REVISED REGIONAL FRASNIAN SEQUENCE STRATIGRAPHIC FRAMEWORK, ALBERTA OUTCROP AND SUBSURFACE
P.K. Wong, John A.W. Weissenberger, Murray G. Gilhooly, 2017. "REVISED REGIONAL FRASNIAN SEQUENCE STRATIGRAPHIC FRAMEWORK, ALBERTA OUTCROP AND SUBSURFACE", NEWADVANCES IN DEVONIAN CARBONATES: OUTCROP ANALOGS, RESERVOIRS AND CHRONOSTRATIGRAPHY, Ted E. Playton, Charles Kerans, John A.W. Weissenberger
Download citation file:
Carbonate strata were widely deposited in the Alberta Basin during the Frasnian. These are well exposed in the Alberta Rocky Mountains and regionally extensive in the adjacent subsurface. This study places many of its classic outcrops from the Cascade (Burnt Timber) Channel to the South Jasper Basin into a single sequence stratigraphic framework for the first time. This framework is correlated from outcrop to subsurface using sequence stratigraphic and biostratigraphic data. Improved confidence in the stratigraphic interpretation is based on new measured sections tied to photographic panoramas, combined with detailed mapping of lithofacies and stratal patterns of continuously exposed platform to basin transitions in outcrop. These data are correlated with new and revised core and well-log interpretations from the Alberta subsurface.
Ten third-order composite sequences and their constituent high-frequency (fourth-order) sequences span the uppermost Givetian through Frasnian strata of the Alberta Basin. They reflect stratigraphic architecture typical of a (second-order) depositional sequence: transgression followed by regression, or basin opening and filling. The eight youngest composite sequences are defined from the Cline Channel and Jasper Basin areas using stratal and facies stacking patterns and regional correlation of sequence boundaries and maximum flooding surfaces, integrated with conodont biostratigraphy. Most sequence boundaries observed are subaerial exposure surfaces, seen in outcrop or inferred from onlap of tidal-flat or reef margin deposits onto foreslope facies.
The basin was filled asymmetrically by mixed carbonate–clay successions that form dominant east to west prograding strata. Two main types of sediment comprise the basin fill: extrabasinal clay and intrabasinal carbonate. Composite sequences (CSs) and high-frequency sequences (HFSs) can be confidently correlated from outcrop to subsurface. A combination of well-log and outcrop cross sections, integrated with biostratigraphy, support these correlations. These regional (time) surfaces allow better understanding of basin evolution and architecture.
The influence of the second-order sequence dominates the accommodation setting and is expressed in the architecture of composite and high-frequency sequences. For example, the tripartite character (lowstand–transgressive–highstand) of CSs in the lower and middle part of the sequence is followed by the appearance of a distinct falling stage component in the upper part of the Frasnian. An increased frequency of truncation surfaces and offlapping strata is consistent with diminishing accommodation. With progressive basin infill and shallowing paleobathymetry, foreslope declivity decreased from a minimum of 10° to less than 1.5° as the depositional system became more ramp-like. This is accompanied by a change of lowstand geometry from wedge to tabular shaped.
Deposition of coarser terrigenous clastics was also limited in most of the basin to the lower part of the second-order sequence, except at CS and HFS. Restricted marine circulation onto the carbonate platforms and basin filling in the upper part of the Frasnian coincided with extensive siliciclastic silt deposition in the study area, particularly in the Jasper Basin, where an influx of terrigenous silt formed mixed carbonate–siliciclastic deposits. Silt was deposited during third- and fourth-order lowstands, bypassed into the basin, and reworked during intermittent inundation of the carbonate platforms.
Beyond the basic transgressive–regressive architecture of the second-order (Givetian–) Frasnian sequence, we document detailed observations such as (1) controls affecting the onset, cessation, and extent of euxinic shale deposition in the mid-Frasnian and its relation to the second-order maximum flooding surface; (2) the relative speed and distribution of illitic basin fill within the second-order highstand; (3) the effect of basin fill and off-platform sediment transport on regional and local carbonate platform architecture, such as the configuration of in situ carbonate lowstands, initiation of reefs along favorable fairways, and overall margin stacking patterns; and (4) the magnitude of relative sea-level falls associated with the development of sequence boundaries. A comparison to previously established Frasnian sequence stratigraphic schemes within the basin is extended to other basins in Europe and Australia.
Figures & Tables
NEWADVANCES IN DEVONIAN CARBONATES: OUTCROP ANALOGS, RESERVOIRS AND CHRONOSTRATIGRAPHY
The Devonian stratigraphic record contains a wealth of information that highlights the response of carbonate platforms to both global-scale and local phenomena that drive carbonate architecture and productivity. Signals embedded particularly in the Middle-Upper Devonian carbonate record related to biotic crises and stressed oceanic conditions, long-term accommodation trends, and peak greenhouse to transitional climatic changes are observed in multiple localities around the world and temporally constrained by biostratigraphy, highlighting distinct and impactful global controls. Devonian datasets also stress the importance of local or regional phenomena, such as bolide impacts, the effects of terrestrial input and paleogeography, syn-depositional tectonics, and high-frequency accommodation drivers, which add complexity to the carbonate stratigraphic record when superimposed on global trends. The unique occurrence of well-studied and pristinely preserved reefal carbonate outcrop and subsurface datasets, ranging across the globe from Australia to Canada, allows for a detailed examination of Devonian carbonate systems from a global perspective and the opportunity to develop well-constrained predictive relationships and conceptual models. Advances in the understanding of the Devonian carbonate system is advantageous considering, not only the classic conventional reservoirs such as the pinnacle reefs of the Alberta Basin, but also emerging conventional reservoirs in Eurasia, and many unconventional plays in North America. The papers in this volume provide updated stratigraphic frameworks for classic Devonian datasets using integrated correlation approaches; new or synthesized frameworks for less studied basins, reservoirs, or areas; and discussions on the complex interplay of extrinsic and intrinsic controls that drive carbonate architectures, productivity, and distribution. The 13 papers in this special publication include outcrop and subsurface studies of Middle to Upper Devonian carbonates of western Canada, the Lennard Shelf of the Canning Basin, Western Australia, and the western USA.