NEWADVANCES IN DEVONIAN CARBONATES: OUTCROP ANALOGS, RESERVOIRS, AND CHRONOSTRATIGRAPHY—INTRODUCTION
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T.E. Playton, J. Weissenberger, C. Kerans, 2017. "NEWADVANCES IN DEVONIAN CARBONATES: OUTCROP ANALOGS, RESERVOIRS, AND CHRONOSTRATIGRAPHY—INTRODUCTION", NEWADVANCES IN DEVONIAN CARBONATES: OUTCROP ANALOGS, RESERVOIRS AND CHRONOSTRATIGRAPHY, Ted E. Playton, Charles Kerans, John A.W. Weissenberger
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INTRODUCTION AND OVERVIEW OF THE VOLUME
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. Data sets around the world exhibit repeatable signals embedded in the Middle–Upper Devonian carbonate record that are related to biotic crises and stressed oceanic conditions, long-term accommodation trends, and peak greenhouse to transitional climatic changes, and these signals are well constrained by biostratigraphy (e.g., Joachimski et al. 2002, Markello et al. 2008). Devonian data sets also stress the importance of local or regional phenomena, such as bolide impacts, the effects of terrestrial sediment input and paleogeography, syndepositional tectonics, and high-frequency accommodation drivers (e.g., Stoakes 1980, Warme and Sandberg 1995, Potma et al. 2001, George et al. 2009). These events and controls 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 data sets, ranging across the globe from Australia to Canada (Fig. 1; e.g., Whalen et al. 2000, Potma et al. 2001, Playford et al. 2009), 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. The studies in this volume have validated the use of paleomagnetics, stable isotope geochemistry, and elemental chemostratigraphy, in conjunction with more traditional biostratigraphy and sequence stratigraphic concepts, as tools with which to construct integrated chronostratigraphic frameworks with both academic and applied utility. The generation of such frameworks not only enables unprecedented interpretation and correlation capability within a single outcrop or reservoir data set, but it also provides reference points that facilitate the comparison of age-equivalent carbonate platforms worldwide. Advances in the understanding of the Devonian carbonate system are relevant for mature conventional reservoirs such as the pinnacle reefs of the Alberta Basin (e.g., Wendte and Stoakes 1982), emerging conventional plays and reservoirs in Eurasia (e.g., Timan–Pechora region; Klimenko et al. 2015, Spina et al. 2015), and unconventional resources in North America (e.g., Bakken–Three Forks Formations [Angulo and Buatois 2012], and the Marcellus Formation [Lash and Engelder 2011]).
The motivation for this volume began with the SEPM (Society for Sedimentary Geology) Research Symposium—an all-day session of oral presentations—held at the American Association of Petroleum Geologists (AAPG) Annual Meeting in Houston, Texas (USA), in April of 2014. Decades earlier, two international symposia were hosted on the Devonian System by one of SEPM’s sister societies, the Canadian Society of Petroleum Geologists (CSPG), the first in 1967 and the second 20 years later. Associated volumes were published (Oswald 1967, McMillan et al. 1987) that comprehensively documented all aspects of the Devonian and a summation of Devonian knowledge up to the late 20th century. The goal of the 2014 symposium was to focus yet again on the Devonian more than two decades later, with particular focus on carbonates, and showcase a suite of recent studies around the world that progress our understanding of these complex systems. The papers in this volume, which stem from the presentations made at the Houston meeting, select solicited submissions, and outside submissions, provide updated stratigraphic frameworks for classic Devonian data sets 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 (Fig. 2). The 13 papers in this special publication include outcrop and subsurface studies of Middle to Upper Devonian carbonates, and they are grouped into sections on (1) outcrops and subsurface of western Canada, (2) Lennard Shelf outcrops, Canning Basin, Western Australia, and (3) outcrops of the western USA. This overview begins with a brief description of each of the contributions, followed by summary sections on Devonian carbonate chronostratigraphy, environmental setting, and hydrocarbon reservoir development, and it concludes with commentary on the advances herein, approaches, and remaining gaps.
STUDIES—OUTCROPS AND SUBSURFACE OF WESTERN CANADA
The Frasnian section of the Western Canadian Sedimentary Basin (WCSB) has long been viewed as a global benchmark for comparison of stratigraphic successions and architecture. Pak K. Wong, John A.W. Weissenberger, and Murray G. Gilhooly deliver a comprehensive synthesis on the Upper Devonian stratigraphic framework of western Canada that is a reference template for future outcrop or subsurface studies in the Alberta Basin in their paper “Revised regional Frasnian sequence stratigraphic framework, Alberta outcrop and subsurface.” The paper integrates a wealth of new regional outcrop data from key and classic exposures in the Alberta Rocky Mountains with an existing subsurface framework (e.g., Potma et al. 2001) to generate a single sequence stratigraphic hierarchy that stretches across the Alberta Basin. They define 10 composite sequences and internal high-frequency sequences using sequence stratigraphic criteria and biostratigraphic constraints, and they document carbonate development and architectures within a long-term transgressive–regressive supersequence. Furthermore, they develop methods for correlating subsurface units by calibrating outcrop expressions with well log signatures, not only enabling them to expand their area of investigation into reservoir data sets, but also establishing useful techniques for reservoir characterization and correlation in the basin. The outcrop-to-subsurface stratigraphic framework allows detailed examination of the evolution of the carbonate system, with thoughtful discussions on: (1) sequence architectures across the supersequence, with special emphasis on the timing, development, and geometries of lowstand and falling stage systems tracts, and associated reconstructed relative sea-level changes; (2) the role of asymmetric basin infill and paleogeography on carbonate platform and margin nucleation, growth, and demise; (3) the development of euxinic shallow basinal conditions that lead to source rock deposition and placement within the hierarchical sequence context; and (4) a broad comparison with previous correlation frameworks of the Alberta Basin outcrops and subsurface, as well as other Devonian data sets around the world.
John A.W. Weissenberger, Pak K. Wong, and Murray G. Gilhooly present detailed outcrop observations and insightful interpretations from the South Jasper Basin, a significant area of Upper Devonian carbonate deposition in the Alberta Basin, western Canada, in “Stratigraphic architecture of the Frasnian South Jasper Basin, north-central Alberta Front Ranges.” Their outcrop localities are classic exposures that have been extensively studied previously, but the authors expand the area’s understanding with additional measured stratigraphic and mapping data, and placement into the regional sequence stratigraphic framework of Wong et al. (2016a). The hierarchical organization of Frasnian composite and high-frequency sequences within the long-term, transgressive–regressive supersequence stacking is a key focus of the study. The authors put forward insightful discussions on relative sea level and external basin infill controls on sequence architectures across the Jasper Basin, and the deposition of organic-rich mudrocks reflective of euxinic basinal conditions that coincide with the supersequence maximum flooding surface. A feature of particular interest is the documentation of higher-frequency deviations from the expected long-term super-sequence architecture trends, the most intriguing of which are shallow carbonate wedges that are perched below former shelf margins and represent lowstand or falling stage deposition. These carbonate geometries are not only rarely documented in steeper reefal margins, but they are observed in both the supersequence transgressive and highstand systems tracts, indicating a highly complex, composite relative sea-level history. Furthermore, the exquisite exposures allow for confident reconstruction of the relative sea-level fall magnitudes and a detailed examination of lowstand or falling stage architecture during different low-frequency accommodation settings. Last, the onset of substantial Upper Frasnian siliciclastic silt deposition on platform tops and in the basin is attributed to accommodation-limited conditions interpreted as the low-frequency falling stage system tract, which affected carbonate deposition largely through the degree of basin fill. In summary, the Jasper Basin work presented here not only represents a critical data set in the generation of the regional Alberta Basin framework (Wong et al. 2016a), but it also progresses our understanding of complex sea-level history in carbonates and resulting lowstand or falling stage architectures in particular—lessons that are useful for improved subsurface correlation and characterization.
