Accumulation rates from well-dated late Neogene carbonate platforms and margins

Abstract

Margins of large carbonate platforms, isolated platforms, and atolls are dynamic settings that both generate and accumulate sediments. With the exceptions of modern platforms, the rates of accumulation are often poorly constrained due to inadequate or unreliable subsurface age data. A compilation is here presented of well-dated late Neogene “ice-house” platform and margin accumulation for the interval from ∼7 to 2 Ma. The rates are grouped according to platform type and setting. Sea-level effects and subsidence-bathymetry control are the significant influences. The six settings are: (1) slow-aggraded platform; (2) partially drowned platform; (3) fast-aggraded platform; (4) atolls; (5) prograded margin; and (6) periplatform. Mean rates are: 10 to 20 m/my for slow-aggraded and partially drowned platforms; 20–40 m/my for atolls and a fast-aggraded platform; ∼40 m/my for a current-dominated prograded margin; and 40–110 m/my for most prograded and periplatform systems. The maximum rates measured from the age-depth plot are generally two- to three-times as high. The mean accumulation rates and maximum rates are primarily related to the influence of sea level and, to a lesser extent, regional subsidence. The shallow platforms and atolls are directly sea-level controlled, whereas the deeper margins are sea-level influenced. The platform-top deposits have the lowest rates of accumulation (10–26 m/my) due to subaerial exposure and only intermittent flooding. The deeper platform margins, below lowstand base level, are influenced by events on the shallow platform, mainly to sediment production and offbank transport. Slope erosion and redeposition are also important. Prograded margins and periplatforms accept large volumes of allochthonous sediment and can have periods of extremely high (>200 m/my) vertical accumulation separated by intervals of non-deposition, erosion, or sediment bypass. These data, despite limitations, provide some basic values for ice-house carbonate accumulation and the relevant extrinsic controls. The late Neogene rates can help constrain the sediment dynamics of the carbonate platform and flank system, data especially important for calibration of computer models and for the resultant reservoir attributes.