancient deposits is less established. Several previous studies of the Bluesky Formation have identified a wide range of depositional affinities; however few of these have focused on the detailed combination of ichnologic criteria outlined within the process ichnological framework. In order to assess the effectiveness of the process ichnological framework to the rock record, high resolution, systematic ichnological characteristics were recorded and combined with sedimentologic data from nine wells containing core from within the Bluesky Formation. These characteristics led to the identification of several inferred physico-chemical stresses within the dataset showing an overall evolution from high energy brackish water deposition to a low energy, marine setting. This study contributes to the well-established brackish-water ichnological model and, in addition, helps establish the utility of the process ichnological methodology in the recognition of physico-chemical stresses in ancient environments.