In the third of a set of works focusing on the regional stratigraphy of the Alberta Basin, “Sequence stratigraphic architecture of the Frasnian Cline Channel, central Alberta Front Ranges,” by Pak K. Wong, John A.W. Weissenberger, and Murray G. Gilhooly, further demonstrates the frequency, hierarchy, and variability of Frasnian carbonate sequence development, as well as the complex response of both ramp and rimmed systems to influx of fine siliciclastics coupled with eustatic forcing. The authors use remarkable outcrops along Cripple Creek, the Cline Channel proper, and Wapiabi Gap in the Alberta Rocky Mountains, key areas that feed into the regional framework of Wong et al. (2016a), to demonstrate the hierarchical framework of sequences and cycles. They interpret seismic-scale dip-oriented outcrop panels to document architectural relationships and illustrate the character of important exposure surfaces and other contacts with additional field data. Correlation between the different subareas highlights the regional variability in platform-margin style across time-equivalent sequences. Within this framework, the authors point out the geometries of lowstand complexes and associated magnitudes of relative sea-level fall, the timing of source rock deposition, the evolution of foreslope declivity and timing of basin fill, and the nature of platform to basin relief and its influence on foreslope grainstone accumulations. Overall, this paper provides key data for demonstrating characteristics of Upper Devonian platform sequence stratigraphy, along with the Jasper Basin outcrop data in Weissenberger et al. (2016), and, in particular, illustrates the complex margin stacking patterns and lowstand/forced regressive depositional elements within this greenhouse system.
The high-resolution sequence stratigraphic analysis of the Carson Creek North field (CCNF) and relationship to the Eastern Shelf of the WCSB in “Paleotopography on the Intra-Swan Hills Formation unconformity in an isolated platform, Carson Creek North field (Upper Devonian, Frasnian), and implications for regional stratigraphic correlation in the Beaverhill Lake Group, southern Alberta, Canada: The case of the missing regression,” by Joel F. Collins, adds subsurface control to the regional Frasnian framework proposed by new studies in this volume (Weissenberger et al. 2016; Wong et al. 2016a, 2016b). This contribution uses a well-documented suite of wireline logs and 40 cored wells to document the sequence framework and stratigraphic evolution of isolated carbonate platforms both within the CCNF and across the Waterways subbasin onto the Eastern Shelf of the WCSB. A detailed depositional facies analysis and well-documented high-resolution depositional and sequence stratigraphic model for the field illustrate a classic platform-top unconformity associated with the Beaverhill Lake Group Sequence 3.1 (BHL 3.1). A key relationship in the framework by Collins is the suggestion that a lowstand prograding wedge up to 100 ft (30 m) thick developed along the Eastern Shelf while the Swan Hills atoll was exposed and being differentially eroded in the CCNF area. The author recognizes this erosional unconformity with up to 13 m of erosional relief atop the Swan Hills atoll as the Intra-Swan Hills Unconformity and interprets it as coincident with the sequence boundary at the base of the Beaver Hills Lake Group Sequence 3 (BHL 3.1). This striking contrast during the BHL 3.1 lowstand of extensive subaerial exposure at CCNF coeval with a downstepped mixed carbonate–siliciclastic prograding complex along the eastern margin of the Waterways subbasin is attributed to differential subsidence histories and basinal input resulting in variable accommodation settings across the basin. The well-developed nature of lowstand complexes, documented in detail both in this study and in the regional studies of Wong et al. (2016a, 2016b) and Weissenberger et al. (2016), challenges our understanding of these Frasnian “greenhouse” successions and suggests that a more careful look should be given to coeval platform to basin successions worldwide.
A well-constrained analysis of the Late Devonian biotic crisis and its impact on carbonate development is accomplished using a high-resolution integrated stratigraphic framework in “Pattern and timing of the Late Devonian biotic crisis in western Canada: Insights from carbon isotopes and astronomical calibration of magnetic susceptibility data,” by Michael T. Whalen, David De Vlee-schouwer, Joshua H. Payne, James E. Day, D. Jeffrey Over, and Philippe Claeys. The authors integrate a robust biostratigraphy, carbon isotope records, and magnetic susceptibility (MS) data collected from Canadian outcrops to unravel the details of the lead up period to the Frasnian–Famennian extinction (F–F). In particular, they utilize the previous work of De Vleeschouwer et al. (2012), who constructed a high-resolution magnetic susceptibility framework with calibration to 405-kyr eccentricity cycles. The Lower and Upper Kellwasser events (e.g., Joachimski et al. 2002), two significant, pre-F–F anoxic events marked by black shale deposition around the world and small-scale extinctions, are examined within the cyclostratigraphic framework. The Kellwasser events are identified from prominent positive shifts in the carbon isotope curve, allowing the authors to meticulously study the initiation, duration, termination, and structure of the excursions within the high-frequency time-calibrated MS stratigraphy. They find that the initiation–termination behavior of the excursions has implications for temperature fluctuations and shallowing of oxygen-poor marine waters in the Late Devonian, possibly related to the rise of terrestrial forests and associated soil formation and weathering. They further comment on how these environmental changes may be linked to decreases in biodiversity as more ecologically resilient species tended to outcompete species with more specialized niches. Their study is not only a robust example of integrated stratigraphic practices to attain a highly constrained, high-resolution framework for detailed analyses, but it also sheds more light on the nature and drivers of stepwise biotic crises and their impact on carbonate factories.
The importance of evaporites within a subsurface shallow-water carbonate reservoir succession is thoroughly documented and examined in “Deposition, diagenesis and reservoir properties of Hondo sulfates in the Grosmont carbonate–evaporite system, Upper Devonian, Canada,” authored by Hans G. Machel, Mary L. Borrero, and B. Charlotte Schreiber. The Grosmont is a significant heavy oil carbonate reservoir that has been historically evaluated in terms of its carbonate fraction; however, Machel and coauthors highlight the impacts of the interbedded evaporites in the system on the porosity–permeability distribution, reservoir compartmentalization, and diagenetic associations that feed hydrocarbon degradation processes. They describe the evaporite depositional features in detail and explore two models for their primary subaqueous deposition: discontinuous salinas (evaporitic ponds) or an extensive restricted lagoon that was later dissected by burial dissolution. Diagenetic modification, the most important being multiple phases of burial dissolution, locally enhanced carbonate porosity and permeability by leaching of evaporite nodules to produce vuggy zones, and the generation of highly conductive brecciated intervals from corrosion of evaporite layers and subsequent collapse. In other regions of the Grosmont, the dissolution was significantly less, and evaporite layers remained intact, forming local baffles or seals, as evidenced by vertical compositional variations of the hydrocarbons. Last, the sulfate evaporites and their diagenetic interactions are speculated to have had a role in the heavy oil–bitumen hydrocarbon system, as the liberation of sulfur promoted sulfur-reducing bacterial processes and hydrocarbon degradation. Although this study focuses on the Hondo evaporite system, the authors underscore the intimate relationship with the Grosmont carbonate system and, more importantly, the significance of evaporite burial diagenesis in the ultimate reservoir quality distribution and hydrocarbon mobility of a supergiant carbonate field.
