Abstract Late Jurassic (Kimmeridgian) to Early Cretaceous (Valanginian) carbonate platforms exhibit "keep-up" and "give-up" types of growth in the Western Atlantic. Twenty seismic lines and four well ties delineate the shelf-edge geometries, depositional systems, and chronostratigraphy. Geometric relations together with paleontologic and lithologic facies data permit interpretation of the primary controlling factors on platform evolution. Atlantic platform "keep-up" growth and development during successive tectono-eustatic fluctuations occur in two stages and vary directly with the supply of siliciclastics. Platform growth stages receiving siliciclastics prograde; those which do not aggrade. During stage I (Kimmeridgian), 8.1 km (5 mi) of margin progradation occurred near the depocenter of mixed carbonate-siliciclastic deposition, while 96 km (60 mi) northwest of the depocenter only 1.6 km (1 mi) of progradation took place across a 1,829-m-deep (6,000 ft) basin. During stage II (Portlandian-Middle Berriasian), margin progradation limited to carbonate deposition decreased to 0.5 km (0.3 mi) and the relief between the platform top and basin floor increased to about 2,134 m (7,000 ft). The switch from progradation to aggradation in the Baltimore Canyon Trough occurred after platform exposure and after the supply of siliciclastics was cut off to the basin seaward of the platform. This variation in platform evolution is found also in equivalent-age platforms in the Sable Island Delta area of the Scotian Basin, but not in coeval platform sequences of the southern Scotian Basin and Lahave Platform. Platform "give-up"-type growth occurs as the last stage in the evolution of the Great Mesozoic Carbonate Bank in the Baltimore Canyon Trough. The cessation of growth is caused by platform drowning, not burial by siliciclastics. During stage III (Late Berriasian-Early Valanginian), more than a hundred meters of mud-dominated carbonates, rich in hexactinellid sponges and planktonic organisms, were deposited at the top of the carbonate bank. These deep-water carbonates succeeded shallow-water shelf carbonates, high-relief, shelf margin pinnacle reefs, and preceded a Valanginian siliciclastic sequence. Drowning resulted because of (1) high-amplitude tectono-eustatic fluctuations, (2) nutrient-suppressed carbonate deposition, and (3) volumetric constraints presented by a high-relief 2,440 m (8,000 ft) platform. Siliciclastic filling of basins seaward of carbonate platforms has several important implications. First, siliciclastic bypassing of active carbonate margins may be contemporaneous with carbonate deposition. Second, the volumetric contribution of siliciclastics to basin fill can be sufficiently large to allow progradation of platforms that are otherwise limited to aggradational growth by their sedimentary carbonate budget.

Abstract Flanks of carbonate platforms steepen as the platform rises higher above the basin floor. Furthermore, platform slopes are steeper on average than siliciclastic slopes. Termination of platform growth through rapid submergence or suffocation by siliciclastics produces an unconformity, because the clastics cannot assume the steep carbonate slope angle and because they are shed from different directions. This "drowning unconformity" resembles the unconformity produced by a lowstand of sea level, even though it is associated with a rise or a highstand of sea level. Examples of drowning unconformities include the mid-Cretaceous unconformity in the Gulf of Mexico, the unconformities on the flanks of the Wilmington platform, the Lahave platform, and the platforms off Morocco, all drowned in the earliest Cretaceous, and the mid-Jurassic unconformities on certain platforms of the High Atlas. Drowning unconformities are best developed on platforms that rise 800 m or more above the basin, have concave upper flanks of 6° or more, and commonly possess an elevated rim. Drowning and burial of smaller platforms with gentle flanks still produce unconformities, because the pattern of sediment input and dispersal is different for carbonates and siliciclastics.