JOURNAL OF SEDIMENTARY RESEARCH,VOL. 70, NO. 4, JULY, 2000, P. 913–936 Copyright  2000, SEPM (Society for Sedimentary Geology) 1073-130X/00/070-913/$03.00 BYPASS MARGINS, BASIN-RESTRICTED WEDGES, AND PLATFORM-TO-BASIN CORRELATION, UPPER DEVONIAN, CANADIAN ROCKY MOUNTAINS: IMPLICATIONS FOR SEQUENCE STRATIGRAPHY OF CARBONATE PLATFORM SYSTEMS MICHAEL T. WHALEN 1 , GREGOR P. EBERLI 2 , FRANS S.P. VAN BUCHEM 3 , ERIC W. MOUNTJOY 4 , AND PETER W. HOMEWOOD 5 1 Department of Geology and Geophysics, University of Alaska Fairbanks, Fairbanks, Alaska 99775, U.S.A. mtwhalen@gi.alaska.edu 2 Division of Marine Geology and Geophysics, Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, Miami, Florida 33149, U.S.A. 3 Institut Franc ¸ais du Pe ´trole, 92852 Rueil-Malmaison, France 4 Department of Earth Science, McGill University, Montreal, Quebec H3A 2A7, Canada 5 Elf Aquitaine Production, 64018 Pau, France ABSTRACT: Carbonate platforms can commonly keep up with relative sea-level rise because of high rates of sediment accumulation and plat- form aggradation. Surrounding basinal environments are commonly starved but can receive variable extrabasinal siliciclastic input and ep- isodically deposited carbonate sediment. If accumulation rates in ba- sinal settings lag behind those of the platform, a bypass or erosional margin can develop. Under these circumstances platform and basin depositional sequences become physically detached and direct corre- lation of basinal and platform sequences is hindered. We report here the results of high-resolution stratigraphic analyses of two Upper Devonian isolated carbonate platforms in western Al- berta that provide insight into the sequence stratigraphy of bypass margins and criteria for accurate correlation of platform and basinal sequences. The slope and basin sequences surrounding the Miette and Ancient Wall platforms consist of basin-restricted, onlapping wedges of fine-grained background sediment deposited dominantly from sus- pension and coarse-grained platform-derived sediment redeposited by a variety of gravity-flow mechanisms. Sequence boundaries are iden- tified within the redeposited carbonate intervals. Identification of se- quence boundaries and differentiation of highstand and lowstand slope and basinal facies was based on the geometry, mineralogy, and clast content of redeposited carbonate units. Highstand carbonates contain sheet-like debris flows and turbidites with abundant slope-derived clasts and background facies with high total carbonate content. Low- stand carbonates contain sheet-like and channelized debris flows and turbidites with abundant platform-derived clasts and background fa- cies with low carbonate content and locally high amounts of organic carbon. Transgressive facies are dominated by initially carbonate-poor and organic-rich background sediments that display a progressive in- crease in carbonate content and decrease in organic carbon content. These patterns are interpreted to record abundant background car- bonate sedimentation during late transgression and highstand when the carbonate factory was robust. Highstand redeposited carbonates re- cord slope erosion due to oversteepening and slope readjustment pro- cesses. Lowstand redeposited carbonates indicate platform and plat- form-margin erosion and low background carbonate sedimentation when the platform was either exposed or under very shallow peritidal conditions. High siliciclastic and organic contents during lowstand and early transgression may partly be the result of reciprocal sedimenta- tion but alternatively may represent continuous siliciclastic supply dur- ing times with little dilution by fine-grained carbonate sediment. Successive stages of platform development at Miette and Ancient Wall were controlled by accommodation changes driven by relative sea-level fluctuations. Backstripping analyses of strata from both plat- forms confirm that significant differential subsidence was a major con- trol on variations in platform thickness and patterns of slope devel- opment. Greater subsidence at Ancient Wall fostered the development of a steeper bypass margin and different slope evolution compared to Miette. Slope oversteepening also initiated a process of slope readjust- ment that eventually reduced the platform-to-basin gradient and fa- cilitated regressive platform progradation. In conventional siliciclastic sequence stratigraphy, basin-restricted wedges are interpreted as lowstand deposits on the basis of their ge- ometry and position relative to an updip margin. Wedge-shaped ba- sinal units along the Miette and Ancient Wall bypass margins contain both highstand and lowstand facies that straddle sequence boundaries. The results of this study provide objective criteria for differentiating systems tracts in carbonate slope and basin environments through min- eralogic and compositional analyses providing more accurate correla- tion of detached platform and basin sequences. Interpretation of car- bonate basin-restricted wedges as purely highstand or lowstand de- posits may lead to erroneous conclusions regarding sequence stratig- raphy, platform-to-basin correlation, and the volumetric partitioning of sediments deposited in different systems tracts. INTRODUCTION The pronounced differences between carbonate and siliciclastic deposi- tional systems prompted Handford and Loucks (1993) to question the ap- plicability of traditional sequence stratigraphic models to carbonate sys- tems. Among the most important differences that affect the geometry of carbonate platform sequences is the ability of carbonate systems to aggrade or sometimes even prograde during all but the most rapid relative sea-level rises (Kendall and Schlager 1981; Schlager 1981). The margins of aggrad- ing platforms can eventually oversteepen (Read 1985; Schlager and Camber 1986), resulting in sediment bypassing across a marginal escarpment and/ or a steep gullied slope (McIlreath and James 1978; Mullins and Neumann 1979; Schlager and Chermak 1979). Sediments deposited seaward of by- pass zones form onlapping wedges that pinch out against the steepened platform margin. Physical detachment of basin-restricted wedges and plat- form sequences renders direct correlation impossible and complicates plat- form-to-basin sequence stratigraphic interpretations. In traditional sequence stratigraphy, predicated on analysis of siliciclastic passive continental margins, such basin-restricted wedges are interpreted as lowstand systems tracts on the basis of their geometry and position relative to an updip margin (Posamentier and Vail 1988; Van Wagoner et al. 1988). In carbonate systems with bypass margins, onlapping basin-restricted wedges can form under both highstand and lowstand conditions, as dem- onstrated along modern Bahamian platform margins (Mullins and Neumann 1979; Schlager and Chermak 1979; Grammer and Ginsburg 1992). Similar relationships were revealed in our high-resolution sequence stratigraphic analysis of two isolated Upper Devonian carbonate platforms (Miette and Ancient Wall) in the Canadian Rocky Mountains (Fig. 1; van Buchem et al. 1996; Whalen et al. 2000). In this study we apply high-resolution se- quence stratigraphy and seismic-scale outcrop analysis to differentiate suc- cessive onlapping sequences, their systems tracts, and platform-margin ge- ometries of the two coeval platforms. This study demonstrates how varia- tions in platform-margin geometry and the relationship between platform