STUDIES—OUTCROPS OF THE LENNARD SHELF, CANNING BASIN, WESTERN AUSTRALIA
Phillip E. Playford, Roger M. Hocking, and Anthony E. Cockbain provide an important summation of the evolution and development of geological understanding of the Upper Devonian Lennard Shelf mixed carbonate–siliciclastic carbonate outcrops of the Canning Basin in Western Australia over nearly a century of study in “Devonian reef complexes of the Canning Basin, Western Australia: A historical review.” Beginning in the 1930s, with the pioneering research of Curt Teichert (e.g., Teichert 1949), studies advanced from paleontology and biostratigraphy of the reefal strata to extensive field mapping by industry and government from the 1940s to 1960s. These studies unraveled the formations and interpreted the dominant depositional environments, facies, and the gross stratigraphic architecture of the system. The spectacular nature of the exposures and textural preservation allowed ever more detailed investigations in the 1970s and 1980s of various aspects of the depositional system and its component parts, such as classic atoll and pinnacle reef development during long-term platform backstepping, the character of Neptunian dikes (syndepositional fractures), reef-derived slope deposits, variations in margin styles, and microbial reef fabrics, including deep-water stromatolites. With Famennian exposures well preserved, this outcrop data set also provided excellent documentation of the changes in carbonate factories across the Frasnian–Famennian extinction interval, with special attention on the overall shift from shallow euphotic metazoan reefal assemblages in the Lower–Middle Frasnian to more resilient microbial assemblages that dominated postextinction. A detailed and objective discussion of an alternative hypothesis for the origin of the carbonate architectures observed in the Lennard Shelf exposures, the “metamorphic model” of Logan and Semeniuk (1976), is also presented to outline the two disparate interpretations; supporting arguments for their preferred explanation are given. Sequence stratigraphic methods were applied to the region beginning in the 1990s, culminating in the Canning Basin Chronostratigraphy Project (CBCP), described by multiple contributors in this volume (Hillbun et al. 2016, Playton et al. 2016, Ratcliffe et al. 2016), which integrates rock-based sequence stratigraphic concepts with isotope records and magnetic reversals constrained within a robust biostratigraphy. Overall, the authors present a comprehensive, yet concise, overview of this classic outcrop belt and provide an excellent starting-point resource for future Lennard Shelf researchers.
A scaled shelf-to-basin reconstruction of the Upper Devonian Lennard Shelf carbonate system and a series of margin and slope sequence stratigraphic models are presented in “Integrated stratigraphic correlation of Upper Devonian platform-to-basin carbonate sequences, Lennard Shelf, Canning Basin, Western Australia: Advances in carbonate margin-to-slope sequence stratigraphy and stacking patterns,” by Ted E. Playton, Roger M. Hocking, Eric Tohver, Kelly Hillbun, Peter W. Haines, Kate Trinajstic, Brett Roelofs, David A. Katz, Joseph Kirschvink, Kliti Grice, Paul Montgomery, Jeroen Hansma, Sergei Pisarevsky, Svenya Tulipani, Kenneth Ratcliffe, Samuel Caulfield-Kerney, and David Wray. The manuscript has two main themes: an explanation of the overall methodology of the CBCP to achieve well-constrained correlations across the Lennard Shelf, and a detailed description of the resulting sequence stratigraphic and depositional framework for the region, with special emphasis on carbonate margin and slope successions and architectures. Extensive outcrop measurements and samples were collected to assimilate multiple independent data sets in order to more accurately correlate geographically separated outcrop localities. Rock-based sequence stratigraphic concepts, magnetic reversal stratigraphy, and stable carbon isotope chemostratigraphy, all constrained within a robust global conodont biostratigraphy, are integrated into a single, cohesive chronostratigraphic framework. Agreement between the various data sets provides confidence in correlating time-significant surfaces across the study area, and from platform-top to slope and basinal settings, in transects that are physically separated and without the direct observation of stratal geometries. As such, systems tracts and sequence boundaries are correlated from platform to basin, allowing a detailed reconstruction of the Lennard Shelf architecture across a long-term transgressive–regressive succession and global biotic crisis (F–F extinction interval). New predictive models for steep-sided carbonate margins and coeval slopes highlight stratal geometries, facies arrangements, and vertical successions as a function of hierarchical accommodation changes, as well as extinction lead-up and recovery periods that impacted carbonate factories. The paper demonstrates an integrated methodology that is particularly useful for data sets with areas of discontinuous data, such as subsurface settings with high-quality logs and core but inadequate seismic records between wells. The authors also highlight the potential value in constructing such a framework in terms of academic understanding or reservoir characterization, with their presentation of a better-constrained Lennard Shelf architecture and carbonate margin-to-slope sequence stratigraphic concepts.
“Correlation and sequence stratigraphic interpretation of Upper Devonian carbonate slope facies using carbon isotope chemostratigraphy, Lennard Shelf, Canning Basin, Western Australia,” by Kelly Hillbun, Ted E. Playton, David A. Katz, Eric Tohver, Kate Trinajstic, Peter W. Haines, Roger M. Hocking, Bret Roelofs, and Paul Montgomery, discusses the carbon isotope chemostratigraphic component of the aforementioned CBCP (Playton et al. 2016). The authors develop a chemostratigraphic framework using six key detailed stratigraphic sections that bracket a complete F–F profile of platform-top and slope facies over a 200-km-strike window of outcrops. The Devonian record contains several distinct δ13C excursions that represent the primary marine signature and reflect the evolving seawater chemistry of the Canning Basin, and likely the Upper Devonian global ocean. More than 1400 analyses at submeter spacing were used to generate section-specific δ13C curves that were tied to and constrained by measured stratigraphic sections within a magnetostratigraphic and conodont biostratigraphic framework. Observed carbon isotope patterns include a general increase in δ13C values during transgressions and a decrease to near-negative values during highstands and approaching sequence boundaries. They document five major excursions within the data that are recognized elsewhere in the world, and six minor excursions that correlate regionally across the Lennard Shelf but are not globally defined. Importantly, the famous Kellwasser events in the latest Frasnian (e.g., Joachimski et al. 2002) are well expressed in this Lennard Shelf data set and are shown to be associated with significant transgressions. Hillbun et al., mainly utilizing late Frasnian relationships, point to a close correlation between fluctuations in δ13C and global sea level, with (1) positive excursions relating to sea-level rise events and associated increased burial or preservation of organic matter, and (2) negative shifts in δ13C linked to increased oxidation of organic material during base-level falls. The authors also shed light on the Famennian, a more sparsely documented interval, by discussing the eustatic driver in a more stable or quiescent oceanic period relative to the late Frasnian. In general, this study provides a chemostratigraphic approach to augment sequence stratigraphic interpretation, especially in carbonate slope settings where systems tracts are poorly defined, and depositional architecture is highly complex.
