Ichnology of an Upper Devonian - Lower Mississippian low-energy seaway: the Bakken Formation of subsurface Saskatchewan, Canada: Assessing paleoenvironmental controls and biotic responses (original) (raw)
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Palaeogeography, Palaeoclimatology, Palaeoecology, 2012
In spite of the high interest on the Late Devonian-Early Carboniferous Bakken Formation as one of the most important oil plays in North America and the numerous studies carried out in this unit, no ichnologic analysis has been presented yet. Based on conventional sedimentologic data, previous studies of the Bakken interpreted the unit as formed entirely under open-marine conditions. However, integration of ichnology has been key to the identification of not only open-marine but also brackish-water marginal-marine conditions. Based on the geometry of the sedimentary bodies and the sequence-stratigraphic framework, the brackishwater interval is interpreted as an embayment with limited or intermittent connection to the open sea. Salinity, oxygen content, and storms are regarded as the most important environmental parameters that controlled the distribution and nature of the trace fossils in the Bakken Formation. Sparse bioturbation, relatively low ichnodiversity, and the "impoverished" Cruziana ichnofacies characterize the marginalmarine deposits, suggesting stressful brackish-water conditions. Marine deposits from the middle member, in contrast, are highly bioturbated, with a moderate ichnodiversity and the "distal" Cruziana ichnofacies that flourished during fully marine well-oxygenated conditions. There is only a slight difference between the total ichnodiversity of the open-marine interval (ten ichnogenera) and that of the brackish-water interval (seven ichnogenera). Ichnodiversity should not be considered at face value and isolated from other aspects when evaluating stress levels in order to detect a brackish-water signal. Lack of bioturbation in the black shale of the lower and upper members resulted from anoxic conditions. The passage from anoxic (lower member) to well-oxygenated conditions (middle member) was gradational, as is revealed by the occurrence of oxygen-deficient trace-fossils at the top of the lower member. Finally, the contrasting styles in tempestite preservation in upper-offshore deposits from the lower and upper open-marine intervals is attributed to variations in the intensity and frequency of storm during deposition of the highstand and transgressive deposits.
The Upper Devonian-Lower Mississippian Bakken Formation hosts one of the most important oil reservoirs in Saskatchewan. Integration of ichnological and sedimentological data indicates that deposition of Bakken strata occurred in two different paleoenvironmental settings: open-marine and brackish-water marginal marine. This paper focuses on the open-marine deposits in the Lower Member, the basal and the upper part of the Middle Member, and the Upper Member. The open-marine deposits embrace shelf, lower and upper offshore, offshore transition, and a transgressive lag facies. The large extension and continuity of the open-marine sedimentary facies point towards deposition in a low-gradient system in a shallow epeiric sea. With the exception of the black shale from the Lower and the Upper Member, all these deposits are characterized by a high bioturbation index and a "distal" Cruziana ichnofacies with Phycosiphon incertum and Nereites missouriensis as dominant elements, and Asterosoma isp., Planolites montanus, and Teichichnus rectus as subordinate elements.
Ichnofabrics and ichnofabric-forming trace fossils in Phanerozoic turbidites
Bulletin of Canadian Petroleum Geology, 2011
Ichnology is commonly used in the palaeoenvironmental analysis and hydrocarbon reservoir characterization of bioturbated shallow marine facies, but is more rarely applied to deep marine and turbiditic successions. This work identifies the key trace fossils that are likely to be of use in ichnological analysis of drill cores, and in the application of ichnological data to the investigation of deep marine turbidite plays. The available data on trace fossil occurrences in turbidites have been analyzed for the whole of the Phanerozoic to identify the principal components of turbidite ichnofabrics at different times. It is found that a limited number of ichnofabric-forming ichnotaxa occur frequently in turbidite-bearing formations. In order of decreasing frequency of occurrence, the key ichnotaxa are Chondrites, Planolites, Scolicia, Zoophycos, Ophiomorpha, Phycosiphon, Thalassinoides and Nereites. Surprisingly, despite well-documented changes in deep marine benthic ecology through the Phanerozoic, ichnotaxonomic components of ichnofabrics show little behavioural change or evolution of new bioturbation styles. A number of ichnotaxa do show changes in their relative abundance through the Phanerozoic, possibly indicating ecological competition or niche replacement (e.g. Early Palaeozoic Dictyodora replaced by Mesozoic and Cenozoic Zoophycos). Ichnofabric-forming trace fossils need to be fully understood before they can be confidently applied to reservoir characterization of turbidite facies, but this study identifies the most important ichnotaxa in such settings and provides a framework for future research.
2004
Estuarine settings are characterised by numerous physical and chemical stresses that can strongly influence the behaviour of burrowing organisms. Although lowered salinity and fluctuating salinity levels normally represent the chief stresses recognised in bays and estuaries, high sedimentation rates, high current energy, turbidity, and low levels of oxygen in bottom and interstitial waters are known to be significant factors that strongly influence the resultant ichnofossil assemblages. This study builds on earlier research and suggests that the effects of each of these parameters can be observed in the rock record through trace fossil analysis. The subsurface lower Albian Bluesky Formation in the Cadotte region of Alberta has been interpreted to represent an estuarine deposit. Examination of the trace fossil assemblages from various facies therein suggests that physicochemical stresses were variable across the ancient estuary and mainly constituted the following: (1) low salinity and fluctuating salinity levels, which are interpreted to have contributed to patterns of low ichnofossil diversity and burrow dimunition proximal to the fluvial point source(s) in the upper and central parts of the depositional system; (2) high sedimentation rates and current energy, evidenced ichnologically by sporadic, penetrative bioturbation and the rare preservation of opportunistic sediment stirring, are most significant in the vicinity of the tidal inlet in the lower estuary and in the bayhead delta of the upper estuary; (3) high turbidity associated with the turbidity maximum in tidal channels of the central reaches of the estuary probably inhibited suspension-feeding behaviours locally; and, (4) low levels of dissolved oxygen in quiescent-water embayments and lagoons, often represented in brackish-water deposits as a Trichichnus, Palaeophycus, and Diplocraterion assemblage.
Ichnofabrics in the sediments drilled at Site 605 reflect environmental changes. Long-term changes allow the section to be differentiated into four units, and short-term fluctuations define cycles in the range of several decimeters. The composition of the ichnofabrics is controlled by the availability of nutrients within the sediment (reflecting productivity in the surface waters and the sedimentation rate) and by oxygen content within the respiration water (related to deepwater circulation patterns as well as to the organic carbon content of the sediment).