Kenneth Ratcliffe, Ted E. Playton, Paul Montgomery, David Wray, Samuel Caulfield-Kerney, Eric Tohver, Roger M. Hocking, Peter W. Haines, Joseph Kirschvink, and Maodu Yan describe in more detail the elemental chemostratigraphic component of the CBCP (Playton et al. 2016) in “Using elemental chemostratigraphy on Mid-Late Frasnian platform-top successions from the Lennard Shelf outcrops, Canning Basin, Western Australia.” While the more regional studies presented in Playton et al. (2016) were not able to utilize elemental signatures over large distances, Ratcliffe et al. present the application and interpretive benefits of this technique within a highly constrained locality that exposes a kilometer-scale carbonate reef flat to platform interior transition. Two measured sections capture this facies transition and are correlated with precision using a walkout correlation and the magnetic reversal stratigraphy presented in Playton et al. (2016). The elemental responses are consequently analyzed within a high-resolution background framework. Zirconium (Zr) was identified as a stable element that is unaffected by diagenesis and weathering, and that primarily reflects land-derived siliciclastic input onto the narrow, attached Lennard Shelf. The authors found that conventional approaches using element ratios to establish isochemical chemozones were not useful; instead, upward trends in single element concentrations, in this case Zr, proved to be of value and agreed with the preexisting framework. The study sections span the long-term maximum flooding surface (MFS) of the Lennard Shelf system in platform-top and reef flat settings, and the analysis demonstrates a change in higher-frequency Zr profile character according to position relative to this significant surface. The authors find that Zr cycles are asymmetric, with increasing concentrations upward, and capped by an abrupt decrease below the MFS. This is followed by more symmetrical Zr cycles above the MFS, with gradual increasing to decreasing trends in concentration. The authors conclude the study with discussion on the complexity of mixed carbonate–siliciclastic systems in the context of the observed Zr signatures relative to position within the supersequence framework. They touch on factors such as carbonate productivity changes, bioherm development, sediment delivery and reworking along the narrow shelf, and windblown external sources to propose controls on Zr trends when compared to carbonate stacking patterns. This study is an excellent example of a detailed analysis that leverages a preexisting framework to unravel complex depositional patterns, and it highlights a rather unconventional approach in elemental chemostratigraphy that has utility as a correlation tool within an integrated data set.
STUDIES—OUTCROPS OF THE WESTERN USA
Benjamin E. Rendall and Leif Tapanila provide the sequence stratigraphic framework and depositional models for a carbonate system that was affected by a substantial bolide impact and discuss the effects on carbonate production in “Sequence stratigraphy across an event deposit: Pre-, syn-, and postimpact accommodation trends and sequence development surrounding the Alamo Impact Breccia.” The Alamo Breccia is a famous outcrop record of a Frasnian impact onto a carbonate shelf in southern Nevada (e.g., Warme and Sandberg 1995), and it has been studied extensively, mostly to understand the nature of the event itself. The authors generate a carbonate depositional model and for the first time divide the precursor, coeval, and postdating carbonates into a sequence stratigraphic framework using sedimentology, facies proportions, and cycle stacking criteria. Their framework constrains the timing of the impact to within one high-frequency systems tract, and it unravels the extent of vertical and lateral deformation of older platform stratigraphy. The impact itself generated localized additional accommodation that formed a minibasin within the larger shelf and shifted the position of platform margins, adding complexity to a stratigraphic record that otherwise indicates long-term accommodation rise. The impact crater was eventually infilled with lowstand siliciclastic deposits, again demonstrating the effect of the deformation on local deposition and architecture. This study documents the carbonate system both inside and outside of the impact region and concludes that sedimentation and accommodation trends were only affected in the immediate vicinity of the target area; regional trends and shallow-water carbonate deposition away from the bolide event were uninterrupted. The authors present an example of local, and in this case, extraterrestrial, effects adding heterogeneity to a rock record otherwise governed by regional or global drivers. They reaffirm the cautions to be exercised when interpreting the rock record, especially in the ways in which features such as shelf-top breccias and disconformities are analyzed, which can have multiple origins. Last, this paper provides a framework for future work to more closely examine the ecological system and the ways in which particular biotic communities existed prior to and recovered from such an event. These analyses can help to better understand threshold sizes for these physical events and the role of bolides in major extinctions.
Another classic region for Devonian and Lower Carboniferous carbonate exposures is the northern USA in Montana and Idaho. George W. Grader, Peter E. Isaacson, P. Ted Doughty, Michael C. Pope, and Michael K. DeSantis summarize several decades of work in the region based on extensive fieldwork covering over 150 outcrops in “Idaho Lost River Shelf to Montana Craton: North American Late Devonian stratigraphy, surfaces, and intrashelf basin.” Detailed observations from previously undescribed remote and poorly accessible localities are added to redescriptions of classic outcrops such as the Logan Gulch type section in southwest Montana. The new descriptions underpin a regional sequence stratigraphic framework that ties the low-accommodation setting on the Montana–Idaho carbonate shelf across the Lemhi Arch to the incipient Central Idaho Trough. Correlation of major sequence boundaries across the region, integrated with biostratigraphy, clarifies some previously disputed stratigraphic relationships and problematic lithostratigraphic terminology. This includes a better understanding of the “Grandview Dolomite” (Upper Jefferson Formation), its reefal buildups on the Idaho Shelf, and time-equivalent shoreface sandstones to the east. The authors also provide new insights into the Famennian depositional system of the region, which is composed of heterogeneous mixed carbonate–siliciclastic depositional environments and extensive evaporite-related collapse breccias. Numerous depositional hiatuses are documented, reflecting a complex tectonic history and resulting stratigraphic architecture, including the significant influence of the Antler Orogeny. Last, the intricacies described in the latest Devonian Sappington and Three Forks formations are particularly useful in understanding the Bakken Formation hydrocarbon system. Overall, this chapter provides a digestible regional framework that ties together an extremely complex depositional and tectonic history over a large area. It closes previous gaps in understanding and explains the way in which well-studied exposures and prolific petroleum basins of the region relate to one another.
The Sappington Formation, a Late Devonian to Early Mississippian outcrop analog for the Bakken Formation unconventional reservoirs, is revisited and divided into sequences with associated facies models in “Sequence stratigraphic framework and facies models for the Late Devonian to Early Mississippian Sappington Formation (Bakken equivalent), southwest Montana,” by Aaron P. Rodriguez, George W. Grader, John C. Hohman, Ted P. Doughty, John Guthrie, and Peter E. Isaacson. Decades of studies on the Sappington Formation outcrops in Montana are expanded with new field data, sequence stratigraphic concepts that clarify previous lithostratigraphy, and improved interpretations of facies and environments that consider the emerging knowledge base of shallow, mixed carbonate–siliciclastic intracontinental basins. Surfaces that are regionally mappable underpin the sequence framework and separate different depositional regimes within the basin. Detailed description and documentation of sediment composition and structures, ichnofacies, and diagenesis provide the observational basis for interpreting complex depositional settings and processes. All of these combine into depositional models that highlight the variability and architecture of sequences within these enigmatic shallow basins. Insightful discussions on the interplay of controls that contributed to different Sappington depositional conditions help to delineate the roles of glacioeustasy, Antler Foreland tectonics and associated external sediment input, climatic perturbations (including anoxic periods), and the rise of land plants and their linkage to marine nutrient levels. This paper provides clarity and utility to a classic set of outcrops that are critical analogs for highly prolific unconventional oil reservoir systems, such as the Bakken and Exshaw Formations.
DISCUSSION—DEVONIAN CARBONATE CHRONOSTRATIGRAPHY
Many of the studies in this volume are founded on the identification and correlation of time-significant surfaces or intervals; time significant in this context means the ability to correlate packages of rock of equivalent age, whether or not the determination of radiometric age dates is achieved or utilized. It is in this context that we use the term “chronostratigraphy” here. The Devonian, especially the late Middle to Upper Devonian, is an exceptional period for which to undertake such efforts, because a well-accepted, high-resolution and globally calibrated biostratigraphic framework is in place as the root foundation for any correlation (e.g., Ziegler and Sandberg 1990, Klapper 1997). Furthermore, studies over the past 15 years have introduced a stable carbon isotope record for the Middle–Upper Devonian that contains numerous recognizable excursions, and these excursions have been demonstrated to be globally significant (e.g., Joachimski et al. 2002, Buggisch and Joachimski 2006). Paleomagnetic reversals are robust global benchmarks for correlation because they are insensitive to setting or environment; early studies showed a high reversal frequency in the Upper Devonian but did not define a repeatable framework (e.g., Hurley and Van der Voo 1987). However, recent studies along the Lennard Shelf in Western Australia (Playton et al. 2016, building from Hansma et al. 2015) have produced a well-constrained, high-resolution reversal stratigraphy across the region for most of the Frasnian and Famennian, adding to the inventory of Upper Devonian global pinning points for stratigraphic correlation. This toolbox, which extends worldwide, allows for direct comparison of different data sets across the globe, and it provides a means with which to decipher local overprints, global signals, and their superimposition to generate complex carbonate architectures.
The contents in this volume exemplify this opportunity for comparison with carbonate studies in three different large-scale settings that share a common set of Upper Devonian global constraints: western Canada, northwestern USA, and Western Australia (Fig. 2). Although the western Canada and northwestern US data sets are geographically close to one another, the tectonic settings are different, and this is reflected in the resultant carbonate stratigraphic expression. All three data sets, as well as a host of others around the world (e.g., Shen et al. 2008), exhibit a long-term evolution from backstepping and aggrading carbonate platforms to prograding configurations—a supersequence (sensu Sarg et al. 1999) with transgressive and highstand systems tracts composed of subordinate sequences (composite sequences). It has also been observed in multiple data sets that the change from backstepping and aggrading to prograding margins (at the supersequence MFS) does not occur at the F–F boundary, but earlier in the Middle to Upper Frasnian; this pattern, observed across the world, argues for a global eustatic control that drove the long-term trajectory of carbonate margins of this age. However, when comparing the different data sets at a higher resolution, the timing of the supersequence MFS within the biostratigraphic framework varies across data sets (Fig. 2). The placement of the supersequence MFS in the Canadian and Lennard Shelf data sets (Wong et al. 2016a and Playton et al. 2016, respectively), although within the same standard biozone of Ziegler and Sandberg (1990), is clearly in different Montagne Noire biozones of Klapper (1997) in the Middle Frasnian. Furthermore, Grader et al. (2016) place the MFS in younger strata in the Upper Frasnian. Further work and interpretation refinement would likely result in a narrowing of the disparities across these studies, but it is equally likely that impacts of more local controls can explain the discrepancies, such as tectonic setting and subsidence rates, hinterland contribution and basin fill, and paleogeography. For example, each of these areas represents highly different tectonic regimes and basinal settings, ranging from passive subsidence within an intracratonic basin (Canada; e.g., Bond and Kominz 1991), to transitional and active convergence in a protoforeland (northwest USA; e.g., Dorobek et al. 1991), to synrift and postrift settings along an open ocean system (Lennard Shelf; e.g., Begg 1987).
Detailed reconstructions of the stratigraphic architecture are presented for the western Canada and Lennard Shelf carbonate systems (Wong et al. 2016a, 2016b and Weissenberger et al. 2016 and Playton et al. 2016, respectively) that allow for a more in-depth comparison (Fig. 3). When comparing the Frasnian data sets, a striking contrast is the difference in thickness of platform-top carbonate accumulation, where Lennard Shelf strata are close to double that of Canadian strata. A likely explanation is elevated subsidence along the Lennard Shelf related to rifting, and a “keep-up” carbonate factory. Overall, greater long-term accommodation from rift subsidence may also account for the transition into progradation along the Lennard Shelf being later than in Canada, where the carbonates took longer to “catch up” with higher subsidence rates superimposed onto the global eustatic signal. Anoxic shales developed around the supersequence MFS in Canada and were deposited fairly high up the slope, reflecting the more restricted nature and subtle underlying basement topography of the intracratonic setting. Conversely, the Lennard Shelf reefal system is perched along the upper fringe of a high-relief rift trough in a likely well-circulated setting, and evidently far removed (and updip) from any anoxic bottom waters (organic-rich rocks are extremely rare throughout the outcrop belt). Wong et al. (2016a) describe the onset of siliciclastic-dominated inner shelf environments in the Late Frasnian, which may indicate large-scale climatic changes and possibly the global transition out of peak greenhouse conditions to transitional settings during this time (e.g., Read et al. 1995, Markello et al. 2008). The Lennard Shelf platform-top system has a considerable siliciclastic component from Lower Frasnian throughout Famennian deposition, attributed to the narrowness of the shelf and proximity to high-relief hinterland sources that continually shed terrigenous sediment into the carbonate system; thus, a possible global climatic signal may be obscured in Lennard Shelf strata due to the paleogeographic configuration. Last, while the Lennard Shelf system accumulated significantly more carbonate thickness in the same time period, the Canadian system expanded laterally through progradation 10–20 times more than its Australian counterpart (10–15 km of Middle–Upper Frasnian progradation versus 500–1000 m, respectively); this is despite the highly productive deep microbial boundstone upper slope factory present in the Lennard Shelf system that never developed in Canada. Undoubtedly, the underlying basement paleotopography was a first-order control on the ability for the margins to prograde, with much gentler gradients and less slope accommodation to fill in the Canadian intracratonic configuration when compared to the high-relief, rift trough geometry of the Lennard Shelf. Additionally, Wong et al. (2016a, 2016b) and Weissenberger et al. (2016) also document a substantial degree of asymmetric, fine-grained basinal deposition starting in the Upper Frasnian that exceedingly aided reef margin progradation by infilling relief by many tens of meters. The hinterland adjacent to the Lennard Shelf also provided large volumes of material into the carbonate system, some of which was stored on the shelf, but much of it likely bypassed across the reefal system into the distal rift trough, and it was thereby detached from and unavailable to support the prograding slope system.
The foregoing comparison between the Frasnian of the Canadian and Lennard Shelf carbonate systems hinges on a common biostratigraphic framework that enables direct examination of many aspects of their coeval evolutions. The Middle–Upper Devonian stable carbon isotope record is another globally correlated and well-defined tool for linking data sets together (e.g., Joachimski et al. 2002, Buggisch and Joachimski 2006). Two papers in this volume use this robust record to further examine the precursor period before the F–F extinction in Canadian outcrops and to supplement sequence stratigraphic interpretation away from platform-top control in Lennard Shelf outcrops (Whalen et al. 2016 and Hillbun et al. 2016, respectively). They utilize the Lower and Upper Kellwasser events (e.g., Joachimski et al. 2002), two globally recognized carbon isotope positive excursions, as the key pinning points in their studies. They both consistently identify the Lower Kellwasser event in Upper Frasnian strata in the upper rhenana biozone of Ziegler and Sandberg (1990) and the Montagne Noire biozone 13a of Klapper (1997). However, there is a discrepancy in the timing of the Upper Kellwasser event: the excursion maxima occurring in the Lower Famennian triangularis biozone (Ziegler and Sandberg 1990) in the Canadian data set, and the full excursion returning to normal values earlier in the Uppermost Frasnian Montagne Noire biozone 13b (Klapper 1997) in the Lennard Shelf. Hillbun et al. (2015) observed and documented this same discrepancy with numerous other data sets around the world, attributing some of the disparity to sampling frequency and resolution. However, there are likely other factors explaining these differences that revolve around the overprint of local phenomena onto global patterns. The occurrence of these perturbations in ocean chemistry documented around the world and constrained to a very narrow window of time reflects a global process, such as significant burial of organic carbon in deep basin depocenters specific to this period, possibly from widespread anoxia and upwelling along many of Earth’s open ocean margins. The Lennard Shelf data set of Hillbun et al. (2015, 2016) consists mostly of transects through carbonate middle slope settings with abundant debris and grain-dominated deposits, interpreted to be high-sedimentation-rate records. One explanation is that higher depositional rates expanded the Upper Kellwasser period in the Lennard Shelf record and accordingly resolved the event and its timing more accurately. Many other isotope records around the world were collected in highly condensed basinal rocks that would be difficult to sample at the spacing required to resolve the timing, and consequently many data sets show the Upper Kellwasser event maxima to be coincident with the F–F boundary and extend into the Early Famennian (see Hillbun et al. 2015). In the case of Whalen et al. (2016), their isotope data were collected in platform-top and reef margin environments, which are also considered to be relatively higher-sedimentation-rate settings, and thus the above explanation does not hold. Other parameters that could influence this timing might include proximity to the major global sinks of organic carbon and ocean-scale paleogeography affecting circulation of the marine signal around Earth; further work is required to better understand these processes.
In general, the preceding discussion revolves around the importance of both global-scale phenomena that provide the reproducible signals that we can utilize for correlation, and more local or regional influences that add complexity to the first-order record and result in disparities when we compare different data sets within a high-resolution stratigraphic framework. The disparities highlight areas of needed refinement and further data collection, but they also allow us to start separating different intrinsic and extrinsic drivers from one another, and they offer opportunity for improved understanding of an intricate interplay of controls that generates the carbonate architectures and heterogeneity that we strive to characterize in both academic and applied settings.
DISCUSSION—DEVONIAN CLIMATE AND SEA LEVEL
The Devonian has traditionally been considered a “greenhouse” period in Earth history (Read et al. 1995), with the Middle Devonian being considered by some a “peak greenhouse” setting on the basis of widespread and diverse reef communities (e.g., Copper and Scotese 2003). The paleoreef reconstructions of Kiessling et al. (1999) draw attention to the Givetian and Frasnian Stages as the apex of reef occurrence and reef diversity in the Phanerozoic record, thus further asserting the warm equitable setting for these units. Historical assessments of a global greenhouse during most of the Devonian are also supported by the systematic, low-amplitude, high-frequency cyclicity, the rarity of significant facies-tract offset and platform-top exposure indicators, and the absence of evidence for downward shifts in coastal onlap (e.g., synthesized by Markello et al. 2008). All of these characteristics are most commonly associated with greenhouse settings.
Adding complexity to this generalization, detailed assessments of Devonian paleotemperature records by Joachimski et al. (2009) suggest that instead of a warm Middle Devonian (as predicted for peak greenhouse conditions), the Eifelian, Givetian, and Early Frasnian were distinctively cooler. This is enigmatic, considering these periods witnessed the most widespread and diverse reefal communities of the Devonian, as well as platform-top stratigraphy bearing the typical greenhouse-like signatures (e.g., Read 1973, Appendices 9 and 14 of Playton et al. 2016). Conversely, Wong et al. (2016a) document forced regressive and in situ carbonate lowstand wedge deposition in the Lower Frasnian, which is more consistent with cooler temperatures and the potential for glacioeustasy as a driver of high-amplitude relative sea-level change.
Furthermore, the work of Joachimski et al. (2009) points to the Early and Late Devonian as being times of warmer relative temperatures, with exceptions. Extreme punctuations of cooling in the latest Frasnian of as much as 5–7° C are documented in association with the Kellwasser events (Buggisch and Joachimski 2006), and these set the stage for biotic crises that preceded and drove one of the most substantial extinctions in Earth history, the F–F (discussed in Whalen et al. 2016). Distinct evidence for latest Devonian glaciation has been noted both from glaciogenic depositional records in South America (Caputo and Crowell 1985) and floral characteristics of the palynomorph record of Bolivia and Europe (Wicander et al. 2011). Elrick et al. (2009) pointed to synchroneity between δ18O and δ13C perturbations at the Early–Middle Devonian boundary in both North America and Europe that indicate two cycles of cooling and sea-level falls that were likely glacioeustatic in origin. Finally, Playton and Kerans (2015) recognized high-frequency sequences in Famennian middle slope deposits and attributed their development to the onset of glaciation during the transition out of greenhouse conditions to icehouse settings of the Carboniferous. Thus, many Early and Late Devonian studies provide evidence for indicators of cooling pulses and glacioeustasy, signals of transitional climates, that somewhat conflict with paleotemperature data suggesting warmer climates more typical of greenhouse conditions.
It is clear that the Devonian was a time of highly complex climatic and environmental processes that greatly impacted carbonate ecology and productivity, and relative sea level. As with other stratigraphic drivers, there were multiple scales of climatic change resulting in more abrupt events within longer trends, such as cooler periods within a “peak greenhouse” time and “events” of global temperature drop within overall warmer stages. These superimpositions can produce conflicting data and challenge our classic interpretations and characterization of greenhouse, transitional, and icehouse settings. The contents herein are largely Frasnian and Famennian studies, which span significant interpreted climatic changes within the Devonian, but mostly focus on the definition of stratigraphic frameworks. Whalen et al. (2016) and Hillbun et al. (2016) begin to discuss the possible explanations behind such climatic signals, but a substantial opportunity remains in the integration of high-resolution, rock-based stratigraphy with a greater understanding of the changing environmental conditions that drive coeval carbonate ecology, productivity, and sedimentation.
DISCUSSION—DEVONIAN CARBONATES AS A PETROLEUM SYSTEM
Much of the study of Devonian carbonates has been due to the economic importance and potential for hydrocarbons found in these strata, especially in the major oil provinces in western Canada, western and northern USA, and western Russia, among others. What was arguably the first commercial oil well in North America was “dug” at Oil Springs, Ontario, Canada, in 1858. It produced from sucrosic dolomites of the Devonian (Eifelian) Lucas Formation (Bailey Geological Services and Cochrane 1985), and oil is still being produced from the Devonian in the region today. The famous Drake well (1859) in Titusville, Pennsylvania, encountered commercial oil in Devonian (Famennian) sands (Dickey 1941). The first major North American oil discovery in a Devonian reef was by Imperial Oil at Norman Wells field in 1920, and the Givetian–Frasnian atoll was still producing 14,000 barrels/day in 2013 (Babiy 2013), for a total of almost 270 MM (million) barrels by 2015, barrel volume = 42 US gallons, 35 imperial gallons, or 159 liters. Similarly, the oil boom that established the WCSB as a major oil province began with the discovery of oil in another reef at Leduc field, which has produced almost 400 MM barrels of oil to date. Frasnian carbonate reservoirs alone, hosting large quantities of oil and gas in Alberta, have conventional reserve estimates to the end of the last century of 11.1 billion barrels of oil and 20 trillion cubic feet (20 Tcf = 566.3e9 m3) of gas (Alberta Energy and Utilities Board 2000). Significant hydrocarbon reserves are also found in the Middle Devonian carbonate–evaporite system (Bebout and Maiklem 1973) and the widespread Famennian carbonate platform (Halbertsma 1994).
Most conventional Devonian hydrocarbons in the WCSB are in stratigraphic traps: atoll and pinnacle reefs with overlying fine-grained basinal sediments or shelf margin traps against shale or evaporite-filled channels or re-entrants. Diagenetic traps have also been described. Commonly, dolomitization or, in some instances, burial dissolution of evaporites modified existing pore networks, following original facies patterns, and/or created vugs and cavernous porosity (e.g., Reinson et al. 1993). The karsted, erosional subcrop of the Alberta Frasnian contains large reserves of heavy oil in the Grosmont Formation carbonates, which has an estimated 508 billion barrels of oil in place (Alberta Energy Regulator 2015; for discussion on role of diagenesis and evaporites on reservoir quality and compartmentalization, see Machel et al. 2016).
Recent years have seen a tremendous increase in the exploitation of unconventional reservoirs, notably basinal mudrocks associated with Devonian carbonates and evaporite systems. For example, the Duvernay Formation organic mudstone, deposited in the southern part of the WCSB, has estimated reserves of 441 Tcf (12.5e12 m3), 11 billion barrels of natural gas liquids, and 62 billion barrels of oil, and the totals increase when age-equivalent mudrock plays to the north are added (B.C. Oil and Gas Commission 2014, 2015). The unconventional hydrocarbon phenomenon has extended exploration and production of these systems into central and eastern Canada, as well as the USA. The Late Devonian to Early Mississippian Three Forks–Bakken play represents a complex carbonate–siliciclastic–mudrock depositional system that has already proven to be a prolific producer in the midcontinent, extending from the Williston Basin proper as far west as southern Alberta. With new drilling, stimulation, and completion technologies, Three Forks–Bakken producible reserves have expanded significantly to estimates on the order of 7.4 billion barrels of oil, 450 million barrels of which were produced between 2008 and 2013 (Gaswirth et al. 2013). The vast Bakken-equivalent system also extends broadly across the USA, from Appalachia, through the midcontinent, and southwest to Texas (U.S. Energy Information Administration 2010), emphasizing an important global depositional control on the development of this style of intracontinental basin. These strata are associated with coeval mixed carbonate–siliciclastic shelf systems and periods of shallow oceanic anoxia. The Marcellus Formation of the Appalachian Basin, New Albany of the Illinois Basin, and Woodford of the midcontinent are also examples that collectively hold approximately 450 Tcf (12.7e12 m3) of natural gas and are being actively exploited (Conant and Swanson 1961).
Beyond North America, significant hydrocarbon production from Devonian carbonates comes from the Timan–Pechora Basin of Russia (Schenk 2015). The basin extends onshore along the west side of the northern Ural Mountains and offshore beneath the Pechora Sea, and it consists of isolated platforms and extensive shelf carbonate reservoirs containing reserves of almost 2.6 billion barrels of oil equivalent (Lindquist 1999). This comprises approximately 24% of all reserves identified in Russia to the end of the last century, with 75–85% of these found in structural traps and stratigraphic traps considered underexplored, leaving significant undiscovered potential in the basin (Lindquist 1999, Schenk 2015). Reefal growth in the basin initiated as early as the Pragian–Emsian (Early Devonian), with large barrier systems up to 1500 m thick (Klimenko et al. 2015). Shallow-water platforms with associated microbial mounds (up to 600 m thick) occur in the Frasnian, which form most of the hydrocarbon reservoirs (Klimenko et al. 2015). A complex interplay of primary porosity overprinted with karst and structure-related faults and fractures controls reservoir quality distribution and performance (Spina et al. 2015). The primary source rocks of the Timan–Pechora Basin and the Pricaspian Basin to the south are the Upper Devonian “Domanik,” thin-bedded, dark siliceous shales and mixed lithology mudstones (Schenk 2015), which are time equivalent to the isolated platforms and carbonate shelves of the basin. The Pricaspian Basin to the south in western Kazakhstan is also a significant oil and gas province, with supergiant fields such as Karachaganak and Tengiz (Katz et al. 2010 and Collins et al. 2013, respectively); however, the main producing intervals in these fields are dominantly in younger Carboniferous sections, with underlying Upper Devonian (Frasnian and Famennian) rocks being the foundations of the isolated buildups, but overall nonproductive.
Despite being a world-class outcrop analog for Middle–Upper Devonian stratigraphic architecture and development of steep, reef-rimmed platforms (e.g., Playford et al. 2009, 2016), the Devonian subsurface of the Canning Basin in Western Australia has historically been poorly or nonproductive with respect to oil and gas (e.g., Wallace et al. 2002). Despite excellent source rocks, potential top seals, and porosity development, it is hypothesized that insufficient charge related to migration pathways or faults, inadequate maturation, hydrocarbon degradation, and leakage collectively led to the observed paucity of economical oil and gas accumulations over the decades (Wallace et al. 2002). Recent oil discoveries in the Canning Basin (e.g., Ungani field; Edwards and Streitberg 2013) have reignited interest in the superbasin; however, these reservoirs tend to be in mixed-system, dolomitic ramp facies of latest Famennian to Tournaisian age, significantly postdating reefal platform development.
The papers in this volume are useful directly, as workflows for, or as analogs to many aspects of Devonian carbonate petroleum geology, from the play to reservoir scale, and from exploration to brown-field development requiring high-resolution reservoir characterization. Some describe methodologies and show results for integrated stratigraphic correlation using multiple independent data sets to arrive at a robust regional- to fine-scale framework: Lennard Shelf outcrops in the Canning Basin of Western Australia (Hillbun et al. 2016, Playton et al. 2016, Ratcliffe et al. 2016), and Canadian Rocky Mountain outcrops of the Alberta Basin in western Canada (Whalen et al. 2016). These methodologies are applicable for basin-scale to interwell subsurface correlation using core or cuttings, as the exposures are mostly isolated or discontinuous and consequently mimic subsurface wells or accumulations, which require correlations across often significant geographic data gaps. The use of well-constrained global events, such as the Lower and Upper Kellwasser carbon isotope excursions (Hillbun et al. 2016, Whalen et al. 2016), underpins such correlations and provides guides in generating better paleogeographic (fairway) maps and breaking out time-equivalent reservoir units within and between hydrocarbon pools. Higher-frequency or more local signals in the rock record, such as magnetic susceptibility and elemental chemostratigraphy (Ratcliffe et al. 2016, Whalen et al. 2016), provide a much finer set of constraints within the overall framework and allow for more detailed, flow-unit-scale characterization in the subsurface that can support decisions on well design and completion, for example. Finally, a better-constrained stratigraphy provides the opportunity for improved conceptual models to influence characterization, as shown by Playton et al. (2016) with the development of new margin and slope sequence stratigraphic interpretations that describe potential reservoir heterogeneity and proposals of predictive tools that link seismic-scale architecture to well-scale facies patterns.
The more regional and subsurface contributions herein on the Devonian of Canada are germane to both hydrocarbon exploration and development, for improving interpretation and exploitation strategy directly in particular fields or providing integrated frameworks that can be utilized across the region. Collins (2016) demonstrates how detailed stratigraphic understanding of a large carbonate pool at Carson Creek North Field can be integrated with regional, exploration-scale mapping and stratigraphic analysis. The characterization of a significant unconformity (Collins 2016), which not only has an impact on reservoir quality distribution, but also marks a surface of differential development across the basin, exemplifies the importance of a sequence stratigraphic approach and rock-based concepts in understanding the subsurface. Machel et al. (2016) provide important insights into the carbonate–evaporite Grosmont Formation depositional system and supergiant heavy oil reservoir, and in particular stress the role of the evaporite component on carbonate reservoir compartmentalization and porosity distribution; it represents yet another example of the complexity and heterogeneity in subsurface carbonates and the value of detailed characterization work that integrates logs and core. The three remaining Canadian submissions (Weissenberger et al. 2016; Wong et al. 2016a, 2016b) together contribute a regional, hierarchical sequence stratigraphic framework for the WCSB, underpinned by biostratigraphy, which may be used as a reference template for building regional exploration programs down to detailed reservoir management. The work seeks to predict both reservoir and source rock distribution, as well as controls on early diagenesis that affect reservoir quality (e.g., Potma et al. 2001). Detailed aspects of stratigraphic architecture are also described, such as the regional and local expression of carbonate lowstands, which are applicable to carbonate reservoir exploitation efforts and may point to new, or underexplored play types.
Subsurface correlations typically assume regional or global controls on stratigraphy, but Rendall and Tapanila (2016) provide a reality check in documenting the effect of a single geologic event (bolide impact) in Nevada, USA, within a regional sequence stratigraphic analysis. This unique geological anomaly had an extreme local impact on facies distribution and stratigraphic architecture, and in many ways created relationships and rock types indicative of subsurface sequence boundaries in the impact realm itself. This provides a further lesson from a well-constrained outcrop on the importance of capturing the range of uncertainty when interpreting the subsurface with limited data points.
Finally, Grader et al. (2016) use an extensive outcrop data set to document the complexity of the Late Devonian and Early Mississippian in Idaho and Montana, northern USA, related to the interplay of carbonate deposition, global environmental phenomena, differential subsidence, and active tectonism. In an impressive effort to clarify decades of disconnected studies and link together a challenging tectono–depositional history, their work both identifies current pitfalls as well as opportunities for further conventional and unconventional hydrocarbon exploration and development in the region. Their detailed stratigraphic analysis can be consequently utilized to reduce risk. Much of the strata described by Grader et al. (2016) are time equivalent to the highly productive Bakken–Three Forks Formations of the midcontinent, and Rodriguez et al. (2016) document in detail a direct outcrop analog to the unconventional oil play, the Sappington Formation in Montana. The Sappington exposures reveal shallow intracontinental basin-fill strata that are very similar to the Bakken–Three Forks system. Rodriguez et al. (2016) subdivide this succession into sequences, develop facies models for, and discuss the controls and predictive distribution of reservoir-prone rock types, serving as an excellent analog data set for Bakken–Three Forks characterization.
DISCUSSION—ADVANCES, APPROACHES, AND FORWARD LOOK
This volume nicely demonstrates the progress made on the understanding of Devonian carbonates over the last decade, with both applications to industry and foundations for further research. Significant improvements have been made to the stratigraphic frameworks of classic Devonian carbonate areas, such as the western USA, Western Australia, and western Canada (Grader et al. 2016, Playton et al. 2016, and Wong et al. 2016a, respectively), in terms of the level of stratigraphic constraint, shelf-to-basin correlation, synthesis of smaller study areas into a regional framework, and reconciliation of different terminologies into a unified nomenclature. These improved frameworks, with greater degrees of control and confidence, allow for more robust interpretations and reconstructions of the depositional and stratigraphic architectures of these complex systems. For example, documentation of well-expressed forced-regressive and lowstand deposits and the development of a substantial erosional subaerial unconformity in the Middle Frasnian (Collins 2016; Weissenberger et al. 2016; Wong et al. 2016a, 2016b) challenges our understanding of carbonate deposition in greenhouse settings. New sequence stratigraphic criteria are presented for settings that are poorly understood with respect to systems tracts and accommodation changes, such as carbonate slopes (Hillbun et al. 2016, Playton et al. 2016) and mixed-sediment, shallow intracratonic basins (Rodriguez et al. 2016). Works such as that by Rendall and Tapanila (2016) highlight the interplay of intrinsic and extrinsic controls on carbonate evolution and the resulting heterogeneity, which can be easily misinterpreted. While many additional questions emerge from the results of the studies herein that will drive future research, a more robust picture of the global Middle–Upper Devonian carbonate system is also provided that will serve as a new foundation to build upon.
A common thread in many of the papers is the practice of integrated stratigraphy, where rock-based and sequence stratigraphic concepts used for correlation are constrained by multiple independent signals extracted from the rock record. While absolute age constraints are not necessarily used, these methodologies are based on defining time-significant intervals or boundaries to establish correlation pinning points that guide the interpretation and definition of sequence stratigraphic surfaces. The use of biostratigraphy to constrain sequence boundaries and systems tracts is heavily stressed and has become somewhat conventional, but the addition of isotopic, paleomagnetic, and elemental data leads to significant refinement and greater resolution of stratigraphic frameworks. A key success factor in these approaches is the recognition and delineation of global signals as the starting points and cornerstones of framework development; the Middle–Upper Devonian period is outstanding in this respect, with a high-resolution biostratigraphic foundation that is calibrated around the world, an oceanic evolution that produced highly recognizable isotopic excursions on a global scale, and apparent frequent magnetic reversals that are environment-insensitive global phenomena, newly defined through the recent work along the Lennard Shelf in Western Australia (Playton et al. 2016, building from Hansma et al. 2015). In addition to carbon isotopes, Whalen et al. (2016) presents a novel approach (building from De Vleeschouwer et al. 2012) that utilizes magnetic susceptibility records in carbonate sediments as proxies for climatic and accommodation oscillations driven by orbital perturbations, in an effort to unravel a complex, high-resolution global signal to add granularity to pre-extinction events and durations. Last, many of the papers clarify historical confusions, inconsistencies, and parallel nomenclatures that resulted from regional formation mapping foundations, lithostratigraphic approaches, and local naming schemes; the newly proposed or synthesized terminologies herein reflect the trend toward time-significant or chronostratigraphic delineation of our data sets and the sequence stratigraphic principles that have guided our studies over the past few decades.
While significant advances in our understanding of the Devonian carbonate system are presented in this volume, especially the basins in western Canada, Western Australia, and the western USA, important gaps remain that should be a focus looking forward. Less-studied regions with substantial Devonian carbonates, such as western Russia and southern China (e.g., Klimenko et al. 2015 and Shen et al. 2008, respectively), would benefit from further refinement and constraint through integrated stratigraphic approaches. Expansion of our global inventory of high-resolution Devonian carbonate sequence stratigraphic frameworks, each with common tie points extracted from the well-defined global signals, allows us to assess different basins and geographical configurations against one another and begin to unravel important relationships, like the impact of tectonic setting, subsidence, and oceanographic conditions on large-scale carbonate platform development. Reconciliation or explanation of stratigraphic discrepancies we observe around the world in coeval systems offers significant learning and opportunity to refine our global frameworks. For example, many future paleomagnetic studies are required to validate and improve the reversal record presented in Playton et al. (2016) and Hansma et al. (2015). Hillbun et al. (2016) points out the need for additional focus on the Famennian carbon isotope record, which is emerging as an important indicator of global climatic change after a major extinction. Whalen et al. (2016) presents a framework based on orbital forcing, which is implicitly a global signal that should be reproducible elsewhere. In general, the starting points of a truly global Middle–Upper Devonian high-resolution framework are available now, and the task of future studies can be to validate, align, reproduce, and synthesize this complex record into a universal set of constraints that is readily accessible for academic research and applied industry projects.